WO2012050438A1

WO2012050438A1 - Satellite communication system for retrieving a plurality of modulated signals having a constant modulus

Info

Publication number: WO2012050438A1
Application number: PCT/NL2011/050684
Authority: WO
Inventors: Hervé PÉTER-CONTESSE; Alex FITIÉ; Wouter Jan Ubbels
Original assignee: Isis - Innovative Solutions In Space B.V.
Priority date: 2010-10-15
Filing date: 2011-10-07
Publication date: 2012-04-19
Also published as: NL2005532C2; EP2628256A1

Abstract

The invention relates to a satellite system for retrieving a plurality of modulated source signals having a constant modulus and comprises a plurality of antennas wherein each antenna is configured to receive the plurality of modulated source signals and convert the received plurality of modulated source signals in an antenna signal vector. The satellite system further comprises a combiner configured to combine a plurality of antenna signal vectors into a single signal matrix and a signal separator configured to separate the single signal matrix into a plurality of separated modulated source signals by means of a constant modulus algorithm. The invention further relates to a tracking system and a method for retrieving the plurality of modulated source signals using the satellite system according to the invention.

Description

SATELLITE COMMUNICATION SYSTEM FOR RETRIEVING A PLURALITY OF MODULATED SIGNALS HAVING A CONSTANT MODULUS

The invention relates to a satellite system, a tracking system and a method for retrieving an amount of N modulated source signals having a constant modulus.

Vessels, such as ships and aircraft, are a vital part of modern economy. They allow transportation of goods and persons all around the world. Over the last decades, vessel traffic has been increasing. Therefore, a strong need for reliable and high capacity vessel traffic systems emerged, e.g. having a high success rate of message decollision. Vessel tracking systems contribute to safety and efficiency of vessel traffic. Different types of vessel tracking systems exist, often dedicated to a certain vessel type.

An example of a vessel tracking system is the Automatic Identification System (AIS). The Automatic Identification System is a short range tracking system used on ships and on shore, for example by Vessel Traffic Services (VTS). It allows for identifying and localizing vessels by electronically exchanging ship data by means of radio communication. A range of AIS is typically 20-30 nautical miles for ship-to-ship and ship-to-shore communication. Ship data for example comprises a position, a speed, a course and an identification of a certain ship. A ship is equipped with an AIS transponder that automatically broadcasts the ship data at regular intervals via a Very High Frequency (VHF) transmitter. AIS offers two frequencies in the Marine VHF band being 161.975 MHz and 162.025 MHz. The AIS transponder uses both frequencies Successive messages are transmitted by the VHF transmitter on alternate VHF channels. The successive messages, comprising the ship data, are modulated onto an AIS signal by means of Gaussian Minimum Shift Keying (GMSK). AIS transmission timing is organized by a Self Organized Time Division Multiple Access (SO-TDMA). In a frame of 60 seconds, a VHF channel is divided in 2,250 time slots wherein each time slots has a time slot duration of 26.67 ms. During each slot a ship is allowed to transmit a ship message with a message length of 256 bits. The time slots are synchronized to Coordinated Universal Time (UTC). The AIS signal can only be received within a line of sight of the AIS transmitter. Other AIS transmitters within the line of sight can reserve time slots and avoid transmitting their ship messages in the same time slots.

Another example of a vessel tracking system is Automatic Dependent Surveillance - Broadcast (ADS-B). ADS-B is a cooperative surveillance technique primarily used in Air Traffic Control (ATC). An aircraft is equipped with an ADS-B transponder and periodically broadcasts aircraft data to ground stations and other aircrafts. Aircraft data for example comprises position, velocity and other relevant information. Acquiring the aircraft data by the aircraft depends on available sensors in the aircraft. For example, position may be acquired using a Global Navigation Satellite System (GNSS) receiver. ADS-B uses a frequency in the L-band of 1090 MHz. Aircraft messages are modulated onto a signal by means of Pulse Position Modulation (PPM). ADS-B transmission time is organized using a form of Time Division Multiple Access (TDM A).

To increase capacity of a vessel tracking system, in particular with AIS, it is proposed in the state of art to provide a satellite-based system, wherein a satellite comprises an AIS receiver. Publication PCT/US2007/070007 discloses such a space-based system for simultaneously tracking and monitoring one or more ships from any point on the Earth. The space-based system comprises a number of satellites in a certain satellite constellation. Each satellite comprises an AIS receiver and the satellites are placed in low Earth orbits (LEO). The AIS signals are strong enough to be detected by the satellites in LEO. Typical orbit altitudes for these satellites range from 600 km to 1000 km.

A drawback of this system is that due to an increased field of view (FOV) AIS signal overlaps occur. A satellite may receive AIS signals from multiple ships at once. Especially, in crowded areas this results in that AIS signals overlap with each other. Due to the increased FOV, AIS signals received by the AIS receiver originate from AIS transponders who are at large distances from each other. At these large distances the SO-TDMA is ineffective in avoiding overlap of AIS signals. The respective AIS transponders are not capable of reserving time slots as they can not see all other AIS transponders in the FOV of the AIS receiver. Therefore, AIS signals transmitted from different AIS transponders may result in overlapping ship messages in one time slot. Additionally, due to a significant increase of the line of sight between AIS transponders and the AIS receiver in a satellite, signal propagation delays may occur resulting also in AIS signals overlapping in one time slot. In other words, subjacent transmitted AIS signals also may at least partially overlap in one time slot. Overlapping signals limit the capacity of the current satellite-based systems. The overlapping signals result in that only a subset of all transmitted signals can be retrieved.

It is an object of the present invention to eliminate one or more of the

abovementioned drawbacks or to at least provide a usable alternative.

In particular, it is an object of the present invention to provide a satellite system that increases the capacity of satellite systems in retrieving modulated signals.

The above object may be achieved by the satellite system and method according to the invention.

