U.S. patent application number 14/380391 was filed with the patent office on 2015-02-19 for method and apparatus for performing handover in c-ran system.
The applicant listed for this patent is LG ELECTRONICS INC.. Invention is credited to Jaehoon Chung, Sunam Kim, Youngsoo Yuk.
Application Number | 20150049623 14/380391 |
Document ID | / |
Family ID | 49006007 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150049623 |
Kind Code |
A1 |
Yuk; Youngsoo ; et
al. |
February 19, 2015 |
METHOD AND APPARATUS FOR PERFORMING HANDOVER IN C-RAN SYSTEM
Abstract
A method and apparatus for performing handover by a user
equipment (UE) in a C-RAN system is disclosed. The method for
performing handover by a user equipment (UE) in a Cloud Radio
Access Network (C-RAN) includes: measuring at least one candidate
remote radio head (RRH); transmitting feedback information caused
by the measurement to a primary RRH; and receiving information
regarding a changed primary RRH according to the measurement result
received from the primary RRH, wherein the measurement is performed
to discriminate each RRH on the basis of a channel state
information-reference signal (CSI-RS) antenna port discriminated
per the at least one candidate RRH.
Inventors: |
Yuk; Youngsoo; (Anyang-si,
KR) ; Kim; Sunam; (Anyang-si, KR) ; Chung;
Jaehoon; (Anyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG ELECTRONICS INC. |
Seoul |
|
KR |
|
|
Family ID: |
49006007 |
Appl. No.: |
14/380391 |
Filed: |
February 25, 2013 |
PCT Filed: |
February 25, 2013 |
PCT NO: |
PCT/KR2013/001478 |
371 Date: |
August 22, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61602088 |
Feb 23, 2012 |
|
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61602605 |
Feb 24, 2012 |
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61623072 |
Apr 12, 2012 |
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Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H04W 36/0055 20130101;
H04W 36/0069 20180801; H04W 88/06 20130101; H04W 36/0094 20130101;
H04W 24/08 20130101; H04W 36/0058 20180801; H04W 36/30
20130101 |
Class at
Publication: |
370/252 |
International
Class: |
H04W 36/00 20060101
H04W036/00; H04W 24/08 20060101 H04W024/08 |
Claims
1. A method for performing handover by a user equipment (UE) in a
Cloud Radio Access Network (C-RAN), the method comprising:
measuring at least one candidate remote radio head (RRH);
transmitting feedback information caused by the measurement to a
primary RRH; and receiving information regarding a changed primary
RRH from the primary RRH according to a result of the measurement,
wherein the measurement is performed to discriminate each RRH on
the basis of a channel state information-reference signal (CSI-RS)
antenna port discriminated per the at least one candidate RRH.
2. The method according to claim 1, further comprising: receiving
list information of the at least one candidate RRH from a base
station (BS) or the primary RRH.
3. The method according to claim 1, wherein the at least one
candidate RRH includes the primary RRH and an RRH communicating
with the UE.
4. The method according to claim 1, wherein the changed primary RRH
corresponds to an RRH having a highest signal intensity from among
the measurement result.
5. The method according to claim 1, further comprising:
transmitting a RACH to the changed primary RRH through RRH
dedicated RACH resources.
6. The method according to claim 1, further comprising: receiving
information regarding a CSI-RS antenna port discriminated per RRH
from a base station (BS).
7. A user equipment (UE) for performing handover in a Cloud Radio
Access Network (C-RAN) comprising: a receiver; a transmitter; and a
processor, wherein the processor measures at least one candidate
remote radio head (RRH), controls the transmitter to transmit
feedback information caused by the measurement to a primary RRH,
and controls the receiver to receive information regarding a
changed primary RRH from the primary RRH according to a result of
the measurement, and the processor performs the measurement to
discriminate each RRH on the basis of a channel state
information-reference signal (CSI-RS) antenna port discriminated
per the at least one candidate RRH.
8. The user equipment (UE) according to claim 7, wherein the
processor receives list information of the at least one candidate
RRH from a base station (BS) or the primary RRH.
9. The user equipment (UE) according to claim 7, wherein the at
least one candidate RRH includes the primary RRH and an RRH
communicating with the UE.
10. The user equipment (UE) according to claim 7, wherein the
processor controls the transmitter to transmit a RACH to the
changed primary RRH through RRH dedicated RACH resources.
11. The user equipment (UE) according to claim 7, wherein the
processor controls the receiver to receive information regarding a
CSI-RS antenna port discriminated per RRH from a base station
(BS).
12. The user equipment (UE) according to claim 7, wherein the
changed primary RRH corresponds to an RRH having the highest signal
intensity from among the measurement result.
Description
TECHNICAL FIELD
[0001] The present invention relates to wireless communication, and
more particularly to a method and apparatus for performing handover
in a Cloud Radio Access Network (C-RAN) system.
BACKGROUND ART
[0002] The 30-years history of mobile communication
commercialization on the basis of AMPS (the first-generation analog
mobile communication systems) has caused a rapid change in society,
such that mobile communication technologies have rapidly come into
widespread use and been commercialized throughout the world.
Specifically, mobile communication networks have generated a
variety of changes throughout modern society in recent times, and
have also rapidly developed.
[0003] Expansion of third-generation communication system has been
considerably delayed due to irregular development of mobile
communication and mobile computing, and fourth-generation
communication systems have been rapidly developed in response to
the computing environment rapidly changing from personal computers
(PCs) such as desktop computers and laptops to personal information
devices such as smartphones and tablets. Particularly, the recent
development of cloud computing environments requires an organic
combination of higher-level communication/computing technologies,
such that the above-mentioned tendency will be accelerated.
[0004] In recent times, as LTE-Advanced and WiMAX-Advanced are
approved as fourth-generation IMT-Advanced standards by the ITU,
many developers and companies are conducting intensive research
into fifth-generation mobile communication systems. For example,
ITU-R WP5D taking charge of IMT serving as one of the international
mobile communication standards has held a local Workshop entitled
"IMT for the Next Decade" in 2011, such that the ITU-R WP5D are
actively carrying out various activities arousing user interest in
fifth-generation communication. Besides, WWRF has investigated
requirements and versions of the NG-Wireless system and a first
report has been published by the WWRF.
[0005] Since the communication service environment has rapidly
changed over the last decade, it is very difficult for users to
predict how much the communication service environment will change
in the next decade. Recently, current communication standard
organizations such as ITU-R and WWRF have considered the following
change factors for 5 G communication up to the year 2020. [0006]
Development of Multimedia Service based on high-quality video
service. [0007] Provision of differentiated User eXperience (UX)
through personalization service: [0008] Provision of services
suited to personal interest, situation, and equipment. [0009]
Communication environment change from device-oriented communication
environment to user-oriented communication environment: A user owns
a plurality of communication devices, provision of user-oriented
services is requested. For example, content charging, provision of
seamless mobility between heterogeneous devices and security
service between heterogeneous devices may be the user-based
services. [0010] Extension of M2M service: Increasing traffic
through increase in the number of M2M devices and provision of new
M2M-based services are requested. [0011] Low Variation of Mobile
Cloud Computing environment: all computing environments are coupled
to the network, such that mobile cloud computing is provided
through provision of lower-latency and higher-performance
communication environments. The following important requirements of
5 G communication systems have been discussed according to the
changing service environment.
