EP0000039B1 - Procédé et système de transmission de données pour la diminution de l'interférence entre deux signaux numériques recus - Google Patents

Procédé et système de transmission de données pour la diminution de l'interférence entre deux signaux numériques recus Download PDF

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Publication number
EP0000039B1
EP0000039B1 EP78100063A EP78100063A EP0000039B1 EP 0000039 B1 EP0000039 B1 EP 0000039B1 EP 78100063 A EP78100063 A EP 78100063A EP 78100063 A EP78100063 A EP 78100063A EP 0000039 B1 EP0000039 B1 EP 0000039B1
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Prior art keywords
digital information
information signal
signal
received
uncoded
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Expired
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EP78100063A
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German (de)
English (en)
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EP0000039A1 (fr
Inventor
Anthony Acampora
Douglas Otto John Reudink
Yu Shuan Yeh
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AT&T Corp
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Western Electric Co Inc
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Publication of EP0000039A1 publication Critical patent/EP0000039A1/fr
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
    • H04L1/0059—Convolutional codes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q25/00—Antennas or antenna systems providing at least two radiating patterns
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/204—Multiple access
    • H04B7/2041—Spot beam multiple access

Definitions

  • the present invention relates to a data transmission process and system for effecting separation and substantial reduction of interference at a receiver between a first and a second received digital signal which use the same frequency spectrum and are received from the same general direction.
  • spot and area coverage beams can be desirable.
  • a separate spot coverage beam can be used for communication between the saltellite and each high traffic ground station while an area coverage beam can be used for communication between the satellite and a plurality of low traffic ground stations under conditions where it might not be desirable to interconnect the individual low traffic ground stations to a nearest high traffic ground station for access to the satellite system.
  • typical prior art techniques would be to use two satellites a few degrees apart in orbit, or, multiple spot beams at thd satellite on both up and down paths, or, separate bandwidths or polarizations, is possible, for the spot coverage beams and the area coverage beam.
  • An alternative technique to enable reception of only one signal of a plurality of signals concurrently received from a plurality of transmitters at an FM receiver would be to modulate the carrier of each transmitter with a separate frequency to provide a unique address that is assigned to an associated receiver as disclosed, for example, in U.S. Reissue Patent Re. 27,478. Such arrangement may be applicable to FM communication systems but does not appear applicable to a digital communication system.
  • the problem remaining in the prior art is to provide an improved technique which permits two digital signals using the same frequency spectrum and general transmission direction to be simultaneously transmitted on one radio channel or overlapping spot and area coverage beams with the ability for the signals to be separated at a receiving station intercepting both signals.
  • a communication system for carrying out the process is characterized in that the receiver is disposed in the path of a first uncoded digital information signal modulated to a predetermined frequency spectrum and a second digital information signal having a different informational content and a lower capacity than said first digital information, said second signal being encoded with a forward error correcting code and modulated to the predetermined frequency spectrum, said receiver comprising a detector capable of generating from the combined received signal of the interfering uncoded first digital information signal and coded second digital information signal the most likely digits representative of the desired first digital information signal and decoded second digital information signal.
  • uncoded digital information signal means that a stream of bits from a digital source is transmitted on a channel using some suitable modulation without being further processed.
  • the present invention has been and will be described primarily in relationship to a satellite communication system to enable the concurrent use of an area coverage satellite radiated beam and a plurality of spot coverage satellite radiated beams where all of the beams use the same frequency spectrum and the spot coverage beams are received within the area encompassed by the area coverage beam.
  • a satellite communication system to enable the concurrent use of an area coverage satellite radiated beam and a plurality of spot coverage satellite radiated beams where all of the beams use the same frequency spectrum and the spot coverage beams are received within the area encompassed by the area coverage beam.
  • inventive concept described can be equally applicable to other radiated wave transmission systems which comprise two or more beams which have different destinations but interfere with each other at one or more of the destinations.
