Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
TDM COMMUNICATION SY5TEM FOR EFFICIENT
SPECTRU~q UTILIZATION
Back~round of the Invention
This invention relates generally to two-way radio
communication and more particularly to time division
multiplexed digital communication and i~ more
particularly directed to a communication system for the
efficient utilization of the ~re~lency speatrum.
Those skilled in the art will appreciate the
congested and crowded nature o~ the available frequency
spectrum. ~he Federal Communication Commission (FCC)
have continually sought ways to reallocate the available
spectrum or as~ign previously reserved spectrum to
relieve thi~ cong~stion. This condition is particularly
noticeable in metropolitan areas where a large number of
radio u~ers are concen~rated in a small geographic area.
ona proposal the FCC is considexing is sharing a portion
of the UHF television spectrum with the land mobile
market (FCC docket 85-172). Another consideration is the
reallocation of the land mobile re erve frequencies in
the 896-902 MHz region to private land mobile uses (FCC
docket 84-1233).
Another alternative for the FCC is to redefine
the standard for land mobile communication channels.
rl~
- 2 - CM-00183N
Currently, the standard for land mobile communication is
a ~hannel having a bandwidth of 25 kHz. However, the FCC
may redefine this standard to use 12.5 kHz (or possibly
15 kHz) channels. The theory behind this "band-split" is
to effectively double the number of channels in any newly
allocated frequency spectrum. Potentially, as "older"
spectrum is reallocated, all communications equipment
will be required to op~rate in the 12.5 kHz channel
bandwidth.
Although ~acially atkractive, a band-split to
double the available number of channels is not without
cost. Present day communication devices operate with a
su~ficient frequency guard-band tha~ protects against
adjacent-channel interference ~given the frequency
stability of the transmitters). Of cour~e, the
band-split would also reduce the frequency guard-band
tending to lead to higher adjacent-channel interference.
Even assuming a greater than a two-to-one improvement in
transmitter ~requency stability, and high selectivity
arystal filters for the receivers, adjacent-channel
performance may be degraded by a band-split. Thus, there
exists sub~tantial teahnological barriers that must be
overcoma to provide a radio with comparable performance
sp~ci~ications at a competitive cost in the marketplace.
Therefore, a sub~tantial need 2xists in the market to
develop a communication system that will provide an
increase in the number of available communication
channels that is compatible with present day 25 kHz
channel bandwidths.
Summary of the Invention
Accordingly, it is an object of the present
invention to provide a spectrally efficient communication
system.
It is a further object of the present invention
to provide a communication sys~em readily adaptable to
improved coding techniques.
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2`~
It is a further object of the present invention
to provide a communication system that operates in a
25 kHz channel bandwidth that maximizes spectral
efficiency.
Accordingly, thQse and other objects are achieved
5 in the present time division multiplex communication
system.
Briefly, according to the invention, a time
division multiplexed (TDM) communicakion system is
disclosed, which apportions radio frequency communication
channels into at least kwo time slots. Voi~e signals for
transmission on this system are analyzPd and vo-coded
into a digital signal that is transmitted during one or
more of the time slots. Received messages are recovered
from at least one of these time slot~ and the voice
message syntheæized from the vo-coded signal. In this
manner multiple voice messages may be transcei.ved in a
time diYision multiplexed manner on a single 25 kHz
bandwidth channèl.
