| The prevalence of high-end smartphones and tablets, served by densely deployed advanced wireless networks, has spurred the explosive growth of high-data-rate wireless services. This tremendous increase in multimedia wireless services has inevitably resulted in an exponentially-increasing energy consumption for both mobile users and the wireless infrastructure. For mobile terminals powered by a limited-capacity battery, the increase in energy consumption is a critical issue since it significantly reduces the operational time. From the network's perspective, the escalation of energy consumption reduces the revenue due to increased energy costs. Thus, energy-efficient wireless network design has become increasingly more important. By utilizing the broadcast nature of the wireless medium and relay nodes distributed in space to form a virtual multiple antenna system, cooperative communications can provide a significant improvement in transmission range, capacity, and quality-of-service (QoS). In addition, when properly designed, a significant improvement in energy efficiency (EE) can be also achieved by user cooperation. Thus, we evaluate cooperative communications from an energy-efficiency perspective. First, we analyze the EE of clustered cooperative beamforming with a maximum transmit power constraint, and compare its performance with that of direct communications. All of the energy consumption overheads incurred are taken into account in the analysis; a constant circuit-power consumption model is assumed. We then determine the number of relay nodes that maximizes the efficiency for a given outage constraint. In addition, we propose a mode switching algorithm which optimally chooses between using cooperative beamforming or direct communications. We show that cooperative beamforming significantly outperforms direct communications in EE, as well as in spectral efficiency, even though a much larger amount of circuit power is consumed. We also investigate the EE of best-select relaying. For the analysis, we propose a simple linear circuit power consumption model where the power consumed by the digital signal processor (DSP) is modeled as a linear function of the transmission bandwidth. Based on this model, we determine the bandwidth and modulation size that maximizes the EE. The evolution of mobile and wireless communications has resulted in the coexistence of multiple radio access technologies (RATs) such as WLAN, WCDMA, WiMax, and LTE-A. Typically, these heterogeneous networks have been operated independently. However, cooperation among multiple RATs can improve the performance by judiciously utilizing the degrees of freedom available in heterogeneous radio access networks. First, we consider the EE of best-select relaying in multi-RAT networks where a mobile device is capable of accessing multiple RATs; each RAT operates on a different carrier frequency with a different transmission bandwidth. Considering the circuit power consumption of each RAT, we propose optimal and various suboptimal energy-efficient best-select relaying schemes and show that the proposed schemes significantly outperform traditional best-select relaying, as well as direct communications. Nowadays, orthogonal frequency division multiple access (OFDMA) is widely adopted in various wireless standards due to its robustness to frequency-selective fading and its efficient hardware implementation. Here, we consider energy-efficient resource allocation for multiuser OFDMA systems in multi-RAT networks. We first investigate the near-optimal resource allocation for parallel transmission among OFDMA-based multiple RATs under the QoS constraint for mobile users and a maximum transmit power constraint in each RAT. We then propose a computationally efficient suboptimal resource allocation algorithm to facilitate implementation. Numerical results show that the proposed suboptimal scheme provides a higher EE compared to the conventional spectral-efficient approach, and it also achieves performance that is close to the optimum with less system complexity. |
