On the Use of Log A Posteriori Probability Ratios for Diversity Combining

The performance of diversity combining based on the Data Quality Metric (DQM) is limited by the challenges of accurately estimating the received Eb/N0 in the demodulator. This paper develops an alternative method for diversity combining that does not require estimates of the received Eb/N0. The development of the method starts with the log a posteriori probability ratio for each bit. The log a posteriori probability ratio depends on Eb/N0, but a scaled version, called relative reliability, depends on the amplitude of the received signal, not on Eb/N0. Computer simulations show the BER performance of combining using relative reliability achieves maximum ratio diversity combining. The BER performance of quantized versions of relative reliability is also examined. It is shown that 5-bit quantizers achieve essentially all of the diversity gain; 4-bit quantizers are within 0.1 dB of unquantized performance.