WO2001065727A1

WO2001065727A1 - Satellite communications with satellite routing according to channels assignment signals

Info

Publication number: WO2001065727A1
Application number: PCT/GB2001/000805
Authority: WO
Inventors: Dennis Roy Mullins
Original assignee: Ico Services Limited
Priority date: 2000-02-29
Filing date: 2001-02-23
Publication date: 2001-09-07
Also published as: GB0004856D0; US20020122408A1; GB2365677A

Abstract

A satellite system comprising at least one satellite (4); at least one Earth station (6), and a plurality of user terminals (2), the satellite (4) being arranged to provide a link between each user terminal (2) and the Earth station (6), via communications channels, each channel comprising one or more timeslots in a repeating time frame on one or more frequencies, carried by a feeder link beam between said satellite (4) and the or each said Earth station (6), and one of a plurality of user terminal link beams (B1-BN) between the satellite (4) and the user terminals (2), the satellite comprising a multiplexer (211) for multiplexing the channels from multiple said terminal link beams onto each said feeder link beam, and a demultiplexer (111) for demultiplexing the channels from onto each said feeder link beam onto multiple said terminal link beams; and further comprising at least one router (112, 212) for assigning channels to and from particular said terminal link beams in response to control signals from said Earth station (6), characterised in that the Earth station (6) is arranged to send, during a first said frame period, channel assignment signals relating to channel assignments in a following said frame period, and in that the satellite (4) is arranged to control the router (112, 212) in accordance with said channel assignment signals in said following frame period.

Description

S_ATELLITE COt_lMUNICATIO_NS WITH SATELLITE ROUTING ACCORDING TO CHANNELS ASSIGNMENT SIGNALS

This invention relates to satellite communications, and particularly to

multipoint satellite communications, such as mobile communications

Satellite mobile communications systems are well known In recent

years, a number of certain s stems

e been proposed, including the recently

launched Iridium system, and the proposed GlobalStar and ICO sy stems,

which are intended for communications with small mobile terminals such as

handsets

As use of information technology increases, there is an increasing

demand for bandwidth which is, however, a scarce resource for satellite

systems since they must avoid conflict with any terrestrial usages in many

different countries.

The present invention is intended to prov ide a bandwidth-efficient

satellite communications sy stem, particularly for relatively low power mobile

terminals

In one aspect, the invention provides a method of TDMA satellite

communications with a user terminal, in which the satellite separately routes

individual TDMA bursts of a given frequency channel and

said routing

from frame to frame. Other aspects and preferred embodiments of the invention, together

with corresponding advantages, will be apparent from the following

description, drawings and claims.

Embodiments of the invention will now be illustrated, by way of

example only, with reference to the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of

example only, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram showing schematically the elements of a

communications system embodying the present invention;

Figure 2 is a block diagram showing schematically the elements of an

Earth station node forming part of the embodiment of Figure 1 :

Figure 3 illustrates schematically the disposition of satellites forming

part of Figure 1 in orbits around the Earth;

Figure 4 illustrates schematically the beams produced by a satellite in

the embodiment of Figure 1 ;

Figure 5 is a cross section through a beam, showing gain against

angular displacement from the antenna boresight axis;

Figure 6a is a diagram showing illustrative a feeder link spectrum in a

first embodiment; and

Figure 6b is a diagram showing illustrative a user link spectrum in that

embodiment over time; Figure 7 is a block diagram illustrating schematically the structure of

the payload of a satellite transponder;

Figure 8 is a block diagram showing the structure of time frequency

demultiplexing apparatus within the payload of Figure 7;

Figure 9 shows schematically the content of a database forming part of

the satellite payload of Figure 7;

Figure 10 is a diagram illustrating the contents of a database forming

part of the Earth station of Figure 2;

Figure 11 is a flow diagram showing the operation of the satellite

payload of Figure 7;

Figure 12a is a flow diagram showing the process of setting up a

signalling connection carried out by the user terminal of the first embodiment;

and

Figure 12b is a flow diagram showing the corresponding process

performed by the Earth station;

Figure 13 is a flow diagram showing the process performed by the

Earth station in allocating TDMA channels; and

Figure 14 is a flow diagram showing the process performed by the

Earth station in the first embodiment in re-allocating TDMA channels.

FIRST EMBODIMENT

Referring to Figure 1 , a satellite communications network according to

this embodiment comprises mobile user terminal equipment 2a, 2b (e.g. handsets 2a and 2b); orbiting relay satellites 4a, 4b; satellite Earth station

nodes 6a, 6b; satellite system gateway stations 8a, 8b; terrestrial (e.g. public

switched) telecommunications networks 10a. 10b; and fixed

telecommunications terminal equipment 12a, 12b.

Interconnecting the satellite system gateways 8a. 8b with the Earth

station nodes 6a, 6b, and interconnecting the nodes 6a. 6b with each other, is a

dedicated ground-based network comprising channels 14a, 14b. 14c. The

satellites 4, Earth station nodes 6 and lines 14 make up the infrastructure of

the satellite communications network, for communication with the mobile

terminals 2, and accessible through the gateway stations 8.

