SE2151554A1

SE2151554A1 - Satellite Communication System, Transceiver Terminal, Main Transceiver, Methods, Computer Programs and Non-Volatile Data Carriers

Info

Publication number: SE2151554A1
Application number: SE2151554A
Authority: SE
Inventors: Anders Ellgardt; Kennet Lejnell; Lars Erup; Maria-Theresa Rieder; Martin Eriksson
Original assignee: Ovzon Sweden Ab
Priority date: 2021-12-17
Filing date: 2021-12-17
Publication date: 2023-06-18
Also published as: WO2023110234A1; SE545756C2

Abstract

A main transceiver communicates source data by contemporaneously transmitting and receiving data over first and second satellite links (L1; L2). A transceiver terminal receives and transmits data in an alternating and temporally non-overlapping manner such that it exclusively receives data during non-transmission intervals. The main transceiver applies a burst error correction encoding scheme on first source data to be transmitted to the transceiver terminal to generate a first set of encoded data. The main transceiver produces a first modulated and amplified signal based on the first set of encoded data; and transmits the first modulated and amplified signal to the transceiver terminal over the first satellite link (L1). The transceiver terminal receives segments of the first modulated and amplified signal over the first satellite link (L1), and reconstructs the data contained in any non-received segments of the first modulated and amplified signal to thus derive a copy of the first source data. Here, the non-received segments arrived at the transceiver terminal during transmission intervals for the transceiver terminal, and the reconstruction is based on the received segments of the first modulated and amplified signal and the burst error correction encoding scheme. The transceiver terminal transmits data over the second satellite link (L2) in bursts and, may also implement a burst error correction scheme. The main transceiver may receive its transmissions and decode the received data as necessary.

Description

Satellite Communication System, Transceiver Terminal, Main Transceiver, Methods, Computer Programs and Non- Volatile Data Carriers TECHNICAL FIELD The invention relates generally to a satellite communication pro- tocol enabling a compact terminal design. ln particular, the pre- sent invention concerns a satellite communication system, a transceiver terminal and a main transceiver. The invention also encompasses methods implemented in hardware or at least part- ly in software, which methods are performed by the transceiver terminal and the main transceiver respectively, as well as corre- sponding computer programs and storage media containing such computer programs.

BACKGROUND Making use of small-sized satellite terminals has been a long- standing trend in satellite communications. New use cases, such as on-the-move communication with man-pack terminals, require even smaller designs. One obstacle in reducing the physical si- ze of the terminals comes from strict requirements on filter solu- tions in the full-duplex terminals, which are commonly used. The filter requirements are strict to protect the receive chain from noise and other interference generated in the transmit chain, or more precisely by the amplifier herein. For operation over satel- lites, the receive and transmit frequencies are often relatively closely spaced. lt is therefore a challenge to prevent the trans- mit-chain noise and other interference from reaching the recei- ver circuitry.

The details of the filter specification depend on the frequency band. This disclosure focuses mainly on the so-called the Ku band, i.e. the portion of the electromagnetic spectrum in the mi- crowave range of frequencies from 10 to 18 GHz (although different definitions exist), which is primarily used for satellite communications. However, of course, the same principles apply generally for any frequency band with similar characteristics and similar or smaller relative separation between transmit and re- ceive bands. For the Ku-band the guard band between transmit and receive frequencies is typically around 1 GHz wide, and the satellite part of Ku-band spectrum spans from 10.7 - 14.5 GHz. Consequently, the guard band is roughly 8% of the 12.6 GHz mid band frequency. Such a tight guard band requires steep filter characteristics, which can only be achieved with filters having fairly large physical dimensions. A typical waveguide filter for this case is 50-100 mm long with a waveguide opening of 19.05 mm x 9.525 mm. This means that the filters will consti- tute a lion part of a terminal sized 200 mm x 200 mm, or smaller. ln other words, the filters pose an important obstacle to further decreasing terminal sizes. Other types of filters, for instance of stripline or microstrip design can be made substantially smaller. However, since these filter types have lower filter performance, additional means are required to ensure sufficient isolation between the receiver and transmit chains in the terminal. This, again, increases the overall terminal size.

Another problem with microstrip-based filters and other compact filters is their poor insertion loss performance compared to tradi- tional waveguide filters. The insertion loss of a filter placed in front of the low-noise amplifier (LNA) directly impacts the noise of the system, as described by Friis formula, and thereby lowers the system gain-to-noise-temperature ratio G/T. An increase in insertion loss of 1 dB may reduce the G/T with more than 2 dB. The aperture needs to increase its area by 58% to compensate for a 2 dB G/T degradation. On the transmit side of the terminal higher insertion losses may also be compensated for by increa- sing the antenna aperture, or by increasing the output power. The latter increases the overall power consumption of the sys- tem and requires more cooling. Both measures lead to increased size and weight of the system.

For the above reasons, it is complicated to design small-sized antennas that are configured to transmit and receive at the Ku band. ln particular, this comes into play for array antennas with distributed amplifiers attached to each antenna element, or each smaller subset of antenna elements. The typical size of an an- tenna element is of the order of half a wavelength. At the Ku band, this measure is roughly 10 mm, which places severe limi- tations on the filter. Namely, the filter block must be arranged physically close to the antenna. A common solution to this prob- lem in electronically steered array antennas, where each receive antenna element port is connected to an LNA, is to omit filters between the elements and the LNAs altogether.

Alternatively, to improve the receiver-to-transmitter isolation and compensate for inadequate filtering, the receiver and the trans- mitter may be physically separated into two different apertures. Naturally, this also results in an overall increase of the terminal size. Yet another method to attain isolation between the receiver and the transmitter is to separate reception and transmission in time, i.e. such that the receiver and the transmitter are active al- ternatingly. This means that the amplifier is turned off whenever the terminal is set to receive. Thereby, the amount of noise en- tering the receiver is reduced drastically, which is equivalent to an increased receiver G/T (gain-to-noise-temperature)_ This mo- de of operating a transceiver is called half-duplex operation, and will be exemplified below.

WO 2019/159165 shows a method for conveying communica- tions within a satellite communication network, by implementing a beam hopping technique for communicating with half-duplex user terminals.

US 2016/0352415 describes a method and apparatus for provi- ding half-duplex communications for a Very Small Aperture Ter- minal (VSAT) operating on a continuous received stream. The method includes: decoding the continuous received stream to establish synchronization with the continuous received stream; locating, in the continuous received stream, a time plan inclu- ding a receiving timeslot and a transmitting timeslot; demodula- ting the continuous received stream by adapting to a timing and frequency variation of the continuous received stream in the re- ceiving timeslot, freewheeling the adapting of the continuous re- ceived stream during the transmitting timeslot, and resuming the adapting of the continuous received stream when the transmit- ting timeslot ends; stopping a receiving of the continuous recei- ved stream during the transmitting timeslot; and transmitting from the VSAT during the transmitting timeslot. The freewhee- ling includes saving a signal acquisition parameter at the start of the transmitting timeslot and restoring the saved signal acqui- sition parameter at the end of the transmitting timeslot.

US 6,477,669 discloses a method for adaptive control of a for- ward error correction code for transmission between a terrestrial cell/packet switch at a first terminal and a satellite/wireless net- work connecting to a second terminal, including the steps of: calculating a byte error rate associated with communication sig- nals received by the first terminal, determining a forward error correction code length based on the byte error rate, and trans- mitting the forward error correction code length to the second terminal.

US 9,397,704 reveals approaches for satellite data transmis- sions, which accommodate for periodic signal blockages without packet loss. Here, a data stream is segmented into packets for wireless transmission, wherein the transmission is subject to a periodic blockage, wherein the periodic blockage comprises two blockages occurring within a time period, and each blockage is of a respective duration and recurs at regular intervals based on the time period. A forward error correction outer code is applied to the packets for recovery of data erasures due to the periodic blockage, wherein the application of the outer code comprises applying an error correction code to each of the packets to ge- nerate a respective code block. Each code block is interleaved to prevent erasure of consecutive parity bits within the code block. The encoded and interleaved code blocks are transmitted over a wireless channel, wherein a number of data erasures oc- cur within each code block due to the periodic blockage.

Thus, half-duplex satellite communication solutions are known. Nevertheless, these designs pose significant restrictions on the entire network and makes for a very inefficient use of spectrum resources. The existing half-duplex designs therefore cannot be incorporated in existing full-duplex networks without adverse ef- fects on the full-duplex users.

Referring now to Figure 7, we see a schematic illustration of a geostationary satellite 710 over the earth 700. ln order to pre- serve the overall system efficiency, traditional half-duplex com- munication systems are typically configured to transmit and re- ceive during intervals that do not overlap at the ground-based terminal. To achieve this, one must consider the applicable pro- pagation delays, inter alia including the variation in delay across the satellite beam footprint D on the earth 700.

