以多項式消除編碼為基礎之OFDM系統頻率估計

Abstract

正交多頻分工(Orthogonal Frequency Division Multiplexing ,OFDM)技術近年來頗受到重視。OFDM系統設計必需要考慮到載波頻率偏移(CFO)補償這個課題，因為載波頻率偏移會造成載波彼此之間的與干擾(ICI)，破壞載波的正交性並將大大的降低系統效能。載波彼此之間的干擾(ICI)會使載波上的信號強度衰減(attenuation)並產生相角旋轉 (phase rotation)。在許多消除ICI的方法中，ICI 自我消除法(ICI self-cancellation) 或稱為多項式消除編碼(polynomial cancellation coding, PCC)因其容易實現以及效能穩定而受到重視。然而，當載波頻率偏移大時，多項式消除編碼只能消除一小部份的ICI。因此，我們提出以多項式消除編碼為基礎的載波頻率偏移估計。首先，以PCC做為事前編碼 (precoding)並解出初步的資料，接下來再以此資料估計載波頻率偏移(CFO)。藉由使用多項式消除編碼，直接估出載波頻率偏移(CFO)並加以補償，可以改善原本多項式消除編碼在載波頻率偏移(CFO)大時只能消除部份ICI的缺失。

Orthogonal Frequency Division Multiplexing (OFDM) is being considered as a promising transmission technique. However, a carrier frequency offset (CFO) between the transmitter and receiver will results in ICI and thus destroy the orthogonality of the subcarrier and degrades the performance. ICI leads to attenuation and phase rotation of desired signal on each subcarrier. These impairments have already motivated several studies to find solutions. Among the several ICI reduction schemes, ICI self-cancellation or polynomial cancellation coding (PCC) scheme has received much attention due to its simplicity and its high robustness to frequency offset errors. However, for large CFO, PCC can only eliminate ICI to a certain extent. Thus, we propose new methods of PCC based CFO estimation to eliminate ICI for either low or high CFOs. First, we use PCC as precoding scheme and get the initial decoded data. Then CFO estimation is done by making use of the decode data. CFO estimation and compensation from PCC can overcome the drawback of PCC that only a certain part of ICI can be eliminated when CFO is large.