On Key Techniques For Uplink Multiuser Data Transmission With Low PAPR

Due to the low peak-to-average power ratio(PAPR)of transmitting signals,single carrier-frequency division multiple access(SC-FDMA)has been adopted as the key technology of multiple access in long term evolution(LTE)uplink.Higher energy-and spectral-efficiencies are required in future mobile communications systems to provide users with higher data rates,lower transmission delay and larger access capacity.The employment of higher-order modulation serves as one possible approach to improve the spectral efficiency of multiuser systems in LTE uplink.However,the PAPR of SC-FDMA signals becomes large with the increase of modulation order.The high PAPR decreases the power efficiency of the power amplifier and causes nonlinear distortion,leading to the loss of energy-and spectral-efficiency,especially for the uplink with stringent demand on power assumption.Starting from the PAPR problem,we deeply analyze and study the key techniques for uplink multiuser data transmission with low PAPR.The main work and contributions are listed as follows.1.The received signal model of uplink multiuser SC-FDMA systems with nonlinear distortion is first presented,followed by the derivation of theoretical error vector magnitude(EVM)for each user.The obtained expression of EVM agrees well with simulation results and helps to evaluate both the self in-band distortion and the interference from out-of-band radiation of nonlinearities of other users.Meanwhile,the obtained EVM result can be applied to the performance analysis of uplink multiuser orthogonal frequency division multiple access(OFDMA)systems.The proposed theoretical analysis can provide benchmarks for the transceiver design and parameter optimization in uplink multiuser data transmission systems with low PAPR.2.To solve the bit error rate(BER)error floor in existing clipping methods,a novel PAPR reduction scheme for SC-FDMA signals is proposed by jointly considering the PAPR reduction and the BER degradation.The constellation points causing large peaks are modified through an additive pre-distortion vector.For effective PAPR reduction,the additive pre-distortion is optimized to minimize the in-band distortion under the constraint of expected PAPR thresholds.The proposed scheme can achieve good PAPR reduction with negligible BER degradation.In addition,it works with low complexity and easy implementation,making it an attractive candidate for future uplink SC-FDMA systems.3.To avoid side information(SI)transmission in conventional selected mapping(SLM)methods,an SI embedding and detection scheme for SLM-based SC-FDMA systems is proposed by exploiting pilot symbols in the uplink subframe.To enable SI embedding at the transmitter,the SI for each SC-FDMA data symbol is represented by an index set of several subcarriers in the pilot symbol.A pilot-aided maximum likelihood(ML)SI detector is first proposed at the receiver.For complexity reduction,a suboptimal SI detector is proposed based on the log-likelihood ratio(LLR).In addition,performance analyses of the proposed SI detectors and the effect of the proposed scheme on channel estimation are provided.The proposed scheme outperforms existing ones in terms of both SI detection and BER and can achieve nearly the same performance as systems with perfect SI.4.Aiming at mitigating the spectral efficiency loss in frequency domain pulse shaping(FDPS)methods,a novel band allocation scheme(BAS)is proposed by spectrum overlapping of edge subcarriers for each user with all its neighbors.To solve the multiuser interference caused by spectrum overlapping in the novel BAS,an iterative multiuser detection(MUD)algorithm termed joint-expectation propagation(J-EP)is first proposed based on the EP principle.For reduction of the increasing complexity with large numbers of allocated subcarriers,a low-complexity MUD algorithm named distributed-EP(D-EP)is then proposed and it performs almost the same as the J-EP.Aided by either of the proposed MUD algorithms,FDPS-based SC-FDMA systems with the novel BAS can achieve a higher spectral efficiency without BER degradation compared to ones with existing BASs,making FDPS more practical in uplink multiuser data transmission with low PAPR.