Method and apparatus for inbound transmission in a two-way radio messaging system

A first portable subscriber unit (122, 602) of a plurality of portable subscriber units transmits (504) an inbound message (400) on an inbound channel. The inbound message is intended for a base receiver (616). The inbound message is received (506) by a second portable subscriber unit (606, 608) of the plurality of portable subscriber units. The second portable subscriber unit retransmits (520) the inbound message on the inbound channel only when a predetermined attribute of the inbound message satisfies (508, 510, 512, 514, 518) a predetermined condition.

FIELD OF THE INVENTION 
This invention relates in general to radio communication systems, and more 
specifically to a method and apparatus for inbound transmission in a 
two-way radio messaging system. 
BACKGROUND OF THE INVENTION 
Modern selective call systems are rapidly evolving into two-way radio 
messaging systems. Two-way systems advantageously provide confirmation of 
message delivery, as well as automatic retry upon a failure of a message 
delivery attempt. In addition, two-way systems allow users of portable 
subscriber units to originate as well as receive messages. 
A problem with a two-way radio messaging system is that the transmitter 
power of the portable subscriber unit, and thus the transmission range, is 
limited by battery life considerations. As a result, multiple, high 
sensitivity base receivers usually are distributed about the system to 
achieve good reliability of inbound (i.e., portable subscriber unit to 
base receiver) transmissions. Such base receivers undesirably add cost to 
the infrastructure of the system, and service providers understandably 
want to minimize the number required. Sometimes, especially in new 
systems, economic constraints result in an insufficient number of base 
receivers for adequate inbound coverage throughout the outbound coverage 
area. 
Thus, what is needed is a reliable method and apparatus for inbound 
transmission in a two-way radio messaging system. Preferably, the method 
and apparatus will provide reliable inbound transmissions using fewer base 
receivers in the system, compared to prior art techniques. 
SUMMARY OF THE INVENTION 
An aspect of the present invention is a method for inbound transmission in 
a two-way radio messaging system including a base transmitter and a base 
receiver, the system also including a plurality of portable subscriber 
units. The method comprises the steps of transmitting, by a first portable 
subscriber unit of the plurality of portable subscriber units, an inbound 
message on an inbound channel, the inbound message intended for the base 
receiver; and receiving the inbound message, by a second portable 
subscriber unit of the plurality of portable subscriber units. The method 
further comprises the step of retransmitting the inbound message on the 
inbound channel, by the second portable subscriber unit only when a 
predetermined attribute of the inbound message satisfies a predetermined 
condition. 
Another aspect of the present invention is a portable subscriber unit for 
inbound transmission in a two-way radio messaging system including a base 
transmitter and a base receiver. The portable subscriber unit comprises an 
antenna for transmitting and receiving messages, and a receiver coupled to 
the antenna for receiving an inbound message intended for the base 
receiver and originally transmitted on an inbound channel by a second 
portable subscriber unit. The portable subscriber unit further comprises a 
transmitter coupled to the antenna for retransmitting the inbound message 
on the inbound channel, and a processing system coupled to the transmitter 
and coupled to the receiver for processing transmitted and received 
messages, the processing system including a memory. The processing system 
is programmed to control the transmitter to retransmit the inbound message 
on the inbound channel only when a predetermined attribute of the inbound 
message satisfies a predetermined condition.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, an electrical block diagram of a two-way radio 
messaging system in accordance with the present invention comprises a 
fixed portion 102 including a controller 112 and a plurality of base 
stations 116, and a portable portion including a plurality of portable 
subscriber units 122, preferably having acknowledge-back capability. The 
base stations 116 communicate with the portable subscriber units 122 by 
radio frequency (RF) signals, and are coupled by communication links 114 
to the controller 112, which controls the base stations 116. 
The hardware of the controller 112 is preferably a combination of the 
Wireless Messaging Gateway (WMG.TM.) Administrator| paging terminal, and 
the RF-Conductor|.TM. message distributor manufactured by Motorola, Inc. 
The hardware of the base stations 116 is preferably a combination of the 
Nucleus.RTM. Orchestra| transmitter and RF-Audience|.TM. receiver 
manufactured by Motorola, Inc. The hardware of the portable subscriber 
units 122 is similar to that of Tenor.TM. voice messaging units and 
Pagefinder.TM. data messaging units, also manufactured by Motorola, Inc. 
The hardware and software of the portable subscriber units 122 are 
modified in accordance with the present invention, as will be described 
herein further below. It will be appreciated that other similar hardware 
can be utilized as well for the controller 112, the base stations 116, and 
the portable subscriber units 122. 
