正交频分复用(Orthogonal Frequency Division Multiplexing ,OFDM) 技术的优势在于,把高速数据流通过串并转换,使得每个子载波上的数据符号持续长度相对增加,从而可以有效减小无线信道的时间弥散所带来的符号间干扰( Inter Symbol Interference ,ISI) ,这样就减少了接收机内均衡的复杂度,有时甚至可以不采用均衡器,仅通过采用插入循环前缀的方法消除符号间干扰的不利影响。
OFDM系统的缺点在于,其信号是多个子载波的叠加,因而存在较大的峰均比(Peak to average Power Ratio ,PAPR) ,此时发射端需要使用功率较高的非线性高功率放大器,增加了硬件实现的成本;此外,当OFDM信号的峰值超出放大器的工作范围时,会产生非线性失真,恶化传输性能. 因此高峰值问题是OFDM技术的一大障碍。
目前,很多文献文提出了降低OFDM信号峰均比的方法,本文主要针对限幅方法抑制峰均比做了着重的介绍，其中限幅(Clipping) 以其较低的实现复杂度得到广泛关注。其基本思想在于:设立一个门限,对OFDM 信号幅度超过门限的部分进行削除,从而得到低峰均比信号. 其缺点在于限幅是一个非线性的处理过程,在降低OFDM 信号峰均比的同时,引入了限幅噪声,限幅噪声会对OFDM信号造成干扰,影响传输性能。
关键词：OFDM，多载波，峰值平均功率比，限幅
ABSTRACT
The advantage of OFDM(Orthogonal Frequency Division Multiplexing) is it can transform the high-speed data into each sub-carrier data symbols, which continued length relative increase can effectively reduce the wireless channel time dispersion of symbol Interference between Inter Symbol Interference(ISI), this reduces the complexity of the balance inside of the receiver, and sometimes use equalizer by inserting prefix circular symbol to eliminate the adverse effects of Interference.
Drawback of OFDM system is that OFDM signal is summed by many sub-carrier signal, so the peak Power than Peak to average (flat, this PAPR), the transmitter needs the high power amplifier , increase the hardware cost.In addition,When the OFDM signal peak beyond the scope of transmitter's work, amplifier can produce nonlinear distortions, deteriorate the transmission performance.Therefore peak is an obstacle of technology OFDM.
At present, many literature propose method of reducing OFDM signal PAPR , this paper mainly aimed at introducing clipping method to the inhibition of the peak, which method with its low complexity. The basic idea is to establish a threshold, reduce the part which over the threshold of the signal amplitude. Its disadvantage is PAPR limit is a non-linear process, the same time when reduce OFDM peak signal, introduces restrictsthe amplitude noise, the noise will make interference to OFDM signal, affect transmission performance.Keywords: OFDM, sub-carrier, PAPR,ISI,Clipping
