Apparatus and method in a radio communication system for distinguishing an identifier of a nearby transmitter from that of a more distant transmitter

An apparatus and a method in a radio communication system repeatedly transmit and receive identifiers including a nearby identifier received by a radio receiver from a nearby transmitter, and distant identifiers received from more distant transmitters. The method and the apparatus distinguish in a probabilistic manner the nearby identifier from the distant identifiers by determining the identifier received most dependably over a predetermined period.

FIELD OF THE INVENTION 
This invention relates in general to radio communication systems, and more 
specifically to an apparatus and a method associated therewith in a radio 
communication system for distinguishing an identifier received from a 
nearby transmitter from identifiers received from other more distant 
transmitters. 
BACKGROUND OF THE INVENTION 
Radio communication systems capable of transmitting and displaying channel 
identification information to confirm proper operation of a portable radio 
receiver on a system at an expected location are well known. One such 
system is described in U.S. Pat. No. 4,981,638 to Davis, entitled 
"Nationwide Display Pager with Location Readout." 
In a system that covers a wide area through a plurality of transmitters, 
location information received by the portable radio receiver preferably is 
that location information transmitted by the one of the plurality of 
transmitters that is nearest the portable radio receiver. Unfortunately, 
time-variant shadowing and multipath effects present in the radio 
communication system can cause the nearest transmitter at times not to be 
dominant over more distant transmitters as the portable radio receiver 
moves about the system. As a result, the location information received by 
the portable radio receiver can sometimes be incorrect. 
Such momentarily incorrect location information can cause improper 
operation of the portable radio receiver if the receiver is near a 
boundary between locations that are intended to control the portable radio 
receiver to function differently from one location to the next. For 
example, if the user is near a time zone boundary, and the location 
information is used to automatically reset a clock in the receiver to 
display the time of day according to the time zone in which the receiver 
is located, the clock can occasionally display the wrong time of day. 
Thus, what is needed is a method and an apparatus for distinguishing 
location information that is transmitted from a nearby transmitter from 
that transmitted by more distant transmitters in the system, even in the 
presence of the aforementioned time-variant shadowing and multipath 
effects. 
SUMMARY OF THE INVENTION 
An aspect of the present invention is a portable radio receiver for 
receiving information comprising location identifiers each having a 
location value. The location identifiers include a nearby location 
identifier transmitted by a nearby transmitter and distant location 
identifiers transmitted by more distant transmitters in a radio 
communication system comprising the portable radio receiver and a 
plurality of the transmitters. The portable radio receiver distinguishes 
the nearby location identifier from the distant location identifiers. The 
location identifiers are transmitted repeatedly in periodic transmissions. 
The portable radio receiver comprises a receiver for receiving over a 
predetermined period a plurality of the periodic transmissions comprising 
the location identifiers. The location identifiers are received multiple 
times throughout the predetermined period. The portable radio receiver 
further comprises a memory coupled to the receiver for storing the 
location identifiers received in the plurality of the periodic 
transmissions, and a processor coupled to the memory for comparing stored 
portions of the location identifiers with one another to determine the 
location value received with greatest dependability relative to all other 
location values received during the predetermined period. The processor 
defines the location value received with the greatest dependability to be 
the location value of the nearby location identifier. The periodic 
transmissions from ones of the plurality of the transmitters are 
synchronized with the periodic transmissions from others thereof, and the 
periodic transmissions are frequency modulated (FM) simulcast 
transmissions. The portable radio receiver has a defined capture ratio, 
and the receiver comprises a demodulator for demodulating a location 
identifier corresponding to a strongest transmitter producing a highest 
signal strength at the portable radio receiver during each of the periodic 
transmissions. The highest signal strength is greater by at least the 
defined capture ratio than a signal strength produced at the portable 
radio receiver by the plurality of the transmitters other than the 
strongest transmitter. The processor comprises a sorter coupled to the 
memory for sorting into like-valued groups the location values that are 
equal to one another, and a counter coupled to the sorter for counting the 
location values sorted into each of the like-valued groups to determine 
one of the like-valued groups having a highest count. The processor 
further comprises a concluder coupled to the counter for concluding 
therefrom that the location value of the one of the like-valued groups 
having the highest count is the location value received with the greatest 
dependability. 
