Subject: [NEXTEL-1] Dispatch mode in code division multiple access systems (CDMA)

From: Marcel <[email-masked]>
Date: Thu, 17 Apr 2003 11:00:04 -0400

Jack,

Here is the patent from one of the originators of CDMA Dispatch:  Dr.
Nagi A Mansour.
This was first developed by Sprint PCS.

See the attached file for Dwgs.
====================================================================================

Inventors:
Mansour; Nagi A. (Overland Park, KS)
Assignee:
Sprint Spectrum L.P. (Kansas City, MO)
Appl. No.:
366609
Filed:
August 3, 1999
Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless telecommunications, and in
particular, to a method of providing dispatch mode calling
in a code division multiple access wireless network.

2. Description of the Prior Art

"Dispatch mode" is a radio communications technique where one radio
communicates to many other radios using short bursts
of communications. Many groups such as taxi drivers and police officers
desire to communicate with dispatch mode radio
devices because it allows them to speak with a number of other people at
the same time without initiating a plurality of separate
phone calls. Until recently, dispatch mode services were only available
in two-way radio systems such as walkie talkies.
Although these radio systems work effectively when all of the users are
located close together, they become ineffective once the
users travel beyond the range of the radio transceivers.

Nextel has recently begun offering dispatch mode operations in its
enhanced specialized mobile radio (ESMR) systems.
ESMR can be deployed on a cellular network and, therefore, supports
network features that enable users to operate in
dispatch mode over greater distances.

ESMR systems use time division multiple access (TDMA) technology which
allocates a
discrete amount of frequency band width to each user of the system to
permit many simultaneous conversations. Each caller is
allowed to transmit in predetermined time slots so that channelization
of users in the same band is achieved through separation
in time. Unfortunately, the capacity of these networks is limited by the
number of available time slots. TDMA networks also
suffer from poor call clarity, "hard" call hand-offs and minimal call
security.

Companies such as Sprint PCS have recently begun offering wireless
communication services using code division multiple
access (CDMA) techniques. CDMA is a digital spread-spectrum modulation
technique which digitizes wireless conversations
and tags them with special codes. The digitized data is spread across
the frequency band in a pseudo random pattern.
Receiving mobile phones are instructed to decipher only the data
corresponding to particular codes to reconstruct the signal.
CDMA networks offer advantages over TDMA networks including increased
network capacity, fewer dropped calls because
of better hand-off methods, improved voice clarity, improved privacy and
transmission security, and enhanced services such as
text messaging and data transmissions. However, until now, CDMA wireless
networks have not included dispatch mode
calling capabilities.

SUMMARY OF THE INVENTION

The present invention solves the above-described problems and provides a
distinct advance in the art of wireless
telecommunications networks. More particularly, the present invention
provides a method of providing dispatch mode calling
in a CDMA wireless network.

The method of the present invention is implemented by first defining at
least one talk-group consisting of a plurality of mobile
phones in a CDMA network operated by users who wish to communicate with
each other in a dispatch mode. To initiate a
dispatch mode call, an originating phone sends a dispatch mode call
request to the network. The dispatch mode call request
is validated, then the location of the destination mobile phones in the
talk-group is determined. Voice channels are then assigned
to the members of the talk-group and a group code or mask is sent to
each destination phone that enables the phones to
demodulate or decode communications from the originating phone. Voice
packets are received from the originating mobile
phone, encoded, and sent to the destination phones. The destination
mobile phones then decipher the encoded voice packets
with the received mask.

The present invention permits dispatch mode calling between a plurality
of users in a talk-group all located within the same
sector of a telecommunications cell without adversely affecting the
capacity of the cell. Specifically, the method assigns a first
Walsh code to the forward traffic channel for the originating mobile
phone that all mobiles listen to if they are located in the
same sector, and a second Walsh code to the forward traffic channel for
all other destination mobile phones that are located in
a different sector. Thus, a dispatch mode call between mobile phones in
the same sector of a telecommunications cell utilizes
the same network capacity as a conventional wireless call between only
two mobile phones.

