Note: Descriptions are shown in the official language in which they were submitted.
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CELL BROADCAST IN A HYBRID GSM/CDMA NETWORK
I. Field of the invention
The present invention relates generally to wireless telecommunications,
and specifically to advanced cellular telephone networks.
II. Background of the invention
The Global System for Mobile (GSM) telecommunications is used in
cellular telephone networks in many countries around the world. Existing GSM
networks are based on time-division multiple access (TDMA) digital
communications technology. GSM offers a useful range of network services and
standards.
One of these GSM network services is a short message service - cell
broadcast (SMSCB), for distributing short messages from a Cell Broadcast
Center
(CBC) via Base Station Subsystems (BSSs) in the network to subscriber units,
or
Mobile Stations (MSs). SMSCB messages may come from various sources, such
as traffic and weather reports. These messages are broadcast to MSs in defined
geographical areas, known as cell broadcast areas, over a dedicated Cell
Broadcast Channel (CBCH) without requiring acknowledgment from the MS.
The messages are received by the MS only in idle mode (i.e., when a telephone
call is not in progress). SMSCB and related interfaces, protocol stacks and
message formats are described, inter alia, in GSM standards 02.03, 03.41,
03.49
and 04.12, which are incorporated herein by reference. It is noted, however,
that
there is no mandatory protocol defined by G5M standards between the CBC and
the BSSs. Rather, the interface protocol is left to be determined by operators
of
the network and cell broadcast services, based on primitives defined by the
03.41
standard.
Code-division multiple access (CDMA) is an improved digital
communications technology using direct sequence spread spectrum modulation
techniques, which affords more efficient use of radio bandwidth than TDMA, as
well as a more reliable, fade-free link between cellular telephone subscribers
and
base stations. The leading CDMA standard is TIA/EIA-95, promulgated by the
Telecommunications Industry Association (TIA), which is incorporated herein
by reference. In the context of the present patent application and in the
claims,
this standard is referred to as IS-95, by which name it is commonly known in
the
cellular communications industry. A recent version of the standard, known as
TIA/EIA-95-B (hereinafter IS-95B), has advanced networking features including
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procedures for broadcast of messages over paging channels, which are
monitored and received by compatible MSs.
PCT patent application PCT/US96/20764 which is based upon U.S.
patent application serial no. 08/575,413 entitled "Wireless Telecommunications
System Utilizing CDMA Radio Frequency Signal Modulation in Conjunction
with the GSM AInterface Telecommunications Network Protocol," filed
December 20, 1995, which are assigned to the assignee of the present patent
application and both incorporated herein by reference, describes a wireless
telecommunications system that uses a CDMA air interface (i.e., basic RF
communications protocols) to implement GSM network services and protocols.
Using this system, at least some of the TDMA base stations (BSSs) and
subscriber
units of an existing GSM network would be replaced or supplemented by
corresponding CDMA equipment. CDMA BSSs in this system are adapted to
communicate with GSM mobile switching centers (MSCs); via a standard GSM
A-interface. The core of GSM network services is thus maintained, and the
changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and
CDMA elements, are also described in PCT patent publications WO 95/24771
and WO 96/21999, and in an article by Tscha, et al., entitled "A Subscriber
Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching
Center," in Proceedings of the 2nd International Conference on Universal
Personal Communications, Ottawa (1993), pp. 181-185, which are incorporated
herein by reference. None of these publications deal with issues of support of
short message or cell broadcast services, such as SMSCB, in such hybrid
networks.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods and apparatus
for use in a mixed GSM/CDMA cellular communications network.
It is a further object of some aspects of the present invention to provide
methods and apparatus enabling realization of GSM Cell Broadcast Service over
a CDMA air interface.
