Return-path: Received: from 26.mail-out.ovh.net ([91.121.27.225]:46997 "HELO 26.mail-out.ovh.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1755029Ab0D0UjJ (ORCPT ); Tue, 27 Apr 2010 16:39:09 -0400 Message-ID: <4BD74B66.3030500@free.fr> Date: Tue, 27 Apr 2010 22:39:02 +0200 From: Benoit PAPILLAULT MIME-Version: 1.0 To: ath9k-devel CC: "linux-wireless@vger.kernel.org" Subject: ath9k: noise floor calibration process Content-Type: text/plain; charset=ISO-8859-1; format=flowed Sender: linux-wireless-owner@vger.kernel.org List-ID: Hello, In order to move forward with noise & signal reporting, I'd like to share my current understanding of the way ath9k HW is working before sending patches (unfortunately, I did the work before the introduction of ar9003... so I need to redo the work). The ultimate purpose of this work is to be able to measure signal levels (and noise if possible) as accurately as a spectrum analyzer or power meter. First, signal level reporting. It is reported in a per packet basis in RX descriptors. There are 7 fields: AR_RxRSSIAnt00 0x000000ff rs_rssi_ctl0 AR_RxRSSIAnt01 0x0000ff00 rs_rssi_ctl1 AR_RxRSSIAnt02 0x00ff0000 rs_rssi_ctl2 AR_RxRSSIAnt10 0x000000ff rs_rssi_ext0 AR_RxRSSIAnt11 0x0000ff00 rs_rssi_ext1 AR_RxRSSIAnt12 0x00ff0000 rs_rssi_ext2 AR_RxRSSICombined 0xff000000 rs_rssi Each value is for a 20 MHz wide channel, on the 3 RX chains. "ctl" is for the primary channel and "ext" is for the secondary channel (using the 802.11n words). The latter rs_rssi is the sum of the 6 previous value. However, since each value is dB, the sum is not an arithmetic sum. Each field is a signed value and the value -128 means that no measurement has been done (no RX chain, RX chain disabled, no secondary channel, ...). It seems that in some cases, the combined value is just plain wrong. Here are few examples: RSSI: ctl=(10,7,-128) ext=(-128,-128,-128) => 12 (11.76) correct RSSI: ctl=(38,29,-128) ext=(69,-84,-101) => -22 incorrect!!! Next, noise floor calibration. From what I understand, signal levels is measured using the AGC + RX amplifiers gain (RF, IF and BB). However, the various gains are not really accurate, only the relative gain are accurate. This means that reading a signal value of -100dBm might not exactly means -100dBm. There is a delta between real signal and measured value. In order to know this value, we need a calibration process with a known signal. One know signal is thermal noise. Thermal noise is generated in any resistor and can be computed using the well know value N = kTB. For a 20 MHz bandwidth, this gives -101dBm. If the HW tries to measure signal strength when the network is supposed to be idle (during SIFS) and with RX/TX switch disabled (?), then it will in fact measure the thermal noise at the RX input. So, we have : Real noise (-101dBm) = Measured noise + delta There are type of registers to control noise floor calibration : - control register at 0x9860 (AR_PHY_AGC_CONTROL) This register allows 3 differents operations : 1. start noise floor measurement write AR_PHY_MAXCCA_PWR (AR_PHY_CCA & 0x000001ff) : this is apparently a max value for noise floor REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_ENABLE_NF); REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NO_UPDATE_NF); REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NF); When channel has been changed however, the noise floor needs to be updated immediately, so AR_PHY_AGC_CONTROL_NO_UPDATE_NF should be cleared in this particular case. Otherwise, the chip is no longer receiving (problem since CCA is defined with noise floor as reference). 2. read noise floor measurement result check REG_READ(ah, AR_PHY_AGC_CONTROL) & AR_PHY_AGC_CONTROL_NF if 0 (noise floor calibration is finished), read AR_PHY_MINCCA_PWR : nf = MS(REG_READ(ah, AR_PHY_CCA), AR_PHY_MINCCA_PWR = 0x0ff80000) 3. write noise floor reference write AR_PHY_MAXCCA_PWR (the value has not the same meaning as operation 1!) REG_CLR_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_ENABLE_NF); REG_CLR_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NO_UPDATE_NF); REG_SET_BIT(ah, AR_PHY_AGC_CONTROL, AR_PHY_AGC_CONTROL_NF); - data register at 0x9864 (AR_PHY_CCA, + more location for other RX chains) The fields are different for AR9280+ chipsets, but the mechanism is the same. AR_PHY_MAXCCA_PWR 0x000001ff (half dBm unit!) AR_PHY_CCA_THRESH62 0x0007f000 AR_PHY_MINCCA_PWR 0x0ff80000 Now, we have : Real signal = Measured signal + delta = RSSI + Noise floor + delta = RSSI + (-101 dBm) Real noise is not thermal noise. There are a lot of definition for noise since noise is NOT signal. Of course, noise includes thermal noise. Since the noise measured by the chip is variable, I think we could do : - Noise floor = minimum (Noise floor measures) - Noise = moving average (Noise floor measures) + delta with delta = (-101 dBm) - Noise floor I'd like to get comments before sending patches. Since ath5k and ath9k are quite close, I'm pretty sure a similar (if not same) process is used on ath5k. Regards, Benoit