U.S. patent application number 13/253833 was filed with the patent office on 2012-05-17 for positioning reference signal assistance data signaling for enhanced interference coordination in a wireless communication network.
This patent application is currently assigned to MOTOROLA MOBILITY, INC.. Invention is credited to Sandeep H. Krishnamurthy, Robert T. Love, Ajit Nimbalker.
Application Number | 20120122472 13/253833 |
Document ID | / |
Family ID | 46048209 |
Filed Date | 2012-05-17 |
United States Patent
Application |
20120122472 |
Kind Code |
A1 |
Krishnamurthy; Sandeep H. ;
et al. |
May 17, 2012 |
Positioning Reference Signal Assistance Data Signaling for Enhanced
Interference Coordination in a Wireless Communication Network
Abstract
A method in a wireless terminal includes receiving a measurement
subframe pattern indicating a first set of subframes on which a
transmission from a first base station must be processed, receiving
Positioning Reference Signal (PRS) subframe pattern information
corresponding to a second base station, determining a subset of the
first set of subframes that overlaps with subframes in the PRS
subframe pattern information, and processing a transmission
received in the subset of the first set of subframes from the first
base station.
Inventors: |
Krishnamurthy; Sandeep H.;
(Arlington Heights, IL) ; Love; Robert T.;
(Barrington, IL) ; Nimbalker; Ajit; (Buffalo
Grove, IL) |
Assignee: |
MOTOROLA MOBILITY, INC.
Libertyville
IL
|
Family ID: |
46048209 |
Appl. No.: |
13/253833 |
Filed: |
October 5, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61413153 |
Nov 12, 2010 |
|
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Current U.S.
Class: |
455/456.1 |
Current CPC
Class: |
H04L 27/2601 20130101;
H04L 27/0012 20130101; H04W 72/082 20130101; H04L 5/0053 20130101;
H04W 24/10 20130101 |
Class at
Publication: |
455/456.1 |
International
Class: |
H04W 4/00 20090101
H04W004/00 |
Claims
1. A method in a wireless terminal, the method comprising:
receiving a measurement subframe pattern indicating a first set of
subframes on which a transmission from a first base station must be
processed; receiving Positioning Reference Signal (PRS) subframe
pattern information corresponding to a second base station;
determining a subset of the first set of subframes that overlaps
with subframes in the PRS subframe pattern information; and
processing a transmission received in the subset of the first set
of subframes from the first base station.
2. The method of claim 1, wherein the PRS subframe pattern
information includes at least one of: PRS occasion time-pattern;
N.sub.PRS, the number of consecutive subframes in one PRS occasion;
PRS bandwidth information; number of cell-specific reference symbol
(CRS) transmit antenna ports; or an OFDM cyclic prefix (CP)
associated with the PRS subframe pattern information.
3. The method of claim 1, wherein the wireless terminal is
connected to or camped on a third base station that is distinct
from the second base station.
4. The method of claim 3, wherein the first base station and the
third base station are identical to the serving base station.
5. The method of claim 1 further comprising receiving the
measurement subframe pattern from a third base station which is a
serving base station, wherein the measurement subframe pattern
indicates the first set of subframes on which the transmission from
the first base station must be processed.
6. The method of claim 1, wherein processing the transmission
received in the subset of the first set of subframes from the first
base station includes performing PRS Interference reduction for
demodulation/processing of at least one desired signal from a
serving cell in an associated subframe.
7. The method of claim 1, processing the transmission received in
the subset of the first set of subframes from the first base
station includes performing at least one of the RLM/RRM/CSI
measurements for the first base station taking into account PRS
IR/IC for at least one resource element containing the PRS subframe
pattern information from the second base station.
8. The method of claim 1, wherein processing the transmission
received in the subset of the first set of subframes from the first
base station includes performing RRM measurements for a third base
station taking into account PRS interference reduction for at least
one resource element containing the PRS subframe pattern
information from the second base station.
9. The method of claim 1 further comprising: identifying a second
subset of the first set of subframes from the first base station
that do not contain PRS subframe pattern information from the
second base station; performing only CRS Interference reduction for
demodulation/processing of at least one desired signal from the
first base station in the second subset of subframes.
10. The method of claim 1, wherein at least one subframe in the
measurement subframe pattern coincides temporally with an Almost
Blank Subframe (ABS) transmission of a third base station.
11. A wireless terminal comprising: a receiver coupled to a
controller, the controller configured to determine a subset of a
first set of subframes, on which a transmission from a first base
station must be processed, that overlaps with subframes in
Positioning Reference Signal (PRS) subframe pattern information
corresponding to a second base station, a measurement subframe
pattern, received by the receiver, indicating the first set of
subframes on which the transmission from a first base station must
be processed; the controller configured to process a transmission
received in the subset of the first set of subframes from the first
base station.
12. The terminal of claim 11, wherein the PRS subframe pattern
information includes at least one of: PRS occasion time-pattern;
N.sub.PRS, the number of consecutive subframes in one PRS occasion;
PRS bandwidth information; Number of cell-specific reference symbol
(CRS) transmit antenna ports; or OFDM cyclic prefix (CP) associated
with the PRS subframe pattern information.
13. The terminal of claim 11 the receiver configured to receives
the measurement subframe pattern from a third base station which is
a serving base station, wherein the measurement subframe pattern
indicates the first set of subframes on which the transmission from
the first base station must be processed.
14. The terminal of claim 11 further comprising: the controller
configured to identify a second subset of the first set of
subframes from the first base station that do not contain PRS
subframe pattern information from the second base station; the
controller configured to perform only CRS reduction for
demodulation/processing of at least one desired signal from the
first base station in the second subset of subframes, wherein the
desired signal is one of CRS, PBCH, PCFICH, PHICH, CSI-RS, PDCCH,
PDSCH, or DM-RS.
15. The terminal of claim 11, wherein at least one subframe in the
measurement subframe pattern coincides temporally with an Almost
Blank Subframe (ABS) transmission of a third base station.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims benefits to provisional
Application No. 61/413,153 filed on 12 Nov. 2010 under 35 U.S.C.
119, the contents of which are incorporated herein by
reference.
FIELD OF THE DISCLOSURE
[0002] The present invention relates generally to wireless
communication networks and, more particularly, to an apparatus and
method for interference measurements in wireless terminals capable
of enhanced inter-cell interference coordination methods.
BACKGROUND
[0003] Wireless communication networks are well known. Some
networks are completely proprietary, while others are subject to
one or more standards to allow various vendors to manufacture
equipment for a common system. One such standards-based network is
the Universal Mobile Telecommunications System (UMTS). UMTS is
standardized by the Third Generation Partnership Project (3GPP), a
collaborative effort by groups of telecommunications associations
to make a globally applicable third generation (3G) mobile phone
system specification within the scope of the International Mobile
Telecommunications-2000 project of the International
Telecommunication Union (ITU). Efforts are currently underway to
develop an evolved UMTS standard, which is typically referred to as
UMTS Long Term Evolution (E-UTRA) or Evolved UMTS Terrestrial Radio
Access (E-UTRA).
[0004] According to Release 8 of the E-UTRA or LTE standard or
specification, downlink communications from a base station
(referred to as an "enhanced Node-B" or simply "eNB") to a wireless
communication device (referred to as "user equipment" or "UE")
utilize orthogonal frequency division multiplexing (OFDM). In OFDM,
orthogonal subcarriers are modulated with a digital stream, which
may include data, control information, or other information, so as
to form a set of OFDM symbols. The subcarriers may be contiguous or
discontiguous and the downlink data modulation may be performed
using quadrature phase shift-keying (QPSK), 16-ary quadrature
amplitude modulation (16QAM), or 64QAM. The OFDM symbols are
configured into a downlink subframe for transmission from the base
station. Each OFDM symbol has a time duration and is associated
with a cyclic prefix (CP). A cyclic prefix is essentially a guard
period between successive OFDM symbols in a subframe. According to
the E-UTRA specification, a normal cyclic prefix is about five (5)
microseconds and an extended cyclic prefix is 16.67
microseconds.
[0005] In contrast to the downlink, uplink communications from the
UE to the eNB utilize single-carrier frequency division multiple
access (SC-FDMA) according to the E-UTRA standard. In SC-FDMA,
block transmission of QAM data symbols is performed by first
discrete Fourier transform (DFT)-spreading (or precoding) followed
by subcarrier mapping to a conventional OFDM modulator. The use of
DFT precoding allows a moderate cubic metric/peak-to-average power
ratio (PAPR) leading to reduced cost, size and power consumption of
the UE power amplifier. In accordance with SC-FDMA, each subcarrier
used for uplink transmission includes information for all the
transmitted modulated signals, with the input data stream being
spread over them. The data transmission in the uplink is controlled
by the eNB, involving transmission of scheduling requests (and
scheduling information) sent via downlink control channels.
