U.S. patent application number 11/528394 was filed with the patent office on 2007-01-25 for method and system of link control.
This patent application is currently assigned to Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Frank Brouwer.
Application Number | 20070021071 11/528394 |
Document ID | / |
Family ID | 20283749 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070021071 |
Kind Code |
A1 |
Brouwer; Frank |
January 25, 2007 |
Method and system of link control
Abstract
The present invention relates to retransmissions in a
communications system and more especially it relates to link load
control in a cellular mobile radio system particularly in a
Universal Mobile Telecommunications System, UMTS. BLER targets are
set depending on load.
Inventors: |
Brouwer; Frank; (GA
Enschede, NL) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ)
Stockholm
SE
|
Family ID: |
20283749 |
Appl. No.: |
11/528394 |
Filed: |
September 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10474062 |
Apr 1, 2004 |
7120448 |
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PCT/SE02/00693 |
Apr 6, 2002 |
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11528394 |
Sep 28, 2006 |
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Current U.S.
Class: |
455/69 ; 370/342;
455/522 |
Current CPC
Class: |
H04W 52/24 20130101;
H04W 52/20 20130101; H04W 52/265 20130101; H04L 1/0001 20130101;
H04W 52/267 20130101; H04W 52/343 20130101; H04W 52/12 20130101;
H04L 1/18 20130101 |
Class at
Publication: |
455/069 ;
370/342; 455/522 |
International
Class: |
H04B 7/00 20060101
H04B007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 6, 2001 |
SE |
0101281-4 |
Claims
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53. A method of link load control in a communications system using
automatic repeat request, the method comprising controlling
transmission power depending on at least a first target and a
second target, the first target being a target to which signal
level or a derivative thereof is compared, and the second target
being a target to which block error rate is compared, and wherein
the second target is set in relation to traffic load level.
54. A method of link load control in a communications system using
automatic repeat request, the method comprising: determining a
first target affected by a second target, and determining the
second target according to a function of traffic load; and
comparing to the first target at least one of (1) signal level, (2)
signal to noise level, (3) signal to interference level, or (4)
signal to noise and interference level, and comparing error rate to
the second target.
55. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a method of link load
control comprising: determining a second target level as a function
of traffic load level; determining a first target level as a
function of the second target level and block error rate; and
controlling transmission power such that at least one of (1) signal
level, (2) signal to noise level, (3) signal to interference level,
and (4) signal to noise and interference level satisfies the first
target level.
56. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a method of link load
control comprising the steps of: determining a traffic load level;
depending on the traffic load level, selecting a second target
level; comparing error rate data to the second target level;
determining a first target level depending on the relation between
the error rate data and the second target level; comparing first
data to the first target level; determining a transmission power
control command depending on the relation between the first data
and the first target level.
57. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a method of link load
control comprising: determining a traffic load level; depending on
the traffic load level, determining whether a second target level
should be changed; comparing error rate data to the second target
level, changed as need be; determining a first target level
depending on the relation between the error rate data and the
second target level; comparing first data to the first target
level; determining a transmission power control command depending
on the relation between the first data and the first target
level.
58. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a method of link load
control comprising: determining a traffic load level; depending on
the traffic load level, determining whether a second target level
should be changed; transferring information on second target level
change as needed to a user equipment unit and changing the second
target level if needed; comparing, in the user equipment unit,
error rate data to the second target level determining a first
target level depending on the relation between the error rate data
and the second target level; comparing first data to the first
target level; determining a transmission power control command
depending on the relation between the first data and the first
target level.
59. The method according to claim 56 wherein the first data
represents signal level, signal to noise level, signal to
interference level or signal to noise and interference level.
60. The method according to claim 53 further comprising determining
the second target level depending on offered traffic load.
61. The method according to claim 53 further comprising determining
the first target depending on the difference between the selected
second target and an actual value, corresponding to the second
target, estimated from data transmissions.
62. The method according to claim 53 further comprising selecting
the second target level from a set of second targets.
63. The method according to claim 53 further comprising sending the
first target from a control node of a radio access network.