Another aspect of the present invention is a probabilistic method in a 
radio communication system comprising a plurality of transmitters and a 
portable radio receiver. The method is for transmitting and receiving 
information comprising location identifiers each having a location value. 
The location identifiers include a nearby location identifier transmitted 
to the portable radio receiver by a nearby transmitter, and distant 
location identifiers transmitted to the portable radio receiver by more 
distant transmitters. The method is further for distinguishing the nearby 
location identifier from the distant location identifiers. The location 
identifiers are transmitted repeatedly in periodic transmissions. The 
method comprises in the portable communication receiver the steps of 
receiving over a predetermined period a plurality of the periodic 
transmissions comprising the location identifiers, wherein the location 
identifiers are received multiple times throughout the predetermined 
period; and storing the location identifiers received in the plurality of 
the periodic transmissions. The method further comprises the steps of 
comparing stored portions of the location identifiers with one another to 
determine the location value received with greatest dependability relative 
to all other location values received during the predetermined period, and 
defining the location value received with the greatest dependability to be 
the location value of the nearby location identifier. The periodic 
transmissions from ones of the plurality of the transmitters are 
synchronized with the periodic transmissions from others thereof, and the 
periodic transmissions are frequency modulated (FM) simulcast 
transmissions The portable radio receiver has a defined capture ratio, and 
the receiving step comprises the step of demodulating a location 
identifier corresponding to a strongest transmitter producing a highest 
signal strength at the portable radio receiver during each of the periodic 
transmissions. The highest signal strength is greater by at least the 
defined capture ratio than a signal strength produced at the portable 
radio receiver by the plurality of the transmitters other than the 
strongest transmitter. The comparing step comprises the steps of sorting 
into like-valued groups the location values that are equal to one another, 
and counting the location values sorted into each of the like-valued 
groups to determine one of the like-valued groups having a highest count. 
The comparing step further comprises the step of concluding from the 
counting step that the location value of the one of the like-valued groups 
having the highest count is the location value received with the greatest 
dependability.

DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS 
Referring to FIG. 1, an electrical block diagram of a radio communication 
system in accordance with the preferred and first and second alternative 
embodiments of the present invention comprises a controller 102 coupled by 
communication links 103 to transmitters 104 for control thereof. The 
transmitters are positioned throughout a first location 106 and a second 
location 108 for providing radio coverage thereto. Each of the 
transmitters 104 preferably transmits a location identifier 204, 208 (FIG. 
2) having a location value associated with the first or second location 
106, 108 in which the transmitter is positioned. A receiver that is 
positioned well inside one of the locations 106, 108, e.g., the portable 
radio receiver 110, is likely to receive a single location identifier 204, 
208 corresponding to the first or second location 106, 108 in which the 
portable radio receiver 110 is positioned. A receiver that is positioned 
near a boundary between the first and second locations 106, 108, e.g., the 
portable radio receiver 112, may receive over time location values 
corresponding to both the first and second locations 106, 108, depending 
upon shadowing and multipath effects at any given instant in time. 
While the portable radio receiver 112 can receive at an instant in time 
either a nearby transmitter 104 in the first location 106 or a more 
distant transmitter 104 in the second location 108, the location 
identifier 204, 208 from the nearby transmitter 104 has a higher 
probability of dependable reception over time. Thus, in accordance with 
the preferred and alternative embodiments of the present invention, the 
portable radio receiver 110, 112 preferably does not react precipitously 
to a single location identifier 204, 208 received during one instant in 
time. Instead, the portable radio receiver 110, 112 preferably monitors 
and stores a plurality of location identifiers 204, 208 received over a 
predetermined period, e.g., sufficient time to receive ten transmissions 
of the location identifiers 204, 208. Then, using probabilistic methods to 
determine the location value that was received the most dependably over 
the predetermined period, as described herein below, the portable radio 
receiver 110, 112 defines the location value to be that transmitted by the 
nearby transmitter. 
