Groups of stations operate in TDMA mode relative to associated frequency-separated transponder segments of a satellite repeater. Stations at radio signaling modes in all groups key to a common frame timing reference. The TDMA frame is partitioned repetitively into IN GROUP and CROSS GROUP intervals, each susceptible of containing multiple demand assignable burst time slots. Each node may transmit TDMA bursts (of time compressed and time multiplexed information signals) in assigned slots in either interval (or both). Such bursts are carried only on the transponder radio frequency associated with the respective group. Station receivers are adaptive to switch local oscillator frequencies in synchronism with transitions between IN GROUP and CROSS GROUP periods, and thereby adaptive to receive signals from stations in both groups. A frame reference signal carried on the transponder frequency associated with one of the groups is receivable by stations in the same group during IN GROUP mode reception and by stations in the other group during CROSS GROUP mode reception. Consequently stations in both groups may key to that reference signal. The transition (crossover time) from IN GROUP mode to CROSS GROUP mode can be varied in time position to efficiently accommodate demand within and between groups.

CROSS REFERENCES TO RELATED PATENTS 
U.S. Pat. No. 4,009,344 granted Feb. 22, 1977 to D. C. Flemming, which is 
assigned to the assignee of the subject invention, describes a system 
employing TDMA and demand assignment operations which are considered 
relevant to the present invention. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to time division multiple access (TDMA) 
communication systems; and particularly to TDMA systems in which multiple 
radio stations communicate through an earth satellite repeater by 
transmitting time-synchronized bursts of radio energy relative to said 
repeater and receiving a time multiplex composite of bursts containing 
corresponding modulation information from said repeater. 
2. Prior Art 
Systems of radio communication using techniques of TDMA operation and 
TDMA/DA operation (DA referring to demand assignment) are well known. 
In TDMA operation multiple transceiver stations associated with radio 
signaling nodes transmit bursts of time concentrated information signals 
on a shared carrier frequency spectrum and receive the same information 
signals after repetition by the satellite on a shifted carrier frequency 
spectrum. Each station is assigned particular time slots in a continuum of 
recurrent frames for transmission of its bursts and for reception of its 
own bursts and bursts of other stations. The bursts interleave at the 
satellite in close time formation without overlapping. 
In DA operation lengths of assigned slots may be varied in accordance with 
the relative distribution of demand at the signaling nodes. 
Various systems have been proposed for enabling stations operating in TDMA 
mode relative to different transponder frequency spectra to 
intercommunicate. Such proposed systems have been rejected for various 
reasons. Systems based upon time domain switching relative to transmission 
frequency spectra have been rejected as overly expensive and inefficient 
because of the magnitudes of transmission power which must be handled. 
Systems based upon simultaneous transmission on plural frequency bands at 
each node have been rejected as inefficient and overly complex. 
The present invention concerns a system for providing intertransponder 
communication in TDMA mode which is efficient, inexpensive to implement 
relative to conventional unitransponder systems, and relatively simple to 
construct and operate. 
SUMMARY OF THE INVENTION 
A principal object of this invention is to provide a system for TDMA 
communication between radio transponders (across time-divided transponder 
frequency bands) in which transmission frequency bands do not have to be 
switched or under-utilized. 
A related general objective is to provide a system for intertransponder 
communication between groups of stations operating in TDMA mode relative 
to separated carrier frequency bands (transponders) which is efficient, 
practical, economical and simple. 
These and other objectives and advantages of this invention are achieved 
presently by partitioning the TDMA frame repetitively into IN GROUP 
(intratransponder) and CROSS GROUP (intertransponder) periods relative to 
plural groups of stations. Stations in any group which are adapted for 
operation in CROSS GROUP (intertransponder) mode are operative to switch 
reception frequencies at time points of transition between IN GROUP and 
CROSS GROUP periods. 
Stations in each group transmit on a single carrier radio frequency band 
exclusively allocated to the group. Station receiving equipment adapted 
for operation in CROSS GROUP mode switches local oscillation frequencies 
at predetermined time points of transition between IN GROUP and CROSS 
GROUP periods. This enables adapted stations to receive information 
signals from stations in both groups. The crossover time T.sub.I/X from IN 
GROUP mode to CROSS GROUP mode is susceptible of being varied to balance 
overall utilization of the satellite repeater by all stations. 
Stations in each group synchronize their transmission and reception burst 
apertures to time bases derived from a common frame timing reference. This 
reference is communicated from a primary reference station at one radio 
transmission node of the system via the transponder frequency associated 
with one of the groups. The frame reference is carried in a time slot 
situated effectively between the end of each CROSS GROUP period and the 
beginning of the next IN GROUP period. Those stations which utilize the 
same transponder as the primary reference station receive the frame 
reference in time continuity with their IN GROUP mode of reception. 
Stations in other groups receive the frame reference in time continuity 
with the end of their CROSS GROUP mode of reception. 
The foregoing and other features, aspects, objectives and advantages of the 
subject invention may be more fully appreciated and understood by 
considering the following detailed description.

DETAILED DESCRIPTION 
INTRODUCTION 
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 carrier frequency 
ft1. FIG. 2 suggests a second group of radio stations 16, which may be 
geographically remote from stations of the first group and 
intercommunicate in TDMA mode through the same satellite repeater 14 using 
transmission carrier frequency ft2 separate from ft1. This invention 
concerns a method of linking stations in both groups. 
