Key Techniques For Multiuser Random Access In OFDMA/SC-FDMA Systems

The LTE (long term evolution) system and the WiMAX (worldwide interoperibility for microwave access) system which both evolve towards the new generation of mobile communication systems, respectively design a random access channel and an intial ranging channel to accomplish the multiuser random access procedure. By using the random access procedure, multiple users could be identified by the base station and their timing offsets, frequency offsets and power could be adjusted as well. The random access procedure involves several issues such as resource allocation of the access channel, waveform design and multiuser detection at the base sation, etc.. However, random access signals of multiple users would be highly distorted by channel fading, timing offsets, frequency offsets as well as the near-far effect, leading to multiple access interference that severely deteriorates the performance of the uplink random access procedure. Therefore, the random access procedure cannot meet the requirement of the internet-of-things that is equipped with a huge number of user equipments. Furthermore, as the random access procedure is only utilized in conventional cells, it cannot be applied into the future distributed coordinated network. To solve these problems, we analyze and study key techniques in the initial ranging procedure of WiMAX systems, the random access procedure of LTE systems, and the random access procedure in distributed coordinated networks. The main research works and innovation points are listed as follows:1. To mitigate the frequency selectivity of the wireless channel in WiMAX systems, the application of general pre-equalization algorithms of multicarrier frequency division multiple access (MC-CDMA) systems into the WiMAX initial ranging procedure is first analyzed, and a general controlled pre-equalization algorithm based on zero forcing (GC-pre-eq-ZF) is proposed, which is able to improve the power boosting effect of the pre-equalization algorithm based on zero forcing, and to mitigate the channel frequency selectivity and the near-far effect, leading to a better multiuser detection and estimation performance.2. In the initial ranging procedure of WiMAX systems, to overcome the shortcoming of available successive interference cancellation (SIC) based multiuser detection and estimation algorithms which suffer from accumulated errors as well as high complexity, an iterative maximum likelihood detection (IMLD) algorithm is proposed. Based on the signal model that is derived by deeming the received signal as many replicas of channel paths, the proposed IMLD algorithm works in a fashion as an adaptive multipath splitter and detector, achieving a better detection and estimation performance at the cost of relatively lower complexity than existing algorithms.3. According to the SC-FDMA structure in LTE uplink, a random access signal model in the transform-domain is first analyzed, and a novel iterative parallel interference cancellation (IPIC) detection algorithm is proposed. The proposed IPIC algorithm could mitigate the multiple access interference that is introduced by non-orthogonality among Zadoff-Chu codes that have different root values, thus leading to a better detection and estimation performance than the existing single user detection (SUD) algorithm at the cost of relatively higher complexity.4. As the subcarrier spacing of the LTE random access channel is pretty narrow, it is easy to be affected by frequency offsets that are introduced by frequency uncertainties at transceivers as well as the Doppler effect. To deal with the LTE random access issue with the existence of frequency offsets, the expression of frequency offsets in the transform-domain is first elaborated, and a multi-steps hybrid multiuser detection (MS-HMUD) algorithm is proposed, which implements a hybrid multiuser detection and interference cancellaton procedure at the trade-off between the multiuser detection performance and complexity. Analyses and simulation results show that the MS-HMUD algorithm is robust against frequency offsets and achieves a better detection and estimation performance than existing algorithms.5. In the study of the random access procedure in distributed coordinated networks, a centralized parallel random access (CP-RA) scheme is proposed for user equipments to access into the distributed coordinated network that is equipped with geographically distributed transmission points. A random access sub-channel allocation scheme is proposed for the central base station or unit to distinguish coordinated users with non-coordinated ones and mitigate mutual interference between them. After that, a centralized multiuser detection algorithm is proposed to improve the multiuser detection performance by exploiting the coordinated diversity of the network structure. The proposed scheme is able to provide knowledge about timing offsets from a user to different coordinated based stations or transmission points, thus lowering the requirement for the length of cyclic prefixes and improving the spectrum efficiency.