U.S. patent application number 14/339485 was filed with the patent office on 2014-11-06 for downlink scheduling in heterogeneous networks.
The applicant listed for this patent is Telefonaktiebolaget L M Ericsson (publ). Invention is credited to Lars Lindbom, Bengt Lindoff, Stefan Parkvall.
Application Number | 20140328328 14/339485 |
Document ID | / |
Family ID | 45329115 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140328328 |
Kind Code |
A1 |
Lindoff; Bengt ; et
al. |
November 6, 2014 |
Downlink Scheduling in Heterogeneous Networks
Abstract
The present invention provides methods to support scheduling of
transmissions from a pico base station or micro base station to a
mobile terminal operating in a link imbalance zone where
interference from macro base station is present. A method is
provided to enable the mobile terminal to detect when it is in a
link imbalance zone, and for triggering scheduling restrictions
when the mobile terminal is in the link imbalance zone.
Inventors: |
Lindoff; Bengt; (Bjarred,
SE) ; Lindbom; Lars; (Karlstad, SE) ;
Parkvall; Stefan; (Bromma, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Telefonaktiebolaget L M Ericsson (publ) |
Stocklholm |
|
SE |
|
|
Family ID: |
45329115 |
Appl. No.: |
14/339485 |
Filed: |
July 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12902205 |
Oct 12, 2010 |
8824383 |
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14339485 |
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Current U.S.
Class: |
370/332 |
Current CPC
Class: |
H04W 16/32 20130101;
H04W 72/1273 20130101; H04W 72/048 20130101; H04L 5/0073 20130101;
H04W 72/1257 20130101; H04W 36/0072 20130101; H04W 72/085 20130101;
H04W 72/042 20130101 |
Class at
Publication: |
370/332 |
International
Class: |
H04W 36/00 20060101
H04W036/00; H04L 5/00 20060101 H04L005/00 |
Claims
1. In a mobile communication network including first and second
base stations with overlapping coverage, a method implemented by
said first base station for scheduling downlink transmission to a
mobile station, said method comprising: determining an estimated
path loss between said first base station and said mobile terminal;
and scheduling downlink transmissions from said first base station
to said mobile terminal based on said estimated path loss.
2. The method of claim 1 wherein determining an estimated path loss
between said first base station and said mobile terminal comprises:
receiving a reference signal transmitted from a mobile terminal
with known transmit power; and estimating path loss based on the
received signal strength of the reference signal.
3. The method of claim 2 further comprising transmitting an
explicit transmit power control command to the mobile terminal to
command the mobile terminal to transmit the reference signal with a
specified transit power.
4. The method of claim 1 wherein scheduling downlink transmissions
from said first base station to said mobile terminal based on said
estimated path loss comprises scheduling the mobile terminal to
receive data only in restricted subframes when said path loss meets
a threshold.
5. A base station in a mobile communication network, said base
station comprising: a transceiver for communicating with one or
more mobile terminals; and a control circuit to control said
transceiver, said control circuit configured to: determine an
estimated path loss between said first base station and said mobile
terminal; and schedule downlink transmissions from said first base
station to said mobile terminal based on said estimated path
loss.
6. The base station of claim 5 wherein the control circuit
determines an estimated path loss between said first base station
and said mobile terminal by: receiving a reference signal
transmitted from a mobile terminal with known transmit power; and
estimating path loss based on the received signal strength of the
reference signal.
7. The base station of claim 6 wherein the control circuit is
further configured to transmit an explicit transmit power control
command to the mobile terminal to command the mobile terminal to
transmit the reference signal with a specified transit power.
8. The base station of claim 5 wherein the control circuit is
configured to schedule the mobile terminal to receive data only in
restricted subframes when said path loss meets a threshold.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/902,205 filed on Oct. 12, 2010, which claims the
benefit of U.S. Provisional Patent Application 61/357,264, filed on
Jun. 22, 2010, the content of which are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates generally to heterogeneous
networks comprising a mixture of low power and high power nodes
with overlapping coverage and, more particularly, to downlink
scheduling in a heterogeneous network.
BACKGROUND
[0003] The new Third Generation Partnership Project (3GPP) standard
known as Long Term Evolution (LTE) (3GPP LTE Rel-10) supports
heterogeneous networks. In heterogeneous networks, a mixture of
cells of different size and overlapping coverage areas are
deployed. For example, a heterogeneous network may deploy pico
cells served by relatively low power nodes within the coverage area
of a macro cell served by relatively high power base stations.
Heterogeneous networks could also deploy relatively low-power home
base stations and relays to provide improved service in indoor
areas. The aim of deploying low power nodes, such as pico base
stations, home base stations, and relays, within a macro cell where
coverage is provided by a high power base station is to improve
system capacity by means of cell splitting gains, as well as to
provide users with wide area experience of very high speed data
access throughout the network. Heterogeneous deployment schemes
represent one alternative to deployment of denser networks of macro
cells and are particularly effective to cover traffic hotspots,
i.e., small geographical areas with high user densities served by
lower power nodes.