The radio communication system of FIG. 1 preferably is coupled to the 
public switched telephone network (PSTN) 116 by telephone trunks 114 for 
communicating with callers using telephones 120 coupled to the PSTN 116 by 
telephone lines 118. In one form of the radio communication system, the 
callers can place calls to the controller 102 to cause radio pages to be 
sent to users of the receiver 110, 112 in a manner well known in the art. 
Preferably, the controller 102 is similar to a model MPS 2000.RTM. paging 
control center, the transmitters 104 are similar to a model C73 PURC 
5000.RTM. transmitter, and the portable radio receiver 110, 112 is similar 
to a model A03KLB5962CA ADVISOR.RTM. pager, all manufactured by Motorola., 
Inc. of Schaumburg, Ill. It will be appreciated that other similar 
equipment may be used as well to construct the radio communication system 
of FIG. 1. 
Three embodiments of the present invention are described herein below. 
Briefly, in accordance with the preferred embodiment, the location 
identifiers 204, 208 are transmitted as simulcast transmissions from the 
transmitters 104, and the bits/symbols of the location identifiers 204, 
208 received over the predetermined period are stored in the portable 
radio receiver 110, 112. The bits/symbols associated with each bit/symbol 
position of the location identifier 204, 208 are then compared with one 
another. If more than a predetermined percentage, e.g., seventy percent, 
of the bits/symbols of the bit/symbol position are the same value, then 
the value becomes the estimated value for the bit/symbol position. The 
resultant location identifier 204, 208 comprising the estimated bit/symbol 
values of all bit/symbol positions is then assumed to be the most 
dependably received location value, and thus to be the location value 
transmitted by the transmitter 104 nearest the portable radio receiver 
110, 112. The preferred embodiment is particularly advantageous for a 
receiver that is moving rapidly in a multipath environment, because the 
preferred embodiment allows the received data to be examined and compared 
over a maximally brief interval, i.e., the interval of a single bit or 
symbol. 
In accordance with the first alternative embodiment of the present 
invention, the location identifiers 204, 208 are transmitted as simulcast 
frequency modulated (FM) transmissions. The portable radio receiver 110, 
112 is an FM receiver having a defined capture ratio, i.e., able to 
accurately receive a first signal in the presence of a second (differing) 
signal if the first signal is stronger than the second signal by at least 
the capture ratio of the receiver, e.g., three decibels. The transmissions 
of the location identifiers 204, 208 are monitored over the predetermined 
period, and the capture effect of the portable radio receiver 110, 112 is 
relied upon to cause the portable radio receiver 110, 112 to receive the 
location identifier 204, 208 of the nearest transmitter 104 more often 
than that of other transmitters during the predetermined period. At the 
end of the predetermined period, the location values are grouped into like 
valued groups, and the largest group is assumed to contain the location 
value transmitted by the nearest transmitter. The first alternative 
embodiment requires less processing power to implement than the preferred 
embodiment, but it is not able to respond as quickly as the preferred 
embodiment in a rapidly changing multipath environment, because the first 
alternative embodiment compares signals received over many bit/symbol 
positions instead of just one. 
In accordance with the second alternative embodiment of the present 
invention, the location identifiers 204, 208 are transmitted sequentially, 
one location identifier 204, 208 per time slot, so that each can be 
received without interference during the predetermined period. The 
portable radio receiver 110, 112 measures the received signal strength of 
each transmission, and stores the location identifiers 204, 208 and 
corresponding signal strengths. At the end of the predetermined period, 
the portable radio receiver 110, 112 determines the mean signal strength 
over the predetermined period of each time slot. The location identifier 
204, 208 corresponding to the time slot having the highest mean signal 
strength is then assumed to contain the location value transmitted by the 
nearest transmitter. The second alternative embodiment is most suitable 
for a system in which simulcast transmission of the location identifier 
204, 208 is impossible or undesirable. 
