Analysis of the Laser Propelled Lightcraft Vehicle

Analysis of the Laser Propelled
Lightcraft Vehicle
Douglas Feikema
Glenn Research Center, Cleveland, Ohio
Analysis of the Laser Propelled
Lightcraft Vehicle
NASA/TM—2000-210240
June 2000
AIAA–2000–2348
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National Aeronautics and
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Prepared for the
31st Plasmadynamics and Lasers Conference
sponsored by the American Institute of Aeronautics and Astronautics
Denver, Colorado, June 19–22, 2000
Douglas Feikema
Glenn Research Center, Cleveland, Ohio
Analysis of the Laser Propelled
Lightcraft Vehicle
NASA/TM—2000-210240 AIAA–2000–2348
Acknowledgments
The author gratefully acknowledges the financial support of NASA MSFC and the American Society of Engineering
Education (ASEE) under the 1998 and 1999 summer faculty fellowship program. The author also gratefully
acknowledges the assistance of Mr. Kevin Buch and Mr. Sandy Kirchendal during the summer of 1999.
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NASA/TM—2000-210240 1
American Institute of Aeronautics and Astronautics
AIAA–2000–2348
ANALYSIS OF THE LASER PROPELLED LIGHTCRAFT VEHICLE
Douglas Feikema
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
ABSTRACT
Advanced propulsion research and technology
require launch and space flight technologies, which can
drastically reduce mission costs. Laser propulsion is a
concept in which energy of a thrust producing reaction
mass is supplied via beamed energy from an off-board
power source. A variety of laser/beamed energy concepts
were theoretically and experimentally investigated since
the early 1970’s. During the 1980’s the Strategic
Defense Initiative (SDI) research lead to the invention of
the Laser Lightcraft concept. Based upon the Laser
Lightcraft concept, the U.S. Air Force and NASA have
jointly set out to develop technologies required for
launching small payloads into Low Earth Orbit (LEO)
for a cost of $1.0M or $1000/lb to $100/lb. The near
term objectives are to demonstrate technologies and
capabilities essential for a future earth to orbit launch
capability. Laser propulsion offers the advantages of
both high thrust and good specific impulse, Isp, in
excess of 1000 s. Other advantages are the simplicity
and reliability of the engine because of few moving
parts, simpler propellant feed system, and high specific
impulse. Major limitations of this approach are the laser
power available, absorption and distortion of the pulsed
laser beam through the atmosphere, and coupling laser
power into thrust throughout the flight envelope. The
objective of this paper is to assist efforts towards
optimizing the performance of the laser engine. In order
to accomplish this goal (1) defocusing of the primary
optic was investigated using optical ray tracing and (2),
time dependent calculations were conducted of the
optically induced blast wave to predict pressure and
temperature in the vicinity of the cowl. Defocusing of
the primary parabolic reflector causes blurring and
Copyright © 2000 by the American Institute of Aeronautics and
Astronautics, Inc. No copyright is asserted in the United States under
Title 17, U.S. Code. The U.S. Government has a royalty-free license
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Governmental Purposes. All other rights are reserved by the
copyright owner.
reduction in the intensity of the laser ignition site on
the cowl. However, because of the caustic effect of raytracing
optics the laser radiation still forms a welldefined
ignition line on the cowl. The blast wave calculations
show reasonable agreement with previously
published calculations and recent detailed CFD
computations.
NOMENCLATURE
C average plasma velocity
D* injector diameter
E laser energy deposited
h enthalpy
I laser intensity
ISP specific impulse
L plasma length
M molecular weight
P gas pressure
q heat addition from laser
r blast wave radius
RU universal gas constant
T gas temperature
t time
u* sonic gas injection velocity
V blast wave velocity
Z gas compressibility factor
Greek
ρ gas density
η percentage of laser energy absorbed
γ ratio of specific heats
Subscripts
CJ Chapman-Jouget Condition
LSD laser supported detonation
o initial or ambient state
p laser pulse
REF reference condition
2D two-dimensional
NASA/TM—2000-210240 2
American Institute of Aeronautics and Astronautics
INTRODUCTION
Advanced propulsion concepts for the 21st century
include the use of beamed energy such as a ground
based laser to provide thrust to a vehicle propelled by a
laser-induced blast wave.1,2 Jointly, the U.S. Air Force
and NASA have set out to develop technologies and
concepts required for launching small payloads into
Low Earth Orbit (LEO) for a total cost of $1.5M or
$1000/lb to $100/lb. This concept is shown
schematically in Fig. 1. Among the technology options
available, one potential high pay off approach is using a
high power pulsed laser as an off board energy source.3
Laser propulsion offers the advantages of both high
thrust and theoretically infinite specific impulse, Isp, at
altitudes less 20 km during the first stage and high
thrust and specific impulse in excess of 1000 s during
the second stage above 20 km. Other advantages are the
simplicity and reliability of the engine because of few
moving parts, simpler propellant feed system, and high
specific impulse. Major limitations of this approach are
the laser power available,4 absorption and distortion of
the pulsed laser beam through the atmosphere,5 and
coupling laser power into thrust throughout the flight
envelope.6
Successful operation of a pulsejet engine relies on
synchronous interplay of complex transient phenomena
and the introduction of new vehicle design concepts,
which integrate the total mission requirements.6-9,19 The
mission requirements include blast wave formation and
propagation, pulsed laser heating, inlet air
replenishment in air-breathing mode, first stage, and
propellant injection in rocket mode, second stage.
The concept of Laser Propulsion originates to at
least 1972 and is accredited to Kantrowitz.3 During the
1960’s after the invention of the laser, scientists
investigated the basic phenomenon of laser-induced
breakdown of gases and plasma ignition which forms
the fundamental basis of pulsed laser propulsion.12-15
During the remainder of the 1970’s, much attention was
directed towards conceptual design and basic research of
using beamed energy from a ground based laser to
assess the possibility and feasibility of laser energy for
rocket propulsion.16 Several of these studies involved
laboratory scale experimentation for proof of concept.
One of the most promising of these concepts at this
time appears to be the repetitively pulsed (RP) laser
concept.1,10-11 Typically, a laser pulse is fired into the
rear of a parabolic reflecting engine/nozzle where the
high-intensity radiation is sufficient to cause an
electrical breakdown of a small propellant volume
creating a high temperature, rapidly expanding plasma.
This plasma, which closely resembles a supersonic
blast wave, rapidly exits a supersonic nozzle to create
thrust.
Two methods for obtaining thrust using the RP
concept from laser radiation have been considered. The
first involves rapid vaporization of a solid propellant
with a first pulse followed by a second pulse, which
forms plasma at the surface, and the formation of a laser
supported detonation (LSD) wave. A second approach
involves focusing the laser intensity into a gas near a
solid surface in order to break down the gas and induce
a detonation wave transferring momentum to the
surface. Previous research has shown that ablation and
pitting of the surfaces occur which can strongly reduce
the efficiency of the laser absorption process. Raizer12
(1977) noted for high temperature air where
I = 105 MW/cm2, ρo = 1.3×10–3 g/cm2, and γ = 1.33
that VLSD = 133 km/s and Tequilibrium = 910,000 K.
These values were in reasonable agreement with
experiment, VLSD = 110 km/s and T = 700,000 K.
Therefore, future advances in pulsed detonation laser
propulsion will require mitigating the adverse effects of
material degradation in the regions of intense laser
radiation, which induces high plasma temperatures 12 in
the vicinity of the surface.
Pirri8 et al. (1974) reported on experiments
utilizing a solid and a gas propellant. They concluded
that the best way to achieve a high specific impulse is
to add laser energy directly to gas rather than to first
vaporize a solid propellant. In order to further improve
performance of the laser engine, a mixture of low
molecular weight propellant and a small percentage of
an easily ionizable seed (i.e., H2 or He plus Li or Cs) as
a seed to the gaseous propellant may provide
improvements. Such improvements would enable
shorter duration laser pulses of higher power, which
would enable high thrust times and higher specific
impulse.
In the past several studies have been conducted to
investigate various effects which include threshold
breakdown laser intensities above various surfaces in
air, the effect of air pressure and density on threshold
energies, and the vaporization of various solid
materials. Maher13 et al. (1973) reported that the
ignition thresholds for laser supported detonation waves
for selected materials were determined in air at one
atmosphere pressure. The type of solid reflecting
material has an important effect. For example, for
aluminum the measured threshold laser intensity is
59 J/cm2 and fused silica it is 310 J/cm2. Therefore,
careful selection of the solid reflecting surface requires
careful consideration.
Hettche15 et al. concluded that the impulse coupled
to the solid surface is controlled by the interaction
between the laser energy and air breakdown products at
the surface. They also noted from the pressure
measurements generated by laser fluxes of 1×108 to
3×108 W/cm2 the peak pressure generated varied from
~20 to 220 atmospheres. However, the measured
pressure relaxes rapidly behind the expanding plasma
front. Coupling of the pressure to the target surface after
the initial reaction is dependent on the complex
interaction between the opacity of the dispersing plasma
and the incident beam intensity.
NASA/TM—2000-210240 3
American Institute of Aeronautics and Astronautics
Currently, laser propelled Lightcraft vehicles10,11 are
being successfully flown in a series of experiments
conducted at the High Energy Laser Systems Test
Facility (HELSTF), White Sands Missile Range, New
Mexico. The axisymmetric Lightcraft vehicles are
propelled by airbreathing, pulsed-detonation engines
(PDE) with an infinite specific impulse. Spin-stabilized,
free-flight launches have been conducted and were
limited to altitudes approaching 30 m. High speed
Schlieren and shadow-graph pictures have shown that
the laser-induced shock wave wraps around the outer
perimeter of the engine’s shroud. Also, testing has
shown the shroud or cowl structure fails after 100 laser
firings into the thrust cavity.
