安達  文幸

IEEE Transactions on Vehicular Technology

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Adachi, Fumiyuki

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Periodic switching diversity effect onco-channel interference performance of adigital FM land mobile radio著者 安達  文幸journal orpublication titleIEEE Transactions on Vehicular Technologyvolume 27number 4page range 220-223year 1978URL http://hdl.handle.net/10097/46501220 IEEE  TRANSACTIONS ON VEHICULAR  TECHNOLOGY,  VOL. VT-27,  NO. 4, NOVEMBER 1978 Periodic  Switching  Diversity  Effect on Co-Channel Interference  Performance of a Digital FM Land  Mobile  Radio FUMIYUKI  ADACHI Absmrcr-The effect  of periodic  switching  diversity on the bit error rate (BER) performance of a binary frequency-shift  keying (FSK) system in the presence of cochannel interference is described. The distribution of the signal-to-interference  energy ratio per bit presented to the FM detector is found and the diversity effect on the  BER performance in a Rayleigh  fading  environment is analyzed. The  diversity effect on the BER performance in a Manchestercoded PSK system with limiterdimhinator detection is v d ~ e d  by laboratory sirnulrtion tests using a Rayleigh  fading  simulator. I.  INTRODUCTION I N UHF OR microwave land mobile radio communication systems, the same carrier frequency is reused in different radio zones for efficient frequency utilization. Since the signal transmission is usually performed via multiple propagation paths, both desired and interfering signals  are subject to Rayleigh fading with vehicle movement [ 11, [2].  In  the presence of fading, the desired  signal may fade  below the received inter- fering signal with a high probability, so that  the signal trans- mission performance is  severely degraded. It is  well known that diversity techniques are  able to improve the cochannel interference performance in a fading signal environment, Many diversity techniques have been proposed and researched [3], [4]. A simple and efficient diversity technique suitable for  a digital FM land mobile radio using a binary FSK system with limiter-discriminator detection has been proposed, and its diversity effect on the bit error rate (BER) performance in a Rayleigh fading signal environment has been verified [5]. This technique receives two RF signals periodically by switching two antenna branches at  a rate moderately higher than the  bit rate (see Fig.  1). This paper  describes the periodic switching diversity effect on the BER performance of a binary frequency-shift keying (FSK) system in the presence  of co-channel interference. The distribution of the signal-to-interference energy ratio per bit (SIR) presented to the FM detector is found, and the diversity effect on the BER performance in the presence  of co-channel interference is analyzed. Its diversity effect in a Manchester- coded FSK system with limiter-discriminator detection is verified by laboratory simulation tests. Manuscript  received  January 3,  1978; revised May 2,1978. The author is with the Mobile Communications Section, Electrical Communication  Laboratories,  Nippon  Telegraph and Telephone Public Corporation, 1-2356, Take,  Yokosuka-shi,  Kanagawa-Ken, 238-03, Japan. 11. DIVERSITY EFFECT ON THE BER  PERFORMANCE  IN THE PRESENCE OF CO-CHANNEL INTERFERENCE A. Distribution of SIR The periodic switching diversity strategy is to switch be- tween two  RF waves received by  the different antenna branches periodically at  a rate moderately higher than the  bit rate of the binary FSK signal (see Fig. 1). The diversity can change the statistics of the SIR presented to the FM detector. The  analysis used in [5] can be also applied to provide the probability density function (pdf) of the SIR. Let us assume that  a single interferer exists and that  both desired and interfering signals exhibit mutually independent slow  Rayleigh  fadings. When the switching rate is much higher than the bit rate, two RF waves received by the respective diversity antennas are periodically switched and combined in a bit interval. Thus the signal energy