Power Allocation in OFDM Based Cognitive Radio System
In this chapter, we investigate one suboptimal power allocation scheme for an OFDM based cognitive radio system. Optical power allocation for secondary user in OFDM subcarrier is complex since it has a power as well as an interference constraint. Mathemat
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Power Allocation in OFDM Based Cognitive Radio System Ishrat Maherin and Qilian Liang
Abstract In this chapter, we investigate one suboptimal power allocation scheme for an OFDM based cognitive radio system. Optical power allocation for secondary user in OFDM subcarrier is complex since it has a power as well as an interference constraint. Mathematical analysis shows that interference between primary and secondary in OFDM based system depends on theirs spectral distance. In our suboptimal method we consider the spectral distance between the secondary and primary user and modify the traditional water-filling algorithm. By applying our suboptimal power allocation method, it is possible that we can get good performance comparable to the optimal scheme. We compare our method with traditional power allocations schemes like equal power allocation and water filling power allocation. Result shows that distance dependent modified water filling (DDMWF) scheme can achieve the highest data rate for the cognitive radio based Secondary user. Keywords Cognitive radio • Water filling • OFDM • Power allocation
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Introduction
Cognitive Radio solves the scarcity of spectrum for wireless communication by dynamically using the available spectrum. Most of the allocated spectrum is underutilized in today’s world as shown by the Federal Communications Commission report [1]. A cognitive radio can sense the spectrum of the licensed user known as primary user (PU) and identify the underutilized spectrum known as white space or spectrum hole. This spectrum hole can be opportunistically used by the secondary user (SU) provided that the interference power introduced to the occupied spectrum is below a certain threshold known as interference temperature limit.
I. Maherin • Q. Liang (*) Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX, USA e-mail: [email protected] Q. Liang et al. (eds.), Communications, Signal Processing, and Systems, Lecture Notes in Electrical Engineering 202, DOI 10.1007/978-1-4614-5803-6_14, # Springer Science+Business Media New York 2012
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I. Maherin and Q. Liang
The overall objective of this chapter is to increase the spectral efficiency by allocating the subcarrier power and increasing the SU’s transmitted data rate and keeping the interference under limit. In order to achieve that goal we propose a CR network that is based on OFDM. Orthogonality of the OFDM system allows high spectral efficiency and guarantees less cross talk between the subcarriers. It simplifies the equalizer and it does not need guard interval between the carriers. In literature, [2] proposes power control schemes for secondary femto cells and outage probability constraints are upper bounded by interference limits. It takes into account the Raleigh fading channel and also proposes an iterative power control schemes. In [3] besides the interference power constraints transmit power constraint for SU is also considered. As in [4], various combinations of peak and average power constraints are considered
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