Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
FAST ADAPTIVE POWER CONTROL FOR A VARIABLE
MULTIRATE COMMUNICATIONS SYSTEM
[0001] This application is a divisional of Canadian patent application Serial
No. 2,569,720, which in turn is a divisional of Canadian patent application
Serial
No. 2,417,242 filed internationally on July 12, 2001 and entered into the
National
Phase in Canada on January 24, 2003.
[0002] The present invention relates to power control for wireless
communication systems and, in particular, fast adaptive power control system
and
methods for a variable multirate communication system.
BACKGROUND OF THE INVENTION
[0003] Various methods of power control for wireless communication
systems are well known in the art. An example of an open loop power control
transmitter system for a single rate data system is illustrated in Figure 1.
An
example of a closed loop power control transmitter system for a single rate
data is
illustrated in Figure 2.
[0004] The purpose of both systems is to rapidly vary transmitter power in
the presence of a fading propagation channel and time-varying interference to
minimize transmitter power while insuring that data is received at the remote
end
with acceptable quality. Typically, in a digital implementation, transmitter
power is
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varied by applying a varying scale factor to the digital data, as opposed, for
example, to varying the gain of an RF amplifier.
[0005] In state-of-the-art communication systems such as Third Generation
Partnership Project (3GPP) Time Division Duplex (TDD) and Frequency Division
Duplex (FDD) systems multiple channels of variable rate data are combined for
transmission. Figures 3 and 4 represent prior art open and closed power
control
transmission systems, respectively. Background specification data for such
systems
are found at 3GPP TS 25.223 v3.3.0, 3GPP TS 25.222 v3.2.0, 3GPP TS 25.224
v3.6 and Volume 3 specifications of Air-Interface for 3G Multiple System
Version
1.0, Revision 1.0 by the Association of Radio Industries Businesses (ARIB).
[0006] Such open and closed loop power control systems for variable
multirate wireless communications systems respond relatively slowly to data
rate
changes, resulting in sub-optimal performance such as relating to excessive
transmitter power and below-quality received signals. It would be desirable to
provide a fast method and system of power control adaption for data rate
changes
resulting in more optimal performance.
SUMMARY OF THE INVENTION
[0007] The invention provides a method of controlling transmitter power in
a wireless communication system in which user data is processed as a multirate
signal having a rate N(t) and in which the user data signal having rate N(t)
is
converted into a transmission data signal having a faster rate M(t) for
transmission.
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The transmission power is adjusted on a relatively slow basis based on quality
of
data received by a receiver of the transmitted data. The transmitter power is
determined as a function of N(t)/M(t) such that a change in the data rate in
the user
data signal or the rate of the transmission data signal is compensated for in
advance
of a quality of data based adjustment associated with such data rate change.
Preferably, the user data signal having rate N(t) is converted into the
transmission
data signal having the faster rate M(t) by repeating selected data bits
whereby the
energy per bit to noise spectrum density ratio is increased in the
transmission data
signal.
[0008] The method is applicable in either an open or closed power control
system where a scale factor is applied to control transmitter power. In
implementing the invention in a transmitter of either an open or closed
system,
preferably /(N(t)/M(0) is applied to the scale factor.
[0009] The method is applicable to an open loop power control system
where the transmitter receives a reference signal, reference signal power
data,
measured interference power data, and target signal to interference ratio
(SIR) data
which SIR data is based on relatively slowly collected received signal quality
data.
The transmitter measures the reference signal to determine received reference
signal power and computes a path loss based on the received reference signal
power data and the determined reference signal power. The transmitter then
computes the scale factor based on the computed path loss, the received
measured
interference power data, the target SIR data and /(N(t)/M(t)).
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[0010] The method is also applicable to a closed loop system where the
transmitter utilizes step up/down data generated by the receiver and computes
the
scale factor based on the step up/step down data and i(N(t)/M(0). Preferably,
the
step up/down data is generated by the receiver by combining measured
interference
power data of the signal received from the transmitter with target signal to
interference ratio (SIR) data based at least in part on relatively slowly
collected
received signal quality data. The target SIR data is preferably computed by
multiplying a nominal target SIR data based on relatively slowly collected
received
signal quality data by a factor N(t)/M(t) so that the target SIR data is
quickly
adjusted when a change in data rate occurs.
