Method and system for efficiently providing two way communication between a central network and mobile unit

A two-way communication system for communication between a system network and a mobile unit. The system network includes a plurality of base transmitters and base receivers included in the network. The base transmitters are divided into zonal assignments and broadcast in simulcast using multi-carrier modulation techniques. The system network controls the base transmitters to broadcast in simulcast during both systemwide and zonal time intervals. The system network dynamically alters zone boundaries to maximize information throughput. The preferred mobile unit includes a noise detector circuit to prevent unwanted transmissions. The system network further provides an adaptive registration feature for mobile units which controls the registration operations by the mobile units to maximize information throughput.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
The present invention relates to methods and systems for providing two-way 
communication capability between a central network and a mobile unit over 
a relatively large area, and more particularly to such methods and systems 
which allow for rapid communication of large messages and efficient use of 
system resources. 
B. Description of the Related Art 
Conventional two-way portable/mobile wireless messaging systems often 
provide a variety of services to subscribers. Conventional messaging 
systems in particular provide one-way services using store and forward 
techniques to mobile receivers carried by the subscriber. A fundamental 
goal of two-way messaging systems is to provide a network of 
interconnected transmitters and receivers which provides sufficient 
transmitted signal strength and receive capability to uniformly cover a 
geographic region. Some conventional messaging systems provide the message 
to the user on a small viewing screen on the mobile unit. 
However, such conventional systems often suffer from problems associated 
with low system throughput, evidenced by slow message delivery and message 
size limitations and do not provide an acknowledgment feature wherein the 
mobile unit transmits an acknowledgment signal to the system to 
acknowledge receipt of the message from the system. Generally, system 
throughput refers to the overall communication capability of a system as 
defined by the total amount of message data from the system to the mobile 
units transferred by the system during a given period of time divided by 
the frequency bandwidth necessary to transmit the message data and may be 
measured in bits transferred per Hz. Further, such conventional systems 
suffer from technical problems preventing consistent wide area coverage 
and would require extremely wide portions of valuable frequency bandwidth 
to achieve acceptable system throughput rates. 
Simulcast technology in communication systems was originally developed to 
extend transmitter coverage beyond that which could be obtained from a 
single transmitter. Over time, however, simulcasting has evolved into a 
technique capable of providing continuous coverage to a large area. 
Generally, simulcast technology provides multiple transmitters, operating 
on substantially the same frequencies and transmitting the same 
information positioned to cover extended areas. As shown in FIG. 1, 
transmitter 100 generally provides coverage over area A, D, and E, 
transmitter 102 generally provides coverage over area B, D, and E, and 
transmitter 104 generally provides coverage over area C, E, and F. In some 
cases, the coverage area of a first transmitter may be entirely enclosed 
within the coverage area of another transmitter, such as in building 
interiors and valleys. In areas where one (and only one) transmitter 
dominates (e.g., areas A, B, and C in FIG. 1), simulcast is effective 
because the other transmitters do not significantly affect receivers in 
those areas. 
However, in "overlap" areas D, E, and F shown in FIG. 1, where the signals 
from two or more transmitters are approximately equal, problems can arise 
because destructive interference of signals occurs in these overlap areas 
such as areas D, E, and F. Destructive interference occurs when the two 
signals are equal in magnitude and 180.degree. out of phase and completely 
cancel each other. While there were some successes, reliable design 
procedures were not available. 
Attempting to precisely synchronize the carrier frequencies of all 
simulcast transmitters does not overcome the problem because points (i.e. 
nodes) at which destructive summing occurred persisted for long periods of 
time. At such points, a mobile receiver can not receive the simulcast 
signal. 
Deliberately offsetting the carrier frequencies of adjacent transmitters 
can ensure that destructive interference does not persist at one point for 
an extended period of time. The slight errors in frequency displayed by 
high quality reference oscillators (e.g., 20 hertz errors in 100 MHz 
signals or a few parts in 10.sup.7) render deliberate offsetting 
unnecessary. Further, merely offsetting the carrier frequencies could not 
guarantee acceptable quality demodulation because proper alignment of the 
modulating signals in time is also required. 
FIG. 2 displays the situation at, for example, point D in FIG. 1 when 
modulating waveforms are synchronized and includes coverage boundary 202 
from a first transmitter and a second transmitter coverage boundary 204 
from a second adjacent transmitter. An equi-signal boundary 200 exists 
where the signals from the first and second transmitters have 
approximately equal signal strengths. A more realistic equi-signal 
boundary would take into account natural and man-made topography and 
propagation conditions, and therefore would probably not be a straight 
line. 
FIGS. 3 and 4 generally illustrate various signals as they may occur at or 
near the equi-signal boundary 200 as shown in FIG. 2. In particular, FIGS. 
3 and 4 illustrate various aspects of modulation synchronization and how 
altering transmission parameters may affect the synchronization. In 
general, there are at least three sources which cause the signals from the 
first transmitter and the second transmitter to be out of synchronization: 
(1) timing shifts in the delivery of the modulating waveform to each of 
the transmitters; (2) timing shifts internal to each transmitter; and (3) 
timing shifts caused by propagation distances and anomalies. From the 
perspective of a receiver located in an overlap area, these three sources 
of timing shifts combine to produce an overall timing shifts between the 
received signals from the first and second transmitters. In current 
commercial practice, the summation of these three components results in 
time shifts of about 200 microseconds. The timing shift present in 
simulcast systems disadvantageously limits the baud rate at which 
information may be transferred. In general, FIGS. 3 and 4 will also 
illustrate how timing shifts prevents high baud rate transmissions. 
A time line representation of a signal 306 from a first transmitter is 
shown in FIG. 3(A) and a signal 308 from a second transmitter is shown in 
FIG. 3(B), both from the perspective of a receiver located in an overlap 
area. Vertical dashed lines 300 represent baud intervals on the time axis. 
As can be seen from FIGS. 3(A) and (B), the signals 306 and 308 are 
frequency modulated between a high and a low frequency value and the 
signals 306 and 308 are exactly in phase. As will be appreciated, the 
timing shift between signals 306 and 308 must be small when compared to 
the baud interval shown in FIGS. 3(A) and (B) since signals 306 and 308 
are in synchronization. Of course, as the baud interval decreases, the 
timing shifts will likely cause signals 306 and 308 to be out of 
synchronization. 
FIGS. 3(C), (D), and (E) show the summation of these two signals 306 and 
308 at an equi-signal boundary, such as boundary 200 in FIG. 2. FIG. 3(C) 
shows a composite signal 310 indicating that the frequency information 
remains unchanged, FIG. 3(D) shows a linear graph 312 of the relative 
phase difference caused by a slight carrier frequency difference between 
the signals from the first transmitter and the second transmitter. FIG. 
3(E) shows a composite amplitude signal 314. A noise threshold is 
indicated by the horizontal dashed line 304 in FIG. 3(E). 
Of interest, FIG. 3(E) shows the composite amplitude signal 314 dipping 
below the noise threshold 304 at an anti-phase condition 302 (e.g., when 
the relative phase angle is .+-.180.degree., as shown in FIG. 3(D)). As 
can be seen from FIG. 3(E), the anti-phase condition 302 caused by the 
slight phase shift between transmitter 1 and transmitter 2 will not cause 
any loss of data because the anti-phase condition persists for only a 
small portion of the baud interval. 
The slight offset of the carrier frequencies between the first and second 
transmitters causes a slow drift of the relative phase of the two signals, 
as shown in FIG. 3(D). When the signals are .+-.180.degree. out of phase, 
the temporary dip in the amplitude signal may cause the loss of a few bits 
in the composite signal, at worst. These errors can be counteracted with a 
conventional error correcting code, such as is commonly known. 
FIG. 4 shows a set of similar signals to those in FIG. 3, but wherein the 
signal 402 from the first transmitter is offset from, or out of 
synchronization with, the signal 404 from the second transmitter by a full 
baud. In particular, signal 404 lags signal 402 by one baud interval. As 
previously discussed, the offset of signals 402 and 404 may be caused by 
various timing shifts in the delivery of both signals 402 and 404 to a 
receiver in an overlap area. FIGS. 4(A) and (B) illustrate the extreme 
case where the sum of these timing shifts is equal to the baud interval 
shown by dashed lines 400. As can be seen in FIG. 4(C), composite signal 
406 includes a period of indeterminate frequency which undesirably covers 
several entire baud intervals and, therefore, successful demodulation is 
impossible during those baud intervals. If the baud interval were 
increased to minimize the effect of these timing shifts, data loss would 
be less likely. Therefore, it can be seen that the baud rate at which good 
data transfer can be accomplished is limited by the timing shifts between 
signals delivered to receivers in overlap areas. 
Through these examples, it can be seen that high degrees of modulation 
synchronization make it possible to obtain good data demodulation in a 
simulcast system. However, the baud rate limitation of simulcast systems 
is a significant drawback and limits system throughput. 
An alternative to simulcast for wide area coverage is assignment of 
orthogonal, non-overlapping subdivisions of the available system capacity 
to adjacent areas. Subdivisions can be made in time (e.g., broadcasting 
the information on the same frequency in different time slots to adjacent 
areas), or in frequency (e.g., broadcasting the information simultaneously 
on different frequencies in adjacent areas). There are several problems 
with such orthogonal systems, however. First, orthogonal assignments 
require tuning the receiver to the assigned frequency or time channel for 
the area in which the receiver currently resides. In the broadcast 
services every traveler has experienced the frustration of finding the 
correct channel for their favorite programs. Simulcast operation avoids 
the need for scanning and re-tuning as the mobile unit moves between 
areas. Such scanning and re-tuning also disadvantageously increases mobile 
unit power consumption. 
Second, and more serious, the orthogonal assignment approach drastically 
reduces the system throughput capacity as measured in bits per Hz because 
anywhere from 3 to 7, or possibly more, orthogonal assignments are 
required to obtain continuous area coverage in most conventional 
orthogonal systems. This waste of capacity is somewhat recouped if the 
same information is not needed throughout the service area because a given 
piece of information is sent only to those cells where it is needed. 
Conventional cellular radio service is a typical example of an orthogonal 
system. In cellular, the same frequencies are reused in spatially 
separated cells to allow different data to be transmitted to different 
mobile units. An example of three cellular arrangements is shown in FIG. 5 
where the number of cells (N) is equal to 3, 4, and 7. Each cell (i.e., A, 
B, C, . . . ) in conventional cellular service usually only includes a 
single transmitter and operates in a different frequency or time division 
within the communication protocol. As shown in FIG. 5, cellular service 
generally locates transmitters utilizing the same division (all the "A" 
transmitters) far enough apart to reduce the likelihood of interference 
between such transmitters. As the number of cells increases, the 
likelihood of interference decreases. For example, with N=3 as shown by 
arrangement 500 in FIG. 3, the distance between the coverage area of "A" 
cells is about 1/2 cell width, with N=4 in arrangement 502, the distance 
between the coverage areas of "A" cells is slightly larger, and with N=7 
in arrangement 504 the distance between "A" cells is larger than the width 
of one cell. 
However, as the number of cells increases, the length of the individual 
time intervals per cell decreases for time division multiplexed systems, 
thereby decreasing the systems total information transfer. In frequency 
division systems, more cells undesirably increases the frequency bandwidth 
required. Therefore, system throughput in bits per Hz is decreased as the 
number of cells increases. Furthermore, cellular systems often require an 
electronic "handshake" between system and mobile unit to identify the 
specific cell (i.e. transmitter) in which the mobile unit is located to 
allow capacity reuse. 
