WO2001084744A1

WO2001084744A1 - Data transmission for a mobile telecommunication system comprising a signal carrier modulated to provide a modified v.110 frame of format used in gsm

Info

Publication number: WO2001084744A1
Application number: PCT/GB2000/001654
Authority: WO
Inventors: Donald James Erskine; Thomas Goerke
Original assignee: Ico Services Ltd.
Priority date: 2000-04-28
Filing date: 2000-04-28
Publication date: 2001-11-08

Abstract

A data signal for a mobile telecommunications system, comprises a signal carrier modulated to provide a 36 bit modified V.110 frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, wherein some of the control bit positions of the frame contain data bits.

Description

DATA TRANSMI SSION FOR A MOBILE TELECOMMUNICATION SYSTEM COMPRISING A SIGNAL CARRIER MODULATED TO PROVIDE A MODIF IED V . l lO FRAME OF FORMAT USED IN GSM

Field of the invention

This invention relates to transmitting data signal in a mobile telecommunications system and has particular but not exclusive application to a satellite mobile telecommunications system in which data such as short text messages, facsimile transmissions or e-mails are transmitted to and from remote user terminals such as mobile telephone handsets or other terminal equipment.

Background

Terrestrial mobile telecommunications systems are well known and a number of different systems have developed which operate according to different standards.

These public land mobile networks (PLMNs) may operate according to analog or digital standards. In Europe, the Far East, excluding Japan and elsewhere, the Global System Mobile (GSM) network has become popular, whereas in USA, the Advanced Mobile Phone Service (AMPS) and the Digital American Mobile Phone System (DAMPS) are in use, and in Japan, the Personal Handiphone System (PHS) and the Personal Digital Communication (PDC) network are used. More recently, proposals have been made for a Universal Mobile Telecommunications System (UMTS). These networks are all cellular and land- based with transmitter/receivers which communicate with mobile user terminals.

Considering for example the GSM system, individual cells of the mobile network are served by a series of geographically spaced, terrestrial base station subsystems (BSS) which each comprise a base transceiver stations (BTS) which are coupled through base station controllers (BSCs) to a mobile switching centre (MSC) which may provide a gateway out of the network to a conventional public switched telephone network (PSTN). The PLMN includes a home location register (HLR) which stores information about the subscribers to the system and their user terminals (UTs). When a UT is switched on, it registers with the HLR. If the user roams to a different GSM network, the user terminal registers with a visitor location register (VLR) of the visited network, which communicates with the HLR of the home network for routing and other purposes. DAMPS, PHS and PDC networks have a generally similar architecture.

The telephone call is routed to the subscriber's UT through the MSC, to the BSC and then to the selected BTS, which provides a full duplex channel to the UT. The channel comprises a downlink channel from the BTS to the UT and an uplink channel from the UT to the BTS. The channels include time division multiple access (TDMA) time slots on different frequencies allocated on initiation of the call.

A number of different mobile telecommunication systems have been proposed that use satellite communication links to the mobile user terminals. One network known as the IRIDIUM ™ satellite cellular system is described in for example EP-A-0365885 and US Patent No. 5 394 561 (Motorola), which makes use of a constellation of so-called low earth orbit (LEO) satellites, that have an orbital radius of 780 km. Mobile user terminals such as telephone handsets are configured to establish a link to an overhead orbiting satellite, from which a call can be directed to another satellite in the constellation and then typically to a ground station which is connected to conventional land-based networks.

Alternative schemes which make use of so-called medium earth orbit (MEO) satellite constellations have been proposed with an orbital radius in the range of 10-20,000 km and reference is directed to Walker J.G. "Satellite Patterns for Continuous Multiple Whole Earth Coverage" Royal Aircraft Establishment, pp 119-122 (1977). These orbits are also known as intermediate earth orbits (TEOs). The ICO ™ satellite cellular system as described in GB-A-2 295 296, is an example. With the ICO™ system, the satellite communication link does not permit communication between adjacent satellites and instead, a signal from a mobile user terminal such as a mobile handset is directed firstly to the satellite and then directed to a ground station or satellite access node (SAN), connected to conventional land-based telephone network. This has the advantage that many components of the system are compatible with known digital terrestrial cellular technology such as GSM.

