From 91ff55ca0eb5f0e5d340ac4ac77e4e81007d82a7 Mon Sep 17 00:00:00 2001
From: Matteo Cypriani <matteo.cypriani@univ-fcomte.fr>
Date: Tue, 19 Jun 2012 17:56:10 +0200
Subject: [PATCH] [Positioner] Measurement: variance in dBm

Along with the variance in mW, compute the variance of the dBm values.
---
 owlps-positioner/src/measurement.cc | 29 +++++++++++++++++++++++------
 owlps-positioner/src/measurement.hh | 20 ++++++++++++++++----
 2 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/owlps-positioner/src/measurement.cc b/owlps-positioner/src/measurement.cc
index 4cf1f86..9756f14 100644
--- a/owlps-positioner/src/measurement.cc
+++ b/owlps-positioner/src/measurement.cc
@@ -108,7 +108,7 @@ void Measurement::merge(const Measurement &source)
 /**
  * - #ap is not deleted, only initialised to NULL.
  * - #ss_list is cleared.
- * - #average_dbm, #average_mw and #variance_mw are reset to 0.
+ * - average and variance variables are reset to 0.
  */
 void Measurement::clear()
 {
@@ -116,7 +116,9 @@ void Measurement::clear()
   average_dbm = 0 ;
   average_mw = 0 ;
   variance_mw = 0 ;
+  variance_dbm = 0 ;
   variance_mw_m2 = 0 ;
+  variance_dbm_m2 = 0 ;
   variance_size = 0 ;
   ap = NULL ;
 }
@@ -186,7 +188,9 @@ void Measurement::recalculate_average()
   average_dbm = 0 ;
   average_mw = 0 ;
   variance_mw = 0 ;
+  variance_dbm = 0 ;
   variance_mw_m2 = 0 ;
+  variance_dbm_m2 = 0 ;
   variance_size = 0 ;
 
   for (map<pkt_id_t, ss_t>::const_iterator
@@ -204,14 +208,19 @@ void Measurement::update_average(ss_t ss_dbm)
   float ss_mw = pow(10, static_cast<float>(ss_dbm) / 10.0) ;
 
   // Update the average:
-  float delta = ss_mw - average_mw ;
-  average_mw += delta / variance_size ;
+  float delta_mw = ss_mw - average_mw ;
+  average_mw += delta_mw / variance_size ;
   average_dbm = 10.0 * log10(average_mw) ;
+  float delta_dbm = ss_dbm - average_dbm ;
 
   // Update the variance:
-  variance_mw_m2 += delta * (ss_mw - average_mw) ;
+  variance_mw_m2 += delta_mw * (ss_mw - average_mw) ;
+  variance_dbm_m2 += delta_dbm * (ss_dbm - average_dbm) ;
   if (variance_size > 1)
-    variance_mw = variance_mw_m2 / (variance_size - 1) ;
+    {
+      variance_mw = variance_mw_m2 / (variance_size - 1) ;
+      variance_dbm = variance_dbm_m2 / (variance_size - 1) ;
+    }
 }
 
 
@@ -229,7 +238,9 @@ Measurement& Measurement::operator=(const Measurement &m)
   average_dbm = m.average_dbm ;
   average_mw = m.average_mw ;
   variance_mw = m.variance_mw ;
+  variance_dbm = m.variance_mw ;
   variance_mw_m2 = m.variance_mw_m2 ;
+  variance_dbm_m2 = m.variance_mw_m2 ;
   variance_size = m.variance_size ;
 
   return *this ;
@@ -294,7 +305,13 @@ ostream &operator<<(ostream &os, const Measurement &m)
         os << static_cast<int_fast16_t>(i->second)
            << '(' << i->first << ')' ;
       }
-  os << " [AVG_dBm=" << m.average_dbm << ";AVG_mW=" << m.average_mw << ";VAR_mW=" << m.variance_mw << ";STD_mW=" << m.get_std_deviation_mw() << "]" ;
+  os << " [AVG_dBm=" << m.average_dbm
+     << ";VAR_dBm=" << m.variance_dbm
+     << ";STD_dBm=" << m.get_std_deviation_dbm()
+     << ";AVG_mW=" << m.average_mw
+     << ";VAR_mW=" << m.variance_mw
+     << ";STD_mW=" << m.get_std_deviation_mw()
+     << "]" ;
 
   return os ;
 }
diff --git a/owlps-positioner/src/measurement.hh b/owlps-positioner/src/measurement.hh
index 59bbfd7..5b31abf 100644
--- a/owlps-positioner/src/measurement.hh
+++ b/owlps-positioner/src/measurement.hh
@@ -31,10 +31,14 @@ protected:
   float average_mw ;
   /// Variance of all the captured signal strengths (mW)
   float variance_mw ;
+  /// Variance of all the captured signal strengths (dBm)
+  float variance_dbm ;
 
 private:
   /// Intermediate variable used to compute the variance (mW)
   float variance_mw_m2 ;
+  /// Intermediate variable used to compute the variance (dBm)
+  float variance_dbm_m2 ;
   /// Number of elements taken into account in the variance computation
   unsigned int variance_size ;
 
@@ -56,14 +60,14 @@ protected:
 public:
   Measurement(const AccessPoint *_ap = NULL):
     ap(const_cast<AccessPoint*>(_ap)), average_dbm(0),
-    average_mw(0), variance_mw(0), variance_mw_m2(0),
-    variance_size(0) {}
+    average_mw(0), variance_mw(0), variance_dbm(0),
+    variance_mw_m2(0), variance_dbm_m2(0), variance_size(0) {}
 
   Measurement(const Measurement &source):
     ap(source.ap), ss_list(source.ss_list),
     average_dbm(source.average_dbm), average_mw(source.average_mw),
-    variance_mw(source.variance_mw), variance_mw_m2(0),
-    variance_size(0) {}
+    variance_mw(source.variance_mw), variance_dbm(source.variance_dbm),
+    variance_mw_m2(0), variance_dbm_m2(0), variance_size(0) {}
 
   ~Measurement(void) ;
 
@@ -79,6 +83,8 @@ public:
   float get_variance_mw() const ;
   /// Returns the standard deviation of the SS list (mW)
   float get_std_deviation_mw() const ;
+  /// Returns the standard deviation of the SS list (dBm)
+  float get_std_deviation_dbm() const ;
   /// Returns the number of SS in the SS list
   int get_nb_ss() const ;
   //@}
@@ -158,6 +164,12 @@ inline float Measurement::get_std_deviation_mw() const
 }
 
 
+inline float Measurement::get_std_deviation_dbm() const
+{
+  return sqrt(variance_dbm) ;
+}
+
+
 inline int Measurement::get_nb_ss() const
 {
   return ss_list.size() ;