U.S. patent application number 14/708586 was filed with the patent office on 2016-11-17 for methods, systems, and computer program products for calibrating phase hardware-induced distortion in a long term evolution communications system.
The applicant listed for this patent is Collision Communications, Inc.. Invention is credited to Sagar Dhakal, Joseph Farkas, Brandon Hombs, Seyedmehdi S. Nokhodberiz.
Application Number | 20160337066 14/708586 |
Document ID | / |
Family ID | 57277256 |
Filed Date | 2016-11-17 |
United States Patent
Application |
20160337066 |
Kind Code |
A1 |
Dhakal; Sagar ; et
al. |
November 17, 2016 |
Methods, Systems, And Computer Program Products For Calibrating
Phase Hardware-Induced Distortion In A Long Term Evolution
Communications System
Abstract
Methods and systems are described for calibrating phase
hardware-induced distortion in a long term evolution (LTE)
communications system. In one aspect, an estimate of a phase
difference at a user equipment (UE) between downlink channels
including signals sent over two or more BS transmitter chains is
received from the UE in an LTE communications system. A phase
difference between uplink channels including signals received over
two or more BS receiver chains is determined at a BS. A relative
phase distortion induced by two or more BS transceiver chains is
determined based on the received estimate of phase difference
between downlink channels and the determined phase difference
between uplink channels.
Inventors: |
Dhakal; Sagar; (Bedford,
NH) ; Farkas; Joseph; (Merrimack, NH) ; Hombs;
Brandon; (Merrimack, NH) ; Nokhodberiz; Seyedmehdi
S.; (Nashua, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Collision Communications, Inc. |
Portsmouth |
NH |
US |
|
|
Family ID: |
57277256 |
Appl. No.: |
14/708586 |
Filed: |
May 11, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 7/0456 20130101;
H04J 11/0053 20130101; H04B 7/0626 20130101; H04W 56/001 20130101;
H04L 27/367 20130101; H04W 88/08 20130101; H04W 84/042 20130101;
H04W 72/0413 20130101; H04W 72/042 20130101; H04B 17/14 20150115;
H04B 7/0452 20130101; H04B 17/12 20150115; H04B 7/0602 20130101;
H04L 5/005 20130101; H04W 88/02 20130101 |
International
Class: |
H04J 11/00 20060101
H04J011/00; H04B 7/06 20060101 H04B007/06; H04W 72/04 20060101
H04W072/04; H04B 7/04 20060101 H04B007/04 |
Claims
1. A method for calibrating phase hardware-induced distortion in a
long term evolution (LTE) communications system, the method
comprising: receiving, from a user equipment (UE) in an LTE
communications system, an estimate of a phase difference at the UE
between estimated downlink channels obtained from signals sent over
two or more base station (BSI transmitter chains; determining, at a
BS, a phase difference between estimated uplink channels obtained
from signals received over two or more BS receiver chains; and
determining, based on the received estimate of phase difference
between estimated downlink channels and the determined phase
difference between estimated uplink channels, a relative phase
distortion induced by two or more BS transceiver chains including
the two or more BS transmitter chains and the two or more BS
receiver chains; wherein at least one of the preceding actions is
performed on at least one electronic hardware component.
2. The method of claim 1 further comprising calibrating for the
determined relative phase distortion.
3. The method of claim 1 wherein receiving an estimate of a phase
difference between estimated downlink channels includes receiving
rank-1 precoding matrix indicator (PMI) feedback from the UE.
4. The method of claim 3 wherein the received rank-1 PMI feedback
is from a pre-defined codebook including one or more PMI codewords
available at the UE and the base station.
5. The method of claim 4 wherein at least one of the one or more
PMI codewords is measured over a set of sub-bands.
6. The method of claim 3 wherein the received rank-1 PMI feedback
is determined from an estimate of downlink channel state
information (DL-CSI) derived from cell-specific reference signals
received at the UE.
7. The method of claim 1 wherein determining a phase difference
between estimated uplink channels includes determining the phase
difference from an uplink channel state information (UL-CSI)
estimate associated with each UE.
8. The method of claim 7 wherein the UL-CSI estimate is determined
by channel estimation using received sounding reference signal
(SRS) symbols at the BS.
9. The method of claim 1 further comprising changing which BS
antennas are associated with a cell ID associated with a cell
served by the BS.
10. The method of claim 9 wherein changing which antennas are
associated with a cell ID includes: dividing antennas into disjoint
cell ID specific sets; assigning a unique sequence to each cell ID
specific set; scrambling cell-specific reference signals by the
assigned unique sequence; transmitting the scrambled cell-specific
reference signals by all antennas within each cell ID specific set;
and hopping one or more antennas from one cell ID specific set to
another cell ID specific set.
11. The method of claim 10 wherein hopping one or more antennas
includes hopping one or more antennas sequentially across cell ID
specific sets.