In preferred embodiments of the present invention, a mixed
GSM/CDMA cellular communications system includes both TDMA and CDMA
base stations (BSSs), which are in contact with a Cell Broadcast Center (CBC)
based on GSM standards. Systems of this type are described generally in U.S.
patent application serial no. 09/119,717 entitled "Base Station Handover in a
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Hybrid GSM/CDMA Network," U.S. Patent Application Serial No. 09/119,717,
filed July 20, 1998, which is assigned to the assignee of the present patent
application and is incorporated herein by reference. A subscriber unit in the
network, referred to herein as a mobile station (MS), is preferably capable of
communicating with both types of base stations by appropriately switching
between TDMA add CDMA air interfaces. GSM network protocols are used over
both types of air interface, so that the MS receives short messages broadcast
from
the GSM CBC whether the MS is in communication with a TDMA BSS or a
CDMA BSS. The present invention thus enables CDMA BSSs and MSs to be
integrated into a GSM network infrastructure without compromising the ability
of the CBC associated with the network to broadcast messages based on GSM
standards to substantially any MS, and with substantially no other
modification
required to existing infrastructure.
In some preferred embodiments of the present invention, GSM short
message service - cell broadcast (SMSCB) messages are broadcast to the MS over
the CDMA air interface using a CDMA paging channel. Preferably, the
messages are broadcast in accordance with broadcast procedures over paging
channels defined by the IS-95B standard.
There is therefore provided, in accordance with a preferred embodiment
of the present invention, in a GSM mobile wireless telecommunications system a
method for broadcasting messages over a CDMA air interface, including:
conveying a message to a base station substantially in accordance with a
GSM cell broadcast service protocol; and
transmitting the message to a mobile station over the CDMA air
interface.
Preferably, transmitting the message includes transmitting a message
substantially in accordance with a CDMA transmission standard, most
preferably IS95B.
Preferably, transmitting the message includes transmitting a message
over a paging channel, wherein transmitting the message includes directing the
message to one or more specified cells.
In a preferred embodiment, transmitting the message includes
transmitting a message so as to have a high likelihood of being received by
the
mobile station while the mobile station is operating in a slotted mode, most
preferably by transmitting a broadcast page followed by transmission of a
message. Preferably, transmitting the broadcast page includes transmitting a
page in a predetermined slot monitored by the mobile station in a periodic
broadcast paging cycle. Optionally, the method includes transmitting a
schedule
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message including information regarding one or more broadcast pages to be
transmitted in a schedule period including one or more periodic broadcast
paging cycles.
Preferably, transmitting the message includes transmitting a single
message multiple times, substantially in accordance with a GSM cell broadcast
command, wherein transmitting the single message includes assigning a
broadcast address to the message, such that when the mobile station receives a
second message having the same broadcast address as a first, earlier message,
the second message is discarded. Preferably, transmitting the single message
multiple times includes repeating transmission of the message at a repetition
frequency determined responsive to the cell broadcast command.
Further preferably, transmitting the message includes encapsulating a
GSM cell broadcast message in an IS-95 message.
Preferably, conveying the message includes receiving a message from a
cell broadcast center.
In a preferred embodiment, transmitting the message includes
transmitting a message header including a message identifier field indicative
of
a characteristic of the message, and the mobile station determines whether to
decode or discard the message responsive to the characteristic.
There is also provided, in accordance with a preferred embodiment of
the present invention, apparatus for broadcasting short messages from a cell
broadcast center over a CDMA air interface, including a base station
subsystem,
which receives the messages from the cell broadcast center substantially in
accordance with a GSM cell broadcast service protocol and transmits the
message to a mobile station over the CDMA air interface.
Preferably, the base station subsystem transmits the message
substantially in accordance with a CDMA transmission standard, most
preferably IS-958.
Preferably, the base station subsystem transmits the message over a
paging channel and directs the message to one or more specified cells.
Preferably, the base station subsystem transmits the message so as to
have a high likelihood of being received by the mobile station while the
mobile
station is operating in a slotted mode, wherein before transmitting the
message,
the base station subsystem transmits a page in a predetermined slot monitored
by the mobile station in a periodic broadcast paging cycle.
Further preferably, the base station subsystem transmits a single message
multiple times, substantially in accordance with a GSM cell broadcast command,
wherein the base station subsystem repeats transmission of the message at a
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repetition frequency determined responsive to the cell broadcast command.
