Research On Adaptive Transmissiontechnology With Imperfect CSI For Distributed Antenna Systems

With the rapid development of modern communication technology, the demand for data communication services with high speed rate and high quality is mounting. Thus, the challenge is that how to develop a new technology to efficiently improve the system spectral efficiency(SE) and link reliability, that the wireless communication will be faced with. Distributed antenna systems, as an emerging technology, has comparable advantages in reducing transmit power, increasing SE, and covering blind spots et al. According to the channel condition, adaptive modulation(AM) technology can realize reasonable wireless resource allocation to satisfy the quality of service(Qo S) requirements. Therefore, this thesis will focus on AM scheme in the distributed antenna systems under imperfect channel state information(CSI), in the hope of meeting Qo S requirements and improving SE or energy efficiency(EE), to provide a theoretical basis for the practical application of AM technology in distributed antenna systems. The main contents of the thesis are summarized as follows:1. The AM scheme in the distributed antenna systems is investigated. According to the target bit error rate(BER) requirement and the accurate BER expression in the additive white gaussian noise channel, the improved thresholds are proposed, similar to the real thresholds. On the basis of these thresholds, the accurate closed-form expressions for the average SE and average BER are derived. Simulation results show that the improved thresholds can avoid the SE loss caused by the traditional approximate thresholds. Furthermore, theoretical values are well consistent with the simulation ones, which provide a valid method for the system performance evaluation.2. The AM scheme based on imperfect CSI in the distributed antenna systems is investigated. In view of the difficulty to obtain the perfect channel information, AM scheme is firstly put forward based on imperfect CSI. Then utilizing the maximum a posteriori(MAP) method and approximate BER, the variable thresholds are proposed to adapt the change of channel condition. These thresholds can reduce the impact of estimation error on the system performance, including a special case of improved thresholds. Based on these thresholds, the approximate closed-form expressions for average BER and average SE are shown by using numerical integral method, which can effectively estimate the system performance under imperfect CSI. The simulation results indicate that the theoretical analyses are valid and the variable thresholds could tolerate larger estimation error than the usual fixed thresholds.3. The energy efficient adaptive transmission technology in the distributed antenna systems is investigated. Considering the influence of imperfect CSI on EE, by means of the fractional programming theory, a suboptimal power allocation scheme is proposed based on numerical iteration to maximize EE, under the constraint conditions of target BER and maximum power. To avoid the iteration algorithm, a closed-form solution for suboptimal power allocation is given by applying the sort method and Lambert W function. It is suggested that the two optimization methods can provide similar EE performances by simulation results. Moreover, the latter can greatly reduce the complexity without iteration calculation, realizing an effective tradeoff between performance and complexity.