Reliable, long-range acoustic communications (LRAC) is an enabling technology for numerous applications of manned and unmanned underwater systems. For example, with the capability of communicating at long ranges of several hundreds or even thousands of kilometers, it will become possible to remotely command and control unmanned underwater vehicles that are otherwise unreachable. As another example, underwater systems will be able to rely on such capability to establish a wide-area undersea network to complete missions in a collaborative fashion.
As an active area of research, LRAC has received a tremendous amount of attention for the past two decades. A number of LRAC schemes have been proposed and tested by seagoing experiments; however, most research done so far has concentrated on the fixed LRAC cases where both the source and the receiver are moored.
In mobile LRAC applications, the source and/or the receiver move at a significant speed. LRAC is made difficult by many factors, including (but not limited to) low signal-to-noise ratios (SNRs) mainly caused by large transmission losses, significant Doppler shifts induced by relative source-receiver motion, environmental factors such as internal waves, and severe inter-symbol interference (ISI) due to large channel delay spread. While these performance-limiting factors exist in both fixed and mobile LRAC, they tend to be more pronounced and therefore more difficult to be dealt with in the mobile cases, making an already challenging LRAC problem even more challenging. While many of the existing LRAC schemes developed for the fixed cases might in theory work well in the mobile cases, only a few have been tested by seagoing experiments.
The new method for mobile underwater acoustic communications encodes a communication signal for transmitting a doubly differential (DD) spread spectrum (SS) communication output signal. The DD-SS method results in a highly reliable and efficient system for LRAC.
The innovative DD-SS encoding methodology increases the SNR through a processing gain, eliminates ISI via multipath suppression, and enables band-width efficiency improvement with data multiplexing. Additionally, the use of DD coding/decoding obviates the need for explicit phase/Doppler tracking and correction. Thus, a system incorporating a DD-SS technique performs robustly against unpredictable fluctuations in underwater communication environments, making it particularly suitable for mobile LRAC.