Single-Receiver Diversity Systems - IEEE Xplore

192

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. VT-22, NO. 4, NOVEMBER 1973

Single-Receiver Diversity Systems J. DAVID PARSONS, PAUL A. RATLIFF, MIGUEL HENZE, AND MICHAEL J. WITHERS

Abstmcr-Diversity reception techniques can help to combat fast fading in urban areas, and this paper describes some predetection combining systems designed to be compatible with exisling standard receivers. Quantitative results show that a worthwhile improvement in quality of reception can be obtained with mall numbers of antennas There are advantages to be gained if, in futurereceivingsystems, the diversity technique is incorporated directly into the receiver design.

INTRODUCTION

A

COMMON situation in mobile radio systems is that of an elevated base station on a good site attempting to control a number of vehicles located in an urban area. For frequencies in and above the VHF range, the direct wave from the transmitter is often not amajor contributing component to the received field, and propagation is principally by way of scattering from buildings and other terrainfeatures. The net signal from any receiving antenna is therefore the resultant of a number of waves that arrive from different directions with different amplitudes and phases. Acomplicatedspatial standing-wave pattern exists,inwhich the maxima and minima are spaced about a quarter-wavelength apart, and in a vehicle travelling at 35 mi/h the fadingrate is typically 10 Hz in the 100 MHz band. The nature of the field suggests that diversity reception techniques can be used to combat the fast-fading caused by multipath propagation, and hence improve the quality of reception. However it remains to decidewhich ofthemany available techniques provides an optimumsolution in given circumstances, and how it can best be integrated into both existing networks and proposed future networks.

to all others. Taking the environment as an example, British cities do not have vast numbers of skyscraper blocks, nor is the arrangement of streets as well-ordered as it is in the United States. As far as frequency and modulation method are concerned, Britain has mobile radio channels in the VHF band at various frequencies between 70 MHz and 173 MHz, where the channel separation is 12.5 kHz and the modulation method is dominantly AM. There are further channels in the 4 2 0 4 7 0 MHz UHF band where the separation is currently 25 kHz and the modulation is FM, but as yet there is no frequencyallocation formobile radio in the 900 MHz band. AU British systems are operated on a double-frequency allocation, which means that mobile-to-mobile communication is only possible via the base station. In considering diversity reception techniques for use in current frequency bands, we have been constrained by the fact that, since there is a large capital investmentin existing equipment, an economically viable system must use this equipment. It is therefore desirable that systems capable of realization in the form of “add-on” units should be designed.

SELF-PHASINGARRAYS Self-phasing antenna arrays are predetection combining systems designed to co-phase the inputs from a number of antennas and pass the sum into a standard receiver. The principle of a system suitable for use on AM radio links is shown in Fig. 1, and uses a phase-perturbing technique proposed by Lewin [ 11 . Addition of a reference signal A , and another signal E , to which a small phase perturbation has been applied, produces AM at the perturbing frequency in their sum. This modulation is in phase with the perturbing signal when B lags A , and in antiBASIC CONSIDERATIONS phase when B leads A . In the co-phased condition only a small One attractive method of impiementing the diversity system second-harmonic component is produced. is in the form of an adaptive antenna array that automatically Fig. 2 shows amuch simplified block diagram of a two-branch adjusts to the prevailing conditions, and space diversity, polardiversity combiner using this principle. One antenna signal ( A ) izationdiversity,and field diversity have been suggested as is passed directly to the summing unit, while the other ( E ) being techniques suitable for this purpose. The optimum numpasses first through a phase shiftercapable of changing the ber of elements in the array, the location of these elements on phase through 277 radians, and then into aphase-perturbing the vehicle, and the manner in which the array is controlled, unit which is a small-deviation phase modulator driven by an are variables that depend upon a number of factors,among oscillator. The resulting AM, (which is at a frequency within which are the environment, the frequency,and the modulation the IF pass band of the receiver, but outside its audio bandmethod. It is not clear that there is one scheme that is superior width) is extracted at the receiver detector and phasedetected with respect to the perturbing oscillator. The polarity of the Manuscript received March 23, 1973. fdtered dc output producedas a result of this process indicates J. D. Parsons and M. Henze are with the Department of Electronic and Electrical Engineering, University of Birmingham, Birmingham, England. the direction in which the main phase-shifter must be changed P. A. Ratliff was with the University of Birmingham,Birmingham, England, and is now with the B.B.C. Research Department, Kingswood, in order to co-phase A and E . In the practical realization of Surrey, England. this system, the phase-shifterhasbeen quantized into eight M. J. Withers was with the University of Birmingham,Birmingham, is driven by abidirectional ring counter.The England, and is now with the British Aircraft Corporation, Stevenage, stepsand counter is actuated by a clock, at a fixed rate, and the control Herts., England.