The grand vision19 is to integrate the Lightcraft
concept into an earth-to-orbit vehicle. This concept
includes launching a 2-meter diameter Lightcraft vehicle
with initial mass of 100 kg into low earth orbit using a
100 MW laser. The Lightcraft vehicle operates in airbreathing
mode to about 20 km, then transitions into
rocket mode using an onboard inert propellant to a
vertical ascent of 100 km. At 100 km the vehicle turns
over and acceleration down range occurs at 5 to
6 degrees from horizontal. Burnout would occur at
~1000 km after ~15 mins from launch with an estimated
payload of 150 kg in orbit.
A cost estimate for a smaller system,4 however,
using a 20 MW laser, 20 kg orbital payload, and
150 kg of propellant would cost $500M to establish the
laser facility, telescope, adaptive optics, tracking, power
generating plant and the structural facility. The 20 MW
– 20 kg system was noted to be the smallest size that
would be cost effective. Larger systems gain linearly in
payload size versus laser power. It is perhaps best to
begin development of a lower power, low payload
system first in order to develop and demonstrate
technology to provide proof of concept before a
(i.e., 1 GW – 1000 kg) system is implemented.
THEORETICAL DESCRIPTION OF THE LASER
SUPPORTED DETONATION (LSD) WAVE
The basic phenomenon occurring inside the nozzle
is the ionization of gas, plasma ignition, and blast wave
propagation.12 During the 1960’s scientists
demonstrated that a focused laser beam from a high
power laser beam in stagnant gas produced electrical
breakdown and the propagation of a blast or detonation
wave.13-15 When the intense laser radiation was focused
next to a solid surface, impulse and momentum
exchange was observed and quantified.
The inverse bremsstrahlung effect (IB)12,17 is the
prime process assisting to achieve the very high plasma
temperatures (104 to 105 K) necessary for laser engine
operation and creation of high thrust. Initial ionization
of the plasma takes place in the gas as a result of
focusing the laser beam to higher laser intensities than
the threshold value, which generates free electrons.
When the plasma is ignited its unobstructed length18
can be estimated by:
L=2VLSDtp (1)
L is the initial length of the plasma, VLSD is the plasma
detonation velocity, and tp is the laser pulse duration.
In this plasma zone the laser beam energy is absorbed
into the gas and L is proportional to the electron mean
free path. The ionization process is faster and therefore
shorter than a single laser pulse. During the remainder
of the laser pulse free electrons are accelerated until the
laser pulse ends. The gas temperature rises, the plasma
length increases, which leads to expansion of the hot
plasma region. After the laser pulse terminates, a blast
wave front exists because of the high energy absorbed in
the plasma, causing further ionization of the cold gas
layers. Raizer17 noted that the majority of the laser
energy is already absorbed into the plasma rather than
in the gas, because the plasma ignited about
9 ns after firing the laser whereas the laser pulse
duration was 30 ns. Because L increases with
temperature and time, the location where high-speed
electrons come into contact with the cold gas front is
eventually shifted away from the focal point. When the
propagation distance of the plasma exceeds the outer
boundaries of the cold gas in the nozzle cavity, the
wave front simply evaporates into the vacuum outside
the gas boundaries.
The initial absorption wave or Laser Supported
Detonation (LSD) wave velocity12 is:
V
I
LSD
o
= −

 

 
2 2 1 2
1
3
(γ ) ()
ρ
where the maximum heat absorbed or energy release by
the wave is:
q I
oVLSD
= ρ (3)
and I is the laser intensity (W/m2), ρo is the initial gas
density, and γ is the ratio of specific heats. The
conservation of mass and energy applied across the
initial LSD wave can be combined to give:17
h
V
q h
LSD VLSD
0
2
0
2 2
2 2
4 + + = +

 

 
η ρ
ρ
( )
where h is the enthalpy and η is the percentage of laser
energy absorbed into the gas. Reilly et. al.21 note that
this conversion efficiency is typically 80 to 90 percent.
The pressure generated directly behind the Laser
Supported Detonation (LSD) wave front corresponds to
the upper Chapman-Jouget point17 and is given by:
NASA/TM—2000-210240 4
American Institute of Aeronautics and Astronautics
P
V
CJ
= LSD
+
ρ
γ 0
2
1
5
( )
( )
By applying the method of characteristics combined
with Raizer’s equations, the blast wave expansion
process can be described. For the case when a flat
surface or plate is placed at the focal point normal to the
axis of the laser beam the maximum pressure on the
surface of the plate is given by:19
P
V
LSD
=  + LSD
 

 
+
γ −
γ
ρ
γ
γ
1 γ
2 1
6
2
1 0
2
( )
( )
The self-similar solutions for the set of differential
equations for axisymmetric and isentropic conditions
were analyzed by Sedov.20 These solutions are used to
determine flow and thermodynamic variables scaled to a
reference point corresponding to a cylindrical blast wave
when the blast wave has evolved into a cylindrical
shape which corresponds to plasma ignition adjacent to
a flat plate. The unpowered Sedov scaling law’s used
for the present set of conditions for pressure behind the
blast wave and the radius of the wave are:
P
P
t
t
r
r
t
REF REF REF tREF
=


=


−1 12
; (7)
/
The reference conditions19 PREF is PLSD, tREF is t2D
21 or
the time for the blast wave to evolve into a completely
cylindrical expansion, and rREF corresponds to the radius
of the wave when wave can be considered completely
cylindrical, rLSD. Shortly after the laser pulse is
completed, a rarefaction wave moves in from the edges
of the blast wave front, according to the method of
characteristics this wave travels at an average plasma
velocity of CLSD
C
V P
LSD
= LSD=
2
8
γ
ρ
( )
When this rarefaction fan first arrives at the centerline of
the surface the blast wave geometry is considered to be
completely cylindrical such that:21
t
r
C D
LSD
LSD
2 = (9)
A simplistic thermodynamic relation for the plasma
expansion is assumed using an equation of state
incorporating the compressibility factor, Z, such that:19
T
C
Z R
M
LSD
u
=
2
10
γ
( )
where M is the gas molecular weight.
RESULTS AND DISCUSSION
Design A pproaches S uggested b y S pecific Impulse
Simmons and Pirri6 derived an analytic expression
for the specific impulse, Isp, of a repetitively pulsed laser
ignited thruster. The payload mass can be maximized
by maximizing the specific impulse:
I
u E
M t t D sp
p s
≈
−
( *) ( )
( ) *
( )
/ /
/ /
1 2 1 2
1 4 1 2 11
where u* is the gas sonic injection velocity in the
thrust cavity, E is the total laser energy deposited, D*
is the nozzle throat diameter, tp is the time between
pulses, M is the propellant molecular weight, and ts is
the ignition time of the blast wave. As noted by this
expression the payload mass fraction can be improved
by (1) Pulsing the laser at a high rate preferably close to
the same rate as the ignition time (i.e. < 1 μsec),
(2) Increasing the total laser energy deposited, (3) Using
low molecular weight propellants, (4) Injecting hot gas
instead of a cold gas during the second stage since u* is
higher, and (5) Injecting propellant through a small
throat diameter. They also noted an important difference
between this expression and that of a chemical rocket.
The specific impulse for a pulsed laser system favors a
low molecular weight propellant but not by as large a
margin as in a chemical rocket where Isp ~ (M)–1/2.
Therefore, the possibility of using heavier propellants
than in chemical systems opens the door for some
revolutionary new opportunities.
Optical R ay T racing: E ffect o f O ff-Axis D efocusing o f
Primary O ptic
One aspect in assessing the performance of the
lightcraft pulsed laser engine involves studying the
effects of defocusing the laser radiation caused by
misalignment of the laser beam with respect to the
primary optic. These effects will affect flight stability
and performance of the laser engine and are more likely
to occur as the distance between the vehicle and the
ground based laser increase because of the atmospheric
effects on the laser radiation.5
An optical ray-tracing program called code V
developed by Optical Research Associates, Inc. was
used to analyze the effectiveness of the lightcraft
parabolic reflector to focus laser light at a laser
wavelength of 10.6 μm with off axis focusing. Figure 2
shows a cross section of one optical set-up under
perfectly focused conditions. The optical system
consists of (1) the point source at 10 miles behind or to
the left of the lightcraft, (2) the aperture or diameter of
the vehicle, 4.4 in. (3) the parabolic reflector with a
focal distance of ~0.5 in. and (4) the cowl or image
surface. Figures 2 and 3 show the optical system
perfectly focused with traced rays in two and threedimensional
perspectives respectively. Only rays of
NASA/TM—2000-210240 5
American Institute of Aeronautics and Astronautics
light originating from the source, which enter through
the aperture or ring in Fig. 3, are reflected off the
100 percent reflective parabolic surface of revolution. All
rays terminate on the image plane or cowl, which is
shown in Fig. 3 as a bent, rectangular strip wrapped
around the axisymmetric parabolic reflector. In this case
the laser beam and the lightcraft are perfectly aligned.