Referring to FIG. 2, a signal timing diagram 200 utilized in the radio 
communication system of FIG. 1 in accordance with the preferred and first 
alternative embodiments of the present invention depicts a simulcast 
signaling protocol. The signal timing diagram 200 includes transmissions 
210 from a first transmitter and transmissions 212 from a second 
transmitter. In a simulcast system the transmitters 104 are synchronized 
with one another such that one of the transmitters 104 transmits a 
synchronization word 202, a location identifier 204, and messages 206 at 
substantially the same time that any other of the transmitters 104 
transmits the synchronization word 202, a possibly different location 
identifier 208, and the messages 206, as indicated by a comparison of the 
transmissions 210, 212. Preferably, the location identifiers 204, 208 are 
transmitted periodically so that the receivers 110, 112 will receive 
multiple transmissions of the location identifiers 204, 208 over the 
predetermined period. 
Referring to FIG. 3, a typical pattern of digital data bits 300 for the 
location identifiers 204, 208 transmitted by the radio communication 
system of FIG. 1 in accordance with the preferred and first and second 
alternative embodiments of the present invention includes a plurality of 
predetermined bit positions 302 containing bits 304 representing a value 
of zero, and bits 306 representing a value of one. The digital data bits 
300 represent the location value transmitted for the location identifiers 
204, 208. 
Referring to FIG. 4, a typical pattern of digital data symbols 400 for the 
location identifiers 204, 208 transmitted by the radio communication 
system of FIG. 1 in accordance with the preferred and first and second 
alternative embodiments of the present invention includes a plurality of 
predetermined symbol positions 402 containing symbols 404, 406, 408, 410 
representing, for example, values of one, two, three, and four, 
respectively. It will be appreciated that either bits or symbols may be 
selected for transmitting the location value of the location identifiers 
204, 208, in accordance with the preferred and first and second 
alternative embodiments of the present invention, the selection being a 
matter of design choice. 
Referring to FIG. 5, an electrical block diagram of the portable radio 
receiver 110, 112 in accordance with the preferred and first and second 
alternative embodiments of the present invention comprises an antenna 502 
for intercepting the radio transmissions comprising the location 
identifiers 204, 208 transmitted by the transmitters 104. The antenna 502 
is coupled to a conventional receiver element 504 for demodulating the 
radio transmissions. The receiver element 504 is coupled to a processor 
506 for decoding and processing information carried in the radio 
transmissions, the information including the location identifiers 204, 
208. The processor 506 is coupled to a memory 508, e.g., a conventional 
random access memory (RAM), for storing the location identifiers 204, 208 
in memory locations 510 therefor. The processor 506 is also coupled to an 
output element 512, such as a conventional liquid crystal display, for 
displaying the received information. 
The processor 506 is further coupled to user controls 514, such as 
well-known switches and buttons, for allowing a user to control the 
portable radio receiver 110, 112. In addition, the processor 506 is 
coupled to an alert element 516, e.g., a conventional piezoelectric 
transducer or lamp for generating an audible or visible alert in response 
to receiving information intended for the portable radio receiver 110, 
112. A conventional clock 518 is also coupled to the processor 506 for 
providing a time signal to the processor 506. 
The processor 506 comprises firmware elements including a receive timer 520 
for measuring the predetermined period over which the portable radio 
receiver 110, 112 monitors the transmissions of the location identifiers 
204, 208. The firmware elements preferably also include a protocol 
processor 522 for processing the location identifiers 204, 208 in 
accordance with a protocol comprising an error-detecting code, an 
error-correcting code, or an error-correcting-and-error-detecting code to 
more accurately determine the most dependably received location value for 
the location identifiers 204, 208, as is explained herein below. 
Preferably, the processor 506 is similar to the MC68HC05, C08, or C11 
series microcomputers manufactured by Motorola, Inc. of Schaumburg, IL. It 
will be appreciated that other similar devices can be utilized for the 
processor 506 as well. It will be further appreciated that the memory 508 
and the clock 518 can be incorporated as integral portions of the 
processor 506 as well. 
Referring to FIG. 6, a firmware diagram 600 depicting firmware elements 
utilized by the processor 506 in the portable radio receiver 110, 112 in 
accordance with the preferred embodiment of the present invention 
comprises an examiner 602 for examining at one of the plurality of 
predetermined bit/symbol positions 302/402 the digital data bits/symbols 
300/400 corresponding to the location identifiers 204, 208 received and 
stored during the predetermined period measured by the receive timer 520. 
