Resource allocation for relay based green communication systems

The relay based cooperative network is one of the promising techniques for next generation wireless communications, which can help extend the cell coverage and enhance the diversity. To deploy relays efficiently with limited power and bandwidth under certain performance requirements, resource allocation (RA) plays an increasingly important role in the system design. In recent years, with the fast growth of the number of mobile phone users, great portion of CO2 emission is contributed by wireless communication systems. The combination of relay techniques and RA schemes reveals the solution to green communications, which aims to provide high data rate with low power consumption. In this thesis, RA is investigated for next generation relay based green wireless systems, including the long-range cellular systems, and the short-range point-to-point (P2P) systems. In the first contribution, an optimal asymmetric resource allocation (ARA) scheme is proposed for the decode-and-forward (DF) dual-hop multi-relay OFDMA cellular systems in the downlink. With this scheme, the time slots for the two hops via each of the relays are designed to be asymmetric, i.e., with K relays in a cell, a total of 2K time slots may be of different durations, which enhances the degree of freedom over the previous work. Also, a destination may be served by multiple relays at the same time to enhance the transmission diversity. Moreover, closed-form results for optimal resource allocation are derived, which require only limited amount of feedback information. Numerical results show that, due to the multi-time and multi-relay diversities, the proposed ARA scheme can provide a much better performance than the scheme with symmetric time allocation, as well as the scheme with asymmetric time allocation for a cell composed of independent single-relay sub-systems, especially when the relays are relatively close to the source. As a result, with the optimal relay location, the system can achieve high throughput in downlink with limited transmit power. In the second contribution, the power consumption in relay based 60 GHz cooperative networks is studied, which is based on three-terminal diversity amplify-and-forward (DAF) and diversity DF (DDF) relaying strategies. A total power consumption model including drive power, decoding power, and power consumption of power amplifier (PA) is proposed, excluding the transmit power, as it is relatively small compared to decoding power and PA power in the indoor environment. This model is formulated as a function of drive power, which gives an easy access to the system level power allocation. To minimise the system total power consumption, the optimal drive power can be allocated to the source node by numerical searching method while satisfying the data rate requirement. The impact of relay locations on the total power consumption is also investigated. It is shown that, with the same data rate requirement, in the small source-relay separation case, DAF consumes slightly less power than DDF; while with larger source-relay separation, DAF consumes much more power than DDF. In the future work, multiuser relay-based short-range communication systems will be considered for the 60 GHz communication in the fading channel scenario, which extends the proposed power consumption model in a more practical way. The power consumption model of other components, such as analog-to-digital converter, data buffer, modulation/demodulation could also be considered to provide more details about green P2P communications.