Method for compressive sensing based LPWAN communication
A LoRa device for communicating sensor signals in a low power wide area network (LPWAN) includes a physical layer using Hamming encoding and Gray indexing with chirp spread signal (CSS) modulation to encode and modulate the sensor signals and a medium access layer (MAC) including a compressive sensing sub-layer which reduces encoded, modulated signals to sparse vectors. A transmission packet is formed by combining the sparse vectors with a selected set of sparse vectors representing past measurements and the incoming velocity of the sensor signals. A receiver decompresses the transmission packet by reconstructing, at a sparse recovery sub-layer of a receiver MAC layer, the encoded, modulated sensor signals. A decoder path removes the CSS modulation and Gray indexing, and Hamming decodes the sensors signals.
1. A method for compressive sensing based communications of low range (LoRa) devices in a low power wide area network (LPWAN) comprising at least one sensor selected from the group consisting of an accelerometer, an altimeter, and a temperature sensor; a GPS module and an antenna, said method comprising:
receiving, by a physical layer of a LoRa device, a bit stream of sensor measurements from the at least one sensor at an incoming velocity;
encoding the bit stream to generate an encoded bit stream;
modulating the encoded bit stream by chirp spread spectrum modulation to generate a chirp modulated encoded bit stream;
forming a first transmission packet including at least a preamble, the chirp modulated encoded bit stream and a cyclic redundancy check (CRC);
receiving the first transmission packet by a multiple access layer (MAC) of the LoRa device;
compressing the chirp modulated encoded bit stream of the first transmission packet into sparse vectors;
combining the sparse vectors with a linear combination of a randomly selected set of sparse vectors representing past measurements and the in-coming velocity of the bit stream to generate a payload;
forming a second transmission packet including the preamble, the payload and the cyclic redundancy check;
transmitting, the second transmission packet through the antenna at a randomly selected time.
2. The method of claim 1 , further comprising:
encoding the bit stream by:
encoding the bits with a Hamming encoder, by using Hamming forward error correction to form a first coded bit sequence;
interleaving the first coded bit sequence to form a second coded bit sequence; and
applying Gray indexing to the second coded bit sequence to prevent off-by-one errors when resolving symbols to generate a third coded bit sequence.
3. The method of claim 2 , further comprising:
modulating the encoded bit stream by combining the third coded bit sequence with a base chirp signal generated by a local oscillator at a chirp rate.
4. The method of claim 3 , further comprising:
selecting a channel bandwidth from the group consisting of 125 kHz, 250 kHz and 500 kHz;
matching a coding rate to a channel coding rate, wherein the coding rate is selected from
R
c
=
4
4
+
n
where
n
=
{
1
,
2
,
3
,
4
}
;
and
encoding the signal bits by using Hamming forward error correction at the coding rate.
5. The method of claim 4 , further comprising:
compressing the chirp modulated encoded bit stream of the first transmission packet into sparse vectors by retaining only non-zero values of the sparse vectors.
6. The method of claim 5 , further comprising:
mapping the second transmission packet to in-phase and quadrature transmission paths; and
transmitting the second transmission packet according to the mapping.
7. The method of claim 6 , further comprising:
transmitting the second transmission packet according to a Class A protocol, in which the LoRa device sleeps until it wakes for transmission at the randomly selected time.