正交頻分復用(Orthogonal Frequency Division Multiplexing ,OFDM) 技術的優勢在于,把高速數據流通過串并轉換,使得每個子載波上的數據符號持續長度相對增加,從而可以有效減小無線信道的時間彌散所帶來的符號間干擾( Inter Symbol Interference ,ISI) ,這樣就減少了接收機內均衡的復雜度,有時甚至可以不采用均衡器,僅通過采用插入循環前綴的方法消除符號間干擾的不利影響。
OFDM系統的缺點在于,其信號是多個子載波的疊加,因而存在較大的峰均比(Peak to average Power Ratio ,PAPR) ,此時發射端需要使用功率較高的非線性高功率放大器,增加了硬件實現的成本;此外,當OFDM信號的峰值超出放大器的工作范圍時,會產生非線性失真,惡化傳輸性能. 因此高峰值問題是OFDM技術的一大障礙。
目前,很多文獻文提出了降低OFDM信號峰均比的方法,本文主要針對限幅方法抑制峰均比做了著重的介紹，其中限幅(Clipping) 以其較低的實現復雜度得到廣泛關注。其基本思想在于:設立一個門限,對OFDM 信號幅度超過門限的部分進行削除,從而得到低峰均比信號. 其缺點在于限幅是一個非線性的處理過程,在降低OFDM 信號峰均比的同時,引入了限幅噪聲,限幅噪聲會對OFDM信號造成干擾,影響傳輸性能。
關鍵詞：OFDM，多載波，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
