WO2017032737A1

WO2017032737A1 - Spot-beam satellite

Info

Publication number: WO2017032737A1
Application number: PCT/EP2016/069793
Authority: WO
Inventors: Anselm Jeyakumar Singarajah
Original assignee: Avanti Communications Group Plc
Priority date: 2015-08-21
Filing date: 2016-08-22
Publication date: 2017-03-02
Also published as: GB2534253A; GB2534253B; GB201514933D0

Abstract

A method of controlling a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving one or more client systems in the respective area; the method comprising: using a first one of the beams to transmit a first signal comprising a sequence of first symbols, by modulating a first carrier; using a second one of the beams to transmit a second signal comprising a sequence of second symbols, by modulating a second carrier; wherein the symbols of the first beam are aligned or at least overlap in time with the symbols of the second beam, and the first and second carrier share a same carrier frequency or at least an overlapping frequency band; and wherein the method comprises controlling the first signal to be out-of-phase relative to the second signal.

Description

Spot-Beam Satellite

Technical Field The present disclosure relates to communicating via a spot-beam satellite, which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface.

BackRround

Some regions of the world such as rural, developing or isolated areas often have limited communication infrastructure, to the extent that it may not be feasible to provide highspeed broadband internet access through traditional, ground-based means such as a wired network or even ground-based cell towers or the like. Providing an internet link via satellite enables such regions to obtain modern standards of internet access without the need to build a large amount of new infrastructure on the ground. Furthermore, satellite-based internet access can even be used as an alternative to ground-based means in regions that do have a developed communication infrastructure, or as backup to such infrastructure in case a ground-based link fails.

To provide internet access via satellite, the satellite provides a satellite link between each of a plurality of client systems and a gateway earth station ("the gateway" for short); and the gateway connects to an internet, i.e. a wide area internetwork such as that commonly referred to as the Internet (capital I). Thus each of the client systems is able to gain access to the internet via the satellite link with the gateway, and the connection between gateway and internet. Each client system could be anything from an individual unit in the home, to a local network serving a whole office, school, hospital, village, community or the like.

Satellites can also be used for other types of communication, not just internet access.

Referring to Figure 2, a satellite can be deployed in a geostationary orbit and arranged so that its field of view or signal covers roughly a certain geographic region 200 on the Earth's surface. Figure 2 shows South Africa as an example, but this could equally be any other country or region within any one or more countries (e.g. a state, county or province, or some other non-politically defined region). Furthermore, using modern techniques the satellite can be configured as a spot-beam satellite based on a beam-forming technology, so that the communications between the satellite and the client systems being served in the covered region 200 are divided amongst a plurality of spatially distinct beams 202. Each beam 202 is directed in a different respective direction such that beams are arranged into a cluster, each beam covering a different respective (sub) area on the earth's surface within the region 200 in question (though the areas covered by the beams 202 may be arranged to overlap somewhat to avoid gaps in coverage). This is a way of increasing capacity, as the limited frequency band of the satellite (e.g. Ka band or Ku band) can be re-used separately in different beams 202 - i.e. it provides a form of directional spatial division multiplexing. By way of example Figure 2 shows five beams 202a-202e which between them approximately cover the area of South Africa, but it will be appreciated that other numbers and/or sizes of beam are also possible.

Although the beams are spatially distinct, covering different respective areas, the beams may nonetheless be deliberately arranged to overlap in space around the edges, in order to try to avoid gaps in coverage. Furthermore, even if not designed to be overlapping, the "edge" of a beam or its coverage area is a matter of definition and hence somewhat notional. Typically it is defined as the contour surface (if talking about the beam) or contour line (if talking about the corresponding coverage area on the ground) at a certain predefined value of some measure of beam power (e.g. intensity or irradiance), for instance a certain fraction of the maximum such as the half-maximum surface or line. Although the drop-off is designed to be relatively sharp around the edge, the beam power does not drop to zero outside the notional edge of the beam, and may in principle taper off indefinitely.

Hence the signals of one beam 202a will encroach to some extent on the area of another beam 202b, and vice versa. Typically to avoid interference, the beams are arranged so that adjacent beams in the cluster (e.g. 202a and 202b) operate on different frequency channels (exclusive frequency bands), and sometimes also different polarization channels (orthogonal polarization). Summary

Nonetheless, the inventors have recognized that even this arrangement is not always adequate, as even the next-but-one adjacent beam may interfere to a non-negligible extent (e.g. 202a and 202c in Figure 2). Alternatively or additionally, it may be desirable to allow adjacent beams in a cluster to share at least a same or overlapping frequency band, and/or a same polarization or non-orthogonal polarization. To address such concerns or similar, according to the present invention the signals of potentially interfering satellite beams are controlled to be out-of-phase with one another, thus mitigating the inter-beam interference.

