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AU2002228208A1 - Communication apparatus and method with links to mobile terminals via a satellite or a terrestrial network with partly reuse of the frequency band - Google Patents

Communication apparatus and method with links to mobile terminals via a satellite or a terrestrial network with partly reuse of the frequency band

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Publication number
AU2002228208A1
AU2002228208A1 AU2002228208A AU2002228208A AU2002228208A1 AU 2002228208 A1 AU2002228208 A1 AU 2002228208A1 AU 2002228208 A AU2002228208 A AU 2002228208A AU 2002228208 A AU2002228208 A AU 2002228208A AU 2002228208 A1 AU2002228208 A1 AU 2002228208A1
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AU
Australia
Prior art keywords
satellite
terrestrial
uplink
downlink
frequency
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
AU2002228208A
Other versions
AU2002228208B2 (en
Inventor
Patrick Chomet
Govindan Vishnu Nampoothiri
Paul Lucian Regulinski
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ICO Services Ltd
Original Assignee
ICO Services Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from EP01301226A external-priority patent/EP1231723B1/en
Priority claimed from GB0108501A external-priority patent/GB0108501D0/en
Priority claimed from US09/835,066 external-priority patent/US6950625B2/en
Application filed by ICO Services Ltd filed Critical ICO Services Ltd
Priority claimed from PCT/GB2002/000458 external-priority patent/WO2002065535A2/en
Publication of AU2002228208A1 publication Critical patent/AU2002228208A1/en
Application granted granted Critical
Publication of AU2002228208B2 publication Critical patent/AU2002228208B2/en
Anticipated expiration legal-status Critical
Expired legal-status Critical Current

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Description

COMMUNICATIONS APPARATUS AND METHOD
FIELD OF THE INVENTION
This invention relates to communications with a mobile user, and
particularly to such communications in which links to mobile users are via a
satellite or satellites, and also a relay station of a terrestrial network.
BACKGROUND OF THE INVENTION
Mobile satellite communication systems (MSSs) providing global
coverage are known. One such is the Iridium™ system, others are the ICO™
system the Globalstar System™, and the Teledesic™ system.
Since such systems operate globally (or at least, over a large part of
the Earth's surface) they need to use a band of frequencies which are available
all round the Earth.
Such MSS systems have inherent limitations in their capability to
provide services to users who are indoors and are in dense urban areas. Thus
the available frequencies for these systems are wasted in dense urban areas
and indoors.
Various Terrestrial mobile communications providing local
geographic coverage are know. Known systems include GSM and its
variants, CDMA, IS-136 and a variety of others using time division multiple
access (TDMA) and code division multiple access (CDMA) techniques. Code division multiple access is a so-called "spread spectrum" system,
in which a given mobile device communicates using a relatively wide band,
produced by multiplying the digital signal with a high bit rate ("chip rate")
code sequence. Each code sequence defines a separate code channel.
Such systems, even though they are efficient and cost-effective in
providing high capacity and coverage indoors and in dense urban areas, are
not efficient and cost effective in terms of providing coverage to vast thinly
populated rural areas.
Ideally, the satellite and terrestrial communications systems could be
allocated completely separately frequency ranges, and they would then not
interfere with each other.
Known systems like Iridium™ and Globalstar™ rely on roaming
between satellite and terrestrial systems, and use completely different
frequency spectrum for accessing the satellite and terrestrial systems.
However, roaming between satellite and terrestrial systems would be a waste
of valuable spectrum, considering that the spectrum used for Satellite
communication system cannot be used in dense urban areas and indoors,
while spectrum allocated for terrestrial use is not deployed in rural and ocean
areas.
Accordingly, the present invention is designed to increase the
possibilities for reusing the same channels (for example frequency channels)
between terrestrial and satellite mobile communication systems. US 5,394,561 discloses a mechanism for networking satellite and
terrestrial networks in which the power levels of the satellite and terrestrial
communications are controlled so as to minimise co-channel interference.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a communications system
comprising a satellite mobile communications network which comprises a
plurality of satellites and a plurality of user terminals communicating on
satellite uplink and downlink bands; and a terrestrial mobile communications
network which comprises a plurality of base stations and a plurality of user
terminal communicating on terrestrial uplink and downlink bands;
characterised in that at least one of the terrestrial bands at least partly reuses at
least one of the satellite bands.
Preferably, an embodiment provides a communication system where
the resource management, allocation and planning functions of the satellite
and terrestrial systems are linked together in such a way as to allow planned
reuse of the spectrum.
In another aspect, the present invention provides a frequency reuse
system comprising means for reducing localised reuse of said satellite uplink
and/or downlink in regions around one of said base stations.
Where the satellite downlink (typically the mobile link) shares the
same frequencies as the land network, dual mode terminals will be able to use the terrestrial network instead of the satellite network, and interference from
the satellite downlink into the land network is reduced. Indirectly,
interference from the satellite uplink is also reduced since terminals cease to
use the satellite service in the absence of downlink.
In another aspect, the present invention provides a frequency reuse
system, comprising means for transmitting a control signal to satellite user
terminals in regions around one of said base stations to cause said user
terminals to reduce (if necessary, to zero) use of said satellite uplink.
In another embodiment, the present invention provides a frequency
reuse system, comprising means for transmitting a control signal to satellite
user terminals in regions around one of said base stations to cause said user
terminals to use channels which are non-interfering with said terrestrial
network.
The control signal may be transmitted by the satellite. It may be a
modified version of a predetermined common control signal.
In another embodiment, the control signal may be transmitted by the
land network. As in the preceding embodiment, where the satellite uplink
shares channels with the terrestrial network, interference from the satellite
uplink is mitigated. Additionally, in this case, since the user terminal
responds to a signal transmitted by the terrestrial network itself, rather than by
the satellite as in the previous aspect, use of the shared spectrum on the satellite uplink is only suppressed when the user terminal is actually within
range of the terrestrial network.
In another aspect, the invention provides a dual mode user terminal in
which the satellite system shares frequencies with the terrestrial system, and
in which the user terminal is arranged to detect downlink or uplink
transmission on the terrestrial network, and to cease use of the shared part of
the satellite spectrum on detection thereof. Again, the terminal may cease
transmission on the satellite system, but alternatively it may be switched to a
non-interfering satellite channel.