  • the present invention can be used to increase the capacity of a radio channel by 50 percent by simultaneously transmitting an uncoded first digital signal and a coded second digital signal with reduced capacity on each radio channel according to the concept to be described hereinafter for the individual area and spot coverage beams.
  • a satellite communication system wherein the present invention is especially useful to permit the concurrent transmission from a satellite 10 of both an area coverage beam 12 and a plurality of spot coverage beams of which, for example, three beams 14a, 14b and 14c are shown with all beams being able to use the same frqeuency spectrum.
  • Spot coverage beams 14a, 14b and 14c are shown radiating from antennae 15a, 15b, and 15c, respectively, and directed at respective ground areas 16a, 16b and 16c which include, for example, high traffic ground stations 17a, 17b and 17c, respectively.
  • Area coverage beam 12 is shown radiating from an antenna 13 and directed at a ground area 18 which includes both the ground areas 16a, 16b and 16c and a plurality of low traffic ground stations of which, for example, four stations 19a-19d are shown.
  • each of the high traffic ground stations 17a-17c communicates with satellite 10 via a separate spot beam 14a-14c, respectively, while the low traffic ground stations 19a-19d communicate with satellite 10 via area coverage beam 12 using any suitable technique to assure that a particular message will be processed by only the appropriate one of stations 19a-19d.
  • Such arrangement permits low traffic ground stations 19a-19d to communicate with satellite 10 under conditions where it is not advantageous to connect a low traffic ground station 19 to a nearby one of high traffic ground stations 17a-17c.
  • each of ground stations 17a-17c will receive both the associated one of spot coverage beams 14a-14c and area coverage beam 12 since these beams emmanate from approximately the same point.
  • prior art arrangements such as, for example, side lobe suppression arrangements to select a wave received from a particular direction over waves received from other directions is not feasible.
  • the concurrent transmission of area coverage beam 12 and a plurality of spot coverage beams 14a-14c which use the same frequency spectrum without interference can be effected in accordance with the present invention by the typical arrangement shown in Fig. 2.
  • a separate source of data 20a-20c generates a digital signals destined to be transmitted via spot coverage beams 14a-14c, respectively.
  • the digital data signals generated by each of data sources 20a-20c are modulated to the desired frequency spectrum for transmission in separate modulators 21 a-21 c, respectively.
  • the outputs from modulators 21 a-21 c are amplified in power amplifiers 22a-22c, respectively, prior to being applied to the respective antennae 15a-15c for transmission via spot coverage beams 14a-14c, respectively.
  • the digital signals to be transmitted via area coverage beam 12 are similarly generated by a data source 20d but at a reduced data rate which is, for example, approximately one-half the rate of sources 20a-20c.
  • These latter signals are, however, first encoded in channel encoder 23 using a forward error correcting code such as, for example, a block or convolutional code prior to being sequentially modulated in modulator 21 d, amplified by power amplifier 22d and transmitted by antenna 13 in area coverage beam 12.
  • a forward error correcting code such as, for example, a block or convolutional code prior to being sequentially modulated in modulator 21 d, amplified by power amplifier 22d and transmitted by antenna 13 in area coverage beam 12.
  • a forward error correcting code such as, for example, a block or convolutional code prior to being sequentially modulated in modulator 21 d
  • power amplifier 22d amplified by power amplifier 22d and transmitted by antenna 13 in area coverage beam 12.
  • data sources 20a-20d, modulators 21 a-21 and power amplifiers 22a-22d can comprise any suitable means capable of providing the function described hereinabove.
  • channel encoder 23 can comprise any suitable means for encoding the digital data signals supplied by data source 20d into a forward error correcting code.
  • the code used is a convolutional code
  • an encoder of any desired constraint length and code rate may be used.
  • digital data signals for the area coverage beam are generated in data source 20d at the rate of one bit every T seconds for transmission over line 24 to encoder 23.
  • the received data signals are shifted into a three-bit shift register 25, or any other suitable means, at the rate of one bit every T seconds.
  • a first modulo-2 adder 26 operates on the information stored in all three bits in register 25 to produce a resultant first binary bit on transmission line 27 while, simultaneously, a second modulo-2 adder 28 operates on the first and third bits in register 25 to produce a resultant second binary bit on transmission line 29.
  • a commutator 30 first selects the signal on first transmission line 27 and then the signal on second transmission line 29 and thereby transmits two binary digits over line 31 to modulator 21 d for each data bit shifted into register 25. Therefore, for each data bit from data source 20d the encoder 23 generates two data bits at its output and the encoded data is then modulated and amplified prior to transmission via area coverage beam 12 to the ground stations disposed within ground area 18.
