Cooperative relaying in OFDMA networks based on the joint use of hierarchical modulation and link adaptation

In cooperative relaying, the base station broadcasts a signal to the destination, the relay overhears it and forwards it in a next time slot to the destination. The latter combines the two received copies of the signal in order to reconstruct the original one. In this study, we consider a relay-based OFDMA network, as is the case of long term evolution advanced systems. We, first, make use of hierarchical modulation to send additional information to the relay so as to enable it to reconstruct a more robust copy of the original signal, and, second, adapt the transmission from the relay to the destination by taking advantage of the typically good radio conditions between them so as to reduce the cost of additional resources needed by relaying and hence improve the overall system capacity. We model such a system at the user level, considering a realistic arrival and departure setting and quantify the gains thus achieved in a cross-layer manner, in terms of throughput and blocking probability. We, eventually, propose an enhancement to the afore-mentioned scheme that takes advantage of the good radio conditions of users who are close to the base station to send them an additional stream using hierarchical modulation.