Orthogonal frequency-division multiplexing (Orthogonal Frequency-Division MulTIplexing, OFDM) technology has been widely used in wired and wireless communication systems, becoming a candidate for a new generation of wireless transmission. The OFDM signal is realized by inverse fast Fourier transform (IFFT), and the receiving end is restored by FFT, so the implementation is relatively simple. However, the receiving end requires good orthogonality between subcarriers. If the frequencies at the receiving and transmitting ends are not accurately synchronized, or the Doppler spread is generated in the high-speed mobile environment at the receiving end, the orthogonality between the subcarriers of the receiver will be destroyed. Inter-carrier interference (ICI) [1] is generated. In order to improve the system performance, it is necessary to adopt an accurate frequency synchronization method to compensate for the residual frequency offset. At the same time, there must be better measures for Doppler frequency expansion.
For the frequency deviation caused by the incomplete synchronization of the frequencies at both ends of the transceiver or the frequency deviation caused by the local oscillator frequency drift, it can be regarded as stable and static in a relatively short period of time. Therefore, frequency synchronization can be achieved by tracking and frequency offset estimation methods [2 ~ 4]. After performing frequency synchronization and frequency offset compensation, there is still inter-carrier interference caused by a small residual deviation, so the inter-carrier interference cancellation method is used to improve the signal-to-noise ratio of the system. At present, the ICI cancellation methods caused by frequency deviation mainly include frequency domain equalization, time domain windowing, ICI self-cancellation schemes and methods for selecting symbol mapping and partial sequence transmission [5 ~ 8], and all can achieve good results. For the Doppler expansion caused by the high-speed movement of the receiver, it changes with the change of the moving speed and the movement environment. It is impossible to perform tracking compensation, and only the method of inter-carrier interference cancellation can be used to improve the performance of the receiver. Applying the ICI self-cancellation scheme to the Doppler expansion environment can also receive good results [7], but it requires two or more subcarriers to transmit one channel of data, and the spectral efficiency is greatly reduced. Literature [9] proposed a frequency domain equalization method, which has a high implementation complexity, but the performance improvement is small, and it is only effective when the signal to noise is relatively high. Aiming at the inter-carrier interference of OFDM under fast movement, this paper will discuss a receiving segment equalization method.
The segmented equalization method divides a time-varying OFDM symbol into multiple sub-symbol segments, and the channel parameters in each small segment do not change or the amount of change is small. The specific operation is as follows: suppose the number of OFDM subcarriers is N, after deleting the guard interval of the correctly synchronized OFDM time domain symbol, the OFDM symbol with a length of N points is cut and divided into m segments, and each segment has a length of N / m. Each small segment is filled with zeros before and after the original OFDM time-domain symbol position to obtain an OFDM symbol of length N, and then N-point FFT is transformed into the frequency domain; using the estimated channel frequency domain parameters corresponding to the segment data To equalize it, add and sum the equalized results, then make a decision, and demodulate to obtain the source signal. Its operation process is shown in Figure 1. In the figure, the OFDM symbol is divided into four sections of equalization, and the received An OFDM symbol is divided into 4 uniform segments by serial-to-parallel conversion, and the zeros of the segmented data in the original OFDM symbol are padded to the symbol length of N, and then the frequency domain data of length N is obtained by FFT transformation. At the same time, the The time-domain channel parameters are also divided into four segments. FFT is performed to obtain the segmented frequency-domain channel parameters H1, H2, H3, and H4. Equalization, the results after equalization are combined, and then sent to the modem for demodulation. Through segmentation, the effect of channel time-varying on inter-carrier interference is reduced.
Figure 1 Schematic diagram of segmented equalization
Inter-carrier interference cancellation for OFDM under high-speed movement
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