Technical Field This disclosure is in the field of satellite communication, and relates in particular to an outdoor unit and a satellite modem for effecting communication with a gateway located on the Earth's surface via a satellite link.

Background

Some regions of the world such as rural, developing or isolated areas often have limited communication infrastructure where high speed broadband through traditional, ground-based (i.e. wired) means is not feasible. 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 links in regions that do have a developed communication

infrastructure, or as backup to such infrastructure in case a ground-based link fails. Figure 1 gives a schematic overview of a system 100 for providing access to a network, which is 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, a satellite 110 in orbit about the Earth, and one or more client systems 112 remote from the gateway 104 and located in a region on the Earth's surface to which internet access is being provided. The gateway 104 comprises a satellite hub 402 connected to the internet 102, and at least one gateway antenna 106 connected to the hub 402. Each of the client systems comprises a so-called outdoor unit, each outdoor unit comprising an antenna 114, connected to a satellite modem 420a. The satellite 110 is arranged to be able to communicate wirelessly with the hub 402 of the satellite gateway 104 via the gateway antenna 106, and with the modems 420a of the client systems 112 via the antennae 114, and thereby provide a satellite link 107 for transmitting internet traffic between the source or destination on the internet 102 and the client systems 112. For example the satellite link 107, hub 402 and modems 420a may operate on the Ka microwave band (26.5 to 40 GHz). The satellite link 107 comprises a forward link 107F for transmitting traffic originating with an internet source to the client systems 112, and a return link 107R for transmitting traffic originating with the client systems 112 to an internet destination.

The hub 402 serves (i.e. provides an internet access service to) 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 link 107 and the hub 402. In one model the operator of the satellite 110 and/or gateway 104 provides bandwidth to a downstream internet service provider (ISP), who in turn provides an internet access service based on that bandwidth to a plurality of end users 116. The end users 116 may be individual people (consumers) or businesses. Depending on implementation, the client systems 112 may comprise a central satellite base station run by the ISP (the base station comprising an antenna 114 and modem 420a), and a local communication infrastructure providing access onwards to the equipment of a plurality of users within the region in question. E.g. the local communication infrastructure may comprise a relatively short range wireless technology or a local wired infrastructure, connecting onwards to home or business routers or individual user devices. Alternatively or additionally, the client systems 112 may comprise individual, private base stations each with its own satellite antenna 114 and modem 420a for connecting to the satellite 110 and local access point for connecting to one or more respective user devices. In this case the ISP does not necessarily provide any extra infrastructure, but acts as a broker for the bandwidth provided by the satellite 110. 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.

Referring to Figure 2 by way of example, 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. 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, using modern techniques the satellite 110 may be 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. A beam refers to a volume of space or "lobe" in which transmission and/or reception of one or more given signals are approximately confined, typically a signal cone. 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) 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. Figure 4 is a block diagram showing part of the gateway 104 in more detail. As shown, the gateway 104 comprises satellite hub 402. For example, the gateway may comprise a building which houses the hubs, in which the satellite hub 402 is installed, and in which various infrastructure is provided so as to connect the hub to the internet 102 and the gateway antenna 106. The hub 402 comprises a modulator 404, a demodulator 406, a network interface 410 and a hub control module 408, to which the network interface 410 and (de)modulator 404 (406) is connected. The gateway 104 has a network infrastructure 109 to which the hub 402 is connected via its network interfaces 410 so as to connect the hub 402 to the internet 102. The gateway 104 also has an RF ("Radio Frequency") infrastructure 105 to which the hub 402 (specifically the hub modulator 404 and hub demodulator 406) is connected so as to connect the hub 402 to the gateway antenna 116.

The hub modulator 404 modulates data, which has been received from the internet 102 and is to be transmitted to a client system 112, into RF signals; the RF signals are supplied to the gateway antenna 116, from which they are transmitted via the forward link 107F to the client system 112. Conversely, modulated data is received from the client system 112 by the gateway antenna 106 via the return link 107R as RF signals, which are demodulated by the hub demodulator 406 to extract the data in its original form; the extracted data is then supplied to the internet 102.

The outdoor unit of each client system 112 needs to be installed in an outdoor environment so that the antenna 114 has an unobstructed line of sight to the satellite 110. The outdoor unit connects to the satellite modem 420a via a so-called inter- facility link (IFL). The IFL comprises a set of one or more coaxial cable(s) via which signals can be relayed between the indoor unit and the outdoor unit. The satellite modem 420a may be in the form of a so-called indoor unit, which is intended for installation in an indoor environment. In some cases the modem may be integrated with the ODU and in that case the IFL is a standard network cable to connect to the indoor equipment.