Further preferably, the base station subsystem assigns a broadcast address to
the
message, such that when the mobile station receives a second message having
the same broadcast address as a first, earlier message, the second message is
5 discarded.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more fully understood from the following
detailed description of the preferred embodiments thereof, taken together with
the drawings in which:
Fig. 1 is a schematic block diagram of a hybrid GSM/CDMA cellular
communications system, in accordance with a preferred embodiment of the
present invention;
Fig. 2 is a schematic block diagram illustrating communications protocols
between elements of the system of Fig. 1, in accordance with a preferred
embodiment of the present invention; and
Fig. 3 is a schematic block diagram illustrating a message format for use
in cell broadcast service messages, in accordance with a preferred embodiment
of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is now made to Fig. 1, which is a schematic block diagram of a
hybrid GSIVI/CDMA cellular communications system 20, in accordance with a
preferred embodiment of the present invention. System 20 is built around a
public land mobile network (PLMN) 22, which is based on the GSM
communications standard, as is known in the art and described briefly
hereinabove. Infrastructure for such networks already exists and is in wide
use
in many countries, and the present invention has the advantage of enabling
gradual introduction of CDMA service in conjunction with such a network
without requiring major changes to the existing switching infrastructure.
PLMN 22 comprises at least one mobile-services switching center (MSC)
24, or possibly a number of such centers (although only one MSC is shown here
for clarity of illustration), which controls network operations within a
geographical area. Among other functions, MSC 24 is responsible for location
registration of subscriber units and handover of subscriber units between base
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stations, as well as linking PLMN 22 to a public switched telephone network
(PSTN) and/or packet data network (PDN) 48. The PLMN also comprises a
network management center (NMC) 26 and a cell broadcast center (CBC) 28.
Other aspects of system 20 and details regarding a mobile station (MS) 40 in
the
system, are described further in the above-mentioned U.S. and PCT Patent
Applications.
System 20 includes a plurality of MSs 40, which communicate with
PLMN 22 via a plurality of base station subsystems (BSS) 30 and 32 over a
wireless RF link at one or more of the accepted cellular communications
frequencies. MS 40, which is also known as a subscriber unit, is preferably
capable of communicating with both GSM BSS 30, using a standard GSM TDMA
radio communications protocol, and CDMA BSS 32, using CDMA-based
communication methods described hereinbelow. Although for the sake of
clarity, only one each of MS 40, GSM BSS 30 and CDMA BSS 32 is shown in Fig.
1, it will be understood that in actuality, system 20 typically comprises a
plurality of each of these system elements.
Communications between CDMA BSS 32 and MS 40 use a CDMA radio
"air interface," which is preferably based on the IS-95 standard for CDMA
communications, and most preferably with the TIA/EIA-95-B version of the
standard, which is incorporated herein by reference, with -necessary
modifications as described herein. BSS 32 is built around a base station
controller
(BSC) 34, which controls and communicates with a number of base station
transceivers (BTS) 36. Each BTS transmits RF signals to and receives RF
signals
from MS 40 when the MS is within a geographical area, or cell, served by the
particular BTS. On the other hand, when MS 40 is within a cell served by GSM
BSS 30, the MS preferably communicates with BSS 30 over a GSM/TDMA air
interface to GSM/TDMA BTSs (not shown in the figure for the sake of
simplicity).
Both GSM BSS 30 and CDMA BSS 32 communicate with and are
controlled by MSC 24, substantially in accordance with GSM standards, i.e.,
via
the GSM standard A-interface, as further described in the above-mentioned U.S.
and PCT Patent Applications. BSS 32 also comprises a radio operation and
maintenance center (OMCR) 38, which communicates with NMC 26 over a GSM-
standard Q3 interface.
BSC 34 communicates with CBC 28 so as to receive short messages to be
broadcast over the air, based on the above-mentioned GSM SMSCB standards.
One CBC 28 is typically connected to BSCs of a plurality of BSSs, which may
include both CDMA BSSs, such as BSS 32, and GSM/TDMA BSSs, such as BSS
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30. The CBC may receive short messages for broadcast from several cell
broadcast entities (not shown), such as weather and traffic report centers, as
described in GSM specifications.
CBC 28 is responsible for management of cell broadcast short messages
and issuing commands to BSC 34 (and other BSCs), including:
~ Allocation of serial numbers for identification of messages, as specified
by GSM standard 03.41. The serial number is included in the header of
each message (described further hereinbelow with reference to Fig. 3). It
typically includes fields specifying a geographical scope of the message,
a message code for differentiating between messages from the same
source and of the same type, and an update number.