The method of the present invention also provides dispatch mode calling
for members of a talk-group even when the members
are located in more than one telecommunications cell. This is achieved
by replicating the encoded voice packets and routing a
copy of the voice packets to each base station serving a cell in which
any destination mobile phone is located. For example, if
destination phones are located in three different telecommunications
cells, the present invention includes the step of replicating
the encoded voice packets and routing the voice packets to each of the
three base stations serving the three cells.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A preferred embodiment of the present invention is described in detail
below with reference to the attached drawing figures,
wherein:

FIG. 1 is a schematic diagram broadly illustrating a code division
multiple access wireless network configured to support
dispatch mode calling in accordance with a preferred embodiment of the
present invention.

FIG. 2 is a schematic diagram of one base station of the wireless
network.

FIG. 3 is a schematic diagram of a dispatch mode controller constructed
in accordance with a preferred embodiment of the
present invention.

FIG. 4 is a flow diagram generally illustrating the steps performed by
the wireless network when establishing a dispatch mode
call.

FIG. 5 is a schematic diagram illustrating the forward traffic channel
structure for dispatch mode calling in the present
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

System Architecture

Turning now to the drawing figures, an example of a wireless
telecommunications network 10 that may be used to implement a
preferred embodiment of the present invention is illustrated. The
illustrated architecture is shown for purposes of disclosing a
preferred embodiment and can be modified as a matter of design choice.
The wireless network is preferably a code division
multiple access (CDMA) PCS wireless intelligent network such as the PCS
network owned and operated by Sprint PCS. As
is well known in the art, the wireless network is coupled with a public
switched telephone network (PSTN) 12. The PSTN
refers to the entire local, long distance, and international landline
phone system used in the United States, which includes well
known components such as central office local exchange carriers (LECs)
and interexchange carriers (IXCs).

The wireless network 10 broadly includes a plurality of base stations
(BTSs) 14, 16, 18, a digital access and cross-connect
system (DACS) 20, a base station controller (BSC) 22, a mobile switching
center (MSC) 24, a signal transfer point (STP) 26,
a wireless intelligent network service control point (WIN SCP) 28, and a
home location register (HLR) 30, all interconnected
by signaling data links and trunk circuits as described below. In
accordance with the present invention, the wireless network
also includes a dispatch mode controller (DMC) 32 operable to provide
dispatch mode calling to mobile phones served by
the network.

The BTSs 14, 16, 18, which are well known in the art, provide wireless
communications to and from mobile phones and other
wireless devices. FIG. 2 illustrates one BTS 14 in more detail. The BTS
includes a plurality of transceivers 34, 36, 38 coupled
with one or more antennas that together provide wireless communications
within a telecommunications cell 40, which is
preferably subdivided into three sectors 42, 44, 46. The BTS serves a
plurality of mobile stations 48, 50, 52, 54, 56 such as
PCS/cellular phones located in the cell. The cell is preferably part of
a CDMA PCS telecommunications network such as the
Sprint PCS network described above. The preferred BTSs are operable to
control transmission and reception of CDMA PCS
traffic independently in the three sectors using selected ones of a
defined set of codes for each sector. The codes may include,
for example, Walsh codes. An example of a BTS that may be used with the
present invention is the Nortel CDMA Outdoor
1900 MHZ base station. Those skilled in the art will appreciate that the
wireless network may include numerous BTSs
positioned in telecommunications cells throughout the country.

Returning to FIG. 1, the DACS 20 is coupled with the BTSs 14, 16, 18
with signaling data links and trunk circuits 58, 59, 60
and is operable for routing and switching control messages between the
BTSs and the other components in the wireless
network. The BSC 22 is coupled with the DACS with signaling data links
and trunk circuits 61 and is operable to control
operation of the DACS and the BTSs 14, 16, 18. The BSC is basically a
high-capacity switch that provides total overview and
control of wireless functions supported by the network such as call
handoff control, cell configuration management, and BTS
and mobile phone power level management. The BSC multiplexes signals
from the BTSs into transmission signals that are sent
to the MSC 24. The BSC also routes network signals and calls from other
components of the wireless network to the
appropriate BTS for transmission to the mobile stations.

