Mitigating reception problems with lower data rates

A new algorithm has been taken in use to calculate the receive window
length and the timing offset involved in opening of the said receive
window. This algorithm performs better than the stock algorthm and
consumes less power.

Author: Hasnain Virk

features/lorawan/lorastack/phy/LoRaPHY.cpp

@@ -32,8 +32,9 @@ SPDX-License-Identifier: BSD-3-Clause
 #define BACKOFF_DC_1_HOUR       100
 #define BACKOFF_DC_10_HOURS     1000
 #define BACKOFF_DC_24_HOURS     10000
-
-#define CHANNELS_IN_MASK  16
+#define MAX_PREAMBLE_LENGTH     8.0
+#define TICK_GRANULARITY_JITTER 1.0
+#define CHANNELS_IN_MASK        16
 
 LoRaPHY::LoRaPHY()
     : _radio(NULL),
@@ -388,23 +389,49 @@ uint8_t LoRaPHY::verify_link_ADR_req(verify_adr_params_t *verify_params,
     return status;
 }
 
-double LoRaPHY::compute_symb_timeout_lora(uint8_t phy_dr, uint32_t bandwidth)
+float LoRaPHY::compute_symb_timeout_lora(uint8_t phy_dr, uint32_t bandwidth)
 {
-    return ((double)(1 << phy_dr) / (double) bandwidth) * 1000;
+    // in milliseconds
+    return ((float)(1 << phy_dr) / (float) bandwidth * 1000);
 }
 
-double LoRaPHY::compute_symb_timeout_fsk(uint8_t phy_dr)
+float LoRaPHY::compute_symb_timeout_fsk(uint8_t phy_dr)
 {
-    return (8.0 / (double) phy_dr); // 1 symbol equals 1 byte
+    return (8.0 / (float) phy_dr); // 1 symbol equals 1 byte
 }
 
-void LoRaPHY::get_rx_window_params(double t_symb, uint8_t min_rx_symb,
-                                   uint32_t rx_error, uint32_t wakeup_time,
-                                   uint32_t *window_timeout, int32_t *window_offset)
+
+void LoRaPHY::get_rx_window_params(float t_symb, uint8_t min_rx_symb,
+                                   float error_fudge, float wakeup_time,
+                                   uint32_t *window_length, int32_t *window_offset,
+                                   uint8_t phy_dr)
 {
-    // Computed number of symbols
-    *window_timeout = MAX((uint32_t) ceil(((2 * min_rx_symb - 8) * t_symb + 2 * rx_error) / t_symb), min_rx_symb);
-    *window_offset = (int32_t) ceil((4.0 * t_symb) - ((*window_timeout * t_symb) / 2.0) - wakeup_time);
+    float min_rx_symbol_overlap_required = float (min_rx_symb);
+    float target_rx_window_offset;
+    float window_len_in_ms;
+
+    if (phy_params.fsk_supported && phy_dr == phy_params.max_rx_datarate) {
+        min_rx_symbol_overlap_required = MAX_PREAMBLE_LENGTH;
+    }
+
+    // We wish to be as close as possible to the actual start of data, i.e.,
+    // we are interested in the preamble symbols which are at the tail of the
+    // preamble sequence.
+    target_rx_window_offset = (MAX_PREAMBLE_LENGTH - min_rx_symbol_overlap_required) * t_symb; //in ms
+
+    // Actual window offset in ms in response to timing error fudge factor and
+    // radio wakeup/turned around time.
+    *window_offset = floor(target_rx_window_offset - error_fudge - wakeup_time);
+
+    // Minimum reception time plus extra time (in ms) we may have turned on before the
+    // other side started transmission
+    window_len_in_ms = (min_rx_symbol_overlap_required * t_symb) - MIN(0, (*window_offset - error_fudge - TICK_GRANULARITY_JITTER));
+
+    // Setting the window_length in terms of 'symbols' for LoRa modulation or
+    // in terms of 'bytes' for FSK
+    *window_length = (uint32_t) ceil(window_len_in_ms / t_symb);
+
+
 }
 
 int8_t LoRaPHY::compute_tx_power(int8_t tx_power_idx, float max_eirp,
@@ -791,7 +818,7 @@ void LoRaPHY::compute_rx_win_params(int8_t datarate, uint8_t min_rx_symbols,
                                     uint32_t rx_error,
                                     rx_config_params_t *rx_conf_params)
 {
-    double t_symbol = 0.0;
+    float t_symbol = 0.0;
 
