Note: Descriptions are shown in the official language in which they were submitted.
CA 02276195 1999-07-14
ENHANCED SLEEP MODE IN RADIOCOMMUNICATION SYSTEMS
This application is related to U.S. Patent 5,603,081 which issued February 1
l,
1997.
BACKGROUND
Applicant's invention relates generally to radiocommunication systems and more
particularly to radiocommunication systems in which evaluations of remote unit
servers
for potential control channel reselection are performed.
The growth of commercial radiocommunications and, in particular, the explosive
growth of cellular radiotelephone systems have compelled system designers to
search for
ways to increase system capacity without reducing communication quality beyond
consumer tolerance thresholds. One way to increase capacity is to use digital
communication and multiple access techniques such as TDMA, in which several
users are
assigned respective time slots on a single radio carrier frequency.
In North America, these features are currently provided by a digital cellular
radiotelephone system called the digital advanced mobile phone service (D-
AMPS), some
of the characteristics of which are specified in the interim standard IS-54B,
"Dual-Mode
Mobile Station-Base Station Compatibility Standard", published by the
Electronic
2 o Industries Association and Telecommunications Industry Association
(EIA/TIA).
Because of a large existing consumer base of equipment operating only in the
analog
domain with frequency-division multiple access (FDMA), IS-54B is a dual-mode
(analog
and digital) standard, providing for analog compatibility in tandem with
digital
communication capability. For example, the IS-54B standard provides for both
FDMA
2 5 analog voice channels (AVC) and TDMA digital traffic channels (DTC), and
the system
operator can dynamically replace one type with the other to accommodate
fluctuating
traffic patterns among analog and digital users. The AVCs and DTCs are
implemented by
frequency modulating radio carrier signals, which have frequencies near 800
megahertz (MHz) such that each radio channel has a spectral width of 30
kilohertz (KHz).
3 0 In a TDMA cellular radiotelephone system, each radio channel is divided
into a
series of time slots, each of which contains a burst of information from a
data source, e.g.,
a digitally encoded portion of a voice conversation. The time slots are
grouped into
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CA 02276195 1999-07-14
successive TDMA frames having a predetermined duration. The number of time
slots in
each TDMA frame is related to the number of different users that can
simultaneously
share the radio channel. If each slot in a TDMA frame is assigned to a
different user, the
duration of a TDMA frame is the minimum amount of time between successive time
slots
assigned to the same user.
The successive time slots assigned to the same user, which are usually not
consecutive time slots on the radio Garner, constitute the user's digital
traffic channel,
which may be considered a logical channel assigned to the user. As described
in more
detail below, digital control channels (DCCs) can also be provided for
communicating
control signals, and such a DCC is a logical channel formed by a succession of
usually
non-consecutive time slots on the radio Garner.
According to IS-54B, each TDMA frame consists of six consecutive time slots
and has a duration of 40 milliseconds (msec). Thus, each radio channel can
carry from
three to six DTCs (e.g., three to six telephone conversations), depending on
the source
rates of the speech coder/decoders (codecs) used to digitally encode the
conversations.
Such speech codecs can operate at either full-rate or half rate, with full-
rate codecs being
expected to be used until half rate codecs that produce acceptable speech
quality are
developed. A full-rate DTC requires twice as many time slots in a given time
period as a
half rate DTC, and in IS-54B, each radio channel can carry up to three full-
rate DTCs or
2 0 up to six half rate DTCs. Each full-rate DTC uses two slots of each TDMA
frame, i.e.,
the first and fourth, second and fifth, or third and sixth of a TDMA frame's
six slots. Each
half rate DTC uses one time slot of each TDMA frame. During each DTC time
slot, 324
bits are transmitted, of which the major portion, 260 bits, is due to the
speech output of
the coded, including bits due to error correction coding of the speech output,
and the
2 5 remaining bits are used for guard times and overhead signalling for
purposes such as
synchronization.
