Different modulation schemes supporting multiple data rates in a Direct Sequence Code Division Multiple Access (DS-CDMA) system are studied. Both AWGN and multipath Rayleigh fading channels are considered. It is shown that the multi processing-gain scheme and the multi-channel scheme have almost the same performance. However, the multi-channel scheme may have some advantages due to easier code design and multiuser receiver construction. The drawback though, is the need for linear amplifiers also in mobile terminals. A multi-modulation scheme is also possible, but the performance for the users with the high data rates are significantly worse than for the other schemes. Furthermore multi chip-rate, parallel combinatory spread spectrum (PC/SS), pulse position modulation (PPM) and variable duty cycle schemes are discussed briefly.

On Schemes for Multriate Support in DS-CDMA Systems

Different modulation schemes supporting multiple data rates in a Direct Sequence Code DivisionMultiple Access (DS/CDMA) system are studied. Both AWGN and multipath Rayleigh fadingchannels are considered. It is shown that the multi processing-gain scheme and the multi-channelscheme have almost the same performance. However, the multi-channel scheme may have someadvantages due to near-far resistance, easier code design and multi-user receiver construction.The drawback though, is the need for linear ampliﬁers also in mobile terminals. A multi-modula-tion scheme is also possible, but the performance for the users with the high data rates are signiﬁ-cantly worse than for the other schemes. Furthermore multi chip-rate, parallel combinatory spreadspectrum (PC/SS) and pulse position modulation (PPM) are discussed brieﬂy.Keywords: DS/CDMA, Spread spectrum, multi-rate schemes, PCS, multi-channel, multi chip-rate, multi processing-gain and multi-modulation schemes, Parallel Combinatory Spread Spec-trum (PC/SS), parallel channels, Amplitude Modulation (AM), Quadrature Amplitude Modula-tion (QAM).

Multi-Rate Performance in DS/CDMA Systems

To support variable data-rates multi-code code-division multiple-access (MC-CDMA) systems have been proposed for mobile communications. They allow the support of variable source data-rates by variation of the set of spreading-sequences assigned to each user. The impact of the sequence-design on the system performance is considered. For the antifading capability and the multiuser interference performance measures are derived. We describe sequence-sets which are almost optimum with respect to these parameters and which allow each user to change its data-rate without affecting the other users.

Adaptive multi-rate multi-code CDMA systems

A parallel combinatory OFDM system (PC-OFDM) is proposed. The proposed system is able to obtain a higher bandwidth efficiency than an ordinary OFDM system. The bit error rate performance is analyzed and simulated. On a Gaussian channel and on known time invariant channels the PC-OFDM system can obtain lower bit error rates than ordinary OFDM. In some cases we can at the same time increase the bandwidth efficiency compared to an ordinary OFDM system. Further, the peak to mean power ratio of the PC-OFDM system is lower than for OFDM, which reduces the problem of designing linear power amplifiers.

A parallel combinatory OFDM system

The multiple access performance of parallel combinatory spread spectrum (PC/SS) communication systems in Rayleigh fading channel is studied. The PC/SS system has the high-speed data transmission capability by transmitting multiple pseudo-noise sequences out of the pre-assigned sequence set. The performance is evaluated in terms of average bit error rate by numerical computation. Focus is put on the frequency-nonselective, slowly fading channel. It is found that the multiple access performance of the PC/SS system including diversity is superior to that of conventional direct-sequence spread spectrum systems. Applying Reed-Solomon coding to the PC/SS system improves of the multiple access performance. >

Performance evaluation of parallel combinatory SSMA systems in Rayleigh fading channel

http://ieeexplore.ieee.org/iel4/5396/14591/00665671.pdf

G. Marubayashi

Papers

Performance of the Parallel Combinatory Spread Spectrum Multiple Access Communication System with the Error Control Technique