To enable multiple, geographically distributed client systems to share return link frequency resources effectively, a channel access method such as Time Division Multiple Access (TDMA) is normally implemented across the multiple client systems. When properly implemented and managed, this minimizes interference on the return link 107R and ensures that the frequency resources of the return link 107R are used efficiently. To achieve this, synchronization of the multiple client systems needs to be maintained, generally by way of a clock signal broadcast via the forward link 107F. In practice, synchronization will not be perfect - this can be accounted for by leaving a small amount of the return link spectrum unallocated where necessary to accommodate this e.g. short guard intervals between transmission slots allocated to different client systems for TDMA.

Summary

Maintaining synchronization of multiple, geographically distributed client systems is complicated by the fact that the client systems are distributed over relatively large distance scales - large enough that the disparity in the transit times of RF signals transmitted between the satellite and the different client systems becomes significant. At present, this is addressed by an installer of a client system measuring, e.g. using a handheld GPS device, and manually inputting the location of the client system during installation e.g. as a set of coordinates. In operation, each installed client system delays its return link transmissions by an amount that matches the manually inputted location so as to maintain synchronization across the multiple client systems as a whole. In some cases this may be aided by the use of additional software to automatically load the GPS data from an external source if such an API is supported by the modem. Not only is the manual inputting of coordinates burdensome for the installer, it also introduces a source of human error, incorrectly inputted coordinates result in asynchronization of client systems, leading to interference on the return link due to overlapping transmissions on the return link if left unaccounted for or waste of return link frequency resources if accounted for by increasing the amount of unallocated spectrum.

According to a first aspect, an outdoor unit is configured to be installed in an outdoor environment, the outdoor unit for effecting communication in a satellite

communication system. The outdoor unit comprises an antenna, an interface coupled to the antenna, and a positioning module. The antenna is configured to, when the outdoor unit is installed in the outdoor environment, transmit signals to a gateway of the system via a satellite link of the system. The interface is configured to receive outgoing data and supply the outgoing data to the antenna for transmission to the gateway via the satellite link. The positioning module is configured to detect a current location of the outdoor unit and generate location data identifying the current location of the outdoor unit.

By including in the outdoor unit a positioning module, e.g. comprising a satellite navigation receiver, which is able to autonomously measure and make available the current location of the outdoor unit, the installer is relieved of the burden of measuring this themselves, which in turn removes a source of human error. The outdoor unit represents an optimal location for the positioning module as the position module is likely to be able to receive the necessary signals needed to measure the current location (e.g. signals from a satellite navigation system) in the outdoor environment in which the outdoor unit is installed. Further, should the location of the outdoor unit change (e.g. if it is reinstalled elsewhere, legitimately or illegitimately e.g. after having been stolen), the location data generated by the positioning module will reflect this, and can be used for resynchronization and/or tracking purposes and also to assist in the mitigation of interference by identifying actual locations of emissions.

In embodiments, the positioning module may comprise a satellite navigation module configured to detect the current location - and in some cases a clock signal - from signals received from a satellite navigation system.

The interface may be connective to a satellite modem external to the outdoor unit and the outgoing data is received by said interface from the satellite modem and in a modulated form. Alternatively, the outdoor unit may comprise a satellite modem, via which the interface is coupled to the antenna, configured to modulate the outgoing data before it is supplied to the antenna.

The positioning module may be configured to output the location data and/or clock signal to the (internal or external) satellite modem, for example via the interface in the case of an external modem. For an external modem, the interface may be an inter-facility link interface by which the satellite modem is connective to the outdoor unit via an inter-facility link, whereby the location data and/or clock signal is outputted to and the outgoing data is received from the satellite modem via the inter- facility link.

The outdoor unit may comprise identification logic embodying an identifier of the outdoor unit, and the positioning module may be configured to provide the location data to a server of a network, the location data provided in association with the identifier. The location data may be provided to the server via the satellite modem.

The location data and/or the clock signal may be outputted externally of the outdoor unit. According to a second aspect, a satellite modem is configured to receive and use the location data (and optionally the clock signal). In particular, the satellite modem may be configured to implement (re)synchronization and/or tracking mechanisms and/or antenna alignment automatically based on the location data generated by the outdoor unit.

According to the second aspect, a satellite modem for effecting communication in a satellite communication system comprises an interface by which a computer is connectible to the satellite modem, a modulator, and an input. The interface is configured to receive from the computer outgoing data to be transmitted to the gateway. The modulator is configured to modulate the outgoing data and supply the modulated data to an antenna of an outdoor unit for transmission to a gateway of the system via a satellite link of the system. The input is configured to receive, from a positioning module of the outdoor unit, location data generated by the positioning module of the outdoor unit which identifies a current location of the outdoor unit.