It can be seen that the TDMA cellular system operates in a buffer-and-burst,
or
discontinuous-transmission, mode: each mobile station transmits (and receives)
only
during its assigned time slots. At full rate, for example, a mobile station
might transmit
3 0 during slot 1, receive during slot 2, idle during slot 3, transmit during
slot 4, receive
during slot 5, and idle during slot 6, and then repeat the cycle during
succeeding TDMA
frames. Therefore, the mobile station, which may be battery-powered, can be
switched
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CA 02276195 1999-07-14
off, or sleep, to save power during the time slots when it is neither
transmitting nor
receiving. In the IS-54B system in which the mobile does not transmit and
receive
simultaneously, a mobile can sleep for periods of at most about 27 msec (four
slots) for a
half rate DTC and about 7 msec (one slot) for a full-rate DTC.
In addition to voice or traffic channels, cellular radiocommunication systems
also
provide paging/access, or control, channels for carrying call-setup messages
between base
stations and mobile stations. According to IS-54B, for example, there are
twenty-one
dedicated analog control channels (ACCs), which have predetermined fixed
frequencies
located near 800 MHz. Two frequency bands, each about 25-MHz wide (the A- and
B-bands), are provided for transmission and reception. The AVCs and DTCs are
also
located within the A- and B-bands. Since these ACCs are always found at the
same
frequencies, they can be readily located and monitored by the mobile stations.
It will be understood that a communication system that uses ACCs has a number
of deficiencies. For example, the format of the forward analog control channel
specified
in IS-54B is largely inflexible and not conducive to the objectives of modern
cellular
telephony, including the extension of mobile station battery life. In
particular, the time
interval between transmission of certain broadcast messages is fixed and the
order in
which messages are handled is also rigid. Also, mobile stations are required
to re-read
messages that may not have changed, wasting battery power. These deficiencies
can be
2 0 remedied by providing a DCC, one example of which is described in U.S.
Patent 5,404,
355 entitled "Digital Control Channel", which issued April 4, 1995. Using such
DCCs,
each IS54B radio channel can carry DTCs only, DCCs only, or a mixture of both
DTCs
and DCCs. Within the IS-54B framework, each radio carrier frequency can have
up to
three full-rate DTCs/DCCs, or six half rate DTCs/DCCs, or any combination in-
between,
2 5 for example, one full-rate and four half rate DTCs/DCCs. As described in
this
application, a DCC in accordance with Applicant's invention provides a further
increase
in fimctionality.
In general, however, the transmission rate of the DCC need not coincide with
the
half rate and full-rate specified in IS-54B, and the length of the DCC slots
may not be
3 0 uniform and may not coincide with the length of the DTC slots. The DCC may
be defined
on an IS-54B radio channel and may consist, for example, of every n-th slot in
the stream
of consecutive TDMA slots. In this case, the length of each DCC slot may or
may not be
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CA 02276195 1999-07-14
equal to 6.67 msec, which is the length of a DTC slot according to IS-54B.
Alternatively
(and without limitation on other possible alternatives), these DCC slots may
be defined in
other ways known to one skilled in the art.
Also, U.S. Patent 5,404,355 also shows how a DCC may be defined alongside the
DTCs specified in IS-54B. For example, a half rate DCC could occupy one slot
and a
full-rate DCC could occupy two slots out of the six slots in each TDMA frame.
For
additional DCC capacity, additional half rate or full-rate DCCs could replace
DTCs. In
general, the transmission rate of a DCC need not coincide with the halfrate
and full-rate
specified in IS-54B, and the length of the DCC time slots need not be uniform
and need
not coincide with the length of the DTC time slots.
FIG. 1 shows a general example of a forward DCC configured as a succession of
time slots 1, 2, .. ., N,... belonging to a particular DCC. These DCC slots
may be defined
on a radio channel such as that specified by IS-54B, and may consist, for
example, of
every n-th slot in a series of N consecutive slots. Each DCC slot has a
duration that may
or may not be 6.67 msec, which is the length of a DTC slot according to the IS-
54B
standard. The DCC slots shown in FIG. 1 are organized into superframes (SF),
and each
super&ame includes a number of logical channels that carry different kinds of
information. One or more DCC slots may be allocated to each logical channel in
the
superframe.