Hence according to one aspect disclosed herein, there is provided a method of controlling a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving one or more client systems in the respective area; the method comprising: using a first one of said beams to transmit a first signal comprising a sequence of first symbols, by modulating a first carrier; using a second one of said beams to transmit a second signal comprising a sequence of second symbols, by modulating a second carrier; wherein the symbols of the first beam are aligned or at least overlap in time with the symbols of the second beam, and the first and second carrier share a same carrier frequency or at least an overlapping frequency band; and wherein the method comprises controlling the first signal to be out-of-phase relative to the second signal.

In embodiments, each of the symbols of the first and second signals may have a common symbol period, and said control of the first signal to be out of phase with the second signal may comprise: controlling the first symbols to be offset in time from the second symbols by a time offset that is a fraction of said symbol period. Preferably said time offset is half the symbol period. Alternatively, said control of the first stream to be out of phase with the second stream may comprise: controlling the first carrier to be out of phase with the second carrier by a phase offset. In embodiments, the modulation of each of the first and second carriers may be by phase modulation, comprising modulating the carrier in units of a phase shift unit; and said phase offset may be a fraction of the phase shift unit. For instance, each of the first and second carriers may comprises an I subcarrier and a Q subcarrier being 90 degrees out of phase with the I subcarrier, and said control of the first stream to be out of phase with the second stream may comprise: controlling the I subcarrier of the first signal to be out of phase with the I subcarrier of the second signal by said phase offset, and also controlling the Q, subcarrier of the first signal to be out of phase with the Q subcarrier of the second signal by said phase offset. Preferably, where the modulation of each of the first and second carriers is by n-fold phase shift keying, with the phase shift unit being 360/n degrees, then said phase offset is 360/(2n) degrees. In further embodiments, relative to the first beam, the second beam may be the nearest beam in the cluster sharing the same carrier frequency or frequency band.

In embodiments, the first carrier may have a same polarization as the second carrier or at least a non-orthogonal polarization relative to the second carrier. E.g. relative to the first beam, the second beam may be the nearest beam in the cluster sharing the same polarization.

In embodiments, the first signal may comprise a signal from a hub of a first vendor, and the second signal may comprise a signal from a hub of a second vendor; and said control may comprise coordinating the hub of the first vendor with the hub of the second vendor in order to control the relative phase of the first and second signals.

In embodiments, the first signal may comprise a signal from a first hub of a given vendor, and the second signal may comprises a signal from a second, separate hub of the same vendor; and said control may comprise coordinating the first and second hubs of said same vendor in order to control the relative phase of the first and second signals. According to another aspect disclosed herein, there is provided a satellite earth station arranged to control a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving a respective one or more client systems in the respective area; wherein the satellite earth station is configured to perform operations according to any of the above methods.

In embodiments, the earth station may also be arranged as a gateway to an internet, and one or both of the first and second signals may comprise a signal communicated from the internet to one or more of the respective client systems.

According to another aspect disclosed herein, there is provided a computer program product for controlling a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving one or more client systems in the respective area; the computer program product comprising code embodied on a computer-readable storage medium and configured so as when run on one or more processors operating a satellite earth station, to perform operations according to any of the above methods. Brief Description of the Drawings

To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made by way of example to the accompanying drawings in which:

Figure 1 is a schematic illustration of a system for providing internet access via satellite,

Figure 2 schematically illustrates the geographic coverage of a cluster of satellite beams, Figure 3 is a schematic illustration of a part of a system for providing internet access via a plurality of satellite beams, Figure 4A is a schematic block diagram of a gateway earth station,

Figure 4B is a schematic block diagram of a plurality of client systems, Figure 5 is a schematic constellation diagram for a phase modulated signal,

Figure 6 shows a pair of respective schematic constellation diagrams for a pair of phase modulated signals that are arranged to be out-of-phase with one another, Figure 7 is a schematic illustration of inter-beam interference,

Figure 8 schematically illustrates the in-phase (I) and quadrature (Q) components of a quadrature phase shift keying (QPSK) scheme, Figure 9 schematically illustrates the in-phase (I) and quadrature (Q) components for each of a pair of beams that are arranged to be out-of-phase with one another,

Figure 10 is a schematic timing diagram in which the symbol periods of a pair of signals are time-shifted to be out-of-phase with one another,

Figure 11 is a schematic timing diagram of a quadrature phase shift keying (QPSK) scheme,

Figure 12 shows a schematic timing diagram for each of a pair of QPSK signals phase-shifted relative to one another, and

Figure 13 is a timing diagram in which a pair of QPSK signals are time-shifted to be out-of- phase with one another.