64. The method according to claim 53 further comprising receiving
the first target in an access point of a radio access network.
65. The method according to claim 53 wherein the step of
controlling transmission power comprises controlling user equipment
transmission power.
66. The method according to claim 64 wherein the access point is a
radio base station of a Universal Mobile Telecommunications
System.
67. The method according to claim 53 further comprising sending one
or more signals carrying traffic load level data from an access
point of a radio access network.
68. A user equipment entity of link load control in a
communications system using automatic repeat request, the user
equipment entity comprising processing means adapted for
transmission power control depending on at least a first target and
a second target, the first target being a target to which signal
level or a derivative thereof is compared, and the second target
being a target to which block error rate is compared, and wherein
the second target is set in relation to traffic load level.
69. An access point of link load control in a communications system
using automatic repeat request, the access point comprising
processing means adapted for transmission power control depending
on at least a first target and a second target, the first target
being a target to which signal level or a derivative thereof is
compared, and the second target being a target to which block error
rate is compared, and wherein the second target is set in relation
to traffic load level.
70. A network element of link load control in a communications
system using automatic repeat request, the network element
comprising: processing means adapted for determining a first target
affected by a second target, and processing means adapted for
determining the second target according to a function of traffic
load; and a comparator for comparing signal level, signal to noise
level, signal to interference level or signal to noise and
interference level to the first target, and a comparator adapted
for comparing error rate to the second target.
71. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a network element of
link load control comprising circuitry arranged for determining a
second target level as a function of traffic load level;
determining a first target level as a function of the second target
level and block error rate; and controlling transmission power such
that at least one (1) of signal level, (2) signal to noise level,
(3) signal to interference level, and (4) signal to noise and
interference level satisfies the first target level.
72. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a network element of
link load control comprising: retrieval means for retrieving a
traffic load level; a selector for selecting a second target level
depending on the traffic load level; a comparator for comparing
error rate data to the second target level; processing means for
determining a first target level depending on the relation between
the error rate data and the second target level; a comparator for
comparing first data to the first target level; processing means
for determining a transmission power control command depending on
the relation between the first data and the first target level.
73. In a wireless communications system wherein transmissions
detected to be in error are retransmitted, a network element of
link load control comprising: retrieval means for retrieving a
traffic load level; processing means for determining whether a
second target level should be changed depending on the traffic load
level; processing means for determining a first target level
depending on a relation between error rate data and the second
target level; a comparator for comparing first data to the first
target level; processing means for determining a transmission power
control command depending on the relation between the first data
and the first target level.
74. The network element according to claim 72 wherein the first
data represents signal level, signal to noise level, signal to
interference level or signal to noise and interference level.
75. The user equipment according to claim 68 further comprising the
second target level being determined depending on offered traffic
load.
76. The access point according to claim 69 further comprising the
first target being determined depending on the difference between
the selected second target and an actual value, corresponding to
the second target, estimated from data transmissions.
77. The network element according to claim 70 wherein the second
target level is selected from a set of second targets.
78. The access point according to claim 69 wherein the access point
is a radio base station of a Universal Mobile Telecommunications
System.
79. The network element according to claim 70 wherein the access
point is a radio network controller of a Universal Mobile
Telecommunications System.
80. The access point according to claim 68, the access point being
part of a radio access network and comprising transmitter means for
sending one or more signals carrying traffic load level data.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to retransmissions in a
communications system, and more especially it relates to link load
control in a cellular mobile radio system, particularly to a
Universal Mobile Telecommunications System, UMTS or WCDMA
system.
BACKGROUND AND DESCRIPTION OF RELATED ART
[0002] Retransmission of data to or from a mobile station, MS, or
user equipment, UE, is previously known. If data is received in
error, it is requested for retransmission. If errors are frequent,
data is requested for retransmission frequently. When the relative
amount of transmissions that are retransmissions of previously
transmitted data increases the effective user data rate reduces,
for a constant channel data rate.
