[PATCH 1/2] Adjust the SNR when comparing BSSes based on Tx power config
Kaidong Wang
kaidong at chromium.org
Fri Sep 22 10:51:12 PDT 2023
The max transmit power of Standard Power (SP) Access Points (AP) in the
6 GHz Band is limited by effective isotropic radiated power (EIRP),
while the max transmit power of Low Power Indoor (LPI) APs in the 6 GHz
Band is limited by power spectral density (PSD). Therefore the max
transmit power of LPI APs grows as the channel width increases.
Adjust the SNR of BSSes when comparing BSSes based on Tx power config.
Signed-off-by: Kaidong Wang <kaidong at chromium.org>
---
src/drivers/driver.h | 2 +
wpa_supplicant/bss.c | 1 +
wpa_supplicant/bss.h | 2 +
wpa_supplicant/events.c | 4 +-
wpa_supplicant/scan.c | 188 +++++++++++++++++++++++++++++++++++++---
wpa_supplicant/scan.h | 14 ++-
6 files changed, 199 insertions(+), 12 deletions(-)
diff --git a/src/drivers/driver.h b/src/drivers/driver.h
index dbe2ad5e4..93b8b5f89 100644
--- a/src/drivers/driver.h
+++ b/src/drivers/driver.h
@@ -334,6 +334,7 @@ struct hostapd_hw_modes {
* @flags: information flags about the BSS/IBSS (WPA_SCAN_*)
* @bssid: BSSID
* @freq: frequency of the channel in MHz (e.g., 2412 = channel 1)
+ * @max_cw: the max channel width supported by both the AP and the STA
* @beacon_int: beacon interval in TUs (host byte order)
* @caps: capability information field in host byte order
* @qual: signal quality
@@ -370,6 +371,7 @@ struct wpa_scan_res {
unsigned int flags;
u8 bssid[ETH_ALEN];
int freq;
+ enum chan_width max_cw;
u16 beacon_int;
u16 caps;
int qual;
diff --git a/wpa_supplicant/bss.c b/wpa_supplicant/bss.c
index 320441426..a33d88c2e 100644
--- a/wpa_supplicant/bss.c
+++ b/wpa_supplicant/bss.c
@@ -286,6 +286,7 @@ static void wpa_bss_copy_res(struct wpa_bss *dst, struct wpa_scan_res *src,
dst->flags = src->flags;
os_memcpy(dst->bssid, src->bssid, ETH_ALEN);
dst->freq = src->freq;
+ dst->max_cw = src->max_cw;
dst->beacon_int = src->beacon_int;
dst->caps = src->caps;
dst->qual = src->qual;
diff --git a/wpa_supplicant/bss.h b/wpa_supplicant/bss.h
index 611da8844..ce9319bfa 100644
--- a/wpa_supplicant/bss.h
+++ b/wpa_supplicant/bss.h
@@ -96,6 +96,8 @@ struct wpa_bss {
size_t ssid_len;
/** Frequency of the channel in MHz (e.g., 2412 = channel 1) */
int freq;
+ /** the max channel width supported by both the AP and the STA */
+ enum chan_width max_cw;
/** Beacon interval in TUs (host byte order) */
u16 beacon_int;
/** Capability information field in host byte order */
diff --git a/wpa_supplicant/events.c b/wpa_supplicant/events.c
index 32cf24c79..b2a75a503 100644
--- a/wpa_supplicant/events.c
+++ b/wpa_supplicant/events.c
@@ -1976,8 +1976,10 @@ wpas_get_est_throughput_from_bss_snr(const struct wpa_supplicant *wpa_s,
int rate = wpa_bss_get_max_rate(bss);
const u8 *ies = wpa_bss_ie_ptr(bss);
size_t ie_len = bss->ie_len ? bss->ie_len : bss->beacon_ie_len;
+ enum chan_width max_cw = CHAN_WIDTH_UNKNOWN;
- return wpas_get_est_tpt(wpa_s, ies, ie_len, rate, snr, bss->freq);
+ return wpas_get_est_tpt(wpa_s, ies, ie_len, rate, snr, bss->freq,
+ &max_cw);
}
diff --git a/wpa_supplicant/scan.c b/wpa_supplicant/scan.c
index c3984a40c..c74709a9c 100644
--- a/wpa_supplicant/scan.c
+++ b/wpa_supplicant/scan.c
@@ -2117,6 +2117,140 @@ struct wpabuf * wpa_scan_get_vendor_ie_multi(const struct wpa_scan_res *res,
return buf;
}
+int wpas_channel_width_offset(enum chan_width cw)
+{
+ switch (cw) {
+ case CHAN_WIDTH_40:
+ return 1;
+ case CHAN_WIDTH_80:
+ return 2;
+ case CHAN_WIDTH_80P80:
+ case CHAN_WIDTH_160:
+ return 3;
+ case CHAN_WIDTH_320:
+ return 4;
+ default:
+ return 0;
+ }
+}
+
+/**
+ * wpas_channel_width_tx_pwr - Calculate the max transmit power at the channel
+ * width
+ * @ies: Information elements
+ * @ies_len: Length of IEs
+ * @cw: the channel width
+ * Returns: the max transmit power at the channel width, TX_POWER_NO_CONSTRAINT
+ * if it is not constrained.
