Cellular specialized mobile radio system

An SMR repeater is made agile by adding a microprocessor controlled frequency selection circuit to enable the repeater to operate on any one of several available frequencies in a band. For dispatch service, the output of the repeater is switched to a power amplifier feeding an omni-directional antenna. The repeater scans several frequencies sequentially. If a signal is detected, scanning is halted to provide repeater service, after which scanning is resumed. The frequency agile repeater is coupled to an antenna system having a narrow beam which can be steered electronically. A control computer scans the azimuth of the beam. The computer divides a service area into a plurality of cells extending radially from the antenna system and assigns channels as needed to service a subscriber. More than one cell can be on the same frequency without interference. The microprocessor includes a table in memory of call signs corresponding to particular transmission frequencies and causes the repeater to transmit the appropriate call sign when transmitting at a particular frequency.

BACKGROUND OF THE INVENTION 
This invention relates to a repeater system for enabling mobile or portable 
radio stations to communicate with each other and, in particular, to a 
specialized mobile radio system that operates like a cellular system. 
A repeater is a receiver-transmitter combination for receiving a signal at 
one frequency and re-transmitting the signal on a second frequency. 
Depending upon application, the transmitted frequency may be relatively 
close to the received frequency, e.g. 600 khz., or greatly displaced from 
the received signal. Depending upon application, frequency, and government 
regulation, the transmitter in a repeater may be relatively powerful, 
hundreds of watts, or may be rated at just a few watts. 
Commercial two-way radio communication has evolved into two different 
techniques for mobile operation, cellular and specialized mobile radio or 
dispatch service. Cellular systems use several repeaters dispersed in a 
geographic area and operating at low power to keep propagation relatively 
short, e.g. within a radius of less than ten miles. The local area covered 
by each repeater overlaps the local areas covered by neighboring 
repeaters, forming overlapping "cells" of coverage. A subscriber traveling 
from one cell to another cell is automatically switched from one repeater 
to another by a computer coupled to the repeaters by microwave link, 
optical fiber, or wire. 
Because propagation is short, the frequencies used by one repeater can be 
used by a non-neighboring repeater without interference. Because 
frequencies can be re-used, more subscribers can be served in a given 
geographic area. The frequency spectrum is allocated by government 
regulation and only a limited number of frequencies or channels are 
available. Thus, re-using assigned frequencies in a geographic area 
provides much more efficient use of a limited resource. 
Specialized mobile radio (SMR) uses a powerful repeater, usually located at 
the highest available elevation in a geographic area. The repeater is 
coupled to an omni-directional antenna to cover the entire geographic 
area, enabling dispatchers to communicate with a fleet of vehicles in the 
geographic area and enabling the vehicles to communicate with each other. 
SMR repeaters are adjustable in frequency but operate at a fixed 
frequency. A problem with SMR repeaters is that the frequency setting 
mechanism, typically a small cluster of switches ("DIP" switches), 
requires that the repeater be turned off, the frequency set, and the 
repeater turned on. Even if the switches can be set while the repeater is 
on, the computer in the repeater must be reset in order to read the new 
settings. Thus, resetting the frequency of a repeater involves significant 
down time. 
There are several differences between cellular radio and SMR. A first 
difference is that an SMR repeater operates on a single frequency, i.e. 
there can be only one user. Another difference is that an SMR repeater 
operates "half duplex," which means that a user can transmit or receive 
but not both, i.e. only one party to a conversation can talk at a time and 
everyone else on that frequency or channel must listen. There is often a 
busy condition where one user occupies a channel needed by another user. 
Trunked specialized mobile radio (TSMR) improves service by using a 
computer to switch users among several channels, typically five to twenty, 
enabling more conversations to take place with fewer busy conditions. 
A problem with cellular systems is the large investment in capital 
equipment because of the number of cells required to cover a geographic 
area. Each cell must have a repeater, an antenna, a favorable site for 
locating the antenna, electrical power, licenses, and other expenses 
including the cost of the control computer and the communication links to 
each repeater. On the other hand, SMR has a lower capital investment but 
serves a limited number of users compared to cellular radio. 
U.S. Pat. No. 4,802,235 (Treatch) describes a mobile transceiver which can 
be used for either cellular operation or trunked dispatch operation. A 
logic controlled frequency synthesizer enables the transceiver to operate 
with either a 25 kc. or a 30 kc. channel separation, as required for the 
different modes of operation. The patent relates to a mobile transceiver, 
not to a repeater, and does not address the problem of increasing the 
capacity of SMR repeater systems. 
In view of the foregoing, it is therefore an object of the invention to 
increase the number of subscribers that can be served by an SMR system. 
Another object of the invention is to provide a low cost SMR system that 
can serve a large number of users. 