[0003] Increasing transmission power can often reduce transmission
errors. Consequently, increasing transmission power can reduce the
relative amount of retransmissions. Correspondingly, transmission
power can be reduced if the relative amount of transmission errors
is sufficiently small.
[0004] U.S. Pat. No. 5,461,639 describes forward link power control
in a CDMA system where error statistics are collected. The error
rate is compared to a threshold. If the error rate exceeds the
threshold, transmission power is increased. A code selector
determines at which redundancy level to transmit. For each level of
redundancy, the selector maintains a predetermined target error
rate.
[0005] International Patent Application WO0013362 describes a
method and device for adapting a transmission data rate or a
transmitter power to the transmission quality of a transmission
channel.
[0006] A channel data rate that gives an acceptable error rate at a
given signal to noise ratio is selected. Transmission power is
controlled.
[0007] It is also known to use medium access control and radio link
control layers of a UMTS protocol structure in acknowledged mode
for dedicated channels and to broadcast system information.
[0008] 3.sup.rd Generation Partnership Project (3GPP): Technical
Specification Group Radio Access Network, Physical Layer
Procedures, 3G TS 25.331 v3.5.0, France, December 2000, specifies
an RRC protocol. Section 8.1.1 describes broadcast of system
information.
[0009] 3.sup.rd Generation Partnership Project (3GPP): Technical
Specification Group Radio Access Network, Physical Layer
Procedures, 3G TS 25.322 v3.5.0, France, December 2000, specifies
the RLC protocol. The RLC layer provides acknowledged data transfer
service.
[0010] 3.sup.rd Generation Partnership Project (3GPP): Technical
Specification Group Radio Access Network, Physical Layer
Procedures, 3G TS 25.301 v3.6.0, France, September 2000, specifies
in chapter 5 Radio Interface Protocol Architecture of a UMTS
system.
[0011] None of the cited documents describes a method and system of
link control where error targets are selected as a function of
transmission load in an ARQ scheme of a radio communications
system.
SUMMARY OF THE INVENTION
[0012] In a power and interference limited system transmission
power cannot be increase beyond all limits. For a high system
traffic load, available power for each user is more restricted than
at a low system traffic load.
[0013] Perceived quality to an individual user needs to be balanced
to overall system performance. Using only one fixed block error
rate target, or one or more block error rate targets independent of
traffic load, results in a too low perceived quality at low traffic
load and a reduced throughput due to traffic being barred during
high traffic load.
[0014] Consequently, an object of this invention is to increase
perceived quality and reduce delay as perceived by a user during
low traffic load.
[0015] It is also an object to increase system throughput during
high traffic load.
[0016] A further object is to present a system and method of
communicating load control commands and block error rate targets
between various system elements.
[0017] Finally, it is an object to adaptively control link load
using double loops including link load estimates and targets for
transmission power control.
[0018] These objects are met by a method and system of
retransmissions such that target block error rate is determined in
relation to traffic load.
[0019] A first embodiment meets the objects for uplink load control
with load measurements in a base station, BS, or Node B and load
control in a radio network controller, RNC, communicating
transmission power control targets, determined from block error
rate targets, to the BS/Node B to be used for uplink transmission
power control of a user equipment, UE. The block error rate
targets, BLER targets, can be either determined to be the same for
all connections or determined individually on a link by link
basis.
[0020] A second embodiment meets the objects for downlink load
control with load measurements in a BS/Node B and load control in
an RNC by communicating a set of BLER targets to the user equipment
and depending on the load situation, which BLER target to use
within the set.
[0021] According to a third embodiment for downlink load control,
only BLER targets to be considered by the UE are communicated to it
and not an entire set of BLER targets.
[0022] As with uplink BLER targets, downlink BLER targets can be
determined individually for each connection, if more than one, of a
UE or only one BLER targets is determined for all one or more
connections of a UE.
[0023] The invention is particularly well suited for high-speed TCP
connections interconnecting the Internet and a user equipment.
[0024] Preferred embodiments of the invention, by way of examples,
are described in detail with reference to the accompanying drawings
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 displays throughput versus channel data rate for an
ARQ system, according to the invention.
