METHODS AND SYSTEMS FOR CONTROLLING HANDOVERS IN A
CO-CHANNEL NETWORK
PRIORITY STATEMENT
This application claims the benefit of U.S. Provisional
Application No. 61/5 1.0,253, filed July 21, 201 1, the entire
contents of which are incorporated herein by reference.
BACKGROUND
The wireless industry is experiencing increasing growth in data
and service traffic. Smart phones and data devices are
demanding ore and more from wireless networks. To off-load
the traffic in densely populated areas and increase indoor
coverage, small cells (e.g., pico cells) have become a feasible
solution. Namely, heterogeneous networks (HetNets) are no
being developed, where cells of smaller footprint size are
embedded within the coverage area of larger macro cells or at
least partially overlapped by the larger macro ceils, primarily to
provide increased capacity in targeted areas of data traffic
concentration. Such heterogeneous networks try to exploit the
spatial variations in user (and traffic) distribution to efficiently
increase the overall capacity of the wireless network. Those
smaller- sized cells are typically referred to as small cells in
contrast to the larger and more conventional macro cells.
In Long Term Evolution (LTE), the handover (HO) is optimized
for a UE moving from One macro cell to another macro cell.
The existing LTE HO is mainly based on an Event A3 in the
3GPP TS 36.331 standard, the entire contents of which are
herein incorporated by reference. Received signal strength
(RSRP) is used a s a metric for an eNodeB (enhanced NodeB) to
make HO decisions. The HQ parameters in the current macro
cellular network are intended for the macro-to-macro HO. The
macro cells have many of the same parameters such as transmit
power and coverage area.
For example, for a macro- macro HO, a late HO initiation is used
to reduce a ping-pong effect or a number of unnecessary HOs.
SUMMARY
The inventors have discovered that when small cells are overlaid
in a co-channel deployment on top of a macro cell, the handover
parameters and procedure should be adjusted considering the
radio-frequency characteristics of the small cells. When cochannel
small cells are deployed with lower transmit powers
and thus, smaller coverage areas, several challenges are
presented for the mobility performance. Consequently, example
embodiments disclose adjusting handover parameters and
procedure to take into account radio-frequency characteristics
of the small cells. In some example embodiments, the handover
parameters are adjusted based on a speed of the user
equipment. In some other example embodiments, handover
parameters are adjusted or pre-configured based on the
handover scenarios including macro -to-small handover, small to
macro handover, macro to macro handover and small to small
handover.
A factor affecting handover performance in co-channel overlay is
co-channel interference. The co-channel interference is quite
severe in the HetNets than the conventional macro network.
For example, i the normal coverage area around the small cell
transmit antenna, interference from the small cell is high for the
macro link. Thus, there is a high possibility for the macro radio
link failure deep inside the small cell coverage.
An example embodiment discloses a method of controlling a
handover of a user equipment. (UE) from a serving base station
to a target base station. The method includes determining, by a
serving base station, a speed of the UE and a type of handover,
the type of handover being one of macro cell to macro cell,
macro cell to small cell, small cell to macro cell and small cell to
small cell, and controlling, by the serving base station, the
handover from the serving base station to the target base
station based on the speed of the UE and the type of handover.
In an example embodiment, the determining the speed of the UE
includes classifying the speed of the UE into one of a low speed,
medium speed and high speed.
In an example embodiment, the controlling includes preventing
the handover if the speed of the UE is the high speed.
In an example embodiment, the controlling includes scheduling
the UE for transmission on almost blank sub-frames (ABS) of
the target base station.
In an example embodiment, the controlling includes increasing
a time-to-trigger (TTT) handover period if the speed of the UE is
the low speed.
In an example embodiment, the controlling includes increasing
a handover threshold if the speed of the UE is the low speed and
handing over the UE if the handover threshold exceeds a
difference between the reference signal received powers (RSRPs)
of th target base station and the serving base station at the UE.
In a example embodiment, different TTT values are set
differently in different handover scenarios including macro to
small, small to macro, macro to macro and small to small
handover scenarios. The controlling includes decreasing a timeto-
trigger (TTT) handover period if the speed of the UE is the
high speed.
I an example embodiment, the controlling includes decreasing
a handover threshold if the speed of the UE s the high speed
and handing over the UE if the handover threshold exceeds a
difference between the reference signal received power (RSRPs)
of the target and serving base stations at the UE
In an example embodiment, the controlling includes increasing
a handover threshold if the speed of the UE is the medium
speed and handing over the UE if the handover threshold
exceeds a difference between the reference signal received power
(RSRPs) of the target and serving base stations a t the UE.
In an example embodiment, the controlling includes changing a
handover threshold based on the speed of the UE and handing
over the UE if the handover threshold exceeds a difference
between the reference signal received power (RSRPs) of the
target and serving base stations at the UE
n an example embodiment, the controlling includes changing a
time-to-trigger (TTT) handover period based on the speed of the
UE.
In an example embodiment, the controlling includes adjusting a
layer 3 filter ' ' value based on the speed of the UE.
In an example embodiment, the method further includes
determining a direction of the UE, and the controlling includes
controlling, by the serving base station, the handover from the
serving base station to the target base station based n a
velocity of the UE, the velocity being the speed and direction of
the UE.
In an example embodiment, the determining the speed of the UE
includes classifying the speed of the UE into one of a low speed,
medium speed and high speed.
I an example embodiment, the controlling includes preventing
the handover if the speed of the UE is the h gh speed.
In an example embodiment, the controlling includes changing a
handover threshold based on the velocity of the UE and handing
over the UE if the handover threshold exceeds a difference
between the reference signal received power (RSRPs) of the
target and serving base stations at the UE,
In an example embodiment, the controlling includes changing a
time-to-trigger (TTT) handover period based on the velocity of
the UE.
In an example embodiment, the controlling includes adjusting a
layer 3 filter K value based on the velocity of the UE.