The MSC 24 is coupled with the BSC 22 and other base station controllers
with signaling data links and trunk circuits 62 and is
operable to coordinate the establishment of calls to and from the mobile
stations 48-56 and to handle transmission facilities
management, mobility management, and call processing. The MSC is also
connected with the PSTN 12 by signaling data links
and trunk circuits 64 to provide switching between the wireless network
and the PSTN.

The MSC 24 either includes an integrated visitor location register (VLR)
66 or is coupled with a stand-alone VLR. The VLR
includes a database that contains information relating to visiting
mobile phones that are roaming outside of their home service
area. When a mobile phone is roaming in a visiting service area, the
local provider in the visiting service area queries the HLR
30 through the STP 26 using Signaling System #7 (SS7) or other signaling
to retrieve information needed to verify the
legitimacy of the mobile phone and to obtain a profile of the features
associated with the mobile phone. The HLR responds to
the query by transferring the necessary data to the VLR. This
information is maintained in the VLR of the local provider as long
as the roaming mobile phone remains active within that coverage area.
The HLR also updates its own database to indicate the
current location of the roaming mobile phone so that it can divert calls
to the phone through the local provider in the visiting
service area. The querying process in the preferred wireless network is
accomplished via SS7 links using the STP and SCP as
described below.

The STP 26 is connected between the MSC 24 and the WIN SCP 28 by
signaling data links 68 and 70 and is operable to
route signaling messages therebetween. STPs are well known in the art
with an example being the DSC Megahub.

The WIN SCP 28, which is well known in the art, preferably uses TCAP
protocols to perform transaction processing for
wireless calls. However, other signaling systems or means to exchange
messages are equally applicable to the present invention.
The WIN SCP is coupled with the STP 26 to exchange signaling messages
with the MSC 24 and other mobile switching
centers. The WIN SCP also includes a plurality of databases for
providing intelligence and certain enhanced services to the
wireless network as described herein.

The HLR 30 may be a database residing on the WIN SCP 28 or may be a
stand-alone database servicing several SCPs. In
either case, the HLR includes a database containing subscriber data and
information used to identify a subscriber of the wireless
network and subscriber data relating to features and services available
to the subscriber. The HLR, which represents the
"home" database for subscribers, may, for example, contain a record for
each home subscriber that includes location
information, subscriber status, subscribed features, and directory
numbers. The HLR is used in conjunction with the VLR 66 as
described above to support mobility management features to which the
user has subscribed when that user is roaming outside
of his home area.

As is well known in the art, the MSC 24, STP 26, WIN SCP 28, HLR 30, and
VLR 66 all communicate via out of band
signaling, typically using SS7 or TCP/IP protocols to facilitate the
routing of calls through the wireless network. The signaling
allows the network elements to exchange information to more quickly and
efficiently route calls over the network.

In accordance with the present invention, the dispatch mode controller
(DMC) 32 is operatively coupled with the other
components of the wireless network 10 to provide dispatch mode calling
capabilities. As illustrated in FIG. 3, the DMC
broadly includes a front end processor (FEP) 72, a metro packet switch
(MPS) 74, and a dispatch application processor
(DAP) 76.

The FEP 72 serves as an interface for coupling the DMC 32 with other
components in the wireless network. In preferred
forms, the FEP is coupled with the MSC 24 with data links and trunk
circuits 78 to provide signaling and call routing
therebetween. The FEP is also coupled with the STP 26 by signaling data
links 80 to provide control signaling therebetween
and with the PSTN 12 by data links and trunk circuits 81 to provide
signaling and call routing therebetween.

