Collaborative techniques for achieving spatial diversity in wireless networks

Recently, there has been lots of interest in techniques that achieve spatial diversity to improve the reliability and/or the rate of the transmission in wireless networks. To achieve spatial diversity the transmitters need to be equipped with more than one antenna. To obtain maximum diversity gain, the fading among these antennas should be uncorrelated and hence the antennas should be well separated. This is usually not possible due to the cost and the small size of the wireless devices. In this case, the only way to achieve spatial diversity is a new technique which is known as cooperative diversity. In this technique, diversity is achieved through collaboration between the transmitting nodes in the network In this thesis, we develop a collaboration protocol and a practical coding strategy for the collaborative communication in a three node network with no knowledge of channel state information (CSI) at the transmitter side. Unlike most other works, we have used a variable time-fraction scheme as the basis of our protocol and will show that this protocol achieves full diversity while providing a noticeable coding gain. Then, assuming the availability of the channel state information at the transmitters via a simple feedback from the destination to the relay or to both the source and the relay, we collaboration protocols for both feedback scenarios. The performance of all of these protocols has been obtained using Monte Carlo simulations and has been compared with the outage probability of a single transmitter scenario. We have also obtained constant gain contours that demonstrate the loci of the relay that guarantees a minimum gain for all three scenarios.