[0004] In heterogeneous networks, there may be a large disparity in
output power of the low power nodes compared to the base stations
serving macro cells. For example, the output power of the base
stations in the macro cells may be in the order of 46 dBm, while
the output power of the low power nodes in the pico cells may be
less than 30 dBm. In some heterogeneous networks, the macro cells
and pico cells operate on the same carrier frequencies, and
inter-cell interference coordination (ICIC) techniques are used to
deal with interference when mobile terminals are operating in areas
served by both macro cells and pico cells. For example, scheduling
restrictions may be applied where a macro base station is prevented
from transmitting in certain subframes, which may be referred to as
"blank" subframes. The pico cells can use these blank subframes to
transmit data to mobile terminals operating in a link imbalance
zone near the cell edge of the pico cells without interference from
the macro base stations.
[0005] Although scheduling restrictions may be applied by the base
station to implement ICIC, there is currently no way to notify the
mobile terminals about such scheduling restrictions. Thus, the
mobile terminals must decode the PDCCH in all subframes, which may
be unnecessary if scheduling restrictions apply to the mobile
terminal. Also, because the received signals from the macro cell
may be up to 10 dB stronger than the received signals from the pico
cell, the mobile terminal might not be able to maintain
synchronization with the pico cell and thus unable to decode the
PDCCH transmitted by the pico cell. Another drawback is that there
is currently no method to detect when a mobile terminal is in a
link imbalance zone and thus should be subject to scheduling
restrictions.
SUMMARY
[0006] The present invention provides mechanisms for detecting when
a mobile terminal is in a link imbalance zone and thus subject to
scheduling restrictions. The detection can be made either by the
mobile terminal, which then informs the network that a change in
scheduling should apply, or by a pico cell. When scheduling
restrictions apply, the downlink transmissions to the mobile
terminal may be restricted to a predetermined subset of the
subframes, referred to herein as the restricted subframes. The
macro cells avoid transmission of the PDCCH and user data in the
restricted subframes. The change between restricted and
unrestricted scheduling modes for mobile terminals served by a pico
cell can be signaled in several ways, for instance through Radio
Resource Control (RRC) or (MAC) signaling.
[0007] Additionally, the present invention provides a mechanism to
identify restricted subframes to the mobile terminal so that the
mobile terminal does not need to decode the PDCCH in unrestricted
subframes when it is operating in the link imbalance zone.
Scheduling information is transmitted to the mobile terminal to
indicate the subframes that can be used to transmit to mobile
terminals when scheduling restrictions apply. Thus, when the mobile
terminal is within the link imbalance zone and thus subject to
scheduling restrictions, the mobile terminal does not need to
monitor the PDCCH in other subframes.
[0008] Exemplary embodiments of the invention comprise methods
implemented by a mobile terminal in a heterogeneous communication
network including first and second base stations with overlapping
coverage. One embodiment comprises a method implemented by a mobile
terminal of decoding a control channel transmitted by a pico base
station cell. The mobile terminal receives scheduling information
indicating one or more restricted subframes for downlink
transmission by said first base station in said area of overlapping
coverage. When the mobile terminal is served by the pico base
station and is within a link imbalance zone, the mobile terminal
decodes the control channel transmitted by said pico base station
only in said restricted subframes.
[0009] Other embodiments of the invention relate to a mobile
terminal configured to operate in a heterogeneous network. In one
embodiment, the mobile terminal comprises a transceiver for
communicating with a base station in a mobile communication
network; and a control circuit to control said transceiver. The
control circuit receives scheduling information indicating one or
more subframes restricted for downlink transmission by said first
base station in said area of overlapping coverage. When the mobile
terminal is served by a pico base station and is in a link
imbalance zone, the mobile terminal decodes the control channel
transmitted by said pico base station in said restricted subframes
when said mobile terminal is in said link imbalance zone.
[0010] Another embodiment of the invention comprises methods
implemented by a mobile terminal in a heterogeneous network of
triggering a subframe scheduling change. In one exemplary method,
the mobile terminal measures a first signal quality of signals
transmitted by a pico base station in subframes restricted for
downlink transmission by the pico base station; measuring a second
signal quality of signals transmitted by a macro base station. The
mobile terminal compares the first and second signal quality
measurements, and transmits a measurement report to a network node
when the comparison meets a predetermined condition.
[0011] Other embodiments of the invention relate to a mobile
terminal in a heterogeneous network and configured to trigger a
subframe scheduling restriction. In one embodiment, the mobile
terminal comprises a transceiver for communicating with a base
station in a mobile communication network; and a control circuit to
control said transceiver. The control circuit measures a first
signal quality of signals transmitted by the pico base station in
subframes restricted for downlink transmission by the pico base
station, and measures a second signal quality of signals
transmitted by a macro base station. The control circuit compares
the first and second signal quality measurements; and transmits a
measurement report to a network node when said comparison meets a
predetermined condition.
