U.S. patent application number 12/563589 was filed with the patent office on 2010-10-14 for inter-cell interference mitigation.
Invention is credited to Jiann-Ching Guey, Christian Hoymann, Kambiz Zangi.
Application Number | 20100261493 12/563589 |
Document ID | / |
Family ID | 42934804 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100261493 |
Kind Code |
A1 |
Guey; Jiann-Ching ; et
al. |
October 14, 2010 |
INTER-CELL INTERFERENCE MITIGATION
Abstract
A method and Coordinated Multi-Point (CoMP) cell controller for
reducing interference in a wireless communication network in which
a first CoMP cell neighbors a second CoMP cell. The CoMP cell
controller in the first CoMP cell gathers scheduling information
from border sub-cells in the neighboring second CoMP cell in which
transmissions to and from UEs cause inter-CoMP cell interference in
at least one sub-cell in the first CoMP cell. The CoMP cell
controller augments intra-CoMP cell scheduling information in the
first CoMP cell with the scheduling information from the border
sub-cells to create augmented scheduling information. The CoMP cell
controller then utilizes the augmented scheduling information to
schedule transmissions to and from UEs within the at least one
sub-cell in the first CoMP cell to reduce the inter-CoMP cell
interference.
Inventors: |
Guey; Jiann-Ching; (Cary,
NC) ; Zangi; Kambiz; (Chapel Hill, NC) ;
Hoymann; Christian; (Aachen, DE) |
Correspondence
Address: |
ERICSSON INC.
6300 LEGACY DRIVE, M/S EVR 1-C-11
PLANO
TX
75024
US
|
Family ID: |
42934804 |
Appl. No.: |
12/563589 |
Filed: |
September 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61167925 |
Apr 9, 2009 |
|
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Current U.S.
Class: |
455/501 |
Current CPC
Class: |
H04W 72/1278 20130101;
H04B 7/024 20130101; H04W 16/02 20130101; H04W 72/1226
20130101 |
Class at
Publication: |
455/501 |
International
Class: |
H04B 15/00 20060101
H04B015/00 |
Claims
1. A method of reducing interference in a wireless communication
network comprising a first Coordinated Multi-Point (CoMP) cell and
a neighboring second CoMP cell, wherein each CoMP cell comprises a
plurality of contiguous sub-cells and a CoMP cell controller
operative to manage transmissions within the sub-cells of the CoMP
cell to minimize intra-CoMP cell interference, the method
comprising the steps of: gathering by the first CoMP cell
controller, scheduling information for an external sub-cell in the
neighboring second CoMP cell, said external sub-cell being located
such that transmissions to and from user equipments (UEs) within
the external sub-cell cause inter-CoMP cell interference in a given
sub-cell in the first CoMP cell; augmenting intra-CoMP cell
scheduling information in the first CoMP cell with the scheduling
information for the external sub-cell to create augmented
scheduling information; and utilizing the augmented scheduling
information to schedule, by the first CoMP cell controller,
transmissions to and from UEs within the given sub-cell in the
first CoMP cell to reduce the inter-CoMP cell interference.
2. The method according to claim 1, wherein the external sub-cell
in the neighboring second CoMP cell is a border sub-cell adjacent
to the given sub-cell in the first CoMP cell.
3. The method according to claim 1, wherein the gathering step
includes gathering the scheduling information through one of: a
switched backbone network that connects the first CoMP cell
controller with the second CoMP cell controller; and a dedicated
point-to-point connection between the first CoMP cell controller
and the second CoMP cell controller.
4. The method according to claim 1, wherein the gathering step
includes gathering a list of the UEs selected to transmit or
receive in the external sub-cell in a next transmit/receive
phase.
5. The method according to claim 4, wherein the augmented
scheduling information also includes one or more of a transmit
vector, a receive vector, a pre-coding matrix, channel matrices, or
statistics of the Additive White Gaussian Noise (AWGN).
6. A method of reducing interference in a wireless communication
network comprising a first Coordinated Multi-Point (CoMP) cell and
a neighboring second CoMP cell, wherein each CoMP cell comprises a
plurality of contiguous sub-cells and a CoMP cell controller
operative to manage transmissions within the sub-cells of the CoMP
cell to minimize intra-CoMP cell interference, the method
comprising the steps of: gathering by the first CoMP cell
controller, scheduling information from a plurality of external
sub-cells in the neighboring second CoMP cell, said external
sub-cells being located such that transmissions to and from user
equipments (UEs) within the external sub-cells cause inter-CoMP
cell interference in at least one sub-cell in the first CoMP cell;
augmenting intra-CoMP cell scheduling information in the first CoMP
cell with the scheduling information from the plurality of external
sub-cells to create augmented scheduling information; and utilizing
the augmented scheduling information to schedule, by the first CoMP
cell controller, transmissions to and from UEs within the at least
one sub-cell in the first CoMP cell to reduce the inter-CoMP cell
interference.