  • data sources 20a-20c generates two bits of data every T seconds for transmission via spot coverage beams 14a-14c, respectively, while data source 20d, generating one data bit every T seconds, in combination with encoder 23, which provides redundancy and generates two data bits for every data bit from source 20d, also generates two bits of data every T seconds for transmission via area coverage beam 12. Therefore, the data rate of all antenna radiated beams is the same with area coverage beam 12 having a lower capacity than each of spot coverage beams 14a-14c.
  • Separation of interfering uncoded spot beam and coded area beam signals is achieved in accordance with the present invention by providing a suitable detector at each of spot beam ground stations 17a-17c and area beam ground stations 1 9a-1 9d.
  • the receiver performs a suitable detection of the spot beam signal received by that ground station plus the received area beam signal, as will be described hereinafter. After the signals are separated the information content of the interfering area beam is discarded.
  • a suitable detection of the desired area beam signal plus the unwanted interfering spot beam signal is again performed and the information content of the interfering spot beam signal is discarded after separation.
  • a suitable detection process can comprise any process which will enable the separation of the two digital signals and the decoding of the forward error correcting coded signal. For example, where the two digital signals have different signal strengths at the receiver, separation may be achieved using a threshold detecting process. Alternatively, where the two received digital signals have approximately the same signal strength, a maximum-likelihood detection process may be performed. The type of detection process employed, however, will depend primarily on the amount of signal degradation which can be tolerated since each of the known detection processes would produce a certain amount of degradation in separating and decoding the two signals described hereinbefore.
  • the preferred method of separating and decoding a simultaneously received uncoded first digital signal and a forward error correcting encoded second digital signal with minimal degradation is accomplished using the technique of joint maximum-likelihood detection as will be described hereinafter.
  • Convolutional decoders and maximum-likelihood detection systems are well known in the art.
  • data is not decoded as soon as it is received from the channel. Instead, a sequence of data, having a -predetermined decoding depth, following the digit to be decoded is first collected. Then, by computing what are known as path metrics, a limited number of possible messages are selected, each extending throughout the decoding depth far beyond the digit presently to be decoded, with one such survivor sequence ending in each of the data states.
  • a correlation between each survivor sequence and the data actually received is computed for the entire decoding depth under consideration.
  • the highest correlated of the survivor sequences is then selected to be the sole survivor sequence.
  • the earliest received digit or digits within the decoding depth is then permanently decoded under the temporary assumption that the sole survivor sequence is the correct sequence.
  • decoding is accomplished by forming the log-likelihood function which hereinafter will be referred to as the path metric. Two samples are taken every T seconds and the path metric is formed for each possible source sequence, and that sequence for which the metric is largest is selected as the best estimate to the true transmitted sequence.
  • metric calculations are an application of dynamic programming techniques and that maximum-likelihood decoding can be performed without actually finding the path metric for each sequence.
  • the procedure for decoding the convolutionally encoded area beam signal is illustrated by the State diagram of Fig. 4.
  • the State is defined as the contents of the first two stages of shift register 25, which changes at a T-second rate.
  • this State can be reached from either of States 00 or 01, both transitions corresponding to a data bit 0 having entered the coder.
  • the input signal to the maximum-likelihood detector comprises two coded area beam channel symbols and two uncoded spot beam channel symbols every T seconds which interfere with each other. Therefore, in the State diagram of Fig. 4, four most- likely paths actually exist for each of the single paths shown for the transitions between States. More particularly, as shown for the transition from State 00 to State 00, the first two symbols for each of the four paths denote the source coding for this particular transition, which is common to each of the possible paths, while the last two symbols denote the four possible data symbols which may exist for the first and second spot beam symbols received during each T seconds. It is to be understood that each of the other transitions between States similarly comprises four possible paths with corresponding symbols to denote the possible received symbols.
  • the arrangement of Fig. 5 is exemplary only and is for purposes of exposition and not for purposes of limitation.
  • inventive concepts described are equally applicable to decode an interfering uncoded first beam and convolutionally coded second beam having different constraint lengths and rates or nonbinary or multilevel alphabets, transmissions, and the like, after the appropriate modification is made as will be easily determined by one skilled in the art once the constraint length and rate is known.