The parabolic reflector is similar in size and shape to
the actual flight test vehicle recently conducted.10,11
Figure 4 shows the parabola used to construct the
primary optic surface for the optical ray-tracing program
results reported here. The equation for this relation in
units of inches is given by:
z(y)=1.9504y−0.4564y2 (12)
The intensity of laser radiation on the top half
perimeter of the cowl, 180°, is shown in Fig. 5 in
arbitrary units. This cowl surface has been flattened out
and the intensity of the laser radiation in arbitrary units
has been plotted as a function of height on this surface.
For the case of perfect alignment, the intensity is
constant along the hemisphere or length of the cowl and
a well-defined ignition line is observed. Figures 6 and 7
show the effect of rotating the lightcraft 5 degrees about
the x-axis. The intensity of the focused laser energy on
the cowl is shown in Fig. 7. The intensity of the
radiation is a factor of 4.5 less than the perfectly focused
case and nearly constant along the ignition line. It is
observed, however, that the ignition line is now curved
rather than being straight. Figure 8 shows the effect of
rotating the lightcraft 5 degrees about the y-axis. The
peak intensity in the center, which corresponds to the
top of the cowl in Fig. 3, is reduced by a factor of
1.4 from the intensity level shown in Fig. 5.
Significant blurring is observed with a high intensity
on the top of the cowl and a line of decreasing intensity
moving in either direction. Figure 9 shows the effect of
rotating the lightcraft both 5 degrees about the x-axis
and the y-axis. The intensity is blurred somewhat and
is reduced in magnitude by a factor of 4.8. The spatial
deviation of the focused laser intensity is observed to
have a 0.5-in. variation in the y-axis and a well-defined
ignition line is observed. This effect is known as the
caustic effect in ray tracing optics whereby the rays tend
to pile up onto a local region on the image plane. This
is a positive observation of this study, namely that a
well-defined ignition line of focused laser radiation is
observed even when the primary optic is defocused.
This implies that laser ignition will still occur under
defocusing conditions but since the laser intensity
varies along the perimeter of the cowl, the local
thermodynamic conditions will also vary along the
perimeter. This will induce multidimensional effects
associated with the gas expansion process.
Blast W ave C alculations o f the E xpanding P lasma
An analysis and computation using MATHCAD
and the NASA Glenn Chemical Equilibrium Code,
CEA, 1999 were used to determine pressure and
temperatures histories in the vicinity of the cowl. This
analysis is described in detail in Refs. 19, 20, and 21
and the preceding section. In the calculation reported
herein the following parameters were used in the model:
γ = 1.2, ρ0 = 1.2 kg/m3, M = 28.97, Compressibility
factor Z = 2.1, Laser pulse duration = 1×10–6 sec, size
of plasma at end of pulse |rLSD = 5 mm, focused laser
intensity I = 5×1011 Watts/m2. Prior to the beginning of
the laser pulse the air was assumed to be at room
temperature and pressure. The surface pressure history
on the cowl in the region of the focused laser radiation
is shown in Fig. 10. The peak surface pressure builds
up during the laser pulse to a peak pressure of about
250 atmospheres. The pressure decays very rapidly
at t–1 as the plasma expands, however, and
reaches atmospheric pressure after 350 μs or
250 nondimensional time units. The blast wave travels
a distance of 7.9 cm when the blast wave has
completely expanded.
In order to more accurately predict the initial state
of the plasma and the temperature and composition of
the gas, the NASA Glenn CEA code, 1999, was
utilized. The maximum heat absorbed by the gas due to
the laser flux is given by Eq. (3) to be 58.7 MJ/kg.
Initially the peak pressure directly behind the wave as
predicted by Eq. (5) is 271.8 ATM. Next, a T-P
calculation using the NASA CEA 1999 code with a
pressure of 271.8 ATM and an assigned temperature
was completed for atmospheric air. The temperature and
composition of the gas were determined by iteratively
solving Eq. (4) with η = 0.86 and the NASA CEA
code. A temperature of 17,400 K and pressure of 271.8
ATM gave good agreement between the computed
enthalpies (7.0×107 J/kg) using Eq. (4) and the NASA
CEA code assuming 86 percent of the laser energy is
absorbed by the gas. The mole fractions of the highly
ionized and dissociated gas as computed by the code are
shown in table I.
In order to model the temperature decay of the
expanding plasma a simple thermodynamic model is
used to predict the gas temperature using Eq. (10). For
a compressibility factor of Z = 2.1, M = 28.97, and
γ = 1.2 a temperature of 17,400 K is computed. Figure
11 shows the gas temperature decay (t-1) behind the
blast wave for a laser intensity of 5×1011 W/m2. Initially
the temperature in the vicinity of the focused laser beam
reaches a maximum of 17,400 Kelvin. The temperature
is observed to decay rapidly to about 1800 Kelvin in
10 t2D or 14 μs after the laser pulse.
Figure 12 shows the radius of the cylindrically
propagating LSD Wave from initiation of the wave at
t2D of 1.41×10–6 seconds from a radius of 5 mm to a
NASA/TM—2000-210240 6
American Institute of Aeronautics and Astronautics
final radius of 79 mm at 3.5×10–4 seconds at which
point in time the blast wave has dissipated to one
atmosphere of pressure. Figure 13 shows the blast
wave velocity decay from initially 7100 m/s
corresponding to a Mach number of 20. The wave
velocity is observed to decay rapidly at t–1/2.
Figure 14 shows the effect of ambient gas density or
altitude on the peak gas pressure for the conditions
described above for a laser intensity of 5×1011 W/m2. At
an altitude of 20 km where the density of the air is
~0.01 kg/m3 the peak surface pressure on the cowl has
decreased from 250 ATM at sea level to 50 ATM at
20 km.
Since the focused laser intensity is such an
important parameter in generating thrust and heating of
the gas, the effect of laser intensity on the peak surface
pressure and the peak equilibrium gas temperature is
shown in Figs. 15 and 16. In Fig. 15 it is shown that
the peak pressure can theoretically attain 10,000 ATM
at 1×1014 W/m2. Also, from Fig. 16 the theoretical peak
gas temperatures approaches 106 °K when the laser
intensity is 2×1014 W/m2.
CONCLUSION
1. Defocusing of the primary parabolic reflector causes
blurring and reduction in the intensity of the laser
radiation at the ignition site on the cowl. However,
because of the caustic effect of ray-tracing optics the
laser radiation still forms a well-defined ignition
line on the cowl. Under off axis focusing of 5°, the
ignition line becomes curved and reduced in
intensity by up to an order of magnitude in
intensity.
2. The blast wave calculations show reasonable
agreement with the values given in Ref. 19 and
recent CFD calculations.22 The pressure and
temperature decay rapidly and the decay process is
completed in 350 μs. The effected radius measured
from the ignition site is 7.9 cm.
REFERENCES
1. Glumb, R.J. and Krier, H.,” Concepts and Status of
Laser-Supported Rocket Propulsion”, J. Spacecraft,
Vol. 21, No. 1, pp. 70–79, 1984.
2. Birkan, M.A.,” Laser Propulsion: Research Status
and Needs”, Journal of Propulsion and Power,
Vol. 8, No. 2, pp. 354–360, March–April 1992.
3. Kantrowitz, A.,” Propulsion to Orbit by Ground
Based Lasers”, Astronautics and Aeronautics, Vol.
10, No. 5, pp. 74–76, May 1972.
4. Kare, J.T.,” Pulsed Laser Propulsion for Low Cost,
High Volume Launch to Orbit”, Space Power
Journal, Vol. 9, No. 1, pp. 6775, 1990.
5. Volkovitsky, O.A., Sedunov, Y.S., and Semenov,
L.P., P ropagation o f Intensive L aser R adiation in
Clouds , American Institute of Aeronautics and
Astronautics, 1992.
6. Simmons, G.A. and Pirri, A.N.,” The Fluid
Mechanics of Pulsed Laser Propulsion”, AIAA
Journal, Vol. 15, No. 6, pp. 835–842, June 1977.
7. Nebolsine, P.E., Pirri, A.N. Goela, J.S., and
Simmons, G.A.,” Pulsed Laser Propulsion”, AIAA
Journal, Vol. 19, No. 1, pp. 127–128, Jan. 1981.
8. Pirri, A.N., Monsler, M.J., and Nebolsine, P.E.,”
Propulsion by Absorption of Laser Radiation”,
AIAA Journal, Vol. 12, No. 9, pp. 1254–1261,
Sept. 1974.
9. Pirri, A.N.,” Analytic Solutions for Laser-Supported
Combustion Wave Ignition above Surfaces”, AIAA
Journal, Vol. 15, No. 1, pp. 83–91, Jan. 1977.
10. Mead Jr., F.B., Myrabo, L.K., and Messitt,
D.G.,”Flight and Ground Tests of a Laser-Boosted
Vehicle, 34th AIAA/ASME/SAE/ASEE Joint
Propulsion Conference and Exhibit, Cleveland, OH,
AIAA 98–3735, July 13–15, 1998.
11. Myrabo, L.N. and Messitt, D.G., and Mead Jr.,
F.B.,” Ground and Flight Tests of a Laser Propelled
Vehicle”, 36th Aerospace Sciences Meeting and
Exhibit, Reno, Nevada, AIAA 98–1001, Jan. 12–15,
1998.
12. Raizer, I.P., L aser-Induced D ischarge P henomen a ,
Plenum Publishing Corporation, New York, NY,
pp. 189–268, 1977.