[0012] 1. Increase of Bandwidth/Transfer Rate
[0013] According to various research reports, it is expected that
the number of mobile devices and the amount of traffic will rapidly
increase over the next decade. In case of mobile devices, whereas
an increase in population is not large, it is expected that low-end
UEs will be replaced with Internet UEs such as smartphones or
tablets and the number of connected UEs such as M2M will rapidly
increase.
[0014] According to a report from Cisco, the amount of overall
mobile traffic has increased by about six fold from 2008 to 2010,
and will increase 26 fold by 2015 year such that monthly mobile
traffic will be 6.3 EB per month. UMTS has predicted that the
overall amount of mobile traffic will change from an annual of 3.8
EB in 2010 to 127 EB (corresponding to about 33 times the annual of
3.8 EB) by 2020 by referring to IDATE documents. For some years
from 2010 to 2020, it is expected that the number of mobile UEs
will be increased about two fold such that the number of mobile UEs
will be increased from 5 billion to about 10 billion.
[0015] It can be recognized that a first object of the 5G mobile
communication system is to increase a transfer rate on the basis of
the above-mentioned tendency. Although various methods can be used
to increase transfer rate, a first method is to find/use an
additional bandwidth that is not presently being used.
[0016] Although current mobile communication systems mainly operate
at 3 GHz or less, many developers and companies are conducting
intensive research into higher bands due to bandwidth limitations.
Specifically, various methods for guaranteeing system performance
in response to frequency characteristics changed within a high
frequency band of 2-6 GHz band have been intensively researched by
many developers and companies.
[0017] 2. Provision of Uniform Service Quality
[0018] 4G communication implements a maximum transfer rate of 1
Gbps such that it has been greatly improved in terms of a transfer
rate. However, the 4 G communication system has serious unbalance
in service quality between a cell edge region and normal cell
regions because a difference between spectrum efficiency of the
cell edge region and average spectrum efficiency of a cell is 30
times or more. Due to the motto "Provision of Desired Service from
anywhere at any time" of the 5 G communication system, improvement
of an unbalanced service quality is of importance to the 5G
communication system.
[0019] In association with the above-mentioned description,
although many developers and companies are conducting intensive
research into performance improvement in the cell edge region
through WiFi offloading, addition of an auxiliary cell such as
Femto BS, eICIC (Enhanced Inter-cell Interference Coordination) and
CoMP (Coordinated Multi-Point) technologies for use in the 4G
system, there is a need to provide a higher-level uniform
service.
[0020] 3. User-Oriented Organic Interoperation
[0021] Although the 4G system has implemented interoperation
between devices on the basis of user handling or predetermined
policy, the next-generation communication system requires
diversification of UEs, organic interoperation of several devices,
and various technologies for providing the same service level in
various communication environments. There are needed various
technologies in which communication technologies such as cellular
or WLAN are organically interoperable without using complicated
processes and user intervention is minimized through seamless
services.
[0022] Measurement establishment for enabling a UE to perform
handover in a cellular network has already been proposed. However,
when the UE performs handover in the C-RAN system, a method for
performing UE measurement, a method for classifying nodes in the
C-RAN system during the UE measurement, and a method for enabling
the UE to perform handover through such measurement have yet to be
proposed.
DISCLOSURE OF INVENTION
Technical Problem
[0023] Accordingly, the present invention is directed to a method
and apparatus for enabling a user equipment (UE) to perform
handover in a Cloud Radio Access Network (C-RAN) system that
substantially obviates one or more problems due to limitations and
disadvantages of the related art.
[0024] An other object of the present invention is to provide a
method for enabling a user equipment (UE) to perform handover in a
Cloud Radio Access Network (C-RAN) system.
[0025] Another object of the present invention is to provide a user
equipment (UE) for performing handover in a C-RAN system.
[0026] It is to be understood that technical objects to be achieved
by the present invention are not limited to the aforementioned
technical objects and other technical objects which are not
mentioned herein will be apparent from the following description to
one of ordinary skill in the art to which the present invention
pertains.
Solution to Problem
[0027] The object of the present invention can be achieved by
providing a method for performing a handover by a user equipment
(UE) in a Cloud Radio Access Network (C-RAN), the method including:
measuring at least one candidate remote radio head (RRH);
transmitting feedback information caused by the measurement to a
primary RRH; and receiving information regarding a changed primary
RRH from the primary RRH according to a result of the measurement,
wherein the measurement is performed to discriminate each RRH on
the basis of a channel state information-reference signal (CSI-RS)
antenna port discriminated per the at least one candidate RRH.
[0028] The method may further include: receiving list information
of the at least one candidate RRH from a base station (BS) or the
primary RRH. The at least one candidate RRH may include the primary
RRH and an RRH communicating with the UE. The changed primary RRH
may correspond to an RRH having a highest signal intensity from
among the measurement result. The method may further include:
transmitting a RACH to the changed primary RRH through RRH
dedicated RACH resources. The method may further include: receiving
information regarding a CSI-RS antenna port discriminated per RRH
from a base station (BS).
[0029] In another aspect of the present invention, a user equipment
(UE) for performing a handover in a Cloud Radio Access Network
(C-RAN) includes: a receiver; a transmitter; and a processor,
wherein the processor measures at least one candidate remote radio
head (RRH), controls the transmitter to transmit feedback
information caused by the measurement to a primary RRH, and
controls the receiver to receive information regarding a changed
primary RRH from the primary RRH according to a result of the
measurement, and the processor performs the measurement to
discriminate each RRH on the basis of a channel state
information-reference signal (CSI-RS) antenna port discriminated
per the at least one candidate RRH.
[0030] The processor may receive list information of the at least
one candidate RRH from a base station (BS) or the primary RRH. The
at least one candidate RRH may include the primary RRH and an RRH
communicating with the UE. The processor may control the
transmitter to transmit a RACH to the changed primary RRH through
RRH dedicated RACH resources. The processor may control the
receiver to receive information regarding a CSI-RS antenna port
discriminated per RRH from a base station (BS). The changed primary
RRH may correspond to an RRH having the highest signal intensity
from among the measurement result.
Advantageous Effects of Invention
[0031] As is apparent from the above description, the UE performs
efficient measurement for handover in the C-RAN system, and
properly selects a primary RRH or a serving RRH so as to performs
communication, resulting in an increase in communication
performance.
[0032] It will be appreciated by persons skilled in the art that
the effects that can be achieved with the present invention are not
limited to what has been particularly described hereinabove and
other advantages of the present invention will be more clearly
understood from the following detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0033] The accompanying drawings, which are included to provide a
further understanding of the invention, illustrate embodiments of
the invention and together with the description serve to explain
the principle of the invention.
[0034] FIG. 1 is a block diagram illustrating a base station (BS)
and a user equipment (UE) for use in a wireless communication
system.