Satellite system according to the invention is arranged for retrieving a plurality of modulated source signals having a constant modulus. The satellite system comprises: a plurality of antennas wherein each antenna is configured to receive the plurality of modulated source signals and convert the received plurality of modulated source signals in a respective antenna signal vector, a combiner configured to combine the antenna signal vectors into a single signal matrix, a signal separator configured to separate the single signal matrix into a plurality of separated modulated source signals by means of a constant modulus algorithm.

The satellite system and the plurality of antennas may be composed by multiple spacecrafts in formation flying with inter-spacecraft distances for example in the range of 100km to 10km. Each spacecraft may be equipped with a plurality of antennas and a transmitter to transmit the antenna signal vector of every antenna on this spacecraft to the others spacecrafts. The combiner may be configured to combine the antenna signal vectors of every antenna of every spacecraft into a single signal matrix.

Receiving the plurality of modulated source signals and combining the plurality of antenna signal vectors into the single signal matrix allows for applying the constant modulus algorithm. Note, that each antenna is generating an antenna signal vector that represents the plurality of modulated source signals. The plurality of modulated source signals may overlap. The constant modulus algorithm allows for separating a plurality of overlapping modulated source signals into a plurality of separated modulated source signals which are non-overlapping. The constant modulus algorithm makes use of a known structural property of the plurality of modulated source signals, namely a constant modulus. Having the satellite system able to retrieve the plurality of separated modulated source signals increases the capacity of vessel tracking systems. The satellite system according to the invention is described in more detail below. It is noted that the antenna may include suitable

amplification and/or processing electronics in order to generate the antenna signal vector.

A constant modulus algorithm is an algorithm (mathematical method) that allows to blindly separate a superposition of constant modulus signals having the same channel and arriving on an antenna array. Blind separation means that no other information than the structural property of a constant modulus signal is needed by the algorithm.

The plurality of modulated source signals, for example AIS signals, have a constant modulus. Note that during ramp up and ramp down of a modulated source signal, the amplitude or modulus of the modulated source signal may not be constant. However, this may be not relevant for applying the constant modulus algorithm as this requires a substantially constant modulus of the modulated source signal between ramp up and ramp down.

The plurality of modulated source signals have not been modulated by means of amplitude-shift keying (ASK) as this would results in a non-constant modulus. The plurality of modulated source signals has been modulated by any other form of modulation, such as phase-shift keying (PSK), in particular Gaussian Minimum Shift Keying (GMSK), frequency- shift keying (FSK). Each modulated source signal originates from a signal source randomly located on Earth within a line of sight with the satellite system and may be synchronized with Coordinated Universal Time (UTC). The signal source may for example be a ship and/or a flying aircraft and/or a terrestrial ground vehicle. Particularly, a corresponding message modulated onto a respective modulated source signal represents characteristics of a respective signal source. The plurality of modulated source signals can be up to an amount of any natural number (any positive integer) and may overlap each other. The plurality of modulated source signals comprises at least one modulated source signals. Preferably, the plurality of modulated source signals comprises at least two modulated source signals For example the plurality of modulated source signals may be 1 , 2, 3, 10 or 50 modulated source signals.

The plurality of modulated source signal may overlap as a result of an increased field of view (FOV) and signal propagations delays. The field of view (FOV) comprises all lines of sight with the satellite system. As the plurality of antennas is comprised in the satellite system, which may be in a low Earth orbit (LEO), it results in a larger FOV compared to a plurality of antennas located on a terrestrial ground or airspace. A typical orbital altitude of the satellite system may be between 600 km and 1000 km where the plurality of antennas can successfully receive the plurality of modulated signal sources originated from Earth.

For example, a first modulated source signal may be transmitting on one side of the FOV and a second modulated source signal may be transmitting on another side of the FOV. Due to a large size of the FOV the first modulated source signal transmitter may be outside a line of sight with the second modulated source signal transmitter. Therefore, they can not see each other and the respective signal source transmits it modulated source signal in a same time slot as the other modulated source signal resulting in the plurality of overlapping modulated source signals.

Additionally, the plurality of modulated source signals could overlap due to signal propagation delays. Modulated source signals transmitted from different geographically located signal sources result in different travel times between the respective signal source and the satellite system. This results in signal overlaps at the satellite system.

The plurality of overlapping modulated source signals can be received by each antenna of the plurality of antennas. Before the constant modulus algorithm is applied, the plurality of modulated sources signals are received and converted into the plurality antenna signal vector. Each antenna signal vector may be linearly combined into the single signal matrix by means of the combiner. The plurality of antennas has, for example, an amount of M antennas. Therefore, M antennas each receive the plurality of modulated source signals and generate an M amount of antenna signal vectors. The plurality of antenna signal vectors are sampled at a fixed sampling rate ( f_s ), wherein the sampling rate can be expressed in a number of samples ( N_s ) per second (s). With M amount of antennas there may be a M amount of antenna signal vectors each supplying N_s samples per second. These antenna signal vectors are combined and collected into the single signal matrix. For a period of time (for example one second, the single signal matrix has a dimension of M x N_s . Where M indicates a number of rows of the single signal matrix and N_s indicates a number of columns of the single signal matrix. The single signal matrix is denoted with X, wherein X is a linear combination of the received plurality of antenna signal vectors.

The plurality of overlapping modulated source signals can now be retrieved by the constant modulus algorithm, which will be explained below. The single signal matrix may be an input to the constant modulus algorithm. The constant modulus algorithm is a form of blind beamforming. Therefore, the constant modulus algorithm may be able to blindly separate the plurality of overlapping modulated source signals. Separation meaning that each individual modulated source signals may be retrieved. Blindly meaning that no other information than the structural property of a constant modulus is needed. The plurality of modulated source signals has, for example, an amount of N overlapping modulated source signals. The constant modulus algorithm solves the equation X = A■ S , where X is the single signal matrix and S is a separated signal matrix of which each row represents a separated modulated source signal. The separated signal matrix has a dimension of N x N_s . The matrix A is an array response matrix and has a dimension of M x N. With a given single signal matrix X the constant modulus algorithm finds the array response matrix A and the separated signal matrix S satisfying certain structural properties. When the array response matrix A has been found so-called weight vectors are given by the rows of a weight vector matrix W, where W is a pseudo-inverse of the array response matrix A. Note, that A may not be a square matrix, therefore the pseudo-inverse of A is used.