[0011] The invention also provides a transmitter for a wireless
communication system in which user data is processed as a multirate signal
having
a rate N(t) and in which the user data signal having rate N(t) is converted
into a
transmission data signal having a faster rate M(t) for transmission. The
transmitter
transmission power is adjusted on a relatively slow basis by applying a scale
factor
to the transmitter power based on quality of data received by a receiver of
the
transmitted data. The transmitter includes a data signal rate converter which
increases the user data signal rate N(t) to a higher data transmission rate
M(t) and a
processor for computing a transmission power scale factor based in part on
data
generated by the receiver related to quality of data received. The data signal
rate
converter is associated with the processor such that the processor computes
the
transmission power scale factor as a function of N(t)/M(t) whereby a change in
the
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data rate in the user data signal or the rate of the transmission data signal
is
compensated for in advance of a receiver quality of data based adjustment
associated with such data rate change.
[0012] Preferably, the data signal rate converter converts the user data
signal
having rate N(t) into the transmission data signal having the faster rate M(t)
by
repeating selected data bits whereby the energy per bit to noise spectrum
density
ratio is increased in the transmission data signal.
[0013] The transmitter is configurable as part of an open loop power control
system where the transmitter receives from the receiver of the transmitted
data: a
reference signal, reference signal power data, measured interference power
data,
and target signal to interference ratio (SIR) data which SIR data is based on
relatively slowly collected received signal quality data. As such, the
transmitter
includes a signal measuring device which measures received reference signal
power and path loss processing circuitry for computing a path loss based on
the
received reference signal power data and the measured received reference
signal
power. The transmitter processor computes the transmission power scale factor
based on the computed path loss, the received measured interference power
data,
the target SIR data and i(N(t)/M(0).
[0014] The transmitter is also configurable as part of a closed loop power
control system where the transmitter receives step up/down data from the
receiver
of the transmitted data. As such, the transmitter processor computes the
transmission power scale factor based on the received step up/step down data
and
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[ [0015] The invention also provides a closed loop
transmission power control
system for a wireless communication system in which user data is processed as
a
multirate signal having a rate N(t), in which the user data signal having rate
N(t) is
converted into a transmission data signal having a faster rate M(t) for
transmission
and in which the transmission power is adjusted by applying a scale factor in
response to step up/down data. The system includes a receiver which receives
the
M(t) rate transmission data signal and generates the step up/down data. The
receiver preferably has a data signal rate converter which decreases the data
rate of
received transmission data M(t) to produce a user data signal having a lower
data
rate N(t), a data quality measuring device for measuring the quality of data
of the
user data signal, and circuitry for computing step up/down data based in part
on the
measured quality of data of the user data signal. The data signal rate
converter is
associated with the circuitry to provide rate data such that the circuitry
computes
step up/down data as a function of N(t)/M(t) whereby a change in the user data
signal rate or the rate of the transmission data signal is compensated for in
advance
of a quality of data based adjustment associated with such data rate change.
[0016] The system also preferably includes a
transmitter having a data signal
rate convertor which converts the user data signal having rate N(t) into the
transmission data signal having a faster rate M(t) by repeating selected data
bits
whereby the energy per bit to noise spectrum density ratio is increased in the
transmission data signal.
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[0017] In a preferred embodiment, the receiver has an interference
measuring device for measuring the power of an interference signal received
with
the M(t) rate transmission data signal. The data quality measuring device
outputs a
nominal target SIR data based on relatively slowly collected received data
quality
data. The receiver circuitry computes the step up/down data by combining
measured interference power data of the signal received from the transmitter
with
target signal to interference ratio SIR data which is computed by multiplying
the
nominal target SIR data by a factor N(t)/M(t) so that the target SIR data is
quickly
adjusted when a change in data rate occurs.
