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RELATED APPLICATION
Related subject matter is disclosed in the following application filed
concurrently herewith and assigned to the same Assignee hereof: U.S.
patent application Ser. No. 087,725 entitled "Radio Communication System
Having Autonomously Selected Transmission Frequencies", inventor J. W.
Smith.
TECHNICAL FIELD
This invention relates to a method and apparatus for synchronously
operating a radio communication system which utilizes a frequency hopping
mode of frequency transmission.
BACKGROUND OF THE INVENTION
The FCC has recently made the 902-928 MHz band and two higher frequency
bands available under Part 15 rules for low-power communications devices
with the proviso that spread-spectrum (SS) modulation be used. One SS
technique permitted is frequency hopping in which information is sent
using a sequence of carrier frequencies that change at set times to
produce a narrow band signal that bounces around in center frequency over
the available spectrum. The FCC specified that at least 75 hopping
frequencies, separated by at least 25 kHz, must be used for each
communication channel and the average time of occupancy on any frequency
must not be greater than 0.4 seconds within a 30-second period.
Since the 902-928 MHz band contains only 1,040 frequency bands of 25 kHz
each, only six simultaneous, independent, two-way links (2 channels) could
be supported without interference among the channels. This is because each
channel requires at least 75 of the 25 kHz frequency bands for its
frequency hopping transmission. A problem arises in applications where a
communication system requires more than six communication links in a small
geographic area. While directional antennas may be utilized to isolate
groups of stations using the same frequencies, interference may still
result.
SUMMARY OF THE INVENTION
In accordance with the present invention, the foregoing problem is solved
using the apparatus of the disclosed frequency-hopping radio communication
system. The system comprises a control unit which transmits to and
receives from each of a plurality of stations using a first group of
hopping frequencies. During a start-up mode, the control unit communicates
a starting message to each slave station using a unique preassigned
frequency selected from a second (start-up) group of frequencies. This
message identifies to each slave station a unique transmitting and
receiving starting frequency. The message also identifies to each slave
station a frequency-hopping sequence to be used to select the frequencies
from the frequency group for transmission to and reception from the
control unit. This ensures that none of the stations use the same
frequencies at the same time for transmission to and reception from the
control unit. Since all the control unit transmitters are locked to a
common clock and all of the stations are locked to the control unit
transmissions, all transmitters at the control unit and stations hop in
synchronism.
BRIEF DESCRIPTION OF THE DRAWING
In the drawings,
FIG. 1 shows a block diagram of a communication system useful for
discussing the operation of the present invention;
FIG. 2 shows a block diagram of a transceiver circuit used to establish a
radio link between the control unit and station locations;
FIG. 3 shows a flow chart of the start-up and communication sequences
utilized at the control unit transceiver location;
FIG. 4 shows a flow chart of the start-up and communication sequences
utilized at the station transceivers;
FIG. 5 shows an illustrative algorithm for selecting start-up and
communication frequencies for the system of FIG. 1;
FIG. 6 shows an illustrative frequency allocation format for use in the
present invention; and
FIG. 7 shows the data format for start messages and system communications.
DETAILED DESCRIPTION
In the following description, each item of each figure has a reference
designation associated therewith, the first number of which refers to the
figure in which that item is first located (e.g., 110 is located in FIG.
1).
Shown in FIG. 1 is a communication system incorporating the present
invention. The system includes a master station or control unit 190
including control module (CM) 100 and transceivers 130 and 180. The CM 100
controls communications made via central office (CO) or private branch
exchange (PBX) lines 101, and via a wire facility or link 111 to station
set 110 and via radio links to slave station sets 120 and 160.
Control module 100 may, illustratively, include a central processor unit
(CPU) 102, program memory 103, data memory 104 and interface circuits (105
and 106). Briefly, control module 100 establishes and controls all
intercom and CO or PBX line communications. Program memory 103 provides
instructions to central processor unit (CPU) 102 for controlling the
various operating features and functions of the system. Data memory 104 is
utilized by the CPU for storing and accessing data associated with
performing the various functions and features programmed in program memory
103. In one embodiment, CPU 102 is a microprocessor, program memory 103 is
read-only-memory (ROM) and data memory 104 is random access memory (RAM).
The interface circuits 105 and 106 contain well-known switching, network
control, and line circuits required by the system to establish, maintain
and terminate communications. Interface circuit 105 also contains
circuitry for interfacing control module 100 to transceivers 130 and 180
to provide radio links, respectively, to stations 120 and 160.
