Resource pooling and effective bandwidths in CDMA networks with multiuser receivers and spatial diversity

Much of the performance analysis on multiuser receivers for direct-sequence code-division multiple-access (CDMA) systems is focused on worst case near -far scenarios. The user capacity of power-controlled networks with multius er receivers are less well-understood. In [1], it was shown that under some conditions, the user capacity of an uplink power-controlled CDMA cell for several important linear receivers can be very simply characterized via a n otion of effective bandwidth. In the present paper, we show that these resu lts extend to the case of antenna arrays, We consider a CDMA system consist ing of users transmitting to an antenna array with a multiuser receiver, an d obtain the limiting signal-to-interference (SIR) performance in a large s ystem using random spreading sequences. Using this result, we show that the SIR requirements of all the users can be met if and only if the sum of the effective bandwidths of the users is less than the total number of degrees of freedom in the system. The effective bandwidth of a user depends only o n its own requirement. Our results show that the total number of degrees of freedom of the whole system is the product of the spreading gain and the n umber of antennas. In the case when the fading distributions to the antenna s are identical, we show that a curious phenomenon of "resource pooling" ar ises: the multiantenna system behaves like a system with only one antenna b ut with the processing gain the product of the processing gain of the origi nal system and the number of antennas, and the received power of each user the sum of the received powers at the individual antennas.