[0012] Other embodiments of the invention comprise methods
implemented by a pico base station in a heterogeneous network to
reduce inter-cell interference. In one exemplary method, the pico
base station determines an estimated path loss between said pico
base station and a mobile terminal, and schedules downlink
transmissions from the pico base station to the mobile terminal
based on said estimated path loss.
[0013] Still other embodiments of the invention comprise a pico
base station in a heterogeneous network configured to reduce
inter-cell interference. In one exemplary embodiment, the pico base
station comprises a transceiver for communicating with one or more
mobile terminals, and a control circuit to control said
transceiver. The control circuit is configured to determine an
estimated path loss between the pico base station and the mobile
terminal; and to schedule downlink transmissions from the pico base
station to the mobile terminal based on the estimated path
loss.
[0014] Other embodiments of the invention comprise methods
implemented by a pico base station in a heterogeneous network to
reduce inter-cell interference. In one exemplary method, the pico
base station receives a measurement report from a mobile terminal.
The measurement report includes measurements of signals transmitted
by the pico base station and at least one neighboring macro base
station. Based on the measurement report, the pico base station
determines whether the mobile terminal is in a link imbalance zone.
If the mobile terminal is in a link imbalance zone, the base
station schedules downlink transmissions to the mobile terminal in
restricted subframes only.
[0015] Still other embodiments of the invention comprise a pico
base station in a heterogeneous network configured to reduce
inter-cell interference. In one exemplary embodiment, the pico base
station comprises a transceiver for communicating with one or more
mobile terminals, and a control circuit to control said
transceiver. The control circuit is configured to receive a
measurement report from a mobile terminal. The measurement report
includes measurements of signals transmitted by the pico base
station and at least one neighboring macro base station. Based on
the measurement report, the control circuit determines whether the
mobile terminal is in a link imbalance zone. If the mobile terminal
is in a link imbalance zone, the base station schedules downlink
transmissions to the mobile terminal in restricted subframes
only.
[0016] Other embodiments of the invention comprise methods
implemented by a pico base station in a heterogeneous network to
reduce inter-cell interference. In one exemplary method, the pico
base station receives an indication from a mobile terminal that
mobile terminal is in a link imbalance zone and, responsive to the
indication, begins scheduling downlink transmissions to the mobile
terminal in restricted subframes only.
[0017] Still other embodiments of the invention comprise a pico
base station in a heterogeneous network configured to reduce
inter-cell interference. In one exemplary embodiment, the pico base
station comprises a transceiver for communicating with one or more
mobile terminals, and a control circuit to control said
transceiver. The control circuit is configured to receive an
indication from a mobile terminal that mobile terminal is in a link
imbalance zone. In response to the indication, the base stations
schedules downlink transmissions to the mobile terminal in
restricted subframes only.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 illustrates macro and pico cell deployment in a
mobile communication network.
[0019] FIG. 2 illustrates an exemplary downlink physical resource
in an exemplary OFDM network.
[0020] FIG. 3 illustrates an exemplary time-domain structure in an
exemplary OFDM network.
[0021] FIG. 4 illustrates an exemplary mapping of LTE physical
control channels, data channels and cell specific reference signals
within a downlink subframe.
[0022] FIG. 5 illustrates link imbalance in a heterogeneous
network.
[0023] FIG. 6 illustrates inter-cell interference coordination
using blank subframes in the downlink.
[0024] FIG. 7 illustrates an exemplary method performed by a mobile
terminal of decoding a control channel.
[0025] FIG. 8 illustrates an exemplary method implemented by a
mobile terminal for triggering subframe scheduling
restrictions.
[0026] FIG. 9 illustrates an exemplary scheduling method
implemented by a network node for determining whether to apply
subframe scheduling restrictions.
[0027] FIG. 10 illustrates another exemplary scheduling method
implemented by a network node for determining whether to apply
subframe scheduling restrictions based on a measurement report from
the mobile terminal.
[0028] FIG. 11 illustrates another exemplary scheduling method
implemented by a network node for determining whether to apply
subframe scheduling restrictions based on an indication from the
mobile terminal.
[0029] FIG. 12 illustrates an exemplary mobile terminal.
[0030] FIG. 13 illustrates an exemplary base station.
DETAILED DESCRIPTION
[0031] Turning now to the drawings, FIG. 1 illustrates an exemplary
heterogeneous communication network 10 according to one exemplary
embodiment of the present invention. The present invention is
described in the context of a Long-Term Evolution (LTE) network,
which is specified in Release 10 of the LTE standard. However,
those skilled in the art will appreciate that the invention may be
applied in heterogeneous networks using other communication
standards.
