U.S. patent application number 15/355421 was filed with the patent office on 2018-05-24 for precoder design for combining high-end rf with constrained rf of massive mimo antennas.
This patent application is currently assigned to Nokia Solutions and Networks Oy. The applicant listed for this patent is Nokia Solutions and Networks Oy. Invention is credited to Berthold Panzner, Rakash Sivasivaganesan, Markus Staudacher, Wolfgang Zirwas.
Application Number | 20180145739 15/355421 |
Document ID | / |
Family ID | 60201824 |
Filed Date | 2018-05-24 |
United States Patent
Application |
20180145739 |
Kind Code |
A1 |
Sivasivaganesan; Rakash ; et
al. |
May 24, 2018 |
PRECODER DESIGN FOR COMBINING HIGH-END RF WITH CONSTRAINED RF OF
MASSIVE MIMO ANTENNAS
Abstract
A method includes mapping a signal desired by at least one
receiver to a projection area based at least on a functionality
corresponding to one or more first radio frequency chains coupled
to a plurality of first antennas; selecting precoding coefficients
for at least one of one or more second radio frequency chains
coupled to a plurality of second antennas to generate a signal
point within the projection area; and compensating for a difference
between the generated signal point and the signal desired by the at
least one receiver using at least one of the first radio frequency
chains, wherein the second radio frequency chains have a reduced
functionality relative to the functionality of the first radio
frequency chains, and wherein the first and second set of antennas
are different.
Inventors: |
Sivasivaganesan; Rakash;
(Unterhaching, DE) ; Zirwas; Wolfgang; (Munchen,
DE) ; Panzner; Berthold; (Holzkirchen, DE) ;
Staudacher; Markus; (Munchen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nokia Solutions and Networks Oy |
Espoo |
|
FI |
|
|
Assignee: |
Nokia Solutions and Networks
Oy
|
Family ID: |
60201824 |
Appl. No.: |
15/355421 |
Filed: |
November 18, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 7/0634 20130101;
H04B 7/022 20130101; H04B 7/024 20130101; H04W 88/08 20130101; H04B
7/0469 20130101; H04B 7/0617 20130101; H04B 7/0691 20130101 |
International
Class: |
H04B 7/06 20060101
H04B007/06; H04B 7/02 20060101 H04B007/02 |
Claims
1. A method comprising: mapping a signal desired by at least one
receiver to a projection area based at least on a functionality
corresponding to one or more first radio frequency chains coupled
to a plurality of first antennas; selecting precoding coefficients
for at least one of one or more second radio frequency chains
coupled to a plurality of second antennas to generate a signal
point within the projection area; and compensating for a difference
between the generated signal point and the signal desired by the at
least one receiver using at least one of the first radio frequency
chains, wherein the second radio frequency chains have a reduced
functionality relative to the functionality of the first radio
frequency chains, and wherein the first and second set of antennas
are different.
2. The method of claim 1, for the case the one or more second radio
frequency chains are not capable of generating a signal point
within the projection area, selecting the precoding coefficients
comprises: selecting precoding coefficients for at least one of the
second radio frequency chains for generating a signal point closest
to the projection area.
3. The method of claim 1, wherein the first antennas and the second
antennas are collocated.
4. The method of claim 1, wherein the first antennas are in a
different location than the second antennas.
5. The method of claim 4, wherein the first antennas are located at
a first base station and the second antennas are located at a
second base station, and wherein the first base station provides a
larger cell than the second base station.
6. The method of claim 4, the method further comprising one of:
receiving, from the first base station via an X2 interface,
configuration information for the one or more first radio frequency
chains for mapping the desired signal to the projection area; and
receiving, from the second base station via an X2 interface,
information for determining the difference between the generated
signal point and the signal desired for compensating for the
difference.
7. The method of claim 1, wherein the functionality of the first
radio frequency chains is based at least in part on a plurality of
features and wherein the second radio frequency chains have a
reduced functionality because one or more features for the second
radio frequency chains are relaxed relative to identical one or
more features for the first radio frequency chains.
8. The method of claim 7, wherein the features correspond to at
least one of: transmission power, amplifier character like
operating region, bit resolution, and analog filters.
9. The method of claim 1, wherein: a shape of the projection area
is based on at least one of: the number of first radio frequency
chains, a total power constraint of the one or more antenna
elements, individual power constraints of each of the one or more
antenna elements, and the channel coefficient of each of the one or
more antenna elements; and a size of the projection area is based
on at least one of: a total power constraint of the one or more
antenna elements, and individual power constraints of each of the
one or more antenna elements, and the channel coefficient of each
of the one or more antenna elements.
10. The method claim 1, wherein the shape of the projection area is
at least one of: a subspace, an ellipsoid, and a polytope.