One radio link includes two transceivers 130 and 140 for
transmitting/receiving data between CM 100 and station set 120. A second
radio link uses transceiver 180 and 170 for communications between CM 100
and station set 160. A radio link may be utilized when a wire or optical
cable facility is not desired or practical to interconnect CM 100 to a
station set. Transceiver 130 may include two antennas 131, 132 that are
switch selectable for space diversity and are operated under control of CM
transceiver 130.
According to one aspect of the present invention, one of the two antennas
(131 or 132, 181 or 182) at each CM 100 transceiver (130, 180,
respectively) is selected for radio transmissions based on the quality of
the radio transmission signal characteristics between CM 100 and the
particular station location. Thus, for example, if a weak signal from
transceiver 130 exists when antenna 131 of transceiver 130 is used,
transceiver 130 circuitry switches to antenna 132 in an attempt to
increase transmission quality. Transceiver 130 would select the antenna
which results in the better signal transmission from station set 120. Such
a situation may occur because of interference caused by physical objects
such as cabinets, doors, etc. or by noise or other interference signals.
A variety of communication systems having different operating
characteristics can be accommodated using the arrangement of FIG. 1. In
one embodiment, the communication system of FIG. 1 may be used for voice
and data signal transmissions. In such an application, while the
information signal (voice or data) may require a non-continuous
transmission, the control signalling (i.e., on-hook, off-hook button
depression signaling, etc.) usually requires a continuous transmission.
Shown in FIG. 2 is an illustrative block diagram of one embodiment of a
control module transceiver (e.g., 130) and a station transceiver (e.g.,
140) for use in the system of FIG. 1. Only those circuit blocks useful in
understanding the operation of the present invention are described.
Obviously, CM 100 may include the CM transceiver apparatus 130 and 180 and
the station terminals 120 and 160 may include station transceiver
apparatus 170 and 140, respectively. It should also be noted that the
basic operation of CM transceiver and of station transceiver are very
similar and utilize identical circuit blocks for their implementation.
Identical components of both transceivers are identified with the same
numbers except that the letter "A" follows numbers of station transceiver
140.
Voice and control information for the remote station are received over
facility 133 and interface 201 to produce a narrow baseband signal 221
about 6 kHz in bandwidth. Under normal circumstances, the baseband signal
221 is passed through switch 202 and low pass filter 203 to modulator 204.
The baseband signal modulates a carrier frequency 227 that is produced
from frequency source 207. The radio frequency (RF) output 224 of
modulator 204 is coupled to the antennas 131, 132 through coupler 211.
The radio signal from the remote station transceiver 140 is picked up by
the antennas 131, 132 and RF output 231 of the coupler 211 is fed to the
mixer 210. The mixer multiplies the received RF signal 231 with the
carrier frequency 227 provided by the frequency source 207. The same
frequency source 207 is used for both transmitter and receiver to reduce
costs. As will be discussed later, the transmitted and received signals
differ in frequency by K(K=13 MHz) which is the intermediate frequency,
IF. The mixer output 230 is filtered by IF filter 209 and demodulated by
demodulator 208 to a baseband signal 228. The baseband signal 228 is fed
to the voice/signaling interface 201 where the voice and control
information are separated. These signals are passed to the control module
100 in the appropriate form over facility 133. In the few millisecond
interval around a frequency transition, the voice and control information
are inhibited by the interface 201.
Transceiver sequence controller 206 includes a microprocessor which uses
the frequency-hopping sequence to select the carrier frequency for any
time slot. The timing clocks 205 and the synchronization signal received
every 30 secs from the control module 100 provide timing for the sequence
controller 206. The possible frequency-hop sequences are stored in a
memory of 206 and one sequence is selected by software or hardware
settings. This setting is translated to frequency identifying signals 226
that are passed to the frequency source 227.
In accordance with one aspect of the present invention, start-up
synchronization is achieved by using a single, predetermined dedicated
frequency (unused by any other channel either at start-up or during
hopping) for each transmitter. With reference to FIG. 3, a typical startup
310 and communication 320 sequence at a CM transceiver (e.g., 130) is
described. All stations are idle on power-up (301) and listen for a poll
302 from the CM transmitter. The poll is a short "are you ready" message
at the dedicated frequency, which could be repeated at 1-second intervals
while satisfying FCC rules. The stations respond with a ready message 303
on their frequency. The CM then sends a start message 304 on its frequency
to each station.