[0032] The communication network 10 comprises a plurality of high
power access nodes providing radio coverage in respective macro
cells 20 of the communication network 10. In the exemplary
embodiment shown in FIG. 1, three pico cells 30 served by low power
access nodes 300 are deployed within the macro cell 20. The low
power access nodes may comprise pico base stations or home base
stations. For convenience, the high power and low power access
nodes 200, 300 will be referred to herein as the macro base
stations 200 and pico base stations 300. The output power of the
high power macro base stations 200 is presumed to be in the order
of 46 dBm, while the output power of the pico base stations 300 is
presumed to be less than 30 dBm.
[0033] In some heterogeneous networks 10, frequency separation
between the different layers, i.e., macro and pico cells 20, 30 in
FIG. 1, is used to avoid interference between the macro cells 20
and pico cells 30, respectively. When frequency separation is used,
the macro base stations 200 and pico base stations 300 operate on
different non-overlapping carrier frequencies to reduce
interference between the macro and pico layers. Cell splitting
gains are obtained when the radio resources in the pico cell 30 can
be simultaneously used when the macro cell 20 is transmitting.
However, frequency separation may lead to resource-utilization
inefficiency. For example, when the pico cell 30 is lightly loaded
so that its resources are not fully utilized, it may be more
efficient to assign all carrier frequencies to the macro cell 20.
However, the split of carrier frequencies between layers is
typically static.
[0034] In some heterogeneous networks 10, radio resources on same
carrier frequencies are shared by the macro and pico layers by
coordinating transmissions in the overlapping macro and pico cells
20, 30. This type of coordination is referred to as inter-cell
interference coordination (ICIC). Certain radio resources are
allocated for the macro cells 20 during specified time periods and
the remaining resources can be accessed by pico cells 30 without
interference from the macro cell 20. Depending on the load
distribution across the layers, the resource split can change over
time to accommodate different load distributions. In contrast to
the splitting of carrier frequencies, sharing radio resources
across layers using some form of ICIC can be made more or less
dynamic depending on the implementation of the interface between
the access nodes. In LTE, an X2 interface has been specified in
order to exchange different types of information between base
stations 200, 300. One example of such information exchange is that
a base station 200, 300 can inform other base stations 200, 300
that it will reduce its transmit power on certain resources.
[0035] Time synchronization between base stations 200, 300 is
required to ensure that ICIC across macro and pico layers will work
efficiently in heterogeneous networks. Time synchronization is
particularly important for time domain based ICIC schemes where
resources on the same carrier are shared by macro and pico base
stations.
[0036] LTE uses Orthogonal Frequency Division Multiplexing (OFDM)
in the downlink and Discrete Fourier Transform (DFT) spread OFDM in
the uplink. The basic LTE downlink physical resource can be viewed
as a time-frequency grid. FIG. 2 illustrates a portion of an
exemplary OFDM time-frequency grid 50 for LTE. Generally speaking,
the time-frequency grid 50 is divided into one millisecond
subframes. Each subframe includes a number of OFDM symbols. For a
normal cyclic prefix (CP) length, suitable for use in situations
where multipath dispersion is not expected to be extremely severe,
a subframe comprises fourteen OFDM symbols. A subframe comprises
twelve OFDM symbols if an extended cyclic prefix is used. In the
frequency domain, the physical resources are divided into adjacent
subcarriers with a spacing of 15 kHz. The number of subcarriers
varies according to the allocated system bandwidth. The smallest
element of the time-frequency grid 50 is a resource element. A
resource element comprises one OFDM subcarrier during one OFDM
symbol interval.
[0037] In LTE systems, data is transmitted to the mobile terminals
over a downlink transport channel known as the Physical Downlink
Shared Channel (PDSCH). The PDSCH is a time and frequency
multiplexed channel shared by a plurality of mobile terminals. As
shown in FIG. 3, the downlink transmissions are organized into 10
ms radio frames 60. Each radio frame comprises ten equally-sized
subframes 62. For purposes of scheduling users to receive downlink
transmissions, the downlink time-frequency resources are allocated
in units called resource blocks (RBs). Each resource block spans
twelve subcarriers (which may be adjacent or distributed across the
frequency spectrum) and one 0.5 ms slot (one half of one subframe).
The term "resource block pair" refers to two consecutive resource
blocks occupying an entire one millisecond subframe.
[0038] Within a cell 20, 30, the base station 200, 300 dynamically
schedules downlink transmissions to the mobile terminals 100 based
on channel state and quality information (CSI, CQI) reports from
the mobile terminals 100 on the Physical Uplink Control Channel
(PUCCH) or Physical Uplink Shared Channel (PUSCH). The CQI reports
indicate the instantaneous channel conditions as seen by the
receiver. In each subframe, the base station 200, 300 transmits
downlink control information (DCI) identifying the mobile terminals
100 that have been scheduled to receive data (hereinafter the
scheduled mobile terminals) in the current downlink subframe and
the resource blocks on which the data is being transmitted to the
scheduled terminals. The DCI is typically transmitted on the
Physical Downlink Control Channel (PDCCH) in the first one, two, or
three OFDM symbols (up to 4 symbols for 1.4 MHz bandwidth systems)
in each subframe.
