Note: Descriptions are shown in the official language in which they were submitted.
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CALL SET-UP ON 800 MHz ANALOG VOICE CHANNEL
FROM 1900 MHz DIGITAL CONTROL CHANNEL
Field of the Invention
This invention relates to digital cellular systems, but more particularly to a
method of setting up a call on an 800 MHz analog voice channel from a 1900 MHz
digital control or traffic channel.
Background of the Invention
In 1994, the United States Federal Communications Commission auctioned
licences to support PCS 1.9 gigaHertz (GHz) radio frequency spectrum for PCS
services in North America. PCS 1900 as it is called, is a variation of the
International
Global System for Mobile Communications (GSM) Standard. It is one of several
new
standards for North American PCS.
In order to facilitate the deployment of PCS 1900, equipment manufacturers
and PCS service providers are working together to provide a suitable
transition of one
system to another by providing multi-mode mobile stations. The use of multi-
mode
mobile stations enables a service provider to overcome limitations inherent in
the
operating frequency of conventional PCS 1900 systems. In particular,
conventional 1.9
GHz systems transmit information only about half as far as existing 800 MHz
cellular
systems because they incur more radio frequency transmission losses. Because
of their
shorter wavelengths, mobile radios are therefore limited to smaller service
areas for
collecting incoming signals. This reduction in range results in a substantial
cost
disadvantage for operators of conventional PCS 1900 systems. A service
provider of
PCS 1900 systems would have to deploy four times as many base stations to
cover the
same area as an existing 800 MHz cellular system. Long range and wide area of
coverage is especially critical for urban areas as well as in suburban and
rural
applications.
By providing multi-mode or multi-standard mobile radios, a service provider
can provide subscribers with the option of using one cellular standard or the
other.
The 1900 MHz air Interface Standard which is described in J-STD-011 enables
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the Mobile Station (MS) operator to request a change in their MS service code.
The
MS service code provides the cellular network with an indication of the
operator's
preference for analog or digital traffic channels. Although the standard
allows a user
or subscriber to use the MS service code to request an analog voice channel
when
served on a 1900 MHz digital traffic channel (DTC), systems that support only
1900
MHz cells cannot provide a handoff to an analog voice channel which might have
been
requested by a MS user. Although subscribers can continue using their handsets
for
voice communication, the ability to offer handoffs between a digital traffic
channel and
an analog voice channel becomes particularly important if a subscriber
operates on the
1900 MHz standard and requires the use of the handset for data services using
FSK
modulation. Unfortunately, data services using FSK modulation is not currently
possible on the 1900 MHz cellular standard. A user that requires the use of a
data
terminal such as a modem connected to a computer or a facsimile machine (other
than
G3 class) which operates on FSK modulation, will require access to an analog
voice
channel in order to complete the call with the data terminal.
Accordingly, a need exists for a system and method for setting up a call on an
analog voice channel when the subscriber's mobile station is being served on a
DCCH
or DTC.
Summary of the Invention
It is therefore an object of the present invention to provide a method of
setting-
up a call on an analog voice channel for users operating on a digital control
channel.
Another object of the present invention is to provide a method of handing off
a call for a mobile station operating on a digital traffic channel to an
analog voice
channel.
Another object of the present invention is to provide a method of setting-up a
call on an analog voice channel operating on an hyperband cell which is
overlaid over
a cell serving a mobile station on a DCCH or DTC.
Another object of the present invention is to provide a method of setting-up a
call on an analog voice channel operating on an hyperband cell which is
collocated with
a cell serving a mobile station on a DCCH or DTC.
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Yet another objet of the present invention is to provide a method of setting-
up
a call on a candidate analog voice channel selected by a mobile switching
controller
from a list of candidates detected by the mobile station.
According to an aspect of the present invention , there is provided a cellular
network having a first cluster of base stations serving a first group of
mobile stations
over digital RF channels and a second cluster of base stations, collocated
with said first
cluster, for serving a second group of mobile stations over analog RF
channels, wherein
some of said base stations in said second cluster have cells which overlap
with cells of
said first cluster, a method of setting up a call on an analog voice channel
of a base
station of said second cluster while a mobile station is being served a
digital control
channel of a base station in said first cluster, comprising the steps of:
receiving at a serving base station of said first cluster, a mobile station
service
code indicative of an analog voice channel preference;
forwarding said mobile station service code to a mobile switching controller;
instructing said serving base station to set up a call with said mobile
station on
a digital traffic channel;
determining if a cell of a base station from said second cluster is
overlapping a
cell which corresponds to said serving base station;
if an overlapping cell is available, determining if an analog voice channel is
available on said overlapping cell; and
if an analog voice channel is available, instructing said serving base station
and
said mobile station to handoff to said analog voice channel on said
overlapping cell.
