Automatic identification system receiver and satellite payload comprising the same
An Automatic Identification System—AIS—receiver comprising at least one processing section (PS 1 , PS 2 ) for synchronizing, demodulating and detecting AIS messages contained in a received signal, said processing steps being carried out separately for a plurality of frequency sub-bands (SB 1 , SB 2 , SB 3 ) spanning an AIS channel (CH 1 , CH 2 ); the receiver being characterized in that: said sub-bands overlap with each others; and said or each processing section is adapted for synchronizing, demodulating and detecting said AIS messages within each sub-band on the basis of timing error and carrier frequency estimations obtained from filtered replicas of said received signal, propagating along respective auxiliary signal paths.
1. An Automatic Identification System (AIS) receiver comprising at least one processing section for demodulating and detecting AIS messages contained in a received signal propagating along a main signal path,
wherein the receiver is configured to carry processing steps separately for a plurality of frequency sub-bands spanning an AIS channel, said sub-bands overlapping with each others; and in that
said or each processing section is configured for demodulating and detecting said AIS messages within each sub-band on the basis of estimations of the timing and carrier frequency of said received AIS signal, said estimation being obtained from filtered replicas of said received signal, propagating along respective auxiliary signal paths parallel to said main signal path.
2. The Automatic Identification System (AIS) receiver according to claim 1 wherein said processing section comprises a bank of parallel demodulators, each of said demodulators being configured for demodulating and detecting said AIS messages within one respective sub-band.
3. The Automatic Identification System (AIS) receiver according to claim 1 wherein said processing section comprises a single demodulator configured for demodulating and detecting said AIS messages successively for a plurality of said sub-bands.
4. The Automatic Identification System (AIS) receiver according to claim 2 , wherein said or each demodulator comprises a synchronization block having an auxiliary signal path parallel to a main signal path, wherein:
said auxiliary signal path comprises a filter for extracting one of said sub-bands from a replica of the signal to be processed; and processing means for obtaining timing and frequency estimations from the filtered replica;
said main signal path comprises means for synchronizing and converting to base-band the signal to be processed by using said timing and frequency estimations;
and wherein no filter for isolating one said sub-band is provided on said main signal path.
5. The Automatic Identification System (AIS) receiver according to claim 1 , wherein each of said sub-bands has a bandwidth equivalent to the bit-rate of said AIS messages, plus or minus 5%.
6. The Automatic Identification System (AIS) receiver according to claim 5 , wherein the distance between the centers of two adjacent sub-bands is 0.3 times the bit-rate of said AIS messages, plus or minus 10%.
7. The Automatic Identification System (AIS) receiver according to claim 6 , wherein three sub-bands span the whole AIS channel.
8. The Automatic Identification System (AIS) receiver according to claim 1 , wherein the overlap of two adjacent sub-bands is at least equal to the separation of their central frequencies.
9. The Automatic Identification System (AIS) receiver according to claim 1 , wherein said synchronization block is configured to obtain said timing and frequency estimations from an AIS message having Continuous Phase Modulation by a non-data-aided feed-forward algorithm.
10. The Automatic Identification System (AIS) receiver according to claim 1 wherein each or said demodulator comprises a detection block for detecting demodulated AIS messages by a non-coherent sequence detection algorithm.
11. The Automatic Identification System (AIS) receiver according to claim 1 wherein each or said demodulator further comprises a frame synchronizing block for determining the timing of a TDMA frame of a detected AIS message by performing sliding-window cyclic redundancy check.
12. The Automatic Identification System (AIS) receiver according to claim 1 , further comprising a message parser (MP) for identifying AIS messages which have been correctly detected in more than one sub-band and discarding replicas thereof.
13. The Automatic Identification System (AIS) receiver according to claim 1 further comprising:
an input buffer for storing a set of samples of said received signal at the input of said processing section;
a re-modulating block for re-modulating the detected AIS messages with the help of available timing and carrier frequency estimation; and
interference cancellation means for subtracting said re-modulated AIS messages from the samples stored in said input buffer.
14. The Automatic Identification System (AIS) receiver according to claim 13 wherein said re-modulating block comprises means for performing data-aided fine frequency estimation.
15. The Automatic Identification System (AIS) receiver according to claim 1 comprising two said processing sections for demodulating and detecting AIS messages contained in a first and a second AIS channels.
16. The Automatic Identification System (AIS) receiver according to claim 1 further comprising a front-end section for receiving, pre-processing and digitizing a signal comprising a plurality of AIS messages, and for providing said digitized signal to said processing section.
17. A satellite telecommunication payload comprising an Automatic Identification System (AIS) receiver according to claim 16 , said payload being adapted for receiving and detecting AIS messages generated by uncoordinated AIS emitters spread over an area having a radius greater or equal than 1,000 nautical miles.