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Description  |
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FIELD OF THE INVENTION
This invention relates generally to telecommunication, and more
particularly to data communication provided in conjunction with voice
communication over radio channels of cellular telephone systems.
BACKGROUND OF THE INVENTION
A cellular telephone system provides radio communication between a
stationary base system and remote units, e.g., mobile radio-telephones
installed in automobiles. The base system typically includes a switching
system, a network of geographically separated basestations, and other
equipment. Each basestation defines a particular cell of the cellular
system, and has at least one transceiver with the capability of calling to
and receiving calls from remote units in the corresponding cell. Such a
cellular telephone system is described in the Bell System Technical
Journal, January 1979, entitled, "The Cellular Concept," page 15 et seq.
Cellular telephone systems are subject typically to governmental
regulations and approvals, and must comply with standards, such as, for
example, those set forth for the United States of America in EIA RS-553 by
Electronic Industries Association, Washington, D.C., U.S.A.
A known cellular telephone system allocates to each basestation a plurality
of radio channels to carry voice signals (called "voice channels"), and at
least one separate signalling or control radio channel. In turn, the
basestation assigns the allocated voice channels to remote units for use
during voice telephone calls; the assignments of the voice channels being
made in response to requests from the remote units for cellular service
for the calls. This preliminary procedure of requesting and assigning a
voice channel occurs over the control channel, and can be referred to as a
"hand-shake" transaction. Hand-shake transactions occupy valuable time on
the control channel.
Furthermore, because of the nature of cellular speech traffic, substantial
voice channel capacity is sometimes idle, e.g., between telephone calls.
This is even more pronounced during certain periods of the day, and during
week-ends and holidays.
It has been proposed to provide data communication service in addition to
voice service over cellular telephone systems. Two general alternatives
exist in the prior art. In one approach, the cellular telephone system
sends data as well as speech information over the voice channels, thereby
purportedly improving radio channel utilization. One such system is
described in U.S. Patent No. 4,887,265. Sharing channels with data in
accordance with that approach, however, degrades voice service insofar as
the data competes with voice traffic for channel utilization, and degrades
overall system performance in that control channels are used to carry both
data and voice communication requests.
A second approach avoids interference with voice services by establishing a
separate cellular data system operating over data-only channels in
frequency bands distinct from those of the voice system. One such system
is described in U.S. Pat. No. 4,914,651. This approach, however, does not
provide for efficient use of the frequency spectrum, and might be subject
to new governmental regulation and approvals, and the promulgation of new
standards for the cellular data system.
SUMMARY OF THE INVENTION
The invention resides in overlaying a packet-switched digital data
communication subsystem on a circuit-switched (e.g., voice) communication
subsystem of a cellular telephone system, with the respective data and
voice services sharing the same voice channels. The voice communication
subsystem provides voice services independently of the operation of the
data communication subsystem.
On the other hand, the data communication subsystem (or data overlay
system, as it can be called) monitors use of the voice channels for voice
communication, and, in response thereto, temporarily allocates one or more
of the available voice channels for data service. By subordinating data
communication in this way, the data service can share the voice channels
without interfering with voice service.
More specifically, the data communication subsystem has a data basestation
in each cell in which data communication service is provided. The data
basestation monitors usage of each data-allocated voice channel for voice
communication by the voice basestation by analyzing, e.g., signal
characteristics represented by voice-channel utilization signals obtained
from the voice basestation or from the airwaves.
For example, the voice-channel utilization signal can be the
radio-frequency output signals of the voice basestation, in which case the
data basestation can examine, e.g., the transitions in the signal's
radio-frequency power levels from substantially zero power (indicative of
the absence of voice transmission) to a non-zero power level indicative of
the presence of voice transmission.
Where the voice-channel utilization signal indicates an absence of voice
communication on the associated voice channel, the data basestation
transmits, from time to time, an available-channel signature to identify
that data-allocated channel as available for data service. Preferably, the
available-channel signature is a special signal broadcasted on the
available data-allocated channels not then being used to transmit voice or
data.
For data communication, a specially-equipped data remote unit monitors
voice channels in the coverage area of the cellular telephone system for
available-channel signatures, selects one of the channels (if any)
identified by the signature thereon as being available for data
communication, and transmits or receives digital information over that
voice channel.