Scheduling grants for uplink transmissions are provided by the eNB
on the downlink and include, among other things, a resource
allocation (e.g., a resource block size per one millisecond (ms)
interval) and an identification of the modulation to be used for
the uplink transmissions. With the addition of higher-order
modulation and adaptive modulation and coding (AMC), large spectral
efficiency is possible by scheduling users with favorable channel
conditions.
[0006] E-UTRA systems also facilitate the use of multiple input and
multiple output (MIMO) antenna systems on the downlink to increase
capacity. As is known, MIMO antenna systems are employed at the eNB
through use of multiple transmit antennas and at the UE through use
of multiple receive antennas. A UE may rely on a pilot or reference
symbol (RS) sent from the eNB for channel estimation, subsequent
data demodulation, and link quality measurement for reporting. The
link quality measurements for feedback may include such spatial
parameters as rank indicator, or the number of data streams sent on
the same resources; precoding matrix index (PMI); rank indicator
(RI) and coding parameters, such as a modulation and coding scheme
(MCS) or a channel quality indicator (CQI). Together MCS or CQI,
PMI and RI constitute elements of the Channel State Information
(CSI) which convey the quality of MIMO channel indicative of the
reliability and condition number of the channel capable of
supporting multi-stream communication between the eNB and the UE.
For example, if a UE determines that the link can support a rank
greater than one, it may report multiple CQI values (e.g., two CQI
values when rank=2 by signaling of the corresponding RI). Further,
the link quality measurements may be reported on a periodic or
aperiodic basis, as instructed by an eNB, in one of the supported
feedback modes. The reports may include wideband or subband
frequency selective information of the parameters. The eNB may use
the rank information, the CQI, and other parameters, such as uplink
quality information, to serve the UE on the uplink and downlink
channels.
[0007] E-UTRA systems must be compliant to regulatory requirements
on spurious emissions on licensed bands in different regions of the
world. E-UTRA follows the "uplink after downlink" principle which
means that a UE must transmit on its uplink only when its downlink
is reliable. In other words, a UE that does not have a reliable
downlink must continuously monitor the quality of the downlink
signal by tracking the downlink signal quality (e.g., based on
channel state estimation) and stop transmission on its uplink if
the downlink signal quality falls below a threshold. In E-UTRA,
this is enabled by means of Radio Link Monitoring (RLM) UE
procedures where a UE continuously monitors the cell-specific
reference signal (CRS) on the downlink and determines the channel
state (including estimating the propagation channel between the eNB
and the UE and the underlying interference on the same carrier).
Qout is defined as the condition that the channel quality between
eNB and the UE is such that the Block Error Rate (BLER) of a first
hypothetical PDCCH transmission exceeds 10%. This event is also
denoted as an "out-of-sync" event. Qin is defined as the condition
that the channel quality between eNB and the UE is such that the
BLER of a second hypothetical PDCCH transmission drops below 2%.
This event is also denoted as an "in-sync" event. The UE monitors
the channel state in RRC_CONNECTED mode continuously or
periodically in both non-discontinuous reception (non-DRX) and
discontinuous reception (DRX) states to evaluate whether Qout or
Qin has occurred. Upon several successive Qout detections, the UE
must determine that a Radio Link Problem (RLP) has occurred. In the
RLP state, the UE must assume that it has lost its downlink with
the serving eNB and start monitoring the link for recovery. If a
Qin is detected within a certain duration of time as configured by
the eNB by means of a Radio Resource Control (RRC) timer, the UE
resumes normal RRC_CONNECTED operation. On the other hand, if a Qin
is not detected within the said duration of time, the UE must
determine that a Radio Link Failure (RLF) has occurred and must
stop all uplink transmission within 40 ms. The RLM procedure
reduces the probability that a UE jams the uplink of a neighbor
cell when the UE has lost the serving cell downlink but has not
been handed over to a different cell by the network due to Radio
Resource Management (RRM) inefficiencies.
[0008] Like other 3GPP standards, E-UTRA supports mobility of UEs
by RRM measurements and associated support for RRC signaling
including specified eNB and UE behavior in both RRC_CONNECTED and
RRC_IDLE states. In the RRC_CONNECTED state, a UE can be configured
to measure and report Reference Signal Received Power (RSRP) and
Reference Signal Received Quality (RSRQ) for both the serving cell
and the neighbor cells (on the serving cell carrier and
inter-frequency carriers). A network element such as the eNB or the
Mobility Management Entity (MME) can perform UE handovers based on
the reported measurements. In RRC_IDLE state, the UE can be
configured to measure RSRP and RSRQ and perform cell reselections
based on these measurements.
[0009] Heterogeneous networks comprise a variety of base stations
serving mobile stations. In some systems, the base stations operate
on the same carrier frequency. The variety of base stations can
include some or all of the following types of base stations:
conventional macro base stations (macro cells), pico base station
(pico cells), relay nodes and femto base stations (also referred to
as femto cells, closed subscriber group (CSG) cells or Home
eNodeBs). Macro cells typically have coverage areas that range from
several hundreds of meters to several kilometers. Pico cells,
relays and femto cells can have coverage areas that are
considerably smaller than the coverage area of typical macro cells.
Pico cells can have coverage areas of about 100-200 meters. Femto
cells are typically used for indoor coverage, and can have coverage
areas in the 10s of meters. Relay nodes are characterized by a
wireless backhaul to a donor base station, and can have coverage
areas similar to pico cells.
[0010] A home-base station or femto-cell or pico-eNB or relay node
(RN) is referred to as hetero-eNB (HeNB) or a hetero-cell or hetero
base station in the sequel. A HeNB can either belong to a CSG as
mentioned earlier or can be an open-access cell. HeNBs are used for
coverage in a small area (such as a home or office) in contrast
with eNBs (also referred to as macro eNBs or macro-cells) which are
typically used for coverage over a large area. A CSG is set of one
or more cells that allow access only to a certain group of
subscribers. HeNB deployments where at least a part of the deployed
bandwidth (BW) is shared with macro-cells are considered to be
high-risk scenarios from an interference point-of-view. When UEs
connected to a macro-cell roam close to a HeNB, the uplink of the
HeNB can be severely interfered with particularly when the HeNB is
far away (for example>400 m) from the macro-cell, thereby,
degrading the quality of service of UEs connected to the HeNB. The
problem is particularly severe if the UE is not allowed to access
the HeNB that it roams close to (for example, due to the UE not
being a member of the CSG of the HeNB). Currently, the existing
Rel-8/9 UE measurement framework can be made use of to identify the
situation when this interference might occur and the network can
handover the UE to an inter-frequency carrier which is not shared
between macro-cells and HeNBs to mitigate this problem. However,
there might not be any such carriers available in certain networks
to handover the UE to. Further, as the penetration of HeNBs
increases, being able to efficiently operate HeNBs on the entire
available spectrum might be desirable for maximizing spectral
efficiency and reducing overall operational cost. Several other
scenarios are likely too including the case of a UE connected one
HeNB experiencing interference from an adjacent HeNB or a macro
cell. The following types of interference scenarios have been
identified. [0011] HeNB (aggressor).fwdarw.MeNB (victim) downlink
(DL) [0012] HUE (aggressor).fwdarw.MeNB (victim)uplink (UL) [0013]
MUE (aggressor).fwdarw.HeNB (victim)UL [0014] MeNB
(aggressor).fwdarw.HeNB (victim)DL [0015] HeNB
(aggressor).fwdarw.HeNB (victim)on DL [0016] HeNB
(aggressor).fwdarw.HeNB (victim)on UL.
[0017] Heterogeneous networks can potentially enable an operator to
provide improved service to users (e.g., increased data rates,
faster access, etc.) with lower capital expenditure. Typically,
installation of macro base stations is very expensive as they
require towers. On the other hand, base stations with smaller
coverage areas are generally much less expensive to install. For
example, pico cells can be installed on roof tops and femto cells
can be easily installed indoors. The pico and femto cells allow the
network to offload user communication traffic from the macro cell
to the pico or femto cells. This enables users to get higher
throughput and better service without the network operator
installing additional macro base stations or provisioning more
carrier frequencies for communication. Thus, heterogeneous networks
are considered to be an attractive path for evolution of wireless
communication networks. 3GPP has commenced work on enabling
heterogeneous E-UTRA networks in 3GPP LTE Release 10.
