Research Of Distributed Antenna And Base Station Cooperation Techniques In Multicell Environment

With the proliferation of wireless communications, the advent of smart terminals, the higher data rate of the new wireless services, and the increased number of wireless users, the demand for additional spectrum to meet the rate requirement grows almost exponentially. In the near future, the data rate is expected to grow even more rapidly, however, the spectrum is an inherently limited resource. Currently, it is managed by government through the means of authorization and most of the spectrum has been allocated to the dedicated users, which makes it unrealistic to assign additional spectrum to the mobile users. Therefore, there is conflict between the ever-increasing data rate requirement and the limited spectrum resource. In this dissertation, we will concentrate on improving the data rates of wireless communication systems without the use of additional spectrum.Distributed antenna system (DAS) is one of the key technologies in B3G/4G communication systems, for its great advantages, and excellent prospects. It has attracted more attention recently, because it can improve the system capacity, reduce the outage probability and bit error rate with no additional spectrum in the single cell environment. The principle of DAS is that the antenna modules are geographically distributed thereby reducing access distance compared with centralized scheme. As a result, the desired signal can be enhanced while the mobile terminals move within the cell for the reduced access distance. In multicell environment, interference is the dominant factor to the system performance, and interference suppression is an active research topic. In this research, the base station cooperation is proposed in order to alleviate interference from the adjacent cells. The interference signals from adjacent cells are exploited as an opportunity in our new scheme instead of a hazard in other no-cooperation method.On the other hand, as the nature environment deteriorates and the greenhouse effect aggravates, energy efficiency in wireless communication is becoming a major concern in the filed of wireless research over the past decades. Energy efficiency has direct relation on the benefits of mobile operators and the battery life of mobile terminal which motivates us to carry out research on the energy efficiency of DAS, and then compare with the other wireless systems.Although a lot of works has been done in the literature, it considers the interference along with noise term as Gaussian random variable with fixed variance by central limit theorem (CLT) method. Nevertheless, the precondition of CLT is that the interference term is large enough. When the interference term is small, the performance analysis of DAS no longer holds. In the high accuracy scenario, CLT method has defect in obtaining the system performances. In light of that, we propose a non-central limit theorem (NCLT) method to overcome that shortcoming.To achieve that goal, we propose several new approaches in this dissertation. Firstly, we introduce the brief history of wireless communications, the development and research status of DAS. Moreover, the wireless communications channel models, DAS models and the procedure of wireless communications is presented in Chapter â…  and â…¡, respectively. The concept of "root" is given in this work, i.e., the transmit power of each remote antenna unit (RAU) multiply the pathloss of mobile terminal to RAU is known as "root". When two or more roots have the same value, it is called repeated-root. Secondly, we propose an non-central limit theorem method to overcome the restricted condition in the CLT method. With the help of the NCLT, the system performance of DAS is re-analyzed when the desired signal and interference signal have repeated-root or not. A novel selection criterion is proposed in DAS for energy efficiency, while base station cooperation is allowed to make good use of interference. The main contributions of our work can be summarized as follows:1) The Performance of Distributed Antenna Systems with the Interference Signal Has Repeated-Root Only:The interference signal will have repeated-root while desired signal is not, when the mobile terminal arrives at some positions in the reference cell. The interference plus noise term is considered as Gaussian random variable with fixed variance in existing work adopting CLT. However, one drawback of the CLT lies in the fact that, repeated-root in denominator of the received SINR into consideration for simplicity. We propose an NCLT method, which treats the interference plus noise term as a random variable affect by short term fading given transmit power and pathloss model in Chapter III. In NCLT method, the variance of interference plus noise is also a random variable but not a constant in CLT method. With no shadowing effect, we derive the closed-form expressions of the performance of DAS by NCLT method, and make comparison with that of the CLT method. When the channels suffer from shadowing effect, the approximated expressions are derived adopting NCLT method. Simulation results demonstrate that the performance of DAS is more accurate with NCLT method, since it can embody the effect of short term fading and reflect the system performance accurate.2) The Performance of Distributed Antenna Systems with Desired Signal and Interference Signal Both Have Repeated-Root:The desired signal and the interference signal of the received signal at mobile terminal both have repeated-root. In this case, we analyze the system performances by NCLT method in Chapter â…£. With reference to conditional expectation and integration by parts, we adopt the NCLT method to analyze the system performance of DAS. The closed-form and approximate expressions are derived with non-and shadowing effect, respectively, which can reveal the system performance more accurately. In this case, the analyzed results can be extended to other cases. The simulation results demonstrate the correctness of the derivation procedure.3) Performance Analysis of RAU Selection Scheme in Multicell Environment for DAS:In DAS, the RAU can be selected to communicate with the mobile terminals, and two selection criteria are compared in Chapter â…¤:the minimum distance criterion and maximum SINR criterion. We derive the closed-form expressions of the performance in DAS by CLT method on the basis of maximum SINR criterion and by NCLT method based on minimum distance criterion. The selection transmission scheme has the advantages of selection diversity and transmitted power conservation, which is in accordance with the standard of green communications at present.4) Capacity Analysis of Distributed Antenna Systems for Base Station Cooperation: The cells are independent between each other when the system performances are analyzed in the previous chapters, and the signal from adjacent cells are considered as noise. The base station cooperation technique is proposed in Chapter â…¥, in which the central units of different cells are connected to a central unit controller (CUC) via optical or dedicated RF links. The CUC has the ability to schedule and manage the RAUs of the cells, and the signals from adjacent cells are no longer considered as interference but as useful signal, because the CUC can distinguish the useful signal from interference. Therefore, there is no pure interference in base station cooperation scheme. In this Chapter, we propose a new system model and derive the channel capacity.At last, Chapter â…¦ summarizes our work and shows the prospect of the developing tendency, which includes interference alignment for adjacent cells in DAS, beamforming, femtocell, and the closed-form expressions of selection transmission scheme based on maximal SINR criterion.