     // Get the datarate, perform a boundary check
     rx_conf_params->datarate = MIN(datarate, phy_params.max_rx_datarate);
@@ -811,9 +838,9 @@ void LoRaPHY::compute_rx_win_params(int8_t datarate, uint8_t min_rx_symbols,
         rx_conf_params->frequency = phy_params.channels.channel_list[rx_conf_params->channel].frequency;
     }
 
-
-    get_rx_window_params(t_symbol, min_rx_symbols, rx_error, RADIO_WAKEUP_TIME,
-                         &rx_conf_params->window_timeout, &rx_conf_params->window_offset);
+    get_rx_window_params(t_symbol, min_rx_symbols, (float) rx_error, RADIO_WAKEUP_TIME,
+                         &rx_conf_params->window_timeout, &rx_conf_params->window_offset,
+                         rx_conf_params->datarate);
 }
 
 bool LoRaPHY::rx_config(rx_config_params_t *rx_conf)

features/lorawan/lorastack/phy/LoRaPHY.h

@@ -597,9 +597,10 @@ class LoRaPHY : private mbed::NonCopyable<LoRaPHY> {
     /**
      * Computes the RX window timeout and the RX window offset.
      */
-    void get_rx_window_params(double t_symbol, uint8_t min_rx_symbols,
-                              uint32_t rx_error, uint32_t wakeup_time,
-                              uint32_t *window_timeout, int32_t *window_offset);
+    void get_rx_window_params(float t_symbol, uint8_t min_rx_symbols,
+                              float rx_error, float wakeup_time,
+                              uint32_t *window_length, int32_t *window_offset,
+                              uint8_t phy_dr);
 
     /**
      * Computes the txPower, based on the max EIRP and the antenna gain.
@@ -632,12 +633,12 @@ class LoRaPHY : private mbed::NonCopyable<LoRaPHY> {
     /**
      * Computes the symbol time for LoRa modulation.
      */
-    double compute_symb_timeout_lora(uint8_t phy_dr, uint32_t bandwidth);
+    float compute_symb_timeout_lora(uint8_t phy_dr, uint32_t bandwidth);
 
     /**
      * Computes the symbol time for FSK modulation.
      */
-    double compute_symb_timeout_fsk(uint8_t phy_dr);
+    float compute_symb_timeout_fsk(uint8_t phy_dr);
 
 protected:
     LoRaRadio *_radio;

features/lorawan/mbed_lib.json

@@ -70,16 +70,16 @@
             "value": true
         },
         "max-sys-rx-error": {
-        	"help": "Maximum timing error of the receiver in ms. The receiver will turn on in [-RxError : + RxError]",
-        	"value": 10
+            "help": "Maximum timing error of the receiver in ms. The receiver will turn on in [-RxError : + RxError]",
+            "value": 1
         },
         "downlink-preamble-length": {
-        	"help": "Number of preamble symbols need to be captured (out of 8) for successful demodulation",
-        	"value": 5
+            "help": "Number of whole preamble symbols needed to have a firm lock on the signal. Default: 5 + 1",
+            "value": 6
         },
         "uplink-preamble-length": {
-        	"help": "Number of preamble symbols to transmit. Must be <= 8",
-        	"value": 8
+            "help": "Number of preamble symbols to transmit. Default: 8",
+            "value": 8
         },
         "fsb-mask": {
         	"help": "FSB mask for upstream [Only for US915 & AU915] Check lorawan/FSB_Usage.txt for more details",

features/lorawan/system/lorawan_data_structures.h

@@ -45,7 +45,7 @@ typedef uint32_t lorawan_time_t;
 #endif
 
 // Radio wake-up time from sleep - unit ms.
-#define RADIO_WAKEUP_TIME                           1
+#define RADIO_WAKEUP_TIME                           1.0
 
 /*!
  * Sets the length of the LoRaMAC footer field.