12. A system for calibrating phase hardware-induced distortion in
an LTE communications system, the system comprising: means for
receiving, from a UE in an LTE communications system, an estimate
of a phase difference at the UE between estimated downlink channels
obtained from signals sent over two or more BS transmitter chains;
means for determining, at a BS, a phase difference between
estimated uplink channels obtained from signals received over two
or more BS receiver chains; and means for determining, based on the
received estimate of phase difference between estimated downlink
channels and the determined phase difference between estimated
uplink channels, a relative phase distortion induced by two or more
BS transceiver chains including the two or more BS transmitter
chains and the two or more BS receiver chains; wherein at least one
of the means includes at least one electronic hardware
component.
13. A system for calibrating phase hardware-induced distortion in
an LTE communications system, the system comprising system
components including: a network interface component configured to
receive, from a UE in an LTE communications system, an estimate of
a phase difference at the UE between estimated downlink channels
obtained from signals sent over two or more BS transmitter chains;
a phase comparator component configured to determine, at a BS, a
phase difference between estimated uplink channels obtained from
signals received over two or more BS receiver chains; and a
calibration component configured to determine, based on the
received estimate of phase difference between estimated downlink
channels and the determined phase difference between estimated
uplink channels, a relative phase distortion induced by two or more
BS transceiver chains including the two or more BS transmitter
chains and the two or more BS receiver chains; wherein at least one
of the system components includes at least one electronic hardware
component.
14. The system of claim 13 wherein the calibration component is
configured to calibrate for the determined relative phase
distortion.
15. The system of claim 13 wherein the network interface component
is configured to receive the estimate of a phase difference by
receiving rank-1 PMI feedback from the UE.
16. The system of claim 15 wherein the received rank-1 PMI feedback
is from a pre-defined codebook including one or more PMI codewords
available at the UE and the base station.
17. The system of claim 16 wherein at least one of the one or more
PMI codewords is measured over a set of sub-bands.
18. The system of claim 15 wherein the received rank-1 PMI feedback
is determined from an estimate of DL-CSI derived from cell-specific
reference signals received at the UE.
19. The system of claim 13 wherein the phase comparator component
is configured to determine [a] the phase difference between uplink
channels by determining the phase difference from an UL-CSI
estimate associated with each UE.
20. The system of claim 19 wherein the UL-CSI estimate is
determined by channel estimation using received SRS symbols at the
BS.
21. The system of claim 13 further comprising an antenna associator
component to change which BS antennas are associated with a cell ID
associated with a cell served by the BS.
22. The system of claim 21 wherein the antenna associator component
is configured to change which antennas are associated with a cell
ID by: dividing antennas into disjoint cell ID specific sets;
assigning a unique sequence to each cell ID specific set;
scrambling cell-specific reference signals by the assigned unique
sequence; transmitting the scrambled cell-specific reference
signals by all antennas within each cell ID specific set; and
hopping one or more antennas from one cell ID specific set to
another cell ID specific set.
23. The system of claim 22 wherein the antenna associator component
is configured to hop one or more antennas by hopping one or more
antennas sequentially across cell ID specific sets.
24. A non-transitory computer readable medium storing a computer
program, executable by a machine, for calibrating phase
hardware-induced distortion in an LTE communications system, the
computer program comprising executable instructions for: receiving,
from a user equipment (UE) in an LTE communications system, an
estimate of a phase difference at the UE between estimated downlink
channels obtained from signals sent over two or more BS transmitter
chains; determining, at a BS, a phase difference between estimated
uplink channels obtained from signals received over two or more BS
receiver chains; and determining, based on the received estimate of
phase difference between estimated downlink channels and the
determined phase difference between estimated uplink channels, a
relative phase distortion induced by two or more BS transceiver
chains including the two or more BS transmitter chains and the two
or more BS receiver chains.
Description
RELATED APPLICATIONS
[0001] This application is related to U.S. patent application Ser.
No. ______ (Attorney Docket No. C161/US), titled "METHODS, SYSTEMS,
AND COMPUTER PROGRAM PRODUCTS FOR CALIBRATING AMPLITUDE
HARDWARE-INDUCED DISTORTION IN A LONG TERM EVOLUTION COMMUNICATIONS
SYSTEM," and U.S. patent application Ser. No. ______ (Attorney
Docket No. C162/US), titled "METHODS, SYSTEMS, AND COMPUTER PROGRAM
PRODUCTS FOR CALIBRATING HARDWARE-INDUCED DISTORTION IN A
COMMUNICATIONS SYSTEM," both filed on even date herewith, the
entire disclosure of both of which are here incorporated by
reference.
BACKGROUND
[0002] The 3GPP long term evolution (LTE) Rel. 8 and later
standards define various transmission schemes for communication
systems equipped with multiple transmit antennas and multiple
receive antennas. Multi-user multiple input multiple output
(MU-MIMO) is a set of advanced multiple-input and multiple-output,
technologies where the available antennas are spread over a
multitude of independent access points, such as base stations, and
independent terminals--each having one or more antennas. To enhance
the communication capabilities of all terminals, MU-MIMO applies an
extended version of space-division multiple access (SDMA) to allow
multiple transmitters to send separate signals and multiple
receivers to receive separate signals simultaneously in the same
band.