+ *
+ * This function is only used to estimate the actual signal RSSI when associated
+ * based on the beacon RSSI at the STA. Beacons are transmitted on 20 MHz
+ * channels, while the data frames usually use higher channel width. Therefore
+ * their RSSIs may be different. Assuming there is a fixed gap between the tx
+ * power limit of the STA defined by the transmit power envelope IE and the tx
+ * power of the AP, the difference in the tx power of X MHz and Y MHz at the STA
+ * equals to the difference at the AP, and the difference in the signal RSSI at
+ * the STA. tx_pwr is a floating point number in the spec, but the error of
+ * casting to int is trivial in comparing two BSSes.
+ */
+static int wpas_channel_width_tx_pwr(const u8 *ies,
+ size_t ies_len,
+ enum chan_width cw)
+{
+#define MIN(a, b) (a < b ? a : b)
+ int offset = wpas_channel_width_offset(cw);
+ const struct element *elem;
+ int max_tx_power = TX_POWER_NO_CONSTRAINT, tx_pwr = 0;
+
+ for_each_element_id(elem, WLAN_EID_TRANSMIT_POWER_ENVELOPE, ies,
+ ies_len) {
+ /*
+ * 80211ax-2021 9.4.2.161 Transmit Power Envelope element
+ * defines Maximum Transmit Power Count (B0-B2), Maximum
+ * Transmit Power Interpretation (B3-B5) and Maximum Transmit
+ * Power Category (B6-B7).
+ */
+ int max_tx_pwr_count = elem->data[0] & 0x07;
+ enum max_tx_pwr_interpretation tx_pwr_intrpn =
+ (elem->data[0] >> 3) & 0x07;
+ enum reg_6g_client_type client_type =
+ (elem->data[0] >> 6) & 0x03;
+ if (client_type != REG_DEFAULT_CLIENT)
+ continue;
+ if (tx_pwr_intrpn == LOCAL_EIRP ||
+ tx_pwr_intrpn == REGULATORY_CLIENT_EIRP) {
+ max_tx_pwr_count = MIN(max_tx_pwr_count, 3);
+ tx_pwr = (signed char)elem->data[MIN(offset,
+ max_tx_pwr_count) + 1];
+ /*
+ * Maximum Transmit Power subfield is encoded as an
+ * 8-bit 2s complement signed integer in the range –64
+ * dBm to 63 dBm with a 0.5 dB step. 63.5 dBm means no
+ * local maximum transmit power constraint.
+ */
+ if (tx_pwr == 127)
+ continue;
+ tx_pwr /= 2;
+ max_tx_power = MIN(max_tx_power, tx_pwr);
+ } else if (tx_pwr_intrpn == LOCAL_EIRP_PSD ||
+ tx_pwr_intrpn == REGULATORY_CLIENT_EIRP_PSD) {
+ tx_pwr = (signed char)elem->data[1];
+ /*
+ * Maximum Transmit PSD subfield is encoded as an 8-bit
+ * 2s complement signed integer. –128 indicates that the
+ * corresponding 20 MHz channel cannot be used for
+ * transmission. +127 indicates that no maximum PSD
+ * limit is specified for the corresponding 20 MHz
+ * channel.