11. An apparatus comprising: at least one processor; and at least
one memory including computer program code, the at least one memory
and the computer program code configured to, with the at least one
processor, cause the apparatus to perform at least the following:
map a signal desired by at least one receiver to a projection area
based at least on a functionality corresponding to one or more
first radio frequency chains coupled to a plurality of first
antennas; select precoding coefficients for at least one of one or
more second radio frequency chains coupled to a plurality of second
antennas to generate a signal point within the projection area; and
compensate for a difference between the generated signal point and
the signal desired by the at least one receiver using at least one
of the first radio frequency chains, wherein the second radio
frequency chains have a reduced functionality relative to the
functionality of the first radio frequency chains, and wherein the
first and second set of antennas are different.
12. The apparatus of claim 11, wherein for the case the one or more
second radio frequency chains are not capable of generating a
signal point within the projection area, selection of the precoding
coefficients comprises: selecting precoding coefficients for at
least one of the second radio frequency chains for generating a
signal point closest to the projection area.
13. The apparatus of claim 11, wherein the first antennas and the
second antennas are collocated.
15. The apparatus of claim 11, wherein the apparatus is abase
station and further comprises at least one of: the first antennas
and the second antennas.
16. The apparatus of claim 15, wherein at least one of the first
antennas and the second antennas are located at another base
station.
17. The apparatus of claim 15, wherein the at least one memory and
the computer program code are configured to, with the at least one
processor, cause the apparatus to perform one of the following:
receive, from the first base station via an X2 interface,
configuration information for the one or more first radio frequency
chains for mapping the desired signal to the projection area; and
receive, from the second base station via an X2 interface,
information for determining the difference between the generated
signal point and the signal desired for compensating for the
difference.
18. The apparatus of claim 11, wherein the functionality of the
first radio frequency chains is based at least in part on a
plurality of features and wherein the second radio frequency chains
have a reduced functionality because one or more features for the
second radio frequency chains are relaxed relative to identical one
or more features for the first radio frequency chains.
19. The apparatus of claim 18, wherein the features correspond to
at least one of: transmission power, amplifier character like
operating region, bit resolution, and analog filters.
20. A computer program product comprising a non-transitory
computer-readable medium storing computer program code thereon
which when executed by a device causes the device to perform at
least: mapping a signal desired by at least one receiver to a
projection area based at least on a functionality corresponding to
one or more first radio frequency chains coupled to a plurality of
first antennas; selecting precoding coefficients for at least one
of one or more second radio frequency chains coupled to a plurality
of second antennas to generate a signal point within the projection
area; and compensating for a difference between the generated
signal point and the signal desired by the at least one receiver
using at least one of the first radio frequency chains, wherein the
second radio frequency chains have a reduced functionality relative
to the functionality of the first radio frequency chains, and
wherein the first and second set of antennas are different.
Description
TECHNICAL FIELD
[0001] This invention relates generally to wireless communication
and, more specifically, relates to massive Multiple In Multiple Out
(mMIMO) antenna systems.
BACKGROUND
[0002] This section is intended to provide a background or context
to the invention disclosed below. The description herein may
include concepts that could be pursued, but are not necessarily
ones that have been previously conceived, implemented or described.
Therefore, unless otherwise explicitly indicated herein, what is
described in this section is not prior art to the description in
this application and is not admitted to be prior art by inclusion
in this section. Abbreviations that may be found in the
specification and/or the drawing figures are defined below, after
the main part of the detailed description section.
[0003] Massive MIMO is a technology where the number of terminals
is much less than the number of base station (mobile station)
antennas, and has been incorporated into wireless broadband
standards like LTE and Wi-Fi. Massive MIMO uses a very large number
of service antennas (e.g., hundreds or thousands) that are operated
fully coherently and adaptively. Extra antennas help by focusing
the transmission and reception of signal energy into ever-smaller
regions of space. This brings improvements in throughput and energy
efficiency, in particular when combined with simultaneous
scheduling of a large number of user equipment (e.g., tens or
hundreds).
BRIEF SUMMARY
[0004] This section is intended to include examples and is not
intended to be limiting.
[0005] According to one example embodiment, a method comprises
mapping a signal desired by at least one receiver to a projection
area based at least on a functionality corresponding to one or more
first radio frequency chains coupled to a plurality of first
antennas; selecting precoding coefficients for at least one of one
or more second radio frequency chains coupled to a plurality of
second antennas to generate a signal point within the projection
area; and compensating for a difference between the generated
signal point and the signal desired by the at least one receiver
using at least one of the first radio frequency chains, wherein the
second radio frequency chains have a reduced functionality relative
to the functionality of the first radio frequency chains, and
wherein the first and second set of antennas are different.
[0006] According to another example embodiment, an apparatus
comprises: at least one processor; and at least one memory
including computer program code, the at least one memory and the
computer program code configured to, with the at least one
processor, cause the apparatus to perform at least the following:
map a signal desired by at least one receiver to a projection area
based at least on a functionality corresponding to one or more
first radio frequency chains coupled to a plurality of first
antennas; select precoding coefficients for at least one of one or
more second radio frequency chains coupled to a plurality of second
antennas to generate a signal point within the projection area; and
compensate for a difference between the generated signal point and
the signal desired by the at least one receiver using at least one
of the first radio frequency chains, wherein the second radio
frequency chains have a reduced functionality relative to the
functionality of the first radio frequency chains, and wherein the
first and second set of antennas are different.
