Hi, I am simulating a single carrier system in which signals are equalized in frequency domain. The concept is similar to OFDM, except both FFT and IFFT are reside in receiver side. Prefix is also used to deal with dispersive channel response. I have implemented the rooted raised cosine filter(Alpha=0.2) in both transmitter and receiver. A static dispersive fading channel is used in each frame. There is no problem with no fading channel, i.e. AWGN channel. The first step I adopt is a simple zero forcing equalizer. The input sampling frequency is two times symbol rate to comply with the bandlimited width channel. FFT is then used to transform signal to frequency domain from time domain. After matched filter processing in frequency domain(including RRC and ideal channel matching), a sampling rate reduction(2 to 1) is used to reduce the date rate to that of the symbol rate. This has considered the aliasing combining effects. As a test procedure of the equalizer, I don't add noise to the system. So, I expect I could get no BER even I use a simple ZF equalizer. Now, the equalizer seems not very well. At the beginning and the end edges(It is about 4 to 8 symbols each end.) of a data block(The total block length is 256), there are some slight signal distortion. In the center part of the data block, signal is very good. Of course, this is after IFFT transformation. The dispersive channel is about 6 symbols long. The RRC tail is about 4 symbols long. I first used the prefix as long as 16 symbols. In order to isolate problem, I have increased the prefix length to 32, even 64. The problem still exists. This is really strange. From the equalized signal waveform. It looks like the initial signal level related. There are some low frequency modulation for the 4 to 8 symbols, i.e. a low frequency but a noticable level fluctuation. I know here it is a signal block based DSP. It is different from the normal time domain processing. What would cause the edges signal worse? The prefix should convert linear convolution to circular convolution. This is like an ideal periodic signal. It shouldn't make the edges differnt from other data. I am sure it is the edges resulting the average error rate about 10^(-3), not 0. But for an ideal circular convolution, this shouldn't be. Could you give me some suggestion? Any comments are highly appreciated.

# Problem with circular convolution in SC-FDE

Started by ●January 8, 2005

Reply by ●January 9, 20052005-01-09

Hi, Sorry, I have to add some information. The simulation is implemented in Matlab. Both transmitter and receiver RRCs are done in time domain. Its oversampling Fs is two times of symbol rate Fd. The symbol is 16 QAM. The dispersive channel is a FIR model who has 12 items in time span. The channel matching is done in frequency domain. Thanks in advance.