Detailed Description of Embodiments

Figure 1 gives a schematic overview of a system 100 for providing access to an internet 102, i.e. a wide area internetwork such as that commonly referred to as the Internet (capital I). The system 100 comprises: a gateway earth station (gateway) 104; at least one satellite 110 in orbit about the earth; and multiple client systems 112 remote from the gateway 104, located in a region on the earth's surface to which internet access is being provided. The gateway 104 comprises one or more satellite hubs 402a, 402b each connected to the internet 102, and at least one antenna 106 connected to the hubs 402. Each of the client systems 112 comprises a satellite modem 420 and an antenna 114 connected to the modem 420. The satellite 110 is arranged to be able to communicate wirelessly with the hubs 402 of the satellite gateway 104 via the gateway antenna 106, and with the modems 420 of the client systems 112 via the respective client antennae 114, and thereby provide a satellite link 107 between the gateway 104 and each of the client systems 112 for transmitting internet traffic between a source or destination on the internet 102 and the client systems 112. For example the satellite links 107, hubs 402 and modems 420 may be arranged to operate on the Ka microwave band (26.5 to 40 GHz), or Ku band (12-18 GHz). Each satellite link 107 comprises a forward link 107F for transmitting traffic originating from an internet source to the client systems 112, and a return link 107R for transmitting traffic originating from the client systems 112 to an internet destination.

Each of the hubs 402a, 402b serves (provides a respective internet access service to) a respective one or more of the client systems 112, so that internet traffic can be transmitted and received between the client systems 112 and the internet 102 via the satellite links 107 with the respective hub 402a, 402b. Note that a client system 112 is referred to herein as a client from the perspective of the satellite system 100, i.e. at least in that it is a client of the satellite-based service provided by the respective hub 402. In most embodiments (at least in the case of providing internet access), most or all of the client systems 112 will also be clients of one or more servers on the internet 102 (e.g. web servers, an email server, a VoIP server, etc.). However, it is not necessarily excluded that one or more of the client systems 112 could alternatively or additionally act as a server from the perspective of the internet 102 (e.g. providing web content to other users on the internet 102). The end users 116 may be individual people, and/or organisations such as businesses.

Depending on implementation, one, some or all of the client systems 112 may each comprise a router, switch, bridge, access point or gateway of a local communication infrastructure, via which internet access is provided to the user equipment of a plurality of end-users within the region in question. E.g. the local communication infrastructure may comprise a relatively short range wireless network or a local wired infrastructure, such as a local area network (LAN), metropolitan area network (MAN), or campus or corporate area network (CAN), connecting onwards to a plurality of home and/or business routers and/or individual user devices in the region. Alternatively or additionally, one, some or all of the client systems 112 may each comprise an individual, private device, each with its own satellite antenna 114 and modem 420 for connecting to the satellite 110 along with a local interface (e.g. a home router or access point) for connecting to one or more respective user devices. For example an individual femtocell or picocell could be located in each home or business, each connecting to a respective one or more user devices using a short range wireless technology, e.g. a local RF technology such as Wi-Fi.

Note also that the gateway 104 is not limited to being implemented as a single earth station at a single geographic site, and the gateway 104 could instead comprise multiple earth stations (each comprising at least one respective antenna 116) networked together over multiple different geographic sites.

Referring to Figure 2, the satellite 110 is deployed in a geostationary orbit and arranged so that its field of view or signal covers roughly a certain geographic region 200 on the Earth's surface. As mentioned, Figure 2 shows South Africa as an example, but this could equally be any other country or region within any one or more countries (e.g. a state, county or province, or some other non-politically defined region). Furthermore, referring to Figures 2 and 3, the satellite 110 is configured as a spot-beam satellite based on a beam-forming technology, so that the communications between the satellite 110 and the client equipment 112 in the covered region 200 are divided amongst a plurality of spatially distinct beams 202. Each beam 202 is directed in a different respective direction such that beams are arranged into a cluster, each beam covering a different respective (sub) area on the earth's surface within the region 200 in question (though the areas covered by the beams 202 may be arranged to overlap somewhat to avoid gaps in coverage). This is a way of increasing capacity, as the limited frequency band of the satellite 110 (e.g. Ka band or Ku band) can be re-used separately in different beams 202 - i.e. it provides a form of directional spatial division multiplexing (though adjacent beams may still use different bands or sub-bands, especially if they overlap in space). By way of example Figure 2 shows five beams 202a-202e which between them approximately cover the area of South Africa, but it will be

appreciated that other numbers and/or sizes of beam are also possible.