Let us assume that a beam footprint D of the satellite 710 ex- tends to an elevation angle on of 5° at a first edge of the beam footprint D and that a first slant range r1 to the satellite 710 is 41,127 km from the first edge. lf the beam footprint D is 1,000 km across, at a second edge of the beam footprint D, which se- cond edge is opposite to the first edge, the elevation angle dz will be 14.2° and a second slant range rg to the satellite 710 is 40,142 km from the second edge. As a result, the one-way pro- pagation delay associated with this beam will vary between 133.81 ms and 137.09 ms over the beam footprint D.

A satellite network commonly employs a shared forward link from the satellite network's HUB, which downlink carries data for all transceiver terminals in the beam footprint D in a time division multiplex structure. The timing of satellite transmissions is therefore typically common to all the transceiver terminals in a beam footprint D. Due to the variation in propagation delay, the satellite transmissions will arrive at the transceiver terminals at times that depend on the respective location of each transceiver terminal.

For a half-duplex transceiver terminal, the transmission from the terminal must be scheduled so that there is no overlap with the periods when the terminal needs to receive. lf the terminal ope- rates in a TDMA mode, its transmissions must be carefully sche- duled so that they arrive in an orderly manner at the satellite 710. Such burst timing control arrangements are always in place for TDMA systems and renders the utilization of the satellite re- sources in terms of power and bandwidth fairly low.

There are several additional complexities in such systems. For example, terminal handover between beams is complicated, even if the time slicing is well coordinated between the beams. Moreover, the transceiver terminals must be capable of "looking ahead" to receive a destination beam, while still receiving servi- ce in an originating beam, constrained by a switching pattern applicable there.

These issues can be addressed by technical means. However, this involves considerable complexity, especially if a system with this functionality shall be augmented. Some examples are given here.

A TDMA terminal transmission, i.e. the return link to the satel- lite, such as the below-mentioned DVB-RCS2 is tightly synchro- nized within itself. However, the TDMA terminal transmission is not tied to the satellite network's HUB transmission, i.e. the for- ward link, or any external reference. lntroducing such synchroni- zation will involve undesired changes to the terminal as well as the infrastructure.

Popular forward link air interfaces such as DVB-S2x do not inhe- rently lend themselves to being synchronized to anything else, and even more so when operated with ACM (Adaptive Coding and Modulation). With ACM the frame duration varies depending on the MODCOD in an unpredictable manner, depending on scheduling decisions that are typically made in isolation. There are constructs such as DVB-S2x superframes that can remedy this and even support bursty transmissions. However, adding this to the scheduling and modulation function is a non-trivial task and may lead to other restrictions. ln order to recoup the satellite resources that are wasted becau- se each beam only operates part of the time, it may be possible to implement a coordination scheme between beams. This is typically not done in ordinary systems and it would also further restrict the flexibility of the satellite.

Hence even if it is possible from a technical perspective to imp- lement a half-duplex scheme by the traditional methods it comes with a substantial amount of complexity and restrictions, which limit the usability of the system for small-sized mobile terminals that need a large degree of flexibility.

SUMMARY One object of the present invention is therefore to offer a solu- tion that enables a compact half-duplex transceiver terminal to co-exist with full-duplex transceiver terminals without causing any disturbances, or requiring careful synchronization between forward and return links.

According to one aspect of the invention, the object is achieved by a satellite communication system containing a main transcei- ver and a transceiver terminal. The main transceiver is configu- red to communicate source data by contemporaneously trans- mitting and receiving data over first and second satellite links. The transceiver terminal is configured to receive and transmit data in an alternating and temporally non-overlapping manner such that the transceiver terminal exclusively receives data du- ring non-transmission intervals for the transceiver terminal. The main transceiver is further configured to apply a burst error cor- rection encoding scheme (BEC) on first source data to be trans- mitted to the transceiver terminal to generate a first set of enco- ded data. The main transceiver is configured to produce a first modulated and amplified signal based on the first set of encoded data; and transmit the first modulated and amplified signal to the transceiver terminal over the first satellite link. The transceiver terminal, in turn, is configured to receive segments of the first modulated and amplified signal over the first satellite link. The transceiver terminal is further configured to reconstruct the data contained in any non-received segments of the first modulated and amplified signal to thus derive a copy of the first source da- ta. Here, the non-received segments arrived at the transceiver terminal during transmission intervals for the transceiver termi- nal, i.e. reception is interrupted due the fact that transmission occurs. The reconstruction is made based on the received seg- ments of the first modulated and amplified signal and the BEC encoding scheme.

This satellite communication system is advantageous because it allows seamless integration of half-duplex transceiver terminals in existing full-duplex networks without any adaptations of the full-duplex transceiver terminals.

The half-duplex transceiver terminals can be made smaller than the full-duplex transceiver terminals due to a simpler antenna and filter design enabled by the fact that transmission and rece- ption never occurs simultaneously.

Preferably, the main transmitter is configured to produce the first modulated and amplified signal according to a procedure in- volving interleaving and block-error coding in a link layer.

This is advantageous because it adds redundancy to the trans- mitted signal by means of long block codes, which are further in- terleaved. As a result, the data contained in relatively long sig- nal segments that are lost due to channel blockage can be suc- cessfully reconstructed. For example, according to one embodi- ment of this aspect of the invention, the main transmitter is con- figured to generate the first set of encoded data according to a procedure in which the first source data is organized in blocks in an n-dimensional matrix of data, where n is an integer larger than or equal to two. Here, each block contains a payload por- tion of the first source data and a parity portion that is calcula- ted based on the payload portion along n-1 dimensions of the n- dimensional matrix of data.

According to another embodiment of this aspect of the invention, the transceiver terminal is configured to apply the BEC encoding scheme to the second source data and thus generate a second set of encoded data containing first and second sets of seg- ments of the second set of encoded data. The transceiver termi- nal is further configured to transmit a second modulated and amplified signal over the second satellite link to the main trans- ceiver, which second modulated and amplified signal is based only on the first set of segments of the second set of encoded data. The second set of segments of the second set of encoded data may thus be discarded. Consequently, a respective silent period separates each segment in the first set of segments from one another during which silent period no signal is transmitted. Additionally, the main transceiver is configured to receive the first set of segments over the second satellite link, and recons- truct the data contained in the second set of segments to thus derive a copy of the of the second source data. The reconstruc- tion is here based on the received first set of segments of se- cond set of encoded data and the BEC encoding scheme. The second set of encoded data need not be transmitted. lnstead, these data are derived on the receiver side, i.e. in the main transceiver.

Preferably, the transceiver terminal contains a transmitter circui- try configured to obtain the second source data from a second data source, and a controller circuitry configured to control the transmitter circuitry. Specifically, the controller circuitry is confi- gured to control the transmitter circuitry to apply the BEC enco- ding scheme to the second source data to generate the second set of encoded data, and transmit the second modulated and amplified signal over the second satellite link to the main trans- ceiver. The second modulated and amplified signal is based ex- clusively on the first set of segments of the second set of enco- ded data. This enables the above economizing of the uplink bandwidth in a highly efficient manner.

According to yet another embodiment of this aspect of the inven- tion, the transmitter circuitry contains a link-layer processor con- figured to encode the second set of encoded data according to a link layer protocol, and a modulator and amplifier circuitry con- figured to receive the second set of encoded data that has been encoded according to the link layer protocol. Moreover, based on the second set of encoded data, the modulator and amplifier circuitry is configured to produce the second modulated and amplified signal in response to a control signal from the control- ler circuitry. As a result, for example the Link Layer FEC (LL- FEC) scheme can be implemented as defined in the DVB-RCS2 standard ETSI EN 301 545-2: "Digital Video Broadcasting (DVB); Second Generation DVB Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite Standard." According to further embodiments of this aspect of the invention, the main transceiver and the transceiver terminal are configured to transmit in a manner whereby transmissions on the first and second satellite links are unsynchronized relative to one an- other. Naturally, this renders the communication protocol very straightforward to put into effect.

According to another embodiment of this aspect of the invention, the transceiver terminal includes a transmitter circuitry configu- red to obtain the second source data from a second data source and a controller circuitry configured to control the transmitter cir- cuitry to transmit a modulated and amplified signal based on the 11 second source data. The modulated and amplified signal is transmitted according to a time table specifying a respective transmission interva| for the transceiver terminal. Thus, in this embodiment, no segments of the encoded data need to be dis- carded.

According to yet another embodiment of this aspect of the in- vention, the transceiver terminal includes a link-layer processing unit and a modulator and amplifier circuitry. The link-layer pro- cessing unit is configured to encode the second source data ac- cording to a link layer protocol in response to a control signal from the controller circuitry. The modulator and amplifier circui- try is configured to receive the second source data that has been encoded according to the link layer protocol, and based thereon produce the second modulated and amplified signal in response to the control signal from the controller circuitry. As a result, for example the LL-FEC scheme can be implemented as defined in the DVB-RCS2 standard. Although the LL-FEC sche- me is defined within the DVB-RCS2 framework, this does not mean that this embodiment of the invention is limited to an DVB- RCS2 network. On the contrary, the embodiment is equally app- licable to other network configurations (for instance a DVB-S2X network), or point-to-point communication.