[0007] According to another example embodiment, a computer program
product comprising a non-transitory computer-readable medium
storing computer program code thereon which when executed by a
device causes the device to perform at least: mapping a signal
desired by at least one receiver to a projection area based at
least on a functionality corresponding to one or more first radio
frequency chains coupled to a plurality of first antennas;
selecting precoding coefficients for at least one of one or more
second radio frequency chains coupled to a plurality of second
antennas to generate a signal point within the projection area; and
compensating for a difference between the generated signal point
and the signal desired by the at least one receiver using at least
one of the first radio frequency chains, wherein the second radio
frequency chains have a reduced functionality relative to the
functionality of the first radio frequency chains, and wherein the
first and second set of antennas are different.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] In the attached Drawing Figures:
[0009] FIG. 1 is a block diagram of one possible and non-limiting
exemplary system in which the exemplary embodiments may be
practiced;
[0010] FIG. 2 shows an example antenna concept including an antenna
array having antenna elements having high end RF chains, and an
antenna array having antenna elements having constrained RF
chains;
[0011] FIG. 3A-3E show example signal spaces in accordance with
exemplary embodiments;
[0012] FIG. 4 is a graph of mean squared error (MSE) as a function
of increasing the number of constrained RFs in accordance with
exemplary embodiments;
[0013] FIG. 5 shows two bar graphs showing distribution of transmit
power for high end RFs in dBW in accordance with exemplary
embodiments; and
[0014] FIG. 6 is a logic flow diagram for precoder design for
combining high-end RF with constrained RF of massive MIMO antennas,
and illustrates the operation of an exemplary method or methods, a
result of execution of computer program instructions embodied on a
computer readable memory, functions performed by logic implemented
in hardware, and/or interconnected means for performing functions
in accordance with exemplary embodiments; and
DETAILED DESCRIPTION OF THE DRAWINGS
[0015] The word "exemplary" is used herein to mean "serving as an
example, instance, or illustration." Any embodiment described
herein as "exemplary" is not necessarily to be construed as
preferred or advantageous over other embodiments. All of the
embodiments described in this Detailed Description are exemplary
embodiments provided to enable persons skilled in the art to make
or use the invention and not to limit the scope of the invention,
which is defined by the claims.
[0016] The exemplary embodiments herein describe techniques for
precoder design for combining high-end RF with constrained RF of
massive MIMO antennas. Additional description of these techniques
is presented after a system into which the exemplary embodiments
may be used is described.
[0017] Turning to FIG. 1, this figure shows a block diagram of an
exemplary system in which the exemplary embodiments may be
practiced. In FIG. 1, UE 110 is in wireless communication with a
wireless network 100. The UE 110 includes one or more processors
120, one or more memories 125, and one or more transceivers 130
interconnected through one or more buses 127. Each of the one or
more transceivers 130 includes a receiver, Rx, 132 and a
transmitter, Tx, 133. The one or more buses 127 may be address,
data, or control buses, and may include any interconnection
mechanism, such as a series of lines on a motherboard or integrated
circuit, fiber optics or other optical communication equipment, and
the like. The one or more transceivers 130 are connected to one or
more antennas 128. The one or more memories 125 include computer
program code 123. Each UE 110 communicates with eNB 170 via a
wireless link 111, and there are N wireless links shown.
[0018] The eNB 170 is a base station that provides access by
wireless devices such as the UE 110 to the wireless network 100.
The eNB 170 includes one or more processors 152, one or more
memories 155, one or more network interfaces (N/W hF(s)) 161, and
one or more transceivers 160 interconnected through one or more
buses 157. Each of the one or more transceivers 160 includes a
receiver, Rx, 162 and a transmitter, Tx, 163. The one or more
transceivers 160 are connected to multiple (e.g., many) antennas
158. The one or more memories 155 include computer program code
153. The eNB 170 includes a MIMO module 150, comprising one of or
both parts 150-1 and/or 150-2, which may be implemented in a number
of ways. The MIMO module 150 may be implemented in hardware as MIMO
module 150-1, such as being implemented as part of the one or more
processors 152. The MIMO module 150-1 may be implemented also as an
integrated circuit or through other hardware such as a programmable
gate array. In another example, the MIMO module 150 may be
implemented as MIMO module 150-2, which is implemented as computer
program code 153 and is executed by the one or more processors 152.
For instance, the one or more memories 155 and the computer program
code 153 are configured to, with the one or more processors 152,
cause the eNB 170 to perform one or more of the operations as
described herein. The one or more network interfaces 161
communicate over a network such as via the links 176 and 131. Two
or more eNBs 170 communicate using, e.g., link 176. The link 176
may be wired or wireless or both and may implement, e.g., an X2
interface.