In satellite communications networks, ground stations are located at different sites around the world in order to communicate with the orbiting satellites. In the ICO™ system and others, a visitor location register is associated with each of the satellite ground stations, which maintains a record of the individual user terminals that are making use of the particular ground station. Each satellite has an antenna which produces an array of spot beams directed towards the earth, that act as cells and correspond to the cells of a conventional terrestrial mobile network, although the cells produced by the spot beams sweep over the surface of the earth as the satellite moves in its orbit.

Referring again to the GSM system, the telephone call is directed over a dedicated traffic channel or TCH. Each TCH has an associated slow rate control channel or SACCH, which is interleaved with the TCH. These channels are described in more detail on pp 195 - 201 of "The GSM System for Mobile Communications" by M. Mouly and M-B. Pautet, 1992 Cell & Sys, ISBN: 2-9507190-0-7.

Also, the TCH can convey traffic other than speech i.e. data such as short messages for a short message service (SMS), e-mails and facsimile transmissions. In order to handle this non-speech data traffic, the GSM system needs to be able to receive an asynchronous data stream from a PSTN or from a mobile user terminal, and convert it into a synchronous stream at a selectible rate for radio transmission in the TCH. The asynchronous data may for example be produced by a personal computing device with a modem which frequency modulates an audio carrier transmitted over the PSTN. In GSM, a data rate conversion scheme has been adopted which is closely based on the rate adaption conventionally used for coupling asynchronous data from a PSTN to an integrated service digital network (ISDN). The International Telecommunication Union (ITU) Recommendation V.llO entitled "Support by an ISDN of data terminal equipments with V-series type interfaces", October 1996 defines a scheme by which an asynchronous data flow of 50 - 19200 bit/s is converted by a rate adaption function RAO into a synchronous flow of 600 - 19200 bit/s. An intermediate rate adaption function RAl converts the resulting synchronous flow to an intermediate synchronous rate of 8 or 16 kbit/s. A rate adaption function RA2 rate adapts the intermediate rate into a 64kbit/s synchronous stream for transmission through the ISDN. The resulting data is configured in 80 bit frames according to the V.llO Recommendations.

The format of the V.llO frame according to the ITU Recommendation is set out in Table 1

Table 1

Frame Structure

Octet number Bit Number

1 2 3 4 5 6 7 8

0 0 0 0 0 0 0 0 0

1 1 Dl D2 D3 D4 D5 D6 SI

2 1 D7 D8 D9 D10 Dll D12 X

3 1 D13 D14 D15 D16 D17 D18 S3

4 1 D19 D20 D21 D22 D23 D24 S4

5 1 El E2 E3 E4 E5 E6 E7

6 1 D25 D26 D27 D28 D29 D30 S6

7 1 D31 D32 D33 D34 D35 D36 X

8 1 D37 D38 D39 D40 D41 D42 S8

9 1 D43 D44 D45 D46 D47 D48 S9 According to N.llO, the bits of the frame are labelled as follows:

D - used to transport data i.e. the data content of the sent message E - used for synchronisation and timing correction of the transmitted bits of the frame

S - status bits i.e. the status of circuits of the sending terminal equipment X - ready to send

Additionally, the 80 bit frame includes a 17-bit frame synchronisation pattern which consists of all 8 bits (set to binary 0) of octet zero and bit one (set to binary 1) of the nine octets shown in Table 1.

In GSM, a modified scheme is used. The 17-bit frame synchronisation pattern of the V.llO frame is dispensed with, due to the fact that the GSM transmission itself provides satisfactory synchronisation. Also the 3 bits El, E2 and E3 are not needed. Furthermore, only half of the data bits D1-D48 are used as each data bit is sent twice in the conventional format and this duplication is not needed. Thus, the modified GSM frame contains a total of 36 bits. Further details of the modified frame are given in "The GSM System for Mobile Communications" supra, pp 177-178. However, this frame format suffers from the disadvantage that only 24 of the 36 bits of the frame are used to transport data and the remainder of bits have hitherto been considered necessary to maintain integrity of the transmitted data. The remainder i.e. the S, E and X bits of the modified N.llO frame, will be referred to herein as "control bits". The provision of such a large number of control bits in the frame results in a large overhead and hence lower data handling capacity than would be desired, particularly for a satellite telecommunications system where the signal handling capacity of the satellites imposes a constraint.

In accordance with the invention, it has been appreciated that at least some of the information conveyed by the control bits is not needed to provide to provide satisfactory conveyance of data in a mobile telecommunications system.

