During the Signal Generator Back-to-Basics Webcast, hosted by Agilent Technologies, on 29 February 2012, we received a number of real-time questions from a broad audience.
If one person asks - someone else will want to know the answer too - so we have captured various questions and answers and are posting them individually here, in this Forum, for your future reference and immediate enjoyment!
Question: I would like to know about the OFDM modulation. How can interference effects be prevented, as in this case the signals are at the same frequency, at the same time, but with different modulation. Some effects such as desensitization or cross modulation could still impair the receiver's front-end? How is it solved?
Answer: The individual subcarriers used in OFDM modulation are each at nearby, but different frequencies. Depending on the particular OFDM implementation being used, the modulation on each subcarrier may or may not be the same. Your question is beyond the scope of what we can answer in this Q&A, but I recommend you take a look at some of the OFDM application notes and technical material on the Agilent website at www.agilent.com. Also, you can find a good primer on OFDM modulation in Wikipedia at http://en.wikipedia.org/wiki/OFDM.
This is used in digital TV sent with COFDM. For a specific channel all carriers will be sent on the same frequency, (as long as it's the same transmission system eg DVB-T 2K, 8K etc). Since the TV-transmitters are locked on GPS signals the frequency will be very accurate. If the COFDM signals are sent in a SFN-network, then two or more transmitters are transmitting the exact same information on the exact same frequency to get a better coverage and usage of the frequencies. This will work for the receiver and the receiver can actually make an advantage of this. (Look for specifications of DVB-T on Internet.)
If it's not the same information transmitted on the same frequency then the receiver won't be able to demodulate the signal or just be able to demodulate the strongest signal. This can happen if a normally distant transmitter are transmitting on the same frequency, as the wanted frequency, and during high preasure the propagation might be better than normal.
I have seen solutions where the transmitt frequency deliberately has been changed a couple of Hz to be able to determine the origin of the signals since it's not always easy to say when the propagation is much better than normal. Then signals can be received on great distances.
In the "old days", with analouge TV, then we saw this as gost images but with digital TV the demodulators will not be able to decode the signal and we'll get a "black screen"...