Coordinating Interference Mitigation Techniques In Distributed Antenna Cellular Systems

Multiple input multiple output (MIMO) systems using multiple transmit and receive antennas are widely recognized as one of the key technologies for the future wireless communications. However, the cellular MIMO systems become interference-limited because of the existence of co-channel interference (CCI) in cellular environments. Many interference mitigation techniques for wireless cellular networks are introduced in this thesis, including conventional approaches, advanced receiver techniques, advanced transmitter techniques and some strategic approaches to the OCI problem. Both of their merit and defect are analyzed, and the base station coordination in distributed antenna system will be the key and strategic technique. The main topic of this thesis is to research the interference mitigation problems in the downlink distributed antenna MIMO cellular coordinating system. Following issues are discussed:In this thesis, a common distributed antenna architecture, which is a kind of many proposed distributed antenna schemes recently, is adopted to study the downlink interference problem in the MIMO cellular system. To keep the fairness between users, the distributed antenna system combining with the base station coordination aims at maximizing the minimum user spectrum efficiency under the constraint of per transmitting antenna power limitation, which is very specific in the distributed antenna configuration. Using the logarithmic barrier function interior-point methods in the convex optimization theory, an iterative numerical arithmetic is derived for solving this optimization problem and the initial values for iteration are given. Furthermore, a user antenna substream off-selection arithmetic is proposed to improve the drawback of the original arithmetic in which the spectrum efficiency drops rapidly when the number of users reaches the maximum. It's shown in the simulation that the user antenna selection cellular coordinating system equipped with distributed antennas can achieve higher equal spectrum efficiency and can keep it increasing near linearly when the user number increases.The bit error rate in a digital wireless communication system, which affects the quality of service directly, is the final criterion to measure the performance of a system. For the similarity characteristic between the approximate equation of bit error rate vs. SNR of quadrature amplitude modulation and the Shannon capacity equation, an optimization problem of substream rate and power allocation arithmetic is proposed to maximize the minimum QAM rate of users at a given target BER under the per antenna power constraint in a uncoded distributed antenna cellular system and make the best to keep the fairness between users. Furthermore, iterative substream power regulation arithmetic is proposed to compensate the error of the approximate bit error rate equation and allocate a kind of QAM rate for each data stream finally. In addition, for the fact that very high order QAM is difficult to be applied due to the limitation of the ADC resolution factor and the complexity, the maximum QAM order is limited at a reasonable level and the saved power can be reallocated to other stream, so that to improve other user's rate. Simulation results show that the proposed algorithm can reach the optimum average rate, maintain a improved fairness betweenusers and increase the minimum rate.All the researches discussed above have the same assumption that the channel canbe got by both the transmitters and the receivers accurately. But in fact, it's nearly impossible to obtain the ideal channel estimation results in the practice systems. Firstly, the channel estimation precision will be damnified due to the channel estimation arithmetic, the length of sequence, the interference or the delay of feedback and so on. So the variance characteristic of the MMSE channel estimation error is analyzed and the spectrum efficiency and fairness loss in the present of unideal channels are given in the simulation. Secondly, for simplicity, the researches always ignore the delay of the signal transmission; but in the distributed antenna system which has large path delay, the signal transmission delay will make the whole system asynchronous and influence the capacity deeply. A timing-advance mechanism is used to keep the desired signal reaches the corresponding user synchronously and the asynchronous interference characteristic from other users is analyzed. An asynchronous symbol interference example is shown to make it easy to be understood. The low bound of the equation of spectrum efficiency in cellular MIMO multi-substream system is derived. The simulation shows the influence from the asynchronous interference to the spectrum efficiency and fairness between users.