[0026] FIG. 2 shows target block error rates versus load, according
to the invention.
[0027] FIG. 3 schematically illustrates uplink control, according
to the invention.
[0028] FIG. 4 shows a block diagram of one embodiment of downlink
load control, according to the invention.
[0029] FIG. 5 displays a block diagram of another embodiment of
downlink load control, according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] Wireless systems, in particular, reach their highest system
throughput when a total amount of transmitted packets partly
includes retransmissions of previously transmitted packets. In FIG.
1, the highest rate with which packets are transmitted on the
channel, the packet transmission rate, and a corresponding
retransmission rate are schematically depicted versus raw channel
bit rate, the gross rate. The packet transmission rate increases
linearly with the gross rate. As the gross rate increases,
transmission errors will be more frequent and the relative amount
of retransmissions increases, everything else being equal. As the
retransmission rate approaches the transmission rate for
successively higher gross rates, the throughput approaches zero.
According to the schematic and exemplary diagram of FIG. 1, there
is a maximum throughput when approximately one third of all
transmissions are retransmissions. Preferably, it is less than one
third in a running system. FIG. 1 only illustrates an exemplary
situation.
[0031] In general, the fraction of retransmissions for which an
optimum throughput is achieved depends on various parameters. One
of these parameters is the block error rate, BLER. With reference
to FIG. 1, as raw channel bit rate (the abscissa) increases
transmitted bit energy decreases and, as a consequence, bits
transmitted at a high raw bit rate, the gross rate, are more error
prone. Therefore, a block or a packet being composed of a number of
bits will also be more error prone as the gross rate increases. At
the receiving end errors are detected by means of error detecting
codes, well known in the art.
[0032] When automatic repeat request, ARQ, is made use of, as is
the case in a WCDMA system, a packet or block detected to be in
error is requested for retransmission. This request could be in the
form of a negative acknowledgement, when an error is detected, or
an omitted positive acknowledgement. The greater the fraction of
packets detected to be in error, the higher the retransmission
rate.
[0033] As can be seen from the simplified diagram in FIG. 1, when
less than the maximum throughput, the same net rate (throughput) is
achieved for more than one gross rate. At the lower gross rate the
relative amount of retransmissions is inferior to the relative
amount of retransmissions at the higher gross rate.
[0034] When retransmissions become a substantial amount of all
transmissions, the users will perceive a low quality of service due
to increased average delays. There will also be a greater delay
variance as each packet may require multiple retransmissions to get
through without errors. For short instances the user might then
imagine the connection being broken.
[0035] A WCDMA system will be designed to interact with e.g. the
Internet. Most applications on the Internet use protocols, such as
TCP (Transport Control Protocol), that controls the transmission
rate, based on link quality in terms of packet loss and delay
characteristics. Consequently, besides the negative effect of
retransmission delays as such on perceived quality, substantial
queuing delay can also lead to secondary effects further reducing
quality of service. An obvious solution to these secondary effects
is to develop a modified TCP less sensitive of the increased number
of retransmissions typical of, e.g., wireless systems. However,
this is greatly undesirable as it may introduce problems to user
applications.
[0036] At high load it is consequently a goal to operate a system
close to its maximum throughput, to make best use of scarce
resources. At low load the system throughput is not a scarce
resource and therefore there is no reason to optimize system for
maximum throughput but for perceived quality.
[0037] FIG. 1 illustrates a situation with a received signal
strength that does not vary, or does not vary very much, in
relation to an overall noise and interference level. A modern
communication system makes use of transmission power control to
keep received signal to interference ratio close to a constant
target ratio, where the target is preset to a fix level.
[0038] As explained above it is advantageous to operate a system at
various retransmission rates depending on load. As packets detected
to be in error are retransmitted the retransmission rate is
reflected into a block error rate, BLER. This invention utilizes
different BLER targets for different load levels. This is
illustrated in FIG. 2. At a high load situation between thresholds
2 and 3, a high BLER target, BLER target 3, is used. BLER target 3
corresponds to an operational setting close to maximum throughput
of FIG. 1. At a low load situation between thresholds 0 and 1, a
small BLER target is used to guarantee a high user quality, where
maximum system throughput is less critical. For intermediary loads
between thresholds 1 and 2 an intermediary BLER target, BLER target
2, is used.