A further object of the invention is to operate an SMR system like a 
cellular system. 
Another object of the invention is to enable a given frequency or channel 
to be used simultaneously for separate transmissions in a give geographic 
area without conflict or interference. 
A further object of the invention is to add multi-user telephone capability 
to an SMR system. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved by this invention, in which an SMR 
repeater is made frequency agile by adding a computer controlled frequency 
selection circuit to enable the repeater to operate on any one of several 
available frequencies in a band. For dispatch service, the output of the 
repeater is switched to a power amplifier feeding an omni-directional 
antenna. The repeater can scan a subset of the available frequencies 
sequentially. If a signal is detected, scanning is halted to provide 
repeater service, after which scanning is resumed. 
In accordance with another aspect of the invention, a frequency agile 
repeater is coupled to an antenna system having a narrow beam which can be 
steered electronically. A control computer scans the azimuth of the beam. 
The computer divides a service area into a plurality of cells extending 
radially from the antenna and assigns channels as needed to service a 
subscriber. More than one cell can be on the same channel at the same time 
without interference. An interface or "patch" to telephones can be 
provided to further enhance the performance of the system.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates the coverage of geographic area 10 by a plurality of 
cellular repeaters. In particular, circles 11, 12, 13, 14, 15, 16, and 17 
approximately cover the area indicated by circle 10 and represents the 
propagation distance, as indicated by radius 23, for a transmitter located 
at the center of a cell, such as transmitter 25 at the center of circle 
17. In typical cellular telephone systems, radius 23 is equal to ten miles 
or less. 
The number of cells and their placement are determined by the actual shape 
of the geographic area in which services is desired. Assuming a circular 
geographic area, as represented by circle 10, seven overlapping cells, 
each having a radius of about eight miles, could cover a geographic area 
having a radius of approximately twenty miles, as indicated by radius 21. 
A frequency or channel in use in one cell could not be used in an 
adjoining cell but could be used in a non adjoining cell in the same area. 
For example, a channel in use in circle 16 could not be used in circle 17 
but could be used in circles 12, 13, or 14. Thus, a large number of 
subscribers can be served without interference. 
FIG. 2 illustrates the coverage provided by a specialized mobile radio 
system in which circle 27 indicates the geographic area served by 
transmitter 29 located at the center of circle 27. A single subscriber 
within geographic area 27 ties up the entire area each time the repeater 
at location 29 is accessed. Cellular repeaters (FIG. 1) typically operate 
on different subsets of the available channels and the repeaters are 
coupled to a control computer for assigning the subsets of frequencies. 
Each centrally located site, such as site 25, includes a plurality of 
repeaters, one for each service channel in the subset. The control 
computer also selects the channel within each subset. 
An SMR repeater servicing the geographic area represented by circle 27 is 
similarly configured, that the repeater is capable of operating on one of 
a plurality of frequencies in a given band. FIG. 3 illustrates a mechanism 
for setting the operating frequency of a commercially available repeater. 
Central processing unit 31 has a plurality of parallel inputs. Each input 
is coupled to ground through a switch, such as switch 32, and is coupled 
to a DC supply voltage through a load resistor, represented by bar 33. The 
particular frequency in a band is chosen by the pattern of open and closed 
switches. The group of switches indicated by reference number 35 selects 
the subset of frequencies available within a repeater. Using a predefined 
coding pattern, one sets the frequency of each repeater to avoid conflict 
with other repeaters. 
In accordance with one aspect of the invention, a repeater for a 
specialized mobile radio system is modified as illustrated in FIG. 4 to 
provide a scanning repeater or a frequency agile repeater. In particular, 
a second central processing unit is substituted for the plurality of 
switches and is controlled by external control lines to the repeater. 
Microprocessor 31 is the existing microprocessor in a repeater and 
microprocessor 36 is added to the circuit, providing the frequency 
encoding data instead of the switches illustrated in FIG. 3. Line 37 is a 
serial data line, such as an RS232 line, for transferring control 
information to microprocessor 36 from a remote location. Input line 38 is 
a repeater network data link (RNDL) which enables a repeater to share a 
site with other repeaters. Line 39 is an antenna network data link (ANDL), 
which enables the repeater to control more than one antenna. 
Microprocessor 36 is programmed to scan pre-selected channels in a 
designated order. For example, microprocessor 36 is programmed to check a 
plurality of channels alternately with channel one. This enables the 
repeater to monitor a "home" channel for activity while scanning a subset 
of all possible channels. 
A number of commercially available repeaters can be modified in accordance 
with the invention to provide the scanning or the frequency agility 
functions. For example, FIG. 5 illustrates the frequency control portion 
of another commercially available repeater. Input switches 41 are coupled 
to the address inputs of EPROM 43. The data outputs from EPROM 43 are 
coupled to programmable divider 45. Programmable divider 45 is part of a 
phase locked loop circuit for controlling the operating frequency of a 
repeater. 