A terminal location database station 15 (equivalent to a GSM HLR) is

connected, via a signalling link 60 (e.g. within the channels 14 of the

dedicated network), to the gateway station and Earth stations 6.

The PSTNs 10a, 10b comprise, typically, local exchanges 16a, 16b to

which the fixed terminal equipment 12a, 12b is connected via local loops 18a.

18b; and international switching centres 20a, 20b connectable one to another

via transnational links 21 (for example, satellite links or subsea optical fibre

cable links). The PSTNs 10a, 10b and fixed terminal equipment 12a, 12b

(e.g. telephone instruments) are well known and almost universally available

today.

For voice communications, each mobile terminal apparatus is in

communication with a satellite 4 via a full duplex channel (in this embodiment) comprising a downlink channel and an uplink channel, for

example (in each case) a TDMA time slot on a particular frequency allocated

on initiation of a call, as disclosed in UK patent applications GB 2288913

and GB 2293725. The satellites 4 in this embodiment are non geostationary.

and thus, periodically, there is handover of each user from one satellite 4 to

another.

Terminal 2

The user terminals (UT's) 2a, 2b may be similar to those presently-

available for use with the GSM system, comprising a digital low rate

coder/decoder, together with conventional microphone, loudspeaker, battery

and keypad components, and a radio frequency (RF) interface and antenna

suitable for satellite communications.

Each UT 2 comprises an omnidirectional antenna, i.e. an antenna

having generally satisfactory communications performance at all directions

above a certain minimum elevation above the horizon (such as ten degrees) so

as not to require pointing or steering to a satellite.

Small satellite communications terminals are currently available (with

omnidirectional antennas) for the Iridium system from Motorola Inc. and

(with steered antennas) for the Inmarsat-M and mini-M systems, for example.

Terminals may be connected, as shown, to data terminal equipment

160a. 160b such as a facsimile machine or a personal computer.

Earth Station Node 6 The Earth station nodes 6 are arranged for communication with the

satellites.

Each Earth station node 6 comprises, as shown in Figure 2. a

conventional satellite Earth station 22 (functioning somewhat equivalentlv to

the Base Station of a cellular system) consisting of at least one satellite

tracking antenna 24 arranged to track at least one moving satellite 4, RF

power amplifiers 26a for supplying a signal to the antenna 24, and 26b for

receiving a signal from the antenna 24; and a control unit 28 for storing the

satellite ephemera data, controlling the steering of the antenna 24. and

effecting any control of the satellite 4 that may be required (by signalling via

the antenna 24 to the satellite 4).

The Earth station node 6 further comprises a mobile satellite switching

centre 42 comprising a network switch 44 connected to the trunk links 14

forming part of the dedicated network. It may be. for example, a commercially

available mobile switching centre (MSC) of the type used in digital mobile

cellular radio systems such as GSM systems.

A multiplexer 46 is arranged to receive switched calls from the switch

44 and multiplex them into a composite signal for supply to the amplifier 26

via a low bit-rate voice codec 50. Finally, the Earth station node 6 comprises

a local store 48 storing details of each mobile terminal equipment 2a within

the area served by the satellite 4 with which the node 6 is in communication.

The local store 48 acts to fulfil the functions of a visited location register (VLR) of a GSM system, and may be based on commercially available GSM

products.

Alternatively, satellite control may be provided from a separate control

station.

Other Network Elements

The gateway stations 8a, 8b comprise, in this embodiment.

commercially available mobile switch centres (MSCs) of the type used in

digital mobile cellular radio systems such as GSM systems. They could

alternatively comprise a part of an international or other exchange forming

one of the PSTNs 10a, 10b operating under software control to interconnect

the networks 10 with the satellite system trunk lines 14.

The gateway stations 8 comprise a switch arranged to interconnect

incoming PSTN lines from the PSTN 10 with dedicated service lines 14

connected to one or more Earth station nodes 6.

The database station 15 comprises a digital data store which contains.

for every subscriber terminal apparatus 2. a record showing the identity (e.g.

the International Mobile Subscriber Identity or IMSI); the service provider

station 8 with which the apparatus is registered (to enable billing and other

data to be collected at a single point) and the currently active Earth station

node 6 with which the apparatus 2 is in communication via the satellite 4. Thus, in this embodiment the database station 15 acts to fulfil the

functions of a home location register (HLR) of a GSM system, and may be

based on commercially available GSM products.

Periodically, the Earth station nodes measure the delay and Doppler

shift of communications from the terminals 2 and calculate the rough

terrestrial position of the mobile terminal apparatus 2 using the differential

arrival times and/or Doppler shifts in the received signal. The position is then

stored in the database 48.

The Earth stations 6 are positioned dispersed about the Earth such that

for any orbital position, at least one Earth station 6 is in view of a satellite 4.

Referring to Figure 3, a global coverage constellation of satellites is

provided, consisting of a pair of orbital planes each inclined at 45 degrees to

the equatorial plane, spaced apart by 90 degrees around the equatorial plane,

each comprising ten pairs of satellites 4a, 4b, (i.e. a total of 20 operational

satellites) the pairs being evenly spaced in orbit, with a phase interval of zero

degrees between the planes (i.e. a 10/2/0 constellation in Walker notation) at

an altitude of about 10,000 km.

