Increased capacity using CDMA for mobile satellite communication

1、IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS. VOL. 8. NO. 4. MAY 1990 503 Increased Capacity Using CDMA for Mobile Satellite Communication Abstract-In this paper the performance of a spread-spectrum CDMA system in a mobile satellite environment is analyzed. Compar- isons to single-channel-per-carrier FDMA systems are also carried out which show that the CDMA approach provides greater capacity. Results from computer simulations, laboratory tests, and field tests of a prototype modem are also

2、presented. The tests results show excellent performance of the modem in the mobile environment and also the feasibility of the spread-spectrum approach to satellite mobile com- munications. INTRODUCTION HIS paper will demonstrate that spread-spectrum T CDMA systems provide an economically superior so- lution to satellite mobile communications by increasing the system capacity compared to single-channel-per-car- rier FDMA systems. Following the comparative analysis of CDMA and FDMA systems, the p

3、aper describes the design of a modem that was developed to test the feasi- bility of the approach and the performance of a spread- spectrum system in a mobile environment. Results of ex- tensive computer simulations as well as laboratory and field tests results are presented. The bit error rate (BER) of the rate 1 /3, constraint length 9, convolutionally encoded BPSK modem proved to be within 0.3 dB from theory in a AWGN channel. A BER of which is adequate for voice was achieved at Eb/No = 2.5 d

4、B. The powerful convolutional code com- bined with interleaving and a robust modulation also pro- vide excellent performance in Rician fading and lognor- mal shadowing. The paper is organized as follows. In Section I, a re- view of previous comparisons of the maximum through- put achievable by CDMA and FDMA systems is given. In Section 11, the comparisons between CDMA and FDMA are rederived for mobile satellite systems. In Sec- tion 111, a description of the spread-spectrum modem and the perform

5、ance results are given. I. SPECTRAL EFFICIENCY OF CDMA AND FDMA In l, Viterbi compared the spectral efficiency of CDMA and FDMA as a function of total carrier-to-noise power ratio. The comparison was carried out for uncoded BPSK (QPSK) as well as convolutionally coded systems Manuscript received February 15, 1989; revised October 2, 1989. This work was supported in part by Hughes Aircraft Company. This paper was presented in part at the Mobile Satellite Conference, Pasadena. CA, May 1988. The au

6、thors are with QUALCOMM. Inc. San Diego, CA 92 12 I. IEEE Log Number 9034779. with various code rates. We review here the main results In a multiple access satellite system uplink from a num- ber M of similar mobile user terminals, each assumed to provide equal incident power at the satellite, the total re- ceived carrier power at the satellite C is given by of l. C = MEl,RI, (1) where E, = Energy per bit of information. Rj, = Each Users information rate. If we divide C by N, W, where No is the

7、single-sided thermal noise spectral density and W, is the total occupied system bandwidth we obtain Link performance is measured by the spectral efficiency 7 of the transponder or link as a function of C/N, W, and is defined as C (3) No In a spread-spectrum system the total noise is deter- mined by the sum of the thermal noise No and the mutual interference noise spectral density Io. The desired bit error rate performance of the link is determined by the Eh/(N, + I,) or the ratio of the bit ener

8、gy to the single-sided total noise power spectral density. The total noise N, + I, is given by the following: N, + I, = N, + (M - 1)E, (4) where E, = energylchip, defined as E, = EhRh/W, re- sulting in Dividing Eh by N, + I, we obtain the following expression: (6) Eh /No - - Elf No + I, M-1 c. 1 +- M NoWs 0733-8716/90/0500-0503$01 .OO O 1990 IEEE 504 IEEE JOURNAI. ON SELkCTED AREAS IN COMMUNICAIIONS. VOL 8. NO 3. MAY 1990 Solving (6) for E,”, and substituting into (3), we ob- tain the following

9、expression for the CDMA spectral ef- ficiency: C WDMA = E No 3 For a system with a large number of users M we obtain: C - bits/sec/Hz. (8) No w, VCDMA No + 41 For a single-channel-per-carrier FDMA system, there is only one user per bandwidth segment. Therefore, Eh/(NO + I,) = &/No and (3) becomes C No for the power limited case, and the following expression for the bandwidth limited case: where r = code rate, m = signal constellation dimension (m = 2 for RI, = each users information rate, and BPSK, m = 4 for QPSK, etc.), GFDMA = FDMA guardband factor. The FDMA guardband factor allows margin for adja- cent channel interference. For the purpose of comparison, the 5 KHz channelization of the L-band spectrum assigned to mobile services, as proposed by the American Mobile Satellite Consortium (AMSC) in 4 is assumed. Thus, in the previous example GFDMA = 0.5, i.e., an octal symbol rate of 2400 sps in the 5 KHz channel. For links that are neither power nor int

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