In one model the operator of the satellite 110 and/or gateway 104 acts as an upstream internet service provider (ISP), i.e. providing bandwidth to multiple different downstream ISPs, each of whom in turn acts as a vendor providing an internet access service to a respective group of end users 116. Furthermore, typically each of the downstream ISPs operates - to an extent - based on its own proprietary technology, and accordingly also therefore acts as a provider of some of the software and/or hardware resources in the satellite system 100. For instance, each downstream ISP may require its own respective hub 402 in the gateway earth station 104, and/or may act as a vendor of its own respective client systems 112 for use by its own respective end-users 116.

This is illustrated in more detail in relation to Figures 1, 4A and 4B.

Figure 4A is a block diagram showing part of the gateway 104 in more detail. As shown, the gateway 104 comprises a plurality of satellite hubs 402, each of a different respective vendor (in this case a different downstream ISP). For example, the gateway 104 may comprise the gateway antenna (or antennae) 106 and one or more buildings which house the hubs 402 along with various infrastructure is provided so as to connect the hubs to the internet 102 and the gateway antenna 106. In Figure 1 and 4a, two sets of hubs 402a and 402b belonging to two different respective vendors A and B are shown by way of illustration, but it will be appreciated that the hubs of other numbers of vendors may be present in any given system (whether only one vendor, or three or more vendors).

In embodiments, each vendor implements a separate hub 402 for each beam 202 in which it provides service. Say for the sake of discussion that the beam labelled 202a in Figure 2 is referred to as beam 1, and the beam labelled 202c in Figure 2 is referred to as beam 2: vendor A may use a first hub 402al serving beam 1, and a second hub 402a2 serving beam 2; and similarly vendor A may use a first hub 402bl serving beam 1, and a second hub 402b2 serving beam 2. Again however, it will be appreciated this is just an example, and in general other numbers of hubs may be used by any one of more of the vendors to server any number of beams. In the case of different hubs being used by a given vendor for different beams, these are typically separate physical units, e.g. different units on a server rack; or at least have separate controllers 408 operating at least somewhat independently.

Whatever the arrangement, each hub 402 comprises: a modulator 404; a demodulator 406; a network interface 410 such as an IP interface for connecting to the internet 102; and a hub control module 408 to which the network interface 410, modulator 404 and demodulator 406 are each connected. The modulator 404, demodulator 406 and control module 408 may be implemented in software, i.e. as code arranged to be executed on one or more processors of the respective hub 402. Alternatively one or more of these components could be implemented in dedicated hardware circuitry of the respective hub 402, or a

combination of hardware and software.

Each of the hubs 402 is connected via its respective network interface 410 to the internet 102. The gateway 104 also has an RF ("radio frequency") front-end to which each of the hubs 402 is connected via its respective modulator 404 and demodulator 406, so as to connect the hubs 402 to the gateway antenna 116.

Figure 4B shows an example of the client systems 112. By way of illustration, Figure 4B shows one client system 112a served by vendor A, and one client system 112b served by vendor B. However, it will be appreciated that generally each vendor may serve a respective subset of one or more client systems 112, and also that other numbers of vendors may be present.

Each client system 112 comprises a respective satellite terminal 412, a respective router 423, and a respective one or more user devices 424a. For example, one, some or all of the client systems 112 may each take the form of a VSAT (very small aperture terminal). Each satellite terminal 412 comprises a respective outdoor unit (ODU) 422, and a respective indoor unit (IDU) 420 connected to the respective ODU 422. The ODU 422 comprises the antenna 114 of the respective client system 112. The IDU 420 operates as a satellite modem, comprising a respective demodulator 414, modulator 416 and control module 418, wherein the respective demodulator 414, modulator 416 and client router 423 are each connected to the control module. The one or more respective user devices 424 are also connected to the respective client router 423. Each of the demodulator 414, demodulator 416 and control module 418 may be implemented in software, i.e. code arranged for execution on one or more processors of the respective IDU; or one or more of these components could instead be implemented in dedicated hardware, or a combination of hardware and software. The user devices 424 take the form of computer devices such as desktop, laptop or tablet computers, smartphones, set-top boxes, and /or smart TVs etc., through which the respective end-users 116 consume the internet access provided via the hubs 402 and satellite(s) 110. The ODUs 422 are situated in an outdoor environment, in which the satellite 110 is visible to their antennae 114. The IDUs 422 are generally situated indoors, e.g. in a residential or business premises, and are each connected to the corresponding ODU 422 via a cable connection.