According to another aspect of the present invention, there is provided a
cellular network having a first cluster of base stations serving a first group
of mobile
stations over digital RF channels and a second cluster of base stations,
collocated with
said first cluster, for serving a second group of mobile stations over analog
RF
channels, wherein some of said base stations in said second cluster have cells
which
overlap with cells of said first cluster, a method of setting up a call on an
analog voice
channel of a base station of said second cluster while a mobile station is
being served
a digital traffic channel of a base station in said first cluster, comprising
the steps of:
receiving at a serving base station a mobile station service code indicative
that
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said mobile station is requesting an analog voice channel;
forwarding said mobile station service code to a mobile switching controller;
determining if a cell of a base station from said second cluster is
overlapping a
cell which corresponds to said serving base station;
if an overlapping cell is available, determining if an analog voice channel is
available on said overlapping cell; and
if an analog voice channel is available, instructing said mobile station and
said
serving base station to handoff to said analog voice channel from said digital
traffic
channel.
Brief Description of the Drawings
Fig. I is a block diagram illustrating cells operating on one standard and
which
are collocated with cells operating on another standard;
Fig. 2 is a block diagram illustrating a data terminal connected to a mobile
station;
Figs. 3a and 3b illustrate a flow diagram for call set-up from a digital
control
channel to an analog voice channel; and
Fig. 4a and 4b illustrate a flow diagram of a user-initiated handoff from a
digital
traffic channel to an analog voice channel.
Description of the Preferred Embodiment
Referring now to Fig. 1, we have shown a cellular network wherein a cellular
service provider is equipped with cell sites operating on two separate
standards. That
is, wherein cells of one standard are collocated with cells of another
standard. It should
be noted that although the method and system of the present invention is
directed to
a network having 1900 MHz and 800 MHz cell sites, the invention is equally
applicable
to cellular networks operating on other standards wherein subscribers being
served on
a DCCH or DTC require access to an Analog Voice Channel (AVC). In order to
allow
for sufficient radio coverage at handoff from a 1900 MHz DTC to an 800 MHz AVC
the collocated hyperband cell concept is introduced.
The collocated hyperband cell refers to a collection of 800 MHz analog voice
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channels, served by a first cluster of cells, which geographically cover
approximately
the same area as a collection of 1900 MHz digital traffic channel, served by a
second
cluster of cells, but which are logically separated from the 1900 MHz channels
(i.e. the
1900 MHz channels and the 800 MIIz channel belong to different cells).
In the illustration of Fig. 1, a cellular service provider has a number of
collocated cell sites. In Fig. 1, cell site A and cell site B are neighbouring
cell sites
operating on one standard, whereas cell site A' and cell site B', also
neighbours,
operate on another standard. The systems operating the cell sites provide
geographically the same coverage, but are independently operated. Although as
a
system, each provide generally the same coverage, cells of one system do not
necessarily overlap. This is caused by the fact that cell sites operating on
different
frequencies, will likely have different coverage. As shown in Fig. 1, although
cell A'
overlaps cell A, cell B' does not overlap cell B. Therefore, although cells of
one
standard have collocated cells in the other standard, not all cells of one
standard will
completely overlap the cells in the other standard. The knowledge of whether a
target
cell overlaps a serving cell is important in determining whether a MS can be
set-up or
can handed off from a cell of one standard to a cell of another standard. In
Fig. 1, cell
A' is considered a collocated hyperband overlaid cell because it completely
overlaps
cell A. On the other hand, cell B' is considered a collocated hyperband
neighbour since
it only partly overlaps cell B.
In the example of Fig. 1, it will be assumed that cells A and B, which are
serviced by base stations BSA and base station BSB, provide digital control
and traffic
channels to MS, and MS2, respectively. Cell A' and B' on the other hand
provide
analog voice channels. Therefore, mobile station MS1 can operate on digital
cellular
service from cell A and request a call set-up or handoff to an analog voice
channel of
collocated hyperband overlaid cell A'. On the other hand, MS2 can operate on
digital
cellular service from cell B, but would only be able to request a call set-up
or handoff
to an analog voice channel of collocated hyperband neighbouring cell A' since
cell B'
is not in range. Other mobile stations will of course access digital or analog
control
channels in accordance with relevant air interfaces supported by the mobile
station
terminal.