The radio antennas in groups 1 and 2 are referred to as radiation access 
nodes N and identified by discrete 2-digit numerical suffixes; N1X for 
group 1 and N2Y for group 2, where X ranges from 1 through m and Y from 1 
through n. In the specific embodiment to be described m and n can each be 
as large as 100. Station equipment associated with each radiation access 
node N is designated by the symbol S and a corresponding two digit suffix. 
Such equipment performs radio transceiving operations, information 
processing operations, through-connection operations and signal conversion 
functions. 
In ordinary TDMA operations stations in both groups transmit bursts of time 
concentrated information signals in each TDMA frame. The information 
signals are carried as modulation on respective group carrier frequencies 
ft1 and ft2. The bursts of individual stations are timed relative to the 
bursts of a reference/master station in each group so as to reach the 
satellite repeater in closely spaced time formation without overlapping. 
The repeater operates as a transponder to shift the carrier frequency 
spectra (ft1 to fr1 and ft2 to fr2) and retransmits the information in a 
time multiplexed mosaic (or composite) of bursts. This mosaic is received 
by each station of the associated group and from it each station extracts 
control information and traffic information pre-scheduled for connective 
routing through ports of that station. 
FIG. 3 illustrates the general organization of access equipment in a 
typical station. The station ports are designated by an ordered series of 
symbols P0, P1 . . . Pk where k is an integer within a predetermined 
range. The station equipment 30 exchanges information signals with the 
ports and provides time compression/decompression (buffer storage) and 
time division multiplex/demultiplex handling of information signals 
relative to transceiver access port 32 which is linked to the associated 
access node antenna 34. 
Referring to FIG. 4 the ports of such a station may be assigned to carry 
telephone (voice) traffic signals and data traffic signals. Voice ports 
are indicated at 40 and data ports at 42. Typically the voice ports 
exchange analog "voice" signals with time shared station circuits 44 which 
convert such signals between analog and digital (e.g., delta modulation) 
forms. "Traffic" signals entering the station equipment at voice telephone 
ports are converted from analog to digital (delta mod) form and traffic 
signals passing from the station equipment to voice ports are converted 
from digital to analog form. Line scanning circuits 46 interface with 
conversion circuits 44 and data ports 42 for exchanging traffic signals 
bit-sequentially between multiple ports 40, 42 and line buffer storage 
arrays 48. 
Buffer arrays 48 exchange bytes (groups of bits) between byte storage 
spaces associated with specific ports and block (channel) storage spaces 
in burst buffer storage arrays 50 through slot interchange switching array 
circuits 52. Spaces in arrays 50 are associated with time division 
channels in the TDMA burst communication path to the satellite. Circuits 
52 operate as a time position switching exchange relative to the ports and 
satellite TDMA channels. 
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 circuits 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 equipment. Facilities 60 
also operate as described below to control station synchronization for 
TDMA operation and to provide interstation communication for slot and 
crossover time assignment processes by which satellite burst time is 
allocated to the stations. 
The organization and operation of station equipment associated with a 
similar single-group TDMA/DA system is described in the above-referenced 
U.S. Pat. No. 4,009,344 to Flemming. To the extent that such description 
is relevant to the system and demand assignment process described herein 
it is incorporated herein by reference. 
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 multiplexing section 54.1 of circuits 
54 composes the outgoing channels of information into burst form. The 
transmitting section 56.1 of transceiver equipment 56 modulates the 
outgoing channels on the carrier ft1 or ft2 of the group associated with 
this station for transmission to the satellite repeater. Typically the 
modulation may be in the form of quadrature phase shift keying (QPSK). The 
satellite shifts the carrier bands of group 1 signals to fr1 and group 2 
signals to fr2, and retransmits composite interleaved bursts on each 
frequency. 
Retransmitted bursts are received at radiation nodes 34, demodulated in 
receiving sections 56.2 of station transceivers 56 and demultiplexed in 
section 54.2 of station equipment 54. Section 54.2 in association with 
common control system 60 selects from among all of the channels of 
information in the received composite only those channels which are 
scheduled for utilization by or connection through the respective station 
(e.g., on the basis of connection tables maintained by system 60 and 
destination intelligence included in the received information). In each 
station selected channels which represent port traffic are passed to burst 
buffers 50 and distributed to ports 40, 42 (via switch 52, buffers 48, 
scanner 46 and circuits 44). The selected information channels which 
represent station control information are forwarded to system 60 and used 
for station synchronization, connection (including telephone line ringing) 
and release of connections. Other common control time assignment functions 
performed at particular reference (assignment) stations in each group will 
be considered and described below. 
INTER-GROUP CONNECTION 
Interconnection between stations of the first group (FIG. 1) and of the 
second group (FIG. 2) is accomplished in accordance with the present 
invention as follows. Referring to FIGS. 5 and 8, both groups use TDMA 
frame intervals T of equal duration and predetermined phase. Each frame is 
partitioned into IN GROUP and CROSS GROUP periods (sub-intervals) relative 
to each group. IN GROUP periods associated with group 1 stations (FIG. 1) 
are designated T11 (FIGS. 5 and 7) and IN GROUP periods associated with 
stations in group 2 (FIG. 2) are designated T22 (see FIGS. 6 and 8). CROSS 
GROUP periods associated with stations in group 1 are designated T12 (see 
FIGS. 5 and 7) and CROSS GROUP periods associated with stations in group 2 
are designated T21 (see FIGS. 6 and 8). The crossover time point T.sub.I/X 
between T11 and T12 (FIG. 5) coincides with crossover time point T.sub.I/X 
between T22 and T21 (FIG. 6). 
