Adaptive Mode Selection for Multiuser MIMO Downlink Employing Rateless Codes with QoS Provisioning

In this paper, the benefit of rateless codes combining with multi-antenna technique is exploited to provide quality-of-service (QoS) guarantee while maximizing the spectral efficiency in a limited feedback multiuser MIMO downlink. Critical to the design of such a system is the achievement of channel state information (CSI) at the base station (BS) to schedule the optimal users and pre-cancel the interuser interference. In order to reduce feedback amount and decrease scheduling complexity simultaneously, we propose to adopt multi-beam opportunistic beamforming (MOBF) and opportunistic space division multiple access (OSDMA) for the cases of noise and interference limited, respectively. Through theoretical analysis, it is found that, in order to satisfy QoS requirement, the admissible number of users has an upper bound, which is a function of data arrival rate and QoS requirement. With the purpose of balancing the achievable rate and feedback amount, we introduce a new concept, namely netput, as the difference of the above focused factors. By maximizing the netput, we derive the optimal feedback thresholds for the two transmission modes, respectively. Furthermore, according to the characteristics of the considered system, we obtain the optimal switch thresholds for the two modes in terms of the number of users and the SNR, respectively. Finally, our theoretical claims are validated by the numerical results.