U.S. patent application number 13/288746 was filed with the patent office on 2012-05-10 for idle state interference mitigation in wireless communication network.
This patent application is currently assigned to MOTOROLA MOBILITY, INC.. Invention is credited to Sandeep H. Krishnamurthy, Ravi Kuchibhotla, Murali Narasimha, Ravikiran Nory.
Application Number | 20120113846 13/288746 |
Document ID | / |
Family ID | 46019557 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120113846 |
Kind Code |
A1 |
Narasimha; Murali ; et
al. |
May 10, 2012 |
Idle State Interference Mitigation in Wireless Communication
Network
Abstract
A mobile station is disclosed wherein the mobile station
determines interference by assessing one or more frames in a first
periodic sequence of frames relative to a reference and, in
response to the assessment, monitors transmissions from a base
station for paging signals during a second periodic sequence of
frames, wherein the second periodic sequence of frames is offset
from the first periodic sequence of frames by a predetermined
amount.
Inventors: |
Narasimha; Murali; (Lake
Zurich, IL) ; Krishnamurthy; Sandeep H.; (Sunnyvale,
CA) ; Kuchibhotla; Ravi; (Gurnee, IL) ; Nory;
Ravikiran; (Buffalo Grove, IL) |
Assignee: |
MOTOROLA MOBILITY, INC.
Libertyville
IL
|
Family ID: |
46019557 |
Appl. No.: |
13/288746 |
Filed: |
November 3, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61412377 |
Nov 10, 2010 |
|
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|
Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H04W 68/02 20130101;
H04W 84/045 20130101; H04W 16/16 20130101 |
Class at
Publication: |
370/252 |
International
Class: |
H04W 24/00 20090101
H04W024/00 |
Claims
1. A mobile station comprising: a wireless transceiver coupled to a
processor, the processor configured to determine interference by
assessing one or more frames in a first periodic sequence of frames
relative to a reference, and the processor configured to monitor,
in response to assessing, transmissions from a base station for
paging signals during a second periodic sequence of frames, wherein
the second periodic sequence of frames is offset from the first
periodic sequence of frames by a predetermined amount.
2. The mobile station according to claim 1, the processor
configured to monitor transmissions from the base station for
paging signals during the second periodic sequence of frames by
monitoring transmissions from the base station for paging signals
during the first periodic sequence of frames and during the second
periodic sequence of frames.
3. The mobile station according to claim 1, the processor
configured to monitor transmissions from the base station for
paging signals during the second periodic sequence of frames by
monitoring transmissions from the base station for paging signals
during the second periodic sequence of frames and not monitoring
for paging signals during the first periodic sequence of
frames.
4. The mobile station according to claim 1, wherein the second
periodic sequence of frames is offset from the first periodic
sequence of frames by a predetermined amount such that each frame
in the second periodic sequence of frames is earlier than a
corresponding frame in the first periodic sequence of frames.
5. The mobile station according to claim 1, wherein the second
periodic sequence of frames is offset from the first periodic
sequence of frames by a predetermined amount such that each frame
in the second periodic sequence of frames is later than a
corresponding frame in the first periodic sequence of frames.
6. The mobile station according to claim 1, the processor
configured to determine interference by determining whether a
non-serving cell can transmit signals in time periods that overlap
one or more of the frames in the first periodic sequence of
frames.
7. The mobile station according to claim 1, the processor
configured to determine interference by determining whether the one
or more of the frames in the first periodic sequence of frames
overlap time periods during which a non-serving cell is configured
to transmit only essential signals.
8. The mobile station according to claim 1, the processor
configured to monitor transmissions from the base station for
paging signals during the first periodic sequence of frames based
on the determination of the interference, and the processor
configured to monitor transmissions from the base station for
paging signals during the second periodic sequence of frames based
on the determination of the interference.
9. The mobile station according to claim 1, the processor
configured to determine the first periodic sequence of frames as a
function of an identifier of the mobile station.
10. A base station comprising: a wireless transceiver coupled to a
processor, the base station configured to determine that a mobile
station is configured to monitor signals from the base station for
paging signals during a first frame, the base station configured to
determine that a coverage of the base station substantially
overlaps a coverage of a second base station, and the base station
configured to cause the transceiver to transmit a page message to
the mobile station during a second frame, wherein the second frame
is offset from the first frame by a predetermined time period, and
wherein the second frame overlaps a time period when a neighbor
cell transmits limited signals.
11. The base station according to claim 10, the base station
configured to cause the transceiver to transmit the page message to
the mobile station during the second frame by transmitting the page
message to the mobile station during the first frame and during the
second frame.
12. The base station according to claim 10 the base station
configured to cause the transceiver to transmit a page message to
the mobile station during the first frame, and the base station
configured to cause the transceiver to transmit the page message to
the mobile station during the second frame if a response to a page
message transmitted to the mobile station during the first frame is
not received.
13. The base station according to claim 10, wherein the second
frame overlaps a time period during which a neighbor cell is
configured to transmit only essential signals.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims benefits to provisional
Application No. 61/421,377 filed on 10 Nov. 2010, the contents of
which are incorporated herein by reference.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates generally to wireless
communications, interference management and interference reduction
in wireless networks.
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
collaboration between 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 (16 QAM), or 64 QAM. 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 continuous 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 control channel 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 control channel 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. The base stations can 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 (also referred to as macro cells), pico base station
(or pico cells), relay nodes and femto base stations (also referred
to as femto cells, 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 10 s 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] 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 base stations can be installed on roof tops and femto
base stations can be easily installed indoors. The pico and femto
base stations allow the network to offload user communication
traffic from the macro cell to the pico or femto cells. This can
enable the 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 LTE networks in
3GPP LTE Release 10.
[0011] 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.