II. SUMMARY OF THE INVENTION 
The systems and methods of the present invention have a wide variety of 
objects and advantages. The systems and methods of the present invention 
have as a primary object to provide a communication system with wide area 
coverage and high message throughput while minimizing frequency bandwidth 
usage. 
It is an object of the invention to provide a simulcast communication 
system with a high data transfer rate which does not exceed the baud rate 
limitations of simulcast transmission. 
It is a further object of the present invention to provide a communication 
system which provides for superior data communication integrity. 
Yet another object of the invention is to provide a mobile transceiver unit 
which prevents unnecessary RF interference, particularly on commercial 
aircraft. 
Still further, it is an object of the invention to provide a zone based 
communication system which may dynamically redefine zone boundaries to 
improve information throughput. 
Another object of the invention is to provide a zone based simulcast 
communication system which can effectively communicate with both mobile 
transceiver units located near the center of each zone as well as mobile 
transceiver units located within the overlap areas between two or more 
zones. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by practicing the invention. The 
objects and advantages of the invention will be realized and attained by 
means of the elements and combinations particularly pointed out in the 
appended claims. 
To achieve the objects and in accordance with the purpose of the invention, 
as embodied and broadly described herein, the invention is directed to a 
method for information transmission by a plurality of transmitters to 
provide broad communication capability over a region of space, the 
information transmission occurring during at least both a first time 
period and a second time period and the plurality of transmitters being 
divided into at least a first and second set of transmitters, the method 
comprising the steps of (a) generating a system information signal which 
includes a plurality of blocks of information, (b) transmitting the system 
information signal to the plurality of transmitters, (c) transmitting by 
the first and second sets of transmitters a first block of information in 
simulcast during the first time period, (d) transmitting by the first set 
of transmitters a second block of information during the second time 
period, and (e) transmitting by the second set of transmitters a third 
block of information during the second time period. 
In another embodiment, the invention is directed to a multi-carrier 
simulcast transmission system for transmitting in a desired frequency band 
a message contained in an information signal, the system comprising a 
first transmitter means for transmitting an information signal by 
generating a first plurality of carrier signals within the desired 
frequency band and by modulating the first plurality of carrier signals to 
convey the information signal, and a second transmitter means, spatially 
separated from the first transmitter, for transmitting the information 
signal in simulcast with the first transmitter by generating a second 
plurality of carrier signals at substantially the same frequencies as the 
first plurality of carrier signals and by modulating the second plurality 
of carrier signals to convey the information signal. 
In another embodiment, the invention is directed to a communication method 
implemented in a computer controlled communication network for locating a 
mobile transceiver within a region of space, the region of space being 
divided into a plurality of zones with each zone serviced by at least one 
base transmitter and at least one base receiver, the network storing data 
corresponding to a zone where the mobile transceiver was last known to be 
located, the communication method comprising the steps of (a) transmitting 
a message signal by a base transmitter servicing a zone where the mobile 
transceiver was last known to be located, (b) transmitting a systemwide 
probe signal by a plurality of base transmitters servicing a plurality of 
zones if the mobile transceiver does not indicate receipt of the message 
signal from the base transmitter, (c) receiving the regional probe signal 
by the mobile transceiver, (d) transmitting an acknowledgment signal by 
the mobile transceiver in response to the received regional probe signal, 
(e) receiving the acknowledgment signal from the mobile transceiver by a 
base receiver, and (f) updating the data to reflect the zone of the base 
receiver that received the acknowledgment signal as the last known 
location of the mobile transceiver. 
In yet another embodiment, the invention is directed to a method of 
communicating messages between a plurality of base transmitters and mobile 
receivers within a region of space divided into a plurality of zones with 
each zone having at least one base transmitter assigned thereto, the 
communication method comprising the steps of (a) transmitting 
substantially simultaneously a first information signal and a second 
information signal to communicate messages to the mobile receivers, the 
first information signal being transmitted in simulcast by a first set of 
base transmitters assigned to a first zone, and the second information 
signal being transmitted in simulcast by a second set of base transmitters 
assigned to a second zone, (b) dynamically reassigning one or more of the 
base transmitters in the first set of base transmitters assigned to the 
first zone to the second set of base transmitters assigned to the second 
zone as a function of the messages to be communicated in an area, thereby 
creating an updated first set of base transmitters and an updated second 
set of base transmitters, and (c) transmitting substantially 
simultaneously a third information signal and a fourth information signal, 
the third information signal being transmitted in simulcast by the updated 
first set of base transmitters, and the fourth information signal being 
transmitted in simulcast by the updated second set of base transmitters to 
communicate additional messages to said mobile receivers. 
In another embodiment, the invention is directed to a mobile transceiver 
unit for transmitting messages to and receiving messages from a network 
comprising input means for allowing the user to input a user message to 
the unit, transmitter means for transmitting a radio frequency signal 
including the user message from the mobile unit to the network, receiver 
means for receiving radio frequency signals having a message from the 
network, signal detector means for detecting at least one type of 
electromagnetic signal generated external to the mobile unit and the 
network, and a circuit, connecting the signal detector means to the 
transmitter means, for disabling the transmitter means upon detection of 
the electromagnetic signal, thereby preventing unwanted radio frequency 
transmission. 
In another embodiment, the invention is directed to a communication method 
for controlling a mobile transceiver which may communicate with a 
communication network controlled by a computer, the network including a 
plurality of base transmitters for transmitting messages from the network 
to the mobile transceiver and base receivers for receiving messages from 
the mobile transceiver, the mobile transceiver being capable of sending a 
registration signal to be received by a base receiver in the network to 
identify the mobile transceiver's location and the plurality of base 
transmitters in the network being capable of sending a probe signal to the 
mobile transceiver to cause the mobile transceiver to transmit a signal to 
a base receiver to identify its location, the method comprising the steps 
of (a) sending a message from the network to the mobile transceiver to 
disable the mobile transceiver's capability to transmit a registration 
signal, (b) storing the number of probe signals sent by the network to the 
mobile transceiver during a first period of time and the number of 
messages successfully delivered to the mobile transceiver by the network 
during a second period of time, (c) processing by the computer the stored 
number of probe signals and number of messages successfully delivered to 
evaluate a likelihood that a probe signal will be required to be sent by 
the network to locate the mobile unit to deliver a message, and (d) 
sending a message to the mobile unit to enable the mobile transceiver's 
capability to transmit a registration signal if the calculated likelihood 
exceeds a selected value. 
Finally, in another embodiment, the invention is directed to a 
communication method for controlling a mobile transceiver which may 
communicate with a communication network controlled by a computer, the 
network including a plurality of base transmitters for transmitting 
messages to the mobile transceiver and base receivers for receiving 
messages from the mobile transceiver, the mobile transceiver being capable 
of sending a registration signal to be received by a base receiver in the 
network to identify the mobile transceiver's location, the network using 
received registration signals to determine a set of base transmitters to 
be operated to transmit a message to the mobile transceiver, the method 
comprising the steps of (a) sending a message from the network to the 
mobile transceiver to enable the mobile transceiver's capability to 
transmit a registration signal, (b) storing the number of registration 
signals from the mobile transceiver to the network during a first period 
of time and the number of messages successfully delivered to the mobile 
transceiver by the network during a period of time, (c) processing the 
stored number of registration signals and number of messages successfully 
delivered to evaluate a likelihood that a registration signal from said 
mobile unit will not be used by the network to determine a set of base 
transmitters, and (d) sending a message to the mobile unit to disable the 
mobile transceiver's capability to transmit a registration signal if the 
likelihood exceeds a selected value. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed.

IV. DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the present preferred embodiments 
and exemplary embodiments of the invention, examples of which are 
illustrated in the accompanying drawings. Wherever possible, the same 
reference numbers will be used throughout the drawings to refer to the 
same or like parts. 
A. Overview of The System Hardware 
FIG. 6 shows an overview of the major elements of a preferred communication 
system according to the present invention. As shown therein, the 
communication system includes a network operations center 600 which is 
connected to a satellite uplink 602 via data path 604. A satellite uplink 
is used to provide data to satellite 606. Satellite 606 redirects the 
received data to several satellite downlink stations including station 608 
and station 610. Conventional satellite technology allows for nominal data 
transfer rates of 24 M bits/second. Further, conventional satellite 
technology allows for accurate delivery of data to stations 608 and 610, 
which allows for precise synchronization between the signals broadcast in 
simulcast by the stations 608 and 610. It should be understood that 
stations 608 and 610 may optionally receive identical data, or may 
individually receive different data simultaneously from the satellite 606. 
Satellite downlink stations 608 and 610 are connected to spatially 
separated base transmitters 612 and 614 via data paths 616 and 618, 
respectively. Base transmitter 612 is connected to antenna 620, and base 
transmitter 614 is connected to antenna 622. Preferably, the base 
transmitters of the present system have a power output capability of about 
350 watts, which will provide an effective transmitter coverage area of 
several tens of miles. Each zone preferably includes multiple transmitter 
stations, shown as, for example, base transmitters 613 and 615 in FIG. 6, 
as will be evident from the following discussion. 
Mobile unit 624 is connected to antenna 626 and, in the preferred 
embodiment, is a small, portable unit capable of being carried easily by a 
user and therefore is similar to conventional pagers in those aspects. 
More preferably, the mobile unit has both receive and transmit capability, 
with a nominal transmit power output of about 1 watt. 
The communication system includes several base receivers 628, 630, 632, and 
634 each connected to antennas 636, 638, 640, and 642, respectively. Base 
receivers 628 and 630 are connected to a regional station 644 via data 
paths 646 and 648, respectively. Base receivers 632 and 634 are connected 
to regional station 650 via data paths 652 and 654, respectively. Base 
transmitters 612, 614 preferably have a large transmit power output 
capability to provide coverage to the mobile unit in areas to which 
communication is typically difficult, such as building interiors, and to 
extend the coverage area of each transmitter. An appropriate number of 
base receivers should be dispersed throughout the geographic area to 
reliably receive the signals from the mobile unit. Due to the difference 
in output power between base transmitters and mobile units, an overall 
ratio of 10 base receivers to 1 base transmitter may be appropriate, and 
the 2 to 1 ratio shown in FIG. 6 is merely shown for ease of illustration. 
Regional station 650 is connected to the network operations center 600 via 
data path 656 and regional station 644 is connected to the network 
operations center 600 via data path 658. The data paths 656 and 658 
preferably include low cost phone lines, but may include any convenient 
and appropriate data transfer technology. 
Generally, the communication system of the present invention roughly 
divides various regions of space into portions called zones. Each zone 
must have one or preferably more base transmitters assigned to it. Zone 
boundaries are roughly defined by the transmitter coverage areas of the 
base transmitters assigned to that zone. For example, FIG. 6 shows a 
dashed zone dividing line 660 roughly dividing a zone 1 from a zone 2. 
Zone 1 includes base transmitter 614, base receivers 632 and 634, regional 
station 650, and mobile unit 624. Zone 2 includes base transmitter 612, 
base receivers 628 and 630, and regional station 644. Dashed line 660 only 
roughly defines the boundary between zones because precise boundaries do 
not exist. For example, to insure adequate coverage of the region, as 
shown in FIG. 1, the range of both transmitter 614 should at least cover 
the region above dashed line 660, and preferably should extend somewhat 
below dashed line 660. Similarly, the range of base transmitter 612 should 
at least cover the region below dashed line 660, and preferably should 
extend somewhat above dashed line 660. As can be seen, an overlap of 
transmitter coverage may occur in the vicinity of dashed line 660. 
Referring back to FIG. 2, it can be seen that boundary 202 and boundary 204 
overlap in an area near the equi-signal 200 and between these boundaries 
which may be termed an "overlap area." In FIG. 6, dashed line 660 is drawn 
near the may be defined as the equi-signal boundary between base 
transmitter 614 and base transmitter 612. Of course, dashed line 660 does 
not represent the overlap area that may occur between base transmitter 614 
and base transmitter 612. 