On initiation of a "voice" telephone call on that channel, however, any
transmission (whether of data or a carrier signal) by the data overlay
system over the channel abruptly terminates, preferably even though, e.g.,
the data transmission has not yet been completed. This prevents the data
service from interfering with voice service provided by the cellular
telephone system. If the data transmission is interrupted prior to
completion of transmission of the data, the remote unit repeats the
channel selection process, and continues data transmission on another
voice channel.
Preferably, the remote unit maintains a list or look-up table stored in
memory of the data-allocated channels associated with each cell. The
look-up table can be static or, preferably, dynamic, with up-dates
furnished (i.e., broadcasted) from time to time by the data basestations.
Accordingly, during channel selection, in order to identify an available
data-allocated channel, the remote unit can monitor only the more likely
available voice channels identified by the updated data-allocated-channel
list to find an available-channel signature, rather than, for example, all
voice channels for the particular cell.
Accordingly, the invention improves channel utilization by sending data on
voice channels. Moreover, the invention assigns a higher priority to voice
service and accordingly controls data service by limiting channel access
by the latter service to only those times when the channels are not being
used for voice service. This approach avoids interference with primary
voice service, yet it provides materially greater utilization of the
overall channel capacity of the system by making use of otherwise inactive
interludes in the voice channels.
Moreover, the invention eliminates hand-shaking over the separate control
channels when data transmission is to take place, since a remote unit can
"seize" and transmit data over an available data-allocated channel, and
need not request and await assignment of a channel from the basestation
prior to data transmission.
Advantageously, the data overlay system in accordance with the invention
can usually be added to existing telephone cellular systems with no
degradation of voice communication services, and with no modifications of
existing voice basestations. In addition, the data overlay system can
economically support a wide range of data communication applications with
accurate and reliable transmission at a sufficiently high data through-put
to accommodate the needs of a sizable number of concurrent users.
Finally, it is expected that the data overlay system in accordance with the
invention can be implemented without requiring the promulgation of new
standards.
BRIEF DESCRIPTION OF THE DRAWING
The above and further advantages of the invention may be better understood
by referring to the following description in conjunction with the
accompanying drawings, in which:
FIG. 1 is a block diagram of a cellular telephone system provided with a
data overlay system in accordance with the invention;
FIG. 2 is a detailed block diagram of the voice basestation, data
basestation and data remote unit of FIG. 1;
FIG. 2a is a detailed block diagram of the central processing unit 58 of
the data basestation of FIG. 2;
FIGS. 3 and 4 are flowcharts depicting the respective operation of the data
remote unit and the data basestation shown in FIG. 2 during
remote-unit-to-basestation transmission of data packets;
FIG. 4a is a block diagram of an available-channel signature;
FIGS. 5 and 6 are flowcharts depicting the respective operation of the data
remote unit and the data basestation shown in FIG. 2 during basestation-to
remote-unit transmission of data packets; and
FIG. 7 is a flowchart depicting another, optional technique of assuring
that data service does not interfere with voice service in the cellular
telephone system of FIG. 1.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
FIG. 1 shows a cellular telephone system 10 including a data communication
subsystem 12 overlaying a voice communication subsystem 14. The voice
communication subsystem 14 has a plurality of voice basestations 16, each
associated with a coverage area or cell 18 of the cellular telephone
system 10.
Each voice basestation 16 provides voice communication services between one
or more conventional voice remote units 20a, which are equipped for voice
communication and present within its coverage area, and a conventional
telephone network 22. A voice switching system 26 connected to the
basestations 16 provides circuit switching for calls from the voice remote
units 20a to the telephone network 22, and vice versa.
The data communication subsystem 12 has a plurality of data basestations
36, each associated with one of the cells 18 of the cellular telephone
system 10. Each data basestation 36 can provide data communication
services between data remote units 20b, which are equipped for data
communication, when such remote units 20b are present with its coverage
area and a conventional data or computer network 42. A data switching
system 46 connected to the data basestations 36 provides packet switching
for calls from the data remote units 20b to the data network 42, or vice
versa.
