FREQUENCY REFERENCES FROM A PRIMARY STATION TO A

PLURALITY OF SECONDARY STATIONS

The present invention refers to a method and a system for distributing time and frequency references from a Primary Station to a plurality of Secondary Stations , using a single Geostationary Satellite with a signal in DVB-S , DVB-S2 or DVB-S2X standard .

In broadcast television broadcasts using satellite distribution, at least one Primary Station transmits a stream containing data, programs and/or services . Said flow is received by a plurality of Secondary Stations , located throughout the territory, which in turn feed the transmitters of the DVB-T/T2 signal to the end users . In order to optimi ze the occupation of the spectrum, the SFN ( Single Frequency Network) methodology is used . This requires DVB-T/T2 transmitters to be synchroni zed in time and frequency . Similarly, other users such as wind and photovoltaic parks, as well as power plants and financial markets, require very precise time and frequency samples.

The current state of the art uses the GNSS network to distribute these samples, or, if the Secondary Stations are connected to an Ethernet network, and this supports the IEEE 1558 protocol, this protocol can be used for synchronization.

The synchronization system using GNSS is not without its drawbacks.

First, the GNSS system can be easily interfered with, voluntarily or not.

Furthermore, these systems depend entirely on each proprietary government which does not give guarantees of continuity of service to other users.

This implies the need to have a synchronization signal in each site (common time and frequency reference) to synchronize all the devices that require it.

In practice, it is evident that it is necessary to have Secondary Stations that are synchronous with each other and able to compensate for the different signal propagation times due to their different geographical location. Patent EP2493094B1 describes a method for synchroni zing at least one Primary Station with one or more Secondary Stations , the method including the following steps : sending a Time Reference signal from a Primary Station to a broadcast distribution Satellite ;

- receiving by said Primary Station said Time Reference Signal retransmitted by said Satellite ; receiving, by said one or more Secondary Stations , said Time Reference Signal retransmitted by the Satellite ;

- modi fying the local time speed in such a way as to synchroni ze the transmission of all signals transmitted by said one or more Secondary Stations to other users with the Time Reference Signal initially transmitted by the Primary Station .

The method according to patent EP2493094B1 solves the problem posed, however it has a drawback in that the time reference is not sent regularly, but is randomly inserted into a series of signal packets (packet multiplexing) . In practice , the Primary Station transmits the payload organi zed as a sequence of packets ( BBI, ₂ , . . . , _N ) , each identi fied by means of a CRC32 ₁ , ₂ , . . . , _N , containing the television programs , and the synchroni zation signal is an Info packet randomly inserted between said packets .

The randomness of the transmission is due to the fact that the time reference cannot be inserted regularly in the sequence of packets containing the television programs , because said insertion is carried out with a Remultiplexer which necessarily alters the temporal position of the packets , between when they are received and when they are re- transmit ted .

Since it is necessary to continuously repeat the sending of the synchroni zation signal to accurately correct the secondary frequency reference , the randomness of the insertion of the synchroni zation signal in the payload packet stream makes the algorithm for updating the time reference of the Secondary Stations complex and slow .

Obj ect of the present invention is providing a system and a method for synchroni zing time and frequency sources , particularly for video data transmissions , suitable for regularly updating the time references of the Secondary Stations .

Another obj ect of the present invention is providing a method for synchroni zing time and frequency sources which uses alternative sources to the GNSS and which is capable of guaranteeing continuous operation regardless of any mal function or degradation, voluntary or involuntary, of the GNSS system .

An innovation and an aim of the present invention is the possibility of using a Geostationary Satellite as a synchroni zation element , but without it being necessary for the information necessary for synchroni zation to pass over it , i f there are other communication channels with the Secondary Stations .

Not least obj ect of the present invention is providing a method for synchroni zing time and frequency sources which is highly reliable , relatively simple to manufacture and at low costs .

These and other obj ects , which will be better defined hereinafter, are achieved by a system and a method for synchroni zing a Primary Station with a plurality of Secondary Stations , according to their respective independent claims .

