Mechanism for deriving accurate frequency reference for satellite communications burst demodulator

Accurate tuning of a satellite system's burst demodulator to a signal which is subject to frequency offset in the course of its transmission over the satellite link is achieved without the use of high precision oscillators at remote sites, or the transmission of a dedicated pilot tone. Instead, a dedicated high precision clock is used for the purpose of establishing both the outlink carrier and the return channel carrier. The modulation of the data on the outlink carrier is also derived from the precision clock source. At each remote station, the outlink channel is monitored to recover the high precision clock. This recovered clock is then used as a reference for generating the return channel carrier. The burst demodulator equipment at the master station monitors both the outlink channel continuous carrier and burst mode transmissions from the remote stations. Each of the continuous and burst mode carriers, having been transmitted through the satellite, will undergo the same frequency offset or modification, so that the frequency difference between the outlink channel carrier and the return link channel carrier will always be constant. The outlink channel carrier, which is available on a continuous basis, is used to derive a local oscillator frequency reference for establishing the burst demodulator's local oscillator reference.

FIELD OF THE INVENTION 
The present invention relates in general to satellite communication systems 
and is particularly directed to a mechanism for providing an accurate 
frequency reference for enabling a burst demodulator to recover 
information signals from a carrier that has been subjected to a frequency 
offset during its transmission over the satellite communication link. 
BACKGROUND OF THE INVENTION 
The successful operation of satellite communication networks, such as 
demand assignment (contention) systems, depends upon the ability of the 
receiver equipment at the respective stations of the network to be 
accurately tuned to the incoming signal from another site. In a demand 
assignment communications scheme, messages from respective contention 
participants of the network are transmitted in a burst format, in which a 
respective station's carrier is turned on for an abbreviated period, or 
time slot, during which a message (e.g. data packet) is transmitted and 
then turned off until that station has a new message to send. 
Because of the intermittent nature of burst mode communications and carrier 
distortion introducing characteristics of the satellite channel, the 
ability to provide an accurate demodulation reference frequency for signal 
recovery becomes a significant problem. One way to solve the problem is to 
provide each station with a high precision oscillator which monitors an 
effectively perfectly stable pilot frequency, that has been transmitted 
from a master site, in order to determine frequency offset through the 
satellite link and to use this information to accurately control the 
characteristics of its burst carrier, so that the message, when received 
at the master site, will be effectively precorrected, permitting 
demodulation and data recovery. The problem with this approach is 
two-fold: on the one hand, the cost of the equipment at each 
burst-sourcing site is increased substantially by the provision of the 
high precision reference oscillator. In addition, because the network uses 
a pilot tone for the purpose of frequency correction, the capacity of the 
satellite link (an extremely precious resource) for data transmission is 
reduced. 
One proposal to eliminate part of the problem, namely to reduce the expense 
of the equipment (high precision oscillator) at the burst-sourcing site is 
described in U.S. Pat. No. 4,509,200 to Luginbuhl et al, entitled 
"Satellite Telecommunications System". Pursuant to the patented scheme, a 
high precision pilot tone oscillator is installed at the master or central 
station, the only purpose of which is to measure frequency offset (drift) 
through the satellite. By monitoring the pilot tone over a loop back to 
itself, the master station is able to measure the offset through the 
satellite, which, as pointed out above, must be corrected. A signal 
representative of the value of this measured offset or error is then 
transmitted as an information signal for use at each remote site. The 
remote site, which does not have the benefit of the high precision 
oscillator, extracts the data to properly tune itself. This operation 
presupposes that the remote site is properly tuned to begin with, 
something that the coarse oscillator used at the remote site cannot 
guarantee. Consequently, the proposed procedure is questionable at best. 