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Fig. 3. Signal amplitudes and relative phasesin an urban environment.

all the information is obtained by using a FM tape recorder, and an ultraviolet recording oscillograph has also been coupled into the system to give a visual display when required. Distance marker pulses are recorded on another tape track, and these are used during the statistical analysis of the data. A series of experiments has been conducted with this modified system coupled to three quarter-wavelength antennas on the roof of a van. Fig. 3 shows the positions of these antennas (0.4 h apart, and coded G, Y , and R), together with some representative signals in an urban area with the van being driven in normal traffic conditions.The individual antenna signals show the characteristic deep fades thatdrop below receiver sensitivity into noise, the peak-to-trough variations in this parFig. 2. Block diagram of the self-phasing array. ticular extract being 27 dB. However, over the same distance the maximum variation in the sum is only 15 dB and not only signal polarity is used to change the direction of count. The does it never drop down to thereceiver noise level, but its rate system therefore “hill-climbs,”and in a static field situation of change is much lower also. In AM systems this means that would ultimately oscillate, atthe clock rate, between two conventional receiver automatic gain control (AGC) is able to phase-shfter positions,one slightly leading and one slightly cope with the fading effects far more successfully than when lagging thetrue co-phased condition. The method is readily only one antenna is used. At the top of the diagram it can be seen that the rate of change of relative phase at the antenna extendable to more thantwo branches, provided aseparate controlloopwithits own perturbing frequency is used for terminals is quite rapid, averaging about 4n radians per waveeach additionalbranch.For FM radiosystemsadual tech- length, and this is consistent with the maximum expected fadnique using amplitude perturbation can be used [2] . ing rate. In Fig. 4, the amplitude information from the previous diaA three-branch self-phasing array using this principle has been constructed for use in the 100 MHz VHF band. Although it gram is presented in the form of a cumulative probability dishas been used under operational conditions as a diversity sys- tribution.It can be seen thatthe individual antenna signals tem, its main function to date has been as a measurement and follow a very close approximation to a Rayleigh distribution, recording facility to assess the effectiveness of diversity recep- (to avoid confusion only one signal, the R-branch, has been tion [3]. For thispurposea numberofmodifications have drawn, since all branches are very similar) while the sum signal been incorporated, and the signal strengths from the individual is never morethan 1 dBaway fromthe ideal three-branch antennas, their co-phased sum, and therelative phases between equal-gain-combining curve given by Brennan [ 4 ] . The 50 percent cumulative distribution level for the sum is about 5 dB them are all recorded by means of atime-division-multiplex sequence. A field-strength measuring receiver is used for am- greater than that for one channel, and the 90 percent level is plitude measurement, and the relative phases are determined about 9 dB greater. These arealmost exactly equal tothe from the positions of the phase shifters. A complete record of theoretical values. To complete the picture, Fig. 5 shows the

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, NOVEMBER 1973

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normahzed autocovariance functionscomputedfrom signals obtained in an urban environment. The shape of the curves is generally similar to the theoretical prediction [ 5 ] without being a close fit at any point after the first minimum. However the value is always sufficiently low to ensure that the diversity improvement is not impaired. In contrast, Fig. 6 shows some results taken in a suburban area, where there are fewer scatterers in the immediate vicinity of the vehicle. The signal strengths are higher than in theurban area, even thoughthe distance fromthetransmitter is considerably greater, and the curves for the individual signals, particularly those of the G and R antennas, which are one behind the other on the offside of the vehicle, are much flatter than the Rayleigh curve, indicating a lower variance. However, althoughthe curve forthesum signal is slightly flatterthan theory predicts, it can be seen that most of the expecteddiversityimprovement is still obtained. The autocovariance functions in this case immediately show the higher correlation that exists, the curves for the G and R antennas being quite similar. One is led to suspect that there may be higher cross-covariance between these signals thanbetweeneither of themandthe Y branch, and confirmation of thisis awaited. In this system we have used a quantized phase shifter together with a technique that enables us to decideinwhich direction the phase shifters must be changed in order to cophase the antenna signals. However the association of quantized phase shifters with sense-of-correction detectors leads to a situation in whichthe phase shifters oscillate abouttheir optimum position. This is due to the inability of such systems to detecttheoptimumposition,thecorrection signal being reversed only after that position has been passed. This oscillation can be eliminated by replacing thesense-ofcorrection detector with a true phase-measurement system, and a new,

Fig. 6 . Cumulativeprobability distributions and normalised autocovariance functions ina suburban area.

