Title of Invention

SYSTEM AND METHOD FOR PERFORMING A HANDOVER IN A BROADBAND WIRELESS ACCESS COMMUNICATION SYSTEM

Abstract A broadband wireless access communication system in which a subscriber station (SS) sends a serving base station (SBS) a handover request to neighbor base stations (NBSs) having a CINR satisfying a handover condition, in response to a handover scan request message, and performs the handover to a particular NBS that transmits a handover response. The SBS transmits the handover scan request message to the SS, sends a handover connection request to NBSs in an order determined by CINRs reported from the SS, and sends, to the SS, information on a particular NBS upon receiving a handover connection response with ACK information from the particular NBS. The particular NBS determines whether it can support handover of the SS, in response to the handover connection request, and sends the handover connection response with the ACK information to the SBS, if it can support handover of the SS.
Full Text BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a broadband wireless access
communication system, and in particular, to a system and method for determining a
handover at the request of a base station in a broadband wireless access communication
system employing Orthogonal Frequency Division Multiplexing (OFDM).
2. Description of the Related Art
In a 4th generation (4G) communication system, active research is being
conducted on technology to provide users with services guaranteeing various qualities of
service (QoSs) at a data rate of about 100 Mbps. The current 3rd generation (3G)
communication system generally supports a data rate of about 384 Kbps in an outdoor
channel environment having a relatively poor channel environment, and supports a data
rate of a maximum of Q. Mbps even in an indoor channel environment having a relatively
good channel environment. A wireless local area network (LAN) system and a wireless
metropolitan area network (MAN) system generally support a data rate of 20 Mbps to 50
Mbps. Therefore, in the current 4G communication system, the active research is being
carried out on a new communication system securing mobility and QoS for the wireless
LAN system and the wireless MAN system supporting a relatively high data rate in
order to support high-speed services the 4G communication system aims to provide.
Due to its broad service coverage and high data rate, the wireless MAN system
is suitable for high-speed communication services. However, because the mobility of a
user or a subscriber station (SS), is not taken into consideration, a handover caused by
fast movement of the subscriber station is also not considered in the system.
A communication system proposed in IEEE (Institute of Electrical and
Electronics Engineers) 802.16a performs a ranging operation between a subscriber
station and a base station (BS), for communication. A configuration of the
communication system proposed in the IEEE 802.16a according to the prior art will now

be described with reference to FIG 1.
FIG 1 is a diagram schematically illustrating a configuration of a broadband
wireless access communication system employing Orthogonal Frequency Division
Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA)
(hereinafter referred to as "OFDM/OFDMA broadband wireless access communication
system"). More specifically, FIG. 1 is a diagram schematically illustrating a
configuration of an IEEE 802.16a/IEEE 802.16e communication system.
Before a description of FIG. 1 is given, it should be noted that the wireless
MAN system is a broadband wireless access (BWA) communication system, and has
broader service coverage and supports a higher data rate compared with the wireless
LAN system. The IEEE 802.16a communication system is a communication system
employing OFDM and OFDMA for supporting a broadband transmission network to a
physical channel of the wireless MAN system. That is, the IEEE 802.16a communication
system is an OFDM/OFDMA broadband wireless access communication system. The
IEEE 802.16a communicution system, applying OFDM/OFDMA to the wireless MAN
system, transmits a physical channel signal using a plurality of subcarriers, thereby
making it possible to support high-speed data communication. The IEEE 802.16e
communication system considers mobility of a subscriber station in addition to
characteristics of the IEEE 802.16a communication system. However, no specification
has been proposed for the IEEE 802.16e communication system. As a result, the IEEE
802.16a communication system and the IEEE 802.16e communication system are both
an OFDM/OFDMA broadband wireless access communication system, and for the
convenience of explanation, the description will be made with reference to both the
IEEE 802.16a communication system and the IEEE 802.16e communication system.
Referring to FIG 1, the IEEE 802.16a/IEEE 802.16e communication system
has a single-cell configuration, and comprises a base station 100 and a plurality of
subscriber stations 110, 120, and 130, which are controlled by the base station 100.
Signal exchanges between the base station 100 and the subscriber stations 110, 120 and
130 are achieved using the OFDM/OFDMA technology.
i
FIG 2 is a diagram schematically illustrating a downlink frame format for an
OFDM/OFDMA broadband wireless access communication system, and in particular,

illustrating a downlink frame format for an IEEE 802.16a/IEEE 802.16e communication
system. Referring to FIG 2, the downlink frame includes a preamble field 200, a
broadcast control field 210, and a plurality of time division multiplexing (TDM) fields
220 and 230. A synchronization signal or a preamble sequence, for acquiring mutual
synchronization between a base station and a subscriber station, are transmitted over the
preamble field 200. The broadcast control field 210 includes a DL (DownLink)_MAP
field 211 and a UL (UpLink)_MAP field 213. The DL_MAP field 211 is a field over
which a DL_MAP message is transmitted, and information elements (IEs) included in
the DL_MAP message are illustrated in Table 1 below.
Table 1

Syntax Size Notes
DL-MAP_Mmajc_Foniiai() j
Management Message Type - 2 8 bits
PHY Synchronization Held Variable Sec appropriate PHY specification.
DCD Count 8 bits
Base Station ID 4$ bits
Number of UL-.MAP Elements H 16 bits
Begin PHY Specific Section { Sec applicable PHY section.
f«r(f = \:i DL_\lA!,JnformatKm_E{fcmcnt() Variable See corresponding P3IY specification.
if Hbyte boundary) i
Padding Nibble 4 bits Padding to reach byte boundary.
. t
>
> \
\
As illustrated in Table 1, IEs of the DL_MAP message include Management
Message Type indicating a type of a transmission message, PHY (PHYsical)
Synchronization Field established based on modulation and demodulation schemes
applied to a physical channel to acquire synchronization, DCD Count indicating a count
corresponding to a change in the configuration of a downlink channel descript (DCD)
message containing a downlink burst profile, Base Station ID indicating a base station
identifier, and Number of DL_MAP Elements n indicating the number of elements
following the Base Station ID. Although not illustrated in Table 1, the DLMAP

message includes information on ranging codes allocated to corresponding rangings
described below.
In addition, the UL_MAP field 213 is a field over which a UL_MAP message is
transmitted, and IEs included in the UL_MAP message are illustrated in Table 2 below.
Table 2

As illustrated in Table 2, IEs of the UL_MAP message include Management
Message Type indicating a type of a transmission message, Uplink Channel ID
indicating an uplink channel ID in use, UCD Count indicating a count corresponding to
a change in the configuration of an uplink channel descript (UCD) message containing
an uplink burst profile, and Number of UL_MAP Elements n indicating the number of
elements following the UCD Count. The uplink channel ID is uniquely assigned by a
media access control (MAC)-sublayer.
Information designating usage of an offset written in an Offset field is included
in a UIUC (Uplink Interval Usage Code) field. For example, if '2' is written in the UIUC
field, it indicates that a starting offset used for initial ranging is written in the Offset field.
Alternatively, if '3' is written in the UIUC field, it indicates that a starting offset used for
bandwidth request ranging or maintenance ranging is written in the Offset field. As

described above, a starting offset used for initial ranging, bandwidth request ranging, or
maintenance ranging, based on the information written in the UIUC field, is written in
the Offset field. Information on a characteristic of a physical channel to be transmitted
over the UIUC field is written in a UCD message.
If a subscriber station has failed to perform successful ranging, it sets a
particular backoff value in order to increase a success rate at the next attempt, and makes
a ranging attempt after a lapse of the backoff time. In this case, information necessary
for setting the backoff value is also included in the UCD message. A configuration of the
UCD message will be described in detail with reference to Table 3 below.
Table 3