The firmware diagram further comprises a determiner 604 coupled to the 
examiner 602 for determining that more than a predetermined percentage of 
the digital data bits/symbols 300/400 at the one of the plurality of 
predetermined bit/symbol positions 302/402 have a single coequal value. 
The firmware diagram also includes a designator 606 coupled to the 
determiner 604 for designating the single coequal value to be an estimated 
bit/symbol value for the one of the plurality of predetermined bit/symbol 
positions 302/402. In addition, the firmware diagram includes a repeater 
608 for repeating the examining, determining, and designating for each of 
the plurality of predetermined bit/symbol positions 302/402, until a 
plurality of estimated bit/symbol values corresponding to the plurality of 
predetermined bit/symbol positions 302/402 have been designated. The 
firmware diagram further comprises a concluder 610 for concluding that the 
plurality of estimated bit/symbol values represent the most dependable 
location value received during the predetermined period. It will be 
appreciated that some or all the firmware elements depicted in the 
firmware diagram 600 can be fabricated in hardware as well, as a custom 
designed integrated circuit, for example. 
Referring to FIG. 7, a flowchart 700 of the operation of the portable radio 
receiver 110, 112 in accordance with the preferred and first and second 
alternative embodiments of the present invention begins with the processor 
506 initializing 702 the receive timer 520 to a predetermined value. Then 
the receiver element 504 receives 704 a next one of the plurality of 
periodic transmissions including the location identifier 204, 208. The 
processor 506 next exits 706 to one of the storage subroutines 800, 1800 
(FIGS. 8, 18) to store in a next available one of the memory locations 510 
the data derived from the received transmission, the data including the 
location identifier 204, 208. More specifically, the storage subroutine 
800 is utilized for the preferred and first alternative embodiments, and 
the alternative storage subroutine 1800 is utilized for the second 
alternative embodiment. 
When the processor 506 returns 708 from the one of the storage subroutines 
800, 1800, the processor 506 checks 710 whether the receive timer 520 has 
expired. If not, the processor 506 returns to step 704 to receive another 
of the periodic transmissions. If, on the other hand, the receive timer 
has expired, then the processor 506 exits 712 to one of the comparing 
subroutines 900, 1300, 1900 (FIGS. 9, 13, 19) to compare portions of the 
stored data with one another to determine a most dependable location value 
corresponding to ones of the periodic transmissions having a highest mean 
dependability of reception. More specifically, the comparing subroutines 
900, 1300, and 1900 are utilized for processing the preferred embodiment, 
the first alternative embodiment, and the second alternative embodiment, 
respectively. 
When the processor 506 returns 714 from the one of the comparing 
subroutines 900, 1300, 1900, the processor 506 checks 716 whether a 
predetermined error value was returned. If so, the processor recognizes 
718 that no reliable location value could be determined, and then returns 
to step 702 to start over. If, on the other hand, a valid most dependable 
location value was returned, the processor 506 then defines 720 the most 
dependable location value to be the location value of the nearby location 
identifier 204, 208, i.e., the location identifier 204, 208 transmitted by 
a transmitter closest to the portable radio receiver 110, 112. Then the 
processor 506 returns to step 702 to begin the process anew. 
Thus, the present invention advantageously can reliably distinguish the 
nearby location identifier 204, 208 from the more distant location 
identifiers 204, 208 of the radio communication system, thereby providing 
a much greater degree of certainty that operational decisions based upon 
the received location identifier 204, 208 will be made correctly. 
Referring to FIG. 8, a flowchart of the storage subroutine 800 in 
accordance with the preferred and first alternative embodiments of the 
present invention begins with the processor 506 selecting 802 a next 
available location 510 in the memory 508 for storing the location 
identifier 204, 208. Then the processor 506 stores 804 the location 
identifier 204, 208 in the selected memory location 510, and returns 806 
to the flowchart 700 (FIG. 7). 
