Title of Invention

METHOD AND APPARATUS FOR EFFICIENT NETWORK SCANNING

Abstract A method and apparatus for efficient network scanning that selects candidate frequencies based upon the frequency bands that were noted as being active during a previous networK scan. A mobile device (10) performs a full network scan on power up or on radio cycle and determines the available active bands. This information is stored as active band information (60) on the device (10). When the device (10) loses coverage on a selected one of the bands, then it identifies a candidate frequency based upon the stored active band information (60) and attempts to locate a suitable network on the candidate band.
Full Text METHOD AND APPARATUS FOR EFFICIENT NETWORK SCANNING
FIELD OF TECHNOLOGY
[0001] The present application relates to wireless mobile devices and, in
particular, to frequency band scanning to locate a suitable network for wireless
communication.
BACKGROUND INFORMATION
[0002] Many wireless devices are capable of communicating with wireless
networks on more than one frequency band. Wireless networks in different countries or
regions often have different frequency bands available for wireless communication
services. In particular, North American systems typically have 850 MHz band and/or
1900 MHz band and European systems typically have 900 MHz band and/or 1800 MHz
band. Devices that are capable of communicating on more than one frequency band
require a mechanism and method for scanning supported frequency bands and
selecting a suitable network.
[0003] One option is to perform an exhaustive network scan to search for suitable
networks on all frequency bands supported by the device. This process is typically
performed by many wireless devices upon power cycle and/or radio cycle; however
scanning all frequency bands exhaustively is expensive in terms of battery power. A
device sometime loses coverage on one frequency band and needs to re-establish a
connection with a wireless network as quickly and efficiently as possible. In these
circumstances, an exhaustive network scan consumes time and battery power.
[0004] Accordingly, some devices base their search for active frequency bands
upon the country in which the device was most recently connected. For example, if the
device had established communications on a 900 MHz band, it may choose to assume it
is located in a country supporting the European network frequency plan. Therefore, it
may assume that the frequency bands that will be available are the usual 900 MHz
and/or 1800 MHz bands. Thus, the device may limit its search for suitable networks to
these two likely co-existing frequency bands, if these two bands are supported by the
device.
[0005] There are some circumstances in which this technique fails to locate the
available networks unless a power cycle or radio cycle is performed. In some countries
or regions suitable networks may be available on frequency bands associated with both
network frequency plans. For example, in the border regions of some countries a
device may be capable of connecting to wireless providers in either of two countries,
each of which may be on a different network frequency plan. Therefore, the device may
have available to it suitable networks providing, for example, 850 MHz, 900 MHz, and
1800 MHz bands. It would be advantageous to have a method and system for efficiently
locating suitable networks on available supported frequency bands in such a situation.
BRIEF SUMMARY
[0006] The present application describes a method and apparatus for efficient
network scanning that searches for suitable networks based upon the frequency bands
that were noted as being active during a previous network scan. A mobile device
performs a full network scan on power up or on radio cycle and determines which of the
frequency bands supported by the device are active. This information is stored as
active band information on the device. When the device loses signal coverage, it scans
for suitable networks beginning with the bands included in the stored active band
information.
[0007] In one aspect, the present application provides a method of efficient
frequency band scanning of multiple supported bands to obtain service for a mobile
device. The mobile device includes a stored list of frequency bands identified as being
active during a previous scan. The mobile device communicates on a current band.
The method includes the steps of selecting a candidate band from the stored list of
bands in response to a loss of coverage on the current band, and scanning the
candidate band to locate a wireless network and, if the wireless network is located,
obtaining service on the candidate band. The current band is a member of a first pair of
frequencies in a first network plan, and the candidate band is a member of a second pair
of frequencies in a second network plan.
[0008] In another aspect, the present application provides a mobile electronic
device. The mobile electronic device includes a communications subsystem for
engaging in wireless communication with a wireless network, the communications
subsystem being capable of communicating on at least two frequency bands, including
a current band. The device also includes memory and a processor connected to the
memory and to the communications subsystem for controlling operation of the
communications subsystem. The device also includes a list stored in the memory,
wherein the list contains frequency bands identified as being active during a previous
scan, and a network locator component. The network locator component selects a
candidate band from the list in response to a loss of coverage on the current band,
scans the candidate band to locate the wireless network and, if the wireless network is
located, obtains service on the candidate band. The current band is a member of a first
pair of frequencies in a first network plan, and the candidate band is a member of a
second pair of frequencies in a second network plan.
[0009] Other aspects and features of the present application will become
apparent to those of ordinary skill in the art upon review of the following description of
specific embodiments in conjunctions with the accompanying figures.