FIGS. 5 and 6 characterize transmission of bursts B from any access node 
N1a in the first group and any access node N2b in the second group. Bursts 
from station S1a (and node N1a) in the first group are designated B11a 
when such bursts occur in IN GROUP time T11 and B12a when coincident with 
CROSS GROUP time T12 (see FIG. 5). Bursts from station S2b (and node N2b) 
in the second group are designated B22b in T22 and B21b in T21. All bursts 
from N1a and all other first group nodes are carried on group carrier 
frequency ft1, and all bursts from N2b and the other second group nodes 
are carried on group carrier frequency ft2. 
FIGS. 7 and 8 characterize the form of signals received at typical stations 
such as S1a and S2b in each group. During IN GROUP time T11, each station 
in the first group, such as station S1a, receives an identical composite 
sequence of multiple bursts B111, . . ., B11a, . . ., B11m (FIG. 7) 
modulated on carrier frequency fr1 which is associated in a transponder 
pairing with ft1. B111 is a frame reference burst. Coincidentally during 
IN GROUP time T22 stations such as S2b receive bursts B111, B221 . . ., 
B22b, . . ., etc. (FIG. 8), where B111 is carried on fr1 and the other 
bursts are carried on fr2 which is associated with ft2. 
Consequently in T11 stations in the first group receive only bursts B11x 
originated at nodes in the first group while coincidentally in T22 
stations in the second group receive the frame reference burst B111 (from 
a reference station in the first group) and bursts B22x from stations in 
the second group. 
In CROSS GROUP time T12 stations in the first group such as S1a receive 
burst sequences B211, B212,. . .B21x (FIG. 7) from stations in the second 
group, while coincidentally in T21 stations in the second group receive 
burst sequences . . .B12x . . . (FIG. 8) from stations in the first group. 
Consequently these stations intercommunicate by receiving bursts originated 
from stations in the same group during the associated IN GROUP time (T11 
or T22) and from stations in the other group during the associated CROSS 
GROUP time (T12 or T21). 
FRAME FORMAT 
A TDMA frame format which sustains IN GROUP and CROSS GROUP communication 
as described above (on separate transponder frequencies) is shown in FIGS. 
9 through 13. Frames (FR) are fifteen milliseconds in duration. Groups of 
twenty consecutive frames comprise a superframe (SF) of 300 milliseconds 
duration. The superframe is the unit of signaling time for exchange of 
demand information. The exchange process will be described later with 
reference to FIG. 21. 
Each frame consists of 1575 channel slots each channel slot comprising 512 
bit slots. Station bursts have various lengths usually encompassing at 
least one-half of a channel. This frame structure is designed to sustain 
bit transmission rates in excess of 53.times.10.sup.6 bits per second. The 
form of a typical frame FR(u) is suggested at 100 (FIG. 9). The time point 
at which the frame begins is designated t0. The first channel slot after 
t0 is allocated for communication of a frame reference burst 102 (FRB). 
This burst is transmitted on ft1 by one predetermined station of the first 
group (FIG. 1) which is designated the primary reference station. The FRB 
burst is received (on fr1) by stations in both groups and utilized as a 
keying reference for synchronizing the burst transmissions of all stations 
relative to the satellite repeater. 
The stations in the first group (group 1) receive the FRB (frame reference 
burst) in time continuity with the beginning of their IN GROUP reception 
mode (see T11, FIG. 7). Stations in the second group (group 2) receive the 
FRB in time continuity with the end of their CROSS GROUP mode of reception 
(see T21, FIG. 8). The form of the FRB will be discussed later. 
The next four and a half channels of frame time are allocated for a group 
assignment burst (G-AB) 104. In this slot one group assignment burst G-AB1 
is sent relative to group 1 stations on frequency ft1 and another group 
assignment burst G-AB2 is sent relative to group 2 stations on carrier 
frequency ft2. G-AB1 is transmitted preferably by the primary reference 
station which transmits the FRB and occupies the entire slot 104. G-AB2 is 
transmitted by a predetermined "assignment station" in group 2 (also 
called the secondary reference station) and also occupies the entire slot 
104. The secondary reference station may be any station in group 2. The 
form of the bursts G-AB will be discussed later. 
The next seven and a half channels of frame time (shown at 106, FIG. 9) are 
allocated for transmission reference bursts (XRB's). There are five XRB 
slots each 1.5 channels wide on each transponder (ft1, ft2). Each XRB is 
allottable to a different station. In successive frames of the superframe 
the XRB slots may be allotted to different sub-groups of five 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 super-frame. The form of the XRB 
burst will be discussed later. 
The next 1559.5 channels of the frame, shown at 108 in FIG. 9, are 
available for demand assignable allocation to multiple stations for 
sustaining exchanges of traffic between ports of separate stations and of 
station control information between station control centers 60 (FIG. 4). 
The burst slots allocated to group 1 stations are carried on frequency 
ft1. Those allocated to group 2 stations are carried on frequency ft2. 
