Analysis and Simulation of the Performance of Space Time Trellis Codes in MIMO Systems

Abstract

The Multiple Input Multiple Output (MIMO) system improves performance by increasing the system capacity as well as improving the reliability of the wireless link without a need to increase the transmission bandwidth or transmission power. Space-Time trellis codes are the most popular channel coding schemes used in MIMO systems since they can offer coding gain along with the ability of exploiting the space and time diversity of MIMO channels. In this thesis the design of these codes are studied and their performance is analyzed to see how they can improve the data rate and the reliability of communications over quasi-static slow fading channels using multiple transmit antennas. Also the challenges and restrictions of applying them in communication systems are analyzed in detail. The design criteria that are used to build reliable trellis codes are also summarized and the performance is shown to be determined by matrices constructed from pairs of distinct code sequences. These matrices represent the Euclidean distances between the decoded received symbols and the reference set of symbols. The minimum rank among these matrices quantifies the diversity gain, while the minimum determinant of these matrices quantifies the coding gain. 4 Data is encoded by a channel code and the encoded data is split into n streams that are simultaneously transmitted using n transmit antennas. The received signal at each receive antenna is a linear superposition of the n transmitted signals perturbed by noise. The internal space time trellis encoder structure with the design criteria that can be used on them are studied with assumption that the fading is slow and frequency nonselective. These study shows that the maximum value of the rank determines whether rank and determinant design criteria are used or trace criterion. Also it shows some important points that must be considered in designing space time trellis codes like the maximum number of antennas can be used, the relationship between the number of antennas and the length of encoder memory that can be used to achieve maximum diversity gain and maximum coding gain. Several examples of good codes are given. An extensive set of performance results for different system model situations are provided for QPSK signal set with data rates of 2 bits /symbol using only 4 state and 8 state encoders. Some basic factors of improving this performance are stated like the affect of increasing the number of antennas or increasing the number of code trellis states. The affect of using transmit diversity in MIMO systems on the capacity of the communication channel is studied and compared with a situation when not using them. A fair comparison between space time trellis codes and space time block codes is provided.