This application claims priority to foreign French patent application No. FR 1700106, filed on Feb. 2, 2017, the disclosure of which is incorporated by reference in its entirety.
The invention relates to a system and a method for transmitting signals in a satellite system. It falls in particular in the context of the satellite user networks for the transmission of radiofrequency signals.
The increase in bit rates, particularly in the context of the so-called VHTS (Very High Throughput Satellite) satellites, is leading to the proliferation of infrastructures with maintenance and interconnection costs which are correlated. For reasons of the requirements of capacity and therefore of frequency resources, it is necessary to use increasingly higher frequency bands (Q/V of the electromagnetic spectrum) comprising attenuation variations linked to the significant meteorological conditions. When the meteorological conditions are too degraded, it is necessary to use a site diversity strategy consisting in passing through another anchorage site known as “gateway”. A gateway is located at various points under the coverage of a satellite. The site diversity consists in using places sufficiently far apart for the correlation between the meteorological conditions to be negligible. The drawback resulting therefrom is the proliferation of the locations and therefore of the infrastructures with the attendant increase in costs.
The idea implemented in the present invention consists notably in locating the hubs at the level of a collection point, better known in the satellite field by the abbreviation PoP, thus reducing the size of the gateways which retain only the RF part, in normal operation.
The invention relates to a data transmission system in a communication network comprising a satellite transmission link, a service provider part, a user part and a satellite access network comprising a collection point PoP adapted to despatch the data flows, several gateways comprising the radiofrequency part, characterized in that it comprises a modulator of data to be transmitted comprising at least two parts:
a first part positioned at the level of the collection point PoP part and comprising the following modules:
a second part positioned at the level of the radiofrequency part of a gateway and comprising the following modules:
the first part and the second part exchange encapsulated data according to the protocol used for the transmission in the system.
The system can comprise a demodulator composed of two parts:
a first part situated at the level of the gateway part of the system comprising the following modules:
a module for reducing the number of bits transporting the information item, before transmission via a transmitter module,
a second part situated at the level of a collection point PoP and comprising at least the following module:
The receiver module of the part situated at the radiofrequency level of a gateway is, for example, adapted to generate the following information: an information item on the latency of the data transmission link, an information item on the triggering of the transmission of the data H.F.Rx trigger, an information item H.F.Rx NCR corresponding to the value of a time corresponding to the start of a hyper frame to be taken into account on reception of the “H.F.Rx trigger” information item in order to reconstitute, locally at the level of the radiofrequency part, a clock corrected of the transmission latency between the first part and the second part of the modulator.
The clock reconstitution module is, for example, adapted to reconstitute a network synchronization and for supplying a current time to be inserted into a data field of a hyper frame under construction.
The “front-end” part can additionally comprise the following elements:
The modulator can be a modulator of DVB-S2 type, or of DVB-RCS2 type. The protocol implemented is for example the CPRI protocol.
The invention relates also to a method for transmitting signals in a communication network comprising a satellite transmission link comprising a service provider part, a user part and a satellite access network comprising a collection point PoP adapted to despatch the data flows, several gateways comprising the RF part, comprising at least the following steps:
a part situated at the level of a collection point of the system constructs a baseband frame comprising a data field including synchronization information and/or an identifier of a user and a field containing data originating from a service provider and intended for the user, in order to generate encapsulated data before transmission to a second part of the system suitable for modulating said data,
the second part reconstructs a local clock from the synchronization information contained in the encapsulated data, before modulation of the data,
the modulated data are then transmitted by satellite.
The method can comprise a step of reconstruction of a clock value by taking account of the value of the latency of the transmission link, of the instant of triggering of transmission of the data, of the value of a time corresponding to the start of a received hyper frame.
The data format is the CPRI format for the transmission of the encapsulated data or even the DVB-S2 frame format.
Other features and advantages of the present invention will become more apparent on reading the following description of exemplary embodiments given in an illustrative and in no way limiting manner, with figures attached which represent:
In order to give a good understanding of the invention, the following example is given in the context of a user network implementing a radiofrequency RF transport and an interface known to the person skilled in the art by the acronym CPRI (Common Public Radio Interface) and for a modulator of DVB-S2(x) type and a demodulator of DVB-RCS2 type that are known to the person skilled in the art. Without departing from the scope of the invention, any person skilled in the art will be able to adapt the modules explained hereinbelow for the implementation thereof in a satellite system whatever the transmission and modulation/demodulation protocol used.
The base idea of the invention is notably to subdivide the modulator and the demodulator used in the satellite system at the level of the collection point PoP and at the level of the gateways of the system in order to minimize the bit rate needed for the transmission of the data.
Since the hub parts 10 are positioned at the level of the collection point PoP, they will be able to be pooled. In effect, a gateway in the prior art is dimensioned by considering the worst case (maximum number of requests and of simultaneous throughputs originating from several users); thus, the sum of the capacity of the various gateways would be far greater than that of the satellite. In the case of the method according to the invention, the colocation of the hubs 10 at the level of the collection point PoP, 21, allows the hubs 10 to be dimensioned strictly to the capacity of the satellite.
