The impairments inherent in any wireless channel include the rate at which received signal power decreases relative to transmitter-receiver distance path loss ,. The impairments determine the types of applications that can be supported in different propagation environments. Applications require different data rates and bit-error rates BER, or the probability that a bit is received in error.
For example, voice applications require data rates on the order of 8 to 32 kbps and a maximum tolerable BER of 10 -3 , whereas database access and remote file transfer require data rates up to 1 Mbps and a maximum tolerable BER of 10 In general, systems designed for the worst-case propagation conditions assume high error rates, which limit their capability to support high-speed data and video teleconferencing applications. The random nature of the radio channel makes it difficult to guarantee quality and performance for demanding applications.
However, a wireless system can be designed to adapt to the varying link quality at both the link and network level, such that the system can support improved data rates and quality. Applications can also be designed to adapt to deteriorating channel conditions to minimize the degradation perceived by the user. The overall system can be optimized by making trade-offs among various performance measures such as BER, outage probability, and spectral and power efficiency.
These trade-offs dictate the choice of modulation, signal processing, and antenna techniques used to mitigate channel impairments. These techniques require fairly intensive digital signal processing at the mobile unit. The extent of the computation that can be performed is limited by the power available to drive the DSP chips and the microprocessor.
Thus, in addition to being power limited, the mobile unit is also complexity limited, which means that trade-offs need to be made in designing the communication link. For example, the transmit power requirements of the mobile unit can be reduced if error-correction coding is used, but then additional power is needed to drive the encoding and decoding hardware.
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In cellular systems it is preferable to place much of the computational burden at the base station, which has fewer power restrictions than do the mobile units. Research aimed at simplifying DSP and antenna processing techniques Section 2. The remainder of this section outlines the characteristics of the wireless channel, focusing on fading and interference problems Section 2.
The characteristics of the radio channel impose fundamental limits on the range, data rate, and quality of wireless communications. The performance limits are influenced by several factors, most significantly the propagation environment and user mobility pattern. For example, the indoor radio channels typically support higher data rates with better reliability than does the outdoor channel used by persons moving rapidly.
Electromagnetic signals can be characterized by the features of the waveform: amplitude the power, or magnitude, of the signal ; phase the timing of the peak or trough of the signal variations ; and frequency the number of repetitions of the signal per second. Large-scale effects involve the variation of the mean received signal power over large distances relative to the signal wavelength, whereas small-scale effects involve the fluctuations of the received signal power over distances commensurate with the wavelength.
Path loss effects are noticeable over large distances i. Signal power variations due to obstacles such as building or terrain features are observable over distances that are proportional to the length of the obstructing object. Very rapid variations result from multipath reflections, which are copies of the transmitted signal that reflect or diffract off surrounding objects before arriving by different paths at the receiver.
These reflections arrive at a receiver later than the nonreflected signal path and are often shifted in phase as well. The multipath reflections either reinforce or cancel each other and the nonreflected signal path depending on the exact position of the receiver if moving or the transmitter if moving.
The overall effects of multipath propagation involving a moving terminal are rapid variation in the received signal power and nonuniform distortion of the frequency components of the signal. The first four subsections below discuss path loss, fading, and various sources of interference as they apply to the path between two terrestrial RF devices. The fifth subsection details the characteristics of satellite RF links.
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Path loss is equal to the received power divided by the transmitted power, and this loss is a function of the transmitter-receiver separation. For a given transmit power, a path loss model 3 predicts the received power level at some distance from the transmitter. The simplest model for path loss, which captures the key characteristics for most channels, is an exponential relationship: The received signal power is proportional to the transmit power and inversely proportional to the square of the transmission.
In environments with dense buildings or trees, path loss exponents can exceed 8. Thus, systems designed for typical suburban or low-density urban outdoor environments require much higher transmit power to achieve the same desired performance in a dense jungle or downtown area packed with tall buildings.
The BER of a wireless link is determined by the received signal power, noise introduced by the receiver hardware, interference, and channel characteristics. The noise is typically proportional to the RF bandwidth. For the exponential path loss model just described, the received signal-to-noise ratio SNR is the product of the transmit power and path loss, divided by the noise power. The SNR required for faithful reception depends on the communications technique used, the channel characteristics, and the required BER.
Because path loss affects the received SNR, path loss imposes limits on the data rate and signal range for a given BER.
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In general, for a given BER, high-data-rate applications typically require more transmit power or have a smaller coverage range sometimes both than do low-data-rate applications. For example, given a transmit power of 1 W, a transmit frequency of 1 GHz, and an omnidirectional antenna, the transfer of data through free space for which the path loss exponent is 2 at 1 Mbps and 10 -7 BER can be accomplished between radios that are m apart, whereas in a jungle for which the path loss exponent is 10 the range can be as low as 4 m.
A received signal is often blocked by hills or buildings outdoors and furniture or walls indoors. The received signal power is in fact a random variable that depends on the number and dielectric properties of the obstructing objects. Signal variation due to these obstructions is called shadow fading. Measurements have shown that the power, measured in decibels dB , of signals subject to shadow fading exhibits a Gaussian i.
The random attenuation of shadow fading changes as the mobile unit moves past or around the obstructing object.
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Because the signal coverage is not uniform even at equal distances from the transmitter, the transmit power needs to be increased to ensure that the received-SNR requirements are met uniformly throughout the coverage region. The power increase imposes additional burdens on the transmitter battery and can cause interference for other users of the same frequency band. Small-scale fading is caused by interference between multiple versions of the signal that arrive at the receiver at different times.