[0018] According to an embodiment of the present disclosure there is
provided a user equipment comprising: a transmitting circuit configured to
transmit
a signal to a base station at a determined transmit power level. The
transmitting
circuit is further configured to derive the determined transmit power level by
at
least combining a first value with a second value. The first value is derived
at least
from a pathloss from the base station to the user equipment and the second
value is
derived at least by dividing a number of data elements for transmission by a
number of elements provided in the transmitted signal for transmission. The
first
value is derived at least from multiplying the pathloss with a parameter
wherein the
parameter is configured to have a value in the range from 0 to 1.
[0019] According to another embodiment of the present disclosure there is
provided a method comprising: determining, by a user equipment, a transmission
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power level wherein the determined transmission power level is derived by at
least
combining a first value with a second value. The first value is derived at
least from
a pathloss from a base station to the user equipment and the second value is
derived
at least by dividing a number of data elements for transmission by a number of
elements provided in the transmitted signal for transmission. The first value
is
derived at least from multiplying the pathloss with a parameter wherein the
parameter is configured to have a value in the range from 0 to 1. The methods
includes transmitting, by the user equipment, a signal to the base station
based on
the determined transmission power level.
[0020] Other objects and advantages will be apparent to those of ordinary
skill in the art based upon the following description of presently preferred
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Figure 1 is a schematic diagram of a conventional open loop power
control system for single rate data wireless communication.
[0022] Figure 2 is a schematic diagram of a conventional closed loop power
control system for single rate data wireless communication.
[0023] Figure 3 is a schematic diagram of a conventional open loop power
control system for variable multirate data wireless communication.
[0024] Figure 4 is a schematic diagram of a conventional closed loop power
control system for variable multirate data wireless communication.
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[0025] Figure 5 is a block diagram of data rate up-conversion from 6 to 8
bits per block using repetition.
[0026] Figure 6 is a block diagram of data rate down-conversion of repeated
data from 8 to 6 bits per block.
[0027] Figure 7 is a schematic diagram of a fast adaptive open loop power
control system for variable multirate data wireless communication made in
accordance with the teaching of the present invention.
[0028] Figure 8 is a schematic diagram of a fast adaptive closed loop power
control system for variable multirate data wireless communication made in
accordance with the teaching of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] Conventional power control methods for wireless systems such as 3G
PP utilize so-called inner and outer loops. The power control system is
referred to
as either open or closed dependent upon whether the inner loop is open or
closed.
The outer loops of both types of systems are closed loops.
[0030] Pertinent portions of an open loop power control system having a
"transmitting" communication station 10 and a "receiving" communication
station
30 are shown in Figure 1. Both stations 10, 30 are transceivers. Typically one
is a
base station and the other a type of user equipment UE. For clarity, only
selected
components are illustrated.
[0031] The transmitting station 10 includes a transmitter 11 having a data
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line 12 which transports a user data signal for transmission. The user data
signal is
provided with a desired power level which is adjusted by applying a transmit
power
scale factor from an output 13 of a processor 15 to adjust the transmission
power
level. The user data is transmitted from an antenna system 14 of the
transmitter 11.
[0032] A wireless radio signal 20 containing the transmitted data is received
by the receiving station 30 via a receiving antenna system 31. The receiving
antenna system will also receive interfering radio signals 21 which impact on
the
quality of the received data. The receiving station 30 includes an
interference
power measuring device 32 to which the received signal is input which device
32
outputs measured interference power data. The receiving station 30 also
includes a
data quality measuring device 34 into which the received signal is also input
which
device 34 produces a data quality signal. The data quality measuring device 34
is
coupled with a processing device 36 which receives the signal quality data and
computes target signal to interference ratio (SIR) data based upon a user
defined
quality standard parameter received through an input 37.
[0033] The receiving station 30 also includes a transmitter 38 which is
coupled with the interference power measuring device 32 and the target SIR
generating processor 36. The receiving station's transmitter 38 also includes
inputs
40, 41, 42 for user data, a reference signal, and reference signal transmit
power
data, respectively. The receiving station 30 transmits its user data and the
control
related data and references signal via an associated antenna system 39.
[0034] The transmitting station 10 includes a receiver 16 and an associated
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receiving antenna system 17. The transmitting station's receiver 16 receives
the
radio signal transmitted from the receiving station 30 which includes the
receiving
station's user data 44 and the control signal and data 45 generated by the
receiving
station 30.