~ Modifying or deleting messages held by the BSC.
~ Initiating broadcast by sending cell broadcast messages to the BSC, and
where necessary padding the messages to a length of 82 octets, based on
GSM standard 03.41.
~ Determining a set of cells or BTSs to which the message should be
broadcast and indicating the geographical scope of the message. A list of
the determined cells or BTSs is conveyed to the BSC, which then
distributes the broadcast accordingly.
~ Determining a time at which the message should commence being
broadcast.
~ Determining a time at which the message should cease being
broadcast, and instructing the BSC accordingly.
~ Determining a rate at which the message broadcast should be repeated.
~ Determining the cell broadcast channel on which the message should
be broadcast.
BSC 34 performs the following function vis-a-vis CBC 28:
~ Receiving and interpreting cell broadcast commands.
~ Storing cell broadcast messages.
~ Scheduling of the messages over CDMA paging channels, as described
further hereinbelow.
~ Providing an indication to the CBC when the desired message
repetition rate cannot be achieved, for example, when the desired rate is
too high.
~ Acknowledging successful execution of CBC commands.
~ Reporting to the CBC when a command cannot be understood or
cannot be executed.
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~ Routing cell broadcast messages to the appropriate BTSs, preferably
using directed paging procedures, as described further hereinbelow.
~ Transferring cell broadcast service (CBS) information to each BTS.
~ Optionally, generating schedule messages, as described further
hereinbelow, indicating an intended schedule of transmissions.
Preferably, when a message conveyed from BSC 34 to CBC 28 pertains to
multiple cells, the message is conveyed only once, together with a list of the
cells
to which it applies.
MS 40 comprises mobile equipment (ME) 42, which preferably includes
either two radio transceivers, one configured for TDMA operation and one for
CDMA, or a single transceiver which can dynamically switch between TDMA
and CDMA. The MS includes mobile termination (MT), which supports terminal
equipment (TE) 46 for voice and data input and output. In addition, MS 40
comprises a subscriber identity module (SIM) 44, in accordance with GSM
standards, which is used in authenticating the identity of a user of MS 40 in
a
manner substantially transparent to and independent of the CDMA air interface.
Although preferred embodiments are described herein with reference to MS 40
having dual CDMA/TDMA air interface compatibility, it will be understood
that the principles of the present invention may similarly be applied to
systems
using mobile stations having only CDMA compatibility or, mutatis mutandis, to
other systems using GSM networking standards.
MS 40 receives the cell broadcast messages transmitted by BTS 36 from
BSC 34. Typically, TE 46 is used to display the messages in alphanumeric
format,
as is known in the art. In addition, MS 40 has the following capabilities with
regard to the 25 cell broadcast messages:
~ Identifying and discarding messages that have a message identifier
indicating that the subject matter of the message is not of interest to a
subscriber using the MS.
~ Ignoring repeated broadcast of messages already received (i.e., a
message broadcast address has not changed).
~ Transferring the message via a R-interface between ME 42 and TE 46,
when such an interface is supported.
Reference is now made to Fig. 2, which is a schematic block diagram
showing protocol stacks used in conveying SMSCB teleservice messages
between CBC 28 and MS 40 via BSS 32, in accordance with a preferred
embodiment of the present invention. The protocols shown in the figure enables
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the substantially unmodified GSM CBC to convey short messages to MS 40,
based on GSM SMSCB standards, using paging channel broadcast procedures
between the BSS and the MS specified by the IS95B standard.
Cell broadcast messages are conveyed from CBC 28 to BSC 34 over a
CBC-BSC interface, which is generally in accordance with GSM specifications.
As noted hereinabove, the protocol to be implemented in this interface is a
mattter to be agreed upon between operators of the CBC and PLMN 22, based
on primitives defined in GSM standard 03.41. Preferably, as illustrated in
Fig. 2,
the stack is in accordance with one of the sample protocol stacks provided in
GSM standard 03.49.
