POWER ALLOCATION IN A WIRELESS SYSTEM WITH BASE
STATIONS HAVING ANTENNA ARRAYS
BACKGROUND OF THE INVENTION
FIELD
Embodiments relate to power allocation in Wireless multiple-inputmultiple-
output (MIMO) systems with base stations having relatively
large antenna arrays.
DESCRIPTION OF THE RELATED ART
Wireless Time Division Duplex (TDD) multiple-input-multiple-output
(MIMO) systems represent an advance in wireless communication.
MIMO systems employ two or more antennas at the transmitting
and/ or receiving ends of a wireless link. The multiple antennas
improve data transmission rates, while holding radio bandwidth and
power constant.
A MIMO transmitter transmits an outgoing signal using multiple
antennas by demultiplexing the outgoing signal into multiple subsignals
and transmitting the sub-signals from separate antennas.
MIMO exploits the multiple signal propagation paths to increase
throughput and reduce bit error rates. Using MIMO techniques the
rate of transmission increases linearly depending on the local
environment.
A typical TDD wireless system may service a relatively large
geographic area organized in relatively smaller geographic units
known as cells. Each cell includes a base station serving n mobiles
also called mobile users, or user equipments, etc. All user
equipments in all cells send pilot signals to the corresponding base
stations. In each cell n user equipments send orthogonal pilot signals
= l,...,n to the associated base station.
As is known, when the number of antennas associated with a base
station in a MIMO system becomes very large the performance of the
system virtually stops depending on additive noise, which is
necessarily present in the receiver of each base station antenna
(e.g., electronic equipment associated with each antenna which may
be processing signals arrived to that antenna). However, increasing
data transmission rates becomes difficult due to inter-cell interference
caused by contamination of downlink signals transmitted to user
equipment located in different cells.
Contamination of downlink signals is caused by the contamination of
pilot signals arriving at a base station from user equipment located in
different cells.
The contamination of the pilot signals is unavoidable due to reuse of
the pilot sequences by user equipments associated with base stations
in different cells. In other words, if a -th user equipment associated
with a base station in cell 1 uses pilot \ k and in a neighboring cell 2
there is a user equipment associated base station in cell 2 that also
uses pilot \ k , then the downlink signal transmitted from the base
station in cell 1 to the k-th user in cell 1 interferes with the downlink
signal transmitted from the base station 2 to the k-th users in cell 2.
SUMMARY OF THE INVENTION
One embodiment includes a method for allocating transmit power in a
wireless network. The method includes determining powers at which
a base station transmits downlink signals to each active user
equipment associated with the base station such that each of the
downlink signals is associated with a different active user and the
base station is permitted to transmit downlink signals with different
powers to the associated active user equipments. The method further
includes transmitting the associated downlink signals, by the base
station, to the active user equipments at the determined powers.
One embodiment includes a base station including a controller. The
controller is configured to determine powers at which the base station
transmits downlink signals to each active user equipments associated
with the base station such that each of the downlink signals is
associated with a different active user and the base station is
permitted to transmit downlink signals with different powers to the
associated active user equipments. The controller is further
configured to transmit the associated downlink signals, using a
plurality of antennas associated with the base station, to the active
user equipments at the determined powers.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description given herein below and the accompanying
drawings, wherein like elements are represented by like reference
numerals, which are given by way of illustration only and thus are not
limiting of the present invention and wherein:
FIG. 1 illustrates a portion of a multiple-input-multiple-output (MIMO)
wireless broadcast system according to example embodiments.
FIG. 2 illustrates a method for allocating power in a multiple-inputmultiple-
output (MIMO) wireless broadcast system according to
example embodiments.
FIG. 3 illustrates a method for allocating power in a multiple-inputmultiple-
output (MIMO) wireless broadcast system according to
example embodiments.
FIG. 4 illustrates a base station according to example embodiments.