+ */
+ if (tx_pwr == 127 || tx_pwr == -128)
+ continue;
+ /*
+ * The Maximum Transmit PSD subfield indicates the
+ * maximum transmit PSD for the 20 MHz channel. Suppose
+ * the PSD value is X dBm/MHz, the tx power of N MHz is
+ * X + 10*log10(N) = X + 10*log10(20) + 10*log10(N/20) =
+ * X + 13 + 3*log2(N/20)
+ */
+ tx_pwr = tx_pwr / 2 + 13 + offset * 3;
+ max_tx_power = MIN(max_tx_power, tx_pwr);
+ }
+ }
+ return max_tx_power;
+#undef MIN
+}
+
+/**
+ * Estimate the RSSI bump of channel width |cw| with respect to 20 MHz channel.
+ * If the tx power has no constraint, it is unable to estimate the RSSI bump.
+ */
+int wpas_channel_width_rssi_bump(const u8 *ies, size_t ies_len,
+ enum chan_width cw)
+{
+ int max_20mhz_tx_pwr = wpas_channel_width_tx_pwr(ies, ies_len,
+ CHAN_WIDTH_20);
+ int max_cw_tx_pwr = wpas_channel_width_tx_pwr(ies, ies_len, cw);
+
+ return (max_20mhz_tx_pwr == TX_POWER_NO_CONSTRAINT ||
+ max_cw_tx_pwr == TX_POWER_NO_CONSTRAINT)
+ ? 0 : (max_cw_tx_pwr - max_20mhz_tx_pwr);
+}
+
+int wpas_adjust_snr_by_chanwidth(const u8 *ies, size_t ies_len,
+ enum chan_width max_cw, int snr)
+{
+ int rssi_bump = wpas_channel_width_rssi_bump(ies, ies_len, max_cw);
+ /*
+ * The noise has uniform power spectral density (PSD) across the
+ * frequency band, its power is proportional to the channel width.
+ * Suppose the PSD of noise is X dBm/MHz, the noise power of N MHz is
+ * X + 10*log10(N), and the noise power bump with respect to 20 MHz is
+ * 10*log10(N) - 10*log10(20) = 10*log10(N/20) = 3*log2(N/20)
+ */
+ int noise_bump = 3 * wpas_channel_width_offset(max_cw);
+
+ return snr + rssi_bump - noise_bump;
+}
/* Compare function for sorting scan results. Return >0 if @b is considered
* better. */
@@ -2129,6 +2263,7 @@ static int wpa_scan_result_compar(const void *a, const void *b)
struct wpa_scan_res *wb = *_wb;
int wpa_a, wpa_b;
int snr_a, snr_b, snr_a_full, snr_b_full;
+ size_t ies_len;
/* WPA/WPA2 support preferred */
wpa_a = wpa_scan_get_vendor_ie(wa, WPA_IE_VENDOR_TYPE) != NULL ||
@@ -2150,10 +2285,21 @@ static int wpa_scan_result_compar(const void *a, const void *b)
return -1;
if (wa->flags & wb->flags & WPA_SCAN_LEVEL_DBM) {
- snr_a_full = wa->snr;
- snr_a = MIN(wa->snr, GREAT_SNR);
- snr_b_full = wb->snr;
- snr_b = MIN(wb->snr, GREAT_SNR);
+ /*
+ * The scan result estimates SNR over 20 MHz, while data frames
+ * usually use wider channel width. The tx power and noise power
+ * are both affected by the channel width.