In one particular aspect, the uplink and downlink frequencies of a
terrestrial network reuse the same frequencies as the satellite downlink but not
the satellite uplink. This has the substantial advantage that no uplink or
downlink transmissions from the terrestrial network are received by the
satellite; such transmissions from a base station or a large number of
terrestrial handsets could be more powerful than the weak signals transmitted
by a satellite handset and hence could potentially cause significant
interference.
In the reverse direction, the satellite downlink is low power because:
firstly, the battery and solar cell power available on the satellite is limited;
secondly, the path length travelled is long; and thirdly, satellite terminals
typically have higher sensitivity. Thus, the total power in the satellite
downlink is low and causes minimal interference. In a particular preferred embodiment, this aspect of the invention is
employed with a satellite using narrowband frequency, or frequency and time,
division multiplexing and a terrestrial network employing CDMA. Where
only a small number of satellite downlink transmissions are taking place, the
effect of these on each CDMA signal is limited since they occupy only a small
part of the CDMA spectrum. The interference from the satellite is thus even
less intrusive in this embodiment.
This aspect is particularly preferably employed with the first aspect of
the invention, in which case because the terrestrial network uses the satellite
frequencies in shadowed areas (such as urban areas and indoors), the satellite
downlink effect is reduced still further since the satellite signal is frequently
shadowed.
In another aspect, the invention provides a satellite system which
reuses radio spectrum with a terrestrial communications network, in which the
satellite uplink shares spectrum with the terrestrial uplink and the satellite
downlink shares spectrum with the terrestrial downlink. In this case, and
particularly when this aspect is combined with the first, the satellite uplink
causes relatively little interference at the terrestrial handset (and particularly
when the terrestrial network uses spread spectrum communication and the
satellite uses narrowband frequency division or frequency and time division
multiplexing). Also, in this embodiment or others it is particularly convenient to
provide a dual mode user terminal having common elements of the radio
frequency transmit and receive circuit, to which a separate terrestrial (for
example CDMA) and satellite (for example FDMA/TDMA) decoder and
demodulator are coupled.
In another aspect, the invention provides a satellite system which
reuses radio spectrum with a terrestrial communications network, in which the
uplink and downlink frequencies of the terrestrial network reuse the same
frequencies as the satellite uplink but not the satellite downlink.
This is particularly advantageous where data terminal equipment is
connected to the mobile terminals, since it is found that typical use of such
data terminal equipment is heavily asymmetrical; that is, much more
information is downloaded on the downlink (for example as a result of
downloading emails, and browsing or downloading files from the Internet)
than is transmitted on the uplink (which typically carries only selection and
navigation commands). There is therefore spare capacity on the satellite
uplink which can be reused for terrestrial communications.
Other aspects and preferred embodiments of the invention are as
described or claimed hereafter, with advantages which will be apparent from the
following.
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
first communications system embodying the present invention;
Figure 2a is an illustrative diagram showing schematically the elements
of mobile terminal equipment suitable for use with the present invention; and
Figure 2b is a corresponding block diagram;
Figure 3 is a block diagram showing schematically the elements of an
Earth station node forming part of the embodiment of Figure 1;
Figure 4a illustrates schematically the beams produced by a satellite in
the embodiment of Figure 1;
Figure 4b illustrates schematically the disposition of satellites forming
part of Figure 1 in orbits around the earth;
Figure 5 shows the arrangement of terrestrial base stations in the first
embodiment;
Figure 6 shows the frequency allocation in the first embodiment;
Figure 7 shows the frequency allocation in the second embodiment;
Figure 8 is a block diagram showing the user terminal of the second
embodiment;
Figure 9 shows the frequency allocation in the third embodiment;
Figure 10 shows the frequency allocation in the fourth embodiment;
and Figure 11 illustrates the uplinks and downlinks present in the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIRST EMBODIMENT
Referring to Figure 1, a satellite communications network according to
this embodiment comprises satellite 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 10; and fixed telecommunications terminal
equipment 12; terrestrial (e.g. public land) mobile telecommunications networks
(PLMNs) 110 and terrestrial mobile terminal equipment 112.
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 central database station 15 is connected, via a signalling link 60 (e.g.
within the channels 14 of the dedicated network) to the gateway station, earth
stations 6, and PLMN 110 (as discussed below). The PSTNs 10 comprise, typically, local exchanges 16... to which the
fixed terminal equipment 12... is connected via local loops 18; and international
switching centres 20... connectable one to another via national and
transnational links 21 (for example, satellite links or subsea optical fibre cable
links). The PSTNs 10 and fixed terminal equipment 12 (e.g. telephone
instruments) are well known and almost universally available today.
The PLMNs 110 comprise, typically, mobile switching centres (MSCs)
116 to which the terrestrial mobile terminals 112 are connected via local radio
paths 118 and base stations 119; and international gateways 20b.
In this embodiment, most or all of the satellite user terminals 2 are dual
mode and hence are also connectable via the terrestrial base stations 119.
For voice communications via the satellite network, each satellite
mobile terminal apparatus 2 is in communication with a satellite 4 via a full
duplex channel (in this embodiment) comprising a downlink (to-mobile)
channel and an uplink (from-mobile) 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 the user terminal from one satellite 4 to another.
For voice communications via the PLMN, each mobile terminal
apparatus 2, 112 is in communication with a mobile switching centre 116 via a base station 119 using an uplink frequency band and a downlink frequency
band.
Mobile Terminal 2
Referring to Figures 2a and 2b, a dual mode mobile terminal handset
equipment 2a of Figure 1 is shown.
It comprises a combination of a satellite handset, similar to those
presently available for use with the GSM system, and a terrestrial handset
suitable for third generation (3G) CDMA, W-CDMA or CDMA 2000.
communications.
The user interface components (microphone 36, loudspeaker 34, display
39 (for example a liquid crystal display) and keypad components 38) and power
supply (battery 40) are shared, i.e. used in both modes.
Apart from such common components (omitted for clarity from Figure
2b), the terminal comprises a CDMA functional unit 200a and a satellite
functional unit 200b. Each comprises a digital coder/decoder 30a, 30b; modem
32a, 32b; control circuit 37a, 37b; radio frequency (RF) interface 32a and 32b,
and antennas 31a and 31b, suitable for satellite and terrestrial mobile
communications respectively. The satellite antenna 31a has some gain in
directions above the horizon (it may be a Quadrifϊlar Helix or QFH antenna).