Of course, due to the fact that the patented scheme dedicates part of the 
satellite link to the offset-controlling pilot tone, the resource 
occupancy problem still exists. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the ability to accurately tune a 
demodulator to an intermittently transmitted (burst) signal, which is 
subject to frequency offset in the course of its transmission over the 
satellite link, is achieved without either of the above-mentioned 
conventional artifices, including the installation of high precision 
oscillators at the remote sites, or the transmission of a separate, 
dedicated pilot tone for the purpose of correcting the problem of 
frequency offset through the satellite link. 
Pursuant to the present invention, a single, dedicated high precision clock 
is used for the purpose of establishing both the outlink carrier from the 
master station to each of the remote stations and the return channel 
carrier through which each remote station transmits burst messages to the 
master site over the satellite link. In addition, the modulation of the 
data on the outlink carrier is derived from the same precision clock 
source. At each remote station, the outlink channel is monitored to 
recover the high precision clock. This recovered clock is then used as a 
reference for generating the return channel carrier. 
Rather than attempt to correct or premodify either the outlink channel 
continuous carrier or the return link channel burst carrier, the system 
permits both channels to be subjected to the drift or offset through the 
satellite. The burst demodulator equipment at the master station monitors 
both the outlink channel continuous carrier (that has been transmitted 
through the satellite and has thereby been subjected to the satellite link 
offset) and the incoming burst mode transmissions from the remote 
stations. Each of the continuous and burst mode carriers, having been 
transmitted through the satellite, will undergo the same frequency offset 
or modification. Consequently, the frequency difference between the 
outlink channel carrier and the return link channel carrier will always be 
constant. The outlink channel carrier, which is available on a continuous 
basis, is used to derive a local oscillator frequency reference for 
establishing the burst demodulator's local oscillator reference. As a 
consequence, regardless of any drift through the satellite, the burst 
demodulator is always referenced to a local oscillator signal that tracks, 
precisely, any variation in the burst carrier. 
Pursuant to a further feature of the present invention, advantage is taken 
of the fact that the outlink channel carrier is modulated with a timing 
signal that governs the occurrence of contention time slots that are used 
by the remote stations to send messages over the return link channel to 
the master station. The availability of this timing signal and the ability 
to adjust the operational frequency characteristics of the programmable 
frequency synthesizers for each of a continuous channel section (from 
which the `continuous` local oscillator frequency reference is obtained) 
and a burst channel recovery section (from which the burst demodulator's 
reference is obtained) of the burst demodulator facilitates substitution 
of a redundant or backup unit in place of another unit.

DETAILED DESCRIPTION 
Before describing, in detail, the particular improved burst demodulator 
frequency reference derivation mechanism in accordance with the present 
invention, it should be observed that the invention resides primarily in a 
novel structural combination of conventional communication circuits and 
components and not in the particular detailed configurations thereof. 
Accordingly, the structure, control and arrangement of these conventional 
circuits and components have been illustrated in the drawings by readily 
understandable block diagrams which show only those specific details that 
are pertinent to the present invention, so as not to obscure the 
disclosure with structural details that will be readily apparent to those 
skilled in the art having the benefit of the description herein. Thus, the 
block diagram illustrations of the figures do not necessarily represent 
the mechanical structural arrangement of the exemplary system, but are 
primarily intended to illustrate the major structural components of the 
system in a convenient functional grouping, whereby the present invention 
may be more readily understood. 
Referring now to FIG. 1, a diagrammatic illustration of a satellite 
communications network employing the burst demodulator frequency reference 
derivation mechanism in accordance with the present invention is 
illustrated as comprising a master or central station 10 which 
communicates by way of a satellite 20 with a plurality of remote stations, 
an individual one of which 30 is shown in the Figure. In effect, the 
network may be considered to be what is normally referred to as a 
star-configured satellite communications network, with the hub of the star 
corresponding to master station 10 and the points of the star 
corresponding to the locations of the remote stations 30. For purposes of 
providing an illustrative example, the network will be assumed to be a Ku 
band system, the outlink channel (master-to-remote) carrier and return 
link channel (remote-to-master) carrier frequencies of which are on the 
order of 14 GHz up to the satellite and 12 GHz down from the satellite. 