In an example embodiment, the serving base station is
associated with a macro cell coverage area and the target base
station is associated with a small cell coverage area, the small
cell coverage area being withi the macro cell coverage area.
In an example embodiment, the serving base station is
associated with a macro cell coverage area and the target base
station is associated with another macro cell coverage area.
In an example embodiment, the serving base station is
associated with a small cell coverage area within a macro cell
and the target base station is associated with another small cell
coverage area within the macro cell.
n an example embodiment, the serving base station is
associated with a small cell coverage area within a macro cell
and the target base station is associated with the macro cell.
In an example embodiment, the controlling further includes,
changing a handover threshold based on the speed of the UE,
the handover threshold being one of a cell specific offset of a cell
associated with the target base station, a hysteresis parameter
for an event and a system wide common offset parameter for the
event.
In an example embodiment, the controlling includes adjusting a
layer 3 filter 'value based on type of handover, the type of
handover being one of macro cell t o macro cell, macro cell to
small cell, small cell to macro cell and small cell to small cell.
In an example embodiment, the controlling includes scheduling
the UE for transmission on almost blank sub-frames (ABS) of
the target base station, and handing over the UE to the target
base station.
In an example embodiment, the controlling includes adjusting
TTT value based on a type of handover, the type of handover
being one of macro cell to macro cell, macro cell to small cell,
small cell to macro cell and small cell to small cell.
Another example embodiment discloses a base station
configured to determine a speed of the UE in an area associated
with the base station and control a handover from the base
station to a target base station based on the speed of the UE
and a type of the handover, the type of the handover being one
of macro cell to macro cell, macro cell to small cell, small cell to
macro cell and small cell to small cell.
Another example embodiment discloses a user equipment (UE)
configured to perform handover measurements regarding a
handover from serving macro cell to a target small cell based
on a velocity of the UE relative to the target small cell.
BRIEF DESCRIPTION OF THE DRAWINGS
Example embodiments will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings. FIGS. 1-7B represent non-limiting,
example embodiments as described herein.
FIG. 1 illustrates a wireless communication system according
to an example embodiment;
FiG. 2 illustrates a macro cell and small cell RSRP profile and a
conventional handover timeline;
FIG. 3 illustrates the macro cell of FIG. 1;
FIG. 4A illustrates a method of controlling a handover of a UE
from a serving base station to a target base station according to
an example embodiment;
FIG. 4B illustrates a method of controlling the handover based
on the speed of the UE;
FIGS. 5A-5C illustrate a method of using Almost Blank Subframes
(ABS) to reduce the amount of handovers according to
an example embodiment;
FIGS. 6A-6B illustrate another example embodiment of a
method of using Almost Blank Sub-frames (ABS) to reduce the
amount of handovers;
FIG. 7A illustrates an example embodiment of a UE shown in
FIG 1; and
FIG. 7B illustrates an example embodiment of a base station
shown in FIG. 1.
DETAILED DESCRIPTION
Various example embodiments will now be described more fully
with reference to the accompanying drawings in which some
example embodiments are illustrated.
Accordingly, while example embodiments are capable of various
modifications and alternative forms, embodiments thereof a e
shown by way of example in the drawings and will herein be
described in detail. It should be understood, however, that
there s no intent to limit example embodiments to the
particular forms disclosed, but on the contrary, example
embodiments are to cover all modifications, equivalents, and
alternatives falling within the scope of the claims. Like numbers
refer to like elements throughout the description of the figures.
It will be understood that, although the terms first, second, etc.
may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are
only used to distinguish one element from another. For
example, a first element could be termed a second element, and,
similarly, a second element could be termed a first element,
without departing from the scope of example embodiments. As
used herein, the term "and/or" includes any a d all
combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as
being "connected" or "coupled" to another element, it can be
directly connected or coupled to the other element or
intervening elements may be present. In contrast, when an
element is referred to as being "directly connected" or "directly
coupled" to another element, there are no intervening elements
present. Other words used to describe the relationship between
elements should be interpreted in a like fashion (e .g ., "between"
versus "directly between," "adjacent" versus "directly adjacent,"
etc.).
The terminology used herein is fo the purpose of describing
particular embodiments only and is not intended to b limiting
of example embodiments. As used herein, the singular forms
"a," "an" and "the" are intended to include the plural forms as
well, unless the context clearly indicates otherwise. It will be
further understood that the terms "comprises," "comprising,"
"includes" and/ r "including," when used herein, specify the
presence of stated features, integers, steps, operations, ele nts
and/or components, but do not preclude the presence or
addition of one or more other features, integers, steps,
operations, elements, components and/ or grou p thereof.
It should also be noted that in some alternative
implementations, the functions/ acts noted may occur out of the
order noted in the figures. For example, two figures shown in
succession may in fa ct be executed substantially concurrently
o may sometimes be executed in the reverse order, depending
upon the functionality /acts involved.
Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as
commonly understood by one of ordinary skill in the art to
which example embodiments belong. It will be further
understood that terms, e.g., those defined in commonly used
dictionaries, should be interpreted as having a meaning that is
consistent with their meaning in the context of the relevant art
and will not be interpreted in an idealized or overly formal sense
unless expressly so defined herein.
Portions of example embodiments and corresponding detailed
description are presented in terms of software, or algorithms
and symbolic representations of operation o data bits within a
computer memory. These descriptions and representations are
the ones by which those of ordinary skill in the art effectively
convey the substance of their work to others of ordinary skill in
the art. An algorithm, as the term is used here, and as it is
used generally, is conceived to be a self-consistent sequence o
steps leading to a desired result. The steps are those requiring
physical manipulations of physical quantities. Usually, though
not necessarily, these quantities take the form of optical,
electrical, or magnetic signals capable of being stored,
transferred, combined, Compared, and otherwise manipulated.
