Asynchronous CDMA Systems With Random Spreading - Part I: Fundamental Limits

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Spectral efficiency for asynchronous Code Division Multiple Access (CDMA) with random spreading is calculated in the large system limit allowing for arbitrary chip waveforms and frequency-flat fading. Signal to Interference and Noise Ratios (SINRs) for suboptimal receivers, such as the linear Minimum Mean Square Error (MMSE) detectors, are derived. The approach is general and optionally allows even for statistics obtained by under-sampling the received signal. All performance measures are given as a function of the chip waveform and the delay distribution of the users in the large system limit. It turns out that synchronizing users on a chip level impairs performance for all chip waveforms with bandwidth greater than the Nyquist bandwidth, e.g., positive roll-off factors.