OverSampling

Media: Radio, Wire | Pins used: 1 / 2


OverSampling has been developed to enable a new software emulated simplex or half-duplex data-link supporting one or many to many communication on a single channel or medium. It can be run on cheap and low performance microcontrollers, it supports communication for many devices connected to the same medium and stable operation in spite of interference. Its procedure has been specified to obtain long range and high reliability using FSK/ASK/OOK 315/433Mhz radio transceivers available on the market. It complies with PJDLR v1.1 specification. Take a look at the video introduction for a brief showcase of its features.

Compatibility

  • ATmega88/168/328 16Mhz (Diecimila, Duemilanove, Uno, Nano, Mini, Lillypad)
  • ATmega2560 16Mhz (Arduino Mega)
  • ATmega16u4/32u4 16Mhz (Arduino Leonardo)

Performance

  • Transfer speed: 202 B/s or 1620 Baud
  • Data throughput: 150 B/s
  • Range: 250 meters in urban environment / 5km with line of sight and ideal atmospheric conditions

How to use OverSampling

Pass the OverSampling type as PJON template parameter to instantiate a new PJON object. All the other necessary information is present in the general Documentation.

  /* Maximum latency can be set defining OS_LATENCY
     before PJON.h inclusion (default 4 milliseconds) */
  #define OS_LATENCY      4000

  /* Set the back-off exponential degree (default 4) */
  #define OS_BACK_OFF_DEGREE 4

  /* Set the maximum sending attempts (default 20) */
  #define OS_MAX_ATTEMPTS   20

  /* The values set above are the default producing a 3.2 seconds
     back-off timeout with 20 attempts. Higher OS_MAX_ATTEMPTS to higher
     the back-off timeout, higher OS_BACK_OFF_DEGREE to higher the interval
     between every attempt. */

  #include <PJON.h>

  PJON<OverSampling> bus; // 2 pin over-sampled data link layer

  void setup() {
    bus.strategy.set_pin(12);      // Set the pin 12 as the communication pin
                                   // or
    bus.strategy.set_pins(11, 12); // Set pin 11 as input pin and pin 12 as output pin
  }  

After the PJON object is defined with its strategy it is possible to set the communication pin accessing to the strategy present in the PJON instance.

Use OverSampling with cheap 433Mhz transceivers

To build a real open-source PJON packet radio able to communicate up to 5km you need only a couple (for PJON_SIMPLEX mode) or two couples (for PJON_HALF_DUPLEX mode) of cheap 315/433Mhz ASK/FSK/OOK transmitter and receiver modules (with a cost around 2/3 dollars each). Please be sure of the regulations your government imposes on radio transmission over these frequencies before use.

PJON Oversampling packet radio

The maximum detected range was experimented with a small packet radio transmitting its position every minute. The maximum range obtained was slightly more than 5 kilometers in line of sight in open area. Testing it instead in an urban environment the range is down to 250 meters. Two couples of STX882 and SRX882 were used as transceivers. If you choose these modules, remember to set HIGH the pin CS on the receiver before starting reception.

If Using OverSampling, the asynchronous acknowledgement is suggested as acknowledgement mechanism because includes in the packet's meta-info a packet id, avoiding duplicated receptions.

Antenna design

Experiments in PJON_HALF_DUPLEX mode have shown that it seems better to keep isolated the two antennas, using two different, not connected elements to transmit and receive. The first suggested antenna design is a wide beam dipole antenna made by two 173mm (quarter wavelength) or 345mm (half wavelength) long conductive elements, one connected to ground and the other connected to the input or output pin:


 173mm (quarter wavelength) / 345mm(half wavelength)

 -------------------|--------------------
                  __|__
                 |TX/RX|
                 |_____|

A more directional, compact and long range antenna design is the wip antenna. Can be easily crafted with two 173mm (quarter wavelength) / 345mm (half wavelength) long insulated wire sections wrapped with each other every 5mm, one is connected to ground and the other to the input or output pin. This design helps because of its strong ground plane, often necessary to have decent results with this sort of hardware.

         5mm
         ||  173mm (quarter wavelength) / 345mm (half wavelength)
 GND   --\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
 RX/TX --/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

Known issues

  • In older versions, OverSampling was affected by ineffective and short range if used in PJON_HALF_DUPLEX mode. This issue has been fixed by handling the gain refresh (see issue 91).

This document is automatically generated from the github repository. If you have noticed an error or an inconsistency, please report it opening an issue here