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Description  |
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The present invention is directed to the introduction of numerous
additional digital communications channels to an existing communications
system already having several communications channels. More specifically,
it is directed to the introduction of such additional channels in a
cellular telephone system, whether in today's primarily analog system or
future primarily digital systems.
BACKGROUND OF THE INVENTION
Because of the crowded electromagnetic spectrum used for communications, it
is useful to, in effect, squeeze extra communications channels into an
already established communications band or system. In the microwave field,
extra data channels are accommodated in an FDM-FM voice carrier known as a
Data Under Voice service, which is provided by AT&T. Other technologies
narrowing channels and allowing closer channel spacing and more capacity
have been found to be valuable technological advancements. In addition,
there is provided apparatus for multiplexing a number of speech and low
speed data channels on a single data multiplex system.
In such techniques as above, technical complexity is required to add the
additional capacity without diminishing service on the previous or
existing system. This includes, of course, non-interference with the
existing channels.
OBJECT AND SUMMARY OF INVENTION
It is therefore a general object of the present invention to provide an
improved communications system either for fixed to fixed site or fixed to
mobile site through additional channels which may be utilized with an
already established communications system.
In accordance with the above object, there is provided an advance mobile
phone service system having a plurality of contiguous cells each with a
cell site having a radio transceiver with voice and setup channels for
communicating with mobile units within the cell. Each of the channels has
a predetermined frequency bandwidth, a predetermined portion of the
channels for each cell having alternate non-used channels between used
channels to provide adequate adjacent channel separation while information
is being transmitted on a used channel at a predetermined power level. The
system is characterized by additional data channels for carrying digital
data, including means for transmitting digital data to and from at least
one field unit in a cell and the cell site, concurrently with information
on a said used channel, each of the additional channels having a frequency
centered substantially midway between the used and unused channels of the
same or adjacent cell and including means for transmitting on the
additional channels at a power at least an order of magnitude lower than
the predetermined power level and for controlling the bandwidth spreading
of the additional channels to prevent interference with adjacent used
channels.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagrammatic view of a cellular communications system embodying
the present invention.
FIG. 2 is a frequency or channel allocation chart showing the additional
channel or extra communications channels provided by the present invention
and their frequency location.
FIG. 3 is a more detailed enlarged view of a portion of FIG. 2 showing the
interaction between standard communications channels and the extra
channels of the present invention.
FIG. 4 is a deployment pattern for a center fed cellular system.
FIG. 5 is an illustration of the repeated use of the deployment pattern of
FIG. 4.
FIG. 6 is a deployment pattern for an edge fed cellular system.
FIG. 7 is a circuit block diagram illustrating the present invention.
FIG. 8 is a more detailed diagram of FIG. 7.
FIG. 9 is a diagrammatic view of a cellular communications system embodying
an alternative embodiment of the invention which provides for locating a
mobile beacon.
FIGS. 10A and 10B are timing diagrams illustrating the operation of a
portion of FIG. 7.
DESCRIPTION OF PREFERRED EMBODIMENT
The data system of the present invention is used in conjunction with what
is known as a cellular mobile radio system or more technically, an
advanced mobile phone service (AMPS) system. Such a system as illustrated
in FIG. 1 has a number of hexagonally shaped contiguous cells (cells A, B,
C, D, E and X are illustrated), each with a cell site 11 which is a base
communications station having a transceiver with the capability of calling
(paging) a mobile phone or receiving (accessing) a call from a mobile
phone. Such a phone system is described in articles in the Bell System
Technical Journal, January 1979, entitled, "The Cellular Concept" on page
15, and a second article entitled "Control Architecture" on page 43.
The Federal Communications Commission (FCC) has allocated to the AMPS
Cellular Service 800 two-way channels with 30 kHz of frequency spacing
between channels. (This is illustrated in FIG. 2.) From a specific
standpoint 800 of the channels are for transmitting from a mobile field
unit (e.g., an automobile) and another 800 (offset by 45 MHz) for
transmitting to the automobile. The channels are used in typical urban
areas in a pattern of cells, as illustrated in FIG. 1, each cell covering
an area of several square miles. The cells are placed to cover the areas
served. There may be ten to twenty such cells covering a typical urban
area. In the AMPS Cellular Service, each cell will have one channel for
control or setup and typically two to twenty channels for voice services.