E,  and the interference energy ei in a bit interval, presented to the FM detector, become one half of the sum of the corresponding energies on the respective  diversity branches, respectively. We obtain where E,, and are the signal energy and the interference energy, in a  bit interval, on the nth branch (n = 1,2), respec- tively. Following the result in [5], the  joint pdf of E,  and ei,  d e , ,  ed ,  is  given by where E, and Ei are the average  signal  energy and the average interference energy on each branch, respectively, and k is the complex fading correlation coefficient between the  two diver- sity branches. The complex fading correlation coefficient is related to the envelope correlation coefficient p. p is very nearly equal to I k l2 [2] . Therefore, defining the  SIRpresented to the FM detector as X = €, /e i ,  the pdf of X, p(X), is easily 0018-9545/78/1100-0220%00.75 0 1978 IEEE Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 07,2010 at 19:46:39 EST from IEEE Xplore.  Restrictions apply. ADACHI:  PERIODIC  SWITCHING  DIVERS TY AND CWHANNEL INTERFERENCE 22 1 A N T E W  method, which adjusts the branch gains to be equal to the RF SWITCH complex conjugate of  the desired signal pilot phasor, and the separate-pilot method, which adjusts the branch gains to be equal to the complex conjugate of the sum of the desired  signal Fig. 1. Periodic  switching diversity. Bond and Meyer 161 have  d rived the probability distribution of the SIR X for  the selection combining using the total-power method. The probability distributions of X for the selection combining using the desired signal power method and for  the {2(1 + I k 12XA + the BER Pe(A) is equal to one half of the cumulative proba- bility at X = 1 as shown in (4). For the selection combining, urn pilot phasor and the interferer pilot phasor. derived from (2): A maximal-ratio combining are  derived  n [7]. Remember that PO) = - (1 - I k 12)2[{A(1 - I k I) + A(l + I k I)}-1 we obtain + I N 1  + I k I) + X(1 - I k 1 13A + 3 2 (A + 1XA + 3X3A + 1)’ (3) pe(A) = - where A = EJEi is the average SIR on each branch. B. BER Perfonnance for  the total-power method With one crucial but not unreasonable assumption, a decep- and tively  simple form may be obtained for  the BER performance on the basis of the FM capture effect. Let us assume the fol- 1 lowing. No error can occur as long as the signal energy eJ = (A + 1XA + 2) presented to the FM detector does not fade below the inter- ference energy ei presented to the FM detector, i.e., X 2 1. If for  the desired-signal-power method. (8) eJ fades below ei,  i.e., X < 1, errors occur with probability 1/2. Let us denote the BER by Pe(A) when the average SIR on For the rnaximal-ratio combining, we obtain each branch is A. From the above assumption, the  BERis equal to one half  of the cumulative probability at X = 1 : 1 1 P,(A) =- - for  the perfect-pilot method (9) 2 (A + 1)2’ 1 3 A + 1  Substituting  (3)  into (4), we obtain P,(A) = - - for  the separate-pilot method. 2 (A+  113’ 3 A + l   + l k l 2 ( A - 1 )  = 2(A + IX(A + 1)2 - 1 k I2(A - 1)2}’ ( 5 )  (10) By comparing ( 5 )  with (7)-(10), it is found that the per- For k = 1, ( 5 )  reduces to the BER without diversity: formance of the periodic switching diversity is just  the same as that of the maximal-ratio combining using the separate-pilot 1 1 method. Equations (5x10) are plotted in  Fig. 2. P e ( A ) = - * -  2 ( A + I ) ’  111. EXPERIMENTAL  PROCEDURE Comparing (5) with (9, it is found that the periodic switching diversity can improve the cochannel interference performance. A blockdiagram of the laboratory simulation test system is C. Comparison shown in Fig. 3. We compare the periodic switching diversity with the well-known selection and maximal-ratio combinings with unconelated  two branch signals. There are several methods of instrumenting the selection and maximal-ratio combiners [4]. For the selection combiner, we consider two methods: the total-power method, which selects the branch with the larger total-power (desired signal power plus interference power), and the desired-signal-power