The structural property may be the constant modulus of the plurality of modulated source signals. Therefore, without loss of generality, the constant modulus of all modulated source signals can be modelled to be equal to 1. Any other value of an amplitude of one of the plurality of overlapping modulated source signal may be absorbed in the A-matrix by a proper scaling of corresponding columns of A and rows of S. Hence, the constant modulus algorithm finds a solution to a factorization problem X = A■ S with A and S having full rank and | = 1 . Lower capital letter /^' denotes the /^'-th entry of its row, where /^'=1 , ... , N. Lower capital j denotes the ;-th entry of its column, where j=^ , ... , N_s .

Note, that the structural property focuses on the S-matrix and the constant modulus. This has as further advantage that it may be more efficient in terms of computational load compared to a structural property focussing on the A-matrix. A well-known approach of blind beamforming focuses the A-matrix instead of the S-matrix. In particular, the columns of the A-matrix are (not always correctly) assumed to be vectors on an antenna array manifold, each associated to a certain direction-of-arrival. The antenna array manifold comprises the plurality of antennas. Identification of directions-of-arrivals necessitates a use of a calibrated geometry of the plurality of antennas or special predefined geometries of the plurality of antennas. This limits the choice of a suitable propagation environment. In principle, a direction of each multipath ray of a modulated source signal may be estimated. To work correctly, rays are not allowed to have identical delays. Moreover, diffuse multipath may be not allowed, which may be the case for the satellite system. For short-delay or diffuse multipath, it might be more accurate to model each column of the A-matrix as the sum of two (or many) vectors on the antenna array manifold. However, estimation of all corresponding directions may be computationally very intensive, if possible at all.

Therefore, the constant modulus algorithm may be particularly useful for a space- based system such as the satellite system according to the invention.

In an embodiment, the satellite system according to the invention further comprises a scheduler configured to generate at least one separation timing signal from a slot length value representative for a length of a signal slot of the plurality of modulated source signals, a maximum partial signal overlap value representative for a maximum partial signal overlap of the modulated source signals, and a synchronization signal which is in a synchronous relation to the signal slot of the modulated source signals, wherein the signal separator is connected to the scheduler and configured to receive the at least one separation timing signal from the scheduler and to enable the separation of the single signal matrix in synchronism with the at least one separation timing signal.

Advantage may be that capacity of the satellite system may be further increased. For the constant modulus algorithm it is preferable that it is applied to the single signal matrix where a constant signal overlap occurs. Note, that the single signal matrix represents the plurality of modulated source signal. The constant modulus algorithm is preferably applied were no shift in a number of signal overlaps occur. Due to the signal propagation delays the plurality of modulated source signals may partly overlap and subjacent transmitted modulated source signals may end up in an adjacent time slot. The maximum partial signal overlap can be determined by knowing the satellite orbital altitude. The higher the satellite orbital altitude, which may be the altitude of the satellite system above Earth ground, the larger a field of view of the satellite system, the larger a distance may be between two modulated source signal transmitters transmitted and the larger the maximum partial signal overlap is. The scheduler generates a separating timing signal such that the single signal matrix may be applied where the signal overlap is constant, meaning away from any partial overlap thus away from a maximal partial overlap located near edges of a signal slot. Therefore, less erroneous separation attempt occur. Preferably, the scheduler is configured to time the separation timing signal so as to enable by the separation timing signal a start of the separation by the separator substantially in a middle of the signal slot.. This may be an optimal location for applying the constant modulus algorithm to the single signal matrix as it may be the furthest away from the edges of the signal slot therefore ensuring a constant signal overlap. This may have as advantage that the capacity of the satellite system may be further increased as less erroneous separation attempts occur.

To ensure a start time and/or an end time of the signal slot with respect to

Coordinated Universal Time (UTC) the scheduler may be configured to receive the synchronization signal such that synchronization with UTC may be possible and the plurality of modulated source signals end up in a correct time slot. For example, the synchronization signal may be representative to a GPST reference provided by a GPS receiver.

In another embodiment of the satellite system according to the invention the satellite system further comprises a demodulator configured to demodulate a separated modulated source signal into separated source messages, wherein the scheduler is configured to generate within the time interval at least one demodulation timing signal for the demodulator, and wherein the demodulator is connected to the scheduler and configured to receive the at least one demodulation timing signal from the scheduler and to enable the demodulation of the separated modulated source signal in synchronism with the at least one demodulation timing signal.

This has as advantage that robustness of the signal separator may be increased. The signal separator applies the constant modulus algorithm to the single signal matrix when the separation timing signal is received and the signal separator generates the plurality of separated modulated source signals. Each of the separated modulated source signals can be demodulated by the demodulator to acquire the separated source message. A

successfully retrieved separated source message indicates that the signal separator applied the constant modulus algorithm correctly. The demodulator is preferably configured to check whether a message start flag and message end flag may be present in the separated source message. In a further preference, the demodulator may be configured to perform a check sum, such as a CRC check. This may be advantageous as it may offer a simple way to check if a separated source message is valid.

In another embodiment of the satellite system according to the invention the satellite system further comprises a synchronizer configured to generate the synchronization signal and to provide the synchronization signal to the scheduler, wherein the synchronizer is configured to generate a trial separation timing signal and a trial demodulation timing signal within a sliding time window, the synchronizer being arranged to slide the time window through a time interval, wherein the synchronizer is further configured to detect an occurrence of successful identified separated demodulated source messages for each sliding time window and is configured to generate the synchronization signal in accordance with the sliding time window or windows that provided a highest number of successful identified separated source messages. The synchronizer provides the trial separation timing signal to the separator, and the trial demodulation timing signal to the demodulator.

