thompson-sampling-wifi-manager.cc

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/*
 * Copyright (c) 2021 IITP RAS
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 * Author: Alexander Krotov <krotov@iitp.ru>
 */
 
#include "thompson-sampling-wifi-manager.h"
 
#include "ns3/core-module.h"
#include "ns3/double.h"
#include "ns3/log.h"
#include "ns3/packet.h"
#include "ns3/wifi-phy.h"
 
#include <cstdint>
#include <cstdlib>
#include <fstream>
#include <iostream>
#include <string>
 
namespace ns3
{
 
/**
 * A structure containing parameters of a single rate and its
 * statistics.
 */
struct RateStats
{
    WifiMode mode;      ///< MCS
    MHz_u channelWidth; ///< channel width
    uint8_t nss;        ///< Number of spatial streams
 
    double success{0.0}; ///< averaged number of successful transmissions
    double fails{0.0};   ///< averaged number of failed transmissions
    Time lastDecay{0};   ///< last time exponential decay was applied to this rate
};
 
/**
 * Holds station state and collected statistics.
 *
 * This struct extends from WifiRemoteStation to hold additional
 * information required by ThompsonSamplingWifiManager.
 */
struct ThompsonSamplingWifiRemoteStation : public WifiRemoteStation
{
    size_t m_nextMode; //!< Mode to select for the next transmission
    size_t m_lastMode; //!< Most recently used mode, used to write statistics
 
    std::vector<RateStats> m_mcsStats; //!< Collected statistics
};
 
NS_OBJECT_ENSURE_REGISTERED(ThompsonSamplingWifiManager);
 
NS_LOG_COMPONENT_DEFINE("ThompsonSamplingWifiManager");
 
TypeId
ThompsonSamplingWifiManager::GetTypeId()
{
    static TypeId tid =
        TypeId("ns3::ThompsonSamplingWifiManager")
            .SetParent<WifiRemoteStationManager>()
            .SetGroupName("Wifi")
            .AddConstructor<ThompsonSamplingWifiManager>()
            .AddAttribute(
                "Decay",
                "Exponential decay coefficient, Hz; zero is a valid value for static scenarios",
                DoubleValue(1.0),
                MakeDoubleAccessor(&ThompsonSamplingWifiManager::m_decay),
                MakeDoubleChecker<double>(0.0))
            .AddTraceSource("Rate",
                            "Traced value for rate changes (b/s)",
                            MakeTraceSourceAccessor(&ThompsonSamplingWifiManager::m_currentRate),
                            "ns3::TracedValueCallback::Uint64");
    return tid;
}
 
ThompsonSamplingWifiManager::ThompsonSamplingWifiManager()
    : m_currentRate{0}
{
    NS_LOG_FUNCTION(this);
 
    m_gammaRandomVariable = CreateObject<GammaRandomVariable>();
}
 
ThompsonSamplingWifiManager::~ThompsonSamplingWifiManager()
{
    NS_LOG_FUNCTION(this);
}
 
WifiRemoteStation*
ThompsonSamplingWifiManager::DoCreateStation() const
{
    NS_LOG_FUNCTION(this);
    auto station = new ThompsonSamplingWifiRemoteStation();
    station->m_nextMode = 0;
    station->m_lastMode = 0;
    return station;
}
 
void
ThompsonSamplingWifiManager::InitializeStation(WifiRemoteStation* st) const
{
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
    if (!station->m_mcsStats.empty())
    {
        return;
    }
 
    // Add HT, VHT or HE MCSes
    for (const auto& mode : GetPhy()->GetMcsList())
    {
        for (MHz_u j{20}; j <= GetPhy()->GetChannelWidth(); j *= 2)
        {
            WifiModulationClass modulationClass = WIFI_MOD_CLASS_HT;
            if (GetVhtSupported())
            {
                modulationClass = WIFI_MOD_CLASS_VHT;
            }
            if (GetHeSupported())
            {
                modulationClass = WIFI_MOD_CLASS_HE;
            }
            if (mode.GetModulationClass() == modulationClass)
            {
                for (uint8_t k = 1; k <= GetPhy()->GetMaxSupportedTxSpatialStreams(); k++)
                {
                    if (mode.IsAllowed(j, k))
                    {
                        RateStats stats;
                        stats.mode = mode;
                        stats.channelWidth = j;
                        stats.nss = k;
 