[0012] HeNB (aggressor).fwdarw.MeNB (victim) downlink (DL)
[0013] HUE (aggressor).fwdarw.MeNB (victim) uplink (UL)
[0014] MUE (aggressor).fwdarw.HeNB (victim) UL
[0015] MeNB (aggressor).fwdarw.HeNB (victim) DL
[0016] HeNB (aggressor).fwdarw.HeNB (victim) on DL
[0017] HeNB (aggressor).fwdarw.HeNB (victim) on UL.
[0018] FIG. 1 illustrates an LTE 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" or "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 LTE RRC connected mode can be connected to, and therefore
associated 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 the base stations in a heterogeneous network share the
frequency spectrum while minimizing interference. Two approaches
can be envisioned:
[0020] A network can configure time periods where 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 to
transmit data to the UE.
[0021] 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
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 interferences 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, not
schedule at all on the second virtual channel when the interference
from the second base station is large.)
[0022] However, it should be noted that the time division
approaches can lead to various problems for UEs in idle mode, some
of which are listed below:
[0023] A UE in idle mode expects to receive paging messages from a
serving cell in certain predefined time periods that occur
periodically. When the paging time periods overlap the time periods
when a strong neighbor cell transmits data, the UE may be unable to
receive paging messages.
[0024] The cell specific reference symbol (CRS) transmissions of
the serving cell may overlap the CRS of a strong neighbor cell.
This can result in the UE being unable to perform correct
measurements of the serving cell and the neighbor cell.
[0025] The physical broadcast channel (PBCH) transmission of the
serving cell may overlap the PBCH transmission of a strong neighbor
cell, resulting in the UE being unable to decode the PBCH of the
serving cell. This can the result in the UE not having up to date
system information of the serving cell, as well as other
undesirable consequences.
[0026] The primary synchronization signal (PSS) and secondary
synchronization signal (SSS) of the serving cell may overlap the
PSS and the SSS of a strong neighbor cell respectively. This can
result in the UE not being able to remain synchronized to the
serving cell.
[0027] Therefore, methods to overcome the problems in idle mode UEs
resulting from the use of time division approaches are needed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 illustrates an example of a Heterogeneous network
comprising macro cells, pico cells and femto cells.
[0029] FIG. 2 illustrates the application of almost blank subframes
for scheduling UEs in a heterogeneous network.
[0030] FIG. 3 illustrates problems related to paging UEs in a
heterogeneous network.
[0031] FIG. 4 illustrates problems related to overlap or collision
of cell-specific reference symbols of different cells in a
heterogeneous network.
[0032] FIG. 5 illustrates problems related to overlap or collision
of physical broadcast channels of different cells in a
heterogeneous network.
[0033] FIG. 6A illustrates changing paging occasion to avoid
interference.
[0034] FIG. 6B is a paging offset determination process in a base
station.
[0035] FIG. 6C is a paging offset determination process in a
UE.
[0036] FIG. 7A illustrates a first embodiment that overcomes
problems related to overlap or collision of cell-specific reference
symbols of different cells in a heterogeneous network.
[0037] FIG. 7B illustrates a second embodiment that overcomes
problems related to overlap or collision of cell-specific reference
symbols of different cells in a heterogeneous network.
[0038] FIG. 8A illustrates a first embodiment that overcomes
problems related to overlap or collision of physical broadcast
channels of different cells in a heterogeneous network.
[0039] FIG. 8B illustrates a second embodiment that overcomes
problems related to overlap or collision of physical broadcast
channels of different cells in a heterogeneous network from the UE
perspective.
[0040] FIG. 8C illustrates a second embodiment that overcomes
problems related to overlap or collision of physical broadcast
channels of different cells in a heterogeneous network from the
femto cell perspective.
DETAILED DESCRIPTION
[0041] 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
Closed Subscriber Group (CSG) cell. FIG. 1 illustrates an example
of Heterogeneous network (100) comprising 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.
[0042] 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 may be
inadequate. Furthermore, in order to maximize offloading of users
to pico cells, a network operator can have an association bias
towards the pico cell. That is, a UE (118) is 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, relative to a reference, due to
transmissions of a neighbor cell (such as a macro cell 102).
[0043] In order to operate multiple cells with overlapping coverage
on a carrier frequency, such as in a heterogeneous network 100, it
is necessary to have coordination between the cells so that the
transmissions don't interfere with one another. LTE 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). In LTE AB
subframes of a base station, the base station can be configured to
not transmit any energy on all resource elements, except for
resource elements used for (a) CRS, (b) PSS and SSS, (c) PBCH, (d)
SIB1, e) paging messages., and (e) Positioning Reference Signal
(PRS). There may be other signals such as Channel State Information
Reference Signal (CSI-RS) in the AB subframes.
[0044] AB subframes of one cell can be used by a neighboring cell
to schedule UEs. FIG. 2 illustrates the use of AB subframes. For
example, each of a femto cell, a macro cell and a pico cell can be
configured with an AB subframe pattern. The patterns can be such
that the AB subframes of different cells can overlap. Alternatively
the patterns can be mutually exclusive, so that AB subframes 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.
[0045] We further illustrate the use of AB subframe patterns. 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). UE 110 represents
such a UE and femto cell 108 represents such a femto cell. Such a
macro UE can experience very high interference from the femto cell,
making communication between the macro UE and the macro cell very
difficult. To overcome the interference, the macro cell can
transmit data to the UE only in the AB subframes of the femto cell.
Since the femto cell only transmits critically important signals in
the AB subframes, the macro cell can avoid most of the interference
from the femto cell and successfully transmit data to the macro UE
in the AB subframes of the femto cell.
[0046] Similarly, a pico UE may be in the cell range expansion area
of the pico cell. UE 118 represents such a pico UE and pico cell
112 represents such a pico cell. Such a pico UE can experience a
very high interference from a neighbor cell (such as macro cell
102), making communication between the pico UE and the pico cell
very difficult. In order to overcome the interference, the pico
cell can transmit data to the UE only in the AB subframes 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.