It has proven convenient at times, principally for reasons of
common usage, to refer to these signals a s bits, values,
elements, symbols, characters, terms, numbers, or the like.
In the following description, illustrative embodiments will be
described with reference to acts and symbolic representations of
operations (e.g., n the form of flowcharts) that may be
implemented a s program modules or functional processes
including routines, programs, objects, components, data
structures, etc., that perform particular tasks or implement
particular abstract data types and may be implemented using
existing hardware at existing network elements or control
nodes. Such existing hardware may include one or more
Central Processing Units (CPUs), digital signal processors
(DSPs), application- specific -integrated- circuits, field
programmable gate arrays (FPGAs) computers or the like.
Unless specifically stated otherwise, or a s is apparent from the
discussion, terms such as "processing" or "computing" or
"calculating" or "determining" or "displaying" or the like, refer to
the action and processes of a computer system, or similar
electronic computing device, that manipulates and transforms
data represented as physical, electronic quantities within the
computer system's registers and memories into other data
similarly represented as physical quantities within the computer
system memories or registers or other such information storage,
transmission or display devices.
As disclosed herein, the term "storage medium", "storage unit"
or "computer readable storage medium" may represent one or
more devices for storing data, including read only memory
(ROM), random access memory (RAM), magnetic RAM, core
memory, magnetic disk storage mediums, optical storage
mediums, flash memory devices and/ or other tangible machine
readable mediums for storing information. The term "computerreadable
medium" may include, but is not limited to, portable or
fixed storage devices, optical storage devices, and various other
mediums capable of storing, containing or carrying
instruction(s) and/or data.
Furthermore, example embodiments may be implemented by
hardware, software, firmware, middleware, microcode, hardware
description languages, or any combination thereof. When
implemented in software, firmware, middleware or microcode,
the program code or code segments to perform the necessary
tasks may be stored in a machine or computer readable medium
such a s a computer readable storage medium. When
implemented in software, a processor or processors will perform
the necessary tasks
A code segment may represent a procedure, function,
subprogram, program, routine, subroutine, module, software
package, class, or any combination of instructions, data
structures or program statements. A code segment may be
coupled to another code segment or a hardware circuit by
passing and /or receiving information, data, arguments,
parameters or memory contents. Information, arguments,
parameters, data, etc. may be passed, forwarded, or transmitted
via any suitable means including memory sharing, message
passing, token passing, network transmission, etc.
As used herein, the term "user equipment" or E" may be
synonymous to a user equipment, mobile station, mobile user,
access terminal, mobile terminal, user, subscriber, wireless
terminal, terminal an /or remote station and may describe a
remote user of wireless resources i a wireless communication
network. Accordingly, a UE may be a wireless phone, wireless
equipped laptop, wireless equipped appliance, etc.
The term "base station" may be understood as a one or more cell
sites, base stations, nodeBs, enhanced NodeBs, access points,
and/or any terminus of radio frequency communication.
Although current network architectures may consider a
distinction between mobile/user devices and access points/cell
sites, the example embodiments described hereafter may also
generally be applicable to architectures where that distinction is
no so clear, such as ad hoc and/or mesh network
architectures, for example.
Communication from the base station to the UE s typically
called downlink or forward link communication. Communication
from the UE to the base station is typically called uplink or
reverse link communication.
Serving base station may refer to the base station currently
handling communication needs of the UE.
FIG. 1 illustrates a . wireless comrminication system according
to an example embodiment. Referring to FIG. 1, the wireless
communication system includes a heterogeneous network 100
(HetNet), where cells of smaller footprint size (e.g., femto cells)
are embedded within the coverage area of a larger macro cell
(e.g., the area served by a macro base station) or at least
partially overlapped by the larger macro cell. As used herein,
the terminology "cell" refers to the coverage area as well as the
base station serving the coverage area. It will be understood
that each cell has an associated base station.
As shown, a plurality of macro cells 105i-105 a e arranged in a
hexagonal grid of cells. ENodeBs l lOi- On serve the plurality of
macro cells lQ5i-lQ5 respectively. Auser equipment (UE) 1 5
within t e cell 105 may communicate with the eNodeB l lO
Since the UE 5 is within the macro cell 105;, the eNodeB
0 may be referred to as a serving base station.
The eNodeB O communicates w th the UE 115 (and vice
versa) via at least one air interface that supports transmission
of data between the eNodeB and the UE 115. Techniques
for establishing, maintaining, and operating the air interfaces
between the UE 1 5 and the eNodeB O to provide uplink
and/or downlink wireless communication channels between the
UE 1 5 and the eNodeB are known in the art and in the
interest of clarity only those aspects of establishing,
maintaining, and operating the air interfaces that are relevant to
the present disclosure will be discussed herein-
Small cells may be overlaid in each of the macro cells 105r
105n. Example embodiments encompass any number and type
of small cell. For example, the phrase "small cell" may refer to
any relatively small cell or access point, such as a femto cell,
femto access point (or base station), pico cell, pico access point
(or base station), micro cell, micro access point (or base station),
metro cell, metro access point (or base station) nano cell, nano
access point (or base station), etc.
For purposes of illustration, a small cell 120 is embedded i the
coverage area of the macro cell 105i. The small cell 120 may be
a pico cell or femto cell. However, small cells are not limited to
being femto or pico cells.
Moreover, the UE 1 5 is travelling at a speed towards the small
cell 120.
The network 100 is a LTE network. However, it should be
understood that example embodiments described herein may be
performed in accordance with System for Mobile
Communications (GSM), General Packet Radio Service (GPRS),
Universal Mobile Telecommunications System (UMTS), High-
Speed Downlink Packet Access (HSDPA), and/ or High-Speed
Uplink Packet Access (HSUPA) cellular standards.