BRIEF DESCRIPTION OF THE Accompanying DRAWINGS
[0010] Embodiments will now be described, by way of example only, with
reference to the attached Figures, wherein:
[0011] Figure 1 shows a block diagram of a mobile electronic device; and
[0012] Figure 2 shows, in flowchart form, a method of efficient network scanning
to locate service.
[0013] Like reference numerals are used throughout the Figures to denote similar
elements and features.
DETAILED DESCRIPTION
[0014] Referring first to Figure 1, there is a block diagram of a mobile electronic
device 10 to which example embodiments of the present application may be applied.
The mobile electronic device 10 communicates with a wireless network 50.
[0015] In the embodiment of Figure 1, the mobile electronic device 10 is a hand-
held two-way mobile communication device 10 capable of obtaining at least one service
from the wireless network 50. In an example embodiment, the device has the capability
to communicate with other computer systems on the Internet. In various embodiments,
mobile electronic devices 10 includes data communication devices, multiple-mode
communication devices configured for both data and voice communication, mobile
telephones, mobile communication devices, PDAs enabled for wireless
communications, 1-way or 2-way pagers, wireless modems operating in conjunction with
computer systems, and any type of mobile wireless communication devices. In the
presently described embodiment, each of the mobile electronic devices 10 is configured
to operate within the wireless network 50. It should be appreciated however that the
present application is in no way limited to these example types of devices and may be
implemented in other wireless devices.
[0016] The device 10 includes a microprocessor 38 and a communication
subsystem 11. The communication subsystem 11 includes a receiver 12, a transmitter
14, and associated components such as one or more, preferably embedded or internal,
antenna elements 16 and 18, and a processing module such as a digital signal
processor (DSP) 20. In some embodiments, the communication subsystem 11 includes
local oscillator(s) (LO) 13, and in some embodiments the communication subsystem 11
and microprocessor 38 share an oscillator. As will be apparent to those skilled in the
field of communications, the particular design of the communication subsystem 11 will
be dependent upon the communication network in which the device 10 is intended to
operate.
[0017] Signals received by the antenna 16 through the wireless communication
network 50 are input to the receiver 12, which may perform such common receiver
functions as signal amplification, frequency down conversion, filtering, channel selection
and the like, and in some embodiments, analog to digital conversion. In a similar
manner, signals to be transmitted are processed, including modulation and encoding for
example, by the DSP 20 and input to the transmitter 14 for digital to analog conversion,
frequency up conversion, filtering, amplification and transmission over the wireless
communications network 50 via the antenna 18.
[0018] The microprocessor 38 controls the overall operation of the device 10. The
microprocessor 38 interacts with communications subsystem 11 and also interacts with
further device subsystems such as a display 22, flash memory 24, random access
memory (RAM) 26, auxiliary input/output (I/O) subsystems 28 (which may include a
thumb-wheel, for example), serial port 30, keyboard or keypad 32, speaker 34,
microphone 36, a short-range communications subsystem 40, a graphics subsystem 44,
and any other device subsystems generally designated as 42.
[0019] Some of the subsystems shown in Fig. 1 perform communication-related
functions, whereas other subsystems may provide "resident" or on-device functions.
Notably, some subsystems, such as keyboard 32 and display 22 for example, may be
used for both communication-related functions, such as entering a text message for
transmission over a communication network, and device-resident functions such as a
calculator or task list.
[0020] The device 10 also includes a security identity module (SIM) 56, or SIM
card. The SIM 56 comprises a memory or storage element containing wireless
communications related data. For example, in some embodiments, the SIM 56 stores
phone numbers, contact names and other data entered by the user. The SIM 56 may
also store the identity of a registered public land mobile network (RPLMN). The RPLMN
identifies the wireless network 50 (i.e. a PLMN) to which the device 10 was most
recently successfully connected to obtain services. RPMLN includes a Mobile Country
Code (MCC) and a Mobile Network Code (MNC). The MCC identifies the country in
which the mobile device 10 is or was located when it last connected to the wireless
provider.
[0021] Operating system software 54 and various software applications 58 used
by the microprocessor 38 are, in one example embodiment, stored in a persistent store
such as flash memory 24 or similar storage element. Software applications 58 may
include a wide range of applications, including an address book application, a
messaging application, a calendar application, and/or a notepad application. Those
skilled in the art will appreciate that the operating system 54, specific device applications
58, or parts thereof, may be temporarily loaded into a volatile store such as RAM 26. It
is contemplated that received communication signals may also be stored to RAM 26.
[0022] The microprocessor 38, in addition to its operating system functions,
preferably enables execution of software applications 58 on the device. A predetermined
set of applications 58 which control basic device operations, including at least data and
voice communication applications for example, will normally be installed on the device
10 during manufacture. Further applications may also be loaded onto the device 10
through the network 50, an auxiliary I/O subsystem 28, serial port 30, short-range
communications subsystem 40 or any other suitable subsystem 42, and installed by a
user in the RAM 26 or a non-volatile store for execution by the microprocessor 38. Such
flexibility in application installation increases the functionality of the device and may