7. The method according to claim 6, wherein the plurality of
external sub-cells in the neighboring second CoMP cell are border
sub-cells adjacent to the at least one sub-cell in the first CoMP
cell.
8. The method according to claim 6, wherein the gathering step
includes gathering the scheduling information through one of: a
switched backbone network that connects the first CoMP cell
controller with the second CoMP cell controller; and a dedicated
point-to-point connection between the first CoMP cell controller
and the second CoMP cell controller.
9. The method according to claim 6, wherein the gathering step
includes gathering a list of the UEs selected to transmit or
receive in each of the external sub-cells in a next
transmit/receive phase.
10. The method according to claim 9, wherein the augmented
scheduling information also includes one or more of a transmit
vector, a receive vector, a pre-coding matrix, channel matrices, or
statistics of the Additive White Gaussian Noise (AWGN).
11. A method of reducing interference in a wireless communication
network comprising a first Coordinated Multi-Point (CoMP) cell
having a plurality of contiguous sub-cells and a first CoMP cell
controller operative to manage transmissions within the sub-cells
of the first CoMP cell to minimize intra-CoMP cell interference, a
neighboring second CoMP cell having a plurality of contiguous
sub-cells and a second CoMP cell controller operative to manage
transmissions within the sub-cells of the second CoMP cell to
minimize intra-CoMP cell interference, and a neighboring third CoMP
cell that neighbors both the first and second CoMP cells and
includes a plurality of contiguous sub-cells and a third CoMP cell
controller operative to manage transmissions within the sub-cells
of the third CoMP cell to minimize intra-CoMP cell interference,
the method comprising the steps of: defining a hierarchical
scheduling order in which the first CoMP cell controller schedules
transmissions in the sub-cells of the first CoMP cell, and then the
second CoMP cell controller schedules transmissions in the
sub-cells of the second CoMP cell, and then the third CoMP cell
controller schedules transmissions in the sub-cells of the third
CoMP cell; after the transmissions in the sub-cells of the first
CoMP cell are scheduled, gathering by the second CoMP cell
controller, scheduling information for sub-cells in the first CoMP
cell that border the second CoMP cell; augmenting intra-CoMP cell
scheduling information in the second CoMP cell with the scheduling
information from the bordering sub-cells in the first CoMP cell to
create augmented scheduling information for the second CoMP cell;
and utilizing the augmented scheduling information for the second
CoMP cell to schedule, by the second CoMP cell controller,
transmissions to and from UEs within the sub-cells of the second
CoMP cell to reduce the inter-CoMP cell interference with the first
CoMP cell; after the transmissions in the sub-cells of the second
CoMP cell are scheduled, gathering by the third CoMP cell
controller, scheduling information for sub-cells in the first and
second CoMP cells that border the third CoMP cell; augmenting
intra-CoMP cell scheduling information in the third CoMP cell with
the scheduling information from the bordering sub-cells in the
first and second CoMP cells to create augmented scheduling
information for the third CoMP cell; and utilizing the augmented
scheduling information for the third CoMP cell to schedule, by the
third CoMP cell controller, transmissions to and from UEs within
the sub-cells of the third CoMP cell to reduce the inter-CoMP cell
interference with the first and second CoMP cells.
12. A Coordinated Multi-Point (CoMP) cell controller in a first
CoMP cell for reducing interference in a wireless communication
network in which the first CoMP cell neighbors a second CoMP cell,
wherein each CoMP cell comprises a plurality of contiguous
sub-cells, the CoMP cell controller comprising: means for gathering
scheduling information for an external sub-cell in the neighboring
second CoMP cell, said external sub-cell being located such that
transmissions to and from user equipments (UEs) within the external
sub-cell cause inter-CoMP cell interference in a given sub-cell in
the first CoMP cell; means for augmenting intra-CoMP cell
scheduling information in the first CoMP cell with the scheduling
information for the external sub-cell to create augmented
scheduling information; and means for utilizing the augmented
scheduling information to schedule transmissions to and from UEs
within the given sub-cell in the first CoMP cell to reduce the
inter-CoMP cell interference.
13. The CoMP cell controller according to claim 12, wherein the
external sub-cell in the neighboring second CoMP cell is a border
sub-cell adjacent to the given sub-cell in the first CoMP cell.