Each of the base stations 116 transmits RF signals to the portable 
subscriber units 122 via a transceiver antenna 118. The base stations 116 
each receive RF signals from the plurality of portable subscriber units 
122 via the transceiver antenna 118. The RF signals transmitted by the 
base stations 116 to the portable subscriber units 122 (outbound messages) 
comprise selective call addresses identifying the portable subscriber 
units 122, and voice and data messages originated by a caller, as well as 
commands originated by the controller 112 for adjusting operating 
parameters of the radio communication system. The RF signals transmitted 
by the portable subscriber units 122 to the base stations 116 (inbound 
messages) comprise responses that include scheduled messages, such as 
positive acknowledgments (ACKs) and negative acknowledgments (NAKs), and 
unscheduled messages, such as registration requests. An embodiment of an 
acknowledge-back messaging system is described in U.S. Pat. No. 4,875,038 
issued Oct. 17, 1989 to Siwiak et al., which is hereby incorporated herein 
by reference. 
The controller 112 preferably is coupled by telephone links 101 to a public 
switched telephone network (PSTN) 110 for receiving selective call message 
originations therefrom. Selective call originations comprising voice and 
data messages from the PSTN 110 can be generated, for example, from a 
conventional telephone 111 coupled to the PSTN 110. It will be appreciated 
that, alternatively, other types of communication networks, e.g., packet 
switched networks, local area networks, and the Internet, can be utilized 
as well for transporting originated messages to the controller 112. 
The protocol utilized for outbound and inbound messages is preferably 
selected from Motorola's well-known FLEX.TM. family of synchronous digital 
selective call signaling protocols. These protocols utilize well-known 
error detection and error correction techniques and are therefore tolerant 
to bit errors occurring during transmission, provided that the bit errors 
are not too numerous in any one code word to be corrected. It will be 
appreciated that other suitable protocols can be used as well. 
Referring to FIG. 2, an electrical block diagram depicts the portable 
subscriber unit 122 in accordance with the present invention, comprising 
an antenna 202 for intercepting outbound transmissions from the base 
stations 116. The antenna 202 is coupled to a conventional receiver 208 
for receiving the transmissions to derive information therefrom. The 
receiver 208 preferably includes a conventional received signal strength 
indicator (RSSI) 204 for measuring and indicating the signal strength 
received by the receiver 208. The receiver 208 is coupled to processing 
system 236, comprising a processor 210 for processing the received 
information, including static and dynamic programming information, as is 
discussed further herein below. The processing system 236 preferably 
controls the frequency of reception of the receiver 208, such that the 
receiver 208 can selectively monitor both the outbound channel for 
receiving an outbound message, and the inbound channel for receiving an 
inbound message from another portable subscriber unit 122, in accordance 
with the present invention. 
The processing system 236 is also coupled to a user interface 214 for 
conveying portions of the received information to a user, via, for 
example, a conventional display 216. The user interface 214 also includes 
a conventional alert 218 for alerting the user that new information has 
arrived. In addition, the user interface includes conventional user 
controls 220 for controlling the portable subscriber unit 122. The 
processing system 236 is further coupled to a transmitter 206 for 
communicating a transmission data stream to a receiver of the base station 
116. The transmitter 206 is coupled to the antenna 202 for emitting a 
radio signal comprising the transmission data stream. It will be 
appreciated that, alternatively, the antenna 202 can be replaced with two 
separate antennas, one for reception and the other for transmission. 
The processing system 236 further comprises a conventional memory 212 for 
storing software and variables for programming the processor 210 in 
accordance with the present invention. The memory 212 comprises a message 
processing element 222 for programming the processor 210 to process 
outbound messages and to maintain synchronization with the synchronous 
communication protocol through well-known techniques. The memory 212 
further comprises an inbound reception element 224 for programming the 
processor 210 to control the receiver 208 to monitor the inbound channel 
during a predetermined time window assigned to the portable subscriber 
unit 122 and synchronized with the protocol. The memory 212 also includes 
an attribute testing element 226 for programming the processor 210 to test 
selected attributes of a received inbound message 228 to determine whether 
to retransmit, i.e., repeat, the received inbound message 228, as will be 
described further below. In addition, the memory 212 includes a location 
for storing the received inbound message 228. The memory 212 also includes 
a retransmission element 230 for programming the processor 210 to control 
the retransmission of the inbound message. The memory 212 further 
comprises a history element 232 for storing a history of message contents 
that have been retransmitted. To conserve memory space the history element 
232 preferably stores a number, such as a conventional check sum, derived 
from the content of each message retransmitted. It will be appreciated 
that, alternatively, other indicators of message content can be stored in 
the history element 232, including the actual characters of the message. 