method, which selects the branch with the larger desired signal power. For the maximal-ratio combiner, we also consider two methods: the perfect-pilot A. Transmitter Both desired and interfering modulating signals are mutually independent 51 1-bit pseudonoise (PN) sequences with a bit rate of fb = 600 bit/s. In an each encoder, after being differen- tially encoded, the above PN sequence is transformed into a Manchestercoded  data signal. Then each !WO” band carrier is frequency modulated with the  Manchestercoded  data signal, resulting in a Manchestercoded FSK signal equivalent to a nonreturn to zero (NRZ) binary FSK signal with a bit rate of 1200  bit/s. Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 07,2010 at 19:46:39 EST from IEEE Xplore.  Restrictions apply. 222 IEEE  TRANSACTIONS  ON VEH CULAR TECHNOLOGY,  V L. VT-27,  NO. 4, NOVEMBER 1978 IO-’ 8 -  6 -  4 -  T W O  BRANCH  DIVERSITY RAYLEIGH FADING C W P L E X  FADING CORRELATION COEFFICIENT k=O 2 -  16‘ - 8 -  WITHOUT DIVERSITY 6 -  4 -  2 -  c k 8 -  : I d ’  PERIODIC SWITCHING DIVERSITY P 4 -  - 2 6 -  8 f 2 - TOTAL-POWER METHOD SELECTION COMBINING DESIREO-SIGNAL-POWER LETMx, los4- MAXIMAL-RATIO  COL(8INING 8 - PERFECT-  PILOT  METHOD - SEPdRATE-PILOT  METWW C Receiver The  receiver consists of an RF switch unit, an FM receiver, and a decoder. The RF switch unit consists of a pi-n diode switch and an astable multivibrator with adjustable free running frequency. The FM receiver  is a conventional double conversion type using a limiter-discriminator. Two Butterworth crystal filters are used as the first and second IF filters of this FM receiver.  Overall 3-dB predetection bandwidth is 16 kHz. The decoder is composed of a postdetection filter,  a timing recovery circuit, decision circuit, and a difference-logic circuit. The post detection filter is a Gaussian-type active filter with 3 d B  bandwidth of 626 Hz. The timing recovery circuit is composed of a digital phase-locked loop with timing error smaller than ?9 degrees. The postdetection filter output is decided upon as “mark” or “space” by the decision circuit. After being differentially decoded by the difference-logic circuit, the BER is measured by the error rate counter. IV. EXI’ERIMENTAL  RESULTS AND DISCUSSION The BER performance was measured for a typical vehicle speed of 48 h/h, and the average camer-to-noise ratio (CNR) - - 5  0 5 IO I5 20 2s 30 35 on each diversity branch of 45 dB. In the following measure- AVERAGE SIR : A Id81 ments, the  SIR on the  two diversity branches were equal to Fig. 2. between periodic switching diversity and each other, and the frequency deviation of the desired signal known selection and  maximal-ratio  c mbining% Af,, and that of the interfering signal Ah, were chosen as Af, = Afi = k5 kHz. -FED INTERFERING Fig. 3. Laboratory simulation test system. The carrier frequency separation between the desired  signal and interfering signal  is set to zero so that  an ideal cochannel situation may be studied. B. Fading  Simulation The two FSK  signals  are fed to the Rayleigh fading simula- tors [ 8 ] ,  respectively. Each fading simulator provides two transmission paths to simulate the correlation which two fading signals may have at  the diversity antennas. The envelope corre- lation coefficient p is  variable from 0 to 1 so that  the influence of correlation on the diversity effect may be measured. The envelope correlations in the respective fading simulators are chosen to be equal to each other. The fading of the desired signal  is independent of that of the interfering signal. Fig. 4 shows the measured BER as a  function of the switching rate f ,  for the envelope correlation of p = 0. The results indi- cate that  the measured BER decreases, then increases  again as the switching rate increases to an excessively high switching rate. Therefore, there exists an optimum value in the switching rate. A possible explanation for the BER increase at high switching rate is as follows [ 5 ]  : when the switching rate is excessively high, the spectrum of the IF fdter  input is so spread that  the signal