As illustrated in FIG. 6, microprocessor 47 is substituted for switches 41 
to provide control data to programmable divider 45. Microprocessor 47 
includes RS232 input 46, RNDL input 48 and ANDL input 49, which enable the 
microprocessor to be controlled remotely and to be trunked with other 
repeaters. Other commercially available repeaters can be similarly 
modified in that a programmed microprocessor is substituted for switches 
or jumpers in the frequency determining portion of the repeater. Although 
any suitable microprocessor can be used, it is preferred that the added 
microprocessor be a single chip microcontroller in order to minimize the 
parts count of the modification. One microcontroller that has been found 
suitable is an MC68HC711E9 microcontroller as sold by Motorola, Inc. This 
microcontroller includes EPROM, RAM, ROM, and programmable I/O lines. 
In a preferred embodiment of the invention, the functions of microprocessor 
45 and 47, or microprocessor 36 and 31, are combined in a single 
microprocessor. FIG. 7 illustrates a communication system including at 
least one repeater constructed in accordance with a preferred embodiment 
of the invention. In repeater 70, microprocessor 71 is coupled to 
frequency synthesizer 73 and to frequency synthesizer 75. Synthesizer 73 
controls the received frequency of the repeater and synthesizer 75 
determines the transmitted frequency of the repeater. The transmitted 
frequency is offset from the received frequency by a predetermined amount, 
depending upon the band being used. For example, in the 800 megahertz 
band, the transmission frequency is 45 megahertz above the received 
frequency. 
Receive synthesizer 73 includes phased lock loop circuit 81, voltage 
controlled oscillator 82, and amplifier 83. Microprocessor 71 controls the 
frequency at which the loop locks by way of data line 85, clock line 86 
and receive enable line 87. Similarly, transmit synthesizer 75 includes 
phase lock loop 91, voltage controlled oscillator 92, and amplifier 93. 
Synthesizer 75 is controlled by data line 85, clock line 86 and 
transmission enable line 95. 
Output 96 from synthesizer 73 is coupled as one input to mixer 98. A second 
input to mixer 98 is coupled to pre-amp 99, which is coupled to antenna 
array 115 and which includes suitable filters, amplifiers, and impedance 
matching networks. Output 101 from mixer 98 is coupled to receiver 100 for 
further amplification, filtering, and detection. Either voice or data or 
both voice and data can be transmitted and received. 
Line 77 is a serial data line, such as an RS232 line, for transferring data 
between microprocessor 71 and control computer 80. Repeaters 155 and 156 
are also coupled to computer 80 by a serial link. Input line 78 is a 
repeater network data link between computer 80 and each of the repeaters 
at the site. Line 79 is an antenna network data link for controlling 
antenna selection and azimuth by way of beam switch 116. 
A voltage indicative of signal strength (the average amplitude of a 
received signal) is provided by receiver 100 on line 105 to level detector 
106. In its simplest form, level detector 106 is a capacitor for smoothing 
the voltage indicative of signal strength. Other circuits, e.g. threshold 
sensing circuits, can be included in level detector 106. The output from 
level-detector 106 is converted into a digital signal by A/D converter 
108. The output of A/D converter is coupled to input 109 of microprocessor 
71. The amplitude information is coupled by microprocessor 71 to the 
control computer for steering the beam in antenna array 115 by way of beam 
switch circuitry 116. For example, repeater 70 tracks a vehicle moving 
from one cell to another by monitoring the amplitude of the received 
signal and briefly switching the beam position to determine if the signal 
level decreases or increases, thereby selecting the appropriate azimuth 
for the beam to maintain contact with a subscriber. 
The output from frequency synthesizer 75 is coupled by line 121 to one 
input of mixer 123. Input 125 to mixer 123 is coupled to a stable local 
oscillator for producing an appropriate frequency on output line 127. 
Output line 127 is coupled to final amplifier 129, which couples the 
frequency determining portion of the repeater to antenna array 115 and 
includes an amplifier, filter, and matching network (not shown). Final 
amplifier 129 includes gain control input 126, which can be coupled to 
microprocessor 71 or to a control computer for adjusting the output power 
of repeater 70. 
As illustrated in FIG. 8, antenna array 115 preferably includes three 
steerable arrays 131, 132, and 133 arranged along the sides of a triangle, 
preferably an equilateral triangle. Beam 137 from array 131 is represented 
somewhat ideally as a single lobe i.e. side lobes are not shown in FIG. 8. 
Beam 137 has a maximum amplitude in direction 141. In a preferred 
embodiment of the invention, each array has a gain of about 19 db. At this 
level, a ten watt output to the antenna produces almost one kilowatt of 
effective radiated power in the beam. 