As illustrated in Table 3, Tfis of the UCD message include Management
Message Type indicating a type of a transmission message, Uplink Channel ID
indicating an uplink channel ID in use, Configuration Change Count being counted in a
base station, Mini-slot Size indicating the number of minislots of an uplink physical
channel, Ranging Backoff Start indicating a backoff start point for initial ranging, i.e.,
indicating a size of an initial backoff window for initial ranging, Ranging Backoff End
indicating a backoff end point for initial ranging, i.e., indicating a size of a final backoff
window, Request Backoff Start indicating a backoff start point for contention data and
requests, i.e., indicating a size of an initial backoff window, and Request Backoff End

indicating a backoff end point for contention data and requests, i.e., indicating a size of a
final backoff window. Here, the backoff value indicates a kind of waiting time during
which a subscriber station should wait for the next ranging if it fails in the rangings
described below. A base station must transmit to a subscriber station the backoff value
that is time information for which the subscriber station should wait for the next ranging
if it fails in the current ranging. For example, if a value given by the Ranging Backoff
Start and the Ranging Backoff End is set to ' 10', the subscriber station must perform the
next ranging after passing an opportunity to perform 210 (=1024) rangings by a truncated
binary exponential backoff algorithm.
In addition, the TDM fields 220 and 230 correspond to time slots assigned to
subscriber stations on a TDM/TDMA (Time Division Multiple Access) basis. The base
station transmits broadcast information to be broadcasted to its subscriber stations over
the DL_MAP field 211 of the downlink frame using a predetermined center carrier.
Upon power-on, the subscriber stations monitor all frequency bandwidths previously
assigned to the subscriber stations, and detect a pilot channel signal having the highest
strength, i.e., the highest pilot carrier-to-interference and noise ratio (CDSTR). A
subscriber station determines a base station that transmitted a pilot channel signal having
the highest pilot CINR, as a base station to which it currently belongs, and acquires
control information for controlling its uplink and downlink and information indicating
an actual data transmission/reception point by analyzing a DLMAP field 211 and a
ULJVIAP field 213 of a downlink frame transmitted from the base station.
FIG 3 is a diagram schematically illustrating an uplink frame format for an
OFDM/OFDMA broadband wireless access communication system, and in particular,
illustrating an uplink frame format for an IEEE 802.16a/IEEE 802.16e communication
system. ,
t
Before a description of FIG. 3 is given, rangings used in the IEEE
802.16a/IEEE 802.16e communication system, i.e., initial ranging, maintenance ranging
(or periodic ranging), and bandwidth request ranging, will be described.
A. Initial Ranging
The initial ranging is performed after a base station request in order to acquire
synchronization the base station with a subscriber station. The initial ranging is

performed to set a correct time offset and control transmission power between the
subscriber station and the base station. That is, the subscriber station performs the initial
ranging in order to receive, upon its power-on, a DL_MAP message and a UL_MAP
message/UCD message, acquire synchronization with a base station, and then control the
time offset and transmission power with the base station. Because the IEEE
802.16a/TEEE 802.16e communication system employs the OFDM/OFDMA technology,
the ranging procedure requires subchannels and ranging codes, and the base station
assigns available ranging codes according to goals, or types, of rangings. This will be
described in more detail herein below.
Ranging codes are generated by segmenting a pseudorandom noise (PN)
sequence having a predetermined length of, for example, 215 bits, in a predetermined
unit. Generally, two 53-bit ranging subchannels constitute one ranging channel, and
ranging codes are created by segmenting a PN code over a 106-bit ranging channel. The
ranging codes generated in this way can be assigned to a maximum of 48 per subscriber
stations, and a minimum of 2 ranging codes per subscriber station are applied by default
to rangings of the 3 goals, i.e., initial ranging, periodic ranging, and maintenance ranging.
Accordingly, different ranging codes are assigned to the rangings of the 3 goals. For
example, N ranging codes are allocated for initial ranging (N RCs (Ranging Codes) for
initial ranging), M ranging codes are allocated for periodic ranging (M RCs for
maintenance ranging), and L ranging codes are allocated for bandwidth request ranging
(L RCs for BW request ranging). The allocated ranging codes are transmitted to
subscriber stations through a DL_MAP message as stated above, and the subscriber
stations perform their ranging procedures by using the ranging codes included in the
DLMAP message according to their goals.
B. Periodic Ranging
The periodic ranging is periodically performed in order for a subscriber station
to control a channel condition to a base station after controlling a time offset and
transmission power with the base station through the initial ranging. The subscriber
station performs the periodic ranging by using the ranging codes allocated for periodic
ranging.
C. Bandwidth Request Ranging
The bandwidth request ranging is performed when a subscriber station requests

allocation of a bandwidth in order to perform actual communication with a base station,
after controlling a time offset and transmission power with the base station through the
initial ranging. The bandwidth request ranging can be performed using a selected one of
the following three methods: Grants, Contention-based Focused bandwidth requests for
Wireless MAN-OFDM, and Contention-based CDMA bandwidth requests for Wireless
MAN-OFDMA. A detailed description of the three methods will now be made herein
below.
(1) Grants
The Grants method requests assignment of a bandwidth when a communication
system to which a subscriber station currently belongs is a single-carrier communication
system. In this method, a subscriber station performs the bandwidth request ranging,
using a default CID (Connection ID) rather than its own CID. If the subscriber station
fails in the bandwidth request ranging, it reattempts the bandwidth request ranging after
a backoff value previously determined according to the last information received from a
base station and a request status of the base station, or determines to discard a received
service data unit (SDU). Herein, the subscriber station has already detected the backoff
value through a UCD message.
(2) Contention-based Focused bandwidth requests for Wireless MAN-OFDM
The Contention-based Focused bandwidth requests for Wireless MAN-OFDM
method requests assignment of a bandwidth when a communication system to which a
subscriber station currently belongs is an OFDM communication system. The
Contention-based Focused bandwidth requests for Wireless MAN-OFDM method is
classified again into two methods. A first method performs the bandwidth request
ranging by transmitting a Focused Contention Transmission message while a subscriber
station uses a default CID as described in the Grants method. A second method performs
the bandwidth request ranging by transmitting a broadcast CTD rather than the default
CID along with an OFDM Focused Contention ID. When the bandwidth request ranging
is performed by transmitting the broadcast CID together with the OFDM Focused
Contention ID, a base station determines a specific contention channel and a data rate for
a subscriber station.
(3) Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA
The Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA

method requests allocation of a bandwidth when a communication system to which a
subscriber station currently belongs is an OFDMA communication system. The
Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA method is
classified again into two methods. A first method performs the bandwidth request
ranging as described in the Grants method, and a second method performs the bandwidth
request ranging by using a CDMA (Code Division Multiple Access)-based mechanism.
In the second method using the CDMA-based mechanism, the communication system
uses a plurality of tones comprised of OFDM symbols, i.e., uses a plurality of
subchannels. Therefore, when a subscriber station performs bandwidth request ranging, a
base station applies the CDMA-based mechanism to each of the subchannels. As a result,
if the base station successfully receives the bandwidth request ranging, a subscriber
station that performed the bandwidth request ranging through a MAC protocol data unit
(PDU) allocates a frequency bandwidth. In a REQ (REQuest) Region-Focused method,
if a plurality of subscriber stations attempt bandwidth request ranging through the same
subchannel using the same contention code, collision possibility is increased undesirably.
Referring to FIG 3, the downlink frame includes an Initial Maintenance
Opportunities field 300 for initial ranging and maintenance ranging (or periodical
ranging), a Request Contention Opportunities field 310 for bandwidth request ranging,
and SS scheduled data fields 320 containing uplink data of subscriber stations. The
Initial Maintenance Opportunities field 300 has a plurality of access burst periods
including actual initial ranging and periodic ranging, and a collision period in case that
collision occurs between the access burst periods. The Request Contention Opportunities
field 310 has a plurality of bandwidth request periods including actual bandwidth request
ranging, and a collision period in case that collision occurs between the bandwidth
request periods. Each of the SS scheduled data fields 320 is comprised of a plurality of
SS schedule data fields (SS #1 scheduled data field to SS #N scheduled data field).
Subscriber station transition gaps (SS transition gap) are located between the SS
scheduled data fields (SS #1 scheduled data field to SS #N scheduled data field).
FIG. 4 is a'diagram schematically illustrating a procedure for performing
communication through the messages illustrated in FIGs. 2 and 3 in a broadband
wireless access communication system. Referring to FIG. 4, upon a power-on, a
subscriber station 400 monitors all previously assigned frequency bandwidths, and
detects a pilot channel signal having the highest strength, i.e., the highest pilot CINR.

The subscriber station 400 determines a base station 420 that transmitted a pilot channel
signal having the highest pilot CINR, as a base station to which it currently belongs, and
acquires system synchronization with the base station 420 by receiving a preamble of a
downlink frame transmitted from the base station 420.
If system synchronization is acquired between the subscriber station 400 and
the base station 420 in this way, the base station 420 transmits a DL_MAP message and
a UL_MAP message to the subscriber station 400 (Steps 411 and 413). The DL_MAP
message, as described in connection with Table 1, transmits, to the subscriber station 400,
information necessary for acquiring synchronization with the base station 420 by the
subscriber station 400 in a downlink and information on a structure of a physical channel
capable of receiving, through the synchronization, messages transmitted to the
subscriber stations 400 in the downlink. The ULJV1AP message, as described in
connection with Table 2, transmits, to the subscriber station 400, information on a
scheduling period of the subscriber station and a structure of a physical channel.
The DL_MAP message is periodically transmitted from a base station to all
subscriber stations, and when a subscriber station can continuously receive the message,
it is said that the subscriber station is synchronized with the base station. That is,
subscriber stations that succeeded in receiving the DLJVIAP message can receive all
messages transmitted through a downlink.
i
As descried in conjunction with Table 3, when a subscriber station fails in
access, a base station transmits to the subscriber station the UCD message containing
information representing an available backoff value.
However, when performing the ranging, the subscriber station 400 transmits an
RNG_REQ message to the base station 420 (Step 415). Upon receiving the RNG_REQ
message, the base station 420 transmits to the subscriber station 400 an RNG_RSP
message containing the above-stated information for correcting frequency, time, and
transmission power (Step 417).
A configuration of the RNG_REQ message is illustrated in Table 4 below.
Table 4


In Table 4, Downlink Channel ID represents a downlink channel ID for a
channel that the subscriber station received from the base station, and Pending until
Complete represents priority of a transmission ranging response. For example, Pending
until Complete=0 indicates that a previous ranging response has priority over other
ranging responses, while Pending until Completed indicates that a currently-transmitted
ranging response has priority over other ranging responses.
In addition, a configuration of the RNG_RSP message responsive to the
RNG_REQ message of Table 4 is illustrated in Table 5 below.
Table 5

i
In Table 5, Uplink Channel ID represents an uplink channel ID for an
RNGREQ message that the base station received.
The OFDMA communication system proposed in an IEEE 802.16a may replace
the RNG_REQ message by using a method of designating a dedicated ranging period to
more efficiently perform the ranging and transmitting ranging codes for the dedicated
period. A communication procedure in the OFDMA broadband wireless access
communication system is illustrated in FIG. 5.

Referring to FIG. 5, a base station 520 transmits a DL_MAP message and a
UL_MAP message to the subscriber station 500 (Steps 511 and 513), and details thereof
are equal to those described in connection with FIG 4. Further, as described above, in
the OFDMA communication system, a ranging code is transmitted instead of the
RNG_REQ message used in FIG 4 (Step 515), and upon receiving the ranging code, the
base station 520 transmits an RNGJR.SP message to the subscriber station 500 (Step
517).
However, new information must be added for writing information responsive to
the ranging code transmitted to the base station in the RNG_RSP message. The new
information that must be added to the RNG_RSP message includes:
1. Ranging Code: received ranging CDMA code;
2. Ranging Symbol: OFDM symbol in the received ranging CDMA code;
3. Ranging subchannel: ranging subchannel in the received ranging CDMA
code; and
4. Ranging frame number: frame number in the received ranging CDMA code.
As descried above, the IEEE 802.16a communication system does not take
mobility of a subscriber station into consideration, i.e., it considers that the subscriber
station is located in a fixed position, and considers only a single-cell configuration.
However, as described above, it is provided that the IEEE 802.16e communication
system considers mobility of a subscriber station in addition to characteristics of the
IEEE 802.16a communication system. Therefore, the IEEE 802.16e communication
system must consider mobility of a subscriber station in a multi-cell environment. In
order to provide mobility of a subscriber station in the multi-cell environment,
modification of operations of the subscriber station and the base station is necessary.
However, the IEEE 802.16e communication system has proposed no specification for
the multi-cell environment and the mobility of a subscriber station. Therefore, in order to
support the mobility of a subscriber station, the IEEE 802.16e communication system
requires a method for performing a handover taking an idle state and also a
communication state into consideration.
SUMMARY OF THE INVENTION
i
It is, therefore, an object of the present invention to provide a system and