Multipath can be helpful if the signals add constructively to produce a higher power a random event , but more often it results in harmful interference. The overall effect is a standing wave pattern of the received signal power. Harmful interference can cause the received signal power to drop by a factor of 1, below its average value at nulls in the standing wave pattern.
Moreover, for practical speeds of wireless terminals, the changes in signal power are extremely rapid: At a frequency of MHz the signal power changes every 30 centimeters, or every 23 milliseconds if the terminal is moving at 50 km per hour. In many practical environments, these changes are referred to as "Rayleigh fading" because the received signal amplitude conforms to a Rayleigh probability density function. Signal fading can be characterized by determining the delay spread of the fading relative to the signal bandwidth.
The delay spread is defined as the time delay between the direct-path signal component and the component that takes the longest path from the transmitter to the receiver. Because the delay spread is a random variable, it is often characterized by its standard deviation, called the root mean square RMS delay spread of the channel.
If the product of the RMS delay spread and the signal bandwidth is much less than 1, then the fading is called flat fading. In this case the received signal envelope has a random amplitude and phase commonly described by a Rayleigh distribution , but there is no additional signal distortion.
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When the product of the RMS delay spread and signal bandwidth is greater than 1, the fading becomes frequency selective. Frequency-selective fading introduces self-interference because the delay spread is so large that multipath reflections corresponding to a given bit transmission arrive at the receiver simultaneously with subsequent data bits. This intersymbol interference ISI establishes an "error floor" in the received bits that cannot be reduced by an increase in signal power because doing so also increases the self-interference. Without compensation, the ISI forces a reduction in the data rate such that the product of the RMS delay spread and signal bandwidth is less than 0.
For a 10 -3 BER and a rural environment, the delay spread is approximately 25 microseconds and the corresponding maximum data rate is only 8 kbps; the data rates for lower BERs are even more limited. Some form of compensation, either signal processing or sophisticated antenna design, clearly is needed to achieve high-rate data transmission in the presence of ISI.
These techniques impose additional complexity and power requirements on the receiver. Movement of a receiver relative to the transmitter or vice versa causes the received signal to be frequency shifted relative to the transmitted signal.
The frequency shift, or Doppler frequency, is proportional to the mobile velocity and the frequency of the transmitted signal. For a transmitted signal frequency of MHz and a receiver or transmitter speed of 96 km per hour, the Doppler frequency is roughly 80 Hz. This Doppler shift creates an irreducible error floor for noncoherent detection techniques which use the previous bit to obtain a phase reference for the current bit.
In general the irreducible BER is not a problem when data are transmitted at high speed faster than 1 Mbps , but it is an issue for moderate-rate slower than kbps data applications. In general the signal changes slowly with time because of path loss, more quickly because of shadow fading, and very quickly because of multipath flat fading; all of these effects are simultaneously superimposed on the transmitted signal. As noted above, the shadow fading needs to be addressed by an increase in transmit power. The deep fades in signal power caused by flat fading also need to be counterbalanced by an increase in transmit power or some other approach see Section 2.
Otherwise the transmitted signal typically exhibits bursts of errors that are difficult to correct. Users of wireless communications systems can experience interference from various sources.
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One source is frequency reuse, a popular technique for increasing the number of users in a given region who can be supported by a particular set of frequencies. Cellular systems reuse frequencies at spatially separated locations, taking advantage of the falloff in received signal power with distance which is indicated by the path loss model. The downside of frequency reuse is the introduction of co-channel interference see Section 2.
Other sources of interference include adjacent channels and narrow bands of problem frequencies. Adjacent-channel interference can be mitigated by the introduction of guard channels between users, although this technique consumes bandwidth. Narrowband interference can be removed by notch filters or spread-spectrum techniques.
Notch filters are simple devices that block the band of frequencies containing the interference; these devices are effective only if the specific frequencies of concern are known. Spread-spectrum techniques see Section 2. Satellite channels the link between a receiver or transmitter on Earth and an orbiting receiver or transmitter have inherent advantages over terrestrial radio channels.
Multipath fading is rare because a signal propagating skyward does not experience much reflection from surrounding objects except in downtown areas with densely packed buildings. Moreover, most satellite systems operate in the gigahertz frequency range, allowing for large-bandwidth communication links that support very high bit rates. The primary limitation of satellite channels is very high path loss, which generally follows the formula described earlier in this chapter.
For satellites the path loss exponent is 2. Because satellites operate at high frequencies and the path distance is long to 2, km for a LEO satellite , much higher transmit power is needed than is the case for terrestrial systems operating at the same data rate. Satellite signals are also subject to attenuation by Earth's atmosphere.
The effects are especially adverse at frequencies above 10 GHz, where oxygen and water vapor, rain, clouds, fog, and scintillation cause random variations in signal amplitude, phase, polarization, and angle of arrival similar to the adverse effects of multipath fading in terrestrial propagation. Satellite systems compensate somewhat for the large path loss and adverse atmospheric effects by using high-gain directional antennas to boost the received power.
The pioneering work of Claude Shannon determined the total capacity limits for simple wired and wireless channel models: These limits established an upper bound on the maximum spectral link efficiency, measured as the data rate per unit of bandwidth as a function of the received SNR. Determining the capacity limits of wireless channels with all the impairments outlined in the previous section is quite challenging.