Developers of wireless systems are continually challenged to find new ways to increase system capacity and improve performance to accommodate the growing number of personal communications users. One method to achieve these goals uses the diversity-combining capabilities of wireless communications receivers.
A popular diversity-combining technique, spatial diversity, uses multiple antennas to capture transmission paths subject to different fading statistics. Various combining techniques use these semi-correlated received paths to optimize signal recovery.
RF Channel Propagation Characteristics
Effective diversity systems cannot be designed without a thorough understanding of the propagation characteristics of the RF channel. Multipath fading, relative path delay, relative path loss, Doppler shift and log-normal shadowing are the typical characteristics encountered on the RF channel.
One or any combination of these may be found on a typical RF communication channel. These characteristics can be demonstrated with a simple transmitter-to-receiver diagram. Figure 1 shows a typical mobile receiver (the car) as it drives along a roadway. Rays A, B, C and D depict just four of the many signal paths from the transmitter to receiver.
Multipath fading or Rayleigh fading
is a rapid fluctuation of the signal power. A Rayleigh faded signal is caused by the summation of a very large number of individual reflected signals.Each of these signals has a random phase and amplitude at the receiver caused by the difference in path length and attenuation. These signals rarely combine to greater than 10 dB above the average signal power. The deep fades (destructive interference) range from just a few dB to 50 dB below the average signal power. The receiver’s front end must be sensitive enough to function properly over these power fluctuations.
Relative path delay
is a phenomenon where individual signals from the transmitter arrive at different times at the receiver. In a digital transmission system, this causes the received symbols to overlap, resulting in inter-symbol interference.
Relative path loss
occurs when individual signal reflections arriving at the receiver are at different absolute power levels. The difference in power levels between signal paths is caused by physical elements in the signal path.Doppler or frequency shift
from the carrier frequency occurs when the distance between the receiver and the transmitter is changing. The Doppler frequency can be either positive or negative, depending on whether the mobile receiver is moving away from or toward the transmitter. The receiver must function properly even though the signal has shifted from the original RF carrier frequency.Log-normal shadowing
is the slow variation of the signal envelope over time. Log-normal shadowing is a loss in the signal strength at the receiver caused by a blockage or absorption of the signal from the transmitter by elements in the environment. This is often called shadowing since the receiver is passing through the signal “shadow” of a large object.Space-Diversity Systems
Space diversity uses two or more spatially separated antennas to obtain multiple copies of a signal. To obtain completely uncorrelated copies of the signals, base antennas are separated by tens of wavelengths. For mobile communications where the multipath is uniformly distributed around the receiving antennas, useful space-diversity benefits can be achieved with antennas separated by just one-half a wavelength.
Actual spacing depends on the disposition of the scatters causing the multipath transmission. With space diversity, N-branches can be used. There is no limitation on N; however, the improvement in immunity to fading realized by adding another branch decreases as N grows larger.
Various combining methods have been developed to enhance the performance of space-diversity systems. The four major combining techniques are switched, selective, maximal ratio and equal gain.Switched combining is used only in dual (N=2) diversity schemes. Based on a given threshold, the receiver continues to monitor a branch until it falls below the defined threshold. At that point, the receiver switches to the other branch regardless of signal power.
Selective combining expands on this concept by adding a receiver for each branch. By doing this, it can select the branch with the highest instantaneous signal-to-noise ratio (SNR).
In contrast to these techniques, maximal-ratio combines signals from the various branch inputs. The signals from each branch are weighted in proportion to their signal-to-noise ratios and then summed.
Unlike maximal-ratio combining, equal-gain combining does not require each branch to be individually weighted. This technique brings all phases to a common point and combines them. The resultant signal is the sum of instantaneous fading envelopes of the individual branches.
Depending on the combining method used, the theoretical performance gains for a multi-branch system will vary. Figure 2 shows the improvement in SNR vs the number of branches for the various combining techniques. The results assume that the fading is uncorrelated. Design and cost trade-offs must be made to determine the appropriate number of branches.
Diversity Test System
In the past, test techniques for evaluating space-diversity receivers suffered from dubious accuracy and repeatability. Driving around in a vehicle is one test method designers have used to evaluate diversity receivers. This approach is not comprehensive because it is nearly impossible to create the full range of conditions an end user would encounter. In addition, this approach is not repeatable, so performance problems cannot be easily diagnosed and solved.
Understanding the test problems associated with diversity systems makes it easy to define the requirements for the test system. The system must:
Emulate the propagation delay, fading and loss characteristics inherent in wireless communication systems.
Provide repeatable and precise control of the fading correlation among four branches.
Imitate a wide range of conditions that a diversity receiver could encounter in the real world.
Support you with a software program that provides an easy-to-use graphical interface.
One system that meets these requirements, FLEX/Diversity™ from Telecom Analysis Systems, consists of two RF channel emulators, specialized software and cabling (Figure 3). The RF transmitter is connected to the four channel inputs provided by the test system with a 1-to-4 power splitter. Each channel output from the test system provides the input signal to each port on the four-branch diversity receiver.
Each branch provides up to six independent paths. For each path, relative delay, relative path loss, Rayleigh fading, Rician fading, Suzuki fading, phase shift, frequency shift and terrain-induced fading characteristics can be programmed.
The test system also provides programmable correlation between Rayleigh faded signals in different branches. It allows you to program the correlation factor between faded paths in any of the four branches. The correlation factor is a measure of the similarity between the output waveforms of two branches. Correlation factors between the extremes of 0.0 and 1.0 describe the degree of the relationship between the output signal amplitudes of the two paths at any instant in time.
The test system contains many intricate features. Test software developed to simplify configuration of the instrument’s various parameters controls the instruments and precisely correlates fading between branches.
The primary application of the system is to test the diversity receivers in a laboratory environment under controllable and repeatable conditions. Typical steps in the test process are:
Run bit error rate and SNR tests on the diversity receivers against a wide range of conditions.
Compare lab data to statistical data to see how close the actual performance tracks the theoretical performance.
Compare field trials to lab results to ensure system is operating properly.
References
1. Jakes, W.C,
Microwave Mobile Communications
, IEEE Press, 1994, ISBN 0-7803-1069-1.2. Hess, G.C.,
Land-Mobile Radio System Engineering,
Artech House, 1993, ISBN 0-89006-680-9.About the Author
Pat Petillo joined Telecom Analysis Systems in 1992 as a Sales Engineer and currently is the Product Manager of Wireless Test Systems. Previously, he worked at Teledyne Brown Engineering and BTG on military communications systems. Mr. Petillo graduated from Rutgers College of Engineering and from Fairleigh Dickinson University with an M.S. degree in electrical engineering. Telecom Analysis Systems, 34 Industrial Way E., Eatontown, NJ 07724, (908) 544-8700.
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Copyright 1996 Nelson Publishing Inc.
September 1996