2 0 FIG. 1 also shows an exemplary downlink superframe, which includes at
least
three logical channels: a broadcast control channel (BCCH) including six
successive slots
for overhead messages; a paging channel (PCH) including one slot for paging
messages;
and an access response channel (ARCH) including one slot for channel
assignment and
other messages. The remaining time slots in the exemplary superframe of FIG. 1
2 5 may be dedicated to other logical channels, such as additional paging
channels PCH or
other channels. Since the number of mobile stations is usually much greater
than the
number of slots in the superframe, each paging slot is used for paging several
mobile
stations that share some unique characteristic, e.g., the last digit of the
MIN.
For purposes of efficient sleep mode operation and fast cell selection, the
BCCH
3 0 may be divided into a number of sub-channels. U.S. Patent 5,404,355
discloses a BCCH
structure that allows the mobile station to read a minimum amount of
information when it
is switched on (when it locks onto a DCC) before being able to access the
system (place
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CA 02276195 1999-07-14
or receive a call). After being switched on, an idle mobile station needs to
regularly
monitor only its assigned PCH slots (usually one in each superframe); the
mobile can
sleep during other slots. The ratio of the mobile's time spent reading paging
messages and
its time spent asleep is controllable and represents a tradeoff between call-
set-up delay
and power consumption. As such hybrid analog/digital systems mature, the
number of
analog users should diminish and the number of digital users should increase
until all of
the analog voice and control channels are replaced by digital traffic and
control channels.
When that occurs, the current dual-mode mobile terminals can be replaced by
less
expensive digital-only mobile units, which would be unable to scan the ACCs
currently
provided in the IS-54B system. One conventional radiocommunication system used
in
Europe, known as GSM, is already an all-digital system, in which 200-KHz-wide
radio
channels are located near 900 MHz. Each GSM radio channel has a gross data
rate of 270
kilobits per second and is divided into eight full-rate traffic channels (each
traffic time
slot carrying 116 encrypted bits).
Digital control and traffic channels are also desirable for other reasons as
described in the aforementioned U.S. Patent 5,603,081, which issued February
11, 1997.
For example, they support longer sleep periods for the mobile units, which
results in
longer battery life. Although IS-54B provides for digital traffic channels,
more flexibility
is desirable in using digital control channels having expanded functionality
to optimize
2 o system capacity and to support hierarchical cell structures, i.e.,
structures of macrocells,
microcells, picocells, etc. The term "macrocell" generally refers to a cell
having a size
comparable to the sizes of cells in a conventional cellular telephone system
(e.g., a radius
of at least about 1 kilometer), and the terms "microcell" and "picocell"
generally refer to
progressively smaller cells. For example, a microcell might cover a public
indoor or
2 5 outdoor area, e.g., a convention center or a busy street, and a picocell
might cover an
office corndor or a floor of a high-rise building. From a radio coverage
perspective,
macrocells, microcells, and picocells may be distinct from one another or may
overlap
one another to handle different traffic patterns or radio environments. Each
of these types
of cells has a base station which transmits at least one control channel.
Thus, a number of
3 o neighboring control channels are present for a mobile or remote unit to
evaluate as a
possible replacement for the current serving control channel to which it is
locked.
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Accordingly, both ACCs and DCCs will be periodically evaluated by the mobile
station for possible control channel reselection. Conventionally, for example,
when in an
idle state (i.e., switched on but not making or receiving a call), a mobile
station in an
IS-54B system tunes to and then regularly monitors the strongest control
channel
(generally, the control channel of the cell in which the mobile station is
located at that
moment) and may receive or initiate a call through the corresponding base
station. When
moving between cells while in the idle state, the mobile station will
eventually "lose"
radio connection on the control channel of the "old" cell and tune to the
control channel
of the "new" cell. The initial tuning and subsequent re-tuning to control
channels are both
accomplished automatically by scanning all the available control channels at
their known
frequencies to find the "best" control channel. The terms "scan" or "scanning"
as used in
this document, can refer to, for example, signal strength measurement, actual
signal
decoding, or any other method of evaluating a signal.