For data transmission, the remote units 20b establish communications via
selected basestations 36 serving the cells in which the remote units 20b
are located. As the remote units 20b move from those cells 18 to others,
communications with those selected basestations 36 are terminated, and the
remote units 20b re-establish communications via other basestations 36.
The remote units 20a, 20b are depicted as mounted in automobiles, although
other portable radio telephones, such as, for example, hand-held units and
even non-portable wireless terminals (for example, security and
control/monitoring units), fall within the scope of the invention.
In accordance with the invention, the data communication subsystem 12 (i)
shares many of the same voice channels with the voice communication
subsystem 14, and yet does not interfere with voice service of the
cellular system 10, i.e., is transparent to users placing calls therein
for voice communication, and (ii) can often be implemented in cells 18
having a voice basestations 16 already installed without requiring
modification or retrofit of those voice basestations 16.
FIG. 2 depicts the components of the cellular telephone system 10
associated with one of the cells 18. Specifically, the voice basestation
16 contains a number of conventional transceivers 50, each of which being
tunable to a particular voice channel, and capable of duplex operation via
antenna 52 for voice communication over that channel in accordance with
cellular telephone standards. The voice basestation 16 also contains audio
lines 53 for connecting the transceivers 50 to the voice switching system
26 (FIG. 1), and thence to an existing land line telephone network 22 for
voice service. The voice basestation 16 also can include other
conventional components, which need not be described herein since they are
not involved in the present invention.
The data basestation 36 includes a number of transceivers 54, each of which
being tunable to communicate over an assigned voice channel and capable of
duplex operation via antenna 60 for data communication over that channel.
The transceivers 54 are each connected by a modem 56 to a central
processing unit ("CPU") 58. The CPU 58 is connected to the data switching
system 46 and, thereby, to the data network 42.
The basestation 36 receives a radio frequency signal containing digital
data over one of the allocated voice channels. The particular transceiver
54 that is tuned to that voice channel converts the received signal into a
baseband signal. The modem 56 coupled with that transceiver 54 demodulates
the baseband signal to recover the data contained therein. The data is
provided to the CPU 58.
Similarly, for transmission of data, the CPU 58 of the basestation 36
receives the data via the switching system 46 from the data network 42, or
generates control or status data, and, in either case, applies the data to
one of the modems 56 for modulation thereof in accordance with any
suitable modulation scheme, thereby generating a baseband signal. The
transceiver 54 coupled to that modem 56 modulates a carrier signal with
the baseband signal, and transmits the resulting modulated radio-frequency
signal via the antenna 60. The CPU 58 controls the operation of the
transceiver 54 and modem 56.
The data basestation 36 also employs a scanner 62, which receives a
voice-channel-utilization signal over line 64 from each of the
transceivers 50 of the voice basestation 16. That signal indicates whether
the voice channels associated with the transceivers 50 of the voice
basestation 16 are being used for voice communication.
For example, the voice-channel-utilization signal can designate the
initiation and termination of voice telephone calls over the voice channel
by representing the power levels or transitions, spectra, or other indicia
of voice transmission from the transceiver 50.
More specifically, the voice-channel utilization signal can indicate the
use of the voice channel by representing radio-frequency ("RF") power
levels or transitions between power levels in an output of the voice
basestation 16, or by representing control signals used within the voice
basestation 16, such as those used to control the variable-gain amplifiers
typically found in the terminal stages of transmitters transceivers 50
thereof.
Another technique is to obtain and demodulate the voice basestation RF
output signals, and check for the presence of certain control information
(e.g., supervisory audio tones) typically included therein in accordance
with the aforementioned standards and indicative of voice communication.
Many commercially available voice basestations have external ports which
permit access to one or another of these signals, in which case, the voice
basestations need not be modified to provide the aforementioned
voice-channel utilization signals to the data basestations.
Alternatively, rather than receiving the voice-channel-utilization signal
over line 64, the scanner 62 of the data basestation 36 can instead
intercept broadcasted voice communication signals on the voice channels,
and use, e.g., the RF power levels or signal spectra or contents of the
intercepted signals as the voice-channel utilization signals. Such
non-intrusive approaches obviously require neither modification of the
voice basestations, nor access to signals within the voice basestations.