Preferably, the communications control mechanism is of the type described 
in copending U.S. patent application Ser. No. 236,756, filed Aug. 26, 
1988, entitled "Link Utilization Control Mechanism for Demand Assignment 
Satellite Communications Network" by E. Gerhardt et al and assigned to the 
assignee of the present application, the disclosure of which is 
incorporated by reference herein. However, it should be observed that the 
present invention is not limited to use with this or any other particular 
type of system, but is applicable to any communication system employing 
burst communications that is subject to a frequency offset over the 
channel. 
In the exemplary system, the outlink channel carrier, which is continuously 
transmitted from master station 10, is modulated with a network timing 
reference for establishing the occurrence of contention timeslots during 
which remote stations 30, each of which continuously monitors the outlink 
channel for messages directed to it from master station 10, transmit 
(burst) messages to the master station over the return link channel. 
Namely, each of remote stations 30 transmits messages to master station 10 
in a demand assignment or contention burst-mode format through the 
satellite 20 by way of the dedicated remote-to-master return link. 
As mentioned briefly above, a burst reference derivation equipment of the 
present invention obviates the need for installing a high precision 
oscillator at each of the remote sites and avoids the necessity of having 
to pre or post correct the frequency offset through the satellite 20. For 
this purpose, the network employs only a single precision reference 
oscillator, located at the master site, through which the outlink channel 
carrier and the return link channel carrier are generated. 
More specifically, at the master station, a baseband clock signal (e.g. 5 
MHz) from a precision source is applied over link 111 to the clock input 
of a (BPSK) digital data modulator 101 and to the reference input of an IF 
translation (up-converter) unit 103. Modulator 101 contains conventional 
frequency reference converter circuitry (frequency multiplier/phase locked 
loop components) for clocking input data on link 113 in accordance with a 
prescribed baud rate (e.g. 112 kb/s) for application to IF up-converter 
103, which is also controlled by high precision 5 MHz clock from link 111. 
Up converter 103 is of conventional configuration, multiplying the high 
precision 5 MHz clock to convert the baseband signal (112 kb/s) to an 
intermediate frequency on the order of 140 MHz. This IF signal is then 
applied to RF transceiver unit 105 which translates the IF signal up to Ku 
band (14 GHz) for application to RF antenna 107 and transmission over the 
outlink channel through the satellite 20 to each of the remote sites 30. 
Each remote site 30 includes an antenna dish 301 and associated RF 
transceiver unit 303 for receiving outlink channel messages and for 
transmitting return link burst messages. The receive output of RF 
transceiver unit 303 is coupled to RF-IF down-converter 305 which outputs 
an IF signal (e.g. at an IF frequency of 950-1450 MHz) to modem 307. Modem 
307 includes a demodulator section which operates off a 50 MHz clock 
source (driving a phase lock loop) to generate a reference 112 KHz for 
recovering the incoming data stream. As mentioned previously, rather that 
employ an expensive precision oscillator to generate the modem reference, 
the remote site derives its reference from the highly accurate clock 
through which the data is modulated and through which the outlink IF 
frequency is produced. 
For this purpose, modem 307 contains conventional a phase locked loop clock 
recovery circuit for recovering the 112 kHz clock contained within the 
incoming data stream. In addition to using the recovered clock for data 
demodulation, this same recovered precision clock is fed to the modulator 
section of modem 307, where it is used as a reference to a narrow 
bandwidth phase lock loop that is driven by an otherwise less precise 
local oscillator (e.g. 50 MHz). This local oscillator is used by the 
up-conversion section of IF translator unit 305 for providing a highly 
accurate local oscillator reference through which data modulation and 
frequency translation (from baseband to an IF frequency on the order of 
950-1450 MHz) are carried out. Thus, because the modulation reference 
frequency is derived from a precision source (located at the master 
station), it is unnecessary to install a separate high precision reference 
frequency oscillator at each remote station. The up-converted signal is 
output from IF stage 305 to transceiver 303 for transmission on the return 
link channel (carrier frequency=14 GHz). 