The terrestrial antenna 3 lb is roughly omnidirectional.
A 'smart card' reader 33 receiving a smart card (subscriber identity
module or SIM) 35 storing user information are also provided, connected to communicate with the satellite control circuit 37b. Specifically, the SIM 35
includes a processor 35a and permanent memory 35b.
The control circuits 37a, 37b (in practice integrated with the respective
codec 30) consist of a suitably programmed microprocessor, microcontroller or
digital signal processor (DSP) chip. Each control circuit 37 performs various
functions including framing speech and data into TDMA time frames for
transmission (and likewise demultiplexing received TDMA frames) or CDMA
sequences respectively; and performing encryption or enciphering.
Separate chipsets may be provided, each for implementing one of the
terrestrial and satellite system functionalities 200a, 200b. Alternatively, a single
processor may be programmed to perform the coding and control for both
functionalities. In each case, in this embodiment separate RF components are
provided, but user interface components are shared.
The mobile phone will then operate either as a satellite telephone or as
terrestrial phone, with the relevant functional unit 200a or 200b working
substantially independently and as it would do in a single mode phone.
The coder/decoder (codec) 30, 30b in this embodiment comprise a
coder, generating a speech bit stream at around 3.6 kilobits per second, together
with a channel coder 30b applying error correcting encoding, to generate an
encoded bit stream, and corresponding decoders.
In this embodiment the modems can support data rates of up to 384 kbps
also. A single mode satellite handset 2 would be as described, but lacking
the section 200b; and a terrestrial handset 112 is as described but lacking
section 200a.
Earth Station Node 6
The earth station nodes 6 are arranged for communication with the
satellites.
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.
Each earth station node 6 comprises, as shown in Figure 3, a
conventional satellite earth station 22 consisting of at least one satellite tracking
antenna 24 arranged to track at least one 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. 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.
Gateway 8
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.
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.
Database Station 15
The database station 15 comprises a digital data store, a signalling
circuit, a processor interconnected with the signalling circuit and the store, and a
signalling link 60 interconnecting the database station 15 with the gateway
stations 8 and Earth stations 6 making up satellite system network, for signalling
or data message communications.
It stores data for terminal apparatus 2, for example position data, billing
data, authentication data and so on, like the Home Location Register (HLR) of a
GSM system.
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 database station 15 in this embodiment also performs frequency
planning, to determine the frequencies to be used for communicating via the
satellites 4 with each of the satellite user terminals 2, and to control the use of
uplink and downlink frequencies thereby as further discussed below. The
database station 15 is accordingly connected additionally with the radio
spectrum allocation components of the PLMN 110 via the signalling link 60; it
can thereby communicate with the Mobile Switching Centre (MSC) or Base
Station Control centre which controls the frequencies used by the base stations
119b of the PLMN.
Periodically, the database station 15 transmits frequency allocation
information to the Earth stations 6 for use in the satellites 4, and PLMNs 110.
Satellites 4
The satellites 4a, 4b comprise generally conventional communications
satellite buses such as the HS601 available from Hughes Aerospace Corp,
California, US, and the payload may be as disclosed in GB 2288913. Each
satellite 4 is arranged to generate an array of beams covering a footprint beneath the satellite, each beam including a number of different frequency channels and
time slots, as described in GB 2293725 and illustrated in Figure 4a.
On each beam, the satellite therefore transmits a set of downlink
frequencies. The downlink frequencies on adjacent beams are different, so as to
permit frequency re-use between beams. Each beam therefore acts somewhat in
the manner of a cell of a conventional terrestrial cellular system. For example,
there may be 61, 121 or 163 beams.
In this embodiment each downlink frequency carries a plurality of time
division channels, so that each mobile terminal 2 communicates on a channel
comprising a given time slot in a given frequency.
The satellites 4a are arranged in a constellation in sufficient numbers
and suitable orbits to cover a substantial area of the globe (preferably to give
global coverage).
Referring to Figure 4b, 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 10,500 km (6 hour orbits). Thus, neglecting blockages, a UT 2 at any position on Earth can
always have a communications path to at least one satellite 4 in orbit ("global
coverage").
Base Station 119
The base station 119 comprises a CDMA base station having transmit
and receive antennas which are arranged to transmit signals on downlink
CDMA channels to mobile terminals, and to receive signals from mobile
tenninals on uplink CDMA channels. The downlink channels are provided, in
this embodiment, in a downlink frequency band and the uplink signals in an
uplink frequency band. At the base station 119, there is further provided a
conventional demodulator for demodulating the uplink signals to provide
digital data and for modulating digital data onto the downlink signals. Each
code channel may spread across the entire uplink or downlink spectrum in
known fashion.
Referring to Fig 5, the base stations 119 of this embodiment comprise
first base stations 119a, each of which define a reception cell around it, which
are deployed in suburban and rural areas as well as an in urban areas. In such
cases, the effective radio coverage of the cell will be of the order of several
kilometres or even tens of kilometres, depending upon the line of sight
visibility.
This embodiment also provides a second set of base stations 119b
which are provided in urban or built up areas. Each defines a "microcell" or "picocell" around it, to provide coverage in heavily shadowed or built up
urban areas. For example, within a building such as an airport or a train
station, or along an underground railway, a number of such picocell base
stations 119b are provided. Cover is therefore provided in areas where the
base stations 119a usually cannot communicate and satellites 4 will almost
never communicate.
The base stations 119 include base station control circuits which
allocate frequencies for communicating with mobile terminals 112.
The transmit and receive antennas at the base stations 119a are
generally constrained to broadcast preferentially in the azimuthal plane, for
example by using a suitable "apple core" torroidal or conical reflector antenna,
or are provided with some other beam shaping or directing means which
reduces the gain above the azimuth (i.e. the horizon) so that the beam shaping
effectively mitigates the interference to the satellites; the transmit and receive
antennas of the terrestrial handsets 112 are generally omnidirectional to
permit the handset to be used in any orientation.
Frequency Allocation
Figure 6 shows the frequency allocations in this embodiment. The
feeder link frequencies will not be discussed further in the following
embodiments, and the terms "satellite uplink" and "satellite downlink"
hereafter will refer to the mobile links. The satellite uplink bands may be within the 1985-2014 MHz range
and the satellite downlink bans within the 2170-2200 MHz band range. These
frequencies are generally referred to as S-band frequencies.