Thus, neglecting blockages, a UT at any position on Earth can always

have a communications path to at least one satellite 4 in orbit ("global

coverage").

Satellites 4 The satellites 4 comprise a bus module and a payload module. The

bus module comprises the elements of the satellite which are common to all

satellite applications.

Specifically, the bus module comprises a propulsion system

comprising thrusters for maintaining the satellite in its assigned orbital

position; a power subsystem comprising, for example, a pair of solar power-

wings pointed at the sun and a storage battery charged from the solar panel

and discharged when the satellite is not in view of the sun; and a thermal

control subsystem to dissipate heat.

Also provided are an attitude control subsystem arranged, in this case,

to direct the body of the satellite towards the Earth and the solar cells towards

the sun as described in our earlier application No. GB 2320232; and a

telemetry and command system by which the satellite transmits data

concerning its operating conditions and receives commands from a satellite

control centre causing it to. for example, adjust its position in orbit.

The bus may be, for example, the HS601 or HS601 high power

satellites, or the HS702 satellite, all available from Hughes Space and

Communications Company, in California, US.

Satellite Payload

Each satellite payload generates a plurality of spatially separated user

link radio frequency beams, B 1 -BN in a manner described in more detail below.

Each satellite also has an array of radiation reception directions which intercept the surface of the Earth; the reception directions roughly coincide with the

beams. The directions of the beams are at defined stereo angles with the

antenna centre axis or "boresight", which (in this embodiment) is directed

vertically towards the centre of the Earth. Each beam is directed towards a

respective user terminal. Thus, as shown in Figure 4. the beams are unevenly

distributed over the satellite footprint - i.e. the portion of the Earth visible from

the satellite (or which has visibility of the satellite above some minimum

elevation angle such as 10°).

Any beam which is not centred on the boresight axis will have a non-

circular profile, derived as the intersection of the conical beam with the

spherical surface of the Earth. The sizes and shapes of the beams therefore vary

with their positions (i.e. the position of the beam centre, referred to here as the

'beam aim point') on Earth.

The satellite also generates global uplink and downlink beams (e.g. a

beam covering the whole satellite footprint area of the Earth) for carrying

signalling traffic for setting up and pulling down calls, and requesting changes

to allocated channel capacity. These may be generated by the same array

antennas or by additional antennas (not shown).

Figure 5 illustrates the beam profile in section. The gain falls away

from a maximum value at the beam centre. Beyond some point (e.g. ldB or

3dB down) the beam may be unsuitable for use; this therefore defines the

"edge" of the beam. However, the beam continues to have an amplitude, and thus to be capable of interfering with other co-channel users, beyond this

"edge".

The satellite payload comprises at least one steerable high gain spot

beam antenna 3 providing a feeder link for communicating with one or more

fixed Earth stations 6 connected to telecommunications networks; a receive

array antenna 1 for receiving the plurality of reception directions Rl-RN; and a

transmit array antenna 200 for generating the plurality of beams B 1 -BN. The

antennas 1-3 are provided on the side of the satellite which is maintained facing

the Earth.

The transmit and receive antennas each comprise two dimensional array

antennas with, for example, a few hundred elements each.

A brief explanation of the access methods employed will now be given,

with reference to Figure 6. The feeder link antenna 3 operates at a transmit

frequency of 7 GHz and a receive frequency of 5 GHz. The receive array

antenna operates at a frequency of 2 GHz and the transmit array antenna at a

frequency of 2.2 GHz.

The bandwidth available for each channel is 4 KHz, which is adequate

for speech. Time Division Multiple Access (TDMA) is employed, with 40mS

frames. In the to-mobile direction, there are 36 timeslots in each repeating

frame, on frequency subcarriers each of 150KHz bandwidth. In the from-

mobile direction, there are 6 timeslots in each frame, on frequency subcarriers

each of 25KHz bandwidth. As is shown in Figure 6a. the feeder length spectrum consists of a set

of frequency channels Ul . U2 which, on the forward (to-mobile) link, are

each of 150 KHz bandwidth. No significant frequency guardbands between

frequency channels are provided, and no Doppler compensation is applied the

channels separately; thus, the spacing between nominal channel centre

frequencies is 150 KHz.

Each of the channels carries (in the forward direction) 36 timeslots.

The channels and timeslots are closely packed, to conserve the feeder

link spectrum and hence reduce interference with other systems.

A signalling channel S is provided within the feeder link spectrum.

The signalling channel carries timeslots permitting random access on

initiation of a call in the from-mobile direction, and paging time slots in the

to-mobile direction for paging a mobile terminal. It also carries timeslots for

instructing mobile terminals to change channel (i.e. change frequency or

timeslot).

Finally, a control channel C is provided within the feeder link beam,

which carries control signals for reconfiguring the satellite payload.

In the feeder downlink spectrum, user channels are similarly-

multiplexed together into a composite beam and the signalling channel carries

signals from mobile terminals (for example indicating signal strength or other

information). No control channel is provided. Referring to Figure 6b, the timing relationship between the control

channel, the signalling channel and the traffic channels is shown (in the to-

mobile direction). On each channel, a repeating frame structure of length

40mS is employed. Successive frames are separated by frame header

sequences for, amongst other things, synchronisation purposes.