The start message of step 304 is shown illustratively as 710 of FIG. 7. The
start message 710 may include header 711 to sync the station to CM 100,
frequency group number 712, FH sequence 713 to be used by that station,
starting frequency 714 and an error detection code 715 for detecting error
in the start message. The start message 710 is transmitted to each station
of the system using an assigned start-up frequency.
The starting frequency 714 may actually be a code which specifies at which
frequency in the hop sequence (a number from 0 to 74) a station is to
begin transmitting. Thus, with reference to FIG. 5 the start frequency
(i.e., at time T0) for station S2 is shown in line 503 as SF1. Note, since
more than one frequency-hopping (FH) sequence may be utilized at the
control module and station transceivers, the control module specifies the
particular FH sequence 713 (or a code representing the FH sequence) to be
utilized at each station. With reference to FIG. 5, the FH sequence
illustrated is one which sequences through frequencies SF0-SF74. The
possible FH sequences can be previously stored at each station (in memory
associated with 206A) and identified and established using a FH code 713
specified by control module CM 100. In FIG. 5, while the FH sequence shown
for the stations (SF0-SF74) is the same as that shown for CM (CF0-CF74)
with a frequency offset K, different FH sequences could be used. A
frequency group number 712 can be utilized to identify one of a number of
predefined groups of frequencies to be utilized for communication (e.g.,
530 of FIG. 5). An example of a frequency-hopping sequence will be
described in detail in a later paragraph.
The start message process of step 304 is repeated with each station
transceiver which has responded with a ready message. Each station is
assigned a unique starting frequency in the hopping sequence at which to
begin normal communication with its associated CM transceiver.
With reference to FIG. 2 again, during the start-up mode, voice and
signaling are blocked by switch 202 and controller 206 provides the
signals 226 to the modulator. Moreover, controller 206 inhibits the output
voice and signaling information to facility 133 using the v/s interface
201.
In step 305, sequence controller 206 of each CM transceiver uses the start
frequency code 714 and FH sequence 713 of start message 710 to select the
proper start frequency and FH sequence for communication with its
associated station transceiver. Each CM and station begin normal
communication 305 at the assigned start frequency in the hop sequence
within a few milliseconds of each other. After 400 ms have elapsed, each
CM transceiver hops to the next frequency in the sequence, 306.
If either end of a radio link (e.g., transceivers 130, 140) loses sync or
signal, transmission stops and the system goes into the start-up mode for
that link. The CM then initiates the start-up sequence 301 to the station
involved in the loss of sync or signal (e.g., station 120).
The following station transceiver description references FIGS. 2, 4 and 7.
The station transceiver operation 140 is identical to the control module
transceiver 130 except that there is no sync signal 225 from an external
source. The station transceiver 140 is a slave to the control module
transceiver 130 and receives sync information via the radio channel. On
power-up or loss of signal 401, controller 206A inhibits radio
transmission 402 and listens for an "are you ready" poll message 403 on
the initialization frequency. When the message is received, the controller
206A sends a "ready" message 404. The "start" message 710 received from
the control module transceiver 130 in step 405 is used by sequence
controller 206A to identify the start frequency 714 for transmissions, the
frequency-hopping sequence 713, and the frequency group 712.
At the station transceiver, the memory associated with sequence controller
206A stores a plurality of frequency groups and frequency-hopping
patterns. The start message 710 is decoded by the sequence controller 206A
and used to identify the particular frequency group and frequency-hopping
pattern which has been selected by CM 100 for use at that station. In
accordance with the present invention, sequence controller 206A uses the
start message 710 and a stored algorithm to identify the start frequency
for receptions from the CM transceiver 130 and the frequency-hopping
pattern to be used for receptions. In the example to be discussed in a
later paragraph, the algorithm used to identify the start frequency for
receptions merely adds K MHz to the start frequency used for transmissions
(as identified by 714). The algorithm also specifies that the
frequency-hopping sequence (as identified by 713) for both receptions and
transmissions should be the same.
Returning to FIG. 4, in step 406 normal transmission and reception begin at
the respective start frequencies. Subsequent frequency hops 407 occur when
frequency hops are detected in the received radio signal and synchronism
is maintained. One way to accurately tie the frequency transitions at
station transceiver 140 is to use the control signals in the received
signal from the CM transceiver 130. This method will be described in a
later paragraph.