Exchanges of station control information in the intervals 108 can be used 
for setting up and releasing connections relative to station ports, and 
for varying the relative timing of the IN GROUP periods (T11 and T22) and 
CROSS GROUP periods (T12, T21). 
T.sub.I/X denotes the time of transition within interval 108 (also termed 
the crossover time) between IN GROUP and CROSS GROUP periods. Traffic 
bursts preceding T.sub.I/X, termed IN GROUP traffic bursts, are 
transmitted only on ft1 by stations in group 1 and only on ft2 by stations 
in group 2; and are receivable only by stations in the respective groups 
on fr1 and fr2 respectively. Traffic bursts following after T.sub.I/X, 
termed CROSS GROUP traffic bursts, are transmitted only on ft1 by stations 
in group 1 and only on ft2 by stations in group 2; and are receivable by 
stations in the opposite groups (see FIGS. 7 and 8). 
The last two and a half channels of each frame shown at 110 in FIG. 9 are 
allocated for CROSS GROUP assignment bursts "CG-AB" which correspond to 
bursts G-AB in intervals 104. The burst CG-AB1 corresponding to G-AB1 is 
sent by the reference station of group 1 on ft1 in T12. Hence it is 
received by the stations in group 2. The burst CG-AB2 corresponding to 
G-AB2 is sent by the assignment (secondary reference) station of group 2 
on ft2 in T21. Hence it is received by the stations in group 1. 
Consequently the bursts CG-AB enable the individual stations of each group 
to determine the time of arrival of CROSS GROUP traffic and establish 
reception apertures for CROSS GROUP traffic. 
The format of the frame reference burst FRB is shown in FIG. 10. This burst 
includes a preamble bit sequence 120 followed by a frame identity bit 
sequence 122 (which may also be used to identify the primary node source 
if the source is variable). This is followed by data and "pad" sequences 
124 and 126. The preamble 120 is 224 bits long and is used by each 
receiving station to establish bit synchronism for reception of the FRB 
information. The frame identity sequence of 32 bits is used to distinguish 
the frame position within the superframe. The data sequence of 128 bits 
contains information for synchronizing burst transmissions of stations in 
both groups and will be explained further below. The pad sequence of 128 
bits serves as a filler which enables stations of group 1 to maintain bit 
synchronism while stations of group 2 switch their reception frequencies 
from fr1 to fr2 as explained later. 
The data sequence 124 contains different information in successive frames 
of the superframe. In frames FR0, FR5, FR10 and FR15 of the superframe the 
data 124 contains delay deviation (range difference information) which 
characterizes the deviation of the signal propagation delay between the 
reference station and the satellite from a predetermined nominal delay 
value. In frames FR1, FR6, FR11 and FR16 the data 124 comprises time of 
day information. In frames FR2, FR7, FR12 and FR17 the data 124 contains 
crossover time information which designates the time position of T.sub.I/X 
relative to t0. In all other frames the slot 124 contains filler bits 
which are not used for information communication in the presently 
described embodiment but are available for future expansion of the system 
to accommodate more reference information. 
The group assignment burst G-AB is shown in FIG. 11. The beginning of this 
burst at 130 coincides with the end of the FRB burst. The burst comprises 
a preamble sequence 132 (224 bits), an identity sequence 134 (32 bits), an 
assignment data sequence 136 (1792 bits) and a guard space 138 (256 bit 
slots). The preamble is used by the receiving stations for bit 
synchronization. The identity field is used to distinguish the node which 
originates this burst (the group 1 primary reference station or group 2 
secondary reference station). The assignment data 136 comprises up to 
twenty node assignments for IN GROUP communication and up to twenty node 
assignments for CROSS GROUP communication which are discussed below. The 
guard space 138 is a quiescent interval (of no signaling) which is used as 
a guard space relative to the beginning of the transmit reference burst 
slots. 
The assignment data 136 indicates to up to twenty specific stations their 
burst assignment times relative to t0 for transmitting their bursts. The 
IN GROUP assignments in G-AB1 indicate to group 1 stations their 
respective burst transmission time assignments in time periods T11 (FIGS. 
5, 7). The IN GROUP assignments in G-AB1 are also used by group 1 stations 
to develop reception apertures for selecting traffic information in time 
portions of T11 which are scheduled for reception at the respective nodes. 
The CROSS GROUP assignments in G-AB1 indicate to the same stations in 
group 1 their transmission time assignments in T12 (FIGS. 5, 7). The IN 
GROUP assignments in G-AB2 indicate to stations in group 2 their 
transmission assignments and enable these stations to establish their 
reception apertures in T22 (FIGS. 6, 8) and the CROSS GROUP assignments 
indicate to the same stations their transmission assignments in T21. 
The form of each transmit reference burst XRB is indicated in FIG. 12. Each 
XRB (there are five XRB's per frame) comprises a preamble sequence 140 
(224 bits), an identity sequence 142 (32 bits), a data sequence 144 (288 
bits) and a guard space 146 (256 bit slots). The guard space is void of 
signals. The five XRB slots in a frame are allocatable to five different 
stations and are used by the respective stations to acquire synchronism 
for burst transmission and to signal status and demand requirements 
relative to the other nodes in the same and other groups. The identity 
sequence 142 identifies the node at which each XRB originates. The data 
sequence 144 contains the status and demand information. 
