Conditional outage performance analysis framework for OFDM channels

The channel state information at the transmitter side (CSIT) is playing a more and more important role in the design of OFDM/OFDMA communication systems. Unfortunately, it is not trivial to conduct a performance analysis of OFDM systems with partial CSIT. Using the saddle-point approximation method, this paper will develop an analytical design and performance analysis framework for OFDM channels with 1 bit CSIT, which we shall refer to as the conditional outage exponent. The proposed framework will then allow us to present the fundamental relationship among the outage probability, transmission rate, SNR, outage capacity, delay-limited capacity, ergodic capacity, diversity-multiplexing tradeoff (DMT), finite-SNR DMT, and the number of diversity branches. It is surprising that the outage performance with 1 bit CSIT (conditional outage exponent) is worse than the corresponding one without CSIT (non-conditional outage exponent) in most cases. This counter-intuitive phenomenon occurs because the observation of the channel at the transmitter side will result in a state space collapse of the channel gains, i.e., from the prior probability space to the posterior probability space. As a result, the conditional outage exponent based framework can be easily used to design and evaluate the performance of existing and upcoming OFDM/OFDMA multichannel systems with 1 bit CSIT.