On the control channel, payload configuration information is also sent

in time frames. The time frames are not aligned with those on the traffic and

signalling channels; each time frame on the control channel carries

configuration information for the next traffic channel time frame to begin after

the end of that signalling channel time frame.

Conveniently, the frequencies allocated to different satellites are such

that no two satellites whose footprints overlap (i.e. who can be seen

simultaneously from any point on the ground) share any common frequencies.

This is conveniently achieved by partitioning the available frequencies

between the two planes of satellites (or. in general. N planes) and then, within

each plane, re-using frequencies only on every alternate satellite (or. where

levels of coverage higher than double coverage are provided by the

constellation, on every Nth satellite).

Referring to Figure 7. the electrical arrangement provided within the

satellite payload comprises a forward link, for communicating from an Earth

station to a terminal, and a return link, for communicating from the terminal to

the Earth station. The forward link begins at the feeder link antenna 3. the feeder uplink

signals from which are bandpass filtered by respectiv e filters 206a-206d and

amplified by respective low noise amplifiers 207a-207d. The amplified signals

are combined and down-converted to an intermediate frequency (IF) by a

combiner/IF downconverter circuit 208. This IF signal is digitised by an

analogue to digital converter (ADC) 210.

The control channel of the feed link signal is separated by a filter (not

shown for clarity in Figure 7), and the information it carries is demodulated and

feed to the digital processor controller 13.

The digitised IF signals are each then frequency-demultiplexed and

down converted into separate 150KHz bandwidth baseband channels by a

demultiplexer 211.

Referring to Figure 8, the demultiplexer 21 1 comprises a frequency

demultiplexer 21 1a, and, fed from the outputs of the frequency demultiplexer, a

plurality of time demultiplexers 211b. 21 1c. ... 21 In. Each of the traffic

channels separated out by the frequency demultiplexer 21 1a is fed to a

respective time demultiplexer 21 1b, which routes each of the 36 timeslots in

round-robin fashion to a corresponding output port so as to produce, at each

output port I a signal burst on the Ith timeslot of each frame (where I ranges

from 1-36).

The separated frequency/time channels output from the output ports of

all of the time demultiplexers 211 b-21 In are fed to input ports of the routing 65727

15 network 212. The routing network 212 has one output port for each of the n

frequency channels and is arranged to route each of the input ports to one of the

output ports under the control of data from the digital processor controller 13.

Several different input ports (up to the number of timeslots; in this case 36) may

be connected to each output port.

Each of the router output ports is connected to -one of the input ports of a

digital beamformer 220, which generates a plurality of energising signals for

energising respective radiating elements 200a-200M of the transmit array

antenna 200.

The digital beamformer network comprises a Fast Fourier Transform

processor which accepts, from the digital control circuit 13, a set of control

parameters for each of the frequency and time channels. The control parameters

comprise:

• The amplitude for the user channel;

• The subcarrier frequency;

• The Doppler shift offset; and

• The angular direction of the beam.

The beamformer is arranged to synthesise beams each in the specified

angular direction with respect to the antenna boresight, at the specified

frequency, with the desired amplitude, by multiplying the signal by the

subcarrier frequency (including Doppler offset) . /65727

16 The energising signals are each converted to an analogue signal by a

respective digital to analogue converter (DAC) 215a-215N. the outputs of which

are up-converted to a beam frequency lying within a 30 MHz range in the 2.2

GHz band by an array of IF/S band converters, amplified by a bank of M RF

power amplifiers 217a-217M, and bandpass filtered by a bank of filters 218a-

218M, prior to being supplied to the respective radiating elements 200a-200M.

The components of the return link are, in general, the reverse of those in

the forward link. A plurality P of receiving elements 118a-1 18P receive

incoming radio signals in the 2 GHz band from user terminals 2 on the Earth.

The signal from each element is filtered and amplified by respective filters

118a-l 18P and low noise amplifiers 1 17a-l 17P, down-converted to a 5 MHz IF

signal by an array of down converters, and digitised by a respective ADC 115a-

115N and fed to the input ports of a digital beamformer 120.

The uplink beamformer 120 is arranged to apply the same direction

control data as the downlink beamformer 220, and amplitude and frequency

offset control data supplied from the digital control circuit 13 (in the latter case,

the Doppler offset is the same, but the frequency channel is different).

The signals at each of the N output ports of the beamformer 120

comprise 25KHz bandwidth channels each carrying 40mS TDMA frames

divided into 6 timeslots. They are time-demultiplexed and routed, under control

of the control circuit 13, through a router switch 112 to a predetermined input

(corresponding to a particular frequency) of a time and frequency multiplexer 1 1 1 generating 25 MHz output signals which are converted to analogue signals

by a DAC 1 10. The analogue signals are up-converted into 7 GHz signals by

an up converter and RF divider network 108.

As in the to-mobile direction, a time demultiplexer is provided on each

channel prior to the router switch 1 12, so that each timeslot can separately be

routed to a different part of the feeder downlink spectrum.