FIG. 2 illustrates a macro cell and small cell RSRP profile and a
conventional handover (HO) timeline. The network 100 is
configured to perform the conventional H timeline shown in
FIG, 2 . While RSRP is illustrated, it should be understood that
reference signal received quality (RSRQ) may be used
interchangeably with RSRP.
As shown in FIG. 2, the RSRP of a macro cell decreases the
farther a UE moves away of an eNodeB. At a certain distance
between the eNodeB and a transmitting antenna of the small
cell, the RSRP from the small cell becomes greater than the
RSRP of the macro. When the RSRP from the small cell is
greater than the RSRP of the macro cell by a HO threshold
HOTHR, the eNodeB triggers the HO process. The HO threshold
HOTHR may be determined based on empirical data. For
example, the HO threshold HOTHR is varied and the HO
performance is monitored to choose the actual value. The HO
threshold HOTHR may be based on HO failure rate and pingpong,
as well as other factors such a s cell coverage range.
In LTE, an event A3 occurs when a measurement metric, such
as RSRP, from a neighbor cell (e.g., small cell) becomes greater
than the RSRP measurement from the serving cell (e.g., macro
cell). Event A3 HO measurement report triggering is specified in
3GPP TS 36.331. While example embodiments discuss HOs
related t 3GPP TS 36.331, it should be understood that
example embodiments should not be limited thereto and may be
implemented in HOs specified i TS 36 300 and other types of
HOs. Moreover, while example embodiments discuss HOs
related to event A3, it should be understood that example
embodiments should not be limited thereto and example
embodiments may be implemented in other event scenarios,
such as event A4 and event AS
In TS 36 33 1, handover procedures are started when:
Mn +Ofn +Ocn - Hys > Ms +Ofs +Ocs +Off (1)
wherein Ocn is the cell specific offset of the neighbor cell, and set to
zero if not configured for the neighbor cell, Ocs is the cell specific
offset of the serving cell and is set to zero if not configured for the
serving cell, Hys is the hysteresis parameter for the event 0 , 0.5... 15
dB) and Off is the system wide common offset parameter for this
event. Mn, Ms are expressed in dBm in case of RSRP, for the neighbor
cell and the serving cell, respectively, or in dB in case of reference
signal received quality (RSRQ). The other handover parameters in the
equation are expressed in dB.
Event A3 occurs and the HO process starts when equation (1) is
met. HO measurement is made by the UE and then reported to
the serving base station.
n TS 36.331, handover procedures end when:
Mn ÷Ofn +Ocn + Hys < Ms +Ofs +Ocs +Off (2)
The HO process ends between two cells when equation (2) is
satisfied.
Equation (1) can be simplified to:
Mn > Ms + HOTHR (3)
wherein Mn is the measurement result (e.g., RSRP or RSRQ) of the
neighboring cell, not taking into account any offsets, Ms is the
measurement result of the serving cell, not taking into account any
offsets, and HOTHR is a handover threshold. The handover threshold
HOTHR is one of parameters Ocn, Hys, Off, Ocs or a combination of
any of the parameters Ocn, Hys, Off and Ocs, which are defined in the
LT standard.
Referring to FIG. 2, the triggering handover process from the
small cell to the macro cell occurs when the RSRP from the
macro cell is greater than the RSRP of the small ce l by the HO
threshold HOTHR.
The serving base station is configured to determine the
parameters Mn, Ms, Ofn, Ocn, Ofs, Ocs, Hys and Off based on
known communications methods or control signaling with the
UE. Thus, for the sake of brevity and clarity no further
description will be provided.
When the UE moves toward a target cell such as the small cell
or another macro cell and the target cell satisfies the entering
condition (1), the target cell is included in a neighbor list for the
HO.
When the UE moves away from the target cel and the leaving
condition (2) is satisfied that target cell is removed from the
neighbor list for the HO.
Management of neighbor lists is well known and therefore, will
not be described for the sake of brevity.
For conventional co-channel cro-to-macro HO, the
parameters Ofn, Ocn, Ofs and Ocs are set to zero. The
parameter Hys is a positive quantity and used to prevent pingponging
and unnecessary HOs. The parameter Off may be the
HO threshold HOTHR and generally is a positive quantity and
common for all target cells.
The UE measures RSRPs at a physical (PHY) layer (Layer 1) an
periodically measured RSRPs are passed through an infinite
impulse response (IIR) filter at layer 3 of the U to remove the
variations due to multipath fading and make the accurate HO
decisions. In standard TS 36.33 , the IIR filter is denoted a s
Fn (1-a) Fn-1 + Mn (4)
where
a = ½ ( /4) (5)
Fn-1 and Fn are the previous and current filtered RSRP values,
respectively.
n TS 36.331, the layer 1 sampling rate is implementation
specific and the layer 3 filtering sampling rate is 200ms.
Referring to FIG. 2 , when the small cell RSRP is greater than the
macro cell RSRP plus the handover threshold HOTHR, the
serving base station decides the UE should be handed over to
the small cells. LTE includes mechanisms to reduce the
unnecessary HOs. These include Time-to-trigger (TTT), HO
thresholds or margins, and layer 3 filtering.
When the RSRP profiles cross each other, the serving base
station considers the HO threshold HOTHR before initiating the
handoff. The HO threshold HOTHR prevents unnecessary HOs
due to shadowing and fading.
The HO is initiated when the UE determines that RSRP of the
small cell exceeds the RSRP of the serving base station b the
HO threshold HOTHR. The UE makes the RSRP measurement
and initiates the TTT. When the TTT expires, the UE sends the
measurement report to the serving base station.
The TTT also prevents unnecessary back and forth HOs.
However, when the small cells with smaller geographical
coverage areas are present, a late HQ and larger TTT may cause
the current serving cell to experience radio link failure (RLF) in
the downlink. If the UE is moving towards /through the small
cell, a HO may not occur until the UE is deep into the small cell.
