Cooperative and cognitive radio systems have been proposed as a solution to improve the quality-of-service (QoS) and spectrum efficiency of existing communication systems. The objective of this dissertation is to propose and analyze schemes for cooperative and cognitive radio systems considering real world scenarios and to make these technologies implementable.
In most of the research on cooperative relaying, it has been assumed that the communicating nodes have perfect channel state information (CSI). However, in reality, this is not the case and the nodes may only have an estimate of the CSI or partial knowledge of the CSI. Thus, in this dissertation, depending on the amount of CSI available, novel receivers are proposed to improve the performance of amplify-and forward relaying. Specifically, new coherent receivers are derived which do not perform channel estimation at the destination by using the received pilot signals directly for decoding. The derived receivers are based on new metrics that use distribution of the channels and the noise to achieve improved symbol-error-rate (SER) performance. The SER performance of the derived receivers is further improved by utilizing the decision history in the receivers. In cases where receivers with low complexity are desired, novel non-coherent receiver which detects the signal without knowledge of
CSI is proposed. In addition, new receivers are proposed for the situation when only partial CSI is available at the destination i.e. channel knowledge of either the source-relay link or the relay-destination link but not both, is available. These receivers are termed as `half-coherent receivers' since they have channel-state-information of only one of the two links in the system.
In practical systems, the CSI at the communicating terminals becomes outdated due to the time varying nature of the channel and results in system performance degradation. In this dissertation, the impact of using outdated CSI for relay selection on the performance of a network where two sources communicate with each other via fixed-gain amplify-and-forward relays is studied and for a Rayleigh faded channel, closed-form expressions for the outage probability (OP), moment generating function
(MGF) and SER are derived. Relay location is also taken into consideration and it is shown that the performance can be improved by placing the relay closer to the source whose channel is more outdated.
Some practical issues encountered in cognitive radio systems (CRS) are also investigated. The QoS of CRS can be improved through spatial diversity which can be achieved by either using multiple antennas or exploiting the independent channels of each user in a multi-user network. In this dissertation, both approaches are examined and in multi-antenna CRS, transmit antenna selection (TAS) is proposed where as in a multi-user CRS, user selection is proposed to achieve performance gains. TAS reduces the implementation cost and complexity and thus makes CRS more feasible.
Additionally, unlike previous works, in accordance with real world systems, the transmitter is assumed to have limited peak transmit power. For both these schemes, considering practical channel models, closed-form expression for the OP performance,
SER performance and ergodic capacity (EC) are obtained and the performance in the asymptotic regimes is also studied. Furthermore, the OP performance is also analyzed taking into account the interference from the primary network on the cognitive network.
|Date of Award||Aug 2013|
|Original language||English (US)|
- Computer, Electrical and Mathematical Sciences and Engineering
|Supervisor||Mohamed-Slim Alouini (Supervisor)|
- Cooperative relay
- Cognitive Radio
- Performance Analysis
- Receiver Design
- multiuser networks
- antenna selection