The azimuth of beam 137 can be changed, e.g. to position 137', by 
controlling the phase of the signals to the elements in array 131, as 
known in the antenna art. Because beam 137 is controlled by digital 
circuitry, the azimuth adjustment is incremental rather than continuous, 
although the increments can be made arbitrarily small. The increments 
should not be larger than the beam width. For example, as illustrated in 
FIG. 9, geographic area 143 is divided into twelve cells extending 
radially from an antenna at the center of the area,,each cell having an 
angular width of about 30.degree.. It is preferred that incremental change 
145 in azimuth (FIG. 8) have maximum value of about 30.degree. also. A 
30.degree. beam width is available at reasonable cost with existing 
technology. A narrower beam increases the number of cells proportionately. 
Arc 138 indicates the maximum azimuth which must be covered by a single 
array. In a preferred embodiment of the invention, this angle is about 
120.degree. and is well within the capability of an electronically 
steerable antenna. 
In a mobile radio system having radially extending cells, non-adjoining 
cells are essentially isolated from each other and can operate at the same 
frequency. For example, in FIG. 9, a subscriber in cell 147 and a 
subscriber in cell 149 can operate at the same frequency without 
interference. The subscriber operating in cell 149 is serviced by repeater 
70 and antenna system 130 and the subscriber in cell 147 is serviced by a 
second repeater, e.g. repeater 155, and by a second antenna system (not 
shown) located at the same site as antenna system 130. A subscriber in 
cell 151 is serviced by repeater 156 and by antenna system 130, using 
array 133. The selection of the antenna system, the array, and the beam 
direction is coordinated by computer 80. Thus, a specialized mobile radio 
system is provided wherein a single subscriber does not tie up the system 
covering a geographic area. Further, a large number of subscribers can be 
serviced on the same frequency without interference. 
A repeater constructed in accordance with the invention can operate in 
several different modes. SMR wide area dispatch operation can be 
omnidirectional as in the prior art but base-to-mobile operations or 
telephone calls require the combination of a steerable beam antenna and 
radially extending cells in order to minimize interference and to increase 
the number of subscribes who can be on the air simultaneously. For wide 
area dispatch service, the output of a synthesizer is coupled to a power 
amplifier feeding an omni-directional antenna in antenna array 115. 
In the steerable beam antenna mode, the beam is electronically rotated like 
an airport beacon. While the beam is rotating, the available channels are 
scanned for activity. A continuous data stream of signal level information 
and direction information is available to enable the control computer to 
select the repeater and the antenna most suited for the service. When a 
signal is detected, the frequency scanning and azimuth scanning are halted 
to provide repeater service and then resumed after a subscriber has been 
served. 
Scan frequencies are programmed into the control computer, which defines 
scanning speed, repeat hold times before resuming scanning, the sequence 
of channels, and azimuth. By combining signal requests from subscribers 
with the status information in the control computer, one avoids 
interfering with other systems by knowing what channels are used in the 
other systems and including this data in the control computer. The control 
computer can skip the potentially conflicting channels or reduce power to 
avoid overlap of propagation. 
FIG. 10 illustrates a lookup table in the memory of microprocessor 71 (FIG. 
7). Each frequency or channel can have its own call sign and the call 
signs are transmitted automatically after a subscriber has been served or 
at timed intervals. The call sign can be transmitted in Morse code (CW) or 
as synthesized voice, depending upon available memory space. A CW call 
sign occupies a few bytes of memory whereas a voice call sign occupies 
tens of thousands of bytes. The system illustrated in FIG. 7 is configured 
in software by downloading channel, call sign, and other information from 
control computer 80 to microprocessor 71. 
Having thus described the invention, it will be apparent to those of skill 
in the art that various modifications can be made within the scope of the 
invention. For example, although a circular service area is shown, it is 
understood that terrain, tall buildings, and government or military usage 
may result in a service area far more complicated than a circle and that 
two or more repeater sites may have to be used to cover a service area. In 
accordance with the invention, frequency agile SMR repeaters can be 
located at more than one site and coordinated by a single computer. 
Overlap between cells radiating from different repeater sites is minimized 
by reducing power for certain azimuths (along a line between the sites) at 
each repeater site. Thus, propagation is reduced and overlap and 
interference are minimized. Instead of monitoring a home channel, a first 
repeater at a site can be coupled to an omnidirectional antenna and scan 
all channels for activity in the area. When activity is found, a second 
receiver, set to the active channel, is coupled to a steerable array to 
locate and lock on to the subscriber. The first repeater can continue to 
scan the remaining channels for activity. Although an electronically 
rotatable antenna is preferred, a mechanically rotated antenna can be used 
instead.