586 IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 26, NO. 4, OCTOBER 2001 Analysis of Channel Effects on Direct-Sequence and Frequency-Hopped Spread-Spectrum Acoustic Communication Lee Freitag, Milica Stojanovic, Sandipa Singh, and Mark Johnson Abstract—Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sam- pling 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 capa- bility, 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 mini- mize 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. Index Terms—Acoustic communication, CDMA, direct-se- quence, frequency-hop, multipath, spread-spectrum. I. INTRODUCTION C ODE-DIVISION multiple-access communication (CDMA) is an important emerging technology for underwater acoustic networks for both civilian and military purposes. CDMA permits random, overlapping access to a shared communication channel as required in an autonomous ocean-sampling network (AOSN) scenario. Drawing from the results in RF wireless communications [1], [2], two code-di- vision spread-spectrum signalling methods are proposed for simultaneous-access communication: phase-modulated, direct-sequence spread-spectrum (DSSS) signaling, and nonco- herent frequency-hopped spread-spectrum (FHSS). DSSS uses phase-shift keying and spreads the data using codes with good Manuscript received April 13, 2000; revised July 6, 2001. L. Freitag, M. Johnson and S. Singh are with the Woods Hole Oceanographic Institution, Applied Ocean and Physics Laboratory, Woods Hole, MA 02543 USA (e-mail: lfreitag@whoi.edu; majohnson@whoi.edu; ssingh@whoi.edu). M. Stojanovic is with the Massachusetts Institute of Technology, Depart- ment of Aeronautics & Astronautics, Cambridge, MA 02139 USA (e-mail: mil- litsa@mit.edu). Publisher Item Identifier S 0364-9059(01)09922-8. auto- and cross-correlation properties, while FHSS methods utilize many frequency bands with hopping patterns chosen for minimum interference between users. While both of these techniques may be suitable for use in the underwater acoustic channel, the best choice depends upon the characteristics of the particular propagation conditions, specif- ically time and frequency spread. Additional considerations in- clude the desired throughput, total transducer bandwidth, and the computational resources available at the receiver. DSSS signaling has been used to increase the SNR per data symbol and resolve multipath components for single-user, short- range applications [3] as well as for long-range, low SNR and multiuser communication [4], [5] . The system described in [3] utilized 10 kHz of bandwidth and short spreading codes in order to combat multipath effects in shallow water, whereas the inves- tigations reported in [4] and [5] include discussions of multiuser access and report favorable results in the 1–2 kHz frequency band at long ranges for a single user. There are tremendous variations in acoustic channels depending on source-receiver geometry, bathymetry and sound-speed profile. In addition, there are many possible system configurations depending on the number of users and the desired link protocol. The large number of possible implementations requires the system design to be as flexible as possible, accom- modating few to many users in a variety of channel conditions. While reconfiguration may or may not be done automatically in the field, an actual system design should allow changing basic system parameters, in particular, the spreading rate. Given the limited bandwidth available for underwater acoustic com- munications systems, increasing the spreading rate inevitably reduces link throughput. In the case of slow-hop frequency-shift keying (FSK) (one tone per symbol), increasing the spreading rate means increasing the symbol time (and thus the energy per symbol), which in turn, reduces the bandwidth per symbol and provides a large set of frequencies from which to select. In a fixed-power system, the SNR will increase as the rate decreases, though the gain in transmitted energy may be offset by loss due to Doppler shift and spread. This results in diminishing returns as the symbol duration increases. In a DSSS system, the spreading rate is increased through use of more chips per data symbol, thus increasing the SNR per symbol in additive white Gaussian noise (AWGN). How- ever, in a time-varying multipath channel, the actual gain de- pends upon the stability of the propagation environment. If the channel changes appreciably during one symbol period, the gain of the receiver will be reduced, ultimately resulting in a net loss 0364-9059/01$10.00 © 2001 IEEE