As explained in the Background of the Invention section, if base 
transmitters 612 and 614 are broadcasting identical signals on the same 
frequencies in simulcast, good reception by a receiver located near the 
dashed line 660, and possibly in an overlap area (not shown), can be 
achieved. Simulcast thus may provide uniform transmitter coverage for the 
region shown in FIG. 6. However, if base transmitter 612 is broadcasting a 
first information signal and base transmitter 614 is broadcasting a 
different, second information signal on identical frequencies 
simultaneously, it will likely be difficult for a receiver located in the 
overlap area to receive either the first or the second information signal. 
In this instance, the overlap area may be referred to as an interference 
area because a receiver in this area would receive a composite signal, 
including the first and second information signal, that would likely be 
unusable. 
The following will be an exemplary discussion of the various interactions 
of the elements of the communication system when delivering a message to 
mobile unit 624. In accordance with the invention, a preferred method 700 
of this interaction is shown in FIG. 7. Network operations center 600 
generates a system information signal of several blocks of information as 
shown in step 702. The blocks of information include an electronic message 
to be delivered to the mobile unit 624. 
In step 704, the system information signal is transmitted to the base 
transmitters. In particular the network operations center 600 provide the 
system information signal and appropriate other data to the satellite 
uplink 602 via data path 604 for transmission to the satellite 606. The 
data is then received and retransmitted by satellite 606 to satellite 
downlink stations 608 and 610. The data received by satellite downlink 608 
is provided to base transmitter 612 through data path 616, and the data 
received by satellite downlink 610 is provided to base transmitter 614 
through data path 618. 
At this point, the exemplary communication system shown in FIG. 6 may 
transfer the message to the mobile unit during one of two time intervals. 
In the first time interval, both base transmitter 612 and base transmitter 
614 transmit data via antenna 620 and antenna 622, respectively, in 
simulcast to be received by mobile unit 624, which corresponds to step 706 
in FIG. 7. This first alternative may be useful to deliver the message if, 
for example, the location of mobile unit 624 in zone 1 or zone 2 is 
unknown and broad coverage is desired. 
In the second time interval, base transmitter 614 transmits a block of 
information including the message data to mobile unit 624 and base 
transmitter 612 transmits another block of information, which corresponds 
to steps 708 and 710 of FIG. 7. This second alternative may be useful if, 
for example, the mobile unit 624 is known to be located in zone 1 and out 
of range of base transmitter 612. Delivery of the message to mobile unit 
624 during the second time interval is advantageous because during message 
delivery to the mobile unit 624 by base transmitter 614, base transmitter 
612 could be delivering a different message to a different mobile unit 
(not shown). As can be seen, this second alternative would increase 
information throughput and system efficiency. 
If the mobile unit 624 has properly received the message via antenna 626, 
then the mobile unit 624 may generate a return signal and broadcast that 
signal via antenna 626. The return signal may be received by any or 
several of the base receivers 628, 630, 632, or 634. For example, the 
return signal could be received by base receiver 632 through antenna 640 
if antenna 640 is located closer to the mobile units than any other 
antenna 636, 638, or 642. In this case, the base receiver would receive 
the return signal and provide it to regional station 650 through data path 
652. The regional station would then provide the return signal to the 
network operations center 600 through data path 656 for further processing 
as appropriate. It should be understood that a return signal may include 
either an autonomous acknowledgment signal which indicates that the mobile 
unit accurately received the message or a user generated reply signal. 
If the mobile unit 624 does not completely receive the message, it can 
generate and broadcast a negative acknowledge signal. The negative 
acknowledge signals when delivered to the network operations center 600, 
indicates that retransmission of the message is necessary. 
It should be understood that the exemplary system shown in FIG. 6 includes 
a modest number of elements for ease of explanation. It is envisioned that 
the system of the present invention include a large number of base 
transmitters, base receivers, regional stations, and mobile units with a 
substantial number of base transmitters assigned to each zone and all base 
transmitters assigned to a particular zone operating in simulcast. 
Further, it is envisioned that the present system could advantageously 
support a large number of zones to cover a wide geographic area. 
B. Overview of the Zonal Simulcast Concepts 
The preferred systems and methods of the present invention variously use 
simulcast techniques within individual zones and over several or all of 
the zones. As previously noted, zones are generally defined by the 
coverage areas of the one or more base transmitters. The network 
operations center 600 assigns each base transmitter in the system to a 
zone. For example, in FIG. 6, base transmitter 614 is assigned to zone 1, 
and the base transmitter 612 is assigned to zone 2 by the network 
operations center 600. To maximize information throughput, the systems and 
methods of the present invention dynamically control zonal assignments and 
the use of simulcast techniques. 
In general, the communication system of the present invention operates by 
repeating a communication cycle to achieve desired information transfer, 
which is more fully discussed infra. The communication cycle is divided 
into a systemwide time interval and a zonal time interval. In the 
systemwide time interval, the base transmitters from at least several 
zones are operated in simulcast to simultaneously transmit identical 
information to a large geographic area. It should be understood that the 
systemwide time merely two or more zones. 
Broadly speaking, the communication system need not know the location of a 
mobile unit to transmit to it during the systemwide time interval. 
Therefore, the systemwide time interval can be used to send a "probe" 
signal that requests a particular mobile unit to broadcast an 
acknowledgment signal to allow the system to determine its approximate 
location by determining which base receiver receives the acknowledgment 
signal. Probe signals, thereby, may be used to track the locations of 
mobile units, or to uncover the location of "lost" mobile units. 
In the zonal time interval, each base transmitter assigned to a particular 
zone transmits identical information in simulcast. However, for mobile 
units at or near the interference areas between adjacent zones, poor 
communication to those mobile units is likely during the zonal time 
interval because transmitters in adjacent zones will be simultaneously 
transmitting different data on the same, or substantially the same, 
frequencies. The zonal time interval provides good communication 
capability for mobile units not located near the zonal boundaries and 
allows the system to "reuse" identical frequencies in adjacent zones. 
Furthermore, if zonal boundaries are selected to be located in areas where 
mobile units are not likely to be located, i.e. unpopulated areas, the 
likelihood of providing good communication capabilities to a large 
percentage of mobile units can be increased. 
As can be seen, from a system perspective, it is desirable to communicate 
with the mobile units in the zonal time interval because information 
throughput is maximized by reusing the transmission frequency band in the 
several zones. In other words, using the zonal time interval allows 
communication with a large number of mobile units in a short amount of 
time. Accordingly, communication during the systemwide time interval 
should be minimized because message transmission during this interval 
requires a large amount of system resources be dedicated to that message. 
For mobile units located near the boundaries between zones where 
interference is likely during the zonal time interval, good communication 
capability can be achieved for these units during the systemwide time 
interval. In the preferred systems and methods, when a mobile unit fails 
to acknowledge a message sent during the zonal time interval or provides a 
negative acknowledgment, the network operations center sends a probe 
signal during a subsequent systemwide time interval to determine the 
location of that mobile unit. If the location of the mobile unit indicates 
that a likely reason for the failure of the mobile unit to receive the 
message is caused by inter-zonal interference, the network operations 
center may simply retransmit the message during the systemwide time 
interval. In other instances, the failure to successfully deliver a 
message may be simply caused by the mobile unit being located in a weak 
signal area within a zone. In these instances, the system may retransmit 
the message during the zonal time interval using an appropriate error 
correcting code or using a stronger error correcting code. 
Alternatively, the network operations center may determine from the probe 
signal that the mobile unit is simply located in a different zone than the 
zone that the message was first sent. In this case, the network operations 
center preferably causes the message to be retransmitted in the 
appropriate zone without again using a portion of the valuable systemwide 
time interval. 
In accordance with the invention, a preferred method 800 for sending a 
probe signal is shown in FIG. 8. In step 802, a message signal is 
transmitted by a base transmitter servicing a zone where the mobile 
transceiver was last known to be located. In particular, this may be 
preferably an attempt by the network to deliver a message to the mobile 
transceiver. 
If the mobile transceiver does not indicate receipt of the message signal 
from the base transmitter transmitted in step 802, the network assumes 
that the mobile transceiver has not received the message and transmits a 
probe signal by a plurality of base transmitters servicing a plurality of 
zones in step 804. The mobile transceiver receives the probe signal in 
step 806. 
Upon receipt of the probe signal by the mobile transceiver, the mobile 
transceiver transmits an acknowledgment signal in step 808. A base 
receiver receives the acknowledgment signal from the mobile transceiver in 
step 810. 
Finally, the data, such as the last location field 2104 shown in user 
database 2100, is updated to reflect the zone of the base receiver, or 
receivers, that receives the acknowledgment signal as the last known 
location of the mobile transceiver in step 812. 
C. The Multi-Carrier Modulation Transmission Format 
The base transmitters of the communication system, such as base 
transmitters 612 and 614 shown in FIG. 6, preferably utilize a 
multi-carrier modulation format as will now be described. In general, a 
multi-carrier modulation format envisions the simultaneous transmission of 
several closely spaced carrier frequencies within a desired frequency 
band, each individually modulated to convey an information signal. The 
multi-carrier modulation format advantageously allows for high data 
transfer rates by providing good bit rate transmission rates while keeping 
below the baud rate limitations of simulcast transmission techniques. 
FIG. 9 shows a frequency representation 900 of an eight carrier modulation 
format. Carrier frequency 902 is shown with side bands 904, carrier 
frequency 906 is shown with side bands 908, carrier frequency 910 is shown 
with side bands 912, carrier frequency 914 is shown with side bands 916, 
carrier frequency 918 is shown with side bands 920, carrier frequency 922 
is shown with side bands 924, carrier frequency 926 is shown with side 
bands 928, and carrier frequency 930 is shown with side bands 932. 
It should be understood that although this exemplary figure shows an eight 
carrier signal modulation format, other different numbers of carrier 
frequencies may be considered for use in the systems and methods of the 
present invention. 
In this exemplary embodiment, the carrier frequencies are spaced 3 KHz 
apart within a desired frequency band of 50 KHz. Dashed line skirts 934 
and 936 represent minimum frequency roll off levels, such as may be 
required by Federal Communication Commission regulations, to prevent 
overlap interference into adjacent frequency bands. 
Because eight unique data streams may be modulated onto the respective 
eight carrier signals in this embodiment, the data transfer rate of the 
transmission from the base transmitters can be greatly increased, while 
keeping the baud rate within acceptable ranges for simulcast transmission. 
It should also be understood that in accordance with good simulcast 
practice, the respective carrier frequencies between adjacent base 
transmitters, such as base transmitter 612 and base transmitter 614 in 
FIG. 6, should be slightly offset to prevent sustained nodes or "dead 
spots" where destructive interference between the signals from each 
transmitter provides an unusable composite signal, as was explained in the 
background section of this application. This frequency offset is 
preferably on the order of 10-20 hertz. 
As previously discussed, each carrier signal may be individually modulated 
to convey a data stream. The following will discuss alternative techniques 
for modulating a plurality of carriers in accordance with the systems and 
methods of the present invention. 
1. Modulated On/Off Keying 
Perhaps the simplest modulation scheme conceptually is modulated on/off 
keying (MOOK). FIG. 10 shows a schematic representation of a MOOK 
modulator 1000. The MOOK modulator 1000 includes a plurality of carrier 
frequency generating devices, such as frequency generator 1002 generating 
frequency F1, frequency generator 1004 generating frequency F2, frequency 
generator 1006 generating frequency F3, frequency generator 1008 
generating frequency F4, and frequency generator 1010 generating frequency 
Fn. As shown in FIG. 10, the MOOK modulator 1000 may include any number 
(i.e. n) of frequency generators, but eight carrier frequencies are 
preferred, as shown in FIG. 9. 