The transceiver modules, illustratively 130 and 140, may be well-known
circuits arranged to communicate analog or digital signals, respectively,
over facilities 133 and 121 with, respectively, interface circuit 105 of
CM 100 and station 120.
In accordance with the present invention, each radio link may use some or
all of the frequency channels of the 902-928 MHz band for the transmission
of analog or digital information using frequency-hopping techniques. The
baseband information consists of an analog voice channel from 0.3 to 3 kHz
and a 1-2 kHz control channel located above 3 kHz that contains the
digital control information. The exact location and width of the control
channel depends upon the economics of the various filtering possibilities.
This combined signal must have a bandwidth of <25 kHz after modulation to
the 915 MHz region to satisfy FCC rules. Because little bandwidth is
available for a control channel, there may be delays in a station radio
frequency response time vis-a-vis cable. For example, the worst-case delay
from button press to response may be longer in the case of a key telephone
system. The use of a higher bandwidth control channel would reduce
response time at the cost of more elaborate and costly filtering.
The baseband signal (voice plus control signals) is modulated on a carrier
whose frequency is switched periodically in what is called a
frequency-hopping mode of transmission. With frequency-hopping
transmission, a given frequency is used for 400 ms (or less) and then the
transmitter switches to another frequency for the next 400 ms. As
specified by the FCC, 75 different frequencies spaced by at least 25 kHz
must be used every 30 seconds. A given sequence of hopping frequencies
must last at least 30 seconds but then the sequence can be repeated. There
are 1,040 frequency channels, each 25 kHz wide, between 902 and 928 MHz.
The purpose of frequency-hopped channels is to allow many users to have
random access to the frequency channels without any coordination among
them. The theory of frequency hopping is that if the frequency-hopping
sequence is random, there is a low probability that more than one
transmitter is using the same frequency at the same time, given a lightly
loaded system. Obviously, with enough simultaneous users and no
coordination, there will be overlap of frequency usage and communications
will deteriorate because of interference.
For a small centralized communication system (e.g., FIG. 1), overlapping
frequencies can be avoided with some coordination. If we assume that each
transmitter needs 76 different frequencies (1 for start-up and 75 for a
30-second transmission cycle), a system can have six simultaneous users
with no common frequencies and no chance for frequency clash. The six
duplex communication links use 12 transmitters, each requiring 76
different frequencies which total 912 frequency channels (out of the total
of 1,040 available channels). While six separate duplex channels should be
sufficient for many arrangements, a problem exists to satisfy the needs of
larger communication systems. Moreover, the potential exists for
interference when multiple small communication systems operate in a close
geographic area.
This problem is solved by the present invention which synchronizes the
frequency-hopping channels of all stations of the system. The
synchronization takes the form of a master/slave message or signal between
CM 100 and each station set (120, 160). The transceivers at CM 100 are the
masters and those at the stations are the slaves. The message insures that
all CM transmitters are locked to a system clock such that all 30-second
frequency-hopping sequences are started simultaneously. Similarly, since
all station transmitters are locked to their associated CM 100
transmitter's cycle, all 30-second frequency-hopping sequences in the
system start together (within the transmission delays of the system-a few
ms). Consequently, all frequency sequences can be chosen so that at any
given time every transmitter is sending a different frequency.
FIG. 5 illustrates one embodiment of the invention in which each
transmitter-i.e., one at CM 100 (e.g., 502) and one at each station set
(e.g., 501) for each 2-way radio link-is assigned one unique frequency for
start-up handshaking (i.e., SC0 and SS0, respectively) and one unique
starting frequency for communications (i.e., CF0 and SF0, respectively, at
time T0). Thus, 1,040 frequency channels provide for 260 simultaneous
users. Half the available frequencies (i.e., a total of 520) are used for
start-up (see 510) and half for communication frequency sequencing (see
590). Each user during each time interval or time slot (T0-T74) requires a
unique pair of frequencies for transmit and receive channels. Thus, during
any one time slot, there can never be more than one transmitter using a
given frequency. In each 400-ms timeslot (T0-T74), the 520 frequencies are
permuted among the CM and station transmitters in accordance with the FCC
constraints. The disclosed method of assigning different frequencies to
different transmitters during each time slot T0-T74 and synchronizing the
frequency hopping prevents frequency interference.