14. The CoMP cell controller according to claim 12, wherein the
means for gathering includes an interface to one of: a switched
backbone network that connects the first CoMP cell controller with
the second CoMP cell controller; and a dedicated point-to-point
connection between the first CoMP cell controller and the second
CoMP cell controller.
15. The CoMP cell controller according to claim 12, wherein the
scheduling information includes a list of the UEs selected to
transmit or receive in the external sub-cell in a next
transmit/receive phase.
16. The CoMP cell controller according to claim 15, wherein the
augmented scheduling information also includes one or more of a
transmit vector, a receive vector, a pre-coding matrix, channel
matrices, or statistics of the Additive White Gaussian Noise
(AWGN).
17. A Coordinated Multi-Point (CoMP) cell controller in a first
CoMP cell for reducing interference in a wireless communication
network in which the first CoMP cell neighbors a second CoMP cell,
wherein each CoMP cell comprises a plurality of contiguous
sub-cells, the CoMP cell controller comprising: means for gathering
scheduling information from a plurality of external sub-cells in
the neighboring second CoMP cell, said external sub-cells being
located such that transmissions to and from user equipments (UEs)
within the external sub-cells cause inter-CoMP cell interference in
at least one sub-cell in the first CoMP cell; means for augmenting
intra-CoMP cell scheduling information in the first CoMP cell with
the scheduling information from the plurality of external sub-cells
to create augmented scheduling information; and means for utilizing
the augmented scheduling information to schedule transmissions to
and from UEs within the at least one sub-cell in the first CoMP
cell to reduce the inter-CoMP cell interference.
18. The CoMP cell controller according to claim 17, wherein the
plurality of external sub-cells in the neighboring second CoMP cell
are border sub-cells adjacent to the at least one sub-cell in the
first CoMP cell.
19. The CoMP cell controller according to claim 17, wherein the
means for gathering includes an interface to one of: a switched
backbone network that connects the first CoMP cell controller with
the second CoMP cell controller; and a dedicated point-to-point
connection between the first CoMP cell controller and the second
CoMP cell controller.
20. The CoMP cell controller according to claim 17, wherein the
scheduling information includes a list of the UEs selected to
transmit or receive in each of the external sub-cells in a next
transmit/receive phase.
21. The CoMP cell controller according to claim 20, wherein the
augmented scheduling information also includes one or more of a
transmit vector, a receive vector, a pre-coding matrix, channel
matrices, or statistics of the Additive White Gaussian Noise
(AWGN).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/167,925 filed Apr. 9, 2009.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM
LISTING COMPACT DISC APPENDIX
[0003] Not Applicable
BACKGROUND
[0004] The present invention relates to wireless cellular
telecommunication systems. More particularly, and not by way of
limitation, the invention is directed to a system and method for
mitigating inter-cell interference in Coordinated Multi-Point
cells.
[0005] Inter-cell interference in a wireless cellular
telecommunication network is one of the most dominant sources for
performance impairment. Traditional approaches to mitigating this
impairment include measures such as frequency reuse and spread
spectrum. More recently, Inter-Cell Interference Coordination
(ICIC) solutions that rely on the ad hoc coordination of multiple
cells have also been proposed. See, for example, the Ericsson
Contribution, "On Inter-cell Interference Coordination Schemes
without/with Traffic Load Indication," 3GPP TSG-RAN WG1
R1-072456.
SUMMARY
[0006] The present invention provides a system and method for
reducing inter-cell interference within a CoMP architecture, which
overcomes the shortcomings of the prior art.
[0007] In one embodiment, the present invention is directed to a
method of reducing interference in a wireless communication network
comprising a first CoMP cell and a neighboring second CoMP cell,
wherein each CoMP cell comprises a plurality of contiguous
sub-cells and a CoMP cell controller operative to manage
transmissions within the sub-cells of the CoMP cell to minimize
intra-CoMP cell interference. The first CoMP cell controller
gathers scheduling information for an external sub-cell in the
neighboring second CoMP cell, wherein the external sub-cell is
located such that transmissions to and from user equipments (UEs)
within the external sub-cell cause inter-CoMP cell interference in
a given sub-cell in the first CoMP cell. Intra-CoMP cell scheduling
information in the first CoMP cell is augmented with the scheduling
information for the external sub-cell to create augmented
scheduling information, and the first CoMP cell controller utilizes
the augmented scheduling information to schedule transmissions to
and from UEs within the given sub-cell in the first CoMP cell to
reduce the inter-CoMP cell interference.
[0008] In another embodiment, the present invention is directed to
a method of reducing interference in a wireless communication
network comprising first, second, and third neighboring CoMP cells.
