Wireless Radio Channel
The first and most fundamental challenge for broadband wireless comes from the transmission
medium itself. In wired communications channels, a physical connection, such as a copper wire
or fiber-optic cable, guides the signal from the transmitter to the receiver, but wireless communication
systems rely on complex radio wave propagation mechanisms for traversing the intervening
space. The requirements of most broadband wireless services are such that signals have to
travel under challenging NLOS conditions. Several large and small obstructions, terrain undulations,
relative motion between the transmitter and the receiver, interference from other signals,
noise, and various other complicating factors together weaken, delay, and distort the transmitted
signal in an unpredictable and time-varying fashion. It is a challenge to design a digital communication
system that performs well under these conditions, especially when the service requirements
call for very high data rates and high-speed mobility. The wireless channel for broadband
communication introduces several major impairments.
Distance-dependent decay of signal power: In NLOS environments, the received signal
power typically decays with distance at a rate much faster than in LOS conditions. This distancedependent
power loss, called pathloss, depends on a number of variables, such as terrain, foliage,
obstructions, and antenna height. Pathloss also has an inverse-square relationship with carrier
frequency. Given that many broadband wireless systems will be deployed in bands above
2GHz under NLOS conditions, systems will have to overcome significant pathloss.
Blockage due to large obstructions: Large obstructions, such as buildings, cause localized
blockage of signals. Radio waves propagate around such blockages via diffraction but incur
severe loss of power in the process. This loss, referred to as shadowing, is in addition to the
distance-dependent decay and is a further challenge to overcome.
Large variations in received signal envelope: The presence of several reflecting and
scattering objects in the channel causes the transmitted signal to propagate to the receiver via
multiple paths. This leads to the phenomenon of multipath fading, which is characterized by
large (tens of dBs) variations in the amplitude of the received radio signal over very small distances
or small durations. Broadband wireless systems need to be designed to cope with these
large and rapid variations in received signal strength. This is usually done through the use of
one or more diversity techniques, some of which are covered in more detail in Chapters 4, 5,
Intersymbol interference due to time dispersion: In a multipath environment, when the
time delay between the various signal paths is a significant fraction of the transmitted signal’s
symbol period, a transmitted symbol may arrive at the receiver during the next symbol period
and cause intersymbol interference (ISI). At higher data rates, the symbol time is shorter; hence,
it takes only a smaller delay to cause ISI. This makes ISI a bigger concern for broadband wireless
and mitigating it more challenging. Equalization is the conventional method for dealing
with ISI but at high data rates requires too much processing power. OFDM has become the solution
of choice for mitigating ISI in broadband systems, including WiMAX, and is covered in
Chapter 4 in detail.
Frequency dispersion due to motion: The relative motion between the transmitter and the
receiver causes carrier frequency dispersion called Doppler spread. Doppler spread is directly
related to vehicle speed and carrier frequency. For broadband systems, Doppler spread typically
leads to loss of signal-to-noise ratio (SNR) and can make carrier recovery and synchronization
more difficult. Doppler spread is of particular concern for OFDM systems, since it can corrupt
the orthogonality of the OFDM subcarriers.
Noise: Additive white Gaussian noise (AWGN) is the most basic impairment present in any
communication channel. Since the amount of thermal noise picked up by a receiver is proportional
to the bandwidth, the noise floor seen by broadband receivers is much higher than those
seen by traditional narrowband systems. The higher noise floor, along with the larger pathloss,
reduces the coverage range of broadband systems.
Interference: Limitations in the amount of available spectrum dictate that users share the
available bandwidth. This sharing can cause signals from different users to interfere with one
another. In capacity-driven networks, interference typically poses a larger impairment than noise
and hence needs to be addressed.
Each of these impairments should be well understood and taken into consideration while
designing broadband wireless systems. In Chapter 3, we present a more rigorous characterization
of the radio channel, which is essential to the development of effective solutions for broadband