~ CROSS REFERENCES TO RELATED PA~ENTS
26 . U. S. Patent 4,009,344 granted February 22, 1977 to 27 D. C. Flemming, which is assigned to the assignee of the subject 28 invention, des~ribes a system employing TD~A and demand assign-29 ment operations which are considered relevant to the present invention.
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~ MA9-77-001 . ~1-i~
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1 BACKGROUND OF T~E INVENTION
2 1. FIELD OF THE INVENTION
3 This invention relates to time division multiple access
4 (TDM~) communication systems; and particularly to TDMA systems -in which multiple radio stations communicate through an earth 6 satellite repeater by transmitting time-synchronized bursts 7 of radio energy rela~ive to said repeater and receiving a time 8 multiplex composite of bursks containing corresponding modula-9 tion information from said repeater.
2. PRIOR ART
11 Systems of radio communication using techniques of TDMA
12 operation and TDM~/DA operation (DA referring to demand assign-13 ment) are well known.
14 In TDMA operation multiple transceiver stations associated with radio signaling nodes txansmit bursts of time concentrated 16 information signals on a shared carrier frequency spectrum and 17 receive the same information signals after repetition by the 18 satellite on a shifted carrier frequency spectrum. Each 19 station is assigned particular time slots in a continuum o~
recurrent frames for transmission of its bursts and for reception 21 of its own bur5ts and bursts of other stations. The bursts 22 interleave at the satellite in close time formation without 23 overlapping.
24 In DA operation lengths of assigned slots may be varied in accordance with the relative distribution of demand at the 26 signaling nodes.
27 Various systems have been proposed for enabling stations 28 operating in TDMA mode relative to dif~erent transponder 29 frequency spectra to intercommunicate. Such proposed systems have been rejected for various reasons. Systems based upon ;
~8~ZO
1 time domain switching relative to transmission frequency spectra 2 have been rejected as overly expensive and inefficient beca~se 3 of the magnitudes of ~ransmission power which must he handled.
4 Systems based upon simultaneous ~ransmission on plural frequency bands at each node have been rejected as inefficient and overly 6 complex.
7 The present invention concerns a system for providing 8 intertransponder communication in TDMA mode which is efficient, 9 inexpensive to implement relative to conventional unitransponder systems r and relatively simple to construct and operate.
12 A principal object of this invention is to provide a 13 system for TDMA communication between radio transponders (across 14 time-divided transponder frequency bands) in which transmission frequency bands do not have to be switched or under-utilized.
16 A related general objective is to provide a system for 17 intertransponder communication between groups of stations 18 operating in TDMA mode relative to separated carrier frequency 19 bands (transponders) which is efficient, practical, economical and simple.
21 These and other objectives and advantages of this inven- ;
22 tion are achieved presently by partitioning the TDMA frame 23 repetitively into IN GROUP (intratransponder) and CROSS GROUP
24 (intertransponder) periods relative to plural groups of stations.
Stations in any group which are adapted for operation in GROS5 26 GROUP (intertransponder) mode are operative to switch reception 27 frequencies at time points o~ transition between IN GROUP and 28 CROSS GROUP periods.
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1 Stations in each group transmit on a single carrier radio 2 frequency band exclusively allocated to the group. Station 3 receiving equipment adapted for operation in CROSS GROUP mode 4 switches local oscilla~ion frequencies at predetermined time points of transition between IN GROUP and CROSS GROUP periods.
6 This enables adapted stations to receive information signals 7 from stations in both groups. The crossover time TI/X from 8 IN GROUP mode to CROSS GROUP mode is susceptible of being 9 varied to balance overall utilization of the satellite repeater by all stations.
11 Stations in each group synchronize their transmission and 12 reception burst apertures to time bases derived from a common 13 frame timing reference. This reference is communicated from a 14 primary reference station at one radio transmission node of the system via the transponder frequency associated with one of the 16 groups. ~he frame reference is carried in a time slot situated 17 effectively between the end of each CROSS GROUP period and the 1~ beginning of the next IN GROUP period. Those stations which 19 utilize the same transponder as the primary reference station receive the frame reference in time continuity with their IN
21 GROUP mode of`reception. Stations in other groups receive the 22 frame reference in time continuity with the end of their CROSS
23 GROUP mode of reception.
24 The foregoing and other features, aspects, objec-~ives and advantages of the subject invention may ~e more fully 26 appreciated and understood by considering the following 27 detailed description.
29 FIGS. 1 and 2 illustrate two TDMA networks formed by discrete groùps of stations operating relative to separate 1 time-divided frequency bands ~transponder segments) of a ~atellite radio repeater;
FIGS. 3 and 4 schematically illustrate the organization of a typical nodal access station suitable for adaptation to operate in a system in accordance with the subject invention;
FIGS. 5 through 8 are schematic signal diagrams for explaining the method of op~ration of the subject invention;
FIGS. 9 through 13 illustrate diagrammatically the frame formats for burst signaling in a system operating in accordance with the sub-ject invention;
FIG. 14 illustrates in diagrammatic form transponder frequency spectra suitable for practicing the subject invention;
FIGS. 15, 16 and 18 shown on the sheet of drawings bearing Fig. 15, schematically illustrate the logic of station reception for operation in accordance with the subject invention;
FIG. 17 illustrates station equipment for producing timing aper-tures for burst transmission in accordance with the invention; ,;
FIGS. 19 and 20 schematically illustrate switching of transponderfrequencies in the satellite (on-board) for explaining an evolutionary capability of the subject invention; and FIG. 21 illustrates the process of crossover time determination conducted by primary and secondary stations.
DETAILED DESCRIPTION
INTRODUCTION
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FIG. 1 suggests a first group of radio stations 10 which are located at geographically separated sites on the surface of the earth 12 and intercommunicate in TDMA mode through geostationary satellite repeater 14 (also designated R) using an associated transmission car- ; `~
rier frequency ftl. FIG. 2 ,. .. . . . .. . .. ..