The MPS 74 is coupled with the FEP 72 and includes a digital packet
switch 82 and one or more packet duplicators 84. The
MPS is operable to receive voice packets from the BTSs 14, 16, 18 via
the MSC 24 to transport voice packets to the BTSs,
and to route control information between the DAP 76 and the BTSs during
a dispatch call.

The DAP 76 is coupled with the FEP 72 and includes a processor 86 and at
least one database 88. The DAP maintains and
tracks subscriber provisioning and mobility information for dispatch
mode calls and is responsible for the overall coordination
and control of dispatch mode services.

Dispatch Mode Operation

Setup

To permit dispatch mode calling in the above-described wireless network
10, an operator must first define or create
"talk-groups" of mobile phone users who wish to at least occasionally
communicate in dispatch mode. For example, one
talk-group may consist of the mobile phones 48, 50, 52, 54, 56
illustrated in FIG. 2. Subsets of these mobile phones and/or
additional phones may be part of another talk-group. This way, a user of
any one of the mobile phones may be a part of a
number of different talk-groups.

A talk group may include only two mobile phones or many phones. The
mobile phones in a talk group may all be positioned
within a "local dispatch location area" serviced by a single BTS, a
"selected service area" served by several base stations
connected to the same MSC, or a "Wide area" served by base stations
positioned throughout the country serviced by a
plurality of different MSCs.

Once the talk-groups have been defined, information relating thereto
such as the identification of each talk-group and an
identification number for each phone in a talk-group is stored in the
database 88 of the DAP 76. For example, a lookup table
indicating that talk-group number 1 consists of the mobile phones 48,
50, 52, 54, 56 illustrated in FIG. 2, talk-group number 2
consists of other mobile phones, etc. is stored in the database. The
mobile identification number (MIN) for each mobile phone
in a talk-group is also stored in the database or in an external
database that can be accessed by the DAP.

The mobile phones 48, 50, 52, 54, 56 must also be configured to permit
dispatch mode calling. Specifically, each phone must
be configured to allow a user to indicate that he or she wishes to
initiate a dispatch mode call rather than a conventional call.
The phones may be equipped with dedicated dispatch mode keys for this
purpose or may be programmed to recognize an
entered code that triggers a dispatch mode call. Each mobile phone is
also preferably programmed to include a menu that
allows a user to select which talk-group he or she wishes to communicate
with. For example, if one of the mobile phones is a
part of three different talk-groups, the mobile phone must be programmed
to allow the user to select with which talk-group he
or she wishes to communicate.

Operation

Before describing the dispatch mode operation of the network 10, it is
helpful to understand a few principles about the normal,
non-dispatch mode operation of the network. To send a call to one of the
mobile phones illustrated in FIG. 2, the BTS 14
transmits in the cell a long code to which the mobile identification
number (MIN) of the phone is attached. Although all the
phones in the cell receive the transmission, only the phone identified
by the transmitted MIN can decode it.

As generally illustrated in FIG. 5, in the dispatch mode operation of
the network, a group code that identifies a group of
mobile phones is substituted forthe MIN of a specific phone in the
forward channel. This permits every phone in the group to
decode a dispatch mode transmission.

In more detail, a preferred embodiment of the dispatch mode operation of
the present invention can be best understood with
reference to FIG. 2, which illustrates a talk-group of mobile phones 48,
50, 52, 54, 56 all positioned within a "local dispatch
location area" such as a company campus served by a single BTS 14.
Assume for this example that the mobile phone 48
(referred to herein as the "originating mobile phone") wishes to
initiate a dispatch call to the mobile phones 50, 52, 54, 56
(referred to herein as the "destination mobile phones"). FIG. 4
generally illustrates the steps in completing such a call. The user
of the originating mobile phone initiates a dispatch call by selecting a
talk-group and then striking a dispatch mode call request
key on his or her phone or entering a dispatch request code as described
above. The mobile phone responds by transmitting a
dispatch mode call request including an identification of the requested
talkgroup to the originating BTS via a control channel as
generally depicted in step 400. The BTS determines if it has available
capacity to handle the call and ques the call if it does not.