It will' be seen that the satellite uplink frequency band occupies
spectrum not shared by the terrestrial network. Thus, the (relatively powerful)
transmission from the terrestrial base stations (and terminals) will not interfere
with the (relatively weak) uplink signals received at the satellite from the
satellite mobile terminals.
Any non-urban base stations 119a may also have allocated uplink and
downlink bands which do not interfere with the satellite uplink or downlink
bands. As is conventional in cellular mobile systems, this spectrum is reused
in geographically separate cells. In urban areas, there is a requirement for
additional capacity since more users are present per square kilometre.
In this embodiment, the additional capacity is provided by reusing the
satellite downlink, as shown in Figure 6, to provide additional terrestrial
uplink and downlink bands. In this embodiment, the terrestrial uplink and
downlink each occupy the same frequencies but are separated by a frequency
space to permit frequency duplex separation within the base stations and the
mobile terminals.
These are used by the base stations 119b in microcells and picocells;
for the purpose of this embodiment, these are cells located inside buildings or
tunnels. (For reasons discussed further below, additional frequencies are present within the satellite downlink band which are not occupied by the
additional terrestrial uplink and downlink frequencies.)
In such areas, the satellite downlink is frequently attenuated by
ceilings or walls. Since the power radiated by the satellite is relatively low
and the path length is relatively long, and the antenna used by a terrestrial user
terminal has a relatively low gain (and/or G/T measure) as it is
omnidirectional, the level of interference from the satellite into the terrestrial
terminal is minimal.
Referring to Figure 5, in this embodiment one or more of the base
stations 119a which are located in urban areas may also make use of the
additional frequency bands used by the pico base stations 119b. This is
because, as shown in Figure 5, the level of shadowing by buildings makes
communication with satellites 4a, 4b difficult; only on the rare occasions
when a satellite 4c is in an unobstructed line of sight to a user terminal will
the user terminal be affected by the satellite downlink.
Thus, to sum up, in this embodiment, the frequencies used by the
terrestrial base stations 119 are allocated so that in urban or other shadowed
areas, the additional terrestrial uplink and downlink bands lying within the
satellite downlink band are utilised. The specific frequency channels within
the satellite downlink band are reused for uplink and downlink of the
terrestrial basestations located in urban and suburban areas. In this embodiment, as the satellite communicates with each user on a
narrow frequency channel, even if communication with a relatively small
number of satellite users continues in the urban area, the interference with the
terrestrial uplink and downlink frequencies will be minor because, due to the
spectrum spreading of CDMA, the interference on one particular frequency is
(up to a certain level) absorbed in the CDMA error correction decoding.
Thus, small numbers of (inherently low) satellite channels merely slightly
raise the noise floor.
Likewise, as the CDMA signals are spread over a wide spectrum, the
noise power contributed by the PLMN 110 into any one of the narrow satellite
communication frequency channels is low.
In this embodiment, however, the effects of such residual interference
are reduced yet further by controlling the broadcast from the satellites 4. The
orbits of each of the satellites 4 are characterised to a high degree of accuracy,
and their inclinations are actively controlled to maintain their beam directions
accurately pointing to Earth. Each of the satellite spot beams has a radius of
the order of some tens or hundreds of kilometres. The spot beams of each
satellite overlap, and those of one satellite overlap with those of another in the
most regions of the Earth and at most times.
In this embodiment, the database station 15 maintains a database
recording the positions of base stations 119b using the additional frequency
bands which lie within the satellite downlink. The frequencies allocated to a given spot beam (which are dictated by a routing table held within each
satellite and periodically reprogrammed from within the data base station 15)
are controlled so that the frequencies first allocated (i.e. preferentially
allocated when available) are those from the region of the satellite downlink
not in use by terrestrial base stations 119. Thus, a number of satellite handsets
may be operated without any possibility of interference with the terrestrial
network.
As the database station 15 communicates with the PLMNs 110
periodically, it is able to vary the frequency allocations depending on the
instantaneous loads (i.e. demand for service) on the terrestrial and satellite
networks.
Frequencies are preferentially allocated from opposite ends of the
shared frequency band; thus, for example, each time a new satellite
communication channel is to be added, the next available frequency down
from the high frequency end of the band may be allocated, whereas where
additional terrestrial capacity is to be allocated, then frequencies may be
allocated from the next available frequency up from the low end of the band.
Where such frequencies are exhausted, the next to be allocated to calls
may be those which overlap with the terrestrial uplink band. In this
embodiment it is easier to mitigate interference on the terrestrial uplink, since
each base station 119b can be provided with sophisticated interference
reduction techniques to reduce the effect of such interference. Finally, the last to be allocated are the frequencies sharing the
terrestrial downlink frequency band in any spot beam which covers the area
overlying one of the base stations 119b.
When the interference due to the above allocation exceeds a certain
limit in such a way that this affects the capacity of the terrestrial network,
dynamic reallocation of the uplink and downlink frequencies used by a
number of terrestrial stations and the satellite network is performed so that the
overall interference is kept to a minimum. For example, the set of frequencies
used by one spot beam of the satellite and its neighbours, and/or by one of the
base stations and its neighbours, are varied to allocate non-interfering
channels to the satellite or the base station or both, exchanging those channels
with those of a neighbour.
The control channels of the satellite and/or the base station may
reallocate channels used on existing calls, by signalling to the terminals to
hand over to a new frequency channel.
Thus, in the preferred arrangement of this embodiment, the satellite
downlink signals are selectively controlled to areas of coverage which include
base stations 119b which are reusing the satellite downlink frequency, so as to
mitigate the interference with the terrestrial system.
Under circumstances where it is impossible to allocate non-interfering
spectrum to satellite users, it would be possible for the satellite system to
signal terminals to cease use of the satellite network. It might be thought that the loss of satellite capacity would preclude economic operation of a satellite
system in this case. However, according to this embodiment it is envisaged
that the vast majority of satellite user terminals 2 will be dual mode terminals
as illustrated in Figure 2. Accordingly, in areas where the satellite has shut
down service, coverage through the terrestrial base stations 119 will be
available.
SECOND EMBODIMENT
Referring to Figure 7, in this embodiment, the satellite uplink is reused
by the second set of base stations 119b in urban areas as a terrestrial uplink,
and the satellite downlink is reused by those base stations as a terrestrial
downlink.