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In the case of the 800 MHz standard, the interface is dictated by the EIA/TIA
IS 54 standard, whereas the 1900 MHz air interface is dictated by the 3-STD-
011
standard. It should be noted that under IS-54, a mobile station can either
access an
analog or a digital 800 MHz control channel. A mobile switching controller
(MSC)
22 can provide service to each cell site and subscriber access to the Public
Switched
Telephone Network (PSTN) 11. The MSC 22 has a list of all cells in the area
near the
mobile station. A collocated hyperband cell does not have to be defined, by
command,
as part of the list of neighbours measured by the MS during a mobile assisted
Handoff
process since it is automatically considered as a neighbour when a Handoff
Request for
an AVC is received in the MSC 22.
As shown in Fig. 2, if a mobile station subscriber 24 desires to make use of a
data terminal 25 such as a modem-connected computer or facsimile machine, the
handset can be provided with an interface cord 26 having a PCMCIA card that
enables
the user to connect a standard RJ-11 jack and cord to the mobile station
handset 24.
However, as indicated earlier, if the data terminal operates with FSK
modulation, in
order to make use of the data terminal 25, the subscriber would have to either
indicate
a preference for analog service or initiate a handoff from a digital control
channel to an
analog voice channel.
Referring now to Figs. 3a, 3b, 4a and 4b, we have shown flow diagrams
illustrating two call set-up scenarios.
In the call set-up of Figs. 3a and 3b, a call set-up for a digital cellular
subscriber to an analog voice channel is attempted.
In this example, a dual mode (analog/digital) dual band (1900MHz/800MHz)
mobile station (MS) camps or is in a standby mode on a Digital Control Channel
(DCCH). While on a DCCH 30, the user can either originate a request 31 for an
analog voice channel or receive a page from a serving base station indicating
an
incoming call. If the subscriber's service code indicates "analog only"
service, the
subscriber's origination message or the Page Response is sent from the MS and
received 32 at the BS. The subscriber's origination message or the Page
Response is
then transferred 33 to the MSC.
The origination or Page Response is processed by the MSC. If the service code
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indicates service on a digital channel 35, the call can proceed as required
36. If it is
determined that the service code indicates a request for an analog voice
channel only,
the call has to be initially set-up on a digital traffic channel, since the MS
is on stand-by
on a DCCH. The call is then returned to the base station to initiate call set-
up from a
DCCH 36.
If a collocated hyperband neighbouring cell which is defined as overlaid is
available 38, then the base station waits for an Initial Traffic Channel
Confirmation
message 39 and the call can proceed to step 36 above.
If a collocated hyperband neighbour is defined 40 for this cell, then the
collocated hyperband channel is included 41 as one neighbour in the DTC
assignment
so that the BS considers it for sending of the Hyperband Measurement Order
towards
the MS.
Note that the BS can be instructed to initiate the Handoff Process at DTC
designation or it can be initiated in the MSC itself without waiting for the
BS to trigger
it.
If a different hyperband neighbour is defined as the serving cell 42, then an
Hyperband measurement order is sent 43. If not defined, then a measurement
order is
sent 44. If candidate channels are provided 45, then the BS selects the best
channels
to handoff to 46. The MS is then instructed to tune to the selected AVC 47.
Once the Mobile Station's presence is verified in the candidate cell, the MS
is
instructed to handoff to the verified candidate channel. Optionally, the
channel quality
can be verified by the mobile station.
The flow diagrams of Figs. 4a and 4b are directed to a call scenario wherein a
user initiates a handoff from a DTC of one cellular service to AVC of another
cellular
service.
In this call scenario, the dual mode (analog/digital) dual band
(1900MHz/800MHz) mobile station (MS) is currently being served by a Digital
Traffic
Channel (DTC) 50. While in this state, the user requests an AVC through the
Service
Request message.
The request message is received in the BS 51 and transferred 52 to the MSC.
Optionally it could be required to perform measurements prior to the selection
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of the AVC and if this is the case, the MS could be instructed to measure on
the
collocated hyperband channel. If a collocated hyperband neighbour defined as
overlaid
is available 53, then the call is transferred to the MSC for processing.
If Collocated Channel Measurement is part of the Neighbour List 54, then the
call is transferred to the MSC for processing. Otherwise, a request is made to
the MS
55 to measure on the Collocated Hyperband Channel by updating the list of
neighbour
"on the fly". That is, while the mobile station is on a DTC.
The BS selects 56 the best channels to handoff out of the allocated candidates
to the serving cell and transfers them to the MSC.
Once the MSs presence is verified 57 in the candidate cell the MS is
instructed
to select an AVC 58 and to handoff 59 to the verified candidate.
