The Internet Encyclopedia (Volume 3)

P1: JDW

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PROPAGATION OFSIGNALS 131

5 10 15 20 25 30 35 40 45 50

10 −^4

10 −^3

10 −^2

10 −^1

Signal-to-Noise Ratio dB

Bit Error Rate

Rayleigh fading

No fading (ideal channel)

17 dB

Figure 9: The bit error rate (BER) in an ideal channel and a fading channel. The in-

creased signal-to-noise ratio required to maintain a BER of 1 in 1,000 when fading is

present is indicated (17 dB).

situations arise even in indoor channels. To account for

the lognormal fading or shadowing, the loss terms in

Equation 7 are modified by introducing a termXto the

equation for the loss.Xis a zero mean normal random

variable with a standard deviation ofσdBto take lognor-

mal fading into consideration.

Loss=Lp(d)+X (16)

The loss calculated now becomes the average of a

normally distributed random variable with a standard

deviation determined by the severity of the lognormal fad-

ing. Long-term fading will also cause outage if the varia-

tion in loss is not taken into account during the design of

the wireless systems. This situation is handled by includ-

ing a power margin in the link budget calculations.

Diversity Techniques

Effects of short-term fading can be mitigated through di-

versity techniques that exploit the randomness existing in

the channel. Consider a scenario for which it is possible to

Doppler Fading

Multipath Fading

Frequency Selective

Signal BW>Channel BW

Flat

Signal BW< Channel BW

Fast

Baseband signal variations

slower than channel variations variations

Slow

Baseband signal variations

faster than channel variations

Figure 10: Overview of short-term fading. Pulse broadening taking place in fre-

quency selective channel can be described in terms of the relationship between the

message bandwidth and channel bandwidth. Doppler fading can be described in

terms of the variations in the channel relative to the signal.