It should be noted that these Figures are intended to illustrate the
general characteristics of methods, structure and / or materials utilized
in certain example embodiments and to supplement the written
description provided below. These drawings are not, however, to scale
and may not precisely reflect the precise structural or performance
characteristics of any given embodiment, and should not be
interpreted as defining or limiting the range of values or properties
encompassed by example embodiments. For example, the relative
thicknesses and positioning of molecules, layers, regions and/ or
structural elements may be reduced or exaggerated for clarity. The use
of similar or identical reference numbers in the various drawings is
intended to indicate the presence of a similar or identical element or
feature.
DETAILED DESCRIPTION OF THE EMBODIMENTS
While example embodiments are capable of various modifications and
alternative forms, embodiments thereof are shown by way of example
in the drawings and will herein be described in detail. It should be
understood, however, that there is no intent to limit example
embodiments to the particular forms disclosed, but on the contrary,
example embodiments are to cover all modifications, equivalents, and
alternatives falling within the scope of the claims. Like numbers refer
to like elements throughout the description of the figures.
Before discussing example embodiments in more detail, it is noted
that some example embodiments are described as processes or
methods depicted as flowcharts. Although the flowcharts describe the
operations as sequential processes, many of the operations may be
performed in parallel, concurrently or simultaneously. In addition,
the order of operations may be re-arranged. The processes may be
terminated when their operations are completed, but may also have
additional steps not included in the figure. The processes may
correspond to methods, functions, procedures, subroutines,
subprograms, etc.
Methods discussed below, some of which are illustrated by the flow
charts, may be implemented by hardware, software, firmware,
middleware, microcode, hardware description languages, or any
combination thereof. When implemented in software, firmware,
middleware or microcode, the program code or code segments to
perform the necessary tasks may be stored in a machine or computer
readable medium such as a storage medium. A processor(s) may
perform the necessary tasks.
Specific structural and functional details disclosed herein are merely
representative for purposes of describing example embodiments of the
present invention. This invention may, however, be embodied in many
alternate forms and should not be construed as limited to only the
embodiments set forth herein.
It will be understood that, although the terms first, second, etc. may
be used herein to describe various elements, these elements should
not be limited by these terms. These terms are only used to
distinguish one element from another. For example, a first element
could be termed a second element, and, similarly, a second element
could be termed a first element, without departing from the scope of
example embodiments. As used herein, the term "and/or" includes
any and all combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as being
"connected" or "coupled" to another element, it can be directly
connected or coupled to the other element or intervening elements
may be present. In contrast, when an element is referred to as being
"directly connected" or "directly coupled" to another element, there are
no intervening elements present. Other words used to describe the
relationship between elements should be interpreted in a like fashion
{e.g., "between" versus "directly between," "adjacent" versus "directly
adjacent," etc.).
The terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of example
embodiments. As used herein, the singular forms "a," "an" and "the"
are intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will be further understood that the
terms "comprises," "comprising," "includes" and/ or "including," when
used herein, specify the presence of stated features, integers, steps,
operations, elements and/ or components, but do not preclude the
presence or addition of one or more other features, integers, steps,
operations, elements, components and/ or groups thereof.
It should also be noted that in some alternative implementations, the
functions/ acts noted may occur out of the order noted in the figures.
For example, two figures shown in succession may in fact be executed
concurrently or may sometimes be executed in the reverse order,
depending upon the functionality/ acts involved.
Unless otherwise defined, all terms (including technical and scientific
terms) used herein have the same meaning as commonly understood
by one of ordinary skill in the art to which example embodiments
belong. It will be further understood that terms, e.g., those defined in
commonly used dictionaries, should be interpreted as having a
meaning that is consistent with their meaning in the context of the
relevant art and will not be interpreted in an idealized or overly formal
sense unless expressly so defined herein.
Portions of the example embodiments and corresponding detailed
description are presented in terms of software, or algorithms and
symbolic representations of operation on data bits within a computer
memory. These descriptions and representations are the ones by
which those of ordinary skill in the art effectively convey the
substance of their work to others of ordinary skill in the art. An
algorithm, as the term is used here, and as it is used generally, is
conceived to be a self-consistent sequence of steps leading to a desired
result. The steps are those requiring physical manipulations of
physical quantities. Usually, though not necessarily, these quantities
take the form of optical, electrical, or magnetic signals capable of
being stored, transferred, combined, compared, and otherwise
manipulated. It has proven convenient at times, principally for
reasons of common usage, to refer to these signals as bits, values,
elements, symbols, characters, terms, numbers, or the like.