[0003] One transmission scheme is downlink beamforming, where
multiple antennas at the enodeB (eNB), also referred to herein as a
base station (BS), transmit to multiple user equipments (UEs) using
the same time-frequency resource. The LTE Rel. 10 standard defines
a codebook for up to eight transmit antennas per BS.
[0004] Beamforming is a signal processing technique involving
directional signal transmission or reception. This is achieved by
combining elements in a phased array in such a way that signals at
particular angles experience constructive interference while others
experience destructive interference. Beamforming can be used at
both the transmitting and receiving ends in order to achieve
spatial selectivity. In a centralized radio access network (C-RAN),
multiple transmit antennas across multiple BS can be utilized to
jointly beamform over a large geographic area. The C-RAN system has
the potential to provide a significant gain in spectral efficiency
and better network coverage. However, co-channel interference, if
not efficiently controlled or cancelled, limits these
advantages.
[0005] Channel state information (CSI), which refers to known
channel properties of a communication link, can be used to control
co-channel interference. This information describes how a signal
propagates from the transmitter to the receiver and represents the
combined effect of, for example, scattering, fading, and power
decay with distance, making it possible to adapt transmissions to
current channel conditions. CSI is typically estimated at the
receiver and quantized and fed back to the transmitter.
[0006] A transmit beamforming algorithm can preemptively remove or
reduce the interference in a MU-MIMO system. Block diagonalization
and zero-forcing are two well-known beamforming techniques that
utilize the downlink channel state information (DL-CSI) of users to
perform linear precoding of their signals before transmission in
order to cancel the co-channel interference. The performance of
these beamforming techniques rely on the accuracy of the DL-CSI.
Phase hardware-induced distortion that is introduced by BS
transceiver chains (which includes the BS transmitter components
and the BS receiver components), however, prevents an accurate
determination of DL-CSI.
[0007] Accordingly, there exists a need for methods, systems, and
computer program products for calibrating phase hardware-induced
distortion in an LTE communications system.
SUMMARY
[0008] Methods and systems are described for calibrating phase
hardware-induced distortion in an LTE communications system. In one
aspect, an estimate of a phase difference at a user equipment (UE)
between downlink channels including signals sent over two or more
BS transmitter chains is received from the UE in an LTE
communications system. A phase difference between uplink channels
including signals received over two or more receiver chains used by
the BSs is determined at a BS. A relative phase distortion induced
by two or more BS transceiver chains is determined based on the
received estimate of phase difference between downlink channels and
the determined phase difference between uplink channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Advantages of the claimed invention will become apparent to
those skilled in the art upon reading this description in
conjunction with the accompanying drawings, in which like reference
numerals have been used to designate like or analogous elements,
and in which:
[0010] FIG. 1 is a block diagram illustrating an exemplary hardware
device in which the subject matter may be implemented;
[0011] FIG. 2 is a flow diagram illustrating a method for
calibrating phase hardware-induced distortion in an LTE
communications system according to an aspect of the subject matter
described herein;
[0012] FIG. 3 is a block diagram illustrating an arrangement of
components for calibrating phase hardware-induced distortion in an
LTE communications system according to another aspect of the
subject matter described herein;
[0013] FIG. 4 is a block diagram illustrating a configuration for a
MU-MIMO communications system according to another aspect of the
subject matter described herein; and
[0014] FIGS. 5A-5C illustrate exemplary matrices used for
calibrating phase hardware-induced distortion in an LTE
communications system according to another aspect of the subject
matter described herein.
DETAILED DESCRIPTION
[0015] Prior to describing the subject matter in detail, an
exemplary hardware device in which the subject matter may be
implemented shall first be described. Those of ordinary skill in
the art will appreciate that the elements illustrated in FIG. 1 may
vary depending on the system implementation. With reference to FIG.
1, an exemplary system for implementing the subject matter
disclosed herein includes a hardware device 100, including a
processing unit 102, memory 104, storage 106, transceiver 110,
communication interface 112, and a bus 114 that couples elements
104-112 to the processing unit 102.
[0016] The bus 114 may comprise any type of bus architecture.
Examples include a memory bus, a peripheral bus, a local bus, etc.
The processing unit 102 is an instruction execution machine,
apparatus, or device and may comprise a microprocessor, a digital
signal processor, a graphics processing unit, an application
specific integrated circuit (ASIC), a field programmable gate array
(FPGA), etc. The processing unit 102 may be configured to execute
program instructions stored in memory 104 and/or storage 106.
[0017] The memory 104 may include read only memory (ROM) 116 and
random access memory (RAM) 118. Memory 104 may be configured to
store program instructions and data during operation of device 100.