provide enhanced on-device functions, communication-related functions, or both. For
example, secure communication applications may enable electronic commerce
functions and other such financial transactions to be performed using the device 10.
[0023] The device 10 further includes a network locator component 62. The
network locator component 62 manipulates and controls the communication subsystem
11 for the purpose of locating service on the wireless network 50. Although the network
locator component 62 is shown as being resident in flash memory 24, those skilled in
the art will appreciate that it may be stored elsewhere, including in RAM 26, in the SIM
56 or within the communication subsystem 11 itself.
[0024] In a data communication mode, a received signal such as a text message
or web page download will be processed by the communication subsystem 11 and input
to the microprocessor 38, which will preferably further process the received signal for
output to the display 22, or alternatively to an auxiliary I/O device 28. A user of device
10 may also compose data items such as email messages for example, using the
keyboard 32 in conjunction with the display 22 and possibly an auxiliary I/O device 28.
Such composed items may then be transmitted over the wireless communication
network 50 through the communication subsystem 11.
[0025] The serial port 30 in Fig. 1 would normally be implemented in a personal
digital assistant (PDA)-type communication device for which synchronization with a
user's desktop computer (not shown) may be desirable, but is an optional device
component. Such a port 30 would enable a user to set preferences through an external
device or software application and would extend the capabilities of the device by
providing for information or software downloads to the device 10 other than through the
wireless communication network 50.
[0026] A short-range communications subsystem 40 is a further component which
may provide for communication between the device 10 and different systems or devices,
which need not necessarily be similar devices. For example, the subsystem 40 may
include an infrared device and associated circuits and components or a Bluetooth™
communication module to provide for communication with similarly enabled systems and
devices.
[0027] Wireless network 50 is, in an example embodiment, a wireless packet data
network, (e.g. Mobitex™ or DataTAC™), which provides radio coverage to mobile
electronic devices 10, although it could be any other types of wireless network. The
wireless network 50 may also be a voice and data network such as GSM (Global
System for Mobile Communication) and GPRS (General Packet Radio System), CDMA
(Code Division Multiple Access), or various other third generation networks such as
EDGE (Enhanced Data rates for GSM Evolution) or UMTS (Universal Mobile
Telecommunications Systems).
[0028] In general, the device 10 may support more than one frequency band for
wireless communications. Such bands are referred to herein as "supported bands".
Two frequency bands that are predominantly used in North America are 850 MHz and
1900 MHz frequency bands. Elsewhere in the world, and in particular in Europe, 900
MHz and 1800 MHz are the two bands primarily in use. A mobile device may be
designed to support multiple frequency bands, including bands in both the European
and North American frequency plans. For example, a wireless device may be designed
to communicate on 900 MHz, 1800 MHz, and 1900 MHz bands. When outside of North
America, such a device must select between 900 MHz and 1800 MHz bands, whereas
inside of North America, the device operates upon the 1900 MHz band.
[0029] In some cases, such a device may be located in a country or region where
both North American and European (World) bands are available. In these
circumstances, all supported frequency bands may be available to the device.
[0030] In the present embodiment, the device 10 includes active band information
60. The active band information 60 may be stored in the flash memory 24, or in any
other memory element. The active band information 60 comprises information
regarding the active frequency bands located during the most recent full network scan
performed by the device 10. The active band information 60 is updated by the device
10 whenever a band is scanned to see if it is active, i.e. to see if there is an available
network.
[0031] When the device 10 is powered on, or otherwise undergoes a power cycle
or radio cycle, the device 10, under the control of the network locator component 62,
performs a full network scan, scanning all the frequency bands that it supports to select
an RPMLN or equivalent PLMN (if it is available). If necessary, it attempts to perform a
location registration. The full network scan involves scanning all of the bands supported
by the device to attempt to identify active frequency bands upon which the device 10
could obtain wireless services. Once the device 10 has successfully registered with a
network, it stores the RPLMN in its SIM 56. For example, the device 10 may find a
network at 900 MHz. It may register with the network and store the corresponding
network identity as RPLMN and engage in communications on the 900 MHz band.
[0032] In addition, the network locator component 62 stores the information
gathered during the network scan as the active band information 60. For example, the
device 10 may have determined that active frequency bands include 900 MHz and 1900
MHz. The active band information 60 reflects these findings.
[0033] At some later point, the device 10 may lose coverage on its selected active
band, and in response it attempts to re-establish contact with a network. If a RPLMN is
available on the device 10, the network locator component 62 is aware that the device
10 was most recently connected to a provider in a particular country based upon stored
country information, which in one embodiment is found in the MCC part of the RPLMN.
Using the MCC, the network locator component 62 is able to determine whether or not it
was in a country affiliated with the North American plan for mobile device frequencies