The GSM primitives defined by the 03.41 standard are generally received
and interpreted by BSC 34 in conformity with the standard, but there are a few
exceptions necessitated in order to accommodate IS-95B broadcast procedures
over the paging channel. GSM standards support two kinds of cell broadcast
channels (CBCH): a basic and an extended channel; but both of these channels
are mapped to the same CDMA paging channel. Furthermore, whereas GSM
specifications define the maximum 20 repetition frequency of a cell broadcast
message as once in every 51 x 8 TDMA frames (1.883 sec), the maximum
repetition frequency over the CDMA air interface, in accordance with IS-95B,
is
once in every periodic broadcast paging cycle, corresponding to about 2.8 sec
at
a minimum. BSS 32 determines a BCAST_INDEX parameter between 1 and 7, as
specified by IS-95B, to set the duration of the periodic broadcast paging
cycles. In
the event of a conflict, such as more than one message to broadcast at a given
time, BSS 32 determines the order of broadcast of the messages in accordance
with predetermined criteria.
BSS 32 manages end-to-end delivery of the messages and provides
synchronization and scheduling services based on the interface primitives used
in the CBC-BSC interface. IS-95B includes several different paging methods,
which are applicable in corresponding preferred embodiments of the present
intention and are described further hereinbelow. Preferably, BSC 34 is capable
of
sending broadcast pages (as described hereinbelow) and messages to specific
cells, so that the cell broadcast messages can be distributed to specified
geographical areas.
MS 40 exchanges signals with CDMA BSS 32 over a CDMA Um interface,
wherein the MS and BSS protocol stacks are modified to accommodate GSM
network services, such as SMSCB, as described herein. The SMSCB messages are
transmitted from BTS 36 to MS 40 over the paging channel of the CDMA air
interface.
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Fig. 3 is a block diagram illustrating a SMSCB message 80 transmitted
over the paging channel to MS 40, in accordance with a preferred embodiment of
the present invention. Message 80 has the form of a standard IS-95 Data Burst
Message (DBM). Message 80 encapsulates an entire GSM cell broadcast message,
5 including a GSM SMSCB header 86 and data 88, preferably in accordance with
GSM standard 03.41. Message 80 also includes a MSG LEN field 82, set by BSS 32
to specify the total length of the message, and a CRC field 92, both in
accordance
with IS-95 specifications, along with a DBM header 84 and a reserved field 90
of
5 bits.
10 DBM header 84 is set by BSS 32 to be generally in accordance with IS-95B
specifications for paging channel messages, with the exception of a broadcast
address field in the DBM header, whose content is preferably set to
accommodate GSM cell broadcast service, as defined in particular by GSM
standard 03.41, section 9.3, "Message Format on BTS-MS interface." The
broadcast address field contains a part of a cell broadcast message header
generated by CBC 28, which uniquely identifies the message so that MS 40 can
detect and avoid decoding messages that it has already received or which are
not of interest to the subscriber. (The CBC message header includes the first
six
octets of the 88 octets of the complete message.) MS 40 ignores messages whose
BURST TYPE and broadcast address fields in DBM header 84 are identical to
those of an earlier message already received in a given periodic broadcast
slot
cycle.
The structure of the broadcast address field is illustrated in Table I
below:
TABLE I
Sub-Field ength alue
(bits)
Message Type 0 - normal message
1 - schedule message (as described
further
ereinbelow)
they - reserved
Serial Number16 or any given Message Identifier, the
Serial
umber is updated every time a new message
is transmitted, to distinguish different
messages
ith a common source and type.
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Message 16 Identifies the source and type of the
message.
Identifier
Data Coding 8 Indicates intended handling of the
message at the
Scheme MS, including alphabet/coding and language
as
defined by GSM standard 03.38.
The structure of the Serial Number sub-field is also defined in
accordance with the above-mentioned section 9.3 of GSM standard 03.41, and is
illustrated in Table II below:
TABLE II
Sub-Field Length Value
(bits)
Geographical2 Indicates a geographical area over
which the
Scope Message Code is unique, as well as
the display
mode.
Message Code10 Varied so as to differentiate between
messages
from the same source and of the same
type
(i.e., having same Message Identifier).
Update 4 Indicates a change of message content
for a given
Number Message Identifier, Geographical Scope
and
Message Code.
Message Codes are allocated by PLMN operators and can be used to
identify different message themes.
