Uplink multiuser MIMO (UL MU-MIMO) is a new feature introduced in 802.11ax (currently under development). This paper investigates the residual carrier frequency offset (CFO) impact on the performance of UL MU-MIMO OFDM systems. We show that if a station (STA) has zero residual CFO, then its packet error rate (PER) will not suffer from other STAs that have residual CFO when a zero-forcing (ZF) or linear minimum mean squared error (LMMSE) MIMO receiver is used at the access point (AP). To reduce the impacts of residual CFO on system PER performance, two CFO correction methods are designed to compensate for the phase shift and cancel the inter-user interference caused by the residual CFO. Several simulation examples are provided to verify the results.

We consider the problem of obtaining channel state information (CSI) via a fast feedback link for transmission on the downlink of a multiuser MIMO system. We examine the relative merits of channel feedback schemes based on Shannon's source-channel separation principle (digital schemes) and non-separation based schemes and show that the latter are preferable in this application when small to moderate bandwidth expansion ratios are used as in many current cellular systems. For comparison, we first compute upper-bounds on the performance of the system as a function of SNR. For the non-separation based schemes, we first consider a simple analog transmission and then develop a hybrid digital-analog transmission scheme which quantizes the CSI using a few bits and sends these bits and also the quantization error using analog transmission. We show that the hybrid scheme achieves a higher throughput compared to both analog and digital transmissions and has a much lower computational complexity compared to a digital scheme.

We consider the problem of channel state information (CSI) transmission on a fast feedback link for a multiuser MIMO system.We examine the relative merits of channel feedback schemes based on Shannon's source-channel separation theorem and non-separation based schemes and show that the latter are preferable in this application. For the non-separation based schemes, we first consider a simple analog transmission and then develop a hybrid digital-analog transmission scheme which quantizes the CSI using a few bits and sends these bits and also the quantization error using analog transmission. We show that the hybrid scheme achieves a higher throughput compared to both analog and digital transmissions and has a much lower computational complexity compared to a digital scheme.