FIG. 13 illustrates the form of the CROSS GROUP assignment bursts CG-AB 
transmitted by the assignment (primary reference) station in group 1 and 
the assignment (secondary reference) station in group 2. Since these 
bursts are sent in coincidence during the CROSS GROUP intervals T12 and 
T21 they are received by the stations in the opposite group. The bursts 
CG-AB1 sent by the reference station in group 1 are received by the 
stations in group 2. The bursts CG-AB2 sent by the assignment station in 
group 2 are received by the stations in group 1. These bursts contain the 
CROSS GROUP assignment information of the corresponding bursts G-AB and 
are utilized by the stations receiving such bursts for developing 
reception apertures for selecting traffic information in T12 and T21 which 
is scheduled for reception at the respective nodes. Each such burst is 2.5 
channels wide and comprises a preamble sequence 150 (224 bits), an 
identity sequence 152 (32 bits), assignment data 154 (896 bits) and a 
quiescent guard space 156 (128 bit slots); a total of two and a half 
channels (1280 bit slots). The assignment information corresponds 
identically to the IN GROUP assignment information contained in the 
corresponding G-AB burst. However since the bursts CG-AB are received 
during CROSS GROUP times T12 and T21 the CROSS GROUP assignment 
information enables the receiving stations to establish selective 
reception apertures for selective handling of inter-group traffic. 
GROUP TRANSPONDER SPECTRA 
FIG. 14 illustrates the spectral distribution of the satellite transponder 
facilities allocatable to the two groups. This is obviously non-limitative 
and is illustrated only for the purpose of indicating the minimal required 
separation and bandwidth of such spectra. The transmission carrier 
frequency bands associated with ft1 and ft2 are 54 megahertz wide and are 
separated by a guard band of at least 7 megahertz as shown. The bands for 
reception associated with fr1 and fr2 are also 54 megahertz wide and 
separated by guard bands of 7 megahertz. The bands for transmission and 
reception may be separated by 2.3 gigahertz. 
STATION RECEPTION 
FIG. 15 illustrates the receiving equipment of a station schematically. The 
incoming signals are passed through wide band rf amplifier 170 to mixing 
circuits 172 and 174. Mixers 172 and 174 are respectively coupled to 
sources of local oscillation 176 and 178. Mixers 172 and 174 feed their 
respective outputs to narrow band filters 180 and 182. Outputs of circuits 
180 and 182, which correspond to the modulation carried on fr2 and fr1 
respectively (i.e., corresponding to the signals sent by group 2 and group 
1 stations respectively), pass to signal taps A and B. Switch 184 (SW) 
alternates in position between taps A and B in each frame, and thereby 
alternately recovers IN GROUP and CROSS GROUP signals. The alternation 
actions of the switch 184 are controlled by line 186 labelled TRP SELECT. 
This action occurs in a time pattern which is dependent on the group 
association of the station. The signals passed through switch 184 are 
applied to carrier recovery circuits 188, clock recovery circuits 190 and 
symbol (bit) recovery circuits 192. The carrier recovery and clock 
recovery circuits operate to recover bit synchronization (bit clock). The 
symbol recovery circuits 192 operate to recover the bit information 
contained in the transmitted signals. 
Unique word detection circuits 194 coupled to the outputs of the clock and 
bit recovery circuits detect unique words contained within the preambles 
of the various bursts. The same unique word may be used in the frame 
reference bursts FRB, the group assignment bursts (G-AB, CG-AB), the 
transmit reference bursts and the traffic bursts. The form of the unique 
word is considered non-relevant to the present invention. Furthermore, the 
utilization of unique words in TDMA communication bursts is well known in 
the art. 
FIG. 16 illustrates the recovery of information for operation of the switch 
184 (FIG. 15). The bit clock, bit symbol and unique word outputs of FIG. 
15 are applied to FRB detection circuit 202 which responds to the unique 
word of the FRB to produce an enabling signal at its output 204. This 
signal enables time base circuits 206 and 208 to respectively generate 
time bases for burst transmission and reception. The reception time base 
generates time signals at 210 and 212 corresponding to the reception times 
of t0 and t0+384. Circuits 214 operate to distinguish the FRB's in frames 
2, 5, 12 and 17 of each superframe. These FRB's contain the crossover time 
data (see FIG. 10). The crossover time data is recovered by circuits 216 
and staticized in register 218. Count/compare circuits 220 count from 
reception time t0 of each frame to reception time T.sub.I/X of each frame 
(the latter time designated digitally by the contents of register 218). 
Signals corresponding to reception times t0-128, t0+384 and T.sub.I/X are 
passed via lines 224 and double pole double throw switch 226 (or the 
logical equivalent of such) to the TRP Select lines 186 (FIG. 15) 
associated with switch 184. In group 1 stations switch 226 is fixed in the 
upward position illustrated in FIG. 16. In group 2 stations switch 226 is 
fixed in the down position opposite to the up position shown in FIG. 16. 