[0019] The one or more buses 157 may be address, data, or control
buses, and may include any interconnection mechanism, such as a
series of lines on a motherboard or integrated circuit, fiber
optics or other optical communication equipment, wireless channels,
and the like. For example, the one or more transceivers 160 may be
implemented as a remote radio head (RRH) 195, with the other
elements of the eNB 170 being physically in a different location
from the RRH, and the one or more buses 157 could be implemented in
part as fiber optic cable to connect the other elements of the eNB
170 to the RRH 195.
[0020] The wireless network 100 may include a network control
element (NCE) 190 that may include MME/SGW functionality, and which
provides connectivity with a further network, such as a telephone
network and/or a data communications network (e.g., the Internet).
The eNB 170 is coupled via a link 131 to the NCE 190. The link 131
may be implemented as, e.g., an Si interface. The NCE 190 includes
one or more processors 175, one or more memories 171, and one or
more network interfaces (N/W I/F(s)) 180, interconnected through
one or more buses 185. The one or more memories 171 include
computer program code 173. The one or more memories 171 and the
computer program code 173 are configured to, with the one or more
processors 175, cause the NCE 190 to perform one or more
operations.
[0021] The wireless network 100 may implement network
virtualization, which is the process of combining hardware and
software network resources and network functionality into a single,
software-based administrative entity, a virtual network. Network
virtualization involves platform virtualization, often combined
with resource virtualization. Network virtualization is categorized
as either external, combining many networks, or parts of networks,
into a virtual unit, or internal, providing network-like
functionality to software containers on a single system. Note that
the virtualized entities that result from the network
virtualization are still implemented, at some level, using hardware
such as processors 152 or 175 and memories 155 and 171, and also
such virtualized entities create technical effects.
[0022] The computer readable memories 125, 155, and 171 may be of
any type suitable to the local technical environment and may be
implemented using any suitable data storage technology, such as
semiconductor based memory devices, flash memory, magnetic memory
devices and systems, optical memory devices and systems, fixed
memory and removable memory. The processors 120, 152, and 175 may
be of any type suitable to the local technical environment, and may
include one or more of general purpose computers, special purpose
computers, microprocessors, digital signal processors (DSPs) and
processors based on a multi-core processor architecture, as
non-limiting examples.
[0023] In general, the various embodiments of the user equipment
110 can include, but are not limited to, cellular telephones such
as smart phones, personal digital assistants (PDAs) having wireless
communication capabilities, portable computers having wireless
communication capabilities, image capture devices such as digital
cameras having wireless communication capabilities, gaming devices
having wireless communication capabilities, music storage and
playback appliances having wireless communication capabilities,
Internet appliances permitting wireless Internet access and
browsing, tablets with wireless communication capabilities, as well
as portable units or terminals that incorporate combinations of
such functions.
[0024] Having thus introduced one suitable but non-limiting
technical context for the practice of the exemplary embodiments of
this invention, the exemplary embodiments will now be described
with greater specificity.
[0025] Exemplary embodiments relate to the addition of massive MIMO
arrays with constrained RFs to existing antenna arrays with high
end RFs in a wireless network, and the distributed implementation
of jointly designing the precoders for antenna elements (AEs)
connected to constrained RF-chains and high-end RF-chains. As
compared to the high-end RF chains, constrained RF-chains may have,
for example, digital-to-analog converters (DACs) with limited bit
resolution, cheaper amplifiers with a small operating region,
and/or relaxed analog filters.
[0026] One document that relates to mMIMO is U.S. Pat. No.
9,231,676, which describes combining low cost RF chains and high
end RF chains, thus providing a low cost implementation of massive
MIMO antenna arrays with potentially hundreds or more antenna
elements. The combination of low cost frontends with high accuracy
RF chains provides high performance due to the high end RF chains
while maintaining low cost due to high number of extremely low cost
RF chains.
[0027] In the existing network infrastructure, a few high end RFs
are already deployed. An mMIMO array with low end RFs may be added
to the high end RFs to boost the performance. Such a `booster`
array could be deployed close to the existing infrastructure or
mounted on a nearby high-rise building. In any case, in order to
perform joint precoding a distributed scheme is necessary with
minimal exchange of information between the high end RFs and the
constrained RFs. Furthermore, the signal transmitted from the high
end RF has very high resolution while the signal transmitted using
constrained RF is limited in resolution e.g. 1 bit of amplitude and
3 bits of phase information. In the simplest case with only 1 bit
of amplitude, the antenna elements are either switched on or
switched off. In order to fully utilize the additional mMIMO array
with constrained RFs, the precoder design for the constrained RFs
should take into account that the high end RFs can compensate for
the limitations in the constrained RFs.
[0028] The following document generally relates to precoder design,
but only for high end RFs: M. Joham, W. Utschick and J. A. Nossek,
"Linear transmit processing in MIMO communications systems," in
IEEE Transactions on Signal Processing, vol. 53, no. 8, pp.
2700-2712, August 2005.
[0029] Embodiments, herein relate to jointly designing the precoder
for the constrained and high end RF in a decentralized way.