[0039] In order to demodulate data transmitted on the downlink, the
mobile terminals 100 need an estimate of the channel over which the
data is transmitted. To facilitate channel estimation by the mobile
terminal 100, the base station 200, 300 transmits reference symbols
on the downlink which are known to the mobile terminal 100. By
correlating the known reference signals with the received signal,
the mobile terminal 100 obtains an estimate of the channel. In LTE,
there are two types of reference signals: cell specific reference
signals and mobile terminal specific reference symbols. The cell
specific reference symbols are usually transmitted in all downlink
subframes. The mobile terminal 100 may use the cell specific
reference signals for both channel estimation and for performing
signal measurements for mobility management purposes. Mobile
terminal specific reference symbols may also be transmitted and
used for channel estimation.
[0040] FIG. 4 illustrates the mapping of the physical control
channels and cell-specific reference signals in a downlink
subframe. As shown in FIG. 4, physical control channels may be
transmitted in the first three symbols of a subframe. User data is
transmitted in the last eleven symbols, except on resource elements
where reference signals are transmitted. In some embodiments, the
physical control channels may use less than three symbols, so data
transmission can begin in the second or third OFDM symbol. The cell
specific reference signals are transmitted on designated
subcarriers in the first, fifth, eighth, and twelfth symbol in each
subframe.
[0041] In order to establish a connection with the LTE network 10,
the mobile terminal 100 needs to find and acquire synchronization
with a cell 20, 30 within the network 10, read system parameters
from a broadcast channel in the cell 20, 30, and perform a random
access procedure to establish a connection with the cell 20, 30.
The first of these steps is commonly referred to as cell search. To
assist the mobile terminal 100 in the cell search procedure, the
base station 200 transmits two synchronization signals on the
downlink: the Primary Synchronization Signal (PSS) and the
Secondary Synchronization Signal (SSS). For LTE radio frame
structure type 1 (used for FDD deployments), the PSS and SSS are
transmitted within the two last OFDM symbols in the first slot of
subframes 0 and 5. For LTE radio frame structure type 2 (used for
TDD deployments), the SSS is transmitted in the last OFDM symbol of
subframe 0 and 5, whereas PSS is transmitted within the third OFDM
symbol of subframe 1 and 6. The synchronization signals are
transmitted in the center of the system bandwidth, occupying six
resource blocks in the frequency domain. The difference in the
positions of the synchronization signals in the case of FDD and TDD
allows for the detection of the duplex scheme used on a carrier if
this is not known in advance.
[0042] The synchronization signals transmitted in each cell 20, 30
comprise a specific set of sequences that define the cell identity.
There are 504 different physical cell identities (PCIs) defined for
LTE, where each PCI corresponds to one specific downlink
reference-signal sequence. The PCIs are further divided into 168
cell-identity groups, with three PCIs within each group. The LTE
standard specifies the location in time and frequency of the
synchronization signals. The time-domain positions of the
synchronization signals within the frame differ somewhat depending
on if the cell 20, 30 is using frequency-division duplex (FDD) or
time-division duplex (TDD). By detecting the synchronization
signals, the mobile terminal 100 will acquire the timing of a cell
20, 30, and, by observing which of multiple sequences the cell is
transmitting, the mobile terminal 100 can identify the cell 20, 30.
Once the mobile terminal 100 has acquired frame timing and the PCI
of the cell 20, 30, the mobile terminal 100 has identified the
cell-specific reference signal and can receive the necessary system
information for accessing the cell 20, 30.
[0043] A mobile terminal 100 does not carry out cell search only at
power-up, i.e., when initially accessing the system. In order to
support mobility, the mobile terminals 100 need to continuously
search for, synchronize to, and estimate the reception quality of
signals transmitted by neighbor cells. The mobile terminals 100 may
evaluate the reception quality of signals from the neighbor cells,
in comparison to the reception quality of the current serving cell,
to determine whether a handover (for mobile terminals 100 in
connected mode) or cell re-selection (for mobile terminals 100 in
idle mode) should be carried out. For mobile terminals 100 in
connected mode, the network 10 makes the handover decision based on
measurement reports provided by the mobile terminals 100. As noted
previously, the cell specific reference signals may be used by the
mobile terminal 100 to perform the measurements.
[0044] The measurement reports provided by the mobile terminal 100
may include measurements of the reference signal received power
(RSRP) and/or reference signal received quality (RSRQ). Depending
on how these measurements, possibly complemented by a configurable
offset, are used, the mobile terminal 100 can be connected to the
cell 20, 30 with the strongest received power, or the cell 20, 30
with the lowest path loss, or a combination of the two. These
selection criteria (received power and path loss) do not
necessarily result in the same selected cell 20, 30. Because the
output power varies for different types of cells 20, 30, it is
possible that, for a given mobile terminal 100, the cells 20, 30
with the highest RSRP and RSRQ measurements and the cells 20, 30
with the lowest path loss are different. This situation is referred
to herein as link imbalance.