According to still another embodiment of this aspect of the in- vention, the main transceiver is configured to transmit the first satellite link on a time-division multiplexed format, which may include frequency hopping. This is beneficial since frequency hopping reduces the number of errors due to frequency selective fading. Frequency hopping also spreads any occurring errors temporally, which improves the chances of correcting these er- rors using a forward error correction scheme.

According to another embodiment of this aspect of the invention, the main transceiver is onboard a satellite. Further, the main transceiver is configured to facilitate direct communication, i.e. relay, between at least two ground based transceiver terminals. 12 Consequently, the main transceiver may function as an intermediary node between two or more ground-based nodes.

According to another aspect of the invention, the object is achie- ved by a transceiver terminal for communicating source data in the above-described system by transmitting and receiving data over first and second satellite links. The transceiver terminal contains transmitter and receiver circuitry configured to transmit and receive data respectively in an alternating and temporally non-overlapping manner, such that the transmitter circuitry ex- clusively receives data during non-transmission intervals for the receiver circuitry. The receiver circuitry is configured to receive segments of a first modulated and amplified signal over the first satellite link, which first modulated and amplified signal repre- sents first source data to which a BEC encoding scheme has been applied to generate a first set of encoded data based upon which the first modulated and amplified signal has been produ- ced. The receiver circuitry is further configured to reconstruct the data contained in any non-received segments of the first set of encoded data to thus derive a copy of first source data, which non-received segments arrived at the transceiver terminal during transmission intervals for the transceiver terminal. Here, the reconstruction is based on the received segments of the first modulated and amplified signal and the BEC encoding scheme.

According to one embodiment of this aspect of the invention, the transceiver terminal has an antenna and filters of a design that is adapted for small-sized terminals. This means that there is a relatively low degree of isolation between the transceiver termi- nal's transmit and receive chains, which, in turn, results in high levels of noise and spurious in the receiver circuitry due to the transmitted signal. Consequently, the receiver circuitry is rende- red unable to receive and decode the above-mentioned non-re- ceived segments of the first set of encoded data.

The advantages of this transceiver terminal, as well as the pre- ferred embodiments thereof, are apparent from the discussion 13 above with reference to the proposed satellite communication system.

According to yet another aspect of the invention, the object is achieved by a main transceiver for communicating source data in the above-described satellite communication system by con- temporaneously transmitting and receiving data over first and second satellite links. The main transceiver contains a main transmitter and a main controller. The main transmitter is confi- gured to obtain first source data from a first data source. The main controller is configured to control the main transmitter to: apply a BEC encoding scheme on the first source data to gene- rate a first set of encoded data, produce a first modulated and amplified signal based on the first set of encoded data, and transmit the first modulated and amplified signal to a transceiver terminal over the first satellite link.

The advantages of this main transceiver, as well as the prefer- red embodiments thereof, are apparent from the discussion abo- ve with reference to the proposed satellite communication sys- tem.

According to still another aspect of the invention, the object is achieved by a method that is implemented in hardware or at least partly computer-implemented, which method is performed in the above-described transceiver terminal for communicating source data by transmitting and receiving data over first and se- cond satellite links. The method involves: transmitting and recei- ving data in an alternating and temporally non-overlapping manner such that the transceiver terminal exclusively receives data during non-transmission intervals for the transceiver ter- minal; receiving segments of a first modulated and amplified sig- nal over the first satellite link, which first modulated and ampli- fied signal represents first source data to which a BEC encoding scheme has been applied; and reconstructing the data contained in any non-received segments of the first set of encoded data to thus derive a copy of first source data, which non-received seg- 14 ments arrived at the transceiver terminal during transmission in- tervals for the transceiver terminal, and the reconstruction is ba- sed on the received segments of the first modulated and amp- lified signal and the BEC encoding scheme.

The advantages of this method are apparent from the discussion above with reference to the proposed transceiver terminal.

According to still another aspect of the invention, the object is achieved by a method that is implemented in hardware or at least partly computer-implemented, which method is performed in the above-described main transceiver for communicating sour- ce data by transmitting and receiving data over first and second satellite links. The method involves: obtaining first source data from a first data source in a main transmitter of the main trans- ceiver; and controlling the main transmitter to: apply a BEC en- coding scheme on the first source data to generate a first set of encoded data; produce a first modulated and amplified signal based on the first set of encoded data; and transmit the first mo- dulated and amplified signal to a transceiver terminal over the first satellite link.

The advantages of this method are apparent from the discussion above with reference to the proposed main transceiver.

According to further aspects of the invention, the object is achie- ved by computer programs loadable into a respective non-volati- le data carrier communicatively connected to a respective pro- cessing unit, where each of the computer programs includes software for executing the above respective methods when be- ing run on the respective processing units.

According to another aspect of the invention, the object is achie- ved by non-volatile data carriers containing the above computer programs.

Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figure 1 Figure 2 Figures 3a-b Figures 4-6 Figure 7 Figure 8 Figure 9 Figure 10 Figures 11-12 shows a block diagram of a main transceiver and a transceiver terminal according to a first embodiment of the invention; shows a block diagram of a main transceiver and a transceiver terminal according to a se- cond embodiment of the invention; show block diagrams of transmitter circuitries of the transceiver terminal according to embo- diments of the invention; illustrate different communication scenarios according to embodiments of the invention; illustrates how a slant range of a satellite may vary over the satellite's footprint on earth; exemplifies the gain-to-noise-temperature as a function of time for a transceiver terminal ac- cording to one embodiment of the invention; exemplifies time intervals relevant for activa- ting and deactivating the transmitter amplifier according to one embodiment of the invention; illustrates, by means of a flow diagram, a ge- neral method of operating the main transceiver and the transceiver terminal; and illustrate, by means of flow diagrams, general methods according to the invention that are performed in the transceiver terminal and the main transceiver respectively. 16 DETAILED DESCRIPTION Figure 1 shows a block diagram of a main transceiver 100 and a transceiver terminal 200 according to a first embodiment of the invention. The main transceiver 100 and the transceiver terminal 200 form part of a satellite communication system, wherein the main transceiver 100 is configured to communicate source data by contemporaneously transmitting and receiving data over first and second satellite links L1 and L2 respectively. As will be des- cribed below, this means that the main transceiver 100 may either be located on the ground, or be placed onboard a satel- lite. The transceiver terminal 200, however, is presumed to be earth based. Consequently, the transceiver terminal 200 either has a fixed location or is mobile. ln the latter case, the transcei- ver terminal 200 may be portable and/or be carried by a ground, water or air vehicle.

According to the invention, the transceiver terminal 200 is confi- gured to receive and transmit data in an alternating and tempo- rally non-overlapping manner such that the transceiver terminal 200 exclusively receives data D1MABEC during non-transmission intervals for the transceiver terminal 200.

Figures 8 and 9 illustrate examples of how this mode of opera- tion can be expressed in the time and frequency domains.

Figure 8 shows a graph representing a gain-to-noise-temperatu- re ratio G/T for a receiver circuitry in the transceiver terminal 200 as a function of time t. ln the ratio G/T, G symbolizes the gain in decibels at a receive frequency f, and T is the equivalent noise temperature of the receiving system in Kelvin. The equiva- lent noise temperature T represents a summation of the antenna noise temperature and the radiofrequency chain noise tem- perature from the antenna terminals to the receiver output. Du- ring periods of reception TR, the gain-to-noise-temperature ratio G/T is high; whereas during periods of transmission TT, the gain-to-noise-temperature ratio G/T is very low. A first switching interval s1 represents a time required to deactivate the receiver 17 circuitry in the transceiver terminal 200, and a second switching interval s2 represents a time required to activate the receiver circuitry in the transceiver terminal 200. Typically, however not necessarily, the first and second switching intervals s1 and s2 have approximately equal lengths. ln any case, the intervals s1 and s2 should be as short as possible, and a transmitter circui- try in the transceiver terminal 200 should be inactive during both these intervals s1 and s2.

Figure 9 shows a graph representing time t along the vertical axis and frequency f along the horizontal axis. Here, a first cen- ter frequency f1 designates a center frequency of a receive band extending from first lower frequency f1L to a first upper frequen- cy f1u, and a second center frequency f2 designates a center frequency of a transmit band extending from a second lower fre- quency f2L to second upper frequency f2u. For example, f1 may be 11 GHz and f2 may be 13 GHz, and a guard band Af of ap- proximately 1 GHz may separate the first upper frequency f1u from the second lower frequency f2L. This may mean that the first lower frequency f1L is around 10.7 GHz, the first upper fre- quency f1u is around 11.45 GHz, the second lower frequency f2L is around 12.75 GHz and the second upper frequency f2u is around 13.25 GHz.

According to the invention, it is possible to use frequency bands with double allocation, i.e. where the same frequency band is employed for both the uplink and the downlink. Referring to Fi- gure 9, this means that f1L is equal to f2L and f1u is equal to f2u.

For a typical solid-state amplifier with a radio frequency output power of 25 W, the noise generated in the receive band needs to be suppressed by approximately 100 dB. lt is possible to achieve 70 dB rejection with a waveguide filter. Further, assu- ming that the antenna isolation between transmit and receive circuitry is around 30 dB, provides a total isolation of 100 dB.

