Oversampling¶
Media: Radio, Wire | Pins used: 1 / 2
Oversampling strategy was initially developed in the PJON_ASK repository, and it was integrated in the PJON repository from version 3.0 beta, as a data link layer strategy. Bits are over-sampled to have high resilience in high interference scenarios, like using an ASK/FSK cheap radio transceivers in an urban environment. It is tested effectively with many versions of the ASK/FSK 315/433Mhz modules available on the market, but it works nominally also through wires and the human body. It complies with PJDLR v1.0 Data link layer 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 PJON object ready to communicate in this Strategy. 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 */
#define OS_BACK_OFF_DEGREE 4
/* Set the maximum sending attempts */
#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 transmitter / receiver modules (with a cost around 2/3 dollars). Please be sure of the regulations your government imposes on radio transmission over these frequencies before use.
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 data link layer, the asynchronous acknowledgment is suggested as default acknowledgment 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 --/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/