An FPGA is a good choice as they have advanced to the point where a 1 MHz wide signal can be processed in a $10-20 device.
The number of points to use in the FFT is related to the multipath spread of the received signal. HF signals with ionospheric propagation tyically have a spread between 1 and 10 milliseconds. VHF and UHF signals travelling long distances over partially obstructed paths often have a spread between 10 and 100 microseconds. The guard interval between symbols should be at least as long as the multipath spread. This interval is often chosen to be about 20% of the symbol period as that limits the loss at the receiver due to ignoring this part othe symbol to 1 dB. Subcarrier spacings of 2-8 kHz are often used on VHF and UHF with guard intervals of 30-120 microseconds. On HF the spacing is usually between 30 and 120 Hz in order to increase the guard interval to 2-8 milliseconds. The number of subcarriers is then chosen to fit the available bandwdth. The percentage of the allocated channel that is used depends on the amount of interference allowed in adjacent channels. An OFDM signal is similar to SSB voice in that there will be third-order intermodulation products on each side of the transmitted signal with a bandwidth equal to that of the desired signal. Depending on the quality of the power amplifier, these will be 30 to 60 dB down. The receiver must correct for doppler shift so that the receiver frequency offset is 1 % or less of the subcarrier spacing. More complex signal constellations require closer tracking. A good book on QAM and OFDM is ISBN 0470 09468 0, "Quadrature Amplitude Modulation." 73, John KD6OZH ----- Original Message ----- From: amman kumar To: digitalradio@yahoogroups.com Sent: Friday, October 24, 2008 03:53 UTC Subject: [digitalradio] Queries reg OFDM Transceiver Implementation Hi, I'm entirely new to the OFDM concepts, Digital radio and we are trying to implement OFDM Transceiver using FPGA. I have the following Queries.Plz clarify 1) How do you determine no.of points in IFFT w.r.to Bandwidth. For example, for 1 MHz bandwidth , what is the maximum amount of bandwidth should be allowed for not having ISI and Cross symbol interference ? 2) How the receiver will interpret Doppler shift? Thanks in Advance. Add more friends to your messenger and enjoy! Go to http://messenger.yahoo.com/invite/