method for performing efficient data communication while guaranteeing mobility of a
subscriber station in a broadband wireless access communication system.
It is another object of the present invention to provide a system and method for
performing a handover between base stations to secure mobility of a subscriber station in
a broadband wireless access communication system.
It is further another object of the present invention to provide a system and
method for receiving, at a handover request of a serving base station, a handover request
message containing handover-related information from a subscriber station, determining
a target base station to which the subscriber station can be handed over from the serving
base station in communication, and transmitting the determined result to the subscriber
station.
In accordance with one aspect of the present invention, there is provided a
method for performing a handover by a subscriber station at a request of a serving base
station in a broadband wireless access communication system including me serving base
station for providing a service to the subscriber station through at least one frequency
bandwidth obtained by dividing an entire frequency bandwidth comprised of a plurality
of subcarriers, and a plurality of neighbor base stations being adjacent to the serving
base station. The method comprises the steps of: receiving information on the neighbor
base stations from the serving base station; measuring carrier-to-interference and noise
ratios (CINRs) of frequency bandwidth signals from the neighbor base stations based on
the information on the neighbor base stations; receiving a handover scan request
message from the serving base station; transmitting to the serving base station a
handover request mdssage containing information on the measured CINRs of the
neighbor base stations; receiving, from the serving base station, information on at least
one target base station capable of supporting handover of the subscriber station among
the neighbor base stations; and performing handover from the serving base station to one
of the at least one target base stations.
In accordance with another aspect of the present invention, there is provided a
method for performing a handover by a serving base station in a broadband wireless
access communication system including the serving base station providing a service to a
subscriber station through at least one frequency bandwidth obtained by dividing an

entire frequency bandwidth comprised of a plurality of subcarriers, and a plurality of
neighbor base stations being adjacent to the serving base station. The method comprises
the steps of: transmitting a handover scan request message to the subscriber station, if a
handover of the subscriber station is required; receiving carrier-to-interference and noise
ratios (CINRs) of the neighbor base stations from the subscriber station in response to
the handover scan request message, and sorting the neighbor base stations in order of
CINR levels; sequentially sending a handover connection request to the neighbor base
stations in order of CINR levels; and transmitting information on a particular neighbor
base station to the subscriber station upon receiving a handover connection response
with ACK (Acknowledgement) information from the particular neighbor base station in
response to the handover connection request.
In accordance with further another aspect of the present invention, there is
provided a broadband wireless access communication system comprising: a subscriber
station for sending a serving base station a request for handover to at least one neighbor
base station having a carrier-to-interference and noise ratio (CINR) satisfying a handover
condition, in response, to a handover scan request message, and performing handover to
a particular neighbor base station that transmits a handover response in response to the
handover request; the serving base station currently in communication with the
subscriber station, for transmitting the handover scan request message to the subscriber
station if handover of the subscriber station is required, sending a handover connection
request to neighbor base stations in order of levels of CINRs reported from the
subscriber station, and sending the subscribe station information on a particular neighbor
base station upon receiving a handover connection response with ACK information from
the particular neighbor base station; and the particular neighbor base station for
determining whether it can support handover of the subscriber station, in response to the
handover connection request, and sending the handover connection response with ACK
information to the serving base station if it can support handover of the subscriber
station.
BRIEF DESCRIPTION OF THE (DRAWINGS ^
The above and other objects, features, and advantages of the present invention
will become more apparent from the following detailed description when taken in
conjunction with the accompanying drawings in which:

FIG 1 is a diagram schematically illustrating a configuration of a broadband
wireless access communication system employing Orthogonal Frequency Division
Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA);
FIG 2 is a diagram schematically illustrating a downlink frame format for an
OFDM/OFDMA broadband wireless access communication system;
FIG 3 is a diagram schematically illustrating an uplink frame format for an
OFDM/OFDMA broadband wireless access communication system;
FIG 4 is a diagram illustrating a ranging procedure between a subscriber station
and a base station in an OFDM broadband wireless access communication system;
FIG 5 is a diagram illustrating a ranging procedure between a subscriber station
and a base station in an OFDMA broadband wireless access communication system;
FIG 6 is a diagram schematically illustrating a configuration of an
OFDM/OFDMA broadband wireless access communication system according to an
embodiment of the present invention;
FIG 7 is a diagram illustrating a procedure for determining a handover by a
serving base station at a handover request of the serving base station in an OFDM
broadband wireless access communication system according to a first embodiment of the
present invention;
FIG. 8 is a diagram illustrating a procedure for determining a handover by a
serving base station at a handover request of the serving base station in an OFDM
broadband wireless access communication system according to a second embodiment of
the present invention;
FIG 9 is a block diagram illustrating a_structure of a subscriber station
according to an embodiment of the present invention;
FIG 10 is a flowchart illustrating a procedure for performing a handover by a
subscriber station in response to a handover scan request from a serving base station
according to an embodiment of the present invention; and
FIG. 11 is a flowchart illustrating a procedure for performing a handover by a
serving base station in response to a handover request from a subscriber station
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Several preferred embodiments of the present invention will now be described

in detail herein below with reference to the annexed drawings. In the following
description, a detailed description of known functions and configurations incorporated
herein has been omitted for conciseness.
FIG 6 is a diagram schematically illustrating a configuration of an
OFDM/OFDMA broadband wireless access communication system according to an
embodiment of the present invention. Before a description of FIG 6 is given, it should
be noted that as stated in the related art section, an IEEE 802.16e communication system
considers the mobility of a subscriber station (SS) in addition to characteristics of the
IEEE 802.16a communication system, but no specification has been proposed for the
communication system. In order to consider the mobility of a subscriber station in
addition to characteristics of the IEEE 802.16a communication system, the IEEE
802.16e communication system can consider a multi-cell configuration and handover of
a subscriber station between multiple cells. Therefore, the present invention proposes a
configuration of an IEEE 802.16e communication system illustrated in FIG 6. The IEEE
802.16e communication system is a broadband wireless access (BWA) communication
system employing Orthogonal Frequency Division Multiplexing (OFDM) and
Orthogonal Frequency Division Multiple Access(OFDMA) (hereinafter, referred to as
"OFDM/OFDMA broadband wireless access communication system"), and for the
convenience of explanation, the description will be made with reference to the IEEE
802.16e communication system.
Referring to FIG 6, the IEEE 802.16e communication system has a multi-cell
configuration, and is comprised of a cell 600, a cell 650, a base station 610 for
controlling the cell 600, a base station 640 for controlling the cell 650, and a plurality of
subscriber stations 611, 613, 630, 651, and 653. Signal exchange between the base
stations 610 and 640 and their associated subscriber stations 611, 613, 630, 651, and 653
is achieved using the OFDM/OFDMA technology. Among the subscriber stations 611,
613, 630, 651, and 653, the subscriber station 630 is located in a boundary region, or a
handover region, between the cell 600 and the cell 650. Therefore, it is necessary to
support a handover of the subscriber station 630 in order to support mobility of the
subscriber station 630.
In a broadband wireless access communication system, a subscriber station
receives pilot channels transmitted from a plurality of base stations. The subscriber

station measures carrier-to-interference and noise ratios (CINRs) of the received pilot
channels. As a result of the measurement results, the subscriber station selects a base
station having the highest CINR among the measured CINRs. That is, the subscriber
station detects a base station to which it belongs, by selecting a base station having the
best channel condition among a plurality of base stations transmitting the pilot channels.
Herein, a base station having the best channel condition with respect to the subscriber
station will be referred to as an "active base station" or a "serving base station."
After selecting the active base station, the subscriber station receives a
downlink frame and an uplink frame transmitted from the active base station. Formats of
the uplink frame and the downlink frame transmitted from the active base station are
similar to the frame formats descried in the related art section. Table 6 below illustrates a
message to be added for the present invention in addition to the DL_MAP message
shown in Table 1.
Table 6