The output from each of the carrier frequency generators 102, 104, 106, 
108, and 110 is applied to a plurality of respective switches SW1 812, SW2 
814, SW3 816, SW4 818, and SWn 820. The output from each switch is 
provided to a combiner 1022. 
Each of the switches SW1 812, SW2 814, SW3 816, SW4 818, and SWn 820 opens 
and closes under the control of a control logic system (not shown) to 
effect the MOOK modulation. The control logic system (not shown) causes 
the desired switches to variously close and open, thereby conveying an 
n-bit binary word. Each carrier frequency transmits a binary "one" if the 
respective switch is closed and a binary "zero" if the respective switch 
is open. 
The summer 1022 combines the modulated carrier frequencies to provide a 
multi-carrier modulated output signal that conveys an n-bit binary word. 
2. Binary Frequency Shift Keying Modulation 
An alternative multi-carrier modulation scheme including frequency shift 
keying (FSK) techniques may be implemented by the modulator shown in FIG. 
11. A frequency shift keying modulator 1100 includes a first frequency 
source 1102, a second frequency source 1104, a third frequency source 
1106, a fourth frequency source 1108, and an nth frequency source 1110. 
The output from each frequency source is provided to a respective 
modulator 1112, 1114, 1116, 1118, and 1120. 
A control logic system (not shown) provides a frequency control signal to 
each modulator to frequency shift modulate the carrier frequencies. In 
particular, the control logic system (not shown) provides frequency 
control signal 1 to modulator 1112, frequency control signal 2 to 
modulator 1114, frequency control signal 3 to modulator 1116, frequency 
signal 4 to modulator 1118, and frequency control signal n to modulator 
1120. In binary frequency shift keying (BFSK), the respective frequency 
control signals provide data corresponding to a binary "one" or "zero" 
which causes the respective modulators to modulate a first or second 
frequency onto the carrier signal. 
A summer 1122 combines the modulated carrier frequencies to produce an 
output signal. 
3. M'ary Frequency Shift Keying Modulation 
A modulation scheme related to binary frequency shift keying is M'ary 
frequency shift keying. M'ary frequency shift keying modulates three or 
more different frequencies onto the respective carrier signals. In 
quaternary frequency shift keying, for example, two bits of information 
may be instantaneously conveyed on a single carrier frequency. Similarly, 
8'ary frequency shift keying may instantaneously convey three bits of 
information per carrier frequency. 
Referring again to FIG. 11, M'ary frequency shift keying may be implemented 
by providing modulators 1112, 1114, 1116, 1118, and 1120 with the 
capability to modulate M different frequencies onto the carrier signal. 
Accordingly, the various frequency control signals must provide data 
indicating which of the M frequencies is to be modulated onto the carrier 
signal. For example, in quaternary frequency shift keying, the frequency 
control signals must each include two bits of information to indicate 
which of the four different frequencies are to be modulated onto the 
carrier frequency. 
The summer 1122 combines the modulated carrier frequencies to produce an 
output signal. 
4. Quadrature Amplitude Multi-Carrier Modulation 
Yet another alternative modulation technique for a multi-carrier 
transmission format is shown in FIG. 12. A quadrature modulator 1200 
includes a first quadrature carrier generator 1202, a second quadrature 
carrier generator 1204, a third quadrature carrier generator 1206, and a 
fourth quadrature carrier generator 1208. As is well known, quadrature 
modulators in general each produce an in-phase carrier signal and a 
quadrature carrier signal that is +/-90.degree. out of phase with 
reference to the in-phase signal. Of course, any number of quadrature 
carrier generators could be envisioned, depending upon data transfer and 
throughput needs. FIG. 12 shows four quadrature carrier generations which 
effectively correspond to eight unique modulator signals. Therefore, 
quadrature amplitude multi-carrier modulation may preferably reduce the 
width of the frequency band necessary to achieve a desired data transfer 
rate. 
Each quadrature carrier generator 1202, 1204, 1206, and 1208 receives a 
control signal from a control logic system (not shown) which provides the 
data to be modulated onto the quadrature carrier signals. In a simple 
implementation, the quadrature carrier generators may amplitude modulate 
the in-phase and quadrature phase output signals to convey two bits of 
information. The in-phase and quadrature signals output from each 
quadrature carrier generators 1202, 1204, 1206, and 1208 are provided to a 
summer 1210 which combines the signals to produce an output signal. 
5. Permutation Frequency Shift Keying (PFSK) 
PFSK may be implemented through control logic systems similar to that used 
in a MOOK or an M'ary FSK modulation scheme. In PFSK, every baud has a 
fixed number of carrier signals present, preferably any 4 of the possible 
8. In a PFSK arrangement, a constant average transmitter power is 
advantageously delivered and the receiver only need decide which 4 carrier 
frequencies contain the most energy. In the case of MOOK, the receiver 
must attempt to determine on a subchannel-by-subchannel basis the presence 
or absence of a signal. This aspect of PFSK may simplify mobile receiver 
design. 
Compared to a binary or M'ary FSK modulation schemes, a higher number of 
bits may be delivered per baud with PFSK. For example, PFSK may generate 
signals that independent FSK subchannels could never generate, such as all 
four carriers being the four highest frequencies, and therefore it can be 
seen that PFSK may advantageously increase information transfer rates. 
D. The Base Transmitter 
Each base transmitter unit, such as base transmitter 612 or 614 shown in 
FIG. 6, receives transmitter control data and message data transmitted 
from the satellite 606. FIG. 13 shows a first preferred embodiment of a 
base transmitter 1300 in accordance with the present invention. The base 
transmitter 1300 receives data from the satellite downlink connected to 
data input 1302 which provides this data to a control logic system 1304 to 
control the operation of the base transmitter unit. The control logic 1304 
provides a control signal to a plurality of modulators 1306, 1308, 1310, 
1312, and 1314. Modulator 1306 produces a carrier signal F1, modulator 
1308 produces a carrier signal F2, modulator 1310 produces a carrier 
signal F3, modulator 1312 produces a carrier signal F4, and modulator 1314 
produces a carrier signal Fn. 
For example, the control logic may generate appropriate control signals to 
modulate the carrier signals in a MOOK, BFSK, M'ary FSK, PFSK, or 
quadrature amplitude modulation scheme, as previously discussed. Each 
modulator then provides the modulated output signal to a combiner 1316 
which combines each of the several modulated carrier frequencies into a 
single output signal. 
The single signal is then applied to a power amplifier 1318 to amplify this 
signal to an appropriate level. The power amplifier 1318 may, for example, 
produce a nominal output signal of 350 watts to antenna 1320. In this 
embodiment, power amplifier 1318 preferably has extremely linear 
characteristics to prevent formation of intermodulation products, and to 
insure that these intermodulation products do not cause signals to be 
generated at undesirable frequencies. Antenna 1320 broadcasts the desired 
signal from power amplifier 1318. 
FIG. 14 shows a second preferred embodiment of a base transmitter unit. The 
second embodiment comprises a base transmitter 1400 which includes a 
satellite downlink connected to data input 1402, control logic 1404, and 
several modulators 1406, 1408, 1410, 1412, and 1414. Each modulator 
receives an appropriate control signal from the control logic 1404, as 
previously discussed with respect to base transmitter 1300. 
The output from each of modulators 1406, 1408, 1410, 1412, and 1414 in base 
transmitter 1400 is provided to respective power amplifiers 1416, 1418, 
1420, 1422, and 1424 to provide an appropriate power output level for 
transmission, such as 350 watts aggregate. 
The output from each of power amplifiers 1416, 1418, 1420, 1422, and 1424 
is provided to combiner 1426 to combine the modulated carrier signals into 
a single output signal which is provided to antenna 1428 for broadcast. 
E. The Mobile Unit 
The mobile unit may be a small, portable mobile transceiver, such as 
pictorially represented in FIG. 16. Referring now to FIG. 15, the mobile 
transceiver 1500 shown therein includes a receiver section for receiving 
signals from the base transmitters of the system, and a transmitter 
section for transmitting replies, or other messages, to the base receivers 
of the system. 
In particular, the mobile transceiver 1500 includes an antenna 1502 which 
is connected to a transmit/receive switch 1504 to switch the antenna 
between the transmit and receive sections of the mobile transceiver 1500. 
A receiver 1506 is provided to receive the messages from the base 
transmitter. Of course, the receiver must be appropriately designed to 
receive the multi-carrier signals from the base transmitters and must be 
appropriately designed to demodulate the particular modulation scheme 
utilized. For example, appropriate analog filters and appropriate 
demodulators could be used. In the preferred embodiment, the receiver 
performs a transform, such as a fast fourier transform, on the received 
signal to separate the data from the various carriers in the multi-carrier 
modulation format. 
The receiver 1506 is connected to a display and storage logic section 1508 
to process the received signal. An annunciator 1510 to alert the user that 
a message has been received is connected to and controlled by the display 
and storage logic 1508. The annunciator 1510 may commonly include a sound 
producing device such as a beeper, or a vibrator, or a flashing light. 
A set of display controls 1512 to control the display of the mobile 
transceiver 1500 is connected to the display and storage logic 1508. A 
display 1514, preferably an LCD display, is also connected to the display 
and storage logic 1508 to display messages and various other information 
to the user. 
Display and storage logic 1508 is connected to transmit logic 1518 via 
connection 1526. Display and storage logic 1508 may generate an autonomous 
acknowledge signal which causes the transmitter 1520 to broadcast an 
appropriately modulated RF signal. As previously discussed, it is 
desirable for the mobile transceiver to transmit an acknowledge signal if 
the message was properly received by the mobile unit, or alternatively to 
transmit a negative acknowledge signal if the message was only partially 
received. The negative acknowledge signal indicates that the network 
operations center should rebroadcast the message to the mobile unit. 
Preferably, the rebroadcast of the message to the mobile unit should occur 
with an appropriate error correcting code which may be decoded by the 
mobile unit to insure complete and accurate reception of the message. Of 
course, error correcting codes should be used only when necessary because 
their use slows data transfer and increases the complexity of the mobile 
unit. Other types of autonomous replies may also be useful, for example, 
to indicate to the network operations center that the user has not viewed 
the message even though the mobile unit properly received it, such as when 
the mobile transceiver is unattended by the user. 
A set of input switches 1516 is provided to allow the user to input a reply 
to a received message, or to otherwise generate a message to be 
transmitted by the mobile transceiver. The input switches are connected to 
transmit logic 1518 which decodes the signal from the input switches 1516 
to generate an output signal to the transmitter 1520. The transmitter 1520 
generates an appropriately modulated RF signal to be broadcast by antenna 
1502. 
The mobile transceiver 1500 also preferably includes a noise detector 1522. 
The noise detector 1522 provides an output signal upon sensing through 
antenna 1502 a threshold level signal. The noise detector 1522 provides an 
output signal to disable the transmitter 1520 via connection 1524, and to 
thereby prevent unwanted transmission by the mobile unit. 
Noise detector 1522 preferably is set to detect electromagnetic signals 
which are generated externally to the communication system and which are 
indicative of a condition when transmissions by the mobile unit are 
undesirable. For example, the noise detector 1522 could be designed to 
serve a threshold level of noise at 400 Hz. When the user enters a 
commercial aircraft, which commonly uses 400 hertz power supply, the 
receipt of this noise by the noise detector 1522 would then disable the 
transmit capability of the mobile transceiver 1500 during operation of the 
aircraft to prevent any unnecessary or unwanted interference with the 
operations of the aircraft by autonomous or intentional transmissions by 
the mobile transceiver 1500. 