[0039] One or more BLER targets are evaluated at the receiving end.
If the number of erroneous packets exceeds the BLER target,
transmission power needs to be increased, everything else being
equal. Consequently, the receiving end should command a
transmission power increase. Correspondingly, if the BLER is less
than the BLER target, transmission power should be commanded to
decrease. BLER targets are set or determined in relation to traffic
load as previously described in relation to FIG. 2. BLER targets
are communicated as needed for closed loop evaluation.
[0040] As described in relation to FIG. 1, the objective of
transmission power control is to maintain receiver signal to
interference ratio, SIR, close to constant. This constant is an SIR
target. In order to achieve the desired transmission power control,
the BLER target selection affects the SIR target to be used for the
transmission power control, i.e. BLER is controlled indirectly.
[0041] FIG. 3 illustrates an embodiment for uplink
control-according to the invention. A radio network controller,
RNC, determines a set of BLER targets, e.g. BLER targets 1-3 of
FIG. 2. As examples only, BLER targets in the range of 1-30%,
depending on traffic load, demonstrate high efficiency. Offered
uplink load is set in the RNC in means <<UL load
monitor>> and received BLER is achieved from uplink data
transmission in means <<BLER estimate>>. Means
<<UL load control>> determines which BLER target from
the set of BLER targets to use by indicating by selector
<<High/low target>> if BLER target should be increased
or decreased. Of course, alternative ways of indicating which
particular BLER target from the set of BLER targets to select also
falls within the scope of this invention. The module <<UL
load control>>, located in the RNC, receives information on
the uplink load, preferably from a base station entity <<Node
B>>. This can originate from measurements, e.g. Received
Signal Strength Indicator, RSSI, or from other traffic
observations, such as counting number of connections. From the BLER
estimate, the BLER target, optionally also including the old BLER
target/old SIR target for which the BLER estimate is determined, a
target <<SIR target>> to be used is calculated or
looked up. This is performed by the uplink outer loop power control
means <<UL outer loop PC>>. A base station <<Node
b>> comprises means for uplink load measurements, as
explained above. It also comprises means <<UL inner loop
PC>> for inner loop uplink transmission power control and
means <<SIR estimate>> for estimating uplink signal to
interference ratio. Means <<UL inner loop PC>> compares
<<SIR target>>, achieved from RNC, and estimated SIR on
uplink, achieved from means <<SIR estimate>>, and
commands a user equipment UE to increase or decrease transmission
power in a power amplifier PA depending on the outcome of the
comparison. BLER and SIR will be estimated in the RNC and
<<Node B>>, respectively, on data transmitted with the
controlled transmission power, closing the inner and outer
loops.
[0042] The set of BLER targets as determined by the RNC, can be
determined individually for each connection or only one set of BLER
targets is determined for all, or a subset of all, connections of
the RNC. Also the SIR target can be determined individually for
each connection or only one SIR target is determined for all, or a
subset of all, connections of the Node B. In UMTS Radio Access
Bearers, RABs, make available radio resources (and services) to
user applications. For each mobile station there may be one or
several RABs. Each RAB is associated with a class of Quality of
Service, QoS. When selecting individual BLER targets it is
preferred to select BLER target and, correspondingly, SIR target in
accordance with the QoS class for an RAB.
[0043] Basically, the load control as described for uplink in
relation to FIG. 3 is also applicable for downlink, with the
obvious interchange of up and down, and <<Node B>> and
UE, respectively. However, there are some differences due to the
fact that BLER targets are determined on the Radio Access Network
side, RAN side, preferably in the RNC and the load measurements are
likewise determined on the RAN side, preferably in the <<Node
B>>. This will have repercussions on how the BLER target for
determining SIR target is communicated to the UE.