It should be noted that the frequency offset through the satellite is not a 
problem for data recovery at the remote stations since the outlink channel 
carrier frequency is continuously transmitted and the high precision 
reference (5 MHz) clock signal from which the carrier is derived is used 
for modulating the data, which is unaffected by the frequency offset 
through the satellite. The problem to which the present invention is 
directed, on the other hand, is the fact that messages from the remote 
stations to the master station are burst mode, rather than continuous mode 
transmissions, so that a data recovery reference frequency that may be 
used by the master station to demodulate received burst traffic is not 
continuously available from the remote stations. 
In accordance with the present invention, however, advantage is taken of 
the fact that the outlink channel carrier is continuously available to the 
master station and undergoes the same offset through the satellite to 
which burst mode transmissions on the return link channel are subjected. 
Because burst mode transmissions on the return link channel and continuous 
mode transmissions on the outlink channel undergo the same offset through 
the satellite, regardless of the frequency differential between the 
outlink channel and the return link channel, the difference between the 
two is always constant, regardless of the magnitude of the offset through 
the satellite (which will vary with time). In accordance with the present 
invention, this constant differential characteristic is employed at the 
master site to derive a reference recovery frequency for a burst mode 
demodulator and thereby obviate the need for transmitting a separate pilot 
tone for correcting the offset through the satellite. 
More particularly, as shown generally in FIG. 1 and in detail in FIG. 2, 
the master station employs a burst mode demodulator 121, coupled to a dual 
RF-IF down-converter stage 115, which down-converts the outlink and return 
link channels from the 12 GHz band to the 950 to 1450 MHz band. The burst 
mode demodulator monitors the IF (950-1450 MHz) outputs (burst and 
continuous mode outputs) of transceiver 105 and provide a pair of further 
down-converted (e.g. 52-88 MHz) outputs on link 123. For purposes of an 
illustrative example, within the passband of interest as output, the 
continuous mode signal may have an IF frequency of a 77.6 MHz, while the 
burst mode signal may have an IF frequency of 77.4 MHz. 
As shown in FIG. 2, demodulator 121 includes a continuous frequency 
reference recovery section 201 and a burst frequency reference recovery 
section 211. Both sections are coupled to link 123, so that each section 
receives the pair of IF frequencies (77.4 and 77.6 MHz) from IF 
down-converter 115. Link 123 is coupled to first inputs of mixers 204 and 
214, second inputs of which are respectively coupled to the outputs of 
frequency synthesizers 203 and 213. Each of frequency synthesizers 203 and 
213 is adjustable in 100 kHz steps over a 36 MHz bandwidth and is driven 
by a local clock reference supplied over link 112. Link 112 is coupled to 
a frequency reference 206 on the order of 12.8 MHz, in response to which 
synthesizer 203 produces a precision output of 88.3 MHz, and synthesizer 
213 produces a precision output of 88.1 MHz. 
For the above discussed outlink and return link channel separation of 200 
KHz (77.6 MHz-77.4 MHz), each of mixers 204 and 214 will produce an output 
of 10.7 MHz in response to the 88.3 MHz and 88.1 MHz signals generated by 
synthesizers 203 and 213, respectively. The first, continuous reference 
frequency of 10.7 MHz produced at the output of mixer 204 is coupled 
through a Costas loop 205, which produces an output frequency on link 207 
that effectively tracks frequency variations of the continuous outlink 
channel carrier. Costas loop 204 operates at a multiple of the 10.7 MHz 
produced by mixer 204 (e.g. four times the input 10.7 MHz rate at 42.8 
MHz). Output link 207 is coupled to a divide-by-four divider 221 which 
controls the operation of a bandpass filter 217 within burst recovery 
section 211. 