  • the present joint maximum-likelihood detector is segmented into four States, each State corresponding to a different one of the possible combinations of one's and zero's in the first two stages of register 25 in encoder 23.
  • a separate sample of the received waveform at each ground station is taken every T/2 seconds, and every T seconds the two samples which may be in digital or analog form, are made available at input 40 of the present detector, each sample comprising elements of the interfering area beam and spot beam signals.
  • the detector recursively computes in processors 41 a-41 d the path metric of the most likely path, of the eight paths, leading to each State. This computation is in the form: where
  • the detector computes the eight path metrics, finds the largest one of the eight path metrics, saves the largest path metric, and stores the path corresponding to the largest metric. This process will now be described in greater detail for processing the path metrics for State 00, and it is understood that a corresponding process is concurrently performed for processing the path metrics for the other States 01, 10 and 11.
  • the largest path metric for States 00, 01, 10 and 11 computed in the previous T second cycle is stored in storage devices 42a-42d, respectively, and have the respective designations M l -M 4 .
  • the outputs from storage devices 42a-42d are normalized in normalization means 43 by, for example, arbitrarily setting one of the four old metrics, M l -M 4 , equal to zero after first having effectively subtracted its value from the remaining three metrics.
  • These normalized old path metrics are designated M n1 -M n4 . This step prevents the successive path metrics from growing linearly with time.
  • the old path metric M n2 associated with State 01 is used together with the appropriate value of the two samples available at input 40 in correlator 44b to compute the path metrics for each of the four possible paths between State 01 at time to and State 00 at to,e t o +T. These four path metrics are indicated by the symbols M" 11 -M" 14 at the output of correlator 44b.
  • the eight path metrics computed in correlators 44a and 44b are compared in comparator 45 and the largest one of the eight metrics is determined.
  • the comparator 45 is strobed by a system clock 46 via a signal on lead 47 to provide the result of comparison at the appropriate sampling instance once every T seconds.
  • the value of the largest path metric for State 00 is transmitted from comparator 45 to storage means 42a via lead 48 where it is stored for use during the next processing cycle T.
  • the one of eight paths leading into a State having the largest value also indicates the most likely digital value for both the decoded area beam signal and the two sequential uncoded spot beam signals generated during a prescribed T second period by the associated data sources 20 at the satellite 10. For example, if comparator 45 determined that the largest path metric corresponded to the uppermost path between State 00 and State 00 in Fig.
  • the most likely value for the bit generated by data source 20d at satellite 10 during the corresponding T second interval would be a zero while the most likely value for the first and second sequential bits received via the interfering spot beam 14 during that same T second interval would be a 0, 0, respectively.
  • Fig. 5 the decoded binary value of the most likely bit received via interfering area beam 12 for State 00 is shown as being stored in a shift register 50a or other suitable means, while the binary values for the most likely first and second sequential bits for State 00 received via interfering spot beam 14 are stored in shift registers 51 a and 52a, respectively, or any other suitable means.
  • the outputs from comparators 45 in processors 41 b-41 similarly load registers 50b-50d, 51 b-51 d and 52b-52d for the most likely binary value for each decoded area beam bit and the first and second sequentially received spot beam bits, respectively, for the respective States 01, 10 and 11.
  • Each of shift registers 50a-50d, 51 a-51 and 52a-52d have a path memory length which preferably is about 4-5 equivalent constraint lengths, implying that, with high probability, all surviving paths have a common prefix.
  • the final state of any one of registers 50a-50d may be selected as the decoded most likely information digits for the received interfering area beam signal.
  • the final stage of any one of registers 51 a-51 d and 52a-52d may be selected as the most likely information digits for the first and second sequential digits, respectively, received via the interfering spot beam signal.
  • each group of registers 50a-50d, 51 a-51 d and registers 52a-52d can be used as a separate input to a separate well-known majority logic gate associated with a particular group of registers which functions to choose the output value indicated by the majority of the final stages of the associated group, and in the event of a tie to output a 0 or a 1.
  • a second alternative would be to select the final stage of the register within each group of registers indicating maximum likelihood.
  • the ground station performing the described joint maximum-likelihood detection process is, for example, a spot beam ground receiving station
  • the ground station performing the described joint maximum-likelihood detection process is, for example, a spot beam ground receiving station
  • registers 50a-50d for storing the decoded most likely area beam digits can be eliminated.
  • registers 50a-50d need be supplied.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Error Detection And Correction (AREA)
  • Radio Relay Systems (AREA)