In various embodiments, memory 104 may include any of a variety of
memory technologies such as static random access memory (SRAM) or
dynamic RAM (DRAM), including variants such as dual data rate
synchronous DRAM (DDR SDRAM), error correcting code synchronous
DRAM (ECC SDRAM), or RAMBUS DRAM (RDRAM), for example. Memory 104
may also include nonvolatile memory technologies such as
nonvolatile flash RAM (NVRAM) or ROM. In some embodiments, it is
contemplated that memory 104 may include a combination of
technologies such as the foregoing, as well as other technologies
not specifically mentioned. When the subject matter is implemented
in a computer system, a basic input/output system (BIOS) 120,
containing the basic routines that help to transfer information
between elements within the computer system, such as during
start-up, is stored in ROM 116.
[0018] The storage 106 may include a flash memory data storage
device for reading from and writing to flash memory, a hard disk
drive for reading from and writing to a hard disk, a magnetic disk
drive for reading from or writing to a removable magnetic disk,
and/or an optical disk drive for reading from or writing to a
removable optical disk such as a CD ROM, DVD or other optical
media. The drives and their associated computer-readable media
provide nonvolatile storage of computer readable instructions, data
structures, program modules and other data for the hardware device
100. It is noted that the methods described herein can be embodied
in executable instructions stored in a computer readable medium for
use by or in connection with an instruction execution machine,
apparatus, or device, such as a computer-based or
processor-containing machine, apparatus, or device. It will be
appreciated by those skilled in the art that for some embodiments,
other types of computer readable media may be used which can store
data that is accessible by a computer, such as magnetic cassettes,
flash memory cards, digital video disks, Bernoulli cartridges, RAM,
ROM, and the like may also be used in the exemplary operating
environment. As used here, a "computer-readable medium" can include
one or more of any suitable media for storing the executable
instructions of a computer program in one or more of an electronic,
magnetic, optical, and electromagnetic format, such that the
instruction execution machine, system, apparatus, or device can
read (or fetch) the instructions from the computer readable medium
and execute the instructions for carrying out the described
methods. A non-exhaustive list of conventional exemplary computer
readable medium includes: a portable computer diskette; a RAM; a
ROM; an erasable programmable read only memory (EPROM or flash
memory); optical storage devices, including a portable compact disc
(CD), a portable digital video disc (DVD), a high definition DVD
(HD-DVD.TM.), a BLU-RAY disc; and the like.
[0019] A number of program modules may be stored on the storage
106, ROM 116 or RAM 118, including an operating system 122, one or
more applications programs 124, program data 126, and other program
modules 128.
[0020] The hardware device 100 may be part of a base station and/or
C-RAN (not shown) configured to communicate with mobile devices in
a communication network. A base station may also be referred to as
an eNodeB, an access point, and the like. A base station typically
provides communication coverage for a particular geographic area. A
base station and/or base station subsystem may cover a particular
geographic coverage area referred to by the term "cell." A network
controller (not shown) may be communicatively connected to base
stations and provide coordination and control for the base
stations. Multiple base stations may communicate with one another,
e.g., directly or indirectly via a wireless backhaul or wireline
backhaul.
[0021] The hardware device 100 may operate in a networked
environment using logical connections to one or more remote nodes
via communication interface 112, including communicating with one
or more mobile devices via a transceiver 110 connected to an
antenna 130. The mobile devices can be dispersed throughout the
network 100. A mobile device may be referred to as user equipment
(UE), a terminal, a mobile station, a subscriber unit, or the like.
A mobile device may be a cellular phone, a personal digital
assistant (PDA), a wireless modem, a wireless communication device,
a handheld device, a laptop computer, a wireless local loop (WLL)
station, a tablet computer, or the like. A mobile device may
communicate with a base station directly, or indirectly via other
network equipment such as, but not limited to, a pico eNodeB, a
femto eNodeB, a relay, or the like.
[0022] The remote node may be a computer, a server, a router, a
peer device or other common network node, and typically includes
many or all of the elements described above relative to the
hardware device 100. The communication interface 112, including
transceiver 110 may interface with a wireless network and/or a
wired network. For example, wireless communications networks can
include, but are not limited to, Code Division Multiple Access
(CDMA), Time Division Multiple Access (TDMA), Frequency Division
Multiple Access (FDMA), Orthogonal Frequency Division Multiple
Access (OFDMA), and Single-Carrier Frequency Division Multiple
Access (SC-FDMA). A CDMA network may implement a radio technology
such as Universal Terrestrial Radio Access (UTRA),
Telecommunications Industry Association's (TIA's) CDMA2000.RTM.,
and the like. The UTRA technology includes Wideband CDMA (WCDMA),
and other variants of CDMA. The CDMA2000.RTM. technology includes
the IS-2000, IS-95, and IS-856 standards from The Electronics
Industry Alliance (EIA), and TIA. A TDMA network may implement a
radio technology such as Global System for Mobile Communications
(GSM). An OFDMA network may implement a radio technology such as
Evolved UTRA (E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11
(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDMA, and the
like. The UTRA and E-UTRA technologies are part of Universal Mobile
Telecommunication System (UMTS). LTE and LTE-Advance (LTE-A) are
newer releases of the UMTS that use E-UTRA. UTRA, E-UTRA, UMTS,
LTE, LTE-A, and GAM are described in documents from an organization
called the "3rd Generation Partnership Project" (3GPP).