Because a given message is not necessarily broadcast by all cells within a
given geographical area, the Geographical Scope may be used to determine
whether two messages having identical Serial Numbers and Message Identifiers
that are received in different cells are indeed identical, as described in the
above-mentioned section 9.3 of the GSM 03.41 standard.
Typically, MS 40 operates in a slotted mode, as specified by IS-95B, and
does not constantly monitor a paging channel. Therefore, in transmitting a
cell
broadcast message, special procedures are preferably used to ensure that MSs
operating in the slotted mode are able to receive the message.
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Thus, in a preferred embodiment of the present invention, BSS 32
transmits a broadcast page message, such as a General Page Message, as
specified by IS-95B, or a Paging Request Message, based on GSM standards, or
another suitable message type, announcing the impending transmission of a
broadcast message. A page record included in the page message (which
normally specifies an identification of the MS to which the message is
addressed) preferably contains a broadcast address, indicating to MSs
receiving
the page message when a cell broadcast message is to be transmitted. The
actual
cell broadcast message is then transmitted once. When one of the MSs decodes
the broadcast address in the page record, it can determine whether the
broadcast
message is a duplicate of a message already received or is not of interest to
the
subscriber and, if so, will avoid decoding the broadcast message itself.
In another preferred embodiment, BSS 32 uses a periodic broadcast
paging cycle, in accordance with the IS-95B specification, so as to reduce
overhead required for sending broadcast messages. In this mode of operation,
MS 40 monitors the paging channel during a predetermined slot in which
broadcast pages or messages are transmitted, so that the pages and messages
need be transmitted only once during each periodic broadcast paging cycle.
Preferably, the broadcast page is sent in the first slot of a given cycle,
which is
monitored by the MS, and the broadcast message itself is transmitted later, in
accordance with IS-95B.
Preferably, broadcast message scheduling, generally as described in the
GSM 03.41 standard, is used in distributing cell broadcast messages to MSs
operating in slotted mode. In this case, the broadcast page message comprises
a
Schedule Message, in which the Message Type field (noted in Table I) inset to
'1'.
The Schedule Message contains information about a number of consecutive cell
broadcast messages that will be transmitted during a Schedule Period,
consisting
of one or more periodic broadcast cycles, immediately thereafter. The Schedule
Message includes a Message Description for each of the messages, along with a
cycle number indicating its time position in the Schedule Period. The Message
Description contains information from the broadcast address, so that MSs can
ascertain the type and source of each scheduled message and whether it has
already received the message.
Each Schedule Message includes fields defining a Begin Cycle Number
and an End Cycle Number, indicating the length of the Schedule Period.
Preferably, a new Schedule Message is transmitted following the last message
of
the Schedule Period, i.e., in a cycle given by the preceding End Cycle Number
+
1. Several Schedule Messages can be broadcast in reference to the same
Schedule
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Period, and the Begin Cycle Number indicates the first cycle immediately
following the last Schedule Message.
When necessary, the BSS may receive instructions to override the
schedule published in the Schedule Message, for example, to transmit a new,
high-priority SMSCB message. Thereafter, the previous schedule of cell
broadcast messages is resumed.
The methods and protocols described hereinabove apply when MS 40 is
in communication with CDMA BSS 32. When MS 40 is in communication with
CBC 28 via GSM BSS 30, the communications protocols between these elements
are in accordance with GSM standards, substantially without modification.
Although preferred embodiments are described hereinabove with
reference to specific TDMA- and CDMA-based communications standards, those
skilled in the art will appreciate that the methods and principles described
hereinabove may also be used in conjunction with other methods of data
encoding and signal modulation. The scope of the present invention
encompasses not only the complete systems and communications processes
described hereinabove, but also various innovative elements of these systems
and processes, as well as combinations and sub-combinations thereof. In
particular, although preferred embodiments are described hereinabove with
reference to hybrid TDMA/CDMA system 20 and MS 40, it will be understood
that the methods and apparatus described may equally be used to convey cell
broadcast messages to a MS in a CDMA system without TDMA capabilities.
It will thus be appreciated that the preferred embodiments described
above are cited by way of example, and the full scope of the invention is
limited
only by the claims.
WE CLAIM:
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