Advantage may be that no heavy or power consuming clock (i.e. clock signal generator) may be needed in the satellite system. The satellite system may not always be provided with an accurate clock for providing UTC. For example, the GPS receiver may not be available in the satellite system to save power and weight. Synchronization may be therefore hazardous. The synchronizer allows for providing the synchronization signal by means of an algorithmic approach, being the sliding time window and counting the number of successful identified separated source messages for each sliding time window. The separating and demodulation is performed for a data set that corresponds to a time window of the signals as received. In case of overlapping signals from neighbouring time slots, a result of the separating and demodulating will exhibit erroneous results. By performing a checksum (such as a Cyclic redundancy check) it may be determined whether or not such errors occur, as in the case of overlap, the resulting demodulated source message will not pass the checksum test. By determining for what time window(s) no, or as little as possible, of such erroneous results occur, it may be determined what part of a time slot is free from partial overlap and thus having a constant amount of overlaps. The separation and demodulation may hence be performed in this time window. A signal slot middle can be determined from a middle of a first window and a last window which resulted in successful identified separated source messages.

In another embodiment of the satellite system according to the invention, it further comprises a buffer configured to buffer the single signal matrix over the time interval, the time interval having a length of at least two times a length of the signal slot added with two times a maximal partial signal overlap.

Advantage may be that computational load may be decreased. The length of at least two times the length of a signal slot added with two times the maximal partial signal overlap ensures that at least one slot plus the maximal partial signal overlap may be present in the time interval and therefore the signal separator can apply the constant modulus algorithm correctly.

In another embodiment of the satellite system according to one of the preceding claims the plurality of antennas may be arranged in a predefined antenna array. The signal separator may be configured to separate the single signal matrix by utilizing polarization differences between the plurality of antennas. Advantage may be that performance may be increased due to the polarization differences. Polarization differences may occur as each modulated source signal travels a different path through the ionosphere. This results in amplitude differences between the received plurality of modulated source signals when the plurality of antennas are arranged in the predefined antenna array. As the constant modulus algorithm focuses on the structural property that the plurality of modulated source signal have constant modulus, the amplitude differences contribute to a more robust constant modulus algorithm.

In another embodiment of the satellite systems according to one of the preceding claims the plurality of antennas may be arranged in a predefined antenna array. The signal separator may be configured to separate the single signal matrix by means of a

measurement of source signal phase differences between the plurality of antennas.

Advantage may be that robustness of the constant modulus algorithm may be increased. Phase differences may occur as each modulated source signal arrives at each of the plurality of antennas with a different angle. The signal separator may be configured to perform a blind beamforming on the source signal phase differences. This means that focus may be on determining the A-matrix instead of the S-matrix. In particular, the columns of the A-matrix are assumed to be vectors on an antenna array manifold, each associated to a certain direction-of-arrival. The antenna array manifold comprises the antenna array.

Identification of directions-of-arrivals necessitates a predefined, meaning a calibrated geometry, of the plurality of antennas or special predefined geometries of the plurality of antennas. Performing this form of blind beamforming parallel to the constant modulus algorithm improves robustness.

According to the invention, the object may be also achieved by providing a tracking system.

The tracking system according to the invention comprises at least one satellite system according to one of the above embodiments. The at least one satellite system comprises a downlink signal transmitter configured to transmit the plurality of separated modulated source signals. The tracking system further comprises a ground segment. The ground segment comprises a downlink signal receiver for receiving the plurality of separated modulated source signals.

This has as advantage that the capacity of current vessel tracking system may be greatly improved. Vessels are for example but not limited to ships, aircrafts or ground based vehicles. By having at least one satellite system according to the invention greatly improves availability and coverage. The ground segment may be able to receive the plurality of separated modulated signals separated by the signal separator. Capacity may be also increased by the signal separator applying the constant modulus algorithm. In an embodiment of the tracking system according to the invention the ground segment comprises an interface configured to supply the plurality of separated modulated source signals for user processing.

This may have as advantage that a high capacity, high availability and high coverage tracking system may be available to an end user at a centralised location. This for example allows for monitoring long haul ships on seas and oceans.

In an embodiment of the tracking system according to the invention the tracking system further comprises a source signal transmitter configured to transmit one or more of the plurality of modulated source signals.

According to the invention, the object may also be achieved by a method for retrieving a plurality of modulated source signals using a satellite system having a plurality of antennas, the method comprising the steps of: receiving the plurality of modulated source signals by each of the plurality of antennas; converting the received plurality of modulated source signals in a respective antenna signal vector; combining the antenna signal vectors into a single signal matrix; separating the single signal matrix into a plurality of separated modulated source signals by means of a constant modulus algorithm.

Receiving the plurality of modulated source signals and combining the plurality of antenna signal vectors into the single signal matrix allows for applying the constant modulus algorithm. The plurality of modulated source signals may overlap. The constant modulus algorithm allows for separating a plurality of overlapping modulated source signals into a plurality of separated modulated source signals which are non-overlapping. The constant modulus algorithm makes use of a known structural property of the plurality of modulated source signals, namely a constant modulus and applies this to the single signal matrix.

Having the step of separating the single signal matrix allows for retrieval of the plurality of separated modulated source signals which may allow for an increase in the capacity of vessel tracking systems.

In an embodiment of the method according to the invention, the at least one of the plurality of modulated source signals may be an Automatic Identification System (AIS) signal.

This may have as advantage that it allows for monitoring and tracking ships.

In an embodiment of the method according to the invention, the at least one of the plurality of modulated source signals may be an Automatic Dependent Surveillance- Broadcast signal.

