## 1 Introduction
Using outage constraint for PU, a power allocation algorithm for OFDM-based cognitive radio is studied in [9]. In [10], optimal power allocation strategies for the secondary user is studied in order to maximize its capacity under transmit power constraint of the SU and outage constraint of the PU. Distributed subcarrier and power allocation technique for the maximization of throughput subject to constraints on the interference and power is studied in [11] for ad hoc cognitive radio networks. Using statistical constraint on interference, a joint admission control, and rate and power allocation for SUs in a CR network is developed in [12]. The authors studied outage probabilities for SUs and interference constraint violation probabilities for PUs for the dynamic fair spectrum sharing scenario. In [13], optimal and suboptimal power loading techniques are studied using statistical interference constraints for CR systems. In [14], adaptive subcarrier allocation and power distribution technique for multi-antenna orthogonal frequency division multiple access (OFDMA) systems that provide multicast service is studied. The problem is formulated as maximizing sum capacity with constraints on the total power. The authors in [15] studied two-dimensional mapping of incoming requests for wireless OFDMA systems, where a base station (BS) is transmitting to a group of subscriber station (SS) using a broadcast channel in downlink. They presented run-time efficient heuristic solutions for the problem, where the objective function is the spatial efficiency. The performance of multimedia streaming over an IEEE\(802.11\)b wireless local area network using quality-oriented adaptive scheme is studied in [16]. The performance is assessed in terms of the average user quality of experience, number of concurrent streaming sessions, loss rate, delay, jitter and total throughput. In [17], the resource allocation problem of assigning a set of subcarriers and determining the number of bits to be transmitted for each carrier in OFDMA-based broadband wireless access systems is considered. The authors considered fairness and efficiency of the optimal and suboptimal algorithms. In [18], an energy efficient structure for beacon signal transmission is proposed for the protection of PU, which is incumbent wireless microphone. The authors in [19] investigate energy-efficient design of CR in order to protect incumbent users, where power allocation and transmission duration are determined by exploiting multiple channels. In [20], the green network planning of single frequency networks based on OFDM schemes is investigated. A genetic algorithm optimization method is proposed to maximize energy-efficiency there. The problem of energy-efficient communication in the downlink OFDMA network is studied in [21]. For multi-cell OFDMA downlink networks with cooperative base stations, the energy-efficiency is discussed in [22]. In [23], a mathematical framework for energy-efficiency is presented. Energy-efficient green communications techniques and network designs are of special importance for future generation communications in order to reduce carbon dioxide gas emission due to exponential increase on energy consumption in our daily lives [24]. To cope with the exponential increase of wireless data traffic in overly crowded consumer radio spectra, CR is identified as a very important technology for future wireless communication networks. CR communications technology will provide a way to opportunistically and efficiently utilize unused piece of spectrum that is currently statically assigned to other services (such as, TV band, amateur radio, _etc._). Since the activity state of the incumbent PUis hidden from the SUs, the main research challenges in CR are to design efficient spectrum sensing, interference-management, and optimized accessing techniques. While wireless handheld devices are playingan ever-increasing integral role in human daily activities, wireless systems as a whole invariably contribute to a significant global carbon footprint.