Summary of the invention

According to the invention there is provided a data signal for a mobile telecommunications system, comprising a signal carrier modulated to provide a modified N.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, wherein some of the control bit positions of the frame contain data bits.

The signal may be used in a satellite system but can also be used in a terrestrial cellular network.

The invention also includes a modulator for a mobile telecommunications system, to modulate a signal carrier so as to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, wherein the modulator is operable to insert data bits into some of the control bit positions of the conventional frame.

Also provided according to the invention is a demodulator for a mobile telecommunications system, to demodulate a signal carrier modulated to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, in which data bits are provided in some of the control bit positions of the conventional frame, the demodulator including means for locating and the demodulating the data bits from both the data bit positions and from the control bit positions of the frame that contain data bits.

The invention further includes a method of modulating a signal carrier to provide a data signal for a mobile telecommunications system, comprising modulating the signal carrier to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, and providing data bits in some of the control bit positions of the frame.

The invention further includes a demodulation method for a mobile telecommunications system, to demodulate a signal carrier modulated to provide 5 a modified N.l 10 frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, in which data bits are provided in some of the control bit positions of the conventional frame, including locating and the demodulating the data bits from both the data bit positions and from the control bit positions of the frame that contain data bits.

10

A method of modulating a carrier to provide sequential modified V.llO frames containing data bits D and control bits SA, SB and X wherein the sampling rates of the control bits relative to the rate of the data bits, are as follows: 1 SA bit every 24 data bits D, through 1 SA bit every Ν2 data bits D; 1 SB bit every N3 is data bits D, through 1 SB bit every N4 data bits D; and 1 X bit every N5 data bits D, through 1 X bit every N6 data bits D.

The modulated carrier may include include SA bits at a rate of the order of 16 bit/s, SB bits at a rate of the order of 40 bit/s and X bits at a rate of the order of 20 20 bit/s and the overall bit rate for the carrier is of the order of 2400 bit/s.

Brief description of the drawings

In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the 25 accompanying drawings, in which:

Figure 1 is a schematic diagram of a satellite telecommunications system together with a local, land-based mobile telecommunications system, in accordance with the invention;

Figure 2 is a schematic frequency/time diagram illustrating time slots for the 30 FDMA transmission scheme;

Figure 3 is a schematic block diagram of the circuits of satellite 3 a;

Figure 4 is a schematic diagram of the cells produced by the spot beams produced by the satellite 3a;

Figure 5 is a schematic diagram of a mobile user terminal;

Figure 6 is a schematic block diagram of the circuits of the user terminal shown in Figure 5; and Figure 7 is a schematic illustration of the rate adaptors used to send a data message through the network.

Detailed description Overview of network Referring to Figure 1, a schematic block diagram of a satellite mobile telecommunication network is shown corresponding to the ICO™ network. A mobile user terminal UT 1 in the form of a mobile telephone handset can communicate on a radio channel over a communication path 1, 2 via an earth orbiting satellite 3 a with a land-based satellite access node SA24. As shown schematically in Figure 1, SAN 1 is provided with an antenna 4 which can track the orbiting satellite. A laptop computer LC is shown connected to UT 1.

A number of the satellite access nodes SAN 1, 2, 3, etc are connected together to form a backbone network 5, which is connected through a number of gateways GW 1, 2, 3, etc to conventional land-based telephone networks. For example, the gateway GWl, is connected to a land-based public switch telephone network (PSTN) 6, which permits connection to be made to a conventional telephone set 7. Also a personal computer PC with a modem card (not shown) is connected to the PSTN 6. The PC is one example of terminal apparatus capable of sending data messages such as e-mails or facsimile transmissions through its modem as a pulse code modulated (PCM) audio tone in an asynchronous bit stream. The sending of such messages to the laptop computer LC connected to UT 1 will be described in more detail hereinafter.

The gateway GWl is additionally connected to a public switch data network (PSDN) 8 which may comprise an ISDN, and a public landl mobile network (PLlVLN) 9. Each of the gateways GWl, 2, 3 may comprise existing International Switching Centres (ISCs) or mobile switching centres (MSCs) of the type used in GSM mobile networks.