                        station->m_mcsStats.push_back(stats);
                    }
                }
            }
        }
    }
 
    if (station->m_mcsStats.empty())
    {
        // Add legacy non-HT modes.
        for (uint8_t i = 0; i < GetNSupported(station); i++)
        {
            RateStats stats;
            stats.mode = GetSupported(station, i);
            if (stats.mode.GetModulationClass() == WIFI_MOD_CLASS_DSSS ||
                stats.mode.GetModulationClass() == WIFI_MOD_CLASS_HR_DSSS)
            {
                stats.channelWidth = MHz_u{22};
            }
            else
            {
                stats.channelWidth = MHz_u{20};
            }
            stats.nss = 1;
            station->m_mcsStats.push_back(stats);
        }
    }
 
    NS_ASSERT_MSG(!station->m_mcsStats.empty(), "No usable MCS found");
 
    UpdateNextMode(st);
}
 
void
ThompsonSamplingWifiManager::DoReportRxOk(WifiRemoteStation* station, double rxSnr, WifiMode txMode)
{
    NS_LOG_FUNCTION(this << station << rxSnr << txMode);
}
 
void
ThompsonSamplingWifiManager::DoReportRtsFailed(WifiRemoteStation* station)
{
    NS_LOG_FUNCTION(this << station);
}
 
void
ThompsonSamplingWifiManager::DoReportDataFailed(WifiRemoteStation* st)
{
    NS_LOG_FUNCTION(this << st);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
    Decay(st, station->m_lastMode);
    station->m_mcsStats.at(station->m_lastMode).fails++;
    UpdateNextMode(st);
}
 
void
ThompsonSamplingWifiManager::DoReportRtsOk(WifiRemoteStation* st,
                                           double ctsSnr,
                                           WifiMode ctsMode,
                                           double rtsSnr)
{
    NS_LOG_FUNCTION(this << st << ctsSnr << ctsMode.GetUniqueName() << rtsSnr);
}
 
void
ThompsonSamplingWifiManager::UpdateNextMode(WifiRemoteStation* st) const
{
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
 
    double maxThroughput = 0.0;
    double frameSuccessRate = 1.0;
 
    NS_ASSERT(!station->m_mcsStats.empty());
 
    // Use the most robust MCS if frameSuccessRate is 0 for all MCS.
    station->m_nextMode = 0;
 
    for (uint32_t i = 0; i < station->m_mcsStats.size(); i++)
    {
        Decay(st, i);
        const auto mode{station->m_mcsStats.at(i).mode};
 
        const auto guardInterval = GetModeGuardInterval(st, mode);
        const auto rate = mode.GetDataRate(station->m_mcsStats.at(i).channelWidth,
                                           guardInterval,
                                           station->m_mcsStats.at(i).nss);
 
        // Thompson sampling
        frameSuccessRate = SampleBetaVariable(1.0 + station->m_mcsStats.at(i).success,
                                              1.0 + station->m_mcsStats.at(i).fails);
        NS_LOG_DEBUG("Draw success=" << station->m_mcsStats.at(i).success
                                     << " fails=" << station->m_mcsStats.at(i).fails
                                     << " frameSuccessRate=" << frameSuccessRate
                                     << " mode=" << mode);
        if (frameSuccessRate * rate > maxThroughput)
        {
            maxThroughput = frameSuccessRate * rate;
            station->m_nextMode = i;
        }
    }
}
 
void
ThompsonSamplingWifiManager::DoReportDataOk(WifiRemoteStation* st,
                                            double ackSnr,
                                            WifiMode ackMode,
                                            double dataSnr,
                                            MHz_u dataChannelWidth,
                                            uint8_t dataNss)
{
    NS_LOG_FUNCTION(this << st << ackSnr << ackMode.GetUniqueName() << dataSnr);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
    Decay(st, station->m_lastMode);
    station->m_mcsStats.at(station->m_lastMode).success++;
    UpdateNextMode(st);
}
 
void
ThompsonSamplingWifiManager::DoReportAmpduTxStatus(WifiRemoteStation* st,
                                                   uint16_t nSuccessfulMpdus,
                                                   uint16_t nFailedMpdus,
                                                   double rxSnr,
                                                   double dataSnr,
                                                   MHz_u dataChannelWidth,
                                                   uint8_t dataNss)
{
    NS_LOG_FUNCTION(this << st << nSuccessfulMpdus << nFailedMpdus << rxSnr << dataSnr);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
 
    Decay(st, station->m_lastMode);
    station->m_mcsStats.at(station->m_lastMode).success += nSuccessfulMpdus;
    station->m_mcsStats.at(station->m_lastMode).fails += nFailedMpdus;
 