When the UE is deep into the small cell, the interference from
the small cell may cause the macro downlink RLF before the HO
is completed. Similarly, for the case when the UE is moving out
of the small cell, the late HO and larger TT may cause the
small cell downlink RLF.
The connected mode HO process is also affected by the radio
link monitoring and failure process (RLM, RLF). Connected
mode means active data transmission takes place between an
eNodeB and UE. Idle mode means the UE is on but no data
transmission takes place and only periodic control channels are
transmitted between the eNB and UE. In example
embodiments described herein, the HO process is during the
connected mode.
The UE continues to monitor the radio link quality. Generally,
the UE checks whether a wideband channel quality indicator
(CQ ) for communications with the serving base station goes
belo a threshold Qout, every 200ms. If CQI goes below the
threshold Qout, then the UE starts an RLF timer (T3 10). When
the RLF timer is running, the UE determines every 100ms if the
wideband CQI goes above a threshold Qin. If the CQI goes
above Qin, the RLF timer is stopped and the radio link is
recovered. Otherwise, when the RLF timer is expired, RLF is
declared by the UE. If the RLF timer is running when the TTT
expires the RLF timer would also be reset.
When the UE receives the HO measurement request, the UE
measures the RSRP periodically for e.g., every 40ms. The
serving base station can request a measurement report or the
UE can send a measurement report without a request from the
serving base station.
After the layer 3 RSRP filtering, the HO process is initiated if
RSRPsma-l > RSRPM + HOTHR (6)
for the macro-to-small HO, where RSRPsmaii is the measured RSRP of
the small cell and RS is the measured RSRP of the macro cell.
In other words, the Event A3 entering condition is satisfied. Once the
HO process is initiated, the serving base station starts the TTT.
The UE continues to measure the RSRPs and wideband CQI
during the TTT period. Then, the UE sends the measurement
report to the serving base station and the serving base station
sends the HO command to the UE after the serving base station
processes the measurement report.
There are at least three possible scenarios for HO failure:
1. When the TTT timer is running, the RLF timer ma expire.
In this situation, the serving cell RLF occurred before HO.
2. When the TTT timer expires the RLF timer is not expired but
is reset. In this situation, the serving cell DL control channel
failure occurred (PDCCH) and results in HO failure.
3. When the TTT expires and the UE receives the HO command
successfully. Then, the UE tries to connect to the target cell
and random access failure may occur (random acces
message 2 failure), leading to HO failure.
Example embodiments reduce the amount of HO failures by
adjusting HO parameters based on speed and/or velocity of the
UE, and using the speed/ velocity to reduce HO failures.
FIG. 3 illustrates the macro cell 105 , shown in FIG. 1. As
shown, an antenna 125 provides communications to equipment
within the small cell 0 .
Both the base station O. and the antenna 125 are configured
to determine a speed and/or velocity of the UE 15. The
antenna 125 may be pico cell base station, femto base station
or any other type of small cell base station. As is known in LTE,
the UE or serving base station estimates in the speed of the UE
based o the number of cells a UE was handed over within a
period of time. Other methods are also known. UE based
methods are called Mobility state estimation, which is specified
in standards. The serving base station could also count and
maintain the accumulated number of HOs to estimate the UE
speed. Other known methods of determining a speed of a U
may be used.
FIG. 4A illustrates a method of controlling a HO of a UE from a
serving base station to a target base station according to an
example embodiment. While the method shown in FIG. 4A is
directed to a macro- small cell HO, it should be understood that
the method of FIG. 4A may be implemented in macro-macro,
small-small and small-macro HOs Moreover, the HO
parameters such as the H threshold, TTT and Kvalue may be
adjusted based on the speed (or velocity) and the type of HO
(e.g., macro-macro, macro-small, small-macro, small-small). It
should be understood that the serving base station may be the
eNodeB l O and the target base station may be the antenna
125. Thus, the UE is in the macro cell coverage area and in
communication with the serving base station.
In one example embodiment, the serving base station uses the
parameter Ocn as the HO threshold. With parameter Ocn,
different HO thresholds could be set depending on the HO
scenario.
In TS 36.331, the layer 3 filtering sampling rate is 200ms.
However, the inventors have discovered that the 200ms
sampling rate affects the HO performance. Thus, in at least one
example embodiment, the serving base station uses a faster
layer 1 to layer 3 sampling period (for example 40 ms, 50ms or
100ms).
Moreover a lower value (for example - 0, 1 or 2 ) is used by
the base station when small cells exist. Additionally, different K
values are used depending on the type of HO, such as macro to
macro, macro to small, small to macro and small to small. The
Layer 3 filter averages out the multipath fading effects on the
S P. The K value in equation (4) is an indication for the
duration of the averaging. A smaller K value means the
averaging is done during a short period of time. Different
values are determined by the network for different handover
scenarios, respectively, based on empirical data.
Similar to the HO threshold HOTHR, the initial TTT time and
initial K value are set based on empirical data. Operators test
the network an determine the HO threshold HOTHR, the initial
TTT time and initial value for certain scenarios/ operating
conditions. The initial values of the UEs are set using dedicated
control messages or a broadcast message.
As shown in FIG. 4A, the serving base station determines a
speed /velocity of the UE and type of HO at S405. The type of
handover is one of macro cell to macro cell, macro cell to small
cell, small cell to macro cell and small ce l to small cell. The
serving base station decides whether to hand over the UE to
another cell. The HO measurements may be based on RSRP.
Cell IDs allocated for the macro cells and small cells are known
to the base stations. From the cell ID of the target base station,
the serving base station determines the type of HO.
At S410, the serving base station controls the HO based on the
speed of the UE. As the HO threshold is adjusted based on the
speed of the UE, the entering condition (1) for Event A3
continuously changes.
FIG. 4B illustrates S410 in more detail according to an example
embodiment.