The data basestation 36 also has a duplicate detector 65, which determines
whether a received data packet is a duplicate of a previously received
packet. The duplicate detector 65 can be implemented as a separate device,
or can be implemented as a software routine within the CPU 58.
FIG. 2a shows the components of CPU 58. CPU 58 has a processor 72, memory
74, clock or timing circuit 76 and one or more input/output ("I/O")
devices 78. These components 72 through 78 are interconnected by a bus 82,
and, along with other conventional components, can be implemented, for
example, in a single integrated circuit. Where the duplicate detector
routine is resident within the CPU 58, the routine can be stored in memory
74, and run by processor 72.
Returning to FIG. 2, the data remote unit 20b is a radio-telephone equipped
for data communication. The data remote unit 20b includes a transceiver
84, which is tunable to communicate over any of a plurality of voice
channels, and an antenna 92 capable of duplex communication in accordance
with cellular telephone standards. The transceiver 84 is connected via a
modem 86 to a conventional data terminal 88, e.g., a portable or other
computer.
For data transmission from the data remote unit 20b, a controller 94 passes
the data to the modem 86, at which the data is modulated pursuant the same
modulation scheme used by the data basestation 36. The modem 86 provides
the modulated data to the transceiver 84, which mixes the modulated data
with a radio-frequency carrier for transmission via the antenna 92 over a
voice channel. The transmitted data can be control or status data
generated by the controller 94, or message data generated by the data
terminal 88. The controller 94 controls operation of the other components
of the data remote unit 20b in accordance with a routine stored in a
memory 96.
On the other hand, when the remote unit 20b receives a broadcast data
signal over a voice channel, the transceiver 84 converts the received
signal into a baseband signal. The modem 86 then demodulates the baseband
signal so as to form data packets, which are then passed to the controller
94. The received data packets can be control or status packets for use by
the controller 94, or can be message packets, which are routed by the
controller 94 to the data terminal 88. A duplicate detector 98 ascertains
whether the received data packets are duplicates of ones previously
received.
The data remote unit 20b further contains a scanner 100, which is used
during channel selection as described hereinafter by causing the
transceiver 84 to tune to a plurality of channels to identify available
channels allocated for data communication. The purpose and operation of
the scanner 100, as well as the procedure for selection of voice channels
for data communication, will be described in greater detail hereinbelow.
An illustrative embodiment of the operation of the data remote unit 20b and
the data basestation 36 will be described next. It should be understood at
the outset that the operation of the data remote unit 20b is controlled
preferably by a program stored in memory 96 and executed by controller 94.
Likewise, the operation of the data basestation 36 is controlled
preferably by a program stored and executed by the CPU 58.
Remote-To-Base Data Transmission
FIGS. 3 and 4 show respective data remote unit and data basestation
operation during remote-to-base transmission of data in accordance with an
illustrative embodiment of the invention. In FIG. 3, in start block 102,
the remote unit 20b performs, e.g., self-diagnostic tests. Then, for data
communication, in block 104, the remote unit 20b performs channel
selection.
For channel selection, the remote unit 20b determines which of the control
channels of all voice basestations 16 in the cellular telephone system 10
carries the strongest signal, and then identifies the voice channels
associated with that control channel. This process assures that, as the
remote unit 20b moves relative to the various cells 18, data transmission
will occur with data basestations 36 and over voice channels that can
carry an adequately strong signal for reliable data communication.
Specifically, the scanner 100 in the remote unit 20b causes the transceiver
84 to scan a plurality of control channels that are stored in a
control-channel-database in memory 96. For each control channel, the
transceiver 84 measures the received-signal-strength and provides a
receive signal strength indicating ("RSSI") signal to the controller 94.
The controller 94 selects the control channel having the strongest signal
and then uses a voice-channel database also stored in memory 96 to
identify all voice channels associated with the identified control
channel, and, thus, associated with the voice and data basestations 16, 36
of the cell 18 that is associated with the identified control channel.
Preferably, the voice channel database is a listing of specific
"data-allocated channels" that have been allocated for data communication
by the cellular telephone system 10 to the data basestations 36. Thus, the
voice-channel database is a look-up table that identifies the
data-allocated channels for use by the remote unit 20b.