Referring to FIG. 9, a flowchart of the comparing subroutine 900 in 
accordance with the preferred embodiment of the present invention begins 
with the processor 506 accessing the examiner 602 to examine 902 at one of 
the plurality of bit/symbol positions 302, 402 the digital data 
bits/symbols corresponding to the location identifiers 204, 208 received 
and stored during the predetermined period. Then the processor 506 
accesses the determiner 604 to determine 904, 906 whether more than a 
predetermined percentage of the digital data bits/symbols at the 
bit/symbol position have a single coequal value. If so, the processor 506 
accesses the designator 606 to designate 910 the single coequal value to 
be an estimated bit/symbol value for the bit/symbol position. Then the 
processor 506 accesses 912 the repeater 608 to repeat 912, 914 the 
examining, determining, and designating steps for each of the plurality of 
bit/symbol positions, by returning to step 902 until an estimated 
bit/symbol value has been designated for all the plurality of bit/symbol 
positions. 
If, on the other hand, in steps 904, 906 the processor 506 does not 
determine that more than a predetermined percentage of the digital data 
bits/symbols at the bit/symbol position have a single coequal value, then 
the processor designates 908 the estimated bit/symbol value for the 
bit/symbol position to be indeterminate. Then the flow continues to step 
912, as before. 
When in step 914 the processor 506 determines that values for all the 
bit/symbol positions have been estimated, the processor 506 accesses the 
protocol processor 522 to process 916 the plurality of estimated 
bit/symbol values to correct or detect errors therein in accordance with 
the type of protocol utilized. That is, if an error-detecting protocol is 
utilized, then the location value represented by the estimated bit/symbol 
values can be checked to determine if it is error free. If an 
error-correcting protocol is utilized, then up to a predetermined number 
of bit/symbol errors can be corrected as well. Error-detecting, 
error-correcting, and error-correcting- and-error-detecting protocols as 
utilized in the protocol processor 522 are conventional and well known to 
one of ordinary skill in the art. 
If, after error detection/correction, the processor 506 determines 918 the 
plurality of estimated bit/symbol values to be error free, then the 
processor 506 accesses the concluder 610 to conclude 922 that the 
plurality of estimated bit/symbol values represent the most dependable 
location value, which value is then returned to the flowchart 700 (FIG. 
7). If, on the other hand, the processor 506 determines in step 918 that 
the plurality of estimated bit/symbol values is not error free, then the 
processor 506 returns 920 the predetermined error value to the flowchart 
700. 
Referring to FIG. 10, an electrical block diagram of a first alternative 
receiver element 1000 in accordance with the first alternative embodiment 
of the present invention comprises a conventional FM demodulator 1002. The 
FM demodulator 1002 is characterized by a defined capture ratio. As 
described herein above for an FM simulcast environment, the first 
alternative receiver element 1000 is able to accurately receive a first 
signal in the presence of a second (differing) signal if the first signal 
is stronger than the second signal by at least the defined capture ratio. 
This means that if a first location identifier 204, 208 transmitted by a 
nearby transmitter is received more strongly by at least the defined 
capture ratio than a second location identifier 204, 208 transmitted by a 
more distant transmitter, then the first location identifier 204, 208 is 
the one that will "capture" the portable radio receiver 110, 112. Due to 
shadowing and multipath, the nearby transmitter may not be received the 
most strongly one-hundred percent of the time. From a probabilistic point 
of view, however, over the predetermined period it is highly likely that 
the nearby transmitter will capture the portable radio receiver 110, 112 
during a majority of the transmissions. It is this fact that provides an 
operational basis for the first alternative embodiment. 
Referring to FIG. 11, a first alternative firmware diagram 1100 depicting 
firmware elements utilized by the processor 506 in the portable radio 
receiver 110, 112 in accordance with the first alternative embodiment of 
the present invention comprises a sorter 1102 for sorting into like-valued 
groups the location values that are equal to one another. The first 
alternative firmware diagram 1100 further comprises a counter 1104 for 
counting the location values sorted into each of the like-valued groups to 
determine one of the like-valued groups having a highest count. The first 
alternative firmware diagram 1100 further comprises a concluder 1106 for 
concluding therefrom that the location value of the one of the like-valued 
groups having the highest count is the most dependable location value. It 
will be appreciated that some or all the firmware elements depicted in the 
first alternative firmware diagram 1100 can be fabricated in hardware as 
well, as a custom designed integrated circuit, for example. 