Brief De~cripkion o~ the ~rawinqs
The features of the present invention which are
believed to be novel are set ~orth with particularity in
the appended claim~. The invention, together with
further ob~ QCt~ and advantages thereof, may be understood
with reference to the following description, taken in
con~unction with the accompanying drawings, and the
several figures of which like referenced numerals
identi~y like elements, and in which:
Figure 1 is a block diagram of a TDM
communication system according to the invention;
Figure 2 is an illustration of the preferred
organization of a communication ¢hannel;
Figure 3a i6 an illustration of the pref~rred
. 35 organization of the slot overhead for a primary to remote
- transmission;
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Figure 3b is an illustration of the pre~erred
organization o~ the slot overhead ~or a remote to primary
transmission;
Figure 4 is a block diagram of a remote unit
according to the invention;
Figure 5 is a block diagram of a primary unit
according to the invention;
Figure 6 is a bloc~ diagram of a single ~requency
primary unit according to the invention;
Figure 7 ~s a block diagram o~ the pre~erred
embodiment o~ the controller of Figure~ 4-~;
Figures 8a-8c are ~low diagrams of the steps
executed by the controller o~ Figure 4;
Figure 9 is a flow diagram o~ khe steps executed
by the controller of Figures 6 or 7D
Detailed Description of the Pre~erred Embodiment
In Figure 1 there is shown a block diagram o~ the
time di~ision multiplexed (TDM) system 100 o~ the present
invention. The system is comprised essentially o~ a
repeater 102, a mobile unit 104, a base station 106 and a
portable 108. As used herein, a portable unit (108) is
de~ined to be a communication unit typically designed to
be carried about the person~ A mobile unit (104) is a
transceiving unit designed to be carried in vehicles, and
a base station (106) is contemplated to be a permanent or
semi-permanent installation at a fixed location. The
mobile 104, the base station 106 and the portable unit
108 arQ hereinafter collectively re~erred to as remote
units, and the repeater 102 is hereinafer raferred to as
the primary station. The remote unit~ communicate via
the primary station using radio freguency tRF) channels
that are divided into at least two time slots. The RF
channels used by the pre~ent inven~ion are contemplated
to be standard narrowband land mobile channels. These
channels are typically understood to be communication
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channels having a bandwidth of 25 kHz (for duplex, the
channel ~r~guency pairs are spaced 45 MHz apart in the
800 MHz band). of course, other channel bandwidths and
spacings are possible, however, the present invention
contemplates the use of standard land mobile channel
requirements thereby obviating the need for any new FCC
allocations or requirements~
Those skilled in the art can appreciate that
human speech contains a large amount o~ redundant
in~ormation. To most ef~iciently utilize the ~requency
spectrum it is desirable to remove as much of the
redundant information as pocsible prior to transmission.
The message i8 then reconstructed at the reaeiving end
Prom the transmitted essential speech infor~ation.
Speech production can be modeled as an excitation signal
(i.e~, air from the lungs) driving a filter (the vocal
tract), which possesses a certain resonant structure.
The spoken sound ahanges with time since the ~ilter
varies with time. The excitation is noise-like ~or
unvoiced sounds (i.e., consonants) and appears as a
periodic excitation ~or voiced sounds (~or example
vowels). Therefore, to reduce the amount o~ bandwidth
required to send a volced 5ignal, the spectral
characteristics of the signal must be analyzed and the
nature o~ the exaitation signal mu~t be determined.
Prior communica~ion systems have employed speech
digitation techniques such as pulse code modulation (PCM)
or continuously variable slope delta (CVSD) modulation to
attempt to replicate the time waveforms of the speech
signals. However, these techniques suffer the detriment
of requiring data rates from 12 kbps to 64 kbps. The
current state of the art in land mobile communications is
a data rate of 12 kbps to 16 kbps on a 25 kHz channel.
This allows the transmission of one voice signal using
CVSD. Those skilled in the art will appreciate that the
combination of more efficient voice coding (for example
coding in the range of 2.4 kbps to 9.6 kbps) and more
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e~ficient data transmission (18 kbp~ to 24 kbps on a 25
kHz channel) would allow the transmission o~ two or more
voice signals in 25 kHz o~ ~re~uency spectrum.
Prior techinques indicate splitting the
communication channels into narrow fre~uency segments,
each being the minimum to allow one digitized voice path.
These techniques have two distinct disadvantages. First,
narrow channel~ and wide channels do not mlx well within
a system so that a gradual tran~ition ~rom wider to
lo narrower channels is accompanied by increased co-channel
and adjacent channel interference. Secondly, any
particular choice of a narrower standard channel
bandwldkh "freezes" the state of the art. That is,
simply redefining and fixing the standard ~andwidth ~or
land mobile communications prohibits advantageous
exploitation of technological improvements without
another reassignment or redefinition of communication
standards.
