Full-Duplex in Massive Multiple-Input Multiple-Output

In-band full-duplex (FD) operation can double the spectral efficiency of massive multiple-input multiple-output (mMIMO) systems by allowing simultaneous transmission and reception over the same time/frequency slot. However, the main challenge encountered in implementing an FD radio wireless transceiver is to greatly suppress the residual self-interference (SI) generated from its own transmitter below the receiver noise floor to avoid performance degradation in the detection of the signal of interest received from remote transmitters. It is pointed out that the single-stage digital beamforming (DBF) scheme would introduce an impractically high complexity unsuitable for implementing FD-mMIMO. This paper considers an FD hybrid beamforming (FD-HBF) scheme for multi-user (MU)-mMIMO systems, in which large SI-suppression is achieved by exploring dynamic transmitter/receiver (Tx/Rx) isolation using RF-beamforming designs, and exploiting the degrees of freedom of a large number of antenna elements in separate Tx and Rx antenna arrays, in conjunction with high-performance baseband (BB) digital fractionally-spaced (FS) SI-cancellation (SIC) techniques. This paper starts with a discussion of the main parameters and impairments (in an mMIMO transceiver) that can impact the SI-suppression performance and the proposed RF-beamforming Tx/Rx isolation and BB SI-cancellation design strategies along with their achieved performance and complexities. Joint-beamforming optimization to maximize both Tx/Rx performance and isolation is discussed with an example of a nature-inspired optimization scheme to achieve an average Tx/Rx RF isolation of 64.4 dB and the best isolation of 76 dB. The paper continues with a description of an FD-mMIMO transceiver prototype using Tx/Rx 8x8-element antenna arrays along with analytical, simulation and measured results. Illustrative results indicate that the proposed combination of RF-beamforming Tx/Rx isolation and BB digital fractionally spaced SI-cancellation can offer an overall SI suppression of more than 131 dB and bring the residual SI below the receiver noise floor.