[0047] When different cells use different patterns of AB subframes,
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. Furthermore, UEs can
perform RRM measurements in idle mode to support idle mode mobility
(i.e., cell selection and cell reselection). UE performs CSI
measurements to support optimal scheduling by the base station. For
example, 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 AB subframes
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 AB subframes of the femto cell.
[0048] Similarly, 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 AB subframes. Alternatively, if the
UE is in idle mode, it can perform a reselection to a cell on
another frequency or RAT, based on the low value of the macro
signal level. This is also an undesirable outcome, as the UE can
remain associated with the macro cell in idle mode.
[0049] Problems related to paging channel reception by UEs in
heterogeneous networks are illustrated in FIG. 3. The paging signal
can comprise two components as described below.
[0050] A control channel signal indicating the Resource Allocation
(RA) corresponding to the data channel carrying the paging message.
In 3GPP LTE, the control channel can be a physical downlink control
channel (PDCCH) and the data channel can be a physical downlink
shared channel (PDSCH). Furthermore, a specific control channel
format can be used for signaling a data channel carrying the paging
message. For example, a PDCCH with a Downlink Control Information
(DCI) format 1A or 1C as per specifications TS 36.212 and TS 36.213
may be used for indicating a PDSCH carrying a paging message. The
DCI is convolutionally coded and the codeword is scrambled with
Paging Radio Network Transaction Identifier (P-RNTI) prior to
transmission. The paging message can include information indicating
a page for one or more UEs and can also include an indication that
a change of broadcast system information of the base station is
impending.
[0051] The paging signal can be transmitted only during a
pre-determined set of subframes. Based on its UE identifier, a UE
determines a paging subframe using a specified formula, during
which it can receive paging signals. The subframe determined based
on the UE identifier is referred to as the UE's paging occasion or
the UE's paging subframe. Details of determining the paging
occasion for LTE UEs are specified in TS 36.304. This mechanism
enables the paging load to be distributed across the predetermined
set of subframes used for paging, while still ensuring that the
base station and the UE have a singular understanding of the UE's
paging occasion.
[0052] A UE may fail to decode the paging signal in the following
two scenarios:
[0053] A UE cannot successfully decode the DCI embedded in the
PDCCH signal and therefore, fails to determine that there is a
PDSCH transmission associated with the paging signal.
[0054] A UE successfully decodes DCI and determines the resource
allocation for the PDSCH, but it fails to decode the Transport
Blocks (TB) in the PDSCH transmission.
[0055] Both of these events lead to a paging failure. If the paging
eNB does not receive a paging response message from the UE within a
certain duration of the time following the paging signal
transmission, the eNB may re-page the UE by a re-transmission of
the paging signal in the next PO. If the UE cannot decode the
paging signal successfully after several paging attempts, this may
lead to a severe paging failure as the eNB may abandon further
paging attempts. In a heterogeneous network, such paging failures
are likely due to interfering transmissions from neighboring base
stations. That is, if the UE is associated with a first cell when a
second cell is a strong interferer, transmissions from the second
cell can cause the UE to be unable to receive its paging signals.
The UE is then said to be in a paging outage condition.
[0056] In LTE, the predetermined set of subframes used for paging
transmission (the paging subframes of the cell) are restricted to
subframes 0, 4, 5 and 9 in FDD and subframes 0, 1, 5 and 6 in TDD.
It may be possible to ensure that the paging subframes of one cell
coincide with AB subframes of a neighbor cell that may pose
interference problem. For example, a femto cell in a TDD network
may configure subframes 0, 1, 5 and 6 to be AB subframes. However,
even in this case, it may not be possible to avoid the neighbor
cell signal transmissions. This is because the (1) the neighbor
cell transmits CRS during AB subframes, and (2) the neighbor cell
transmits criticially important signals such as PSS, SSS, PBCH,
SIB1, paging signals, PRS and CSI-RS during AB subframes. Note that
the SIB1 signal includes a PDCCH component and a PDSCH component;
consequently the PDCCH component of the SIB1 signal from a neighbor
cell can interfere with a PDCCH component of the paging signal from
the serving cell to a UE (as shown in FIG. 3), resulting in a
paging failure.
[0057] Problems related to interference from CRS transmissions and
interference to CRS transmissions are illustrated in FIG. 4. The
following interference scenarios must be considered in a
heterogeneous network.
[0058] Neighbor cell CRS interference to serving cell CRS
[0059] Neighbor cell CRS interference to serving cell PDCCH
[0060] Neighbor cell PDCCH interference to serving cell PDCCH
[0061] Neighbor cell PDCCH interference to serving cell PDSCH
[0062] Neighbor cell PDSCH interference to serving cell PDCCH
[0063] Neighbor cell PDSCH interference to serving cell PDSCH.
[0064] A first cell and a neighbor cell of the first cell can
select PCID such that the CRS resource elements are substantially
non-overlapping. This PCID planning where serving cell and neighbor
cell use substantially different CRS frequency offsets leading to
non-overlapping CRS can mitigate problem (i) above. However, this
scheme can lead to problem (ii) which cannot be avoided. Also
problem (iii) is not possible to avoid due to the dependence of
codeword to RE mapping on the sub-block interleaver and PCID. By
configuring the number of symbols used for control channel
transmissions of the neighbor cell to be smaller than the number of
symbols used for control channel transmissions of the serving cell,
it is possible to reduce the impact of (iv). However, such an
approach may be difficult to use in a heterogeneous network
comprising macro cells, pico cells and femto cells. Moreover, such
a restriction leads to not being able to avoid problem (v) (e.g.,
SIB1 transmission from a femto cell can interfere with macro cell's
PDCCH in a paging subframe). Problem (vi) can be avoided by
frequency domain orthogonalization where the serving and neighbor
cells use non-overlapping RBs and this can be achieved by network
planning.