As shown in Figure 1, the handset UT 1 can also communicate with the conventional land-based mobile network PLMN 9, which is shown schematically to include a transceiver station 10 that establishes a duplex link 11 with the user terminal UT 1. In this example, the PLMN 9 is a GSM network. For a fuller understanding of GSM, reference is directed to the various GSM Recommendations issued by the European Telecommunications Institute (ETSI). Also reference is directed to "The GSM System for Mobile Communications" supra, for a more readable overview.

The satellite network is designed to provide worldwide coverage and the satellites 3a, 3b form part of a constellation of satellites, which may be arranged in several orbits. In one example, two orbits of five satellites are used, which can be shown to provide coverage of a major part of the surface of the earth, in which for a 10° satellite elevation angle, one satellite can be accessed by the mobile handset all of the time and two satellites can be accessed for at least 80% of the time, thereby providing system diversity. Additional satellites may be included in the constellation in order to provide further diversity and redundancy.

The satellites may be arranged in a MEO constellation, for example with an orbital radius of 10,355 km, although the invention is not restricted to a particular orbital radius. In this embodiment, satellites 3a, 3b are shown in a common orbit and the satellites are tracked by the antenna arrangement of each SAN. The SANs are spaced around the earth in order to provide continuous coverage. In the example shown, SAN 1 may be located in Europe whereas SAN 2 may be located in Africa, SAN 3 in America and other SANs may be located elsewhere.

SAN 1 consists of a satellite base station SBS 1 which is coupled to the five dish antennas 4 for tracking the satellites, the SBS 1 including transmitter and receiver circuits with amplifiers, multiplexers, demultiplexer and codecs. A mobile satellite switching centre MSSC 1 is coupled to SBS 1 and includes a satellite visitor location register VLR_SAT 1. MSSC 1 couples communication signals to the backbone network 5 and to the SBS 1, so as to allow individual telephone calls to be established through the backbone network 5 and the duplex communication link 1, 2 via the satellite 3a, to the mobile terminal UT 1. Also, MSSC 1 is connected to the gateway GWl so as to provide an output connection to PLMN 9, together with PSDN 8 and PSTN 6. The interchange of signals between SBS 1 and MSSC 1 is controlled by a terrestrial network manager TNM 1 in SAN 1.

The NLR_SAT 1 maintains a record of each of the subscribers currently registered, namely the identity of each user that is making use of the SAΝ 1 for signal communication. It will be understood that all the SANs are of similar construction with a respective NLR_SAT to maintain a record of the subscribers registered.

In Figure 1, the SAΝ 2 is shown communicating with user terminal UT 2 via satellite 3b. For further details of the network, reference is directed to GB-A- 2 295 296 and EP-A-0869698.

The satellites 3a, 3b are in non-geostationary orbits and comprise generally conventional hardware such as the Hughes HS 601. They may include features disclosed in GB-A-2 288 913. As explained in more detail hereinafter, each satellite 3a, 3b is arranged to generate an array of radio spot beams covering a footprint on the earth beneath the satellite, each beam including a number of different frequency channels and time slots as described in GB-A-2 293 725. The spot beams thus provide adjacent cellular areas which correspond to the cells of a conventional land-based mobile telephone network.

The satellites are controlled by means of a satellite control centre (SCC) 12 and a - 77 -

telemetry tracking and control station (TT&C) 13, which are connected to a network management centre (NMC) 14 through a digital network 15 that is coupled to the backbone network 5. The SCC 12 and the TT&C 13 control operations of the satellites 3a, 3b, e.g. for setting the general transmission power levels and transponder input tuning, as directed by the NMC 14. Telemetry signals for the satellites 3a, 3b are received by the TT&C 13 and processed by the SCC 12 to ensure that the satellites are functioning correctly.

Channel Configuration Each of the handsets communicates during a telephone call, with a respective SAN via one of the satellites. Thus for example, handset UT 1 communicates with SAN 1 via satellite 3a via paths 1, 2 shown in Fig. 1. A full duplex communication path is provided between the UT and the SAN. Communication from the SAN to the UT via the satellite is referred to as a "downlink", and communication directed from the UT via the satellite to the SAN is referred to as an "uplink".

The general configuration of the transmitted signals is in some respects similar to those used for conventional GSM transmissions in a PLMN and makes use of a frequency diverse time division multiple access (TDMA) scheme. Referring to Figure 2, the basic unit of transmission between the SAN and UT is a series of about 100 modulated digital bits, which is referred to as a burst. Bursts each have a finite duration and occupy a finite part of the radio spectrum. Thus, they are sent in time and frequency windows which are referred to as slots. The slots are positioned every 25 kHz and recur in time every 40/6 ms. In the time domain, the slots are given a timeslot number TN The duration of each slot is referred to as a burst period or BP. A graphical representation of a slot in the time and frequency domain is shown in Figure 2.

