Improving the performance of IS-54/136 through the use of the MDM scheme

In this paper, we analyze the performance of the multiply-detected macrodiversity (MDM) scheme over a correlated-error channel that uses a convolutional coder and a soft decision Viterbi decoder. The MDM scheme is a post-detection combining technique; i.e., the signal from the target mobile is detected by three base-stations simultaneously and the digital data are sent to a central point for combining, which is based on the maximum likelihood criterion. In particular, we have applied our evaluation framework to the IS-54/136 standard, examining the MDM improved performance in its speech channel and in its fast associated control channel (FACCH). These performance figures were then compare with the performance of the traditional selection-based macrodiversity schemes. Our results show that through the use of the MDM scheme, significant improvement can be achieved in the IS-54/136 cellular systems. As few points of reference, for power loss exponent of 4.0 and shadow fading standard deviation of 4.0 dB, the MDM scheme can eliminate about 50% of the FACCH outage events, as compared with the (S/I)-diversity. This corresponds to an increase in the user capacity of 20%-60%. In addition, the word error rate of the FACCH can be decreased, on average, by at least one order of magnitude throughout 80% of the cell area for the power loss exponent of 3.5 and the shadow-fading standard deviation of 8.0 dB.