This look-up table can be either static or, preferably, dynamic, with
updates furnished from time to time by the data basestation 36. When
dynamic, the data-allocated channels identified in the look-up table can
be reduced in number so as to be only those channels which are not then,
at the time of the up-dates, as indicated by the voice-channel-utilization
signals, being used for voice communication. Methods for providing the
up-dates will be described shortly.
In blocks 106, 108, the remote unit 20b determines which of the
data-allocated channels identified in the stored voice channel database
are available, at that time, for data communication. Specifically, in
block 106, the remote unit 20b tests whether a first of the data-allocated
channels carries an "available-channel signature."
Preferably, the available-channel signature is a special digital signal
generated by the modem 56 in response to commands from the CPU 58, and
broadcasted by the data basestation 36 on any data-allocated channel which
is not at that time being used for voice or data communication.
As shown in FIG. 4a, the available-channel signature can have a first field
of digital data providing identification information that will be
recognized by the controller 94 as indicating that this particular signal
is an available-channel signature, thus indicating that the channel over
which it is being sent is available for data transmission. Preferably, and
as shown, the available-channel signal also has a second field of digital
data providing the voice-channel-database-update information that was
mentioned above.
(Alternatively, the update information for the voice-channel database need
not be contained in the available-channel signal, but, rather, can be
furnished by separate update signals sent by the data basestation 36,
e.g., periodically or whenever a change is to be made to the voice channel
database.)
Returning to FIG. 3, if the first data-allocated channel does not carry an
available-channel signature, then, in block 108, then the remote unit 20b
switches to a next data-allocated channel, and repeats the "detect
signature" test of block 106. This process continues until a channel
carrying the signature is identified, in which case the transceiver 84 has
been tuned to a voice channel available for data transmission.
Accordingly, in the preferred embodiment, the remote unit 20b itself
selects an available data-allocated channel for data transmission without
involving a hand-shake transaction (e.g., an exchange of control packets
on a control channel) with potential basestations.
Subsequently, in block 110, the remote unit 20b sends a data packet over
the identified, available data-allocated voice channel.
In block 112, the remote unit 20b tests whether, within a period T1 after
the data packet has been transmitted, the transceiver 84 stops receiving
the available-channel signature over the identified data allocation
channel used for that data packet transmission. Normally, in response to
receipt of a data packet from a remote unit 20b, as will be described
below, the data basestation 36 disables the available-channel signal on
that channel. Thus, this test provides a first indication of whether the
data basestation 36 has in fact received the data packet. This is
analogous to the "busy idle bit" routine specified in the above referenced
EIA RS-553 standards.
If no loss of the available-channel signature is detected within the time
T1, the remote unit 20b then returns to block 104 for channel selection.
On the other hand, if a loss of the available-channel signature is
detected within that period of time, the remote unit 20b next goes to
block 114.
In block 114, the remote unit 20b checks whether the transceiver 84 has
received an acknowledgement from the data basestation 36. Using
acknowledgements to confirm receipt of data packets finds utility herein
because, even though there has been a detected loss of the
available-channel signature, that loss of signature may have resulted from
receipt by the basestation 36 of a data packet from another remote unit
20b, and not from the one whose operation is being described herein.
Accordingly, if the remote unit 20b receives an acknowledgement, then the
data basestation 36 has in fact received the data packet broadcasted by
the remote unit 20b, and the remote unit 20b next returns to block 104 for
channel selection.
On the other hand, if no acknowledgement is received, then, in block 116,
the remote unit 20b again checks whether a available-channel signature has
been received from the data basestation within a time T2, thus indicating
that the same voice channel can be used for further data transmission. If
the signature is not detected, then the remote unit 20b returns to block
104, and channel selection is repeated.
On the other hand, if the signature is detected within T2, then in block
118, the remote unit 20b resends the data packet and, in block 122,
increments a count to indicate that the data packet has been resent. After
incrementing the count, the remote unit 20b then, in block 124, tests
whether the count is greater than a preselected number, N1. If the count
variable is not greater than the N1 limit, then the remote unit 20b
rechecks for loss of signature in block 112, as indicated by the circled
"B" in the drawing. However, if the count variable is greater than the N1
limit, then the remote unit 20b repeats the channel selection step at
block 104.