The signal transmission scheme includes channels for providing communication between the UTs individually and one of the SANs via a satellite, together with a number of common channels for all UTs within the cell defined by a particular - 72 -

satellite spot beam.

For speech transmission, data is sent on an individual traffic channel TCH. Each TCH is provided with an associated slow rate control channel or SACCH. Each 5 TCH consists of one slot every 6 BP and comprises a cycle of 25 slots. The SACCH is multiplexed in blocks each consisting of 12 consecutive slots. Two SACCH blocks are transmitted for every 25 slot cycle of the TCH and the remaining slot is used to transmit a timing reference so as to make up a 25 slot SACCH message for each 25 slot cycle of TCH. The TCH/SACCH is 10 transmitted when the UT is in dedicated mode i.e. during a call.

A TCH/SACCH is also used for the communication of data messages between UT 1 and SAN 1, such as facsimile transmissions, e-mails and SMS as will be explained in more detail later.

75

In addition to the channels TCH/SACCH for the individual UTs, a number of common channels are provided for all UTs within each cell corresponding to one of the satellite's spot beams, For example, a broadcast control channel BCCH is broadcast from the satellite to all UTs within a particular cell. The 20 BCCH provides information which identifies the cell to the UT, which is received by the UT in idle mode i.e. when no call is being made.

Satellite

A schematic diagram of the major signal processing components of the satellite is 25 given in Figure 3. Signals transmitted from one of the SANs are received by antenna 16 and directed to a detector/multiplexer/demultiplexer circuit 17. It will be understood that the signals transmitted from the SAN to the satellite contains a large number of TCH/SACCHs that are to be directed to individual UTs by the satellite. To this end, the satellite includes a plurality (for example 30 163) of antennas 18 that produce individual spot beams that correspond to a cellular configuration as previously described. A beam forming processor circuitry configuration 19_down receives the various TCH/SACCHs that are demultiplexed by circuit 17 and assembles them into multiplexed signals directed on 163 outputs to the spot beam antennas 18. The circuitry 19_down is also configured to include the downlink common control channels for each of the spot beams, including the BCCH, on the basis of data transmitted to the satellite from the SAN. The signals transmitted between the satellite and the cells may be of a frequency in the 2GHz range.

An example of cells CO - C6 produced by the footprints of seven of the 163 spot beams from antenna 18 is shown schematically in Figure 4. The user terminal UT 1 lies within cell CO but also can monitor the BCCHs of adjacent cells Cl - C6 and cells produced by other satellites that are within range (not shown).

It will be understood that the foregoing description of the satellite circuitry is schematic and for further details, reference is directed to GB-A-2 288 913 supra.

User terminal (UT 1)

The mobile user terminal UT 1 is shown in more detail in Figures 5 and 6. It comprises a hand held device which is generally similar to a mobile telephone used for conventional terrestrial digital networks. It is powered by a rechargeable battery (not shown) and is configured to operate with either the local terrestrial cellular network or to roam to the satellite network. As shown in Figure 4, the handset comprises a microphone 20, a loudspeaker 21, a battery 22, a keypad 23, antennas a, 24b and a display 25, typically a liquid crystal display panel. The handheld unit UT 1 also includes a subscriber identification module (SIM) smartcard 26. The circuit configuration of the handset UT 1 is shown in block diagrammatic form in Figure 5. The SIM card 26 is received in an SIM card reader 27 coupled to a controller 28, typically a microprocessor. The microphone and loudspeaker 20, 21 are coupled to first and second codecs 29a, 29b, coupled to conventional radio interfaces 30a, 30b connected to the antennas 24a, 24b so as to transmit and receive communication signals, in a manner well known per se. The handset is capable of dual mode operation, with the codec 29a, radio interface 30a and antenna 24a being used with the GSM PLMN 9, and corresponding circuits 29b, 30b and 24b being used with the satellite network. The user terminal UT 1 can thus handle speech through either network using the microphone and loudspeaker 20, 21.