CDMA2000.RTM. and UMB are described in documents from an
organization called the "3rd Generation Partnership Project 2"
(3GPP2). The techniques described herein may be used for the
wireless networks and radio access technologies mentioned above, as
well as other wireless networks and radio access technologies.
[0023] Other examples of wireless networks include, for example, a
BLUETOOTH network, a wireless personal area network, and a wireless
802.11 local area network (LAN). Examples of wired networks
include, for example, a LAN, a fiber optic network, a wired
personal area network, a telephony network, and/or a wide area
network (WAN). Such networking environments are commonplace in
intranets, the Internet, offices, enterprise-wide computer networks
and the like. In some embodiments, communication interface 112 may
include logic configured to support direct memory access (DMA)
transfers between memory 104 and other devices.
[0024] In a networked environment, program modules depicted
relative to the hardware device 100, or portions thereof, may be
stored in a remote storage device, such as, for example, on a
server. It will be appreciated that other hardware and/or software
to establish a communications link between the hardware device 100
and other devices may be used.
[0025] It should be understood that the arrangement of hardware
device 100 illustrated in FIG. 1 is but one possible implementation
and that other arrangements are possible. It should also be
understood that the various system components (and means) defined
by the claims, described below, and illustrated in the various
block diagrams represent logical components that are configured to
perform the functionality described herein. For example, one or
more of these system components (and means) can be realized, in
whole or in part, by at least some of the components illustrated in
the arrangement of hardware device 100. In addition, while at least
one of these components are implemented at least partially as an
electronic hardware component, and therefore constitutes a machine,
the other components may be implemented in software, hardware, or a
combination of software and hardware. More particularly, at least
one component defined by the claims is implemented at least
partially as an electronic hardware component, such as an
instruction execution machine (e.g., a processor-based or
processor-containing machine) and/or as specialized circuits or
circuitry (e.g., discrete logic gates interconnected to perform a
specialized function), such as those illustrated in FIG. 1. Other
components may be implemented in software, hardware, or a
combination of software and hardware. Moreover, some or all of
these other components may be combined, some may be omitted
altogether, and additional components can be added while still
achieving the functionality described herein. Thus, the subject
matter described herein can be embodied in many different
variations, and all such variations are contemplated to be within
the scope of what is claimed.
[0026] In the description that follows, the subject matter will be
described with reference to acts and symbolic representations of
operations that are performed by one or more devices, unless
indicated otherwise. As such, it will be understood that such acts
and operations, which are at times referred to as being
computer-executed, include the manipulation by the processing unit
of data in a structured form. This manipulation transforms the data
or maintains it at locations in the memory system of the computer,
which reconfigures or otherwise alters the operation of the device
in a manner well understood by those skilled in the art. The data
structures where data is maintained are physical locations of the
memory that have particular properties defined by the format of the
data. However, while the subject matter is being described in the
foregoing context, it is not meant to be limiting as those of skill
in the art will appreciate that various of the acts and operation
described hereinafter may also be implemented in hardware.
[0027] To facilitate an understanding of the subject matter
described below, many aspects are described in terms of sequences
of actions. At least one of these aspects defined by the claims is
performed by an electronic hardware component. For example, it will
be recognized that the various actions can be performed by
specialized circuits or circuitry, by program instructions being
executed by one or more processors, or by a combination of both.
The description herein of any sequence of actions is not intended
to imply that the specific order described for performing that
sequence must be followed. All methods described herein can be
performed in any suitable order unless otherwise indicated herein
or otherwise clearly contradicted by context.
[0028] FIG. 4 illustrates one configuration for a MU-MIMO
communications system 400. The C-RAN 402 communicates with one or
more BSs 404-1 to 404-N, which in turn transmit downlink signals
(shown with solid lines) to UEs 406-1 to 406-N via BS transmit
chains 408-1 to 408-N and 412-1 to 412-N and receive uplink signals
(shown with dashed lines) from UEs 406-1 to 406-N via BS receive
chains 410-1 to 410-N and 414-1 to 414-N, both via respective
associated antennas 409-1 to 409-N and 413-1 to 413-N,
correspondingly. UEs 406-1 to 406-N receive downlink signals from
BSs 404-1 to 404-N via UE receive chains 416-1 to 416-N and
transmit uplink signals to BSs 404-1 to 404-N via UE transmit
chains 418-1 to 418-N, both via respective associated antennas
417-1 to 417-N, correspondingly. A transmit chain includes the
components necessary to transmit a signal and a receive chain
includes all the components necessary to receive a signal. Transmit
and receive chains together make up a transceiver chain 407. For
example, BS transmit chain 408-1, antenna 409-1, and BS receive
chain 410-1 together make up a BS transceiver chain 407-1.
