We present information theoretic results associated with digital cellular radio with special emphasis on time division multiple access (TDMA) protocols. In the considered model, the message is transmitted in K blocks (slots) each of duration T. where the signal occupies a bandwidth of W (Hz). The value of K is dictated by the decoding delay constraints, and we assume that 2WT much greater than 1: that is, in each block, large block-length codewords (or large constraint length convolutional codes) are accommodated. In each block, it is assumed that the signal is received through a two (or a single, as a special case) ray propagation mechanism, where the scattered components are modeled by independent complex Gaussian processes of bandwidth B (Hz). It is also assumed that BT much less than 1, and, therefore, the scattered components in each block are approximated by complex random variables which are in general correlated. The receiver is supposed to be fully aware of the channel parameters in all K blocks, while the transmitter does not possess this knowledge. For stringent decoding delay constraints (i.e., K is small) the system has no Shannon capacity in the strict sense, and a natural information-theoretic performance measure is based on the capacity versus outage probability characteristics, where the latter is determined by the probability that the instantaneous channel parameters cannot support the considered data rate. Average capacities are also considered, and it is shown that these are of little value when used to predict performance for small K and acceptably low outage probabilities. For K --> infinity (absolutely relaxed delay constraints), however, and under mild conditions of decrease in correlation between the scattered gains in separated blocks, the average capacity yields the Shannon capacity in the usual sense. It is demonstrated that for small outage probabilities, which are of practical interest, the double ray propagation mechanism yields an advantage in the capacity versus outage performance sense when compared to a single ray propagation model, under the same signal-to-noise ratio (rho) conditions. For K = 2, the performance of the optimal communication method is compared to that achieved by a simple time-diversity (two-fold repetition) with either selection or optimum ratio combining, and it is concluded that even these simple time-diversity methods are useful and enhance performance considerably. The advantage is performance for K = 2 over K = 1 is mitigated to a serious degree only for large values of the correlation (tau) between the scattered components in the two blocks. Space diversity with two diversity branches is also addressed, reinterpreting the results developed for the two-fold time-diversity method.
