TABLE OF CONTENTS A spread-spectrum signal is a signal that has an extra modulation that expands the signal bandwidth beyond what is required by the underlying data modulation. Spread-spectrum communication systems [1], [2], [3] are useful for suppressing interference, making interception difficult, accommodating fading and multipath channels, and providing a multiple-access capability. The most practical and dominant methods of spread-spectrum communications are direct-sequence modulation and frequency hopping of digital communications.
At first it might seem that a spread-spectrum signal is counterproductive insofar as the receive filter will require an increased bandwidth and, hence, will pass more noise power to the demodulator. However, when any signal and white Gaussian noise are applied to a filter matched to the signal, the sampled filter output has a signal-to-noise ratio (SNR) that is inversely proportional to the noise-power spectral density. The remarkable aspect of this result is that the filter bandwidth and, hence, the output noise power are irrelevant. Thus, we observe that there is no fundamental barrier to the use of spread-spectrum communications.
A direct-sequence signal is a spread-spectrum signal generated by the direct mixing of the data with a spreading waveform before the final carrier modulation. Ideally, a direct-sequence signal with binary phase-shift keying (PSK) or differential PSK (DPSK) data modulation can be represented by
where A is the signal amplitude, d(t) is the data modulation, p(t) is the spreading waveform, f c is the carrier frequency, and ? is the phase at t
