Joint user scheduling, link adaptation and beam-forming for distributed antenna systems

This paper proposes a novel management algorithm for distributed antenna systems (DASs) that exploits the spatial diversity of the architecture in order to schedule (over the same frequency band) as many transmissions as possible with the most appropriate modulation and coding schemes (MCSs). This goal is achieved by means of a joint user scheduling, link adaptation and beam-forming algorithm that maximizes capacity. The adaptation scheme includes power control and adaptive modulation and coding (AMC). The core of the algorithm is based on an iterative least squares (LS) optimization, where power levels and beam-forming vectors are jointly calculated so as to comply with a different SINR (signal-to-interference-plus-noise ratio) for each scheduled user. This SINR level ensures the transmission of the selected MCS with a given value of BLER (block error rate). In comparison with previous approaches, the proposed scheme ensures not only the existence of a solution for systems with power constraints, but it also allows for a smooth integration of scheduling and AMC schemes. Each LS stage is solved by the method of gradient steepest descent, whose convergence speed is improved by reusing the solution of previous LS stages. An iterative method is also proposed for a more efficient calculation of inter-cell interference. Imperfect channel state information is used in all simulations. Results show considerable throughput gains when compared to previous solutions.