The memory 212 also includes a signal quality index (SQI) element 234 for 
programming the processor 210 to maintain a current SQI indicative of a 
received signal quality of an outbound transmission received from a 
transmitter of the base station 116. The SQI preferably is maintained by 
the portable subscriber unit 122 using well-known techniques described 
further below. 
FIG. 3 is a protocol timing diagram 300 in accordance with the present 
invention. Preferably, the protocol comprises a plurality of contiguous 
frames of outbound data transmitted on an outbound channel. The frames 
preferably are synchronized to a time reference, such as a Global 
Positioning Satellite (GPS) time reference. A frame 302 is depicted in the 
diagram 300. The frame 302 begins with an outbound synchronization signal 
304, which the portable subscriber units 122 monitor periodically to 
acquire synchronization with the protocol. The frame 302 further comprises 
an address field 306 for identifying portable subscriber units 122 for 
which messages about to be sent are intended. The frame 302 also includes 
a vector field 308 for identifying where in a subsequent message field 
each message corresponding to an address is to be transmitted. The frame 
302 further comprises a message field 310, which contains the messages. 
The frame 302 is followed by the outbound synchronization signal 304 of a 
next frame 302. Preferably an integer number of the frames 302, e.g., 128 
frames, form one cycle of the protocol. For the preferred FLEX.TM. 
protocol, one cycle has a duration of 4.0 minutes. 
The diagram 300 also depicts an inbound frame 314, comprising a plurality 
of synchronous inbound message packets PO-Pn. Note that the first inbound 
message packet 316 of the inbound frame 314 is transmitted immediately 
after the end of the outbound synchronization signal 304, so that the 
inbound message packets also are synchronized to the protocol. The last 
inbound message packet 318 of the inbound frame 314 ends before, but not 
necessarily exactly at, the end of the outbound synchronization signal 304 
of the next subsequent frame 302. At the end of the outbound 
synchronization signal 304 of the next subsequent frame 302, the first 
inbound message packet 320 of a next subsequent inbound frame 314 begins. 
Preferably, a first portion 322 of the inbound message packets is assigned 
exclusively to scheduled messages, while a second portion 324 is assigned 
to both unscheduled (ALOHA) messages and retransmitted inbound messages, 
in accordance with the present invention. It will be appreciated that, 
alternatively, the inbound message packets PO-Pn can be divided into three 
portions, the first portion assigned exclusively to scheduled messages, 
the second portion assigned exclusively to unscheduled messages, and the 
third portion assigned to retransmitted inbound messages. Preferably, the 
inbound frames 314 are transmitted on an inbound channel that operates on 
an RF frequency different from that utilized by the outbound channel. It 
will be appreciated that, alternatively, the outbound channel and the 
inbound channel can also operate on a single RF frequency by employing a 
time division technique. 
FIG. 4 is a protocol timing diagram detailing the inbound message 400 in 
accordance with the present invention. The inbound message 400 comprises a 
ramp up and synchronization portion 408 for allowing the transmitter 206 
to stabilize and for synchronizing the receiver 208 and the base receiver 
616 (FIG. 6) with the inbound message 400 from another portable subscriber 
unit 122. The diagram 400 further comprises a message data portion 402. In 
accordance with the present invention, the inbound message 400 includes a 
signal quality index 404. The SQI 404 preferably is a number incode was 
many times a transmitter color code was received during the last N, e.g., 
8, transmissions of the outbound synchronization signal 304. 
Alternatively, the signal quality index can be a number representing an 
average signal strength indicated by the RSSI 204 during the last N 
outbound transmissions. 
The inbound message 400 also includes a life count 406, which has a 
predetermined value, e.g., 2, when transmitted by a portable subscriber 
unit 122 which is originating an inbound message. Before a portable 
subscriber unit 122 retransmits a received inbound message originated by 
another portable subscriber unit, the retransmitting portable subscriber 
unit 122 changes the life count 406 by a predetermined amount, e.g., 
decrements the life count by unity. Each time a portable subscriber unit 
122 receives an inbound message, it checks the value of the life count 
406. If the value of the life count 406 is outside of a predetermined 
range, e.g., less than 1, then the portable subscriber unit 122 does not 
retransmit the inbound message. This feature prevents an inbound message 
from being retransmitted more times than are considered useful for 
improving the reliability of an inbound transmission. 