In the following description, illustrative embodiments will be described
with reference to acts and symbolic representations of operations (e.g.,
in the form of flowcharts) that may be implemented as program
modules or functional processes include routines, programs, objects,
components, data structures, etc., that perform particular tasks or
implement particular abstract data types and may be implemented
using existing hardware at existing network elements. Such existing
hardware may include one or more Central Processing Units (CPUs),
digital signal processors (DSPs), application-specific-integratedcircuits,
field programmable gate arrays (FPGAs) computers or the
like.
It should be borne in mind, however, that all of these and similar
terms are to be associated with the appropriate physical quantities
and are merely convenient labels applied to these quantities. Unless
specifically stated otherwise, or as is apparent from the discussion,
terms such as "processing" or "computing" or "calculating" or
"determining" of "displaying" or the like, refer to the action and
processes of a computer system, or similar electronic computing
device, that manipulates and transforms data represented as physical,
electronic quantities within the computer system's registers and
memories into other data similarly represented as physical quantities
within the computer system memories or registers or other such
information storage, transmission or display devices.
Note also that the software implemented aspects of the example
embodiments are typically encoded on some form of program storage
medium or implemented over some type of transmission medium. The
program storage medium may be magnetic (e.g., a floppy disk or a
hard drive) or optical (e.g., a compact disk read only memory, or "CD
ROM"), and may be read only or random access. Similarly, the
transmission medium may be twisted wire pairs, coaxial cable, optical
fiber, or some other suitable transmission medium known to the art.
The example embodiments not limited by these aspects of any given
implementation .
As used herein, the term "user equipment" (UE) may be considered
synonymous to, and may hereafter be occasionally referred to, as a
client, mobile unit, mobile station, mobile user, mobile, subscriber,
user, remote station, access terminal, receiver, etc., and may describe
a remote user of wireless resources in a wireless communication
network.
Similarly, as used herein, the term "base station" may be considered
synonymous to, and may hereafter be occasionally referred to, as a
Node B, extended Node B, base transceiver station (BTS), etc., and
may describe a transceiver in communication with and providing
wireless resources to mobiles in a wireless communication network.
As discussed herein, base stations may have all functionally
associated with conventional, well-known base stations in addition to
the capability to perform the methods discussed herein.
FIG. 1 illustrates a portion of a multiple-input-multiple-output (MIMO)
wireless broadcast system according to example embodiments.
Example embodiments will be described in the general sense with
respect to the conventional MIMO system in FIG. 1. The MIMO system
of FIG. 1 may use time division duplexing (TDD) or frequency division
duplexing (FDD). However, it will be understood that example
embodiments may be implemented in other MIMO systems as well as
other wireless communication systems.
Referring to FIG. 1, cells 105- 1 and 105-2 include base stations 115-
1, 115-2 having antennas 120- 1 and 120-2 respectively. Cell 105- 1
includes user equipments 110- 1, 110-3, 110-n and cell 105-2
includes user equipment 110-2. As shown in FIG. 1, antenna 120- 1
transmits a downlink signal to user equipment 110- 1 and that
downlink signal may also propagate to user equipment 110-2. As is
described above, the downlink signal transmitted by antenna 120- 1 to
user equipment 110-2 may cause inter-cell interference with signals
transmitted by antenna 120-2 to user equipment 110-2.
For the purposes of discussing example embodiments associated with
FIG. 1 and the subsequent equations, cell 105- 1 is the j-th cell and
cell 105-2 is the Z-th cell. Further, for the purposes of discussing
example embodiments associated with FIG. 1 and the subsequent
equations, user equipment 110- 1 is the k-th mobile with regard to the
j-th cell 105- 1 and user equipment 110-2 is the k-t mobile with
regard to the Z-th cell 105-2. Further, the k-th mobile in the j-th cell
and the k-th mobile in the Z-th cell use the same pilot sequence (e.g.,
.