    UpdateNextMode(st);
}
 
void
ThompsonSamplingWifiManager::DoReportFinalRtsFailed(WifiRemoteStation* station)
{
    NS_LOG_FUNCTION(this << station);
}
 
void
ThompsonSamplingWifiManager::DoReportFinalDataFailed(WifiRemoteStation* station)
{
    NS_LOG_FUNCTION(this << station);
}
 
Time
ThompsonSamplingWifiManager::GetModeGuardInterval(WifiRemoteStation* st, WifiMode mode) const
{
    if (mode.GetModulationClass() == WIFI_MOD_CLASS_HE)
    {
        return std::max(GetGuardInterval(st), GetGuardInterval());
    }
    else if ((mode.GetModulationClass() == WIFI_MOD_CLASS_HT) ||
             (mode.GetModulationClass() == WIFI_MOD_CLASS_VHT))
    {
        auto useSgi = GetShortGuardIntervalSupported(st) && GetShortGuardIntervalSupported();
        return NanoSeconds(useSgi ? 400 : 800);
    }
    else
    {
        return NanoSeconds(800);
    }
}
 
WifiTxVector
ThompsonSamplingWifiManager::DoGetDataTxVector(WifiRemoteStation* st, MHz_u allowedWidth)
{
    NS_LOG_FUNCTION(this << st << allowedWidth);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
 
    auto& stats = station->m_mcsStats.at(station->m_nextMode);
    const auto mode = stats.mode;
    const auto channelWidth = std::min(stats.channelWidth, allowedWidth);
    const auto nss = stats.nss;
    const auto guardInterval = GetModeGuardInterval(st, mode);
 
    station->m_lastMode = station->m_nextMode;
 
    NS_LOG_DEBUG("Using mode=" << mode << " channelWidth=" << channelWidth << " nss=" << +nss
                               << " guardInterval=" << guardInterval);
 
    const auto rate = mode.GetDataRate(channelWidth, guardInterval, nss);
    if (m_currentRate != rate)
    {
        NS_LOG_DEBUG("New datarate: " << rate);
        m_currentRate = rate;
    }
 
    return WifiTxVector(
        mode,
        GetDefaultTxPowerLevel(),
        GetPreambleForTransmission(mode.GetModulationClass(), GetShortPreambleEnabled()),
        GetModeGuardInterval(st, mode),
        GetNumberOfAntennas(),
        nss,
        0, // NESS
        GetPhy()->GetTxBandwidth(mode, channelWidth),
        GetAggregation(station),
        false);
}
 
WifiTxVector
ThompsonSamplingWifiManager::DoGetRtsTxVector(WifiRemoteStation* st)
{
    NS_LOG_FUNCTION(this << st);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
 
    // Use the most robust MCS for the control channel.
    auto& stats = station->m_mcsStats.at(0);
    WifiMode mode = stats.mode;
    uint8_t nss = stats.nss;
 
    // Make sure control frames are sent using 1 spatial stream.
    NS_ASSERT(nss == 1);
 
    return WifiTxVector(
        mode,
        GetDefaultTxPowerLevel(),
        GetPreambleForTransmission(mode.GetModulationClass(), GetShortPreambleEnabled()),
        GetModeGuardInterval(st, mode),
        GetNumberOfAntennas(),
        nss,
        0, // NESS
        GetPhy()->GetTxBandwidth(mode, stats.channelWidth),
        GetAggregation(station),
        false);
}
 
double
ThompsonSamplingWifiManager::SampleBetaVariable(uint64_t alpha, uint64_t beta) const
{
    double X = m_gammaRandomVariable->GetValue(alpha, 1.0);
    double Y = m_gammaRandomVariable->GetValue(beta, 1.0);
    return X / (X + Y);
}
 
void
ThompsonSamplingWifiManager::Decay(WifiRemoteStation* st, size_t i) const
{
    NS_LOG_FUNCTION(this << st << i);
    InitializeStation(st);
    auto station = static_cast<ThompsonSamplingWifiRemoteStation*>(st);
 
    Time now = Simulator::Now();
    auto& stats = station->m_mcsStats.at(i);
    if (now > stats.lastDecay)
    {
        const double coefficient = std::exp(m_decay * (stats.lastDecay - now).GetSeconds());
 
        stats.success *= coefficient;
        stats.fails *= coefficient;
        stats.lastDecay = now;
    }
}
 
int64_t
ThompsonSamplingWifiManager::AssignStreams(int64_t stream)
{
    NS_LOG_FUNCTION(this << stream);
    m_gammaRandomVariable->SetStream(stream);
    return 1;
}
 
} // namespace ns3