(i.e. 850 MHz and 1900 MHz) or the European plan for mobile device frequencies (i.e.
900 MHz and 1800 MHz). Prior art devices base their attempts to re-locate coverage
solely upon the likely co-existing bands as indicated by the MCC of the RPLMN, if
available. In other words, if the device was most recently connected to a provider in a
country affiliated with the European plan on a 900 MHz band, then the device would
only seek to attain coverage on the 1800 Mhz frequency band, if supported by the
device 10.
[0034] The device 10 shown in Figure 1 includes the active band information 60,
which informs the device 10 as to the active frequency bands that were noted during the
most recent network scan. The network locator component 62 uses the active band
information 60 to determine where to look for reconnection to the wireless network 50
when it loses coverage on its first selected band. The active band information 60
provides the network locator component 62 with promising candidate frequencies.
Since these frequencies were recently noted as active, the network locator component
62 presumes that they may still be available and attempts to determine if they are still
active. If one of the active bands in the list is also a likely co-existing band, i.e. it is on
the same network frequency plan as the first selected band, then the device 10 will
begin to look for coverage on the likely co-existing band. As an example, if the active
band information 60 indicates that the 900 MHz and 1900 MHz bands were active, and
the device 10 just lost coverage on 900 MHz, then rather than checking 1800 MHz,
which is the European plan pair frequency to 900 MHz, the device 10 first scans the
1900 MHz band to see if it can select a network to obtain service.
[0035] If the device 10 exhausts the candidate bands listed in the active band
information 60 without obtaining service from a network successfully, then it may utilize
the MCC in the RPLMN information to identify a further candidate band, which would be
the likely co-existing band based on the network frequency plan if that likely co-existing
band was not already included in the active band information 60.
[0036] In the event that none of the bands in the active band information 60 are
still active and the likely co-existing band is not active, then the device 10 may continue
to scan for any other supported and unscanned bands to attempt to locate a suitable
network.
[0037] Reference is now made to Figure 2, which shows, in flowchart form, a
method 100 of efficient network scanning to locate service. The method 100 begins in
step 102, wherein a mobile wireless device performs a full scan for a suitable network.
In step 104, the mobile wireless device stores a list of active bands as active band
information. The list may be stored in any suitable manner in a memory element
associated with the mobile wireless device. The list is populated with the active bands
located during the scan performed in step 102.
[0038] The wireless device obtains service on a selected one of the active bands
in step 106. This may include registering the wireless device with the network provider
and storing the identity as the RPLMN of the network provider.
[0039] In step 108, coverage on the selected active band is lost. As a result, in
step 110 the wireless device identifies a candidate band from the list of active bands
stored as active band information. The candidate band may or may not be the
corresponding band from the same frequency plan as the selected active band, i.e. the
likely co-existing band. For example, if the selected active band was 1900 MHz, the
candidate band is not necessarily 850 MHz (the other North American plan frequency),
but may be 900 MHz or 1800 MHz if supported. However, in one embodiment, if the list
of active bands includes the likely co-existing band, then the device selects this as the
first candidate band.
[0040] In step 112, the wireless device scans the candidate band in an attempt to
locate a suitable network. In step 114, the device evaluates whether or not a suitable
network was located during the scan in step 112. If a suitable network is located on the
candidate band, then the wireless device updates the active band information in step
116 and obtains service in step 118. If a suitable network is not available on the
candidate band, then in step 120 the device updates the active band information. In
step 122, the device assesses whether any additional unscanned bands remain in the
list of active bands, i.e. if there are any more candidate bands in the active band
information. If there are other candidate frequency bands, then the method 100 returns
to step 110 wherein the device attempts to identify another candidate band in the list of
active bands.
[0041] If the device exhausts the list of active bands without locating a suitable
network, then it proceeds to step 124 wherein it determines whether or not it has already
scanned for a likely co-existing band. Based on the country information (i.e. MCC) in
the RPLMN stored in the device, it may identify the likely co-existing band related to the
band on which the device has lost coverage. Provided that the device supports the
likely co-existing band, it may be able to locate a suitable network on this band even
though it was not previously noted as being active. If the device has not already
scanned this band in step 112 - in other words, if the likely co-existing band was not
included in the list of active bands - then the method 100 proceeds to step 126, wherein
the likely co-existing band is identified as a candidate band. From there the method 100
returns to step 112 to scan the likely co-existing band. By way of example, suppose that
a device supports the 850 MHz, 900 MHz, and 1900 MHz bands, and lost coverage on
the 850 MHz band. The active band information may direct the device to scan the 900
MHz band. If service is not located on this band, then the device would determine in
step 124 that it has not scanned the likely co-existing band, namely 1900 MHz.
Accordingly, the device would next scan the 1900 MHz band.
[0042] If the device does not support the likely co-existing band, or if it has been
scanned already, then the method 100 proceeds to step 128, wherein the device