When a control channel with good reception quality is found, the mobile
station
remains tuned to this channel until the quality deteriorates again. In this
way, mobile
stations stay "in touch" with the system. The analog (non-slotted) control
channels
specified in IS-54B require the mobile stations to remain continuously (or at
least 50% of
the time) "awake" in the idle state, at least to the extent that they must
keep their receivers
switched on. Thus, these conventional systems typically evaluate candidate
control
2 0 channels for reselection purposes at some predetermined, fixed intervals.
According to a more recent innovation in cell reselection disclosed in U.S.
Patent
5,353,332 to Raith and Muller, each control channel in each cell is configured
to
broadcast information about the presence, if any, of other cells and the
characteristics of
those cells including minimum quality criteria, power requirements, etc.
Typically,
2 5 information about the presence of other cells is broadcast about
neighboring cells. For
instance, a neighboring cell may be adjacent to, overlapping, or non-
contiguous from the
broadcasting cell. A mobile periodically scans during idle mode the
neighboring control
channels in the coverage area that the mobile is located in to determine which
cell it
should be locked to. Each control channel includes a neighbor list. The
neighbor list
3 0 identifies other control channels which mobiles locked to that control
channel should
periodically evaluate. Thus, a mobile may continuously select cells to be
locked to based
on the existing location of the mobile and quality criteria (e.g., received
signal strength)
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associated with the cells. The cell to which the mobile may be locked is the
cell in which
the mobile satisfies the quality criteria associated with the cell.
While in the idle state, and in addition to evaluating control channels as
potential
reselection candidates, a mobile station must monitor the control channel for
paging
messages addressed to it. For example, when an ordinary telephone (land-line)
subscriber
calls a mobile subscriber, the call is directed from the public switched
telephone network
(PSTN) to a mobile switching center (MSC) that analyzes the dialed number. If
the dialed
number is validated, the MSC requests some or all of a number of radio base
stations to
page the called mobile station by transmitting over their respective control
channels
paging messages that contain the mobile identification number (MII~ of the
called
mobile station. Each idle mobile station receiving a paging message compares
the
received MIN with its own stored MIN. The mobile station with the matching
stored MIN
transmits a page response over the particular control channel to the base
station, which
forwards the page response to the MSC.
Upon receiving the page response, the MSC selects an AVC or a DTC available to
the base station that received the page response, switches on a corresponding
radio
transceiver in that base station, and causes that base station to send a
message via the
control channel to the called mobile station that instructs the called mobile
station to tune
to the selected voice or traffic channel. A through-connection-for the call is
established
2 0 once the mobile station has tuned to the selected AVC or DTC.
As noted above, one of the goals of a digital cellular system is to increase
the
user's "talk time", i.e., the battery life of the mobile station. To this end,
U.S. Patent
5,404,355 discloses a digital forward control channel (base station to mobile
station) that
can carry the types of messages specified for current analog forward control
channels
2 5 (FOCCs), but in a format which allows an idle mobile station to read
overhead messages
when locking onto the FOCC and thereafter only when the information has
changed; the
mobile sleeps at all other times. In such a system, some types of messages are
broadcast
by the base stations more frequently than other types, and mobile stations
need not read
every message broadcast.
3 0 While the innovative system described in the aforementioned patent
application
provides many benefits in terms of efficient sleep mode operation with respect
to paging,
additional features may be advantageous. For example, it would be advantageous
to
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CA 02276195 1999-07-14
provide the capability to temporarily reassign a mobile or remote station to a
different
paging class to provide the flexibility necessary for changing system
conditions in a cell
specific manner.
SUMMARY
Accordingly to exemplary embodiments of the present invention, mobile or other
remote units are able to sleep for longer periods of time, and battery life is
extended, by
optimizing the periodicity at which the remote unit measures control channels
on a
neighboring list to determine if a new control channel should replace the
serving control
channel. According to other exemplary embodiments of the present invention, a
paging
frame class which has been assigned to the remote unit can be modified so that
the remote
unit monitors the paging channel at intervals which are more battery
conservative based
on current communication conditions.
Therefore, in accordance with a first aspect of the present invention there
is provided a method for commanding a remote station to listen for pages in a
radiocommunication system comprising the steps of: providing a plurality of
paging
frame classes, each class having a different repeat period for listening for
paging
messages; assigning one of the plurality of paging frame classes to the remote
station
when the remote station registers with the system; and transmitting, from the
system, a
2 0 paging frame modifier which commands the mobile to either use the assigned
paging
frame class or to use a different paging frame class.