As illustrated in FIG. 1, the downlink signals transmitted by antenna
120- 1 and received by users 110- 1 and 110-2, respectively, may be
mathematically described as Pj kj
n d Pj ki respectively. Where
Pjk i the power with which the j-th base station (e.g., base station
115- 1) transmits to the k-th mobile (e.g., user equipment 110- 1) in the
j-th cell (e.g., cell 105- 1), - - are the slowly fading coefficients
between the j-th base station (e.g., base station 115- 1) and the k-th
mobile (e.g., user equipment 110- 1) in the j-th cell (e.g., cell 105- 1),
and j l are the slowly fading coefficients between the j-th base
station (e.g., base station 115- 1) and the k-th mobile (e.g., user
equipment 110-2) in the k-th cell (e.g., cell 105-2).
Based on the above equations, the Signal to Interference Ratio (SIR) of
the t-th user equipment (e.g., user equipment 110-2) in the 1-th cell
(e.g., cell 105-2) is:
Equation 1:
2
Based on equation 1, intuitively the smaller the power p , the smaller
the interference created by the j-th base station to any user equipment
in other cells. However, if p is too small the downlink signal
transmitted to the k-th user equipment in the j-th cell by a base
station in the j-cell will be too weak for the k-th user equipment to use
the downlink signal.
The maximum possible data transmission rate for the k-th user
equipment in the 1-th cell may be approximately equal to:
Equation 2 :
R « log(l + SIR + additive noise)
As is known, if a base station includes a relatively large number of
base station antennas, the additive noise may be relatively small.
Therefore, equation 2 becomes:
Equation 3 :
log(l + ¾ )
A known performance measure in wireless systems is the smallest
transmission data rate among N% (e.g., N=95%) of the best user
equipments (e.g., the smallest SIR among N% of user equipments). In
other words we drop (100-N)% of the worst user equipments and
analyze the smallest SIR of the remaining N% users (e.g., 95% of the
user equipments). This rate may be represented as SIRN . Example
embodiments provide at least one method for choosing base station
transmit powers pl that maximize SIRN (e.g., the N user equipments
with the lowest SIR).
Example embodiments described below with regard to FIGS. 2 and 3
illustrate a method for choosing optimal powers pl for a fixed k and
for 1=1, ...,L. L is the total number of cells in the wireless network.
The method also applies to the j-t ,j k user equipments in the L
cells.
The coefficients jki may change slowly. As a result, coefficients 
may be estimated by the corresponding base stations and user
equipments. Coefficients jki may be updated regularly. Therefore,
example embodiments assume that all l , j,l=l,...L, are available to
all L base stations.
As is known, base stations may be connected with each other by a
backbone transmission medium. Therefore, the base stations may
send each other the corresponding coefficients j ki making each of
the corresponding coefficients jki available to all other base stations
in the wireless network. Further, the backbone transmission medium
provides a mechanism by which the base stations may exchange with
other base station in the wireless network user equipment
information. For example, transmit powers ¾ and user equipment
status (e.g., active or passive) may be exchanged via the backbone
transmission medium.
The backbone media may also be used for transmitting coefficients
to a central controller and for transmitting allocated powers
p lk r vs the controller back to base stations. The network may include
up to n controllers (n is the length of pilot signals) . Further, for a given
k the coefficients 1 for all j,l =\,...,L, (L is the total number of cells)
may be transmitted to one and the same k-th controller. In other
words, the coefficients 1 corresponding to the k-th user equipment,
that are the users that share the same pilot signal v in all cells, may
be collected in the k-th controller.
At relatively the same time the coefficients , j,l =\,...,L, (where t k )
may be collected by the t-th controller. The k-th controller, as it is
described below, may select N% of active users among the k-th users
in L cells, and may compute optimal powers p kl for the active users.
The k-th controller may then transmit these powers p kl to the
corresponding base stations. Note that any base station may be the kth
controller. Alternatively, the k-th controller may organized as a
separate unit connected to all base stations. Note also that several or
all controllers may be combined in one and the same network unit.
