IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 03 Special Issue: 07 | May-2014, Available @ http://www.ijret.org 474 AN EFFICIENT APPROACH FOR SQUARED CREST FACTOR REDUCTION IN OFDM SYSTEMS USING TONE RESERVATION WITH CUSTOMIZED CONVEX OPTIMIZATION V.Kiruthika 1 , V.Nandalal 2 1 PG Scholar, Electronics and Communication Engineering, Sri Krishna College of Engineering and Technology, Tamilnadu, India 2 Assistant Professor, Electronics and Communication Engineering, Sri Krishna College of Engineering and Technology, Tamilnadu, India Abstract Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. This scheme uses a number of sub-carriers and converts a frequency selective fading channel to a set of parallel flat fading channels. The OFDM signal is having a drawback that some signal values can be much higher than the average value. This results in high Squared Crest Factor (SCF). This paper deal with the Tone Reservation (TR) technique for reducing SCF. In this scheme few subcarriers are reserved to reduce the peak signal. These reserved carriers are called as Peak Reduction Carriers (PRCs) and these PRCs are used to reduce the SCF of the original signal. Similarly the Bit Error Rate also gets lowered. In contrast to previous methods, the TR method with Customized Convex Optimization implies a very quick convergence of minimum SCF and the bit error rate solution at a lower computational cost. The proposed method is compared with the SCF reduction techniques like Differential Scaling. The proposed OFDM system also discusses about the effects of oversampling at different rates. Keywords: Convex Optimization, Orthogonal Frequency Division Multiplexing (OFDM), Squared Crest Factor (SCF), Tone Reservation (TR), Peak Reduction Carriers (PRCs), Complementary Cumulative Distribution Function (CCDF). ---------------------------------------------------------------------***--------------------------------------------------------------------- 1. INTRODUCTION Orthogonal frequency Division Multiplexing is a technique in which high speed data is split into low speed parallel channels. One of the main practical issues of OFDM is the high Squared Crest Factor of the transmit signal. There are many techniques for reducing the SCF but these techniques either need side information or not needed. Depending on the necessity of side information the SCF reduction technique can be classified into two groups. The main two techniques which need side information is the Partial Transmit Sequence (PTS) [1], [10] and the Selected Mapping (SLM) [2], [9] technique. These techniques modify the symbols of both frequency domain and time domain. The side information is generally used for preventing the OFDM system from burst errors. The other techniques which do not need side information include Clipping and Filtering (CF) [3], Peak Windowing (PW) [4], Active Constellation Extension (ACE) [5], and Tone Reservation [6], [7]. The TR method reserves a minimum number of tones and assigns appropriate values to reduce SCF. The TR approach is also an iterative one, but it does not change the data symbols. 2. TONE RESERVATION In the TR approach, a small number of unused subcarriers are reserved by the transmitter for SCF control. These unused subcarriers are known as Peak Reduction Carriers (PRCs) which are designed to be orthogonal with the data bearing subcarriers thereby causing no distortion. At the receiver side the contents of the PRCs will be ignored since it does not contain any useful information and also no side information needs to be transmitted. In general, there are two schemes to reduce the SCF in the TR technique. The first is to select the PRC set for reducing the SCF and the second is to design the proper values on these PRCs to generate an optimal peak-canceling signal that minimizes the SCF of a transmitted OFDM signal. Several methods [11-13] have been proposed to search a nearly-optimal PRC sets in the TR technique for the improvement of the SCF performance of OFDM signals. 3. DIFFERENTIAL SCALING Differential Scaling is another technique which is used for the reduction of SCF. In this technique different amplitude levels are scaled in a different manner and so it is called as Differential Scaling. We have considered three types of scaling as described below.