Spectral Efficiency Optimization Of Full-duplex MU-MIMO Based On SLNR Scheme

Recently,multi-user MIMO(MU-MIMO)systems operate in a half-duplex(HD)mode,which implies that uplink(UL)and downlink(DL)channel users communicate in both directions by separating the frequencies or time slots.This shows that the resources are not exploited efficiently.Thus,full-duplex(FD)mode of communication is one of the key techniques for the future generation because it significantly improves or approximately doubles the spectral efficiency(SE)compared to HD system.FD based MU-MIMO(FD-MU-MIMO)system is a system in which DL and UL channel users receive and transmit data streams,respectively,from/to base station(BS)at the same resources.This system severely suffers due to interferences such as SI in the UL channel,co-channel interference(CCI)in the DL channel and multi-user interferences(MUI)in both channels.To mitigate these interferences,it is necessary to rely on sub-optimal and lower complex linear transmission schemes such as zero-forcing(ZF)precoder,block diagonalization(BD)precoder,ZF receiver(ZF-R)and a BD receiving filter.However,these schemes have restrictions on the number of antennas at the base station and both channel users.To be specific,FD is one of the promising modes of transmission for the next generation wireless communication.Therefore,in the future,the numbers of antennas equipped by users grow and the number of antennas at base station should be increased,accordingly.However,ZF and BD approaches cannot exploit the whole channel degree of freedom for some applications of MIMO in communication systems.Thus,the shortcomings of these schemes can be overcome by making use of a signal-to-leakages ratio(SLR)approach.This approach maximizes the desired power at every user meanwhile;it minimizes the power of the leakage set off by this user for all other users.Further,the precoding variables for all users are simultaneously optimized using the signal-to-leakage-and-noise ratio(SLNR)metric.This metric has the ability in decoupling the optimization problem and giving closed-form solutions.Thus,the employment of SLNR precoding scheme for FD mode of communication is a strategic technique to substantially increase the SE without any limitation on the number of antenna configurations.Besides,the SE of FD-MU-MIMO systems for Gaussian distribution and independent and identically distributed(i.i.d.)Rayleigh fading environment were widely analyzed.Nevertheless,in a real circumstance,there is a line-of-sight(LOS)link between the transmitter and receiver(e.g.,in short range and mmWave communications).To include this scenario,we use a Rician flat fading channel.For the FD based communications of interest that account residual SI and MUI,however ignores CCI,we propose SLNR-based precoding.The development and analysis are focused on multi-user beamforming for the cases of single stream and multi-streams.Specifically,for the DL channel,we apply SLR scheme to design the optimized beamformer.For the UL channel,we propose a new SI-plus noise covariance matrix aware SLNR-based precoding.This model uses a suppression filter at the receiver for the interference cancellation.For both cases,the problem is formulated as maximizing SLNR to obtain the optimized precoding coefficients for all users simultaneously.However,for the multiple streams case,there is additional design constraint that leads to decoupling among the multi-streams during decoding.Then,the objective function is carried out using generalized eigenvalue decomposition(GEVD).Next,the total spectral efficiencies of the FD-MU-MIMO systems for both cases are obtained,by adding the sum rates of both UL and DL channels,which are found by inserting the optimized precoder coefficients.For the system models of FD of interest,which consider residual SI,CCI and MUI for both single stream case and multiple streams case,we have developed an enhancement to the SLNR-based precoder design to greatly improve the SE of FD systems on the Rician fading environment.Particularly,for the DL channel,we propose a new CCI-aware enhancement to SLNR-based precoding to design the optimized precoder.This system makes use of a Principal Component Analysis(PCA)whitening filter at the receiver for interference suppression.For the UL communication,we propose a new SI-aware improvement to the SLNR based precoding approach.This model also uses a PCA suppression filter at the receiver for interference cancellation and a precoder design using only SI-plus-noise covariance matrix for each user.The optimization problem is formulated as maximizing SLNR for both cases to obtain the optimized precoding coefficients.In the objective function,there is additional design constraint for the case of multiple streams transmissions.This is obtained from the simple decoding that we applied for the sake of decoupling the multiple streams.Then,the objective function is optimized by making use of GEVD.By inserting the optimized precoder coefficients into the total sum rates of both UL and DL channels,a new total SE of the FD-MU-MIMO system is yielded.Simulation results validate that the SE of FD-MU-MIMO system is greatly enhanced relative to HD-MU-MIMO system for fixed number of antennas at the BS and when the transmit powers of the base station and users are small.These are due to the fact that smaller transmit powers create a smaller signal power of the SI and CCI.Additionally,the SE of the downlink channel for multi-streams case based on the proposed SLNR precoding increases for all SNR values,growing the number of antennas per user and fixed number of antennas at the base station.However,in the cases of BD and ZF precoding schemes,as the number of antennas per user increases,the SE degrade for the same number of antennas at the BS.This is because of the dimensional restrictions on both schemes,consequently,they cannot use all the channel degrees-of-freedom each user can only send a data stream.Therefore,the capacity loss is obvious.