As in the preceding embodiment, the between the satellite system and
the terrestrial system is similarly small because of the blocking and
shadowing effects of buildings.
In this embodiment, additional measures are taken to limit the
interference from the satellite user terminals into terrestrial base stations
(satellite uplink into terrestrial uplinks), by providing that the satellite user
terminals detect a signal indicating the possibility of interference, and in
response cease to transmit satellite signals on the interfering channels and use
non-interfering channels where available.
Again, in this embodiment, it is envisaged that most of the satellite
user terminals are dual mode terminals. Referring to Figure 8, in this embodiment, as distinct from the preceding embodiment, since the satellite
uplink and downlink spectra are the same as the additional terrestrial uplink
and downlink spectra, some of the radio frequency components can be reused.
Figure 8 is based on Figure 2b, and like components are omitted from Figure
8 for clarity.
In this embodiment, separate satellite and terrestrial antennas 31a, 31b
are maintained, since although the area of spectrum occupied is the same, the
satellite antenna preferably has a higher gain above the horizon whereas the
terrestrial antenna will generally be omnidirectional.
A common RF amplifier block 52 comprising a low noise amplifier
54b on the downlink and power amplifier 54a on the uplink is provided,
connected switchably to either of the antennas 31a, 31b. The amplifier
section 52 is connected to a common up/down converter block 58 consisting
of an up converter converting from baseband to RF and a down converter
converting from RF to base band with a pair of switchable bandwidths
corresponding to those of the satellite communications channels (which are
relatively narrow) and terrestrial communications channels (which are
relatively broad).
At base band frequency, the signal is then routed between the
converter block 58 and the separate codecs etc as discussed in relation to
Figure 2b. Thus, the expensive RF components need not be duplicated, resulting in reduced cost, weight and power consumption. Single mode
satellite handsets 2 would omit the CDMA codec portion shown in Figure 8.
In this embodiment, a special code indicating the frequency channels
used and the location of the terrestrial base station is defined for transmission
on the broadcast common control channel in each spot beam. When a spot
beam overlies a base station 119b, which reuses the satellite frequencies, the
code is broadcast. When it is received by any satellite user terminal 2, the
user terminal 2 responds by ceasing all uplink transmissions in shared
channels by the satellite codec, until a control channel is detected in a satellite
downlink spot beam on which the control signal is not being broadcast or the
contents of the control signal indicates a different frequency (indicating that
the user terminal is now within coverage of a spot beam that does not overlap
the terrestrial base stations 119b).
The above described embodiment has the effect of causing all satellite
user terminals 2 which can receive the downlink on a beam which overlies
one of the base stations 119b to cease to generate satellite signals on the
shared frequency channels. However, firstly, since the beam may cover a
wider area than the cell surrounding the base station 119b, many satellite user
terminals 2 which could otherwise communicate with the satellite without
interfering with the terrestrial base station 119 are adversely affected.
Secondly, satellite user terminals 2 which cannot receive the signal concerned
(for example because of fading or blockage) may nonetheless broadcast on the satellite uplink channel and hence interfere with the terrestrial base station
119.
To resolve the first of these problems, rather than sending a broadcast
mode control signal which is to be acted upon by all satellite user terminals 2
within the beam, the position of each satellite user terminal 2 is registered
with the data base station 15 (either by incorporating a GPS receiver within
each user terminal which reports its data to the satellite periodically, or by
using a range and Doppler position sensing technique as described above).
The data base station 15 compares the position of each to data defining
the coverage area of each of the base stations 119b, and when a satellite user
termmal 2 is detected to be within one of the coverage areas, a control signal
of the type discussed above is transmitted specifically to that satellite user
terminal on a dedicated control channel therefore, or on a broadcast channel
with a user terminal address decodable thereby. Thus, only those terminals
which are detected as being likely to interfere cease to be able to use the
satellite system.
Alternatively, where the handsets are aware of their own positions (for
example each is equipped with a GPS receiver), the control signal may specify
the co-ordinates of the coverage area of the base station 119b and each
satellite user terminal 2 may be arranged to terminate uplink transmissions on
shared frequency channels only if it lies within that coverage area. To deal with the second problem identified above, in an alternative
embodiment, rather than making the satellite user terminals responsive to a
control signal broadcast in the downlink from the satellite to cease satellite
mode transmissions, the terrestrial base stations 119b which reuse the satellite
uplink and downlink are equipped with a transmitter arranged to transmit the
control signals.
The satellite codec within each satellite user terminal 2 which can
receive transmissions from the base station 119b (and hence might generate
uplink transmissions which would interfere with reception by that base
station) is arranged, on detecting the control signal, to cease transmissions by
the satellite system codec on shared frequency channels.
The control signal might simply be a beacon, broadcast at a
predetermined frequency. Alternatively, it might emulate one of the satellite
broadcast control channels.
Thus, in this embodiment, with some small modification to the base
stations 119b, only those satellite user terminals 2 which are actually within
range of the base station 119b are made unable to communicate with the
satellite 4 using shared frequency channels, and this is achieved regardless of
whether the satellite downlink can be received by them or not. The terrestrial
mobile codec of the dual mode terminal 2 in this embodiment (as in the last)
does not require modification. This embodiment is effective not only with
dual mode satellite terminals sets 2 but also with satellite user terminals which lack a terrestrial mobile codec, because the broadcast signal from the base
station 119b is received and acted upon by the satellite system codec.
Finally, rather than modifying the base stations 119b, it is possible
instead to modify the terrestrial mobile codecs of dual mode satellite user
terminals 2, so that such terminals continuously monitor the downlink for
signals from a terrestrial base station 119b. On detection of a CDMA
downlink signal, the terrestrial codec sends a control signal to the satellite
system codec indicating the detected terrestrial frequencies, to cause the
satellite system codec to cease it's transmission on shared channels, and
switch to free satellite channels. On loss of signal from the base station 119,
after a predetermined time without signal from the base station, the terrestrial
system codec issues a control signal to the satellite system codec permitting
use once more on the shared channels when necessary.
This embodiment therefore has the advantage that minimal
modifications to the terrestrial base stations 119 are required.
It will be seen that this embodiment, in which the satellite uplink
spectrum is also available for terrestrial mobile uplink and the satellite
downlink spectrum is also available for terrestrial mobile downlink, provides
more bandwidth to the terrestrial network for sharing than the preceding
embodiment, and enables common RF components to be used in the satellite
user terminal 2. Unlike the preceding embodiment there is also the possibility of
interference on the satellite uplink and the terrestrial uplink.