1 suggests a second group of radio stations 16, which may be 2 geographically remote from stations of the first group and 3 intercommunicate in TDMA mode through the same satellite 4 repeater 14 using transmission carrier frequency ft2 separate from ftl. This invention concerns a method of linking stations 6 in both groups.
7 The radio antennas in groups 1 and 2 are referred to as 8 radiation access nodes N and identified by discrete 2-digit 9 numerical suffixes; NlX or group 1 and N2Y for group 2, where X ranges from 1 through m and Y from 1 through n. In the 11 specific embodiment to be described m and n can each be as 1~ larye as 100. Station equipment associated with each radiation 13 access node N is designated by the symbol S and a corresponding 14 two digit suffix. Such equipment performs radio transceiving operations, information processing operations, through-16 connection operations and signal conversion functions.
17 In ordinary TDMA operations stations in both groups trans-18 mit bursts of time concentrated information signals in each 19 TDMA frame. The information signals are carried as modulation on respective group carrier frequencies ftl and ft2. The 21 bursts of individual stations are timed relative to the bursts 22 of a reference/master station in each group so as to reach the 23 satellite repeater in,closely spaced time formation without 24 overlapping. The repeater operates as a transponder to shift the carrier frequency spectra (ftl to frl and ft2 to fr2) and 26 retransmits the information in a time multiplexed mosaic (or - -27 composite) of bursts. This mosaic is received by each station 28 of the associated group and from it each station extracts 29 control information and traffic information pre-scheduled for connective routing through ports of that station.
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1 FIG 3 illustrates the general organization of access 2 equipment in a typical station. The station ports are desig-3 nated by an ordered series of symbols P0, Pl...Pk where k is 4 an integer within a predetermined range. The sta~ion equipment 30 exchanges information signals with the ports and provides 6 time compression~decompression (buffer s~orage~ and time 7 division multiplex/demultiplex handling of information signals 8 relative to transceiver access port 32 which is linked ~o the 9 associated access node antenna 34. "
Referring to FIG. 4 the ports of such a station may be 11 assigned to carry telephone (voice) traffic signals and data 12 traf~ic signals. Voice ports are indicated at 40 and data ports 13 at 42. Typically the voice ports exchange analog "voice" signals 14 with time shared station circuits 44 which convert such signals between analog and digital (e.g., delta modulation) forms.
16 'ITraffic" signals entering the station equipment at ~oice tele 17 phone ports are converted from analog to digital (delta mod) 18 form and traffic signals passing from the station ~quipment to 19 voice ports are converted from digital to analog form. Line scanning circuits 46 interface with conversion circuits ~4 and 21 data ports ~2~for exchanging traffic signals bit-sequentially 22 between multiple ports 40, 42 and line buffer stoxage arrays 48.
23 Buffer arrays 48 exchange bytes (groups of bits) betwean 24 byte storage spaces associated wi~h specific ports and block (channel) storage spaces in burst bu~fer storage arrays S0 26 through slot interchange switching array circuits 52. Spaces 27 in arrays 50 are associated with time division channels in the 28 TDMA burst communication path to the satelli~e. Circuits 52 2~ operate as a time position switching exchange relative to the ports and satellite TDMA channels.
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1 Burst buffer arrays 50 exchange multi-byte blocks (channels) of burst traffic with burst multiplex/demultiplex process circuits 54. Circuits 54 exchange burst information signals with modulation/
demodulation circuits of transceiver equipment 56 which links to the satellite access node 34.
Connection request sensing circuits 58 interface with ports 40, 42 for sensing connection request signals (e.g., "off-hook" signals at ports 40), initiating setup of connections and terminating (releasing) connections. Common control facilities 60 (e.g., a programmed general purpose data processing system) interface with connection sensing cir-cuits 58 and multiplex/demultiplex circuits 54 for exchanging information (including connection request, connection acknowledgment and connection release information) with other stations via access node 34 and the satellite repeater. Facilities 60 also connect with slot interchange circuits 52 for setting up connections in the respective station equip-ment. Facilities 60 also operate as described below to control station synchronization for TDMA operation and to provide interstation communi-cation for slot and crossover time assignment processes by ~hich satel-lite burst time is allocated to the stations.
The organi~ation and operation of station equipment associated with a similar single-group TDMA/DA system is described in the above- ;
referenced patent 4,009,344 to Flemming.
The information channels exchanged between buffer arrays 50 and satellite access node 34 are time concentrated into TDMA bursts which occupy small fractions of a TDMA time frame. The - ... ..
a ~ zo 1 multiplexing section 54.1 of circuits 54 composes the outgoing 2 channels of information into burst form. The transmitting 3 section 56.1 of transceiver equipment 56 modulates the outgoing 4 channels on the carrier ftl or ft2 of the group associated with this station for transmission to the satellite repeater. Typi-6 cally the modulation may be in the form of quadrature phase 7 shift keying (QPS~). The satellite shifts the carrier bands 8 of group 1 signals to frl and group 2 signals to fr2, and 9 retransmits composite interleaved bursts on each frequency.
Retransmitted bursts are received at radiation nodes 34, 11 demodulated in receiving sections 56.2 of station transceivers 12 56 and demultiplexed in section 54.2 of station equipment 54.
13 Section 54.2 in association with common control system 60 14 selects ~rom among all of the channels of information in the received composite only those channels which are scheduled 16 for utilization by or connection through the respective sta~ion 17 (e.g., on the basis of connection tables maintained by system 18 60 and destination intelligence included in the received 19 information). In each station selected channels which represent port traffic are passed to burst buffers 50 and distributed to 21 ports 40, 42 (via switch 52, buffers 48, scanner 46 and circuits 22 44). The selected information channels which represent station 23 control information are forwarded to system 60 and used for 24 station synchronization, connection (including telephone line ringing) and release of connections. Other common control 26 time assignment functions performed at particular reference 27 (assignment~ stations in each group will be considered and 28 described below.