FIG. 5 is an exemplary flow chart 500 depicting operation of the two-way 
radio messaging system in accordance with the present invention. The flow 
chart 500 begins with each of the plurality of portable subscriber units 
122 maintaining 502 a signal quality index (SQI). Next, a first portable 
subscriber unit 122 transmits 504, i.e., originates, an inbound message 
400 including the SQI 404 maintained by the portable subscriber unit 122, 
along with the life count 406. A second portable subscriber unit 122, 
which has been assigned a predetermined time window that covers the 
reception time of the inbound message 400, monitors 506 the inbound 
channel during the predetermined time window. Preferably, the 
predetermined time window for monitoring the inbound channel is assigned 
to the portable subscriber unit 122 by the controller 112 through the base 
station 116. Preferably the assignment of the predetermined time window is 
made via a new command added to the FLEX.TM. two-way protocol, utilizing 
well-known techniques for adding a command. Alternatively, the 
predetermined time window can be pre-programmed into the portable 
subscriber unit 122. In addition, a new block information word (BIW) 
preferably is added to the protocol to allow the controller 112 to 
dynamically adjust the size of the predetermined time window according to 
how many portable subscriber units 122 are present in the two-way radio 
messaging system. This adjustment is preferably based upon the number of 
portable subscriber units 122 registered in the system. Alternatively, the 
adjustment can be based on other factors, such as time of day, day of 
week, or measured traffic in the system. 
Because the inbound time slot (a timing attribute of the inbound message) 
used by the first portable subscriber unit 122 falls within the 
predetermined time window of the second portable subscriber unit 122, the 
second portable subscriber unit 122 receives the inbound message 400. The 
second portable subscriber unit 122 then compares the received SQI 404 
with its own SQI, which is maintained by the processor 210 in cooperation 
with the SQI element 234. The processing system 236 of the second portable 
subscriber unit 122 next checks 508 whether the received SQI 404 is at 
least X, e.g., 2, less than its own SQI. If not, the second portable 
subscriber unit 122 concludes that it is probably not much closer to the 
base station 116 than the first portable subscriber unit 122, and thus it 
should not retransmit the inbound message, and the process ends. 
If, on the other hand, the received SQI 404 is at least X less than the SQI 
maintained by the second portable subscriber unit 122, then the processing 
system 236 of the second portable subscriber unit 122 cooperates with the 
RSSI 204 to check 510 whether the inbound message was received by the 
second portable subscriber unit 122 with a signal strength less than Y, 
e.g., a signal strength corresponding to 60-70 dB of propagation loss. If 
the inbound message was not received with a signal strength less than Y, 
the processing system 236 concludes that the first and second portable 
subscriber units 122 are too close to one another for a retransmission to 
be of much help, and the process ends. (The value of Y preferably can be 
adjusted by the controller 112 using a new block information word, added 
through well-known techniques, to achieve a desired level of 
retransmission probability for a given radio messaging system population.) 
If, on the other hand, the inbound message was received by the second 
portable subscriber unit 122 with a signal strength less than Y, then the 
processing system 236 of the second portable subscriber unit 122 checks 
512 whether the life count 406 of the received message is out of range, 
e.g., less than 1. If so, the inbound message has been retransmitted the 
maximum number of times allowed, and the process ends. 
If, on the other hand, the processing system 236 determines that the life 
count 406 is not out of range, then the processing system 236 of the 
second portable subscriber unit 122 checks 514 whether an error count 
maintained by the processing system 236 using well-known error detection 
and correction techniques is less than Z, e.g., less than 1 (indicating no 
errors after error correction in accordance with the error correcting 
protocol). If not, the inbound message 400 was received with too many 
errors, and the process ends. If, on the other hand, the processing system 
236 determines that the error count is less than Z, then the processing 
system 236 compares 516 the message content with any message contents in 
the history element 232. The processing system 236 of the second portable 
subscriber unit 122 then checks 518 whether the inbound message 400 
matches a message content (or indication thereof) in the history element 
232. If so, the processing system 236 concludes that the second portable 
subscriber unit 122 has already retransmitted the inbound message once, 
and the process ends. If, however, the inbound message 400 does not match, 
then the processing system 236 decrements 520 the life count, retransmits 
the received inbound message 400 (after error correction), and stores the 
content (or indication thereof) of the inbound message in the history 
element 232. 
While the exemplary flow chart 500 tests many attributes of the inbound 
message before retransmitting the inbound message, it will be appreciated 
that, alternatively, some of the tests can be changed or omitted 
altogether. For example, one can decide to eliminate the history element 
232 and the history match check step 518 to reduce processing and storage 
requirements in the portable subscriber unit 122. These elements are of no 
use when the initial value of the life count 406 is chosen, for example, 
such that a maximum of two retransmissions can be performed on each 
inbound message 400. 