In operation, outgoing data received from the internet 102 via the network interface 410 and intended for the satellite terminal 412 (and therefore ultimately for the user device (s) 424 connected to that terminal), is supplied from the internet 102 to the hub control module 408. The control module 408 performs any preliminary processing of the data that may be required, such as encryption, differential encoding, error protection and/or reformatting to the relevant protocol for transmission, then supplies the data to the modulator 404 of the hub 402 for modulation into RF signals, which are transmitted via the RF front-end 109 and forward satellite link 107F. These RF signals are received at the satellite terminal 412 of the client system 112, via its antenna 114. The received signals are demodulated by the demodulator 414 at the satellite terminal 414 to extract the original data, which is supplied to the terminal's control module 418 to perform any further processing such as re-formatting, error detection or correction, decoding and/or decryption (to compliment or reverse the corresponding processing operation performed at the transmit side, as appropriate). The data is then output from the control module 418 to the router 423 for routing to the relevant user device(s) 424. In the other direction, data originating with one of the user devices 424 and destined for the internet 102, is received by the control module 418 of the relevant client satellite terminal 412 via the corresponding router 423, and the control module 418 performs any preliminary processing that may be required, such as encryption, differential encoding, error protection and/or reformatting to the relevant protocol for transmission. The control module 418 then supplies the data to the modulator 416 of the terminal 412, which modulates this data into RF signals which are transmitted via the return satellite link 107R and received at the gateway antenna 106. The received signals are passed to the demodulator 406 of the hub 402 of the relevant vendor via the RF front-end 105, and demodulated by the vendor's demodulator 406 to extract the original data, which is supplied to the respective hub control module 408. The control module 408 of the vendor's hub performs any further processing such as re-formatting, error detection or correction, decoding and/or decryption (to compliment or reverse the corresponding processing operation performed at the transmit side, as appropriate), and then outputs this data to the internet 102 via the network interface 410.

Thus according to an arrangement such as described in relation to Figures 4A and 4B, each of one or more vendors A, B serves its respective client system(s) 112 via the operator's satellite front-end 106 and satellite(s) 110 of the operator. The service is provided over multiple beams 202 of the spot-beam arrangement, with each vendor A, B serving client systems 112 in a respective one or more beams. Different vendors may serve client systems in the same beam 202, and/or any given one of the one or more vendors may serve different beams 202.

In the case of different vendors A, B providing service in a given beam (say 202a, beam 1), then to avoid the signals of the different vendors interfering with one another within the beam 202a, the signals of the different vendors in a given beam 202a are multiplexed by using different, exclusive frequency sub-bands - i.e. by frequency division multiple access. Alternatively the signals of the different vendors in a given beam could be interleaved into different time-slots of a combined signal - i.e. by time-division multiplexing. However, regardless of whether one or multiple vendors operate in given beam 202a, the system is susceptible to inter-beam interference.

In embodiments, to try to minimize this, one or more adjacent or neighbouring beams 202 in the cluster (those closest to one another) are arranged to use different, exclusive frequency bands as the carrier for their respective signals (i.e. a different frequency channel); and/or to use a carrier with a different, orthogonal polarization (whether linear or circular/elliptical polarization). These techniques are preferably applied at least on the downlink 107F (forward link from satellite 110 to client system(s) 112). So taking beam 202a as an example, in Figure 2 the closest beams are 202b and 202e (closeness may be judged e.g. in terms of the shortest distance between the circumferences of the two beam edges as projected onto the earth's surface, i.e. the coverage area, with the edge being the contour defined as discussed previously; or in terms of the shortest distance between the beam axes or centres of the coverage areas). In this example, one or more of the closest beams 202b, 202e are arranged to transmit and/or receive on a different frequency band than beam 202a, and/or one of the closest beams 202b, 202e is arranged to transmit and/or receive using a polarization orthogonal to that of beam 202a. For instance, beam 202b may transmit (107F) on a different frequency band than beam 202a but with the same polarization, while beam 202e transmits on the same frequency band as beam 202a but with a polarization orthogonal to that of beam 202a. Or even, beams 202a, 202b and 202e may transmit in three different respective frequency bands (perhaps even with 202b and 202e transmitting with the same polarization as beam 202a).