13. Maher, W.E., Hall, R.B., and Johnson, R.R.,”
Experimental Study of Ignition and Propagation of
LSD Waves”, Journal of Applied Physics, Vol. 45,
pp. 2138–2145, May 1974.
14. Pirri, A.N., Schlier, R., and Northam, D.,”
Momentum Transfer and Plasma Formation above a
Surface with a High-Power CO2 Laser”, Applied
Physics Letters, pp. 79–81, August 1972.
15. Hettche, L.R., Schriempf, J.T., and Stegman,
R.L.,”Impulse Reaction Resulting from the in-air
Irradiation of Aluminum by a Pulsed CO2 Laser”,
J. Appl. Physics, Vol. 44, No. 9, pp. 4079–4085,
Sept. 1973.
16. Caveny, L.H., Orbit Raising and Maneuvering
Propulsion: Research Status and Needs, Vol. 89,
Progress in Astronautics and Aeronautics, AIAA,
New York, NY, 1984.
17. Raizer, I.P.,” Breakdown and Heating of Gases
Under the Influence of a Laser Beam”, Soviet Physics
Uspekhi, Vol. 8, No.5, pp. 650–673, March–April
1966.
18. Brandstein, A. and Levy, Y.,” Laser Propulsion
System for Space Vehicles”, Journal of Propulsion
and Power, Vol. 14, No. 2, pp. 261–269,
March–April 1998.
NASA/TM—2000-210240 7
American Institute of Aeronautics and Astronautics
19. Myrabo, L.N., “Transatmospheric Laser
Propulsion: Lightcraft Technology Demonstrator”,
Final Technical Report, SDIO Laser Propulsion
Program, Contract No. 2073803, pp. 117–142,
1989.
20. Sedov, L.L., S imilarity a nd D imensional M ethods
in M echanic s , Fourth Russian Ed., Academic Press,
New York, NY, 1959
21. Reilly, J.P., Ballantyne, A., Woodroffe, J.A.,
“Modeling of Momentum Transfer to a Surface by
Laser-Supported Absorption Waves,” AIAA Journal,
Vol. 17, No. 10, p. 1098–1105, 1979.
22. Personal Communication.
TABLE 1—Initial State of the Laser Ignited Air Plasma as
Computed by NASA CEA, 1999, Equilibrium Air Mole
Fractions at 271.8 ATM and 17,400 K.
*e- 1.0830-1 *N+ 9.1863-2
*Ar 3.3503-3 *N- 7.4395-4
*Ar+ 8.5684-4 NCO 3.782-11
*C 1.0079-4 *NO 3.6288-4
*C+ 4.1572-5 *NO+ 1.8947-4
*C- 1.8139-7 NO2 1.0435-8
*CN 4.5023-7 NO2- 1.307-11
*CN+ 1.0521-7 NO3 1.380-14
CN- 7.833-10 *N2 1.5659-3
CNN 4.311-11 *N2+ 5.6956-4
*CO 1.5655-7 N2- 2.1197-6
*CO+ 4.9732-8 NCN 9.153-11
*CO2 8.008-12 N2O 2.7163-8
*CO2+ 6.324-12 N2O+ 4.1142-8
*C2 3.579-11 N3 4.7608-8
*C2+ 2.597-11 *O 1.7189-1
C2- 1.130-13 *O+ 1.5759-2
CCN 2.694-14 *O- 2.4471-4
CNC 3.885-14 *O2 1.5923-5
C2O 1.534-14 *O2+ 8.6215-6
*N 6.0414-1 O2- 3.8408-8
O3 4.5254-9
Figure 1.—The Laser Lightcraft Vehicle Propulsion Concept.
NASA/TM—2000-210240 8
American Institute of Aeronautics and Astronautics
Figure 2.—Cross Sectional view of one Code V Optical System
of Lightcraft Parabolic Reflector showing Ray Tracing
for a perfectly focused laser beam.
Figure 3.—Code V Optical System of Lightcraft Parabolic Reflector
showing Ray Tracing for a perfectly focused laser beam.
NASA/TM—2000-210240 9
American Institute of Aeronautics and Astronautics
Figure 4.—Plot of Parabola used for Primary Focusing Optic.
Figure 5.—Laser Intensity in Arbitrary units along upper half, 180°,
of Cowl of the focused laser radiation for Figure 3. Near Constant Intensity.
Spatial Dimensions of Focused Projected Area (Black):
X: 5 inches, Y: 0.25 inches.
Figure 6.—Code V Optical System of Lightcraft Parabolic Reflector
showing Ray Tracing for a Rotation of 5 degrees about X-axis.
z(y)
y
0 0.55 1.1 1.65 2.2
0
0.5
1
1.5
2
2.5
Parabola for Primary Optic
Distance from Central Axis (inches)
Distance (inches)
NASA/TM—2000-210240 10
American Institute of Aeronautics and Astronautics
Figure 7.—Five Degree Rotation about the x-axis only. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Near Constant Intensity. Spatial Dimensions: X: 5 inches,
Y: 0.25 inches. Peak intensity decreased by a factor of 4.5 from perfectly focused case,
well formed ignition line but curved. The intensity is near constant along the arc.
Figure 8.—Five Degree rotation about the y-axis only. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Peak Intensity decreased by a factor of 1.4 in the
center from perfectly focused case. Spatial Dimensions: X: 5 inches, Y: 0.25 inches.
Figure 9.—Five Degree rotation about both the x and y axes. Laser Intensity in arbitrary units of the
Focused laser radiation on the Cowl. Peak intensity decreased by a factor of 4.8 from
perfectly focused case, well formed ignition line but curved.
Spatial Dimensions: X: 5 inches, Y: 0.5 inches.
NASA/TM—2000-210240 11
American Institute of Aeronautics and Astronautics
Figure 10.—Surface Pressure History of Blast Wave For Laser Intensity of 5×1011 W/m2.
Figure 11.—Equilibrium Temperature behind Shock Wave Decay
as a function of time for Laser Intensity of
f
5×1011 W/m2.
P(t)
t
t2D
1 10 100
0
50
100
150
200
250
Surface Pressure History of LSD Wave
Non-Dimensional Time
Surface Pressure (ATM)
T(t)
t
t2D
1 10 100
0
1800
3600
5400
7200
9000
1.08 10
4
1.26 10
4
1.44 10
4
1.62 10
4
1.8 10
4 Temperature Behind Wave
Non-Dimensional Time
Temperature (Kelvin)
NASA/TM—2000-210240 12
American Institute of Aeronautics and Astronautics
Figure 12.—Shock Wave Radius as a function of time for
Laser Intensity of 5×1011 W/m2.
Figure 13.—Shock Wave Velocity as a function of time for
Laser Intensity of 5×1011 W/m2.
VS(t)
t
t2D
1 10 100 1 10
3
0
750
1500
2250
3000
3750
4500
5250
6000
6750
7500
Velocity of LSD Wave
Non-Dimensional Time
Blast Wave Velocity (m/s)
radius(t)
t
t2D
1 10 100 1 10
3
0
0.008
0.016
0.024
0.032
0.04
0.048
0.056
0.064
0.072
0.08
LSD Wave Radius Versus Time
Non-Dimensional Time
LSD Wave Radius (Meters)
NASA/TM—2000-210240 13
American Institute of Aeronautics and Astronautics
Figure 14.—Effect of Gas Density or Altitude on the Peak Surface Pressure
for a Laser Intensity of 5×1011 W/m2.
Figure 15.—Effect of Focused Laser Intensity on Peak Surface Pressure,
ρ0 = 1.2 kg/m3.
PLSDATM (ρ)
ρ
0 0.5 1 1.5 2
0
50
100
150
200
250
300
Effect of Gas Density on Peak Pressure
Gas Density (kg/m3)
Peak Gas Pressure (ATM)
PLSDATM (I)
I
1 10
9
1 10
10
1 10
11
1 10
12
1 10
13
1 10
14
1 10
15
1
10
100
1 10
3
1 10
4
1 10
5
Effect of Laser Focused Intensity
Laser Intensity (Watts/m2)
Peak Surface Pressure (ATM)
NASA/TM—2000-210240 14
American Institute of Aeronautics and Astronautics
Figure 16.—Effect of Focused Laser Intensity on Peak Equilibrium Gas Temperature,
ρ0 = 1.2 kg/m3.
TLSD(I)
I
1 10
9
1 10
10
1 10
11
1 10
12
1 10
13
1 10
14
1 10
15
100
1 10
3
1 10
4
1 10
5
1 10
6
1 10
7 Effect of Laser Intensity on Peak Temp.
Laser Intensity (W/m2)
Peak Equilibrium Plasma Temperature (K)
This publication is available from the NASA Center for AeroSpace Information, (301) 621–0390.
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20
A03
Analysis of the Laser Propelled Lightcraft Vehicle
Douglas Feikema
Propulsion; Future flight
Unclassified - Unlimited
Subject Categories: 20, 15, and 36 Distribution: Nonstandard
Prepared for the 31st Plasmadynamics and Lasers Conference sponsored by the American Institute of Aeronautics and
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(216) 433–5707.
