Study On The Joint Estimation Algorithms Of Symbol Timing And Carrier Frequency Offset For OFDM System

1. Introduction For several years, OFDM technology has been the hotspot of people's research, which because of the high utilization efficiency on system frequency, low cost, ability of mitigating inter-symbol interference (ISI) caused by frequency-selective channels and multi-path fading distortion. It is also considered as one of the promising candidates for wireless communication standard. Many world standard bodies are selecting the OFDM as the physical layer technique for the next generation of wireless communication system.Synchronization technology is a practical problem of every communication system; its performance has close relationship with the communication quality. If there is no accurate synchronization algorithm, there is no orthogonal to protect credible data transmission. So it is very important to research in OFDM system. There are three demands in synchronization: signal synchronization that decides the start moment of OFDM signal for receiver and estimates the accurate position of FFT window; carrier synchronization that guarantees the equality of oscillation frequency of the receiver to that of sender; sampling synchronization that sustain the same sampling frequency for both receiver and sender.Comparatively, sampling synchronization cannot influence OFDM seriously. So the joint estimation algorithms of symbol timing and carrier frequency offset for OFDM system are mainly discussed in this paper. They can estimate both symbol timing and carrier frequency offset, be suitable to practical application.There are two kinds of synchronization aspect for the joint estimation algorithms of symbol timing and carrier frequency offset for OFDM system: one is blind estimation utilizing structure of signal such as cyclic prefix and virtual sub-carrier, which overcomes disadvantages of resources waste. The other is non-blind estimation, based on pilot symbol, which can lead to the waste of band width and power resource, thus decreases efficiency of the system, but the precision is high. In this paper, a blind estimation algorithm based on cyclic prefix and a non-blind estimation algorithm based on selective superimposed pilot sequences are presented.2. A joint estimation algorithm of symbol timing and carrier frequency offset based on cyclic prefixCyclic prefix is the copy of the last part of OFDM symbol. Van de Beek provided ML algorithm to estimate symbol timing and carrier frequency offset, which according to the correlation between cyclic prefix and data symbol. This algorithm has several advantages such as saving resource, small amount of calculation, low redundancy, easy to be implemented. But it cannot be used in multi-path fading channels, because of the destroyed correlation. In this case, the precision is bad and hardly to be utilized in practical OFDM system. Therefore, the task of this paper is to sufficiently modify this algorithm through exerting its advantages and overcoming its disadvantages.The modified algorithm makes the likelihood function of ML synchronization algorithm be suitable to Jakes multi-path channels. The autocorrelation function for this channel model satisfies zero-order Bessel function. The model's simulation contains two parts: one is to make the signal envelope depend on Rayleigh distribution; the other is to make the multi-path signal which has known time delay. After multiplying them and adding together, the multi-path channels model can be got, which has Doppler frequency domain spread.The length of cyclic prefix can usually overcome the ISI/ICI caused by multi-path fading, so the modified algorithm ignores the correlation outside the cyclic prefix, which is caused by multi-path dispread. Another simple coarse synchronization is involved to compensate, it is a coarse to delicate process. The simulation results prove the process is in reason and the comparisons between ML algorithm and modified algorithm in SNR, length of cyclic prefix, carrier frequency offset and so on can indicate that the modified algorithm is effectively suitable for multi-path channels and could estimate the synchronization exactly.3. A joint estimation algorithm of symbol timing and carrier frequency offset based on selective superimposed pilot sequencesIn order to utilize data to obtain more accurate synchronization sufficiently in OFDM system, the synchronization process is divided into two segments: seizing and tracing. The dividing makes it easy that only inter-segment tasks be considered without considering counterparts of other segments, therefore designing the synchronization structure can be more liberate.The superimposed pilot technology transmits PN sequences with OFDM symbol together, and the average power of system contains the same. This technology has a high spectral efficiency. In the timing seizing period, only the PNsequences can be transmitted. After in the tracing period, PN sequences superimpose on the OFDM symbol, the sum power of the two periods is the system's average power.This paper focus on the algorithms in the period of tracing, providing a joint estimation algorithm of symbol timing and carrier frequency offset based on selective superimposed pilot sequences. In this scheme, the pilot sequence with the minimum correlation with the data symbol sequence is selected from the set of the pilot sequences assigned to the transmitter, the correlation is low, and the synchronization estimation performance is superior to the one pilot superimposed system. The receiver correlates the received signal sequence with the pilot symbol sequence to select the pilot sequence which is needed in synchronization estimation, based on maximum correlation principle. The synchronization process is done by the strong autocorrelation between pilot sequences. This technology requests the length of PN sequence is equal to the length of the OFDM symbol or its multiple, in order to make sure the correlation peak value is on the boundary between OFDM symbols. Contrast to the power of the OFDM symbol, the power of PN sequence is low to deteriorate the influence.The simulation results indicate that the synchronization correlation has sharp peak value, it can provide accurate symbol timing, and the range of carrier frequency offset is large about to 1/2 carrier interval. The precise can be highly improved because of the pilot selectivity.