The system for the synchroni zation of a Primary Station with one or more secondary Stations , in which said Primary Station transmits television programs (payl oad) by sending them continuously m the form of signal packets (BB ₁ , ₂ ,..., _n ) , identified through the correspondents CRC321 , 2 , . . . , n, to a satellite that retransmits them to the ground stations, or called Primary Station and one or more Secondary Stations, is characterized in that:

(a) said Primary Station includes:

- a multiplexer capable of inserting Info packets into a Main Data Flow, generating a Main Data Flow with Info; a DVB-S/S2 (x) Modulator, suitable for transmitting said Main Data Flow with Info, sending it continuously to the Satellite;

- a Primary Reference, comprising a Primary Counter and a Primary Reference Oscillator designed to generate a Primary Time and Frequency Reference;

- a Primary Packet Processor, capable of generating said Info Packets on the basis of the arrival times at the Primary Station of said packets (BB ₁ , ₂ , ... , _n ) and related CRC32 _1,2 , ... , _n calculated on any packets temporally equidistant; a DVB-S/S2 (x) demodulator, suitable for receiving said Main Data Flow with Info, transmitted by the Satellite and for demodulating it ; (b) said one or more Secondary Stations include : a Demodulator designed to demodulate the Main Data Flow with Info received from the Satellite ; a Secondary Packet Processor, capable of calculating the CRC321 , 2 , . . . , n of the packets of the Flow and detecting the instant of arrival of the packets of said Flow, said instant of arrival being detected with respect to a Secondary Reference which includes a Secondary Counter and a Secondary Reference Oscillator designed to generate a Secondary Time and Frequency Reference;

- a Memory, suitable for storing the values CRC32 _1,2 , ... , _n of the packets (BB ₁ , ₂ ,..., _n ) of the Main Data Flow with Info and the instant of arrival (AT1 _{1,2, . .. ,n} ) of each of them.

The method to synchronize a Primary Station with one or more Secondary Stations, in which said Primary Station transmits television programs (payload) by continuously sending them in the form of packets (BB ₁ , ₂ ,..., _n ) of signals to a satellite which retransmits them to the ground stations, i.e. the Primary Station itself and the various Secondary Stations, is characterized in that said Info packets are sent regularly to the Secondary Stations , in any mode , or inserted in the Main Data Flow or via a line telephone or other communication channel such as , for example , a radiomodem or the Internet .

Information contained in the Info package are used to synchroni ze by means of a Time Shi ft of the Secondary References of the Secondary Stations and a Frequency Correction of the Secondary References of the Secondary Stations .

Preferred embodiments and non-trivial variants of the present invention form the subj ect matter of the dependent claims .

As will become clear later in the description, the fact that any packet of the payload, identi fied in subsequent processes by its CRC32 _{( n )} can be used as a time reference , means that it is not essential to enter any information in the traf fic to the Satellite , but that any other communication channel , for example a telephone line , a radiomodem or the Internet network, can be used to describe the temporal position of the packet used as a reference and identi fied with its CRC32 . In other words , this means that the satellite transmission of the information necessary for the functioning o f the system is a mere matter of convenience . Furthermore , the packets chosen as a time reference can be selected with regular intervals , not dependent on the randomness that is intrinsic to the packet multiplexer of the Remultiplexer .

The lock time and response time to external events such as temperature changes or satellite movements depend on the frequency with which time samples are transmitted . A greater number of time samples transmitted implies a higher bandwidth occupation, while a regular transmission of the reference packets simpli fies the latching algorithm . Furthermore , the possibility of transmitting Info packet data on a transmission channel that is not that of the reference packets , does not need a precise time constraint , does not have the costs of a satellite transmission and makes the present solution adaptable toneeds and inexpensive .

It is understood that all attached claims form an integral part of the present description .

It will be immediately obvious that innumerable variations and modi fications can be made to what has been described, for example relating to the method of identi fication of the _p ackets CRC32I, ₂ , ... ,nr to the contents of the Info package, with equivalent functions without departing from the protection field of the invention as appears from the attached claims.

The present invention will be better described by a preferred embodiment, provided by way of nonlimiting example, with reference to the attached drawings, in which:

Fig. 1 shows the functional diagram of a Primary Station/Secondary Station system according to the present invention;

- Figs. 2 (a, b) show the internal functions of the Primary and Secondary Frequency/Time References ;

Fig. 3 shows the timing diagram of the synchronization of the Primary Station with the Secondary Stations.

With reference to Fig. 1, (1) designates a satellite telecommunications system in which a Primary Station (M14) transmits television programs (payload) , or a Main Data Flow (M6) organized as a sequence of packets (BB ₁ , ₂ ,..., _n ) , each identified through its own CRC32 _1,2 , ... , _n , to a Geostationary Satellite (Ml) which retransmits them to a plurality of Secondary Stations (M13 _{1,2, . . . ,N} ) , and towards the Primary Station (M14) itself.

