Precoding Design And Performance Analysis For Massive MIMO Relay Systems

Massive multiple-input multiple-output(MIMO)is well-acknowledged as a promising technology for 5G(5th Generation)mobile communication due to its capability of providing substantial gain in terms of both spectral and energy efficiency.When the number of BS(Base Station)antennas grows unboundedly,both noise effect and inter-user interference disappear theoretically,and simple linear signal processing techniques become asymptotically optimal.On the other hand,relay technology is able to enhance wireless coverage,re-duce transmit power,and improve system reliability.Motivated by both advantages,massive MIMO relaying are attracting more and more attentions recently.However,there still exist some key challenges for massive MI MO implementation,such as high power consumption and hardware cost brought by massive RF(Radio Frequency)chains and full-resolution ADCs(Analog-to-Digital Converters).Therefore,it is necessary to find solutions for implementing massive MIMO with low cost.This thesis focuses on precoding design and per-formance analysis in massive MIMO relay systems considering practical RF-chain constraint and receiving with mixed-ADC structure.Firstly,the thesis considers massive MIMO relaying with conventional full RF-chain structure and in-vestigates the system performance based on full digital precoding.Particularly for full-duplex relaying,we propose to mitigate loop interference(LI)by deploying a large antenna array and use low transmit power.Under imperfect channel state information(CSI),tight approximations for achievable sum rates are derived,which are later numerically verified for full-duplex and half-duplex relaying.Further,a hybrid duplex mode is proposed,which adaptively switches between full-duplex and half-duplex relaying according to channel conditions.The proposed hybrid duplex mode is shown to outperform any single duplex mode significantly.Secondly,the thesis focuses on hybrid analog/digital precoding schemes for massive MIMO relaying.With massive relay antennas driven by a small number of RF chains,we propose hybrid detection/precoding schemes for both uplink and downlink transmission of the relaying system.The analog precoder is designed by extracting channel phases to harvest array gain,while simple zero-forcing(ZF)precoding is conducted for baseband equivalent channel to mitigate multiuser interference(MUI).We provide an approximate expression for the achievable rate under imperfect CSI and explicitly derive the constraint condition between the number of RF chains and the number of relay antennas.Specifically,power scaling laws for two typical scenarios are presented.Both theoretical and numerical results reveal that the proposed hybrid processing scheme is able to asymptotically achieve the performance of full digital precoding.Concerning practical analog phase shifters(APSs)with discrete phases,the impact of phase quantization is also considered.Further,energy efficiency(EE)of the system is investigated with a proper power consumption model.It is proved that EE is quasi-concave with respect to both transmit power and the number of RF chains,which allows the optimal solutions to be obtained effectively.Finally,in order to reduce the high power consumption and expenditure caused by full-resolution ADC-s,we consider massive MIMO relaying with a mixed-ADC structure.The widely used additive quantization noise model(AQNM)is firstly introduced.Based on the quantization model,a tight approximation for uplink achievable rate is derived with perfect CSI and MRC(Maximal Ratio Combining)at the receiver.Additional-ly,the effects of various system parameters are characterized,including number of antennas,transmit power of users and relay,as well as ratio of full-resolution ADCs.Asymptotic analysis under typical power scaling cases reveal that the performance of the mixed-ADC structure can be well characterized by the "effective"(average)precision of all the ADCs.It is verified that the mixed-ADC architecture strikes a well balance between system performance and power consumption,which provides solid support for massive MIMO im-plementation.