29-09-2012, 05:01 PM
PAPR Reduction on OFDM system using LUT
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ABSTRACT:
OFDM or orthogonal frequency division multiplexing can be viewed as a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower rate subcarriers. The main reason of using OFDM is to increase the robustness against frequency selective fading or narrowband interference. Apart from these OFDM posses some advantages which include Immunity to delay spread and multipath, simple equalization and efficient bandwidth usage. Besides these factors OFDM has some drawbacks such as Synchronization,
Need of FFT units at transmitter and receiver, Sensitive to carrier frequency offset and High peak to average power ratio. This paper presents PAPR reduction technique using lookup table in which the selection of input sequences are carried out in a proper manner.
INTRODUCTION:
OFDM symbol consists of a sum of subcarriers that are modulated by using phase shift keying (PSK) or quadrature amplitude modulation (QAM).The number of subcarriers is denoted by N.The frequency spacing of subcarriers is ∆f. The total available bandwidth B will be divided into N equally spaced subcarriers .Each subcarrier has exactly an integer number of cycles in the interval T and the number of cycles between adjacent subcarriers differs by exactly one. This accounts for the orthogonality between the subcarriers. Each subcarrier can be modulated independently with the complex modulation symbol, X_(m,n)where m is a time index and n is a subcarrier index. Then within the time interval T of the following signal of the m-th OFDM block period can be described by equation (1)
Peak-to-Average Power Ratio:
OFDM system exhibits a very high peak-to average power ratio (PAPR) when using a number of subcarriers. This is the major drawback of OFDM. As a result, power amplifiers are not operated with large power back-offs, hence the out-of-band power is below the specified limits. Hence amplification is very inefficient and transmitters become expensive. So reducing the PAPR is highly desirable. A detailed mathematical analysis for PAPR is presented. The Root Mean Square(RMS) magnitude of the OFDM signal is defined as the root of the time average of the envelope power√(P ̅ ) where P ̅ is defined by equation(6).