[0035] FIG. 2 is a conceptual diagram illustrating modification of
BS/RAN structures through an RRH concept and RRH.
[0036] FIG. 3 is a conceptual diagram illustrating a cloud network
based on C-RAN.
[0037] FIG. 4 is a conceptual diagram illustrating a user-oriented
cell.
[0038] FIG. 5 is a conceptual diagram illustrating an RRH and
virtual BS (VBS) shared scenario in C-RAN.
[0039] FIG. 6 is a conceptual diagram illustrating a model of
multiple control layers.
[0040] FIG. 7 is a conceptual diagram illustrating a
next-generation network supporting situation-cognition based
intelligence interoperation.
[0041] FIG. 8 is a flowchart illustrating a conventional X2 based
handover procedure.
[0042] FIG. 9 is a conceptual diagram illustrating a method for
supporting mobility through RRH node switching in a C-RAN according
to the present invention.
[0043] FIG. 10 is a flowchart illustrating a primary RRH switching
process in response to UE movement in a C-RAN according to the
present invention.
[0044] FIGS. 11A and 11B illustrate a Cell Reference Signal
Received Power (RSRP) changing with UE movement.
[0045] FIGS. 12A and 12B illustrate a cell RSRP changing with UE
movement.
[0046] FIG. 13 shows a table showing the connected RRH MAC control
element.
[0047] FIG. 14 shows a table illustrating primary RRH switching MAC
control
BEST MODE FOR CARRYING OUT THE INVENTION
[0048] Reference will now be made in detail to the preferred
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings. The detailed description,
which will be given below with reference to the accompanying
drawings, is intended to explain exemplary embodiments of the
present invention, rather than to show the only embodiments that
can be implemented according to the present invention. The
following detailed description includes specific details in order
to provide a thorough understanding of the present invention.
However, it will be apparent to those skilled in the art that the
present invention may be practiced without such specific details.
For example, the following description will be given centering upon
a mobile communication system serving as a 3GPP LTE system, but the
present invention is not limited thereto and the remaining parts of
the present invention other than unique characteristics of the 3GPP
LTE system are applicable to other mobile communication
systems.
[0049] In some cases, in order to prevent ambiguity of the concepts
of the present invention, conventional devices or apparatuses well
known to those skilled in the art will be omitted and be denoted in
the form of a block diagram on the basis of important functions of
the present invention. Wherever possible, the same reference
numbers will be used throughout the drawings to refer to the same
or like parts.
[0050] In the following description, a terminal may refer to a
mobile or fixed user equipment (UE), for example, a user equipment
(UE), a mobile station (MS) and the like. Also, the base station
(BS) may refer to an arbitrary node of a network end which
communicates with the above terminal, and may include an eNode B
(eNB), a Node B (Node-B), an access point (AP) and the like.
Although the embodiments of the present invention are disclosed on
the basis of IEEE 802.16 for convenience of description, contents
of the present invention can also be applied to other communication
systems.
[0051] In a mobile communication system, the UE may receive
information from the base station (BS) via a downlink, and may
transmit information via an uplink. The information that is
transmitted and received to and from the UE includes data and a
variety of control information. A variety of physical channels are
used according to categories of transmission (Tx) and reception
(Rx) information of the UE.
[0052] The following embodiments of the present invention can be
applied to a variety of wireless access technologies, for example,
CDMA (Code Division Multiple Access), FDMA (Frequency Division
Multiple Access), TDMA (Time Division Multiple Access), OFDMA
(Orthogonal Frequency Division Multiple Access), SC-FDMA (Single
Carrier Frequency Division Multiple Access), and the like. CDMA may
be embodied through wireless (or radio) technology such as UTRA
(Universal Terrestrial Radio Access) or CDMA2000. TDMA may be
embodied through wireless (or radio) technology such as GSM (Global
System for Mobile communication)/GPRS (General Packet Radio
Service)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may be
embodied through wireless (or radio) technology such as Institute
of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE
802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). UTRA is a
part of UMTS (Universal Mobile Telecommunications System). 3GPP
(3rd Generation Partnership Project) LTE (long term evolution) is a
part of E-UMTS (Evolved UMTS), which uses E-UTRA. 3GPP LTE employs
OFDMA in downlink and employs SC-FDMA in uplink. LTE-Advanced
(LTE-A) is an evolved version of 3GPP LTE.
[0053] It should be noted that specific terms disclosed in the
present invention are proposed for convenience of description and
better understanding of the present invention, and the use of these
specific terms may be changed to other formats within the technical
scope or spirit of the present invention.
[0054] FIG. 1 is a block diagram illustrating a base station (BS)
105 and a user equipment (UE) 110 for use in a wireless
communication system 100 according to the present invention.
[0055] Although FIG. 1 shows one UE 105 and one UE 110 (including a
D2D UE) for brief description of the wireless communication system
100, it should be noted that the wireless communication system 100
may further include one or more BSs and/or one or more UEs.
[0056] Referring to FIG. 1, the BS 105 may include a transmission
(Tx) data processor 115, a symbol modulator 120, a transmitter 125,
a transmission/reception antenna 130, a processor 180, a memory
185, a receiver 190, a symbol demodulator 195, and a reception (Rx)
data processor 197. The UE 110 may include a Tx data processor 165,
a symbol modulator 170, a transmitter 175, a transmission/reception
antenna 135, a processor 155, a memory 160, a receiver 140, a
symbol demodulator 155, and an Rx data processor 150. In FIG. 1,
although one antenna 130 is used for the BS 105 and one antenna 135
is used for the UE 110, each of the BS 105 and the UE 110 may also
include a plurality of antennas as necessary. Therefore, the BS 105
and the UE 110 according to the present invention support a
Multiple Input Multiple Output (MIMO) system. The BS 105 according
to the present invention can support both a Single User-MIMO
(SU-MIMO) scheme and a Multi User-MIMO (MU-MIMO) scheme.
[0057] In downlink, the Tx data processor 115 receives traffic
data, formats the received traffic data, codes the formatted
traffic data, interleaves the coded traffic data, and modulates the
interleaved data (or performs symbol mapping upon the interleaved
data), such that it provides modulation symbols (i.e., data
symbols). The symbol modulator 120 receives and processes the data
symbols and pilot symbols, such that it provides a stream of
symbols.
[0058] The symbol modulator 120 multiplexes data and pilot symbols,
and transmits the multiplexed data and pilot symbols to the
transmitter 125. In this case, each transmission (Tx) symbol may be
a data symbol, a pilot symbol, or a value of a zero signal (null
signal). In each symbol period, pilot symbols may be successively
transmitted during each symbol period. The pilot symbols may be an
FDM symbol, an OFDM symbol, a Time Division Multiplexing (TDM)
symbol, or a Code Division Multiplexing (CDM) symbol.
[0059] The transmitter 125 receives a stream of symbols, converts
the received symbols into one or more analog signals, and
additionally adjusts the one or more analog signals (e.g.,
amplification, filtering, and frequency upconversion of the analog
signals), such that it generates a downlink signal appropriate for
data transmission through an RF channel. Subsequently, the downlink
signal is transmitted to the UE through the antenna 130.
