Research On Diversity Combining Techniques In Wireless Communications

Radiowave propagation through wireless channels is a complicated phenomenon characterized by various paths from transmitter to receiver. When a received signal experiences this multipath fading during transmission, the fading effects on the amplitude, phase and angle can severely degrade performance. Moreover, deep fading can lead to communication interruption. So, how to combat fading in order to offer a reliable and high-efficient wireless communication is a hot topic. Recently, there are some technologies to combat fading, such as, RAKE receiver in spread spectrum communication system, error correction coding, interleave techonoly, adaptive channel equalization and diversity combining. Among these technologies, diversity combining is more efficient to combate deep fading and widly used. And, lots of diversity combining strategies has been proposed, such as Maximal Ratio Combining, Equal Gain Combining, Selective Combining and Switched Combining. But, there are some problems in diversity combining schemes, such as the contradiction between performance and complexity, incomplete research for combining performance and extendable applications of diversity combining technology. Based on this, we research on diversity combining from three concepts:combining strategy, combining performance and application. The main innovations in this dissertation are outlined as following:(1) Two novel hybrid combining technologies named SEC/MRC and SSC-SC are presented. MRC has high complexity and is sensitive to channel estimation errors. On the other hand, SEC can eliminate the bad branches. So, we compose MRC and SEC to form a novel hybrid combining named SEC/MRC. The performance of SEC/MRC scheme is studied. The theoretical analysis and the simulation results show that SEC/MRC which requires a smaller number of receive chains and a less complex structure performs almost as well as MRC. In addition, SC requires the simultaneous and continuous monitoring of all diversity branches which leads to the waste of resources, and SSC only needs one receiver chain and one switch logic circuit to save resources. So, we compose SSC and SC to form a novel hybrid combining named SSC-SC. The analysis of the first-order and second-order statistical signal properties at the output of the SSC-SC receiver is proposed. It is shown that SSC-SC combines the advantages of both SC and SSC, and its first-order and second-order statistical properties are improved to some extent. Consider both system performance and realization, the two hybrid combining technologies proposed have good practical value. (2) The general closed expressions for the second-order statistical properties of switched combining schemes are obtained. The second-order statistical signal properties are important performance measure for communication system design, and conventional Rice formula is very complexity. So, the closed-form expressions of the second-order statistical properties for DB-SSC, SEC and SECps over multiple independent generalized fading paths which are not necessarily identically distributed are presented through the discrete analysis method. These closed-form expressions are maked up of the LCR and CDF of each branch, which is easy to implement and applies to any form fading channels. The closed-form expressions for SEC and SECps are obtained for the first time.(3) The equivalent relationship between multiuser power allocation and diversity combining systems is built, and power allocation strategies based on statistical properties are proposed. Power allocation is one of the most important techniques in multiuser mobile communication system. We find that the power allocation system is similar to diversity combining system, and built the equivalent relationship between them. Moreover, based on the equivalence, the idea of blind ratio combining is introduced in power allocation system, and new statistical power allocation strategies are proposed. The theoretical analysis and the simulation results show that the proposed power allocation strategies can reduce the feedback information and system complexity and do not affected by the imbalance of user subchannels, which can be efficiently applied in the time-varying channel environment. Otherwise, the equivalent relationship between two systems can help for the crossover study of these two systems and forming new power allocation strategy.(4) A rapid increment recurrence power allocation strategy is proposed. Optimal water-filling power allocation requires repetitive iterative process, and is difficult to be realized in time-varying fading channels. So, we do some improvement on the algorithm, and a novel power allocation strategy which can update power allocation for all users based on an increment recurrence formula is presented. The theoretical analysis and the simulation results show that this strategy with low complexity has a good performance as WF algorithm in time-varying environment, and can be used in joint optimization schemes.