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AssigneeQinetiq Limited
InventorsJonathan James Davies, Shaun Michael Dunn, Stephen Arthur Pointer, Samuel William Downer
Estimating the term…
For digital underwater transmission an information sequence (17) of digital bits is partitioned into a succession of symbols each comprising n bits where n is equal to or greater than one (2n potential symbols), each symbol (19) is replaced (20) with a corresponding base code sequence of length m bits and the result (21) is passed to an acoustic transducer after bi-phase modulation (22) on a carrier. The code has good correlation properties, e.g. a pseudo-random maixmal length sequence, a Gold code or a Kasami code, and the value of m/n is at least (32). Preferably the base sequence is cyclically extended at one or both ends prior to modulation, and Solomon Reed redundancy coding (18) is initially applied to the information sequence. The output (23) of modulator (22) may be prefaced (26) by a pair of chirp waveforms (24). Provision is made for Doppler and multi-path correction.
A digital underwater transmission device (1) comprising means for receiving or generating an information sequence (17) of digital bits, means (18) for partitioning said sequence into a succession of symbols (19) each comprising n bits where n is equal to or greater than one, so that there are 2n potential symbols, coding means (20) for replacing each said symbol with a corresponding base code sequence (21) of length m chips (bits), wherein the code has good correlation properties and the value of m/n is at least 32, modulating means (22) for modulating a carrier with the signal from the coding means, and transmission transducer means (3) coupled to the output (23) of the modulating means for transmitting an acoustic signal through water.
A transmission device according to any preceding claim and including means for providing a predetermined symbol at the head of said information sequence, or for providing a predetermined code sequence at the start of the coded information. sequence representative of a predetermined symbol.
A transmission device according to any preceding claim wherein the code has good auto-correlation properties or good cross-correlation rejection properties.
A transmission device according to any preceding claim wherein said modulating means is arranged to perform bi-phase modulation.
A transmission device according to any preceding claim and including means for incorporating redundancy into said information sequence for error correction.
A transmission device according to any preceding claim, including means for prefacing said modulated carrier with two pulse signals (24) of known timing relative to each other and the modulated signal.
A digital underwater receiving device adapted for responding to a signal transmitted from a transmission device of the type according to any preceding claim, the receiving device comprising means (4, 33, 34, 35) for receiving and demodulating or down-converting said signal, means for deriving from the demodulated signal a plurality of received code sequences (21′) each related to a respective one of said transmitted code sequences first correlating means (31, 32) for correlating each said received code sequence with each of said 2′ base code sequences, identification means (31, 32) for determining which of the code sequences provides a maximum likelihood result, and decoding means (31, 32) for deriving from each maximum likelihood result the corresponding decoded symbol thereby to synthesise a received information sequence (17′).
A method of transmitting a sequence (17) of digital bits representing information, comprising the steps of partitioning said sequence into a succession of blocks (19) each comprising n bits where n is equal to or greater than one, replacing each said block with a transmitted series (21) of bits consisting of or comprising a code sequence of length m bits having a predetermined correspondence with the bit or pattern of bits in the block, wherein the code is selected to have good correlation properties, and the value of m/n is at least 32, modulating a carrier with the resulting signal, and transmitting (4) an acoustic wave. signal corresponding to said modulated carrier through water.
Dependencies are inferred from the claim text.