Mitigation Techniques for Multipath Fading

In analog radiolink systems, multipath fading results in an increase in thermal noise as the RSL drops. In digital radio systems, however, there is a degradation in BER during periods of fading that is usually caused by intersymbol interference due to multipath. Even rather shallow fades can cause relatively destructive amounts of intersymbol interference. This interference results from frequency-dependent amplitude and group delay changes. The degradation depends on the magnitude of in-band amplitude and delay distortion. This, in turn, is a function of fade depth and time delay between the direct and reflected signals.

Five of the most common methods to mitigate the effects of multipath (Ref. 20) on digital radiolinks are:

1. System configuration (i.e., adjusting antenna height to avoid ground reflection; implementation of space and/or frequency diversity).

2. Use of IF combiners in diversity configurations.

3. Use of baseband switching combiners in diversity configuration.

4. Adaptive IF equalizers.

5. Adaptive transversal equalizers.

System configuration techniques have been described previously in Chapter 2, such as sufficient clearance to avoid obstacle diffraction, high-low antennas to place a reflection point on "rough" ground, and particularly the use of diversity. We will narrow our thinking here to space diversity. We now expand on several of the items listed above for mitigation of the effects of multipath.

┬╗Section 3.5.1 is based on Section 3.2/3.3 of ITU-R F.1093-2 (Ref. 11).

An optimal IF combiner for digital radio receiving subsystems can be designed to adjust adaptively to path conditions. One such combining technique, the maximum power IF combiner, vectorially adds the two diversity paths to give maximum power output from the two input signals. This is done by conditioning the signal on one path with an endless phase shifter, which rotates the phase on this path to within a few degrees of the signal on the other diversity path prior to combining. The output of this type of combiner can display in-band distortion that is worse than the distortion on either diversity path alone, but functions well to keep the signal at an acceptable level during deep fades on one of the diversity paths.

A minimum distortion IF combiner operation is similar in most respects to the maximum power IF combiner but uses a different algorithm to control the endless phase shifter. The output spectrum of the combiner is monitored for flatness such that the phase of one diversity path is rotated and, when combined with the second diversity path, produces a comparatively flat spectral output. The algorithm also suppresses the polarity inversion on the group delay, which is present during nonminimum phase conditions. One disadvantage of this combiner is that it can cancel two like signals such that the signal level can be degraded below threshold.

Reference 12 suggests a dual algorithm combiner that functions primarily as a maximum power combiner and automatically converts to a minimum distortion combiner when signal conditions warrant. Using space diversity followed by a dual-algorithm combiner can give improvement factors better than 150.

Adaptive IF equalizers attempt to compensate directly at IF for multipath passband distortion. Digital radio transmitters emit a transmit spectra of relatively fixed shape. Thus various points on the spectrum can be monitored, and when distortion is present, corrective action can be taken to restore spectral fidelity. The three most common types of IF adaptive equalizers are shape-only equalizers, slope and fixed notch equalizers, and tracking notch equalizers.

Another equalizer is the adaptive transversal equalizer, which is efficient at canceling intersymbol interference due to signal dispersion caused by multipath. The signal energy dispersion can be such that energy from a digital transition or pulse arrives both before and after the main bang of the pulse. The equalizer uses a cascade of baud delay sections that are analog elements to which the symbol or baud sequence is inputted. The "present" baud or symbol is defined as the output of the Mh section. Sufficient sections are required to encompass those symbols or bauds that are producing the distortion. These transversal equalizers provide both feedforward and feedback information. There are both linear and nonlinear versions. The nonlinear version is sometimes called a decision feedback equalizer. Reference 12 reports that both the IF and transversal equalizers show better than three times improvement in error rate performance over systems without such equalizers.

3.5.2 ITU-R Guidelines on Combating Propagation Effects Space Diversity. Space diversity is one of the most effective methods of combating multipath fading. For digital radio systems, where performance objectives are difficult to meet owing to waveform distortion caused by multipath effects, system designs must often be based on the use of space diversity.

By reducing the effective incidence of deep fading, space diversity can reduce the effects of various types of interference. In particular, it can reduce the short-term interference effects from cross-polar channels on the same or adjacent channel frequencies, and the interference from other systems and from within the same system.

Linear amplitude dispersion (LAD) is an important component of waveform distortion and quadrature crosstalk effects and can be reduced by the use of space diversity. Diversity combining used specifically to minimize LAD is among the methods that are particularly effective in combating this type of distortion.

The improvement derived from space diversity depends on how the two signals are processed at the receiver (combiner). Two examples of techniques are "hitless" switching and variable phase combining. The "hitless" switch selects the receiver with the greatest eye opening or the lowest error rate, and the combiners use either co-phase or various types of dispersion-minimizing control algorithms. "Hitless" switching and co-phase combining provide very similar improvements. Adaptive Channel Equalizing. Some form of receiver equalizing is usually necessary in the radio channel(s). As propagation conditions vary, an equalizer must be adaptively controlled to follow the variations in transmission characteristics. Such equalizers work in either the frequency domain or the time domain.

frequency domain equalization. Equalizers operating in the frequency domain are comprised of one or more linear networks that are designed to produce amplitude and group delay responses. They compensate for transmission impairments, which are most likely to produce system performance degradation during periods of multipath fading. Table 3.2 shows several alternative equalizer structures that may be considered by the system engineer.

time domain equalization. Time domain equalizers combat intersymbol interference directly. With these equalizers control information is derived by correlating the interference that appears at the instant of decision with the various adjacent symbols producing it, and this result is used to adjust tapped delay line networks to provide appropriate cancellation signals. Such an equalizer is able to handle simultaneous and independent types of distortion

TABLE 3.2 Comparison of Adaptive Equalizers

Fade Characteristic and Position of Maximum Attenuation3

Minimum Nonminimum Phase Phase

TABLE 3.2 Comparison of Adaptive Equalizers

Fade Characteristic and Position of Maximum Attenuation3

Minimum Nonminimum Phase Phase


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