Example embodiments maximize data rates for N% of the user
equipments (e.g., N% of user equipments with the relatively best SIR).
Therefore, example embodiments may partition k-th user equipments
in L cells into active and passive users. For example, the k-th user
equipment in the 1-th cell may be determined to be passive if p l = 0
for the k-th user equipment (e.g., the 1-th base station is not currently
transmitting to the k-th user equipment).
FIG. 2 illustrates a method for allocating power in a multiple-inputmultiple-
output (MIMO) wireless broadcast system according to
example embodiments. The example embodiment described below
with regard to FIG. 2 is described with regard to a base station
controller. However, example embodiments are not limited thereto.
Referring to FIG. 2, in step S205 a controller, for example a controller
(e.g., controller 4 10 described below with regard to FIG. 4 ) associated
with a base station (e.g., base station 115- 1) determines the active
user equipments associated with each base station in a wireless
network. For example, as described above, the k-th user equipment
(e.g., user equipment 110- 1) in the j-th cell may be determined to be
passive if p k = 0 . By contrast, the k-th user equipment in the 1-th cell
(e.g., user equipment 110-2) may be determined to be active if 0 .
In step S2 10, the controller determines powers at which base stations
in different cells transmit downlink signals to the k-th active user
equipments associated with the corresponding base stations, such
that each of the downlink signals is associated with a different active
user and the base station is permitted to transmit downlink signals
with different powers to the associated active user equipments.
For example, referring to FIG. 1, the controller may determine that the
k-th user 110-2 in cell 1 is active and further determines the power
p_{lk}, with which the base station in cell 1 (e.g., base station 115-2)
will transmit to this user. While the k-th user 110- 1 in the j-th cell
may be passive.
The power determination may be based on maximizing a power
associated with the associated active user equipments and minimizing
interferences for user equipments associated with another base
station. The signal to interference ratio for a given user may be based
on the transmitted associated downlink signals and the downlink
signals transmitted by another base station sharing a same pilot
sequence.
The signal to interference ratio may also be based on fading
coefficients associated with the transmitted associated downlink
signals and fading coefficients associated with downlink signals
transmitted by the another base station.
Determining powers at which a base station transmits downlink
signals may also include calculating a variable based on a maximum
signal to interference ratio and a minimum signal to interference ratio,
and determining a minimum power solution of a linear function of
powers subject to a constraint based on the variable.
Each of the above examples is described in more detail with regard to
FIG. 3 below.
Returning to FIG. 2, in step S2 15 the controller determines whether or
not a number of active user equipment is less than or equal to a
predetermined percentage. If a number of active user equipments is
less than or equal to a predetermined percentage, then processing
continues to step S220. Otherwise, processing moves to step S225.
Determining if a number of active user equipments is less than or
equal to a predetermined percentage is described in more detail above.
For example, the predetermined percentage may be N = 95%.
In step S220, the controller transmits the downlink signals associated
with each active user at the powers determined in step S 10. For
example, in a MIMO wireless system (as described above) base station
115- 1, 115-2 transmits an outgoing signal using multiple antennas by
demultiplexing the outgoing signal into multiple sub-signals and
transmitting the sub-signals from separate antennas. Transmitting
downlink signals is known to those skilled in the art and will not be
described further for the sake of brevity.
If, in step S2 15, the controller determines a number of active user
equipments is greater than a predetermined percentage, processing
continues to step S225. In step S225, the controller drops the active
user equipment with the largest determined power.
For example, the controller may set the transmission power for one of
the associated user equipments (e.g., user equipment 110- 1) to zero
(0). Other methods for dropping a user equipment are known to those
skilled in the art are known to those skilled in the art and will not be
described further for the sake of brevity. After the user equipment is
dropped, processing returns to step S2 10.
As one skilled in the art will appreciate, by dropping the user
equipment with the largest power, SIR effects on other user equipment
may be relatively reduced the most because the downlink signal
transmitted at the highest power creates the strongest interference to
other user equipments.
FIG. 3 illustrates a method for allocating power in a multiple-inputmultiple-
output (MIMO) wireless broadcast system according to
example embodiments.