2 Intereonnection between stations of the first group (FIG.
3 1) and of the second group (FIG. 2) is accomplished in accor-4 dance with the present invention as follows. Referring ~o FIGS.
5 and 8, both groups use TDMA frame intervals T of equal
6 duration and predetermined phase. Each frame is partitioned
7 into IN &ROUP and CROSS GROUP periods (sub-intervals) relative
8 to each group. IN GROUP periods associated with group 1
9 stations (FIG. 1) are designated Tll (FIGS. 5 and 7) and IN
GROUP periods associated with stations in group 2 (FIG. 2) are 11 designated T22 (see FIGS. 6 and 8). C~OSS GROUP periods 12 associated with stations in group 1 are designated ~12 (see 13 FIGS. 5 and 7) and CROSS G~OUP periods assoeiated with stations 14 in group 2 are designated T21 (see FIGS. 6 and 8). The erossover time point TI/X between Tll and T12 (FIG. 5) coincides with 16 erossover time point TI/X between T22 and T21 (FIG. 6).
17 FIGS. 5 and 6 characterize transmission of bursts B from 18 any access node Nla in the first group and any access node 19 N2b in the seeond group. Bursts from station Sla (and node Nla) in the first group are designated Blla when such bursts 21 oecur in IN GROUP time Tll and B12a when coineident with CROSS
22 GROUP time T12 (see FIG. 5). Bursts from station S2b (and node 23 N2h) in the seeond group are designated B22b in T22 and B21b in 24 T21. All bursts from Nla and all other first group nodes are earried on group earrier frequeney ftl, and all bursts from 26 N2b and the other seeond group nodes are earried on group 27 earrier frequeney ft2.
28 FIGS. 7 and 8 eharaeterize the form of signals received 29 at typieal stations sueh as Sla and S2b in each group. During MA9-77-001 ~10-2~
1 IN GROUP time Tll, each station in the first group, such as 2 station Sla, receives an identical composite sequence of 3 multiple bursts Blll,...,Blla,...,Bllm (FIG. 7) modulated on 4 carrier frequency frl which is associated in a transponder pairing with ftl. Blll is a frame reference burst. Coinciden-6 tally during IN GROUP time T22 stations such as S2b receive 7 bursts Blll, B221,...,B22b,....,etc. ~FIG. 8); where Blll is 8 carried on frl and the other bursts are carried on fr2 which 9 is associated with ft2.
Consequently in Tll stations in the first group receive 11 only bursts BllX originated at nodes in the first group while 12 coincidentally in T22 stations in the second group receive the 13 frame reference burst Blll (from a reference station in the 14 first group) and bursts B22X from stations in the second group.
In CROSS GROUP time T12 stations in the first group such 16 as Sla receive burst sequences B211, B212,...B21X (FIG. 7) from 17 stations in the second group, while coincidentally in T21 18 stations in the second group receive burst sequences ... B12X...
19 (FIG. 8) from stations in the first group.
Consequently these stations intercommunicate by recei~ing 21 bursts originated from stations in the same group during the 22 associated IN GROUP time (Tll or T22) and from stations in the 23 other group during the associated CROSS GROUP time (T12 or T21).
24 FR~ME FORMAT
. . _ A TDMA frame format which sustains IN GROUP and CROSS
26 GROUP communication as described above ton separate transpondèr 2-7 frequencies) is shown in FIGS. 9 through 13. Frames (FR) are 28 fifteen milliseconds in duration. Groups of twenty consecutive 29 frames comprise a superframe (SF) of 300 milliseconds duration.
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l The superframe is the unit of signaling time for exchange of 2 demand information. The exchange process will be described 3 later with reference to FIG. 21.
4 Each frame consists of 1575 channel slots each channel slot comprising 512 bit slots. Station bursts have various 6 lengths usually encompassing at least one-half of a channel.
7 This frame structure is designed to sustain bit transmission 8 rates in excess of 53 X 106 bits per second. The form of a 9 typical frame FR(u) is suggested at 100 (FIG. 9). The time point at which the frame begins is designated t0. The first 11 channel slot after t0 is allocated for communication of a frame 12 reference burst 102 (FRB). This burst is transmitted on ftl 13 by one predetermined station of the first group ~FIG. 1) which 14 is designated the primary reference station. The FRB burst is received (on frl) by stations in both groups and utilized 16 as a keying reference for synchronizing the burs~ transmissions 17 of all stations relative to the satellite repeater.
18 The stations in the first group (group l) receive the 19 FRB (frame reference burst) in time continuity with the beginning of their IN GROUP reception mode (see Tll, FIG. 7). Stations 21 in the second group (group 2) receive the FRB in time continuity 22 with the end of their CROSS GROUP mode of reception (see T21, 23 FIG. 8). The form of the FRB will be discussed later.
24 The next four and a half channels of frame time are allo-cated for a group assignment burst (G-AB) 104. In this slot one 26 group assignment burst G-ABl is sent relative to group l stations 27 on frequency ftl and another group assignment burst G-~B2 is sent 28 relative to group 2 stations on carrier frequency ft2. G-AB1 MA9-77-001 . -12-2~
1 is transmitted preferably by the primary reference station 2 which transmits the FRB and occupies the ~ntire slot 104O
3 G-AB2 is transmitted by a predetermined "assignment station"
4 in group 2 (also called the secondary reference station) and also occupies the entire slot 104. The secondary reference 6 station may be any station in group 2. The form of the bursts 7 G-AB will be discussed later.
