2
$\begingroup$

The OFDM baseband signal can be demodulated since each subcarrier is orthogonal to another. But In SFN system, i.e., Single Frequency Network, all OFDM signals from several transmitters are in the same frequency domain, which breaks the orthogonality.

So without some technology like CDMA, the OFDM sigals from each transmitter must be time-divided.

Is that right?

$\endgroup$
2
$\begingroup$

In a multi-user scenario where every transmitter uses the same center frequency and bandwidth, but transmits different data, time-division multiplex has to be applied. Otherwise, interference between the multiple transmit signals will make successful reconstruction of a specific transmit signal at the receiver virtually impossible.

However, in typical SFNs, all transmitters are part of a broadcast network and transmit identical data. Examples are Digital Audio Broadcast (DAB) and terrestrial Digital Video Broadcast (DVB-T). In such networks, the guard interval of OFDM symbols can compensate for the propagation delay between different transmitters and there is no need for time-division multiplex.

Also, I don't think CDMA and OFDM can be combined. As a multiple-access scheme, OFDM(A) allows sharing of the same frequency band by frequency multiplex while CDMA applies code multiplex.

$\endgroup$
0
$\begingroup$

There is research that shows promise for self-cancellation of a signal. This allows two OFDM transceivers to operate in full-duplex mode (transmitting and receiving at once). This might be used in a cabled scenario or possibly in a satellite link. Most of the research is based around using the known transmit signal and characterizing the feedback channel to perform an adaptive cancellation of the transmit signal prior to sampling the received signal. As far as I know, this is still in the research phase.

As you mentioned, and contrary to Deve's comment, one could apply CDMA techniques to OFDM by applying a chipping sequence across the subcarriers. This is often referred to as Multicarrier Code Division Multiple Access (MC-CDMA). Different transmitters can then coexist on the same subcarriers to transmit different messages to a single receiver or to independent receivers. This would require tight synchronization between the transmitters (like you might have in the downlink channel). The different transmissions would need to coordinate their transmission times, cyclic prefix times, and be synchronized well enough to maintain orthogonality of the subcarriers.

As Deve points out, OFDMA is another approach to sharing the spectrum among multiple transceivers. OFDMA is a scheme with different subcarriers allocated to different users. This also requires tight timing and frequency synchronization between radios. They must coordinate their transmission times, cyclic prefix times, and ensure that the signals transmitted by different users are orthogonal (i.e., they have negligible frequency and sampling time offsets). Again, this is fairly easy to do in the downlink where the transmitters are likely to be colocated, but much harder with untethered radios.

Another approach is spatial multiplexing where different OFDM transceivers apply beam forming techniques to target a specific location. In this case, the other locations in the network see negligible leakage, and so, the same subcarriers can be used again at these locations (with another beam former). Following the same pattern as the other techniques, this requires tight synchronization between the transmitters. Once again, it is far easier to accomplish this synchronization in a downlink where the different transmit sequences can be coordinated.

Time division multiplexing (TDM) is the most forgiving approach if you don't require very tight synchronization between a pair of radios. On the other hand, if synchronization is possible, then these other approaches can yield large gains in network capacity.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.