Spectral Efficiency of Direct-Sequence Spread-Spectrum Multiaccess with Random Spreading - Information Theory Workshop, 1998

Information theoret ic aspects of C o d e Division Mult iple Access ( C D M A ) r a n d o m directsequence spread-spectrum (DSSS) are investigated. T h e CDMA-DSSS channel wi th randomly a n d independent ly chosen spreading sequences accurately models the s i tuat ion where pseudo-noise sequences span many symbol per iods. We analyze t h e spec t ra l efficiency ( total capaci ty p e r chip) as a funct ion of t h e number of users, spreading gain a n d signal-to-noise ratio, a n d we quant i fy t h e loss i n efficiency relative to a n opt imal ly chosen s e t of s igna ture sequences a n d to a n opt imal multiaccess sys tem without spreading. Par t icu lar a t tent ion is given to t h e l imit ing spec t ra l efficiency as the number of users grows without bound. W h i t e Gaussian background noise a n d equal-power synchronous users a r e assumed. T h e analysis compr ises the following receivers: a ) opt imal j o i n t processing, b) single-user matched filtering; c) decorrela t ion a n d d) minimum m e a n s q u a r e e r r o r l inear processing. I. CHANNEL MODEL Coded direct-Sequence Spread-Spectrum (DSSS) Code Division Multiple Access (CDMA) has well-known advantages over time/frequency division multiple access: dynamic channel sharing, robustness to channel impairments, graceful degradation, ease of cellular planning, etc. These advantages result from the assignment of “signature waveforms” with large timebandwidth products to every potential user of the system. The coded DSSS IC-user model is as in [l], [2]