Analysis of channel effects on direct-sequence and frequency-hopped spread-spectrum acoustic communication

Abstract

Abstract-Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles,moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance.

Rate-range for an FH-FSK acoustic modem

“…Frequency hopping can be exploited in a multi-user environment in order to share the available bandwidth by allowing a number of users to transmit simultaneously on different hopping patterns. However, frequency hopping is also exploited in a single-user system, allowing the channel to clear between successive transmissions in the same bin; thereby eliminating ISI at the receiver [4]. Table 1 gives a summary of the frequency hopping characteristics that we will use in our singleuser analysis.…”

Adaptive sampling in autonomous marine sensor networks

“…Some of these advances are described in [22] and [23]. Advances have been made both in terms of energy efficiency and in data rates.…”

Prospects and problems of wireless communication for underwater sensor networks

“…Multiple arrivals at the receiver side can be separated via the despreading operation which suppresses the timespreading induced interference, thanks to the nice auto-correlation properties of the spreading sequence. Channel estimation and tracking are needed if phasecoherent modulation such as phase shift keying (PSK) is used to map information bits to symbols before spreading [18]. For noncoherent DSSS, information bits can be used to select different spreading codes to be used, and the receiver compares the amplitudes of the outputs from different matched filters, with each one matched to one choice of spreading code.…”