Turning again to Figure 1, according to the invention, the main 18 transceiver 100 is configured to apply a burst error correction (BEC) encoding scheme on first source data D1 to be transmit- ted to the transceiver terminal 200 to generate a first set of en- coded data representing the first source data D1. The main transceiver 100 is further configured to produce a first modula- ted and amplified signal DliviAßec based on the first set of enco- ded data, and transmit the first modulated and amplified signal DWABEC to the transceiver terminal 200 over the first satellite link L1.

Additionally, the transceiver terminal 200 is configured to recei- ve segments of the first modulated and amplified signal DliviAßec over the first satellite link L1, which segments are here exempli- fied by s11, s1s, s1s and sw respectively.

The main transceiver 100 basically presumes that there is a full- duplex connection to the transceiver terminal 200. The main transceiver 100 therefore transmits the first modulated and amp- lified signal DliviAßec over the first satellite link L1 in an uninter- rupted manner, i.e. as a continuous stream of data. Consequent- ly, since, however, the transceiver terminal 200 is configured to receive and transmit data in an alternating and temporally non- overlapping manner so that the transceiver terminal 200 exclu- sively receives data DWABEC during non-transmission intervals for the transceiver terminal 200, any segments n1o, n12, n14 and me of the first modulated and amplified signal DmAsec that arrived at the transceiver terminal 200 from the main transceiver 100 during one of the transmission intervals for the transceiver terminal 200 cannot be received by the transceiver terminal 200.

For that reason, the transceiver terminal 200 is configured to re- construct the data contained in the non-received segments n1o, n12, n14 and ms respectively of the first modulated and amplified signal DWABEC to thus derive a copy "D1" of the first source data D1. The reconstruction is based on the received segments s11, s1s, s1s, and sn of the first modulated and amplified signal DWABEC and the BEC encoding scheme. Preferably, a demodu- 19 lator/decoder unit 250 in a receiver circuitry of the transceiver terminal 200 is configured to carry out the reconstruction of the data contained in the non-received signal segments n1o, n12, n14 and me.

According to one embodiment of the invention, the receiver cir- cuitry of the transceiver terminal 200 includes a receiver chain 240 containing an antenna and an amplifier, which receiver chain 240 is configured to receive the segments s11, s1s, s1s and sw of the first modulated and amplified signal D1MAsEc, and pro- vide the demodulator/decoder unit 250 with a signal based upon which the demodulator/decoder unit 250 is configured to derive the copy "D1" of the first source data D1.

According to one embodiment of the invention, the transceiver terminal 200 includes a receiver circuitry 240 configured to re- ceive the segments s11, s1s, s1s and sw of the first modulated and amplified signal D1MABEC. Based thereon, and the BEC en- coding scheme, the receiver circuitry 240 is configured to derive the copy "D1" of the first source data D1. ln the embodiment shown in Figure 1, the transceiver terminal 200 is configured to transmit a second modulated and amplified signal DZMA over the second satellite link L2 to the main trans- ceiver 100. The second modulated and amplified signal D2|v|A is transmitted during the periods of transmission TT rendering the receiver circuitry 240 and 250 of the transceiver terminal 200 unable to receive and decode data, i.e. equivalent to periods when the non-received signal segments n1o, m2, n14 and ms arrive at the transceiver terminal 200. The second modulated and amplified signal DZMA is based on a complete set of second source data D2 from a second data source 210, which either generates the second source data D2, or obtains the second source data D2 via an input channel (not shown).

According to one embodiment of the invention, the main trans- ceiver 100 contains a main transmitter 120 and a main controller 130. The main transmitter 120 is configured to obtain the first source data D1 from a first data source 110, which either gene- rates the first source data D1, or obtains the first source data D1 from an external source via an input channel (not shown). The main controller 130 is configured to control the main transmitter 120, e.g. via commands or control messages CTRL, to apply the BEC encoding scheme to the first source data D1, so as to generate the first set of encoded data.

Preferably, the main transmitter 120 is configured to produce the first modulated and amplified signal D1MABEc according to a procedure that involves interleaving and block-error coding in a link layer.

For example, according to this embodiment of the invention, the link layer FEC (LL-FEC) as defined in the DVB-RCS2 standard ETSl EN 301 545-2: "Digital Video Broadcasting (DVB); Second Generation DVB Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite Standard" may be applied in com- bination with additional explanations provided in the guidelines for mobile systems according to the standard ETSl TR 102 768, "Digital Video Broadcasting (DVB); Interaction channel for Satel- lite Distribution Systems; Guidelines for the use of EN 301 790 in mobile scenarios" ln its essence, the link-layer error coding scheme adds redun- dancy to the signal by means of long block codes combined with interleaving. This allows reconstruction of the data contained in the parts of the signal that are lost due to channel blockage. The use of LL-FEC is defined separately for each so-called (Generic Stream Encapsulation) GSE-FEC stream in the overall generic stream. ln the following we will omit details regarding the GSE stream, focusing instead on a description of the basic LL-FEC principles.

The implementation of LL-FEC entails organizing the data to be protected (the "application data") in tables, referred to as LL- 21 FEC frames, together with parity bits (the "FEC data"). The LL- FEC frame is a construct used to generate LL-FEC parity sec- tions from a sequence of higher-layer PDU's. lt is conceptually arranged as a matrix, which is sub-divided "vertically" into two parts known as the "Application Data Table" (ADT) and the "FEC Data Table" (FDT), respectively.

Conceptually, the frame may be organized so that the ADT and FDT have the same number of rows, however both tables have a flexible number of columns. The respective maximum number of columns in each of the tables depends on the type of code used. The number of columns in the ADT signaled in each parity sec- tion/packet transmitted along with the LL-FEC frame.

The matrix has a flexible number of rows. (The number of rows is signaled in the forward link as part of the configuration of the GSE-FEC stream. The transmitter does not need previous know- ledge of the matrix size). Each position in the matrix can hold an information byte. The ADT is used to hold higher-layer PDU's and possibly padding. The FDT holds the parity information of the FEC code. The number of columns in the ADT and FDT can vary frame-by-frame. Higher-layer PDU's are inserted consecuti- vely, starting with the first byte of the first datagram in the upper left corner of the ADT matrix; going downwards in the first co- lumn and wrapping to the next column when the last row in a co- lumn has been filled. The length of the datagrams may vary. Once the ADT is filled, parity data columns for the FDT can be computed by applying the selected coding technique to each row of the frame.

The standard ETSI EN 301 545-2: "Digital Video Broadcasting (DVB); Second Generation DVB Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite Standard" offers two coding schemes, Reed-Solomon and Raptor codes. The de- cision on the completeness of an ADT table may depend on la- tency considerations, the LL-FEC code rate and other parame- ters. There will typically be a timeout that prevents the frame 22 from going "stale" in the event of a pause in transmissions.

The ADT and FDT are both transmitted column-wise. Therefore, loss of data will tend to erase a few symbols in each of several code words; hence, the risk of exceeding the correcting capacity of the code is minimized. The ADT contents can be transmitted at the same time they are entered into the table; there is there- fore no additional latency on the data proper. However, the pari- ty can only be computed once the ADT is full, or has timed out.

On the receive side, equivalents of the ADT and FDT are filled with the received data. Once the frame is complete, or timeouts have occurred, the row-wise code words are decoded and any erased/erroneous symbols are re-constructed. Like the under- lying block code, LL-FEC is a systematic code; i.e., the encoder output consists of the original data, plus parity bits/symbols. lt is therefore valid simply to receive the ADT portion of the frame. Not only does this eliminate the additional latency, it also elimi- nates any protection afforded by the LL-FEC.

The dimensions of the LL-FEC frame are determined by three main parameters, namely: -the data rate Rsefvice to be protected (bits/second) -the maximum fraction of time fi during which the signal can be interrupted, and -the maximum duration lmax of an interruption (seconds) The so-called "latency" A, which is equivalent to the frame size, needs to cover a period longer than lmaX/fi.

This is needed to support a maximum-length interruption without exceeding the allowed fraction. Thus, the latency A is: A = lmaX/fi (seconds) The total required frame size, i.e. the total of ADT and FDT, is 23 therefore: NLLFEC = Rservice'|max/(8fi) (byteS) The erasure correcting capability of the Reed-Solomon code is equal to the number of parity symbols (bytes) in each code word. The number of columns in the ADT and FDT must therefo- re be adjusted such that the fraction of parity symbols in each code word is at least the fraction of the latency that can be cor- rected, i.e.: FB? X n» ._ .ivu~t.tàišïï'š.c.:.« " t f" . - .s .- fr' _ F31* twsrztmras "i" *iQ i tu- -t .vàﬁ .rig Where FDTcoiumns designates the number of columns in the FDT, ADTcoiumns designates the number of columns in the ADT and nc designates the coding efficiency of the respective error correc- tion code used. For Reed-Solomon codes nc may be 1, and for other codes nc may be smaller than 1, such as for Raptor codes.