The present invention keeps the current standard
~or land mobile communication channels while splitting
the time among users according to tlle ~raction of the
chann~l bit rate required ~or on voice signal. This
method has the advantage~ of pre~erving the present level
o~ inter~erQnce protection and allowing splitting (in
time) as often a~ needed to take ~ull advantage of
advances in the state of the art of coding and data
transmission.
The present invention contemplates vo-coding the
voice signal to minimize the speech data rate. As used
herein, vo-coding mean~ the analysis and synthesis of
voice, which either utilizes a vocal track model, or
quantize~ sub-bands of a speech wave~orm to remove
redundant speech information thereby enabling the
transmission of the required voice information in a
reduced bandwidth.
~ typical example o~ a vo-coder employing a vocal
track model is a linear predictive coder ~LPC). An LPC
~ 7 - CM-00183N
analyzer typically operates on blocks of digitized voice,
detarmining the model parameters that are applicable
during a paxticular block, and transmitting thQse
parameters to a synthesizer at the receiving unit. The
synthesizer reconstructs the speech signal by using the
parameters received. Since the model parameters vary
slowly with time compared to the speech waveform, the
redundancy of the speech is removed.
A typical example of a vo-coder employing speech
sub-band quantitization is a sub-band coder (SBC). In an
SBC analyzer, sub-bands of a speech waveform are
quantized and a determinakion is made concerning the
amoun~ Or speech ener~y in each sub-band. Only those
sub-bands having an energy content above a predetermined
threæhold are transmitted thereby enabling transmission
in a reduced bandwidth. Accordingly, vo-coding provides
a further reduction in the speech data rate by using a
coding technique basQd upon specific speech
characteristics, transmitting only ~he perceptually
important in~ormation contained in a speech signal.
Vo-coding allows a su~ficiently low speQch coding rate to
enable the division of a 25 kHz channel bandwidth,
thereby providing a spectrally e~iciant communication
system.
Re~erring now to Figure 2, there is shown an RF
communication channel 200 subdivided into 8 time
sub-slot~. Each time sub-slot 1-8 has associated with it
an overhead data portion 202 which contains a signalling
protocol to be hereina~ter defined. Once the RF channel
is divided into a predetermined number of time sub-slots
(8 in the preferred embodimen~) they are grouped into
subsets that ~orm communication time slots employed by
the actual system users.
Those skilled in the art will appreciate that
vo-coding a voice at various coding rates may affect the
perceived quality of the received speech. Accordingly,
spaech vo-coded in a 9.6 kbps sub-band coder may be o~
~ CM-00183N
higher perceived quality than 2.4 kbps LPC coded speech.
There~ore, the present invention contemplates grouping
~he 8 tlma sub-slots into subsets as required by the
particular vo-coder utilized. An exemplary arrangement
of slot assignments i8 illustrated in Figure 2 (reference
202). Sub-slot~ 1-4 have been combined to form slot la,
which may provide toll quality speech for the users of a
system. Slot lb and slot lc are formed by combining two
sub slots (5~6 and 7~8 respectively) that may provide
speech o~ a lesser quality that i~ still acceptable to a
particular user. Accordingly, the air-time billing rate
may vary depending upon the quality of speech required in
a particular user environment. Moreover, as technology
improves and the quality of speech for a lower bit rate
vo-coder is enhanced, ~urther ~ubdivisions may be readily
employed since the system wa~ designed originally to
operate with a greater number of time slots (i.e.,
ultimately the ~.time sub-slots would be communication
time slots).
Re~erring now to Figures 3a and 3b, there is
shown the preferred embodiment of ths overhead data
in~ormation (202 o~ Figure 2) for both the
primary-to-remote, and remote-to-primary transmissions.
