DSP implementation of direct-sequence spread spectrum underwater acoustic modems with networking capability

This paper presents the hardware implementation of a Direct-Sequence Spread Spectrum (DSSS) modem with networking capability for underwater acoustic sensor networks. The hardware platform uses the Texas Instrument Digital Signal Processor (DSP) for physical layer transmission/receiving and microcontroller for network layer control and node ID detection. The system uses 125 kHz single carrier with Binary Phase Shift Keying (BPSK) for ID code modulation and DSSS for payload data. For payload data detection, adaptive channel estimation is implemented to track the time-varying frequency-selective channel and rake receiver is used with simple Zero-Forcing (ZF) equalization. For node ID detection, dual Pseudo-random Noise (PN) sequences are inserted with gaps of variable length that relates with the ID code. The cross-correlation of the two PN blocks is used to detect the ID code while providing carrier and symbol synchronization. The system is tested under real-world underwater channels with severe multipath up to 12 ms. The performance of the design is satisfactory and the computational complexity is affordable by a single DSP.