Referring to FIG. 3, in step S305 the controller assigns a constant
power level to each active user associated with each base station in
each cell of the wireless network. Alternatively, the controller can
assign a constant power to each active user of some subset of the base
stations and cells in the network. For example, each active user
equipment 110- 1 to 110-2 may be assigned p lk = p k =P , where is
any constant (e.g., P= l).
In step S3 10, the controller calculates a Signal to Interference Ratio
(SIR) for each active user equipment associated with each base
station. For example, SIR may be calculated using equation 1
described above. In step S3 15, the controller calculates the minimum
SIR (Smin) for the set of active user equipment. Smm may be calculated
with equal powers p l = P . Therefore, with optimal (unequal) powers
p l , SIRs for active users will be at least as large as Smm . For
example, Smm may be calculated as follows:
Equation 4 :
In step S320, the controller calculates an ideal maximum SIR (Smax) .
For example, any Smax that is greater than any SIR that would
optimally be achieved with optimal powers p l . For example Smax may
be calculated to be:
Equation 5 :
max 000 • S m m
In step S325, the controller chooses a linear function of powers. The
linear function of power is a design choice decision. For example, the
linear function of powers may be chosen to be:
Equation 6 :
( t P k ) ~ Plk Plk •" PKk , where K is
the number of active users sharing the same pilot sequence Vk.
In step S330, the controller calculates a variable s based on SIR. The
formula for calculating s is a design choice decision. For example, the
formula for calculating s may be:
Equation 7 :
max — mmmm /
In step S335, the controller determines a minimum power solution to
the linear power function chosen in step S325 (e.g., equation 6 )
subject to a constraint based on s and equation 1. For example, any
linear programming method (e.g., the known simplex method) may be
used to check feasibility of the solution of the following problem:
Minimize P k P k ) P\k + Plk + P
Subject to:
Equation 8 (based on equation 1):
Equation 9 :
p l 0, 1= 1,..., K
If, in step S340, the controller determines a solution to the minimum
power solution in step S335 is found, processing continues to step
S345. Otherwise, processing moves to step S350. In step S345, the
controller sets Smm is to the value of s and processing moves to step
S355. In step S350, the controller sets Smax set to the value of s and
processing moves to step S355. In step S355 the controller calculates
Smax - Smin and compares Smax - Smin to a value. If Smax - Smin is less
than the value, processing moves to step S360. Otherwise, processing
returns to step S330.
For example, If Smax - Smin is relatively small, e.g., if
smax - sm <0.5 dB , processing stops and the controller uses the
found powers p ik =\,...,K, otherwise the process is repeated (e.g.,
processing returns to step S330). In step S360, the controller uses
the powers equal the determined minimum powers. For example, the
determined powers of step S205 above are equal to the powers
determined in step S335.
Although FIGS. 2 and 3 illustrate methods for allocating power in a
multiple-input-multiple-output (MIMO) wireless broadcast system,
example embodiments are not limited thereto. It will be understood
by one of ordinary skill in the art that variations for allocating power
based on for example alternative equations and formulas from those
described above with regard to FIGS. 2 and 3.
FIG. 4 illustrates a base station according to example embodiments.
Referring to FIG. 4, a base station 405 includes a controller 4 10. The
controller 4 10 includes a processor (P) 420 that may be configured to
perform the steps as described above with regard to FIGS. 2 and 3.
Antennas 4 15 may transmit the associated downlink signals as
described above with regard to, for example, step S220 as shown in
FIG. 2. Although three antennas 4 15 are illustrated, example
embodiments are not limited thereto. For example, Time Division
Duplex (TDD) multi-cell multi-user (MIMO) wireless systems may
include 100 to 400 and more antennas 4 15.
Although FIGS. 2-4 illustrate methods and an apparatus for allocating
power in a multiple-input-multiple-output (MIMO) wireless broadcast
system with reference to a base station controller, example
embodiments are not limited thereto. For example, alternatively, each
base station in the wireless broadcast system may make the power
allocation determinations for each user equipment having an index k
across each cell in the wireless broadcast system. In other words this
base station will serve as a controller for the k-th users across all cells
in the wireless broadcast system.