[0018] FIG. 1 illustrates an E-UTRA Heterogeneous network
comprising a macro cell, pico cells and femto cells operating on a
single carrier frequency. A mobile station, also referred to as
"user equipment" (UE), may be associated with one of the cells
based on its location. The association of a UE to a cell can refer
to association in idle mode or connected mode. That is, a UE is
considered to be associated with a cell in idle mode if it is
camped on the cell in idle mode. Similarly, a UE is considered to
be associated with a cell in connected mode if it is configured to
perform bi-directional communication with a cell (for example, a UE
in E-UTRA radio resource control (RRC) connected mode can be
connected to and therefore associate with a cell). A UE associated
with a macro cell is referred to macro UE, a UE associated with a
pico cell is referred to as a pico UE, and a UE associated with a
femto cell is referred to as a femto UE.
[0019] Various time-division approaches are possible for ensuring
that base stations in heterogeneous networks share the frequency
spectrum while minimizing interference. Two approaches can be
envisioned. A network can configure time periods when different
base stations are required to not transmit. This enables cells that
can interfere with one another to transmit in mutually exclusive
time periods. For example, a femto cell can be configured with some
time periods during which it does not transmit. If a macro UE is
located within the coverage of the femto cell, the macro cell can
use the time periods during which the femto cell does not transmit
data to the UE.
[0020] The network can configure time periods where a first base
station transmits on all available time periods (e.g., pico eNBs),
while a second base station (e.g., macro eNB) transmits only on a
subset of the available time periods. A UE connected to the first
base station can therefore have two "virtual" channels at different
channel qualities depending on how much the second base station's
transmission interferes with that for the first (i.e., signal
geometry of the first base station relative to the second). The
first virtual channel is where only the first base station
transmits data while the second base station does not transmit
data. The second virtual channel is one where both the first and
the second base stations transmit data. The first base station can
use adaptive modulation and coding and schedule at different MCS
levels on the two virtual channels. In the extreme case, the first
base station may not schedule at all on the second virtual channel
when interference from the second base station is large.
[0021] However, it should be noted that time division approaches
can lead to inaccurate or inconsistent RRM, RLM and CSI
measurements. For example, if a macro UE located near a femto cell
performs measurements during time periods when the femto cell
transmits, the measured values can be significantly different from
measured values obtained from measurements made during time periods
when the femto cell does not transmit. Such measurements can lead
to erratic behaviors, such as failed connections, unnecessary
handovers and unnecessary cell reselections. In addition, such
inaccuracies can lead to suboptimal scheduling on UE downlink
leading to inefficient utilization of spectral resources.
Therefore, methods are needed for performing measurements of cells
that overcome the problems mentioned above.
[0022] The various aspects, features and advantages of the
disclosure will become more fully apparent to those having ordinary
skill in the art upon a careful consideration of the following
Detailed Description thereof with the accompanying drawings
described below. The drawings may have been simplified for clarity
and are not necessarily drawn to scale.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The accompanying figures, where like reference numerals
refer to identical or functionally similar elements throughout the
separate views and which together with the detailed description
below are incorporated in and form part of the specification, serve
to further illustrate various embodiments and to explain various
principles and advantages all in accordance with the one or more
embodiments of the present invention.
[0024] FIG. 1 illustrates a prior art Heterogeneous network
comprising macro cells, pico cells and femto cells.
[0025] FIG. 2 is an electrical block diagram of a wireless
communication system providing wireless communication service to a
wireless communication device.
[0026] FIG. 3 illustrates electrical block diagrams of an exemplary
base station usable in the wireless communication system of FIG. 2
and a wireless communication device.
[0027] FIG. 4 shows a positioning reference signal transmission
from a base station in accordance with E-UTRA Rel-9
specification.
[0028] FIG. 5 illustrates the flowchart for an exemplary embodiment
of the present invention for Positioning Reference Signal
Interference Rejection or Interference Cancellation.
[0029] FIG. 6 illustrates the flowchart for another exemplary
embodiment of the present invention for enabling Interference
Cancellation or Interference Rejection based on a UE-autonomous
trigger or a network initiated trigger.
[0030] Skilled artisans will appreciate that elements in the
figures are illustrated for simplicity and clarity and have not
necessarily been drawn to scale or to include every component of an
element. For example, the dimensions of some of the elements in the
figures may be exaggerated alone or relative to other elements, or
some and possibly many components of an element may be excluded
from the element, to help improve the understanding of the various
embodiments of the present invention.
DETAILED DESCRIPTION
[0031] Femto cells are generally used in homes and offices and
their precise location and configuration is not entirely under the
network operator's control. For example, two femto cells located in
nearby homes can have the same physical layer cell identifier
(PCID). A femto cell can be a restricted access cell such as a CSG
cell. In FIG. 1, a Heterogeneous network 100 comprises a macro cell
102, femto cells 104, 108, 122, pico cells 112, 124 and mobile
stations 106, 110, 116, 118, 120, 126. If a UE 110 is not a member
of the CSG to which the femto cell 108 belongs, the UE may be
unable to access the femto cell. Even if the UE 110 is very close
to such a femto cell 108, the UE may be associated with the macro
cell. The UE may then experience significant interference to its
communication with the macro cell due to transmissions of the femto
cell.
[0032] Pico cells generally do not restrict access to specific
users. However, some operator configurations can allow pico cells
to restrict access to certain users. Pico cells are generally fully
under the network operator's control and can be used to enhance
coverage in locations where the macro cell signal quality may be
inadequate. Furthermore, in order to increase offloading of users
to pico cells, a network operator can have an association bias
towards the pico cell. In FIG. 1 for example, a UE 118 may be made
to associate with a pico cell even if the pico cell 112 is not the
strongest cell at the UE's 118 location. This is referred to as
"cell range expansion" of the pico cell. A UE is said to be the
cell range expansion area of a pico cell, if it associates with the
pico cell only if an association bias is used, and associates with
another cell (e.g., a macro cell 102) if the association bias is
not used. If a UE 118 is in the cell range expansion area of the
pico cell 112 and is associated with the pico cell 112, it can
experience significant interference due to transmissions of a
neighbor cell (such as a macro cell 102).
[0033] In order to operate multiple cells with overlapping coverage
on a carrier frequency, such as in a heterogeneous network 100 in
FIG. 1, it is necessary to have coordination between the cells so
that the transmissions from the different cells do not interfere
with one another. E-UTRA heterogeneous networks will use time
division techniques to minimize interference. Specifically, a cell
can be configured with patterns of subframes during which it does
not schedule user data. Such subframes are referred to as "Blank
subframes". Furthermore, it may be necessary to transmit some
critically important information in all subframes. For example, it
may be necessary to transmit cell-specific reference symbols (CRS)
to enable UEs to perform measurements during the subframe. It may
also be necessary to transmit primary and secondary synchronization
signals (PSS and SSS), primary broadcast channel (PBCH) and System
Information Block 1 (SIB1), Paging Channel and the Positioning
Reference Signal (PRS). Such information is essential for proper
operation of functions such as cell search and maintenance of
up-to-date system information. Blank subframes which are not used
for scheduling data but can be used for transmission of a
restricted set of information (such as the critically important
information described above) are referred to as "Almost blank
subframes" (AB subframes or ABS or ABSF, and ABSs and ABSFs
indicate the plural form of an "almost blank subframe"). In each
ABS being transmitted from a base station, the base station can be
configured to not transmit any energy on all resource elements,
except for resource elements used at least one of (a) CRS, (b) PSS
and SSS, (c) PBCH, (d) SIB1, and (e) paging messages.
[0034] The time corresponding to the ABS transmission of one cell
can be used by a neighboring cell to schedule UEs connected to that
(i.e. neighboring) cell. For example, each of a femto cell, a macro
cell and a pico cell can be configured with an ABS pattern (i.e., a
sequence of subframes with a certain time reuse, where a subset of
the subframes are configured as ABSs and the remainder configured
for normal downlink scheduling). The patterns can be such that the
ABSs of different cells can overlap. Alternatively the patterns can
be mutually exclusive, so that ABSs of two cells do not overlap.
Also, some cells may not be configured with an AB subframe pattern.
