A system and method of driect-sequence spread-spec trum TDMA (or TDD) digital communication, wherein acquisition and tracking occurs for a plurality of frames having a predetermined number of time slots. One of the predetermined number of time slots in each frame is assigned for acquisition purposes and carries acquisition and sync digital information. The remaining time slots in each frame have assigned header bytes for tracking purposes. The assigned acquisition time slot is spread with an acquisition direct-sequence spreading code. The remaining time slots are spread with a con munication direct-sequence spreading code. During acquisition, the signal strength in each successive time slot for each spread frame is measured for a given num ber of time frames. The peak is located within the given number of time frames through a major acquistion sweep and a refinement sweep. The major acquisition sweep and refinement acquisition sweep locate the frame and time slot boundaries of the transmitted signal. Once acquired, tracking occurs during the header bytes of each successive remaining time slot. Acquisition, tracking and demodulation of the digital data is per formed with the same circuitry.

Spread-spectrum signal acquisition: Methods and technology

A receiver in a spread-spectrum communication system is adapted to receive a signal which is modulated by a binary code modulated by a binary code or a data, and includes a matched filter which detects a correlation between the received signal and a reference signal modulated by a reference code produced by a code generator. When the received code supplied from the matched filter coincides with or is slightly displaced from the reference code, a pulse is produced from a correlation spike waveform with a relatively large amplitude to extract a desired pulse from the pulse and use it to initialize the reference code generator.

Spread-spectrum communication system

A pseudo-noise (PN) correlation and demodulation method and apparatus, in which received signals are correlated with locally generated PN codes and correlation measurements are maintained for multiple time bins corresponding to signals received over possible multiple paths. Instead of selecting one or a small number of significant correlation measurements to control data demodulation, the method of the invention performs data demodulation over the multiple time bins, and then selects data values corresponding to significant correlation measurements and combines the selected data values, weighting them in accordance with their relative strengths a determined during correlation. Use of the invention provides improved performance in terms of signal quality and continuity in the presence of multipath conditions. Careful selection of design parameters allows the improved performance to be obtained in a compact and easily portable implementation including transmitter and receiver modules.

Pseudo-noise modem and related digital correlation method

Acoustoelectric (AE) convolvers for spread-spectrum communication applications are described with input bandwidth capacities to 200 MHz. These devices offer a unique combination of large processing gain, high dynamic range, small size and weight, and low drive power requirements. The programmable feature of convolvers allows the encoding waveform to be changed from bit-to-bit, thereby providing resistance to repeat jamming and enabling secure communications. The basic concepts of a convolver-based spread-spectrum communications system are reviewed, current convolver capabilities are discussed, and projections are made for future device performance. Deviations from nonideal convolver performance are considered. Special techniques which must be used in the system implementation and evaluation of convolvers are described, and the performance level achieved in a state-of-art convolver subsystem is given.

Acoustoelectric Convolver Technology for Spread-Spectrum Communications

http://ieeexplore.ieee.org/iel5/10280/32713/01534295.pdf

Wideband Elastic Convolvers

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.

http://ieeexplore.ieee.org/iel5/10278/32711/01534043.pdf

J.E. Laynor

Papers

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