In general, it is imperative to understand the target operating environment in order
to optimize the transmitter and receiver design of the communication systems. There
is no such thing as a universal communication systems that is effective for all communication
environments. For examples, both wireless LAN systems and 3G networks
are designed to offer high bit rate. For IEEE 802.11g system, the highest bit rate
is 54Mbps. For UMTS systems (Re1 99), the highest bit rate is just 2Mbps. Obviously,
the cost of the UMTS system is much higher than the wireless LAN systems.
Hence, one might query about why we still require UMTS system when wireless
LAN systems cost only less than US$lOO. Figure 1.8 illustrates a more comprehensive
comparison between Wi-Fi and UMTS systems. We do not just look at the
maximum bit rate offered by both systems but we also have to compare the target
environment of operations. For instance, while Wi-Fi systems offer higher bit rate,
they are designed to operate at pedestrian mobility and indoor coverage. For 3G/4G
cellular systems, although their maximum bit rate is lower than Wi-Fi systems, they
are designed to operate at very high mobility and macroscopic coverage.
Figure 1.8 A comparison between WiFi systems and 3G cellular systems.
1.6 SUMMARY
In this chapter, we have elaborated on the modeling of wireless channels. We have
introduced the 3 levels of channel models, namely the large scale path loss model, the
medium scale shadowing model as well as the microscopic scale fading model. The
path loss model deals with the variation of received signal strength with respect to
variation of distance between the transmitter and the receiver and is focused on the time
scale of seconds. The path loss between a transmitter and a receiver is characterized
by a path loss exponent. In free space or line of sight propagation condition, the path
loss exponent is 2 meaning that the signal power will be reduced by 4 times for every
double in the distance separation. In non line-of-sight propagation environment, the
path loss exponent can reach 4 or above. The larger the path loss exponent, the faster
the signal attenuates as it propagates.
Shadowing model deals with medium scale variation of received signal strength
when the distance is fixed. This is contributed by the variation in the terrain profile
such as obstacles, hills and buildings. Due to law of large number, the shadowing
effects (received signal strength variations) can be modeled by log-normal shadowing
component and parameterized by the standard derivation a(dB).
Finally, microscopic fading deals with small scale variation of received signal
strength due to constructive and destructive multipath superpositions. The time scale
of interests can be of millisecond order. Microscopic fading can be parameterized by
the delay spread (coherence bandwidth) for the multipath dimension as well as the
Doppler spread (coherence time) for the time variation dimension. Note that the multipath
dimension and the time variation dimension are two independent dimensions.
The number of resolvable multipaths is given by L, = [Wtl/Bc]W. hen there is only
one resolvable multipath (Wt, < B,), the signal experiences flat fading channels.
Otherwise, the signal experiences frequency selective fading channels. Similarly,
when T, > T,, the signal experiences fast fading channels. Otherwise, the signal
experiences slow fading channels. Figure 1.9 summarizes the concept of frequency
flat fading, frequency selective fading, fast fading and slow fading channels.
PROBLEMS
1.1 A wideband signal is more difficult to transmit than a narrowband signal
because of the inter-symbol interference problem faced by the wideband signal.
[ TrueFalse]
1.2 In flat fading channels, there is no multipath effect at all as there is only a single
resolvable echo at the receiver. [TruelFalse].
Figure 1.9 Summary of classifications of fading channels.
The End