Advanced propulsion research and technology require launch and space flight technologies, which can drastically reduce mission costs. Laser
propulsion is a concept in which energy of a thrust producing reaction mass is supplied via beamed energy from an off-board power source. A variety of
laser/beamed energy concepts were theoretically and experimentally investigated since the early 1970’s. During the 1980’s the Strategic Defense
Initiative (SDI) research lead to the invention of the Laser Lightcraft concept. Based upon the Laser Lightcraft concept, the U.S. Air Force and NASA
have jointly set out to develop technologies required for launching small payloads into Low Earth Orbit (LEO) for a cost of $1.0M or $1000/lb to
$100/lb. The near term objectives are to demonstrate technologies and capabilities essential for a future earth to orbit launch capability. Laser propulsion
offers the advantages of both high thrust and good specific impulse, Isp, in excess of 1000 s. Other advantages are the simplicity and reliability of
the engine because of few moving parts, simpler propellant feed system, and high specific impulse. Major limitations of this approach are the laser
power available, absorption and distortion of the pulsed laser beam through the atmosphere, and coupling laser power into thrust throughout the flight
envelope. The objective of this paper is to assist efforts towards optimizing the performance of the laser engine. In order to accomplish this goal
(1) defocusing of the primary optic was investigated using optical ray tracing and (2), time dependent calculations were conducted of the optically
induced blast wave to predict pressure and temperature in the vicinity of the cowl. Defocusing of the primary parabolic reflector causes blurring and
reduction in the intensity of the laser ignition site on the cowl. However, because of the caustic effect of ray-tracing optics the laser radiation still forms
a well-defined ignition line on the cowl. The blast wave calculations show reasonable agreement with previously published calculations and recent
detailed CFD computations.

4G Wireless Systems

4G Wireless Systems
Next-Generation Wireless Working Group
Jawwad Ahmad, Ben Garrison, Jim Gruen, Chris Kelly, and Hunter Pankey
May 2, 2003
4G Wireless Systems 1
Contents
1 Introduction 3
2 Economic Impact 3
2.1 Advantages of 4G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Problems with the Current System . . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Barriers to Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Wireless Security 6
3.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Stakeholders in Wireless Security . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Information Security Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Wireless Security Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5 Security Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.2 Threats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.3 Security Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4 Current Technology 10
4.1 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5 4G Hardware 13
5.1 Ultra Wide Band Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2 Smart Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6 4G Software 15
6.1 Software Defined Radio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2 Packet Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3 Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3.1 Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3.2 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.4 Implementation of Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.4.1 Current System: IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.4.2 Recommended System: IPv6 . . . . . . . . . . . . . . . . . . . . . . . 19
6.4.3 Voice over IP (VoIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.5 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.6 Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.7 Anti-Virus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7 Conclusion 21
4G Wireless Systems 2
List of Figures
1 Cellular Provider System Upgrades . . . . . . . . . . . . . . . . . . . . . . . 4
2 Information Security Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 Time Division Multiple Access . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Sending Data using Code Division Multiple Access . . . . . . . . . . . . . . 12
5 Receiving Data using Code Division Multiple Access . . . . . . . . . . . . . 12
6 UWB Spectrum Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7 Switched Beam Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8 Adaptive Array Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9 Packet with 896-bit payload . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
List of Tables
1 Cellular Providers and Services . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 The Cellular Industry as a Game . . . . . . . . . . . . . . . . . . . . . . . . 5
4G Wireless Systems 3
1 Introduction
Consumers demand more from their technology. Whether it be a television, cellular phone,
or refrigerator, the latest technology purchase must have new features. With the advent
of the Internet, the most-wanted feature is better, faster access to information. Cellular
subscribers pay extra on top of their basic bills for such features as instant messaging, stock
quotes, and even Internet access right on their phones. But that is far from the limit of
features; manufacturers entice customers to buy new phones with photo and even video
capability. It is no longer a quantum leap to envision a time when access to all necessary
information — the power of a personal computer — sits in the palm of one’s hand. To
support such a powerful system, we need pervasive, high-speed wireless connectivity.
A number of technologies currently exist to provide users with high-speed digital wireless
connectivity; Bluetooth and 802.11 are examples. These two standards provide very highspeed
network connections over short distances, typically in the tens of meters. Meanwhile,
cellular providers seek to increase speed on their long-range wireless networks. The goal
is the same: long-range, high-speed wireless, which for the purposes of this report will be
called 4G, for fourth-generation wireless system. Such a system does not yet exist, nor will
it exist in today’s market without standardization. Fourth-generation wireless needs to be
standardized throughout the United States due to its enticing advantages to both users and
providers.
2 Economic Impact
2.1 Advantages of 4G
In a fourth-generation wireless system, cellular providers have the opportunity to offer data
access to a wide variety of devices. The cellular network would become a data network on
which cellular phones could operate — as well as any other data device. Sending data over
the cell phone network is a lucrative business. In the information age, access to data is the
“killer app” that drives the market. The most telling example is growth of the Internet over
the last 10 years. Wireless networks provide a unique twist to this product: mobility. This
concept is already beginning a revolution in wireless networking, with instant access to the
Internet from anywhere.
2.2 Problems with the Current System
One may then wonder why ubiquitous, high-speed wireless is not already available. After
all, wireless providers are already moving in the direction of expanding the bandwidth of
their cellular networks. Almost all of the major cell phone networks already provide data
services beyond that offered in standard cell phones, as illustrated in Table 1.
Unfortunately, the current cellular network does not have the available bandwidth necessary
to handle data services well. Not only is data transfer slow — at the speed of analog
modems — but the bandwidth that is available is not allocated efficiently for data. Data
transfer tends to come in bursts rather than in the constant stream of voice data. Cellular
providers are continuing to upgrade their networks in order to meet this higher demand by
4G Wireless Systems 4
Table 1: Cellular Providers and Services
Cellular provider Features
Sprint e-mail, pictures, games, music, Internet
AT&T e-mail, games, music
Verizon e-mail, pictures, games, music, Internet
Nextel e-mail, pictures, games, music, Internet
T-Mobile (VoiceStream) e-mail, pictures, games, music, Internet
Cingular text messaging
Figure 1: Cellular Provider System Upgrades
switching to different protocols that allow for faster access speeds and more efficient transfers.
These are collectively referred to as third generation, or 3G, services. However, the
way in which the companies are developing their networks is problematic — all are currently
proceeding in different directions with their technology improvements. Figure 1 illustrates
the different technologies that are currently in use, and which technologies the providers plan
to use.
Although most technologies are similar, they are not all using the same protocol. In
addition, 3G systems still have inherent flaws. They are not well-designed for data; they are
improvements on a protocol that was originally designed for voice. Thus, they are inefficient
with their use of the available spectrum bandwidth. A data-centered protocol is needed.
If one were to create two identical marketplaces in which cellular providers used 3G and
4G respectively, the improvements in 4G would be easy to see. Speaking on the topic of 3G,
one of the worlds leading authorities on mobile communications, William C.Y. Lee, states
4G Wireless Systems 5
Table 2: The Cellular Industry as a Game
Players The cellular providers
Strategies Upgrade to 4G, or make small incremental changes
Outcome This can be simplified to the cost of conversion. The cost
of conversion (because of economies of scale), depends on
the number of companies that actually convert to 4G —
networking equipment and wireless access equipment will
get cheaper as more of them are produced and bought by
the cellular providers.
that 3G would be “a patched up system that could be inefficient”, and it would be best if
the industry would leapfrog over 3G wireless technology, and prepare for 4G (Christian ).
4G protocols use spectrum up to 3 times as efficiently as 3G systems, have better ways of
handling dynamic load changes (such as additional cellular users entering a particular cell),
and create more bandwidth than 3G systems. Most importantly, fourth-generation systems
will draw more users by using standard network protocols, which will be discussed later, to
connect to the Internet. This will allow simple and transparent connectivity.
2.3 Barriers to Progress
This begs the question: Why are cellular providers not moving to 4G instead of 3G? A
marketplace like the cellular industry can be modeled as a game, as seen in Table 2.
There are three basic paths the game can take:
Nobody makes the conversion to 4G All end up upgrading to 2.5G and 3G services.
The upgrades are incremental, and don’t require a complete reworking of the system,
so they are fairly cheap — the equipment required is already developed and in mass
production in other places in the world.
Everyone makes the conversion to 4G The equipment and technology needed for 4G
will be cheap, because of all of the cellular manufacturers investing in it. Cellular
providers will market additional services to its customers.
Some of the players make the conversion to 4G Because not all of the players have
chosen 4G, the equipment will be more expensive than the second scenario. Even
though converters will be able to sell more services to their customers, it will not be
enough to cover the higher costs of converting to 4G.
Therefore, if a player chooses the 4G strategy, but nobody else follows suit, that player
will be at a significant disadvantage. No cellular provider has incentive to move to 4G unless
all providers move to 4G. An outside agent — the national government — must standardize
on 4G as the wireless standard for the United States.
Of course, legitimate concerns can be posed to the idea of implementing 4G nationwide.
A common concern is the similarity of this proposal to the forced introduction of HDTV
4G Wireless Systems 6
in the US, which has (thus far) failed miserably. There are two key differences, however,
between 4G and HDTV. The first is the nature of the service providers. There are many
small television broadcasters in rural areas whose cost of conversion would be as much as 15
years of revenue. The cellular industry, however, does not have this problem. The players are
multi-billion dollar companies, who already have enough capital; continual network upgrades
are part of their business plan. Our proposal is simply choosing a direction for their growth.
An often overlooked area of financial liability for cellular providers is in the area of
information security. Providers could lose money through fraudulent use of the cellular
system or unauthorized disclosure of user information over the airwaves. Both of these cases
could be caused by an insecure wireless system. This lesson was learned during the use of the
first generation of cellular phones in the United States: If a standard is to be set nationwide,
it must be secure.