[0045] FIG. 5 illustrates how link imbalance can occur in a
heterogeneous network 10. It is realistically presumed for purposes
of this example that the output power of a pico base station 300 in
the pico cell 30 is in the order of 30 dBm or less, while the
output power of the macro base station 200 is in the order of 46
dBm. Consequently, when the mobile terminal 100 is operating near
the cell edge of the pico cell 30, the received signal strength
from the macro cell 20 can be much larger than that of the pico
cell 30.
[0046] However, the path loss to the base station 200 in the macro
cell 20 may be greater than the path loss to the pico base station
300 in the pico cell 30.
[0047] In FIG. 5, the downlink border indicates the point at which
the received signal strengths from the macro cell 20 and pico cell
30 are equal. The uplink border indicates the point at which the
path losses to the base stations 200, 300 in the macro cell 20 and
pico cell 30 respectively are equal. The region between the DL and
UL borders is the link imbalance zone. From a downlink perspective,
it may be better for a mobile terminal 100 in the link imbalance
zone to select a cell 20, 30 based on downlink received power; but
from an uplink perspective, it may be better to select a cell 20,
30 based on the path loss because the transmit power of the mobile
terminal 100 is limited. In this scenario, it might be preferable
from a system perspective for the mobile terminal 100 to connect to
the pico cell 30 even if the macro downlink is up to 10-20 dB
stronger than the pico cell downlink. However, inter-cell
interference coordination (ICIC) between macro and pico layers is
necessary when the mobile terminal 100 is operating within the link
imbalance zone.
[0048] One approach of providing ICIC across layers is illustrated
in FIG. 6, where an interfering macro cell 20 does not transmit
PDCCHs, and thus no data, in some subframes. The pico cell 30 is
aware of the locations of these "blank" subframes and can schedule
downlink transmissions to cell edge mobile terminal 100 (mobile
terminals 100 operating within the link imbalance zone) in
subframes aligned with the blank subframes at the macro layer 30.
For legacy mobile terminals, the macro cell 20 will still need to
transmit cell specific reference symbols in all subframes so the
blank subframes will not be completely empty. Pico cell users
operating inside the DL border can be scheduled in all
subframes.
[0049] One drawback to the ICIC approach shown in FIG. 6 is that
the mobile terminal 100 may not be aware of scheduling restrictions
and does not know what downlink subframes to monitor. Thus, the
mobile terminal 100 typically decodes the PDCCH in all subframes,
which may be unnecessary if subframe scheduling restrictions apply
to the mobile terminal 100. Also, because the received signals from
the macro cell may be up to 10 dB stronger than the received
signals from the pico cell 30, the mobile terminal 100 might not be
able to maintain synchronization with the pico cell 30 and thus
unable to decode the PDCCH transmitted by the pico cell 30. Another
drawback is that there is currently no method to detect when the
mobile terminal 100 is in a link imbalance zone and therefore when
scheduling restrictions should be applied.
[0050] In embodiments of the present invention, scheduling
restrictions may be applied to a mobile terminal 100 when the
mobile terminal 100 is operating in a link imbalance zone. When
subframe scheduling restrictions apply, the downlink transmissions
to the mobile terminal 100 may be restricted to a predetermined
subset of the subframes, referred to herein as the restricted
subframes. The macro cells 20 avoid transmission of the PDCCH and
user data in the restricted subframes. Scheduling information is
transmitted to the mobile terminal 100 to indicate the subframes
that can be used to transmit to mobile terminals 100 when
scheduling restriction apply. When the mobile terminal 100 is
within the link imbalance zone, and thus subject to scheduling
restrictions, the mobile terminal 100 does not need to monitor the
PDCCH in other subframes. Also, a mechanism is provided for
detecting when a mobile terminal 100 is in the link imbalance zone.
The detection can be made either by the mobile terminal 100, which
then informs the network that a change in scheduling should apply,
or by the pico base station 300 itself. The change of scheduling
can be signaled in several ways, for instance through Radio
Resource Control (RRC) or (MAC) signaling.
[0051] FIG. 7 illustrates an exemplary method 400 implemented by a
mobile terminal 100 of decoding a control channel. The procedure
begins when the mobile terminal 100 is connected to a base station
200, 300 in the network (block 410). The mobile terminal 100 could
be connected to either a macro base station 200 or pico base
station 300. While the mobile terminal 100 is connected, the mobile
terminal 100 receives scheduling information identifying restricted
subframes used by a pico cell 30 to transmit downlink data to
mobile terminals 100 operating in a link imbalance zone (block
420). The scheduling information can be transmitted from either the
macro cell 20 or the pico cell 30. For example, the scheduling
information may be received by the mobile terminal 100 as an RRC
message before or during a handover from a macro cell 20 to a pico
cell 30. The mobile terminal 100 could also read the scheduling
information on a broadcast channel (MIB or SIB) broadcast by the
macro cell 20 or pico cell 30.