8 The next seven and a half channels of frame time (shown at 9 106, FIG. 9) are allocated for transmission reference bursts (XRB's). There are five XRB siots each 1.5 channels wide on 11 each transponder ~ftl, ft2). Each XRB is allottable to a 12 different station. In successive frames of the superframe 13 the XRB slots may be allotted to different sub-groups of five 14 stations in each group so that each station of a group (of up to 100 stations) has at least one XRB slot available to it per 16 super-frame. The form of the XRB burst will be discussed later.
17 The next 1559.5 channels of the frame, shown at 108 in FIG.
18 9, are available for demand assignable allocation to multiple 19 stations for sustaining exchanges of traffic between ports of separate stations and of station control information between 21 station contr~l centers 60 (FIG. 4). The burst slots allocated 22 to group 1 stations are carried on frequency ftl. Those ~3 allocated to group 2 stations are carried on frequency ft2.
24 Exchanges of station control information in the intervals 108 can be used for setting up and releasing connections relative 26 to statian ports, and for varying the relative timing of the ~
27 IN GROUP periods (Tll and T22) and CROSS GROUP periods (T12, T21).
28 TI/X denotes the time point ofS~ransition within interval 29 108 (also termed the crossover time) between IN GROUP and CROSS
GROUP periods. Traffic bursts preceding TI/X, termed IN GROUP
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1 traffic bursts, are transmitted only on ftl by stations in 2 group 1 and only on ft2 by s~ations in group 2; and are 3 receivable only by stations in the respective groups on frl 4 and fr2 respectively. Traffic bursts following after TI/X, termed CROSS GROUP traffic bursts, are transmitted only on 6 ftl by stations in group 1 and only on ft2 by stations in 7 group 2; and are receivable by stations in the opposite groups 8 (see FIGS. 7 and 8).
9 The last two and a half channels of each frame shown at 110 in FIG. 9 are allocated for CROSS GROUP assignment bursts 11 "CG-AB" which correspond to bursts G-AB in intervals 104. The 12 burst CG-ABl corresponding to G-ABl is sent by the reference 13 station of group 1 on ftl in T12. Hence i~ is received by the 14 stations in group 2. The burst CG-AB2 corresponding to G-AB2 is sent by the assignment (secondary reference) station of group 2 16 on ft2 in T21. Hence it is received by the stations in group 17 1. Consequently the bursts CG-AB enable the individual stations 18 of each group to determine the time of arrival of CROSS GROUP
19 traffic and establish reception apertures for CROSS GROUP trafficO
Tha format of the frame reference burst FRB is shown in FIG.
21 10. This bur~t includes a preamble bit sequence 120 followed 22 by a frame identity bit sequence 122 (which may also be used 23 'to identiy the primary node source if the source is variable).
24 Ths is followed by data and ~/pad~ sequences 124 and 126.
The pre~amble 120 is 224 bits long and is used by each receiving 26 station to establish bit synchronism for reception of the FRB
27 information The frame identity sequence of 32 bits is used 28 to distinguish the frame position within the superframe. The 29 data sequence of 128 bits contains information for synchronizing :.
.
-92~1 1 burst transmissions of stations in both groups and will be 2 explained further below. The pad sequ~nce of 128 bits serves 3 as a filler which enables stations of group 1 to maintain bit 4 synchronism while stations of group 2 switch their reception frequencies from frl to fr2 as explained later.
6 The data sequence 124 contains different information in 7 successive frames of the superframe~ In frames FR0, FR5, FR10 8 and FR15 of the superframe the data 12~ contains delay deviation 9 (range difference information) which characterizes the deviation of the signal propagation delay between the reference station 11 and the satellite Erom a predetermined nominal delay value. In 12 frames FRl, FR6, FRll and FR16 the data 124 comprises time of 13 day information. In frames FR2, FR7, FR12 and FR17 the data 14 124 contains crossover time infornation which designates the time position of TI/X relative to tO. In all other frames the 16 slot 124 contains filler bits which are not used for information 17 communication in the presently described embodiment but are 18 available for future expansion of the system to accommodate 19 more reference information.
The group assignment burst G-AB is shown in FIG. 11. The 21 beginning of this burst at 130 coincides with the end of the 22 FRB burst. The burst comprises a preamble sequence 132 (224 23 bits), an identi~ sequence 134 (32 bits), an assignment data 24 sequence 136 (1792 bits) and a guard space 138 (256 bit slots).
The preamble is used by the receiving stations for bit syn~
26 chronization. The identity field is used to distinguish the 27 node which originates this burst (the group 1 primary reference 28 station or group 2 secondary reference station). The assign- ;
29 ment data 136 comprises up to twenty node assignments for IN
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1 GROUP communication and up to twenty node assignments for 2 CROSS GROUP communication which are discussed below. The guard 3 space 138 is a qulescent interval (of no signaling3 which is 4 used as a guard space relative to the beginning of the transmit reference burst slots.
6 The assignment data 136 incdicates to up to twenty specific 7 stations their burst assignment times relative to t0 for trans-8 mittin~ their bursts. The IN ~,R~UP assiynments in G-ABl indicate 9 to group 1 stations their respectlve burst transmission time assi~nments in time periods Tll ~FIGS. 5, 7). The IN GROUP
11 assignments in G-ABl are also usPd by group 1 stations to 12 de~elop reception apertures for selecting traffic information 13 in time portions of Tll which are scheduled for reception at the 14 respective nodes. The CROSS GROUP assignments in G-AEl indicate to the same stations in group 1 their transmission time assign-16 ments in T12 (FIGS. 5, 7). The IN GROUP assiynments in G-AB2 17 indicate to stations in group 2 their transmission assignments 18 and enable these stations to establish thelr reception apertures 19 in T22 (FIGS. 6, 8) and the CROSS GROUP assiynments indicate to the same stations their transmission assignments in T21.
