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Long-distance wireless comm with low power devices

Posted: 21 Apr 2014     Print Version  Bookmark and Share

Keywords:wireless communication  3GPP LTE  4G  Continuous Wave  CW 

These days wireless communication has become a crucial means to enable applications and services which otherwise would be impractical to implement. Wireless communication has evolved to deliver data rates that were unheard of in the last decade. Under ideal conditions, 3GPP LTE / 4G technology is expected to deliver 1Gbps of data rate over small cellular hotspots. For all practical purposes though, the quality and reliability of the service varies case-to-case, with data being delivered on a 'best-effort' basis.

These technologies however do not cater to the other end of the application spectrum where reliable, low-latency and low-power communication is required over long distances. A case in point could be having a 'homing' beacon embedded on life-jackets for search and rescue missions in the sea. The technology should support low power consumption, as it is expected to be battery powered, and searches could go on for days together before a rescue is made. It should also communicate over long distances as survivors could drift apart by many kilometres.

This article explores the application of Continuous Wave (CW) to achieve long distance (15-20Kilometers) low-bandwidth wireless coverage range in Line of Sight (LOS) with low power devices. We shall explain efficient methods which involve conversion of time domain data into frequency domain data using Fast Fourier Transforms (FFT).

Using continuous wave in RF communications
All RF transceivers have feature to turn the transmission On and Off. When data needs to be transmitted, usual sequence of operation is:

At Transmitter:

1. Packetise data as per protocol
2. Turn On RF Transmission
3. Send out data as a modulated signal
4. Turn Off RF Transmission

At the receiver
1. RF Receive On
2. Read modulated data
3. Unpack the data as per protocol

Based on the number of codes we need for communication we define the length of unitcode symbol. Considering that we have alphanumeric content, we need alphabets (26) and numbers (10) i.e. 36different codes. Using 6 bits we can have 64 different combinations, so length of a unit code symbol will be 6.

We define the symbols as:
A = 0 0 0 0 0 11 = 0 1 1 0 1 1
B = 0 0 0 0 1 02 = 0 1 1 1 0 0
C = 0 0 0 0 1 13 = 0 1 1 1 0 1
..
Y = 0 1 1 0 1 0 9 = 1 0 0 1 0 0

**Here 0 means OFF and 1 means ON

Now suppose we would like to transmit a '9' as information. '9' is sent as ON-OFF-OFF-ON-OFF-OFF

Adjoining figure shows '9' being transmitted using CW Operation.

Duration of ON and OFF signals is important parameter in determining range. There is a trade-off between range and data rate.

Data rate (symbols per second) = [Number of Signals per second] / [Length of Unit Symbol]

Say we define duration of a signal as 10ms, then data rate = [(1000/10)]/[6] = 16 symbols/second for above mentioned scenario.

It is important to identify number of symbols needed for an application to optimise length of unit symbol. Say if the application needs only numbers then unit symbol length will be reduced to 4 and thus symbols per second will increase.

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