The satellite modem further comprises:

• a synchronization module configured to delay the outputting of the modulated outgoing data to the outdoor unit based on the location data by an amount which substantially matches the current location of the outdoor unit; and/or

• a tracking module configured to transmit the location data to a server of a network, the location data transmitted in association with an identifier of the outdoor unit; and/or

• an antenna control module, wherein the antenna has a controllable orientation and the antenna control module is configured to change the orientation of the antenna based on the location data.

The satellite modem may comprise an access module configured to instigate a location request signal to the outdoor unit, the location data supplied by the outdoor unit in response to the location request signal. The location request signal may be instigated in response to the satellite modem entering an active state from an inactive state. Multiple such location request signals may be instigated over time. The multiple location request signals may be instigated periodically. The input may be configured to receive a clock signal from the positioning module of the outdoor unit, and the received clock signal may be used to modulate the outgoing data.

The identifier may be received from identification logic of the outdoor unit.

In response to the access module receiving further location data from the satellite modem identifying a new location of the outdoor unit, the synchronizer module may be configured to change said amount to match the new location.

In response to the access module receiving further location data from the satellite modem identifying a new location of the outdoor unit, the tracking module may be configured to transmit the new location data to the server in association with the identifier.

The satellite modem may comprise an inter-facility link interface by which the outdoor unit is connectible to the satellite modem via an inter-facility link, wherein the location data is received from and the outgoing data is outputted to the satellite modem via the inter-facility link.

A system may comprise an outdoor unit of the first aspect and a satellite modem according to the second aspect - the satellite modem may be external to or integrated in the outdoor unit.

According to a third aspect, a method is performed in a satellite communication system. The system comprises an outdoor unit installed in an outdoor environment, the outdoor unit comprising an antenna for transmitting signals to a gateway of the system via a satellite link of the system. Location data is received, from a positioning module of the outdoor unit, which location data has been generated by the positioning module and identifies a current location of the outdoor unit.

Outgoing data is supplied, for transmission to a gateway of the system, in a modulated form to the antenna of the outdoor unit, wherein the supplying of the outgoing data to the antenna is delayed based on the location data by an amount which substantially matches the current location of the outdoor unit.

Alternatively or in addition, the location data is transmitted to a server of a network, the location data transmitted in association with an identifier of the outdoor unit.

Alternatively or in addition, the orientation of the antenna may be changed based on the location data. According to a fourth aspect, a computer program product comprises a computer readable storage medium storing code which is configured when executed on a processor to implement the method.

Brief Description off Figures

To aid understanding of the present subject matter, reference will be made by way of example to the following drawings in which:

Figure 1 is a schematic diagram of a system for providing internet access via satellite;

Figure 2 is a schematic diagram showing geographic coverage of a cluster of satellite beams; Figure 3 is a schematic diagram of a part of a system for providing internet access via satellite beams;

Figure 4 is a schematic block diagram representing a gateway Earth station; Figure 5A show schematic block diagrams of an indoor unit and an outdoor unit in a first embodiment;

Figure 5B show schematic block diagrams of parts of an indoor unit and an outdoor unit in a second embodiment; Figure 5C show schematic block diagrams of parts of an indoor unit and an outdoor unit in a third embodiment; Figure 5D shows an outdoor unit which comprises a satellite modem;

Figure 6 schematically illustrates a TDMA scheme;

Figure 7 shows the Earth, a satellite in orbit thereof, and a collection of client systems geographically distributed thereon.

Detailed Description of Embodiments

Figure 5A is a schematic block diagram of a client system 112 in a first embodiment. As shown, the client system 112 comprises a satellite modem in the form of an indoor unit (IDU) 420a, an outdoor unit (ODU) 422a, a router 423, and one or more computers 424.

The indoor unit 420a comprises a network interface (first interface) 500 - by which the computer(s) 424 are connected to the indoor unit 420a via the router 423 - and a second interface 510. The outdoor unit 422a comprises a third interface 522.