In the up position the signals transferred by switch 226 to TRP SELECT 
lines 186 (FIG. 15) cause the switch 184 (FIG. 15) to transfer from 
position A to position B at reception times t0-128 and from position B to 
position A at reception times T.sub.I/X ; thereby enabling group 1 
stations to receive group 1 signals (carried on fr1) after t0 and group 2 
signals (carried on fr2) after T.sub.I/X +128. When fixed in the down 
position switch 226 passes signals to TRP Select line 186 transferring 
switch 184 from position B to position A at t0+384 and from position A to 
position B at T.sub.I/X ; thereby enabling stations in group 2 to receive 
group 2 signals (carried on fr2) after t0+512 and group 1 signals (carried 
on fr1) after T.sub.I/X +128. Since t0+384 occurs after arrival of the 
useful information of the FRB the stations in group 2 will also receive 
the FRB sent by the reference station of group 1. Since the FRB occupies 
the time between t0 and t0+512 the "pad" space of 128 bits between t0+384 
and t0+512 allows time to complete the switchover transition. A similar 
transition time for switchover should be allowed relative to T.sub.I/X. 
Consequently the signal associated with T.sub.I/X on lines 186 (FIG. 15) 
should precede the arrival time of useful information in the CROSS GROUP 
intervals (T12 and T21) by at least 128 bits. 
TRAFFIC BURST RECEPTION 
Traffic burst reception is illustrated in FIG. 18. Circuits 240 operate to 
recover the IN GROUP assignment data 136 (FIG. 11) in the received group 
assignment bursts G-AB. Circuits 240 may be integrated in the common 
control system 60 (FIG. 4). The assignment data recovered by the circuits 
240 is applied to timing circuits 242 which generate reception apertures 
relative to the IN GROUP portions of the composite incoming traffic bit 
stream (i.e., in T11 or T22). 
The IN GROUP traffic reception time period spans the time space between 
t0+6656 (beginning thirteen channels after t0; see FIG. 9) and T.sub.I/X. 
Signals defining this time period are received from the receive time base 
circuits 208 (FIG. 16) IN GROUP connection data of the respective station 
is presented at 244. Such data is maintained as previously indicated by 
the common control system 60 of the associated station. This connection 
data, in combination with the IN GROUP assignment data, is sufficient to 
establish the channel portions of the incoming traffic bit stream which 
are scheduled for utilization at the respective station. The IN GROUP 
traffic stream is processed selectively through circuits gated by pulse 
outputs of circuits 242. If the traffice stream contains station control 
information in "unapertured" slots such control information may be 
recovered by circuits (not shown) sensitive to the control informaton 
signals. The specific form and mode of recovery of such control 
information is not considered relevant to the present invention, and will 
not be considered further in this description. 
The CROSS GROUP traffic bit stream is treated similarly. Processing 
circuits 246 which may be integral to the common control system 60 (FIG. 
4) detect and recover the CROSS GROUP assignment data contained in the 
CROSS GROUP assignment bursts CG-AB. Such data is applied to aperture 
generating circuits 248 which are enabled during the CROSS GROUP interval 
which extends from T.sub.I/X to t0 of the next frame. Circuits 248 
produced timed reception aperture signals for recovery of specific traffic 
slot/channel portions of the CROSS GROUP bit stream. These aperture 
signals are applied to not-shown gate circuits which operate to select out 
of the CROSS GROUP bit stream the relevant traffic information. As 
indicated previously if the incoming stream contains relevant station 
control information in unapertured slots/channels respective station 
circuits should be adapted to recover such control information separately. 
BURST TRANSMISSION 
Burst transmission involves a process of synchronization acquisition which 
is presently well understood in the art of TDMA communication. In the 
present system synchronization acquisition is acquired in three phases: 
reception acquistion, IN GROUP transmission acquisition and CROSS GROUP 
transmission acquisition. 
At system start-up time the primary reference station in group 1 begins to 
cyclically transmit FRB's on ftl keyed to an internal frame clock. While 
doing so the primary reference station reception circuits monitor the 
signals returning on FR1 for FRB's. When FRB signals are detected the 
timing of the receive apertures is adjusted to correct for doppler effects 
until the incoming FRB's are appropriately "centered" in time. 
The propagation delay of FRB's, from transmission to reception, is 
monitored by not-shown common control circuits of the primary reference 
station and used to calculate a delay deviation factor relative to the 
nominal propagation delay for that station for the particular time of day. 
The primary reference station includes the delay deviation factor and time 
of day information in its outgoing FRB's. It also includes crossover time 
data associated with t.sub.I/X in its FRB's. Initially the crossover time 
may be set arbitrarily (e.g., at the midpoint of the frame). 
Stations other than the primary reference station may acquire reception 
synchronism by recovering the primary reference FRB information in 
circuits 250 (FIG. 17) and register 218 (FIG. 16). The switch 184 (FIG. 
15) may be positioned initially to pass only signals carried on frl, until 
the FRB's being sent by the primary reference station are being detected 
repeatedly in successive frame periods in a stable mode. Thereafter the 
switch 184 may be operated in the "normal" alternating mode described 
previously; transferring to the position for CROSS GROUP reception at 
T.sub.I/X and to the position for intra-group reception at the associated 
group time (t0-128 in group 1 stations and t0+384 in group 2 stations). 
Using the delay deviation, time of day and crossover time information in 
the FRB a station seeking to acquire synchronization for burst 
transmission operates its transmission timing circuits 254 (FIG. 17) to 
send "self-synchronizing" signals in a predetermined IN GROUP slot 
assigned to that station. Initially such self-synchronizing signals are 
sent in a traffic slot assigned to the station. After transmit 
synchronization has been achieved these signals are sent in the XRB slot 
assigned to the station. 