It should be reemphasized at this point that, because the second portable 
subscriber unit 122 is synchronized with the synchronous protocol, as 
depicted in the diagram 300, the steps of receiving the inbound message 
and retransmitting the inbound message occur at predetermined times in a 
cycle of the protocol. FIG. 6 is an exemplary propagation diagram 600 
depicting inbound message retransmission in accordance with the present 
invention. The diagram 600 depicts a first portable subscriber unit 602 
having a first transmission range 604 for an inbound message. Due to the 
transmission environment of the first portable subscriber unit 602, e.g. 
signal attenuated by building walls, the first transmission range 604 is 
not sufficient to reach a base receiver 616 of the nearest base station 
116. Second and third portable subscriber units 606, 608 are within the 
first transmission range 604 of the first portable subscriber unit 602. 
The second and third portable subscriber units 606, 608 have second and 
third transmission ranges 610, 612, respectively, each sufficient to reach 
the base receiver 616. 
In accordance with the present invention, the second and third portable 
subscriber units 606, 608 have been assigned, for example, the same 
predetermined time window for monitoring the inbound channel. The time 
window assignment preferably is accomplished by a new outbound protocol 
command (similar to an existing FLEX.TM. family command for establishing 
ALOHA boundaries) sent from the controller 112. The new command can, for 
example, assign at least one inbound frame 314 of the cycle and at least 
one of the inbound packets PO-Pn of the inbound frame 314 to be monitored 
by a portable subscriber unit 122. Preferably, a predetermined mapping 
exists between a monitored inbound packet and the packet slot to be 
utilized for retransmitting the packet. For example, a portable subscriber 
unit assigned to monitor frame 1, inbound packets PO-P4, would always 
retransmit a P0 message in ALOHA slot 1, a P1 message in ALOHA slot 2, et 
cetera. 
The first portable subscriber unit 602 now transmits an inbound message 400 
intended for the base receiver 616. The inbound message 400 does not reach 
the base receiver 616. The inbound message 400 does, however, reach the 
second and third portable subscriber units 606, 608 during the 
predetermined time window assigned to the second and third portable 
subscriber units 606, 608 for monitoring the inbound channel. Thus, the 
second and third portable subscriber units 606, 608 both receive the 
inbound message. When the second and third portable subscriber units 606, 
608 have determined that the attributes of the inbound message (as 
discussed herein above) meet all requirements for retransmission, the 
second and third portable subscriber units 606, 608 will retransmit the 
inbound message in the ALOHA slot to which the message is mapped by the 
rules of the system. 
Because the same retransmission time slot is used by all retransmitters of 
a given inbound message 400, and because the radio messaging system uses a 
synchronous protocol to which all the portable subscriber units 122 are 
synchronized, the second and third portable subscriber units 606, 608 will 
retransmit the inbound message 400 simultaneously, as a simulcast 
transmission. As is well known in the art, a simulcast transmission, 
properly performed, can create additional reinforcement of the signal 
strength, thereby further improving reception reliability. In the 
preceding example, either of the portable subscriber units 606, 608 alone 
possesses a transmission range 610, 612 capable of reaching the base 
receiver 616. Thus, even if only one of the second and third portable 
subscriber units 606, 608 had retransmitted the inbound message 400, it 
likely would have been received by the base receiver 616. 
Additional portable subscriber units 618, 620, 622 are within range of the 
first portable subscriber unit 602. The additional portable subscriber 
units 618, 620, 622 do not retransmit the inbound message 400, however, 
because one or more of the attributes of the inbound message 400 do not 
satisfy a predetermined condition of the additional portable subscriber 
units 618, 620, 622. For example, the inbound message 400 might be 
transmitted outside the predetermined time windows assigned to the 
additional portable subscriber units 618, 620, 622, or the received signal 
strength of the inbound message could be too high, or the reported SQI 404 
could be too large compared to the SQIs currently maintained by the 
additional portable subscriber units 618, 620, 622. 
While the foregoing example depicts the base receiver 616 to be collocated 
with a base transmitter 614, it will be appreciated that there can be 
additional base receivers 616 remote from the base transmitter 614 for 
obtaining a wider coverage area for receiving inbound messages. The 
present invention, however, advantageously minimizes the number of such 
additional base receivers required. 
Thus, it should be clear from the preceding disclosure that the present 
invention provides a method and apparatus for performing reliable inbound 
transmissions in a two-way radio messaging system. Advantageously, the 
method and apparatus can perform reliable inbound transmissions using 
fewer base receivers in the system, compared to prior art techniques. 
Many modifications and variations of the present invention are possible in 
light of the above teachings. Thus, it is to be understood that, within 
the scope of the appended claims, the invention can be practiced other 
than as described herein above.