At S4 , the serving base station classifies the speed of the UE.
For example, the serving base station classifies the speed into
one of a low speed, medium speed and high speed. As an
example, a low speed ma be less than 40 kmph, a medium
speed may be 40-60 kmph and a high speed may be greater
than 60 kmph.
Based on the classified speed and the type of the HO, the
serving base station adjusts the HO threshold at S420. In one
example embodiment, the HO threshold for high speed UEs is
set so that all HO to the small cells are blocked. In another
example embodiment, the HO threshold for the high speed UE is
lowered t trigger a quicker HO. For the low speed UEs which
are allowed to HO to the small cells, the HO threshold is
increased and is realitvely higher than the HO threshold for
medium speed UEs and the HO threshold for high speed UEs
that are permitted to HO to the small cells. The HO threshold
may be Ocn, HYS, Off or a combination thereof.
In a further example embodiment, the HO threshold is scaled
based on the speed of the UE. For example, the HO threshold
may be scaled by .25 for high speed, .75 for medium speed and
1 for low speed.
Referring back to FIG. 4B, the TTT is then adjusted by the
serving base station based on the speed classification at S4 5.
In an example embodiment, the T T for the high speed UE is
lowered to trigger a quicker HO. For the low speed UEs which
are allowed to HO to the small cells, the TTT is increased and is
realitvely higher than the TTTs for medium and high speed UEs.
In a further example embodiment, the TTT is scaled based on
the speed of the UE. For example, the TTT ma be scaled by .25
for high speed, .75 for medium speed and 1 for low speed.
At S430, the serving base station adjusts the K value based on
the speed classification and the type of the HO. As discussed
above, different K values are determined by the network for
different handover types, respectively, based on empirical data.
n an example embodiment, the Kvalue for the high speed UE is
lowered. For the low speed UEs which are allowed to HO to the
small cells, the value is increased and is realitvely higher than
the Kvalues for medium and high speed UEs.
In other words, for faster moving UEs, more responsive filtering
is performed to support quicker HO decisions. Higher UE
speeds use a lower LI filtering time (which is LI to L3 reporting
time). Higher UE speeds also use a smaller value of the L3
filter.
At S435, the UE determines whether a HO should proceed
based on the adjusted HO threshold, TTT and K values. More
specifically, after the RSRP (or RSRQ) measurement exceeds the
HO threshold, the UE triggers the TTT, and the UE transmits a
measurement report to the serving base station once the TIT
expires is triggered, after waiting for TTT. The UE then receives
an HO command from the target cell at S44G.
If the serving base station determines that a HO i not required,
the method proceeds to S405. It should be also be understood
that the serving base station may continuously monitor HQ
situations while adjusting HO parameters.
The UE speed dependent HO configuration change could be
realized by 1) the network detecting the UE speed and based on
the speed, the network sending a reconfiguration unicast
message to the UE; and 2 ) the UE estimating its own speed a d,
based on the estimation results, adjusting the HQ configuration
parameters by itself. The unicast message carries the values of
the HO parameters. The network may broadcast related
information to assist this activity. This approach could be used
for not only UEs in CONNECTED mode but also UEs in IDLE
mode.
t should be understood that the steps may be implemented in
various order and are not limited to the order shown in FIG. 4B.
In another example embodiment, the serving base station
makes the HO decision based on the UE's velocity, which
includes the speed and direction. It should be understood that
the HO parameters may adjusted in the manner described in
FIG. 4B.
Multiple location estimations of the UE in a given period of time
could be used for UE speed and moving direction estimation.
The serving base station makes the HO decision based on the
speed and the relative angle of the moving UE. For example, the
serving base station may implement the following:
(1) If the speed is very high, do not HO to the small cell.
(2) If the speed is medium, allow the HO to the small cell if the
angle is less than or equal to (shown in FIG. 3).
Otherwise do not allow the HO.
(3) If the speed is low, allow the HO to the small cell if the
angle is less than or equal to (shown in FIG. 3)
Otherwise do not allow the HO
The angles a and may be determined based on empirical data.
n another example embodiment, the UE decides whether to
search a neighboring small cell for HO. For example, the UE
decides whether to perform the measurement of the neighboring
small cell based on the following logic similar to the HO
decision:
(1) If the speed is very high, no search of the neighboring
small cel is needed.
(2) f the speed is medium and the angle is less than or equal
to , small cell measurement is performed to search the
small cell. Otherwise, no small cell measurement is
performed
(3) If the speed is low and the angle is less than or equal to ,
allow small cell measurement. Otherwise, no
measurement for the small cell is performed.
Moreover, while speeds are classified in FIG. 4B, it should be
understood that HO parameters may be adjusted based on the
speed of the UE without a classification.
FIGS. 5A-5B illustrate a method of using Almost Blank Subframes
(ABS) to reduce the amount of handover failures
according to an example embodiment.
As shown in FIG 5A, a macro cell 105IA includes a cell border
Bios and a small cell 2 includes a border . It should be
understood that the macro ce l I A and the small cell 2QA
may b e the same a s the macro cell 105i and the small cell 120.
Thus, the discussion of FIG. 5A will discuss features not
previously discussed.
The border between the macro cell 10 Aand the small cell 120
is in terms of the relative power measurement at the coverage
area. If at a certain coverage area, the measured power from
the macro cell 1Q5IA and the small cell 120A are the same or
close, the coverage area is at the border of to two cells. The
macro cell 105 A and the small cell 1.20A configure the UEs to
determine the boarder border B 120 based on the measured
powers received from the h e macro cell 105 and the small cell
The method of FIGS. 5A-5B use inter-cell interference
coordination, a s described in the 3GPP Release 8 and 9 Inter-
Cell Interference Coordination (ICIC) and Release- 10 (enhanced
Inter-Cell Interference Coordination (e C C) the entire contents
of which are incorporated by reference.