The synchronization of the Secondary Stations (M13 _{1,2, . . . ,N} ) with the Primary Station (M14) takes place by sending to the Secondary Stations (M13 _{1,2, . . . ,N} ) Info packets (M2 _{1,2, . . . ,N} ) , containing all information necessary for synchronization.

In accordance with the present invention, the sending of said Info packets (M2 _{1,2, . . . ,N} ) is carried out regularly, so as not to make the algorithm for updating the time reference of the Secondary Stations complex and slow.

According to the embodiment described, said Info packets (M2 _{1,2, . . . ,n} ) are inserted in the Main Data Flow (M6) . Alternatively, they can be sent via a telephone line, or a radiomodem or the Internet, or another communication channel.

The Primary Station (M14) includes: a multiplexer (M20) able to insert said Info packets (M2 _{1,2, . . . ,n} ) in the Main Data Flow (M6) , generating a Main Data Flow with Info (M9) ;

- a DVB-S/S2 (x) Modulator (M3) , able to carry out the transmission of said Main Data Flow with Info (M9) sending it continuously to the Satellite (Ml) ;

- a Primary Reference (M4) , comprising a Primary Counter (PIO) and a Primary Reference Oscillator (Pll) (Fig.2a) designed to generate a Primary Time Reference (P12) (for example one pulse per second - 1PPS) and a Frequency Reference (P13) (for example 10 MHz) ; a Primary Packet Processor (M5) , capable of generating said Info Packets (M2 _{1,2, . . . ,n} ) on the basis of the arrival times (MO1 _{1,2, . . . ,n} ) at the Primary Station (M14) of said packets (BBi _;2 ,..., _n ) and related CRC32 _1,2 , ... , _n calculated on any temporally equidistant packets (BB _(n) ) ; a DVB-S/S2 (x) demodulator (M7) , adapted to receive said Main Data Flow with Info (M9) , transmitted by the Satellite (Ml) and to demodulate it .

The Secondary Stations (M13 ₁ , ₂ , ... , _n ) include:

- a Demodulator (M8) suitable for demodulating the Main Data Flow with Info (M9) received from the Satellite (Ml ) ;

- a Secondary Packet Processor (M15) , able to calculate the CRC32 _1,2 , ... , _n of the Flow packets (M9) and to detect the instant (AT1 _{1,2, . .. ,n} ) of arrival of the Flow packets (M9) , said arrival instant being detected with respect to a Secondary Reference (Mil) which includes a Secondary Counter (P15) and a Secondary Reference Oscillator (P16) (Fig.2b) suitable for generating a Secondary Time Reference (P17 ) (for example one pulse per second - 1PPS) and a frequency reference (P18) (for example 10 MHz) ; in the Secondary Packet Processor (M15) there is also a Low Pass Filter (P20) , necessary to stabilize the correction loop typically made with a PID (Proportional, Integral, Derivative) circuit; a Memory (Wl) , suitable for storing the values CRC32 _1,2 , ... , _n of the packets (BB ₁ , ₂ ,..., _n ) of the Flow (M9) and the instant of arrival (AT1 _{1,2, . .. ,n} ) of each of them.

The object of the invention is synchronizing the Secondary Reference (Mil) of each of the Secondary Stations (M13 _{1,2, . . . ,N} ) to the Primary Reference (M4) . Said synchronization is carried out through a correction of the Secondary Reference (Mil) .

Fig. 3 indicates the time sequence of the actions necessary for synchronizing the Secondary Reference (M11) with the Primary Reference (M4) .

The Primary Reference (M4) through the Primary Reference Oscillator (Pll) and the Primary Counter (P10)) , generates said Primary Time Reference (P12) (1PPS) .

Similarly, the Secondary Reference (Mil) , through the Secondary Reference Oscillator (P16) and the Secondary Counter (P15) , generates said Secondary Time Reference (P17) (1PPS) .

The Primary Counter (PIO) counts at a rate dictated by the Primary Frequency Reference (M4) (typically 10 MHz) , resetting each time reference (one pulse per second - 1PPS) .

The Secondary Counter (P15) counts at a rate dictated by the Secondary Frequency Reference (P16) (typically 10 MHz) , resetting each time reference (one pulse per second - 1PPS) .