The second interface 510 and third interface 522 are cable interfaces, specifically IFL interfaces (i.e. coaxial cable interfaces). An IFL 511 , comprising a set of one or more coaxial cables, connects the second interface 510 and the third interface 522 to one another, and thereby connects the IDU 420a and the ODU 422a to one another. Thus signals outputted to the ODU 522 by the IDU 420a via the second interface 510 are received at the third interface 522 of the ODU 422a and vice versa. The ODU 422a is installed in an outdoor environment, outside of a building 506 (e.g. a residential or business premises). The IDU 420a is located inside of the building 506, and the IFL 511 runs between the interior and exterior of the building 506. The ODU 422a and IDU 420a constitute a satellite terminal, specifically a Very Small Aperture Terminal ("VSAT"). The "ODU", "IDU" and "VSAT" terminology are known in the art. The ODU 422a comprises an antenna 114, specifically a parabolic dish antenna which is typically less than about 3 metres in diameter, and is typically greater than about 0.6 metres in diameter. The ODU 422a also comprises various electronic components which include: a frequency down-converter 512 and a frequency up- converter 514, which have an output and an input connected to the third interface 522 respectively; a receive-path band pass filter 524 having an input connected to the antenna 114, and a low noise amplifier 520 having an input connected to an output of the receive-path filter 524 and an output connected to an input of the up- converter 512; and a power amplifier 522 having an input connected to an output of the up-converter 514, and a transmit-path band pass filter 526 having an input connected to an output of the power amplifier 522 and an output connected to the antenna 114. In this manner, the third interface 510 is coupled to the antenna via the frequency converters 512, 514, amplifiers 520, 522 and filters 524, 526. The various electronic components are housed in a casing of the ODU 522 (which casing also supports the third interface 522) so as to protect them from the outdoor environment. When the ODU 422a is correctly installed in the outdoor environment, the antenna 114 has an unobstructed line of sight to the satellite 110 and is thus able to transmit and receive RF signals to/from the gateway 104 via the satellite link 107 (specifically, via the return link 107R and forward link 107F respectively). The IDU 420a comprises: a modem processing module 418; a demodulator 414, which has an input connected to the second interface 510 and an output connected to a first input of the modem processing module 418; a data buffer 417, which has an input connected to a first output of the modem processing module 418; and a modulator 416, which has a first input connected to an output of the buffer 417, a second input connected to a second output of the processing module 418, and an output connected to the second interface 510.

The modem processing module 418 is connected to the network interface 500 and thus to the computers) 424 via the router 423. The router 423 may be integrated in the IDU 420a or it may be a separate device. The connection between the

processing module 418 and the network interface 500 is bi-directional i.e. the modem processing module can both send and receive data to/from the interface 500.

The computers) 424 supplies outgoing data that is to be transmitted via the satellite link 107 to the modem processing module 412 via the router 423 and first interface 500. The outgoing data is received by the modem processing module 418 from the router 412 as one or more outgoing bit streams. The modem processing module 418 performs processing functions such as:

• encryption of the outgoing data;

• where there are multiple outgoing bit streams, multiplexing of the multiple streams (e.g. from different ones of the computers) 424) into a single outgoing bit stream;

• scrambling of the outgoing stream e.g. to remove or at least reduce the number of long "0"-only or T-only bit sequences in the outgoing data stream to provide transmission energy dispersal - for example, by pseudo randomizing the stream in a manner that is reversible by a corresponding descrambler of at the gateway 104;

• coding of the outgoing data stream, for example differential coding - whereby each bit is transformed into another bit determined from that bit and a previously transmitted bit(s) - and forward error correction (FEC) coding - whereby redundancy bits are intentionally added to the outgoing data stream which can be used at the gateway 104 to correct errors introduced in transmission.

Outgoing data which has been processed by the processing module 418 is held is the buffer 417 so that it is accessible to the modulator 416.

The modulator 416 modulates the buffered outgoing data i.e. converts it to a modulated form, specifically IF (Intermediate Frequency) signals - labelled O in figure 5A - according to a TDMA scheme, details of which are described below. The IF signals are outputted to the ODU 422a via the second interface 510 and IFL 511 , and supplied from the third interface 522 of the IDU 422a to the up-convertor 514. The up-con verter 412 up-converts the IF signals to generate higher frequency RF signals, which are amplified by the power amplifier 522, filtered by the transmit-path band-pass filter 526, and outputted to the antenna 114, from which they are transmitted to the gateway 104 via the return link 107R.

Modulated incoming data, received via the forward link 107F at the antenna 114 as RF signals and intended for one of the computer(s) 424, is filtered by the receive- path band pass filter 524, amplified by the low noise amplifier 520, and supplied to the down-converter 512. The down-converter 512 down-converts the RF signals into lower frequency IF signals- labelled I in figure 5A - which are outputted to the IDU 420a via the third interface 522 and IFL 511. The IF signals are then supplied from the second interface 510 to the demodulator 414, which demodulates the IF signals to extract the incoming data.

