Spread spectrum communication system utilizing differential code shift keying

DETAILED DESCRIPTION The present invention is a spread spectrum data communication system that utilizes a modulation technique referred to as differential code shift keying (DCSK) to increase the number of bits transmitted per symbol, decrease synchronization requirements and to improve performance.
The data is transmitted in the form of time shifts between consecutive circularly rotated waveforms of length T which are referred to as spreading waveforms.
The spreading waveforms can comprise any type of waveform that has suitable auto correlation properties.
In the example presented herein, standard CEBus chirps can be used as the spreading waveform in order to permit the coexistence of standard CEBus transmissions and transmissions generated by the communication system of the present invention.
During each symbol period which is also referred to as a unit symbol time (UST, a plurality of bits are transmitted.
The symbol period is divided into a plurality of shift indexes with each shift index representing a particular bit pattern.
The waveform is rotated by an amount in accordance with the data to be transmitted.
The data is conveyed in the amount of rotation applied to the chirp before it is transmitted.
Alternatively, the data may be conveyed in the shift differential between consecutive symbols.
A correlator is used to decode the received waveform.
The correlator employs a matched filter having a template of the chirp waveforn pattern to detect the amount of rotation of the chirp within the received signal for each symbol.
The received data is fed into a shift register and circularly rotated.
For each rotation shift, the matched filter generates a correlation sum.
The shift index chosen for each UST corresponds to the shift index that yields the maximum (or minimum) correlation sum.
Differential shift indexes are generated by subtracting the currently received shift index from the previously received shift index.
The differential shift index is then decoded to yield the originally transmitted data