Shown in FIG. 6 is an illustrative example of an allocation of frequencies
for both the start-up 602 and active communication 601 phases of
operation. The frequency F is approximately 902.0125 MHz. Note, according
to one aspect of the present invention, the active communication
frequencies are interleaved with the start-up frequencies to simplify the
filter designs. The bandwidth B1 of the active communication channels 601
may be approximately 25 kHz to accommodate a modulated 6 kHz signal needed
for voice (3 kHz) and control channel signaling (1-2.0 kHz). The narrow
bandwidth B2 for the start-up channels 602 may be about 10 kHz, providing
a >1K-bit per second data rate during start-up.
The parameters of a start-up protocol may be chosen so that synchronization
can be achieved quickly (within a few seconds) rather than waiting for the
beginning of a new 30-second cycle. Note that the protocol messages are
very narrow bandwidth signals 602 so the likelihood of interference with
adjacent hopping frequencies should be very low. This is shown in FIG. 6
which illustrates that the bandwidth B2 for the start-up frequencies 602
is less than the bandwidth B1 of the communication frequencies 601.
With joint reference to the tables of FIG. 5 and FIG. 6, an illustrative
frequency allocation algorithm and the resulting frequency-hopping
sequence is described. In the disclosed algorithm, each station (e.g.,
S1-S75) is assigned to a group of stations which utilize a first group of
frequencies for receiving 540 and a second group of frequencies for
transmitting 530. Each station uses the frequency group number 712 in
start message 710 to select the group. Thus, for example, the frequency
group number 712 may select group 530 for station transmissions and group
540 for receptions from the control unit.
The start message 710 is also used to identify the start frequency number
714. The start frequency number 714 could be a code for identifying one of
the frequencies of groups 530 and 540 or could be the actual encoded
frequency. As previously noted, each station sequence controller 206A may
utilize an algorithm to determine the receive start frequency and receive
frequency group from the transmit start frequency and transmit frequency
group (i.e., by adding K to the transmit start frequency to define the
receive start frequency).
Each station transceiver is programmed to sequentially and synchronously
step (i.e., frequency-hop) to the next communication frequency following
the identified frequency-hopping sequence 713, beginning with the start
frequency 714 identified in start message 710. Thus, for example, station
S1 at time slot T0 receives at frequency CF0 and transmits at frequency
SF0. Thereafter station S1 would synchronously hop to frequency CF1 and
SF1 at time slot T1 and so forth. When frequency SF74 is reached, station
S1 recycles back to SF0 on the next time slot. Similarly, the associated
transceiver at CM 100 is preprogrammed to step sequentially from a
starting frequency CF0 through CF74. Note there are a total of 520
start-up frequencies (510) and 520 communication frequencies (590). While,
as previously discussed, the start-up frequencies 510 are interleaved with
the communication frequencies 590, other frequency assignments are
contemplated within the scope of the present invention. Thus, for example,
one pair of start-up frequencies (e.g., SS0 and SC0) could be encoded and
used for all start message transmissions to and responses from stations.
However, this would slow the start-up procedure for the system.
In the following example, each CM transceiver/station transceiver pair
(e.g., 130 and 140) has corresponding start-up and communication
frequencies separated by a fixed frequency offset K (13 MHz) which could
serve as the intermediate frequency (IF). Thus, for example, SC0 is offset
from SS0 and CF0 is offset from SF0 by K. Moreover, the described
frequency-hopping (FH) sequence at CM 100 transceiver 130 (CF0-CF74) is
identical to that of the station transceiver 140 (SF0-SF74) with a
frequency offset of K, as shown in FIG. 5. The use of identical sequences
means that one frequency source (e.g., 207) serves both modulator (e.g.,
204) and mixer (e.g., 210) to reduce costs.
The following paragraphs describe a frequency-hopping technique according
to the present invention for a system, such as shown in FIG. 1, having two
radio frequency coupled station sets S1 (120) and S2 (160). The station
start-up frequency assigned to the station set S1 to CM 100 channel is
designated SS0 and has a frequency of F+25 kHz, as shown in 501.
Similarly, for station S2 and CM 100 the start frequency SS1 and SC1,
respectively, are shown in lines 503 and 504. During the start-up mode,
station S1 is assigned a start frequency SF0 and station S2 the start
frequency SF1.
