First of all, it is important to note that, with a concern for clarity, the disadvantages of prior art are presented here in the particular case of the GSM standard. However, an embodiment of this invention applies to any type of cellular network, such as for example those defined by the 3GPP project (for “Third Generation PartnerShip Project”). Those skilled in the art can easily implement an embodiment of this invention in a network of the UMTS type or other type.
The increasing popularity of the GSM system across the entire world has led the operators to deploy this service not only in the metropolitan regions, but also more and more in rural areas and in more isolated or remote areas. In this latter type of regions, a land infrastructure is often insufficient or poorly adapted in providing good network coverage. A radio link system via satellite is then a very good way to extend the GSM service and this type of system is commonly used today in many regions of the world.
However, satellite radio resources are still costly today, and the problem with this type of application resides in the techniques for reducing the bandwidth needed to transmit data via satellite radio.
Such a problem remains particularly valid in the case where two users are located in the same geographic cell, or at least located in cells that are close to one another. In such a case, it is understood that the conventional techniques for transmission in a GSM network, by definition centralised, consume traffic resources that are much higher than what an optimised routing would make possible.
For more clarity, the disadvantages of the prior art are described hereinafter in the specific case of a GSM system implemented through the intermediary of a satellite link, and wherein two users located in the same cell or in two cells that are sufficiently close in the GSM network, downstream of the satellite link, are in communication.
1. Architecture of GSM
In relation with FIG. 1, the conventional architecture of a cellular network of the GSM type comprises a mobile service switch 10, called MSC (for “Mobile Switching Centre”), a base station controller 11, called BSC (for “Base Station Controller”) and finally one or several base stations 12, called BTS (for “Base Transceiver Station”).
Each BTS provides the GSM radio coverage in one or several cells. By way of example, in relation with FIG. 1, the BTS 121 is controlled by the BSC 11 and covers the geographic cell 13, wherein is located a certain number of users having a Mobile station (MS) for radio communication 14.
More precisely, the MSC controls the configuration of calls for each incoming or outgoing call, and it has the role of an interface with the other telecommunication networks. Each communication goes through the MSC, which controls several BSC.
The BSC is in charge of allocating the radio channels needed for each call. It handles the intercellular transfers between two BTS. A single BSC supports several BTS which provides coverage for a large geographic zone.
Finally, a BTS has for role to carry out the GSM radio transmission with the users of Mobile Stations. The BTS are located in the vicinity of towers 122 supporting antennas, and distributed in the geographic space of coverage of the cellular network.
The GSM standard and its evolutions, such as defined by the 3GPP group (for “Third Generation Partnership Project”), make use of voice compression. This compression is carried out by a transcoder also called TC. According to the GSM standard, the TC can be implemented at the MSC site, at the BSC site or at the BTS site. Economic considerations lead to implementing more preferably the TC at the MSC site, so as to reduce transmission costs.
Several types of codecs have been defined by the 3GPP group. The codec GSM FR “full rate” codec operates at a rate of 13 kbit/s. The HR “half rate” and EFR “enhanced full rate” codecs operate at 5.6 kbit/s and 12.2 kbit/s respectively. After transcoding, speech at 64 Kbit/s compressed to 13/12.2 kbit/s (respectively 5.6 kbit/s) is carried to the base station BTS over a time slot at 16 kbit/s (respectively 8 kbit/s). According to the 3GPP TS 08.60 (respectively TS 08.61) specification, the compressed speech is transmitted to the BTS every 20 ms according to the frame format TRAU (for “Transcoder and Adaptation Unit”).
These same principles apply to the AMR (“Adaptive Multi Rate”) full rate FR and reduced rate HR codings.
The TRAU frame carries, in addition to compressed speech data, signalling data of the “control bits” type making it possible to optimise the quality of the communications between the transcoding entity TC and the channel coding/decoding unit CCU (for “Channel Codec Unit”) with the BTS. These control bits make it possible in particular to provide the synchronisation of the data exchanged, to define the type of codings used (FR, EFR, HR or AMR), and also to indicate the discontinuity of the transmission linked to the silence in the speech (DTX).
In such a way as to introduce the implementation of a satellite link within a cellular network, in relation with FIG. 2, the interfaces implemented are now described succinctly and their denomination between the main entities introduced previously.
The PSTN (for “Public Switched Telephone Network”) is denoted as PSTN 22.
The interface between the MSC 10 and a BSC 11 is referred to as interface A.
The interface between a BSC 11 and the BTS 121 is referred to as the interface Abis.
In the case where the TC 21 is implemented at the MSC site 10, the interface between the TC 21 and the BSC 11 is called Ater.
A satellite link can be used within the transmission chain for each of these interfaces. The main problem with inserting a satellite link on one of these interfaces is then to determine how to effectively transmit the necessary data while minimising the radio band needed for the transmission via satellite.