Claims (6)

1. Un processus de transmission de données permettant la séparation et une réduction importante du brouillage, à un récepteur, entre des premier et second signaux d'information numériques qui utilisent le même spectre de fréquence et qui sont reçus par le récepteur en arrivant approximativement de la même direction, caracterise en ce que
- à un émetteur éloigné:
on émet le premier signal d'information numérique sous une forme non codée dans une bande de fréquence prédéterminée, dans une première direction, tandis que simultanément
on émet le second signal d'information numérique, contenant une information différente, sous forme codée et avec une capacité de transmission d'information inférieure à celle du premier signal d'information, dans la bande de fréquence prédéterminée et dans la première direction, le code utilisé consistant en un code correcteur d'erreur direct, et
- au récepteur:
on reçoit le premier signal d'information numérique non codé et le second signal d'information numérique codé, et
on accomplit un processus de détection sur le premier signal d'information numérique non codé et sur le second signal d'information numérique codé pour générer les symboles les plus vraisemblables représentatifs du premier signal d'information numérique reçu et du secofd signal d'information numérique décodé.
2. Le processus selon la revendication 1 caracterise par son utilisation dans un système de télécommunication dans lequel le premier signal d'information numérique se trouve dans un faisceau rayonné qui couvre une première zone de réception et le second signal d'information numérique se trouve dans un second faisceau rayonné qui couvre une seconde zone de réception qui est en recouvrement au moins partiel avec la première zone de réception.
3. Le processus selon la revendication 1 caracterise en ce que les premier signal d'information numérique et le second signal d'information numérique sont situés dans la même voie radioélectrique reçue par le récepteur.
4. Le processus selon la revendication 1 caracterise en ce que le code correcteur d'erreur direct est un code convolutionnel et, pour accomplir le processus de détection,
(1) on calcule des fonctions de vraisemblance pour toutes les transitions possibles vers chaque état du code convolutionnel en utilisant des premier et second échantillons du signal reçu, qui comprennent des éléments des premier et second signaux d'information numériques qui se brouillent, et la fonction de vraisemblance la plus probable calculée la plus récemment, pour chaque état possible du code convolutionnel, afin de générer des signaux représentatifs de la fonction de vraisemblance calculée,
(2) on compare les signaux représentatifs des fonctions de vraisemblance calculées qui sont associées à chacun des états, tels qu'ils ont été générés à l'étape (1), pour déterminer la transition la plus probable vers chaque état, et
(3) sous l'effet du signal généré à l'étape (2), on mémorise simultanément (a) la valeur de ce signal pour une utilisation ultérieure dans la réitération de l'étape d'accomplissement du processus de détection et (b) la séquence de données la plus vraisemblable dans chaque état, .sur une longueur prédéterminée, pour n'importe quelle combinaison désirée du premier signal d'information numérique reçu brouilleur et du second signal d'information numérique reçu décodé, pour produire un train de sortie de données numériques correspondant à l'estimation la plus vraisemblable du signal numérique désiré.
5. Un système de télécommunication destiné à mettre en oeuvre le processus de la revendication 1 caracterise en ce que le récepteur est disposé dans le chemin d'un premier signal d'information numérique non codé (14a) qui est modulé dans un spectre de fréquence prédéterminé et d'un second signal d'information numérique (12) qui contient une information différente et qui a une capacité Inférieure à celle de la première information numérique, ce second signal étant codé avec un code correcteur d'erreur direct et modulé dans le spectre de fréquence prédéterminé, ce récepteur comprenant un détecteur (figure 5) capable de générer à partir du signal reçu combiné formé par le premier signal d'information numérique non codé brouilleur et par le second signal d'information numerique codé, les symboles les plus vraisemblables représentatifs du premier signal d'information numérique et du second signal d'information numérique décodé désirés.