The number of rows NR required is then simply: *f D' ^ 'tuffa-fia å' ß U' T.§<:<štu:z:1-=*:.s ~:"-'ï 'i ._ 'F 'm __; There are some restrictions on the choices of number of rows and columns. ln particular, the Reed-Solomon code used in the DVB-RCS2 standard, ETSI EN 301 545-2, is (255,191) over GF(2^8). Therefore, it has a maximum of 191 ADT columns and 64 FDT columns. Within that envelope, adjustments are made using shortening (i.e. reduction of ADT column count) and punc- turing (reduction of FDT column count). Some examples are pro- vided below. ln the above description, the first source data D1 is organized in a two-dimensional matrix, i.e. the ADT is arranged in rows and 24 columns. Nevertheless, the first source data D1 may be organi- zed any number of higher dimensions. According to one embo- diment of the invention, the main transmitter 120 is configured to generate the first set of encoded data according to a procedure in which the first source data D1 is organized in blocks in an n- dimensional matrix of data, where n is an integer larger than or equal to two. Here, each block contains a payload portion of the first source data D1 and a parity portion that is calculated based on the payload portion along n-1 dimensions of the n-dimensio- nal matrix of data. ln the following, we give examples of how the proposed LL-FEC tables may be dimensioned in the half-duplex mode according to embodiments of the invention. This may be effected in many ways. Here, we take a total bitrate for the forward and return link Rtono and Rtom respectively as reference points. Namely, for a full-duplex user terminal these would be the applicable data ra- tes. lt is important to note that in contrast to standard LL-FEC implementations, where the interruption parameters are determi- ned by external circumstances such as the interruption pattern of the channel; according to this invention there is some free- dom in setting the parameters.

The dimensioning of the LL-FEC and half-duplex operation may be done with respect to either a wanted service data rate for the forward link RM or the forward link Rbi. The procedures are slightly different: When dimensioning for a minimum throughput in the forward link, the following procedure may be applied.

Step 1: Determine required bitrate in the forward link RM. This determines the required error coding rate rE = Rbp/Rtono.

Step 2: Determine a maximum latency A being acceptable for the service in question.

Step 3: Dimension LL-FEC tables for an interruption fraction fu > 1 -rE. The inequality is due to an overhead included to account for re-synchronization. The total table size is NLL = ARtOLO/S. The SiZe Of the ADT and FDT iS NADT = nA_co|nrows 2 (1 - f||)N|_|_ and NFDT = ngcoinrows S fnNLL respectively. Determine number of rows and columns while considering possible restrictions or conside- rations on stability for individual error codes.

Step 4: Determine half-duplex parameters. The time required for interference-free reception in the half-duplex user terminal per latency period is TRX 2 rEA. Accounting for an overhead Ts to switch the amplifier on and off, the half-duplex transceiver ter- minal may transmit for a time TTX = A - TRX - Ts.

Step 5: Determine data rate for return link. The possible service bitrate over the return link is bound by the fraction of time allo- cated to transmission in the half-duplex transceiver terminal Rim < (TTX/A)Rt0t,i. The inequality is due to an overhead included to account for re-synchronization.

When dimensioning for a minimum throughput in the return link, the following procedure may be applied.

Step 1: Determine required bitrate in return link RbJ. This deter- mines the required usage ratio of the return link resources under external circumstances gr = RbJ/Rtoni.

Step 2: Determine maximum latency A acceptable for the servi- ce in question.

Step 3: Determine half-duplex parameters. The active transmit time is TTX > gTA. Again, the inequality is due to overhead to account for re-synchronization. Accounting again for the ampli- fier's switching time, the resulting time during which the half- duplex transceiver terminal can receive data on the forward link iS TRx = Å - TTx _ TS.

Step 4: Dimension LL-FEC table for an interruption ratio fil > 1 - TRX/A. The inequality is for re-synchronization. The total table 26 size is NLL = ARt0L0/8. The size of the ADT and the FDT is NADT = nA_colnrows 2 (1'fll)NLL and NFDT = nF_colnrows S fllNLL fespeCtiVely. Determine number of rows and columns while considering possible restrictions or considerations on stability for individual error codes.

Step 5: Determine throughput in forward link. The resulting error coding rate is rE = nA_c0|/(nA_C0| + ngcoi) and the resulting protected data rate is Rb,o = rERtOLO.

For the LL-FEC implementation, the receive/transmit time does not need be a continuous time interval for every latency period, but due to the overhead for switching time in the half-duplex transceiver terminal, it may be advantageous to design it that way. Thus, the amplifier would generally be scheduled to switch on and off exactly once during one latency period.

Below, we give two concrete examples for introducing a half- duplex transceiver terminal to a network. As a starting point, a number of full-duplex terminals N =36, 20, 10, 5 are assumed to be in a network with equal usage of the spectrum resources m = 1/N. At a given symbol rate Rs and modcod with efficiency n, each terminal receives a certain data rate Rao = nmRS. Next, one of the transceiver terminals is turned into a half-duplex transceiver terminal according to the present invention. The interruption ratio in RX, f, is determined by the need for trans- mission and is considered here to be either 25%, 50%, or 75%. We distinguish two cases, wherein in a first case, we require the throughput per transceiver terminal to be constant - both for half-duplex Rbﬁo = rEUmHoRs and full-dupleX Rbfo = FEUmFDRs With Rbﬁo = Rbfo = Rb,0- and calculate the number of full-duplex transceiver terminals supported besides the half-duplex trans- ceiver terminal NFD = (1-mHD)/m. ln a second case, we require the resource allocation per terminal to stay constant, i.e. mHD = mFo = 1/N, and calculate the resulting throughput for the half-du- plex transceiver terminal. lt is important to note that these calcu- lations do not take into account any restrictions on the error 27 coding rate in the above-described LL-FEC scheme.

Symbol Rate (MS/s) 51 Modcod QPSK 1/2 Spectral Efficiency 1 # terminals 36 20 10 5 Equal allocation 0.03 0.05 0.10 0.20 Baseline throughput (Mb/s)1.42 2.55 5.10 10.20 Case 1: lntroducing one Half-duplex, throughput per terminal constant Rx b|ockage\# Full-duplex terminals 0.25 34 18 8 3 0.5 34 18 8 3 0.75 32 16 6 1 Case 2: lntroducing one Half-duplex, allocation per terminal constant Rx blockage\Throughput for half-duplex (l\/lb/s) 0.25 1.06 1.91 3.83 7.65 0.5 0.71 1.28 2.55 5.10 0.75 0.35 0.64 1.28 2.55 According to one embodiment of the invention, the transceiver terminal 200 includes a transmitter circuitry 220 and a controller circuitry 230. The transmitter circuitry 220 is configured to ob- tain the second source data D2 from the second data source 210. The controller circuitry 230 is configured to produce a con- trol signal CTRL, which, in turn, is arranged to control the trans- mitter circuitry 220 to transmit a modulated and amplified signal D2MA based on the second source data D2. The modulated and amplified signal D2MA is transmitted according to a time table specifying a respective transmission interval for the transceiver terminal 200. Here, reception via the receiver circuitry 240 is in- terrupted whenever the modulated and amplified signal D2|v|A is transmitted.

Referring now to Figure 3b, in the light of the above discussion, it is further preferable if the transceiver terminal 200 contains a link-layer processing unit 311. The link-layer processing unit 311 28 is configured to encode the second source data D2 according to a link layer protocol in response to a first control signal CTRL1 from the controller circuitry 230. For example, the link layer pro- tocol may be implemented according to the LL-FEC scheme de- fined in the DVB-RCS2 standard ETSI EN 301 545-2: "Digital Vi- deo Broadcasting (DVB); Second Generation DVB Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite Standard." A modulator and amplifier circuitry 320 in the transceiver termi- nal 200 is configured to receive the second source data D2 that has been encoded according to the link layer protocol. Based thereon, the modulator and amplifier circuitry 320 is configured to produce the second modulated and amplified signal DZMA in response to the first control signal CTRL1 from the controller cir- cuitry 230.

Figure 2 shows a block diagram of a main transceiver 100 and a transceiver terminal 200 according to a second embodiment of the invention. ln contrast to the embodiment illustrated in Figure 1, the transc- eiver terminal 200 does not interrupt feeding the second source data D2 to the transmitter circuitry 220 when the transmitter cir- cuitry is inactivated. Consequently, some of the segments szo, szz, s24 and szß - typically half of the second source data D2 - will be discarded in the transceiver terminal 200, namely those segments that are fed to the transmitter circuitry 220 when the transmitter circuitry is inactivated. Specifically, according to this embodiment of the invention, the transceiver terminal 200 is configured to apply the BEC encoding scheme to the second source data D2, and thus generate a second set of encoded data DZMABEC. The second set of encoded data DZMABEC contains a first set of segments and a second set of segments, which are illustrated as s21, szs, szs, sz? and szo, szz, s24, sze respectively in Figure 2. The first set of segments represents the portion of the second source data D2 that is transmitted over the second satel- 29 lite link L2 to the main transceiver 100, whereas the second set of segments represents the portion of the second source data D2 that is discarded in the transceiver terminal 200.