As known, ICIC reduces inter-cell interferences by radio
resource management (RRM) methods. ICIC is a multi-cell RRM
function that takes into account information (e.g., the resource
usage status and traffic load situation) from multiple cells.
e C C is an enhancement of the inter- cell interference
coordination that effectively extends the ICIC to DL control in
the time domain. This requires synchronization at least
between a macro eNodeB and the lower power eNodeB's in its
coverage. e C C is realized by means of 3GPP Release Releases
8 and 9 (Multicast Broadcasts on a Single Frequency Network)
or Release 10 Almost Blank Sub-frames (ABS).
In one example embodiment, the macro cell eKodeB ( 0 }uses
MBSFN to provide interference management for macro cel and
small cell mobility. The macro cell eNodeB reduces high
interference by 'muting' in near-blank sub-frames configured by
operations and management. The use of OAM i known and,
therefore, will not be described for the sake of brevity.
The small cell 125) knows which macro sub-frames are nearblank
and transmits in these sub-frames for the UEs in the
border.
In a radio frame configuration, e.g., for frequency division
duplex (FDD), sub -frames 1, 2, 3, 6, 7, 8, are available for
MBSFN. MBSFN (for near-blank sub-frames) reduce downlink
interferences.
MBSFN for elCIC impacts a downlink layer 2 scheduler only.
The layer 2 downlink scheduler is mute and. layer 2 ensures the
configuration of the MBSFN sub-frames.
There are two types of ABS sub-frames: MBSFN ABS (in releases
8 and 9 ) sub-frames and non-MBSFN ABS sub-frames (available
in release 10 and after). In MBSFN sub-frames, only the data is
not transmitted (entire control signal is transmitted). In non-
MBSFN sub-frames control is transmitted on only o first
subframe. The blanking information is shared between macro
cell and small cells via X .
The network decides which and how many sub -frames to be
blanked. The blanking of a sub -frame is determined based on
the based on the control information it carries. The network
notifies the small cell the ABS pattern of the umbrella macro
cell. The information is delivered to the UE covered by the small
cell. In general, these area details of elCIC and known art.
FIGS. 5B-5C illustrate a method of using ABS.
As shown in FIG. 5B eight sub- ram s are shown. ABS' are
created in the macro ce l and the macro cell does not use ABS
for data transmission. In FIG. 5B, the macro cell uses subframes
1, 2, 4, 5, 7 and 8 for data transmission. The ABS's are
located in sub-frames 3 and 6 . A base station (eNodeB)
scheduler decides the ABS's based on the load and mobility of
the UEs around the coverage area related to the small cell and
the macro cel Only pilots and broadcast signals are
transmitted by the macro cell during an ABS.
As shown in FIG. 5C the antenna 125 schedules its cell border
mobiles during the ABS of the macro cell. More specifically, the
antenna 125 transmits information to the UEs in the border
B120 during the ABS's The antenna 125 may know a UE is
within the border 1 0 based on the HO measurement report or
CQI report from the UE. The antenna 125 knows whether the
UE is at the border B120 and whether it should be handed over
to the macro cell. Additionally, if a UE is just handed over to
the small cell, that UE is at the edge of the small cell. The small
cell may use information to decide whether a UE is still at the
border such as UL received signal strength or UL CQI or DL
CQI.
By scheduling transmission to border UEs during the ABS's, the
small cell reduces interference from the macro cell and UE may
not experience LF.
As described above, macro cell sub-frame blanking mitigates
macro /small cell interferences.
n another example embodiment small cells transmit to the UEs
at the border in the blanked sub-frames of the umbrella macro
and transmit to the other in -ce UEs in all sub-frames.
FIGS. 6A-6B illustrate another example embodiment of a
method of using Almost Blank Sub-frames (ABS) to reduce the
amount of handovers. The method of FIGS. 6A-6B achieves fast
hand-in and hand-out of low and medium speed UE's and keeps
high speed users on the macro cell. The serving base station is
configured to determine the speed of the UE and classify the
speed of the UE, as described in FIGS. 4A-4B.
In the method of FIGS, 6A-6B, the serving base station keeps
high speed UEs on the macro cell by scheduling the high speed
UEs on the interference-free macro cell sub frames that are
blanked at the small cell. As shown in FIG. 6B, the small cell
has an ABS in sub-frame 9. Therefore, the macro cell schedules
transmission for the high speed UEs during sub-frame 9. The
ABS pattern of the small cell is also delivered to the macro cell
by X2.
The base station scheduler may also rely on an observed
difference in channel quality indicator (CQI) reported b the UE
over the interference-free subframe (the ABS sub frame of the
small cell) versus the remaining frames. If the base station
scheduler gets the CQ s values every sub-frame, the scheduler
can determine which sub -frames are blanked in the small cell
because the CQI values will be higher in blanked sub-frames o
the small cells.
For medium and low speed UE's, the macro cell schedules
measurements and sends RRC handover commands over the
interference-free sub-frames, i.e., corresponding to the almost
blank sub-frames at the metro cell.
A UE's signal to interference plus noise ratio (SIKR) at the
macro cell is acceptable within range of a small cell coverage
area because the UE communicates with the macro cell in the
ABS sub-frames and may not experience RLF.
Another example embodiment discloses early hand-in or
handover of a UE to a small cell and late hand-out from the
small cell. Macro cell sub-frame blanking could reduce the
small cell interferences during HO in elCIC operation with the
cell selection bias towards the small cells. The cell selection
bias towards the small cells, results in larger small ce l coverage
area. As a result, the early hand-in to the small cells and late
hand-out from the small cells occur. For early hand-in or
handover of a UE to a small cell, the serving base station
determines that the target small cell power strength is still too
weak just after the handover. Therefore, the small cell base
station schedules transmission on the ABS of the macro cell to
avoid the interference from the macro cell to . For a macro to
small HO, the early HO is initiated with a negative HO threshold
and for small to macro HO positive HO threshold would initiate
the late HO. For late handover or hand-out of a UE to a macro
cell, the serving small cell base station determines that its
power strength is too weak due to the large positive HO
threshold just before the handover. Therefore, to handover from
the small cell to the macro cell, the small cell base station
schedules transmission on the ABS of the umbrella macro cell
to avoid the interference from the macro cell to the small cell
(serving cell) before the HO taking place.