At the conclusion of the start-up operation, transceiver 140 of station S1
uses the assigned frequency SF0 and station S2 the assigned frequency SF1
during the first 400-millisecond (ms) time interval T0. In accordance with
the disclosed illustrative algorithm during the second time interval T1,
station S1 switches to frequency SF1(F+50 kHz) and station S2
synchronously switches to frequency SF2(F+100 kHz). Thereafter stations S1
and S2 step along in 50 kHz frequency increments for each time slot
interval (T2 through T74) until on the 75th time interval (T74) station S1
utilizes SF74 equal to F+3.70 MHz and station S2 utilizes SF0. The method
of synchronizing the switching of time intervals will be discussed in a
later paragraph.
With reference to 502, control module 100 similarly causes its transceiver
130 to start, illustratively, at frequency CF0 equal to F+K at time T0 and
to step-along in 50 kHz intervals until it reaches F+K+3.70 MHz at time
interval T74 (interval 75). At the transition to the next time interval,
the frequency oscillator at transceiver 130 is reset to frequency CF0.
Thus, transceiver 140 at station 120 and transceiver 130 at CM 100
communicate with each other using different frequencies without ever
overlapping frequencies during the 75 time intervals. Similarly,
transceiver 170 at station S2 and transceiver 180 of CM 100 are arranged
to start (time interval T0) at frequencies SF1 equal to F+50 kHz and CF1
equal to F+K+50 kHz, respectively, and step along in 50 kHz intervals.
Thus, transceiver 180 starts at frequency CF1 equal to F+K+50 kHz and
steps along until frequency CF74 equal to F+K+3.70 MHz is reached during
time interval T73. On the next transition to time interval T74, the
frequency clock is reset to SF0 equal to F+K MHz. Again, notice that,
because of the synchronous switching, stations S1, S2 and transceivers 130
and 180 never overlap frequencies with each other.
The above illustrated frequency allocation algorithm can accommodate 75
facilities within the frequency groups 530 and 540 without frequency
overlap. If more than 75 radio links are required, then a second group of
75 frequencies can be assigned to station (e.g., 550) and CM transceivers
(e.g., 560). The frequencies for group 550 would extend from F+3.75 MHz
through F+7.45 MHz in 50 kHz steps. The frequencies for group 560 would
extend from F+K+3.75 MHz through F+K+7.45 MHz in 50 kHz steps. This
process can continue until six groups of 75 links each or a total of 450
radio links. The last 70 frequencies (451 through 520) cannot be utilized
in the same manner since it would violate the FCC requirements of a
minimum of 75 frequency hopping frequencies.
It should be obvious that other algorithms for allocation of hopping
frequencies may be utilized without deviation from the present invention.
Thus, for example, each station can be made to step along in a 50 kHz
increment from frequency F until frequency F+12.950 MHz is reached at
which time the frequency returns to frequency F at the next time interval.
Such an arrangement would spread the transmissions from any station over a
wider frequency spectrum. The resulting diversity may offer advantages in
certain systems.
The selection of an algorithm for selecting hopping frequency may also
consider that the frequency separation between a transmit and receive
signal to any station should be different from the 13 MHz(K) utilized in
the illustrative algorithm. Moreover, to more evenly distribute the
frequencies across the band, stations of a system may be assigned
frequencies which are separated by much more than the 50 kHz utilized in
the illustrative example. Thus, it is contemplated that any of a variety
of algorithms may be utilized for allocating hopping frequencies and/or
startup frequencies.
Additionally, depending upon the requirements of the communication system,
the start-up messages for each station may be multiplexed together and
thereby utilize less than two starting frequencies (510) for each
station-to-control module link. The result is that more frequency channels
would be available for use as communication channels (590).
In accordance with another aspect of the present invention, the system may
need to use space diversity to combat multipath transmission. In addition,
the demodulation of the frequency-hopped carrier may be very noisy at the
transitions of the carrier. That means that voice may have to be muted for
a few ms every 400 ms (not perceptible) and data transmission may be
confined to the nontransition interval. This is shown in FIG. 7 which
illustrates carrier transition time 701 and muting interval 702 at each
end of the 400-ms time interval 703. Each station transceiver (e.g., 140)
detects a header 704 to determine the start of each time interval. The
muting interval 702 takes care of transition errors, such as those caused
by transmission delays between the CM 100 and station terminals. The time
interval 705 following the header 704 can be used to time the frequency
transitions at the station transceiver 130 to maintain system
synchronization.
What has been described is merely illustrative of the application of the
principles of the present invention. Other methods and circuits can be
implemented by those skilled in the art without departing from the spirit
and scope of the present invention.
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