Cross-layer combining of adaptive pilot-symbolassisted modulation with truncated automatic repeat request in the presence of imperfect channel state information at both the receiver and the transmitter

In this study the authors provide an adaptive cross-layer design that maximises the spectral efficiency under prescribed quality of services (QoSs) such as error rate and delay. The considered scheme combines adaptive modulation at physical layer with a truncated automatic repeat request at data link layer in the presence of both channel estimation and prediction errors. Both the channel estimation and prediction are done based on adaptive pilot-symbol-assisted modulation in which both pilot spacing and power allocation on pilot symbols are optimised in order to maximise the spectral efficiency. We provide closed-form expression for average spectral efficiency and average packet error rate over Rayleigh fading channel. Numerical results demonstrate that our adaptive cross-layer design improves the spectral efficiency while meeting the prescribed QoS. We have shown that the channel prediction is more important than the channel estimation in order to improve the spectral efficiency in a practical situation. Also we have found that the increment of spectral efficiency decreases on increasing the maximum number of retransmissions, and thus in order to reduce the delay and buffer size at the data link layer, the maximum number of retransmissions are limited to a small number in practice.