Analogous to the above, transceiver terminal 200 is configured to transmit a second modulated and amplified signal DZMABEc over the second satellite link L2 to the main transceiver 100. However, the second modulated and amplified signal DZMABEC is based only on the first set of segments sz1, szs, szs and sz? of the second set of encoded data DZMABEC.

The segments in the first set of segments sz1, szs, szs and sz? are separated from one another by a respective silent period nzo, nzz, nz4 and nze during which silent period no signal is trans- mitted. Each silent period may have a duration of the same or- der of magnitude as a respective period during which each of the segments in the first set of segments sz1, szs, szs and sz? is transmitted. However, according to the invention, any other in- terrelationship is equally possible.

The main transceiver 100 is configured to receive the first set of segments sz1, szs, szs and sz? over the second satellite link L2; and reconstruct the data contained in the second set of seg- ments szo, szz, sz4 and szß to thus derive a copy "D2" of the se- cond source data D2. The reconstruction is here based on the received first set of segments sz1, szs, szs and sz? of the second set of encoded data DZMABEC and the BEC encoding scheme.

According to one embodiment of the invention, the main trans- ceiver 100 contains a main receiver 140 and a demodulator/de- coder 150. The main receiver 140 is configured to receive the first set of segments sz1, szs, szs and sz? over the second sa- tellite link L2. The demodulator/decoder 150 is configured to de- rive a copy "D2" of the source data D2 based on the received first set of segments sz1, szs, szs, and sz? and the BEC encoding scheme.

Preferably, analogous to the above, the transceiver terminal 200 includes a transmitter circuitry 220 configured to obtain the se- cond source data D2 from the second data source 210.

The controller circuitry 230 is configured to produce a second control signal CTRL2, which is arranged to control the trans- mitter circuitry 220 to: apply the BEC encoding scheme to the second source data D2 to generate the second set of encoded data D2BEc; and transmit the second modulated and amplified signal D2MABEC over the second satellite link L2 to the main transceiver 100. Here, the second modulated and amplified sig- nal D2MABEC is based exclusively on the first set of segments s21, szs, szs and S27 of the second set of encoded data DZBEC.

Figure 3a shows a block diagram of transmitter circuitry 220 ac- cording to one embodiment of the invention. ln this embodiment, the transmitter circuitry 220 contains a link-layer processor 310 and a modulator and amplifier circuitry 320.

The link-layer processor 310 is configured to encode the second set of encoded data D2BEC according to a link layer protocol, for example the LL-FEC scheme defined in the DVB-RCS2 standard ETSI EN 301 545-2: "Digital Video Broadcasting (DVB); Second Generation DVB Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite Standard." The modulator and amplifier circuitry 320 is configured to recei- ve the second set of encoded data D2BEC that has been encoded according to the link layer protocol. Based thereon, the modula- tor and amplifier circuitry 320 is configured to produce the se- cond modulated and amplified signal D2MABEC in response to the second control signal CTRL2 from the controller circuitry 230.

Preferably, to keep level of signalling moderate, the main trans- ceiver 100 and the transceiver terminal 200 are configured to transmit in such a manner that the transmissions on the first and second satellite links L1 and L2 respectively are unsynchronized relative to one another. 31 Figure 4 illustrates a first communication scenario according to an embodiment of the invention, where a transceiver terminal 200 on the ground is connected to a main transceiver 100, which also is located on the ground, and a satellite facilitates direct communication between the first and second satellite links L1 and L2. The main transceiver 100 is operated under the presumption of a full-duplex connection. This means that the first satellite link L1 transmits an uninterrupted stream of data from the main transceiver 100 via the satellite 410 to the transceiver terminal 200. The transceiver terminal 200 is operated under the presumption of a half-duplex connection, where the transceiver terminal 200 receives and transmits data in an alternating and temporally non-overlapping manner. This means that the transceiver terminal 200 exclusively receives data during those intervals when it does not transmit any data. Consequently, the transceiver terminal 200 must reconstruct the first source data D1 as described above; and if the transceiver terminal 200 discards data segments as explained with reference to Figure 2, main transceiver 100 also must re- construct the second source data D2.

Figure 5 illustrates a second communication scenario according to an embodiment of the invention, where the main transceiver 100 on the ground communicates with a number of transceiver terminals in network operation. This means that the main trans- ceiver 100 acts as a hub/gateway to a number of transceiver ter- minals, which are exemplified by first and second terminals 200 and 205 respectively. Typically, in such a scenario, the first sa- tellite link L1 is of TDl\/I type, whereas the second satellite link L2 is of FDMA or TDMA type.

Thus, according to one embodiment of the invention, the main transceiver 100 is configured to transmit the first satellite link L1 on a time-division multiplexed format.

Additionally, for improved robustness, the time-division multip- lexed access format may further involve frequency hopping. 32 ln any case, the main transceiver 100 is operated under the pre- sumption of a full-duplex connection, where the first satellite link L1 transmits an uninterrupted stream of data from the main tran- sceiver 100 via the satellite 410 to the transceiver terminals 200 and 205. Each of the transceiver terminals 200 and 205 is ope- rated under the presumption of a half-duplex connection, where they receive and transmit data in an alternating and temporally non-overlapping manner, so that they exclusively receive data during those intervals when they do not transmit any data. The- refore, the transceiver terminal 200 must reconstruct the first source data D1 as described above. lf the transceiver terminal 200 discards data segments as explained with reference to Figu- re 2, the main transceiver 100 must also reconstruct the second source data D2.

For instance, the second communication scenario may provide internet connections to the transceiver terminals 200 and 205.

Figure 6 illustrates a third communication scenario according to an embodiment of the invention, where at least one first trans- ceiver terminal 200 on the ground communicates with at least one second transceiver terminal 205 on the ground via a main transceiver 100 carried by a satellite 410. Here, the main trans- ceiver 100 is configured to facilitate communication between at least one first and second ground-based transceiver terminals 200 and 205. The main transceiver 100 demodulates the signal received from the first ground-based transceiver terminal 200, processes the signal data content and modulates a new signal that is transmitted to at least the second ground-based transcei- ver terminal 205. ln this scenario, .the main transceiver 100 is configured to facili- tate communication between at least one first and second ground-based transceiver terminals 200 and 205. The main transceiver 100 demodulates the signal received from the first ground-based terminal 200, processes the signal data content and modulates a new signal that is transmitted to at least the 33 second ground-based terminal 205.

Analogous to the above, the main transceiver 100 is operated under the presumption of a full-duplex connection by transmit- ting an uninterrupted stream of data on the first satellite link L1, and the first and second transceiver terminals 200 and 205 are operated under the presumption of half-duplex connections. As a result, the transceiver terminal 200 must reconstruct the first source data D1 as described above. Further, the main tran- sceiver 100 may reconstruct the second source data D2, if the transceiver terminal 200 discards data segments as explained with reference to Figure 2.

A control station 420 may be arranged on the ground, which control station 420 is configured to coordinate the transmissions to and from the first and second transceiver terminals 200 and 205 via control signals cc sent to the main transceiver 100 and status signals ss received from the main transceiver 100.

Figure 10 illustrates, by means of a flow diagram a general me- thod according to which the main transceiver and the transceiver terminal may be operated to accomplish the proposed communi- caüon. ln a first step, an interruption sequence is programmed into the controller circuitry 230 of the transceiver terminal 200. ln a step 1020 thereafter, a BEC encoding scheme is dimensioned and implemented in the main controller 130 of the main transceiver 100. ln following parallel steps 1030 and 1040, the main transceiver 100 and the transceiver terminal 200 exchange data in a manner where the main transceiver 100 transmits a continuous stream of data while the transceiver terminal 200 transmits data accor- ding to the interruption sequence.

During the exchange of data, a step 1050 repeatedly checks if the encoding scheme is acceptable, i.e. enables satisfying re- 34 construction of the data contained in the non-received data seg- ments. lf the encoding scheme is acceptable, the procedure loops back to steps 1030 and 1040. Otherwise, a step 1060 fol- lows in which the dimensioning of the BEC encoding scheme is updated, and the updated version is implemented in the main controller 130 of the main transceiver 100. Subsequently, the procedure loops back to steps 1030 and 1040. ln order to sum up, and with reference to the flow diagrams in Figures 11 and 12, we will now describe methods according to the invention that are performed in the transceiver terminal 200 and the main transceiver respectively 100, and which methods may either be implemented in hardware or at least partly in soft- ware. ln the flow diagram of Figure 11, in a first step 1110 data is transmitted from the transceiver terminal 200 via a second satel- lite link L2. ln a subsequent step 1120, segments of a first modulated and amplified signal are received over a first satellite link L1. The first modulated and amplified signal represents first source data D1 and said segments are received during non-transmission in- tervals for the receiver of the transceiver terminal 200.

Then, in a step 1130, the data contained in any non-received segments of the first set of encoded data are reconstructed ba- sed on the received segments and a BEC encoding scheme. Thereafter, the procedure loops back to step 1110. ln the flow diagram of Figure 12, in a first step 1210, first source data D1 are obtained from a first data source in a main trans- mitter 120 of the main transceiver 100.