Each RF signal is amplified by an RF power amplifier (e.g. a travelling

wave tube or solid state amplifier device) 107a-107d; filtered by a bandpass

filter 1 16a-l 16d; and supplied to a feeder link antenna 3 for transmission to a

respective Earth station.

Thus, the system shown will be seen to consist of a feeder link

communication subsystem comprising the elements 3, 106-109 and 206-209; a

channel separation and combination subsystem comprising the elements 21 1-

212 and 1 1 1-112; and a mobile link communication subsystem comprising the

elements 215-218, 1 15-1 18. and antennas 1 and 200.

The digital control circuit 13 comprises a store 502 and a digital

processor 504. The store 502 is shown in Figure 9, and comprises static store

502a and a dynamically updated store 502b, each of which has an entry for each

beam. Each beam is associated with a user terminal 2, with which each entry is

therefore also associated. Each entry in the static table 502a comprises fields

storing: the beam number; data defining the position of the beam aim point on

Earth; the channels used (defined as frequency subcarriers of the forward and reverse link channels for the user the timeslots of the forward and reverse link

channels; and the power for the forward and reverse link channels for the user).

Each entry in the dynamic table 502b comprises the beam number; data

defining the beam direction (relative to the antenna) and the Doppler shift to

apply.

The digital processor 504 connected is to the store 502, and receives

control data from the Earth Station 6 in the control channel, in each time frame.

The control data specifies the user terminal positions, and time and frequencies

to be used for each, to be written to the store 502.

Referring to Figure 10, the database 48 of the Earth station node 6 in

this embodiment, comprises, for each terminal 2. a field defining the terminal

position on Earth (e.g. in latitude and longitude, or as a three dimensional

position relative to the centre of the Earth), a beam aim point position (which

will be discussed in greater detail below) in similar dimensional co-ordinates;

a beam power level specifying the power transmitted towards the user

terminal; a beam frequency field specifying the frequency of the beam

transmitted to the terminal (for example by specifying the frequency channel

used); and a time slot field specifying the time slot used for communication

by the terminal.

Operation of Satellite 4

The satellite 4 payload performs, essentially, two processing loops; a

first in which new beam control data is received from the Earth station 6, and /65727

19 a second in which beam directions and Doppler compensations are

periodically re-estimated to maintain direction and frequency accuracy.

Accordingly, as shown in Figure 1 1. in each uplink frame in a step

1002 the satellite 4 determines whether new user beam data is being received

from the Earth station 6 (on the control channel C) and. if so. in step 1004.

data is received, and written to the store 502 in step 1006.

In step 1008, a first beam is selected from those listed in the store and

in step 1010 it is determined whether the current beam is the last beam. If not.

in step 1012, the processor 504 calculates the Doppler shift to the user

terminal from the satellite, utilising the user terminal position data stored in

the table 502a, and the current satellite position (calculated from the satellite

orbital data, or from other sources such as a GPS receiver on the satellite) and

the satellite orbital speed (which is calculated from its orbit and position).

The Doppler shift information is then stored in the store 502b.

Next, in step 1016. the direction in which the beam is to be pointed

(from the satellite) is calculated by reading the beam aim point position data

from the table 502a and using the satellite position data as calculated above.

This too is written to the store 502b in step 1018.

On having processed the last beam (step 1010) in the table 502, the

control circuit 504 amends the router 212, 1 12 to take account of any new-

beam assignments from the Earth station 4, and sends the Doppler offset,

direction, and power control data to the beamforming network 220, 120. The 1/65727

20

process then returns to step 1002 to detect further uplinked beam data from

the Earth station 6.

The beamformers 220, 120 are operative thereafter to synthesise

transmission and reception beams with the designated power, direction and

frequency, towards the user terminal, allowing data transmission to take the

place in conventional fashion.

The process of Figure 8 needs to be repeated on each occasion when

data is received from the Earth station 6, since the beam aim points on the

ground may have changed, or frequency or timeslot allocations may alter as

more or less bandwidth is required.

Doppler shift and aim point recalculation needs to be repeated

sufficiently frequently to track the movement of the satellite in orbit; in other

words, sufficiently frequently that the movement of the satellite footprint on

the ground (determined by satellite altitude) in-between successive executions

of the process of Figure 8 is small compared to the width of the beams, so that

the gain of the link to the user is essentially unchanged between repetitions.

The processes of determining the beam aim points on the ground an

the frequencies to be used for particular users are described fully in our co-

pending UK Patent Application No filed on the same day as the present

application and carrying agents reference (J41746 GB). Accordingly, for

clarity, those processes will be omitted from the description of the present invention; they will be understood to be incorporated by reference herein in

their entirety.

Referring to Figures 12 on, the processes of channel allocation will

now be described. Where a mobile terminal 2 is to open a communications

session then, in step 1202, it generates a hailing signal on a random access

signalling frequency slot within the from-mobile signalling frequency S,

which passes through the satellite 4 and is relayed to the Earth station 6 where

it is received in the feeder downlink in step 1212. The Earth station 6

allocates a TDMA logical signalling channel for use setting up in the traffic

channels to be used by the mobile terminal subsequently.