The back-end part 30BE comprises at least the following elements:
The operation of these various modules will be described later.
The front-end part 30FE comprises the following modules:
The management of the clock and synchronization information (time inserted into the data frame and latency) for the modulator part situated at the level of the back-end and for the modulator part situated at the level of the front-end will now be described. The CPRI transmitter module, 304, will encapsulate the data Di to be transmitted by observing the CPRI format. This format comprises a first part comprising an NCR information item (information for the synchronization of the various receivers) and data Di to be transmitted to the front-end part to be modulated.
The encapsulated data are transmitted via a CPRI link at the level of one or more gateways as a function of the user requests. An example of a scheme for pooling of the hubs and of the gateways is given hereinbelow in the description.
The transmission of the digitized RF signals will take place over the CPRI link thus avoiding the use of RF cables for the transmission of the data, the CPRI standard in its version 6.1 being defined by the consortium of the same name on the website www.cpri.info.
On the back-end side (30BE), a signal triggering reception of the Rx data (H.F. Rx trigger) is generated by the CPRI reception module 310 at the start of the hyper frame, 401. It is sent by the CPRI transmitter block to the NCR reconstruction module 311. This signal is accompanied by the value of the NCR time recovered from the part “emanating from the provider” extracted from the data to form the “H.F. Rx NCR” signal.
The instant of arrival ta of the data packet is signalled by a signal “H. F. Rx trigger”, 402, as well as the corresponding NCR time value by “H.F. Rx NCR”, 403. These two signals (value of the time and instant of arrival of the data packet) allow the reconstruction module “local NCR reconstruction” 311 to locally reconstitute the NCR clock using a loop of digital PLL type, for example.
By using the native function of the CPRI that makes it possible to measure the latency between the two ends of the transmission chain (instant of transmission—instant of reception on the receiver side of the modulator), this local value of the NCR clock NCRlocal, can be compensated NCRcomp to represent a value identical to the value of the clock at the level of the back-end. This information is passed from the “CPRI receiver” block to the “local NCR reconstruction” block by the “link latency” signal, τl, 404.
Ultimately, this reconstituted NCR clock will be able to be used for the insertion of an NCR time into the DVB-S2(x) frames according to the recommendations of the DVB-RCS2 standard (§ 6.2.1) known to the person skilled in the art.
The CPRI format is for example used in the eight-bit I&Q format (phase and phase quadrature). The data from a back-end to a front-end are encapsulated in a 16-bit A×C container, for example, by using the format indicated. This will make it possible in particular to be able to transport the data from several back-ends to the respective front-ends over one and the same physical CPRI link by multiplexing, in the same CPRI flow, the data intended for several front-ends. The containers of the data are for example coded consecutively in the CPRI hyper frames and filling or “padding” data are added at the end of the hyper frame if it is not completely filled.
For each A×C container, two control words are added to the “vendor specific” data defined by the CPRI standard.
These two words are for example subdivided as follows, starting from the least significant bit LSB of the second word:
The descriptor of a DVB-S2(x) A×C container can be represented as follows:
The indication of the number of bits remaining makes it possible to add and remove, on the fly, on a CPRI link, the flows originating from different back-ends.
The size of the A×C containers makes it possible to delimit them in the hyper frame. Finally, the distance to the next header BB-header allows, upon the starting up of the link, a synchronization on the boundaries of the BB-frames. In effect, when the first BB-frame is found, the DFL fields of the BB-header make it possible to determine the next frame and so on to the end of the A×C container.
At the level of the front-end part 50FE, the device for example comprises the following elements:
On the side of the back-end 50BE at the level of the collection point PoP of the system, the demodulator is composed of the following modules:
In the example, six BE are considered in the PoP part and use, to transmit their respective flows, the identifiers from 1 to 6. A CPRI multiplexer, 60, according to the prior art of this standard, is configured to multiplex the flows from the back-ends BE1, BE3 and BE4 in the CPRI link to the first gateway 61. Likewise, the flows from the back-ends BE2, BE5 and BE6 are sent to the second gateway 62.
On the side of the first gateway 61, three front-ends are configured respectively to process the flows identified F1, F3 and F4. Likewise, the second gateway 62 has three front-ends configured to process the flows identified F2, F5 and F6.
By virtue of the identifier of the back-end transmitter “back-end transmitter identifier”, the flows are indeed multiplexed, transmitted to the respective gateways, then extracted.
When a front-end fails, it is then easy to place another front-end processing the same back-end identifier in service and to substitute this second front-end for the first.
Finally, in the case of need for geographic diversity, the CPRI multiplexer has to be reconfigured in order for the flow from a back-end to be rerouted to change destination and a front-end in the new destination gateway has to be configured with the right identifier to take account of this new flow.
The system according to the invention makes it possible to reduce the infrastructure and maintenance costs of the gateway sites. The site diversity no longer entails changing the routing of the signals between the hub and the radiofrequency RF part.
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17 00106 | Feb 2017 | FR | national |
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