Then, in a step 1220, a BEC encoding scheme is applied on the first source data D1 to generate a first set of encoded data.

Thereafter, in a step 1230, a first modulated and amplified sig- nal is produced based on the first set of encoded data; and ln a following step 1240, the first modulated and amplified signal is transmitted to the transceiver terminal 200 over the first satellite link L1; and subsequently, the procedure loops back to step 1210.

The process steps described with reference to Figures 10 to 12 may be controlled by means of a programmed processor. More- over, although the embodiments of the invention described above with reference to the drawings comprise processor and processes performed in at least one processor, the invention thus also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the process according to the invention. The program may either be a part of an operating system, or be a separate application. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a Flash memory, a ROM (Read Only Memory), for example a DVD (Digital Video/Versatile Disk), a CD (Compact Disc) or a semi- conductor ROM, an EPROM (Erasable Programmable Read- Only Memory), an EEPROM (Electrically Erasable Program- mable Read-Only Memory), or a magnetic recording medium, for example a floppy disc or hard disc. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or by other means. When the program is embodied in a signal, which may be conveyed, directly by a cable or other device or means, the carrier may be constituted by such cable or device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the re- levant processes. 36 Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed in- vention, from a study of the drawings, the disclosure, and the appended claims.

The term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, inte- gers, steps or components or groups thereof. The indefinite ar- ticle "a" or "an" does not exclude a plurality. ln the claims, the word "or" is not to be interpreted as an exclusive or (sometimes referred to as "XOR"). On the contrary, expressions such as "A or B" covers all the cases "A and not B", "B and not A" and "A and B", unless otherwise indicated. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. lt is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

1. A satellite communication system comprising a main trans- ceiver (100) configured to communicate source data by contem- poraneously transmitting and receiving data over first and se- cond satellite links (L1; L2), characterized in that the communication system further comprises a transceiver terminal (200) configured to receive and transmit data in an al- ternating and temporally non-overlapping manner such that the transceiver terminal (200, 205) exclusively receives data (DWABEC) during non-transmission intervals for the transceiver terminal (200, 205), and the main transceiver (100) is further configured to: apply a burst error correction (BEC) encoding scheme on first source data (D1) to be transmitted to the transceiver termi- nal (200, 205) to generate a first set of encoded data; produce a first modulated and amplified signal (Dm/mac) based on the first set of encoded data; and transmit the first modulated and amplified signal (D1MABEc) to the transceiver terminal (200, 205) over the first satellite link (L1), and the transceiver terminal (200, 205) is further configured to: receive segments (s11, s1s, s1s, sw) of the first modulated and amplified signal (DWABEC) over the first satellite link (L1); and reconstruct the data contained in any non-received seg- ments (mo, m2, m4, me) of the first modulated and amplified signal (DWABEC) to thus derive a copy (“D1”) of the first source data (D1), which non-received segments arrived at the transcei- ver terminal (200, 205) during transmission intervals for the transceiver terminal (200, 205), and the reconstruction is based on the received segments (s11, s1s, s1s, sw) of the first modula- ted and amplified signal (DWABEC) and the burst error correction (BEC) encoding scheme.

2. The satellite communication system according to claim 1, wherein the main transceiver (100) comprises:a main transmitter (120) configured to obtain the first sour- ce data (D1) from a first data source (110), and a main controller (130) configured to control (CTRL1, CTRL2) the main transmitter (120) to apply the burst error cor- rection (BEC) encoding scheme to the first source data (D1) to generate the first set of encoded data.

3. The satellite communication system according to claim 2, wherein the main transmitter (120) is configured to produce the first modulated and amplified signal (D1MABEc) according to a procedure involving interleaving and block-error coding in a link layen

4. The satellite communication system according to claim 3, wherein the main transmitter (120) is configured to generate the first set of encoded data according to a procedure in which the first source data (D1) is organized in blocks in an n-dimensional matrix of data, where n is an integer larger than or equal to two, and each block contains a payload portion of the first source data (D1) and a parity portion that is calculated based on the payload portion along n-1 dimensions of the n-dimensional mat- rix of data.

5. The satellite communication system according to any one of the preceding claims, wherein the transceiver terminal (200, 205) is further configured to: apply the burst error correction (BEC) encoding sche- me to the second source data (D2) and thus generate a second set of encoded data (DZMABEC) comprising first and Second Sets Of segments (S21, S23, S25, S27; S20, S22, S24, S26) of the second set of encoded data (D2MABEc); transmit a second modulated and amplified signal (DZMABEC) over the second satellite link (L2) to the main transceiver (100), which second modulated and amplified signal (DZMABEC) is based only on the first set of segments (s21, s2s, s2s, s27) of the second set of encoded data(DZMABEC), wherein a respective silent period (nzo, nzz, n24, nze) separates each segment in the first set of segments (s21, szs, szs, szv) from one another during which silent pe- riod no signal is transmitted, and the main transceiver (100) is further configured to: receive the first set of segments (s21, szs, szs, szv) over the second satellite link (L2); and reconstruct the data contained in the second set of segments (szo, szz, s24, sze) to thus derive a copy (“D2”) of the of the second source data (D2), wherein the reconst- ruction is based on the received first set of segments (s21, szs, szs, szw) of second set of encoded data (DZMABEC) and the burst error correction (BEC) encoding scheme.

6. The satellite communication system according to claim 5, wherein the transceiver terminal (200, 205) comprises: a transmitter circuitry (220) configured to obtain the se- cond source data (D2) from a second data source (210), and a controller circuitry (230) configured to control (CTRL2) the transmitter circuitry (220) to: apply the burst error correction (BEC) encoding sche- me to the second source data (D2) to generate the second set of encoded data (D2BEc); and transmit the second modulated and amplified signal (DZMABEC) over the second satellite link (L2) to the main transceiver (100), which second modulated and amplified signal (DZMABEC) is based exclusively on the first set of segments (s21, szs, szs, szw) of the second set of encoded data (D2BEc).

7. The satellite communication system according to claim 6, wherein the transmitter circuitry (220) comprises: a link-layer processor (310) configured to encode the se- cond set of encoded data (DZBEC) according to a link layer proto- col, and a modulator and amplifier circuitry (320) configured to re- ceive the second set of encoded data (DZBEC) that has been en- coded according to the link layer protocol, and based thereon, produce the second modulated and amplified signal (DZMA, DZMABEC) in response to a control signal (CTRL2) from the con- troller circuitry (230).

8. The satellite communication system according to any one of the preceding claims, wherein the main transceiver (100) and the transceiver terminal (200, 205) are configured to transmit in a manner whereby transmissions on the first and second satelli- te links (L1; L2) are unsynchronized relative to one another.

9. The satellite communication system according to any one of claims 1 to 4, wherein the transceiver terminal (200, 205) comprises: a transmitter circuitry (220) configured to obtain the se- cond source data (D2) from a second data source (210), and a controller circuitry (230) configured to control (CTRL1) the transmitter circuitry (220) to transmit a modulated and ampli- fied signal (D2MA) based on the second source data (D2), which modulated and amplified signal (D2MA) is transmitted according to a time table specifying a respective transmission interval for the transceiver terminal (200, 205).

10. The satellite communication system according to claim 9, wherein the transceiver terminal (200, 205) comprises: a link-layer processing unit (311) configured to encode the second source data (D2) according to a link layer protocol in response to a control signal (CTRL1) from the controller circuitry (230),and a modulator and amplifier circuitry (320) configured to re- ceive the second source data (D2) that has been encoded according to the link layer protocol and based thereon produce the second modulated and amplified signal (DZMA) in response to the control signal (CTRL1) from the controller circuitry (230).

11. The satellite communication system according to any one of the preceding claims, wherein the transceiver terminal (200, 205) comprises a receiver circuitry (240, 250) configured to re- ceive the segments (s11, s1s, s1s, sw) of the first modulated and amplified signal (D1MABEC), and based thereon, derive the copy (“D1”) of the first source data (D1).

12. The satellite communication system according to any one of the preceding claims, wherein the main transceiver (100) is configured to transmit the first satellite link (L1) on a time-divi- sion multiplexed format.

13. The satellite communication system according to claim 12, wherein the time-division multiplexed access format further in- volves frequency hopping.

14. The satellite communication system according to any one of the preceding claims, wherein the main transceiver (100) is comprised in a satellite (410), and the main transceiver (100) is configured to facilitate direct communication between at least two ground based transceiver terminals (200; 205).

