Reducing Peak To Average Power Ratio Based On Low Complexity Companding Technique

OFDM (Orthogonal frequency division multiplexing, OFDM) is a parallel multi-carrier transmission scheme, high spectrum efficiency, using a number of mutually orthogonal subcarriers to transmit information, it has a good ability of anti-multipath interference. However, one main disadvantage of OFDM is the higher peak-average power ratio (Peak-to-Average-Power Ratio, PAPR), this requires a high linear power amplifier system and high dynamic range in order to avoid the nonlinear distortion and the spectrum proliferation of transmission signal, it will increase the cost and the realization of the difficulty.This paper uses a companding technique to reduce the PAPRFirstly introduced the theoretical knowledge of PAPR, introduced the question of the origin of PAPR , the OFDM frequency domain signal performances N orthogonal sub-carrier signal of the superposition in the time domain, when the N signals are just the sum of the peak point at the same phase , OFDM signal will have the greatest peak power , the peak power is N times the average power . Then introduced the definition of PAPR and the probability distribution, analyses the impact of nonlinear amplifier on the PAPR, in order to enable the transmission amplifier model features close to the actual amplifier , Smoothing factor p will generally be of limited value range 2-4.Then analyses the impact of over-sampling on the PAPR, the simulations show that different sampling rates come to the PAPR distribution is not the same as usual. When the sampling rate is too low, often misses some of the peak signal, PAPR value will be lower than the actual system. In order to approximately describe the probability distribution of the PAPR in OFDM systems, over-sampling is necessary. Focused on the three main types of technique of reducing the peak-average power ratio (PAPR): signal distortion techniques, signal encoding and scrambling code technique. Signal distortion technology directly did the non-linear operation on the peak signal, simple, direct, but the non-linear operation will bring about in-band noise and out-band interference. Signal coding techniques using the original information encoded code word is mapped to a good PAPR characteristics of the transmission code, in order to avoid the the code word appearing the peak signal, this technology is the linear process, will not lead to signal distortion, but has the high computational complexity, coding and encoding are too much trouble, and information rate of such technique reduces quickly, is only applicable to the situation of fewer carrier. Signal scrambling code technology, such technology uses the method used linear process, will not occur signal distortion, can effectively reduce the PAPR, but has too much computational complexity. These technologies exist some limitations, at home and abroad there is not yet a technology that can simultaneously reduce the PAPR, reduce the computational complexity and computation, such as to improve system performance to achieve very good results.In this paper, followed by the problem of lower PAPR study, companding algorithm was used to reduce the PAPR. Discussed the principle of companding algorithm to reduce the PAPR and several common companding function, as well as the impact on system performance, focus on an analysis of the impact of system BER on companding transform,cited to introduce the use of companding transform OFDM system bit error rate formula,pointed out that on the situation of maintaining the signal power before and after companding unchanged, a certain signal to noise ratio, the companding transform can reduce the system bit error rate, at the same time obtain the corresponding expansion of benefits. Based on OFDM signals and voice signals are similar in a sense, the emergence of substantial value signal not cyclical,for voice signal companding technique can be used to the PAPR of OFDM signal processing of companding,μ-law companding and A-law companding method was often used, they were non-linear companding in log-companding. Traditionalμ-law companding maked large signals compressed,small signals were enlarged, A turning point value was signal peak value, simulation showed that the use of companding in SNR can certainly reduce the non-linear noise, when the SNR was low, the low value ofμhad better BER performance, whenμvalue was low,system of the PAPR was high , the higher value ofμwould reduce the system performance, how to select the value ofμthere is no specific discussion. Therefore, improved theμ-law companding method, A value of the inflection point checked for the average value, were not peak value, and ensured that the average power are relatively unchanged before and after companding, the simulation proved that the algorithm can effectively reduce the PAPR, and compared with the PTS algorithm was not only a small amount of calculation, but the effect was more obvious compression. Results from the simulation that the increasing value ofμleaded to the deterioration of system performance, value ofμmust be less than 25,in the actual system , in the case of BER system allowed, select the best value ofμto gain the maximum compression of the PAPR.Finally, according toμ-law improved companding Algorithm, A-law companding algorithm was improved. Traditional A-law companding maked large signals compressed, small signals were enlarged, A turning point value was signal peak value, the simulations showed that the PAPR after companding was obviously much smaller than the original signal.A small signal during transmission was vulnerable to the effects of interference,A small decrease in signal showed that companding signal during transmission had a stronger ability of noise resistance. In the improved algorithm, A value check for the average value of amplitude, and ensured that the average power are relatively unchanged before and after companding. The improved algorithm, indentified the best value of k. Simulation showed that the improved algorithm could effectively reduce the PAPR, new companding signal was similar to uniform distribution, amplitude of signal mainly concentrated in the vicinity of the average range of amplitude. Enhanced small amplitude signals the anti-interference ability, would not make small-amplitude signals relatively expand much, resulting in severe signal distortion.