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I'm using WARP for Angle Of Arrival estimation (802.11 ref + 4-ant buffer). But the incoming signal is very weak and varies. So I want to use AGC to balance the signal. But I'm wondering if AGC affects the phase of each antenna? If it does, what should I modify the AGC so that it does not affect the signal phase (or keep the phase shift of each antenna equal)?
Thanks.
Vu
Last edited by vutran (2018-Sep-06 08:32:37)
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This thread may be of interest.
The 802.11 AGC core (wlan_agc.mdl) implements four functions:
-Controls the MAX2829 RF and baseband gains
-Controls the MAX2829 RXHP control signal
-Estimates and subtracts Rx DC offset
-Digital high-pass filter to remove near-DC terms
As discussed in the thread above, you will observe phase/delay changes when the radio LNA changes gains. The MAX2829 RXHP control signal en/disables an analog high-pass filter. I'm not sure how this affects phase/delay, but would not be surprised if it does.
The AGC core's Rx DCO estimation/subtraction may affect phase. This block averages over preamble symbols, computing separate averages for the I and Q paths, then subtracts these averages from the I/Q signals for the reset of the packet. Typically these DC estimates are small relative to the I/Q of a received waveform.
The digital high-pass filter has a very low corner frequency and I believe it has nearly linear phase in the passband.
The AGC disables RXHP and enables the DCO subsystem after gains are locked. The gains lock before the channel training symbols (LTF) and remain locked until the end of the packet.
I would recommend using WARPLab to experiment with the effects of these parameters. For example you can use WARPLab to directly control the Rx gains and RXHP state, then measure the impact these parameters have on your waveform processing.
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Another problem I faced is that sometimes the relative phase shift between 2 Tx antenna changes. I measure the Tx phase difference (of the carrier) in the morning, in the evening, it automatically changes to a different value.
My question is that: is there any cases the AP switches the Tx/Rx channel? Are there any other situations that the Tx/Rx is relocked?
Thank you.
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The AP code does not re-tune the radio autonomously (unlike the IBSS and STA applications which implement active scanning).
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