[0060] Configuration of the UE 110 will hereinafter be described in
detail. The antenna 135 of the UE 110 receives a DL signal from the
BS 105, and transmits the DL signal to the receiver 140. The
receiver 140 performs adjustment (e.g., filtering, amplification,
and frequency downconversion) of the received DL signal, and
digitizes the adjusted signal to obtain samples. The symbol
demodulator 145 demodulates the received pilot symbols, and
provides the demodulated result to the processor 155 to perform
channel estimation.
[0061] The symbol demodulator 145 receives a frequency response
estimation value for downlink from the processor 155, demodulates
the received data symbols, obtains data symbol estimation values
(indicating estimation values of the transmitted data symbols), and
provides the data symbol estimation values to the Rx data processor
150. The Rx data processor 150 performs demodulation (i.e.,
symbol-demapping) of data symbol estimation values, deinterleaves
the demodulated result, decodes the deinterleaved result, and
recovers the transmitted traffic data.
[0062] The processing of the symbol demodulator 145 and the Rx data
processor 150 is complementary to that of the symbol modulator 120
and the Tx data processor 115 in the BS 205.
[0063] The Tx data processor 165 of the UE 110 processes traffic
data in uplink, and provides data symbols. The symbol modulator 170
receives and multiplexes data symbols, and modulates the
multiplexed data symbols, such that it can provide a stream of
symbols to the transmitter 175. The transmitter 175 receives and
processes the stream of symbols to generate an uplink (UL) signal,
and the UL signal is transmitted to the BS 105 through the antenna
135.
[0064] The BS 105 receives the UL signal from the UE 110 through
the antenna 130. The receiver processes the received UL signal to
obtain samples. Subsequently, the symbol demodulator 195 processes
the symbols, and provides pilot symbols and data symbol estimation
values received via uplink. The Rx data processor 197 processes the
data symbol estimation value, and recovers traffic data received
from the UE 110.
[0065] A processor 155 or 180 of the UE 110 or the BS 105 commands
or indicates operations of the UE 110 or the BS 105. For example,
the processor 155 or 180 of the UE 110 or the BS 105 controls,
adjusts, and manages operations of the UE 210 or the BS 105. Each
processor 155 or 180 may be connected to a memory unit 160 or 185
for storing program code and data. The memory 160 or 185 is
connected to the processor 155 or 180, such that it can store the
operating system, applications, and general files.
[0066] While the UE processor 155 enables the UE 110 to receive
signals and can process other signals and data, and the BS
processor 180 enables the BS 105 to transmit signals and can
process other signals and data, the processors 155 and 180 will not
be specially mentioned in the following description. Although the
processors 155 and 180 are not specially mentioned in the following
description, it should be noted that the processors 155 and 180 can
process not only data transmission/reception functions but also
other operations such as data processing and control.
[0067] The processor 155 or 180 may also be referred to as a
controller, a microcontroller), a microprocessor, a microcomputer,
etc. In the meantime, the processor 155 or 180 may be implemented
by various means, for example, hardware, firmware, software, or a
combination thereof. In a hardware configuration, methods according
to the embodiments of the present invention may be implemented by
the processor 155 or 180, for example, one or more application
specific integrated circuits (ASICs), digital signal processors
(DSPs), digital signal processing devices (DSPDs), programmable
logic devices (PLDs), field programmable gate arrays (FPGAs),
processors, controllers, microcontrollers, microprocessors,
etc.
[0068] In a firmware or software configuration, methods according
to the embodiments of the present invention may be implemented in
the form of modules, procedures, functions, etc. which perform the
above-described functions or operations. Firmware or software
implemented in the present invention may be contained in the
processor 155 or 180 or the memory unit 160 or 185, such that it
can be driven by the processor 155 or 180.
[0069] Radio interface protocol layers among the UE 110, the BS
105, and a wireless communication system (i.e., network) can be
classified into a first layer (L1 layer), a second layer (L2 layer)
and a third layer (L3 layer) on the basis of the lower three layers
of the Open System Interconnection (OSI) reference model widely
known in communication systems. A physical layer belonging to the
first layer (L1) provides an information transfer service through a
physical channel. A Radio Resource Control (RRC) layer belonging to
the third layer (L3) controls radio resources between the UE and
the network. The UE 110 and the BS 105 may exchange RRC messages
with each other through the wireless communication network and the
RRC layer.
[0070] While the UE processor 155 enables the UE 110 to receive
signals and can process other signals and data, and the BS
processor 180 enables the BS 105 to transmit signals and can
process other signals and data, the processors 155 and 180 will not
be specially mentioned in the following description. Although the
processors 155 and 180 are not specially mentioned in the following
description, it should be noted that the processors 155 and 180 can
process not only data transmission/reception functions but also
other operations such as data processing and control.
[0071] 5G communication network technology is largely classified
into a radio access network field and a core network field.
Generally, two technical fields are present in the radio access
network field. First, a centralized access network may be used
through introduction of a network cloud. Three core technologies
for implementing the network cloud are Remote Radio Head
(RRH)/Coordinated Multi-Point (CoMP) technology, software modem
technology, and cloud computing technology.
[0072] The most important core technology for implementing the
network cloud in the radio access network field is the introduction
of RRH. Although RRH is very important in terms of radio
transmission, the RRH may greatly change a radio access network
structure.
[0073] FIG. 2 is a conceptual diagram illustrating modification of
BS/RAN structures through RRH concept and RRH.
[0074] Although RRH has been developed as one of optical repeaters,
RRH has recently been used as a core element to implement a
centralized base station. The most important core technology for
implementing the network cloud in the radio access network field is
the introduction of RRH. RRH is very important in terms of radio
transmission, and may greatly change the radio access network
structure. With the introduction of RRH, a conventional base
station is no longer physically distributed due to physical
distribution of Radio Frequency Units (RFUs) and Baseband Units
(BBUs). Recently, a cloud access network is interoperable with
several hundreds of RRHs through only one device so as to implement
network operation/management, resulting in formation of a cell
different from a typical cell.
[0075] Although communication systems to the 4 G communication
system have defined all radio access operations on the basis of a
cell, a new cell concept needs to be established through the above
structural change. Presently, 3GPP has intensively proposed various
scenarios on the condition that RRH and a macro BS are present
through Release 11 CoMP (Coordinated Multi-Point) Work Item. In
recent times, various research into enabling several cells to share
one RRH as in a Shared Antenna System (SAS) have also been
proposed. In addition, a method for dynamically changing a cell
region by adjusting an RRH cluster according to a situation has
also been recently proposed. Due to these situations, user interest
in the Cloud Radio Access Network (C-RAN) project is rapidly
increasing.
[0076] FIG. 3 is a conceptual diagram illustrating a cloud network
based on C-RAN.
[0077] FIG. 3 is a conceptual diagram of a C-RAN. The C-RAN may
include a plurality of RRHs, a software-based virtual base station
(VBS), an access control server for controlling RRHs and VBSs, and
a core network cloud server (including a resource management
server, an accounting/authentication server, etc.). As described
above, as elements of the core network are gradually changed to the
open IP network, C-RAN elements are organically interoperable with
the core network elements.
