Cooperative Communications : Link Reliability and Power Efficiency

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Demand for high data rates is increasing rapidly for the future wireless generations, due to the requirement ofubiquitous coverage for wireless broadband services. More base stations are needed to deliver these services, in order tocope with the increased capacity demand and inherent unreliable nature of wireless medium. However, this would directly correspond to high infrastructure costand energy consumption in cellular networks. Nowadays, high power consumption in the network is becoming a matter of concern for the operators,both from environmental and economic point of view.Cooperative communications, which is regarded as a virtual multi-input-multi-output (MIMO) channel, can be very efficient in combating fading multi-path channels and improve coverage with low complexity and cost. With its distributed structure, cooperativecommunications can also contribute to the energy efficiency of wireless systems and green radio communications of the future. Using networkcoding at the top of cooperative communication, utilizes the network resources more efficiently.Here we look at the case of large scale use of low cost relays as a way of making the links reliable, that directly corresponds to reductionin transmission power at the nodes. A lot of research work has focused on highlighting the gains achieved by using network codingin cooperative transmissions. However, there are certain areas that are not fully explored yet. For instance, the kind of detectionscheme used at the receiver and its impact on the link performance has not been addressed.The thesis looks at the performancecomparison of different detection schemes and also proposes how to group users at the relay to ensure mutual benefit for the cooperating users.Using constellation selection at the nodes, the augmented space formed at the receiver is exploited for making the links more reliable. Thenetwork and the channel coding schemes are represented as a single product code, that allows us to exploit the redundancy present in theseschemes efficiently and powerful coding schemes can also be designed to improve the link performance.Heterogeneous network deployments and adaptive power management has been used in order to reduce the overall energy consumption in acellular network. However, the distributed structure of nodes deployed in the network, is not exploited in this regard. Here we have highlightedthe significance of cooperative relaying schemes in reducing the overall energy consumption in a cellular network.  The role of differenttransmission and adaptive resource allocation strategies in downlink scenarios have been investigated in this regard.It has been observed that the adaptive relaying schemes can significantly reduce the total energy consumption as compared to the conventionalrelaying schemes. Moreover, network coding in these adaptive relaying schemes, helps in minimizing the energy consumption further.The balance between the number of base stations and the relays that minimizes the energy consumption, for each relaying scheme is also investigated.

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