RSS measurement

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Introduction

Radio Signal Strength (RSS) refers to the transmitter power output as received by a reference antenna (i.e. receiver) placed at a distance from the transmitting antenna.

In this tutorial, we will learn how to measure the Radio Signal Strength received by a Tmote Sky module. The tutorial has two nodes, a transmitter and a receiver. The receiver node continuously receives packets from the transmitting node, measures its signal strength and displays the RSS value of the packet received.

A step-by-step description of the simulation process and the C source code can be found in the subsequent sections.

RSS Measurement Program on Cooja

Step 1

Create New Simulation on Cooja. Let's say we call our simulation as "RSS".

Step 2

Create Motes.

Step 3

Browse the firmware. Note: You can find the firmware later in this tutorial.

Step 4

Compile the firmware.

Step 5

Select number of motes. You can select any number of motes, but for simplicity we are considering only 2 motes. Note: We are uploading same firmware on both the motes. One will act as Sender and another will act as Receiver.

Step 6

Start the simulation.

Step 7

You can see the RSSI value in the mote output window. These RSSI values are in dBm.

Code

<HEADER FILES>

static struct collect_conn tc;

PROCESS(example_collect_process, "RSS Measurement");
AUTOSTART_PROCESSES(&example_collect_process);

static void recv(const rimeaddr_t *originator, uint8_t seqno, uint8_t hops)
{
  static signed char rss;
  static signed char rss_val;
  static signed char rss_offset;
  printf("Sink got message from %d.%d, seqno %d, hops %d: len %d '%s'\n",originator->u8[0], originator->u8[1],seqno, hops,packetbuf_datalen(),
         (char *)packetbuf_dataptr());
  rss_val = cc2420_last_rssi;
  rss_offset=-45;
  rss=rss_val + rss_offset;
  printf("RSSI of Last Packet Received is %d\n",rss);
}

static const struct collect_callbacks callbacks = { recv };

PROCESS_THREAD(example_collect_process, ev, data)
{
  static struct etimer periodic;
  static struct etimer et;
  
  PROCESS_BEGIN();

  collect_open(&tc, 130, COLLECT_ROUTER, &callbacks);

  if(rimeaddr_node_addr.u8[0] == 1 && rimeaddr_node_addr.u8[1] == 0) 
  {
    printf("I am sink\n");
    collect_set_sink(&tc, 1);
  }

  /* Allow some time for the network to settle. */
  etimer_set(&et, 120 * CLOCK_SECOND);
  PROCESS_WAIT_UNTIL(etimer_expired(&et));

  while(1) 
  {

    /* Send a packet every 1 seconds. */
    if(etimer_expired(&periodic)) 
    {
      etimer_set(&periodic, CLOCK_SECOND * 1 );
      etimer_set(&et, random_rand() % (CLOCK_SECOND * 1));
    }

    PROCESS_WAIT_EVENT();


    if(etimer_expired(&et)) 
    {
      static rimeaddr_t oldparent;
      const rimeaddr_t *parent;
      if(rimeaddr_node_addr.u8[0] != 1 )
      {
        printf("Sending\n");
        packetbuf_clear();
        packetbuf_set_datalen(sprintf(packetbuf_dataptr(),"%s", "Fight On") + 1);
        collect_send(&tc, 15);

        parent = collect_parent(&tc);
        if(!rimeaddr_cmp(parent, &oldparent)) 
        {
           if(!rimeaddr_cmp(&oldparent, &rimeaddr_null))
           {
              printf("#L %d 0\n", oldparent.u8[0]);
           }
           if(!rimeaddr_cmp(parent, &rimeaddr_null)) 
           {
              printf("#L %d 1\n", parent->u8[0]);
           }
           
           rimeaddr_copy(&oldparent, parent);
        }
      }
    }

  } //end of while

  PROCESS_END();
} //end of process thread
 

Understanding the Code

1-static const struct collect_callbacks callbacks = { recv };

callbacks = { recv } calls the recv() function when a packet is received.

2-collect_open(&tc, 130, COLLECT_ROUTER, &callbacks);

Opens a connection.

3-collect_set_sink(&tc, 1);

Sets the node as sink. Note that we are setting node 1.0 as the sink here.

4-collect_send(&tc, 15);

Sends data through the link.

5-etimer_set(&et, CLOCK_SECOND * 1);

This will set the timer to repeat the iterations every 1 seconds.

6-if(rimeaddr_node_addr.u8[0] != 1 )

This if condition will work only for the source nodes i.e. node id != 1.

To understand the calculations performed in recv() function, see page 49 on this link http://web.stanford.edu/class/cs244e/papers/cc2420.pdf

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Edited by : Nitin