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 communi cation 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 distance dependent 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.
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.
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 communi cation 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 distance dependent 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.
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.