Note: Descriptions are shown in the official language in which they were submitted.
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TRANSMITTING DIGITAL DATA USING
MULTIPLE SUBCARRIERS
BACKGROUND OF THE INVENTION
This invention relates generally to wireless data
transmission and more particularly a system for
increasing transmission capacity by using multiple
subcarriers on FM broadcast radio stations.
It is well known that information can be
transmitted by means of a subcarrier contained on FM
broadcast radio waves. FIG. 1 shows spectral
components of FM signals used in transmissions of both
analog radio station broadcasts and digital paging
messages. Most FM broadcast radio stations only use
the baseband frequencies from 50 hertz (Hz) to 53
kilohertz (kHz) to transmit stereo program material.
In such systems, a first component 12 transmits
left plus right channel audio material. A component 14
then transmits left minus right channel audio material.
A stereo tone signal 18 is also transmitted at 19 kHz.
A mono-channel receiver processes the analog signal
from component 12. Alternatively, a dual channel
stereo receiver processes both component 12 and
component 14. In some geographical locations the FM
baseband also includes additional subcarriers.
The remaining baseband frequencies from 53 kHz up
to the legal maximum are typically available for the
transmission of other data. In the United States,
information can be transmitted at frequencies up to 100
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kHz. In other geographical locations, such as in Europe,
FM stations can only be transmitted at frequencies up to
75 kHz. Thus, different portions of the FM baseband may be
available on any station depending on baseband bandwidth
restrictions or the presence of other subcarrier signals.
Various issued patents, including U.S. Patents
4,713,808 and 4,897,835 (both by Gaskill) and
U.S. Patent 5,187,470 (King), show systems where digital
data is transmitted on an FM subcarrier 16 modulated on the
FM baseband shown in FIG. 1. Subcarrier 16 has a center
frequency at 66.5 kHz and a bandwidth that extends from
approximately 57 kHz to 75 kHz.
SUMMARY OF THE INVENTION
In a system which operates with subcarrier
channels of a given bandwidth, the invention significantly
increases the amount of payload data that can be
transmitted. Two separate subcarrier channels are utilized.
The amount of payload data transmitted utilizing the
invention is more than twice the amount which can be
transmitted utilizing one subcarrier channel. The timing
and synchronization information utilized by the first
channel can include information such as bit synchronization
information, packet synchronization (i.e. flags), slot and
frame synchronization information.
The second channel does not need to transmit this
same information. Instead, at the modulator, the
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transmission of the bit stream in the second channel is
synchronized to the transmission of the bit stream in
the first channel. Thus, at the receiver, the timing
and synchronization information extracted from the
first~channel can be used to accomplish the timing and
synchronization of information in the second channel.
Furthermore, utilizing this invention the data decoder
for the secondary channel can be simplified by
utilizing resources which are utilized for decoding the
first channel.
This invention is particularly useful in a system
designed to (a) operate on a large number of stations
some of which have subcarrier channels used for other
purposes and (b) which is designed to transmit
information to receivers that can receive signals on
either one or two channels each or which is located at
a particular region of the subcarrier. In such a
system, in a situation where one of the channels is
used for an unrelated purpose, information can be
transmitted on the signal channel which is available.
The transmission capacity of the system will be
decreased; since all of the information will have to be
transmitted in one channel instead of two; however, by
providing two relatively narrow channels rather than a
single wide channel, the system achieves significant
advantages in diversity. With the present invention,
which makes the sum of information transmitted on two
narrow channels more than would be expected by
utilizing two independent channels of similar width, a
two channel system becomes particularly advantageous.
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The foregoing and other objects, features and
advantages of the invention will become more readily
apparent from the following detailed description of a
preferred embodiment of the invention which proceeds with
reference to the accompanying drawings.