21 The form of each transmit reference burst XRB is indicated 22 in FIG. 12. Each XRB (there are five ~R~'s per frame) comprises 23 a preamble sequence 140 (224 bits), an identit~ sequence 142 24 (32 bits), a data sequence 144 (288 bits) and a guard space 146 (256 bit slots). The yuard space is void of signals. The five 26 ~RB slots in a Erame are allocatable to five different stations 27 and are used by the respective stations to acquire synchronism 28 for burst transmission and to signal status and demand require-2g ments relative to the other nodes in the same and other yroups.
The identity scquence 142 identifies the node at which each ~9-77-001 -16-g20 1 XRB oriyinates. The data sequence 144 contains the status and 2 demand informa~ion.
3 FIG. 13 illustrates the form of the CROSS GROUP assignment 4 bursts CG-AB transmitted by the assignment (primaxy reference) station in yroup 1 and the assignment (secondar~v reference) 6 station in group 2. Since these bursts are sent in coincidence 7 during the CROSS GROUP intervals T12 and T21 they are received by the stations in the opposite group. The bursts CG-ABl sent 9 by the reference statlon in group 1 are received by the stations in group 2. The bursts CG-AB2 sent by the assignment station 11 in group 2 are received by the stations in group 1. These burstc 12 contain the CROSS GROUP assignment information of the corres-13 ponding bursts G-AB and are utilized by the stations receiving 14 such bursts for developing reception apertures for selecting traffic information in T12 and T21 which is scheduled for 16 reception at the respective nodes. Each such burst is 2.5 17 channels wide and comprises a prean~le sequence 150 (224 bits), 18 an identity sequence 152 (32 bits), assignment data 154 (896 19 bits) and a quiescent guard space 156 (128 bit slots); a total of two and a half channels (1280 bit slots). The assigmnent 2.1 information corresponds identically to the CROSS GROUP assign-22 ment information contained in the corresponding G-AB burst~
23 However since the bursts CG-AB are received during CROSS GROUP
24 times T12 and T21 the CROSS GROUP assiynment informa~ion enables the receiviny stations to establish selective reception aperture_ 26 for selective handling of inter-yroup traf~ic.
27 GROUP TR~NSPONDER SPE:CTR~
28 FIG. 14 illustrates the spectral distribu~ion of the 29 satellite transponder facilities allocatable to the two groups.
This is obviously non-limitative and is illustrated only for ~9-77-001 -17-:
1 the purpose of indicating the minimal required separation and 2 bandwidth of such spectra. The transmission carrier frequency 3 bands associated with ftl and ft2 are 54 megahertz wide and 4 are separated by a guard band of at least 7 megahertz as shown.
The bands for reception associated with frl and fr2 are also 6 54 megahertz wide and separated by guard bands of 7 megahertz~
7 The bands for transmission and reception may be separated by 8 2.3 gigahert2.
FIG. 15 illustrates the receiving equipment of a station 11 schematically. The incoming signals are passed through wide 12 band rf amplifier 170 to mixing circuits 172 and 174. Mixers 13 172 and 174 are resp~ctively coupled to sources of local oscil-14 lation 176 and 178. Mi~ers 172 and 174 feed th~ir respective outputs to narrow band filters 180 and 182. Outputs of cir-16 cuits 180 and 182, which correspond to the modulation carried 17 on fr2 and frl resp~ctively (i.e., corresponding to the signals 18 sent by group 2 and group 1 stations respectively), pass to 19 signal taps A and B. Switch 184 (SW) alternates in position between taps A and B in each frame, and thersby alternately 21 recovers IN GROUP and CROSS GROUP signals. The alternation 22 actions of the switch 184 are controlled by line 186 labelled 23 TRP SELECT. This action occurs in a time pattern which is 24 dependent on the group association of the station. The signals passed throu~h switch 184 are applied to carrier recovery circuit~
26 188, clock recovery circuits 190 and symbol (bit) recovery 27 circuits 192. The carrier recovery and clock recovery circuits 28 operate to recover bit syncl)ronization (bi-t clock). The symbol 29 recovery circuits 192 operate to recover the bit information contained in the transmitted signals.
1 Unique word detection circuits 194 coupled to the outputs 2 of the clock and bit recovery circuits detect unique words 3 contained within the preambles of the various bursts. The 4 same unique word may be used in the frame reference bursts FRB, the group assignment bursts (G-AB, CG-AB), the transmit reer-6 ence bursts and the traffic bursts. The form of the unique 7 word is consiaered non-relevant to the present invention.
8 Furthermore, the utilization of unique words in TDMA communi-9 cation bursts is well known in the art.
FIG. 16 illustrates the recovery of information for opera-11 tion of the switch 184 (FIG. 15). The bit clock, bit sy~bol and 1~ unique word ou~puts of FIG. 15 are applied to FRB detection 13 circuit 202 which responds to the unique word of the FRB to 14 produce an enabling signal at its output 204. This signal enables time base circuits 206 and 208 to respectively generate 16 time bases for burst transmission and reception. The reception 17 time base generates time signals at 210 and 212 corresponding 18 to the reception times of tO and tO + 384. Circuits 214 operate 19 to distinguish the FRBIs in frames 2, 5, 12 and 17 of each -, superframe. These FRB's contain the crossover time data (see 21 FIG. 10). The crossover time data is recovered by circuits 216 22 and staticized in register 218. Count/compare circuits 220 count 23 from reception time t~ of each frame to reception time TI/X of 24 each frame (the latter time designated digitally by the contents of register 218). Signals corresponding to reception times 26 tO-128, tO + 384 and TI/X are passed via lines 224 and double 27 pole double throw switch 226 (or the logical equivalent of ,. ~. .