Not all stations in each group need be equipped for inter-group 
communication. Stations not so equipped will receive only the associated 
group frequency (frl in group 1 and fr2 in group 2) and acquire 
synchronism by detecting FRB signals passed through the associated group 
transponder. Group 2 stations operating in this manner will receive 
secondary FRB signals sent by the secondary reference station in a manner 
detailed below. 
Receiving its own self-synchronizing signals in circuits 256 (FIG. 17) a 
station seeking to acquire transmission synchronism for mixed 
communication (intra-group and inter-group) adjusts the transmission 
timing of its self-synchronizing signals to the leading edge of its 
assigned slot. The "self-synchronizing" signal is timed initially to 
occupy a central position in the assigned slot (to avoid interference with 
other slots) and thereafter adjusted incrementally in timing (in "small" 
increments) until it is consistenly positioned at the leading edge of the 
same slot (over multiple frames); whereupon the station may begin to 
utilize the XRB slot for transmission of the self-synchronizing signals, 
demand data, etc. 
The primary reference station of group 1 utilizes the XRB's of stations in 
its group to determine status and connection requirements of said group. 
The primary reference station sends data in its G-AB1 bursts (FIG. 11) 
Assigning IN GROUP traffic slots to each synchronized station in group 1. 
This data is received in circuits 258 (FIG. 17) and utilized to control 
circuits 254 for transmission of group 1 traffic information. 
The secondary reference station of group 2 begins its acquistion of 
reception synchronization detecting the primary FRB signals. When 
reception synchronization is achieved the secondary reference station may 
begin its aquisition of transmission synchronization using the delay 
deviation and time of day information forwarded by the primary reference 
station and its assigned slots on ft2, fr2. It also acquires crossover 
synchronization by monitoring the crossover time information in the 
primary FRB's. The other stations in group 2 may acquire reception 
synchronism similarly, using the primary FRB data, and thereafter acquire 
transmission synchronism; initially using assigned portions of the traffic 
space on the associated transponder as described previously to circulate 
self-synchronizing signals, and thereafter maintaining synchronism by 
using respective XRB slots to circulate self-synchronizing signals. 
Stations in both groups may acquire CROSS GROUP synchronization by 
monitoring the crossover time information in the primary reference FRB and 
switching respective switches 184 (FIG. 15) at appropriate time points as 
described previously The stations may then use CROSS GROUP assignments (in 
G-AB1 and CG-AB2 for group 1 stations and G-AB2 and CG-AB1 for group 2 
stations) to carry on inter-group communications. 
For stations not equipped for CROSS GROUP operation the above 
synchronization system may be modified as follows. Note (FIG. 10) that the 
primary reference FRB occupies the interval between t0 and t0+512 but is 
carried only on ftl, frl. Hence there is effectively a vacancy in time on 
ft2, fr2 between the same time points t0 and t0+512. This "vacant" slot 
may be used by the secondary reference station to transmit secondary FRB's 
(frame reference bursts) which are identical to previously received FRB's 
sent by the primary station (i.e., primary FRB's). Stations in group 2 
equipped to receive only fr2 will thereby receive and use data in the 
secondary FRB's and assigned XRB slots on ft2, fr2 to acquire transmission 
synchronization. 
As explained previously stations in both groups adapted for inter-group 
communication will synchronize directly to the primary FRB signals. This 
is preferred inasmuch as synchronization to the secondary FRB signals in 
group 2 stations introduces a potential double "jitter" effect relative to 
the primary reference source. However it is not essential if the sources 
of primary and secondary FRB's are sufficiently stable. In systems having 
sufficiently stable FRB sources satisfactory operation may be achieved if 
all stations in group 2, other than the secondary reference station, 
synchronize to the secondary FRB signals and the secondary reference 
station and all stations in group 1 synchronize to the primary FRB's. In 
such systems the stations in group 2, other than the secondary reference 
station, would switch to IN GROUP reception mode at t0-128 (i.e., at the 
same time as group 1 stations) and only the secondary reference station 
would switch at t0+384 . 
DEMAND ASSIGNMENT 
The present system in its preferred mode of operation utilizes two 
processes of demand assignment. In one process termed crossover time 
assignment the primary reference station determines a crossover time 
associated with T.sub.I/X which is communicated in the primary FRB. In 
another process of assignment the primary and secondary reference stations 
assign "available" time slots within the IN GROUP and CROSS GROUP periods 
delimited by T.sub.I/X to stations of respective groups. The primary 
reference station assigns slots to group 1 stations in IN GROUP time T11 
and CROSS GROUP time T12, and communicates the assignments on assignment 
bursts G-AB1. The secondary reference station assigns slots in IN GROUP 
time T22 and CROSS GROUP time T21 to group 2 stations and communicates the 
assignments on assignment bursts G-AB2. 
This procedure is characterized in FIG. 21. The primary and secondary 
reference stations receive their respective XRB transmissions at 260 and 
262, from respective group stations, and extract demand information as 
suggested at 264 and 266 respectively. The reference stations allot 
transmission time slots in respective IN GROUP and CROSS GROUP periods (of 
respective transponder frequencies ft1 and ft2) in accordance with 
existing demand as suggested at 268 and 270. The assignments are based on 
conventional algorithms for TDMA/DA operation which are not relevant to 
the present invention. The objective is to maximize utilization of the 
available time and avoid under-utilization of time by some stations while 
other stations have a need for the same time. 