As indicated above, a cell can be configured to only transmit
critically important information during its AB subframes. While ABS
are typically defined for the Downlink, ABS or Blank subframes can
also be defined for uplink transmissions wherein the UE is
requested to not transmit any uplink data or uplink control
information. Thus, any uplink transmissions in the ABS may be
suspended until the next transmission opportunity for the
corresponding information. For data, the next transmission
opportunity may be determined by the Hybrid Automatic repeat
request (HARQ) timing, and for uplink control information such as
CQI/RI/PMI/Scheduling Request (SR), the next transmission
opportunity may be determined based on the corresponding schedule
as indicated to the UE by the eNB, based on dynamic, semi-static
and/or higher layer signaling.
[0035] The use of AB subframe patterns is described further below.
A macro UE may be in the coverage of a non-allowed femto cell, such
as a CSG cell whose CSG the UE is not a member. In FIG. 1, UE 110
represents such a UE and femto cell 108 represents such a femto
cell. Such a macro UE can experience interference from the femto
cell, making communication between the macro UE and the macro cell
difficult. To overcome the interference, the macro cell can
transmit data to the UE only in the ABSs of the femto cell. Since
the femto cell only transmits critically important signals in the
ABSs, the macro cell can avoid most of the interference from the
femto cell and successfully transmit data to the macro UE in the
ABSs of the femto cell.
[0036] Similarly, a pico UE may be in the cell range expansion area
of the pico cell. In FIG. 1, UE 118 represents such a pico UE and
pico cell 112 represents such a pico cell. Such a pico UE can
experience a high interference from a neighbor cell, such as macro
cell 102), making communication between the pico UE and the pico
cell difficult. In order to overcome the interference, the pico
cell can transmit data to the UE only in the ABSs of the macro
cell. Since the macro cell only transmits critically important
signals in the AB subframes, the pico cell can avoid most of the
interference from the macro cell and successfully transmit data to
the pico UE in the AB subframes of the macro cell. Further, the
pico cell can also transmit in the non-ABSs of the macro cell, but
can schedule a lower MCS to account for the degraded signal quality
in such subframes. The pico cell may request the UE to report two
CQI/PMI/RI values, one for the subframes where the macro cell is
transmitting ABS, and another for when the macro cell is
transmitting regular subframes. Alternatively, the pico cell may
require the UE to report the CQI/PMI/RI values based on a
restricted set of resources that is signaled to the UE via dynamic
or higher layer signaling. The pico cell may then apply suitable
filtering and/or processing to keep track of the multiple
CQI/PMI/RI levels for the subframes with and without the
interference.
[0037] When different cells use different patterns of ABSs, the
RRM, RLM and CSI measurements performed by UEs in the heterogeneous
network can result in unpredictable and undesirable behavior. UEs
perform RLM measurements in connected mode to ensure that the
serving cell signal conditions are adequate to schedule the UE. UEs
perform RRM measurements to support handovers in connected mode and
reselections in idle mode. UE performs CSI measurements to support
optimal scheduling by the base station. For example, in FIG. 1,
macro UE 110 in the coverage of a non-allowed femto cell 108 may be
performing RLM measurements of the macro cell 102 signal. Due to
interference from the femto cell 108 in subframes during which the
femto cell schedules (i.e., not the ABSs of the femto cell), the
macro UE can conclude that the radio link between the macro cell
and the macro UE has failed. The UE can make such a conclusion even
if it can be successfully scheduled by the macro cell during the
ABSs of the femto cell.
[0038] Similarly, in FIG. 1, the macro UE 110 in the coverage of a
non-allowed femto cell 108 may be performing RRM measurements of
the serving cell and neighbor cells. Due to interference from the
femto cell, the UE may measure a low value the macro cell signal
level and transmit a measurement report indicating the low value to
the network. As a result of the measurement report, the network can
perform a handover of the UE to another frequency or to another
radio access technology, such as UMTS or GSM. This is an
undesirable outcome, as the UE can be successfully scheduled by the
macro cell in the femto cell's ABSs.
[0039] Similarly, in FIG. 1, the macro UE 110 in the coverage of a
non-allowed femto cell 108 may be performing CSI measurements of
the serving cell. Due to interference from the femto cell, the UE
may measure a low value of the macro cell's channel quality and
transmit a low value of CQI (and potentially a low value of RI or a
suboptimal value of PMI) to the network. As a result of the low
value of CQI, the base station can avoid scheduling the UE or
transmit a very small amount of data to the UE. Thus, the data rate
experienced by the UE is reduced, although it may be possible to
maintain a high data rate for the UE by scheduling during the femto
cell's ABSs.
[0040] Similar observations can be made for pico UEs. In FIG. 1,
for example, a pico UE 118 in the coverage expansion area of a pico
cell 112 can conclude that the radio link between the pico UE and
the pico cell has failed due to interference from the macro cell
102. The pico UE 118 in the coverage expansion area of a pico cell
112 can report low measured values for the pico cell signal level
resulting in a handover away from the pico cell. In order to
overcome these problems, it is necessary to restrict measurements
performed by the UE to certain subframes.
[0041] Given that different cells can be configured with different
ABS patterns, methods are needed for determining which subframes
should be used by a UE to perform various measurements under
different scenarios. In the foregoing, the embodiments described in
the context of ABSs. However, it should be clear that the same
methods are applicable to blank subframes and subframes that are
only partially used for scheduling. That is, subframes in which
only some of the time-frequency resources are used for scheduling.
In the context of the disclosure, measurements can include, but are
not limited to, one or more of (a) measurements required to perform
cell identification, (b) RRM measurements such as RSRP and RSRQ
measurements of cells detected by the UE, (c) measurements required
for performing radio link monitoring, or (d) channel state
measurements, such as measurements needed for performing channel
state information reporting and channel quality indication
reporting.
[0042] FIG. 3 illustrates electrical block diagrams of a UE and an
exemplary eNB usable in the wireless communication system. Each
base station 301 can include one or more transmit antennas 304-307
(four shown for illustrative purposes), one or more receive
antennas 309, 310 (two shown for illustrative purposes), one or
more transmitters 312 (one shown for illustrative purposes), one or
more receivers 314 (one shown for illustrative purposes), one or
more processors 316 (one shown for illustrative purposes), and
memory 318. Although illustrated separately, the transmitter 312
and the receiver 314 may be integrated into one or more
transceivers as is well understood in the art. By including
multiple transmit antennas 304-307 and other appropriate hardware
and software as would be understood by those of ordinary skill in
the art, the base station 301 may support use of a multiple input
and multiple output (MIMO) antenna system for downlink (base
station-to-wireless communication device) communications. The MIMO
system facilitates simultaneous transmission of downlink data
streams from multiple transmit antennas 304-307 depending upon a
channel rank, for example as indicated by the wireless
communication device 201 or as preferred by the base station 301. A
rank supplied by the UE or enables the base station 301 to
determine an appropriate multiple antenna configuration (e.g.,
transmit diversity, open loop spatial multiplexing, closed loop
spatial multiplexing, etc.) for a downlink transmission in view of
the current downlink channel conditions.
[0043] The processor 316, which is operably coupled to the
transmitter 312, the receiver 314, and the memory 318, can be one
or more of a microprocessor, a microcontroller, a digital signal
processor (DSP), a state machine, logic circuitry, any combination
thereof, or any other device or combination of devices that
processes information based on operational or programming
instructions stored in the memory 318. One of ordinary skill in the
art will appreciate that the processor 316 can be implemented using
multiple processing devices as may be required to handle the
processing requirements of the present invention and the various
other functions of the base station 301. One of ordinary skill in
the art will further recognize that when the processor 316 has one
or more of its functions performed by a state machine or logic
circuitry, the memory containing the corresponding operational
instructions can be embedded within the state machine or logic
circuitry as opposed to being external to the processor 316.
[0044] The memory 318, which may be a separate element as depicted
in FIG. 3 or may be integrated into the processor 316, can include
random access memory (RAM), read-only memory (ROM), FLASH memory,
electrically erasable programmable read-only memory (EEPROM),
removable memory, a hard disk, and/or various other forms of memory
as are well known in the art. The memory 318 can include various
components, such as, for example, one or more program memory
components for storing programming instructions executable by the
processor 316, one or more address memory components for storing an
identifier associated with the base station 301 as well as for
storing addresses for wireless communication devices currently in
communication with the base station 301, and various data storage
components. The program memory component of the memory 318 may
include a protocol stack for controlling the transfer of
information generated by the processor 316 over the data and/or
control channels of the E-UTRA. It will be appreciated by one of
ordinary skill in the art that the various memory components can
each be a group of separately located memory areas in the overall
or aggregate memory 318 and that the memory 318 may include one or
more individual memory elements.
