Joint Frequency Offset Compensation and Detection for Multi-User MIMO-OFDM Systems with Frequency Asynchronous User Access

Multi-user (MU) MIMO-OFDM systems with aggressive spatial multiplexing are promising to enhance throughput and enable massive connectivity. In such systems, residual carrier frequency offsets (CFOs), due to the instability of oscillators and doppler shifts, can substantially degrade the achievable uplink throughput, especially when the number of connected devices becomes large. Existing approaches to mitigate CFOs in MU scenarios, typically involve closed-loop feedback that can result in high signaling overhead and/or significant residual CFO. Being able to compensate for the CFO of the multiple users at the receiver side, can enable the joint transmission of frequency asynchronous users, can obviate the need for high overhead synchronization procedures, can enable the use of cheaper oscillators, and can potentially unlock new user access schemes. However, as we discuss here in detail, compensating for the multiple user CFOs at the receiver is currently impractical due to the corresponding exponential complexity requirements. At the same time, methods that are typically used in single-user MIMO-OFDM systems are inappropriate for MU-MIMO scenarios and, as we show, can result in substantial (e.g., 80%) throughput degradation. To fill this gap, for the first time, we propose a joint CFO compensation and MU detection scheme that can support a large number of spatially transmitted information streams with practical processing complexity and latency requirements. We show that the proposed scheme enables frequency asynchronous user transmission and approaches the performance of perfectly synchronized systems with complexity requirements that are comparable to current MU-MIMO detection schemes that assume perfect synchronization.