6. Le système selon la revendication 5 caracterise en ce que le code correcteur d'erreur direct est un code convolutionnel ayant un ensemble prédéterminé d'états possibles et un taux d'émission arbitraire; et le détecteur est un détecteur à vraisemblance maximale conjointe comprenant
des premiers moyens (44a, 44b) capables de calculer des fonctions de vraisemblance pour toutes les transitions possibles vers chaque état de l'ensemble d'états possibles du code convolutionnel, encombinaison avec toutes les valeurs possibles du permier signal d'information numérique non codé brouilleur, ces premiers moyens utilisant à la fois des valeurs sélectives d'un premier et d'un second échantillons séquentiels (40) du signal reçu, qui comprennent des éléments du premier signal d'information numérique non codé et du second signal d'information numérique codé, et des signaux (MI,-MI4) représentatifs des états les plus probables calculés les plus récemment du code convolutionnel et de la valeur possible la plus vraisemblable associée pour le premier signal d'information numérique reçu, afin de calculer les fonctions de vraisemblance;
des seconds moyens (45) connectés à la sortie des premiers moyens pour comparer les signaux représentatifs des fonctions de vraisemblance associées à chaque état du code convolutionnel et les valeurs possibles associées pour le premier signal d'information numérique brouilleur non codé, et pour générer un signal de sortie indiquant la transition la plus probable vers chaque état parmi l'ensemble d'états possibles, et la valeur la plus probable pour le premier signal d'information numérique;
des troisièmes moyens (42a-42d) branchés entre les seconds et premiers moyens et capables de mémoriser temporairement le signal de tension calculé le plus récemment généré par les premiers moyens et représentatif de la fonction de vraisemblance la plus probable pour chaque état et la valeur la plus probable associée du premier signal d'information numérique, pour une utilisation ultérieure par les premiers moyens pour calculer les fonctions de vraisemblance pour chaque transition possible pendant l'intervalle de temps suivant; et
des quatrièmes moyens (50a-50d; 51 a-51 d; 52a-52d) qui réagissent au signal de sortie des seconds moyens de façon à mémoriser n'importe quelle combinaison de la séquence de données la plus vraisemblable vers chaque état du code convolutionnel (50a-50d) et la séquence de données la plus vraisemblable pour chaque état pour le premier signal d'information numérique, sur une longueur prédéterminée et de façon à produire sur une borne de sortie de ces quatrièmes moyens un train de données correspondant aux données pour le premier signal d'information numérique non codé (51 a-51 d) et le second signal d'information numérique décodé (52a-52d) désirés.
EP78100063A 1977-06-03 1978-06-01 Procédé et système de transmission de données pour la diminution de l'interférence entre deux signaux numériques recus Expired EP0000039B1 (fr)

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US05/803,152 US4178550A (en) 1977-06-03 1977-06-03 Method and apparatus to permit substantial cancellation of interference between a received first and second signal
US803152 1977-06-03

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EP0000039A1 EP0000039A1 (fr) 1978-12-20
EP0000039B1 true EP0000039B1 (fr) 1982-07-07

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US (1) US4178550A (fr)
EP (1) EP0000039B1 (fr)
JP (1) JPS542614A (fr)
AU (1) AU520073B2 (fr)
CA (1) CA1102880A (fr)
DE (1) DE2861935D1 (fr)

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Also Published As

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AU520073B2 (en) 1982-01-14
AU3669478A (en) 1979-12-06
DE2861935D1 (en) 1982-08-26
CA1102880A (fr) 1981-06-09
US4178550A (en) 1979-12-11
EP0000039A1 (fr) 1978-12-20
JPS542614A (en) 1979-01-10

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