[0078] Referring to FIG. 3, several RRGs are connected to a virtual
base station (VBS) through an optical access device. The VBS is
implemented by software, and may be implemented by various radio
access technologies such as Long Term Evolution (LTE), HSPA,
WiMAX/WiFi, etc., and one or more RRHs are grouped and controlled
by a single VBS. While the cell region is fixed in the related art,
C-RAN dynamically changes RRH clusters so that cells can be
dynamically allocated. Such dynamic allocation may also be adjusted
according to distribution of users present in the region.
Therefore, there is a need to consider a method for removing the
cell concept and constructing a cell per user.
[0079] As can be seen from FIG. 3, several RRHs are connected to
the VBS through an optical access device. The VBS may be
implemented by software, and may also be implemented by various
radio access technologies such as LTE, HSPA, WiMAX/WiFi, etc. One
or more RRHs are grouped and controlled by a single VBS. While the
cell region is fixed in the related art, RRH clusters are
dynamically changed in the C-RAN system so that cells can be
dynamically allocated.
[0080] FIG. 4 is a conceptual diagram illustrating a user-oriented
cell.
[0081] In C-RAN, RRH clusters are dynamically changed so that cells
are dynamically allocated. Such dynamic allocation may also be
adjusted according to distribution of users present in the region.
Some developers and companies have conducted intensive research
into a method for removing the cell concept and constructing a
per-user cell on the basis of a current user.
[0082] FIG. 5 is a conceptual diagram illustrating an RRH and VBS
shared scenario in C-RAN.
[0083] A virtual structure such as a C-RAN has proposed a new
possibility in consideration of network opening and network sharing
characteristics. Individual countries have executed various
policies [e.g., MVNO, BS(Base Station)/AP(Access Point)] to
introduce competitive elements to a communication market. For
example, MVNO, BS(Base Station)/AP(Access Point), etc. If the VBS
is introduced to the market, it is possible to use various service
scenarios, for example, a virtual enterprise may construct a VBS
under the condition that an interface with an RRH is maintained,
and the same radio resources may be shared by a plurality of
VBSs.
[0084] In FIG. 5, enterprises A, B and C may share the same RRH
pool so as to provide services. Especially, the enterprise B and
the enterprise C may provide different services using the same VBS.
The VBS enterprise may charge a usage fee to each of the
enterprises B and C according to the amount of radio resources
used. Each enterprise applies a unique resource allocation policy
to predetermined radio resources so as to support a subscriber.
[0085] Services for selling frequency resources in real time have
recently been activated in the United States of America, and a new
frequency-associated business capable of leasing some frequency
resources may also be possible in the VBS environment.
[0086] The second important flow of the radio access network field
is to strengthen a distributed layer. Although the centralized and
central processing function of the radio access network has been
strengthened through C-RAN, there is a limit to the capabilities of
the VBS and access server of the C-RAN. Specifically, assuming that
a base station (BS) such as a femto BS is connected through a
private IP network, it is very difficult to manage the BS in real
time. In addition, it is very difficult for the network to control
all operations of Device-to-Device (D2D) communication. As a
result, while a common access network based on C-RAN evolves in the
form of central control processing, localized communication such as
femto BS and D2D communication may have a distributed control
structure.
[0087] FIG. 6 is a conceptual diagram illustrating a model of
multiple control layers.
[0088] Referring to FIG. 6, a model of the multiple control layers
(hereinafter referred to as a multi-control layer model) includes a
central control layer controlled through a cloud access network and
a distributed control layer. The distributed control layer is
controlled by each communication entity whereas it is partially
controlled by the central control layer. Studies into interference
control for coexistence between layers are of importance to the
multi-control layer model.
[0089] In the core network aspect, it is expected that the core
network will evolve into an All IP-based Open Network in view of 4G
system continuity. In recent times, various services have a
tendency to be changed from network enterprise based services such
as IMS to Web/application layer based services. The above-mentioned
fact can be confirmed through a comparison between the enterprise
based Rich Communication Suite (RCS) and the service enterprise
based Over The Top (OTT) service. Although it is difficult for the
user to make a hasty conclusion of the confirmation result, SMS-,
MMS-, and IMS-based RCS services of communication companies will be
replaced with OTT services such as Kakao Talk and Skype.
[0090] Specifically, the aforementioned tendency will be
accelerated through activation of the next-generation Web standards
such as HLML5 and mobile cloud services. In response to change of
service fields, the core network function will focus on provision
of IP transport to a radio network, and it is expected that the
core network will be developed from the legacy voice-service-based
hierarchical network to a more horizontal IP network. As a result,
many conventional network elements are simplified in structure, and
network elements are gradually changed from the large-sized
server-dependent structure to a structure implemented through a
plurality of core network cloud servers, such that the
above-mentioned network development can be achieved through lower
CAPEX/OPEX.
[0091] The latent principal issue of the core network other than
service provision is provision of IP flow mobility. Heterogeneous
network support technology such as Multi-Access PDN connectivity
(MAPCON) or IP Flow Mobility (IFOM) has been standardized in 3GPP
SA2. Specifically, offloading through interaction with a WLAN is of
importance to 3GPP SA2. In contrast, with the introduction of
C-RAN, an interaction structure between heterogeneous networks is
simplified so that it is expected that IP-based mobility control
will also be greatly simplified. Assuming that the related art has
proposed services through two different PDNs, the above-mentioned
services can be implemented by only one method through interaction
between the C-RAN access control server and the core-network
integrated mobility control server, irrespective of the radio
access scheme employed. In addition, an Access Node Discovery and
Selection Function (ANDSF) being introduced for the offloading
policy may be easily contained in the core network (CN) mobility
control server region and then simplified.
[0092] In recent times, as information technology devices become
highly intelligent, management elements that have been confined to
UE radio characteristics, traffic types, accounting,
authentication, etc. will evolve in more various ways. A smartphone
of the user collects basic information, personal location
information, use and movement pattern, user interest, biomedical or
vital information, etc., and the network will provide user-oriented
services using the above-mentioned information. For this purpose,
it is expected that a function for collecting/processing large
volumes of data will be added to the core network and then
strengthened. While the legacy core network provides necessary
services on the basis of the enterprise policy, a method for
collecting, analyzing, and processing large volumes of data so as
to provide the user-oriented service will be intensively discussed
and studied in the next-generation communication network.
[0093] Specifically, interaction between heterogeneous networks is
facilitated, such that providing optimum radio access suitable for
a current situation through analysis of the corresponding big data
is of importance to the network enterprise and users.
[0094] The importance of research into associated technical fields
will gradually increase.
[0095] FIG. 7 is a conceptual diagram illustrating a
next-generation network supporting situation-cognition based
intelligence interoperation.
[0096] Referring to FIG. 7, various information collected by a UE
is transferred to the access network server and the core network
server, such that an optimum access environment of the UE can be
controlled on the basis of the collected information.