In certain applications, acknowledgements of transmissions are not required
and the acknowledgement steps just described can be omitted, in which
case, after sending the data packet in block 110, the remote unit 20b
returns to block 104.
FIG. 4 depicts data basestation operation during remote-to-base
transmission. As represented by block 132, the data basestation 36
initially performs, e.g., self-diagnostic tests. Then, in block 134, the
basestation 36 checks whether a first of the data-allocated channels is
being used for data or voice communication. The data-allocated voice
channels are identified in a data-allocated voice channel look-up table
maintained in memory 74.
More specifically, the CPU 58 determines whether the data basestation 36 is
itself using the voice channel for data communication, and determines
whether the basestation 36 has received a voice-channel-utilization signal
indicating that the voice channel is being used by the voice basestation
16 for voice communication.
If that channel is being so used, then, in block 136, the data basestation
36 switches to a next of the data-allocated voice channels and repeats the
test of block 134 for the next channel.
On the other hand, if the data basestation 36 determines that the
data-allocated voice channel is, not being used at that time for either
voice or data transmission, the basestation 36 next, in block 138, enables
an available-channel signature indicating that channel as available for
data communication. More specifically, the CPU 58 causes the transceiver
54 to broadcast, e.g., the special digital signal on that data-allocated
channel to identify that channel as being available for data transmission.
Further in block 138, the data basestation initializes and starts a data
channel use timer 72a, which is shown in FIG. 2a as a part of the
processor 72. The timer 72a responds to timing signals from the clock 76,
and measures the total duration of the use of the channel for data
transmission. The purpose of the timer 72a will be described below.
The data basestation 36 next, in block 142, checks whether the transceiver
54 has received a packet from any remote unit 20b. If no packet has been
received, the data basestation 36 waits until a data packet is received by
continuously performing the test of block 142. On receipt of a data
packet, in block 144, the data basestation 36 disables the
available-channel signature, i.e., stops broadcasting the signature, as
mentioned hereinabove during the description of remote unit operation.
In block 146, the data basestation 36 sends an acknowledgement of receipt
of the data packet.
After receiving the data packet, the data basestation 36 determines whether
the data packet is a duplicate of one that was earlier received. This test
is performed in block 148. If the packet is a duplicate, the data
basestation 36 ignores the duplicate packet per block 152. On the other
hand, if the data packet is not a duplicate, then, in block 154, the data
basestation 36 sends the packet to the data switching system 46 for
routing to its destination.
In either case, the data basestation 36 next, in block 156, once again
enables the available-channel signature, and then the above-described
procedure is repeated, as indicated by the circled "C" in the drawing,
starting with block 142.
As explained above, the cellular telephone system 10 provides data services
in such a way as to assuredly not interfere with voice services. To that
end, the data basestation 36 takes certain measures during remote-to-base
transmission to prevent such interference by appropriately controlling
data service. These measures will be described next.
After the data basestation enable the signature and starts the data channel
use timer in block 138, in addition to the test performed in block 142,
the data basestation 36 performs parallel tests in block 162 and 164.
Based on the outcomes of these tests, the CPU 58 causes the basestation 36
to step through the operations of the blocks 142 through 156 as just
described, or terminates, at any time, the operations of those blocks.
In block 162, the data basestation 36 continuously tests whether the voice
channel is being used for voice communication. The initiation of voice
communication is determined by the data basestation 36 through the use of
scanner 62 and CPU 58 respectively obtaining and recognizing a signal
(described hereinabove as the voice channel utilization signal) from the
voice basestation 16, which indicates that voice communication has begun.
On initiation of voice communication, the data basestation 36 immediately,
as indicated in block 166, disables the available-channel signature, stops
sending any acknowledgement that is being sent in block 146, and turns off
the transmitter 54 so that not even the carrier is being sent. Afterwards,
as indicated by the circled "D" in the drawing, the data basestation 36
switches to the next channel at block 136. On the other hand, as mentioned
above, | | |
