Effective spectral efficiency for adaptive QAM with diversity and pilot assisted channel estimation

This paper investigates the effects of non-ideal channel state information in adaptive M-ary quadrature amplitude modulation systems with subset diversity. The slow adaptive modulation (SAM) technique, which adapts the modulation parameters to slow channel variations, is considered. With respects to fixed scheme modulation systems, SAM achieves a good gain in terms of spectral efficiency (SE) and outage probability and, compared to fast adaptive modulation, reduces the system complexity by lowering the feedback rate. Resources dedicated to channel estimation affect both the system performance, in terms of error and outage probability, and the effective SE for which a proper definition is needed. Here, we evaluate the performance of SAM technique when pilot-assisted channel estimation errors are considered. Taking into account the number of pilot symbols transmitted, an effective SE is obtained to address the tradeoff between channel estimation quality and SE.