[0065] In summary, interference mitigation methods at the very
least must address problems (i), (ii), (iii) and (iv).
[0066] In particular, for TDD deployments and synchronous FDD
deployments, the frame time is aligned for all base stations within
a geographical area. If a macro UE roams close to a CSG femto cell,
femto cell's PDCCH/PDSCH associated with SIB1 in subframe 5 can
interfere with paging messages for such macro UEs. A similar
problem can arise for a pico UE in the range expansion area of a
pico cell, due to the macro cell's SIB1 transmission. The
interference can be large enough to result in increased paging
failures or to result in a paging outage.
[0067] FIG. 5 illustrates problems related to overlap of PBCH
transmissions from neighboring cells. The PBCH delivers the Master
information block (MIB), which is a fundamental component of the
cell's broadcast system information. The MIB indicates essential
information for system operation (such a operating bandwidth,
system frame number, number of antennas used, etc). The UE needs to
successfully decode the PBCH and use the information contained in
the MIB to receive other parts of the system information such as
SIB1, SIB2 (system information block 2) etc. A UE is expected to
maintain up-to-date system information of a serving cell. Changes
in broadcast system information of the cell are indicated in paging
messages, wherein an indication that a system information change is
impending is transmitted. Upon receiving an indication that a
system information change is impending, the UE decodes the PBCH in
a predefined time interval and then goes on to receive other system
information. The PBCH is transmitted using fixed resources. In LTE,
the PBCH is transmitted in the center 6 resource blocks of every
subframe 0.
[0068] In a heterogeneous network, since cells operating on a
frequency are synchronized, PBCHs of neighbor cells can overlap.
This can lead to UEs being unable to decode the PBCH and being
unable to maintain up-to-date system information. For example, if a
macro UE is under the coverage of a non-allowed femto cell, the
PBCH transmissions of the non-allowed femto cell can overlap the
PBCH transmissions of a macro cell the UE is associated with. The
UE can then be unable to decode the PBCH of the macro cell. A
similar problem can occur when a pico UE is in the range expansion
area of a pico cell the UE is associated with, and also in the
coverage of a macro cell. In this situation, the UE can be unable
to receive the PBCH of the pico cell. In an FDD system, a time
offset can be applied by some cells on a frequency while still
maintaining time synchronization and alignment of subframe
boundaries across all cells on the frequency. Such a time offset is
referred to as a subframe offset. A subframe offset can avoid the
problem of overlapping PBCH transmissions between different cells
on a frequency. However, a subframe offset cannot be applied in TDD
systems, due to rigidly defined patterns of subframes that are used
for uplink and downlink transmissions.
[0069] Problems related to the overlap of PSS and SSS transmissions
from neighbor cells in a frequency can also result in significant
problems for UEs in idle mode. As in the case of the PBCH, the PSS
and the SSS are transmitted using predefined resources. In an FDD
LTE system, the PSS is transmitted in the last symbol in slots 0
and 10 and in a TDD LTE system it is transmitted in the 3rd symbol
in subframes 1 and 6. In an LTE FDD system, the SSS is transmitted
two symbols before the last symbol in slots 0 and 10 and in a TDD
LTE system it is transmitted the penultimate symbol in slots 1 and
11. The PSS and the SSS are used by UEs to remain synchronized to
the serving cell and to identify cells. The PSS and SSS together
indicate the PCID. Therefore being able to reliably receive the PSS
and the SSS is crucial to proper system operation.
[0070] If a UE is in the coverage of a femto cell, the PSS and SSS
transmissions from the femto cell can interfere with the PSS and
SSS transmissions of a macro cell operating on the same frequency.
Consequently, the UE may be unable to remain synchronized to the
macro cell. This can result in service outage, paging failures and
other undesirable consequences. Similar problems can occur when a
pico UE associated with a pico cell is in the range expansion area
of the pico cell. As in the PBCH case, a subframe offset can avoid
the problem of overlapping PSS and SSS transmissions between
different cells on a frequency. However, a subframe offset cannot
be applied in TDD systems, due to rigidly defined patterns of
subframes that are used for uplink and downlink transmissions.
[0071] Several embodiments are described to address the problems
described above.
[0072] According to a first embodiment of the invention a mobile
station comprises a wireless transceiver coupled to a processor.
The processor is configured to determine interference by assessing
one or more frames in a first periodic sequence of frames relative
to a reference. The processor is further configured to monitor, in
response to assessing, transmissions from a base station for paging
signals during a second periodic sequence of frames, wherein the
second periodic sequence of frames is offset from the first
periodic sequence of frames by a predetermined amount. This
embodiment is illustrated in FIG. 6. The paging occasion of a UE
can be changed if the UE experiences significant interference in
the normal paging occasion. For example, a UE can have a paging
occasion in a first subframe. The subframe that corresponds to the
paging occasion is typically predetermined. For example, in LTE the
subframe corresponding to the paging occasion is determined as a
function of an identifier or the UE. The UE may be camped on a
macro cell but be in the coverage of a non-allowed femto cell. In
such a situation, the UE may experience interference during its
normal paging occasion due to transmissions from the femto cell,
and be unable to receive paging messages. Upon determining that it
can experience interference, relative to a reference, during its
paging occasion, the UE can change its paging occasion to a new
subframe. The new subframe for the paging occasion can be a
predetermined time offset later than the subframe corresponding to
the normal paging occasion. The new subframe for the paging
occasion can be chosen so that the likelihood of experiencing
interference from the femto cell in the new subframe is low. This
is illustrated in FIG. 6A.
