Distributed space-time coding including the golden code with application in cooperative networks

This thesis presents new methodologies to improve performance of wireless cooperative networks using the Golden Code. As a form of space-time coding, the Golden Code can achieve diversity-multiplexing tradeoff and the data rate can be twice that of the Alamouti code. In practice, however, asynchronism between relay nodes may reduce performance and channel quality can be degraded from certain antennas. Firstly, a simple offset transmission scheme, which employs full interference cancellation (FIC) and orthogonal frequency division multiplexing (OFDM), is enhanced through the use of four relay nodes and receiver processing to mitigate asynchronism. Then, the potential reduction in diversity gain due to the dependent channel matrix elements in the distributed Golden Code transmission, and the rate penalty of multihop transmission, are mitigated by relay selection based on two-way transmission. The Golden Code is also implemented in an asynchronous one-way relay network over frequency flat and selective channels, and a simple approach to overcome asynchronism is proposed. In one-way communication with computationally efficient sphere decoding, the maximum of the channel parameter means is shown to achieve the best performance for the relay selection through bit error rate simulations. Secondly, to reduce the cost of hardware when multiple antennas are available in a cooperative network, multi-antenna selection is exploited. In this context, maximum-sum transmit antenna selection is proposed. End-to-end signal-to-noise ratio (SNR) is calculated and outage probability analysis is performed when the links are modelled as Rayleigh fading frequency flat channels. The numerical results support the analysis and for a MIMO system maximum-sum selection is shown to outperform maximum-minimum selection. Additionally, pairwise error probability (PEP) analysis is performed for maximum-sum transmit antenna selection with the Golden Code and the diversity order is obtained. Finally, with the assumption of fibre-connected multiple antennas with finite buffers, multiple-antenna selection is implemented on the basis of maximum-sum antenna selection. Frequency flat Rayleigh fading channels are assumed together with a decode and forward transmission scheme. Outage probability analysis is performed by exploiting the steady-state stationarity of a Markov Chain model

Secrecy Enhancement in Cooperative Relaying Systems

Cooperative communications is obviously an evolution in wireless networks due to its noticeable advantages such as increasing the coverage as well as combating fading and shadowing effects. However, the broadcast characteristic of a wireless medium which is exploited in cooperative communications leads to a variety of security vulnerabilities. As cooperative communication networks are globally expanded, they expose to security attacks and threats more than ever. Primarily, researchers have focused on upper layers of network architectures to meet the requirements for secure cooperative transmission while the upper-layer security solutions are incapable of combating a number of security threats, e.g., jamming attacks. To address this issue, physical-layer security has been recommended as a complementary solution in the literature. In this thesis, physical layer attacks of the cooperative communication systems are studied, and corresponding security techniques including cooperative jamming, beamforming and diversity approaches are investigated. In addition, a novel security solution for a two-hop decode-and-forward relaying system is presented where the transmitters insert a random phase shift to the modulated data of each hop. The random phase shift is created based on a shared secret among communicating entities. Thus, the injected phase shift confuses the eavesdropper and secrecy capacity improves. Furthermore, a cooperative jamming strategy for multi-hop decode-and-forward relaying systems is presented where multiple non-colluding illegitimate nodes can overhear the communication. The jamming signal is created by the transmitter of each hop while being sent with the primary signal. The jamming signal is known at the intended receiver as it is according to a secret common knowledge between the communicating entities. Hence, artificial noise misleads the eavesdroppers, and decreases their signal-to-noise-ratio. As a result, secrecy capacity of the system is improved. Finally, power allocation among friendly jamming and main signal is proposed to ensure that suggested scheme enhances secrecy