Similarly, UE transmit chain 418-1, antenna 417-1, and UE receive
chain 416-1 together make up a UE transceiver chain 415-1. As can
be appreciated, in a MU-MIMO system, multiple input and out
antennas are employed and thus multiple transceiver chains are
employed.
[0029] Each BS 404 is communicatively coupled to C-RAN 402 such
that certain operations may be performed by the BS 404, by the
C-RAN 402, or by a combination of both. Each BS 404 may, in some
embodiments, be considered a part of the C-RAN 402 such that
operations performed at a BS 404 are performed by the C-RAN 402 by
extension.
[0030] Various operations are described herein with respect to a
particular BS 404. It should be understood that these operations
may be performed by multiple BS's 404 each with respective or the
same UEs 406 and that these multiple operations may be aggregated
by the C-RAN 402 for the purposes of hardware-induced phase
distortion calibration or any other operation described herein.
[0031] In order to perform MU-MIMO beamforming for an LTE based
C-RAN, an accurate estimate of DL-CSI is needed. Generally
speaking, DL-CSI can be determined roughly from knowledge of the
uplink channel state information (UL-CSI) due to reciprocity
between the uplink and downlink air propagation channels. The
UL-CSI from UEs 406 to BSs 404 is obtained at the BSs 404 by
channel estimation using received sounding reference signal (SRS)
symbols. In order to derive an accurate DL-CSI from the UL-CSI,
however, compensation for the effects of hardware-induced phase
distortion that is introduced by transceiver chains at one or more
BSs should be considered. More particularly, each BS 404 antenna
suffers from a random phase distortion induced by its transceiver
chain. By way of example, respective to BS-1 404-1, the transmit
chain 408-1 can introduce a random phase distortion .delta..sub.1
to a signal transmitted via its associated antenna 409-1 and the
receive chain 410-1 can introduce a random phase distortion
.tau..sub.1 to a signal received via its associated antenna 409-1.
Similarly, the transmit chain 408-N can introduce a random phase
distortion 6N to a signal transmitted via its associated antenna
409-N and the receive chain 410-N introduces a random phase
distortion TN to a signal received via its associated antenna
409-N. Unless the phase distortion at each BS antenna is
calibrated, both block diagonalization and the zero-forcing (ZF)
beamformer will project each UEs 406 downlink signal in random
sub-spaces and the co-channel interference between UEs 406
increases as a result.
[0032] Turning now to FIG. 2, a flow diagram is illustrated
illustrating a method for calibrating phase hardware-induced
distortion transceiver chains in an LTE communications system
according to an exemplary aspect of the subject matter described
herein. FIG. 3 is a block diagram illustrating an arrangement of
components for calibrating phase hardware-induced distortion in an
LTE communications system according to another exemplary aspect of
the subject matter described herein. FIG. 1 is a block diagram
illustrating an arrangement of components providing an execution
environment configured for hosting the arrangement of components
depicted in FIG. 3. The method in FIG. 2 can be carried out by, for
example, some or all of the components illustrated in the exemplary
arrangement in FIG. 3 operating in a compatible execution
environment, such as the environment provided by some or all of the
components of the arrangement in FIG. 1. The arrangement of
components in FIG. 3 may be implemented by some or all of the
components of the hardware device 100 of FIG. 1.
[0033] With reference to FIG. 2, in block 202 an estimate of a
phase difference at a UE between downlink channels including
signals sent over two or more BS transmitter chains 408 is received
from the UE 406 in an LTE communications system. Accordingly, a
system for calibrating phase hardware-induced distortion in an LTE
communications system includes means for receiving, from a UE 406
in an LTE communications system, an estimate of a phase difference
at a UE between downlink channels including signals sent over two
or more BS transmitter chains 408. For example, as illustrated in
FIG. 3, a network interface component 302 is configured to receive,
from a UE in an LTE communications system, an estimate of a phase
difference at a UE between downlink channels including signals sent
over two or more BS transmitter chains 408.
[0034] In one aspect, the received rank-1 PMI feedback can be
determined from an estimate of DL-CSI derived from cell-specific
reference signals (CS-RS) received at the UE 406. Each UE 406
measures downlink channels based on CS-RS and computes the phase
difference between the measured downlink channels. The phase
difference between any two downlink channels is the sum of phase
difference due to propagation delay as well as phase difference due
to distortion at the BS transmitter chain associated with each
transmit antenna of the BS 404. For example, as depicted in FIG. 4,
the phase difference .alpha..sub.1N between the first downlink
channel between antenna 409-1 associated with BS-1 404-1 to antenna
417-1 associated with UE-1 406-1 and the N-th downlink channel
between antenna 409-N associated with BS-1 404-1 and antenna 417-1
associated with UE-1 406-1 is given by:
.alpha..sub.1N=.phi.+(.delta..sub.1-.delta..sub.N), (1)
[0035] where .phi. is the phase difference caused by a difference
in propagation times of signals travelling from antenna 409-1
associated with BS-1 404-1 to antenna 417-1 associated with UE-1
406-1 and from antenna 409-N associated with BS-1 404-1 to antenna
409-1 associated with UE-1 406-1.