It can be understood from Table 6 that the active base station transmits the
DL_MAP message along with information on neighbor base stations. The "neighbor
base station" refers to a base station to which a subscriber station can be handed over
from the active base station. A parameter MAX_T represents a maximum time for which
CUSTRs measured by the subscriber station using pilot channels received from the
neighbor base stations remain below a threshold set by a subscriber. The MAX_T is set
in order for the subscriber station to measure CINRs of pilot signals received from the
neighbor base stations and determine whether there are neighbor base stations of which
CINRs remain below the threshold for the set time. As a result of the determination, the
neighbor base stations with CINRs lower than the threshold suspend CINR measurement
even though they are included in the neighbor base station list. This excludes an
unnecessary operation of measuring CINRs of the neighbor base stations whose CINRs
are lower than the threshold. However, the neighbor base stations for which CINR
measurement is suspended can be newly included in the neighbor base station list at a
user's option. That is., when the time set by the subscriber has passed, the subscriber
station again performs CINR measurement for the neighbor base stations with CTNRs
lower than the threshold.
A parameter MTN_T represents a minimum time for which a CINR of a
neighbor base station is higher than CINR of the active base station when the subscriber
station sends a handover request message to the neighbor base station. The MIN_T is set
in order to prevent a ping-pong phenomenon in which the subscriber station sends a
handover request to the base station each time a CINR of a received pilot channel is
higher than a CINR of the active base station. The MAX_T and the MTN_T can be
changed according to conditions of the base stations and channel conditions.
i
A parameter Measurement mode represents a method of measuring a pilot
CINR of the neighbor base station and reporting the measured pilot CINR to the active
base station on a periodical basis or an event basis. When the measured pilot CINR is
reported on a periodic basis, a value of a parameter 'report period' is set. When the
measured pilot CINR is reported on an event basis, possible events are divided into an
'event a', where the subscriber station requests handover, and an 'event b', where the
active base station requests handover. In a mode where the report is made on an event
basis, the subscriber station requests handover when the measured pilot CINR of the
neighbor base station is higher than a CINR of the active base station, while the active

base station requests handover when the active base station transmits a handover scan
request message to the subscriber station.
A parameter Measurement command represents information based on which the
subscriber station newly determines whether to set, reset or release information on a
method of measuring a pilot CINR of the neighbor base station. Every frame, the active
base station can command the subscriber station to set, reset, or release a new
measurement method through a DL_MAP message.
A parameter Measurement configuration represents information on a method of
measuring a pilot CINR of the neighbor base station by the subscriber station. The
parameter Measurement configuration includes information on a measurement start
frame where the subscriber station should measure a pilot CINR of the neighbor base
station, and information on measurement period for which the subscriber station
performs the measurement. Based on this information, the subscriber station must
periodically measure a pilot CINR of the neighbor base station.
A process in \vhich the subscriber station sends a ranging request to the active
base station upon receiving the DLJVIAP message and the UL_MAP message, and a
process in which the active base station transmits a ranging response message to the
ranging-requesting subscriber station, are equivalent to the corresponding processes
described above. Therefore, these processes will not be described again herein. Upon
receiving the ranging response message, the subscriber station performs wireless access
communication with the active base station.
With reference to FIG. 7, a description will now be made of a handover process
during wireless access communication between the subscriber station and the active base
station when the active base station sends a handover scan request to the subscriber
station. More specifically, FIG. 7 is a diagram illustrating a procedure for determining a
handover by an active base station at a handover request of the active base station in a
broadband wireless access communication system according to a first embodiment of the
present invention, wherein the active base station sends a handover scan request to a
subscriber station, and the subscriber station then transmits a handover request
containing a measured pilot CINR to the active base station.

In FIG 7, the OFDM broadband wireless access communication system
includes a subscriber station 701, a serving base station 702, and target base stations 703
and 704. A description will now be made of i) a process in which the subscriber station
701 measures pilot signals from neighbor base stations, ii) a process in which the serving
base station 702 sends a handover scan request to the subscriber station 701, iii) a
process in which the serving base station 702 determines a target base station to which
the subscriber station 701 will be handed over in response to a handover request from
the subscriber station 701, and iv) a process of setting up ranging between the target base
station and the subscriber station 701. A description will first be made of a process in
which the subscriber station 701 measures pilot signals from neighbor base stations.
In steps 711 and 712, the subscriber station 701 receives a DL_MAP message
and a ULJVIAP message from the serving base station 702. The detailed configurations
of the DL_MAP message and the ULJVIAP message have been described with reference
to Table 2 and Table ,6. The DLJV1AP message is transmitted to the subscriber station
701 along with the information shown in Table 6. The subscriber station 701 receives the
DL_MAP message and manages a neighbor base station list transmitted from the serving
base station 702. In step 731, the subscriber station 701 measures CINRs of pilot signals
received from the neighbor base stations using the neighbor base station list transmitted
from the serving base station 702.
The process in which the subscriber station 701 measures CINRs of pilot
signals received from the neighbor base stations is achieved by suspending by the
subscriber station 701 reception of data transmitted from the serving base station 702.
That is, the subscriber station 701 suspends reception of data transmitted from the
serving base station 702 and measures CINRs of pilot signals received from the neighbor
base stations for the suspended time. In this case, it is preferable to measure only CINRs
of pilot signals received from neighbor base stations not excluded by the MIN_T value,
rather than measuring CINRs of pilot signals received from all neighbor base stations
included in the neighbor base station list.
When the serving base station 702 desires that another base station should
process a call of the subscriber station 701, the serving base station 702 sends a
handover scan request message to the subscriber station 701 in step 724. A configuration
of the handover scan request message transmitted from the serving base station 702 to

the subscriber station 701 is illustrated in Table 7 below.
Table 7

As illustrated in Table 7, the serving base station 702 sends the subscriber
station 701 the handover scan request message containing Measurement IE and
Activation time. The Measurement IE indicates a measurement and report method
requested by the serving base station 702. The Activation time indicates a maximum
frame time for which the serving base station 702 desires to perform handover. When
sending the handover scan request to the subscriber station 701, the 'event b' is set as
defined in Table 6.
In step 713, the subscriber station 701 sends a handover request message to
message to the serving base station 702. An example of the handover request message
transmitted from the subscriber station 701 to the serving base station 702 is illustrated
in Table 8 below.
Table 8