3 Wireless Security
3.1 History
The original cellular phone network in the United States was called the Analog Mobile
Phone System (AMPS). It was developed by AT&T and launched in 1983. AMPS operated
in the 800 MHz range, from 824-849 MHz and 869-894 MHz. The lower band was used for
transmissions from the phone to the base station, and the upper band was for the reverse
direction (Leon-Garcia and Widjaja 2000). This allows full duplex conversation, which is
desirable for voice communications. The bands were divided into 832 subchannels, and each
connection required a pair: one each for sending and receiving data. Each subchannel was
30 KHz wide, which yielded voice quality comparable to wired telephones. The subchannels
were set up so that every subchannel pair was exactly 45 MHz apart (Leon-Garcia and
Widjaja 2000). Several of the channels were reserved exclusively for connection setup and
teardown. The base station in a particular cell kept a record of which voice subchannel pairs
were in use.
Though usable, this system included a number of security flaws. Because each phone
transmitted (like any radio transmitter) in the clear on its own frequency, the phones in this
system “were almost comically vulnerable to security attacks” (Riezenman 2000, 40). The
crime of service theft plagued cellular service providers, as individuals with radio scanners
could “sniff” the cellular frequencies and obtain the phone identification numbers necessary
to “clone” a phone (Riezenman 2000, 39). The abuser could then use this cloned phone
to make free telephone calls that would be charged to the legitimate user’s account. In an
attempt to stem these attacks, service providers worked with Congress to punish such abuse.
Congress passed a law in 1998 to make owning a cellular scanner with intent to defraud a
federal crime (Riezenman 2000, 40). Unfortunately, punitive legislation was not enough to
solve the problem; a new standard was needed. To create a new standard, engineers needed
to start anew, examining each part of the current system.
4G Wireless Systems 7
3.2 Stakeholders in Wireless Security
In attempting to avoid security problems like those that plagued the first-generation cellular
systems, engineers must design security into any new technology it cannot be added as an
afterthought. Unfortunately, this is no easy task. Implementing good security requires that
security be designed into every aspect of the system; otherwise, a security leak exists. Thus,
the following entities must cooperate to create the secure wireless system:
• Government regulator
• Network infrastructure provider
• Wireless service provider
• Wireless equipment provider
• Wireless user (Russell 2001, 172)
3.3 Information Security Model
Before seeking to design and implement wireless security, however, one first needs to understand
what this elusive concept of security really means. In this case, wireless security is
really a combination of wireless channel security (security of the radio transmission) and network
security (security of the wired network through which the data flows). These collectively
can be referred to as “wireless network security” (Russell 2001, 173). But this still does not
explain the security aspect. In a digital realm, security almost always means “information
security.” Therefore, we can use the information security model proposed by the National
Security Telecommunications and Information Systems Security Committee (NSTISSC), as
seen in Figure 2: Along the top edge of the cube are the three states information can be in,
while the rows on the left side of the cube are the information characteristics that the security
policy should provide. The columns on the right side of the cube detail the three broad
categories of security measures that can be pursued to protect the information. The cube
is thus split into 27 smaller cubes, each of which must be examined for risks and solutions
in any extensive security audit. This document, on the other hand, is not meant to contain
such an audit, but rather to present the major issues of wireless security, the objectives of
future wireless technology, and the security measures needed to reach those goals.
3.4 Wireless Security Issues
Wireless systems face a number of security challenges, one of which comes from interference.
As more wireless devices begin to use the same section of electromagnetic spectrum, the
possibility of interference increases. This can result in a loss of signal for users. Moreover, an
abuser can intentionally mount a denial-of-service attack (lowering availability) by jamming
the frequencies used. Iowa State University professor Steve Russell comments that “an RF
engineer using $50 worth of readily-available components can build a simple short-range
jammer for any of the common microwave frequencies” (Russell 2001, 174).
4G Wireless Systems 8
Figure 2: Information Security Model
Physical security can pose problems as well. Cellular phones and other handheld devices
were designed to be small and mobile, but this also means that they are more likely than other
pieces of technology to get lost or stolen, and thieves can easily conceal them. Because of
their size, these devices often have extremely limited computing power. This could manifest
itself in lower levels in the encryption that protects the information (NIST, U.S. Dept. of
Commerce , 5-26). As encryption is improved in the same device, speed is consequently
lowered, as is available bandwidth (Russell 2001, 174).
Other software issues can open security holes as well. For example, many handheld
wireless devices include the ability to download and run programs, some of which may not
be trustworthy. Even the core operating system software may not be secure; engineers may
have rushed to release it in order to offer new features in the competitive handheld device
market. Perhaps most damaging, the users typically lack awareness that any of these security
issues may be present in their wireless handheld device (NIST, U.S. Dept. of Commerce ,
5-27).
These security issues serve as a reminder that designing for security is never a finished
process. Every new technology must be analyzed for security issues before it is fully implemented.
Even then, one must keep a careful eye on any new issues that may develop.
4G Wireless Systems 9
3.5 Security Analysis
3.5.1 Objectives
The first step in analyzing cellular wireless security is to identify the security objectives.
These are the goals that the security policy and corresponding technology should achieve.
Howard, Walker, and Wright, of the British company Vodafone, created objectives for 3G
wireless that are applicable to 4G as well:
• To ensure that information generated by or relating to a user is adequately protected
against misuse or misappropriation.
• To ensure that the resources and services provided to users are adequately protected
against misuse or misappropriation.
• To ensure that the security features are compatible with world-wide availability...
• To ensure that the security features are adequately standardized to ensure world-wide
interoperability and roaming between different providers.
• To ensure that the level of protection afforded to users and providers of services is considered
to be better than that provided in contemporary fixed and mobile networks...
• To ensure that the implementation of security features and mechanisms can be extended
and enhanced as required by new threats and services.
• To ensure that security features enable new ‘e-commerce’ services and other advanced
applications(Howard, Walker, and Wright 2001, 22)
These goals will help to direct security efforts, especially when the system is faced with
specific threats.
3.5.2 Threats
Because instances of 4G wireless systems currently only exist in a few laboratories, it is
difficult to know exactly what security threats may be present in the future. However, one
can still extrapolate based on past experience in wired network technology and wireless
transmission. For instance, as mobile handheld devices become more complex, new layers
of technological abstraction will be added. Thus, while lower layers may be fairly secure,
software at a higher layer may introduce vulnerabilities, or vice-versa. Future cellular wireless
devices will be known for their software applications, which will provide innovative new
features to the user. Unfortunately, these applications will likely introduce new security
holes, leading to more attacks on the application level (Howard, Walker, and Wright 2001,
22). Just as attacks over the Internet may currently take advantage of flaws in applications
like Internet Explorer, so too may attacks in the future take advantage of popular applications
on cellular phones. In addition, the aforementioned radio jammers may be adapted to use
IP technology to masquerade as legitimate network devices. However, this would be an
extremely complex endeavor. The greatest risk comes from the application layer, either
from faulty applications themselves or viruses downloaded from the network.
4G Wireless Systems 10
3.5.3 Security Architecture
The above topics merely comprise a brief overview of some of the issues involved in wireless
handheld device security. They by no means define a complete security solution for 4G
wireless security. Rather, these topics serve as examples of some of the more prominent
security problems that currently exist or may exist in future wireless systems. A more
thorough security analysis is needed before a 4G wireless system can be implemented. This
should lead to a 4G security architecture that is:
Complete The architecture should address all threats to the security objectives. Unfortunately,
it may be difficult to avoid missing some features when there are so many
independent parts of the 4G system.
Efficient Security functionality duplication should be kept to a minimum. Again, this may
be difficult given the number of independent functions.
Effective Security features should achieve their purpose. However, some security features
may open up new security holes.
Extensible Security should be upgradeable in a systematic way.
User-friendly End users should have to learn as little about security as possible. Security
should be transparent to the user; when interaction must be involved, it should be easy
to understand (Howard, Walker, and Wright 2001, 26).
These objectives were taken into account when the current generation of cellular technology
was designed. This generation, referred to as 2G, has worked well; though it is showing its
age, it is still in use.
4 Current Technology
Most modern cellular phones are based on one of two transmission technologies: time-division
multiple access (TDMA) or code-division multiple access (CDMA) (Riezenman 2000, 40).
These two technologies are collectively referred to as second-generation, or 2G. Both systems
make eavesdropping more difficult by digitally encoding the voice data and compressing it,
then splitting up the resulting data into chunks upon transmission.
4.1 TDMA
TDMA, or Time Division Multiple Access, is a technique for dividing the time domain up into
subchannels for use by multiple devices. Each device gets a single time slot in a procession
of devices on the network, as seen in Figure 3. During that particular time slot, one device
is allowed to utilize the entire bandwidth of the spectrum, and every other device is in the
quiescent state.
The time is divided into frames in which each device on the network gets one timeslot.
There are n timeslots in each frame, one each for n devices on the network. In practice,
every device gets a timeslot in every frame. This makes the frame setup simpler and more
4G Wireless Systems 11
Figure 3: Time Division Multiple Access
efficient because there is no time wasted on setting up the order of transmission. This has
the negative side effect of wasting bandwidth and capacity on devices that have nothing to
send (Leon-Garcia and Widjaja 2000).