distortion is caused by IF filter band restriction. Thus in the following measurements the switching rate was chosen to be the optimum value off ,  = 2 kHz. B. Measured  BER  Performance Fig. 5 shows the measured BER performance with the envelope correlation coefficient p as a parameter. Although differential encoding is used in the  laboratory simulation test, each measured BER value should be equal to the theoretical value predicted in Section I1 because we assume that errors occur with probability 1/2 when the signal energy fades below the interference energy. For comparison, the theoretical BER performances are plotted as solid curves. Reasonably good agreement is obtained. V.  CONCLUSION This paper describes the improvement made by periodic switching diversity on  the  cochannel interference performance of a binary FSK system with limiter-discriminator detection and Rayleigh  fading.  The distribution of the SIR presented to Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 07,2010 at 19:46:39 EST from IEEE Xplore.  Restrictions apply. ADACHI:  PERIODIC  SWITCHING  DIVERSITY  AND  CO-CHANNEL INTERFERENCE 223 lo-‘ 8 -  6 -  AVERAGE CNR=45d0 fb = 6 0 0  b/s 4 -  af, = af,:i5kHr p =o 8 EXPERIMENTAL POINTS 0 A = 14.5dE x A =19.5dB 2 SWITCHING  RATE: 1, (kHz) to-’ - 8 -  6 -  4 -  2 -  10-2 - 8 -  6’ 4 -  r 2 -  2 k 1 6 ’ ~  - B 8 :  Y 6  Li 4 -  2 -  1 6 ~  - 8 -  6 -  4 -  2 -  I 6 50- AVERAGE CNR=45dB f b = 6 0 0  b/s A f, Xnf, = t 5 k H z  f,=PkHz SOU0 CURVES SHOW THE THEORETICAL RESULTS Fig. 4. Bit error rate versus  witching rate with average SIR as Fig. 5 .  Bit error rate versus  average  SIR with p as parameter. parameter. the FM detector is found  and the diversity effect on the BER performance in a Rayleigh fading environment is analyzed. The experimental verification by the laboratory simulation tests is also presented. The ability to combat  the cochannel interference has been surely verified. However, the  adjacentchannel interference performance may be degraded because periodically switching the  RF stage  would tend to cause the  adjacentchannel  spectrum to fold over into the desired channel band unless adjacent- channel selectivity is provided before switching. ACKNOWLEDGMENT The author wishes to thank Mr. S. Ito, Chief of Mobile Radio Communication System Section and Mr. K. Hirade, Staff Engineer of Mobile Radio Communication Equipment Section for their useful  discussions. [2] R. H. Clarke, “A Statistical Theory of Mobile Radio Reception,” BeUSys Tech. J., voL 47, pp. 957-1000, July-Aug. 1968. [3] J. D. Parsons, M. Heme, P. A. Ratliff, and M. J. Withers,  “Diversity Techniques for Mobile Radio Reception,” The Radio and Efec- tronic Engineer, vol. 45, pp. 357-367, July 1975. [4] W. C. lakes, Jr., Microwe Mobile Communications, John Wiley and Sons Inc., New York, 1974, ch. 5 and  ch. 6. [5] F. Adachi,  T. Hattori, K. Hirade, and T. Kamata, “A Periodic Switching  Diversity Technique for a Digital FM Land  Mobile Radio,” ZEEE Tmn. Veh. TechnoL, voL VT-27, no. 4, pp. 000- 000, Nov. 1978. [6] F. E. Bond and H. F. Meyer, “The Effect of Fading on Com- munication Circuits Subject to Interference,” Roc. IRE, vol. 45, pp. 636-642, May 1957. [7] Ibid. [4],  pp. 362-377. [ 81 K. Hirade, K. A b ,  T. Hanazawa, and F. Adachi, “Fading Simulator for Land Mobile Radio Communication,” Trans. ZECE of Japan, VOL 58-B, pp. 449-456, Sept. 1975. REFERENCES [ I ]  Y. Okumura, E. Ohmori, T. Kawano, and K. Fukuda, “Field Strength and its Variability in VMF and UHF Land  Mobile  Service,” R d y ~ k i  Ad.chi for a photograph and biography see Page 219 of Rev. Elec. Comm Imb., vol. 16, pp. 825-873, Sept.-Oct. 1968. this issue. Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 07,2010 at 19:46:39 EST from IEEE Xplore.  Restrictions apply. 

Periodic switching diversity effect on co-channel interference performance of a digital FM land mobile radio

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Periodic switching diversity effect on co-channel interference performance of a digital FM land mobile radio

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