Referring to FIG. 12, a first alternative memory utilization chart 1200 for 
the portable radio receiver 110, 112 in accordance with the first 
alternative embodiment of the present invention depicts group identifiers 
1202 for identifying the like-valued groups. The first alternative memory 
utilization chart 1200 further depicts count locations 1204 corresponding 
to the group identifiers 1202 for storing a count of the location values 
sorted into each of the like-valued groups, and corresponding location 
identifier slots 1206 for storing the location value corresponding to each 
of the like-valued groups In addition, the first alternative memory 
utilization chart 1200 depicts match flags 1208 corresponding to each of 
the memory locations 510 containing a location identifier 204, 208 for 
indicating that the location identifier 204, 208 has been compared with 
the other location identifiers 204, 208 stored in the memory 508. 
Referring to FIG. 13, a flowchart of the first alternative comparing 
subroutine 1300 in accordance with the first alternative embodiment of the 
present invention begins with the processor 506 accessing the protocol 
processor 522 to process 1302 the location values of the location 
identifiers 204, 208 stored in the memory 508. The processing 
corrects/detects errors in the location values in accordance with the 
protocol utilized. A location value that contains an error after the 
processing is replaced with the predetermined error value. 
Next, the processor 506 accesses 1304 the sorter 1102, the counter 1104, 
and the concluder 1106 to sort and count the location identifiers 204, 208 
and to determine the most dependable location value. More specifically, 
the processor 506 initializes 1306 a group tracking number to zero, counts 
of all the count locations 1204 to zero, and all the match flags 1208 to 
"unmatched." Next, the processor 506 selects 1308 from the memory 508 an 
"unmatched" location identifier 204, 208 from one of the memory locations 
510. The processor 506 sets the corresponding match flag 1208 to 
"matched," and increments by one the group tracking number and the count 
of the count location 1204 corresponding to the group identifier 1202 that 
matches the group tracking number. The processor also writes the location 
value of the selected location identifier 204, 208 into the corresponding 
location identifier slot 1206. 
The processor 506 then compares 1310 the location value of the selected 
location identifier 204, 208 with that of all "unmatched" location 
identifiers 204, 208 stored in the memory 508. Whenever a location value 
matching that of the selected location identifier 204, 208 is encountered, 
the processor 506 increments the count in the count location 1204 
corresponding to the group identifier 1202 and sets the match flag 1208 
corresponding to the memory location 510 containing the matched location 
value to "matched." 
The processor next checks 1312 to determine whether all the match flags 
1208 corresponding to the memory locations 510 that contain a location 
value are set to "matched." If not, the processor 506 returns to step 1308 
to select a next "unmatched" location identifier 204, 208 for matching. 
If, on the other hand, in step 1312 the processor 506 determines that the 
all match flags 1208 corresponding to the memory locations 510 that 
contain a location value are set to "matched," then the processor 506 
compares 1314 the counts contained in the count locations 1204 to 
determine the group having the highest count. The processor 506 then 
concludes that the location value contained in the location identifier 
slot 1206 corresponding to the group having the highest count is the most 
dependable location value, which value is then returned to the flowchart 
700 (FIG. 7). 
Referring to FIG. 14, an alternative signal timing diagram 1400 utilized in 
the radio communication system in accordance with the second alternative 
embodiment of the present invention is similar to the signal timing 
diagram 200, the essential difference being that the transmission of the 
synchronization word 202 and the location identifier 204, 208 is 
sequential. That is, each synchronization word 202 and each location 
identifier 204, 208 corresponding to a transmission 1402, 1404 preferably 
is transmitted in a dedicated transmission time slot that does not overlap 
with any other transmission time slot utilized by the system in the same 
general receiving area. It will be appreciated that the messages 206 may 
be simulcast, as depicted in FIG. 14, or sent sequentially, as well. 