The demodulated incoming data is then supplied to modem processing module 418 as an incoming bit stream and the processing module 418 processes outgoing data for transmission by performing processing functions such as:

• decoding of the outgoing data stream, e.g. FEC and Differential decoding to reverse any FEC and Differential encoding applied at the gateway 104;

• descrambling of the outgoing stream to reverse any scrambling applied at the gateway 104;

• where the incoming bit stream is a multiplexed stream formed of multiple incoming bit streams, demultiplexing of the multiple streams;

· decryption of the incoming data;

• Clock recovery (NCR) to synchronise modem

The lower frequencies of the IF signals are better suited to propagation via the cable- based IFL 511 , whereas the higher-frequency RF signals are more suited to propagation in air or vacuum to/from the satellite.

Frequency resources on the link 107 are finite, and thus mechanisms need to be put in place to ensure that these resources can be shared between multiple client systems. For the return link 107F, this requires the coordination of transmissions by multiple, geographically distributed client systems. To this end, a TDMA scheme is implemented across the multiple client systems. Figure 6 schematically illustrates a TDMA scheme, which shows how frequency resources in the form of a frequency channel (band) f on the return link 107R may be shared between multiple client systems - in this case first, second and third client systems (112a, 112b, 112c in figure 7) but this is just by way of example. In the time-domain, sequential TDMA frames F are divided into non-overlapping slots - in this example, three slots Sa, Sb and Sc which are allocated to the first second and third client systems 112a, 112b, 112c respectively. Typically, the frames are of the same fixed length in time though this is not essential. For each frame F,

transmission by each client system 112a, 112b, 112c on the return link channel f is restricted to that client system's time slot Sa, Sb, Sc in that frame. Adjacent time slots in the same or adjacent frames are separated by short guard intervals G in which the channel f is unallocated so as to provide some resilience to imperfect synchronization of the client systems 112a, 112b, 112c which might otherwise lead to interference caused by transmissions from different client systems overlapping in time when they reach the satellite 110.

To implement the TDMA scheme, the gateway 104 broadcasts to the client systems 112a, 112b, 112c, via the forward link 107F, a burst time plan (schedule) which allocates slots Sa, Sb, Sc within each frame F to different client systems. For example, each frame F may be 1000 ms in length, and the burst time plan may allocate a first slot running from 0 ms to 380 ms (Sa) to the first client system 112a; a second slot running from 400 ms to 580 ms (Sb) to the second client system 112b; and a third slot (Sb) running from 600 ms to 980 ms, thereby leaving 20 ms guard intervals G between each pair of adjacent slots. The burst time plan may be static or pseudo-static e.g. so that the slot allocations only changes on a time-scale of order hours, days, weeks etc. or it may be varied dynamically e.g. on a time scale of order minutes, seconds etc. e.g. in response to changing demands of the clients systems 112a, 112b, 112c. The gateway also broadcasts to the client systems 112a, 112b, 112c, via the forward link 107F, a clock signal which is used by the client systems 112a, 112b, 112c to determine the start/end of their allocated slot(s) within each frame.

Returning to figure 5A, for each client system 112, the buffer 417 holds yet to be transmitted outgoing data outputted by the modem processing module 418. The modulator receives the clock signal ("elk") and scheduling information ("schdl") derived from the forward link broadcasts and, based on these, modulates the buffered outgoing data so as to effect transmission of the outgoing data the antenna 114 via the forward link 107F in bursts which are restricted to the client system's allocated time slots. That is, data outgoing data is buffered until a timeslot in which it can be transmitted becomes available.

