Research On Cooperative Diversity Technology In Wireless Relay Networks

Multiple input multiple output (MIMO) technology can take full advantage of spatial resources to combat wireless channel fading, therefore it can improve the reliability and frequency spectrum utilization of communication systems without additional system bandwidth and transmit power. However, owing to the limitation of mobile terminals or network nodes with regard to the size, complexity and power consumption, MIMO technology hardly can be applied in some practical situations, such as the uplink of cellular system, wireless ad hoc networks and wireless sensor networks. Hence, a new spatial diversity technology, cooperative diversity technology, has been proposed. The basic idea is that multi terminals or nodes compose a virtual MIMO system and share the link resources with each other to obtain spatial diversity gain. Cooperative diversity technology has broken the restriction of the traditional MIMO technology on mobile terminal devices and provided a new method for its utilization in engineering practice.Motivated by the previous studies on the diversity technology and MIMO technology, we propose the following cooperative transmission schemes in this dissertation.1.Differential cooperative transmission schemeA differential space-time cooperative transmission (DSTCT) scheme using quadrature amplitude modulation (QAM) has been presented. Although it is required to estimate the channel power (CP) and normalized power factor (NPF) in the differential demodulation, the proposed scheme has the advantage of signal-to-noise ratio (SNR) gain compared with the conventional DSTCT scheme using PSK modulation. Simulation results show that when the modulation order is larger than 8 in time-varying fading channels, the DSTCT-QAM scheme has better symbol error rate (SER) performance than that of the DSTCT-PSK scheme. Specially, when the modulation order is 64, the DSTCT-QAM scheme obtains a 5.35 dB SNR gain. Although the SNR gain decreases as the Doppler frequency increases, the DSTCT-QAM scheme still can achieve a better performance than the DSTCT-PSK scheme, provided the CP estimation interval is selected suitably.In addition, a double differential cooperative transmission (DDCT) scheme using QAM modulation has been proposed to overcome carrier frequency offset in cooperative transmission. A closed-form SER approximation formulation, an SER upper bound as well as an asymptotic approximation at high SNR have been derived. Based on the asymptotic approximation, an approximate solution of optimum power allocation (OPA) at high SNR has been derived. The simulation results show that the approximate solution of the OPA also provides a good solution to a realistic moderate SNR scenario. Moreover, the SER of the DDCT scheme with QAM modulation and that with PSK modulation have been compared. Simulation results show that the DDCT- QAM scheme has better SER performance than that of the DDCT-PSK scheme, when the modulation order is larger than 8. Specially, when the modulation order equals 64, the DDCT-QAM scheme obtains a 4.5dB SNR gain.2.Differential modulation and relay selection schemeThe performance of a multi-relay cooperative communication scheme with differential modulation and relay-selection (DM-RS) has been studied. The scheme consists of a source, N_R relays, and a destination, where the best relay is selected to forward the source node's signal with detect-and-forward (DetF) protocol. The performance of the DM-RS scheme with PSK modulation (which can be called the DPSK-RS-DetF scheme) has been investigated first. In particular, a closed-form outage probability and an approximation at large SNR have been derived using the SNR approximation of the equivalent relay link. The outage probability approximation shows that the scheme can achieve a full diversity order of N_R+ 1. Furthermore, a closed-form SER approximation and an upper bound have been derived. Simulation results confirm the presented analytical results.In addition, a cooperative communication scheme using QAM modulation in the DM-RS scheme has been proposed, which can be named as the DQAM-RS-DetF scheme. Since the CP and NPF are required in the differential demodulation, one CP estimation method and two NPF estimation methods have been developed forgoing training symbols. In addition, the mean square error (MSE) of the estimation methods have been investigated, and the performances of the two NPF estimation methods have been compared in time-varying fading channels. Moreover, assuming the CP knowledge at the destination, a closed form SER approximation of the DQAM-RS-DetF scheme has been derived. Simulations results show that when the modulation order is larger than 8 in time-varying channels, the DQAM-RS-DetF scheme has better SER performance than that of the DPSK-RS-DetF scheme.3.Quasi-orthogonal space-time coded cooperative transmission schemeA cooperative transmission scheme using quasi-orthogonal space-time block codes (QOSTBC) for MIMO relay networks has been proposed. Comparing with the conventional cooperative transmission scheme using orthogonal space-time block codes (OSTBC), the proposed scheme can achieve higher bandwidth efficiency with the same decoding complexity. Moreover, an adaptive decode-and-forward (ADF) relaying protocol has been proposed based on one-bit channel state information (CSI) feedback. According to the CSI feedback, a better transmission mode can be selected adaptively between the direct transmission and decode-and-forward (DF) cooperative transmission. In addition, the outage performance of the proposed scheme has been investigated and a closed-form upper bound on the outage probability has been derived. The analytical results show that the proposed scheme can achieve a full diversity order, which is higher than that of the direct and DF cooperative transmissions. Simulation results verify the obtained analytical results.4.Transmit antenna selection (TAS) schemes in MIMO relaying systemThe outage performances of three TAS schemes in amplify-and-forward (AF) MIMO relaying over independent and identically distributed (IID) Nakagami-m fading channels are investigated. In particular, asymptotic lower bounds on the outage probabilities of the schemes are derived and the obtained diversity orders are compared. Moreover, the outage performances of three TAS schemes in DF MIMO relaying over IID Nakagami-m fading channels are investigated. In particular, the asymptotic outage probabilities of the schemes are derived and the obtained diversity orders are compared. Both of the presented expressions are general for the three TAS schemes with an arbitrary antenna configuration and a fading environment with arbitrary values of severity parameters m?0.5. Simulation results confirm the presented analytical results.