In this chapter a new closed-loop scheme for distributed extended or- thogonal space time block coding (EO-STBC) with one-bit feedback based on selection cyclic phase rotation for two relay nodes each equipped with two antennas is proposed. Furthermore, this approach is applied to asynchronous cooperative relay networks using orthogonal frequency division multiplexing (OFDM) type transmission over frequency flat channels.
6.1 Introduction
Distributed space time block codes (STBCs) were proposed to provide both full cooperative diversity and full data transmission rate for wire- less relay node networks, each relay node equipped a single antenna. However, higher order STBCs have been proposed but either coop- erative diversity gain or full data transmission rate must be relaxed. A distributed quasi orthogonal STBC (QO-STBC) [43] for four relay nodes, each having only one antenna, which achieves full data transmis- sion rate at the expenses of loss in cooperative diversity gain has been
Section 6.1. Introduction 165 designed [7]. In order to fully realize the benefits of distributed STBC for cooperative relay networks, channel state information (CSI) should be available at the relay nodes through feedback from the destination node. This would provide a good strategy to improve the performance of many physical layer techniques, including distributed STBC. There- fore, a closed-loop (CL EO-STBC) with two-bit feedback scheme was presented in [15] and [16] (See Appendix A) to achieve full data trans- mission rate and full diversity gain for point-to-point systems, however, these two closed-loop schemes required more than a single bit of feed- back for each transmission block.
In this chapter a new distributed CL EO-STBC for wireless relay network with two relay nodes, each equipped with two antenna us- ing only one-bit feedback based on selection of cyclic phase rotation is proposed to enhance cooperative diversity and improve the system per- formance. In particular, a new one-bit scheme is developed to eliminate the self interference from adjacent symbols in the distributed EO-STBC scheme. In this scheme, phases of symbols are rotated from relay node antennas in a prescribed way based upon CSI, which is fed back from the destination node. The rotation is effectively equivalent to rotating the phases of the corresponding channel coefficients.
As a result, the proposed one-bit feedback scheme achieves full coop- erative diversity order of four for two relay nodes, each being equipped with two antennas, resulting in a significant improvement in the bit error rate (BER) performance with a significantly reduced amount of feedback compared to previous closed-loop schemes in [15] and [16] in distributed STBC for synchronous cooperative relay networks. The assumption of synchronization is not realistic due to the distributed na-
Section 6.1. Introduction 166 ture of each relay node, therefore, recently, asynchronous cooperative diversity has been discussed in [25], [24] and, [94].
Most of these approaches were proposed for flat fading channels and using the DF strategy at the relay nodes, where each relay has a sin- gle antenna. In [27] a simple distributed A-STBC transmission scheme based on OFDM type transmission was proposed to combat the timing error between relay nodes, however this is only valid for the case of two relay nodes as mentioned earlier. Therefore, in this chapter, the proposed one-bit feedback EO-STBC is applied along with OFDM type transmission for asynchronous relay networks over frequency flat fad- ing channels by implementing OFDM type transmission at the source node and using amplify-and-forwared (AF) type transmission [7], time reversal (TR) and complex conjugating at two relay nodes, each being equipped with two antennas to reduce timing error among the relay nodes.
The organization of this chapter is as follows, a complete character- ization of the distributed EO-STBC scheme for relay nodes with two antennas is introduced in Section 6.2. In Section 6.3 a new distributed one-bit feedback algorithm based on selection cyclic phase rotation is presented. The case of imperfect synchronization using the proposed feedback scheme along with OFDM type transmission for two relay nodes with two antenna on each relay node over frequency flat fading channels is considered in Section 6.4. In Section 6.5 the performance of the proposed feedback scheme is simulated and compared with that of existing feedback schemes. Finally, Section 6.6 concludes and sum- marizes this chapter.
Section 6.2. Complete Characterization One-Bit Feedback EO-STBC 167
6.2 Complete Characterization One-Bit Feedback EO-STBC
6.2.1 System Models
As shown in Figure 6.1 the proposed distributed CL EO-STBC scheme with one-bit feedback based on selection of cyclic rotation for two relay nodes with two antennas on each relay node and only a single antenna located at the source node and the destination node, without direct transmission (DT) connection between the source node and the desti- nation node, is presented. As mentioned in Section 5.5, the distance between each pair of antennas in each relay node is assumed to be equal to half of the transmitted wavelength and all the relay nodes are subjected to half duplex constraint.
ܴͳ Destination Source Relay Nodes Second phase First phase ܵ ܴʹ ܦ 12 τ 22 τ ݃ଵଵ ݃ଶଵ ݃ଵଶ ݃ଶଶ ݂ଶଶ ݂ଵଶ ݂ଵଵ ݂ଶଵ ߨ 0 ߠଵ ߠଶ
Figure 6.1. Basic structure of distributed CL EO-STBC scheme using
one-bit feedback based on selection cyclic phase rotation for wireless relay networks with two antennas at each relay node and one antenna at the source and the destination node with two phases for the cooperative transmission process.
All wireless channels are assumed to be flat fading channel with zero- mean and unit-variance complex Gaussian random variables, where fik
denotes the channel coefficient between the source node and each an- tenna of each relay node and gikdenotes the channel coefficient between