Nonetheless, the set of available frequency bands and polarizations is finite, and the inventors have realized that there are still beams such as non-adjacent beam 202c which can interfere to a non-negligible extent with a given beam such as 202a, and/or vice versa. I.e. there are still spatially overlapping signals being transmitted at the same time, using the same frequency band, and with the same polarization (frequency band means herein an undivided band, i.e. no distinct frequency channels within it). In absence of any other multiple access technique being used (e.g. CDMA), i.e. if the spatial division is the only multiple access technique separating the signals in two different beams 202a, 202c, then due to the non-perfect confinement of the signals to the beam edges (see discussion above), these signals are liable to interfere with one another.

By way of illustration, Figure 7 shows a component 700 of a second beam 202c ("beam 2") as received at the location X of a client system 112 in the coverage area of a first beam 202a ("beam 1").

To mitigate such effects, the present invention controls the transmitted (downlink) signals in two beams such as 202a and 202c so as to be out-of-phase with one another, preferably maximally so. As will be discussed in the following, this could mean offsetting the phase of the carriers relative to one another, and/or offsetting the symbol periods relative to one another in time. Preferably this phase difference is applied selectively between only certain pairs of beams 202: not between adjacent (neighbouring) beams which preferably already use different frequency bands and/or polarities, but rather between one or more other, non-adjacent (non-neighbouring) pairs of beams like 202a and 202c where these pairs are the closest that share at least a same frequency band and potentially also a same polarization. I.e. for a certain first beam 202a, the one or more other beams 202c in which the phase difference is applied relative to the first beam 202a are those that are the closest to the first beam 202a without being so close that they have been arranged to share a same frequency band (and in embodiments a same polarization).

In embodiments, the phase of the signals transmitted from satellite 110 to client system 112 may be controlled by one or more of the hubs 402, according to whichever hub or hubs is/are the source of the signals to be transmitted in the beams in question 202a, 202c. Each of these signals in the different beams 202a, 202c may comprise a signal of a different respective vendor (e.g. A and B), and/or each of these signals in the different beams 202a, 202c may comprise a different respective signal of a same vendor (e.g. A). In the case of different vendors, note that different vendors typically use different respective hubs 402a, 402b. And even in the case of the same vendor, a given vendor often uses different distinct hubs 402al, 402a2 to serve different beams. Either way, the different hubs will comprise separate controllers 408 governing the timing of the modulation of the signals transmitted from satellite 110 to client system 112. Therefore in such cases, whereas they would traditionally operate independently of one another, the controllers 408 of the different hubs, e.g. 408a 1 and 408b2, and/or 408a 1 and 408a2, are now arranged to coordinate with one another to coordinate the phases of the signals they are causing to be sent from the satellite 110 via the respective beams 202a ("beam 1") and 202c ("beam 2") to the respective one or more client systems 112 in each beam.

A first embodiment of the present disclosure is now described in relation to Figures 5 to 9 and 11 to 12. As will be familiar to a person skilled in the art, a common form of modulation is phase modulation. An example of this is phase shift keying (PSK), e.g. quadrature phase shift keying (QPSK). As shown in Figure 8, to implement QPSK, the bits of the input data to be modulated are split between two branches: an in-phase (I) branch and a quadrature branch (Q). The I branch corresponds to a cosine wave and the Q branch corresponds to a sine wave (or vice versa), i.e. two oscillating subcarriers that are 90 degrees out of phase with one another. An input bit on the I branch is mapped to a +1 or a -1 depending on whether its value is 0 or 1, and similarly an input bit on the Q branch is mapped a +1 or a -1 depending on whether its value is 0 or 1. I.e. to represent a bit value of 0 on the I branch the cosine wave subcarrier is modulated by -1 and to represent a bit value of 1 on the I branch the cosine wave subcarrier is modulated by +1 (or vice versa), and to represent a bit value of 0 on the Q branch the sine wave subcarrier is modulated by -1 and to represent a bit value of 1 on the Q branch the sine wave subcarrier is modulated by +1 (or vice versa). The waves from the I and Q branches are mixed together to produce a composite modulated waveform comprising a sequence of symbols, each having a symbol period T, and each of which can take the form of any one of four different variants: the negative cosine wave multiplied by the negative sine wave, the negative cosine wave multiplied by the positive sine wave, the positive cosine wave multiplied by the negative sine wave, or the positive cosine wave multiplied by the positive sine wave. Each of these combinations represents a different one of the four possible combinations of bits, 00, 01, 10 and 11.