[0045] In one embodiment, the base station transmitter 312,
receiver 314, and processor 316 are designed to implement and
support a wideband wireless protocol, such as the Universal Mobile
Telecommunications System (UMTS) protocol, the E-UTRA protocol, the
3GPP Long Term Evolution (E-UTRA) protocol, or a proprietary
protocol, operating to communicate digital information, such as
user data (which may include voice, text, video, and/or graphical
data) and/or control information, between the base station 301 and
the UE over various types of channels. In an E-UTRA system, an
uplink data channel may be a Physical Uplink Shared Channel
(PUSCH), an uplink control channel may be a physical uplink control
channel (PUCCH), a downlink control channel may be a physical
downlink control channel (PDCCH), and downlink data channel may be
a physical downlink shared channel (PDSCH). Uplink control
information may be communicated over the PUCCH and/or the PUSCH and
downlink control information is communicated typically over the
PDCCH. The UE may further transmit uplink sounding reference
signals to assist the eNB on scheduling uplink (for frequency
division duplex (FDD)) and for one or both uplink and downlink for
time-division duplex (TDD). In the Rel-8 LTE and beyond LTE systems
such as Rel-10 (also known as LTE-Advanced), the base station
transmits using an OFDM modulation scheme on the downlink and the
UEs transmit on the uplink using a single carrier frequency
division multiple access (SC-FDMA) scheme and/or Discrete Fourier
Transform Spread OFDM (DFT-SOFDM). On the UL, the UE may transmit
using contiguous or non-contiguous resource allocations and the UE
may also transmit data and control on the uplink simultaneously
using the so-called simulateneous PUCCH and PUSCH transmission
scheme. In a Frequency Division Duplex (FDD) operation, the frame
structure in the uplink and downlink, each comprises of a 10
millisecond (ms) Radio frame, which is in turn divided into ten
subframes, each of 1 ms duration, wherein each subframe is divided
into two slots of 0.5 ms each, wherein each slot contains a number
of OFDM symbols. The downlink and uplink bandwidth are subdivided
into resource blocks, wherein each resource block comprises of one
or more subcarriers in frequency and one or more OFDM symbols in
the time domain (12 subcarriers.times.7 OFDM symbols for normal
Cyclic Prefix (CP)). In LTE resource blocks are defined on a slot
basis. A resource block (RB) is typical unit in which the resource
allocations are assigned for the uplink and downlink
communications. Furthermore, the eNB configures appropriate
channels for uplink and downlink control information exchange. For
the DL the physical downlink control channel (PDCCH) is used for
sending the uplink and downlink control information to the UEs. The
PDCCH is sent in the beginning portion of a subframe on a
potentially variable number of OFDM symbols, and this number
(typically 0 to 3 for large system bandwidths such as 5 MHz, etc
and 0 to 4 for smaller system bandwidths such as 1.25 MHz) is
signaled on the Physical Control Format Indicator Channel (PCFICH)
or sent via higher layer signaling. However, in other scenarios,
the PDCCH may also be located in certain fixed or variable
time/frequency/spatial resources i.e., spanning one or more
subcarriers in one or more subframes and/or one or more spatial
layers. For example, it may occupy a subset of resource blocks
instead of spanning the entire DL system bandwidth. The Physical
Hybrid ARQ Channel (PHICH) is the Acknowledgment indicator channel
used to send the HARQ feedback on the DL for the uplink data
transmissions from the UE. The PCFICH, PHICH, PDCCH are sent on
OFDM symbols at the beginning of the DL subframes. In some
subframes such as ABS or when the eNB has no UEs scheduled (i.e.
very low or no load cases), these channels may be absent. In LTE
Release-8, the master information block (MIB) is sent on the
Physical Broadcast CHannel (PBCH), the MIB comprises of system
frame number (SFN), downlink system bandwidth, number of signaled
downlink transmit antennas (or the number of CRS ports), and
Physical Hybrid ARQ Channel (PHICH) configuration (i.e. duration).
In LTE Release-8, the PBCH is sent on subframe 0 (each subframe
comprising of two slots, each slot corresponding to a 0.5
milli-second). The Synchronization signals are transmitted on the
inner six PRBs or inner 72 subcarriers (i.e. 1.25 MHz) on subframes
0 and 5. The exact location of the Synchronization signals depends
upon the duplex type, Cyclic Prefix length, etc.
[0046] If an MBSFN subframe is configured, the subframe may contain
an initial portion (near the beginning of the subframe) containing
a unicast region and the rest of the subframe may be configured
differently based on the higher layer signaling. If the MBSFN
subframe is used for transmission of multicast transmission channel
(MTCH), then the rest of the subframe may contain multicast OFDM
symbols with cyclic prefix (CP) that may be distinct (and likely
larger) than the CP used for the initial transmission. In Rel-8/9,
the MBSFN subframe configuration is typically sent in the SIB2
message, wherein the SIB2 message is a higher layer message sent on
the PDSCH by the eNB. The schedule of SIB2, SIB3, (and other SIB
messages) is indicated in SIB1. The System information typically
changes on the order of SIB transmission window (e.g. in multiples
of 8 ms) i.e. the system information update only when a new
SI-transmission window begins and the UEs are paged to indicate a
SI update so that they can re-acquire system information. Typically
SIB transmissions are not allowed in MBSFN subframes as there may
be no CRS in the data region of the MBSFN subframes.
[0047] It is possible to schedule new System information
transmissions within the MBSFN region by transmitting a PDSCH
associated with one or more dedicated reference signal (DRS) ports
that UEs of newer release are able to process. For instance,
signaling though higher layers (e.g. in MIB or SIB1) can be used to
indicate UEs of newer release if they can receive SI in MBSFN
subframes based on Rel-8 Downlink Control Information (DCI) Formats
1A or 1C or enhanced 1A or 1C. Such Rel-8 DCI 1A/1C or enhanced
1A/1C grants are transmitted in the common search space of the
MSBFN subframes, with the CRC masked by SI-RNTI or another special
RNTI (RNTI-specific search space) which is also signaled (or
pre-configured) in the system information accessed prior to SIB1.
The enhanced DL grants may indicate whether the UE can rely on
dedicated reference ports or cell-specific reference ports for
demodulating the corresponding PDSCH to recover SI information (or
recover LLRs for soft combining across multiple SI-x receptions
within the SI-x reception window) The enhanced DL grants may also
indicate the number of DRS ports used to demodulate the SI-x PDSCH.
In Rel-8/9, for Format 1C, the redundancy version determination
(for HARQ channel encoding/decoding) is typically tied to the
subframe number within the transmission window wherein the MBSFN
subframes are skipped. However, for SI transmission in MBSFN
subframes, the RV determination section may be different than that
in Rel-8/9. Thus, the UE may keep track of two Redundancy Versions
(RVs) or two Subframe Numbering, one for the existing LTE Rel-8/9
SI transmission opportunities, and a second one for SI
transmissions within MBSFN subframes. For instance, the second RV
may always be set to 0 or may be cyclically cycled through the
available set of Redundancy versions. The same technique (i.e.
transmissions in MBSFN subframes) as above may be applicable to
Paging messages received via P-RNTI, or Random-Access messages
received via RA-RNTI.
[0048] The SIB1 typically includes cell access related information
such as Public Land Mobile Network (PLMN) identity, tracking area
code, frequency band indicator, etc. SIB1 may also include cell
selection information such as receive signal levels. The SIB1 also
includes the scheduling information for other sytem information
blocks such as the number of other SIBs, the sequence of
transmission, the transmission window size, the periodicity (e.g.
how many transmissions of each SIB within the transmission window),
etc. The other SIBs include SIB2, SIB3, etc. These SIBs include
additional system information that is required to get full service
from the base station. For example, the additional system
information can comprise of uplink system bandwidth, radio resource
configuration information common to the cell, the MBSFN and/or
other subframe configurations, mobility related parameters, cell
selection parameters, neighbor cell information,
intra/inter-frequency cell re-selection parameters, inter
Radio-Access Technology (inter-RAT) reselection parameters,
etc.
[0049] In Rel-10 and beyond systems, the signaling of the ABS
patterns can be included in the SIB1 message, or in SIB2 message
(re-using the same description as MBSFN subframe signaling), other
SIBs, or sent on a UE-specific basis via RRC signaling such as a
MAC message. The broadcast of this information is advantageous as
it allows UEs in both connected and idle modes to access the ABS
information and use it for handling interference for channel
measurements, channel quality estimations, etc.