[0073] In the base station process of FIG. 6B, at 602, the base
station transmits a paging message for the UE in the regular paging
occasion. At 604, if a response to the paging message is received
by the bas station, the procedure ends at 606. If no response is
received, then at 608 the base station pages the UE in the subframe
that is a determined time offset relative to the regular paging
occasion. In the UE process of FIG. 6C, at 612, the UE determines
that it is experiencing strong interference. At 614, the UE
determines whether interference to reception of paging messages is
likely. If not, the procedure ends at 616. If interference of the
reception of paging messages is likely, at 618, the UE monitors for
paging in the a subframe that is at a predetermined time offset
relative to the regular paging occasion.
[0074] Upon determining that a page message needs to be transmitted
to the UE, the base station can first transmit the page message in
the subframe corresponding to the UE's normal paging occasion. If
the base station does not receive a response to the page message
from the UE, the base station can transmit the page message in the
subframe corresponding to a new paging occasion. The new paging
occasion can be a predetermined time offset later than the subframe
corresponding to the normal paging occasion.
[0075] The determination by the UE that it can experience
interference during its paging occasion can be performed by
determining that its paging occasion coincides with a subframe
during which a signal that can interfere, such as SIB1 may be
transmitted. Alternatively, the UE can perform measurements during
its paging subframe and determine that the interference during the
paging subframe is unacceptably high.
[0076] Furthermore, the choice of the new paging occasion can be
such that a subframe with specific characteristics is chosen for
the new paging occasion. For example, a macro UE may be associated
with a macro cell whose coverage overlaps the coverage of one or
more femto cells. The macro UE can be configured to choose a new
paging subframe that is at least a predetermined time offset later
than the normal paging occasion, and corresponds to the first AB
subframe of the femto cell after the predetermined time offset, if
the macro UE is in the coverage of a non-allowed femto cell. In
another example, the macro UE can be configured to choose a new
paging subframe that is at least a predetermined time offset later
than the normal paging occasion, and corresponds to the first AB
subframe of the femto cell after the predetermined time offset,
wherein the femto cell does not transmit SIB1 in the first AB
subframe of the femto cell, if the macro UE is in the coverage of a
non-allowed femto cell. Based on the rule specified, the macro cell
can uniquely determine the new paging occasion of the UE.
[0077] Furthermore, UEs that are implemented according to a legacy
specification (such as LTE Release 8 and Release 9 UEs) can be
paged on the normal paging occasions. On the other hand, UEs that
are implemented according to a newer specification (such as LTE
Release 10 UEs) can be paged on both their normal paging occasions
and the new paging occasions. Furthermore, in order to ensure that
interference to the paging signals are minimized, the new paging
occasion can correspond to an AB subframe of one or more neighbor
cells. If a UE determines that its original paging occasion
coincides with subframe 5 of an interfering cell, and it determines
that the new paging occasion coincides with a CRS-only AB subframe,
it may decide to change its paging occasion to the new paging
occasion.
[0078] According to another embodiment, a UE can avoid a paging
outage scenario if interference to its paging signal is very
likely. For example, a macro UE may be associated with a macro cell
and be in the coverage of a non-allowed femto cell. If the UE
determines that its paging occasion overlaps, all or most of the
time, a signal from the femto cell, the UE can perform an
inter-frequency or inter-RAT reselection. Specifically, in LTE, if
the paging occasion of the UE overlaps a subframe 5 in an even
numbered radio frame of the femto cell, any paging signal from the
macro cell to the UE will be interfered by SIB1 transmissions from
the femto cell. Consequently, the UE can perform an inter-frequency
or inter-RAT reselection if the paging occasion of the UE overlaps
a subframe 5 in an even numbered radio frame of the femto cell. As
a further simplification, and to ensure that the UE does not need
to first determine the system frame number of radio frames of the
femto cell, the UE can perform an inter-frequency or inter-RAT
reselection if the paging occasion of the UE overlaps any subframe
5 of the femto cell.
[0079] According to another embodiment, the interference caused by
an interfering cell to the paging signals can be substantially
reduced by adjusting the number of symbols used for the control
channel transmissions. The interference to the PDCCH component of
the paging signal may be more significant than the interference to
the PDSCH component of the paging signal. Furthermore, the
interference to the PDCCH component of the paging signal is likely
to be from the PDCCH component of another signal, such as SIB1. In
LTE the number of control channel symbols can be semi-statically
configured to 1, 2 or 3. In the presence of interfering neighbor
cells, a cell can always use a value of 3 for the number of control
channel symbols in paging subframes that can experience
interference. That is, the semi-statically configured value of the
number of control channel symbols can be overridden for paging
subframes that can experience interference, and a value of 3 can be
used. Using the largest possible number of symbols for the control
channel transmissions ensures that the interference to the control
channel is minimized. For example, a macro UE may be in the
coverage of a non-allowed femto cell and be associated with a macro
cell. The UE may experience interference from the femto cell during
its paging subframe. The macro cell can override the
semi-statically configured value of the number of control channel
symbols and use a value of 3 for the number of control channel
symbols in some or all of the paging subframe. The femto cell can
use a value of 1 for the number of control channel symbols in the
subframes that correspond to the paging subframe of the macro cell.
The number of control channel symbols is a function of the load in
the cell. Femto cells are generally lightly loaded and a smaller
number of control channel symbols may be adequate for control
channel transmissions in a femto cell. Thus, the interference
experienced by the UE in the PDCCH component of the paging signal
is restricted to a single subframe, increasing the likelihood of
correctly decoding the PDCCH component of the paging signal.
Furthermore, the femto cell can use a low value of the number of
control channel symbols in all subframes, based on the coverage of
the femto cell overlapping the coverage of a macro cell.
[0080] In some embodiments, details of the new paging occasion can
be broadcasted by the network as part of system information. System
information is typically signaled in the MIB or one of the SIBs.