In accordance with a second aspect of the invention there is provided a base
station comprising: a transmitter for transmitting supervisory data messages
on a control
channel and for providing neighbouring lists to remote units; the supervisory
data
2 5 messages including an instruction for remote stations to modify a default
frequency rate
at which at least one control channel identified in the neighbouring lists is
to be scanned.
In accordance with a fiu-ther aspect of the invention there is provided a
method for
controlling scanning of a plurality of control channels by a remote station
comprising the
steps of providing a default scanning frequency for the control channels; and
transmitting
3 0 a parameter which indicates whether the default scanning frequency is to
be modified.
BRIEF DESCRIPTION OF THE DRAWINGS
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CA 02276195 1999-07-14
The foregoing, and other, objects, features and advantages of the present
invention
will be more readily understood upon reading the following detailed
description in
conjunction with the drawings in which:
Figure 1 depicts an exemplary forward digital control channel;
Figure 2 shows an example of a hierarchical cell structure;
Figure 3 is a flowchart illustrating control channel measurement according to
an
exemplary embodiment of the present invention;
Figure 4 is an illustration of different paging frame classes;
Figure S depicts an exemplary SPACH header; and
Figure 6 represents a block diagram of an exemplary cellular mobile
radiotelephone
system.
DETAILED DESCRIPTION
According to exemplary embodiments of the present invention, the measurement
periodicity for evaluating control channels for potential reselection of the
serving control
channel can be optimized to increase "sleep time" of the remote unit and
extend battery
life. Briefly, control channels are used for setting up calls, informing the
base stations
about locations and parameters associated with mobile stations, and informing
the mobile
stations about locations and parameters associated with the base stations. The
base
2 0 stations listen for call access requests by mobile stations and the mobile
stations in turn
listen for paging messages.
Future systems will employ additional cells. For example, new systems may
include any combination of macrocells, indoor microcells, outdoor microcells,
public
microcells and restricted or private microcells. New systems therefore will
likely be
2 5 designated to incorporate an increasing number of control channels.
Currently, there are
approximately twenty-one analog control channels available for a cluster in a
typical
system employed, for example, in the United States.
FIG. 2 is an exemplary hierarchical, or multi-layered, cellular system. An
umbrella macrocell 10 represented by a hexagonal shape makes up an overlying
cellular
3 0 structure. Each umbrella cell may contain an underlying microcell
structure. The umbrella
cell 10 includes microcell 20 represented by the area enclosed within the
dotted line and
microcell 30 represented by the area enclosed within the dashed line
corresponding to
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CA 02276195 1999-07-14
areas along city streets, and picocells 40, 50, and 60, which cover individual
floors of a
building. The intersection of the two city streets covered by the microcells
20 and 30 may
be an area of dense traffic concentration, and thus might represent a hot
spot.
Each of the cells illustrated in Figure 2 will include a base station
transmitting on
at least one control channel. Consider the remote station, e.g., a portable
unit, which is
travelling down the city streets and up into the building including picocells
40, S0, and
60. During various portions of this transit, potentially all of the control
channels
associated with the umbrella cell 10, the microcells 20 and 30 and the
picocells 40, 50,
and 60 may be transmitted on the neighbor list of control channel candidates
for
reselection. Evaluating those control channels to determine if reselection is
desirable is
performed by having the remote station scan those channels (e.g., measure the
received
signal strength (RSS) or decode portions of the signal).
The periodicity with which a mobile or remote station scans the control
channels
in the neighbor list during the idle mode can be controlled according to
exemplary
embodiments of the present invention as follows. Two information elements can
be
transmitted on the broadcast control channel (BCCH) for the mobile or remote
unit to
receive that are related to this scanning process. A SCANFREQ information
element can
be sent in the control channel selection message on the BCCH to inform the
mobile or
remote station about a default minimum number of signal strength measurements
to be
2 0 made per time period, for example, per superframe. Alternatively, some
default value
could be provided without transmitting this information element. For a
complete
discussion of super&ames and hyperframes, the interested reader is referred to
the
aforementioned US Patent 5,603,081. Briefly, each superframe includes a
complete set of
F-BCCH information (i.e., a set of Layer 3 messages), using as many slots as
are
2 5 necessary, and that each superframe begins with a F-BCCH slot. After the F-
BCCH
slot(s), the remaining slots in each superframe include one or more (or no)
slots E-BCCH,
S-BCCH and SPACH logical channels. A hyperframe consists of two superframes.
