| Orthogonal Frequency Division Multiplexing (OFDM) has its own advantages, such as efficient spectral bandwidth, the good performance against multipath fading and low system complexity and cost. So OFDM has been used in various high-speed wireless data transmission systems and also been proposed as a standard for the next-generation mobile radio communication system. But one major drawback is that the OFDM signals of transmitter have high Peak-to-Average Power Ratio (PAPR). It's the key point which limits the wide applications of OFDM.The principle of OFDM system is that high-speed serial data is switched into N parallel low-speed data, then the low-speed data is modulated N mutual orthogonal tones, and cyclic prefix is used to eliminate inter symbol interference. When many subcarriers are in-phase, high PAPR is produced. Tone Reservation algorithm is mainly studied in this paper.Traditional TR algorithm can be easily realized, but the effect is not good and it needs more reserved tones. Combining the traditional TR algorithm with clipping method, an improved method is presented—clipping TR algorithm. Although its effect is obvious, its signal distortion is too serious to apply in actual system. So another improved TR method is presented—controlled clipper method. Making use of kernel generated by the reserved tones, the kernel is circularly shifted to the maximum peak value in time domain to reduce the peak. This method is very simple, and the computing capacity is also very low, while it only needs few iterative times to get better effect of reducing PAPR. Therefore, this algorithm can be applied in actual system. The effectiveness of the proposed algorithm is demonstrated by simulations.Based on the IEEE 802.16e protocol, an OFDM baseband system is designed. The system realizes 18Mbit/s baud rate in 10MHz widthbreadth (contain frequency protection partition), and it is made up of Pseudo Random Binary Sequence, CTC coding, interleaving, IFFT and reducing PAPR. The system is realized by C language programme. The system does not increase the bit error rate, and the effectiveness of controlled clipper algorithm has been verified. |