One optimization that makes TDMA much more efficient is the addition of a registration
period at the beginning of the frame. During this period, each device indicates how much
data it has to send. Through this registration period, devices with nothing to send waste
no time by having a timeslot allocated to them, and devices with lots of pending data can
have extra time with which to send it. This is called ETDMA (Extended TDMA) and can
increase the efficiency of TDMA to ten times the capacity of the original analog cellular
phone network.
The benefit of using TDMA with this optimization for network access comes when data is
“bursty.” That means, at an arbitrary time, it is not possible to predict the rate or amount
of pending data from a particular host. This type of data is seen often in voice transmission,
where the rate of speech, the volume of speech, and the amount of background noise are
constantly varying. Thus, for this type of data, very little capacity is wasted by excessive
allocation.
4.2 CDMA
CDMA, or Code Division Multiple Access, allows every device in a cell to transmit over the
entire bandwidth at all times. Each mobile device has a unique and orthogonal code that is
used to encode and recover the signal (Leon-Garcia and Widjaja 2000). The mobile phone
digitizes the voice data as it is received, and encodes the data with the unique code for that
phone. This is accomplished by taking each bit of the signal and multiplying it by all bits
in the unique code for the phone. Thus, one data bit is transformed into a sequence of bits
of the same length as the code for the mobile phone. This makes it possible to combine
with other signals on the same frequency range and still recover the original signal from an
arbitrary mobile phone as long as the code for that phone is known. Once encoded, the data
is modulated for transmission over the bandwidth allocated for that transmission. A block
diagram of the process is shown in Figure 4.
4G Wireless Systems 12
Figure 4: Sending Data using Code Division Multiple Access
Figure 5: Receiving Data using Code Division Multiple Access
4G Wireless Systems 13
Figure 6: UWB Spectrum Usage
The process for receiving a signal is shown in Figure 5. Once the signal is demodulated, a
correlator and integrator pair recovers the signal based on the unique code from the cellular
phone. The correlator recovers the original encoded signal for the device, and the integrator
transforms the recovered signal into the actual data stream.
CDMA has been patented in the United States by Qualcomm, making it more expensive
to implement due to royalty fees. This has been a factor for cellular phone providers when
choosing which system to implement.
By keeping security in mind while designing the new system, the creators of 2G wireless
were able to produce a usable system that is still in use today. Unfortunately, 2G technology
is beginning to feel its age. Consumers now demand more features, which in turn require
higher data rates than 2G can handle. A new system is needed that merges voice and data
into the same digital stream, conserving bandwidth to enable fast data access. By using
advanced hardware and software at both ends of the transmission, 4G is the answer to this
problem.
5 4G Hardware
5.1 Ultra Wide Band Networks
Ultra Wideband technology, or UWB, is an advanced transmission technology that can be
used in the implementation of a 4G network. The secret to UWB is that it is typically
detected as noise. This highly specific kind of noise does not cause interference with current
radio frequency devices, but can be decoded by another device that recognizes UWB and
can reassemble it back into a signal. Since the signal is disguised as noise, it can use any
part of the frequency spectrum, which means that it can use frequencies that are currently
in use by other radio frequency devices (Cravotta ).
An Ultra Wideband device works by emitting a series of short, low powered electrical
pulses that are not directed at one particular frequency but rather are spread across the entire
spectrum (Butcher ). As seen in Figure 6, Ultra Wideband uses a frequency of between 3.1
to 10.6 GHz.
The pulse can be called “shaped noise” because it is not flat, but curves across the
spectrum. On the other hand, actual noise would look the same across a range of frequencies
— it has no shape. For this reason, regular noise that may have the same frequency as the
4G Wireless Systems 14
Figure 7: Switched Beam Antenna
pulse itself does not cancel out the pulse. Interference would have to spread across the
spectrum uniformly to obscure the pulse.
UWB provides greater bandwidth — as much as 60 megabits per second, which is 6 times
faster than today’s wireless networks. It also uses significantly less power, since it transmits
pulses instead of a continuous signal. UWB uses all frequencies from high to low, thereby
passing through objects like the sea or layers of rock. Nevertheless, because of the weakness
of the UWB signal, special antennas are needed to tune and aim the signal.
5.2 Smart Antennas
Multiple “smart antennas” can be employed to help find, tune, and turn up signal information.
Since the antennas can both “listen” and “talk,” a smart antenna can send signals back
in the same direction that they came from. This means that the antenna system cannot only
hear many times louder, but can also respond more loudly and directly as well (ArrayComm
2003).
There are two types of smart antennas:
Switched Beam Antennas (as seen in Figure 7) have fixed beams of transmission, and
can switch from one predefined beam to another when the user with the phone moves
throughout the sector
Adaptive Array Antennas (as seen in Figure 8) represent the most advanced smart antenna
approach to date using a variety of new signal processing algorithms to locate
and track the user, minimize interference, and maximize intended signal reception
(ArrayComm 2003).
Smart antennas can thereby:
• Optimize available power
• Increase base station range and coverage
• Reuse available spectrum
• Increase bandwidth
• Lengthen battery life of wireless devices
4G Wireless Systems 15
Figure 8: Adaptive Array Antenna
Although UWB and smart antenna technology may play a large role in a 4G system, advanced
software will be needed to process data on both the sending and receiving side. This
software should be flexible, as the future wireless world will likely be a heterogeneous mix of
technologies.
6 4G Software
4G will likely become a unification of different wireless networks, including wireless LAN
technologies (e.g. IEEE 802.11), public cellular networks (2.5G, 3G), and even personal area
networks. Under this umbrella, 4G needs to support a wide range of mobile devices that
can roam across different types of networks (Cefriel ). These devices would have to support
different networks, meaning that one device would have to have the capability of working
on different networks. One solution to this “multi-network functional device” is a software
defined radio.
6.1 Software Defined Radio
A software defined radio is one that can be configured to any radio or frequency standard
through the use of software. For example, if one was a subscriber of Sprint and moved into
an area where Sprint did not have service, but Cingular did, the phone would automatically
switch from operating on a CDMA frequency to a TDMA frequency. In addition, if a new
standard were to be created, the phone would be able to support that new standard with a
simple software update. With current phones, this is impossible. A software defined radio in
the context of 4G would be able to work on different broadband networks and would be able
to transfer to another network seamlessly while traveling outside of the user’s home network.
A software defined radio’s best advantage is its great flexibility to be programmed for
emerging wireless standards. It can be dynamically updated with new software without any
changes in hardware and infrastructure. Roaming can be an issue with different standards,
but with a software defined radio, users can just download the interface upon entering new
territory, or the software could just download automatically (Wang 2001).
Of course, in order to be able to download software at any location, the data must be
formatted to some standard. This is the job of the packet layer, which will split the data
into small “packets.”
4G Wireless Systems 16
6.2 Packet Layer
The packet layer is a layer of abstraction that separates the data being transmitted from the
way that it is being transmitted. The Internet relies on packets to move files, pictures, video,
and other information over the same hardware. Without a packet layer, there would need
to be a separate connection on each computer for each type of information and a separate
network with separate routing equipment to move that information around. Packets follow
rules for how they are formatted; as long they follow these rules, they can be any size and
contain any kind of information, carrying this information from any device on the network
to another.
Currently, there is little fault tolerance built into cellular systems. If a little bit of the
voice information is garbled or lost in a transfer between locations, or if interference from
other devices somehow affects the transmission, there is nothing that can be done about it.
Even though the loss is usually negligible, it still can cause major problems with sensitive
devices and can garble voice information to a point where it is unintelligible. All of these
problems contribute to a low Quality of Service (QoS).
6.3 Packets
6.3.1 Advantages
There are many advantages of packets and very few disadvantages. Packets are a proven
method to transfer information. Packets are:
More Secure Packets are inherently more secure for a variety of reasons:
• A predictable algorithm does not split packets — they can be of any size and
contain any amount of data. Packets can also travel across the network right
after each other or separated by packets from other devices; they can all take the
same route over networks or each take a different route.
• The data in packets can be encrypted using conventional data encryption methods.
There are many ways to encrypt data, including ROT-13, PGP, and RSA; the
information in a packet can be encoded using any one of them, because a packet
doesn’t care what kind of data it carries. Within the same packet, no matter how
the data segment is encrypted, the packet will still get from one place to the other
in the same way, only requiring that the receiving device know how to decrypt
the data.
• There is no simple way to reconstruct data from packets without being the intended
recipient. Given that packets can take any route to their destination, it is
usually hard to piece them together without actually being at their intended destination.
There are tools to scan packets from networks; however, with the volume
of packets that networks receive and the volume of packets per each communication,
it would take a large amount of storage and processing power to effectively
“sniff” a packet communication, especially if the packets were encrypted.
More Flexible Current technologies require a direct path from one end of a communication
to the other. This limits flexibility of the current network; it is more like a large number
4G Wireless Systems 17
of direct communication paths than a network. When something happens to the path
in the current system, information is lost, or the connection is terminated (e.g. a
dropped call). Packets only require that there is an origin, a destination, and at least
one route between them. If something happens to one of the routes that a packet is
using, the routing equipment uses information in the packet to find out where it is
supposed to go and gives it an alternate route accordingly. Whether the problem with
the network is an outage or a slowdown, the combination of the data in the packet and
the routing equipment lead to the packet getting where it needs to go as quickly as
possible.
More Reliable Packets know general things about the information they contain and can
be checked for errors at their destination. Error correction data is encoded in the last
part of the packet, so if the transmission garbles even one bit of the information, the
receiving device will know and ask for the data to be retransmitted. Packets are also
numbered so that if one goes missing, the device on the receiving end will know that
something has gone wrong and can request that the packet(s) in question be sent again.
