Research On Several Key Problems In MIMO Communication Systems

Future wireless communication systems are characteristic of the higher rate of data transmission and the higher reliability of transmission method. Because MIMO can significantly increase system capacity and improve communication ? quality to satisfy the requirement of high-rate wireless operation, it has been one of the key technologies for future wireless communication systems. The central issue in MIMO system is achieving or approach to the capacity of MIMO channel through space-time coding and space-time signal processing.Space-time code design is always one of the primary topics in MIMO study. Without obvious loss of system transmission efficiency, a realizable space-time code design with high performance should achieve maximum diversity gain and coding gain at the same time. Simple coding structure and low-complexity decoding algorithm are also expected.Because of the open wireless channel, safely transmission becomes an important problem in MIMO communication systems. However, the inherent connection between different transmitted data flows in MIMO is not considered by the most present encryption technologies. Further, the effective combination of MIMO system taking advantage of the capacity with spread spectrum system capable of multiple accesses is another problem deserving deeply study. In the view of above key questions, the main contents and contributions of this dissertation are as follows:1. As a special space-time coding technique, time-varying linear transformation (TVLT) is deeply analyzed. The ability of TVLT in space-time coding system to obtain full diversity is proved, and the mathematic expression of the relationship between TVLT and coding gain is given. Aim at the requirement for channel codes used in conjunction with TVLT, Turbo code is compared with LDPC code, and a tight upper limit to minimum Hamming distance of LDPC codes is demonstrated based on the generated matrix. The sufficient and necessary conditions of Tanner codes without girth 4 are proposed and proved, according to which the girth 4 in Tanner codes can be verified and good LDPC codes are generated consequently.Based on the combination of TVLT with Tanner code, a novel TVLT-LDPC-MIMO system architecture is proposed, and corresponding joint iterative detection and decoding algorithm is described. Superior to the common STBC-LDPC-MIMO architecture, the new system with simple structure can achieve both full diversity gain and maximum coding gain, and takes the advantage of higher data transmission rate and lower error rate.2. The feasibility of TVLT as an encryption technique is proved. Using the stream cipher with one-time pad in combination with the randomly varying unitary matrix in TVLT, the paper proposes the concept of MIMO secret communication system based on TVLT. Multiple independently generated stream ciphers form the random element of unitary matrix in TVLT.Through the united application of multiple symmetric cryptographies and asymmetric cryptographies, the encryption/decryption flowsheet and the high-speed algorithm are introduced in detail. The encryption algorithm almost has no effect on transmission rate. Complying with the Kirchhoff guideline, all processes operate with software algorithm. The theoretical security and realistic security are proved for the new system which is very propitious to future wireless wide-band networks.3. A new low-complexity MIMO precoding scheme and corresponding algorithm for downlink in CDMA systems are proposed along with its error rate analysis. In contrast to present MIMO CDMA schemes, great decrease in system complexity and better bit error performance are obtained in the new scheme. With the application of smart antenna and Rake technique to MIMO CDMA system, the paper brings forward a simplified space-time receiver scheme for base station in TD-SCDMA system. The digital signal processing software implement of our scheme can not be achieved in the other schemes. Founded on beam forming technique, the space-time receiver algorithm similar to the 2-D signal filter is proposed which can be used to identify mobile station with signal waveform and spatial signal, and the concept of MIMO subsection is also given.The definition of generalized frequency-hopping pattern with wide interval and the concept of S distance for frequency-hopping sequence are provided on the basis of the combination of MIMO and frequency-hopping communication. A scheme of fast frequency-hopping MIMO system with noncoherent demodulation is provided. Compared with general frequency-hopping communication system, the new scheme can achieve much higher transmission rate and lower bit error rate. The efficiency and reliability are greatly improved.