This may have as advantage that it allows for monitoring and tracking aircraft.

The method according to the invention may further comprise: generating at least one separation timing signal from a slot length value representative for a length of a signal slot of the plurality of modulated source signals, a maximum partial signal overlap value representative for a maximum partial signal overlap of the modulated source signals, and a synchronization signal which is in a synchronous relation to the signal slot of the modulated source signals, and enabling the separation of the single signal matrix in synchronism with the at least one separation timing signal.

Still further, the method according to the invention may comprise: demodulating a separated modulated source signal into separated source messages, generating within the time interval at least one demodulation timing signal, and enabling the demodulation of the separated modulated source signal in synchronism with the at least one demodulation timing signal

Still further, the method according to the invention may comprise: generating a trial separation timing signal and a trial demodulation timing signal within a sliding time window, sliding the time window through a time interval, detecting an occurrence of successful identified separated demodulated source messages for each sliding time window and generating the synchronization signal in accordance with the sliding time window or windows that provided a highest number of successful identified separated source messages.

With the above embodiments of the method, the same or similar effects may be achieved as with the satellite system according to the invention. Furthermore, the same or similar preferred embodiments as described with reference to the satellite system according to the invention, also apply to the method according to the invention, thereby achieving same or similar effects.

These and further embodiments of the satellite system, tracking system and method according to the invention are captured in the dependent claims.

These and other aspects, characteristics and advantages of the present invention will be explained in more detail by means of the following description of a non limiting embodiment of the satellite system, tracking system and method according to the invention, in which identical reference numerals denote identical components, and in which:

fig. 1 shows an overview of a tracking system, a satellite system, a ground segment and a source signal transmitter;

fig. 2 shows a high level architecture of a plurality of antennas, a combiner and a signal separator;

fig. 3 shows a detailed architecture of the satellite system according to a first embodiment of the invention;

fig. 4 shows an overview of a plurality of modulated source signals and a maximal partial signal overlap; fig. 5 shows an overview of the plurality of modulated source signal being a plurality of overlapping modulated source signals including partial signal overlaps and a time interval of a single signal matrix to be separated by the signal separator; and

fig. 6 shows an architecture of a satellite system for separating a plurality of overlapping modulated source signals transmitted over a dual channel.

Fig. 1 shows a general overview of a satellite system 1 for retrieving a plurality of modulated source signals 3. The plurality of modulated source signals 3 are Automatic Identification System (AIS) modulated source signals. The plurality of modulated source signals 3 shown comprises in this embodiment three AIS modulated source signals 3a, 3b, 3c each transmitted from a source signal transmitter 5a, 5b, 5c. The source signal transmitter 5a, 5b, 5c is an AIS transponder. Also a ground segment 10 is shown which is able to receive a plurality of separated modulated source signals 7 from the satellite system 1. The satellite system 1 comprises a plurality of antennas 6, a combiner 1 1 and a signal separator 2 which allow a separation of the plurality of modulated source signals 3 into the plurality of separated modulated source signals 7. The combiner 1 1 and signal separator 2 are shown in the high level architecture of fig. 2.

Note, that AIS uses two frequencies in the Marine VHF band: 161.975 MHz and 162.025 MHz. The source signal transmitters 5a, 5b, 5c, are located on a ship 4a, 4b, 4c and each of the source signal transmitters 5a, 5b, 5c transmits one of the AIS modulated source signals 3a, 3b, 3c using both frequencies resulting in successive source messages being transmitted on alternating channels. The source messages are modulated onto one of the AIS modulated source signals 3a, 3b, 3c by means of Gaussian Minimum Shift Keying (GMSK) and transmission timing is organized by means of Self Organized Time Division Multiple Access (SO-TDMA).

A timeline is divided in signal slots. A frame covers a time span of 60 seconds and comprises 2550 signal slots. Each signal slot therefore has a duration of 26.67 ms. During each signal slot the source signal transmitter 5a, 5b, 5c is allowed to transmit a 256 bit source signal message. Note, that one source signal message has a length approximately equal to one signal slot. A frame start is synchronized with Coordinated Universal Time (UTC) or Global Positioning System (GPS) time minutes.

Now, referring to fig. 2, the plurality of antennas are shown. In this first embodiment the plurality of antennas comprises four antennas 6w, 6x, 6y, 6z. Each antenna 6w, 6x, 6y, 6z receives the plurality of modulated source signals 3 and converts them into a respective antenna signal vector 12w, 12x, 12y, 12z. As there are four antennas 6w, 6x, 6y, 6z a plurality of antenna signal vectors 12 comprises four antenna signal vectors 12w, 12x, 12y, 12z. The combiner 1 1 combines the plurality of antenna signal vectors 12 into one single signal matrix 9 that is suitable to be processed by the signal separator 2. The signal separator 2 is configured to receive the single signal matrix 9 and is able to retrieve and separate the single signal matrix 9 into a plurality of separated source signals 7. The plurality of separated source signals in this embodiment comprises three separated source signals 7a, 7b, 7c and correspond to the plurality of modulated source signals 3a, 3b, 3c transmitted by the source signal transmitters 5a, 5b, 5c.

In fig. 3, a more detailed architecture of the satellite system 1 is shown. The satellite system further comprises a buffer 14 and a scheduler 19. The signal separator 2 applies a constant modulus algorithm to generate the plurality of separated modulated source signals 7. Therefore, it is necessary that the plurality of modulated source signals 3 have a constant overlap, resulting in that no amplitude changes occur. For example, a signal overlap may occur when a first modulated source signal 3a is transmitting from a first Earth location and a second modulated source signal 3b is transmitting from a second Earth location in a same signal slot. This is the case when the first Earth location is out of a line of sight with the second Earth location such that the SO-TDMA is ineffective. A field of view (FOV) of the satellite system 1 is sufficient large to receive both transmitted modulated source signals 3a, 3b resulting in a signal overlap. Additionally, the plurality of modulated source signals 3a, 3b, 3c may overlap due to signal propagation delays. This results in partial signal overlaps between subjacent signal slots.