Fig. 7. Principle of the phase-measurement technique using singlesideband-withcarrier (ssbwc) modulation.

compatible, direct-reading method has been devised for this purpose [6] . The method employs a single-sideband modulation technique which is illustrated in Fig. 7. Assume that S1 and S2 are the two signals whose relative phase is to bemeasured. If S1 is single-sideband modulated without carrier suppression by alow-frequency signal M1 which has initial phase cpl, then the resultant phasor diagram is shown in Fig. 7(c). If S2 is added to S1, the modulation is transferred to the sum S, and the angle between M1 and S contains 0 1 , the angle between S1 and the sum. Amplitude detection and filtering will recover the modulatingsignal affected by the angle 0 1 , and a low-frequency phase measurement between M1 and Dl yields the angle e l . If atthe same time, S2 is modulated in the same way by another signal at a different frequency, then this modulation will also be transferred to the sum, and similar signal processing will yield 0 2 , the angle between Sz and the sum. It is a feature of this method that the angles O 1 and O 2 can just as easily be extracted by phase or frequencydemodulation,duetothenature of the singlesideband-with-carrier (ssbwc) signal. In the application of this technique to a pre-detection combining diversity system, the sum signal is used as a reference, and the phase angles O and O 2 are used directly to shift the

PARSONS et a l . : SINGLE-RECEIVER DIVERSITY SYSTEMS

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signals towards that reference. A block diagram of one branch of a diversity system using this principle is shown in Fig. 8, the receiver being a perfectly standard AM or FM receiver, without any changes. A prototype of t h s system is currentlyunder construction. AN ALTERNATIVE TECHNIQUE

In addition to self-phasing arrays, there is another technique which is compatible with single receivers and which uses the principle of theserrodynefrequency translator [7] . It is a technique that shows promise of being applicable to forms of modulation,such as single-sideband anddouble-sideband reduced carrier, and this is of particular interest, since the latter method is being considered by some British mobile-radio users

PI. Fig. 9 shows the basic system that has been used as an experimental AM diversity receiver, together with mathematical expressions atselected points for the case whenequal-strength signals are received. This is, of course, a special case but it helps to illustrate the principle of operation. The phase shifter is caused to sweep continuously and linearly through 271 radians. In every excursion through 271, there will be a value at w h c h the signals are co-phased, and at this point the receiver output will be a maximum. In order to make use of this property to produce an improved output, the filter following thedetectormust be designed so that it performsa peak detection at the sweep repetition rate, but passes the information frequencies. The sweep rate should be at least twice the highest informationfrequency in the received signal, to satisfy the Nyquist criterion. The operation of this system can also be explained in other ways. First, the two antennas can be considered as forming an array whose directional pattern is smoothed by continuously changing the relative phase as a function of time [9] . Although a wide range of phase modulations may be used for this purpose, a linear change seems most suitable for communication purposes, since it minimizes spectrum spreading. Secondly, the continuous phase change can be considered as a frequency