The method includes the steps of defining a hierarchical scheduling
order in which the first CoMP cell controller schedules
transmissions in the sub-cells of the first CoMP cell, and then the
second CoMP cell controller schedules transmissions in the
sub-cells of the second CoMP cell, and then the third CoMP cell
controller schedules transmissions in the sub-cells of the third
CoMP cell; and after the transmissions in the sub-cells of the
first CoMP cell are scheduled, gathering by the second CoMP cell
controller, scheduling information for sub-cells in the first CoMP
cell that border the second CoMP cell. The method also includes
augmenting intra-CoMP cell scheduling information in the second
CoMP cell with the scheduling information from the bordering
sub-cells in the first CoMP cell to create augmented scheduling
information for the second CoMP cell; and utilizing the augmented
scheduling information for the second CoMP cell to schedule, by the
second CoMP cell controller, transmissions to and from UEs within
the sub-cells of the second CoMP cell to reduce the inter-CoMP cell
interference with the first CoMP cell. After the transmissions in
the sub-cells of the second CoMP cell are scheduled, the third CoMP
cell controller gathers scheduling information for sub-cells in the
first and second CoMP cells that border the third CoMP cell.
Intra-CoMP cell scheduling information in the third CoMP cell is
augmented with the scheduling information from the bordering
sub-cells in the first and second CoMP cells to create augmented
scheduling information for the third CoMP cell; and the third CoMP
cell controller utilizes the augmented scheduling information for
the third CoMP cell to schedule transmissions to and from UEs
within the sub-cells of the third CoMP cell to reduce the
inter-CoMP cell interference with the first and second CoMP
cells.
[0009] In another embodiment, the present invention is directed to
a CoMP cell controller in a first CoMP cell for reducing
interference in a wireless communication network in which the first
CoMP cell neighbors a second CoMP cell. The CoMP cell controller
includes means for gathering scheduling information from a
plurality of external sub-cells in the neighboring second CoMP
cell, wherein the external sub-cells are located such that
transmissions to and from UEs within the external sub-cells cause
inter-CoMP cell interference in at least one sub-cell in the first
CoMP cell. The CoMP cell controller also includes means for
augmenting intra-CoMP cell scheduling information in the first CoMP
cell with the scheduling information from the plurality of external
sub-cells to create augmented scheduling information; and means for
utilizing the augmented scheduling information to schedule
transmissions to and from UEs within the at least one sub-cell in
the first CoMP cell to reduce the inter-CoMP cell interference.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0010] In the following section, the invention will be described
with reference to exemplary embodiments illustrated in the figures,
in which:
[0011] FIG. 1 is an illustrative drawing of a simple example of a
Coordinated Multi-Point (CoMP) cell with a schedule of users;
[0012] FIG. 2 is an illustrative drawing of three adjacent CoMP
cells;
[0013] FIG. 3 is an illustrative drawing of a grouping of CoMP
cells in a system deployment in an embodiment of the present
invention;
[0014] FIGS. 4A and 4B are illustrative drawings of a scheduling
hierarchy among a grouping of CoMP cells in an exemplary embodiment
of the present invention;
[0015] FIG. 5 is a flow chart illustrating the steps of an
embodiment of the method of the present invention;
[0016] FIG. 6 is a simplified block diagram of an exemplary
embodiment of a CoMP cell controller according to the teachings of
the present invention; and
[0017] FIG. 7 is a graphical representation of the uplink
performance of the present invention compared to a conventional
method.
DETAILED DESCRIPTION
[0018] One solution to mitigate inter-cell interference is to
connect multiple cells to a central controller unit, which
coordinates the transmission and reception to and from the User
Equipments (UEs) so that interference can be avoided by scheduling,
or can be actively suppressed using signal processing techniques.
This type of solution may be referred to as Coordinated Multi-Point
(CoMP) transmission and reception. In this context, a "CoMP cell"
is a collection of geographically contiguous cells, referred to as
sub-cells, connected to the same central control unit.
[0019] FIG. 1 illustrates a simple example of a CoMP cell 10
comprising seven sub-cells numbered 0 through 6, a CoMP cell
controller 11, and a "schedule" 12 generated by the CoMP cell
controller. The schedule is essentially a list of users being
served at different times. The triangles represent the UEs in
different cells scheduled in a first time interval, the squares
represent the UEs scheduled in a second time interval, and the
circles represent the UEs scheduled in a third time interval. There
is no particular relationship among UEs designated by the same
shape in this example; they just happen to be scheduled to be
served at the same time.
[0020] The information to be communicated to the CoMP cell
controller may be as simple as the path gains between the UEs and
the base stations. However, the optimal schedule list under certain
constraints may be difficult to determine since the typical number
of users in the system makes the hypothesis space prohibitively
large for an exhaustive search.