Figura 3a illustrate~ the primary-to-remote data overhead
300. The data overhead begins with a propagation delay
302. Typically, the maximum propagation time delay will
be de~ined by the particular system coverage designed
into a particular implementation. Typically, system
range is predominately responsible for determining the
propagation delay. For example, the two-way propagation
delay for distant remote units (60 miles) may be twelve
bits with 18 kbps signalling. If the vo-coded signal
recaived at the primary station (repeater) were simply
repeated, the message delay would become a function of
3~ the distance o~ the transmitting remote unit. The
receiving remote uni~s would be required to correctly
detexmine where the message information resided within
.
- g - CM-00183N
the slot to correctly recover the voice messaye.
Accordingly, the present invention contemplates a system
wherQin the primary ~tation repeat~ the information at a
fixed point in the slot. All remote units synchronize to
the primary station's transmitted signal.
Following the propagation delay 302 is the
transmit key time 304. The transmit key time 304
representq khe time required to switch a unit between thP
transmit and receive frequency. This is typically
considered to be a hardware limitation, and in the
preferred embodiment is lo 22 milli-seconds (ms) in
duration. Those skilled in the art will appreciate ~hat
the actual number of bitR transmitted will depend on the
data rate used. Of course, as improved power amplifiers
and ~re~uency synthe6izers are designed, the transmit key
time may decrease to a lesser duration. The bit
synchronization pa~tern 306 follows the transmit key 304.
The bit sync portion o~ the data overhead 300 rapresents
a digital pattern re~uired to obtain bit synchronization
between a transmitting unit and a receiving unit. In the
preferred embodiment, the bit sync portion 306 consists
o~ 1.22 ms of an alternating logic-one logic-zero
pattern. After acquiring bit synchronization, the
receiving unit must al80 have ~rame synchronization to
properly decods one or more time slot~. In the pre~erred
embodiment o~ the present invention the ~rame
synchronization portion 308 consists of a predetermined
digital word~ The receiving unit must correctly receive
the frama sync portion 308 in a majority decision fashion
(3 out of 5 in the preferred embodiment) in order to
properly acquire ~rame synchronization. Synchronizing in
this manner allows an acceptabla system falsing rate
utilizing a minimized number o~ data bit~ to ~orm the
synchronization word. ~fter frame synchroniza~ion, the
receiving remote unit receives the sub~rame ID code 310.
The sub~rame ID code contains information which is used
by a remote unit to control and direct the receiving
-- 10 -- CM--00183N
~_ ~s ~_D ~
circuitry to operate on at lea~ one TDM 510~. Of
aour~, a~ uBtrated in Figure 2, ths receiving remote
unit may be informed, via the subframe ID 310, that it
will group a plurality of time sub-slots into a single
user slot. ~ter correctly synchronizing and decoding an
a~signment to at least ona TDM ~lot, the remote receives
the vo~coded speech 312, which follows the data ov~rhead
300.
In Figure 3b, the data overhead 314 for the
remote-to-primary station transmission is illustrated.
lo The daka overhead 314 i8 similar to the data overhead 300
o~ Figure 3a except that the pxopagation delay 302 is not
requirad since the primary station repeats all messages
at the same point in the time slot, and the subframe ID
310 is not required since slot assignment i~ performed by
the primary station (repeater). Following the frame
synchroni2ation portion 308 (of the remote-to-primary
station datA overhead 314) the remote unit transmits the
vo-coded VoicQ me~sage.
In Figure 4 there i5 shown a block diagram of a
remote unit 400. The heart of the remote unit 400 is the
controller 402 (a more detailed illustration and
disaus~ion of which follows hereinafter). To transmit, a
~peech signal i~ first input via a microphone 404. The
speech i~ analyzed by a vo-coder analyzer 406, which is
enabled by ~he controller 402 via connection 407. The
vo-coder analyzer may be any suitable coder and in the
preferred embodiment is an LPC or SBC vo-coder. The
controller 402 takes the vo~coded information, which is
in digital form, and routes i~ to the transmit buf~er 408
via data line 410. The digitized speech information is
stoxed in the transmit buffer 408 at whatever coding rate
is selected for th~ vo-coder analyzer 406. Typical
examples of vo-coding data rates include, but are not
limited to, 9~6, 4.8, and 2.4 kbps. When the transmit
buffer 408 has reached a predetermined capacity limit,
the informa~ion is extracted by the con~roller 402 via
~ CM-00183N
connection 412 and routed to the transmitter 414. Of
course, the controller 402 preambles the speech
in~ormation by the data overhead portion 202 as
illustrated in Figure 2. The controller 402 couples the
transmi~ter 414 to an antenna 416 via the switch 418.
