Johnathan, Thanks for the clarifications. I've been using a Costas loop for synchronization, which more or less eliminates the imaginary part of the correlation. Is this how you would do things?
Jordan On Mon, Jul 13, 2009 at 10:58 AM, Johnathan Corgan < [email protected]> wrote: > On Mon, Jul 13, 2009 at 08:48, Jordan J Riggs<[email protected]> wrote: > > > I'm not an expert on the matter, but I suspect that the XCVR2450 board > > requires some sort of command to select the 5GHz band, which would need > to > > be implemented in the sounder's FPGA code. > > Actually, in gr-sounder the host is used to tune the daughterboard, so > nothing different is needed for the XCVR2450. > > > At a chip rate of 32MHz, you have a resolution of 3e8/32e6 meters. > > As the PN autocorrelation function is not a true delta, but > triangular, the effective resolution is about half this (twice the > distance). This is about 20 meters per bin; really only useful for > outdoor propagation studies. > > > The > > length of the PN sequence determines the maximum unambiguous distance at > > which multipaths can be detected. So with a degree-12 sequence, you have > > (2^12 - 1)*3e8 meters to work with. > > You're missing the chip rate as a divisor in the above. > > A full-length degree 12 code is 4095 chips, which at 32 Mcps, is about > 12.8 us in length. The maximum measurable path delay then is about 38 > km. > > > With long sequences, however, you are > > limited by the capabilities of the USRP. Depending on your DBoard, you > have > > ~20mW of output. > > Yes. The SNR for each of the delay bins will decrease as the path > loss increases for each of the reflected signals, so you need to do > your transmitter power and receiver dynamic range calculations based > on the expected maximum path delay you wish to measure. > > > In my experience (with a sounder of my own design), averaging multiple > IR's > > appreciably reduces the noise. > > You can increase the SNR against uncorrelated noise by doing so. But > this is limited in practice (see below.) > > > While I'm at it, can anyone explain why a complex correlation vector is > > coming out of the sounder FPGA? Doesn't a mathematical correlation return > > real values? > > This is a correlation of a real reference PN code with a complex > baseband IQ signal, so the result is complex. This preserves the > phase of the delay bin. The received signal, in addition to > multipath, interference, and additive noise, will have frequency > offset, possible Doppler spread, and timing offset (the latter due to > the difference in sample frequency between TX and RX USRPs.) These > all result in a complex channel impulse response. If you are only > interested in the delay power, you can do I^2+Q^2 on the impulse > response vector. > > Due to the fact that gr-sounder uses a simple O(N^2) serial correlator > without synchronization, the impulse response vectors are very > sensitive to timing offset between transmitter and receiver. This > results in the correlation peaks being separated by more or less than > the PN code length number of samples, and makes it difficult to > coherently add them to reduce noise. Using an external frequency > reference on both ends would make a dramatic difference. > > I plan eventually (read: someday) to reimplement/republish gr-sounder > with synchronization (I have already done something similar for a > commercial customer), but it will be done on the USRP2. > > Johnathan >
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