Since satellite uplink channels are on narrow frequency bands, the
effect on the broadband terrestrial CDMA uplink channels of any residual
satellite uplink transmissions is merely to increase slightly the noise floor
experienced.
THIRD EMBODIMENT
Referring to Figure 9, in this embodiment, the frequency reuse of the
previous embodiment is reversed. That is to say, the satellite uplink is reused
by the terrestrial downlink and vice versa. Thus, transmissions in the
downlink from the satellite do not affect the terrestrial handsets, but could be
received by the terrestrial base stations 119b. Each such base station can,
however, be protected from transmission from above by an overlying metal
plate, or by suitably designing the antennas to reduce the gain and sidelobes in
higher elevation angles and also by pointing the antennas tilted down from the
horizontal.
Thus, such shielding or beam shaping, in addition to the shadowing
and blockage caused by the deployment of the base stations 119b indoors and
in urban areas, substantially reduces the power levels on the satellite downlink
reaching the base stations 119b. For similar reasons, and because the antennas of the base stations 119b
are intended to broadcast predominantly in the azimuthal plane, the impact of
the terrestrial downlink on the satellite uplink is minimal.
As in the preceding embodiments, the antennas of the base stations
may either broadcast preferentially in the azimuthal plane or in all directions
other than above the azimuth, so as to reduce the power broadcast towards,
and reduce the sensitivity to signals from, the satellite 4.
Although the signals transmitted on the terrestrial and satellite uplink
by user terminals are of lower amplitude due to the lower power available in
the user terminals, it is noted that the terrestrial uplink signals transmitted by
terrestrial handsets could interfere with the satellite downlink signals received
by satellite mode handsets, and vice versa, where active terrestrial 112 and
satellite mode 2 handsets are close to each other.
Accordingly, in this embodiment, the techniques discussed in the
above first and second embodiments in reducing satellite transmissions on the
satellite downlink and handset transmissions on the satellite uplink are
preferably employed.
Alternatively, each dual mode handset 2 of this embodiment may be
arranged to detect CDMA transmission on the terrestrial uplink (i.e. the
terrestrial uplink frequency used by other terrestrial handsets)through the
satellite receiver . On detection of the frequency of transmissions from a
terrestrial handset 112, the satellite system codec is instructed to cease transmissions on any shared frequency channels on the satellite uplink. Thus,
where a dual mode terminal 2 is close enough to a terrestrial mode terminal
112 to detect transmission from it (and hence is likely to interfere with it)
potentially interfering transmissions from the dual mode handset 2 are
terminated.
This embodiment has the advantage that the relatively powerful
satellite and terrestrial downlink transmissions are received at the satellite 4
and the base station 119, rather than at the user terminals 2, 112, making
interference at the user terminals less likely than interference at the satellite 4
and base station 119. Since it is easier to provide sophisticated interference
mitigation and cancellation techniques, of the type described in our earlier
applications WO 00/48333, WO 00/49735 or WO 00/35125 for example, at
the network side rather than within the user terminals, the effects of any such
interference can more easily be mitigated.
FOURTH EMBODIMENT
Referring to Figure 10, in this embodiment, as in the first, the
terrestrial uplink and downlink frequency bands both occupy one of the
satellite frequency bands. In this embodiment, however, it is the satellite
uplink frequency band which is shared. This can be accomplished by placing
a frequency gap between the terrestrial uplink and downlink bands, allowing a
frequency division duplexer to separate the bands in the handsets and the base
stations. This embodiment is advantageous in situations where many satellite
user terminals 2 are connected to data terminal equipment such as personal
computers, personal digital assistants or other devices. Typically, such
devices are used to download emails; or to download files via the Internet
using either file transfer protocol (FTP) or hyper text transfer protocol (i.e.
"web browsing").
In such uses, the uplink needs to carry only occasional control and
navigation commands specifying files to be downloaded, or acknowledging
receipt of data, relative to the heavy usage of the satellite downlink. There is,
therefore, considerable scope for reusing the satellite uplink.
As the data rate on satellite uplink channels will be low, they are
inherently more immune to the additional noise generated by the wide band
CDMA PLMN traffic if each satellite uplink channel is allowed to occupy the
same bandwidth as the satellite downlink channel. Alternatively, the satellite
uplink channels may be allocated a narrower bandwidth, for example by time
division multiplexing a higher number of uplink channels together. The
unused uplink channel frequencies thus released are available for terrestrial
reuse.
Summary Of Interference Modes And Effects
Figure 11 shows the satellite and terrestrial uplink and downlinks.
Table 1 : Interference Modes
Referring to Table 1, the potential modes of interference in each of the
above embodiments are briefly discussed, together with the techniques
preferred for mitigation thereof. It will be seen that in the first embodiment
there is potential interference from the satellite into the terrestrial base station
119 and user terminal 112; and from the terrestrial base station 119 and user
terminal 112 into the satellite user terminal 2. In the second embodiment there is potential interference from the satellite 4 into the terrestrial user
terminal 112 and vice versa, and from the base station 119 into the satellite
user terminal 2 and vice versa.
In the third embodiment there is potential interference from the
satellite 4 into the terrestrial base station 119 and vice versa, and from the
satellite user terminal 2 into the terrestrial user terminal 112 and vice versa.
In the fourth embodiment there is potential interference in the satellite
user terminal 2 into the terrestrial base 119 and user terminal 112, and from
the terrestrial base station 119 and user terminal 112 into the satellite 4.
Satellite 4 into Base Station 119
Using representative figures, providing a substantial number of
continuous satellite downlink channels (of the order of 40) would increase the
noise level, and hence reduce the effective cell size allowable for the
terrestrial base station 119 by the order of 60%. To reduce this impact on the
PLMN 110, the following measures are proposed:
Providing suitable gain reduction above azimuth (for example by
shielding, beam shaping or both) as discussed above can provide up to 25 dB
discrimination, halving the reduction in cell size.
As disclosed above, initial allocation by the database station 15 of
non-interfering channels to the satellite and terrestrial networks reduces the
impact of the interference. Subsequently, shared frequency channels are only
allocated up to a predefined limit, which is decided by the amount of interference. Finally, dynamically controlling the number of channels which
overlap the PLMN bandwidth which can be allocated to satellite uplinks and
downlinks depending on the relative loading of the two networks, further
assists in reducing the impact. As most or all satellite terminals 2 will be dual
mode, they can also operate on the terrestrial PLMN 110 where needed.