[0035] The transmitting station processor 15 is associated with the
transmitting station's receiver 16 in order to compute the transmit power
scale
factor. The transmitter 11 also includes a device 18 for measuring received
reference signal power which device 18 is associated with path loss computing
circuitry 19.
[0036] In order to compute the transmit power scale factor, the processor 15
receives data from a target SIR data input 22 which carries the target SIR
data
generated by the receiver station's target SIR generating processor 36, an
interference power data input 23 which carries the interference data generated
by
the receiving station's interference power measuring device 32, and a path
loss data
input 24 which carries a path loss signal that is the output of the path loss
computing circuitry 19. The path loss signal is generated by the path loss
computing circuitry 19 from data received via a reference signal transmit
power
data input 25 which carries the reference signal transmit power data
originating
from the receiving station 30 and a measured reference signal power input 26
which
carries the output of the reference signal power measuring device 18 of the
transmitter 11. The reference signal measuring device 18 is coupled with the
transmitting station's receiver 16 to measure the power of the reference
signal as
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received from the receiving station's transmitter 38. The path loss computing
circuitry 19 preferably determines the path loss based upon the difference
between
the known reference power signal strength conveyed by input 25 and the
measured
received power strength conveyed by input 26.
[0037] Interference power data, reference signal power data and target SIR
values are signaled to the transmitting station 10 at a rate significantly
lower than
the time-varying rate of the propagation channel and interference. The "inner"
loop
is the portion of the system which relies on the measured interface. The
system is
considered "open loop" because there is no feedback to the algorithm at a rate
comparable to the time-varying rate of the propagation channel and
interference
indicating how good the estimates of minimum required transmitter power are.
If
required transmit power level changes rapidly, the system cannot respond
accordingly to adjust the scale factor in a timely manner.
[0038] With respect to the outer loop of the open loop power control system
of Figure 1, at the remote receiver station 30, the quality of the received
data is
evaluated via the measuring device 34. Typical metrics for digital data
quality are
bit error rate and block error rate. Computation of these metrics requires
data
accumulated over periods of time significantly longer than the period of the
time-
varying propagation channel and interference. For any given metric, there
exists a
theoretical relationship between the metric and received SIR. When enough data
has been accumulated in the remote receiver to evaluate the metric, it is
computed
and compared with the desired metric (representing a desired quality of
service) in
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processor 36 and an updated target SIR is then output. The updated target SIR
is
that value (in theory) which applied in the transmitter inner loop would cause
the
measured metric to converge to the desired value. Finally, the updated target
SIR is
passed, via the receiving station transmitter 38 and the transmitting station
receiver
16, to the transmitter 11 for use in its inner loop. The update rate of target
SIR is
bounded by the time required to accumulate the quality statistic and practical
limits
on the signaling rate to the power-controlled transmitter.
[0039] With reference to Figure 2, a communication system having a
transmitting station 50 and a receiving station 70 which employs a closed loop
power control system is illustrated.
[0040] The transmitting station 50 includes a transmitter 51 having a data
line 52 which transports a user data signal for transmission. The user data
signal is
provided with a desired power level which is adjusted by applying a transmit
power scale factor from an output 53 of a processor 55 to adjust the power
level.
The user data is transmitted via an antenna system 54 of the transmitter 51.
[0041] A wireless radio signal 60 containing the transmitted data is received
by the receiving station 70 via a receiving antenna system 71. The receiving
antenna system will also receive interfering radio signals 61 which impact on
the
quality of the received data. The receiving station 70 includes an
interference
power measuring device 72 to which the received signal is input which device
72
outputs measured SIR data. The receiving station 70 also includes a data
quality
measuring device 73 into which the received signal is also input which device
74
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produces a data quality signal. The data quality measuring device 73 is
coupled
with a processor 74 which receives the signal quality data and computes target
signal to interference ratio (SIR) data based upon a user defined quality
standard
parameter received through an input 75.
[0042] A combiner 76, preferably a substracter, compares the measured SIR
data from the device 72 with the computed target SIR data from the processor
74,
preferably by subtracting, to output an SIR error signal. The SIR error signal
from
the combiner 76 is input to processing circuitry 77 which generates step
up/down
commands based thereon.