[0050] When the base station 301 implements the E-UTRA standard,
the base station processor 316, in one embodiment, includes a
logical channel coding and multiplexing section for implementing
channel coding and multiplexing of control information and
positioning reference signals destined for transmission over a
downlink subframe 340. The channel coding and multiplexing section
is a logical section of the base station processor 316, which
performs the coding and multiplexing responsive to programming
instructions stored in memory 318. The channel coding and
multiplexing section may include one channel coding block for
encoding control channel information (e.g., channel quality
indicators, cell-specific reference symbols (CRS), rank indicators,
and hybrid automatic repeat request acknowledgments (HARQ-ACK/NACK)
into associated transmission resources (e.g., time-frequency
resource elements) and another block for encoding positioning
reference signals and other information typically communicated over
the primary/secondary synchronization channel (e.g., P/S-SCH) into
associated transmission resources. The channel coding and
multiplexing section of the processor 316 may include additional
coding blocks for encoding various other types of information
and/or reference symbols used by the wireless communication device
201 for demodulation and downlink channel quality determination.
The channel coding and multiplexing section of the processor 316
also includes a channel multiplexing block that multiplexes the
encoded information generated by the various channel coding blocks
into a subframe, which is supplied to the transmitter 312 for
downlink transmission.
[0051] Each wireless communication device 201 can include one or
more transmit antennas 320 (one shown for illustrative purposes),
one or more receive antennas 322, 323 (two shown for illustrative
purposes), one or more transmitters 325 (one shown for illustrative
purposes), one or more receivers 327 (one shown for illustrative
purposes), a processor 329, memory 331, a local oscillator 332, an
optional display 333, an optional user interface 335, and an
optional alerting mechanism 337. Although illustrated separately,
the transmitter 325 and the receiver 327 may be integrated into one
or more transceivers as is well understood in the art. By including
multiple receive antennas 322, 323 and other appropriate hardware
and software as would be understood by those of ordinary skill in
the art, the UE may facilitate use of a MIMO antenna system for
downlink communications.
[0052] The wireless communication device transmitter 325, receiver
327, and processor 329 are designed to implement and support a
wideband wireless protocol, such as the UMTS protocol, the E-UTRA
protocol, the 3GPP E-UTRA protocol or a proprietary protocol,
operating to communicate digital information, such as user data
(which may include voice, text, video, and/or graphical data)
and/or control information, between the UE and a serving base
station 301 over control and data channels. In an E-UTRA system, an
uplink data channel may be a PUSCH and an uplink control channel
may be a PUCCH. Control information may be communicated over the
PUSCH and/or the PUCCH. Data is generally communicated over the
PUSCH.
[0053] The processor 329 is operably coupled to the transmitter
325, the receiver 327, the memory 331, the local oscillator 332,
the optional display 333, the optional user interface 335, and the
optional alerting mechanism 337. The processor 329 utilizes
conventional signal-processing techniques for processing
communication signals received by the receiver 327 and for
processing data and control information for transmission via the
transmitter 325. The processor 329 receives its local timing and
clock from the local oscillator 332, which may be a phase locked
loop oscillator, frequency synthesizer, a delay locked loop, or
other high precision oscillator. The processor 329 can be one or
more of a microprocessor, a microcontroller, a DSP, a state
machine, logic circuitry, or any other device or combination of
devices that processes information based on operational or
programming instructions stored in the memory 331. One of ordinary
skill in the art will appreciate that the processor 329 can be
implemented using multiple processors as may be required to handle
the processing requirements of the present invention and the
various other included functions of the UE. One of ordinary skill
in the art will further recognize that when the processor 329 has
one or more of its functions performed by a state machine or logic
circuitry, the memory containing the corresponding operational
instructions can be embedded within the state machine or logic
circuitry as opposed to being external to the processor 329.
[0054] The memory 331, which may be a separate element as depicted
in FIG. 3 or may be integrated into the processor 329, can include
RAM, ROM, FLASH memory, EEPROM, removable memory (e.g., a
subscriber identity module (SIM) card or any other form of
removable memory), and/or various other forms of memory as are well
known in the art. The memory 331 can include various components,
such as, for example, one or more program memory components for
storing programming instructions executable by the processor 329
and one or more address memory components for storing addresses
and/or other identifiers associated with the wireless communication
device 201 and/or the base stations 203-205. The program memory
component of the memory 331 may include a protocol stack for
controlling the transfer of information generated by the processor
329 over the data and/or control channels of the E-UTRA system, as
well as for controlling the receipt of data, control, and other
information transmitted by the different cells in the E-UTRA
system. It will be appreciated by one of ordinary skill in the art
that the various memory components can each be a group of
separately located memory areas in the overall or aggregate memory
331 and that the memory 331 may include one or more individual
memory elements.
[0055] The display 333, the user interface 335, and the alerting
mechanism 337 are all well-known elements of wireless communication
devices. For example, the display 333 may be a liquid crystal
display (LCD) or a light emitting diode (LED) display and
associated driver circuitry, or utilize any other known or
future-developed display technology. The user interface 335 may be
a key pad, a keyboard, a touch pad, a touch screen, or any
combination thereof, or may be voice-activated or utilize any other
known or future-developed user interface technology. The alerting
mechanism 337 may include an audio speaker or transducer, a tactile
alert, and/or one or more LEDs or other visual alerting components,
and associated driver circuitry, to alert a user of the wireless
communication device 302. The display 333, the user interface 335,
and the alerting mechanism 337 operate under the control of the
processor 329.
[0056] In E-UTRA Rel-10 methods for supporting enhanced inter-cell
interference coordination (eICIC) techniques will be specified.
Such methods are targeted towards increasing the spectral
utilization of licensed (and unlicensed) bands by the deployment of
Heterogeneous networks. Small to large handover bias has been
considered for the macro/pico case where a pico UE in the coverage
extension region of a pico cell (i.e., pico cell is not the
strongest cell) is forced to associate with the said pico cell.
Such a UE connected to a pico cell can experience elevated
interference due to macro cell transmission in ABSs when scheduled
by the pico cells in the macro cell's ABSs relative to when the
macro cell transmission is absent (i.e., macro cells is configured
for blank subframe transmission). This is because, a ABS always
contains CRS and can potentially contain other channels such as
P/S-SCH, PBCH, PCFICH, PHICH, PDSCH (associated with Paging and
SIB1) and Positioning Reference Signal (PRS). Although, the pico
cell transmission is received at a better signal quality over the
macro cell's ABSs relative to non-ABSs, the quality of pico cell
transmission in the macro cell's ABSs may still be inadequate to
maintain association with the pico cell specially when legacy
Rel-8/9 receivers are implemented in the UE. Several interference
mitigation techniques for rejecting or cancelling interference of
various signals in the ABSs is known in prior art. Among such
methods are:
[0057] CRS interference rejection by suitable modification to LLRs
including nulling REs assoicated with the pico cell transmission
that overlap with the macro cell CRS transmission in a given
subframe.
[0058] Processing of pico cell P/S-SCH post subtraction of the
estimated macro cell P/S-SCH from the received signal.
[0059] Decoding of pico cell PDCCH post blind detection of macro
cell PDCCH transmission followed by subtraction of the macro cell
PDCCH component from the received signal. This method might require
higher-layer assistance signal associated with the macro cell PDCCH
transmission.
[0060] PCFICH/PHICH interference rejection by suitable modification
to LLRs including nulling REs assoicated with the pico cell
transmission that overlap with the macro cell PCFICH/PHICH
transmission in a given subframe. This method might require
higher-layer assistance signal associated with the macro cell
PCFICH/PHICH transmission.
[0061] For the macro/femto case where a macro UE roams close to a
CSG femto cell, a macro UE both in RRC_CONNECTED and RRC_IDLE
states similarly experiences elevated interference due to femto DL
transmission even when the macro cell is transmitting on the
femto's ABSs.
[0062] For both the macro/pico and the macro/femto cases, the
serving cell transmits a measurement pattern comprising subframes
on which the UE is expected to perform RRM/RLM/CSI measurements.
The restricted subframe measurement pattern is configured such that
the UE performs measurements mostly on the ABSs of the dominant
neighbor cell (i.e., the macro cell in the macro/pico case for a
pico UE and a femto cell in the macro/femto case for the macro UE).
Since ABSs contain at least the CRS and possibly other downlink
signals transmitted by the dominant neighbor cell, the RLM/RRM/CSI
measurements as defined in E-UTRA Rel-9 specification will likely
be inadequate in supporting efficient deployment of Heterogeneous
networks.
[0063] Specifically, E-UTRA Rel-9 measurements and procedures as
described in TS 36.213, TS 36.214 and TS 36.133 are likely
inadequate to cope with large interference signals present in ABSs.
This invention is focused on addressing such issues.