The setup channels are for the purpose of paging mobile phones, giving
access to the phones, and avoiding collisions between competing mobile
phones. The cellular system requires transmission of control signals that
fully occupy 20 kHz of spectrum each; they even contain significant
amounts of energy over a bandwidth of 40 kHz. As a result, a normal
Cellular receiver will experience an unacceptable amount of interference
if it is spaced only 30 kHz from an interfering channel. For this reason,
cellular frequency planning guidelines suggest a spacing of at least two
channels or 60 kHz between channels within the same cell or between a
channel in one cell and a channel and any cell immediately adjacent to it.
This requirement for acceptable cellular operation leaves space, in
frequency and area, for a properly designed system.
The foregoing is shown in FIG. 2 where, for example, cell A occupies
channels 1 and 3, cell B, which is adjacent to cell A, channels 5 and 7,
and then cell E, which is non-adjacent to either cell A or cell B, is
allowed to occupy channels 2 and 4 which are channels immediately adjacent
in frequency to the respective channels 1 in cell A and 5 in cell B.
In addition, the setup channels, since they relate to the control aspects
of an AMPS cellular system, generally have fixed channel allocations. On
the other hand, the voice channels may from time to time be moved from one
cell to another to accommodate change in demand. Thus, in view of the
fixed nature of the setup channels, it may be preferred initially to
utilize the system of the present invention in conjunction with the setup
channels.
The cellular data system of the present invention, in its preferred
embodiment, uses 2 kHz bandwidth 2 kb/sec data carriers operating at
positions .+-.15 kHz off the cellular channels in each cell. The spectral
width and the transmitter power of each digital channel is closely
controlled to prevent interfering energy from entering the receiver of the
adjacent "standard" cellular channel. Thus, a pair of channels, one 15 kHz
higher and one 15 kHz lower, as illustrated in FIG. 2, can be used next to
every channel which is allocated in the cellular system, either voice or
setup channel.
This channel location, .+-.15 kHz off the cellular channels in each cell,
is guaranteed to also be noninterfering with standard channels located at
+30 kHz and -30 kHz since by AMPS standards such channels will not be
located within the central cell or within any immediately adjacent cell.
In the AMPS Cellular System, there are 800 two-way channels. Thus, as
illustrated, there may be 1600 extra two-way data channels. In addition,
as illustrated with respect to cell X, the same frequency can be used,
that is at 12' and 13', if cell X is more than 10 miles from the other
remaining cells. This may be possible in two or three other cells also in
the urban area. Thus, in the same geographical urban areas, the entire
system may accommodate as many as 4800 two-way data channels. Thus, the
total data carrying capacity of all channels in an urban area is as much
as 9.6 Mb/sec.
FIG. 3, in an enlarged format, indicates the additional channels 12 through
15 in conjunction with "standard" channels 1, 2 and 3 and shows the
relative power levels which have been found ideal to maintain an adequate
separation and prevent interference. Thus, the nominal power level of
channels 1 and 2 is approximately 3 watts in the AMPS system, as
indicated, with the power level of the additional channels 12-15 being
substantially one-third of a watt; in other words, it is a power level
which is an order of magnitude lower.
Then, in conjunction with a technique for controlling bandwidth spreading,
interference does not occur between the additional channels and the
existing channels 1 and 3 until a low dB level 81 has been reached. This
is a low enough level so as not to interfere with an existing operating
channel of the cellular system.
In addition to the very necessary elimination of interference between the
additional channel and the existing used channel on which the additional
channels ride as if on the "shoulder" of the used channel, interference
must be prevented with the receivers of the adjacent channel #2, as
illustrated in FIG. 3. Referring to FIG. 1, such interference would occur
when a field unit is transmitting in cell A and that signal, for example,
reaches cell E, where channel 2 is an active used channel. In the existing
cellular system, interference is prevented by the relatively low power
level indicated at 82 at which the transmitter bandwidth characteristic of
channels 1 and 3 overlap into channel 2 and intersect with the receive
bandwidth characteristic of channel 2 indicated by the dashed outline 84.
This receive bandwidth is relatively narrow (20 kHz, for example, as
opposed to the nominal 30 kHz spacing of the channels) and thus intersects
the transmissions of channels 1 and 2 at a fairly low power point on their
transmission bandwidth characteristics.
In a similar manner the additional channels provided by the present
invention, namely, channels 13 and 14, as illustrated in FIG. 3, have an
intersection point with the receive bandwidth characteristic curve 84 at a
point equal to power level 82, or as indicated at a lower point 83.