The invention may be summarized as a radio paging
system comprising: means for transmitting a paging message
from an FM transmitter to paging receivers via signals
carried on an FM baseband radio signal, said baseband radio
signal comprising: a first subcarrier containing both a
first portion of said message and control data, and a second
subcarrier containing a second portion of said message, the
second portion of the message identified by said control
data in the first subcarrier thereby increasing the
proportion of message data transmitted in the second
subcarrier.
According to another aspect the invention provides
a method for transmitting information from an FM transmitter
to at least one receiver, comprising: transmitting a first
subcarrier on an FM radio baseband containing control data
that identifies a target receiver for receiving a digital
data; transmitting a second subcarrier on the FM radio
baseband, the second subcarrier containing at least a
portion of the digital data; and reading the digital data in
the second subcarrier with the identified target receiver
according to the control data in the first subcarrier.
According to another aspect the invention provides
a method for transmitting digital time division multiplexed
messages from an FM transmitter to a receiver via signals
carried on an FM radio signal, comprising: transmitting a
first subcarrier on the FM radio signal, the first
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subcarrier including a time division multiplexed data stream
having a first set of data packets positioned at different
temporal locations, the data packets containing both message
data containing at least a portion of a message and control
data for identifying an associated receiver for receiving
the message and identifying the temporal locations of the
data packets; and transmitting a second subcarrier having a
second set of data packets containing a second portion of
the message, the control data from the first subcarrier
identifying the locations of the second set of data packets
to the identified receiver.
According to another aspect the invention provides
a radio paging system comprising': an FM transmitter and a
plurality of paging receivers, said FM transmitter being
adapted to transmit paging messages on an FM baseband radio
signal, said baseband radio signal comprising: a first
subcarrier containing first time slot that contains both a
first portion of a message and control data, and a second
subcarrier containing time slots that contains a second
portion of said message, said second portion being related
to the control data in the first time slot in said first
subcarrier thereby increasing the amount of message data
transmitted in the second subcarrier.
According to another aspect the invention provides
a radio paging receiver comprising: means for receiving data
packets on a first FM subcarrier, said packets containing
both a first portion of a message and control data, and
means for receiving a second data packet on a second
FM subcarrier, said second data packet containing a second
portion of said message, said second data packet being
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associated with the control data in the first subcarrier,
whereby the amount of message data that can be received via
said second subcarrier can be increased due to the absence
of said control data.
According to another aspect the invention provides
a radio paging transmitter comprising: means for
transmitting a first data packet on a first FM subcarrier,
said data packet containing both a first portion of a
message and control data, and means for transmitting a
second data packet on a second FM subcarrier, said second
data packet containing a second portion of said message,
said second data packet being identified by said control
data in the first subcarrier, whereby the amount of message
data that can be transmitted in said second subcarrier can
be increased due to the absence of said control data.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a prior art diagram of the baseband
spectrum of a typical FM broadcast station including a first
subcarrier for transmitting digital paging data.
FIG. 2 is a diagram of the FM baseband shown in
FIG. 1 including an additional synchronized subcarrier
signal according to the invention.
FIG. 3 is a diagram showing the format for control
and message data transmitted by the system in FIG. 2.
FIG. 4 is a block diagram showing a method for
transmitting digital messages over dual subcarriers
according to the invention.
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FIG. 5 is a block diagram showing a method for
receiving digital messages over dual subcarriers according
to the invention.
FIG. 6 is a block diagram of the transmission
system used for transmitting the FM baseband shown in
FIG. 2.
FIG. 7 is a block diagram of the receiver system
used for receiving messages over multiple subcarriers.
DETAILED DESCRIPTION
The present invention is designed to operate
within the baseband spectrum of a typical FM broadcast
station as previously shown in FIG. 1. The specific format
of the transmitted messages and the hardware used for
transmitting and receiving the messages as discussed below
are described in detail in U.S. Pat. No. 4,713,808 to
Gaskill.