The display and storage logic 1508 of the mobile transceiver 1500 further 
preferably includes a timing circuit (not shown) which may be used to turn 
the receiver section 1506 on or off, as desired. The timing circuit (not 
shown) advantageously allows the mobile transceiver to "power down" during 
periods of time when messages are not anticipated to be transmitted. For 
example, in a preferred communication protocol, the receiver could simply 
power up at the beginning of each cycle to receive data to determine if a 
message will be transmitted to that mobile transceiver during that cycle 
or when information concerning message availability will be transmitted. 
If the mobile transceiver is to receive a message, the timing circuit 
could power up at the appropriate time to receive the message, and then 
power down after receipt. The timing circuit, therefore, advantageously 
prolongs the battery life of the mobile transceiver 1500. Of course, it 
should be understood that the timing circuit could control the other 
elements of the mobile transceiver, such as the display 1514, and the 
transmit logic 1518. 
In an alternate implementation, the receiver 1506 may adaptively change its 
demodulation techniques to accommodate various formats. For example, each 
zone may advantageously use a different modulation format depending on 
message traffic levels, and other considerations. In particular, the 
receiver may receive a signal indicating the modulation scheme utilized in 
a given zone via a modulation format message contained in an overhead 
portion of the data stream. The demodulation of FSK, M'ary FSK, PFSK, and 
MOOK formats all begin with the determination of the energy levels 
detected at each of the carrier frequencies, and thus require identical 
processing of the received RF energy. The logic (not shown) in the 
receiver interprets the meaning of these measured energy levels based upon 
the modulation scheme selected as indicated by the received modulation 
format message. In this manner simpler and more economical transmitters, 
with a decreased capacity for information transfer, can be used in zones 
that have decreased traffic loads and more expensive, high-throughput 
transmitters can be used only in those areas where they are needed. 
A pictorial representation of the mobile transceiver is shown in FIG. 16. 
The mobile transceiver 1600 shown therein includes a case 1602, a pair of 
display control buttons 1604, a display 1606, and a set of six reply 
buttons 1608, 1610, 1612, 1614, 1616, and 1618. As indicated previously, 
display 1606 is preferably an LCD display and a set of display control 
buttons 1604 may be used to scroll text up or down on the display 1606. 
The message "will you be home for dinner?" is shown on display 1606. 
The set of six reply buttons 1608, 1610, 1612, 1614, 1616, and 1618 provide 
a flexible system for user generated replies to received messages. The 
display and storage logic 1508 provides information immediately above each 
button indicating a possible reply message by the user. In the simple 
example shown in FIG. 16, the user may reply "yes," "no," or "?" to the 
message 1620 displayed on the screen 1606. The transmit logic 1518 
generates an appropriate signal based upon which button the user presses. 
In this simple scenario, buttons 1614, 1616, and 1618 are unused. 
In alternate applications, up to six possible reply messages may be shown 
on the screen 1606. Of course, other particularized applications may be 
envisioned for the reply feature of the mobile transceiver 1500. For 
example, if the user is a stockbroker, the display 1606 could display the 
terms "buy," "sell," or "hold" above the appropriate buttons. A variety of 
other applications may be envisioned. 
With the six button reply option provided by mobile transceiver 1500, a 
three bit message may be transmitted by the mobile transceiver to the base 
receivers. The two remaining states of the three bit message may be used 
by the transmit logic 1518 for the autonomous acknowledgment signal which 
indicates that the message has been properly received, and for the 
autonomous negative acknowledgment signal which indicates that the message 
has not been completely or properly received. 
Of course, the mobile transceiver 1500 shown in FIG. 16 could be configured 
differently to provide more or less reply buttons, different display 
control buttons, and different display formats as desired or needed by the 
user. 
Further, the mobile transceiver 1500 could additionally include a data 
output port (not shown) for connection to other electronic devices of the 
user. For example, the mobile transceiver could be connected through an 
output port to a laptop or palmtop PC, or could be incorporated therein. 
The PC could display the message on its screen, thereby obviating the need 
for the display 1606, and the keyboard could be used to generate any 
appropriate reply messages from the user, thereby obviating need for the 
reply buttons and allowing free form messages to be sent by the mobile 
transceiver. A user selected reply would be transferred to the mobile 
transceiver 1500 from the PC for transmission to the base receiver. 
Alternatively, the mobile transceiver could be connected to a voice data 
replay device, such as a speaker, thereby allowing the user to receive 
messages from a voice mailbox, for example. Of course, a voice data 
generation device, such as a microphone, could be connected to the mobile 
transceiver 1500 to allow the user to reply to the voice mail message he 
has received or to initiate voice data communication from the mobile 
transceiver to the base receivers. Similarly, facsimile transmissions 
could be supported. 
An alternate embodiment of the mobile unit includes only receive 
capabilities, but does not include any transmit capabilities. FIG. 17 
shows a mobile receiver 1700. The various components of the mobile 
receiver generally correspond in functionality to the similar elements 
shown in FIG. 15. Of course, the mobile receiver 1700 cannot generate 
replies, which includes user initiated replies, an autonomous 
acknowledgment signals or negative acknowledgment signals, because of the 
lack of transmit capability. Also, the location of this alternate 
embodiment cannot be tracked by the network control center because of the 
lack of transmit capability. Generally, because of these reasons, the 
mobile receiver 1700 embodiment of the mobile unit is less preferable than 
the mobile transceiver embodiment 1500. Further, it should be appreciated 
that the mobile transceiver embodiment may include circuitry for 
generating various autonomous responses without interaction by the user. 
F. The Base Receiver 
The base receivers of the present system receive the low power output 
signal from the mobile transceiver unit. As is shown in FIG. 6, mobile 
receivers are dispersed throughout the geographic service area. Base 
receivers need not be associated with zonal boundaries per se, but will 
always be located to service at least one zone, of course. A few base 
receivers may exist in the overlap region between zones. 
During transmission of the return signal by the mobile transceiver unit, it 
is possible that several base receivers could receive this return signal. 
In this instance, the network operations center 600 preferably selects the 
data from the base receiver with the highest received signal strength 
(i.e. the signal with the lowest probability of errors) to maximize the 
likelihood of receiving accurate data. The signal strength approach is 
preferred and can be satisfactorily implemented if the base receiver 
locations are carefully selected to insure adequate signal strength 
reception from the mobile transceiver units and to minimize the overlap 
between base receiver coverage areas. Alternately, the network operations 
center 600 could use "voting" techniques by comparing each data set from 
the several base receivers to arrive at the most likely return signal data 
using conventional voting receiver technology. 
FIG. 18(A) shows a first embodiment of an analog base receiver. Analog 
receiver 1802 is connected to an antenna 1800. The analog receiver 1802 
simply receives the signal from the antenna 1800 and removes the modulated 
waveform from the carrier frequency and outputs this waveform in analog 
format to a regional demodulator 1804 via data path 1806. Data path 1806 
is preferably a 4 KHz analog telephone channel. 
The regional demodulator 1804 receives signals from several analog 
receivers included in several base receivers. Preferably, the regional 
demodulator 1804 is located in the regional station, such as regional 
station 650 shown in FIG. 6. The demodulated signal from the regional 
demodulator 1804 is then transferred to the regional processing circuitry 
1808, and then onto the network operations center 600. 
The analog receiver 1802 could generate identification data to be 
transmitted with each received message so the network operations center 
600 can determine the source of each message received. Alternatively, and 
preferably, dedicated communication paths are used for each base receiver 
and therefore, the source of the message can be inferred from the 
communication path that is activated. 
FIG. 18(B) shows a digital base receiver embodiment which includes an 
antenna 1800 attached to an analog receiver 1802. As in the previously 
discussed embodiment, the analog receiver 1802 removes the modulated 
waveform from the carrier signal transmitted by the mobile transceiver 
unit. The analog receiver 1802 outputs the modulated waveform to a 
demodulator 1810 included in the base receiver. The demodulator 1810 
produces a digital output signal corresponding to the data stream 
transmitted by the mobile transceiver unit. The demodulator 1810 provides 
the digital output signal to the regional processing circuitry 1808 in the 
regional station via data path 1812. Data path 1812 may be any 
conventional data path which can satisfactorily convey the digital data 
from the demodulator 1810 to the regional processing center 1808. The 
regional processing circuitry 1808 then passes the data to the network 
operations center 600. 
FIG. 19 shows a digital base receiver including error correction and store 
and forward features. An antenna 1900 is connected to an analog receiver 
1802 which is connected to a demodulator 1810, as previously described 
with reference to FIG. 18(B). The demodulated digital signal is output 
from demodulator 1810 to error correction circuitry 1906 which may perform 
error correction algorithms to insure the integrity of the return signal 
received from the mobile transceiver unit. Of course, the error correction 
circuitry should decode and correct data which have been compatibly 
encoded by the mobile transceiver. 
The error corrected data output from the error correction circuitry 1906 is 
provided to a store and forward circuit 1908. The store and forward 
circuit 1908 stores the received data to allow it to be transmitted later 
at a convenient time and at a convenient data transmission rate. 
For example, in the present system it is likely that the return signal 
traffic received by the base receiver will occur in short bursts at a 
relatively high data transfer rate. However, it is also likely that the 
average data transfer rate from the base receivers is substantially lower 
than the instantaneous data transfer rate during traffic bursts. The store 
and forward circuit 1908 may preferably act as a buffer to allow the 
return signal data to be communicated from the store and forward circuit 
1908 to the regional processing circuitry 1808 at a lower (and less 
expensive) data transfer rate. Store and forward circuit 1908 is, 
therefore, preferably connected to regional processing circuitry 1808 via 
data path 1910 which may include a low cost telephone line. 
G. The Network Operations Center 
1. Overview 
The network operations center 600 is shown in schematic form in FIG. 20. 
The network operations center 600 includes a base receiver input system 
2000 which receives data from the various regional stations throughout the 
system (e.g., regional stations 644 and 650) via various data paths, such 
as data paths 656 and 658 as shown in FIG. 6. The data received by the 
base receiver input system 2000 includes reply data from users with 
various control data. Base receiver input system 2000 may include 
appropriate conventional signal processing equipment. Control data may 
include data identifying the base receiver (i.e. location of the mobile 
unit) which received the associated reply. Preferably, the base receiver 
input section 2000 receives data from the regional stations via phone 
lines. However, other appropriate data paths may be considered 
The base receiver input system 2000 then provides the received data to a 
central computer 2002. The central computer 2002 may also receive input 
from a user input system 2004. For example, the user input system 2004 may 
receive data from users via phone lines who may access and interact with 
the central computer via voice, DTMF, or modem transmission and may 
include appropriate conventional signal processing equipment. A user may 
interact with the central computer 2002 to modify his service, to initiate 
or receive messages, or to perform other desirable functions. 
Generally, the central computer 2002 processes the data received from the 
base receiver input system 2000 and from the user input system 2004 to 
perform various operations on the data, to update various database entries 
for use by the central computer 2002, and to generate data for 
transmission to a satellite uplink output system 2006. 
It should be understood that, although FIG. 20 shows the central computer 
as existing at a single location in the network operations center 600, a 
distributed computing system may be used to perform the necessary 
functionality of the central computer 2002. Presently, however, a single 
location for the central computer 2002 is preferred. 
Satellite uplink output system 2006 receives data from the central computer 
2002 and provides it to satellite 606, shown in FIG. 6, for transmission 
to base transmitters within the system (e.g., base transmitters 612 and 
614 in FIG. 6). 