The interface A, used between a MSC and a BSC, is constituted of one or several 2 Mbit/s links (ITU G703/G704 standard). Each 2 Mbit/s link supports 30 uncompressed voice channels—at 64 kbit/s—and one signalling channel SS7. The number of 2 Mbit/s links depends on the sizing of the BSS subsystem. The signalling channel contains messages indicating in particular the traffic needs according to the number of communications.
The interface Abis connects a BSC with a BTS and is constituted of one or several 2 Mbit/s links (ITU G703/G704 standard). It is one of the interfaces which is conventionally implemented with a transmission via satellite.
This interface Abis carries traffic data, such as compressed voice and signalling data.
On the interface Abis, two types of signalling data circulate:                signalling messages exchanged with the BTS, transported in a specific signalling channel, which make it possible to control the BTS equipment itself as well as the mobile station (MS) which are in relation with it. The corresponding messages are specified by the GSM in the TS 08.58 specification.        control “in band” data which is transmitted in the same flow as the traffic data. This data is transmitted within TRAU frames. This data is “control bits”, complementary to the “data bits”, of which the meaning is explained in the TS 08.60/08.61 specifications.        
The signalling data of the first type, constituted of protocol messages, is carried over dedicated time slots, with typically over the interface Abis a rate of 64 kbit/s.
Each 2 Mbit/s link of the interface Abis has 31 time slots (TS) which are allocated to the signalling channels or to the speech channels. According to the typology of the network and coding choices for the speech, a 2 Mbit/s link on the interface Abis can typically be used to support up to ten radio transmission access channels, called TRX (“Transceiver”). Each TRX in turn supports eight GSM channels dedicated to speech at full rate FR or sixteen GSM channels at half rate HR. The corresponding reservation of the speech channels on the interface Abis represents for each TRX an allocation of 2 TS at 64 Kbit/s (8*16 Kbit/s=16*8 Kbit/s=128 Kbit/s.
According to the sizing of the GSM network, the BTS is equipped with a number N of TRXs, which induces a proportional occupation of the number of TS on the interface Abis.
2. Satellite Applications
A conventional GSM network implementing a radio link of the satellite type is described in relation with FIG. 3.
The GSM connecting network then comprises, conventionally, a MSC 30, a BSC 31 as well as a base station BTS 32, providing the communications to users having a mobile terminal 34 and located in the coverage area of the BTS 32.
In addition, a radio link 36 is implemented on the interface Abis, between the BSC 31 and the BTS 32. This radio link 36 is provided by a radio system via satellite containing two antennas 331 and 332 for emitting-receiving on each side of the interface Abis, and a satellite 35.
Note that it is possible in fact to insert a radio link via satellite on each of the interfaces implemented in the GSM system: A, Abis, Ater. But the insertion of such a satellite link on the interface Abis, i.e. between a BSC and BTSs, is very often preferred in order to extend the GSM service to remote geographic locations and of a low density of users with minimal infrastructural costs.
So as to avoid any confusion, it is important to note that in such an implementation, two types of radio systems are implemented, but that they do not have the same role:                The GSM network itself uses a first radio link to communicate, and in particular to carry out the transmission between the BTSs and the users of mobile station.        The satellite system consists of a second radio transmission link. Conventionally, a device called Hub allocates the radio resources needed for the transmission of data by satellite between BSC and BTS.        
In what follows of the description, radio resources are referred to: this denomination thus concerns the radio transmission link via satellite, but it can be extended according to an embodiment of the invention to any other type of radio link with shared resources, as for example links via radio beams (“microwaves”), or systems of the LMDS type (“Local Multipoint Distribution Systems”), or other land transmission systems of the WiFi, WiMAX (for “Wireless Microwave Access”) type, etc.
An embodiment of this invention applies in particular to configurations using a satellite channel managed in DVB-S/DVB-RCS mode.
Concretely, when two users are in communication, the usual realisation in a GSM network demands that the flow of speech transit through the BSC, as well as through the MSC. This then requires the allocation of resources on two channels of the satellite link: the upstream connection and the downstream connection. This in particular remains valid regardless of the position of the users (caller and recipients), and particularly when the two users are located in the same cell or in two nearby cells.
3. Disadvantages of Prior Art
To date, the implementation of a radio link, via satellite in particular, between a BTS and the corresponding BSC of a cellular network systematically results in, during a communication between two users each served by a BTS connected via satellite, the allocation of two radio channels: a first for the called party and a second for the caller.
Indeed, the usual application demands that the speech be transmitted “all the way up” to the MSC of the GSM network. The flow of speech thus passes twice via the satellite, even if the communication at hand is of a local nature. So a local communication suffers needlessly from the addition of twice the transfer time via satellite, of a magnitude of 250 milliseconds. The existence of this double satellite link therefore adds not only non-negligible transmission time which is reflected in the quality of the communication as perceived by the users, but it is in addition very costly.
This situation has been accepted up until now.
So, to date there are no means making it possible to specify the local nature of a call. Current techniques therefore do not handle such a configuration in an optimised manner.