In step 1215, the Earth station 6 sends a signal on the paging slot of

the forward link signalling frequency S channel indicating, to the mobile

terminal 2, the TDMA logical signalling channel it should use in future

communications. The signalling channel allocated with typically represent

one of a number of timeslots on the signalling frequency.

In step 1215, the logical channel identification is transmitted, via the

satellite, to the mobile terminal 2 where it is received in step 1204.

Subsequently (shown as stages 1206 and 1216), the mobile terminal and Earth

station 6 transmit signalling data over allocated signalling channel (strictly, a

forward link signalling channel and a return link signalling channel, for

communications to and from the mobile terminal respectfully). In particular, channel allocation data is transmitted to the mobile terminal on its allocated

signalling channel, as will be described in greater detail below.

Where a voice or data communications session is to take place (either

as a result of an incoming call or and outgoing call from the mobile terminal)

then in step 122, the mobile terminal 2 requests allocation of one or more

channels (e.g. one channel for a normal speech call:- two channels for a full

rate speech call; multiple to-mobile channels for a database access session; or

multiple from-mobile channels for a data upload session).

In step 1224, the requested frequency and timeslots are allocated as

disclosed in out above referenced co-pending application filed on the same

day as this application. In step 1224, the newly-allocated channels are

signalled, on the signalling channel, to the mobile terminal 2, to be used by

the mobile terminal 2 in the next time frame.

Having allocated user link time and frequency channels and signalled

this allocation to the user terminal 2. the Earth station 6 then allocates feeder

link time and frequency channels, by examining those frequency channels

already in use for vacant timeslots, and allocating such vacant timeslots. Only

where insufficient timeslots are available on the frequency channels currently

in use on the feeder link are new frequency channels used in the feeder link;

such channels are selected conveniently to be adjacent existing channels, so as

to control the total bandwidth from uppermost frequency edge to lower most

frequency edge of the feeder link. Likewise, when channels are de-allocated, the outermost frequency

channels in the feeder link are examined and re-allocated to the now vacant

timeslots further within the feeder link spectrum. Thus, the bandwidth

required by the feeder link spectrum is no greater than required at any time.

Having thus determined the frequency and timeslot on the user link

beams and the frequency and timeslots within the feeder link beams to be used

in the communication session, the routings to be performed by the routers

1 12. 212. in the satellite 4 in routing between the feeder link and service link

spectra are derived.

In step 1228, the channels are transmitted up to the satellite 4 on the

control frequency, for use by the controller 13 on the next time frame.

As described above, the satellite is arranged to read the channel

allocations and to set up the routers 212, 1 12 during the frame header period

before the next traffic channel, so that the change over from one channel to

another can take place simultaneously and immediately (next frame), at the

satellite and the mobile terminal and the Earth station.

The volume of re-allocation data carried on the control channel will

normally be relatively modest. This is because, relative to the volume of

other traffic, the number of new calls being set up or pulled down is small.

Changes in the channel allocation may also be performed by the Earth station

6 where a handover is to be performed, for example, due to motion of the

satellite or the appearance of an interferer or blockage. , ,,„„„ PCT/GB01/ 1/65727

24

Referring to Figure 14, during an existing call the channel allocation

may also be varied; for example, because the call had closed (in which case

the channel is cleared down).

Equally, for "bursty" data communications, where the data rate may

change within a call session, the terminal equipment 160, mobile terminal 2.

or Earth station 6 may accordingly vary the number of channels to be

allocated to the user terminal mid-session.

Where a mobile terminal 2 is arranged to handle variable bandwidth

communications, this may conveniently be by varying the number of timeslots

used by the mobile terminal.

As it is anticipated that the bandwidth in the to-mobile direction will

usually be large (to enable the mobile terminal to receive high bandwidth data

such as Internet or multimedia files, or television), more timeslots are

provided in the to-mobile direction to enable, in this case, 36 different

multiples of the basic 4KHz bandwidth to be employed by the mobile

terminal.

Alternatively, the mobile terminal 2 could be provided with a variable

acceptance width filter, to receive several adjacent frequency channels, or

with multiple RF receivers to receive several non-adjacent frequency channels

simultaneously.

Referring to Figure 14, accordingly in step 1232, the Earth station re¬

allocates channels in the same manner as the initial channel allocation, to avoid or reduce interference with other terminals; in step 1234 signals the new-

channel allocation on the signalling channel on the mobile terminal 2; and in

step 1236 signals the new allocation on the control channel to the satellite 4.

SECOND EMBODIMENT

In this embodiments, the invention is arranged to be capable of

altering the TDMA frame time; for example between 40mS and 240mS.

Accordingly, the time demultiplexers 21 1b. 21 1b ... are configurable to vary

the number of output ports they use. and the control channel is arranged to

carry TDMA length control data causing the digital control processor 30 to

control the number of time channels between which the time demultiplexers

distribute TDMA bursts.

Separate control information on the TDMA frame length (i.e. number

of bursts per frame) is transmitted on the control channel for each of the

frequency channels, enabling each time demultiplexer to be controlled

separately.