[0052] When the mobile terminal 100 connects with a base station
200, 300, the mobile terminal 100 determines whether it is
connected to a macro base station 200 or pico base station 300
(block 430). This determination should be made each time the mobile
terminal 100 moves between cells 20, 30. If the serving base
station 200, 300 is a macro base station 300 and scheduling
restriction apply because of nearby pico cells 30, the mobile
terminal 100 may decode the PDCCH only in the unrestricted
subframes (block 440). Because the PDCCH is not transmitted by the
macro cells 20 in the restricted subframes, there is no need to
decode the PDCCH in the restricted subframes. If there are no
scheduling restrictions imposed on the macro cell 20, the mobile
terminal 100 should decode the PDCCH in all subframes.
[0053] If the mobile terminal 100 is served by a pico base station
300, the mobile terminal 100 next determines if it is in a link
imbalance zone (block 450). The mobile terminal 450 may determine
itself whether it is in a link imbalance zone or, alternatively,
the network may make the determination and transmit a control
signal to the mobile terminal 100 to indicate when it is in a link
imbalance zone. In the later case, the mobile terminal 100 makes
the determination based on the control signals received from the
serving pico base station 300. If the mobile terminal 100 is not in
a link imbalance zone, the mobile terminal 100 decodes the PDCCH in
all subframes (block 460). On the other hand, if the mobile
terminal 100 is in a link imbalance zone, the mobile terminal
decodes the PDCCH only in the restricted subframes (block 470).
[0054] FIG. 8 illustrates an exemplary measurement reporting
procedure 500 for implementing subframe scheduling restrictions.
This procedure 500 is performed by a mobile terminal 100 when the
mobile terminal 100 is being served by a pico cell 30 (block 510).
The general idea is for the mobile terminal 100 to perform signal
quality measurements on both macro cells 20 and pico cells 30 and
trigger scheduling changes by sending a measurement report if
certain conditions are met. The measurement procedure shown in FIG.
8 begins with the assumption that scheduling restrictions apply,
although the reverse could be assumed in some circumstances as
described below. The mobile terminal 100 measures on a regular
basis (example once every 40 ms) the received signal strength (RSRP
or RSRQ for instance) of signals transmitted by the pico cell 20 in
the restricted subframes (block 520). The mobile terminal 100 also
on a regular basis measures the received signal strength on
neighboring cells, especially macro cells 20 having coverage areas
that overlap with the pico cell 20 (block 530). Preferably the
measurements are done in unrestricted subframes; however, the
invention is not limited to that case.
[0055] The mobile terminal 100 compares the signal strength of the
received signals from the pico cell 30 to the received signal
strength of the signals from the macro cell 30 (block 540). For
mobile terminals 100 served by pico cells 30, reasonable downlink
performance in the unrestricted subframes can be achieved if the
RSRP of the pico cell 30 is about 3-4 dB lower than RSRP of the
macro cell 20. Thus, the value of .DELTA. should be in the range of
2-5 dB. If the signal strength of the received signals from the
pico cell 30 is sufficiently large compared to the received signal
strength of the signals from the macro cell 30, the mobile terminal
100 sends a measurement report to the network to trigger a
scheduling change (block 550). The measurement report could be
transmitted, for example, as part of Medium Access Control (MAC) or
RRC signaling. The pico base station 300 receiving the measurement
report may change the scheduling strategy and inform the mobile
terminal 100 via RRC or MAC signaling. If the signal strength of
the pico cell 30 is not strong enough based on the decision
criteria, the procedure ends without the mobile terminal 100
sending a measurement report, in which case no change in scheduling
will be made (block 560).
[0056] If the mobile terminal 100 is operating without scheduling
restrictions, the mobile terminal 100 may continue to monitor the
signal strength of the macro cells 20 and pico cells 30. The same
decision criteria as shown in FIG. 8 may be applied. However, in
this case, the mobile terminal 100 sends a measurement report when
the decision criteria is no longer met to trigger another
scheduling change.
[0057] FIG. 9 illustrates an exemplary scheduling procedure 600
implemented by a network node, e.g., pico base station 300. This
procedure 600 is performed when the mobile terminal 100 is being
served by a pico cell 30 (block 610). The general idea is for the
network node to measure or otherwise determine the path loss
between the mobile terminal 100 and pico cell 30 (block 620), and
to trigger scheduling changes based on the path loss (block 630).
When the mobile terminal 100 is served by a pico cell 30, the pico
base station 300 measures the path loss from the mobile terminal
100 (block 620). The path loss could be estimated in several ways.
For example, the mobile terminal 100 may transmit sounding
reference symbols on the uplink. The pico base station 300 may send
transmit power control commands (TPC) to the mobile terminal 100 to
control the transmit power of the sounding reference signals. In
LTE, the TPC could include explicit values for the mobile terminal
transmit power. The pico base station 300 can then determine the
path loss based on the received signal strength. Based on the path
loss, the pico base station 300 determines whether the mobile
terminal 100 could be scheduled without restrictions. For example,
the pico base station 300 may compare the path loss to a threshold
(block 640). If the path loss is less than the threshold, the pico
base station 300 may schedule the mobile terminal in any subframes
(block 650). On the other hand, if the path loss is greater than
the threshold, the pico base station 300 may decide to schedule the
mobile terminal 100 only in the restricted subframes (block 660).