Still further, alternatively, a central network element or controller
associated with the wireless broadcast system may make the power
allocation determinations for each user equipment in all cells
associated with the wireless broadcast system. Such variations will be
understood by those skilled in the art and are not to be regarded as a
departure from the invention, and all such modifications are intended
to be included within the scope of the invention.
Alternative embodiments of the invention may be implemented as a
computer program product for use with a computer system, the
computer program product being, for example, a series of computer
instructions, code segments or program segments stored on a tangible
or non-transitory data recording medium (computer readable
medium), such as a diskette, CD-ROM, ROM, or fixed disk, or
embodied in a computer data signal, the signal being transmitted over
a tangible medium or a wireless medium, for example, microwave or
infrared. The series of computer instructions, code segments or
program segments can constitute all or part of the functionality of the
methods of example embodiments described above, and may also be
stored in any memory device, volatile or non-volatile, such as
semiconductor, magnetic, optical or other memory device.
While example embodiments have been particularly shown and
described, it will be understood by one of ordinary skill in the art that
variations in form and detail may be made therein without departing
from the spirit and scope of the claims.
The invention being thus described, it will be obvious that the same
may be varied in many ways. Such variations are not to be regarded
as a departure from the invention, and all such modifications are
intended to be included within the scope of the invention.
WE CLAIM:
1. Amethod for allocating transmit power in a wireless network
105- 1, 105-2, the method comprising:
Determining S2 10 powers at which a base station 115- 1, 115-2,
405 transmits downlink signals to each active user equipment
associated with the base station such that each of the downlink
signals is associated with a different active user and the base station
is permitted to transmit downlink signals with different powers to the
associated active user equipments; and
transmitting the associated downlink signals, by the base
station, to the active user equipments at the determined powers.
2. The method of claim 1, wherein the determined powers are
based on a power associated with the associated active user
equipments and a signal to interference ratio for user equipment
associated with another base station.
3. The method of claim 2, wherein the signal to interference ratios
are based on downlink signals transmitted by the another base station
and on the transmitted associated downlink signals.
4. The method of claim 3, wherein the signal to interference ratio is
reduced between the transmitted associated downlink signals and the
downlink signals transmitted by the another base station sharing a
same pilot sequence.
5. The method of claim 3, wherein the signal to interference ratio is
based on fading coefficients associated with the transmitted
associated downlink signals and fading coefficients associated with
downlink signals transmitted by the another base station.
6. The method of claim 5 wherein
where,
SIR is the signal to interference ratio for the k h user in the 1
cell,
K is the number of active users that share the same pilot
sequence with the k user in the Ith cell,
P is the transmission power for the k 1 user in the 1 cell,
is the fading coefficient between the Ith cell transmission
antennas and the the user in the Ith cell,
is the transmission power for the k h user in the j th cell, and
jkl is the fading coefficient between the j cell transmission
antennas and the the k 1 user in the Ith cell.
7. The method of claim 1, wherein the determining powers at
which a base station transmits downlink signals step includes,
calculating a variable based on a maximum signal to
interference ratio and a minimum signal to interference ratio, and
determining a minimum power solution of a linear power
function subject to a constraint based on the variable.
8. The method of claim 7, wherein
s = (SIRmax - SIRmin) / 2 under the constraint of
PlkPlkl
,. .,, where
PjkPjkl
p 0,1 =1,. K , where
s is the variable,
Kis the number of active users,
P is the transmission power for the k user in the 1th cell,
is the fading coefficient between the 1th cell transmission
antennas and the the k user in the 1th cell,
is the transmission power for the k h user in the j t h cell, and
jkl is the fading coefficient between the j t h cell transmission
antennas and the the k user in the 1th cell.
9. The method of claim 1, further comprising:
dropping an active user equipment having a highest determined
power if a number of active user equipments associated with the
wireless network is greater than a percentage of users.
10. The method of claim 9, wherein the percentage of users is based
on a number of active user equipments having a relatively low signal
to interference ratio and a number of active user equipments having a
relatively high signal to interference ratio.