Within the burst recovery section 211, the output of mixer 214 is connected 
to a X2 multiplier 215. As in conventional modulator design, the output of 
the X2 multiplier is a signal at twice the IF frequency which is 
effectively stripped of the BPSK modulation on the IF signal. To be 
employed as a reference for the demodulator, it is necessary to bandpass 
filter this signal, and to then divide it by two, back to the 10.7 MHz 
frequency where it is available to provide a reference signal for 
demodulating the burst data signal. The output of X2 multiplier 215 is 
therefore coupled to the input of a tracking bandpass filter 217. Tracking 
bandpass filter 217 includes a splitter 223 to which respective in-phase 
(I) and quadrature (Q) channel filter sections are coupled. The in-phase 
filter section includes a mixer 225 to which the output of splitter 223 
and the in-phase component of the 21.4 MHz output of divider 221 is 
coupled. The output of mixer 225 is coupled to a low pass filter 231, the 
output of which is coupled to a further mixer 235 which is driven by the 
21.4 MHz output of divider 221. The quadrature channel includes a mixer 
227, which is coupled to the quadrature output of 223 and is driven by the 
quadrature component of the 21.4 MHz output of divider 221. The output of 
mixer 227 is coupled through a lowpass filter 223 to a further quadrature 
mixer 237, which receives the 21.4 MHz output of divider 221. The outputs 
of mixers 235 and 237 are summed and then coupled to a divide-by-two 
divider 241, the output of which is the actual 21.4 MHz reference to be 
used for recovery of burst channel data. Each of low pass filters 231 and 
233 has a bandwidth that is as narrow as possible, so as to recover the 
10.7 MHz reference for demodulating the burst signal with as high a signal 
to noise ratio as possible. If fixed low pass filters were to be used for 
elements 231 and 233, their bandwidth would have to be at least twice the 
total frequency offset or drift through the satellite and down converter. 
By using tracking filters for elements 231 and 233, the possibility exists 
of using a narrower filter bandwidth, provided that the tracking filter 
has a control mechanism to allow it to properly track the incoming burst 
signal. It is for this reason that the reference signal from the outlink 
demodulator is generated (at 21.4 MHz). This reference signal exactly 
tracks any variation of the frequency of the burst signal. The bandwidth 
of the tracking filter can therefore be made much narrower than the 
frequency offset or drift through the satellite and downconverter. Its 
bandwidth can then be freely chosen to optimize the data recovery 
mechanism of the burst demodulator, (i.e. the bandwidth of the filter may 
be made just wide enough so that filter transients settle out during the 
burst preamble time.) The 21.4 MHz output of tracking filter 217 is 
divided by two, so that it may be used in the data recovery process. 
Demodulation of the data is effected by coupling the 10.7 MHz reference 
output of divider 241 to one input of a mixer 243 the second input of 
which is coupled to receive the burst modulation signal produced at the 
output of mixer 214. The output of mixer 243 is coupled to a lowpass 
filter 245 from which the burst data is recovered. 
In operation, each return link channel burst transmission from a remote 
station that passes through the satellite and is thereby subjected to its 
associated frequency offset or drift is accompanied by an outlink channel 
frequency that is being continuously transmitted and monitored by the 
master station's receiver equipment. Specifically, continuous frequency 
reference recovery section 201 produces an output frequency at twice the 
10.7 MHz reference frequency, variations in which (as a result of the 
frequency offset through the satellite repeater) are the same as those of 
a burst mode signal on the return link channel whose IF signals are 
coupled to burst frequency reference recovery section 211. The center 
frequency of bandpass filter 217 of burst section is thereby controlled by 
a 10.7 MHz reference that tracks the 10.7 MHz component of the burst 
signal, so that an accurate reference for recovering the data from the 
incoming burst IF signal may be obtained. 