[0021] FIG. 2 is an illustrative drawing of three adjacent CoMP
cells (A, B, and C), each comprising a number of sub-cells. In a
CoMP architecture, the interference between sub-cells is only
coordinated or suppressed within each CoMP cell. In the context of
CoMP, this type of interference between sub-cells in a single CoMP
cell is referred to as intra-CoMP-cell interference. However, there
is no coordination between the larger CoMP cells, and thus the
border sub-cells, indicated by shading, may experience interference
from transmissions in the border sub-cells of neighboring CoMP
cells. The interference among multiple CoMP cells along the shaded
border areas shown in FIG. 1 is referred to as inter-CoMP-cell
interference. This type of interference still remains a major
source of performance impairment.
[0022] An ultimate solution is to connect all of the sub-cells in
all of the CoMP cells to a single central unit. However, this is
not feasible in practice, especially for a large system deployment.
The present invention provides an alternative solution that
approximates the optimal solution without having a global central
unit.
[0023] Within each CoMP cell, the interference is determined by the
path gains between the UEs and their surrounding sub-cells. The
central control unit scheduler attempts to group and order UEs to
be served simultaneously by their attached sub-cells in a way such
that a certain Signal-to-Interference-Ratio (SIR) target is
met.
[0024] It is assumed that each cell knows the path gain between the
base station and the UEs attached to it. The cells then communicate
this information to the central unit, which then forms a path gain
matrix:
G = [ g 11 g 12 g 1 N g 21 g 22 g 2 N g M 1 g M 2 g MN ] ,
##EQU00001##
where g.sub.mn is the path gain between the m'th UE and the n'th
base station. For each UE i.epsilon.{1, . . . , M}, there is a
corresponding strongest base station .sigma.(i), where
.sigma. ( i ) = argmax j .di-elect cons. { 1 , , N } g ij .
##EQU00002##
For each base station j.epsilon.{1, . . . , N}, there is a
corresponding strongest UE .mu.(j), where
.mu. ( j ) = argmax i .di-elect cons. { 1 , , M } g ij .
##EQU00003##
[0025] With this gain matrix, the central unit then performs the
scheduling. The schedule essentially instructs each base station
whether to transmit or not, and if so, to which UE and whether to
schedule an uplink UE.
[0026] The present invention extends this concept by gathering
additional information related to the scheduled UEs that are
outside the CoMP cell under consideration but close enough to cause
interference in the border sub-cells. The CoMP cell controller
subsequently processes the augmented information to reduce the
interference. The information communicated among neighboring CoMP
cells can be as simple as the schedule, which is a list of the UEs
selected to transmit or receive in the next transmit/receive phase.
The description below describes the contents of the additional
information, the way it is acquired, and how the augmented
information is processed.
[0027] From a signal processing perspective, a CoMP system can be
formulated as a linear system:
[ r 1 r 2 ] = [ h 11 h 12 h 21 h 22 ] [ s 1 s 2 ] + [ z 1 z 2 ] , (
1 ) ##EQU00004##
or more concisely,
r=Hs+z (2)
[0028] where r is the received vector, H is the instantaneous
channel state information, s is the transmit signal, and z is the
Additive White Gaussian Noise (AWGN).
[0029] In many applications, the transmit signal s may be, but is
not limited to, a linear function of the users' data symbols d
given by:
s=Wd, (3)
where W is sometimes referred to as a pre-coding matrix.
[0030] Note that these values are also a function of the frequency.
This dependency is omitted for simplicity. The extension of what is
disclosed here to a frequency selective system is
straightforward.
[0031] This simple linear equation formulation will be used for
both uplink and downlink calculations. For the uplink, the received
vector r is available at the central controller and can therefore
be jointly processed to detect the transmit vector s. For the
downlink, on the other hand, r is available at the UEs that are not
communicating with each other. The linear formulation is then used
by the central controller to determine the transmit vector s such
that interference can be mitigated without any UE's
coordination.
[0032] In either case, the extent to which the elements in this
formulation are known to the central controller will determine how
well the system performs. One extreme case is that all sub-cells
are connected to one single central controller at which the entire
H matrix is available. Since there is only one CoMP cell, there is
no inter-CoMP cell interference to deal with. In practice, however,
there are multiple interfering CoMP cells that can only observe a
subset of the elements in equation (1). This subset of observation
can also be expressed in a matrix representation:
r.sub.D=H.sub.Ds.sub.D+z.sub.D. (4)
[0033] If, in addition to equation (4), the CoMP cell also has
access to some information from the neighboring CoMP cells, an
augmented linear equation can be formed:
r A .ident. [ r D r I ] = [ H D H DI H ID H II ] [ s D s I ] + [ z
D z I ] , .ident. H A s A + z A ( 5 ) ##EQU00005##
where r.sub.I is a subset of the received vector in neighboring
CoMP cells, Z.sub.I is the corresponding AWGN, and (H.sub.DI,
H.sub.ID, H.sub.II) are the associated channel matrices. The
present invention is concerned with such a basic architecture and
may be implemented in an exemplary embodiment as follows.
Information to be Communicated
[0034] The information communicated among neighboring CoMP cells
can be as simple as the schedule, which is a list of the UEs
selected to transmit or receive in the next transmit/receive phase.
The schedule essentially identifies the active elements in the
channel matrices. It can also be any part of the linear equation
including the channel matrices, the transmit vector, the pre-coding
matrix, the receive vector, or the statistics of the AWGN.
[0035] On the uplink, for example, the interfering CoMP cells may
inform the desired CoMP cell of the uplink UEs the interfering CoMP
cells have scheduled. Thus, the desired CoMP cell has:
r D = [ H D H DI ] [ s D s I ] + z D ( 6 ) ##EQU00006##
to work with. The matrix H.sub.DI is the uplink channel between the
sub-cells in the desired CoMP cell and the scheduled UEs in the
neighboring CoMP cells. The matrix H.sub.DI can be estimated from
the pilot signal transmitted by the UEs on the uplink.
[0036] On the downlink, the interfering CoMP cells may inform the
desired CoMP cells the UEs they have scheduled along with their
corresponding channel matrix H.sub.ID so that it has
[ r D r I ] = [ H D H ID ] s D + [ z D z I ] ( 7 ) ##EQU00007##
to work with. The desired CoMP cell can then design the transmit
signal s.sub.D to avoid causing interference to the scheduled UEs
in the neighboring CoMP cells. Note that the matrix H.sub.ID is the
downlink channel between the scheduled UEs in the neighboring CoMP
cells and the sub-cells in the desired CoMP cell. The matrix
H.sub.ID needs to be explicitly communicated in a Frequency
Division Duplex (FDD) system. However, if the system is a Time
Division Duplex (TDD) system, the matrix H.sub.ID can be measured
on the uplink based on the theory of reciprocity.
[0037] Finally, the desired CoMP cell may obtain all the channel
matrices in equation (5)Error! Reference source not found. from
methods to be described later and has access to the full augmented
linear equation:
r.sub.A=H.sub.As.sub.A+z.sub.A. (8)
[0038] The downlink transmission and uplink reception can then be
designed accordingly as if it were a self-contained, single CoMP
cell system.
Information Gathering
[0039] FIG. 3 is an illustrative drawing of a grouping of CoMP
cells in a system deployment in an embodiment of the present
invention. The schedules and channel matrices mentioned above may
be exchanged among the involved CoMP cells through an existing
backbone network or other suitable connections. In some cases, the
UEs may also serve as relays by communicating channel information
within the desired CoMP cell to neighboring non-serving CoMP cells.
Alternatively, the bordering sub-cells may be connected to multiple
neighboring CoMP cells as shown in FIG. 3.
[0040] Conventionally, each CoMP cell in FIG. 3 consists of the
non-overlapping sub-cells marked by different patterns. In
embodiments of the present invention, an architecture is utilized
in which the sub-cells in the border areas are connected to more
than one neighboring CoMP cell controllers. For example, sub-cells
{s.sub.02, s.sub.12, s.sub.22} are connected to CoMP cells
{C.sub.0, C.sub.1, C.sub.2}, and sub-cells {s.sub.01, s.sub.21} are
connected to CoMP cells {C.sub.0, C.sub.2}. This essentially
extends the observation beyond the original CoMP cell boundary into
neighboring CoMP cells. Therefore, from the observation point of
view, the extended CoMP cells have borders marked by the
overlapping thick lines. However, even though an extended CoMP cell
has access to the sub-cells in neighboring CoMP cells, it still
only processes the transmissions and receptions of the UEs within
its original border.
[0041] There are occasions when the scheduling and transmission
decisions of a CoMP cell depend on the decisions made by the
neighboring CoMP cells. For example, a CoMP cell may be able to
determine the optimal set of UEs to schedule if the UEs scheduled
in neighboring CoMP cells are known. In another example, a CoMP
cell may be able to best design its pre-coding matrix if the
pre-coding matrix of neighboring CoMP cells are known. In these
cases, the present invention may establish among the CoMP cells, a
hierarchical relationship in which scheduling information from a
first set of CoMP cells is passed to neighboring CoMP cells to
assist in their scheduling.
[0042] FIGS. 4A and 4B are illustrative drawings of a scheduling
hierarchy among a grouping of CoMP cells in an exemplary embodiment
of the present invention. In order to mitigate the inter-CoMP-cell
interference among border sub-cells in neighboring CoMP cells, the
multiple CoMP cells in a system deployment are grouped into a
number of mutually exclusive subsets such as A, B, and C. The CoMP
cells in each subset are sufficiently separated from each other
that no inter-cell interference occurs among them and therefore
they can be scheduled independently. Each subset then takes turn
scheduling transmissions in its sub-cells in a certain order (for
example, subset A, followed by subset B, and then subset C as shown
in the illustrated example). As each subset schedules its
transmissions, it avoids causing interference to subsets that have
already scheduled, and then passes sufficient information to the
remaining subsets so that the same interference avoidance measures
can be taken. The scheduling and information passing preferably all
transpire before the data transmission phase, which occurs once
every TTI.
[0043] Referring to FIG. 4A, the CoMP cells in subset A send
information regarding the scheduling of their border sub-cells to
all six of their neighboring CoMP cells so that the neighboring
CoMP cells can take adequate action to avoid generating
interference to the A cells. Once this process is completed, the B
cells broadcast such information to the C cells as shown in FIG.
4B. Because of the action that the B cells have taken, little or no
interference is generated to the A cells. The B cells only need to
broadcast the appropriate information to the C cells so that the C
cells can take the necessary action to avoid generating
interference to both the A cells and B cells. The C cells do not
need to broadcast any information since they are the last to
perform the scheduling action.
[0044] Referring again to FIG. 3, assume that the scheduling
hierarchy is left cross-hatched cells (C1,C3,C5) first, right
cross-hatched cells (C2,C4,C6) second, and then the vertically
cross-hatched cell (C0) third. CoMP-controller C1 may schedule the
sub-cells of its CoMP cell first. In particular, the weights of
sub-cell S12 are optimized in order to mitigate intra-COMP-cell
interference. Second, the CoMP-controller C2 schedules. Because
CoMP-controller C2 is connected to S12, C2 knows the weights of S12
and schedules neighbor sub-cell S22 and all other sub-cell S2x so
that inter-COMP-cell interference is mitigated. Of course the
performance of the COMP-cell is maximized and intra-COMP-cell
interference is considered in the scheduling decision as well.
Finally, the weights of S22 are set. Third, CoMP controller C0
schedules. COMP-controller C0 knows the weights of S12 and S22. So
the weights for S02 and all other sub-cell S0x are optimized taking
the schedules/weights of S22 and S12 into account. Finally the
sub-cells S02, S22 and S12 are cooperatively scheduled.
[0045] In the given example, the border sub-cell, e.g., a radio
head, will have a buffer that stores the commands (weights) from
the CoMP controllers so that they can be shared afterwards. The
weights and the associated schedule of bordering sub-cells may be
passed in the hierarchical order, for example, through a
switched/routed backbone network or through a dedicated
point-to-point or point-to-multipoint connection between a border
sub-cell and its neighboring CoMP cell controller.
Information Processing--Uplink
[0046] On the uplink, the pre-coding matrix is usually an identity
matrix since there is no coordination among the UEs to reshape the
transmit signal. The information processing then lies on the
receiver design based on the augmented observation. One simple
example is the MMSE receiver given by:
s.sub.D=E{s.sub.Ds.sub.A.sup.H}H.sub.A.sup.H(H.sub.AE{s.sub.As.sub.A.sup-
.H}H.sub.A.sup.H+E{z.sub.Az.sub.A.sup.H}).sup.-1r.sub.A (9)
where E{.} and ( ).sup.H denote the expected value and Hermitian
transpose respectively.
Information Processing--Downlink
[0047] On the downlink, the information processing depends on the
pre-coding matrix design. In one embodiment, each CoMP cell designs
the pre-coding matrix based on the augmented observation using
methods such as zero-forcing. For those sub-cells that are
connected to more than one CoMP cell, their transmission weights
are then set to the sum of the weights from the CoMP cells to which
they are connected.
[0048] FIG. 5 is a flow chart illustrating the steps of an
exemplary embodiment of the method of the present invention. The
illustrated embodiment is based on a scenario involving three CoMP
cells, each of which includes a plurality of contiguous sub-cells
and a CoMP cell controller operative to manage transmissions within
the sub-cells of each respective CoMP cell to minimize intra-CoMP
cell interference. At step 21, a hierarchical scheduling order is
defined in which the first CoMP cell controller schedules
transmissions in the sub-cells of the first CoMP cell, and then the
second CoMP cell controller schedules transmissions in the
sub-cells of the second CoMP cell, and then the third CoMP cell
controller schedules transmissions in the sub-cells of the third
CoMP cell. At step 22, the first CoMP cell controller schedules
transmissions in the sub-cells of the first CoMP cell. At step 23,
after the transmissions in the sub-cells of the first CoMP cell are
scheduled, the second CoMP cell controller gathers scheduling
information for sub-cells in the first CoMP cell that border the
second CoMP cell. At step 24, the intra-CoMP cell scheduling
information in the second CoMP cell is augmented with the
scheduling information from the bordering sub-cells in the first
CoMP cell to create augmented scheduling information for the second
CoMP cell. At step 25, the second CoMP cell controller utilizes the
augmented scheduling information for the second CoMP cell to
schedule transmissions to and from UEs within the sub-cells of the
second CoMP cell to reduce the inter-CoMP cell interference with
the first CoMP cell.
[0049] At step 26, after the transmissions in the sub-cells of the
second CoMP cell are scheduled, the third CoMP cell controller
gathers scheduling information for sub-cells in the first and
second CoMP cells that border the third CoMP cell. At step 27, the
intra-CoMP cell scheduling information in the third CoMP cell is
augmented with the scheduling information from the bordering
sub-cells in the first and second CoMP cells to create augmented
scheduling information for the third CoMP cell. At step 28, the
third CoMP cell controller utilizes the augmented scheduling
information for the third CoMP cell to schedule transmissions to
and from UEs within the sub-cells of the third CoMP cell to reduce
the inter-CoMP cell interference with the first and second CoMP
cells.
[0050] It should be noted that the augmented scheduling information
may also include one or more of a transmit vector, a receive
vector, a pre-coding matrix, channel matrices, or statistics of the
AWGN. This enables the second and third CoMP cell controllers to
output transmit waveform information for further minimizing
interference in addition to scheduling the transmissions.
[0051] FIG. 6 is a simplified block diagram of an exemplary
embodiment of the CoMP cell controller 11 according to the
teachings of the present invention. The CoMP cell controller is
communicatively linked to the plurality of base stations in the
sub-cells of the associated CoMP cell. A scheduling processor 31
schedules transmissions to and from UEs in the sub-cells of the
CoMP cell to minimize intra-CoMP-cell interference among the
sub-cells within the CoMP cell. As noted above, the processor may
also output transmit waveform information.
[0052] The scheduling processor 31 runs computer program
instructions stored on a memory 32. When the instructions are run,
the scheduling processor schedules transmissions such that no UE in
the CoMP cell experiences interference greater than a defined
threshold. Alternatively, the processor may schedule transmissions
such that the total interference experienced by a given UE does not
exceed a defined threshold. In other words, the scheduling
processor schedules transmissions such that the total interference
experienced by any of the UEs does not result in a
signal-to-interference ratio (SIR) budget being exceeded.
[0053] As previously noted, the CoMP cells in the network are
divided into mutually exclusive subsets, and the CoMP cells in the
different subsets may rotate the order in which they schedule their
transmissions. Thus, any given CoMP cell controller 11 may be in
the first, second, or third subset to schedule, given the scenario
described above. If the CoMP cell is in the first or second subset
to schedule, a first interface (Interface-1) 33 is used to send
scheduling information from the CoMP cell controller to neighboring
CoMP cells that have not yet scheduled. If the CoMP cell is the
second or third subset to schedule, a second interface
(Interface-2) 34 is used to receive scheduling information in the
CoMP cell controller from neighboring CoMP cells that have finished
scheduling. The scheduling processor 31 may also accept path gains,
a channel matrix, and pre-coding matrix as inputs. The processor
then generates the scheduling information and transmit waveform
information for transmissions in its associated CoMP cell.
[0054] The present invention, as described in embodiments above,
improves CoMP performance by extending the observation set over
relevant bordering sub-cells. The improvement is most significant
for UEs experiencing greater inter-CoMP cell interference along the
border. One-to-many associations between a sub-cell and neighboring
CoMP cells enables fast information sharing.
[0055] FIG. 7 is a graphical representation of simulation results
for the uplink processing utilizing Equation (9) above. The CoMP
cell in the simulation consists of 21 sub-cells. One additional
ring of sub-cells are included as augmented observations similar to
FIG. 3. The baseline prior art processes only the observation made
by the 21 sub-cells. The improvement in user data rate is
approximately 0.75 bit/sec/Hz.
[0056] As will be recognized by those skilled in the art, the
innovative concepts described in the present application can be
modified and varied over a wide range of applications. Accordingly,
the scope of patented subject matter should not be limited to any
of the specific exemplary teachings discussed above, but is instead
defined by the following claims.
* * * * *