, ~8920 1 such) to the TRP Select lines 186 (FIG. 15) associated with 2 switch 184. In group 1 stations switch 226 is fixed in the 3 upward position illustrated in FIG. 16. In group 2 stations 4 switch 226 is fixed in the down position opposite to the up position shown in FIG. 16. In the up position the signals 6 transferred by switch 226 to TRP SELECT lines 186 (FIG. 15) 7 cause the switch 184 (FIG. 15) to transfer from position A to 8 position B at reception times t0 - 128 and from position B to 9 position A at reception times TI/X; thereby enabling group 1 stations to receive group 1 signals (carried on frl) after 11 t0 and group 2 signals (carried on fr2) after TI/X + 128~ When 12 fixed in the down position switch 226 passes signals to TRP
13 Selec~ line 186 transferring switch 184 from position ~ to 14 position A at t0 + 384 and from position A to position B at TI/X;
thereby enabling stations in group 2 to receive group 2 signals 1~ tcarried on fr2) after t0 ~ 512 and group 1 signals (carried on 17 frl) after TI/X ~ 128. Since t0 + 384 occurs after arrival of the 18 useful information of the FRB the stations in group 2 will also 19 receive the FRB sent b~ the reference station o group 1. Since the FRB occupies the time between t0 and t0 + 512 the "pad"
21 space of 128 ~its between t0 + 384 and t0 ~ 512 allows time to 22 complete the switchover transition. A similar transition time 23 for switchover should be allowed relative to TI/X. Consequently 24 the signal associated with TIJX on lines 186 ~FIG. 15) should precede the arrival time of useful information in the CROSS
26 GROUP intervals (T12 and T21) by at least 128 bits.
28Traffic burst reception is illustrated in FIG. 18. Cir-29cuits 240 operate to recover the IN GROUP assignment data 136 MA9-77-001 -2~-;;
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1 (FIG. 11) in the received group assignment bursts G-As.
2 Circuits 240 may be integrated in the common control system 3 60 (FIG. 4). The assignment data rec~vered by the circuits 4 240 is applied to tlming circuits 242 which generate reception apertures relative to the IN GROUP portions of the composite 6 incoming traffic bit stream (i.e., in Tll or T22).
7 The IN GROUP traLfic reception time period spans the time 8 space between t0 ~ 6656 (b~ginning thirteen channels after t0;
9 see FIG. 9) and TI/y. Signals defining this time period are received from the receive time base circuits 208 (FIG. 16).
11 IN GROUP connection data of the respective station is presented 12 at 244. Such data is maintained as previously indicated by the 13 common control system 60 of the associated station. This 14 connection data, in combination with the IN GROUP assi~nment ;~
data, is sufficient to establish the channel portions of the 16 incoming traffic bit stream which are scheduled for utili~ation 17 at the respective station. The IN GROUP traffic stream is 18 processed selectively through circuits gated by pulse outputs 19 of circuits 242. If the traffic stream contains station control information in "unapertured" slots such control infor-21 mation may be recovered by circuits (not shown) sensitive to 22 the control information signals. The specific form and mode 23 of recovery of such control information is not considered 24 relevant to the present invention, and will not be considered further in this description~
26 The CROSS GROUP traffic bit stream is treated similar~y.
27 Processing circuits 246 which may be integral to the common~
28 control system 60 (FIG. 4) detect and recover the CROSS GROUP
29 assignment data contained in the CROSS GROUP assignment ~892~
l bursts CG-AB. Such data is applied to aperture generating 2 circuits 248 which are enabled during the CROSS GROUP interval 3 which extends from TI/X to t0 of the next frame. Circuits 248 4 produce timed receptioll aperture signals for xecovery of specific traffic slot/channel portions of the CROSS GROUP bit stream.
6 These aperture signals are applied to not-shown gate circuits 7 which operate to select out of the CROSS GROUP bit stream the 8 relevant traffic information. As indicated previously if the 9 incoming stream contains relevant station control information in unapertured slots/channels respective station circuits ll should be adapted to recover such control information separately.
-13 Burst transmission involves a process of synchronization 14 acquisition which is presently well understood in the art of lS TDMA communication. In the present system synchronization 16 acquisition is acquired in three phases: reception acquisition, 17 IN GROUP transmission acquisition and CROSS GROUP transmission 18` acquisition.
19 At system start-up time the primary reference station in group l begins to cyclically transmit FRB's on ftl keyed to 21 an internal frame clock. While doing so the primary reference ~`
22 station reception circùits monitor the signals returning on 23 FRl for FRB's. When FRB signals are detected the timing of 24 the receive apertures is adjusted to correct for doppler effects until the incoming FRB's are appropriately "centered"
26 in time.
27 The propagation delay of FRB's, from transmission to 28 reception, is monitored ~y not-shown comrnon control circuits 29 of the primary reference station and used to calculate a .
~9-77-001 -22-.
1 delay deviation fac~or relative to the nominal propagation 2 delay for that station for the particular time of day. The 3 primary reference station includes the delay deviation factor 4 and time of day information in its outgoing FRB's. It also includes crossover time data associated with TI/x in its 6 FRB's. Initially the crossover time may be set arbitrarily 7 (e.g., at the midpoint of the frame~.
8 Stations other than the primary reference station may 9 acquire reception synchronism by recovering the primary reference FRB information in circuits 250 (FIG. 17) and reg-11 ister 218 (FIG. 16). The switch 184 (FIG. 15) may be positioned 12 initially to pass only signals carried on frl, until the FRB's 13 being sent by the primary reference station are being detected 14 repeatedly in successive frame periods in a stable mode. There-after the switch 184 may be operated in the "normal" alternating 16 mode described previously; transferring to the position for 17 CRO~S GROUP reception at TI/X and to the position for intra-18 group reception at the associated group time (t0 - 128 in group 19 1 stations and t0 ~ 384 in group 2 stations).
