DISTRIBUTED SPACE-FREQUENCY CODING IN WIRELESS RELAY NETWORKS

Abstract

Recently, there has been much interest in modulation techniques to achieve transmit diversity motivated by the increased capacity of multiple-input multiple-output (MIMO) channels. To achieve transmit diversity the transmitter needs to be equipped with more than one antenna. The antennas should be well separated to have uncorrelated fading among the different antennas; hence, higher diversity orders and higher coding gains are achievable. It is affordable to equip base stations with more than one antenna, but it is difficult to equip the small mobile units with more than one antenna with uncorrelated fading. In such a case, transmit diversity can only be achieved through user cooperation leading to what is known as cooperative diversity. Cooperative diversity provides a new dimension over which higher diversity orders can be achieved. An ad-hoc network with a sender, a destination and a third station acting as a relay is analyzed. Different combining methods and diversity protocols are compared. The simulation results have shown that amplify and forward protocol gives a better performance than decode and forward protocol. To combine the incoming signals the channel quality should be estimated as well as possible. Information about the average quality shows nice benefits, and a rough approximation about the variation of the channel quality increases the performance even more. Whatever combination of diversity protocol and combining method is used second level diversity is observed. The problem with the multi-node decode-and-forward protocol and the multi-node amplify-and-forward protocol is the loss in the data rate as the number of relay nodes increases. The use of orthogonal subchannels for the relay nodes transmissions, either through Time-Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA), results in a high loss of the system spectral efficiency. This leads to the use of what is known as distributed space-frequency coding, where relay nodes are allowed to simultaneously transmit over the same channel by emulating a space-frequency code. The term distributed comes from the fact that the virtual multi-antenna transmitter is distributed between randomly located relay nodes. Relay nodes iii are used to form a virtual multi-antenna transmitter to achieve diversity through the use of distributed space-frequency codes. The design of distributed space frequency codes (DSFCs) for wireless relay networks employing the amplify and forward (AAF) protocol is considered. DSFCs are designed to achieve the multipath (frequency) and cooperative diversities of the wireless relay channels. The DSFC can achieve full diversity of order LN, where L is the number of paths of the channel and N is the number of relay nodes. Simulation results are also presented. iv