Referring to FIG. 15, an electrical block diagram of a second alternative 
receiver element 1500 in accordance with the second alternative embodiment 
of the present invention comprises a conventional received signal strength 
indicator (RSSI) 1502 for measuring and reporting to the processor 506 the 
signal strength of received transmissions. 
Referring to FIG. 16, a second alternative memory utilization chart 1600 
for the portable radio receiver 110, 112 in accordance with the second 
alternative embodiment of the present invention depicts RSSI locations 
1602 for storing a received signal strength corresponding to location 
values stored in the memory locations 510. For each of the transmission 
time slots, there is a time slot identifier 1604, a location identifier 
space 1606 corresponding to the time slot, and a corresponding mean signal 
strength space 1608. 
Referring to FIG. 17, a second alternative firmware diagram 1700 depicting 
firmware elements utilized by the processor in the portable radio receiver 
110, 112 in accordance with the second alternative embodiment of the 
present invention comprises a calculator 1702 for calculating from the 
received signal strengths stored in the RSSI locations 1602, for each of 
the transmission time slots, a mean received signal strength value of the 
periodic transmissions occurring therein over the predetermined period. 
The second alternative firmware diagram 1700 further comprises an examiner 
1704 for examining the mean received signal strength values calculated to 
determine the transmission time slot having a highest mean received signal 
strength value. The second alternative firmware diagram 1700 also includes 
a designator 1706 coupled to the examiner for designating the location 
value received during said transmission time slot to be the most 
dependable location value. It will be appreciated that some or all the 
firmware elements depicted in the second alternative firmware diagram 1700 
can be fabricated in hardware as well, as a custom designed integrated 
circuit, for example. 
Referring to FIG. 18, a flowchart of the alternative storage subroutine 
1800 in accordance with the second alternative embodiment of the present 
invention begins with the processor 506 selecting 1802 a next available 
memory location 510 and RSSI location 1602 for storing the location 
identifier 204, 208 and the corresponding received signal strength. Then, 
the processor 506 stores 1804 the location identifier 204, 208 and the 
received signal strength corresponding to the received periodic 
transmission in the selected memory location 510 and RSSI location 1602. 
The processor 506 then returns 1806 to the flowchart 700 (FIG. 7). 
Referring to FIG. 19, a flowchart of the second alternative comparing 
subroutine 1900 in accordance with the second alternative embodiment of 
the present invention begins with the processor 506 accessing the protocol 
processor 522 to process 1902 the location values stored in the memory 
locations 510 to correct/detect errors therein. Any location value 
containing an error after the processing is eliminated. Next, the 
processor 506 checks 1904 whether any location values remain. If not, the 
processor returns 1906 the predetermined error value to the flow chart 700 
(FIG. 7). 
If, on the other hand, in step 1904 the processor 506 finds that at least 
one location value remains, then the processor 506 accesses the calculator 
1702 to calculate 1908 from the received signal strengths stored in the 
RSSI locations 1602 during the predetermined period a mean received signal 
strength corresponding to each time slot, i.e., corresponding to each 
unique location value stored in the memory locations 510. The processor 
then stores the calculated mean signal strength in the mean signal 
strength space 1608 corresponding to the time slot. The processor 506 also 
stores the location value corresponding to the time slot in the 
corresponding location identifier space 1606. The processor then accesses 
the examiner 1704 and the designator 1706 to compare 1910 the calculated 
mean received signal strengths stored in the mean signal strength slots 
1608 to determine the highest mean received signal strength stored. The 
processor 506 then concludes that the location value stored in the 
location identifier slot 1606 corresponding to the time slot having the 
highest mean received signal strength is the most dependable location 
value, which value is then returned to the flowchart 700. 
Thus, the present invention advantageously provides a method and an 
apparatus for distinguishing location information that is transmitted from 
a nearby transmitter from that transmitted by more distant transmitters in 
the system, even in the presence of time-variant shadowing and multipath 
effects. The present invention advantageously provides a much greater 
degree of certainty that operational decisions based upon received 
location information will be made correctly.