[0064] In accordance to 3GPP RAN Working Group 1 (i.e., RAN1)
Liaison Statement R1-105793, ABSs are defined as follows: [0065]
UEs can assume the following about ABSs: [0066] All ABSs carry CRS
[0067] If PSS/SSS/PBCH/SIB1/Paging/PRS coincide with an ABS, they
are transmitted in the ABS (with associated PDCCH when SIB1/Paging
is transmitted) [0068] Needed for legacy support [0069] Channel
State Information Reference Signal (CSI-RS) transmission on ABS is
For Further Study (FFS) [0070] No other signals are transmitted in
ABSs [0071] If ABS coincides with Multicast-Broadcast Single
Frequency Network (MBSFN) subframe not carrying any signal in data
region, CRS is not present in data region [0072] MBSFN subframe
carrying signal in data region shall not be configured as ABS
[0073] Although, RAN1 has primarily considered restricted subframe
measurements (i.e., UE performing RRM/CSI/RLM measurements over set
of subframes signaled by the serving eNB) for RRC connected mode,
in RAN4, two contributions R4-103790 and R4-103738 that were
presented discussed extending this concept to idle mode RRM
measurements to address the macro/femto interference problem.
[0074] In cell range expansion (CRE) for the macro/pico case, a
macro UE may be scheduled only on a subset of all possible
subframes that corresponds to non-ABSs of the macro cell and the
macro cell may not schedule any UE in the ABSs. In this set up, the
pico cell may schedule its UEs both on subframes that coincide with
the macro cell ABSs and subframes that coincide with macro cell
non-ABSs. When medium to large HO bias (>4 dB) is used, this can
lead to two sets of subframes each with different DL signal quality
levels or two "virtual" channels with different downlink signal
qualities.
[0075] In order that medium to large CRE (i.e., 4+ dB cell
association bias) can be supported, a UE must implement a
interference rejection (IR) receiver or a interference cancellation
(IC) receiver to eliminate the interference from one or more of the
above-listed signals present in the ABS. IR and IC are referred to
herein generically as interference reduction. Such a receiver
capability may also be necessary in the macro/femto case to enable
a macro UE to remain connected to the macro cell under strong
interference from a nearby non-allowed CSG femto cell. With such a
capability, a UE will be able to remain connected to the desired
cell (i.e., pico cell in the macro/pico case and macro cell in the
macro/femto case) that is 4+ dB weaker than the strongest cell
(i.e., macro cell in the macro/pico case and femto cell in the
macro/femto case).
[0076] IR/IC techniques known in prior art as applicable to
rejecting/cancellation of the different signals transmitted in ABSs
are summarized below. The desired signal is can be one of CRS,
P/S-SCH, PBCH, PCFICH, PHICH, CSI-RS, PDCCH, PDSCH, Demodulation
Reference Signal (DM-RS or DRS), User Equipment-Specific Reference
Signal (UE-RS), and Channel State Information Reference Signal
(CSI-RS).
[0077] P/S-SCH: If PSS/SSS in the ABS interferes with the desired
signal, the UE must cancel the strong cell interference using
typical interference cancellation techniques known in the art,
including for example the method taught in US20100029262.
[0078] PBCH: If PBCH in the ABS interferes with the desired signal,
the UE may decode the PBCH first, subtract the reconstructed PBCH
signal corresponding to the decoded the MIB codeword from the
received signal and then process the received signal.
[0079] PDCCH: If PDCCH in ABS interferes with the desired signal, a
method similar to PBCH IC which includes decoding followed by
cancellation can be used (typical interference cancellation
techniques known in the art) for example, the method taught in
US20100190447 may be used.
[0080] CRS: If CRS in the ABS interferes with the desired signal,
one method is CRS IC which involves first estimating the neighbor
cell channel, reconstructing the CRS component using the CRS
template, subtracting the estimated CRS component from the received
signal and estimating the serving cell channel from the residual
signal. A second method is CRS IR which involves nulling REs that
overlap with neighbor cell CRS prior to processing of the desired
signal.
[0081] PCFICH/PHICH: If PCFICH or PHICH in ABS interferes with the
desired signal, it is generally much difficult to perform IC
although IR is feasible.
[0082] PRS: If PRS in ABS interferes with the desired signal, PRS
IR or IC can be carried out in a manner similar to CRS IR or
IC.
[0083] Once the UE detects the PCID and frame/OFDM symbol timing of
a neighbor cell and receives the ABS configuration corresponding to
the neighbor cell from the serving cell, the UE generally knows the
following:
[0084] P/S-SCH time/frequency location and signal structure of the
interferer.
[0085] Therefore, if the interfering signal is P/S-SCH, together
with the PCID and the frame/OFDM symbol timing of the neighbor
cell, the UE sufficient information to estimate the component of
P/S-SCH in the received signal and cancel it.
[0086] But, if the interfering signal is not P/S-SCH, the UE may
require further information pertaining to the neighbor cell
transmission in order to perform IR or IC such DL channels in a
computationally feasible manner. Such information pertaining to the
said DL channels may be included a assistance data signaled to the
UE. The assistance data may be included in the system broadcast
messages such as System Information Blocks or in Radio Resource
Configuration (RRC) messages or other UE-specific messages. The
signaling of such assistance information to enable UE IR/IC is
further discussed below.
[0087] PCFICH/PDCCH/PHICH: If the interfering signal is one of
PCFICH, PHICH, and PDCCH, the UE may be made aware of at least the
number of OFDM symbols in the control region (i.e., PCFICH codeword
transmitted by the neighbor cell), the number of OFDM symbols on
which PHICH is transmitted, and the set of REs over which one or
more PDCCH codewords are transmitted (i.e., the set of occupied
CCEs). If the interfering signal is PHICH, the UE may be made aware
of the PHICH duration which preferably remains constant over all
ABS subframes for the strong interferer.
[0088] PRS: The UE may or may not have positioning capability i.e.
the capability to process Positioning Reference Signals (PRS).
Further, the serving cell may not be a participant in E-UTRA
positioning or have the capability of supporting E-UTRA Location
Based Services (LBS) including support for LTE Positioning Protocol
(LPP). As a result, the UE may not have access to PRS assistance
information which is normally transported in LPP that identifies
the time-frequency resources (i.e., radio frames, subframes,
bandwidth, etc.) used for transmission of PRS by the different
cells in a certain geographical area. In these cases, the UE must
be made aware of PRS pattern and the associated assistance
information to enable the UE to perform IR or IC of the PRS signal.
Just like other signals transmitted from a neighbor cell, neighbor
cell's PRS can also severely interfere with desired signals. Note
that the neighbor cell PCID and coarse timing are typically known
after cell detection. But, this is usually insufficient for
determining whether or not neighbor cell is transmitting PRS and
whether or not PRS interference is a potential problem.
[0089] According to TS 36.211, the PRS is transmitted in resource
blocks in downlink subframes configured for positioning reference
signal transmission. If both normal and MBSFN subframes are
configured as positioning subframes within a cell, the OFDM symbols
in a MBSFN subframe configured for positioning reference signal
transmission shall use the same cyclic prefix as used for subframe
#0. If only MBSFN subframes are configured as positioning subframes
within a cell, the OFDM symbols configured for positioning
reference signals in these subframes shall use extended cyclic
prefix length. In a subframe configured for positioning reference
signal transmission, the starting positions of the OFDM symbols
configured for positioning reference signal transmission shall be
identical to those in a subframe in which all OFDM symbols have the
same cyclic prefix length as the OFDM symbols configured for
positioning reference signal transmission. Positioning reference
signals are transmitted on antenna port 6. The positioning
reference signals shall not be mapped to resource elements (k,l)
allocated to PBCH, P/S-SCH regardless of their antenna port p.
Positioning reference signals are defined for subcarrier spacing
.DELTA.f=15 kHz only. The bandwidth for positioning reference
signals and N.sub.RB.sup.PRS is configured by higher layers and the
cell-specific frequency shift is given by
.nu..sub.shift=N.sub.Cell.sup.ID 6 where N.sub.Cell.sup.ID is the
PCID.
[0090] FIG. 4 shows a schematic of PRS transmission one subframe
showing 1 PRB for a subframe with normal CP.
[0091] According to TS 36.211, the cell specific subframe
configuration period T.sub.PRS and the cell specific subframe
offset .DELTA..sub.PRS for the transmission of positioning
reference signals are listed in the Table 1 below.