Also, UT 1 can handle non-speech data messages such as SMS, facsimile and e- mail, through either the satellite network or the PLMN 9, with received messages from the radio interfaces and codecs 29, 30 being processed by the controller 28. SMS messages can be displayed on the display 25. Outgoing SMS messages can be composed using the keypad 23, and displayed on the display 25 prior to transmission, in a manner known per se.

Received data messages for the laptop computer LC are ported onto connection 31 from the controller 28 and similarly, data from the LC to be transmitted by the UT is fed through connection 31 to the controller 28 and then to the appropriate codec and radio interface 29, 30. As will be explained in more detail hereinafter, the controller 28 performs a rate adaption function on the data fed to and from the laptop LC.

Service provision The described network can provide service to customers in a number of different ways. For example, communication may be provided from UT 1 to UT 2 using the satellite backbone network 5. Alternatively, telephone communication can be established between the telephone set 7 and UT 1 either via SAN 1 and satellite 3a, or, through PLMN 9, antenna 10 and link 11. For a further description of service options, reference is directed to our EP-A-0869628.

The network can be used to be used to send data messages. In the following, the transmission of data message such as a facsimile or e-mail, from the PC through PSTN 6, SAN 1, satellite 3a to UT 1 and the laptop LC will be considered in detail with reference to Figure 7. - 75 -

Data message transmission

As previously explained, the modem in the PC shown in Figure 1, produces an asynchronous PCM stream of data in a manner known per se. The signals are directed through the PSTN 6, the gateway GW 1 to the MSSC 1 of SAN 1 shown in Figure 1. Referring to Figure 7, the signals are received on line 32 and typically comprise a 64kbit/s PCM data stream. This is fed to a voice band modem VBM which demodulates the audio tone to produce a corresponding asynchronous 300-33.4 kbit/s data stream.

The MSSC 1, as previously explained, is operative to switch calls and hence has the capability to perform data processing on signals routed through it. The data processing capability is used to perform rate adaption functions according to the V.llO ITU recommendation. As shown in Figure 7, rate adaption function RAO and RAl are performed on the incoming data. Rate adaption function RAO produces a synchronous 600-19200 bit/s stream, which is converted to an intermediate synchronous rate of 8/16/32 kbit/s data stream by the rate adaption function RAl. In a modification where the incoming data is already synchronous, the rate adaption function RAO can be omitted.

The V.llO rate adaption function RA2 is then performed to convert the intermediate rate data into synchronous 64kbit/s data stream, which is fed over an ISDN link 33 to TNM 1. Here, the synchronous 64kbit/s stream is subject to an inverse rate conversion RA2 back to an intermediate bit rate stream of 8/16/32 kbit/s. The intermediate stream is fed on line 34 to TMN 1. The data is thus configured in 80 bit frames according to the the V.llO ITU Recommendations .

At TNM 1, the synchronous intermediate rate data is subject to a rate conversion in accordance with the invention into a modified N.110 frame at an intermediate rate of 3.6, 6 or 12 kbit/s, referred to herein as rate adaption RAl'. The resulting data is fed on line 35 to the transmitter Tx of SBS 1, which assembles an appropriate TCH/SACCH for transmission via satellite 3a to user terminal UT 1.

As shown in Figure 7, user terminal UT 1 performs a terminal adaptation function (TAF). Receiver circuits Rx of the user terminal UT 1 receive and decode the data stream at the intermediate rate in the modified N.llO frame according to the invention and supply this synchronous data on line 36 to a rate adaptor which performs an inverse rate adaption RAl', to convert the data into a synchronous stream at 600-19200bit/s, in the conventional N.llO 80 bit frame format. The resulting synchronous data is then subject to an inverse rate adaption function RAO according to the V.llO Recommendations, so as to produce an asynchronous 300-33.4kbit/s output, which is fed to line 31 for onward transmission to the laptop computer LC. The data on line 31 is thus in a format compatible with the processing circuits of the LC, suitable for conversion to TCP-IP or other data processing protocols. It will be understood that for messages transmitted in a reverse direction from the laptop computer LC to the PC, the rate conversion scheme just described will be performed in an inverse manner.

