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ie: Fix up HT/VHT data rate calculation

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The current calculation was giving erroneous results when it came to VHT
MCS index 4 and VHT MCS index 8 & 9.

Switch to a precomputed look up table and add a multiplication factor
for short GI.
This commit is contained in:
Denis Kenzior 2021-05-21 22:31:23 -05:00
parent 0dca744d4b
commit c3b1425edd
1 changed files with 49 additions and 28 deletions
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77
src/ie.c
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@ -1744,20 +1744,50 @@ static const int32_t ht_vht_base_rssi[] = {
-82, -79, -77, -74, -70, -66, -65, -64, -59, -57
};
struct ht_vht_rate {
uint64_t rate;
uint64_t sgi_rate;
};
/*
* Data Rate for HT/VHT is obtained according to this formula:
* Nsd * Nbpscs * R * Nss / (Tdft + Tgi)
*
* Where Nsd is [52, 108, 234, 468] for 20/40/80/160 Mhz respectively
* Nbpscs is [1, 2, 4, 6, 8] for BPSK/QPSK/16QAM/64QAM/256QAM
* R is [1/2, 2/3, 3/4, 5/6] depending on the MCS index
* Nss is the number of spatial streams
* Tdft = 3.2 us
* Tgi = Long/Short GI of 0.8/0.4 us
*
* Short GI rate can be easily obtained by multiplying by (10 / 9)
*
* The table was pre-computed using the following python snippet:
* rfactors = [ 1/2, 1/2, 3/4, 1/2, 3/4, 2/3, 3/4, 5/6, 3/4, 5/6 ]
* nbpscs = [1, 2, 2, 4, 4, 6, 6, 6, 8, 8 ]
* nsds = [52, 108, 234, 468]
*
* for nsd in nsds:
* rates = []
* for i in xrange(0, 10):
* data_rate = (nsd * rfactors[i] * nbpscs[i]) / 0.004
* rates.append(int(data_rate) * 1000)
* print('rates for nsd: ' + nsd + ': ' + rates)
*/
static const struct ht_vht_rate ht_vht_rates[] = {
[HT_VHT_CHANNEL_WIDTH_20MHZ] = { .rate = 6500000,
.sgi_rate = 7200000 },
[HT_VHT_CHANNEL_WIDTH_40MHZ] = { .rate = 13500000,
.sgi_rate = 15000000 },
[HT_VHT_CHANNEL_WIDTH_80MHZ] = { .rate = 29300000,
.sgi_rate = 32500000 },
[HT_VHT_CHANNEL_WIDTH_160MHZ] = { .rate = 58500000,
.sgi_rate = 65000000 },
static const uint64_t ht_vht_rates[4][10] = {
[HT_VHT_CHANNEL_WIDTH_20MHZ] = {
6500000ULL, 13000000ULL, 19500000ULL, 26000000ULL,
39000000ULL, 52000000ULL, 58500000ULL, 65000000ULL,
78000000ULL, 86666000ULL },
[HT_VHT_CHANNEL_WIDTH_40MHZ] = {
13500000ULL, 27000000ULL, 40500000ULL, 54000000ULL,
81000000ULL, 108000000ULL, 121500000ULL, 135000000ULL,
162000000ULL, 180000000ULL, },
[HT_VHT_CHANNEL_WIDTH_80MHZ] = {
29250000ULL, 58500000ULL, 87750000ULL, 117000000ULL,
175500000ULL, 234000000ULL, 263250000ULL, 292500000ULL,
351000000ULL, 390000000ULL, },
[HT_VHT_CHANNEL_WIDTH_160MHZ] = {
58500000ULL, 117000000ULL, 175500000ULL, 234000000ULL,
351000000ULL, 468000000ULL, 526500000ULL, 585000000ULL,
702000000ULL, 780000000ULL,
}
};
/*
@ -1771,29 +1801,20 @@ static bool calculate_ht_vht_data_rate(uint8_t index,
int32_t rssi, uint8_t nss, bool sgi,
uint64_t *data_rate)
{
const struct ht_vht_rate *rate = &ht_vht_rates[width];
uint64_t rate;
int32_t width_adjust = width * 3;
if (rssi < ht_vht_base_rssi[index] + width_adjust)
return false;
rate = ht_vht_rates[width][index];
if (sgi)
*data_rate = rate->sgi_rate;
else
*data_rate = rate->rate;
rate = rate / 9 * 10;
/* adjust base for spatial streams */
*data_rate *= nss;
/*
* As with HT, the VHT rates multiplier jumps up
* by 2 after MCS index 4
*/
if (index < 4)
*data_rate *= index + 1;
else
*data_rate *= index + 3;
rate *= nss;
*data_rate = rate;
return true;
}