Thus, the unique location of the additional channels midway between the
existing channels of the cellular system is guaranteed to interfere with
an adjacent channel less than the already existing interference vcaused by
the out of band power of the channels 1 and 3. And both of these power
levels, namely, 82 and 83, are below the receive threshold level so
indicated in FIG. 3.
FIG. 4 shows a common technique of assigning the AMPS channels in a pattern
following the rules that no adjacent channel can be used in the same or
adjacent cells and that no channel may be used again any closer than 21/2
cell diameters. In FIG. 4 there are twelve cells grouped in a cluster;
that is, the twelve cells within the darkened outline 20. Each cell is
served by an antenna mounted in its center and radiating power equally in
all directions. This is called a center-fed cell. In this pattern, twelve
consecutive channels, spaced 30 kHz from each other, are assigned to the
twelve cells. Note that no consecutive numbers are placed in adjacent
cells and that channel 1 is not adjacent to channel 12.
If more area coverage is required than covered by the twelve cells, the
pattern is repeated to the left and to the right with the same exact
pattern of assignment. The pattern can also be repeated up and down,
shifted by 60.degree., as shown in FIG. 5. With this repeat pattern, an
arbitrarily large area can be covered reusing the same twelve channels
many times. The twelve channels are so organized within the basic pattern
outlined in dark 20 such that with the repeated expansion still no
channels (even at borders between the basic pattern) are assigned with
adjacent channels in adjacent cells. Also, in the repeated pattern a
channel always appears at least 21/2 cell diameters away from any other
cell using the same channel.
If more than one channel is needed in each cell, then a second set of
twelve channels can be assigned in the same pattern, with channel 13 being
in the same cell as channel 1, channel 14 being in the same cell as
channel 2, and so on until channel 24 is in the same cell as channel 12.
Similarly, if a third channel is required in each cell, channels 25
through 36 can be assigned in the same manner.
The 800 channels can be divided into groups of twelve and assigned as
described to cover the projected service needs. If thirty-six cells were
sufficient to cover an urban service area, the 800 channels would each be
used three times in the area. That is a total of 2,400 channels that would
provide service.
Using the technique of the present invention, for each of the normal
service channels described above two 2 kb/s data channels could be placed
on its edges, one at +15 kHz and one at -15 kHz from the normal service
frequency. Control of the power as described above would allow the new
service to be offered with no interference to the old service.
The above pattern or similar ones meeting the same adjacent channel and
co-channel interference criteria are used in AMPS systems. The patented
technique is designed to work on any and all such patterns of frequency
assignment.
Another technique used to prevent interference in the AMPS system is shown
in FIG. 6. This technique employs edge-fed cells. The base station towers
are placed on the borders between the cells. On each tower, three
antennas, each with its own unique channels, are used to broadcast into
the three adjoining cells. Each antenna is directional, concentrating its
energy or sensitivity into its cell and restricting energy or sensitivity
into the other two adjacent cells. Each antenna covers only 120.degree. of
angle. As shown, three antennas cover each cell from three of its corners.
In FIG. 6, twenty-one channels are assigned to a group of seven cells,
three to each cell, i.e., one on each of the three towers at the cell's
corners. The pattern is repeated to the upper right at 60.degree. angles
and to the upper left at 30.degree. angles to cover an entire area.
In this allocation technique, the protection is dependent both on antenna
pattern directivity and on cell spacing. Note that adjacent cells are
assigned adjacent channels, i.e., channels separated by only 30 kHz from
each other. In the basic set of seven cells in FIG. 6 note that channel 5
is serving a cell that is adjacent to a cell above served by channel 6 and
a cell to its lower right served by channel 4. This is allowed because the
antennas radiating the energy for channel 4 and channel 6 are pointing
away from the area served by the antenna radiating the energy for channel
5. A mobile receiver listening to signals from channel 5 in the central
cell will have the extra protection from the directivity of the antennas
radiating energy on channels 4 and 6 in the adjacent cells. These antennas
do not radiate full signal strength into the cell using channel 5.
Similarly, the mobile transmitting its signals back to the tower receiving
channel 5 will be received on that tower's antenna with high sensitivity.
Other mobiles in the two adjacent cells on channels 4 or 6 will be
received at the central tower (channel 5) with lower sensitivity because
they are outside of the main antenna pattern.
Note also that when the pattern is repeated right or left or up or down the
same channel is reused at a distance that is only a little more than 2
cell diameters. However, again note that the two antennas are not facing
towards each other. This added antenna selectivity allows the spacing for
reuse to be reduced.