Using the delay deviation, time of day and crossover time 21 information in the FRB a station seeking to acquire synchron-22 ization for burst transmission operates its transmission timing 23 circuits 254 (FIGo ~7) to send "self-synchronizing" signals in 24 a predetermined IN GRO~P slot assigned to that sta~ion. Initial-ly such self-synchronizing signals are sent in a traffic slot 26 assigned to the station. After transmit synchronization has 27 been achieved these signals are sent in the XRB slot assigned~
28 to the station.
~9-77-001 -23-.. . ...
1 Not all stations in each group need be equipped for 2 inter-group communication. Stations not so equipped will 3 receive only the associated group frequency (frl in group 1 4 and fr2 in group 2) and acquire synchronism by detecting FRB
signals passed through -the associated group transponder.
6 Group 2 stations operating in this manner will receive secon-7 dary FRB signals sent by the secondary reference station in 8 a manner detailed below.
9 Receiving its o~n self-synchronizing signals in circuits 256 (FIG. 17) a station seeking to acquire transmission 11 synchronism for mixed communication (intra-yroup and inter-12 group) adjusts the transmission timing of its self-synchronizing 13 signals to the leading edge of its assigned slot. The "self-14 synchronizing" signal is timed initiall~ to occupy a c~ntral position in the assigned slot (to avoid interference with 16 other slots) and thereafter adjusted incrementally in timing 17 ~in "small" increments) until it is consistently positioned 18 at the leading edge of the same slot (o~er multiple frames);
19 whereupon the station may begin to utilize the XRB slot for transmission of the self-synchronizing signals, demand data, 21 etc.
22 The primary reference station of group 1 utilizes the XRB's 23 of stations in its rJroup to determine status and connection 24 requirements of said group. The primary reference sta~ion sends data in its G-AB1 bursts (FIG. 11) assiyning IN GRO~P
26 traffic slots to each synchronized station in group 1. This 27 data is received in circuits 258 (FIG. 17) and utilized to 28 control circuits 254 for transmission of group 1 traffic 29 information.
M~9-77-001 -24-39Z(~
1 The secondary reference station of group 2 begins its 2 acquisition of reception synchronization by detecting the 3 primary FRB signalsO When reception synchronization is achieved 4 the secondary reference station may begin its acquisition of transmission synchroni~a~ion using the delay deviation and 6 tim~ of day information forwarded by the primary reference 7 station and its assigned slots on ft2, fr2. It also acquires 8 crossover synchronization by monitoring the crossover time 9 information in the primary FRB's. The other stations in group 2 may acquire reception synchronism similarly, using 11 the primary FRB data, and thereafter acquire transmission 12 synchronism; initially using assigned portions of the traffic 13 space on the associated transponder as described previously 14 to circulate self-synchroniziny signals, and thereafter maintaining synchronism by using respective XRB slots to 16 circulate self-synchronizing signals. ~ `
17 Stations in both groups may acquire CROSS GROUP synchron-18 ization by monitoring the crossover time information in the 19 primary reference FRB and switching respective switches 184 (FIG. 15) at appropriate time points as described previously.
21 The stations may then use CROSS GROUP assignrnents (in G-ABl 22 and CG-AB2 for group 1 stations and G-~B2 and CG-ABl for group 23 2 stations) to carry on inter-group communications.
24 For stations not equipped for CROSS GROUP operation the above synchronization system may be modified as follows.
26 Note (FIG. 10) that the primary reference FRB occupies the~
27 interval between t0 and t0 + 512 but is carried only on ftl"
28 frl. Hence there is effectively a vacancy in time on ft2, M~9-77-001 -25-l fr2 between the same time points tO and tO + 512. This 2 "vacant" slot may be used by the secorldary reference station 3 to transmit secondary FRB's (frame reference bursts) which 4 are identical to previously received FRB's sent by the primary station (i.e., primary FRB's). Stations in group 2 equipped 6 to receive only fr2 will thereby receive and use data in the 7 secondary FRB's and assigned XRB slots on ft2, ~r2 to acquire 8 transmission synchronization~
9 As explained previously stations in both groups adapted for inter-group communication will synchronize directly to ll the primary FRB signals~ This is preferred inasmuch as 12 synchronization to the secondary FRB signals in group 2 stations 13 introduces a potential double "jitter" effect relative to the 14 primary reference source. However it is not essential if the sources of primary and secondary FRB's are sufficiently stable.
16 In systems having sufficiently stable FRB sources satisfactory 17 operation may be achieved if all stations in group 2, other l~ than the secondary reference station, synchronize to the - ;
l9 secondary FRB signals and the secondary reference station and all stations in group l synchronize to the primary FRB's.
21 In such systems the stations in group 2, other than the 22 secondary reference station, would switch to IN GROUP reception 23 mode at tO - 128 (i.e., at the same time as group 1 stations) 24 and only the secondary reference station would switch at 25 tO + 38~. -27 The present system in its preferred mode of operation 28 utilizes two processes of demand assignment. In one process 29 termed crossover time assiynment the primary reference station determines a crossover time associated with TI/X which is ~113920 1 communicated in the primary FRBo In another process of assign~
2 ment the primary and secondary reference stations assign 3 "available" time slots within the IN GROUP and CROSS GROUP
4 periods delimited by TI/X to stations of respective groups.
The primary reference station assigns slots to group 1 stations 6 in IN GROUP time Tll and CROSS GROUP time T12, and communicates 7 the assignments on assi~nment bursts G-ABl. The secondary 8 reference station assigns slots in IN GROUP time T22 and CROSS
9 GROUP time T21 to group 2 stations and communicates the assign-ments on assignment bursts G-AB2.
11 This procedure is characterized in FIG. 21. The primary 12 and secondary reference stations receive their respective XRB
13 transmissions at 260 and 262, from respective group stations, 14 and extract demand information as suggested at 264 and 266 respectively. The reference stations allot transmission time 16 slots in respective IN GROUP and CROSS GROUP periods (of 17 respective transponder frequencies ftl and ft2) in accordance 18 with existing demand as suggested at 268 and 270. The assign-19 ments are based on conventional algorithms for TDMA/DA opera-tion which are not relevant to the present invention. The 21 objective is to maximize utilization of the available time and22 avoid under-utilization of time by some stations while other 23 stations have a nee~d for the same time.
