How far should we spread using DS-CDMA in time and frequency selective fading channels?

Spreading is used to provide diversity in frequency and results in a number of benefits. However, recent literature has shown that for a channel that decorrelates in time and frequency, the capacity of a wideband system in a fading channel goes to zero in the limit of infinite bandwidth (given second and fourth moment constraints that decay with the spreading bandwidth). We first extend to the two-user case the upper bound from Medard and Tse (Ref.5). Together with a lower bound based on binary transmission, meaningful bounds to the optimal spreading bandwidth using DS-CDMA are obtained. Given typical system and channel parameters, the minimum spreading bandwidth behaves as a linear function of channel strength. We will show that this result can be meaningfully applied as guidelines in the design of future communication systems.