Details of the new paging occasion can comprise a frame index or
sub frame index or a system frame number. In embodiments where the
new paging occasion is using a time offset later than the subframe
corresponding to the normal paging occasion, the time offset value
can also be broadcasted by the network. In some embodiments, the
specific PDSCH resource allocation (resource block indices,
Modulation and coding scheme) for the paging message in the new
paging location can also be broadcasted by the network.
Alternately, the specific PDSCH resource allocation for the paging
message in the new paging location can be based on pre-specified
values known a priori to the base station and the UE. In
embodiments where the specific resource allocation for the paging
message in the new paging location is either broadcasted or known a
priori, the UE can read the paging message directly on the PDSCH
without decoding PDCCH in the new paging location.
[0081] The UE can receive the PDCCH component of the paging signal
in a first subframe. The UE may be unable to decode the PDSCH
component of the paging signal in the first subframe. The UE can
the attempt to decode the PDSCH component of the paging signal in a
second subframe, without attempting to receive a PDCCH component of
the paging signal in the second subframe. For example, the first
subframe can have little or no interference in the symbols used for
control channel transmissions, but can have significant
interference in the symbols used for PDSCH transmissions. Thus, the
UE may successfully decode the PDCCH component of the paging
signal, but be unable to decode the PDSCH component of the paging
signal, in the first subframe. If the UE is unable to decode the
PDSCH component of the paging signal in the first subframe, it can
monitor the second subframe for the PDSCH component of the paging
signal. The second subframe can overlap an AB subframe of a
neighbor cell. Alternatively, the UE can experience significant
interference in the symbols used for control channel transmissions
in its normal paging subframe. Therefore the UE can monitor an
alternate subframe for the PDCCH component of the paging channel
and its normal paging subframe for the PDSCH component of the
paging subframe.
[0082] According to a second embodiment, the UE can modify its cell
reselection behavior based on whether overlap of resources reserved
for CRS transmission can occur. In a first approach illustrated in
the process 700 FIG. 7A, at 710, the UE may be camped on a macro
cell but be in the coverage of a non-allowed femto cell. In such a
situation, the resource elements used by the macro cell and the
femto cell for their respective CRS transmissions can overlap,
resulting in the UE being unable to perform correct measurements of
both the macro cell and the femto cell and possibly other cells.
For example, the PCID of the macro cell and the femto cell can be
such that the resource elements used for their respective CRS
transmissions overlap. At 720, the UE determines whether the CRS
transmissions of the macro cell and the femto cell can overlap.
This determination can be done by (a) first detecting the PCID of
the femto cell, (b) then, based on the PCID, determining the
resource elements used for CRS transmissions of the femto cell, and
(c) comparing the resource elements used for CRS transmissions of
the femto cell and the resource elements used for CRS transmissions
of the macro cell.
[0083] In addition to the PCID, information pertaining to the
number of CRS transmission ports in the neighbor cell and the
serving cell can be used. The UE can determine the number of CRS
transmission ports of the serving cell based on PBCH decoding. On
the other hand, the number of CRS transmission ports for the
neighbor cell can be determined based on either neighbor cell PBCH
decoding or by means of assistance data signaled by the serving
cell that includes this information. When the number of CRS
transmission ports for the serving and neighbor cells are
different, different situations may arise.
[0084] When both serving cell and neighbor cell use 1 Tx, CRS
collision occurs when mod(PCID.sub.serving,
6)=mod(PCID.sub.neighbor, 6).
[0085] When both the serving cell and the neighbor cell have 2 Tx,
CRS collision occurs when mod(PCID.sub.serving,
3)=mod(PCID.sub.neighbor, 3).
[0086] When the serving cell has 4 Tx and neighbor cell has 2 Tx,
CRS ports #2 and #3 for the serving cell will not experience any
CRS interference from the neighbor cells as ports #0 and #1 are
mapped to a different set of OFDM symbols relative to ports #2 and
#3.
[0087] Two reselection thresholds may be configured in the UE by
the network (e.g., by signaling in SIB or in a RRC message as part
of RRC connection release), one applicable to the case when the CRS
transmissions of a femto cell overlaps the CRS transmissions of a
macro cell, and another applicable to the case when the CRS
transmissions of the femto cell does not overlap the CRS
transmissions of the macro cell. A macro UE with CRS Interference
Cancellation (IC) or Interference Rejection (IR) receiver
capabilities may be able to stay on the same frequency even when
the interference from the femto cell is large, if there is no CRS
collision. The UE may be able to remain attached to and remain
schedulable by the macro cell even when the RSRP difference between
the serving cell and the neighbor cell is as low as, say, -20 dB.
However, CRS interference rejection/cancellation capabilities may
be limited when there is CRS collision. The UE may be able to
remain attached to the macro cell only up to, say, -6 dB in this
case. Therefore, different reselection thresholds matched to the
receiver capabilities in the non-colliding and colliding CRS cases
may be necessary.
[0088] In FIG. 7A, at decision block 730, if there is no overlap of
resource elements used for CRS, then the UE follows normal idle
mode relesction procedures. If overlap occurs, at 740 the UE
determines whether the neighbor cell signal level is greater than
the serving cell signal level plus a threshold. At 742, if the
neighbor cell signal level is not greater than the serving cell
signal level plus a threshold, the UE follows normal idle mode
reselection procedures. At 750, if the neighbor cell signal level
is greater than the serving cell signal level plus the threshold,
the UE performs inter-frequency or inter-RAT reselection.
[0089] If the UE determines that the resource elements used for the
CRS transmissions of the macro cell and the femto cell overlap, it
can perform an inter-frequency reselection or an inter-RAT
reselection. According to a further embodiment, the UE can perform
the inter-frequency or inter-RAT reselection only if a signal level
of the femto cell is no less than a signal level of the macro cell
plus a threshold. The signal level metric used to determine whether
the signal level of the femto cell is no less than the signal level
of the macro cell plus a threshold can be obtained by measurements
of resources other than the CRS.