This edge-fed cellular frequency plan is used in many AMPS installations.
In most applications irregularities in geography and in service
requirements in each cell have resulted in a mix of edge-fed and
center-fed cells being used.
In the edge-fed application, the invention is implemented by radiating the
2 kb/sec new data channels located .+-.15 kHz from the normal service
channels with the same antenna patterns used by the normal AMPS service.
This is done either by using the same antennas (through duplexer antenna
couplers) or by using a separate antenna with the same pattern as the
normal antenna.
With this technique, the new data service has the same interference
improvement from antenna directivity as the normal AMPS service. The
protection between new service and the normal service is the same as in
the center-fed case.
The preferred embodiment of the invention is to use the same mix of
edge-fed and center-fed cell coverages as used in the AMPS service,
placing two data carriers, one at +15 kHz and one at -15 kHz from the
normal channel in each cell and using the same coverage patterns as the
normal channels.
Two other strategies (outlined below) are claimed as usable under some
circumstances, but not normally as effective as the preferred embodiment
described above.
In a few AMPS applications, because of very high demand or special
geographic conditions, some cells will use adjacent channels in adjacent
cells even though the cells may be center-fed. This does cause extra
service loss in the normal system. Loss is reduced by control of power in
the two cell sites, but usually it is not reduced to the levels
encountered with the normal frequency plans.
In these non-standard AMPS installations, the invention is used just as in
the standard application. That is, two data channels are used at .+-.15
kHz from the normal channel and at an order of magnitude less power. This
will provide some interference to the services in the adjacent normal
channels in the adjacent cells. However, this interference, for the
reasons illustrated in FIG. 3, will be no worse than the interference
caused by the normal system itself.
A second alternative embodiment is also shown in FIG. 2. In this
embodiment, the new data channels 13a, 13b, 13c, etc., are located at
greater than .+-.15 kHz separation from the normal channel, for example,
at .+-.17.5 kHz, .+-.20 kHz, .+-.22.5 Hz or .+-.25 kHz. The power of the
data carriers is more than an order of magnitude less than the normal
carriers. By spacing the channels farther from the normal channel and
using directional antennas on the remote data transceivers, the base
station data transmitters can be reduced to even lower values. This
reduction will in many cases prevent interference to mobiles receiving the
normal channels at .+-.30 kHz since they typically are located more than
21/2 cell diameters away from the service cell. Similarly, the directional
antenna of the remote data transceiver prevents power of its transmission
from interfering with the normal base station receivers at .+-.30 kHz
located at least 21/2 cell diameters away.
While it is possible to add the data channels in this way, the application
is complicated by the need to closely coordinate directionality of both
the base station and remote transceiver antennas of the data system. It is
also very difficult to the point of impracticality for mobile data users.
It is also subject to added complexities when the normal AMPS channels are
moved to meet changing demand. For these reasons, while this technique is
an alternative embodiment, it is not preferred in normal applications.
FIG. 7 illustrates a typical circuit block diagram of the equipment
necessary to implement, for example, an extra channel 12. Within a cell is
a field unit 16 having a directional antenna 17 aimed at the
unidirectional antenna 18 of the cell site or base station 11. Referring
briefly to FIG. 1, two directional antennas are shown one at 17 and one at
17' which is at the periphery of a cell. The closer directional antenna
(since it is fixed and not mobile in one particular context of the present
invention), has a gain of only three decibels whereas the peripheral
antenna 17' may have a gain of 8 to 15 decibels. Thus, the power of the
antenna is adjusted to that amount necessary for adequate transmission and
reception. The closer it is to the base station the less power is
necessary. Thus, this preserves the power relationship illustrated in FIG.
3 where the power level for the additional channel is substantially an
order of magnitude lower than the normal power of the standard channel.
Referring again to FIG. 7, in the field unit 16, which has its individual
identification code, there is an input/output unit 21 which either
receives or transmits information and with a power level regulated by an
automatically adjustable input 22 connected to the output unit 21. Input
data from a unit 23 is typically converted to digital format by a minimum
phase shift keying (MSK) modulation technique in unit 24 and coupled to
antenna 17 via output unit 21. Unit 24 also acts as a homodyne demodulator
for received information. Finally, field unit 16 includes a base station
frequency lock unit 25 which locks on to the signal frequency from the
cell site 11 to thus allow each field unit to utilize as a carrier
frequency reference the frequency reference of the base station 11, which
is provided by a precision crystal.
