Abstract
The principle of superposition does not hold in non-linear systems where response to one input is modified in the presence of other inputs. So we are unable to determine the response of this system to an arbitrary input signal based on its response to an impulse or a sinusoid. We may however employ the system's transfer characteristic, which gives its output as a polynomial function of input over a specified range. In so doing we find that non-linearity not only distorts the wanted signal, but it also introduces harmonic and intermodulation products into the output, and these may interfere with other signals in adjacent channels or frequency bands. This is undesirable in transmission channels and must be minimised by ensuring that HPA operations are linear, for example by using a larger (and heavier) HPA than necessary and employing sufficient input power back-off to set its operating point within the linear region. In the next chapter we begin an in-depth study of the efficient representation of digital signals for transmission, and explore how the conflicting goals of minimising transmission bandwidth requirement and maintaining high fidelity with the source signal are achieved in practice.
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