[0090] According to another embodiment illustrated in FIG. 7A at
734 the UE applies different reselection biases for the serving
frequency or serving cell based on whether overlap occurs. In one
implementation, if the UE determines that the resource elements
used for the CRS transmissions of the macro cell and the femto cell
overlap, it can apply a negative bias to the serving frequency. The
negative bias can result in cells on another frequency or another
RAT being viewed by the UE as suitable candidates for reselection,
and the UE can perform a inter-frequency or inter-RAT reselection.
If the UE determines that the resource elements used for the CRS
transmissions of the macro cell and the femto cell do not overlap,
it can apply a positive bias to the serving frequency. The positive
bias can ensure that the UE remains camped on the macro cell even
if a signal level of the femto cell (such as RSRP) is higher than
that of the macro cell. Additionally, or alternatively, the UE can
apply a negative bias to the serving cell if the UE determines that
the CRS transmissions of the macro cell and the femto cell overlap
substantially. The UE can apply a positive bias to the serving cell
if the UE determines that the CRS transmissions of the macro cell
and the femto cell do not overlap substantially.
[0091] In a second approach illustrated in the process 701 of FIG.
7B, at 711, a femto cell can determine that its coverage overlaps
the coverage of one or more neighbor cells. At 721, the femto cell
can determine the resource elements used by the one or more
neighbor cells. At 731, if the femto cell determines that the
resource elements used for CRS transmissions by at least one of the
one or more neighbor cells overlaps the resource elements used for
the femto cell's CRS transmissions, the femto cell can use a
different set of resource elements for its CRS transmissions. The
femto cell can use the different set of resource elements for its
CRS transmissions in some or all subframes. For example, 741, the
femto cell can use the different set of resource elements for its
CRS transmissions only during its AB subframes. Additionally, the
different set of resource elements for the femto cell's CRS
transmissions can be obtained by applying offsets in time and
frequency to the original resource elements, as shown in FIG. 7.
The offsets in time and frequency can be signaled to UEs in the
network, for example by macro cells.
[0092] According to another embodiment, a femto cell can modify
access restrictions based on whether its CRS transmissions
substantially overlap the CRS transmissions of a macro cell. For
example, a femto cell may be a CSG cell and allow access only to a
certain group of users. The femto cell can determine that its
coverage may overlap one or more macro cells. The femto cell can
further determine that the resource elements it uses for
transmissions of CRS can substantially overlap the resource
elements used for CRS transmissions by at least one of the one or
more macro cells. The femto cell can then modify access
restrictions such that all users can access the femto cell. The
modifying of access restrictions can ensure that a UE does not
remain in the coverage of the femto cell without being able to
connect to the femto cell. That is, if all femto cells in the
network perform such a procedure, UEs do not encounter non-allowed
femto cells whose CRS transmissions overlap the CRS transmissions
of macro cells. The modification of access restrictions can be
performed by changing the status of the femto cell from a CSG cell
to a "hybrid access" cell or "open access" cell.
[0093] According to a third embodiment, a base station can use
alternate resources to transmit PBCH contents or PBCH related
information. The alternate resources used to transmit PBCH contents
can be predetermined so that UEs can receive the PBCH contents or
the PBCH related information in these resources. In a first
approach illustrated in FIG. 8A, at 810, a base station determines
that its system information needs to be changed. At 820, the base
station determines whether its coverage overlaps the coverage of
one or more neighbor cells, such as femto cells. If not, at 822,
the base station resumes normal operation. At 830, in the presence
of overlap, the base station transmits a system information change
indication message, wherein the message includes the PBCH contents.
UEs that are associated with the base station (in connected mode or
idle mode) and are in the coverage of non-allowed femto cells can
receive the system information change indication message and the
included PBCH contents. This can enable the UEs to receive other
system information such as system information block 1 (SIB1).
Alternatively, at 840, the base station can transmit a system
information indication message, wherein the message includes an
indication of alternate resources uses for PBCH transmission. UEs
that are associated with the base station (in connected mode or
idle mode) and are in the coverage of non-allowed femto cells can
receive the system information change indication message and the
indication of alternate resources for PBCH transmission. The UEs
can then decode the PBCH in the alternate resources. This can
enable the UEs to receive other system information.
[0094] The PBCH contents transmitted in the paging message can
include one or more of the information elements in the MIB. For
example, the paging message can include one or more of the downlink
bandwidth, information related to the system frame number and
information related to the physical HARQ indicator channel
(PHICH).
[0095] According to another embodiment, the UE can receive the PBCH
of a macro cell and a femto cell by transmitting the PBCH of the
femto cell in alternate resource blocks in a predetermined manner.
For example, a macro UE may be associated with a macro cell. In a
second approach illustrated in the process 801 of FIG. 8B, at 811,
the UE decodes the PBCH of the macro cell in a first set of
resource blocks of the macro cell. The UE can receive additional
system information of the macro cell after decoding the PBCH. At
821, the UE receives a `Femto cell PBCH resource offset` parameter
from the macro cell. At 831, the UE detects a physical cell
identifier of a femto cell. The UE then roam into the coverage of a
femto cell and detect the PCID of the femto cell. At 841, the UE
determines a second set of resource blocks by applying an offset
equal to the `Femto cell PBCH resource offset` to the first set of
resource blocks. At 851, the UE attempts to decode the PBCH of the
femto cell in the resource blocks of the femto cell that overlap
the second set of resource blocks. The UE can continue to decode
the PBCH of the macro cell in the first set of resource blocks as
needed.
[0096] The femto cell may be configured to transmit its PBCH using
a normal set of resource blocks. In process 802 of FIG. 8C, at 811,
the femto cell determines that its coverage may overlap the
coverage of a macro cell. At 822, the femto cell transmits its PBCH
in an alternate set of resource blocks. The alternate set of
resource blocks can be offset from the normal set of resource
blocks by an amount equal to the `Femto cell PBCH resource offset`.