Alternatively, the switch 418 could be replaced with a
duplexer (or the like) to continually couple the
transmitter and receiver to the antenna. In this manner,
the data overhead and speech information are transmitted
at a selected transmission data rate, which must be at
least twice that of the vo-coding data rate.
Alternately, data information (already in digital form)
may be transmitted in the same manner via data source
420. Moreover, a combination of vo~coded speech and data
may alternatively be sent as determined by a particular
usar.
To receive information from a time slot, the
controller 402 couples the antenna 416 to a receiver 422
via the switch 418. The receiver ~2 is coupled both to
the controller 402 and a clock recovery means 424, which
may be any suitabla clock recovery means that will
synchronize the controller 402 to the received
in~ormation using the bit syna or frame sync portions.
once synchronized, the controller 402 takes the received
vo-coded speech ~or digital data) and routes it to the
receive bu~fer 426 via connection 428. This information
is cloc]ced into the receive bu~fer 426 at a suitable data
rate, which typically may be the transmission data rate.
The in~ormation is extracted from receive buffer 426 via
connection 430 and routed through the controller 402 to
the vo-coding synthesizer 432. Of course, the
information must be ex~racted at a data rate identical to
that which the speech information was vo-coded. The
synthesizer 432, enabled by the controllar 402 by
connection 433, operates on the essential speech
components to synthesize the voice signal. This signal
is applied to a speaker 434 that allows the message to be
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received by the operator. If, however, data was
tran~mitted during a TDM slot, the data sink 436, which
may be a printer or monitor device, accepts the data and
di~plays it for the operator.
Referring now to Figure 5, there is shown a
repeater 500 suitable for usQ in the TDM communication
system of the present invention. The controller 502
controls the operation of the repeater 500. The system
re~erence 504 provides the aontroller 502 with the clock
signal, which is used to determine the transmission data
rate. Operationally, a vo-coded signal iR received from
at least one time slot on a first frequency and travels
from the antenna 506 through the duplexer 508 to a
receiver 510. The receiver 510 is coupled to a clock
recovery device 512 and the controller 502. The
controller accepts the received data signal from the
receiver 510 at the data rate determined by the clock
recovery davice 512 and supplies it to the transmitter
514. Tha transmitter 514 repeats the signal including
the overhead 202 in at least one time ~lot on second
frequency (at a transmission da~a rata determined by the
controller 502) through the duplexer 508 to the antenna
506.
Re~erring now to Figure 6, a single frequency
repeater ~SFR) suitable for use in the TDM system of the
present invention is shown. The repaater 600 is
controlled by the controller 602, which takes a master
clock signal from the system reference 604. A signal is
received via antenna 606 and routed via the switch 608 to
thQ receiver 610. The receiver 610 supplies signals to
the clock recovery means 612 and the controller 602. The
received vo-coded ignal is stored in a bu~fer 618 via
connection 620 at the received data rate as de~ermined by
the clock recover means 612. The vo-coded message is
stored in the buf~er 618 until a subsequent time slot, at
which time the buffer 618 is emptied by the controller
602 via connection 622 at a predetermined data rate,
13 - CM-00183N
which i ~ypically the tran~mission data rate. The
conkroller 602 then routes the buf~ered signal to the
tranRmitter 614. The transmitter 610 sends the signal to
the antenna 606 via the switch 608, which has been
S coupled to the transmitter via the controller 602 through
connection 624. Accordingly, in an SFR, the transmitter
614 and receiver 610 aro multiplexed to the antenna 606 a
duplexer is not required. Those skillad in the art will
appreciate that either the multiple frequency repeater or
19 the single fxequency repeater may be used alternately or
in combination in any particular TD~ system.