As illustrated in Table 8, the subscriber station 701 sends the serving base
station 702 carrier frequencies of neighbor base stations included in the neighbor base
station list and the measured CINRs. In addition, the subscriber station 701 informs the
serving base station 702 of a channel of the subscriber station 701 located in the
handover region by transmitting an identifier (ID) of an uplink channel over which it
exchanges data with the serving base station 702. Moreover, the subscriber station 701
designates a desired quality of service (QoS) and a desired bandwidth (BW). The QoS
can be classified into Unsolicited Grant Service (UGS), Real-Time Polling Service
(rtPS), Non-Real-Time Polling Service (nrtPS), and Best Effort Service (BE).
A description will now be made of a process in which the serving base station
702 determines a target base station in response to the handover request from the
subscriber station 701. Upon receiving the handover request message from the
subscriber station 701, the serving base station 702 sorts neighbor base stations included
in the handover request message in step 732. There are several possible methods for
sorting the neighbor base stations, and in the embodiment of the present invention, the
neighbor base stations are sorted in order of their CINR level, by way of example. Of
course, the neighbor base stations can be sorted in other methods. CTNRs of neighbor
base stations, CINR measurement for which is suspended by the subscriber station 701,
have a value of '0'. The serving base station 702 can store information on the sorted
neighbor base stations in a list.
After sorting the neighbor base stations depending on CINR information
included in the received handover request message, the serving base station 702
sequentially transmits a handover connection request message to the neighbor base
stations in sorted order. In step 714, the serving base station 702 transmits the handover
connection request message to a first target base station 703 having the highest CINR.
An example of the handover connection request message is illustrated in Table 9 below.
Table 9



As illustrated in Table 9, the handover connection request message is
transmitted along with the QoS and the BW desired by the subscriber station 701.
Therefore, the serving base station 702 must determine whether the first target base
station 703 selected for handover can meet the QoS and the BW requested by the
subscriber station 701. The serving base station 702 transmits the handover connection
request message along with information on the QoS and the BW, and receives a response
message corresponding thereto, in order to determine whether the first target base station
703 is an available target base station.
Upon receiving the handover connection request message, the first target base
station 703 transmits to the serving base station 702 a handover connection response
message in response to the received handover connection request message, in step 715.
An example of the handover connection response message is illustrated in Table 10
below.
Table 10

As illustrated in Table 10, the first target base station 703 determines whether it
can support the QoS and the BW requested by the subscriber station 701, included in the
received handover connection request message. If the first target base station 703 can
support the QoS and the BW requested by the subscriber station 701, the first target base
station 703 transmits the handover connection response message along with ACK
(Acknowledgement) information. However, if the first target base station 703 cannot
support the QoS and the BW, it transmits the handover connection response message

containing NACK (Negative Acknowledgement) information. For example, in step 715,
the first target base station 703 transmits the handover connection response message to
the serving base station 702 along with the NACK information. That is, the first target
base station 703 cannot support the QoS and the BW requested by the subscriber station
701.
Upon receiving the handover connection response message from the first target
base station 703, in step 716, the serving base station 702 transmits a handover
connection request message to a second target base station 704 having a second highest
CESTR. The handover connection request message transmitted in step 716 is identical to
the handover connection request message transmitted in step 714 except that a Target BS
ID included therein. Upon receiving the handover connection request message, the
second target base station 704 sends a response message to the serving base station 702
in response to the received handover connection request message. That is, in step 717,
the second target base station 704 delivers a handover connection response message to
the serving base station 702. Similarly, the second target base station 704 determines
whether it can support the QoS and the BW requested by the subscriber station 701, and
then transmits a handover connection response message containing the determined result.
For example, in FIG. 7, the second target base station 704 can support the QoS and the
BW requested by the subscriber station 701.
Upon receiving the handover connection response message from the second
target base station 704, the serving base station 702 transmits a handover response
message to the subscriber station 701 in step 718. The handover response message
includes information on the selected target base station and a frequency bandwidth used
by the selected target base station. An example of the handover response message is
illustrated in Table 11 below.
i
Table 11


After transmitting the handover response message to the subscriber station 701
in step 718, the serving base station 702 transmits a handover connection confirmation
message to the second target base station 704 in step 719. An example of the handover
connection confirmation message is illustrated in Table 12.
Table 12

After transmitting the handover connection confirmation message to the second
target base station 704, the serving base station 702 releases a call connected to the
subscriber station 701 in step 734.
In steps 720 and 721, the second target base station 704 transmits a DL_MAP
message and a UL_MAP message to the subscriber station 701. The DL_MAP message
and the ULJvIAP message are transmitted after information on the subscriber station
701 included therein is updated. Upon receiving the DLJV1AP message and the
UL_MAP message, the subscriber station 701 transmits a ranging request message to the
second target base station 704 in step 722. Upon receiving the ranging request message,
the second target base station 704 transmits a ranging response message to the subscriber
station 701 in step 723. A detailed process performed in the steps 720 to 723 is identical
to the process performed in the steps 411 to 417 illustrated in FIG. 4. Therefore, a
detailed description of these step will not be given again herein.
FIG 8 is a diagram illustrating a procedure for determining handover by a
serving base station at a handover request of the serving subscriber station in an
OFDMA broadband wireless access communication system according to a second
embodiment of the present invention. The elements used in FIG. 8 are identical to those
in FIG. 7. In addition, steps 811 to 834 of FIG. 8 are identical in operation to the steps
711 to 734 illustrated in FIG 7. Further, steps 820 to 823 are identical in operation to the
steps 511 to 517 illustrated in FIG 5. Therefore, a detailed description of FIG 8 will be
omitted for simplicity.

FIG 9 is a block diagram illustrating a subscriber station according to an
embodiment of the present invention. As illustrated in FIG 9, the subscriber station is
comprised of a matched filter 900, a reception power measurer 910, a reception power
comparator 920, a controller 930, and a transmitter 940. A PN code for synchronization
detection received at a receiver (now shown) is applied to the matched filter 900, and the
matched filter 900 outputs a specific energy value according to whether synchronization
is acquired. A correlator, or a correlation detector, can be used in place of the matched
filter 900. The matched filter 900 compares the received PN code for synchronization
detection with a unique PN code stored in the receiver, and outputs a specific value when
they are identical. That is, the matched filter 900 sequentially inputs received signals into
a particular window, and bit-operates the window with the unique PN code on a parallel
basis, and sums up the bit-operated values. If the received signal is identical to the
unique PN code stored in the receiver, it indicates an autocorrelated state and the
matched filter 900 outputs a maximum value. However, if the received signal is not
identical to the unique PN code, it indicates a non-autocorrelated state and the matched
filter 900 outputs a relatively low value. Generally, the output values are compared with
a given threshold to determine whether autocorrelation is detected. That is, whether
autocorrelation is detected or not can be determined based on the output value of the
matched filter 900.
If it is determined by the matched filter 900 that a pilot channel received from a
neighbor base station is autocorrelated, the reception power measurer 910 measures
reception power of the received pilot channel. That is, the reception power measurer 910
measures a CINR of the received pilot channel, and delivers information on the
measured CINR for the received pilot channel to the reception power comparator 920.
The reception power comparator 920 compares the CINRs of neighbor base stations,
provided from the reception power measurer 910, with a predetermined threshold. If one
or more CINRs among the measured CINRs of the neighbor base stations are higher than
the threshold, the reception power comparator 920 compares CINRs of the neighbor
base stations, being higher than the threshold, with a CINR of a serving base station, to
determine whether there is any CINR that is higher than the CINR of the serving base
station. The reception power comparator 920 provides the determined result to the
controller 930. When a handover scan request message is received from the serving base
station, the controller 930 performs a control operation of transmitting a handover
request message containing the determined result to the serving base station.

