# A Receiving Antenna Allocation Scheme for Downlink MU-MIMO-OFDM Transmission
###### Abstract  
This paper proposes a novel receiving antenna allocation scheme for downlink multiuser multiple input multiple output orthogonal frequency division multiplexing (MU-MIMO-OFDM) transmission; an access point (AP) simultaneously transmits data frames to a combination of allocated receiving antennas on a subcarrier basis at each station (STA). The proposed scheme combines a limited channel state information (CSI) feedback sequence with a receiving antenna decision method. In the proposed scheme, each STA estimates the channel responses of all receiving antennas by using the training preamble transmitted from an AP, and then feeds the channel response of the antenna with maximum norm back to the AP when the spatial correlation value between receiving antennas is higher than a threshold. Otherwise, each STA feeds full channel responses back to the AP. This scheme decreases the amount of CSI fed back while exploiting the spatial diversity gain, and the AP's computational complexity is also decreased regarding the antenna allocation. Moreover, the receiving antenna decision method eliminates the overhead to notify the allocated antenna information from the AP to each STA by simply comparing its own receiving antenna powers. We clarify the effectiveness of the proposed scheme in our computer simulations using channel responses measured in an indoor environment. The results show that the proposed scheme maintains the channel capacity of the downlink MU-MIMO-OFDM transmission while greatly reducing the overhead and computational complexity. precoding and vector perturbation [11; 12]. Conventional papers assume scenarios where the number of transmit antennas exceeds the total number of receiving antennas. In such scenarios, ISI can be greatly mitigated by using the degree of freedom (DoF) of transmit antennas. However, if the number of receiving antennas at each STA increases, the DoF of transmit antennas will be insufficient. Thus, several studies have proposed coordinated transmitter-receiver (Tx-Rx) BD technique [9; 13], wherein one data stream is transmitted to each user, an initial set of received weights is assumed, and the optimal Tx/Rx weights are alternatively recomputed until the solution converges to the one with minimum power and acceptable performance. This technique achieves higher channel capacity, but the computational complexity at the transmitter to calculate the Tx/Rx weights and notification overhead created by the received weights becomes problematic. As a more practical solution, the downlink MU-MIMO transmissions with receiving antenna allocation or selection schemes have been proposed for single carrier systems [14; 15]. In these schemes, the transmitter simultaneously transmits data frames to an antenna combination, including the allocated antenna of each receiver. They attain the spatial diversity gain by exploiting the multiple antennas at each receiver, but the computational complexity at the transmitter to allocate the antenna combination with the maximum channel capacity from all combinations is high. In addition, it is necessary that each receiver must acquire information of the allocated receiving antenna before receiving the actual data frame. Since this overhead is proportional to the numbers of receivers, receiving antennas, and subcarriers, further degradation of the transmission efficiency occurs. For this problem, some papers have proposed techniques for reducing the computational complexity and overhead from a theoretical point of view [16; 17; 18; 19]. However, there is no proposal that considers them in an integrated manner based on the existing wireless LAN systems. This paper proposes a high-efficiency and low-complexity receiving antenna allocation scheme for the downlink MU-MIMO-OFDM transmission on IEEE 802.11 ac [6]. In the assumed system, an AP simultaneously transmits data frames to the antenna combination allocated on a subcarrier basis at each STA.