A maximum partial signal overlap can be calculated from an orbital altitude value, wherein the orbital altitude value represents the orbital altitude of the satellite system 1. Typical values for the orbital altitude are 600 km to 1000 km which corresponds to Low Earth Orbits (LEO). Transmitted AIS modulated source signals 3a, 3b, 3c can be received by satellites in LEO. The maximum partial signal overlap can be calculated from a minimal travel time and a maximal travel time, respectively a middle of the FOV and an edge of the FOV. The difference between the minimal travel time and the maximal travel time is approximately 9 ms for a 1000 km orbital altitude. As the signal slot duration is 26.7 ms, this means that at least one third of the signal slot duration is not subjected to partial overlaps for an orbital altitude smaller than 1000 km. This is also illustrated in fig. 4 where the maximal partial signal overlap is shown for 5 modulated source signals. To ensure that a constant signal overlap is present one should stay away from the maximal partial signal overlaps that occur on both edges of a signal slot being a signal slot start and a signal slot end.

Therefore, a scheduler is shown in fig. 3 to generate a first separation timing signal 30 for the signal separator 2. The separation timing signal 30 determines a CMA buffer over which the signal separator 2 can successfully apply the constant modulus algorithm. In this embodiment use is made of a time interval of the single signal matrix 21 shown in fig. 5. The time interval of the single signal matrix 21 also called buffer comprises a buffered single signal matrix with a length of three signal slots referred to as a first slot buffer 32, a second slot buffer 33 and a third slot buffer 34. Buffered meaning stored in the buffer 14 for processing. Shown in fig. 5 are a first CMA buffer 42, a second CMA buffer 43 and a third CMA buffer 44 with their middle coinciding with a middle of respectively the first slot buffer 32, the second slot buffer 33 and the third slot buffer 34. This ensures a constant overlap of the modulated source signals.

The signal scheduler 21 keeps track of the buffer by means of a circular buffer pointer which runs through the buffer. Whenever the buffer pointer exceeds a slot buffer middle augmented with half the length of the CMA buffer a first separation timing signal 30 is generated by the scheduler 19 such that the signal separator 2 shall apply the constant modulus algorithm on the corresponding CMA buffer. This results in the signal separator 2 to generate the plurality of separated modulated source signals 7.

In this embodiment the separated modulated source signals 7 are demodulated by means of the demodulator 13 as shown in fig. 3. Each of the separated modulated source signals 7a, 7b, 7c is demodulated by a respective first sub-demodulator 13a, second sub- demodulator 13b and third sub-demodulator 13c. This results in a plurality of separated source messages 15 which were modulated on the plurality of modulated source signals 3. The scheduler 19 is configured to generate a first demodulation timing signals 40 to determine when and what is demodulated. In this embodiment, whenever the buffer pointer exceeds a slot buffer middle augmented with half the length of the maximal partial overlap a first demodulation timing signal 40 is sent to the demodulator 13.

In this embodiment the satellite system 1 also comprises a synchronizer 17 as shown in fig. 3. Due to the random nature of the modulated source signal overlaps around a synchronized signal slot the signal slot starts and signal slot ends are determined by the scheduler 19. This knowledge allows for storing the single signal matrix 9 into the slot buffers 32, 33, 34 correctly with respect to time. The scheduler 19 is configured to receive synchronization signals 18 for this purpose. Synchronization signals 18 may for example be timing data provided by an accurate UTC clock or a GPS receiver. In this embodiment the synchronization signals 18 are provided by the synchronizer 17.

The synchronizer 17 is configured to generate a trial separation timing signal 31 and a trial demodulation timing signal 41 within a sliding time window arranged by the

synchronizer 17 to slide through a subset of the time interval of the single signal matrix 21. In this embodiment the length of the subset is equal to the length of two times a signal slot augmented with two times the length of the maximal partial signal overlap. This ensures that at least one signal slot augmented with partial signal overlaps is present for the window to slide through. The sliding window itself has a length of one signal slot augmented with two times the length of the maximal partial signal overlap. Note that the length can be both expressed in bits or in ms. For each window the signal separator 2 applies the constant modulus algorithm. Therefore the trial separation timing signal 31 is send to the signal separator 2. The synchronizer 17 is also configured to generate the trial demodulation timing signal 41 such that possible separated modulated source signals in the window are demodulated into separated source messages. The synchronizer 17 is configured to count a number of successful identified separated source messages for each sliding time window and is configured to generate the synchronization signal depending on the number of successful identified separated source messages. Preferably, the synchronizer 17 checks if an identified separated source message has a proper checksum. This is performed for each window that slides with a predefined number of steps through the subset of the time interval of the single signal matrix. The synchronization signal is determined from a middle of the first and last windows which resulted in successful received messages. This is

representative for the signal slot middle and can be used by the scheduler 19 to schedule its first separator timings signals 30 and its first demodulation timing signals 40. Once the first separator timing signal 30 and the first demodulator timing signal 40 have been generated by the scheduler using the first synchronization signal 18 provided by the synchronizer, the synchronizer may use the average position of the start flags of the successful identified separated source messages from the demodulator to compensate for clock drift over time and thus may provide updated synchronization signals 18 to the scheduler 19 over time.

The signal separator 2 comprises a rank estimator 24 and a beamformer 25 as shown in fig. 3. The rank estimator 24 and the beamformer 25 are configured to receive a time interval of the single signal matrix 21 . The scheduler 19 determines the length of this time interval or in other words what part of the buffer needs to be separated. This can be the CMA buffer, but can also be a time interval corresponding with the sliding window.