That is, the controller 930 of the receiver receiving the handover scan request
message from the serving base station generates a handover request message and
provides the generated handover request message to the transmitter 940. The transmitter
940, under the control of the controller 930, transmits the handover request message to
the serving base station.
FIG 10 is a flowchart illustrating an operation of a subscriber station according
to an embodiment of the present invention. In steps 1000 and 1002, the subscriber
station reads a DL_MAP message and a UL_MAP message received from the serving
base station. In step 1004, the subscriber station reads a neighbor base station list
included in the DL_MAP message. The neighbor base station list includes information
on neighbor base stations, received from the serving base station.
In step 1006, the subscriber station measures CINRs of pilot channels
transmitted from the neighbor base stations. Unique numbers of the neighbor base
stations, provided from the serving base station, are BS_1 to MAX_BS_NUM. At first,
the subscriber station measures in step 1006 a CINR of a neighbor base station BS_1,
and then proceeds to step 1008. The subscriber station determines in step 1008 whether a
unique number of ;the CINR-measured neighbor base station is smaller than
MAX_BS_NUM. If it is determined that a unique number of the CINR-measured
neighbor base station is greater than or equal to MAX_BS_NUM, the subscriber station
proceeds to step 1010. However, if a unique number of the CINR-measured neighbor
base station is smaller than MAX_BS_NUM, the subscriber station returns to step 1006.
In step 1006, the subscriber station increases a unique number of the neighbor base
station by one, and measures a CINR of a neighbor base station having the increased
unique number.
In step 1010, the subscriber station compares CINRs of the neighbor base
stations with a CINR of the serving (or active) base station. Of course, before comparing
CINRs of the neighbor base stations with a CINR of the serving base station, the
subscriber station first compares CINRs of the neighbor base stations with a threshold. If
the highest CINR among the CINRs of the neighbor base stations is lower than the
CINR of the serving base station, the subscriber station determines in step 1026 whether
the handover scan request message is received from the serving base station. If the

serving base station requests handover, the subscriber station will receive the handover
scan request message from the serving base station. However, if the handover scan
request message is not received, the subscriber station returns to step 1000 to receive the
DL_MAP message which the serving base station transmits. However, if the highest
CTNR among the CINRs of the neighbor base stations is greater than or equal to the
CINR of the serving base station, the subscriber station determines to send a handover
request, and then proceeds to step 1012.
According to an embodiment of the present invention, the subscriber station can
be handed over at the request of the serving base station even if it is determined in step
1010 that the highest CINR among the CINRs of the neighbor base stations is lower than
the CINR of the serving base station.
Upon receiving the handover scan request message from the serving base
station, the subscriber station transmits a handover request message to the serving base
station in step 1012. The detailed configuration of the handover request message is
illustrated in Table 6. (
After transmitting the handover request message, the subscriber station receives
a handover response message in step 1014. The detailed configuration of the handover
response message is illustrated in Table 11. In FIG. 10, the handover response message
includes ACK information for the handover requested by the subscriber station. In step
1016, the subscriber station reads an ID of a target base station and a carrier frequency
used in the target base station, included in the handover response message.
In step 1018, the subscriber station changes its frequency to a frequency of the
target base station. As a result, the subscriber station suspends data exchange with the
serving base station and performs data exchange with the target base station. For that
i
purpose, the subscriber station reads a DLJVIAP message and a UL_MAP message
transmitted from the target base station in steps 1020 and 1022, and performs the data
exchange with the target base station in step 1024.
FIG. 11 is a flowchart illustrating an operation of a serving base station
according to an embpdiment of the present invention. In steps 1100 and 1102, the
serving base station transmits a DLJVIAP message and a UL_MAP message to the

subscriber station. When the serving base station desires to hand over the subscriber
station as occasion demands, it transmits a handover scan request message to the
subscriber station in step 1122. The handover scan request message has been described
above with reference to Table 7.
Upon receiving the handover scan request message from the serving base
station, the subscriber station transmits a handover request message to the serving base
station. The serving base station then receives the handover request message from the
subscriber station in step 1104. The configuration of the handover request message has
been described above with reference to Table 8.
Thereafter, in step 1106, the serving base station sorts the neighbor base stations
included in the handover request message, in order of their CINR level. As described
with reference to FIG 9, unique numbers of the neighbor base stations include BS_1 to
MAX_BS_NUM. After completion of sorting the neighbor base stations, in step 1108,
the serving base station transmits a handover connection request message to a neighbor
base station (i.e., target base station) having the highest CINR among the sorted
neighbor base stations. The configuration of the handover connection request message
has been described above with reference to Table 9.
i
After transmitting the handover connection request message, serving base
station receives in step 1110a handover connection response message from the neighbor
base station that transmitted the handover connection request message. The
configuration of the handover connection response message has been described above
with reference to. Table 10.
i
Upon receiving the handover connection response message, the serving base
station determines in step 1112 whether the target base station can support handover, i.e.,
received an ACK. If it is determined that the target base station can support a handover,
the serving base station proceeds to step 1116. However, if the target base station cannot
support a handover, the serving base station proceeds to step 1114. In step 1114, the
serving base station selects a neighbor base station having the second highest CINR, and
then transmits the handover connection request message to the selected neighbor base
station.

In step 1116, the serving base station transmits a handover connection
confirmation message to an available target base station capable of supporting handover.
The configuration of the handover connection confirmation message has been described
above with reference to Table 12. After transmitting the handover connection
confirmation message, the serving base station transmits a handover response message,
which has been described with reference to Table 11, to the subscriber station in step
1118. The order of the steps 1116 and 1118 is changeable at a user's option. After steps
1116 and 1118, the serving base station releases a link (or call) connected to the
subscriber station in step 1120.
As described above, in the proposed broadband wireless access communication
system having a multi-cell configuration to support mobility of subscriber stations, a
serving base station requests a subscriber station to perform handover to another base
station in order to distribute calls of subscriber stations located in a overloaded cell. In
the existing single-cell configuration, call distribution to another cell is unavailable, so
an overloaded cell cannot accommodate subscriber stations. However, in the
embodiment of the present invention, when a cell is overloaded, its serving base station
requests subscriber stations to perform handover to another base station to distribute
their calls, thereby making it possible to accommodate more subscriber stations.
While the present invention has been shown and described with reference to
certain preferred embodiments thereof, it will be understood by those skilled in the art
that various changes in form and details may be made therein without departing from the
spirit and scope of the present invention as defined by the appended claims.