It should be noted that the procedure can be applied to any
combinations of cells instead of the macro cell and femto cell
combination described above. The macro cell can indicate a PBCH
resource offset for any specific cell or set of cells (for example
as part of a neighbor list). When a PCID of a neighbor cell is
detected, the UE can apply the corresponding PBCH resource offset
(if indicated), to obtain the alternate resource blocks used by the
neighbor cell for PBCH transmission.
[0097] In synchronous networks, interference PSS transmissions of
two cells on the same frequency can interfere with each other and
SSS transmissions of two cells on the same frequencies can
interfere with each other. The following solutions mitigate this
problem.
[0098] The PSS and/or the SSS can be transmitted using alternate
resource elements. The alternate resource elements can be protected
from interference. Furthermore, the PSS and/or the SSS can be
transmitted using both the resource elements used normally for PSS
and/or SSS transmission and the alternate resource elements. For
example, a macro UE may be in the coverage of a non-allowed femto
cell, and thus may be unable to receive the PSS and/or SSS of the
macro cell. To overcome such a problem, the femto cell can first
recognize that its coverage overlaps that of the macro cell. The
femto cell can then transmit the PSS and/or the SSS in alternate
resource elements. The femto cell can choose the alternate resource
elements such that the alternate resource elements do not overlap
some critically important transmissions from the macro cell.
Furthermore, the femto cell can also ensure that some or all of the
resource elements that overlap the PSS and/or the SSS transmissions
of the macro cell do not carry any transmissions from the femto
cell.
[0099] The alternate resource elements used for PSS and/or SSS
transmissions can be offset by a time duration from the normal
resource elements used for PSS and/or SSS transmissions.
Preferentially, the offset can be a certain number of subframes.
That is, the PSS and/or SSS can be transmitted in alternate
subframes but in the same OFDM symbol. For example, the PSS may
normally be transmitted in the 3.sup.rd symbol of subframe 1 and
the 3.sup.rd symbol of subframe 6; and the SSS may normally be
transmitted in the 13.sup.th symbol in subframe 1 and the 13.sup.th
symbol in subframe 6. The femto cell can instead transmit the PSS
in the 3.sup.rd symbol of subframe 3 and the 3.sup.rd symbol of
subframe 8.
[0100] A UE that receives the PSS and/or SSS from the femto cell
may be unaware that the femto cell is using alternate resources for
PSS and/or SSS transmissions. Hence the UE can interpret the frame
timing of the femto cell assuming the PSS and/or SSS are being
transmitted in the normal resource elements. Such a UE may be
either a macro UE that is in the coverage of the femto cell, a
associated with the femto cell, or a UE attempting to associate
with the femto cell. Thus, in the above example, such a UE can
assume that the subframes in which the PSS transmissions are
received are subframe 1 and 6, and the subframes in which the SSS
transmissions are received are subframe 1 and 6. The UE will then
be unable to decode the PBCH or critical transmissions such as
SIB1, paging etc from the femto cell. In order to prevent such
problems, the time duration offsets used for the PSS and/or the SSS
can be made known to the UE apriori. For example, a macro cell can
signal the time duration offset to the UEs. The time duration
offset can also be a fixed value for a class of cells (such as
femto cells or CSG cells) and may not need to be signaled. The UEs
can use the time duration offset to correct their interpretation of
the frame boundary of the femto cell. Thus, in the above example, a
time duration offset equal to two subframes, for the femto cell, is
made known apriori to the UE. Based on this interpretation, the UE
can determine that the subframes in which the PSS transmissions are
performed by the femto cell are subframes 3 and 8, and the
subframes in which the SSS transmissions are performed by the UE
are subframes 3 and 8. Thus, the functions and apparatus in the UE
related to PSS/SSS reception (such as cell search) can have a first
interpretation of the frame timing; and the other functions and
apparatus in the UE (for example, other physical layer functions,
medium access control functions and measurements related functions)
can have a second interpretation of frame timing. The second
interpretation of frame timing can be an offset by a time period
from the first interpretation of frame timing. Furthermore, it
should be noted that such an approach can be used to avoid overlap
of PBCH transmissions of neighbor cells.
[0101] According to another embodiment, if a macro UE determines
that PSS and/or SSS of a non-allowed femto cell overlaps the PSS
and/or SSS transmissions of a serving macro cell, the UE can
perform an inter-frequency or inter-RAT reselection. Thus, the UE
can remain associated with the macro cell even if the non-allowed
femto is a strong interfering cell, unless the PSS and/or SSS
transmission of the serving macro cell overlap the PSS and/or SSS
transmissions of the non-allowed femto cell. Similarly, a pico UE
in the range expansion area of the pico cell may experience high
P/S-SCH interference due to interference from a macro cell. Such a
UE can perform inter-frequency or inter-RAT reselection if the PSS
and/or SSS of the macro cell and the pico cell overlap.
[0102] According to another embodiment, a UE can recognize its
proximity to a femto cell and determine that it needs to attempt
reception of PSS and/or SSS in alternate resource elements. For
example, a UE may determine that it is close to a femto cell based
on RRM measurements on the frequency. RRM measurements (such as
RSRP) can indicate that the UE is close to the femto cell. The UE
can also determine that the coverage of the femto cell overlaps the
coverage of a macro cell. The UE can then attempt to receive the
PSS and/or SSS in the alternate resource elements. The UE can apply
such a procedure when it roams into the coverage of a femto cell
whose coverage overlaps that of a macro cell. The UE can also apply
such a procedure when it is powered up in the coverage of a femto
cell. Other means of recognizing proximity to a femto cell can be
used, including positioning methods such as global positioning
system (GPS) and Enhanced observed time difference (E-OTD).
[0103] 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.
* * * * *