Re~erring now to Flgure 7 there i5 shown a block
diagram of a controller 700 suitable ~or use in either a
primary or remote unit. The controller 700 is comprised
o~ a microprocessor 702, such as an MC6801 manu~actured
by Motorola, Inc. The microprocessor 702 is supplied a
clock signal by clock source 704. The system reference
(see Figs. 5 an~ 6) is routed to the frame marker 706 and
tha Synchronous Serial Data Adaptor ~SSDA) 708.
Microprocessor 702 is coupled to the frame marker 706 and
the SSDA 708 via an address bus 710 and a data bus 712.
The frame marker 706 i9 used to generate ~he ~rame
synchronization in~ormation contained in the data
overhead a~ was de~cribed in con~unc~ion with Figure 2.
The frame marker 706 can be any convenient device and may
be, for example, a programmable timer module (PTM), such
ag an MC6840 manu~actured by Motorola, Inc. The SSDA 708
is used in the controller 700 to accept da~a ~rom the
mioroprocessor 702 and communicate the data serially to
the transmitter 714. In the pre~erred embodiment, the
SSDA is an MC685~ manu~actured by Motorola, Inc. The
SSDA 708 is also coupled to the clock recovery and data
detector 716. The clock recovery data detector 716 is
coupled to the receiver 718 and is used to supply the
received synchroniza~ion information and received
- vo=coded voice signals to the SSDA 708. Thus, the SSDA
is used in both the transmit and receive modes to route
- 14 - CM-00183N
data accordingly. The clock recovery and data detector
716 is also coupled to the frame sync detec~or 720. The
frame sync detector 720 receives data from the data
datector and clock rscovery device 716 and is used to
look for the frame sync marker in the received vo-coded
signal. When frame synchronization is achieved, the
frame sync detector 720 alerts the microprocessor 702 via
connection 722. Once the clock recovery device and the
frame ~ync detector have both synchronized, the vo-coded
signal can be either repeated (a~ in the primary stations
of Figs S or 6), or received and routed to the vo-coder
synthe~izer to recover the voice signal (as in the remote
u~it of Fig~ 4).
Referring now to Figures 8a-8c, there is shown a
flow diagram of the steps executed by a controller
utilized in a remote unit. In Figure 8a, the routine
begins with the initialization step 800, which is
executed during first time operation or after a reset.
The initialization step 800 programs any ~re~lency
synthesizers and loads various ID codes that may be
employed during the operation of the controller. The
routine next proceeds to deci~ion 802, which checks to
see if the repeater is active. The remote unit
determines i~ tha repeater is active via the bik Syllc
circuity that operatss on the bit sync portion of the
data overhead (see Figure 3). ~ positive bit sync
indication occurs if tha repeater i9 operatiny (i.e.,
transmitting). Of course, if the repeater were inactive,
the remote unit would not be abla to obtain bit sync.
Referring again to Figure 8a, if the repeater is
not active the routine proceeds to decision 804 to detect
whether the push-to-talk (PTT) switch has been actuated
to initiate a communication. If the determination of
decision 804 is that the PTT switch is not actuated, the
routine returns to reference letter A and decision 80~.
The routine will continue in ~his loop un~il the PTT
switch is actuated at which time the routine procaeds to
- 15 - CM-00183N
step 805. In step 805, the predetermined repeater key-up
code i5 transmitter to activate the repeater. The key-up
code may be any suitable code and, of course, if a
particular implementation the repeater is always
activated, step 805 could be omitted. In the preferred
embodiment of ~he present invention, the repeaters are
inactive (i.e., off the air) if no remote unit is
transmitting. This conserves energy and increases the
mean time between failure (MTBF) of the primary station.