As indicated above, a critical factor with regards to interference on the channel f is the extent to which transmission bursts on the channel f overlap when they reach the satellite 110 as the satellite 110 is the focal point of return link transmission for all of the client systems. Figure 7 shows the clients systems 112a, 112b, 112c as they might be geographically distributed on the surface of the Earth E. Due to the differing geographic locations, the distances La, Lb, Lc between the client systems 112a, 112b, 112c and the satellite 110 in the geostationary orbit (path lengths) may not be the same as one another; and due to the relatively large distance scales involved, the path length differences are, in practice, large enough to introduce non- negligible differences in the propagation times for RF signals between the satellite 110 and the different client systems 112a, 112b, 112c thereby causing miss-matched delays in the clock signal as received by different client systems and in the return link transmissions. Left unchecked, this could lead to a situation in which, despite each client system adhering to the burst time plan and only transmitting RF bursts within its allocated slots on the channel f accordingly, the bursts would nonetheless overlap and therefore interfere at the satellite 110. In other words, the differing path delay on the satellite link as observed by different client systems introduce a level of asynchronization across the geographically distributed client systems as a whole. Whilst this could be accounted for my making the guard bands G large enough to accommodate this level of asynchronicity, that would be an inefficient use of the time domain and hence frequency spectrum. Existing return link interference mitigation techniques address the asynchronicity issue by each indoor unit adjusting its own transmission scheduling based on the geographic location of the client system. Specifically, a delay time ("frame delay" or equivalents "frame offset", denoted "fd") is determined based on the geographic location, and each transmission burst is delayed by that amount of time relative to the received clock signal i.e. if the burst time schedule allocates a slot [to, t1] to a client system, the client system times its transmissions in that slot to begin when or after the clock signal reaches time tO+fd and end when or before the clock signal reaches t1+fd. The frame delay is computed according to a pre-calibrated procedure which ensures that those client systems closest to (resp. furthest from) the satellite 110 introduce the longest (resp. shortest e.g. zero) frame delay. The frame delay computed by a given client system matches the path length between that client system and the satellite 110 i.e. so that any two transmission bursts from any two given client systems client systems which the burst time plan dictates should be separated by a certain amount of time do indeed arrive at the satellite 110 separated by that amount of time when the two client systems apply their respective frame delays. This ensures that transmission burst from different client systems on the same channel f do not collide when they arrive at the satellite 110. This means that the size of the guard intervals G can be reduced, leaving a greater amount of spectrum free to be allocated for return link transmission.

To date, the operation of such interference mitigation techniques has relied on an installer of a VSAT manually configuring the IDU with location coordinates of the ODU, typically measured using hand-held (e.g. GPS) equipment The frame delay is then computed by the IDU from the manually inputted coordinates.

However, sometimes the coordinates are measured or inputted incorrectly; for example the format in which the coordinates are outputted by the hand-held device may not match the format to which the IDU is configured. This leads to incorrect frame delays being computed - short of identifying and manually reconfiguring each offending VSAT (which may be impractical), the only option to prevent colliding transmissions on the return link in this scenario is to increase the size of the guard intervals G in the burst time plan to accommodate the errors, which is a waste of finite spectrum. Returning to figure 5A, in accordance with the present subject matter, the ODU 522 is shown to comprise a positioning module, embodied in a silicon chip 502 and integrated in the ODU 422a. The chip 502 is also housed within the casing of the ODU 522 along with the other electronic components of the ODU 422a and is thus similarly protected from the outdoor environment. The chip 502 comprises a satellite navigation system receiver. As is well known, a satellite navigation system is a system of satellites that provides autonomous global or regional geo-spatial positioning. For example, the Global Positioning System (GPS) is a global navigation satellite system (GNSS) maintained by the United States government which can provide location and time information to a GPS receiver at any point on the surface of the Earth from which there exist unobstructed lines of sight to at least four GPS satellites. The chip 502 has its own antenna (not shown) by which it is configured to receive time signals broadcast by navigation satellites in orbit of the Earth, from which signals it detects a current location of the chip 502 and thus of the ODU 522 and generates location data - labelled L in figure 5A - identifying the current location, e.g. in the form of a latitude, longitude coordinate pair. It can also generate a clock signal, which indicates the current time to a high level of accuracy from time data included in the signals. The positioning module has a data interface (not shown) by which the location data and (where applicable) clock signal are accessible. The chip 502 has an output connected to the third interface 502, by which it can output the location data L to the IDU 420a via the existing IFL 511. Utilizing the existing IFL infrastructure is advantageous in that it minimizes the amount of additional infrastructure that is needed to implement the present subject matter. Any suitable multiplexing technique can be used to this end. For example, the location data L may be modulated onto a particular frequency sub-carrier reserved for communicating control data between the ODU and modem.

The IDU 420a further comprises an access module 519, connected to the second interface, for accessing data held by the outdoor unit 422a. The access module 519 has a first input connected to the second interface 510, by which it can receive the location data L as generated by the chip 502. The IDU 420a also comprises a synchronization module 516 having an input connected to a first output of the access module by which it receives the location data L. The synchronization module 516 automatically computes a frame delay fd from the location data L as received form the ODU 422a. The frame delay fd is computed according to the same pre-calibrated procedure described above, but because the location data L has been automatically generated a source of human error has been removed which ensures more accurate synchronization across the client systems as a whole. In turn, this means that the lengths of the guard intervals G can be reduced, freeing up spectrum for allocation to client systems and thus increasing the capacity of the return link 107F. The automation also simplifies and speeds-up the installation procedure of the VSAT by relieving the installer of the burden of measuring and inputting the location.

The ODU represents an optimal location for the chip 502: when properly installed in the outdoor environment, the ODU 422a is located so as to have a clear line of sight to the satellite 110; generally, this will also mean that the chip 502 located in the ODU 422a is likely to have the necessary clear lines of sight to the relevant navigation satellites, and in particular is free from obstruction by the building 506 itself.