assesses whether there are any other supported bands that have not been scanned. If
so, then in step 130 an unscanned supported band is identified as a candidate band and
the method 100 returns to step 112 to scan the candidate band.
[0043] If the device scans all its supported bands without locating a suitable
network, then the method ends at step 132 with the conclusion that service is
unavailable. At this stage the device has essentially performed a full network scan for
coverage. If no network is available, then the device would perform a full network scan
at the next appropriate time such as when a timer expires to scan for network again.
[0044] Those of ordinary skill in the art will appreciate that the foregoing steps
and functions attributed as being performed by the device may be performed by a
microprocessor, microcontroller, application specific integrated circuit, or other
programmable logic device capable of manipulating and controlling memory elements
and the communications subsystem. The operations may be performed by these
elements operating under stored program control. The suitable programming of such
devices will be within the ability of one of ordinary skill in the art, having regard to the
description herein.
[0045] Although the foregoing embodiments describe the active band information
60 (Fig. 1) as being stored in the flash memory 24 (Fig. 1), those of ordinary skill in the
art will appreciate that the active band information 60 may be stored in other memory
elements, including the SIM 56 (Fig. 1) or RAM 26 (Fig. 1). The location or manner in
which the active band information 60 is stored is not intended as a limitation of the
present application.
[0046] Although the foregoing embodiments describe the RPLMN information as
being stored in the SIM 56 (Fig. 1), those of ordinary skill in the art will appreciate that
the RPLMN information may be stored in other memory elements, including the flash
memory 24 (Fig. 1) or RAM 26 (Fig. 1). The location or manner in which the RPLMN
information and, in particular, the MCC information, is stored is not intended as a
limitation of the present application. Moreover, those of ordinary skill in the art will also
appreciate that in some embodiments the country information may be stored other than
in RPLMN information. Accordingly, it will be appreciated that the device may store and
read country information in any suitable manner.
[0047] The above-described embodiments of the present application are intended
to be examples only. Alterations, modifications and variations may be effected to the
particular embodiments by those skilled in the art without departing from the scope of
the application, which is defined by the claims appended hereto.
WE CLAIM:
1. A method of efficient frequency band scanning of multiple supported bands to obtain
service for a mobile device (10), wherein the mobile device comprises a stored list of
frequency bands (60) identified as being active during a previous scan, the mobile device
(10) communicating on a current band, the method comprising the steps of:
selecting a candidate band from the stored list of bands (60) in response to a loss of
coverage on the current band; and
scanning said candidate band to locate a wireless network (50) and, if the
wireless network (50) is located, obtaining service on said candidate band, characterized
in that the current band is a member of a first pair of frequencies in a first network plan,
and wherein said candidate band is a member of a second pair of frequencies in a second
network plan.
2. The method as claimed in claim 1, wherein the method comprises a step of performing
said previous scan and a step of creating the stored list of bands (60) based upon active
bands located in said previous scan.
3. The method as claimed in any one of claims 1 to 2, wherein if said step of scanning
concludes that said candidate band is inactive, the method comprises the steps of
identifying another candidate band and scanning said another candidate band.
4. The method as claimed in claim 3, wherein said another candidate band comprises a band
in the stored list of bands (60).
5. The method as claimed in claim 3 or claim 4, wherein said another candidate band
comprises a likely co-existing band associated with the current band as a frequency pair
in a network plan.
6. The method as claimed in any one of claims 3 to 5, wherein said another candidate band
is one of said plurality of supported bands and is not included in the stored list of bands
(60) and is not a likely co-existing band.
7. The method as claimed in any one of claims 1 to 6, wherein the method comprises a step,
following said step of scanning, of updating the stored list of bands.
8. A mobile electronic device (10), comprising:
a communications subsystem (11) for engaging in wireless communication with a
wireless network (50), said communications subsystem (11) being capable of
communicating on at least two frequency bands, comprising a current band; memory
(24, 26, 56);
a processor (38) connected to said memory (24, 26, 56) and to said communications
subsystem (11) for controlling operation of said communications subsystem (11),
a list (60) stored in said memory (24, 26, 56), wherein said list (60) contains
frequency bands identified as being active during a previous scan;
a network locator component (62) for selecting a candidate band from said list (60) in
response to a loss of coverage on said current band, scanning said candidate band to
locate the wireless network (50) and, if the wireless network (50) is located, obtaining
service on said candidate band; and
said current band is a member of a first pair of frequencies in a first network plan, and
said candidate band is a member of a second pair of frequencies in a second network
plan.
9. The device as claimed in claim 8, wherein said network locator component (62)
comprises a full scan component for performing said previous scan and creating said
list (60) based upon active bands located in said previous scan.
10. The device as claimed in claim 8, wherein said network locator component (62)
updates said list (60) after scanning said candidate band.

