Fast Synchronization in a Spread-Spectrum System Based on Acoustoelectric Convolvers

TLDR: A wideband PN spread-spectrum modem has been implemented using a pair of acoustoelectric convolvers as programmable matched filters in combination with digital delay and coincidence gating to achieve coarse synchronization to a precision of one PN chip using only four preamble bits.

Abstract: A wideband PN spread-spectrum modem has been implemented using a pair of acoustoelectric convolvers as programmable matched filters in combination with digital delay and coincidence gating. The convolvers provide the capability for fast synchronization of a burst-format message during a short 13-bit preamble. The spreading code is different for every preamble and data bit of a message. (Conventional receivers often require long synchronization times for non-repeating preamble codes or require the same code to be repeated for every preamble bit.) Efficient algorithms have been developed for coarse synchronization and bit alignment of the received signal and reference in each convolver. The modem achieves coarse synchronization to a precision of one PN chip using only four preamble bits with an effective correlation loss only 1.5 dB from ideal. Alignment of the coded bits within the convolver interaction region for subsequent DPSK data detection is achieved by shifting the reference time base. Fine synchronization of the receiver for a data sampling precision of one-eighth of a PN chip is completed at the end of the 13-bit preamble. Message data modulation is DPSK and spreading modulation is MSK. Receiver performance in the presence of wideband interference has been measured to be within 1.5 dB of theoretical predictions.