[0044] FIGS. 4 and 5 show two embodiments of downlink load control.
In FIG. 4, a complete set of BLER targets is transferred to the UE
and, depending on the downlink load control, a target selector is
communicated, indicating which BLER target of the stored set of
BLER targets to use. Since the set of BLER targets only need to be
transferred at connection setup or is updated infrequently,
signaling could be reduced as compared to a solution where the BLER
target to use is communicated. A block diagram of such a solution
is illustrated in FIG. 5. If the respective BLER target elements of
the set of BLER targets are changed frequently, or the DL load
control is designed to only infrequently switch between the various
BLER targets of a set of BLER targets, the solution according to
FIG. 5 will perform more efficiently.
[0045] According to both FIGS. 4 and 5, offered downlink load is
set in the RNC in means <<UL load monitor>>. The RNC
determines a set of BLER targets. As for uplink load control,
<<Node B>> comprises means <<DL load
measurement>> for determining actual downlink load. The
downlink load can e.g. be estimated from the downlink Transmitted
Carrier Power or from traffic observations. Means <<DL load
control>> determines whether BLER target should increase or
decrease, depending on downlink load measured, as described above
in relation to FIG. 2. Means <<DL outer loop PC>>
calculates a target signal <<SIR target>> input to
means <<DL inner loop PC>> determining downlink inner
loop power control. If means <<SIR estimate>> indicates
a downlink SIR less than target <<SIR target>>, means
<<DL inner loop PC>> commands the power amplifier PA of
<Node B>> to increase transmission power. If estimated
downlink SIR is greater than target <<SIR target>>, PA
is commanded to decrease transmission power. Means <<DL outer
loop PC>> receives a signal indicating estimated BLER from
means <<BLER estimate>>.
[0046] FIG. 4 shows an embodiment where the entire set <<BLER
targets>> of BLER targets is transferred from the RNC to the
UE. For selection of the particular BLER target to use in selecting
an SIR target also a selector <<High/low target>> is
communicated to the UE. Preferably the signal carrying selector
<<High/low target>> is broadcast in a system
information message when the same BLER target is used for all
connections. In the UE, means <<DL outer loop PC>>
determines which BLER target to use from the set of BLER targets as
indicated by the selector <<High/low target>>. This
BLER target is further compared with the estimated downlink BLER.
If estimated BLER is less than the selected BLER target, the SIR
target is reduced. If estimated BLER is greater than the selected
BLER target, the SIR target is increased.
[0047] The embodiment of FIG. 5 transfers a selected BLER target
from RNC to UE via <<Node B>. In the RNC a target signal
BLER target>> is determined in means <<DL BLER
control>>. Target signals <<High/low target>> and
<<BLER targets>>, determined as described above, are
input to means <<DL BLER control>>. This means for
downlink BLER control selects a particular BLER target from the set
of BLER targets accordingly. In the UE the BLER target carried by
signal <<BLER target>> is input to means <<DL
outer loop PC>> for comparison with estimated BLER from means
<<BLER estimate>>. Depending on the outcome of this
comparison, an SIR target is determined as described above.
[0048] A person skilled in the art readily understands that the
receiver and transmitter properties of a <<Node B>> or
a UE are general in nature. The use of concepts such as
<<Node B>>, UE or RNC within this patent application is
not intended to limit the invention only to devices associated with
these acronyms. It concerns all devices operating correspondingly,
or being obvious to adapt thereto by a person skilled in the art,
in relation to the invention. As two explicit non-exclusive
examples, the invention relates to mobile stations without a
subscriber identity module, SIM, as well as user equipment
including one or more SIMs and physical entities, base stations, as
well as the logical nodes, Nodes B, they represent. Further,
protocols and layers are referred to in relation to UMTS
terminology. However, this does not exclude applicability of the
invention in other systems with other protocols and layers of
similar functionality.
[0049] The invention is not intended to be limited only to the
embodiments described in detail above. Changes and modifications
may be made without departing from the invention. It covers all
modifications within the scope of the following claims.
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