Thus,
the mobile station will perform a total of SCANFREQ signal strength
measurements per
superframe regardless of the size of the neighbor list. Alternatively,
SCANFREQ can
3 o indicate measurements per time interval, e.g., superframe, for each entry
in the neighbor
list.
CA 02276195 1999-07-14
The number of measurements made by the mobile station on each control channel
in the neighbor list per time interval raises an interesting tradeoff. On the
one hand, the
more measurements which are made, the more accurate the measurement
information will
be. On the other hand, the greater the number of measurements, the greater the
drain on
the mobile or remote unit's battery. Since it may be desirable to allow the
mobile to take a
greater or lesser number of measurements on particular control channels in the
neighbor
list to finely balance these competing factors, the default SCANFREQ frequency
or rate
can be modified by a second parameter, denoted HL FREQ, as described below.
The HL FREQ information element is transmitted in the neighbor list. For each
entry in the neighboring list there is an associated HL FREQ information
element. If the
HL FREQ is set to HIGH, this particular control channel is to be measured
using the
default frequency or rate defined by the SCANFREQ parameter. If, on the other
hand, the
HL FREQ information element is set to LOW, this particular control channel can
be
measured, for example, at half the frequency or rate required by the default
SCANFREQ
parameter. Of course those skilled in the art will recognize that the
particular assignment
of HIGH and LOW values of HL FREQ to the attributes described above is
arbitrary and
could be reversed.
For example, consider that the neighbor list contains 16 entries, eight of
which
have HL FREQ set to HIGH and eight of which have HL FREQ set to LOW. If the
2 0 SCANFREQ parameter is set to be 12 measurements per superframe, then the
number of
measurements for entries marked as HIGH can be measured at a minimum rate of
12/16
per superframe. For entries marked as LOW, the mobile or remote shall measure
these
control channels at a minimum rate of (12/16)/2 per superframe. Thus, the
total number of
measurements in this example per superframe would then be:
2 5 (12/16)*8 + ((12/16)/2)*8 = 9
To facilitate sleep mode efficiency for the mobile or remote stations, the
basic
procedure outlined above can be adjusted by the mobile or remote station to
reduce the
frequency or rate of measurements required. The following discussion describes
three
exemplary techniques which may be used by the mobile or remote station to
reduce the
3 0 measurement frequency and thereby minimize battery drain.
The flowchart in Figure 3 illustrates three exemplary tests for reducing the
measurement frequency of control channels in the neighboring list as a
cumulative
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procedure such that the measurement frequency can be cumulatively reduced by
some
factor for each test that is validated. However, those skilled in the art will
readily
appreciate that these three tests could be used on their own or in various
combinations
rather than being cumulative as set forth in this exemplary embodiment.
At decision block 600, the first test is implemented by determining if the
current
control channel has been serving the mobile or remote station for more than a
predetermined period of time, for example, one hour. This test can be used,
for example,
to reduce measurements when the remote station is not moving. If so, then the
flow
moves to block 610 where the measurement frequency on control channels
identified by
1 o the neighbor list can be reduced by some predetermined factor, for
example, a factor of
two.
If not, then no reduction in the frequency measurement is indicated by this
test
and the flow moves to 620.
The second exemplary test is based on an average received signal strength
(RSS)
for the serving control channel and the control channels in the neighbor list.