In embodiments, the access module 519 obtains the location data L by "pinging" the chip 502 via the IFL 511 i.e. the access module 519 instigates a location request signal to the chip 502 via the IFL 511 , in response to which the chip 502 returns the location data L.

In some such embodiments, the access module 519 so pings the chip 502 upon start-up of the IDU 420a i.e. in response to the IDU 420a entering an active state from an inactive state. Alternatively or additionally the access module 519 repeatedly (e.g. periodically) pings the chip 502 overtime so as to receive updated location data whilst in the active state. In response to a change in the location data L, the frame delay fd is updated automatically by the synchronization module 518 based on the new location data. By repeatedly pinging the chip 502 over time and updating the frame delay as needed, the IDU 420a is able to adapt automatically to movement of the ODU 422a - for instance, as might occur when the ODU 422a and IDU 420a are re-installed at a different location (previously this would require manual reconfiguration of the IDU with newly-measured coordinates).

Moreover, the chip 502 also provides an opportunity for automatic tracking of the ODU 522. As shown in figure 5A, the ODU 422a also comprises identification logic 504 embodying an identifier ("id") of the ODU 422a, for example a unique serial number of the ODU 422a. The identification logic 504 may for instance comprise a (e.g. write once) memory to which the identifier is (e.g. permanently) written during manufacture of the ODU 422a. The identification logic 504 has an output connected to the third interface 522, by which it outputs the identifier id to the IDU 420a via the IFL 511.

The IDU 420a further comprises a tracking module 420a, which has a first input connected to receive from the access module 519 the location data L as supplied by the chip 502. The access module 519 has a second input connected to the second interface 510, by which it receives the identifier id as supplied by the identification logic 504. The identifier id may be supplied in response to an identifier request instigated by the access module 519 to the ODU 422a. The tracking module 518 has a second input connected to a second output of the access module 519, by which it receives the identifier id. The tracking module 518 transmits the location data L, in association with the identifier id, to a server the internet 102 via the return satellite link 107R by supplying the location data L and associated identifier id to the modem processing module 418 for transmission in the same manner as the outgoing user data (although the possibility of the location data/identifier being transmitted via a separate, non-satellite based link is not excluded). This may be done every time the ODU is pinged for location data (e.g. at each IDU start-up, periodically etc.), or the IDU may store the most recently received location data and transmit an update to the server only when the location data changes. The server can receive such location/identification data from multiple client systems, which it stores in a database accessible to an operator of the system 100. This enables the operator to keep track of the various ODUs in the system 100, even if they are moved. This is particularly useful in countries/regions in which the operator has a regulatory responsibility to know where the client equipment is being used e.g. for the purpose of monitoring interference power levels. Such monitoring can also provide visibility of equipment theft or misuse.

Second and third embodiments will now be described with reference to figures 5B and 5C. Note, like reference signs in figures 5A, 5B, 5C denote like components and all the relevant description of the first embodiment applies equally to the second and third embodiments.

Figure 5B show a satellite modem 420b and ODU 422b in the second embodiment. In the second embodiment, in addition to the location data L, a clock signal elk is generated by the positioning module 502 integrated in the ODU 422b, and supplied to the modem 420a ^* . The clock signals elk is generated based on timing information embedded in the signals from the visible positioning satellites. Rather than deriving the clock signal to be used by the modulator 416 from forward link transmissions, the clock signal elk generated by the chip 502 is used instead. The clock signal elk generated by the chip 502 may be converted into a different format compatible with the modulator 416, though this is not shown explicitly in figure 5B.

Advantageously, this removes the need to send the clock signal on the forward link, freeing up bandwidth and simplifying the transmission process at the gateway 102. It may also be possible to derive a more accurate clock signal in this manner.

Figure 5C shows an ODU 422c and modem 420a" in the third embodiment, which is particularly useful for mobile applications, for example where the ODU 422c and modem 420a" are installed on a vehicle such as a boat or train. The ODU 422c comprises an antenna tracking system, which itself comprises a primary antenna 114 (equivalent to that of the first and second embodiments), and a secondary antenna 564. As is known in the art, signals form the primary and secondary antenna are fed back ("fdbk" in figure 5C) to an antenna control module 560 of the modem 420a". The two signal strengths are compared by the antenna control module 560, and based on the comparison the antenna control module controls a drive mechanism 566 ("ctrl" in figure 5C) of the antenna tasking system 564 to adjust the roll and/or pitch of the antenna 114 as necessary to maintain alignment with the satellite 110 as the vehicle moves.