24 This process operates recursively as indicated by return lines at 272 and 274 to respective processes of XRB recovery.
26 Concurrently the primary and secondary reference stations 27 determine the overall utilization of IN GROUP and CROSS GROUP
28 time in the respective groups as shown at 276 and 278. The 29 secondary reference station utilizes signaling channels of . . . ~ ".
, `' :` ` '', , ,, . j . . ;. . :
. - ., ., ; ~: , .. . . ...
92~) 1 the traffic bursts to send messages to the primary station as 2 shown at 280 and suggested by line 281. The group 1 prirnary 3 station determines the relative utilization of IN GROUP and 4 CROSS GROUP time on the transponders associated with both groups 1 and 2. With this information the primary reference 6 station determines a crossover time suitable for balanced 7 utilization of both transponders. If this time is different 8 from the time currently being communicated in the FRB the 9 FRB data is updated as suggested at 282 and the updated cross-over time information is communicated to all stations as 11 suggested at 284. The crossover time is changed only on super-12 frame boundaries. All assignments G-AB by the primary and 13 secondary reference stations are based upon time periods de-14 limited by the current (updated) crossover time.
15 DESTINATION (PORT) ADDRESSIMG .
16 In the foregoing system traffic signal channels may be 17 directed to the ports 40, 42 (FIG. 4) by means of address 18 information in the signal channels. An interesting aspect of 19 the present system is that sùch address signals in IN GROUP
and CROSS GROUP time slots need not be relatively differentiated 21 since slots are received only by stations in one group.
22 For the situation in which some group 2 stations are 23 adapted only for unitransponder (one-frequency) operation it 2~ should be apparent that signals sent to such stations during CROSS GROUP time will originate only at group 2 stations and 26 occupy only slots on fr2 which are not in use relative to group 27 1 stations. Hence common destination addressing presents no 28 problem of ambiguity.
~1139;2 [) ADAPT~TION FOR MORE TIIAN TWO GROUPS
_ . ~
2 The crossover time partitioning technique described above 3 extends in an obvious mode to serve three groups of stations 4 using three transponder frequencies (ftl/frl, ft2/fr2 and ft3/fr3). It is merely necessary to define three crossover 6 times in the primary FRs; for respectively delimiting periods 7 for communication between stations of the first and second 8 groups, second and third groups and first and third groups.
9 Obviously the circuits of FIGS. 15-18 would be modified to allow for recovery and utilization of the three crossover 11 time factors.
13 Should a primary or secondary reference station become 14 unavailable it will be desirable to be able to establish a new primary or secondary reference station. For this purpose any 16 of the existing stations may be used as a reference station.
17 If time synchronization is not lost the "new" primary or 18 secondary reference station may begin to broadcast the FRB in 19 the FRB slot after the "old" station is "silenced". A new reerence station will also transmit a "new" group assignment 21 burst in the appropriate burst assignment slot. A new secondary 22 reference station may as indicated above also transmit a copy 23 of the primary FRB in the initial (FRB) time slot ft2.
The system described above should adapt very simply and 26 economically to future satellite repeater technologies invol~ing 27 the use of more sophisticated "on-board" equipment in the 28 satellite.
1 FIG. 19 illustrates a hypothetical capability of future 2 satellite repeaters for performing frequency switching on an 3 "intelligent" basis during IN GROUP and CROSS GROUP periods of 4 a basic TDMA frame shared by multiple groups of transceiver nodes. FIG. 19 suggests transposition of carrier frequencies 6 ftl and ft2 to carrier frequencies frl and fr2 respectively 7 during IN GROUP periods, and to fr2 and frl respectively during 8 CROSS GROUP periods. Ovbiously this would be the equivalent of 9 the functions presently performed by the multiple earth station receivers using more conventional "active" satellite repeaters.
11 FIG. 20 suggests "on-board" satellite "logic" for shift-12 ing the time point of TI/X in accordance with earth station `
13 demand. Satellite receiver 300 passes information received by 14 the satellite on ftl and ft2 to on-board processor 302.
Facilities 304 in said processor recover the FRB and facilities 16 306 recover and staticize the crossover time data contained in 17 the FRB. This data is compared in compare circuit 308 to 18 earlier crossover time data in register 310~ ~hen inequality 19 exists switch 312 is operated to replace the contents of register 310 with the new crossover time data provided by 21 facilities 306. The data in register 310 is applied to 22 generator circuit 316 to produce time signals which represent 23 the transition point TI/X from IN GROUP to CROSS GROUP reception 2~ periods "on-board" the satellite. Time base circuits 31~
coupled to FRB detector 304 provide signals corresponding to 26 the initial frame time t0 relative to on-board reception at 27 the satellite. The signals produced by circuits 316 and 318 .
28 are applied to the satellite txansmission equipment to determine 29 the transition points in time for switching between IN GROUP
and CROSS GROUP displacements of the "repeated" frequency.
3L~ ZO
1 It should be apparent that only minor modifications of 2 ground station transceiver equipment would be required to adapt 3 to such on~board frequency shifting capability. The receiver 4 switches such as 184 (FIG. 15) should be fixed in positions such that the respective station receives only the transponder 6 frequency frl or fr2 associated with its own station group 7 during both IN GROUP and CROSS GROUP intervals. Quite 8 apparently future stations not equipped with switches 184 9 would be inherently adaptive to CROSS GROUP operation in such a system.
11 While the invention has been particularly shown and 1~ described with reference to preferred embodiments thereof, 13 those skilled in the art will recognize that the above and 14 other changes in form and details may be made therein without departing from the spirit and scope of the invention.
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