Abstract-This paper aims to minimize the total transmit power consumption for Massive MIMO (multiple-input multipleoutput) downlink cellular systems when each user is served by the optimized subset of the base stations (BSs). We derive a lower bound on the ergodic spectral efficiency (SE) for Rayleigh fading channels and maximum ratio transmission (MRT) when the BSs cooperate using non-coherent joint transmission. We solve the joint user association and downlink transmit power minimization problem optimally under fixed SE constraints. Furthermore, we solve a max-min fairness problem with user specific weights that maximizes the worst SE among the users. The optimal BS-user association rule is derived, which is different from maximum signal-to-noise-ratio (max-SNR) association. Simulation results manifest that the proposed methods can provide good SE for the users using less transmit power than in small-scale systems and that the optimal user association can effectively balance the load between BSs when needed. By 2017 there will be seven trillion wireless devices and the fast growth will increase the global CO 2 -equivalent emissions significantly [1], [2] . Moreover, 80% of the power in current networks is consumed at the BSs [2]. The BS technology therefore needs to be redesigned to reduce the power consumption. Many researchers have investigated how the physical layer transmissions can be optimized to reduce the transmitted signal power, while maintaining the qualityof-service (QoS); see [3]-[5] and references therein. However, the papers [3]- [5] are all optimizing the power with respect to the small-scale fading, which is practically questionable since the fading coefficients change rapidly (i.e., the powers must be reoptimized every few milliseconds) and since the fading can be efficiently mitigated by channel coding. In contrast, the small-scale fading has negligible impact on Massive MIMO systems, thanks to favorable propagation [6] , and closed-form expressions for the ergodic SE are available for linear precoding schemes [7] . The power allocation can be optimized with respect to the large-scale fading in Massive MIMO [8] , which makes advanced power control algorithms computationally feasible. A few recent works have considered power allocation for Massive MIMO systems [9] , [10] , but none of them has considered the BS-user association problem. Massive MIMO has demonstrated high energy efficiency in homogeneously loaded scenarios [7] , where an equal number
Furthermore, in REF , a spectral efficiency expression was formulated for Rayleigh fading channel and accordingly a power control algorithm was designed using max-min QoS optimization.
Downlink power control for massive MIMO cellular systems with optimal user association
2016 IEEE International Conference on Communications (ICC)
2016 IEEE International Conference on Communications (ICC)
Mathematics Computer Science