FIG. 7A illustrates an example embodiment of the UE 15. It
should be also understood that the UE 115 may include
features not shown in FIG. 2A and should not be limited to
those features that are shown.
The UE 115 is configured to determine speed and direction
information relative to the small cell 120 and determine whether
to HO to the small cell 120.
The UE 15 may include, for example, a transmitting unit 10,
a UE receiving unit 220, a memory unit 230, a processing unit
240, and a data bus 250.
The transmitting unit 210, UE receiving unit 220, memory u ni
230, and processing unit 240 may send data to and/ or receive
data from one another using the data bus 250. The
transmitting unit 2 .10 is a device that includes hardware and
any necessary software for transmitting wireless signals on the
uplink (reverse lin k including, for example, data signals, control
signals, and signal strength/quality information via one or more
wireless connections to other wireless devices (e.g., base
stations).
The UE receiving unit 220 is a device that includes hardware
and any necessary software for receiving wireless signals on the
downlink (forward link) channel including, for example, data
signals, control signals, and signal strength /quality information
via one or more wireless connections from other wireless devices
(e.g., base stations). The UE receiving unit 220 receives
information from the serving base station 10i and the antenna
125.
The memory unit 230 may be any storage medium capable of
storing data including magnetic storage, flash storage, etc.
The processing unit 240 may he any device capable of
processing data including, for example, a microprocessor
configured to carry out specific operations based on input data,
or capable of executing instructions included in computer
readable code. The processing unit 240 may determine
reception parameters based on the transmission parameters.
FIG. 7B illustrates an example embodiment of the base station
0 . It should be also understood that the base station l lOi
may include features not shown in FIG. 7B and should not be
limited to those features that are shown.
Referring to FIG. 5B, the base station 1 0 i may include, for
example, a data bus 259, a transmitting unit 252, a receiving
unit 254, a memory unit 256, and a processing unit 258.
The transmitting unit 252, receiving unit 254, memory unit 256,
and processing unit 258 may send data to and/ or -eceive data
from one another using the data bus 259. The transmitting
unit 252 is a device that includes hardware and any necessary
software for transmitting wireless signals including, for
example, data signals, control signals, and signal
strength/ quality information via one or more wireless
connections to other network elements in the wireless
communications network 100,
The receiving unit 254 is a device that includes hardware and
any necessary software for receiving wireless signals including,
for example, data signals, control signals, and signal
strength/ quality information via one or more wireless
connections to other network elements in the network 100.
The memory unit 256 may be any device capable of storing data
including magnetic storage, flash storage, etc.
The processing unit 258 may be any device capable of
processing data including, for example, a microprocessor
configured to carry out specific operations based o input data,
or capable of executing instructions included in computer
readable code.
For example, the processing unit 258 is capable of determining
a speed of the UE and controlling a HO from the serving base
station to a second base station based on the speed of the UE,
a s described above. Furthermore, the processing unit 258 is
configured to perform handover measurements regarding a
handover from a serving macro cell to a target small cell based
o a velocity of the UE relative to the target cell.
Example embodiments being thus described, it will be obvious
that the same may be varied in many ways. Such variations
are not to be regarded as a departure from the spirit and scope
of example embodiments, and all such modifications as would
be obvious to one skilled in the art are intended to be included
within the scope of the claims.
What is claimed is:
1. A method of controlling a handover of a user equipment (UE) from
a serving base station to a target base station in a heterogeneous
network, the method comprising:
determining (S405), by a serving base station, a speed of the UE
and a the type of the handover, the type of the handover being one of
macro cell to macro cell, macro cell to small ceil, small cell to macro
cell and small cell to small cell; and
controlling (S410), by the serving base station, the handover
from the serving base station to the target base station based on the
speed of the UE and the type of the handover.
2. The method of claim 1, wherein the controlling (S410) includes,
classifying S4 .5) the speed of the UE into one of a low speed,
medium speed and high speed.
3. The method of claim 2, wherein the controlling S 10) includes,
preventing the handover to a small cell if the speed of the UE is
the high speed, the target base station serving the small cell.
4. The method of claim 3, wherein the controlling S 10) includes,
scheduling the U for transmission on almost blank sub-frames
(ABS) of the target base station.
5. The method of claim 1, wherein the controlling (S410) includes,
changing (S420) a handover threshold based on the speed of the
UE, and
handing over the UE if the handover threshold exceeds a
difference between the reference signal received power (RSRPs) of the
target and serving base stations at the UE.
6 . The method of claim 1, wherein the controlling S4 0) includes,
adjusting (S430) a layer 3 filter value based on the speed of
the UE.
7 . The method of claim 1, further comprising:
determining a direction of the UE, and the controlling includes,
controlling, by the serving b a s station, the handover
from the serving base station to the target base station based on a
velocity of the UE, the velocity being the speed and direction of the
UE.
8 . The method of claim 7 , wherein the controlling (S 0 ) includes,
classifying the speed of the UE into one of a low speed, medium
speed and high speed
9 . The method of claim 1, wherein the controlling S 0 ) further
includes,
changing a handover threshold based on the speed of the UE,
the handover threshold being one of a cell specific offset of a cell
associated with the target base station, a hysteresis parameter for an
event and a system wide common offset parameter for the event.
1C5. The method of claim 1, wherein the controlling includes,
adjusting time-to-trigger (TTT) handover period based on the
type of the handover.