[0030] In general, embodiments described herein iteratively
construct a desired signal at the receivers by determining the
precoding coefficients for the constrained RFs one after another
taking into account at each iterative step that the high end RFs
are capable of producing high resolution signals and could
compensate for certain errors. For example, first the desired
signal may mapped to a signal subspace. Alternatively, an ellipsoid
or polytope may be used depending on the transmit power constraint
of the high end RFs. The subspace represents the union of the
desired signal and all the errors that can be compensated by the
high end RFs. Secondly, the constrained RFs' precoding coefficients
are used to generate a signal point lying in this subspace. Within
this subspace the high end RFs can fully compensate for the
difference between the desired signal and the signal point
constructed by constrained RFs.
[0031] In case there are not a sufficient number of constrained RFs
to generate a signal point within this signal subspace (e.g., the
number of simultaneously served users is larger than the number of
full RFs) then a signal point closest to the signal subspace is
generated by the constrained RFs. Furthermore, the order of
choosing the precoding coefficients for the constrained RFs
influence the resulting error. An exhaustive search can be
performed through all the combinations of the constrained RF's AEs
to find the best precoding coefficients that minimize the error,
but this tends to be computationally expensive. Even in the case of
an exhaustive search, it is sufficient to search through only the
constrained RFs to find the optimum order because all the high end
RFs are taken into account in each step by considering the signal
subspace instead of just the desired signal point.
[0032] The set of constrained and high end RFs may be either
co-located or distributed. For example, the high end RFs may be
placed at typical macro sites, while the constrained RF panels may
be placed somewhere within the cell, e.g., similar to how small
cells are placed. In addition to proper synchronization for the
case of backhaul delays, it may be necessary to coordinate the
independently running precoding algorithms for the high end and the
constrained RFs, i.e., to exchange information about the achievable
subspace region of the high end RF AEs, defined, e.g., by the
maximum full RF power, and vice versa to inform the precoder at the
high end RF site about the precoding strategy being used to reach
the subspace or to get close to the subspace. This information may
be exchanged over the X2 interface for proper precoder alignment to
allow for a distributed simultaneous calculation of the precoder
weights.
[0033] In some examples, the high end RFs may share channel
coefficients to the constrained RFs through, e.g., the X2
interface. The constrained RFs may then determine their precoding
coefficients based on this information. Afterwards, the constrained
RFs may communicate the error or difference in signal that the high
end RFs need to compensate for. In some embodiments, the
constrained RFs could communicate the error to the desired signal
that the high end RFs need to use, while other embodiments
communicate the optimum precoder coefficients.
[0034] Referring now to FIG. 2, this figure shows an example first
antenna array 202 having constrained antenna elements (AE) and an
example second antenna array 210 having high end AEs in accordance
with exemplary embodiments.
[0035] As illustrated by FIG. 2, there are thirty antenna elements
205-1 through 205-30 (i.e., a 5.times.6 array of antennas in this
example) for the first antenna array 202, and four AEs 215-1
through 215-4 (i.e., a 2.times.2 array of antennas in this example)
for the first antenna array 202. In FIG. 2, the antenna arrays 202
and 210 may be located at different locations, for example, at
different base stations. However, it is noted that the antenna
array may also be collocated as shown by antenna array 211 for
example. It is noted that the number of AEs in the example shown in
FIG. 2 is not intended to be limiting, and more or less AEs may be
used. Herein, the antenna elements are also referred to as antennas
(e.g., each antenna element is an antenna), and any antenna
configuration for such antenna elements may be used. FIG. 2 also
indicates that antenna elements 205-1 to 205-30 are connected to
constrained RF chains, and that the antenna elements of antenna
array 210 are connected to four high end RF chains 215-1 through
215-4. Embodiments discussed herein provide an optimal way of
integrating the high end RF chains into any algorithm that
determines the precoding coefficients of the constrained RF
chains.
[0036] The benefits of the proposed concept are a very high
performance in terms of number of served users as well as the
residual mean squared error (MSE), even in case of a very limited
number of high end RF frontends. One application could be to use
the high end RFs from an available macro site having, for example,
just 4 RF front ends as shown in FIG. 2, and adding a low cost
constrained RF panel directly at the site or somewhere in the cell.
In such an example configuration up to ten user equipment could be
served. In addition the Knapsack algorithm as well as the subspace
approach will lead to limited processing overhead.
[0037] The concept of over the air signal generation aims at
transmitting signals from antenna elements such that when
multiplied by the channel response, they result in the desired
signal at the receiver (Quadrature Amplitude Modulation (QAM)
symbols or time samples of OFDM symbol). In case of multiple
receivers, the desired signals need to be generated simultaneously
at all the receivers, and each receiver may have multiple antenna
elements. Cascading the signal samples from all the AEs of all the
UEs at any particular time instant may be represented by a combined
signal (vector) of dimension n. In an n-dimensional signal space
this combined signal is denoted by a point. Switching on any one of
the AE results in a signal point in this n-dimensional space, which
corresponds to the channel response at that time instant.
