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Cognitive radio (CR) is considered one of the prominent techniques for improving the utilization of the radio spectrum. A CR network (i.e., secondary network) opportunistically shares the radio resources with a licensed network (i.e., primary network). In this work, the spectral-energy efficiency trade-off for CR networks is analyzed at both link and system levels against varying signal-to-noise ratio (SNR) values. At the link level, we analyze the required energy to achieve a specific spectral efficiency for a CR channel under two different types of power constraint in different fading environments. In this aspect, besides the transmit power constraint, interference constraint at the primary receiver (PR) is also considered to protect the PR from a harmful interference. Whereas at the system level, we study the spectral and energy efficiency for a CR network that shares the spectrum with an indoor network. Adopting the extreme-value theory, we are able to derive the average spectral and energy efficiency of the CR network. It is shown that the spectral efficiency depends upon the number of the PRs, the interference threshold, and how far the secondary receivers (SRs) are located. We characterize the impact of the multi-user diversity gain of both kinds of users on the spectral and energy efficiency of the CR network. Our analysis also proves that the interference channels (i.e., channels

Abstract-In this paper, we propose a new concept called mobile Femtocell (MFemtocell) network, which can be considered as a practical implementation of mobile relays (more precisely, moving networks). MFemtocells can be deployed in moving vehicles, such as trains, buses, or private cars to provide enhanced user throughput, extended coverage, and reduction of the signaling overhead and drop calls. We investigate the spectral efficiency of cellular systems with MFemtocell deployment and two resource partitioning schemes. Simulation results demonstrate that with the deployment of MFemtocells, the spectral efficiency and average user throughput can significantly be increased while the signaling overhead is reduced.

Abstract-Energy efficiency is becoming increasingly important for wireless communication systems in order to minimize carbon footprint of wireless networks and to increase the battery life of mobile terminals. The spectral-energy efficiency trade-off is of primary significance to determine how much energy per bit is required in a wireless communication system to achieve a specific spectral efficiency. In this paper, we study energyefficient resource allocation scheme for Orthogonal FrequencyDivision Multiple Access (OFDMA) systems with multiple users. The trade-off between spectral and energy efficiencies is analyzed under the constraint of maintaining the fairness among users. We first formulate the energy-efficient optimization problem as an integer fractional programming. We then apply an iterative fractional method to simplify the problem to integer linear programming (ILP) problem. Simulation results demonstrate that the impact of user's quality of service (QoS) is minor on the energy efficiency when a large spectral efficiency is required.

Mobile femtocell (MFemtocell) is a new concept that has been proposed recently to be a potential wireless communication technology for the next generation cellular systems. The essence of MFemtocell lies in adopting the femtocell technology inside vehicles such as trains, buses, or private cars, to provide better coverage and good Internet experience whilst on the move. In this paper, we investigate the spectral and energy efficiency for the MFemtocell-assisted network with different resource partitioning schemes. Closed-form expressions for the relationships between the spectral efficiency and energy efficiency are derived for a link-level MFemtocell network. We also investigate the spectral efficiency for multi-user system-level MFemtocells with opportunistic scheduling schemes. Our analysis shows that MFemtocells can provide better spectral and energy efficiency compared to direct transmission scheme. Index Terms--Mobilefemtocell, spectral efficiency, energy efficiency, resource partitioning schemes, scheduling schemes. 0018-9545 (c) , IEEE Transactions on Vehicular Technology IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY 12Cheng-Xiang Wang (S'01-M'05-SM'08) received the B.Sc. and M.Eng. degrees in communication and information