The central computer 2002 is also connected to a database system 2008 which 
stores various data such as message data, user status data, system status 
data, and message status data, for example, for use by the central 
computer 2002 in processing. 
Also, a control access 2010 is provided to allow systems engineers or 
programmers to access the central computer 2002 to observe and modify its 
operations and system performance. 
2. Database Structure 
The database 2008 of the network operations center includes several 
database structures necessary for the operation of the system. While a 
preferred partitioning of these databases is described below, it should be 
understood that other partitionings could be considered, such as moving 
the various "user traffic" fields from the traffic statistics database to 
the user database. 
a. The User Database 
For example, the user database structure shown in FIG. 21 includes a record 
for each user of the system who possesses a mobile unit. The record for 
user 1 2100 includes various fields, such as an ID number field 2102 which 
indicates a unique number associated with that particular user. The 
transmit capability field 2106 indicates whether the mobile unit assigned 
to the user has the capability to transmit. The last location field 2104 
includes data which indicates the last known location of the user. The 
last location field may be updated when the central computer recognizes 
that a new base receiver has received a return signal from the mobile 
unit, thereby indicating the mobile unit has moved since the last return 
signal. Of course, if the mobile unit only includes a mobile receiver 
without transmit capability, the last location field 2104 cannot be 
updated and the mobile unit may be given a default location. 
The service area field 2108 includes data corresponding to the area in 
which the user has subscribed to. For example, if a user desires service 
in geographic areas less than the total system service area, the central 
computer could use the data in the service area field 2108 to cause only 
selected base transmitters to attempt to transmit messages to a mobile 
unit. 
The button format field 2110 includes data indicating the format of reply 
buttons the user may access on the mobile transceiver. Of course, for 
mobile units with only receive capabilities, the button format field will 
not be used. 
The message field 2112 includes data representing one or more messages 
which are intended for the user. A receive flag is set when the central 
computer has received data indicating that the message has been received 
by the mobile unit via an acknowledgment signal. If the mobile unit does 
not have transmit capability, the receive flag is set upon transmission of 
the message by the appropriate base transmitters. The user database 
structure may include other fields for each user of the communication 
system of the present invention as needed to provide various desired 
services. 
b. The Receiver Database 
Database 2008 of FIG. 20 includes a receiver database (not shown) which 
includes an entry with several associated fields for each base receiver in 
the system. A first field for each base receiver preferably includes the 
total number of mobile units which have last communicated with this 
receiver. A second field for each base receiver preferably includes a list 
of base transmitters which may cover all or a portion of the receiver 
coverage area of that base receiver. 
c. Traffic Statistics Database 
Database 2008 of FIG. 20 should also include preferably a traffic 
statistics database as shown in FIG. 22 which includes various fields 
containing statistics calculated by the central computer 2002 concerning 
traffic patterns for the system. For example, the traffic database 2200 
preferably includes a user field 2202 for data indicating a user of the 
network. Several fields are preferably associated with the user field 
2202. Field 2204 includes data representing the number of probe signals 
sent by the network to locate the mobile unit associated with the user 
field 2202. Field 2206 includes data representing the number of 
registration signals received by the network from the mobile unit 
associated with the user field 2202. Field 2208 includes data representing 
the number of messages from the network that have been successfully 
delivered to the mobile unit associated with the user field 2202. Field 
2210 may be used for other traffic related data, such as data indicating 
the average traffic per cycle, and data indicating a time average (i.e. 
for the last hour) traffic amount. 
Further, the traffic database 2200 could include fields (not shown) for 
data concerning overall system performance and, in particular, each zone 
in the network. Such area specific traffic data may be useful in 
optimizing system performance by allowing intelligent redefinition of 
zonal boundaries. 
d. The Service Queue 
Database 2008 of FIG. 20 also includes a service queue 2300 as shown in 
FIG. 20. The service queue 2300 includes a current messages queue and a 
probe list queue. The current messages queue includes a system wide list 
of messages to be delivered by the system. The current messages queue 
includes, for example, a series of ID number fields 2302, 2304, and 2306 
with associated data location fields 2308, 2310, and 2312, respectively. 
The data location fields 2308, 2310, and 2312 include pointers to the 
appropriate fields in the user database structure shown in FIG. 21. The ID 
number fields 2302, 2304, and 2306 include data indicating the ID number 
of the user to which the message is to be delivered. 
In operation, the central computer retrieves the ID number 2302 and data 
location 2308 from the top of the current messages queue and retrieves the 
appropriate data from the user database 2100 to process and transmit a 
message to the user. 
The probe list queue includes a ID number fields 2314, 2316, and 2318 and 
data location fields 2320, 2322, and 2324 similar in form to those in the 
current messages queue. The probe list queue contains a list of users 
which the system has previously attempted unsuccessfully to deliver a 
message to. In other words, the users listed in the probe list are 
considered to be "lost" by the system. The central computer 2002 then 
initiates a probe routine for the ID number 2314 and data location 2320 
located at the top of the probe list. 
After successful execution of the probe routine, the last location field 
2304 in the user database structure 2100 will have been updated to provide 
an accurate last location of the user from the base receiver that received 
the mobile unit's acknowledgment to the probe signal. After the last 
location field 2304 has been updated, the message can then be replaced in 
the current messages queue for delivery to the user via the appropriate 
base transmitters located near the mobile unit. 
Preferably, the network operations center gives priority to the delivery of 
all messages in the current message queue, and then sends probe signals to 
the users listed in the probe list queue after delivery has been attempted 
for all messages in the current message queue. If the message volume in 
the current message queue remains high for an extended period of time, the 
network operations center preferably begins to periodically send probe 
signals to the users listed in the Probe List, even though undelivered 
messages remain in the current messages queue. For example, in this 
instance of persistent filled current messages queue, the network 
operation center preferably transmits three probe signals in every cycle 
transmitted. 
e. Base Transmitter Assignment List 
The database 2008 of the network operations center also includes a base 
transmitter database 2400 as shown in FIG. 24. The base transmitter 
database 2400 includes a zonal assignment field 2404 for data representing 
a zone assignment associated with a base transmitter field 2402 in the 
system. Also, a field 2406 for data representing the base receivers in the 
transmitter coverage area, and a field 2408 for other data associated with 
a base transmitter, are associated with base transmitter field 2402. As 
can be seen in FIG. 24, each base transmitter in the network has a base 
transmitter field and associated fields as described above. 
In normal operating conditions of the system with low amounts of message 
traffic being transmitted, each base transmitter will remain assigned to 
its particular zone. However, the systems and methods of the present 
invention provide for dynamically changing the zonal assignments of 
various base transmitters to improve information throughput. These dynamic 
zone allocation concepts dynamically reassign base transmitters to new 
zones generally based upon the volume of messages transmitted during the 
systemwide time interval, and more particularly based upon the localized 
volume of messages to mobile units. In general, dynamic zone allocation 
may be used to deliver messages to mobile units in overlap areas (i.e. 
"zonal dithering"), or to balance the volume of message traffic between 
zones. 
FIG. 25 is useful to explain these concepts. Various base transmitters, 
each designated as an "X," are dispersed throughout a region of space 
shown in FIG. 25. Also, various base receivers are dispersed throughout 
this region of space 2500, each being designated by an "R." The normal 
zonal boundary for zone 1 in FIG. 25 is shown by solid line 2502. A normal 
boundary for zone 2 is represented by solid line 2504 during normal load 
traffic operation conditions. As can be seen, base transmitters 2506, 
2508, and 2510 are located near the zonal boundary of zone 2, and base 
transmitters 2512, 2514, and 2516 are located near the boundary of zone 1. 
Base receivers 2518 and 2520 are located in an overlap area 2521 between 
zones 1 and 2. As previously discussed, mobile units located in this 
overlap area 2521 near base receivers 2518 and 2520 must be communicated 
with during the systemwide time interval because of the interference 
created during the zonal time interval by adjacent base transmitters. 
During normal, low to moderate volume system operations, the zonal overlap 
area 2521, i.e., interference area, near base receivers 2518 and 2520 will 
preferably have a small number of mobile units located therein. Therefore, 
communication with these mobile units will not significantly consume 
system resources by occasionally communicating with them during the 
systemwide time interval. 
However, if the traffic volume from the overlap area 2521 near base 
receivers 2518 and 2520 increases, such as because additional mobile units 
enter this overlap area 2521, the handling of this traffic in the 
systemwide time interval can significantly consume system resources. For 
example, communication with a large number of mobile units during the 
systemwide time interval may significantly delay delivery of messages to 
units in this and other regions. 
In this instance, the zonal boundaries are changed to remove this high 
traffic region from a zonal overlap area. For example, system efficiency 
is restored if the zone 1 boundary were moved to dashed line 2522 and the 
zone 2 boundary were moved to dashed line 2524. 
The central computer 2002 may dynamically accomplish this zonal 
redefinition by assigning one or more base transmitters to a new zone to 
reduce systemwide time interval messages. In the present example shown in 
FIG. 25, the central computer updates the base transmitter zonal 
assignment list to reassign base transmitters 2512, 2514, and 2516 to zone 
2 while removing these base transmitters from zone 1. In view of this 
zonal redefinition, the new zone 1 boundary is shown by dashed line 2522, 
and the new zone 2 boundary is shown by dashed line 2524. The high traffic 
region near base receivers 2518 and 2520 is now squarely within zone 2 and 
messages to these units may be efficiently delivered during subsequent 
zonal time interval(s). 
In accordance with the invention, a preferred method 2600 for accomplishing 
zonal redefinition is shown in FIG. 26. In accordance with the method, 
step 2602 provides for transmitting substantially simultaneously a first 
information signal and a second information signal, the first information 
signal being transmitted in simulcast by a first set of base transmitters 
assigned to a first zone, and the second information signal being 
transmitted in simulcast by a second set of base transmitters assigned to 
a second zone. For example, as shown in FIG. 25, the base transmitters in 
zone 1 defined by boundary line 2502 could be the first set of base 
transmitters, and the base transmitters located in zone 2 defined by 
boundary line 2504 could be the second set of base transmitters. 
Step 2604 of the method provides for dynamically reassigning one or more of 
the base transmitters in the first set of base transmitters assigned to 
the first zone to the second set of base transmitters assigned to the 
second zone, thereby creating an updated first set of base transmitters 
and an updated second set of base transmitters. For example, base 
transmitters 2512, 2514, and 2516 could be reassigned from zone 1 to zone 
2. As shown in FIG. 25, new zonal boundaries would be defined by dashed 
lines 2512 for zone 1 and 2524 for zone 2. 
Step 2606 provides transmitting substantially simultaneously a third 
information signal and a fourth information signal, the third information 
signal being transmitted in simulcast by the updated first set of base 
transmitters and the fourth information signal being transmitted in 
simulcast by the updated second set of base transmitters. For example, as 
shown in FIG. 25, the base transmitters assigned to zone 1 defined by 
dashed line 2522 (i.e. not including base transmitters 2512, 2514, and 
2516) could transmit during a subsequent communication cycle a third 
information signal, and base transmitters in zone 2 defined by dashed line 
2524 (i.e. including base transmitters 2512, 2514, and 2516) could 
transmit a fourth information signal during that same subsequent 
communication cycle. 
Further, it is desirable that during the redefinition of the zonal 
boundaries, it is insured that the new overlap area 2525 near base 
receiver 2526 and between dashed lines 2522 and 2524 is an area that is 
not likely to produce, or is not currently producing a high volume of 
message traffic. Generally, zonal boundaries should be preferably 
redefined to maximize information throughput by minimizing the data that 
must be transferred during the systemwide time interval. A network manager 
could review the overall traffic patterns and tendencies to determine an 
optimum redefinition of zonal boundaries. Of course, the central computer 
2002 could also implement an algorithm accessing the traffic statistics 
database 2200 to determine optimal zonal boundary redefinition. 