As illustrated by the constellation diagram of Figure 5, in the complex plane where the imaginary axis represents the in-phase (I) component of the signal and the imaginary axis represents the quadrature component (Q), the four possible symbol values correspond to points at four different angles relative to the vertical I axis: -135 degrees, -45 degrees, +45 degrees and +135 degrees. Hence as shown in Figure 11, the modulation may be considered to consist of a sequence of symbols (of period T) which, from symbol-to-symbol, can shift in phase in discrete units of 90 degrees. According to the first embodiment of the present disclosure, the phase difference between the two signals from beam 1 (202a) and beam 2 (202c) is implemented by offsetting the carrier of beam 2 relative to beam 1 (or vice versa) by a phase offset that is a fraction (i.e. non-integer multiple) of the discrete phase shift units used by the phase modulation scheme - preferably half so that the carrier of beam 2 is maximally out of phase with that of beam 1

So as shown in Figures 6 and 8, in the case of QPSK, this means that the I subcarrier of beam 2 is out of phase with the I subcarrier of beam 1 by 45 degrees, and similarly the Q subcarrier of beam 2 is out of phase with the Q subcarrier of beam 1 by 45 degrees in the same direction.

Figure 7 shows how the interfering component 700 from beam 2 travels the same path to point X as does the component of beam 1 with which it interferes. As they travel the same path, this means the received phase offset at the receiving point X will be the same as the phase offset with which the signals were transmitted from the satellite 110. Hence a client system 112 at point X in the coverage area of beam 1 will experience reduced interference from the interfering component 700 from beam 2 (and vice versa for a client system at some point in the coverage area of beam 2).

As will be appreciated by a person skilled in the art given the disclosure herein, an analogous technique would also work for other dimensions of PSK scheme. For instance, with a binary PSK scheme where there is only one branch or "subcarrier" (only a single carrier wave) modulated by corresponding to symbols of 0 and 180 degrees, then the technique disclosed above may be implemented by offsetting the carrier of beam 2 by 90 degrees relative to that of beam 1. Or for an 8-PSK scheme using four subcarriers each out of phase with one another by 45 degrees and modulated by +/-1, the technique disclosed above may be implemented by offsetting each subcarrier of beam 2 by 22.5 degrees relative to its counterpart in beam 1. Or in general for an n-ary PSK scheme using n/2 subcarriers each out of phase with one another by 360/n degrees and modulated by +/-1, the technique disclosed above may be implemented by offsetting the each subcarrier of beam 2 by 360/(2n) degrees relative to its counterpart in beam 1. A second embodiment of the present disclosure is illustrated in Figure 10 and 13. Here the phase of the carriers is not necessarily offset, but rather the symbols themselves are offset by a fraction (i.e. non-integer multiple) of the symbol period T, such that the symbol periods of beam 1 are out-of-phase with the symbol periods of beam 2. That is, each symbol Sn(l) transmitted in beam 1 is offset from a corresponding symbol Sn(2) transmitted in beam 2 by a time offset that is a fraction of the symbol period T (both beams having the same symbol period, and the symbol period being constant over multiple symbols in each beam). This technique also has the effect of mitigating interference between beams.

Note that the first and second embodiments may be used alone or in combination. I.e. in embodiments, the carriers of the two signals may be offset relative to one another but not the symbol periods, or the symbol periods may be offset relative to one another but not the carriers, or both the carriers and symbol periods may be offset.

It will be appreciated that the above embodiments have been described only by way of example.

For instance, while it is preferred that the signals from the first and second beams are maximally out-of-phase with one another (i.e. by half the phase shift unit and/or symbol period), this is not absolutely necessary in all possible embodiments. An arrangement in which the carrier of one beam is out of phase by only, say, 1/3 of the phase shift unit (e.g. 30 degrees in the case of QPSK), and/or only 1/3 of the symbol period T, would still reduce interference to some extent and hence may still be of some use relative to no phase offset at all. Further, the applicability of the disclosed techniques is not strictly limited to cases where the beams in question share an exact same frequency band, have a completely non- orthogonal polarization, and/or have no other multiple access technology separating their signals. For example, beams having different but overlapping frequency bands, and/or different but non-orthogonal polarization, will still experience interference to some extent. Or even if orthogonal polarization and/or another multiple access technology is used (e.g. CDMA), in practice this may not be perfect at avoiding interference between channels. Hence there may still be a desire to apply an additional technique for mitigating

interference, such as that disclosed herein.

