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Hi all...
Currently I am working on synchronization issue in OFDM systems. I am a beginner so would you please help me with these basics questions.
I have several questions in modelling the synchronization task.
1. In my OFDM systems model, i don't use I/Q mod. Before sending through the channel I assign frequency offset to the signal. I use the frequency offset block fr simulink blockset. But when i see your model (SISO), it generated it's own offset using constant block and DDS. what's the difference between these things? can i still use the offset block coz I m just need to do simulation not hardware implementation.
2. In your model there's no block to give time offset. I thought that I need to put a delay block in the model to give a time offset to the signal and this delay need to estimate (preamble detection) later using correlation of the received signal . Or am I misunderstanding abt this thing?
3. This last question is about the most difficult problem that I am facing now. It seems quite simple to understand the teory of how to estimate the coarse time and coarse frequency offsets. In coarse estimation, after detecting the peak corr which is the indication of the start of the preamble then the angle of this peak correlation is the estimation of the frequency offset. But it's very complex when come to the modelling. I don't have that such idea of how to model it before and I made a very simple model which didn't picture out the complexity of the synchronization task. Could you please explain in a simpler form the details of the data flow needed for these coarse estimation. I've seen your model but frankly i couldn't follow it.
Thanks in advance
Uma
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hi Uma- the synchronization schemes in our OFDM model are mostly based on standard algorithms presented in the literature. We used a few papers when developing the model (listed below; you'll need access to IEEE Xplore to view them).
Robust Frequency and Timing Synchronization for OFDM by Schmidl and Cox
Optimum Receiver Design for Wireless Broad-Band Systems Using OFDM - Part I by Speth, Fechtel, Fock & Meyr
Optimum Receiver Design for Wireless Broad-Band Systems Using OFDM - Part II by Speth, Fechtel, Fock & Meyr
We use the algorithms described in these papers for both symbol timing and frequency offset recovery. Both synchronization steps are based on auto- and cross-correlation of each packet's preamble.
For your other questions:
1) The DDS block at the receiver is used to correct the carrier frequency offset. The DDS generates a complex sinusoid whose frequency is derived from the preamble-based carrier offset estimates. The DDS is not involved in the OFDM-modulation process; an IFFT at the transitter and FFT at the receiver are used to load individual subcarriers with data symbols.
2) The delay between the transmitter and receiver is not a parameter the receiver needs to estimate. The receiver derives its timing from the packet's preamble, indepenent of the actual propogation delay from Tx to Rx.
3) The algorithms are best described in the papers I listed above and in other tutorials on the web (here and here, for example).
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