In a preferred embodiment in the instance where an entire region is 
saturated with mobile units, such as a large metropolitan area repetitive 
reassignments of base transmitters may be used to reduce message traffics 
during the systemwide time interval. There may exist no appropriate 
overlap area, such as overlap area 2525, with a low traffic level to 
facilitate a long term reassignment of base transmitters with the 
resulting redefinition of zonal boundaries. In this case, the preferred 
embodiment alternates between a first and second set of zonal boundaries 
over each communication cycle and does not attempt to deliver messages 
during the systemwide time interval. 
For example, in FIG. 25 this preferred embodiment would utilize the zonal 
boundaries defined by lines 2502 and 2504 during a first zonal time 
interval and would not attempt to deliver messages to mobile units in 
overlap area 2521. In a subsequent cycle, this preferred embodiment 
redefines the zonal boundaries to dashed lines 2522 and 2524 and delivers 
messages to the mobile units in previous overlap area 2521 during the 
zonal time interval using zone 2 base transmitters. During this cycle, the 
network would not attempt to deliver messages to mobile units in overlap 
area 2525. In yet a later cycle, this preferred embodiment would switch 
back to zonal boundaries 2502 and 2504 which would allow message delivery 
to mobile units in the now previous overlap area 2525 during the zonal 
time interval using zone 1 base transmitters. As can be seen, alternating 
between a first and second set of zonal boundaries advantageously reduces 
the need for communication during the systemwide time interval, but slows 
message delivery somewhat by only allowing communication to mobile units 
in overlap areas during zonal time intervals on alternating communication 
cycles. 
H. The Preferred System Communication Protocol 
The system communication protocol is preferably a time division protocol 
organized within repetitive communication cycles of preferably 30 seconds 
in duration. 
The blocks of data transmitted by the network are preferably formed by a 
bit interleaving process to prevent loss of data during bursts of 
interference. Bit interleaving may be envisioned as stacking two or more 
blocks of data (which read from left to right), and then transmitting a 
bit stream in a column-by-column, top-to-bottom sequence. As can be seen, 
a burst of interference will likely only cause the loss of a few bits per 
word at most, which can be corrected by error correction techniques, 
rather than the loss of entire words. Of course, the mobile unit must 
appropriately deinterleave the data prior to processing. 
FIG. 27 generally illustrates a variety of preferred time intervals which 
may variously be used for communication between the system and various 
sets and subsets of mobile units. An adaptable schedule for these time 
intervals is preferably generated, and may be revised according to system 
demands. The scheduling of the time intervals advantageously allows a 
mobile unit to "power down" during inactive time periods when the mobile 
unit will not transmit or receive any messages, thereby conserving battery 
power. Similarly, messages or information for delivery to a subset of the 
total number of mobile units will preferably be transmitted during time 
intervals which minimize the delivery of those messages or information to 
unintended mobile units not included in the subset to further conserve 
battery power. 
A preferred cycle protocol 2700 is shown in FIG. 27(A). The cycle protocol 
2700 includes a cycle header time interval 2702, a systemwide forward 
(FWD) batch time interval 2704, a systemwide response time interval 2706, 
a zonal forward (FWD) batch time interval 2708, a zonal reverse time 
interval 2710, and a reverse contention time interval 2712. Other 
arrangements, such as moving the systemwide reverse interval next to the 
zonal reverse interval may be considered if transmitter turn on time is 
significant. 
The cycle protocol generally schedules time slots for systemwide and zonal 
forward channel information transfer from the network to the mobile units 
and for systemwide and zonal reverse channel information transfer from the 
mobile transceiver units to the network. Briefly, the cycle header 2702 
field includes overhead or "housekeeping" information, the systemwide 
forward batch field 2704 and the zonal forward batch field 2708 provide 
forward communication capability through the base transmitters to the 
mobile units in a systemwide time interval and a zonal time interval, 
respectively. The systemwide response field 2706 and zonal reverse field 
2710 provide a return signal period for the mobile transceivers to respond 
to messages generated during the systemwide and zonal forward batch 
periods 2504 and 2508, respectively. Finally, the reverse contention 2712 
field allows the mobile transceiver to initiate access to the network. 
Each of the fields shown, except the cycle header 2702 field, is preferably 
variable in duration, and may be changed by the central computer 2002, 
depending on message traffic requirements. The beginning of the cycle is 
synchronized by the central computer to a time standard and preferably 
coincides with the start of minute or half minute intervals. Each mobile 
unit preferably includes timing circuitry, as previously described, which 
allows for the mobile unit to power up at the beginning of each cycle to 
receive communication. 
For each cycle, the central computer 2002 calculates the amount of time 
required for each field to maximize information throughput by the network. 
For example, for the cycle protocol 2700 shown in FIG. 27(A), the central 
computer will calculate the amount of time necessary for the systemwide 
forward batch field 2704, the systemwide response interval 2706, the zonal 
forward interval 2708, the zonal reverse interval 2710, and the reverse 
contention interval 2712. The cycle header 2702 will preferably include 
timing offset data which will indicate the timing offset from the cycle 
header until the beginning of the systemwide response interval 2706, the 
beginning of the zonal forward interval 2708, the beginning of the zonal 
reverse interval 2710, and the beginning of the reverse contention 
interval 2712. 
The cycle header 2702 starts preferably with an 8 digit long preamble (not 
shown) for digit synchronization purposes. The preamble allows for the 
mobile unit to synchronize its timing circuitry with the network. For 
example, the timing circuitry of the mobile unit could become offset from 
the network due to commonly caused inaccuracies. The preamble is followed 
by a "start of header" string of four digits and all timing offsets within 
the cycle are calculated as a number of predefined intervals beginning 
from the start of the last header digit. The start of header string is 
followed by an 8 digit string grouped into two words, each of which is 
protected against errors by encoding it using a forward error correcting 
code, preferably a Bose, Chaudhuri, and Hocquenghem (BCH) code or a Reed 
Solomon code. These error correcting codes add additional digits to the 
information digits in a code word, where the additional digits are a 
specific function of the information digits, so that if certain common 
error events occur, a decoding step involving all of the transmitted 
digits, both information and additional, can recover the original 
information digits. The first code word will contain a count of the 
current cycles executed for that day. The second code word will contain 
the necessary timing offsets for the beginning of the time intervals in 
the cycle protocol 2700. Further information regarding error correcting 
codes may be found in Gallagher, "Information Theory and Reliable 
Communication," Wiley 1968, which is hereby incorporated by reference. 
The systemwide forward batch 2704 field generally includes a zonal header 
time interval including overhead information and a series of 64 batches. 
Also, the zonal forward interval 2710 similarly includes a zonal header 
time interval with overhead information and a series of 64 batches. Each 
batch is a string of data containing information specifically directed to 
a single group of mobile units. Each batch preferably contains information 
directed to a certain class of mobile units with the classes divided by 
the types of service provided. For example, a first batch could be 
directed to all mobile transceiver units, and a second batch could be 
directed to all mobile receiver units. Further, each batch may contain 
several messages, each intended for different mobile units within the 
particular class of unit to which that batch is directed. Generally, FIG. 
27(B) shows the forward batch interval protocol 2750 preferred for both 
the systemwide forward interval 2704 and the zonal forward interval 2708. 
The systemwide forward interval 2704 is preferably used only for sending a 
probe signal to a mobile transceiver unit which does not respond to zonal 
messages (i.e. a "lost" unit). However, when necessary, the systemwide 
forward interval 2704 may be used to deliver messages to mobile units 
located in overlap areas. The ID number, or address, of the lost mobile 
unit is preferably followed by data indicating a timing offset which is a 
time delay amount until the beginning of the time slot designated for the 
return signal of that mobile unit. An alternative implementation, which 
may be useful for mobile units that have not responded for a period of 
time, could have mobile units that have received a probe signal respond 
during the reverse contention interval. 
After the end of the broadcast on the systemwide forward batch time 
interval 2704, all network base transmitters shut down until the beginning 
of the zonal forward batch time interval 2708. 
The forward batch interval protocol 2750 includes a forward channel header 
interval 2714 which includes data to allow the timing circuitry of the 
mobile units to synchronize themselves with the incoming data stream. The 
forward channel header 2714 also preferably includes data indicating a 
timing offset scheduling a reverse channel time interval for each batch, 
as may be required. Of course, the forward channel header 2714 for the 
systemwide forward interval 2704 would indicate a timing offset for 
reverse channel transmission during the systemwide response interval 2706, 
and the forward channel header 2714 for the zonal forward interval 2708 
would indicate a timing offset for reverse channel transmission during the 
zonal reverse interval 2710. 
The forward channel header 2714 further includes a data stream to the 
mobile unit listing which of the 64 batches will follow and the timing 
offsets indicating when those batches will be transmitted. Again, this 
feature advantageously allows the mobile unit to "power down" during the 
systemwide and zonal forward intervals 2704 and 2708 until the appropriate 
time for receiving its batch information, thereby conserving the battery 
power of the mobile unit. The remaining fields batch i 2720, batch j 2722, 
and batch k 2724 are the individual batches directed to the mobile units. 
It should be understood that different classes of mobile units can follow 
different desirable batch protocols, depending on the type of service, 
processing power, battery capacity, or other factors. 
The individual batch protocol 2780 is shown in FIG. 27(C). The batch header 
field 2726 is similar to the header fields discussed above for FIGS. 27(A) 
and (B). The batch header 2726 includes a list of particular mobile units 
to receive messages within the batch and includes timing offsets 
indicating when such messages will be broadcast. Further, the batch header 
2726 includes data indicating a timing offset scheduling a reverse channel 
interval in the system reverse interval, the zonal reverse interval, or 
the reverse contention interval, as appropriate. Again, this information 
allows the mobile unit to extend its battery life because the mobile unit 
need only power up at the appropriate time to receive or transmit the 
appropriate message. Further, it is preferred that the reverse channel 
timing offset data be transmitted using error correction codes to insure 
accurate receipt thereof by the mobile unit. Accurate receipt of the 
reverse channel timing offset data will prevent unwanted or untimely 
transmissions by the mobile unit and insure that a mobile unit may 
properly transmit a negative acknowledgment signal if it fails to properly 
receive an unencoded message. 
The individual message interval 2732 includes the individual message 
intended for a particular mobile unit or units. The duration of each 
message and number of messages within a batch may be varied by the network 
operations center 600 and is traffic dependent. 
Each mobile unit with transmit capability that has received a message in 
the immediately previous systemwide forward interval 2704 or the zonal 
forward interval 2708 will have an appropriate time slot for transmission 
scheduled in the systemwide response interval 2706, or the zonal reverse 
interval 2710, respectively. The timing circuit in the mobile transceiver 
unit determines the assigned time slot for transmission. For example, if 
the mobile unit simply intends to transmit an acknowledgment signal, which 
indicates that the mobile unit has properly received the message from the 
network, an 8 bit preamble followed by the address of that mobile unit 
need only be transmitted and a 3 bit acknowledgment. However, if a more 
extensive reply from the mobile unit is required, additional data could be 
transferred during this time slot. In particular, long reverse messages 
could be scheduled in response to a request from the mobile unit sent 
during the contention interval 2712, as discussed hereafter. 
Due to the low power transmit capability of the mobile transceiver units, 
there is an increased likelihood of data transmission errors for reply 
signals. The extended Golay code for error protection may be utilized for 
reverse channel messages from mobile transceiver units to the network. 