When a scheduling change is made, the mobile terminal 100 is
informed via RRC or MAC signaling as described above.
[0058] FIG. 10 illustrates an alternate procedure 700 implemented
by a pico base station 300 for determining when scheduling
restrictions should be applied to a mobile terminal 100 served by
the pico base station 300. This procedure 700 is performed when the
mobile terminal 100 is being served by a pico cell 30 (block 710).
The basic idea in this embodiment is for the mobile terminal 100 to
monitor the signal strength of signals from the macro base station
200 and pico base station 300, and transmit a measurement report to
the pico base station (block 720). Based on the measurement report,
the pico base station 300 may determine whether the mobile terminal
is in a link imbalance zone (block 730). If so, the As previously
noted, the pico base station 300 may schedule downlink
transmissions to the mobile terminal 100 only in the restricted
subframes when the mobile terminal 100 is in the link imbalance
zone (block 740). Otherwise, the pico base station 300 may schedule
the mobile terminal in any subframes. When a scheduling change is
made, the mobile terminal 100 is informed via RRC or MAC signaling
as described above.
[0059] FIG. 11 illustrates an alternate procedure 800 implemented
by a pico base station 300 for determining when scheduling
restrictions should be applied to a mobile terminal 100 served by
the pico base station 300. This procedure 700 is performed when the
mobile terminal 100 is being served by a pico cell 30 (block 810).
The basic idea in this embodiment is for the mobile terminal 100 to
monitor the signal strength of signals from the macro base station
200 and pico base station 300, and transmit an indication to the
pico base station 300 when the mobile terminal is in a link
imbalance zone (block 820). As previously noted, the indication may
comprise or be included in a measurement report sent by the mobile
terminal 100 to the pico base station 300. In response to the
indication, the pico base station 300 may schedule downlink
transmissions to the mobile terminal 100 only in the restricted
subframes (block 830). When a scheduling change is made, the mobile
terminal 100 is informed via RRC or MAC signaling as described
above.
[0060] FIG. 12 illustrates an exemplary mobile terminal 100 that
implements the extended cell search procedure described herein. The
mobile terminal 100 comprises a transceiver 110, control circuit
120, and user interface 130. The transceiver 110 comprises a
standard cellular transceiver according to the LTE standard, or
other standard now known or later developed, which supports
extended cell search procedures. The control circuit 120 controls
the operation of the mobile terminal 100 based on instructions
stored in memory (not shown). The control circuit 120 may comprise
one or more processors, hardware, firmware, or a combination
thereof. The control circuit 120 is configured to implement the
procedure as shown in FIGS. 4 and 5. Program instructions to
implement the extended cell search procedure may be stored in some
form of persistent memory (e.g., read-only memory, flash memory,
etc.). The control circuit 120 may also include random access
memory to store temporary data. The user interface 130 typically
comprises a display and one or more input devices to enable the
user to interact with and control the mobile terminal 100. The user
input devices 130 may include a keypad, touchpad, function keys,
scroll wheels, or other similar input devices. If the mobile
terminal 100 includes a touchscreen display, the touchscreen
display may also function as a user input device.
[0061] FIG. 13 illustrates an exemplary base station 200, 300 for
communicating with the mobile terminal 100. The base station 200,
300 comprises an antenna 210, 310 coupled to a transceiver 220, 320
and a control circuit 230, 330. The transceiver 220, 320 comprises
a standard cellular transceiver operating according to the LTE
standard, or other standard now known or later developed,
supporting extended cell search procedures. The control circuit
230, 330 controls the operation of the base station 200, 300. The
functions performed by the control circuit 230, 330 include radio
resource control and mobility management functions. The control
circuit 230, 330 may be implemented by one or more processors,
hardware, firmware, or a combination thereof. The control circuit
230, 330 is configured to implement the procedure as shown in FIG.
3. Program instructions to implement the extended cell search
procedure may be stored in some form of persistent memory (e.g.,
read-only memory). The control circuit may also include random
access memory to store temporary data.
[0062] The present invention provides a method for identifying the
restricted subframes to the mobile terminal 100. The invention also
provides a method for detecting when the mobile terminal is in a
link imbalance zone and for signaling scheduling changes to the
mobile terminal 100. Thus, the mobile terminal 100 in the link
imbalance zone can decode the PDCCH only in the restricted
subframes.
[0063] The present invention may, of course, be carried out in
other specific ways than those herein set forth without departing
from the scope and essential characteristics of the invention. The
present embodiments are, therefore, to be considered in all
respects as illustrative and not restrictive, and all changes
coming within the meaning and equivalency range of the appended
claims are intended to be embraced therein.
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