In the third embodiment, the antenna control module 560 also received the location data L as generated by the position module 502 of the ODU 422c, which can be also be used to change the orientation of the antenna 114. For example, an initial orientation of the antenna 114 may be determined based on the location data L automatically, which is straightforward as the location data L indicates the location of the antenna 114 relative to the satellite 110. Once the antenna has been configured to the initial orientation, e.g. by adjusting the roll and/or pitch of the antenna, thereafter tracking based on signal feedback can be used to maintain alignment. Further, should the vehicle undergo a sudden movement that tracking is unable to account for so that communication with the satellite is temporarily lost, the location data L can be used to re-configure the orientation of the antenna 114 accordingly to resume communication. In addition to the location data L, data such as compass and/or velocity data may be used to determine the correct orientation.

Note that in the third embodiment, the synchronization module 416 can also update the frame delay automatically based on the location data L as the vehicle moves relative to the satellite 110 to ensure that the correct value of the frame delay fd is used at all times.

Note that only selective components of the modem 420a7420a" and/or ODU

422b/422c are shown in figures 5A/5B to avoid unnecessary duplication; any or all of the omitted components described in relation to the first embodiment may

nevertheless be present in the ODU 420b/420c and/or modem 422b/422c of the second/third embodiments. The clock signal in the third embodiment may be derived as in the first or second embodiments.

Note, in an alternative configuration, an ODU 422 may comprise an internal modem 420, as shown in figure 5D. That is, the modem 422 may instead be integrated in the ODU 420 in the same manner as the other components of the ODU 422. In this case, the IFL 511 and first/second interfaces 510, 522 are replaced with internal interface logic 570 comprising one or more internal interfaces. The first interface 500, which is an external interface of the ODU 422 in this configuration, is then connected from the outside to the indoor network by a standard (outdoor rated) network cable (e.g. CAT5/6) or possibly by a wireless link. That aside, the modem 420 and ODU 422 can be configured in the same manner, and comprise the same components, as any of the modems 420a, 420b, 420c and outdoor units 422a, 422b, 422c respectively of the first second and third embodiments. In this configuration, the first interface 500 is connected to the antenna 114 via the internal modem 420, which is itself connected to the antenna 114 via filter(s), amplifiers) etc. (not shown in figure 4D) in some cases as described above.

That is, outgoing data may be received by the third interface 522 from a computer 424 via an external satellite modem and in a modulated form (having been

. modulated by the external modem). Alternatively, (unmodulated) outgoing data may be received from a computer 424 by the third interface 522, and supplied to an internal satellite modem integrated in the ODU; the internal modem modulates the received data and supplies the modulated data to the antenna (via filter(s), amplifiers) etc. in some cases).

Note that the various described components, modules etc. of the IDU 420a and ODU 422a described with reference to figure 5A - including the synchronization module 516 and tracking module 518 - may be implemented in software (i.e. as code executed on a processor of the IDU 420a), dedicated hardware/firmware of the IDU 420a or any combination of both.

The above presents GPS as an example of a satellite navigation system, but this is not exhaustive. For example, at the time of writing, various other regional/global navigations systems are currently being operated by Russia, China and Europe with additional such systems currently in development by other countries/regions such as Japan and India. The positioning module 502 could, for instance, take the form of so-called multi-GNSS receiver which receives and uses signals from multiple global and/or regional navigation satellite systems, or any other type of satellite navigation positioning module which receives navigation signals from a satellite navigation system and detects the current location (and optionally generates the clock signal) from the received satellite navigation signals. Moreover, this possibility of using non- satellite based positioning techniques as an alternative or in addition are not excluded.

In the above, the existing IFL infrastructure is used to communicate the location data between the ODU and IDU. Whilst this is advantageous for reasons discussed, the possibility of communicating the location data by other means - e.g. via separate wireless interface of the ODU either to the IDU or to another apparatus external to the ODU - is not excluded. Moreover, whilst in the above the location data is supplied for tracking purposes from the ODU to a server via the IDU, the possibility of the location data being so supplied by some route which does not involve the IDU is not excluded, e.g. via a different apparatus external to the ODU, or the ODU may comprise additional internal logic to enable it transmit to the network 102 directly e.g. via the satellite 110. Further, whilst in the above a TDMA scheme is employed, alternative channel access methods may be applied to similar effect i.e. to enable multiple client systems to share resources of the return satellite link. Further, whilst in the above an external satellite modem is installed inside of a building, this is not essential.

Whilst the above has been described in terms of specific embodiments, these are not exhaustive. The scope is not limited by the described embodiments but only by the following claims.