| 摘要 |
A spread spectrum communications system comprises a transmitter (40-50) and a receiver (52-64). The transmitter includes mixing means (42) having a first input for connection to a data source (40) and a second input for connection to means (44, 46) for providing a dc free encoded spreading sequence. The output from the mixing means (42) is applied to a pulse shaping filter (48), the output of which controls the frequency of a voltage controlled oscillator (50). The signal which is received at the receiver is applied to a frequency down converter (52) which produces quadrature related signals (I, Q) which are applied to first inputs of mixers (56, 58). A dc free encoded spreading sequence is provided by a spreading sequence source (60) and encoder (62) and is applied to second inputs of the mixers (56, 58); the applied encoded spreading sequence corresponds to that applied to the mixing means (42) in the transmitter. The despread outputs from the mixers (56, 58) are applied to a symbol estimator (64) which recovers the received data. Optionally, a plurality of dc free encoded spreading sequences may be stored in a ROM, and by applying an appropriate address signal to the ROM, a predetermined dc free encoded spreading sequence is applied to the filter (48). |