Referring to FIG. 2, subcarrier 16 contains a time
division multiplexed (TDM) digital message that is
transmitted to a specific target receiver. A second
subcarrier 22 is transmitted in conjunction with the first
subcarrier 16 to increase the overall frequency bandwidth
available for transmitting raw message data. In one
embodiment, data in subcarrier 16 is transmitted to a one-
way wireless pager receiver. However, the invention can be
utilized in any system used to transmit wireless digital
information.
The FM baseband in FIG. 2 includes an existing
57 kHz subcarrier 20 which exists on some FM stations. As
explained above, some FM transmission frequencies cannot
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exceed 75 kHz. Thus, subcarrier-16 is located between the
upper limit of the audio programming and the common baseband
frequency limit of 75 kHz. Subcarrier 16 has a center
frequency at 66.5 kHz.
The second subcarrier 22 is located somewhere
between 75 kHz and the 100 kHz upper frequency limit for
FM transmissions. The center frequency of subcarrier 22 is
typically located at 85.5 kHz but could vary according to
available baseband frequencies. In general, subcarriers
16 and 22 have the same bandwidth which is equal to the
frequency of the FM stereo pilot frequency 18
(i.e., 19 kHz). Further, the center frequencies of each
subcarrier are selected, if possible, at some multiple of
the stereo pilot frequency (e. g., 3.5 or 4.5).
Subcarrier 22 is then transmitted in synchrony to
subcarrier 16. Thus, after the portion of the message
transmitted in subcarrier 16 is located by the receiver,
remaining portions of the message located in the second
subcarrier are also known. Therefore, bandwidth overhead
required to synchronize the first subcarrier message with
the receiver is not required in the portion of the message
transmitted in the second subcarrier. Additionally, reduced
synchronization hardware is required for the second
subcarrier.
The second subcarrier 22 doubles the amount of the
FM baseband spectrum used by the digital transmission
system. However, all synchronization and addressing
overhead is located in subcarrier 16. Therefore,
subcarrier 22 more than doubles the amount of bandwidth
available for transmitting raw message data while only using
twice as much of the FM baseband.
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For example, in one embodiment, the first
subcarrier 16 carries 260 bits of data per packet. However,
only approximately 240 bits contain raw message data. Out
of these 240 bits, there is time slot information, format
data, error checking data, etc. After subtracting out this
overhead data, only approximately 75% of the bandwidth of
subcarrier 16 is
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used for transmitting raw message data. Alternatively,
almost 100 of the bandwidth of the second subcarrier
can be used for transmitting raw message data.
To explain further, FIG. 3 shows one format for
transmitting digital data. Subcarrier L6 carries a
master frame containing multiple subframes similar to
subframe 24. Subcarrier 22 carries additional extended
data in subframes similar to subframe 28. The first
three time slots 26 in subframe 24 contain control data
used for master frame synchronization, time of day,
station identification, etc. Subsequent message time
slots 0 through 1023 contain data packets which each
contain, data addresses, format fields, error
correction bits, etc. Subcarrier 22 also carries
multiple subframes similar to subframe 28. The first
few time slots in subframe 28 are control time slots
for the extended data. Each subframe in subcarrier 22
also includes message time slots that only carry
additional extended data. The format of the master
frame, subframes and message time slots are described
in Gaskill and are, therefore, not explained in detail.
The second subcarrier 22 carries an additional 260
bits for each data packet from the first subcarrier.
Thus, any packet read from the first subcarrier is
"followed" synchronously with 260 bits on the second
subcarrier. One major advantage of transmitting two
subcarriers is that all synchronization receiver
identification, message identification, format, error
correction codes, and cyclic redundancy checks, etc.,
are identified in the data on the first subcarrier 16.
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For example, message time slot 7 in subframe 24
tells the receiver that an additional portion of the
message is located in either subframe 24 or 28. The
remaining times slots in subframe 28 are message time
slots containing data. Thus, message time slot 7 in
subframe 24 directs the receiver to message time slot 7
in subframe 28. The receiver then reads the remaining
portion of the extended data contained in subframe 28.