[0097] The above-mentioned description has disclosed the network
evolution for the next-generation communication system in view of
the radio access network and the core network. The radio access
network evolves into the cloud network such that extensibility and
network flexibility will be improved. In addition, as the control
region is gradually extended due to centralization, a large number
of functions associated with legacy core network communication will
be connected to the access network. In contrast, as the core
network is gradually developed in consideration of network
intelligence, it will be possible to provide an intelligent access
control function and an interaction control function based on
various situation information collected by a user, an access
network and other environmental elements. The development direction
of 5 G communication has just started to be laid out. It is
expected that 5 G communication technology will be developed to a
new wireless communication technology accompanied with appearance
of IT technology and bio- or nano-technology.
[0098] The principal change in a Cloud RAN (C-RAN) is that an
inter-cell handover (HO) concept is changed to the switching
between RRH nodes. The handover (HO) procedure in the C-RAN may be
different from a legacy inter-cell handover procedure due to such
C-RAN change.
[0099] FIG. 8 is a flowchart illustrating a conventional X2 based
handover procedure.
[0100] Referring to FIG. 8, a serving eNB (S_eNB) may provide a
user equipment (UE) with measurement configuration information in
step S810. If measurement of each neighbor cell is configured by
such measurement configuration, and if a specific situation is
satisfied, the UE begins measurement in step S820, and reports the
measurement result to the serving eNB (S_eNB) under a specific
situation in step S830. The serving eNB (S_eNB) may transmit a
handover request to a target eNB (T_eNB) according to the reporting
result in step S840. Upon receiving a response to the handover
request, the target eNB (T_eNB) transmits a handover request
response message including a handover (HO) command to the serving
eNB (S_eNB) in step S850. Thereafter, the serving eNB (S_eNB)
transmits a message including the HO command to the UE in step
S860. As a result, a handover (HO) message is generated through
negotiation between the serving eNB (S_eNB) and the target eNB
(T_eNB). Thereafter, the UE attempts to perform random access to
the target eNB (T_eNB) in step S870. Thereafter, the UE transmits
an HO confirmation message to the target eNB (T_eNB) in step
S880.
[0101] The above-mentioned process is classified into a S1-based
handover on the basis of MME and a X2-based handover on the basis
of negotiation between eNBs. For convenience of description, FIG. 8
assumes X2-based handover.
[0102] C-RAN Based RRH Switching
[0103] Handover (HO) in the C-RAN is also achieved in a similar
manner as described above. Several RRHs are connected to a virtual
base station (VBS) on a C-RAN, and the UE changes only the RRH
without changing the eNB or BS. Basically, since the RRH situation
assumes Joint Transmission(JT)/Joint Reception (JR), the UE can
perform seamless handover.
[0104] FIG. 9 is a conceptual diagram illustrating a method for
supporting mobility through RRH node switching in a C-RAN according
to the present invention.
[0105] Referring to FIG. 9, associated RRH sets and connected RRH
sets are contained in the cell. Cell configuration may be comprised
of two scenarios (Scenario 1 and Scenario 2) on the basis of
identifiers (IDs). In Scenario 1, `PHY cell id=MAC cell id` is
achieved, and the cell is identified by a synchronous channel of
the BS, and RRH may be identified for each antenna node according
to a CSI-RS. In Scenario 2, `PHY cell id !=MAC cell id` is
achieved, and the cell may be identified by a logical cell ID, and
RRH may be identified by a synchronous channel.
[0106] One primary RRH has the same concept as the serving cell of
the legacy cellular system. Generally, one primary RRH will be set
to an RRH having the highest reception signal intensity on
downlink.
[0107] 1.about.n (for example, 2 or 3) connected RRH(s): Connected
RRH(s) may be RRH(s) participating in data transmission/reception,
and may be allocated by the BS. Generally, activation/deactivation
is achieved using a MAC control element (for example, a signaling
header), and the UE always performs MIMO-associated
measurement/report using CSI-RS or the like.
[0108] 1.about.m (m(4.about.8).gtoreq.n) associated RRH sets:
Associated RRH set is a set of RRHs configured to periodically
perform monitoring according to situations, and indicates candidate
RRHs to serve as the connected RRHs. The primary RRH may inform the
UE of the RRH list associated with the associated RRHs.
Alternatively, a specific Global CSI-RS port is allocated to each
RRH, such that the UE may identify each RRH using the corresponding
port. The BS performs measurement configuration in units of this
divided allocation unit, and performs measurement and report
according to the configuration result.
[0109] The RRH switching handover operation in the C-RAN
environment is similar to the legacy handover operation. As shown
in FIG. 9, a connected RRH set and an associated RRH set are
present in the UE. In the present invention, the connected RRH set
is an aggregate of RRHs that are capable of performing JT/JR to
communicate with another party. An associated RRH set is an
aggregate of RRHs, each of which includes a connected RRH and
performs measurement. The associated RRH set is defined as
candidate RRHs.
[0110] Measurement for such RRHs may be achieved in various ways,
and a detailed description thereof will be given below. A primary
RRH from among connected RRHs always communicates with the UE in
the same manner as in the serving cell. As can be seen from FIG. 9,
if the UE moves to another region, the primary RRH and some
connected RRHs participate in communication, and one RRH from among
the connected RRHs may be changed to a new primary RRH according to
the UE movement. The associated RRH set may be allocated by the BS,
and may be broadcast from the primary RRH to the UE.
[0111] Primary RRH Switching in C-RAN
[0112] FIG. 10 is a flowchart illustrating a primary RRH switching
process in response to UE movement in a C-RAN according to the
present invention.
[0113] As the UE moves to another region in the C-RAN, the UE needs
to be handed over to the region, and this process corresponds to
the primary RRH switching process. Referring to FIG. 10, RRH1 is a
primary RRH, and the UE transmits and receives data to and from
RRH1 in step S1005. In this case, it is assumed that RRH1 is a
connected RRH. If the UE needs to be handed over, the primary RRH
informs the UE of the list of associated RRHs indicating candidate
RRHs in step S1010. The UE measures the associated RRHs according
to measurement information configured by the primary RRH in step
S1015. The UE transmits the measurement result as feedback
information to an RRH (RRH1). Thereafter, the primary RRH transmits
the feedback information to the VBS in step S1020. Cell
selection/reselection is performed according to such feedback
information. The UE includes an activation command of RRH2 in
feedback information on the basis of the measurement result in step
S1020, and transmits the resultant feedback information to the
VBS.
[0114] The BS transmits a MAC signal indicating activation of RRH2
to the RRH1, and may transmit the MAC signal to the UE in step
S1025. RRH1 and RRH2 are contained in the connected RRH. During
activation, the UE performs additional measurement such as Channel
Quality Information (CQI) and Precoding Matrix Index (PMI) and may
feed back the measurement result to the primary RRH indicating
RRH1, and the RRH1 may transmit the feedback result to the BS in
step S1030.
[0115] Thereafter, the BS transmits a signal indicating that the
primary RRH is changed from RRH1 to RRH2 to the RRH1, and may
transmit the signal to the UE in step S1035. The primary RRH may be
used as RRH2, and the connected RRH includes RRH1 and RRH2.