Documents:

837-KOL-2005-(05-03-2012)-FORM-27.pdf

837-kol-2005-abstract.pdf

837-kol-2005-amanded pages of specification.pdf

837-kol-2005-claims.pdf

837-KOL-2005-CORRESPONDENCE-1.1.pdf

837-kol-2005-correspondence.pdf

837-kol-2005-description (complete).pdf

837-kol-2005-examination report.pdf

837-kol-2005-form 1.pdf

837-kol-2005-form 13.pdf

837-kol-2005-form 18.pdf

837-kol-2005-form 2.pdf

837-kol-2005-form 26.pdf

837-kol-2005-form 3.pdf

837-kol-2005-form 5.pdf

837-kol-2005-gpa.pdf

837-KOL-2005-OTHERS-1.1.pdf

837-kol-2005-specification.pdf

837-kol-2005-translated copy of priority document.pdf


Patent Number 248062
Indian Patent Application Number 837/KOL/2005
PG Journal Number 24/2011
Publication Date 17-Jun-2011
Grant Date 14-Jun-2011
Date of Filing 12-Sep-2005
Name of Patentee RESEARCH IN MOTION LIMITED
Applicant Address 295, PHILLIP STREET, WATERLOO, ONTARIO, N2L3 W 8
Inventors:
# Inventor's Name Inventor's Address
1 YIU MING LAM 522 WINTERBURG WALK, WATERLOO, ONTARIO N2V 2S8
PCT International Classification Number H04W 76/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 EP 04104702.8 2004-09-27 EPO