TABLE-US-00001 TABLE 1 PRS configuration PRS periodicity T.sub.PRS
PRS subframe offset .DELTA..sub.PRS Index I.sub.PRS (subframes)
(subframes) 0-159 160 I.sub.PRS 160-479 320 I.sub.PRS-160 480-1119
640 I.sub.PRS-480 1120-2399 1280 I.sub.PRS-1120 2400-4095
Reserved
[0092] The PRS configuration index I.sub.PRS is configured by
higher layers. Positioning reference signals are transmitted only
in configured DL subframes. Positioning reference signals shall not
be transmitted in special subframes. Positioning reference signals
shall be transmitted in N.sub.PRS consecutive downlink subframes,
where N.sub.PRS is configured by higher layers.
[0093] The positioning reference signal instances, for the first
subframe of the N.sub.PRS downlink subframes, shall satisfies
(10.times.n.sub.f+.left brkt-bot.n.sub.s/2.right
brkt-bot.-.DELTA..sub.PRS)mod T.sub.PRS=0, where n.sub.f is the
System Frame Number (SFN, in the range 0, 1, . . . , 1023) and
n.sub.s is the slot number (in the range 0, 1, . . . , 19).
[0094] As indicated earlier, the higher-layer signaling of
I.sub.PRS, N.sub.PRS and PRS BW may not be available to a UE that
does not have positioning capability or LPP support. But, such a UE
when it is capable of eICIC support must be capable of rejecting or
canceling PRS interference to the desired signal to ensure reliable
operation in E-UTRA Rel-10.
[0095] In the absence of UE positioning capability, PRS assistance
data may be transmitted to the UE indicative of the PRS
transmission from a neighbor cell in ABS containing occasional PRS
transmissions. The PRS assistance data may contain one or more of
the following elements. [0096] Neighbor cell's PRS occasion pattern
in time domain, for example the I.sub.PRS parameter which signals
the periodicity of PRS occasions and the time offset of this
pattern relative to reference cell SFN#0 (i.e., both the subframe
offset and the PRS occasion periodicity can be signaled). [0097]
Neighbor cell's N.sub.PRS--the number of consecutive subframes in
one PRS occasion [0098] Neighbor cell's PRS BW [0099] Neighbor cell
OFDM symbol Cyclic Prefix (CP) associated with the PRS transmission
[0100] Neighbor cell's number of CRS transmit antenna ports
[0101] The PRS periodicity is 160, 320, 640 or 1280 ms as indicated
in Table 1, while the ABS pattern has a 40 ms periodicity.
Therefore, only a subset of ABS contains PRS signals.
[0102] The parameters listed above together with neighbor cell
PCID, number of CRS transmit antenna ports and CP length uniquely
determine the structure and sequence associated with the neighbor
cell PRS signal in ABS. This information should therefore be
provided to (or determined by) a UE that is expected to process a
neighbor cell ABS with PRS for rejecting or canceling
interference.
[0103] PRS IR may comprise simply nulling the REs or Log-likelihood
Ratios (LLRs) associated with REs of the desired signal that
overlap with the neighbor cells PRS REs. The nulled REs or nulled
LLRs lead to the rejection of neighbor cell's PRS component in the
received signal. LLR nulling can be applied prior to decoding of a
codeword carrying the desired information, where the codeword may
one of a block code (PCFICH and PHICH), a convolutional code (PBCH
and PDCCH), and a turbo-code (PDSCH).
[0104] PRS IR/IC may be applied to demodulation/processing of at
least one desired signal among CRS, P/S-SCH, PBCH, PCFICH, PHICH,
CSI-RS, PDCCH, PDSCH, DM-RS, and UE-RS.
[0105] FIG. 5 shows a flowchart 500 of UE operations illustrating
an exemplary embodiment in accordance with the present disclosure.
At 510, the UE receives an ABS pattern for a neighbor cell. At 520,
the UE receives PRS assistance data from the serving cell. At 530,
the UE receives serving cell and neighbor cell signals. At 540, the
UE determines a subset of ABS that contains a neighbor cell PRS
transmission and the UE determines the REs associated with such a
PRS transmission. At 550, the UE determines whether IR or IC
triggering is used. At 560, for IR, the UE nulls the desired signal
REs or LLRs of REs that overlap with neighbor cell PRS prior to
processing. At 570, for IC the UE estimates the neighbor cell PRS
component received signal and cancellation occurs.
[0106] When the neighbor cell interference to a desired signal is
degrading the demodulation/decoding performance of the desired
signal, the UE can either (1) autonomously trigger IR or IC for one
or more of the DL channels in the ABS or (2) can do so based on a
network trigger.
[0107] In the first approach (i.e., autonomous triggering of IR or
IC), the UE determines that IR or IC is necessary based on a signal
strength threshold such as the event that the RSRP difference
between the strongest neighbor and the serving cell exceeds a
certain threshold. The said threshold can be a fixed quantity, say,
4 dB or it can be a network signaled threshold transported from the
serving cell to the UE via higher layers such as through a. RRC
message or System Information Broadcast message. Alternately, the
CQI dropping below a certain threshold can be used as an indication
that the neighbor cell interference is strong and IR or IC must be
triggered. In this case, the UE may indicate to the network that it
has triggered IR or IC so that the network can use that information
while scheduling the UE. It is also possible for an eNB to
implicitly deduce that the UE is using IR or IC based on one or
more other signals from the UE (CQI reports, UL SRS transmissions,
A/N transmissions, etc).
[0108] In the second approach (i.e., network triggered IR or IC),
the UE reports the RSRP or RSRQ or other measurements such as CSI
(including one or more of a CQI, PMI, RI) corresponding to the
serving cell. Furthermore, the UE can report RSRP or RSRQ or CSI
for one or more neighbor cells. Based on these reports from the UE,
the network can send a RRC message instructing the UE to perform IR
or IC for one or more channels in the ABS. The list of channel for
which the UE must perform IR or IC for may also be signaled to the
UE in the control message. In a typical example, when the UE goes
near a closed femto cell, the UE reports to its serving eNB that it
is close to a closed femto cell, and then the eNB can inform the UE
to start IR/IC to be able to receive the messages such as Paging,
Synchronization Signals, etc.
[0109] FIG. 6 shows a flowchart 600 of UE operations illustrating
an exemplary embodiment in accordance with the present invention.
At 610, the UE receives an ABS pattern for a neighbor cell. At 620,
the UE receives PRS assistance data from the serving cell. At 630,
the UE receives serving cell and neighbor cell signals. At 640, the
UE determines whether a trigger for enabling IR or IC is network
based or UE based. At 650, for network initiated triggering, the UE
sends RSRP/RSRQ/CSI reports to the serving cell and receives a
control message indicating the IR or IC must be enabled. At 660,
for autonomous UE triggering, the UE determines if an interfering
signal in ABS poses a significant risk by comparing neighbor cell
RSRP/RSRQ/CSI against a threshold. At 670, IR or IC is enabled for
the desired signal.
[0110] CRS: If the interfering signal is the neighbor cell CRS
transmission, the UE may be made aware of the following: [0111] a.
neighbor cell bandwidth (BW) for example expressed as the number of
Physical Resource Blocks (PRBs) [0112] b. neighbor cell's number of
CRS transmit antenna ports [0113] c. an indication of the type of
subframes in the ABS pattern for example including an indication of
which subframes in the ABS pattern are normal, MBSFN or fake UL
subframes.
[0114] PBCH: If the neighbor cell PBCH is the interfering signal,
the UE may be made aware of neighbor cell's number of transmit
antenna ports so that the PBCH encoding structure is known. This
can facilitate the UE receiver in demodulating and decoding of the
Master Information Block (MIB) codeword embedded in the PBCH. After
decoding the MIB, the UE may reconstruct the component of the
neighbor cell PBCH signal and cancel it from the received signal so
that interference to the desired signal is mitigated. The PBCH
encoding can be one of Single Input Multiple Output (SIMO),
Space-Frequency Block Coding (SFBC) and Space-Frequency Block
Coding with Frequency-Switched Transmit Diversity (SFBC-FSTD).
[0115] In the foregoing specification, specific embodiments of the
present invention have been described. However, one of ordinary
skill in the art will appreciate that various modifications and
changes can be made without departing from the scope of the present
invention as set forth in the claims below. Accordingly, the
specification and figures are to be regarded in an illustrative
rather than a restrictive sense, and all such modifications are
intended to be included within the scope of present invention. The
benefits, advantages, solutions to problems, and any element(s)
that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as critical,
required, or essential features or elements of any or all the
claims. The invention is defined solely by the appended claims
including any amendments made during the pendency of this
application and all equivalents of those claims as issued.
* * * * *