Packet data capacity in a wideband CDMA system

An analytical formula similar to the Qualcomm capacity formula for speech is theoretically derived for cellular packet data with ARQ. The value of the target E_b/N₀ that gives the highest system capacity can from the formula together with the E_b/N ₀ to BLER (block error rate) mapping be found. (We define a block to be the smallest unit that is re-transmissible on the air-interface.) The system capacity can be found except for a multiplicative constant which depends on the reuse and on the path loss function. System simulations aiming for confirming the theory has been performed for a downlink 32 kbps data channel in a CDMA test system with non-orthogonal DL. Results for the alpha-group concept (WCDMA) in ETSI are also shown. The optimal value of the target E_b/N₀ found by simulations is very close to the one given by the theory. Furthermore, the ratio of the capacity in the two cases investigated (with and without fast PC) is very close to what was predicted by the theory. The result in this paper indicate that using the theory makes it possible to optimize coding, interleaving and block size with respect to system capacity by only performing link simulations. Hence, system simulations are probably not needed for this optimization procedure