[0038] Referring now to FIGS. 3A and 3B, these figures shows
example signal spaces in accordance with exemplary embodiments. In
FIG. 3A, a signal space 300 of received signals for a transmitter
with 3 constrained RFs and one high end RF and 2 single antennas
UEs. The x- and y-axis represent the signal received at UE1 and
UE2, respectively. The channel seen by the constrained RFs is
represented as h1, h2, and h3; the channel seen by the high end RF
is represented as h4; and the desired signal point is denoted d. In
the following description of FIGS. 3A and 3B, the constrained RFs
h1, h2, and h3 are assumed to have one bit resolution (i.e. antenna
elements connected with constrained RFs are either switched on or
switched off). In this example, h1 to h4 correspond to AE 1 to AE
4. However, this example description can be directly extended to
multi-bit amplitude and phase. Referring now to FIG. 3B, in the
absence of the high end RF h4, exhaustive search will switch on AE
1 and 3 (represented by the circles surrounding h1 and h3
respectively) to construct a signal as close as possible to d. As
can be seen, the vector 302 and the vector 304 correspond to h1 and
h3, respectively in this example.
[0039] Referring now to FIG. 3C, this figure shows example signal
space 300 and amplitude values that can be constructed at the
receiver when a high end RF is present. In this example, the high
end RF corresponds to point h4. In FIG. 3C, the amplitude values
that can be constructed at the high end RF are represented by line
320 through the origin of the signal space 300. The length of the
line 320 varies depending on the transmit power constraint for this
AE. The dotted line 325 in FIG. 3C shows the line 320 shifted to
desired signal point d. The dotted line 325 represents the errors
that can be corrected by the high end RF h4. Since from any point
on this line 325, the desired signal point d can be reached by
using the high end RF, it is sufficient for the constrained RFs
during each iteration to switch on the AE that will result in a
point close to the line. In this example, the AE 2 is switched on
as shown by vector 330.
[0040] In general, a few number of amplitude and phase bits can be
transmitted from each constrained RF AE and hence, the points h1,
h2, and h3 can be scaled by a discrete complex value to construct
d. Furthermore, there can be more than one high end RFs. In case of
two high end RFs, the line 320 in FIG. 3C would be a parallelogram
or ellipse for case the AEs have individual or total power
constraints, respectively. An example with two AEs namely, AE 4 and
AE 5 each connected to one high end RF is shown in FIG. 3D and FIG.
3E for individual and total power constraints, respectively. The
points h4 and h5 in FIGS. 3D and 3E correspond to AE 4 and AE 5
respectively. The size of the parallelogram 340 shown in FIG. 3D
and the size of the size of the ellipse 345 shown in FIG. 3E are
proportional to the power available at the transmitter. The length
of the sides of the parallelogram 340 which are parallel to line
341 are proportional to the individual transmit power of AE 5, and
the length of the sides of the parallelogram 340 that are parallel
to line 342 are proportional to the individual transmit power of AE
4. The size of the ellipse 345 shown in FIG. 3E is proportional to
the total transmit power of AE 4 and AE 5. Without the transmit
power constraint (i.e. with infinite power) the projection area
will be the complete two dimensional subspace spanned by the signal
received at UE1 and UE2.
[0041] One example implementation of the determination of the order
in which the constrained RFs precoders are chosen is based on the
following iterative process: out of all available constrained RFs'
AE, at each step, precoding coefficients corresponding to one AE
are determined such that the chosen AE together with all the
previously chosen AE's precoding coefficients the error is
minimized between the desired signal subspace and the signal
generated at the receiver. This is commonly referred to as the
Knapsack algorithm, where the best order out of many combinations
is found. According to exemplary embodiments, each search step
further includes accounting for the fact that the high end RFs can
be used to produce high resolution signals to compensate for the
difference between the desired signal, d, and the signal generated
over the air using the constrained RFs. It is noted that the
Knapsack algorithm is suboptimal as it looks for the best AE in
each step sequentially; whereas combining the high end RF is
optimal as the influence of all the high end RFs are taken into
account in each step.
[0042] Referring now to FIG. 4, this figure shows MSE as a function
of increasing the number of constrained RFs in accordance with
exemplary embodiments. MSE performance according to exemplary
embodiments described herein is indicated by line 408 for the case
of 10 UEs served by a base station with 4 high end RFs and varying
number of constrained RFs. The line 404 shows the case where only
the constrained RFs is used and the AEs are chosen based on
knapsack (KS) algorithm. Line 406 shows the MSE for the case where
first the AE with constrained RFs are chosen based on KS and then
the high end RFs are used to minimize the remaining MSE in a single
step. Line 402 shows the algorithm where the ZF coefficients are
quantized for the constrained RFs (e.g. as described in U.S. Pat.
No. 9,231,676). It can be seen that line 408 shows an 11 dB better
performance than line 402 corresponding to the reference algorithm
when 96 constrained RFs and 4 high end RFs are utilized. It is
noted that no power constraint has been considered in FIG. 4 with
respect to line 402.