In combination, these techniques greatly mitigate the impact of
interference between the terrestrial and satellite systems.
Satellite 4 into Terrestrial Mobile Terminal 112
Using similar figures, it is estimated that up to 40 satellite downlink
channels could reduce the effective terrestrial cell size by of the order of 50%
where these interfere with the terrestrial downlink.
To mitigate this, as above, the data base station 15 initially
preferentially allocates channels to the satellite terminals 2 which do not
overlap with the terrestrial spectrum, and dynamically controls the number of
satellite channels sharing the spectrum where it is not possible to avoid
overlap. The combination of these techniques effectively mitigates potential
interference.
Satellite User Terminal 2 into Terrestrial Base Station 119
The radio horizon experienced between the base station antenna and
the user terminal will prevent interference from user terminals more than, say,
30 km from the base station. However, within that distance, and to the extent not obstructed by obstacles, user terminals 2 can interfere into the terrestrial
base station 119b where the uplink spectra are shared.
To mitigate the interference, firstly, the satellite terminals 2 are made
dual mode, and are controlled as described above to operate as terrestrial
mobile terminals and consequently to inhibit satellite uplink transmissions
while within the coverage of terrestrial base stations 119. This virtually
eliminates the interference except where a satellite user terminal 2 is outside
the coverage of a base station 119b but close enough to interfere with it, or
where the satellite terminal is single mode only.
Secondly, as discussed above, the database station 15 takes advantage
of its knowledge of the locations of the base stations 119 and the user
terminals 2, to dynamically limit the uplink frequency assignments for user
terminals 2 close to a base station 119, to channels which do not overlap the
base station receive band. As the maximum interference range is estimated to
be around 30 km, different allocations can be made within different parts of
each satellite beam.
The combination of these techniques virtually eliminates the potential
interference in this mode. Satellite User terminal 2 into Terrestrial Mobile Terminal 112
The radio horizon between two handheld terminals is only around 8
km, so that terminals further away from this will not interfere with each other
even where the uplink spectra are shared.
To mitigate this interference, the same techniques as in the previous
interference mode are operated, with the same results.
Terrestrial Base Station 119 into Satellite 4
As noted above, shadowing substantially reduces the direct line of
sight from urban base stations to the satellite. Further, as discussed above, the
base station antennas are preferably designed to minimise gain at angles above
the horizon, giving up to 25 dB discrimination in the direction of the satellites.
Further, the database station 15 initially assigns satellite uplink channels
which do not overlap with the base station emission bandwidth for satellite
beams which overlap terrestrial base stations 119b; shared channels are only
assigned as necessary.
Thirdly, as discussed above, dynamic control of the number of shared
channels allocated to satellite uplinks depending on relative loading of the two
networks is performed.
Terrestrial Base Station 119 into Satellite User Terminal 2
The interference situation is essentially the reverse of that for satellite
user terminal interference into the terrestrial base station, and the same
techniques are used to mitigate interference. Terrestrial Mobile Terminal 112 into Satellite 4
To mitigate this interference, the data base station 15 initially assigns
satellite uplink channels which do not overlap with the base station downlink
bandwidth in areas where satellite beams overlap cells of base stations 119b;
shared channels are only assigned as the satellite or terrestrial systems reach
capacity. Secondly, the number of channels shared is dynamically controlled
in dependence on the relative loading of the two networks and thereby the
interference between both the networks are minimised.
Terrestrial Mobile Terminal 112 into Satellite User Terminal 2
As above, the maximum interference range between the two user
terminals is only 8 km. To mitigate interference, firstly, as above, the fact that
the satellite terminals 2 are dual mode causes them to operate whenever
within range of a base station 119b as terrestrial mobile terminals, which
eliminates most of interference except where the terrestrial terminal 112 is in
communication with a base station 119b and is within range of a satellite user
terminal 2 which is blocked or otherwise prevented from communicating with
the terrestrial network 119b.
Secondly, the data base station 15 in conjunction with the MSC 116
dynamically allocates the satellite and terrestrial uplink and downlink
frequency assignments for satellite user terminals near a base station 119 to
channels that do not overlap each other. OTHER EMBODIMENTS
It will be clear from the foregoing that the above described embodiments
are merely a few ways 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.
For example, although the above-described embodiments mention base
stations sited indoors or in urban areas, and thus make use of the potential
shadowing there, it will be clear that the various interference mitigation
techniques and spectrum reuse techniques described could also be used with
base stations sited additionally or alternatively in suburban or rural areas.
Although in the above embodiments a limit is set on the number of
interfering frequencies used, a limit based on other criteria such as the total
interfering power (calculated for example taking into account path loss and
power used on each channel, and/or other criteria), may instead be used.
Whilst in certain of the above embodiments, frequency duplexing is
used to share satellite spectrum between the terrestrial uplink and downlink,
time division duplex between the terrestrial uplink and downlink could
alternatively be used (as in certain existing terrestrial networks).
It will be clear that other possibilities for reuse by terrestrial networks
of the satellite spectrum exist, making use of the above described observations
and techniques. Further, the above described techniques may be combined. Further, it will be clear that each of the above described techniques for
reducing the interference between the satellite and terrestrial systems, or for
detection techniques to do so, may be employed separately of the others, in
other similar interference scenarios.
Whereas in the above described embodiments a dual mode user
terminal comprises a common housing and user interface containing separate
satellite system and terrestrial codecs, other constructions are possible; for
example it could comprise separate single mode terminals interconnected by a
wire or a wireless interface.
The CDMA can be third generation wide band CDMA (W-CDMA) or
CDMA 2000.
Whereas a TDMA/FDMA satellite system and CDMA terrestrial system
are described above, in principle the satellite system could be CDMA and the
terrestrial system TDMA/FDMA or FDMA.
