Capacity of CDMA Fading Channels

The capacity of multiple-access channels subject to fading has attracted much recent interest exemplified by [I, and references therein]. The spectral efficiency (total capacity per chip) of the additive white Gaussian noise synchronous direct-sequence spread spectrum channel with perfect power control and randomly chosen spreading sequences was obtained in [2] as a function of the system load and signal-to-noise ratio. The loss in efficiency incurred by random spreading relative to both optimally chosen signature sequences and to multiaccess with no spreading as well as the loss in efficiency incurred by several linear multiuser detectors are studied in [2]. See also [3] and [4] for various results on suboptimal linear detectors. CDMA spectral efficiency under homogeneous (chip-independent) fading [3] was already addressed in [2]. The key aspects of the fading multiaccess model considered in the present paper are: synchronous users with common processing gain N; random spreading sequences known at the receiver; slow independent fading, fading amplitudes remaining constant per information symbol and otherwise ergodic, with arbitrary first order distribution; the fading is independent and equally distributed for all transmitters; the realizations of the fading amplitudes are perfectly known at the receiver; additive white Gaussian noise; fixed and arbitrary system load p = K / N , while K,N The total capacity of the multiaccess fading channel and with equal normalized power (SNR) for all users is 00.