[0116] RRH2 updates the list of associated RRHs and may transmit
the list of associated RRHs to the UE in step S1040. In step S1050,
the UE reports measurement information (S1045) and feedback
information to an RRH2 on the basis of the updated associated RRH
list information as in step S1010. Therefore, the UE can perform
RRH handover (or switching). In this case, feedback information may
include a specific command incapable of deactivating the RRH1 on
the basis of the measurement result so that the resultant feedback
information can be transmitted. The BS transmits a MACK signal
indicating deactivation of RRH1 to an RRH2, and the RRH2 may
transmit the MACK signal to the UE in step S1055. Only the RRH2 is
contained in the connected RRH.
[0117] A measurement process for cell selection/reselection (or RRH
selection/reselection) and inter-cell handover (or handover between
RRHs) will hereinafter be described in detail.
[0118] Cell Selection/Reselection (or RRH,
Selection/Reselection)
[0119] Assuming the presence of both a cell and an RRH, the
following three cell selection procedures can be used. Basically,
cell selection is based on a common reference signal (CRS). Instead
of cell selection, a random access channel (RACH) may be
RRH-specifically used for high efficiency. The following table 1
shows various cell selection(reselection) methods. As can be seen
from FIG. 10, the BS transmits a message indicating RRH2 selection
to the primary RRH (RRH1), and the RRH1 may transmit the message to
the UE.
[0120] Table 1
TABLE-US-00001 TABLE 1 Method 1 Method 2 Method 3 Cell Selection
CRS based CRS based Global CSI-RS Criteria based for RRH RACH Cell
common RACH resources for RRH dedicated transmission RACH RRH
having the RACH resources(Cell ID highest power(RRH resources(RRH
based) ID based) ID based) Primary RRH Allocated by BS The
strongest RRH The strongest RRH BCH/Paging Cell Common Cell Common
Reselection Cell Common Cell Common RRH specific Parameter
[0121] Referring to Table 1, when Method 1 and Method 2 are
performed according to the cell selection criteria, the cell
selection is based on a CRS. In case of Method 2, cell selection is
performed for Global CSI-RS (Channel State Information-Reference
Signal) for RRH.
[0122] For allocation of the primary RRH, the BS allocates the
primary RRH in case of Method 1, and the RRH having the highest
reception signal intensity is used as the primary RRH in case of
Method 2 and Method 3.
[0123] In case of Method 1 in RACH transmission, the UE transmits a
RACH through the cell common RACH resources allocated based on a
cell ID. Meanwhile, according to Method 2, the UE determines an RRH
having the highest reception power to be the primary RRH, transmits
a RACH through RACH resources (RRH ID-based allocation) of an RRH
having the highest reception power. In case of Method 3, the UE
transmits an RACH to the RRH having the highest reception power
through RRH dedicated RACH resources based on an RRH ID.
[0124] Method 1 and Method 2 are cell-commonly applied to
BCH(Broadcast Channel)/paging transmission. Method 1 and Method 2
are cell-commonly applied to the reselection parameter. In case of
Method 3, the reselection parameter can also be specifically
established for each RRH.
[0125] Measurement for Inter-Cell Handover (or Handover Between
RRHs)
[0126] Measurement in the C-RAN handover situation assumes that an
RRH is allocated a global CSI-RS and the allocated result is used.
Assuming that all RRHs in the same cell transmit the same CRS,
individual RRHs are allocated different global CSI-RS ports and can
be measured independently.
[0127] 1. Handover Measurement Method
[0128] Cell RSRP (Reference Signal Received Power) can be
determined to be a sum of CRSs of several RRHs. RRH RSRP may be
measured using RSs (such as Global CSI-RS) separately allocated to
individual RRHs. In the C-RAN handover situation, a handover result
is compared with inter-cell handover in the legacy cellular network
such that a new measurement entity or a new measurement allocation
method must be defined.
[0129] 2. Handover Trigger Condition
[0130] In Method 1, RSRP_cell (i.e., an RSRP value of the measured
serving cell) of the serving cell is compared with another
RSRP_cell (i.e., an RSRP value of the measured target cell). If the
RSRP of the target cell is higher than the RSRP of the serving
cell, triggering can be performed. However, if the numbers of
per-cell RRHs are different from each other, there may occur
unfairness.
[0131] In Method 2, an RSRP value (RSRP_RRH1) is compared with an
RSRP value (RSRP_RRH2). Generally, Method 2 is achieved by
extending the legacy cell-based concept to an RRH. However, since
an RRH radius is very small in size, there is a high possibility
that handover abruptly and frequently occurs. In Method 2,
measurement complexity reduction or reliability must be guaranteed
in response to the increasing number of RRHs to be measured.
[0132] The sum of RSRPs of N optimum reception RRHs contained in
each cell is compared in Method 3. Assuming the CoMP JT/JR scheme,
the sum of optimum RSRPs corresponding to the number of currently
received RRHs may be defined. The above-mentioned scheme has high
complexity, and has difficulty in measuring a combining gain of
several RRHs.
[0133] 3. New Measurement Configuration for C-RAN Handover
[0134] Items different from those of the related art must be added
to measurement configuration so as to perform handover in the C-RAN
environment. For example, the following items may be added to the
measurement configuration.
[0135] Information for measuring an RRH of a neighbor cell is
needed. Global CSI-RS associated information (typically including
the list of cell IDs) may be used as information for measuring an
RRH of a neighbor cell. In addition, a new triggering condition and
measurement object information must be defined.
[0136] Conventionally, the UE has measured an RSRP and an RSRQ
(Reference Signal Received Quality).
[0137] FIGS. 11A and 11B illustrate a Cell Reference Signal
Received Power (RSRP) changing with UE movement.
[0138] FIG. 11A shows an RSRP change within one macro cell, and
FIG. 11B shows an RSRP change when 19 RRHs are present in one macro
cell. As can be seen from FIG. 11B, the cell RSRP value (RSRP_cell)
moves to largely swing in the vicinity of RRH.
[0139] The correlation between RSRP_RRH and RSRP_cell in FIGS. 11A
and 11B can be used for new purposes. If a high measurement value
is obtained, this means that a small number of RRHs contained in
the same cell may be located at a cell edge. If a low measurement
value is obtained, this means that a large amount of RRHs are
present in a peripheral region and may be located at a cell center.
If the proposed value is a predetermined value or higher, this
value may be used as triggering for measuring a CRS of another
cell.
[0140] FIGS. 12A and 12B illustrate a cell RSRP changing with UE
movement.
[0141] FIG. 12A shows a difference between RSRP_cell and RSRP_RRH
in a macro cell, and FIG. 12B shows not only RSRP_cell of the macro
cell but also a difference between RSRP_cell and RSRP_RRH. A
difference value (RSRP_cell-RSRP_RRH) between RSRP_cell and
RSRP_RRH may be used for new purposes. If the measurement value
(i.e., a difference between RSRP_cell and RSRP_RRH) is high, this
means that a small number of RRHs is present in the same cell and
may be located at a cell edge. If the measurement value (i.e., a
difference value between RSRP_cell and RSRP_RRH) is small, this
means that a large number of RRHs are present in a peripheral
region and may be located at a cell center. If the proposed
measurement value (i.e., a difference value between RSRP_cell and
RSRP_RRH) is a predetermined value or higher, this value may be
used as triggering for measuring a CRS of another cell.