[0036] In another aspect, the network interface component 302 can
be configured to receive an estimate of a phase difference by
receiving rank-1 precoding matrix indicator (PMI) feedback from the
UE 406. Each UE reports a PMI, which is an index to the precoding
matrix in a codebook, thereby providing a set of recommended
transmission properties to BS 404. For example, the received rank-1
PMI feedback can be from a pre-defined codebook including one or
more PMI codewords available at the UE 406 and the BS 404. The
rank-1 PMI estimates the phase difference observed at the UE 406
across the transmit antennas. Using a codebook based rank-1 PMI, an
estimate of phase difference measured in the downlink channels is
reported by the UE 406 to the C-RAN 402. At least one of the one or
more PMI codewords can be measured over a set of sub-bands.
[0037] Returning to FIG. 2, in block 204 a phase difference between
uplink channels including signals received over two or more BS
receiver chains 410 is determined at the BS 404. Accordingly, a
system for calibrating phase hardware-induced distortion in an LTE
communications system includes means for determining, at a BS, a
phase difference between uplink channels including signals received
over two or more BS receiver chains 410. For example, as
illustrated in FIG. 3, a phase comparator component 304 is
configured to determine, at a BS 404, a phase difference between
uplink channels including signals received over two or more BS
receiver chains 410.
[0038] In one aspect, the phase comparator component 304 can be
configured to determine a phase difference between uplink channels
by determining the phase difference from a UL-CSI estimate
associated with each UE 406. For example, the UL-CSI estimate can
be determined by channel estimation using received SRS symbols at
the BS 404. In the C-RAN 402, a BS 404 measures the uplink channels
between a UE 406 and the receive antennas at the BS 404, and
determines the phase difference between the measured uplink
channels. The phase difference between any two uplink channels from
a UE 406 is the sum of phase difference due to propagation delay
and phase difference due to distortion introduced by the receiver
chain associated with each receive antenna 409 associated with the
BS 404. For example, as depicted in FIG. 4, the phase difference
.beta..sub.1N between the first uplink channel from antenna 417-1
associated with UE-1 406-1 to antenna 409-1 associated with BS-1
404-1 and the N-th uplink channel from antenna 417-1 associated
with UE-1 406-1 to antenna 409-N associated with BS-1 404-1 is
given by:
.beta..sub.1N=.phi.+(.tau..sub.1-.tau..sub.N), (2)
[0039] where .phi. is the phase difference caused by a difference
in propagation times of signals travelling from antenna 417-1
associated with UE-1 406-1 to antenna 409-1 associated with BS-1
404-1 and from antenna 417-1 associated with UE-1 to antenna 409-N
associated with BS-1 404-1. It is to be noted from Equations (1)
and (2) that the phase difference .phi. caused by propagation
delays is reciprocal in the downlink and the uplink of a
time-division duplex (TDD) system.
[0040] Returning to FIG. 2, in block 206 a relative phase
distortion induced by two or more BS transceiver chains 407 is
determined based on the received estimate of phase difference
between downlink channels and the determined phase difference
between uplink channels. Accordingly, a system for calibrating
phase hardware-induced distortion in an LTE communications system
includes means for determining, based on the received estimate of
phase difference between downlink channels and the determined phase
difference between uplink channels, a relative phase distortion
induced by two or more BS transceiver chains 407. For example, as
illustrated in FIG. 3, a calibration component 306 is configured to
determine, based on the received estimate of phase difference
between downlink channels and the determined phase difference
between uplink channels, a relative phase distortion induced by two
or more BS transceiver chains 407.
[0041] In one aspect, relative phase distortion at the BS
transceiver chains 407 can be determined based on the phase
difference of the uplink channels and the PMI-based estimate of the
phase difference of the downlink channels. The UL-CSI is calibrated
using the measured relative phase distortion to derive the DL-CSI.
More particularly, each BS 404 receives the rank-1 PMI feedback
from its designated UE 406 and also estimates the UL-CSI feedback
based on the received SRS symbols. Using the rank-1 PMI and the
UL-CSI estimate obtained from each UE 406, the C-RAN 402 can
compute the relative phase-distortion induced by the transceiver
chains 407 across multiple BS 404 antennas at the C-RAN 402. The
relative phase-distortion can be averaged across multiple UE 406
reports to improve the accuracy.
[0042] In another aspect, the calibration component 306 can
calibrate for the determined relative phase distortion. For
example, a relative phase-distortion matrix can be constructed by
forming a diagonal matrix having relative phase distortion factors
as diagonal elements, where relative phase distortion of each BS
transceiver chain 407 is computed with respect to a first BS
transceiver chain 407-1. For example, the relative phase distortion
between the transceiver chain 407-N of BS-1 404-1 with respect to
transceiver chain 407-1 can be obtained as given in Equation (3)
below
.rho..sub.1N=.alpha..sub.1N-.beta..sub.1N (3)
[0043] Next, a relative phase-distortion matrix is given by the
matrix shown in FIG. 5A.
