INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS Int. J. Satell. Commun., 16, 87–93 (1998) PERFORMANCE OF DS/CDMA SYSTEMS WITH DIFFERENTIAL M-ARY ORTHOGONAL MODULATION AND RS-CODING FOR LEO SATELLITE COMMUNICATIONS a. c. iossifides and f.-n. pavlidou * Telecommunications Division, Department of Electrical and Computer Engineering, School of Engineering, Aristotle University of Thessaloniki, PO Box 1641, 54006 Thessaloniki, Greece SUMMARY This paper presents a coded modulation scheme based on M-ary orthogonal modulation by means of Walsh–Hadamard (WH) sequences, suitable for low-earth-orbit (LEO) direct sequence/code division multiple access (DS/CDMA) satellite communication systems. Based on the IS-95 scheme, we consider Reed–Solomon (RS)-coded M-ary orthogonal modulation with error or erasures decoding, which presents good performance enhancement with low complexity. LEO satellite links are characterized by large Doppler frequency shifts caused by the difference in velocity between the satellite and the earth mobile terminal, which make conventional non-coherent detection ineffective. In order to overcome the phase shift variations during the symbol period, which result in orthogonality loss of the WH sequences, we applied a differential encoding process to the spreading sequences or the WH chips prior to transmission. A special diversity process suitable for the environment under consideration is also applied. Simulation results show that the proposed diversity/coding/modulation scheme attains very good performance at low transmitter/receiver complexity.  1998 John Wiley & Sons, Ltd. key words: differential encoding; DS/CDMA; LEO satellite communications; RS codes; M-ary orthogonal modulation 1. INTRODUCTION M-ary orthogonal modulation by means of WH sequences is a well-established scheme for DS/CDMA applications. It has been investigated in several configurations for terrestrial cellular mobile, either indoor or outdoor, and LEO satellite communications. 1–10 The increased spectral efficiency that M-ary orthogonal modulation offers, compared with conventional DS/CDMA systems, has placed it among the most promising schemes for future wireless communications. In conjunction with proper modulation, effective channel coding is very important for CDMA systems in order to ach- ieve higher bandwidth efficiency and user capacity. Lately, RS-coded M-ary orthogonal modulation with error decoding has been proposed and found to present good performance with low complexity (bounded distance decoding). 1,2 In this paper, eras- ure decoding is also considered resulting in further performance enhancement without significant com- plexity increase, owing to a simple erasure criterion. The special character of the LMS (land mobile satellite) channel originates crucial problems in the *Correspondence to: F. N. Pavildou, Telecommunications Division, Department of Electrical and Computer Engineering, School of Engineering, Aristotle University of Thessaloniki, PO. Box 1641, 54006 Thessaloniki, Greece. Email: niovivergina.eng.auth.gr CCC 0737–2884/98/020087–7$17.50 Received December 1997  1998 John Wiley & Sons, Ltd. Revised 27 January 1998 Accepted 18 March 1998 application of M-ary orthogonal modulation. The most important one is the large performance degra- dation that M-ary orthogonal modulation faces in high-Doppler-shift environments such as the LMS channel when envelope detection is applied. This is due to the phase shift of the transmitted wave- forms, which destroys orthogonality at the receiver. 10,11 Thus envelope detection without any special device to remove the Doppler shift (which takes values of the order of the symbol rate or even higher) is applicable only for low Doppler shifts (lower than 0.3 times the symbol rate). Instead of using such a device and in order to keep the com- plexity and cost as low as possible, we have recently applied two techniques based on chip-by-chip differ- ential encoding. Chip-by-chip differential encoding after spreading was first proposed in Reference 12. In this paper the differential process is applied either to the resulting wave-form after coding and spread- ing (DDS M-ary) 13 or at the WH chip level (DM- ary). 14 This allows effective non-coherent detection when the Doppler shift is lower than the chip rate. Thus the system performs almost independently of the Doppler shift, but this is achieved at the expense of the signal-to-noise-plus-interference ratio (SNIR) increase that is required to attain the desired bit error probability (BEP). In high-Doppler-shift environments where the path amplitude may vary during the symbol period, the selection or combination of two or more diversity