[0142] Measurement in C-RAN RRH Switching Situation
[0143] It is assumed that all RRHs of the same cell transmit the
same CRS and respective RRHs are allocated different global CSI-RS
ports and can be measured independently from each other.
[0144] 1. RRH Measurement Method
[0145] It is assumed that the UE measures Global CSI-RS of an RRH
to measure an RSRP of the RRH. The UE can measure associated RRHs.
For this purpose, measurement configuration for each associated RRH
must be configured. In this case, a new report trigger condition
different from the report trigger condition according to the legacy
inter-cell handover measurement must be defined and used as
follows.
[0146] Definition of New Report Trigger Condition
[0147] If RSRP_RRH>T (where T is a predetermined threshold
value) and `RSRP_RRH>a.times.RSRP_RRH_serving (0<a<1)` are
satisfied, the report triggering condition can also be satisfied.
"RSRP_RRH>a.times.RSRP_RRH_serving" indicates that the RSRP
ratio of a current serving RRH is equal to or higher than a
predetermined value [this may be
"RSRP_RRH(dB)>RSRP_RRH_serving(dB)-TH(dB)" (where TH is a
predetermined threshold value)].
[0148] If a predetermined report triggering condition from among
associated RRHs is satisfied, the report triggering condition is
satisfied. According to the reporting result, the BS may change the
connected RRH and the primary RRH.
[0149] Method for Changing Connected RRH
[0150] An associated RRH is mapped to each global CSI-RS port such
that the UE can measure the associated RRH. A MAC control element
denoted by a 1-bit bitmap can be generated at each global CSI-RS
port. As a result, the MAC control element may include as much
bitmap data as the number of global CSI-RS ports in the associated
RRH. Each bitmap may include an RRH activation(1)/deactivation(0)
command. In case of UE activation, additional operations associated
with the corresponding global CSI-RS port can be carried out. In
case of UE activation, additional measurement of CQI and PMI can be
carried out, and a process for participating in the
transmission/reception process can be performed. Meanwhile, for the
connected RRH, CSI-RS may be allocated independently.
[0151] FIG. 13 shows a table showing the connected RRH MAC control
element.
[0152] In FIG. 13, the Ci field may include the RRH
activation(1)/de-activation(0) command denoted by bitmap in the
i-th global CSI-RS port. In case of UE activation, the UE can
perform additional operations associated with the corresponding
global CSI-RS port. In case of UE activation, the UE may perform
additional measurement of CQI and PMI, and may participate in the
transmission/reception processes.
[0153] Primary RRH Switching
[0154] In case of primary RRH switching, the global CSI-RS port
number or bitmap of the primary RRH is transmitted to the MAC
control element. FIG. 14 shows a table illustrating primary RRH
switching MAC control elements.
[0155] In FIG. 14, the Ci field may include the RRH
activation(1)/de-activation(0) command denoted by bitmap in the
i-th global CSI-RS port.
[0156] Primary RRH change needs to be confirmed. This primary RRH
change confirmation can be implemented by transmitting a primary
RRH port number received by the UE, and this operation can be
carried out by the MAC control element.
[0157] Associated RRH Set Update
[0158] If the primary RRH is changed, the associated RRH set can
also be changed. Some methods for changing the associated RRH set
can be used. There is a bitmap for the entire global CSI-RS port, a
global CSI-RS port including an allocated bit is used as the
associated RH, an additional bitmap for the associated RRH also
exists, such that activation/deactivation can be notified using the
additional bitmap.
[0159] All signaling processes may also be carried out using an ID
of the RRH, or may be blind-processed by the global CSI-RS port
number.
[0160] The following Table 2 shows the comparison result between a
cell (S1, X2)-based handover and a C-RAN handover.
[0161] Table 2
TABLE-US-00002 TABLE 2 Cell based(S1, X2) HO C-RAN HO Inter-Cell
Inter-cell IF based on S1 Implementation issue in Inter- and X2 is
needed cell is controlled in MAC Signaling RRC message based MAC
control element based reception ACK is needed
(Activation/De-activation, and message transmission/ Primary node
change) reception reception delay occurs. ACK is not always needed.
RACH Full RACH procedure is Execution only in UL needed.-UL
synchronization synchronization- Transmission/reception Dedicated
RACH resource of Messages 3 and 4 are not needed HO Type Break
before Make Make before Break Update Cell Radio Network C-RNTI (if
required) Temporary Identifier(C-RNTI), logical channel ID(LCID),
Security Context etc Etc Large delay in HO failure Legacy
connection can be maintained in HO failure
[0162] Referring to Table 2, in case of the cell (S1, X2)--based
handover technology, S1- or X2-based inter-cell IF is needed,
signaling is based on RRC messages, reception ACK for message
transmission is needed, and message transmission/reception delay
may occur. The entire RACH procedure for handover is needed (UL
synchronization and Dedicated RACH resources are needed). C-RNTI,
LCID, Security Context, etc. are updated. In addition, a large
delay occurs in handover failure.
[0163] In contrast, in case of the C-RAN based handover technology,
activation/deactivation release and primary RRH change are signaled
on the basis of the MAC control element, but it should be noted
that reception ACK message of the signaled information are not
always needed. The RACH procedure is performed only when UL
synchronization is needed, and transmission/reception of Messages 3
and 4 in the RACH procedure is no longer required. C-RNTI may be
updated as necessary. Although a handover failure occurs, the
legacy connection can be maintained.
[0164] Exemplary embodiments described hereinbelow are combinations
of elements and features of the present invention. The elements or
features may be considered selective unless mentioned otherwise.
Each element or feature may be practiced without being combined
with other elements or features. Further, an embodiment of the
present invention may be constructed by combining parts of the
elements and/or features. Operation orders described in embodiments
of the present invention may be rearranged. Some constructions of
any one embodiment may be included in another embodiment and may be
replaced with corresponding constructions of another embodiment.
Also, it will be obvious to those skilled in the art that claims
that are not explicitly cited in the appended claims may be
presented in combination as an exemplary embodiment of the present
invention or included as a new claim by subsequent amendment after
the application is filed.
[0165] It will be apparent to those skilled in the art that the
present invention can be embodied in other specific forms without
departing from the spirit and essential characteristics of the
invention. Thus, the above embodiments are to be considered in all
respects as illustrative and not restrictive. The scope of the
invention should be determined by reasonable interpretation of the
appended claims and all changes which come within the equivalent
scope of the invention are within the scope of the invention.
INDUSTRIAL APPLICABILITY
[0166] The method for enabling a UE to perform a handover in a
Cloud Radio Access Network (C-RAN) system can be applied to various
communication systems for industrial purposes.
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