With reference to Fig. 3, the method for synchronizing to the Primary Station (M14) called one or more Secondary Stations (M13i, ₂ ,...,n) is described .

This method includes the following steps:

- the Primary Station (M14) receives, through the Demodulator (M7) , the Main Data Flow (M6) including the Signal Packets (BB ₁ , ₂ ,..., _n ) transmitted by the Satellite (Ml) and, through the Packet Processor Primary (M5) , occasionally and periodically records, on the basis of its Primary Reference (M4) , the instant (MOI) in which it receives any of said Data Packets (BB (n) ) , of which it calculates the CRC32 _(n) ; 1 1he same Primary Packet Processor (M5) composes its own Info packet (M2 (n) ) which contains the following data: the just calculated CRC32 _(n) of the received packet (BB _(n) ) ; the instant of receipt (MO1 _(n) ) of the packet (BB _(n) ) of which the CRC32 _(n) referred to the Primary Reference (M4) was calculated;

- possibly the geographical position data of the Primary Station (M14) and of the Satellite (Ml) ;

- the Primary Station (M14) sends the Info packet (M2 _(n) ) to the Secondary Stations (M13 ₁ , ₂ ,..., _n ) (through the same Satellite or other communication channels (for example a telephone line) ;

- said one or more Secondary Stations (M13 ₁ , ₂ , ... , _N ) store in the memory (Wl) :

- the reception instants, or temporal positions, (AT1 _{1,2, . .. ,n)} referred to the Secondary Reference (Mil) of all packets (BB _{1;2,.. N} ) received;

- the related CRC32 ₁ , ₂ ,..., _n , calculated on site by the Secondary Packet Processor (M15) ; whenever the Secondary Packet Processor (M15) receives an Info packet (M2) , it scans backwards the memory (Wl) in order to find an element with the same CRC32 _(n) contained in the Info packet (M2 _(n) ) , and, once found the element, extracts the associated temporal information (AT1 _(n) ) ;

- the Secondary Stations (M13 _{1,2, . . . ,n} ) use the time position (AT1 _(n) ) of the packet received with the same CRC32 _(n) of the one contained in the Info packet (M2 _(n) ) , to calculate the time difference between the two instants, i.e. the time error between the Primary Reference (M4) and the Secondary Reference (Mil) unless a propagation time difference (CGI) in the path between the Primary Station (M14) and the Satellite (Ml) and the Secondary Station (M13 _{1,2, . . . ,n} ) and the Satellite (Ml) itself; said one or more Secondary Stations (M13 _{1,2, . . . ,n} ) use said time difference (CGI) to correct the error of their own Secondary Reference (Mil) , taking into consideration the propagation time difference (CGI) in the path between the Satellite (Ml) and the Primary Station (M14) and between said one or more Secondary Stations (M13 _{1,2, . . . ,n} ) and the Satellite (Ml) itself, said propagation time difference (CGI) being calculated on the basis at the position of the Primary Station (M14) and of the Satellite (Ml) .

A generic Info packet (M2 _(n) ) may or may not contain the geographic data of the position of the Primary Station (M14) and the position of the Satellite (Ml) . Since the position of the Primary Station (M14) is fixed, this data can be sent only once, or set on site when starting up the Secondary Stations (M13 _{1,2, . . . ,n} ) , while the position of the Satellite (Ml) , as it varies slowly, the related data may not be sent in each Info packet (M2) , but only for very long time intervals. Finally, if the quality of the requested synchronization is not very high, it can never be sent and regardless of the geographical correction (CGI) .

The Secondary Packet Processor (M15) carries out the operation described above only at start-up or if the Secondary Reference (Mil) is judged to be excessively incorrect; in steady state conditions, the Secondary Reference Frequency (P16) is corrected by adjusting the same by variations in the value of the Frequency Corrector (P21) so as to obtain a soft variation of the frequency and a constant number of cycles of the Secondary Frequency Reference (P18) with respect to the Secondary Time Reference (P17) , said frequency being increased, if the Time Reference must be brought forward (P17) , or decreased in the opposite _c ase .

A further correction of the various Secondary References (M11 _{1,2, ... ,n} ) must be made to take into account any delay difference between the Demodulators (M8) of the various Secondary Stations (M13 _{1,2, . . . ,n} ) •