An average
received signal strength can be used to provide a statistically accurate
representation, as
opposed to any one instantaneous measurement which could be skewed due to, for
example, Rayleigh fading. For the purposes of this example, assume that the
mobile or
remote station keeps a running average of the last five signal strength
measurements for
2 o each measured frequency. Then, the compound test expressed by decision
block 620 and
630 is as follows. If the rate of change of the average received signal
strength on the
serving channel is not less than some predetermined threshold rate, for
example, 7 dB
over a previous five minutes, then no reduction in the measurement frequency
is
warranted and the flow moves down to block 650. Otherwise, the flow moves to
decision
2 5 block 630 and it is determined whether the change in the average received
signal strength
on all of the control channels in the neighbor list is less than some
predetermined
threshold rate, for example, 7 dB over a previous five minutes. If so, then
the
measurement frequency or rate of the control channels in the neighbor list can
be reduced
by some predetermined factor, e.g., two. Otherwise, no reduction in the rate
of frequency
3 o is warranted. In either case, the flow moves on to the third test at block
650.
Therein, it is determined whether or not the rate of change of a difference
between
the average received signal strength of the serving control channel and of
some specific
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entry in the neighbor list is less than some predetermined threshold rate, e.
g., 10 dB over
the last five minutes. If so, then the measurement frequency or rate of the
control
channels in the neighbor list can then be reduced by a factor, for example,
two at block
660. Otherwise, no reduction is warranted and in any event the procedure is
then
completed.
The aforedescribed process illustrates exemplary ways in which the number of
measurements taken by the mobile or remote station on channels in the
neighboring list
may be reduced to conserve battery power. As the conditions which triggered
the
reduction in measurement frequency change so that those conditions no longer
hold, then
the corresponding reduction in measurement frequency can be revoked. For
example, the
process illustrated y in Figure 3 can be performed periodically, e.g., every
minute. Thus,
if a mobile had previously reduced its measurement frequency of the control
channels on
the neighbor list based on satisfaction of the criteria in block 600 and, in a
subsequent
iteration the mobile had changed its serving control channel, the reduction of
block 610
would be revoked.
In addition to being "awake" to measure control channels, a mobile or remote
station is also awake periodically so that it can receive a page.
Specifically, the mobile
may be paged at any time, therefore the mobile must be locked to a particular
cell in a
location area so that the mobile may receive the page. For example, if the
mobile has
2 0 moved out of the location area of a first cell to which the mobile was
locked to a second
cell in a different location area, a paging request for the mobile will not be
heard or
received because the mobile switching center, or MSC, will page the mobile
over a
paging channel available to the location area in which the mobile is
registered. Thus, a
paging request would not be received by the mobile in the distant location
area if it is not
2 5 registered in that location area. Therefore, the mobile should register
with a new base
station when entering a new location area.
According to exemplary embodiments of the present invention, pages are
repeated
periodically and the mobile station can be assigned to a paging frame class at
registration
to take advantage of knowledge of this periodicity to remain asleep longer.
Exemplary
3 0 paging frame classes are illustrated in Figure 4. Therein note that a
first paging frame
class is defined as being every hyperframe, a second paging frame class spans
two
hyper&ames, a third paging frame class spans three hyperframes, and a fourth
paging
13
CA 02276195 1999-07-14
frame class spans four hyperframes. When the mobile station registers, it can
temporarily
change its paging frame class to PF1, until it receives a registration
response. If the
registration response contains a new paging frame class assignment, then that
shall be the
assigned paging frame class for the mobile. Otherwise, the mobile can retain
the default
paging frame class of PF1.
Having been assigned a paging frame class, either by default or through the
registration response, the mobile station shall then awaken to determine if it
is being
paged more or less frequently based upon its paging frame class. According to
exemplary
embodiments of the present invention, the system can adjust the frequency with
which the
mobile station awakens to monitor the paging channel without actually changing
the
mobile station's assigned paging frame class. This provides a solution to
another dilemma
facing system designers, i.e., the tension between paging frequency and call-
setup delay.
Consider that the more frequently a mobile or remote unit is paged, the less
call-setup
delay there will be. However, the more frequently a mobile can be paged, the
more often
a mobile must "awaken" to potentially receive a page, resulting in greater
battery drain.
Thus, during periods such as nighttime hours or other times when communication
with
the mobile or remote station is expected to be less frequent, exemplary
embodiments of
the present invention provide a mechanism for allowing the mobile or remote
station to
sleep longer and take advantage of greater tolerance for call-setup delay.
