Research On Key Techniques Of Full-duplex And Cooperative Relaying Wireless Transmission

With the spread of smart phones,mobile Internet has developed rapidly in recent years.People's demands for high/super-definition voice,images,and videos keep growing.How to boost data rate and network capacity is an urgent problem to be solved in the future.Nevertheless,the scarcity of spectrum resources has become a bottleneck to achieve this goal.At present,academia and industry world mainly solve this problem from two aspects:one is to improve the efficiency of existing frequency bands;the other is to exploit higher communication frequency bands.By simultaneous transmission and reception at the same time over the same frequency,full-duplex technique can theoretically double the spectral efficiency compared to traditional half-duplex system,thus becoming a potential method for solving the capacity requirements of next-generation wireless communication system.On the other hand,the application of new frequency signals,such as millimeter-wave or terahertz,mainly faces the coverage and penetration problem.Relay cooperation technology can effectively mitigate the effects of fading,widen the coverage,provide cooperative diversity,and improve spectral efficiency.As two important technologies for future wireless communication,both full-duplex and relay cooperation are very worthy of study.This dissertation mainly studies the key technologies of future wireless communication systems from three aspects:full-duplex,relay cooperation,and full-duplex relay cooperation.The specific work and main contributions are as follows:(1)In full-duplex orthogonal frequency-division multiplexing(OFDM)systems with in-phase and quadrature-phase(IQ)imbalances,a frequency-domain least-square(FD-LS)channel estimator is proposed to estimate both intended and residual self-interference channels.Following this,an optimal closed-form pilot matrix is derived to minimize the sum of mean square errors(Sum-MSE)of the proposed FD-LS channel estimator with equal power constranit.Then,an improved DFT-based channel estimator is presented to further enhance the performance of FD-LS by exploiting the time-frequency property of wireless channel.Simulation results show that for the proposed FD-LS estimator,the optimal pilot matrix performs best when the conditional number(CN)of its Gram matrix equals to one.To be specific,the pilot Gram matrix of CN=1 achieves about 10dB signal-to-noise ratio(SNR)gain over the case of CN=10.And the DFT-based improved FD-LS channel estimator provides a higher-precise estimation than the FD-LS one.(2)For full-duplex OFDM systems with white Gaussian noise,the DFT-based improved channel estimator can achieve N/L-fold Sum-MSE performance gain compared to the FD-LS one,where N and L denote the number of total subcarriers and the length of cyclic prefix,respectively.How about the Sum-MSE performance gain in the colored interference plus noise(IPN)scenarios?In order to solve this problem,we use matrix analysis and statistical theory to derive the closed-form expression of Sum-MSE gain.And its simple upper and lower bounds are given using the matrix eigenvalue inequality.Through simulation analysis,it can be found that the exact value of Sum-MSE gain depends heavily on the correlation factor of the IPN covariance matrix.The Sum-MSE performance gain grows from 1 to N/L as the correlation factor gradually decreases from 1 to 0.In addition,the exact Sum-MSE gain degenerates into N/L and 1 in two extreme scenarios:white independent and fully-correlated IPN,respectively.Numerical simulation results further validate above conclusions.(3)To solve the uplink-downlink pairing problem in multi-pair two-way OFDM relay networks,a space-frequency channel pairing(SFCP)method is proposed based on maximizing the sum-rate.First,block diagonalization and singular value decomposition beamforming are utilized at the relay station(RS)to eliminate inter-user interference and intra-user interference,respectively.Then,the closed-form low-complexity solution of SFCP matrix is obatined according to the arrangement inequality of product.Following this,a space-frequency joint water-filling power allocation(WFPA)scheme is designed to further improve the BER performance.The simulation results illustrate that,compared to only spatial channel pairing and no pairing schemes,the proposed SFCP method can achieve substantial performance gains,including improvements in the sum-rate and BER.In addition,the SFCP-WFPA can improve BER performance in the low and medium SNR regimes,compared to equal power allocation(EPA).(4)In wideband multi-pair two-way relay networks,the performance of beamforming at RS is intimately related to the accuracy of the channel state information(CSI)available.The accuracy of CSI is determined by Doppler spread,delay between beamforming and channel estimation,density of pilot symbols,and transmit power of pilot symbols.The coefficient of the CSI error model is established as a function of CSI delay,Doppler spread,and SNR at pilot symbols,which can be estimated in real time.In accordance with the real-time estimated coefficients of the error model,an adaptive robust maximum signal-to-interference-and-noise ratio(Max-SINR)plus maximum signal-to-leakage-and-noise ratio(Max-SLNR)beamformer at RS is proposed to track the variation of CSI error.From simulation results and analysis,it is shown that:compared to existing non-adaptive beamformers,the proposed adaptive beamformer is more robust and performs much better in the sense of BER;as the density or power of transmit pilot symbols increase,its BER and sum-rate performances tend to those of Max-SINR plus Max-SLNR beamformer with ideal CSI.(5)In OFDM-based full-duplex relay networks with IQ imbalances,the optimal pilot pattern and power allocation scheme are designed using the criterion of minimizing the Sum-MSE for the FD-LS channel estimator.First,the design problem of pilot pattern is casted as a convex optimization.From the Karush-Kuhn-Tucker conditions,the optimal analytical expression is derived.Under the total transmit power sum constraint of source node and RS,an optimal power allocation(OPA)strategy,based on the Lagrange multiplier method,is proposed to further alleviate the effect of Sum-MSE.Simulation results show that the proposed OPA performs better than EPA in terms of Sum-MSE.And the Sum-MSE performance gain increases with deviating p from the value of ?° minimizing the Sum-MSE,where ? is defined as the average ratio of the residual SI channel gain at RS to the intended channel gain from source to RS.