shift, and this frequency shift is equal to twice the highest modulating frequency in theinformationband. Thiseffectively translates one of the signals t o an adjacent frequency band that is still within the pass-band of the receiver. Signal combination then takes place at the detector. Static tests conducted with thissystem have shown that even when the phase shifter produces only a stepped approximation to thelinear phase change, an improvement of over 2 dB can be obtained with equal signals on the antennas. Mobile tests have indicated that a worthwhde improvement in intelligibility can be obtained when weak speech signals are received. CONCLUSIONS Some single-receiver diversity reception systems have been described,and quantitative results have shown thatunder Rayleigh-fading conditionsthe self-phasing array provides a significant improvement in the quality of reception. Subjective tests using speechtransmission have indicatedthatthreebranch diversity has little to offer over two-branch; however where data messages are transmitted, lugher orders of diversity may be desirable to avoid large error rates. All the systemsdescribed can be realized in the form of "add-on" units suitable for use with existing receivers. However when new receiving systems are being considered, a number of advantages can be obtained by incorporating the diversitysystemdirectly intothe receiver design. Inthe selfphasing systems the phase shifters could be replaced by mixers coupled to variable-phase local oscillators. In the other system two local oscillators separated by at least 6 kHz can be used and the signals combined at the intermediate frequency. If, in addition, a narrow-band RF amplifier is used in each branch, this will eliminate the susceptibility of the system to adjacentchannel interference. It should be mentioned that an identical system for use in HF systems [ l o ] was described shortly after the original disclosure of thesystemdescribedinthispaper [ 1 11 . However there is no restriction on frequency, and there is no reason why the techniqueshould not be readily applicable in both situations.

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NOVEMBER TECHNOLOGY, VEHICULARTRANSACTIONS ON IEEE

REFERENCES L. Lewin, “Diversity reception and automatic phase correction,” Proc. Inst. Elec. E n g , vol. 109 B, pp. 295-304, July 1962. J. D. Parsons and P. A. Ratliff, “Self-phasing aerial array for FM communication links,” Electron. Lett., vol. I , pp. 380-381, July 1971. [31 P. A. Ratliff, J. D. Parsons, and E. D. R. Shearman, “SpacediversityreceptionforVHF mobile radio,” Electron. Lett., vol. 7, pp. 655-656, Nov. 1971. D. G. Brennan, “Linear diversity combining techniques,” Proc. IRE, vol. 47, pp. 1075-1102, June 1959. R. H. Clark, “A statistical theory of mobileradioreception,” Bell Syst. Tech. J . , vol. 47, pp. 957-1000, July-Aug. 1968. J. D. Parsons and M. Henze, “A proposed predetection combining system for mobile radio,” in Proc. Institution of Electronics and Radio Engineers Conf Radio Receivers and Assoc. Systems, July 1972. [7 1 M. J. Withers, “A diversity technique for reducing fast-fading,” in Proc. Institution of Electronics qnd Radio Engineers Conf. Radio Receivers and Assoc; Systems, July 1972. [81 E. W. Crompton, ‘The development of s i g n a l processing tech; niques to meet home officetelecommunicationrequirements, in Roc. Institution-ofElectrical EL’gineers Con5 S@al Processing Methods for Radio Telephony,London, May 1970. S . Kazel, “Antennapattern smoothing by phase modulation,” Proc. IEEE (Corresp.), vol. 5 2 , p. 435, Apr. 1964. 0. G. Villard, Jr., J. M. Lomasney,and N. M. Kawachika, “A mode-averaging diversity combiner,” IEEE Trans. Antennas Prop agat., vol. AP-20, pp. 4 6 3 4 6 9 , July 1972. M. J. Withers, “Single-receiver diversity system for reducing the effect of fast-fading in mobile radio,” Electron. Lett., vol. 7, pp. 721-729, Dec. 1971.

1973

Paul A. Ratliff was born in Bristol, England in 1948. He graduated from Birmingham University, Birmingham, England, in 1969 with a firstclass honors degree in electronic engineering, and was awarded a British Broadcasting Corp. (BBC) research scholarship. He stayed onat Birmingham University to work on diversity reception techniques for mobeing in a bile radio, his particularinterest study of VHF multipath fading in urban areas. He is now with the BBC Research Department. Kingswood, Surrey, England.

hiiguel Henze was born in Rio de Janeiro, Brazil, in 1939. He graduated in electronic engineering from the Instituto Tecnologico de Aeronautica, Brazil, receiving the degrees of B.Sc. in 1962 and M.Sc. in 1970. After graduation he spent eight months with Standard Electrica S.A., an I.T.T. subsidiary in Rio de Janeiro, and one year at the Technische Hochschule, Darmstadt, Germany, supportedby theRotaryFoundation. Since 1964 he has held an appointment as Lecturer at the Instituto Tecnologico de Aeronautica. MI. Henze is now on leave of absence from his University, and has joined the Mobile Radio Research Groupin the Department of Electronic and Electrical Engineering, University of Birmingham, Birmingham, England, where he is working on diversity reception techniques for UHF mobile radio.