The multiplexers and demultiplexers at the Earth station 6 and user

terminal 2 (not shown) are correspondingly of controllably variable frame

length, and the Earth station 6 transmits reconfiguration information on the

signal channel to cause each mobile terminal 2 to vary its frame length

accordingly.

Thus, in this embodiment, the data rate may be varied even more

substantially than in the first, since not only may the number of timeslots within a frame used by a given user terminal may be varied by also the

number of timeslots available for use in the frame may also be varied.

Summary of Embodiments

The channel allocation signalling and set up methods described herein

will be seen to have various advantages when compared with those proposed

in the past.

Firstly, the separate demultiplexing of each timeslot within the

satellite allows more efficient use of the feeder band spectrum. This is

because feeder band frequency channels may be closely packed with

timeslots, even though, to reduce co-channel interference, such timeslots

could not all co-exist in the user link beams.

In a conventional satellite communications system, the required feeder

link spectrum corresponds to the number of frequency channels actually in use

in the user link spectrum (counting each re-use of a frequency channel

separately) so that, for example, where 2.000 frequency channels are in use

then the feeder link spectrum would require a bandwidth of 2,000 times the

bandwidth of a single frequency channel.

By contrast, according to the present invention if. say, only a single

timeslot is being used on each frequency channel in the user link then the

feeder link spectrum required can be divided by the number of timeslots

present, requiring in the above embodiment merely /₃₆ of the total bandwidth

which would be required in the prior art. By providing that the satellite 4 is capable of routing any

communications channel (i.e. frequency link/timeslot combination) through a

different frequency and/or timeslot in each frame, by signalling in one frame

to cause a changeover in the allocation in the following frame, the invention is

capable of handling bursty communications, and of performing rapid

handover.

It is also able to make highly efficient use of the satellite resources for

the transmission of packet switched data, by using spare traffic channel

capacity on a burst-by-burst basis, and thus accommodating the transmission

of packet switched messages of widely variable length and different

bandwidths.

Providing the Doppler correction at the satellite enables the channel

spacing on the feeder link to the Earth station node 6 to be reduced, since it is

not necessary to provide for the possibility of Doppler correction within the

feeder link; accordingly, the channels are closely multiplexed together in the

feeder link on adjacent frequency bands without substantial frequency guard

bands.

Since the satellite is calculating the Doppler compensation to be

applied for each channel, it is also convenient for the satellite to calculate the

beam directions as the satellite moves in orbit. Thus, it is only necessary for

the Earth station node 6 to transmit beam aim points on Earth, on a relatively

infrequent basis, rather than continually uplinking beam steering commands. This reduces the volume of signalling on the uplink control channels from the

Earth station node 6.

In other respects, however, the satellite is able to act as a transparent

transponder, repeating the signal from the feeder link on to the user terminal

beams and vice versa without needing to demodulate the signals (which

would require substantial on-board processing and could introduce additional

signal delays).

Other Embodiments

It will be clear from the foregoing that the above described

embodiment is merely one way of putting the invention into effect. Many

other alternatives will be apparent to the skilled person and are within the

scope of the present invention.

It would be possible to use the allocation methods described in the first

and second embodiments where a fixed grid of beams were provided, as an

alternative to a rigid frequency re-use pattern.

Whilst single beams allocated to each user have been described, it

would be possible (for example, where a large number of users are known to

be at almost exactly the same position on Earth) to provide a single beam

serving multiple users on a single or common frequency, allocating different

time slots to each.

The numbers of satellites and satellite orbits indicated are purely

exemplary. Smaller numbers of geostationary satellites, or satellites in higher altitude orbits, could be used; or larger numbers of low Earth orbit (LEO)

satellites could be used. Equally, different numbers of satellites in

intermediate orbits could be used.

Although TDMA has been mentioned as suitable access protocol, the

present invention may be applicable to other access protocols, such as code

division multiple access (CDMA) in which a limited number of codes, or non-

orthogonal codes are re-used or pure frequency division multiple access

(FDMA).

Equally, whilst the principles of the present invention are envisaged

above as being applied to satellite communication systems, the possibility of

the extension of the invention to other communications systems (e.g. digital

terrestrial cellular systems such as GSM) is not excluded.

It will be understood that components of embodiments of the

invention may be located in different jurisdictions or in space. For the

avoidance of doubt, the scope of the protection of the following claims

extends to any part of a telecommunications apparatus or system or any

method performed by such a part, which contributes to the performance of the

inventive concept.

Claims

1/6572730CLAIMS

1. A satellite system comprising at least one satellite (4); at least one

Earth station (6), and a plurality of user terminals (2).

the satellite (4) being arranged to provide a link between each user

terminal (2) and the Earth station (6). via communications channels.

each channel comprising one or more timeslots in a repeating time

frame on one or more frequencies, carried by a feeder link beam between said

satellite (4) and the or each said Earth station (6), and one of a plurality of

user terminal link beams (Bl-BN) between the satellite (4) and the user

terminals (2),

the satellite comprising a multiplexer (21 1) for multiplexing the

channels from multiple said terminal link beams onto each said feeder link

beam, and a demultiplexer (1 1 1) for demultiplexing the channels from onto

each said feeder link beam onto multiple said terminal link beams;

and further comprising at least one router (1 12. 212) for assigning

channels to and from particular said terminal link beams in response to control

signals from said Earth station (6),

characterised in that the Earth station (6) is arranged to send, during a

first said frame period, channel assignment signals relating to channel

assignments in a following said frame period, /65727

31 and in that the satellite (4) is arranged to control the router (1 12, 212)

in accordance with said channel assignment signals in said following frame

period.

