페이딩 채널에서 OFDM신호의 개선된 PAPR 감소기법

Abstract

Orthogonal frequency division multiplexing (OFDM) is a promising technique for the next generation wireless communications. OFDM is a special multi-carrier modulation technology for high speed data transmission and efficient spectrum applications over the multipath fading channels. It also provides the probability to eliminate the need for equalizers, where efficient hardware implementation can be realized using fast Fourier transform (FFT). However, the high peak-to-average power ratio (PAPR) of the transmit signal is one of the main disadvantages of OFDM. An OFDM signal with high PAPR may cause nonlinear distortion and require significant back-off of the average operating power of a RF power amplifier if the signal is to be linearly amplified. The typical distortion technique for PAPR reduction is clipping and filtering which maybe the simplest technique to solve the PAPR problem. Clipping can be easily implemented using a nonlinear amplifier that limits the amplitude of the OFDM signal to a predefined value. However, use of the clipping technique may cause undesirable distortion of the original OFDM signal and reduces the spectral efficiency of transmissions. The filtering is normally performed after the clipping in order to reduce out-of-band radiation; oversampling four times before clipping can effectively prevent the PAPR regrowth that is caused by the filtering. In addition, a special receiver that utilizes a decision-aided reconstruction method can be implemented to mitigate the clipping noise. One representative distortionless technique for PAPR reduction is adaptively symbol selection scheme (ASSS); such as, selected mapping (SLM) and interleaving technique. ASSS generates a set of sufficiently different candidate data blocks representing the same information and selects the most suitable data block with the lowest PAPR for transmission. It does not cause distortion on the OFDM signal; however, that is at the cost of increase of the system complexity. In addition, side information representing the index of the candidate data blocks for ASSS is needed to be transmitted to the receiver for demodulation. In the conventional scheme, side information is transmitted by using the data subcarriers; hence, the transmission data rate will be reduced in some extent. Comparing the advantages and disadvantages of clipping and ASSS in terms of PAPR reduction and system complexity, in some previous works, the deliberate clipping and ASSS are implemented together in the OFDM system to obtain an effective PAPR reduction technique and moderate system complexity. Considering the ASSS OFDM signals are transmitted over fading channels in wireless communications, we propose a side information transmission technique in which the pilot tones are used not only for channel estimation but also for transmission of side information. In this technique, the side information are equally spaced in the subcarriers which are originally used for pilot tones, therefore the side information are not inserted into the subcarriers used for data sequence that can improve the transmitted data rate. In addition, a special receiver with twice channel estimations is implemented to avoid the degradation of the channel estimation result caused by using less pilot tones. Numerical analysis and computer simulations show that the proposed side information transmission technique and the special receiver can improve the transmitted data rate and bit error rate (BER) performance, meanwhile the output signals maintain the same PAPR reduction ability as that of the conventional ASSS OFDM signals. Combining with deliberate clipping and ASSS, the side information transmission technique is then implemented in the conventional OFDM system. In this improved OFDM system, the clipping technique is modified by using a decision aided reconstruction method, and the ASSS technique is improved by using the side information transmission technique. In the numerical analysis of this OFDM system, we derive a more accurate probability density function (pdf) of amplitude of the clipped-adaptively-selected (CAS) OFDM signal, which shows more accurate PAPR reduction ability of this OFDM system than the previous works. In the computer simulation, the OFDM system is tested in the realistic fading channels. The simulation results show the proposed OFDM system has an effective PAPR reduction ability, a high transmission data rate and a reliable BER performance.