If the BTS 14 has capacity available to handle the call, it routes the
call request and talk-group identifier to the DACS 20. The
DACS connects the request to the MSC 24, which recognizes that the call
request is for a dispatch mode call and forwards
the request to the DMC 32 as depicted in step 402. If the call request
was not for a dispatch mode call, the MSC would
handle the call request in a conventional manner.

Once the DMC 32 receives the dispatch mode call request, it validates
the request and coordinates the dispatch mode call.
Specifically, the processor 86 of the DAP 76 first accesses the database
88 to identify the members of the requested
talk-group as depicted in step 404. The DAP then determines the status
and current location of each destination mobile phone
in the talk-group by sending a status and location query to the HLR 30
via the STP 26. The HLR responds with current cell
and sector information for each mobile phone as depicted in steps 406
and 408.

If at least one of the destination mobile phones in the talk-group is
available to receive the dispatch call, the DAP 76 signals
the BTS 14 via the MSC 24 to provide voice channels to the originating
mobile phone 48 and the destination phones 50, 52,
54, 56 that are available as depicted in step 410. Destination phones
that are not initially available may be added to a dispatch
call as they become available as they will be able to decode the signal.

While setting up voice channels, the DMC 32 instructs the BTS 14 to
assign a first Walsh code to the forward traffic channel
for the originating mobile phone 48 and a second Walsh code to the
forward traffic channel for all destination mobile phones in
one sector of the cell. If the destination mobile phones are in more
than one sector as illustrated in FIG. 2, the DMC and BTS
assign a separate Walsh code for each sector in which a destination
phone is located.

During call setup, the DMC 32 and BTS also send each active member of a
talk-group a group code that serves as a mask to
allow all of the mobile phones in the group to demodulate or decode the
dispatch mode call. This enables all members of a
talk-group to participate in a dispatch mode call.

When the user of the originating mobile phone 48 begins to talk, the
phone sends digitized voice packets to the BTS 14, which
forwards them to the DMC 32 via the MSC 24 for processing. The packet
switch 82 and packet duplicator 84 replicate the
voice packets and distribute a copy to each cell in which one of the
destination mobile phones is currently located as depicted
in steps 412 and 414. In the example illustrated in FIG. 2, only one
copy is required because all of the destination phones are in
the same cell. However, if the DAP 76 determines that destination phones
are in three different cells, the packet switch and
packet duplicator make three copies of the voice packets and send a copy
to the BTS serving each cell.

Similarly, the DAP 76 may determine that one of the destination mobile
phones is currently roaming outside of the home
coverage area of the MSC 24. Then, the DMC 32 would route the voice
packets to the PSTN 12 for routing to the MSC and
BTS in which the destination mobile phone in the talk group is currently
roaming.

Once the BTS or BTSs receive the copied voice packets from the DMC 32,
they transmit the voice packets in the cell or cells
to the mobile phones in a conventional manner as depicted in step 416.
Because all of the phones in a talk-group have been
provided with the mask necessary to demodulate the transmitted voice
packets, each destination mobile phone can decode the
voice packets so that all the destination mobile phones can communicate
with the originating mobile phone.

To regulate the power of mobile phones in a talk-group that are in the
same cell, the BTS uses the closed loop power control
to set the transmitted power of the mobile group that is usually in
close by locations to an average value that will correspond to
an acceptable FER value.

In addition to voice communications between members of a talk-group, the
DMC 32 may also provide for the sending of call
alerts, emergency calls, and status inquiries between members of a
talk-group.

Although the invention has been described with reference to the
preferred embodiment illustrated in the attached drawing
figures, it is noted that equivalents may be employed and substitutions
made herein without departing from the scope of the
invention as recited in the claims.