2 0 This is accomplished, for example, by transmitting a paging frame modifier
(PFM) from the system to the mobile station in a routinely transmitted
overhead message
via the digital control channel. One exemplary mechanism for transmitting the
PFM is in
a header of the SPACH channel, e.g., in every PCH subchannel. The SPACH
channel, as
described in more detail in the above-identified U.S. Patent 5,603,081,
carries layer 2
2 5 messages which are used to carry point-to-point SMS, paging or ARCH
information. An
exemplary SPACH Header including the PFM bit is illustrated as Figure 5. By
adjusting
the paging frame class of a mobile or remote unit in this way, a cell specific
adjustment
can be made (rather than changing paging classes upon registration which
requires an
entire registration area, i.e., potentially many cells, change) and
synchronization can be
3 0 maintained. Thus, the PFM field is unaddressed in the sense that it is
transmitted to every
mobile or remote in a cell as opposed to a page which is directed to a
particular remote or
mobile station.
14
CA 02276195 1999-07-14
According to exemplary embodiments of the present invention, the paging frame
modifier can comprise a single bit which, if set to binary "0", indicates that
the mobile
unit should continue to use its assigned paging flame class. If, however, the
PFM bit is set
to binary " 1" then the mobile station will adjust its paging frame class, for
example, to
the next higher paging frame class. As an example, assume that a mobile
station has been
assigned to paging frame class 2 (PF2) such that it monitors its paging
channel for a page
every other hyperframe. Then, assume that the paging frame modifier bit
transmitted on
the SPACH channel is subsequently set to binary " 1". At this point, the
mobile will act as
if its assigned paging frame class is PF3 and will sleep longer until the PFM
reverts to
"0". Mobiles or remote stations which are asleep when the PFM is changed will
nonetheless become quickly aware in their change of paging frame class since,
according
to this example, the PFM is transmitted as a bit on the SPACH header.
The foregoing has described mobile and base station operation in terms of
functional qualities. While specific hardware implementations of such stations
per se are
known to those skilled in the art, a brief example will now be described. FIG.
6 represents
a block diagram of an exemplary cellular mobile radiotelephone system,
including an
exemplary base station 110 and mobile station 120. The base station includes a
control
and processing unit 130 which is connected to the MSC 140 which in turn is
connected to the PSTN (not shown). General aspects of such cellular
radiotelephone
2 0 systems are known in the art, as described by the above-cited U. S.
patents and by the
additional U.S. Patents 5,175,867 entitled "Neighbor-Assisted Handoff in a
Cellular
Communication System" which issued December 29, 1992, and U.S. Patent
5,745,523
entitled "Multi-Mode Signal Processing", which issued April 28, 1998.
The base station 110 handles a plurality of voice channels through a voice
channel
2 5 transceiver 150, which is controlled by the control and processing unit
130. Also, each
base station includes a control channel transceiver 160, which may be capable
of handling
more than one control channel. The control channel transceiver 160 is
controlled by the
control and processing unit 130. The control channel transceiver 160
broadcasts control
information over the control channel of the base station or cell to mobiles
locked to that
3 o control channel. It will be understood that the transceivers 150 and 160
can be
implemented as a single device, like the voice and control transceiver 170,
for use with
DCCs and DTCs that share the same radio carrier frequency.
CA 02276195 1999-07-14
The mobile station 120 receives the information broadcast on a control channel
at
its voice and control channel transceiver 170. Then, the processing unit 180
evaluates the
received control channel information, which includes the characteristics of
cells that are
candidates for the mobile station to lock on to, and determines on which cell
the mobile
should lock. Advantageously, the received control channel information not only
includes
absolute information concerning the cell with which it is associated, but also
contains
relative information concerning other cells proximate to the cell with which
the control
channel is associated, as described in U.S. Patent No. 5,353,332.
The above-described exemplary embodiments are intended to be illustrative in
all
respects, rather than restrictive, of the present invention. Thus the present
invention is
capable of many variations in detailed implementation that can be derived from
the
description contained herein by a person skilled in the art. All such
variations and
modifications are considered to be within the scope and spirit of the present
invention as
defined by the following claims.
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