Of course, the repeaters could be designed to operate
continuously thereby eliminating the need of an
activation code. After transmitting the repeater key-up
code, ~he routine proceeds to decision 806. Decision 806
determines whether or not synchronization has been
achieved. Both bit synchronization and frame
synchronization are required for an affirmative
determination in decision 806 (however, bit sync may have
already been established in decision 802). Frame sync is
determined by a ma~ority determination based on a
threa-o~-~ive correct receptions of the frame sync word
(see Figure 3). If synchronization i9 established, the
routine proceeds to step 808, which enables the analyzer
of the particular vo-coder employed. Following the
enabling of the vo-coding analyzer, the routine proceeds
to decision 810, which determines whether the PTT switch
has been activated. If the switch has been activated,
the routine routes to reference letter B of Figure 8b (to
transmit). If the PTT ~witch is not activated, the
routine proceeds to reference letter C of Figure 8c (to
receiye).
Referring now to Figure 8b, the steps involved
during the transmit mode of the controller are shown.
The routine begins in step 812, which takes the digitized
speech informa~ion from the vo-coding analyzer. The
vo-coded speech is s~ored in the buffer t408 of Figure 4)
in step 814 at the vo-coding data ra~e. Decision 816
determines whether the ~uffer is sufficiently full to
- 16 - CM-00183N
begin transmitting. In the preferred embodiment, the
buffar is deem~d to bs full (ready) when at least
one-hal~ of one slot of vo-coded data has been buffered.
If declsion 816 determines that the buffer is not
sufficiently full, the routine returns to the reference
letter B to receive more vo-coded speech from the
analyzer in step 812. I~ the determination of decision
816 is that the buffer i5 sufficiently full, the routine
proceeds to decision 818 to determine whether the present
time lot is the assigned slot of a particular unit.
The time slots must be assigned so that the mobile
controller knows how many of the sub-slots (1-8) to
combine for this particular communication slot. If the
pr~ent time slot is not the unit's assigned time slot,
the routine proceeds to decision 817 to check for sync.
If decision 817 determines that sync has been lost, the
routine proceeds to reference letter A. Otherwise, the
routine proceeds to reference letter B. I~ decision 818
determines that the present time ~lot is the unit's
assigned time slot, the routine proceeds to step 819 to
determine whether the unit is still in frame sync. The
unit will have a valid ~rame sync if it has correctly
recelved five of the past nine frame sync words. If
decision 819 datermines that the unit has dropped ~rame
sync, control returns to reference letter B. If the unit
has held sync, tho routine proceeds to step 820, which
formats the data overhead preamble as previously
described in conjunction with Figure 3. Following the
data overhead formatting of step 820, step 822 transmits
a single burst on the TDM channel by transmitting ~he
overhead and vo-coded speech taken from the buffer at the
transmission data rate. After this single slot is burst
on to the TD~ channel, decision 824 determines whether
the buffer is empty. I~ the bu*fer is not empty, the
routine returns to ra~erence le~ter B which takes more
speech and continues to transmit. If the buffer is
empty, the routine returns to reference letter A of
i ~;r~
- 17 - CM-00183N
FigurQ 8a which det~rmines whether the repeater is
active.
In Figure 8c, the steps executed by the mobile
controller for the receive operation are shown. Routine
begins in step 826 which receives the vo-coded signal
from one or more time slots in the TDM channel. Step 828
update~ the slot assignments for the device employing the
controller. In the preferred embodiment, thi represen~s
updating a memory location which sontain~ the number of
sub-slots (1-8) that may be combined in various
arrangements to form communication slots for the TDM
device. ~he routine next proceeds to decision 830 to
determine whether or not synchronization has been
maintained. An affirmative dete~mination results if the
unit has correctly received five of the past nine frame
sync words. If there is synchronization, the routine
proceeds to decision 832 to determine whether the
communication davice is muted or whether the squelch is
open to allow reception of the message. ThosQ skilled in
the art will appreciate various metho~s of squelch are
known. One technique would consist of detecting whether
khe received signal i~ valid data or noise. An
alternative would be to use a form of continuous squelch,
commonly re~erred to as "digital private line" (DPL).