The plurality of antenna signal vectors 12 are sampled at a fixed sampling rate ( f_s ), wherein the sampling rate can be expressed in a number of samples ( N_s ) per second (s). With an amount of for example 4 antennas 6w, 6x, 6y, 6z there is an amount of 4 antenna signal vectors 12w, 12x, 12y, 12z each supplying N_s samples per second. These antenna signal vectors 12w, 12x, 12y, 12z are combined and collected into the single signal matrix 9. The single signal matrix has a dimension of 4 x N_s . Where a number of rows of the single signal matrix and N_s indicates a number of columns of the single signal matrix 9. The single signal matrix 9 can mathematically be denoted with X, wherein X is a linear combination of the received plurality of antenna signal vectors which can mathematically be denoted with Xi , x₂, X3 and x₄ respectively. See also fig. 2. Note, that a bit rate of the AIS modulated source signals is 9600 bits per second. The number of sample (N_s ) per second (s) shall be higher than the bit rate of the AIS modulated source signals of 9600 bps.

The plurality of modulated source signals 3 have an amount of N constant overlapping modulated source signals. As seen in fig. 4, the number of constant overlapping modulated source signals 3a, 3b, 3c in the left signal slot is three. The constant modulus algorithm solves the equation X = A■ S , where X is the single signal matrix and S is a separated signal matrix of which each row represents a separated modulated source signal 7a, 7b, 7c. The separated signal matrix has a dimension of 3 x N_s . The matrix A is an array response matrix and has a dimension of 4 x 3 in this embodiment. With a given single signal matrix X the constant modulus algorithm finds the array response matrix A and the separated signal matrix S satisfying the structural property that the modulated source signals have a constant modulus. When the array response matrix A has been found so-called weight vectors or beamforming vectors are given by the rows of a weight vector matrix W, where W is a pseudo-inverse of the array response matrix A.

Without loss of generality, the constant modulus of all modulated source signals 3a,

3b, 3c are modelled to be equal to 1. Any other value of an amplitude of one of the plurality of overlapping modulated source signal is absorbed in the A-matrix by a proper scaling of corresponding columns of A and rows of S. Hence, the constant modulus algorithm finds a solution to a factorization problem X = A^■ S + N_noise with A and S having full rank and

= 1 . Lower capital letter /^' denotes the /^'-th entry of its row, where /^'=1 , ... , N. Lower capital j denotes the ;-th entry of its column, where j=^ , ... , N_s .Note, that N_noise represents added noise to the data.

This is solved by a so-called analytical constant modulus algorithm. The application of the analytical constant modulus algorithm comprises four steps.

The first step is estimating a number of rows of S. In other words determining the rank of X or the number of separated modulated source signals 50. This done by the rank estimator 24, which for example is done by means of a singular value decomposition. Here, the rank is equal to the number of singular values of X that are significantly larger than zero. It is noted that the rank estimation step may be omitted. The second step is applying a constant modulus condition to a matrix forming an orthonormal basis of the row span of X. This matrix can be obtained for example by means of a singular value decomposition of X. The thirds step is solving a simultaneous diagonalization problem. And the fourth step is recovering the separated modulated source signals by first calculating the beamforming vectors followed by calculating the rows of S. The second, third and fourth step are performed by the beamformer 25. Fig. 6 shows a second embodiment of a satellite system. This second embodiment has similar elements compared to the first embodiment however is provided with dual channels. Shown is a first combiner 1 1 a and a first signal separator 2a. The first combiner 11 a and the first signal separator 2a are respectively identical to the combiner 1 1 and the signal separator 2 of the first embodiment. Further shown is a second combiner 1 1 b and a second signal separator 2b. Again these are respectively identical to the combiner 1 1 and the signal separator 2. The first combiner 1 1a and the first signal separator 2a are configured to process AIS modulated source signals having a frequency of 161.975 MHz referred to as AIS1 signals. The second combiner 1 1 b and the second signal separator 2b are configured to process AIS modulated source signals having a frequency of 162.025 MHz referred to as AIS2 signals. Similar to the first embodiment, the plurality of separated modulated source signals are demodulated by the sub-demodulators of a demodulator 13 for every separated modulated source signal.

In this second embodiment a plurality of antennas 106 comprises three antennas (the figure of three antennas is an example only). The three antennas receive a plurality of modulated source signals 3 comprising of three AIS modulated source signals 3a, 3b, 3c. Each antenna generates an antenna signal vector which is received by a frontend 60. The frontend 60 downshifts a carrier frequency from 161.975 MHz and 162.025 MHz to and intermediate AIS frequency of -25 kHz and +25 kHz and provides a digital representation (with a certain word length) of an inphase and quadrature component of the different antenna signal vectors. Noise power may for example have a value of -125 dBm. Maximal received signal power at an orbital altitude below 1000 km is approximately -85 dBm, which results in a maximal signal to noise ratio of 40 dB.

The frontend 60 supplies an intermediate AIS frequency signal to the decimator, baseband converter and channel splitter 70. Channel splitting and baseband conversion are performed in one single step. A complex multiplier with a sinusoidal signal having a frequency of 25kHz and its counter phase representation, allow to remove the intermediate carrier frequencies such that baseband channels are obtained. A filter is applied to fillter away undesired remaining channel image which is then located at 50kHz. A maximal Doppler frequency of 4kHz is expected at an altitude of 600km. A filter cut-off frequency filter away signal components with frequencies exceeding the maximal AIS signal bandwidth augmented with an offset of 4kHz. A passband is designed to have a minimal amplitude and phase change, such that beamforming coefficients remain constant over the baseband signal bandwidth. The decimator, baseband converter and channel splitter 70 provides an antenna signal vector for each channel having a baseband frequency.

In the described embodiments, the satellite system is suitable for retrieving a plurality of AIS modulated source signal. However, in a further embodiment or an alternative embodiment the satellite system is also suitable for retrieving other types of modulated source signals, such as Automatic Dependent System-Broadcast (ADS-B) signals. Typically, these modulated source signals have different carrier frequencies, modulation types and signal slot length. These modulated source signals have in common that time

synchronization is in a form of Time Division Multiple Access (TDMA) and modulation is in a form of constant modulus modulation, thereby excluding a form of Amplitude Shift Keying (ASK). TDMA is mandatory for the scheduler to work correctly and a constant modulus is mandatory for the signal separator to work correctly. Other properties of the modulated source signal may however differ. For example, the carrier frequency and signal slot length may differ and the scheduler and the signal separator are configured to process these modulated source signals.

It will be understood that some of the elements of the satellite system, such as the separator, scheduler, combiner, etc may be implemented partially or fully in a form of software executable on a processing device such as a microcontroller.

Claims

C L A I M S

Satellite system for retrieving a plurality of modulated source signals having a constant modulus, comprising:

a plurality of antennas wherein each antenna is configured to receive the plurality of modulated source signals and convert the received plurality of modulated source signals in a respective antenna signal vector;

a combiner configured to combine the antenna signal vectors into a single signal matrix;

a signal separator configured to separate the single signal matrix into a plurality of separated modulated source signals by means of a constant modulus algorithm.

Satellite system according to claim 1 , further comprising

a scheduler configured to generate at least one separation timing signal from a slot length value representative for a length of a signal slot of the plurality of modulated source signals,

a maximum partial signal overlap value representative for a maximum partial signal overlap of the modulated source signals, and

a synchronization signal which is in a synchronous relation to the signal slot of the modulated source signals, and

- wherein the signal separator is connected to the scheduler and configured to receive the at least one separation timing signal from the scheduler and to enable the separation of the single signal matrix in synchronism with the at least one separation timing signal.

Satellite system according to claim 2, further comprising:

a demodulator configured to demodulate a separated modulated source signal into separated source messages,

- wherein the scheduler is configured to generate within the time interval at least one demodulation timing signal for the demodulator, and

- wherein the demodulator is connected to the scheduler and configured to

receive the at least one demodulation timing signal from the scheduler and to enable the demodulation of the separated modulated source signal in synchronism with the at least one demodulation timing signal

4. Satellite system according to claim 3, wherein the satellite system further comprises:

a synchronizer configured to generate the synchronization signal and to provide the synchronization signal to the scheduler, wherein the synchronizer is configured to generate a trial separation timing signal and a trial demodulation timing signal within a sliding time window, the synchronizer being arranged to slide the time window through a time interval, wherein the synchronizer is further configured to detect an occurrence of successful identified separated demodulated source messages for each sliding time window and is configured to generate the synchronization signal in accordance with the sliding time window that provided a highest number of successful identified separated source messages.

5. Satellite system according to one of the claims 2-4, wherein the scheduler is configured to time the separation timing signal so as to enable by the separation timing signal a start of the separation by the separator substantially in a middle of the signal slot.

6. Satellite system according to one of the claims 2-5, further comprising a buffer configured to buffer the single signal matrix over the time interval, the time interval having a length of at least two times a length of the signal slot added with two times a maximal partial signal overlap.

7. Satellite systems according to one of the preceding claims, wherein the plurality of antennas is arranged in a predefined antenna array, the signal separator is configured to separate the single signal matrix by means of a measurement of polarization differences between the plurality of antennas.

8. Satellite systems according to one of the preceding claims, wherein the plurality of antennas is arranged in a predefined antenna array and the signal separator is configured to separate the single signal matrix by means of a measurement of source signal phase differences between the plurality of antennas.

9. Tracking system comprising;

at least one satellite system according to one of the claims 1-8, wherein the at least one satellite system comprises a downlink signal transmitter configured to transmit the plurality of separated modulated source signals; a ground segment, comprising a downlink signal receiver for receiving the plurality of separated modulated source signals.

10. Tracking system according to claim 9, wherein the ground segment comprises an interface configured to supply the plurality of separated modulated source signals for user processing.

1 1. Tracking system according to one of the claims 9-10, further comprising a source signal transmitter configured to transmit one or more of the plurality of modulated source signals.

12. Method for retrieving a plurality of modulated source signals using a satellite system having a plurality of antennas, comprising the steps of:

receiving the plurality of modulated source signals by each of the plurality of antennas;

converting the received plurality of modulated source signals in a respective antenna signal vector;

combining the antenna signal vectors into a single signal matrix;

separating the single signal matrix into a plurality of separated modulated source signals by means of a constant modulus algorithm.

13. Method for retrieving a plurality of modulated source signals according to claim 12, wherein at least one of the plurality of modulated source signals is an Automatic Identification System (AIS) signal.

14. Method for retrieving a plurality of modulated source signals according to one of the claims 12-13, wherein at least one of the plurality of modulated source signals is an Automatic Dependent Surveillance-Broadcast signal. 15. The method according to any of claims 12 - 14, further comprising:

generating at least one separation timing signal from a slot length value representative for a length of a signal slot of the plurality of modulated source signals, a maximum partial signal overlap value representative for a maximum partial signal overlap of the modulated source signals, and a synchronization signal which is in a synchronous relation to the signal slot of the modulated source signals, and enabling the separation of the single signal matrix in synchronism with the at least one separation timing signal.

16. The method according to claim 15, further comprising:

demodulating a separated modulated source signal into separated source messages, generating within the time interval at least one demodulation timing signal, and enabling the demodulation of the separated modulated source signal in synchronism with the at least one demodulation timing signal

The method according to claim 16, further comprising:

generating a trial separation timing signal and a trial demodulation timing signal within a sliding time window, sliding the time window through a time interval, detecting an occurrence of successful identified separated demodulated source messages for each sliding time window and generating the synchronization signal in accordance with the sliding time window or windows that provided a highest number of successful identified separated source messages.

Method according to any of claims 12 - 17, wherein the satellite system comprises a satellite system according to any of claims 1 - 8.