[0043] Referring now to FIG. 5, this figure shows two bar graphs
showing distribution of transmit power for high end RFs in dBW in
accordance with exemplary embodiments. The first bar graph 502 is
for 20 constrained RFs and the second graph 504 is for 100
constrained RFs. In each of the graphs 502, 504 in FIG. 5, 0 dBW
corresponds to maximum transmit power per AE. It can be seen that
for most of the cases the transmit power constraint is satisfied
automatically.
[0044] Grouping the high end RFs together and representing their
signal space at the receiver in the form of a subspace provides
opportunity to group the AEs of constrained RFs into capacity
providing AEs and energy saving AEs: The capacity providing AEs may
corresponds to AEs whose instantaneous channel coefficients help in
moving towards the high end RF subspace during the iterative steps.
The energy saving AEs are the AEs whose instantaneous channel
coefficients help in moving within the high end RF subspace there
by reducing the power required by the high end RFs. The
coefficients of the different groups can be weighted differently in
order to either save energy of the high end RFs or increase the
capacity of the system. Furthermore, this grouping could be used to
reduce the computational complexity by choosing the AEs in each
group after the other group AEs. E.g. if the base station is
operating in energy saving mode, then the energy saving AEs are
designed first and then the capacity providing RFs. For the
capacity maximization mode, the order is reversed. In capacity
maximization mode, the channel coefficients from the energy saving
AEs could be updated less often than that of other AEs, which can
be triggered by the base station.
[0045] Alternatively, one can use a weighted optimization criterion
in each iteration of the Knapsack algorithm so that capacity and
power saving are balanced constantly and all antenna elements can
be used all the time.
[0046] FIG. 6 is a logic flow diagram for precoder design for
combining high-end RF with constrained RF of massive MIMO antennas.
This figure further illustrates the operation of an exemplary
method or methods, a result of execution of computer program
instructions embodied on a computer readable memory, functions
performed by logic implemented in hardware, and/or interconnected
means for performing functions in accordance with exemplary
embodiments. For instance, the MIMO module 150 may include
multiples ones of the blocks in FIG. 6, where each included block
is an interconnected means for performing the function in the
block. The blocks in FIG. 6 are assumed to be performed by a base
station such as eNB 170, e.g., under control of the MIMO module 150
at least in part.
[0047] With reference to FIG. 6, in one example embodiment a method
may comprise: mapping a signal desired by at least one receiver to
a projection area based at least on a functionality corresponding
to one or more first radio frequency chains coupled to a plurality
of first antennas as indicated by block 602; selecting precoding
coefficients for at least one of one or more second radio frequency
chains coupled to a plurality of second antennas to generate a
signal point within the projection area as indicated by block 604;
and compensating for a difference between the generated signal
point and the signal desired by the at least one receiver using at
least one of the first radio frequency chains, wherein the second
radio frequency chains have a reduced functionality relative to the
functionality of the first radio frequency chains, and wherein the
first and second set of antennas are different as indicated by
block 606.
[0048] For the case the one or more second radio frequency chains
are not capable of generating a signal point within the projection
area, selecting the precoding coefficients may include: selecting
precoding coefficients for at least one of the second radio
frequency chains for generating a signal point closest to the
projection area. The first antennas and the second antennas may be
collocated. The first antennas may be in a different location than
the second antennas. The first antennas may be located at a first
base station and the second antennas may be located at a second
base station, and the first base station may provide a larger cell
than the second base station. The method may include receiving,
from the first base station via an X2 interface, configuration
information for the one or more first radio frequency chains for
mapping the desired signal to the projection area. The method may
include receiving, from the second base station via an X2
interface, information for determining the difference between the
generated signal point and the signal desired for compensating for
the difference. The functionality of the first radio frequency
chains may be based at least in part on a plurality of features and
the second radio frequency chains may have a reduced functionality
because one or more features for the second radio frequency chains
are relaxed relative to identical one or more features for the
first radio frequency chains. The features may correspond to at
least one of: transmission power, amplifier character like
operating region, bit resolution, and analog filters. A shape of
the projection area may be based on at least one of: the number of
first radio frequency chains, a total power constraint of the one
or more antenna elements, individual power constraints of each of
the one or more antenna elements, and the channel coefficient of
each of the one or more antenna elements; and a size of the
projection area may be based on at least one of: a total power
constraint of the one or more antenna elements, and individual
power constraints of each of the one or more antenna elements, the
channel coefficient of each of the one or more antenna elements.
The shape of the projection area may be at least one of: a
subspace, an ellipsoid, and a polytope.
[0049] According to another example embodiment, an apparatus may
comprise at least one processor; and at least one memory including
computer program code, the at least one memory and the computer
program code configured to, with the at least one processor, cause
the apparatus to perform at least the following: map a signal
desired by at least one receiver to a projection area based at
least on a functionality corresponding to one or more first radio
frequency chains coupled to a plurality of first antennas; select
precoding coefficients for at least one of one or more second radio
frequency chains coupled to a plurality of second antennas to
generate a signal point within the projection area; and compensate
for a difference between the generated signal point and the signal
desired by the at least one receiver using at least one of the
first radio frequency chains, wherein the second radio frequency
chains have a reduced functionality relative to the functionality
of the first radio frequency chains, and wherein the first and
second set of antennas are different.
