Chirp Slope Keying: A Practical Benchmark Modulation Scheme for Underwater Acoustic Channel Replay Simulation
Performance bench-marking of underwater acoustic modems manufactured by various vendors are not straightforward due to the unavailability of standard channel models for Underwater Wireless Acoustic channels (UWA). Recognizing the difficulty for accurately modeling the UWA channels, channel replay simulation is often resorted to as a realistic simulation strategy. In this approach, Time-Varying Impulse Response (TVIR) of the channel is measured continuously for a duration using channel sounding technique. Such databases are created for benchmark sites and the recorded channel TVIR are used for channel replay simulation. One such database of TVIR developed by Norwegian Defence Research Establishment (FFI) is named as underWater AcousTic channEl Replay benchMARK (WATERMARK) is adopted by many authors as a benchmarking tool for acoustic modem physical layer technologies. Popular modulation in communication literature (such as BPSK and QPSK etc.) will not work in UWA channels without the support of additional processing blocks such as complex adaptive equalizers, synchronization schemes, and Doppler compensation techniques due to the double-spread and wide-band characteristics of UWA channels. In this paper, we have demonstrated Chirp Slope Keying (CSK) as a simple and robust modulation scheme which will work on almost all UWA channels encountered in practice by suitably choosing the data rate, without the support of additional processing blocks such as complex adaptive equalizers, synchronization schemes and Doppler compensation techniques. Hence CSK provides a "practical performance benchmark for channel replay simulation"against which the performance of other advanced physical layer schemes can be compared.
Underwater acoustic communications: Design considerations at the physical layer based on field trials
An overview of the unique challenges in underwater acoustic (UWA) communication in ocean environment is presented. The UWA channel is compared with conventional RF channels. Several design considerations applicable to the physical layer design of a UWA communication system is described based on field measurements. The unique problems in synchronization under high Doppler are described highlighting the key differences in the way Doppler affects the RF and UWA channels. A novel timing synchronization mechanism has been introduced in this study which caters for both the sampling skew offset and the Doppler shift between the transmitter and the receiver. © 2012 IEEE.
Impact of Doppler on narrowband and wideband waveforms
The matched filter output performance of wave forms affected by delay and Doppler are often analyzed by narrowband ambiguity functions in RADAR and radio communication literature. The same ideas are often adopted in the active SONAR and Underwater Wireless Acoustic (UWA) communication designs. UWA channels are wide band in nature and hence many of the simplifying assumptions commonly used for narrowband channels may not be applicable. Different types FM waveforms are analyzed using narrowband and wide band definition of ambiguity function to illustrate the effects of Doppler in narrowband and wide band channels on these wave forms.
Performance Comparison of JANUS and CSK using WATERMARK Channel Replay Simulator
JANUS is a robust digital underwater signalling standard for network node discovery and interoperability. Chirp Slope Keying (CSK) is a simple and robust broadband modulation scheme for practical Underwater Wireless Acoustic (UWA) communications at relatively low data rates where information is modulated in the slope of a chirp signal. In this paper, we have compared the performance of JANUS and CSK using channel replay simulation. The results were further compared with theoretical and simulated results of other standard modulation schemes such as Binary Phase/ Frequency Shift Keying Modulations. The role of Forward Error Correction (FEC) coding and Frequency Hopping (FH) in the performance JANUS are brought out by channel replay simulation.