By the time the initial portion of the message in
time slot 7 is read, the receiver has already
synchronized with the data transmitted in subcarrier
16. As a result, bits used in the message time slots
of subframe 24 to identify the correct target receiver
are not needed in the message time slots transmitted in
subframe 28. Thus, substantially more of the bits in
the extended data packets located in subframe 28 are
available for storing raw data.
FIG. 4 shows a method for transmitting digital
messages on multiple subcarriers. Decision block 30
determines whether a first subcarrier is available for
transmission on the FM baseband. If the first
subcarrier is available, decision block 31 then
determines whether a second subcarrier is also
available for transmission on the FM baseband. A
second subcarrier cannot be transmitted in some
locations. In this situation, block 35 encodes data
into a master frame only on the first subcarrier 16
(FIG. 1). Block 36 then transmits only the first
subcarrier on the FM baseband.
When a second subcarrier 22 is also available for
FM transmission in addition to the first subcarrier,
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decision block 31 jumps to block 34. Block 34 encodes
the message data into subframes transmitted on multiple
subcarriers. Block 36 then transmits the message on
both subcarriers 16 and 22 (FIG. 2) on the FM baseband.
Referring back to decision block 30, a situation
may occur where the first subcarrier is not available
for transmission on the FM baseband. Accordingly,
block 33 encodes the entire message on the second
subcarrier. The message is then transmitted on the FM
baseband in the second subcarrier 22 block 36.
The proportions of the message transmitted on the
first and second subcarriers depend on the number of
other messages being transmitted to other receivers and
on the length of the message. For example, during
times of low utilization, a larger proportion of the
message may be transmitted on the first subcarrier.
However, during high traffic conditions, higher
proportions of a message may be transmitted on the
second subcarrier.
The dual subcarriers can also transmit messages to
multiple receivers. For example, the dual subcarriers
could be used to transmit stock market reports, scores
of sporting events, etc. to multiple paging receivers
at the same time.
FIG. 5 shows the method for receiving a digital
message over multiple subcarriers in an FM baseband.
Block 38 first determines whether a first subcarrier is
available. If the first subcarrier is available, block
40 synchronizes the receiver to data on the first
subcarrier and block 42 then decodes data packets in
the first subcarrier.
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Decision block 44 determines whether a second
subcarrier is available for transmitting data. If the
second subcarrier is not available, the receiver
returns for receiving the next data transmission. If
decision block 44 determines that the second subcarrier
is available, block 46 decodes packets in the second
subcarrier utilizing synchronization from the first
subcarrier.
Referring back to decision block 38, if the first
subcarrier is not available, decision block 48 then
determines if the second subcarrier is available. If
neither the first or second subcarrier are available,
decision block 48 generates an error message. If the
second subcarrier is available, the receiver in block
50 synchronizes to the data in the second subcarrier.
Block 52 then decodes packets in the second subcarrier
and then returns for receiving the next message.
A system of the general type shown in FIGS. 6 and
7, is utilized to generate the FM baseband spectrum
shown in FIG. 2. The transmission system shown in FIG.
6 includes a digital data signal 50 that is passed
through a digital to analog converter (DAC) 54. The
analog signal is then passed through a set of filters
56 creating the first and second analog subcarrier
signals 16 and 22, respectively (FIG. 2). Subcarrier
signal 16 and 22 are then applied to the subcarrier
input port 60 of a broadcast FM transmitter 62.
As is conventional, transmitter 62 includes a
summation circuit 70, a voltage controlled oscillator
72 and an amplifier 74. Radio frequency energy from
the FM transmitter 62 is then broadcasted as FM radio
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waves by an antenna 76. Contained within the
transmitter 62 is the baseband signal 68 which consists
of the summation of the subcarriers 16 and 22, the
stereo program material 64, and any other subcarriers
that may be present 66. Such transmitters and antennas
are commercially available.
