The Internet Encyclopedia (Volume 3)

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186 RADIOFREQUENCY ANDWIRELESSCOMMUNICATIONS

for any given locations of the Tx and Rx is given by

h(t,τ)=

∑M

k= 1

αk(t)δ(t−τk(t))e−jφk(t) (4)

whereαk,(t),τk(t), andφk(t) represent the time-varying

amplitude, delay, and phase of thekth path signal. This

shows that in general, the received signal is a series of

time-delayed, phase-shifted, attenuated versions of the

transmitted signal. The variablesh(t,τ),αk(t),φk(t), and

τk(t) are also random.

WIRELESS COMMUNICATION

TECHNIQUES

Because the wireless channel is not a reliable propaga-

tion medium, techniques to achieve reliable and efficient

communication are necessary. In mobile channels, for ex-

ample, the Rx has to constantly track changes in the prop-

agation environment to ensure optimal extraction of the

signal of interest. As the receiver moves, the surrounding

environment changes, affecting the received signal’s am-

plitude, phase, and delay. The multipath received signals

are combined at the antenna either constructively or de-

structively. During destructive combining the received sig-

nal may not be strong enough to produce reliable commu-

nication because of the degradation in the signal-to-noise

ratio (SNR). It is not uncommon in shadowed signals for

the amplitude of the received signal to drop by 30 dB or

more within a distance of a fraction of a wavelength (Eng,

Kong, & Milstein, 1996). Hence, achieving reliable com-

munication over a wireless channel is a daunting task.

To counter this problem, techniques have been devel-

oped for efficient wireless communication. These include

spread spectrum, multiple access, diversity, equalization,

coding, and related techniques such as multicarrier mod-

ulation, orthogonal frequency division multiplexing, mul-

ticode and multirate techniques, and multiple input mul-

tiple output system, to mention just a few. All these tech-

niques are aimed at increasing the reliability of the chan-

nel and the performance of the system. Discussion of some

of these techniques is beyond the scope of this paper. How-

ever, a summary of the major wireless communication

techniques is given below.

Spread Spectrum

Spread spectrum (SS) is a modulation technique where

the transmitted bandwidthBssis much greater than the

data bandwidthBs. The idea is to transform a signal with

bandwidthBsinto a noise-like signal of much larger band-

widthBss. Spreading is usually achieved by modulating

the data with a pseudo-random noise (PN) sequence called

the “chip” at a rate that is much higher than the data rate.

The significance of SS is evident from the capacity equa-

tion, given by

C=Blog 2 ( 1 +SN R) (5)

whereCis the channel capacity in bits andBis the band-

width in hertz. Observe that by increasing the bandwidth

B, we may decrease the SNR without decreasing the ca-

pacity and, hence, the performance.

The main parameter in SS systems is the processing

gain,Gp, defined as

Gp=

Spr ead Bandwi dth

Infor mation Bandwidth

=

Bss

Bs

=

Tb

Tc

(6)

whereTbandTcare the bit period and the chip period, re-

spectively.Gpis sometimes known as the “spreading fac-

tor” (Rappaport, 2002). From a system viewpoint,Gpis

the performance increase achieved by spreading. It deter-

mines the number of users that can be allowed in a system,

and hence the amount of multipath reduction effect. It is

used to describe the signal fidelity gained at the cost of

bandwidth. It is throughGpthat increased system perfor-

mance is achieved without requiring a higher SNR. For SS

systems, it is advantageous to haveGpas high as possible,

because the greater theGp, the greater the system’s ability

to suppress interference. SS techniques are used in cellu-

lar mobile telephones, global positioning satellites (GPS),

and very-small-aperture satellite terminals. The strength

of this system is that whenGpis very large, the system

offers great immunity to interference.

There are two major methods of SS modulation,

namely direct sequence spread spectrum (DSSS) and fre-

quency hopping spread spectrum (FHSS). In DSSS the

frequency of the given signal is spread across a band of

frequencies as described above. The spreading algorithm

changes in a random fashion that appears to make the

spread signal a random noise source. FHSS is the repeated

switching offcfrom one band to another during transmis-

sion. Radio signals hop from onefcto another at a specific

hopping rate and the sequence appears to be random. In

this case, the instantaneous frequency output of the Tx

jumps from one value to another based on the pseudo-

random input from the code generator. The overall band-

width required for FHSS is much wider than that required

to transmit the same information using only one carrier.

However, eachfcand its associated sidebands must stay

within a defined bandwidth.

Diversity

Diversity is one of the techniques widely used to increase

system performance in wireless communication systems.

Diversity combining refers to the system in which two

or more closely similar copies of some desired signal are

available and experience independent fading. In diversity

systems, the received signals from several transmission

paths, all carrying the same information with individual

statistics, are combined with the hope of improving the

SNR of the decision variables used in the detection pro-

cess. Diversity combining techniques could be based on

space (antenna), frequency, angle of arrival, polarization,

and time of reception (Eng et al., 1996; Yacoub, 1993).

For example, in space diversity the transmitted signal is

received viaNdifferent antennas with each multipath re-

ceived through a particular antenna. This can be regarded

as communication overNparallel fading channels. Diver-

sity reception is known to improve the reliability of the

systems without increasing either the transmitter power