Analysis And Suppression Of Multiple Access Interference And Inter-Cell Interference In Mobile Communication Systems

With the development of mobile communication technology, the users'demand for communication efficiency and reliability keeps increasing, which brings up the need for improving the signal to interference and noise ratio (SINR) of users. Besides raising the signal power, the reduction of various interference becomes effective method to improve SINR in mobile communications. In mobile communication systems, there are two types of major interference, i.e. the intra-cell multiple access interference and inter-cell interference. Thus it is very significant both theoretically and practically to anlyze and to suppress these tow kinds of interferece. In this thesis, with the employment of retransmission diversity and the multiuser detection techniques, the random access random spreading code division multiple access (CDMA) system model and large system analysis tool are firstly adopted to reveal the basic and limiting traits of the multiple access interference. Then, by analyzing the inter-cell interference in the cellular systems, some methods are proposed to reduce the inter-cell interference based on the signal autocorrelation and the non-overlapping in the autocorrelation domain between the signals from different cells.CDMA is an interference limited system. The multiuser detection techniques can significantly suppress the multiple access interference and improve the system performance due to the fact that it takes into account the interference from other users when recovering the desired signals. In this thesis, the performance of CDMA random access systems employing the decorrelator and minimum mean square error (MMSE) detectors over Rayleigh fading channels is firstly investigated. When both the number of users and spreading gain tend to infinity with their ratio converging to a constant, the output SINR is shown to converge almost surely to a constant which depending only on the traffic load, transmission probability, channel gain and the distribution of transmission power rather than the spreading sequences. By analyzing the throughput and spectrum efficiency, it is found that the MMSE detector outperforms the decorrelator since the background noise is considered in the MMSE detector. Through the research on the stability region, the condition of the stable system is obtained as the arrival rate less than the service rate. By the simulation of the stability region, it is demonstrated that the stability region gets narrower as the traffic load increases.Since wireless CDMA channels are characterized by multiuser interference and multipath fading, a packet needs to be transmitted many times before successfully detected. Likewise, the received packet in each retransmission is different due to that the multiple access interference, noise and fading are random and time-variant. If the packets in all the retransmissions are combined to detect, some extra retransmission diversity can be obtained. The multiple access interference and system performance of CDMA random access systems with retransmission diversity employment are investigated. The converging values of low bounds of the SINR after the matching filter and MMSE detector are obtained based on the large system analysis, which are independent with the spreading sequences. Both the theoretical and simulation result shows that the retransmission diversity and multiuser detection techniques can significantly improve the throughput and spectrum efficiency. Based on the analysis of energy efficiency, it is found that the spectrum efficiency increases or decreases as energy efficiency increases when SNR is larger or smaller respectively than some optimum value.Furthermore, the uplink performance of the CDMA cellular systems is investigated. Both the large-scale fading containing the propagation loss and log-normal distributed shadowing and the small-scale fading adopting the Rayleigh fading model are considered as the channel model. The multiple access interference and inter-cell interference at the receiver of the base station are analyzed firstly. Then the output SINR after matching filter and MMSE detector are obtained by the large system analysis, which is dependent only on the cell load, transmission power and channel gain rather than the spreading sequences. Based on the SINR, the bit error rate (BER) and throughput are further analyzed, which are verified by the simulation results. Meanwhile, the capacity of the cellular systems with finite users is analyzing by numerical results.In the practical mobile communication systems, the main interference preventing system performance enhancement is the inter-cell interference. Therefore an inter-cell interference reduction scheme based on signal autocorrelation for downlink cellular systems is proposed. In this scheme, the transmitted signals from different base stations are processed to be non-overlapping in the autocorrelation domain by different pre-filters at the transmitter. At the receiver, the inter-cell interference can be eliminated by designing appropriate autocorrelation matching filters. To ease the requirement of multiple receiving antennas at the mobile station, a modified scheme is proposed to suppress the strongest ICI from the nearest adjacent cell, which only requires 2 antennas equipped at the mobile station. The simulation results show that the modified scheme is especially efficient for the cell edge users.In the last part, an uplink cross-slot interference suppression method of the time division duplex (TDD) cellular systems is proposed. This method can be accomplished by four steps, (1) the cellular system is divided into multiple non-overlapping cell groups each with N adjacent cells, then find N linearly independent sequences and allocate them to N cells of each group; (2) each base station determines the uplink cross-slot region based on the traffic situation and counts the number of downlink adjacent cells in the current uplink cross-slot region; (3) the autocorrelation sequences of the transmission signals from the same cell are made to be equal to the sequence allocated by the pre-processing of the transmission signal; (4) the uplink cross-slot interference can be eliminated by the adaptive processing of the received signals by the detection based on the signal autocorrelation. Then the transmission signals can be recovered by the reverse processing. It is shown that this method has high spectrum efficiency and good uplink cross-slot inference-resistance effect, especially for the strong uplink cross-slot interference.