Algorithm Research On Multi-User Random Access For OFDMA/SC-FDMA Systems

As two different multiple access technologies, OFDMA/SC-FDMA, based onOFDM, are adopted as uplink data transmission multiple access method by IEEE802.16e and3GPP LTE respectively. Random access is an important physical layerprocedure by which base station (BS) detects all the user equipment (UE) andestimates their transmission delays and signal powers. UE adjusts their transmissioninstances and signal powers according to the information sent back by BS to achievesynchronization in the FFT window with same power to maintain orthogonality. Afterobtained uplink synchronization BS would allocate corresponding time and frequencyresources to UE and finally establish the data communication link.The random access procedure is consisted of three basic parts: BS distinguishesdifferent UE by performing the access code detection; BS estimates the transmissiontime delay (TTD) for all the UEs detected; BS estimates the transmitted signal powers.The first issue comes most difficult among the above three parts.In this paper we focus on the algorithm of uplink random access multi-userdetection and time estimation for OFDMA/SC-FDMA system, and finally proposed anew random access scheme. At first, the system model of random access isestablished, and we analyze the effect of frequency and time synchronization error forsignal detection. Secondly, as an example of typical OFDMA/SC-FDMA uplinkaccess communication systems, we further studied on signal design, time andfrequency allocation and multi-user detection for LTE system. According to theparameters in the protocol, the performance of existing methods under differentchannel models especially for double fading (frequency-selective fading and fastfading) channel is evaluated.On the basis of above studies, we analyzed the influence of double-fadingchannel over the coherent detection algorithm and proposed a new random accessscheme. The essential flow, access signal and the structure of transmitter and receiverare introduced, and then we established the system model. Finally the non-coherent energy detection algorithm and timing offset estimation algorithm are proposed. Thesimulation results show that occupying the same frequency and time resources ourscheme has better detection performance than LTE ranging process in double-fadingchannels. Our random access scheme is suitable to implement in a high-speed cellwith small radius due to its robustness and low-complexity.