In addition, when something does go wrong, the rest of the packets will find a way
around the problem, requiring that only the few lost during the actual instant of the
problem will need to be resent.
Proven Technology Packets are the underlying technology in essentially all data based
communication. Since the beginning of the Internet over 30 years ago, packets have
been used for all data transmission. Technologies have evolved to ensure an almost
100% QoS for packet transmission across a network.
Easier to Standardize Current technologies use a variety of methods to break up voice
communication into pieces. None of these are compatible with each other. Packets,
however, are extremely compatible with various devices. They can carry different
types of information and be different sizes, but still have the same basic makeup to
travel over any network using any of the methods of transmission. Essentially, this
enables different technologies to be used to handle the same fundamental information
(Howstuffworks.com ). An example of the format of a packet carrying 896 bits of actual
information can be seen in Figure 9: The “Protocol” section would contain whatever
information was needed to explain what type of data was encoded; in the case of voice
using Voice over IP (VoIP), it would read: H.323 (Protocols.com ).
Extensible As shown by the growth of the Internet over the past few years, the capacity of
packets is expandable. They have moved from carrying short text messages to carrying
video, audio, and other huge types of data. As long as the capacity of the transmitter
is large enough, a packet can carry any size of information, or a large number of packets
can be sent carrying information cut up into little pieces. As long as a packet obeys
the standard for how to start and end, any data of any size can be encoded inside of
it; the transmission hardware will not know the difference.
4G Wireless Systems 18
Figure 9: Packet with 896-bit payload
6.3.2 Disadvantages
Unfortunately, to use packet, all cellular hardware will need to be upgraded or replaced.
Consumers will be required to purchase new phones, and providers will need to install new
equipment in towers. Essentially, the communication system will need to be rebuilt from
the ground up, running off of data packets instead of voice information. However, given
the current pace of technological development, most consumers buy new phones every six
to twelve months, and providers are constantly rolling out new equipment to either meet
expanding demand or to provide new or high-end services. All networks will be compatible
once the switch is completed, eliminating roaming and areas where only one type of phone
is supported. Because of this natural pace of hardware replacement, a mandated upgrade
in a reasonable timeframe should not incur undue additional costs on cellular companies or
consumers.
The technological disadvantage of using packets is not really a disadvantage, but more of
an obstacle to overcome. As the voice and data networks are merged, there will suddenly be
millions of new devices on the data network. This will require either rethinking the address
space for the entire Internet or using separate address spaces for the wireless and existing
networks.
6.4 Implementation of Packets
6.4.1 Current System: IPv4
Currently, the Internet uses the Internet Protocol version 4 (IPv4) to locate devices. IPv4
uses an address in the format of xxx.xxx.xxx.xxx where each set of three digits can range from
0 to 255 (e.g 130.207.44.251). Though combinations are reserved, but this address format
allows for approximately 4.2 billion unique addresses. Almost all IP addresses using IPv4
have been assigned, and given the number of new devices being connected to the Internet
every day, space is running out. As people begin to connect refrigerators, cars, and phones
to the Internet, a larger address space will be needed.
4G Wireless Systems 19
6.4.2 Recommended System: IPv6
The next generation addressing system uses the Internet Protocol version 6 (IPv6) to locate
devices. IPv6 has a much larger address space. Its addresses take the form x:x:x:x:x:x:x:x
where each x is the hexadecimal value that makes up one eighth of the address. An example
of this is: FEDC:BA98:7654:3210:FEDC:BA98:7654:3210 (The Internet Engineering Task
Force Network Working Group ). Using this address format, there is room for approximately
3.40 1038 unique addresses. This is approximately 8.05 1028 times as large as the IPv4
address space and should have room for all wired and wireless devices, as well as room for
all of the foreseeable expansion in several lifetimes. There are enough addresses for every
phone to have a unique address. Thus, phone in the future can use VoIP over the Internet
instead of continuing to use their existing network.
6.4.3 Voice over IP (VoIP)
Voice over IP is the current standard for voice communication over data networks. Several
standards already exist for VoIP, the primary one being International Multimedia Telecommunications
Consortium standard H.323. VoIP is already in use in many offices to replace
PBX-based systems and by several companies that offer cheap long distance phone calls over
the Internet, such as Net2Phone and Go2Call. VoIP allows for flexibility the same way that
data packets do; as far as the network is concerned, VoIP packets are the same as any other
packet. They can travel over any equipment that supports packet-based communication and
they receive all of the error correction and other benefits that packets receive. There are
many interconnects between the data Internet and the phone network, so not only can VoIP
customers communicate with each other, they can also communicate with users of the old
telephone system.
One other thing that VoIP allows is slow transition from direct, connection based communication
to VoIP communication. Backbones can be replaced, allowing old-style telephone
users to connect to their central office (CO) the same way. However, the CO will then
connect to an IPv6 Internet backbone, which will then connect to the destination CO. To
the end user, there will not seem to be any difference, but the communication will occur
primarily over a packet-based system, yielding all of the benefits of packets, outside of the
short connections between either end of the communication and their CO.
Of course, in order to keep curious users from listening in by “sniffing,” all data, including
voice, should be encrypted while in transit.
6.5 Encryption
Two encryption/decryption techniques are commonly used: asymmetric and symmetric encryption.
Symmetric encryption is the more traditional form, where both sides agree on a
system of encrypting and decrypting messages — the reverse of the encryption algorithm
is the decryption algorithm. Modern symmetric encryption algorithms are generic and use
a key to vary the algorithm. Thus, two sides can settle on a specific key to use for their
communications. The problem then is the key transportation problem: How do both sides
get the key without a third party intercepting it? If an unauthorized user receives the key,
then he too can decrypt the messages.
4G Wireless Systems 20
The solution to this problem is asymmetric encryption. In symmetric encryption, the
encryption and decryption algorithms are inverses, but the key is the same. In asymmetric
encryption, the keys are inverses, but the algorithm is the same. The trick is that one
cannot infer the value of one key by using the other. In an asymmetric (also called publickey)
system, an end user makes one key public and keeps the other private. Then all
parties know the algorithm and the public key. If any party wishes to communicate with
the users, that party can encrypt the message using the public key, and only the user (with
her private key) can decrypt the message. Moreover, the user can prove that she generated
a message by encrypting it with her private key. If the encrypted message makes sense to
other parties when decrypted with the public key, then those parties know that the user
must have generated that message (Dankers, Garefalakis, Schaffelhofer, and Wright 2002,
181).
Situations exist in cellular wireless systems where either symmetric or asymmetric keys
are particularly useful. Asymmetric keys are useful for one-time connections, especially when
used to create a symmetric key for an extended connection. Meanwhile, symmetric keys are
smaller and faster, and thus are strongly preferred if key transportation is not a problem.
An excellent example of this is the GSM system’s subscriber information card placed into
each phone. The card holds a unique symmetric key for each subscriber.
6.6 Flexibility
In reality, however, the usage of different encryption schemes depends on many factors,
including network data flow design. Thus, it is important that the encryption method
be able to change when other determining factors change. Al-Muhtadi, Mickunas, and
Campbell of University of Illinois at Urbana-Champaign showed great foresight in admitting
that “existing security schemes in 2G and 3G systems are inadequate, since there is greater
demand to provide a more flexible, reconfigurable, and scalable security mechanism as fast
as mobile hosts are evolving into full-fledged IP-enabled devices” (Al-Muhtadi, Mickunas,
and Campbell 2002, 60).
Unfortunately, IPv6 can only protect data in transmission. Individual applications may
contain flaws in the processing of data, thereby opening security holes. These holes may
be remotely exploited, allowing the security of the entire mobile device to be compromised.
Thus, any wireless device should provide a process for updating the application software as
security holes are discovered and fixed.
6.7 Anti-Virus
As wireless devices become more powerful, they will begin to exhibit the same security
weaknesses as any other computer. For example, wireless devices may fall victim to trojans or
become corrupt with viruses. Therefore, any new wireless handheld device should incorporate
antivirus software. This software should scan all e-mail and files entering through any port
(e.g. Internet, beaming, or synchronizing), prompting the user to remove suspicious software
in the process. The antivirus software should also allow secure, remote updates of the
scanning software in order to keep up with the latest viruses (NIST, U.S. Dept. of Commerce
, 5-34).
4G Wireless Systems 21
7 Conclusion
Consumers demand that software and hardware be user-friendly and perform well. Indeed, it
seems part of our culture that customers expect the highest quality and the greatest features
from what they buy. The cellular telephone industry, which now includes a myriad of wireless
devices, is no exception.
Meanwhile, competition in the industry is heating up. Providers are slashing prices while
scrambling for the needed infrastructure to provide the latest features as incentives, often
turning to various 3G solutions. Unfortunately, this will only serve to bewilder customers in
an already confusing market.
Customers want the features delivered to them, simple and straightforward. Wireless
providers want to make money in a cutthroat industry. If the U.S. government wants to
help, the best way to help all parties is to enforce 4G as the next wireless standard. The
software that consumers desire is already in wide use. The transmission hardware to take it
wireless is ready to go. And we have the security practices to make sure it all works safely.
The government need only push in the right direction; the FCC need only standardize 4G
in order to make the transition economically viable for all involved.
This is a need that demands a solution. Today’s wired society is going wireless, and it
has a problem. 4G is the answer.
4G Wireless Systems 22
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