[0043] The receiving station 70 also includes a transmitter 78 which is
coupled with the processing circuitry 77. The receiving station"s transmitter
78
also includes an input 80 for user data. The receiving station 70 transmits
its user
data and the control related data via an associate antenna system 79.
[0044] The transmitting station 50 includes a receiver 56 and an associated
receiving antenna system 57. The transmitting station"s receiver 56 receives
the
radio signal transmitted from the receiving station 70 which includes the
receiving
station"s user data 84 and the control data 85 generated by the receiving
station.
[0045] The transmitting station"s scale factor processor 55 has an input 58
associated with the transmitting station"s receiver 56. The processor 55
receives
the up/down command signal through input 58 and computes the transmit power
scale factor based thereon.
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[0046] With respect to the inner loop of the closed loop power control
system, the transmitting station' 's transmitter 51 sets its power based upon
high-
rate "step-up'" and 'step-down' commands generated by the remote receiving
station 70. At the remote receiving station 70, the SIR of the received data
is
measured by the measuring device 72 and compared with a target SIR value
generated by the processor 74 via combiner 76. The target SIR is that value
(in
theory) which, given that the data is received with that value, results in a
desired
quality of service. If the measured received SIR is less than the target SIR,
a
'step-down' command is issued by the processing circuitry 77, via the
receiving
station"s transmitter 78 and the transmitting station"s receiver 56, to the
transmitter 51, otherwise a "step-up" command is issued. The power control
system is considered 'closed-loop' because of the high-rate feedback of the
"step-up'" and 'step-down' commands which can react in real time to the time-
varying propagation channel and interference. If required transmit power level
changes due to time varying interference and propagation, it quickly responds
and
adjusts transmit power accordingly.
[0047] With respect to the outer loop of the closed loop power control
system, the quality of the received data is evaluated in the receiving station
70 by
the measuring device 73. Typical metrics for digital data quality are bit
error rate
and block error rate. Computation of these metrics requires data accumulated
over
periods of time significantly longer than the period of the time-varying
propagation
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channel and interference. For any given metric, there exists a theoretical
relationship between the metric and received SIR. When enough data has been
accumulated in the remote receiver to evaluate the metric, it is computed and
compared with the desired metric (representing a desired quality of service)
by the
processor 74 and an updated target SIR is then output. The updated target SIR
is
that value (in theory) which applied in the receiver algorithm would cause the
measured metric to converge to the desired value. The updated target SIR is
then
used in the inner loop to determine the direction of the step up/down commands
sent to the transmitting station's power scale generating processor 55 to
control the
power of the transmitter 51.
[0048] Figures 1 and 2 illustrate power control systems for single rate data
transmissions. However, in a digital communications system, data can be
processed
in blocks with a given bit rate and given block size, or alternatively, a
given number
of bits per block and given block rate. In such systems, for example, 3GPP FDD
and TDD systems, more than one data rate can exist at any given time within
the
communications system, and such data rates can vary over time. Figure 3
illustrates
a modified open-loop power control system and Figure 4 illustrates a modified
closed-loop power control system for wireless systems which communicate
multiple data channels having variable data rates.
[0049] To accommodate multichannel variable rate data transmission, the
open loop power control system illustrated in Figure 1 is modified, as shown
in
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Figure 3, to include an up converter 27 in the transmitting station 10 and a
down
converter 47 in the receiving station 30.
[0050] The user data for transmission is combined into a signal having a data
rate N(t). The data stream having the rate N(t) is converted to a data stream
having
a higher rate M(t) by data up converter 27 which has an output 28 which
carries the
transmission data signal having the rate M(t).
[0051] At the receiving station 30, the user data signal having the rate M(t)
is
received and down converted by the converter 47 to the original rate N(t). The
interference power measuring device 32 measures the interference of the signal
as
received with its higher M(t) rate. The data quality measuring device 34 is
coupled
to the user data path down stream the converter 47 and measures the quality of
the
data after it has been down converted to the N(t) rate.
