How much time does it take to demodulate one OFDM symbol? In 802.11a standard the Symbol Interval Time is 4us. Does this mean that each symbol is demodulated within this period or there is pipelining involved? I understand that the answer may depend on the hardware platform, but I would like to know the range (us/ms) in the fastest implementation. Moreover, a breakdown of time needed at various steps would be nice.
Once you have the various I+Q samples, you need to execute an FFT. Plus other "cleanup" steps.
The Wikipedia page states 20MegaSamples/second for 4 us, thus 80 samples. Round up to 128 samples.
If I recall rightly, the computational load for FFT is 4 * N * log2N, thus 4 * 128 * 7 ~~ 3,500 operations. At 1 nanosecond per multiple/accum/butterfly, the time is 3.5uS.
There is a lower limit to the time it takes, which is the 4uS it takes to collect all of the data. Obviously you can't start handling the symbol until all the data is in.
There is an upper limit to the time it is allowed to take in a conforming implementation. This is the turn-around time associated with ACK/NACK response. It will be specified in the physical layer protocol somewhere, but I don't know it off the top of my head, and am not about to go digging. At least you now know what you are looking for if you can access a copy.
In between those limits, any time is permitted, and, given the large number of different implementations using FPGAs, DSPs, and custom hardware that exist out there, you can be assured that most possibilities will have been covered.
A better question would be 'how much time does this particular implementation take'? A more meaningful question would be 'how many operations is needed for a symbol decode'?