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. In this thesis, we consider the design of protocols that allow several terminals to cooperate via forwarding each others' data, which can increase the system reliability by achieving spatial cooperative diversity. We consider the problem of "how to achieve and where to exploit diversity in cooperative networks?"

We first propose a cooperation protocol for the multi-node amplify-and-forward protocol. We derive symbol error rate (SER) and outage probability bounds for the proposed protocol. We derive an upper-bound for the SER of any multi-node amplify-and-forward protocol. We prove that the proposed protocol, where each rely only forwards the source signal, will achieve the SER upper-bound if the relays are close to the source node. Then, we consider the problem of power allocation among the source and relay nodes based on the derived SER and outage probability bounds to further enhance the system performance.

We consider the design of distributed space-time and distributed space-frequency codes in wireless relay networks is considered for different schemes, which vary in the processing performed at the relay nodes. We consider the problem of whether a space-time code that achieves full diversity and maximum coding gain over MIMO channels will achieve the same if used in a distributed fashion. Then, we consider the design of diagonal distributed space-time code (DDSTC) which relaxes the stringent synchronization requirement by allowing only one relay to transmit at any time slot. Then, we consider designing distributed space-frequency codes for the case of multipath fading relay channels that can exploit the multipath as well as the cooperative diversity of the channel.

Then, we consider studying systems that exhibit diversity of three forms: source coding diversity (when using a dual description encoder), channel coding diversity, and user-cooperation diversity. We derive expressions for the distortion exponent of several source-channel diversity achieving schemes. We analyze the trade-off between the diversity gain (number of relays) to the quality of the source encoder and find the optimum number of relays to help the source. Then, we consider comparing source coding diversity versus channel coding diversity.

Finally, we will consider the use of relay nodes in sensor networks. We will consider the use of relay nodes instead of some of the sensor nodes that are less-informative to the fusion center to relay the information for the other more-informative sensor nodes. Allowing some relay nodes to forward the measurements of the more-informative sensors will increase the reliability of these measurements at the expense of sending fewer measurements to the fusion center. This will create a trade-off between the number of measurements sent to the fusion center and the reliability of the more-informative measurements.