Voice Link Over Spread Spectrum Radio


Posted on Feb 5, 2014

Until quite recently spread spectrum techniques were almost exclusively in the military domain. Their use in GPS and the latest cellular phones will be followed by many other civil applications. This article, the first of [two] parts, examines the technology by describing an experimental direct sequence voice transmission system as a worked example.


Voice Link Over Spread Spectrum Radio
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Most Communication Engineers are used to minimising transmission bandwidths. The trend has been to use narrower bandwidths, as with the transition from double sideband to single sideband modulation. It is quite obvious that narrower bandwidths permit more communication channels to be packed into a defined frequency band. However the rationale of using the very wide bandwidths required by Spread Spectrum systems needs explanation. Claude Shannon produced a ground breaking paper on the mathematical theory of communication in 1949. Shannon`s resulting theorem can be expressed as: where C = data rate in bits per second, W = bandwidth (Hz), S = average signal power (W), N = mean white gaussian noise power (W). It can be seen from the equation that the only options available to increase a channel`s capacity are to increase either the bandwidth (W) or the signal to noise ratio (S/N). An increase in the signal to noise ratio requires an increase in transmitter power as the noise within the channel is beyond our control! Thus we can either trade power or bandwidth to achieve a specified channel data rate. Because of the logarithmic relationship, increasing the power output is often unrealistic. However if frequency allocation constraints permit, the bandwidth can be increased. An appreciable increase in data capacity or signal to noise ratio (for a fixed data rate) can then be achieved. From the derived relationship it can be...




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