[0044] The UL-CSI estimate can be multiplied by a relative
phase-distortion matrix for calibrating the hardware phase effects
and the DL-CSI estimate is obtained. For example, the UL channel
between antenna 417-1 of UE-1 406-1 to the j-th antenna of BS-1
404-1 is given by the product e.sup.i.gamma..sup.j
e.sup.i.phi..sup.1j e.sup.i.lamda., where .lamda. is the random
phase distortion introduced by the transmit chain of antenna 417-1
of UE 1 406-1, .phi..sub.1j is the phase shift resulting from
signal propagation from antenna 417-1 of UE-1 406-1 to antenna-j of
BS-1 404-1 and .tau..sub.j is the random phase distortion
introduced by the j-th receive chain of BS-1 404-1. Therefore, the
UL-CSI between UE-1 406-1 and all N antennas of BS-1 404-1 is given
by the channel vector shown in FIG. 5B.
[0045] Multiplying the UL-CSI channel vector shown in FIG. 5B by
the relative phase distortion matrix shown in FIG. 5A and
substituting for .rho., .alpha. and .beta. using Equations (1), (2)
and (3), results in a scaled DL-CSI vector shown in FIG. 5C, where
a downlink channel from the j-th antenna of BS-1 to antenna 417-1
of UE-1 is e.sup.i.kappa.e.sup.i.phi..sup.1j e.sup.i.delta..sup.j,
.kappa. is the random phase distortion introduced by the receive
chain 416-1 of UE 1, .phi..sub.1j is the phase shift resulting from
signal propagation from antenna-j of BS-1 to antenna 417-1 of UE-1
and .delta..sub.j is the random phase distortion introduced by the
j-th transmit chain of BS-1, and .eta. is a scalar quantity given
by
.eta. = ( .lamda. - .kappa. ) ( .delta. 1 - .gamma. 1 ) .
##EQU00001##
[0046] The DL beamformer can be designed based on the scaled DL-CSI
vector shown in FIG. 5C.
[0047] In another aspect, as described above, CS-RS is received at
the UE 406. More particularly, the BS 404 transmit antennas 409 for
the C-RAN 402 can be partitioned into sets, where each set is
assigned to a certain CS-RS. Each BS 404 transmit antenna 409 may
be assigned to a different set, as required by the C-RAN 402.
[0048] Returning to FIG. 3, an optional antenna associator
component 308 can be included in system 300 to change which
antennas are associated with a cell ID. In one aspect, the antenna
associator component can be configured to change which antennas are
associated with a cell ID by dividing antennas into disjoint cell
ID specific sets, assigning a unique sequence to each cell ID
specific set, scrambling CS-RSs by the assigned unique sequence,
transmitting the scrambled CS-RSs by all antennas within each cell
ID specific set, and hopping one or more antennas from one cell ID
specific set to another cell ID specific set. For example, one or
more antennas can be hopped sequentially across cell ID specific
sets.
[0049] The approach described above does not require hardware
calibration, since the phase-induced hardware distortion is
compensated for, and the approach is based on existing PHY signals
defined by LTE R8 and beyond standard. Accordingly, an advantage of
this approach is that it can be transparent to the UEs 406, as only
the CRAN 402 performs calibration steps periodically.
[0050] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the subject matter
(particularly in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by context.
Recitation of ranges of values herein are merely intended to serve
as a shorthand method of referring individually to each separate
value falling within the range, unless otherwise indicated herein,
and each separate value is incorporated into the specification as
if it were individually recited herein. Furthermore, the foregoing
description is for the purpose of illustration only, and not for
the purpose of limitation, as the scope of protection sought is
defined by the claims as set forth hereinafter together with any
equivalents thereof entitled to. The use of any and all examples,
or exemplary language (e.g., "such as") provided herein, is
intended merely to better illustrate the subject matter and does
not pose a limitation on the scope of the subject matter unless
otherwise claimed. The use of the term "based on" and other like
phrases indicating a condition for bringing about a result, both in
the claims and in the written description, is not intended to
foreclose any other conditions that bring about that result. No
language in the specification should be construed as indicating any
non-claimed element as essential to the practice of the invention
as claimed.
[0051] Preferred embodiments are described herein, including the
best mode known to the inventor for carrying out the claimed
subject matter. One of ordinary skill in the art should appreciate
after learning the teachings related to the claimed subject matter
contained in the foregoing description that variations of those
preferred embodiments may become apparent to those of ordinary
skill in the art upon reading the foregoing description. The
inventor intends that the claimed subject matter may be practiced
otherwise than as specifically described herein. Accordingly, this
claimed subject matter includes all modifications and equivalents
of the subject matter recited in the claims appended hereto as
permitted by applicable law. Moreover, any combination of the
above-described elements in all possible variations thereof is
encompassed unless otherwise indicated herein or otherwise clearly
contradicted by context.
* * * * *