+ */
+ ies_len = wa->ie_len ? wa->ie_len : wa->beacon_ie_len;
+ snr_a_full = wpas_adjust_snr_by_chanwidth((const u8 *)(wa + 1),
+ ies_len, wa->max_cw,
+ wa->snr);
+ snr_a = MIN(snr_a_full, GREAT_SNR);
+ ies_len = wb->ie_len ? wb->ie_len : wb->beacon_ie_len;
+ snr_b_full = wpas_adjust_snr_by_chanwidth((const u8 *)(wb + 1),
+ ies_len, wb->max_cw,
+ wb->snr);
+ snr_b = MIN(snr_b_full, GREAT_SNR);
} else {
/* Level is not in dBm, so we can't calculate
* SNR. Just use raw level (units unknown). */
@@ -2603,10 +2749,9 @@ static unsigned int max_he_eht_rate(const struct minsnr_bitrate_entry table[],
prev->bitrate, entry->bitrate);
}
-
unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
const u8 *ies, size_t ies_len, int rate,
- int snr, int freq)
+ int snr, int freq, enum chan_width *max_cw)
{
struct hostapd_hw_modes *hw_mode;
unsigned int est, tmp;
@@ -2659,6 +2804,7 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (hw_mode && hw_mode->ht_capab) {
ie = get_ie(ies, ies_len, WLAN_EID_HT_CAP);
if (ie) {
+ *max_cw = CHAN_WIDTH_20;
tmp = max_ht20_rate(snr, false);
if (tmp > est)
est = tmp;
@@ -2670,6 +2816,7 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
ie = get_ie(ies, ies_len, WLAN_EID_HT_OPERATION);
if (ie && ie[1] >= 2 &&
(ie[3] & HT_INFO_HT_PARAM_SECONDARY_CHNL_OFF_MASK)) {
+ *max_cw = CHAN_WIDTH_40;
tmp = max_ht40_rate(snr, false);
if (tmp > est)
est = tmp;
@@ -2681,7 +2828,8 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
ie = get_ie(ies, ies_len, WLAN_EID_VHT_CAP);
if (ie) {
bool vht80 = false, vht160 = false;
-
+ if (*max_cw == CHAN_WIDTH_UNKNOWN)
+ *max_cw = CHAN_WIDTH_20;
tmp = max_ht20_rate(snr, true) + 1;
if (tmp > est)
est = tmp;
@@ -2690,6 +2838,7 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (ie && ie[1] >= 2 &&
(ie[3] &
HT_INFO_HT_PARAM_SECONDARY_CHNL_OFF_MASK)) {
+ *max_cw = CHAN_WIDTH_40;
tmp = max_ht40_rate(snr, true) + 1;
if (tmp > est)
est = tmp;
@@ -2716,6 +2865,7 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
}
if (vht80) {
+ *max_cw = CHAN_WIDTH_80;
tmp = max_vht80_rate(snr) + 1;
if (tmp > est)
est = tmp;
@@ -2725,6 +2875,7 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
(hw_mode->vht_capab &
(VHT_CAP_SUPP_CHAN_WIDTH_160MHZ |
VHT_CAP_SUPP_CHAN_WIDTH_160_80PLUS80MHZ))) {
+ *max_cw = CHAN_WIDTH_160;
tmp = max_vht160_rate(snr) + 1;
if (tmp > est)
est = tmp;
@@ -2757,7 +2908,8 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
boost = 3;
}
}
-
+ if (*max_cw == CHAN_WIDTH_UNKNOWN)
+ *max_cw = CHAN_WIDTH_20;
tmp = max_he_eht_rate(he20_table, snr, is_eht) + boost;
if (tmp > est)
est = tmp;
@@ -2767,6 +2919,10 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (cw &
(IS_2P4GHZ(freq) ? HE_PHYCAP_CHANNEL_WIDTH_SET_40MHZ_IN_2G :
HE_PHYCAP_CHANNEL_WIDTH_SET_40MHZ_80MHZ_IN_5G)) {
+ if (*max_cw == CHAN_WIDTH_UNKNOWN ||
+ *max_cw < CHAN_WIDTH_40) {
+ *max_cw = CHAN_WIDTH_40;
+ }
tmp = max_he_eht_rate(he40_table, snr, is_eht) + boost;
