I’m currently trying to wrap my head around optical OFDM. I’m reading various tutorials and articles, but I’m hitting a stumbling block trying to understand Hermitian Symmetry and how it is applied. I’m taking the block diagram of an OFDM system from Figure 3 in this paper, and working through it step by step starting from the input to the transmitter. It’s probably best that I go step-by-step up until the point where I get confused.
Step 1 - serial to parallel
For the sake of simplicity I’m skipping the “Coding” and “Interleaving” steps from that paper. Let’s start with a serial bit stream we want to send: 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1…etc.
We will map this to parallel bit streams, one for each subcarrier. I’m using 4 subcarriers in this example, C1 to C4. The parallel streams are now:
Step 2 - QAM modulation
Modulate each subcarrier. Here I'll use 4-QAM. Taking 2 bits at a time from each of the subcarrier columns I get:
|1 +1j||1 +1j||-1 +1j||-1 -1j|
|1 -1j||-1 +1j||-1 -1j||-1-1j|
|-1 -1j||1 -1j||1 +1j||-1 +1j|
Step 3 - IFFT
This is where my understanding starts to break down. I understand for “normal” OFDM we’d pass these values to the IFFT to generate the time-domain OFDM signal. However, what exactly are we sending to the IFFT block? Do we send it each row of that table in turn, i.e. the values of each subcarrier at a particular sample time?
Step 4 - Hermitian Symmetry
This is where my understanding really breaks down! I understand that Optical-OFDM require purely real-valued time-domain OFDM signals. I also follow that if the input to the IFFT block has the property of "Hermitian symmetry", then the IFFT output will be the desired real signal.
Hermitian symmetry is "imposed" on Xk symbols by ensuring that:
X0 = XN/2 = 0 and
Xk = X*N-k for 0 < k < N/2
Where N is the IFFT length and k is the carrier number (I assume?)
My confusion here is if we have to "impose" that certain carriers must take a given value (e.g. carrier 0 and carrier N/2 must equal 0) then isn't that somehow "overwriting" the value for that carrier that we have applied at the QAM modulation stage? That seems like it must be wrong to me, but I can't understand why!
So, apologies for the long-winded question, but just to clarify I'm asking if my understanding of the IFFT at Step 3 is correct, and what exactly we are doing when we "impose" Hermitian Symmetry onto our signal prior to the IFFT in Step 4.