wiki:WARPLab/AGC

Version 4 (modified by chunter, 8 years ago) (diff)

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WARPLab: Automatic Gain Control

WARPLab includes the option to automatically adjust receiver gains based on the power and structure of an incoming waveform. Additionally, the AGC optionally provides built-in mechanisms for subtracting DC offset (DCO) terms in the received waveform before the waveform is presented to the host MATLAB environment. Both AGC and DCO make certain assumptions about the received waveform. This document clarifies these assumptions so that users can make an informed decision on whether the use of the AGC and/or DCO correction subsystems is appropriate given their specific use case.

Gain Selection based on RSSI and Received I/Q

The MAX2829 transceiver in WARP's radios has two stages of Rx gain: a low noise amplifier (LNA) that can provide up to 30 dB of amplification followed by a baseband gain that can provide up to 63 dB of additional amplification. Together, these gain stages can provide up to 153 dB of gain to a received signal: a multiplicative factor of around 1015 the power of the minimum gain selection. The AGC core provided with WARPLAB selects these two gains in three sequential stages:

  1. When triggered, the AGC first selects an LNA gain based upon the RSSI signal provided by the transceiver. It does this by first converting the digital RSSI measurement into an receive power estimate in dBm. With this value, the AGC chooses one of the three possible LNA gain settings that minimizes EVM. The MAX2829 datasheet provides graphs that show EVM for each LNA gain setting as a function of receive power.
  2. The act of changing the LNA gain affects the RSSI measurement. After RSSI has a chance to settle from the previous LNA gain adjustment, the value is re-read and is used to make an initial, coarse, update to the baseband gain stage.
  3. After the both the LNA gain and the baseband gain have been adjusted based on RSSI, a final refinement to the baseband gain is made by the AGC based upon waveform's I and Q values themselves. Prior to this stage, the waveform cannot be trusted for an accurate power measurement since it is likely saturating the radio's ADCs. Using the waveform's I and Q values to refine the baseband gain introduces an important waveform dependence -- during the window that the AGC inspects the I/Q waveform, the magnitude of the signal must be periodic in 16 40-MHz samples. A series of 20 MHz-wide 802.11 STS symbols meets these requirements.

DC Offset Correction based on Received I/Q

After the final gain adjustments are made, the AGC core can optionally subtract any detected DC offset. This also produces an [#waveformDependence waveform dependence -- the transmitted I and Q waveforms must be zero mean over a period of 32 samples. A series of 20 MHz-wide 802.11 STS symbols meets these requirements.

Dependence on 802.11's STS

AGC Characterization

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