The input data unit 23 is also designated "alarm" to indicate one of the
preferred uses of the present invention. That is, as a system to monitor
on a continuing basis the status of associated alarm units, for example,
either for antitheft purposes or for safety purposes. In other words, the
field unit 16 would be paged by the base station perhaps once every 30
seconds to ascertain whether or not it is in an alarm state or if it is
functioning normally. For example, a binary `0` might indicate `O.K.` and
a binary `1` an alarm. In a co-pending application entitled "Method of
Radio Data Communication" filed Sept. 9, 1988, Ser. No. 242,958, such a
paging technique is described.
The foregoing co-pending application also discusses an efficient use of the
communications system of the present invention where, as illustrated in
FIG. 7, effective response time in a time division multiplexing mode, is
provided by an adjustable time delay register 71. This register is loaded
by instructions from time delay instruction unit 72 of the base station 11
so that each field unit 16 is given a unique time delay in which to
respond in sequence to a request from the base station. In addition, the
time delay register has a fixed portion which is keyed into the ID number
of the field unit so that the initial response binary width response of a
`0` or `1` is provided in the proper time division multiplex sequence
without one field unit in a cell interfering with another. This provides
for very efficient utilization of the additional communications channels
provided and allows a relatively low data rate to be used to thus minimize
frequency spreading, which is of course a critical requirement of the
present invention.
Input data unit 23 may also be utilized for any type of home or office
management system monitoring such as remote meter reading, etc. In
addition, any type of digital communication and even digitized voice,
which is compressed, may be used. All that is required is that the
relatively low digital data rate, such as 2 kb/sec be sufficient. The
technology is especially useful with the paging technique of the above
co-pending application in sporadic data applications. One of these might
be automatic teller machines where the communication is in short blocks
separated by large time intervals. Others include answer back paging and
access to data terminals in vehicles.
Another application might be as illustrated in FIG. 9 where the modified
cell site antennas 17' are utilized, for example, in cells B and D, to
sense by well known angular tracking techniques (for example, see the
Loran Navigation System) the location of a truck or car 30 (in cell C)
carrying a locating beacon. Here the cell sites 11 would respond both to
the angle and the frequency and time of transmission for proper
correlation of data. This application has use in vehicle theft monitoring
and location.
And finally, although the use of stationary field units 16 is illustrated,
mobile field units could also be accommodated for the same reasons that
they're accommodated in a normal cellular systems. But the power levels
would still be maintained as above and the data transmission rate and
frequency spreading would be carefully controlled by the above technology
to meet the non-interference criteria set out above.
Still referring to FIG. 7, at the base station or cell site 11, there is a
circuit almost identical to field unit 16. An input/output unit 21' feeds
mod/demod 24'. Crystal frequency reference 28 is oven controlled to
provide a precise carrier frequency reference for all field units. A
microprocessor 31 provides overall control of the system--at least the
additional channel and the accessing and recording of needed information.
For example, if the input data is an alarm indication for security
purposes, then the microprocessor would cause an immediate investigation
to take place at the location of that particular field unit.
FIG. 8 is a more detailed block diagram of both the field unit 16 in solid
outline and shows in dashed outline at least part of the modification for
the base station or cell site 11. First, considering the field unit 16
only, the antenna 17 receives input data which is amplified by an
amplifier 51 and is fed to a homodyne type demodulator generally indicated
at 24, which includes the mixers 52 and 53. The other mixing input comes
from a receive local oscillator 54 which drives a multiplier 56, which in
turn is directly connected to mixer 52 but has its signal delayed by
90.degree. in unit 57 for the mixer 53. This then provides via amplifiers
58 and 59 signals (with a carrier frequency suppressed--in other words, a
homodyne technique) of I and Q components. A demodulator unit 59 then
processes the signal which, as discussed above, has been quadrature phase
shift modulated and this is coupled to the microprocessor 31.
In operation, demodulator 59 gives a net count of the average frequency of
the received waveform. This count is processed by microprocessor 31 and is
used as a frequency error detector to control via the lines 61 and 62 the
receive oscillator 54 and the transmit oscillator 63.