The receiver system shown in FIG. 7 includes an
antenna 77 for converting radio waves into electrical
signals which are then amplified and demodulated by a
conventional or unconventional FM receiver 80. The FM
receiver is capable of tuning over the range of the
international FM broadcast bands. The output 86 of
receiver 80, consists of the summation of the baseband
signal 68 of the FM broadcast station.
The baseband signal is then digitized by an analog
to digital converter 88. After digitization, the
signal is applied to one or more filters 90 which have
appropriate passbands so as to produce the desired
digital data from subcarriers 16 and 22 on its output
92. The details of the functional blocks in the
transmitter and receiver are understood by one skilled
in the art and are, therefore, not described in detail.
Because the two subcarriers are modulated in
essentially the same manner, most of the decode
circuitry in the receiver is the same. For example, as
previously shown in FIG. 2, the first subcarrier has a
center frequency of 66.5 kHz which is 3.5 times the FM
pilot frequency (19 kHz). The second subcarrier has a
center frequency of 85.5 kHz which is 4.5 times the FM
pilot frequency. The second subcarrier can be passed
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through the same filter 90 and then combined with the
digital data from the first subcarrier. Thus, common
circuitry can be used to perform the analog to digital
conversion, clock extraction, filtering, and data
decoding for both subcarriers.
The reduction in hardware is especially
advantageous for receivers with space constraints and
portable power supplies. For example, the redundancy
in circuitry for a dual subcarrier receiver allows the
data payload to increase by more than a factor of two
while using essentially the same hardware. Because the
receiver hardware doesn't have to operate at a higher
speed or power more circuitry, the receiver power
supply can operate for longer periods of time.
By transmitting multiple subcarriers, the receiver
also processes data in a more efficient manner. For
example, to conserve energy, a paging system polls at
predetermined times to determine if a message is being
transmitted. Thus, a large amount of time and energy
is spent simply trying to identify messages.
After the receiver successfully identifies a
message, the receiver can be quickly and efficiently
directed to additional portions of the message
contained in either the first or second subcarrier.
For example, as described above, the receiver is
directed by an address to additional time slots
containing portions of a message. The present
invention allows the receiver to activate only for
those time slots containing additional portions of the
message. Thus, minimal additional energy is required to
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read large portions of a message located in the second
subcarrier
Therefore, the system described above
substantially increases the available bandwidth of
wireless digital data transmissions with minimal
changes in existing hardware. Further, multiple
subcarriers can be transmitted without changing present
digital data transmission formats.
Because one or more subcarriers are capable of
transmitting data, the system also has the advantage of
being more adaptable to various subcarrier arrangements
in the baseband frequency spectrum. For example, if a
first subcarrier is already allocated to another
system, a message can be sent on one of the alternative
subcarriers. Alternatively, if each subcarrier is
available, larger messages can be sent in a shorter
amount of time.
Having described and illustrated the principles of
the invention in a preferred embodiment thereof, it
should be apparent that the invention can be modified
in arrangement and detail without departing from such
principles. I claim all modifications and variation
coming within the spirit and scope of the following
claims.
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CLAIMS
1. A radio paging system wherein paging messages
are transmitted from an FM transmitter to paging
receivers via signals carried on an FM baseband radio
signal, comprising:
a first subcarrier containing both a first portion
of the message and control data, the control data
identifying the message with a given paging receiver;
and
a second subcarrier containing a second portion of
said message directed to the given paging receiver, the
second portion of the message identified by the paging
receiver according to the control data in the first
subcarrier thereby increasing the proportion of message
data transmitted in the second subcarrier.
2. A radio paging system according to claim 1
wherein the FM baseband signal includes a stereo pilot
frequency and the first subcarrier includes a frequency
bandwidth substantially equal to the stereo pilot
frequency.
3. A radio paging system according to claim 1
wherein the first and second subcarriers have
substantially the same bandwidth.
4. A radio paging system according to claim 1
wherein each paging receiver includes a single circuit
for decoding both the first and second subcarriers into
digital data.
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