This process operates recursively as indicated by return lines at 272 and 
274 to respective processes of XRB recovery. Concurrently the primary and 
secondary reference stations determine the overall utilization of IN GROUP 
and CROSS GROUP time in the respective groups as shown at 276 and 278. The 
secondary reference station utilizes signaling channels of the traffic 
bursts to send messages to the primary station as shown at 280 and 
suggested by line 281. The group 1 primary station determines the relative 
utilization of IN GROUP and CROSS GROUP time on the transponders 
associated with both groups 1 and 2. With this information the primary 
reference station determines a crossover time suitable for balanced 
utilization of both transponders. If this time is different from the time 
currently being communicated in the FRB the FRB data is updated as 
suggested at 282 and the updated crossover time information is 
communicated to all stations as suggested at 284. The crossover time is 
changed only on superframe boundaries. All assignments G-AB by the primary 
and secondary reference stations are based upon time periods delimited by 
the current (updated) crossover time. 
DESTINATION (PORT) ADDRESSING 
In the foregoing traffic signal channels may be directed to the ports 40, 
42 (FIG. 4) by means of address information in the signal channels. An 
interesting aspect of the present system is that such address signals in 
IN GROUP and CROSS GROUP time slots need not be relatively differentiated 
since slots are received only by stations in one group. 
For the situation in which group 2 stations are adapted only for 
unitransponder (one-frequency) operation it should be apparent that 
signals sent to such stations during CROSS GROUP time will originate only 
at group 2 stations and occupy only slots on fr2 which are not in use 
relative to group 1 stations. Hence common destination addressing presents 
no problem of ambiguity. 
ADAPTATION FOR MORE THAN TWO GROUPS 
The crossover time particularly technique described above extends in an 
obvious mode to serve three groups of stations using three transponder 
frequencies (ft1/fr1, ft2/fr2 and ft3/fr3). It is merely necessary to 
define three crossover times in the primary FRB; for respectively 
delimiting periods for communication between stations of the first and 
second groups, second and third group and first and third groups. 
Obviously the circuits of FIGS. 15-18 would be modified to allow for 
recovery and utilization of the three crossover time factors. 
REFERENCE STATION RECONFIGURATION 
Should a primary or secondary reference station become unavailable it will 
be desirable to be able to establish a new primary or secondary reference 
station. For this purpose any of the existing stations may be used as a 
reference station. If time synchronization is not lost the "new" primary 
or secondary reference station may begin to broadcast the FRB in the FRB 
slot after the "old" station is "silenced". A new reference station will 
also transmit a "new" group assignment burst in the appropriate burst 
assignment slot. A new secondary reference station may as indicated above 
also transmit a copy of the primary FRB in the initial (FRB) time slot 
ft2. 
ACCOMMODATION OF FUTURE SATELLITE TECHNOLOGIES 
The system described above should adapt very simply and economically to 
future satellite repeater technologies involving the use of more 
sophisticated "on-board" equipment in the satellite. 
FIG. 19 illustrates a hypothetical capability of future satellite repeaters 
for performing frequency switching on an "intelligent" basis during IN 
GROUP and CROSS GROUP periods of a basic TDMA frame shared by multiple 
groups of transceiver nodes. FIG. 19 suggests transposition of carrier 
frequencies ft1 and ft2 to carrier frequencies fr1 and fr2 respectively 
during IN GROUP periods, and to fr2 and fr1 respectively during CROSS 
GROUP periods. Ovbiously this would be the equivalent of the functions 
presently performed by the multiple earth station receivers using more 
conventional "active" satellite repeaters. 
FIG. 20 suggests "on-board" satellite "logic" for shifting the time point 
of T.sub.I/X in accordance with earth station demand. Satellite receiver 
300 passes information received by the satellite on ft1 and ft2 to 
on-board processor 302. Facilities 304 in said processor recover the FRB 
and facilities 306 recover and staticize the crossover time data contained 
in the FRB. This data is compared in compare circuit 308 to earlier 
crossover time data in register 310. When inequality exists switch 312 is 
operated to replace the contents of register 310 with the new crossover 
time data provided by facilities 306. The data in register 310 is applied 
to generator circuit 316 to produce time signals which represent the 
transition point T.sub.I/X from IN GROUP and CROSS GROUP reception periods 
"on-board" the satellite. Time base circuits 318 coupled to FRB detector 
304 provide signals corresponding to the inital frame time t0 relative to 
on-board reception at the satellite. The signals produced by circuits 316 
and 318 are applied to the satellite transmission equipment to determine 
the transition points n time for switching between IN GROUP and CROSS 
GROUP displacements of the " repeated" frequency. 
It should be apparent that only minor modifications of ground station 
transceiver equipment would be required to adapt to such on-board 
frequency shifting capability. The receiver switches such as 184 (FIG. 15) 
should be fixed in positions such that the respective station receives 
only the transponder frequency fr1 or fr2 associated with its own station 
group during both IN GROUP and CROSS GROUP intervals. Quite apparently 
future stations not equipped with switches 184 would be inherently 
adaptive to CROSS GROUP operation in such a system. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, those skilled in the art will 
recognize that the above and other changes in form and details may be made 
therein without departing from the spirit and scope of the invention.