Further, the disclosed techniques are not limited to phase modulation or any particular PSK scheme. E.g. only one carrier wave (one subcarrier or branch) could be used, but this could be shifted by more than two different phases to modulate data into the carrier (i.e. using phase shift units finer than 180 degrees). Or even if the modulated property of the carrier is a property other than phase, e.g. amplitude modulation, then the phase of the carriers and/or symbol periods can still be arranged to be out of phase and thus mitigate interference.

Furthermore, the scope of the present disclosure is not limited to the case where the satellite beams 202 are used to provide internet access. As will be appreciated, a satellite can also be used to provide other types of communication services such as a telephone service, a live video link service or a television service, and such services may also benefit from mitigating inter-beam interference.

Other variants may become apparent to a skilled person once given the disclosure herein. The scope of the present disclosure is not limited by the example embodiments, but only by the accompanying claims.

Claims

1. A method of controlling a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving one or more client systems in the respective area; the method comprising:

using a first one of said beams to transmit a first signal comprising a sequence of first symbols, by modulating a first carrier;

using a second one of said beams to transmit a second signal comprising a sequence of second symbols, by modulating a second carrier;

wherein the symbols of the first beam are aligned or at least overlap in time with the symbols of the second beam, and the first and second carrier share a same carrier frequency or at least an overlapping frequency band; and

wherein the method comprises controlling the first signal to be out-of-phase relative to the second signal.

2. The method of claim 1, wherein each of the symbols of the first and second signals have a common symbol period, and said control of the first signal to be out of phase with the second signal comprises: controlling the first symbols to be offset in time from the second symbols by a time offset that is a fraction of said symbol period.

3. The method of claim 2, wherein said time offset is half the symbol period.

4. The method of claim 1, 2 or 3, wherein said control of the first stream to be out of phase with the second stream comprises: controlling the first carrier to be out of phase with the second carrier by a phase offset.

5. The method of claim 4, wherein the modulation of each of the first and second carriers is by phase modulation, comprising modulating the carrier in units of a phase shift unit; and wherein said phase offset is a fraction of the phase shift unit.

6. The method of claim 5, wherein each of the first and second carriers comprises an I subcarrier and a Q subcarrier being 90 degrees out of phase with the I subcarrier, wherein said control of the first stream to be out of phase with the second stream comprises:

controlling the I subcarrier of the first signal to be out of phase with the I subcarrier of the second signal by said phase offset, and also controlling the Q subcarrier of the first signal to be out of phase with the Q subcarrier of the second signal by said phase offset.

7. The method of claims 5 or 6, wherein the modulation of each of the first and second carriers is by n-fold phase shift keying, the phase shift unit being 360/n degrees; and wherein said phase offset is 360/(2n) degrees.

8. The method of any preceding claim, wherein relative to the first beam, the second beam is the nearest beam in the cluster sharing the same carrier frequency or frequency band.

9. The method of any preceding claim, wherein the first carrier has a same polarization as the second carrier or at least a non-orthogonal polarization relative to the second carrier.

10. The method of claim 9, wherein relative to the first beam, the second beam is the nearest beam in the cluster sharing the same polarization.

11. The method of any preceding claim, wherein the first signal comprises a signal from a hub of a first vendor, and the second signal comprises a signal from a hub of a second vendor; and wherein said control comprises coordinating the hub of the first vendor with the hub of the second vendor in order to control the relative phase of the first and second signals.

12. The method of any preceding claim, wherein the first signal comprises a signal from a first hub of a given vendor, and the second signal comprises a signal from a second, separate hub of the same vendor; and wherein said control comprises coordinating the first and second hubs of said same vendor in order to control the relative phase of the first and second signals.

13. A satellite earth station arranged to control a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving a respective one or more client systems in the respective area; wherein the satellite earth station is configured to perform operations according to any of claims 1 to 12.

14. The earth station of claim 13, wherein the earth station is also arranged as a gateway to an internet, one or both of the first and second signals comprising a signal communicated from the internet to one or more of the respective client systems.

15. A computer program product for controlling a spot-beam satellite which provides coverage in the form of a cluster of spatial beams each covering a respective area on the earth's surface, and each serving one or more client systems in the respective area; the computer program product comprising code embodied on a computer-readable storage medium and configured so as when run on one or more processors operating a satellite earth station, to perform operations according to any of claims 1 to 12.