Multicast or broadcast in multiuser OFDM systems
Résumé
The situation where the same signal has to be sent to a number of wireless receivers arises more and more often, due to the increasing use of wireless internet including TV. Classically, this is done either by establishing several one to one communications, which is obviously a waste of resources, or by establishing a broadcast service, addressing potentially all users of a given area. However, the broadcast service cannot be tuned to a specific set of users, and also results in a waste of power in a number of circumstances. This could be avoided if the transmitter knew the parameters of the users willing to attend the service, in a situation similar to "multicast". This paper provides the tools that are necessary to evaluate the performance improvement brought by wireless multicast over the classical broadcast situation, and provides numerical evaluations when the transmission is done over realistic OFDM channels. We developed a power allocation scheme for multiuser SISO-OFDM system based on the optimization of the achievable sum-rate, and we analyze the proposed allocation scheme. Performance evaluation is obtained through histograms of the throughput increase in several realistic scenarios, thus allowing to decide when the power optimization is not efficient enough, and that transmission with uniform power allocation would be a better choice (in terms of complexity). The results show that with our multiuser SISO-OFDM scheme, the multicast optimization presents an important improvement in a number of circumstances.