if (tmp > est)
est = tmp;
@@ -2774,6 +2930,10 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (!IS_2P4GHZ(freq) &&
(cw & HE_PHYCAP_CHANNEL_WIDTH_SET_40MHZ_80MHZ_IN_5G)) {
+ if (*max_cw == CHAN_WIDTH_UNKNOWN ||
+ *max_cw < CHAN_WIDTH_80) {
+ *max_cw = CHAN_WIDTH_80;
+ }
tmp = max_he_eht_rate(he80_table, snr, is_eht) + boost;
if (tmp > est)
est = tmp;
@@ -2782,6 +2942,10 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (!IS_2P4GHZ(freq) &&
(cw & (HE_PHYCAP_CHANNEL_WIDTH_SET_160MHZ_IN_5G |
HE_PHYCAP_CHANNEL_WIDTH_SET_80PLUS80MHZ_IN_5G))) {
+ if (*max_cw == CHAN_WIDTH_UNKNOWN ||
+ *max_cw < CHAN_WIDTH_160) {
+ *max_cw = CHAN_WIDTH_160;
+ }
tmp = max_he_eht_rate(he160_table, snr, is_eht) + boost;
if (tmp > est)
est = tmp;
@@ -2795,6 +2959,10 @@ unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
if (is_6ghz_freq(freq) &&
(eht->phy_cap[EHT_PHYCAP_320MHZ_IN_6GHZ_SUPPORT_IDX] &
EHT_PHYCAP_320MHZ_IN_6GHZ_SUPPORT_MASK)) {
+ if (*max_cw == CHAN_WIDTH_UNKNOWN ||
+ *max_cw < CHAN_WIDTH_320) {
+ *max_cw = CHAN_WIDTH_320;
+ }
tmp = max_he_eht_rate(eht320_table, snr, true);
if (tmp > est)
est = tmp;
@@ -2821,8 +2989,8 @@ void scan_est_throughput(struct wpa_supplicant *wpa_s,
if (!ie_len)
ie_len = res->beacon_ie_len;
- res->est_throughput =
- wpas_get_est_tpt(wpa_s, ies, ie_len, rate, snr, res->freq);
+ res->est_throughput = wpas_get_est_tpt(wpa_s, ies, ie_len, rate, snr,
+ res->freq, &res->max_cw);
/* TODO: channel utilization and AP load (e.g., from AP Beacon) */
}
diff --git a/wpa_supplicant/scan.h b/wpa_supplicant/scan.h
index 30f43951c..26eebd0ef 100644
--- a/wpa_supplicant/scan.h
+++ b/wpa_supplicant/scan.h
@@ -30,6 +30,13 @@
*/
#define GREAT_SNR 25
+/*
+ * IEEE 80211ax-2021 9.4.2.161 Transmit Power Envelope element indicates no max
+ * tx power limit if Maximum Transmit Power field is 63.5 dBm.
+ * The default tx power if it is not constrained by Transmit Power Envelope IE
+ */
+#define TX_POWER_NO_CONSTRAINT 64
+
#define IS_2P4GHZ(n) (n >= 2412 && n <= 2484)
#define IS_5GHZ(n) (n > 4000 && n < 5895)
@@ -87,12 +94,17 @@ void scan_est_throughput(struct wpa_supplicant *wpa_s,
struct wpa_scan_res *res);
unsigned int wpas_get_est_tpt(const struct wpa_supplicant *wpa_s,
const u8 *ies, size_t ies_len, int rate,
- int snr, int freq);
+ int snr, int freq, enum chan_width *max_cw);
void wpa_supplicant_set_default_scan_ies(struct wpa_supplicant *wpa_s);
int wpa_add_scan_freqs_list(struct wpa_supplicant *wpa_s,
enum hostapd_hw_mode band,
struct wpa_driver_scan_params *params,
bool is_6ghz, bool only_6ghz_psc,
bool exclude_radar);
+int wpas_channel_width_offset(enum chan_width cw);
+int wpas_channel_width_rssi_bump(const u8 *ies, size_t ies_len,
+ enum chan_width cw);
+int wpas_adjust_snr_by_chanwidth(const u8 *ies, size_t ies_len,
+ enum chan_width max_cw, int snr);
#endif /* SCAN_H */
--
2.42.0.515.g380fc7ccd1-goog
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