On Thu, Jun 9, 2011 at 3:01 PM, Marcus D. Leech
<address@hidden> wrote:
On Wed, Jun 8, 2011 at 11:24 PM, Morgan Redfield<
address@hidden> wrote:
I found that centering my FFT on a frequency that's offset from what
I'm transmitting at will remove that central spike. I was able to
finally see the gap in the center of the OFDM boxcar and adjust that.
It looks like in my setup I have an offset of about 6kHz.
My OFDM signal never seems to be more than about 10 dB above the noise
floor though. When I bump up the gain or tx-amplitude, everything gets
raised by the same amount. I'm still not able to demodulate packets,
and I think this is why. Do you have any advice about this?
Thanks,
Morgan
Try changing the receiver gain instead. If the noise floor is moving with changes in the transmitter, then you are seeing non-linear effects in the transmit chain, which is bad. This is the chief problem of OFDM in that you need a good, linear PA to transmit with higher power for greater distance (which is one reason LTE is using SC-FDMA in the handsets).
If changing the *TX* amplitude doesn't improve things, then perhaps the frequency offset is the problem.
I'm not much of an OFDM guy, but it seems to me if your OFDM "bins" aren't where they're supposed to be,
to less than a fraction of a bin-width, then there could be problems.
The synchronization algorithms in OFDM correct for both fractional (inner subcarrier) offset and integer (greater than a subcarrier) offset, but only to an extent. So you can be off by a few subcarriers from the desired frequency and have those corrected (I think we put in +/- 5 or 10), and the fractional offset is also taken care of. The analysis of this shows that you get a significant increase in BER if you are even slightly off carrier after sync, so it's a very important part of the process (since OFDM depends on things being orthogonal, any frequency offset destroys the orthogonality).
Tom
Also, to confirm that your RX is sensitive enough, if there's a way you could generate a single-tone signal at
about -110dBm, directly connected to the RX, and see if you can see the tone in an FFT display. If not then
you have RX sensitivity issues.
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