As pointed out above, with the satellite communications network typically 
servicing a multiplicity of users, the master station will normally 
contain a plurality of transceiver equipments (of the type 
diagrammatically shown in FIGS. 1 and 2), and, in addition, contain one or 
more back-up or redundant units to be substituted or switched over in 
place of a previously on-line unit in the event of a malfunction or 
failure. Conventionally, substitution of redundant units has been 
accomplished by a (hardware intensive) controlled interconnect arrangement 
containing auxiliary switching components and transmission lines between 
dedicated redundant unit(s) and each on-line equipment. However, pursuant 
to a further feature of the present invention, by virtue of the 
availability of a system timing signal on the outlink channel carrier and 
the adjustability or programmability of frequency characteristics of the 
burst demodulator, substitution of a new unit simply requires an 
appropriate adjustment of the settings of the frequency synthesizers 
203/213 (and a synthesizer for IF translator 305 which may be physically 
installed within the modem) in the redundant demodulator unit (configured 
in the manner shown in FIG. 2) and then placing that unit on line, while 
disabling the unit to be taken out of service, in accordance with the 
system timing signal that is modulated onto the outlink channel carrier. 
More particularly, as diagrammatically shown in FIG. 3, master station 10 
may contain a plurality of transceiver units TU1, TU2, . . . , TUn, each 
of which comprises the continuous demodulator and burst demodulator 
sections described above with reference to FIGS. 1 and 2. In addition, as 
redundant equipment, the master station includes one or more auxiliary 
burst demodulators ABD of the type shown in FIG. 2, coupled in tandem with 
the burst demodulator in each of the transceiver units TU. Normally, the 
auxiliary burst demodulator ABD unit is off-line or in a quiescent state. 
However, in the event of the need to make an equipment substitution for an 
on-line demodulator, the master station communications control processor, 
through which the operation of the master station's communication 
equipment is controlled (as described in the above-referenced copending 
application), adjusts the frequency parameters of synthesizers 210 and 213 
and then places the redundant demodulator unit on-line in synchronism with 
the system timing signal. At the same time it disables the unit to be 
removed from service. In other words, because of the availability of a 
network time slot-defining timing signal and the ability to adjust the 
frequency control parameters of the frequency synthesizer of each burst 
demodulator at the master station, the need for complex interface 
transmission lines and switching circuitry for effecting a backup 
replacement of the demodulator is avoided. 
As will be appreciated from the foregoing description, the ability to 
accurately tune a burst demodulator to an intermittently transmitted 
(burst) signal, which is subject to frequency offset in the course of its 
transmission over a satellite link, is achieved in accordance with the 
present invention without the use of conventional mechanism, such as the 
installation of high precision oscillators at the remote sites, or the 
transmission of a separate, dedicated pilot tone for the purpose of 
correcting the problem of frequency offset through the satellite link. 
Pursuant to the invention, through the use of a single, dedicated high 
precision clock for establishing both the outlink channel carrier and the 
return channel carrier and a novel dual continuous mode/burst mode 
demodulator configuration, it is possible to provide a burst recovery 
reference signal that is optimally filtered independent of frequency 
offset through the satellite. 
In addition, the availability, at the master station, of a system timing 
signal and the ability to adjust the operational frequency characteristics 
of the burst demodulator's frequency synthesizers for each of a continuous 
channel section, from which the `continuous` local oscillator frequency 
reference is obtained, and a burst channel recovery section, from which 
the burst demodulator's reference is obtained, facilitates substitution of 
a redundant or backup unit in place of another unit. 
While we have shown and described an embodiment in accordance with the 
present invention, it is to be understood that the same is not limited 
thereto but is susceptible to numerous changes and modifications as known 
to a person skilled in the art, and we therefore do not wish to be limited 
to the details shown and described herein but intend to cover all such 
changes and modifications as are obvious to one of ordinary skill in the 
art.