The numbers of satellites and satellite orbits indicated are purely
exemplary. Smaller numbers of geostationary satellites (for regional coverage),
or satellites in higher altitude orbits, could be used; or larger numbers of low
earth orbit (LEO) satellites could be used, as disclosed in EP 0365885, or
publications relating to the Iridium or Teledesic systems, for example. Equally,
different numbers of satellites in intermediate orbits could be used. In principle,
even flying platforms such as balloons or aircraft are 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)

  1. WE CLAIM:
    1. A communications system comprising a satellite mobile
    communications network which comprises a plurality of satellites and a
    plurality of user terminals communicating on satellite uplink and downlink
    bands; and a terrestrial mobile communications network which comprises a
    plurality of base stations and a plurality of user terminals communicating on
    terrestrial uplink and downlink bands; characterised in that at least one of the
    terrestrial bands at least partly reuses at least one of the satellite bands.
    2. A system according to claim 1, in which said base stations
    comprise second base stations reusing said satellite bands, said second base
    stations being provided only in areas where the path from said satellites to the
    user terminals will be shadowed.
    3. A system according to claim 2, in which said areas are
    enclosed spaces.
    4. A system according to claim 2, in which said areas are urban
    areas.
    5. A system according to claim 1, in which said satellite mobile
    communications network communicates in frequency-divided fashion, using
    relatively narrow frequency channels within said bands.
    6. A system according to claim 1, in which said terrestrial mobile
    communications network communicates in code-divided fashion, using
    relatively wide frequency channels within said bands.
    7. A system according to claim 1, in which said terrestrial uplink
    and downlink bands at least partly reuse said satellite downlink band.
    8. A system according to claim 7, in which said terrestrial bands
    do not reuse said satellite uplink band.
    9. A system according to claim 1, in which said terrestrial uplink
    and downlink bands at least partly reuse said satellite uplink band.
    10. A system according to claim 9, in which said terrestrial bands
    do not reuse said satellite downlink band.
    11. A system according to claim 1, in which said terrestrial uplink
    band reuses said satellite uplink band, and said terrestrial downlink band
    reuses said satellite downlink band.
    12. A system according to claim 1, in which said terrestrial
    downlink band reuses said satellite uplink band, and said terrestrial uplink
    band reuses said satellite downlink band.
    13. A system according to claim 1, further comprising a channel
    allocator allocating channels to be used by at least one of said networks, in
    dependence upon the frequencies allocated to the other.
    14. A system according to claim 13, in which the channel allocator
    is arranged to control the frequencies allocated to both said networks.
    15. A system according to claim 13, in which the channel allocator
    is arranged to allocate a channel for use by a terminal to communicate with
    one of said networks initially from a set of frequencies not used by the other
    said network in the region of the terminal, where such a non-interfering
    frequency is available.
    16. A system according to claim 13, in which the channel allocator
    is arranged to allocate a channel for use by a terminal to communicate with
    one of said networks from a set of frequencies also used by the other said
    network in the region of the terminal, provided that less than a predetermined
    measure of interference is thereby reached.
    17. A system according to claim 16, in which said level is
    determined by a number of said channels.
    18. A system according to claim 16, in which, when said level is
    reached, the channel allocator is arranged to use frequency planning and
    terminal and network location information to dynamically allocate shared
    frequency channels.
    19. A dual mode user terminal for use in a system according to any
    claim 1.
    20. A terminal according to claim 19, in which there is provided a
    common radio frequency circuit shared by a satellite system control circuit
    and a terrestrial system control circuit.
    21. A terminal according to claim 19, arranged to cease usage of
    frequencies shared between the satellite and terrestrial systems on detection of
    predetermined conditions associated with the proximity of said terrestrial
    mobile communications network, to prevent interference therewith.
    22. A terminal according to claim 21, in which the predetermined
    conditions comprise detection of a control signal transmitted by a said
    satellite.
    23. A terminal according to claim 21, in which the predetermined
    conditions comprise detection of a signal transmitted by a said base station.
    24. A terminal according to claim 21, in which the predetermined
    conditions comprise detection of a signal transmitted by a user terminal in the
    terrestrial uplink band.
    25. A satellite communications network for use in the system of
    claim 1.
    26. A network according to claim 25, comprising a control station
    arranged to reduce use of said satellite downlink and/or uplink in regions
    around one of said base stations.
    27. A network according to claim 25, comprising a control device
    arranged to transmit a control signal to satellite user terminals in regions
    around one of said base stations to cause said user terminals to reduce use of
    said satellite uplink.
    28. A terrestrial communications network for use in the system of
    claim 1.
    29. A network according to claim 28, comprising a control device
    arranged to transmit a control signal to satellite user terminals in regions
    around one of said base stations to cause said user terminals to reduce use of
    said satellite uplink.
    30. A method of allocating communications spectrum to base
    stations of a terrestrial mobile communications network, in which a frequency
    band interferes with channels of a satellite communications system,
    comprising allocating said frequency band preferentially to base stations in
    areas where shadowing will reduce the level of communications with the
    satellites of said satellite communications system.
    31. A method of reusing frequency bands between base stations of
    a terrestrial mobile communications network and a satellite communications
    network, comprising allocating said frequency bands using integrated resource
    management and other mitigation techniques in a way to minimise
    interference between both the systems and thus making optimum usage of
    valuable frequency spectrum.
AU2002228208A 2001-02-12 2002-02-04 Communication apparatus and method with links to mobile terminals via a satellite or a terrestrial network with partly reuse of the frequency band Expired AU2002228208B2 (en)

Applications Claiming Priority (7)

Application Number Priority Date Filing Date Title
EP01301226A EP1231723B1 (en) 2001-02-12 2001-02-12 Dual mode terrestrial satellite mobile communications apparatus and method
EP01301226.5 2001-02-12
GB0108501A GB0108501D0 (en) 2001-04-04 2001-04-04 Communications apparatus and method
GB0108501.8 2001-04-04
US09/835,066 US6950625B2 (en) 2001-02-12 2001-04-16 Communications apparatus and method
US09/835,066 2001-04-16
PCT/GB2002/000458 WO2002065535A2 (en) 2001-02-12 2002-02-04 Communication apparatus and method with links to mobile terminals via a satellite or a terrestrial network with partly reuse of the frequency band

Publications (2)

Publication Number Publication Date
AU2002228208A1 true AU2002228208A1 (en) 2003-02-20
AU2002228208B2 AU2002228208B2 (en) 2007-05-10

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US (2) US7155161B2 (en)
AU (1) AU2002228208B2 (en)
CA (1) CA2440609C (en)
EA (1) EA005472B1 (en)
WO (1) WO2002065535A2 (en)

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