15. A transceiver terminal (200, 205) for communicating sour- ce data in the system of claim 1 by transmitting and receiving data over first and second satellite links (L1; L2), the transceiver terminal comprising transmitter and receiver circuitry (220; 240, 250) configured to transmit and receive data respectively in an alternating and temporally non-overlapping manner such that the transmitter circuitry (220) exclusively receives data (D2iviA, DZMABEC) during non-transmission intervals for the receiver cir- cuitry (240, 250), the receiver circuitry (240, 250) being further configured to: receive segments (s11, s1s, s1s, sw) of a first modulated and amplified signal (D1MABEc) over the first satellite link (L1), which first modulated and amplified signal (D1MABEc) represents first source data (D1) to which a burst error correction (BEC) en-coding scheme has been applied to generate a first set of enco- ded data based upon which the first modulated and amplified signal (D1MABEc) has been produced; and reconstruct the data contained in any non-received seg- ments(n1o, n12, n14, me) of the first set of encoded data (D1BEc) to thus derive a copy (“D1”) of first source data (D1), which non- received segments arrived at the transceiver terminal during transmission intervals for the transceiver terminal, and the re- construction is based on the received segments (s11, s1s, s1s, sw) of the first modulated and amplified signal (D1MAsEc) and the burst error correction (BEC) encoding scheme.

16. The transceiver terminal (200, 205) according to claim 15, comprising: antenna and filters of a design adapted for small-sized ter- minals with a relatively low degree of isolation between a trans- mit chain and a receive chain resulting in high levels of noise and spurious in a receiver circuitry of the transceiver terminal (200, 205) due to the transmitted signal, rendering the receiver circuitry unable to receive and decode the non-received seg- ments(n1o, n12, n14, me) of the first set of encoded data (D1BEc).

17. The transceiver terminal (200, 205) according to any one of the claims 15 or 16, wherein the receiver circuitry (240, 250) is configured to reconstruct the data contained in said non- received segments (n1o, n12, n14, me) of the first set of encoded data (D1BEC) assuming that the first modulated and amplified signal (DWABEC) has been produced according to a procedure involving interleaving and block-error coding in a link layer.

18. The transceiver terminal (200, 205) according to claim 17, wherein the receiver circuitry (240, 250) is configured to recon- struct the data contained in said non-received segments (n1o, n12, n14, me) of the first set of encoded data (D1BEc) further assuming that the first set of encoded data has been generated according to a procedure in which the first source data (D1) isorganized in blocks in an n-dimensional matrix of data, where n is an integer larger than or equal to two, and each block contains a payload portion of the first source data (D1) and a parity portion that is calculated based on the payload portion along n-1 dimensions of the n-dimensional matrix of data.

19. The transceiver terminal (200, 205) according to any one of the claims 15 to 18, wherein the transmitter circuitry (220) is configured to obtain the second source data (D2) from a second data source (210), and the transceiver terminal comprises a controller circuitry (230) configured to control the transmitter circuitry (220) to apply the burst error correction (BEC) encoding scheme to the second source data (D2) to generate the second set of encoded data (D2BEc); and transmit the second modulated and amplified signal (D2MABEC) over the second satellite link (L2) to the main trans- ceiver (100), which second modulated and amplified signal (DZMABEC) is based exclusively on the first set of segments (s21, szs, szs, szv) of the second set of encoded data (D2BEc).

20. The transceiver terminal (200, 205) according to claim 19, wherein the transmitter circuitry (220) comprises: a link-layer processor (310) configured to encode the se- cond set of encoded data (D2BEC) according to a link layer proto- col, and a modulator and amplifier circuitry (320) configured to re- ceive the second set of encoded data (D2BEC) that has been en- coded according to the link layer protocol and based thereon produce the second modulated and amplified signal (D2MA, D2MABEC) in response to a control signal (CTRL2) from the con- troller circuitry (230).

21. The transceiver terminal (200, 205) according to any one of the claims 15 to 18, wherein the transceiver terminal compri- ses:a transmitter circuitry (220) configured to obtain the se- cond source data (D2) from a second data source (210), and a controller circuitry (230) configured to control (CTRL2) the transmitter circuitry (220) to transmit a modulated and amplified signal (DZMA) based on the second source data (D2), which modulated and amplified signal (DZMA) is transmitted ac- cording to a time table specifying a respective transmission in- terval for the transceiver terminal.

22. A main transceiver (100) for communicating source data in the system of claim 1 by contemporaneously transmitting and re- ceiving data over first and second satellite links (L1; L2), characterized in that the main transceiver comprises: a main transmitter (120) configured to obtain first source data (D1) from a first data source (110), and a main controller (130) configured to control the main transmitter (120) to: apply a burst error correction (BEC) encoding sche- me on the first source data (D1) to generate a first set of encoded data; produce a first modulated and amplified signal (Dm/mac) based on the first set of encoded data; and transmit the first modulated and amplified signal (D1MABEc) to a transceiver terminal (200, 205) over the first satellite link (L1).

23. The main transceiver (100) according to claim 22, wherein the main transmitter (120) is configured to produce the first mo- dulated and amplified signal (DWABEC) according to a procedure involving interleaving and block-error coding in a link layer.

24. The main transceiver (100) according to claim 23, wherein the main transmitter (120) is configured to generate the first set of encoded data according to a procedure in which the first source data (D1) is organized in blocks in an n-dimensional matrix of data, where n is an integer larger than or equal to two, and each block contains a payload portion of the first source data (D1) and a parity portion that is calculated based on the payload portion along n-1 first dimensions of the n-dimensional matrix of data.

25. The main transceiver (100) according to any one of the claims 22 to 24, comprising: a main receiver (140) configured to receive segments (S21, szs, szs, szv) of a modulated and amplified signal (DZMABEC) over a second satellite link (L2), which received segments (s21, szs, szs, szv) represent second source data (D2), and a demodulator/decoder (150) configured to derive a copy (“D2”) of the source data (D2) based on the received segments (s21, szs, szs, szv) and the burst error correction (BEC) encoding scheme.

26. A hardware or at least partly computer-implemented me- thod performed in the transceiver terminal (200, 205) according to claim 15 for communicating source data by transmitting and receiving data over first and second satellite links (L1; L2), characterized by: transmitting and receiving data in an alternating and temporally non-overlapping manner such that the transceiver terminal exclusively receives data (D1MA, D1MAsEc) during non- transmission intervals for the transceiver terminal, receiving segments (s11, s1s, s1s, sw) of a first modulated and amplified signal (D1MABEc) over the first satellite link (L1), which first modulated and amplified signal (D1MABEc) represents first source data (D1) to which a burst error correction (BEC) en- coding scheme has been applied; and reconstructing the data contained in any non-received seg- ments (nio, n12, n14, me) of the first set of encoded data (D1BEc) to thus derive a copy (“D1”) of first source data (D1), which non- received segments arrived at the transceiver terminal during transmission intervals for the transceiver terminal, and the reconstruction is based on the received segments (s11, s1s, s1s,sw) of the first modulated and amplified signal (D1MAsEc) and the burst error correction (BEC) encoding scheme.

27. The method according to claim 26, further comprising: obtaining second source data (D2) from a second data source (210), controlling (CTRL1) a transmitter circuitry (220) in the transceiver terminal to apply the burst error correction encoding scheme (BEC) to the second source data (D2) to generate the second set of encoded data (D2BEc); and controlling (CTRL1) the transmitter circuitry (220) to trans- mit the second modulated and amplified signal (DZMABEC) over the second satellite link (L2) to the main transceiver (100), which second modulated and amplified signal (DZMABEC) is based exclusively on the first set of segments (s21, szs, szs, szv) of the second set of encoded data (D2BEc).

28. The method according to claim 27, further comprising: encoding the second set of encoded data (D2BEc) accor- ding to a link layer protocol, and modulating and amplifying the second set of encoded data (D2BEC) that has been encoded according to the link layer proto- col to produce the second modulated and amplified signal (DZMABEC) in response to a control signal (CTRL1) from the con- troller circuitry (230).

29. The method according to claim 26, further comprising: obtaining second source data (D2) from a second data source (210), and controlling (CTRL1) a transmitter circuitry (220) a trans- mitter circuitry (220) in the transceiver terminal to transmit a modulated and amplified signal (D2MA) based on the second source data (D2), which modulated and amplified signal (D2MA) is transmitted according to a time table specifying a respective transmission interval for the transceiver terminal (200).

30. A computer program (138) loadable into a non-volatile data carrier (137) communicatively connected to a processing unit (135), the computer program (138) comprising software for exe- cuting the method according to any of the claims 26 to 29 when the computer program (138) is run on the processing unit (135).

31. A non-volatile data carrier (137) containing the computer program (138) of the c|aim

32. A hardware or at least partly computer-implemented me- thod performed in the main transceiver (100) according to c|aim 22 for communicating source data by contemporaneously trans- mitting and receiving data over first and second satellite links (L1; L2), the method comprising: obtaining first source data (D1) from a first data source (110) in a main transmitter (120) of the main transceiver (100), characterized by controlling the main transmitter (120) to: apply a burst error correction (BEC) encoding sche- me on the first source data (D1) to generate a first set of encoded data; produce a first modulated and amplified signal (Dm/mac) based on the first set of encoded data; and transmit the first modulated and amplified signal (D1MABEc) to a transceiver terminal (200, 205) over the first satellite link (L1).

33. A computer program (238) loadable into a non-volatile data carrier (237) communicatively connected to a processing unit (235), the computer program (238) comprising software for exe- cuting the method according to c|aim 26 when the computer pro- gram (238) is run on the processing unit (235).

34. A non-volatile data carrier (237) containing the computer program (238) of the c|aim 33.