WE CLAIM
1. A method for performing a handover by a subscriber station in a broadband
wireless access communication system, the method comprising:
receiving information on a plurality of neighbor base stations from the
serving base station;
measuring carrier-to-interference and noise ratios (CINRs) of frequency
bandwidth signals from the neighbor base stations based on the information
about the neighbor base stations;
receiving a handover scan request message from the serving base station;
transmitting to the serving base station a handover request message
including information about the measured CINRs of selected neighbor base
stations satisfying a predetermined handover condition in response to the
handover scan request message;
receiving, from the serving base station, a handover response message
including information on at least one target base station among the selected
neighbor base stations, the at least one target base station capable of supporting
the handover of the subscriber station among the neighbor base stations; and
performing the handover from the serving base station to one of the at least one
target base station,

wherein the predetermined handover condition comprises a first
handover condition and second handover condition,
wherein the first handover condition includes a CINR of a particular neighbor
base station should not remain lower than a minimum CINR for a maximum
holding time; and the second handover condition, in which the CINR of the
particular neighbor base station should remain higher than a measured CINR of
the serving base station for a minimum holding time..
2. The method as claimed in claim 1, wherein the information about the
neighbor base stations is included in a downlink message received from the
serving base station.
3. The method as claimed in claim 1 or 2, comprising the step of stopping the
CINR measurement of neighbor base stations with CINRs not satisfying the
first handover condition, among the measured CINRs of the neighbor base
stations.

4. The method as claimed in claim 1, 2 or 3, wherein CINRs of the neighbor
base stations not including the neighbor base stations which are stopped by
the subscriber station, are set to v0'.
5. The method as claimed in claim 4, wherein the handover request message is
transmitted to the serving base station along with the CINRs of the neighbor
base stations, a quality of service (QoS), and a bandwidth.
6. A subscriber station for performing a handover in a broadband wireless
access communication system comprising:
the subscriber station for receiving information on a plurality of neighbor base
stations from the serving base station, measuring carrier-to-interference and noise
ratios (CINRs) of frequency bandwidth signals from the neighbor base stations
based on the information on the neighbor base stations, receiving a handover scan
request message from the serving base station, transmitting to the serving base
station a handover request message including information about measured CINRs of
selected neighbor base stations satisfying a predetermined handover condition in
response to the handover scan request message; receiving, from the serving base
station, a handover response message including information on at least one target
base station among the selected neighbor base stations, the at least one target base

station capable of supporting the handover of the subscriber station among the
neighbor base stations; and performing the handover from the serving base station
to one of the at least one target base station,
wherein the predetermined handover condition comprises a first
handover condition and second handover condition,
wherein the first handover condition includes a CINR of a particular
neighbor base station should not remain lower than a minimum CINR for a
maximum holding time; and the second handover condition, in which the
CINR of the particular neighbor base station should remain higher than a
measured CINR of the serving base station for a minimum holding time.
7. The subscriber station as claimed in claim 6, wherein the handover condition
comprises:
wherein when there is a CINR satisfying the second handover condition
among measured CINRs of the at least one neighbor base stations, the
subscriber station requests a handover to a neighbor base station
corresponding to a CINR satisfying the first handover condition.

8. The subscriber station as claimed in claim 7, wherein CINR measurement on
neighbor base stations with CINRs not satisfying the first handover condition
among the measured CINRs of the neighbor base stations, is suspended.
9. The subscriber station as claimed in claim 6, wherein CINRs of the neighbor
base stations not having the neighbor base stations which are stopped by
the subscriber station, are set to '0'.

Documents:

01557-kolnp-2005-claims.pdf

01557-kolnp-2005-description complete.pdf

01557-kolnp-2005-drawings.pdf

01557-kolnp-2005-form 1.pdf

01557-kolnp-2005-form 2.pdf

01557-kolnp-2005-form 3.pdf

01557-kolnp-2005-form 5.pdf

01557-kolnp-2005-international publication.pdf

1557-kolnp-2005-abstract.pdf

1557-KOLNP-2005-AMANDED CLAIMS.pdf

1557-kolnp-2005-claims.pdf

1557-KOLNP-2005-CORRESPONDENCE 1.1.pdf

1557-kolnp-2005-correspondence-1.1.pdf

1557-KOLNP-2005-CORRESPONDENCE.pdf

1557-kolnp-2005-correspondence1.2.pdf

1557-kolnp-2005-description (complete).pdf

1557-kolnp-2005-drawings.pdf

1557-kolnp-2005-examination report.pdf

1557-kolnp-2005-examination report1.1.pdf

1557-kolnp-2005-form 1.pdf

1557-kolnp-2005-form 18.1.pdf

1557-kolnp-2005-form 18.pdf

1557-kolnp-2005-form 2.pdf

1557-kolnp-2005-form 3.1.pdf

1557-kolnp-2005-form 3.pdf

1557-kolnp-2005-form 5.1.pdf

1557-kolnp-2005-form 5.pdf

1557-KOLNP-2005-FORM-27.pdf

1557-kolnp-2005-gpa.pdf

1557-kolnp-2005-gpa1.1.pdf

1557-kolnp-2005-granted-abstract.pdf

1557-kolnp-2005-granted-claims.pdf

1557-kolnp-2005-granted-description (complete).pdf

1557-kolnp-2005-granted-drawings.pdf

1557-kolnp-2005-granted-form 1.pdf

1557-kolnp-2005-granted-form 2.pdf

1557-kolnp-2005-granted-specification.pdf

1557-KOLNP-2005-OTHERS 1.1.pdf

1557-kolnp-2005-others.pdf

1557-kolnp-2005-others1.2.pdf

1557-kolnp-2005-reply to examination report.pdf

1557-kolnp-2005-reply to examination report1.1.pdf

1557-kolnp-2005-specification.pdf


Patent Number 248535
Indian Patent Application Number 1557/KOLNP/2005
PG Journal Number 30/2011
Publication Date 29-Jul-2011
Grant Date 22-Jul-2011
Date of Filing 05-Aug-2005
Name of Patentee SAMSUNG ELECTRONICS CO. LTD., a Korean company
Applicant Address 416, MAETAN-DONG, YEONGTONG-GU, SUWON-SI, GYEONGGI-DO
Inventors:
# Inventor's Name Inventor's Address
1 So-Hyun KIM #531-1402, SHINAN APT., YEONGTONG-DONG, PALDAL-GU, SUWON-SI, GYEONGGI-DO
2 Jung-Je SON #401 905, 181, SANGNOKMAEUL BOSEONG APT., JEONGJA-DONG, BUNDANG-GU, SEONGNAM-SI, GYEONGGI-DO
3 Chang-Hoi KOO 2ND FLOOR, 241-8, JEONGJA-DONG, BUNDANG-GU, SEONGNAM-SI, GYEONGGI-DO
PCT International Classification Number H04B 7/26
PCT International Application Number PCT/KR2004/000470
PCT International Filing date 2004-03-05
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 10-2003-0014643 2003-03-08 Republic of Korea