The systemwide response interval 2706 and the zonal reverse interval 2710 
provide communication capability from the mobile transceiver units to the 
network (i.e. the reverse channel). 
Still further, a preferred embodiment accommodates mobile terminals with 
extensive reverse message generation capabilities (e.g., a laptop computer 
connected to a radio transceiver) by allowing for contention messages that 
request extended reverse channel time for the transmission of a long 
reverse message. The reverse contention interval 2712 is located after the 
zonal reverse interval 2710 and provides for unscheduled messages from the 
mobile unit to the network. For example, the mobile transceiver unit could 
send a message to the network during the reverse contention interval 2712 
indicating that the user no longer wishes to receive messages, thereby 
terminating service. Also, the user could transmit a message to the 
network during the reverse contention interval 2712 indicating that the 
user now desires to reestablish services and begin receiving messages from 
the network. Further, a "registration signal," which is discussed infra, 
could be transmitted during the reverse contention interval 2712. 
The reverse contention interval preferably utilizes a so-called "slotted 
ALOHA" protocol, which allows the mobile unit to randomly select a 
predefined time slot within the contention interval to transmit a message. 
A mobile station wanting to transmit will first divide the contention 
interval into slots, preferably 5.33 ms in length, and then choose 
randomly any of them to start transmitting. The slotted ALOHA protocol is 
preferred because of the low likelihood of data "collisions" (i.e. 2 or 
more mobile units transmitting during the same time slot). 
I. Registration of the Mobile Unit 
Because the network operations center 600 stores the location of each 
mobile unit in the system in the user database 2100, it is preferred that 
each mobile transceiver unit have the capability to "register" with the 
network operations center 600 by sending a registration signal to a base 
receiver into the network to update the location data. 
The mobile transceiver unit preferably registers by simply transmitting its 
identification number to a base receiver, which forwards this data and 
data representing the location of the base receiver to the network 
operations center 600. 
The mobile transceiver preferably registers upon crossing zonal boundaries 
to alert the network operation center that the mobile transceiver has left 
one zone and entered another. For example, the mobile unit could receive 
information from the nearest base transmitter identifying which zone that 
base transmitter is assigned to at the beginning of each communication 
cycle. Upon receipt of such information from a base transmitter indicating 
that a nearby base transmitter is assigned to a new zone, the mobile 
transceiver then preferably transmits a registration signal. 
The mobile transceiver unit may also transmit a registration signal in 
other desirable instances. For example, if the mobile transceiver unit has 
moved away from the transmitter coverage areas of the network for a period 
of time, the mobile transceiver unit may preferably transmit a 
registration signal upon returning to a coverage area. The display and 
storage logic 1508 of the mobile transceiver unit preferably recognizes 
that the unit has left the coverage area of the network upon failure to 
receive data from a base transmitter in the network during the cycle 
header time interval 2702, for example. The mobile unit may leave the 
coverage area of a base transmitter of the network when the user takes the 
unit out of the country, or enters the basement of a building, for 
example. 
The mobile unit may also preferably transmit a registration signal when 
power is restored to the mobile unit after having power removed, such as 
after being turned off by the user. Of course, the power may be restored 
to the unit by replacing or recharging a dead battery, which may also 
cause transmission of a registration signal. 
In general, the network must balance the need for frequent registrations by 
the mobile transceiver units, and the desirable result of accurately 
knowing the location of each mobile unit, thereby preventing the need for 
probe signals, with the undesirable overhead costs of too frequent 
registration, which sacrifices data throughput by utilizing valuable 
transmit time. 
In the preferred embodiment, the central computer 2002 of the network 
operations center 600 can achieve desirable performance by implementing 
one or more algorithms to evaluate the need for registration by a mobile 
unit, and then appropriately controlling the registration performance of 
that mobile unit. If the central computer determines that registration of 
a particular mobile unit is useful, then the mobile unit preferably should 
receive a message from the network to cause the mobile unit to send 
registration signals at appropriate times. Conversely, if the central 
computer determines that the registration signals from the mobile unit are 
too frequently not useful, the mobile unit preferably should receive a 
message from the network to cause the mobile unit not to transmit 
registration signals. 
To implement this feature, the mobile transceiver unit further preferably 
includes a registration flag (not shown) in the display and storage logic 
section 1508. If the registration flag is set, the display and storage 
logic section 1508 causes the mobile transceiver to autonomously send a 
registration signal to the network operations center on a desired basis. 
If the registration flag is not set, the display and storage logic section 
1508 prevents any registration signals from being sent. The registration 
flag may be set or removed upon command from the network operations center 
by transmission of an appropriate signal from a base transmitter near the 
mobile unit. A variety of algorithms, possibly regarding individual users 
or groups of users, can be used to determine whether or not the 
registration flag should be set. It should be appreciated that the present 
invention provides two distinct algorithms for implementing these 
registration concepts depending upon whether the registration flag is set 
or not in the mobile unit (i.e. the state of the mobile unit). 
FIG. 28(A) shows a flow chart describing a preferred method 2800 for 
implementing the registration concepts of the present invention wherein 
the registration feature of the mobile unit is disabled. In step 2802, the 
network sends a message to disable the registration feature (i.e. set the 
registration flag to zero) of the mobile unit to disable the mobile 
transceiver's capability to transmit a registration signal. As can be 
seen, step 2802 determines the initial state for the method set forth in 
FIG. 28(A). 
In step 2804, the network stores the number of probe signals sent to the 
mobile transceiver during a first period of time, and the number of 
messages successfully delivered to the mobile transceiver by the network 
during a second period of time. Preferably, the first and second time 
intervals are identical. The traffic statistics database 2200 of the 
database 2008 is preferably used to store the number of probe signals and 
successful messages for each mobile unit. As explained hereinafter, these 
two statistics from the operation of the network are preferably used to 
determine whether registration by the mobile unit is useful. 
In step 2806, the stored number of probe signals and number of messages 
successfully delivered is processed to evaluate a likelihood that a probe 
signal will be required to be set by the network to locate the mobile unit 
to deliver a message. The preferred embodiment of the invention processes 
the stored number of probe signals and messages successfully delivered in 
accordance with the method set forth in FIG. 29(A). 
Referring now to FIG. 29(A), therein is shown a series of substeps which 
are preferably performed during the implementation of the processing step 
2804 shown in FIG. 28(A). In particular, steps 2902 and 2904 are event 
driven and only proceed to the next step after an input has been received 
by the network. Step 2902 determines if the network sent a probe signal to 
a lost mobile transceiver unit and if a reply to the probe signal was 
received by a base receiver in the network. If this event occurs, a 
counter (not shown) is incremented by a value P by the central computer 
2002. 
In step 2904, if a message was successfully delivered to a mobile 
transceiver, preferably including an acknowledgment signal return from the 
mobile transceiver to the network, the counter (not shown) in the central 
computer 2002 is decremented by a value D. 
After the occurrence of either of the events tested for in step 2902 or 
step 2904, the algorithm proceeds to step 2906. In step 2906, if the 
counter value is greater than a predetermined value J, this indicates that 
the likelihood that a probe signal will be necessary to locate the mobile 
transceiver is greater than a selected value. 
As can be seen, the process of substeps in FIG. 29(A) balances the 
frequency of probe signals sent to a particular unit against the number of 
successfully delivered messages to that unit. If the system must send a 
large number of probe signals, it would be useful to enable the 
registration feature by setting the registration flag on that mobile unit 
to enable the registration feature. In contrast, if many messages have 
been successfully delivered without requiring a probe signal, it is 
unnecessary to enable the registration feature by setting the registration 
flag. 
In step 2808, a message is sent to the mobile unit to enable the mobile 
transceiver's capability to transmit a registration signal if the 
calculated likelihood in step 2804 exceeds a selected value. As can be 
seen, step 2808 preferably sets the registration flag in the mobile 
transceiver unit. 
FIG. 28(B) shows a flow chart describing a method 2810 for implementing the 
registration concepts of the present invention wherein the registration 
feature of the mobile unit is enabled. In step 2812, the network sends a 
message to enable the registration feature (i.e. set the registration flag 
to 1) of the mobile unit to enable the mobile transceiver's capability to 
transmit a registration signal. As can be seen, step 2812 determines the 
initial state for the method set forth in FIG. 28(B). 
In step 2814, the network stores the number of registration signals 
received by the network during a first period of time, and the number of 
messages successfully delivered to the mobile transceiver by the network 
during a second period of time. Preferably, the first and second time 
intervals are identical. The traffic statistics database 2200 of the 
database 2008 is preferably used to store the number of registration 
signals and successful messages for each mobile unit. As explained 
hereinafter, these two statistics from the operation of the network are 
preferably used to determine whether the registration by the mobile unit 
is useful. 
In step 2816, the stored number of registration signals and number of 
messages successfully delivered is processed to evaluate the likelihood 
that a registration signal will be received by a base receiver in the 
network that will not be used by the network to determine a set of base 
transmitters to be operated to transmit a message to the mobile 
transceiver. The preferred embodiment of the invention processes the 
stored number of registration signals received and number of messages 
successfully delivered in accordance with the method set forth in FIG. 
29(B). 
Referring now to FIG. 29(B), therein is shown a series of substeps which 
are preferably performed during the implementation of the processing step 
2814 shown in FIG. 28(B). In particular, steps 2912 and 2914 are event 
driven and only proceed to the next step after an input has been received 
by the network. Step 2912 determines if a registration signal was received 
by a base receiver in the network. If so, a counter (not shown) in the 
central computer 2002 is incremented by a value A. 
In step 2914, if a message was successfully delivered to a mobile 
transceiver, preferably including an acknowledgment signal return from the 
mobile transceiver to the system, the counter (not shown) in the central 
computer 2002 is decremented by a value M. 
It should be understood that the counter referred to with regard to steps 
2912 and 2914 is different then the counter referred to with regard to 
steps 2902 and 2904 since each counter is only necessary when the 
registration feature is enabled or disabled in the mobile transceiver. 
However, the same physical or logical device may be used to implement both 
counters. 
After the occurrence of either events in the step 2912 or step 2914, the 
algorithm proceeds to step 2916. In step 2916, the process determines if 
the counter value is greater than a predetermined value T. The value of T 
can be varied to meet the needs of a particular network. When the counter 
value exceeds T, it is indicated that the likelihood that a registration 
signal from that mobile unit will not be used by the network to determine 
a new set of base transmitters, and therefore the registration status for 
that mobile unit needs to be changed to disable the registration feature. 
In other words, the process in FIG. 29(B) balances the frequency of 
registration signals sent by a particular unit against the number of 
successfully delivered messages to that unit. As can be seen, if the 
mobile unit sends a large number of registration signals without the 
system using these registration signals, it would be useful to have the 
registration feature on that mobile unit disabled. In contrast, if many 
messages have been successfully delivered without too many registration 
signals being sent by the mobile unit, it is unnecessary for the 
registration feature to be disabled. 
In step 2818, a message is sent to the mobile unit to disable the mobile 
transceiver's capability to transmit a registration signal if the 
calculated likelihood in step 2814 exceeds a selected value. As can be 
seen, step 2818 may preferably remove the registration flag in the mobile 
transceiver unit. 
Of course, it should be understood that the variables P, D, and J used in 
FIG. 29(A), and the variables A, M, and T used in FIG. 29(B) can be 
adjusted as desired to enhance system performance, as will be apparent to 
one of ordinary skill in the art. The counters can be implemented with 
so-called "reflective boundaries" so that if a counter reaches a minimum 
value (e.g., zero), it will continuously reset to that minimum value when 
further decremented. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the systems and methods of the present 
invention without departing from the scope or spirit of the invention. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the following claims.