Another alternakive would be to employ begin-of-message
(BOM) and end-of-message (EOM) data words pre-ambled and
post-ambled to the mes~age, respectively. Basically, any
suitable squelch system is acceptable to the presenk
invention to operate as decision 832. If the squelch is
muted, the routine returns to reference letter D of
Figure 8a. However, if the squelch is unmuked the
routine proceeds to set 834 where the vo-coded signal is
placed in the buf~er (426 of Figure 4) at the received
data rate. Step 836 remove~ khe buffered ~ignal from khe
buffer at the vo-coding data rate and presents it to the
vo-coding synthesizer (432 in Figure 4). The vo-coding
synthesizer reconstructs the original voice message and
~ CM-00183N
presents i~ to ~he operator either via a speaker of othsr
means. Following the completion of the synthesized
me~sage, the routine returns to reerence letter D of
Figure ~a.
Re~erring now to Figure 9, the steps executed by
the primary controller trepeater) are illustra~ed. The
routine beglns in decision 900, which determines whether
the key up code has been received from a particular
remote unit. I~ the key up code is not received, the
lo repeater waits (i.e., off the air) until a key up code is
received. Assuming, however, that the key up code was
received, the routine proceeds to step 902, which starts
the frame marker and keys up the transmitter. Step 904
transmits ona burst o~ the data overhead defined in
Figure 3 containing the TDM slot assignmen~ ~or ~he
remote unit. A~ter the remote unit receives sync and a
slot assignment, the remote unit transmits the data
overhead and TDM.vo-coded data message to the repeater.
Accordingly, declsion 906 determines whether the
synchronization (both bit and ~rame) from the mobile has
been received in the pres~nt time slot. I~ sync has been
received, the routino proceeds to step 908, which resets
a transmitter time--out-timer, which may be present to
prevent tha transmitter from transmitting either
permanenkly or for prolonged periods. The routine then
proceeds to ~tep 910, whiah receives the TDM vo-coded
data ~rom ths particular slot (or group of slots)
assigned by tha repeater. Step 912 retransmits or
repeats the TDM data in another time slot on either the
same ~requency or in the same or dif~erent time slot on a
second fre~uency depending upon which type of repeater is
employed. Following the retransmission of step 912, the
routine returns to reference letter A, which again sends
one burst o~ data overhead wi~h the time slo~ assignment
and continues in this loop until there is no more
vo-coded data to transmit.
- 19 - CM-00183N
Referring again to decision 906, if the decision
of step 906 i5 that the synchronization was not received
in the present time 910t, the routine proceed~ to
deci~ion 914, which determine~ whether or not the
repeater transmitter i5 still key~d. The repeater
transmitter may not be keyed if th~ time-out-timer has
expired or a dekey code has been received (i~ any such
code is employed). I~ the determination of decision 914
i~ that the repeater is still keyed, an alternating
loyical one and logical zero pattern are transmitted in
the flrst sub-slot in step 916. Following step 916 The
data over-head and slot assignment are transmitted in
each of the sub-slots that form ~he particular time slot
used. Since th~ data over-head will not fill a sub-slot,
an alternating logical one and logical zero pa~tern is
5 US2 to ~ill each sub-slot. Following step 918 the
routine returns to re~erQnce letter A, which will again
send one burst of data overhead with the time slot
assignment to thQ mobile unit, and then to decision 906
to recheck if the repeatar has properly received
synchronization ~rom the remote unit. I~ the
determination o~ dQcision 914 i~ that the repeater is no
longer keyed, the routine returns to reference letter B,
which again will await the kQy Up code before the
repeater is operational again.
While a particular embodiment o~ the invention
has been described and shown, it should be understood
that the invention i9 not limited thereto since many
modi~ications may be made. It is therefore contemplated
to cover by the present application any and all such
modificationR that may fall within the true spirit and
scope of the basic underlying principles disclosed and
claimed herein.
What is claimed is:
