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A few questions are listed in the follow after I read WARP SISO AGC control page.
(A) The difference between dB and dBm. I couldn't find 'dBm' in the description. 'dB' is not a power unit!
void ofdm_AGC_Initialize(int noise_estimate)
Resets and initializes the AGC peripheral with all required parameters. The initial baseband gain is computed from the noise estimate, so an accurate guess of the noise floor is required. This value is generally near -85 dB.
Upon completing this routine, the AGC is enabled and ready to be triggered by the packet detector.
ofdm_AGC_SetTarget(signed int c)
Sets the target signal strength value to c dB. After the AGC completes its execution, the receiver will see a signal of this magnitude.
I think you mean dBm in above functions, right?
(B) How do you calculate the initial background noise power, -19 dB (or dBm)? Over what bandwidth?
Before the AGC runs, the gain values in the radio are calibrated such that background noise appears to the receiver at -19 dB.
(C) All the functions have names start with 'ofdm_', do they really relate with OFDM?
(D) Radio_slot_ID is not a parameter for the AGC API. Does that mean VGA controls on all four radio slots are wired to the same pin sets of FPGA? I feel this is NOT the case, and we should be able to control individual AGC on each radio board, right?
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I have some answers for your questions...
(a) Yes, we mean dBm. That is an oversight in the documentation which we have fixed. Thanks for pointing that out!
(b) The -19 dBm is chosen based on specifications from our radio manufacturer, and applies to the full receive bandwidth.
(c) The functions are named ofdm_ to conform with our naming convention, but they don't necessarily relate exclusively to OFDM modulation schemes, since the AGC acts on the front-end only.
(d) The AGC core as supplied is meant to act on only one receive path. Which radio slot it acts on is chosen at system compilation time, in the System Assembly view of XPS. Take a look at which radio bridge the AGC gain values are wired to, and adjust as necessary. At this time, we do not have a switch within the radio bridge to allow "on-the-fly" switching of the AGC radio target.
If you require more AGCs, simply instantiate more AGC cores and connect them to the other radio slots. You can use the same API for the different cores, but with different base addresses.
Last edited by gbmidd (2007-Mar-14 18:26:08)
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A few more notes:
(c) The AGC core relies on a certain packet structure to function. The input signal is assumed to be wideband and spectrally representative of the packat payload which follows. The packet must also start with a periodic sequence with zero mean, period of 16 and at least 4 repetitions. This sequence is used to calibrate some values after the AGC selects gains. We use the 802.11a standard short training symbols for this purpose.
(d) A MIMO AGC core is forthcoming which will support two radios in parallel. A four radio core could also be created, but isn't immediately useful without a 4-antenna PHY, which we haven't yet designed.
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gbmidd wrote:
The -19 dBm is chosen based on specifications from our radio manufacturer, and applies to the full receive bandwidth.
So what is the full receive bandwidth? I guess it is the RF filter bandwidth, what exactly is the value? Thanks.
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The receive bandwidth is nominally 20 MHz, though the radios can support 40 MHz in a special mode. For our purposes, we operate in the 20 MHz regime.
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