Abstract: In this paper, the capacity region of downlink DS CDMA system under frequency selective fading channels is derived. Closed form solution of the optimum resource allocation schemes for multi user is obtained assuming that the channel information is known by both transmitter and receiver. The equation shows that a nonuniform service distribution, which is obtained by allocating more resource to a better channel quality, leads to a higher capacity. Numerical results show that the capacity of CDMA system changes with different diversity combining schemes and different priorities. MRC has the best system capacity.

Key words: optimum power allocation, Shannon capacity, CDMA system