[0050] For case the one or more second radio frequency chains are
not capable of generating a signal point within the projection
area, selection of the precoding coefficients may include:
selecting precoding coefficients for at least one of the second
radio frequency chains for generating a signal point closest to the
projection area. The first antennas and the second antennas may be
collocated. The apparatus may be a base station and may further
comprise at least one of: the first antennas and the second
antennas. At least one of the first antennas and the second
antennas may be located at another base station. The at least one
memory and the computer program code may be configured to, with the
at least one processor, cause the apparatus to perform: receiving,
from the first base station via an X2 interface, configuration
information for the one or more first radio frequency chains for
mapping the desired signal to the projection area. The at least one
memory and the computer program code may be configured to, with the
at least one processor, cause the apparatus to perform: receive,
from the second base station via an X2 interface, information for
determining the difference between the generated signal point and
the signal desired for compensating for the difference. The
functionality of the first radio frequency chains may be based at
least in part on a plurality of features and wherein the second
radio frequency chains may have a reduced functionality because one
or more features for the second radio frequency chains are relaxed
relative to identical one or more features for the first radio
frequency chains. The features may correspond to at least one of:
transmission power, amplifier character like operating region, bit
resolution, and analog filters.
[0051] According to another embodiment, a computer program product
may comprise a non-transitory computer-readable medium storing
computer program code thereon which when executed by a device
causes the device to perform at least: mapping a signal desired by
at least one receiver to a projection area based at least on a
functionality corresponding to one or more first radio frequency
chains coupled to a plurality of first antennas; selecting
precoding coefficients for at least one of one or more second radio
frequency chains coupled to a plurality of second antennas to
generate a signal point within the projection area; and
compensating for a difference between the generated signal point
and the signal desired by the at least one receiver using at least
one of the first radio frequency chains, wherein the second radio
frequency chains have a reduced functionality relative to the
functionality of the first radio frequency chains, and wherein the
first and second set of antennas are different.
[0052] Without in any way limiting the scope, interpretation, or
application of the claims appearing below, a technical effect of
one or more of the example embodiments disclosed herein is 10 UEs
can be supported using only 4 high end RFs and MSE up to -17 dB is
achievable with 96 constrained RFs. Another technical effect of one
or more of the example embodiments disclosed herein is, in
comparison to having only constrained RFs, few high end RFs with
high precision helps in achieving minimizing the error to a large
extent. Another technical effect of one or more of the example
embodiments disclosed herein is, in comparison to having only Full
RF, now less power will be necessary due to the diversity gain
introduced due to the constrained RF chain AEs. Another technical
effect of one or more of the example embodiments disclosed herein
is different power constraints namely, maximum power constraint or
total power constraint can be addressed. Depending on the power
constraint the projection area where the desired signal is
projected will change. The proposed invention can be used in all
these cases.
[0053] Embodiments herein may be implemented in software (executed
by one or more processors), hardware (e.g., an application specific
integrated circuit), or a combination of software and hardware. In
an example embodiment, the software (e.g., application logic, an
instruction set) is maintained on any one of various conventional
computer-readable media. In the context of this document, a
"computer-readable medium" may be any media or means that can
contain, store, communicate, propagate or transport the
instructions for use by or in connection with an instruction
execution system, apparatus, or device, such as a computer, with
one example of a computer described and depicted, e.g., in FIG. 1.
A computer-readable medium may comprise a computer-readable storage
medium (e.g., memories 125, 155, 171 or other device) that may be
any media or means that can contain, store, and/or transport the
instructions for use by or in connection with an instruction
execution system, apparatus, or device, such as a computer. A
computer-readable storage medium does not comprise propagating
signals.
[0054] If desired, the different functions discussed herein may be
performed in a different order and/or concurrently with each other.
Furthermore, if desired, one or more of the above-described
functions may be optional or may be combined.
[0055] Although various aspects of the invention are set out in the
independent claims, other aspects of the invention comprise other
combinations of features from the described embodiments and/or the
dependent claims with the features of the independent claims, and
not solely the combinations explicitly set out in the claims.
[0056] It is also noted herein that while the above describes
example embodiments of the invention, these descriptions should not
be viewed in a limiting sense. Rather, there are several variations
and modifications which may be made without departing from the
scope of the present invention as defined in the appended
claims.
[0057] The following abbreviations that may be found in the
specification and/or the drawing figures are defined as follows:
[0058] eNB (or eNodeB) evolved Node B (e.g., an LTE base station)
[0059] ADC analog-to-digital converter [0060] AE antenna element
[0061] I/F interface [0062] LTE long term evolution [0063] MIMO
multiple input multiple output [0064] MME mobility management
entity [0065] NCE network control element [0066] N/W network [0067]
RF-Chain radio frequency chain [0068] RRH remote radio head [0069]
Rx receiver [0070] SGW serving gateway, [0071] Tx transmitter
[0072] UE user equipment (e.g., a wireless, typically mobile
device)
* * * * *