Specifically, the transmitter data is encoded with a net of a zero
frequency offset over a ten second interval to provide the resultant
demodulated signals to be used for a simple automatic frequency control
(AFC) correction. In other words, the receive oscillator 54 is adjusted to
provide a zero difference frequency via the mixers 52 and 53 during this
test period. And local oscillator 54 includes a varicap control which
adjusts the low cost crystal 54a, which is a part of the receive
oscillator. Such control provides an error of .+-.160 Hz. The transmit
frequency is generated by the same receive oscillator 54, plus an offset
transmit oscillator 63 (which is offset a nominal 45 megahertz from the
receive oscillator). It also includes a low cost crystal 63a which is also
adjusted with the same correction on line 62 as was done for the receive
oscillator. This may also be done by a controller table lookup in
conjunction with the correction factor. Thus, a transmit mixer unit 64
provides via adjustable gain amplifier 66 the transmit frequency of the
field unit 16.
Finally, the adjustable gain amplifier 66 receives from microprocessor 31 a
proper voltage gain indication so that the signal is maintained at the
relatively low power level discussed in FIG. 3; that is, a power that is
significantly below the power of the standard signals of the cellular
system. Microprocessor 31 via line 62 to the transmit oscillator 63, of
course, also provides either quadrature phaseshift modulation or minimum
shift keying modulation to provide digital data transmission back to the
base station 11.
When the transceiver of FIG. 8 is utilized as a base station, since the
base station will provide a frequency reference for all of the field
units, the crystals 54a and 63a are oven controlled with a precision of
.+-.0.2 ppm. Thus, there is no AFC. In addition, there is a relatively
large scale computer designated PC 67 which provides for necessary data
processing functions. Of course, the transmit and receive frequencies are
reversed from the field unit 16. Other portions of the more detailed
aspects of the circuit of FIG. 8 have not been shown since they are well
known such as the use of filters, etc.
By the use of precision oven controlled crystal frequency references at
only the base station 11, and then utilizing this precise frequency in all
of the field units, low cost crystals may be used in the field units. This
provides for very precise additional digital channels and is especially
important in the context of the present invention where, as is clear from
FIG. 3, a slight shift in frequency would cause undesired interference
with, for example, an active or used cellular voice or control channel.
In addition, to provide the necessary low power signal, a homodyne
demodulation technique is utilized where the carrier is suppressed. This,
in combination with a minimum shift keying modulation (MSK) technique
provides in the preferred embodiment a low cost and robust system which
can operate effectively at the low powers required.
FIGS. 10A and 10B illustrate an alternative staggered quadrature phase
shift keying modulation (SQPSK) technique. The digital data to be
transmitted may be in any one of the four indicated quadrants. The Q
direction is the horizontal direction and the I direction is the vertical
direction. In a normal quadrature phase shift phase shift modulation
technique, when, for example, one is at the data point 32 and wishes to
proceed to data point 33, this is done via the indicated dashed line.
However, this will cause additional bandwidth spreading. By using a
staggered system, as illustrated in FIG. 10A, first the Q shift is made as
indicated by the solid arrow to point 34 and then the I shift made to the
destination point 33. And the data is sensed as indicated by the dashed
line in FIG. 5B after both shifts have been made. This two step staggered
shift also reduces bandwidth spreading, as indicated in FIG. 3, and thus,
interference occurs only at a very low decibel level.
Thus, the use of minimum shift keying (MSK) modulation will provide a rapid
decrease of spectrum away from the digital data carrier. SQPSK is also
suitable. Other techniques are BPSK (binary phase shift keying),
quadrature AM or merely AM modulation techniques. However, QPSK or MSK
modulation is believed to be optimum.
In summary, the present invention has provided a cellular communications
system while not incurring the high cost of normal cellular channels. In
other words, monitoring a transceiver with a cellular call every 30
seconds would use scarce capacity in many cell sites and be prohibitively
expensive. The solution of the present invention in providing an extra or
additional channels without interfering with existing channels is simple,
elegant and low cost.
With the frequency coordination scheme discussed above, the space made
available by the necessary clearance strategies required in the cellular
channels is effectively utilized without interference because of the low
power, narrow bandwidth and assignment pattern of the extra channels. The
invention is even more effective because of its ability to transmit
digital data at a fairly high data rate, but with low bandwidth and rapid
response time.
In any communications system employing standard channels and allocated in
an area so as to reuse the frequency without adjacent channel interference
or co-channel interference, the present technique can be used to implement
an additional lower bandwidth, lower power service without interference.
This may even occur in the evolution of the present day cellular system
where, due to the advancements in digital technology, the voice channels
may become digital and have narrower channels than the current system.
Such a conversion of existing cellular systems to all digital may
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