2. A system according to claim 1 , in which said following frame period

is the next following frame period.

3. A system according to claim 2. in which the number of said slots in a

said frame in the from-terminal direction is different to that in the to-terminal

direction.

4. A system according to claim 3, in which the length of a said frame in

the from-terminal direction is the same as that in the to-terminal direction.

5. A system according to claim 3 or claim 4, in which the bandwidth

provided by each of said slots in a said frame in the from-terminal direction is

the same as that in the to-terminal direction.

6. A system according to any of claims 3 to 5. in which there are more

said timeslots in the to-terminal direction than in the from-terminal direction. /65727

32

7. A system according to any preceding claim, in which the number of

slots in a said frame is variable.

8. A system according to any preceding claim, in which the satellite (4)

comprises means (211b) for time-demultiplexing said slots of each frame and

said router (1 12, 212) is arranged to route slots of a -single frame to different

frequencies, or vice-versa, and to vary the routing of slots of a said frame on a

said frequency.

9. A system according to claim 8 appended to claim 7, in which the

length of the time-demultiplexer means (21 1b) is variable to accommodate

said variable number of slots.

10. A satellite system comprising at least one satellite (4); at least one

Earth station (6). and a plurality of user terminals (2),

the satellite (4) being arranged to provide a link between each user

terminal (2) and the Earth station (6), via communications channels.

each channel comprising one or more timeslots in a repeating time

frame on one or more frequencies, carried by a feeder link beam between said

satellite (4) and the or each said Earth station (6), and one of a plurality of

user terminal link beams (Bl-BN) between the satellite (4) and the user

terminals (2), J J the satellite comprising a multiplexer (21 1 ) for multiplexing the

channels from multiple said terminal link beams onto each said feeder link

beam, and a demultiplexer ( 1 1 1) for demultiplexing the channels from onto

each said feeder link beam onto multiple said terminal link beams;

and further comprising at least one router (1 12. 212) for assigning

channels to and from particular said terminal link beams in response to control

signals from said Earth station (6),

characterised in that there are more said timeslots in the to-terminal direction

than in the from-terminal direction.

11. A satellite system comprising at least one satellite (4); at least one

Earth station (6). and a plurality of user terminals^' (2),

the satellite (4) being arranged to provide a link between each user

terminal (2) and the Earth station (6), via communications channels,

each channel comprising one or more timeslots in a repeating time

frame on one or more frequencies, carried by a feeder link beam between said

satellite (4) and the or each said Earth station (6), and one of a plurality of

user terminal link beams (Bl-BN) between the satellite (4) and the user

terminals (2).

the satellite comprising a multiplexer (21 1) for multiplexing the

channels from multiple said terminal link beams onto each said feeder link beam, and a demultiplexer (1 1 1 ) for demultiplexing the channels from onto

each said feeder link beam onto multiple said terminal link beams;

and further comprising at least one router (1 12. 212) for assigning

channels to and from particular said terminal link beams in response to control

signals from said Earth station (6),

characterised in that the number of slots in a said frame can be varied.

12. A system according to claim 1 1, in which said number can be varied

independently for each said frequency channel.

13. A system according to any preceding claim, in which a single said

beam is provided for each said user terminal (2).

14. A system according to any preceding claim, comprising a plurality of

said satellites covering a region of the Earth.

15. A system according to claim 14. in which said satellites form a non-

geostationary constellation.

16. A system according to claim 15, in which said constellation provides

global coverage.

17. A system according to any of claims 14 to 16, in which the or each

satellite (4) comprises means for applying a Doppler shift correction to each

said beam.

18. A system according to any preceding claim, in which said user

terminals (2) comprise handheld terminals.

19. Channel allocation apparatus for use in the system of any preceding

claim.

20. Apparatus according to claim 19, said apparatus being provided at a

said Earth station (6).

21. A satellite for use in the system of any of claims 1 to 20.

22. A user terminal for use in the system of any of claims 1 to 20.

23. A method of TDMA satellite communications with a user terminal, in

which the satellite separately routes individual TDMA bursts of a given

frequency channel and varies said routing from frame to frame.

24. A method of TDMA satellite communications with a user terminal, in

which the number of said slots in a said frame in the from-terminal direction

is different to that in the to-terminal direction.

25. A method according to claim 24, in which the bandwidth of said slots

is the same in the from-terminal direction to that in the to-terminal direction.

26. A method according to claim 24, comprising varying the number of

said slots allocated to a user terminal.

27. A method according to claim 24, comprising varying the number of

said slots in a said TDMA frame.

28. A method according to claim 27, comprising varying the number of

said slots in a said TDMA frame on a first frequency differently to that on a

second frequency.