Intermediate rate adaptation RAl' As previously explained, the rate adaptor RAl' used in TΝM 1 and UT 1 makes use of a modified V.llO frame. The modified frame uses as its starting point the conventional GSM modified V.llO frame, which, as previously explained, consists of 36 bits rather than 80 in the conventional, unmodified V.llO frame. The conventional, modified V.llO GSM frame is set out below:

- 77-

Conventional, modified V.llO GSM frame Dl D2 D3 SI D4 D5 D6 X

D7 D8 D9 S3 D10 Dll D12 S4

E4 E5 E6 E7 D13 D14 D15 S6

D16 D17 D18 X D19 D20 D21 S8

D22 D23 D24 S9

The number of bits of different categories per frame is thus as follows: total D bits = 24 (used to transport data) total S bits = 6 (SI, S3, S4, S6, S8, S9) total E bits = 4 (Used for timing correction) total X bits = 2

Thus, out of a total of 36 bits of the frame, 24 D bits are used to transport data and the remainder, namely the S, E and X bits referred to herein as control bits, are used for control purposes and constitute an unwanted, large overhead.

Considering the control bits in more detail, the mapping between the S and X bits to control circuits for the standard GSM 2400bit/s data service is shown in Table 2. Table 2 also includes the characteristics of the data bits D for comparison purposes. The circuit numbers provided in Table 2 together with the circuit names, correspond to those used in the ITU Recommendations concerning data transfer between a terminal or computer (referred to as DTE in the ITU Recommendations) and a modem (or DCE). The data flow direction is defined with respeα to the DTE and Table 2 covers data flow in both directions. The various circuit names and numbers shown in Table 2 can be summarised as shown in Table 3. The rate at which the values of the circuits are sampled or an approximate average sampling rate is shown in Table 2. 78 -

Table 2 - conventional frame

Table 3 - ITU circuit nomenclature

For further details, reference is directed to " Data Communications, Computer Networks and Open Systems" F. Halsall, Fourth Edition [ISBN 0-201-42293X] pp 82-83. In accordance with the Recommendations, the S bits can be conveniently considered in terms of subgroups SA and SB as follows: ^■ 79 -

SA <» SI, S3, S6, S8 (circuits CT 107, 108) SB <» S4, S9 (circuits CT 105, 109)

Referring again to Table 2, it can be seen that the fixed format, conventional, GSM modified V.llO frame gives rise to predetermined sampling rates for circuits 105, 106, 107, 108 and 109.

In accordance with the invention, it has been found that the data produced by these circuits changes at a much slower rate than the predetermined sampling rates provided by the conventional GSM frame. Thus, in accordance with the invention, it is not actually necessary to use such high sampling rates for the S and X bits.

Experiments carried out by the present inventors have determined that when sending e-mail messages, much lower sampling rates of the circuits can be used, for the SA, SB and X bits without loss of data integrity in transmitted messages.

In a practical example of an improved frame structure according to the invention, sampling rates were adopted as shown in Table 4. Table 4

Thus, the sampling rates for the bit groups SA, and SB is significantly lower that the rates adopted in the prior art, modified GSM V.llO frame shown in Table 2. The same is true for the sampling rate of the X bits. The E bits shown in Figure 5 are sampled at much the same rate as hitherto. The E bits, it will be recalled, are used for timing correction to ensure that the modem circuits at each end of the message transfer path are maintained in synchronism. In this example of the invention, it has been assumed that the sampling rate for the E bits needs to be maintained at a conventional rate although the present invention envisages the possibility of reducing the sampling rate for the E bits.

Considering again the conventional, modified V.llO GSM frame shown above, when the sampling rates shown in Table 4 are adopted, it is not necessary to send samples of the S bits and X bits with every frame. Thus, in accordance with the invention, the frame structure can be modified from that shown above to the arrangement shown below.

Example of a modified V.llO GSM frame according to the invention Dl D2 D3 54D* D4 D5 D6 X-D*

D7 D8 D9 5*D* D10 Dll D12 S4

E4 E5 E6 E7 D13 D14 D15 56D*

D16 D17 D18 XD* D19 D20 D21 S«D*

D22 D23 D24 S9

D* = extra data bit

In this example, extra data bits D* are inserted into the control bit positions which are no longer required to contain S bits and X bits due to the reduced sampling rate adopted in accordance with the invention. In this example, no X or SA bits need to be transmitted in the frame and so the relevant bit positions include data bits D*. In the example shown, it is assumed that SB bits need to be transmitted and so bits S4 and S9 are shown in the frame. It will however be appreciated that for other frames in accordance with the invention, none of the SA, SB and X bits will need to be transmitted in which case all of the SA, SB and X bit positions can contain extra data bits D*.

Thus, in accordance with the invention, the control bits SA, SB and X are - 27 -

transmitted within successive ones of the frames at their respective, improved, reduced sampling rates. Furthermore, they are replaced by data bits D* at the relevant bit positions where the bits are not transmitted.

The improved data capacity of the channel can be seen from Table 5. For a 2400 bit/s channel only half of the bit rate can be used for data bits according to the conventional GSM scheme. Thus, 50% of the bit rate of the channel was occupied by control bits, namely 1200 bit/s. In the comparable example according to the invention, the control bits only occupy 96 bit/s of the channel rate, so that the remainder - 2304 bit/s can be used for transmitting data.

Table 5

From the foregoing, it will be understood that according to the invention, the sampling rates for the SA, SB and X bits can be reduced compared to the conventional frame over a range of values and not just the specific example given in Table 4. It is convenient to measure the sampling rates of the SA, SB and X bits in terms of their occurence per number of data bits D. Satisfactory results can be obtained in accordance with the invention if the rates are within the following limits:

1 SA bit every 24 data bits D, through 1 SA bit every N2 data bits D 1 SB bit every N3 data bits D, through 1 SB bit every N4 data bits D 1 X bit every N5 data bits D, through 1 X bit every N6 data bits D

The invention is not restricted to the described satellite channels and can be used with conventional GSM, or other known mobile protocols, for example with the PLMN 9 over the link 11 shown in Fig. 1. The invention also can be used with other satellite systems such as the IRIDIUM system discussed hereinbefore. Whilst the user terminals have been described as mobile, it will be understood that they may be semi-mobile, e.g. mounted on a ship or aircraft, and may actually be stationary during operation.

Claims

1. A data signal for a mobile telecommunications system, comprising a signal carrier modulated to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, characterised in that some of the control bit positions of the frame contain data bits.

2. A data signal according to claim 1 wherein the control bit positions that contain data bits comprise S or X bit positions of the conventional frame.

3. A modulator for a mobile telecommunications system, to modulate a signal carrier so as to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, characterised in that the modulator is operable to insert data bits into some of the control bit positions of the conventional frame.

4. A demodulator for a mobile telecommunications system, to demodulate a signal carrier modulated to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, in which data bits are provided in some of the control bit positions of the conventional frame, the demodulator being configured to locate and the demodulate the data bits from both the data bit positions and from the synchronisation bit positions of the frame that contain data bits.

5. A user terminal including a demodulator according to claim 4.

6. A method of modulating a signal carrier to provide a data signal for a mobile telecommunications system, comprising modulating the signal carrier to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number of data bit and control bit positions, characterised by providing data bits in some of the control bit positions of the frame.

7. A method according to claim 6 wherein the control bits include SA bits with a sampling rate related to the rate of data bits D, as follows:

1 SA bit every 24 data bits D, through 1 SA bit every N2 data bits D

5

8. A method according to claim 6 or 7 wherein the control bits include SB bits with a sampling rate related to the rate of data bits D, as follows:

1 SB bit every N3 data bits D, through 1 SB bit every N4 data bits D

10 9. A method according to claim 6, 7 or 8 wherein the control bits include X bits with a sampling rate related to the rate of data bits D, as follows:

1 X bit every N5 data bits D, through 1 X bit every N6 data bits D.

10. A demodulation method for a mobile telecommunications system, to

75 demodulate a signal carrier modulated to provide a modified V.llO frame of the format conventionally used in GSM, with a predetermined number data bit and control bit positions, characterised in that data bits are provided in some of the control bit positions of the conventional frame, the method including locating and the demodulating the data bits from both the data bit positions and from the

20 control bit positions of the frame that contain data bits.

11. A method of modulating a carrier to provide sequential modified V.llO frames containing data bits D and control bits SA, SB and X characterised in that the sampling rates of the control bits relative to the rate of the data bits, are

25 as follows:

1 SA bit every 24 data bits D, through 1 SA bit every 80 data bits D 1 SB bit every 48 data bits D, through 1 SB bit every 240 data bits D 1 X bit every 48 data bits D, through 1 X bit every 240 data bits D.

30 12. A method according to claim 11 wherein the modulated carrier includes SA bits at a rate of the order of 16 bit/s, SB bits at a rate of the order of 40 bit/s and X bits at a rate of the order of 20 bit/s and the overall bit rate for the carrier is of the order of 2400 bit/s.