[0052] To accommodate multichannel variable rate data transmission, the
closed loop power control system illustrated in Figure 2 is modified, as shown
in
Figure 4, to include an up converter 67 in the transmitting station 50 and a
down
converter 87 in the receiving station 70. The user data for transmission is
combined
into a signal having a data rate N(t). The data stream having the rate N(t) is
converted to a data stream having a higher rate M(t) by data up converter 67
which
has an output 68 which carries the transmission data signal having the rate
M(t).
[0053] At the receiving station 70, the user data signal having the rate M(t)
is
received and down converted by the converter 87 to the original rate N(t). The
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interference power measuring device 72 measures the interference of the signal
as
received with its higher M(t) rate. The data quality measuring device 73 is
coupled
to the user data stream down stream the converter 87 and measures the quality
of
the data after it has been down converted to the N(t) rate.
[0054] In both types of multichannel variable rate systems, the user data
input to the transmitter 11, 51 for transmission to the remote receiver 30, 70
has the
data rate denoted N(t) and the user data output from the remote receiver is at
that
same rate. Data rate N(t) can be the composite of several data rates of
different
data channels which have been multiplexed for transmission over a common
bearer.
That N is a function of (t) indicates that the rate may vary, that is, be
different from
time to time, or from block to block. Reasons for this variation include the
addition
and/or deletion of data channels and actual data rate changes in existing
channels,
as is typical for packet services.
[0055] Also in both systems, illustrated in Figures 3 and 4, in the transmit
data path, the date rate is changed from N(t) to M(t) and change back to N(t)
in the
remote receiver. Data rate N(t) is the user data rate and the data rate M(t)
is over-
the-air data rate, which can be quite independent of each other.
[0056] In a 3GPP TDD system, for example, M(t) is the number of bits per
msec. frame in a given number of time slots and orthogonal variable spreading
factor codes at given spreading factors. That M is a function of (t) indicates
that the
rate may vary, that is, be different from time to time, or more specifically,
from
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frame-to-frame. Varying M is equivalent to varying the spreading factors
and/or
number of physical channels used per frame, varying N is equivalent to a data
rate
change in one or more transport channels. Rate M(t) is equivalent to Ndataj
bits per
msec. frame and N(t) is equivalent to
PL.1 / RMõõn E RM,=1Vi3
TrCH
bits per 10 msec. frame, during the time t when TFCj is in effect, where, as
defined
in 3GPP:
Nõ, is the number of bits in a radio frame before rate matching on
TrCH i with transport format combination j.
RM, is the semi-static rate matching attribute for TrCH i which is
signaled from higher layers.
PL is the puncturing limit which value limits the amount of
puncturing that can be applied in order to minimize the number
of physical channels and is signaled from higher layers.
Ndatcy is the total number of bits that are available for a coded
composite TrCH in a radio frame with transport format
combination j.
TF,O) is the transport format of TrCH i for the transport format
combination j.
TB or Transport Block is defined as the basic data unit exchanged
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between Li and MAC. An equivalent term for Transport
Block is "MAC PDU".
TBS or Transport Block Set is defined as a set of Transport Blocks
that is exchanged between Layer 1 and MAC at the same time
instance using the same Transport Channel.
TrCH or Transport Channel are the channels offered by the physical
layer to Layer 2 for data transport between peer Layer 1
entities are denoted. Different types of Transport Channels are
defined by how and with which characteristics data is
transferred on the physical layer, e.g. whether using dedicated
or common physical channels.
TF or Transport Format is defined as a format offered by Layer 1
to MAC for the delivery of a Transport Block Set during a
Transmission Time Interval on a Transport Channel. The
Transport Format constitutes of two parts -- one dynamic part
and one semi-static part.
TFC or Transport Format Combination is defined as the
combination of currently valid Transport Formats on all
Transport Channels, i.e. containing one Transport Format from
each Transport Channel.
TFCS or Transport Format Combination Set is defined as a set of
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Transport Format Combinations.
MAC or Medium Access Control is a sub-layer of radio interface
Layer 2 providing unacknowledged data transfer service on
logical channels and access to Transport Channels.
PDU or Protocol Data Unit is a unit of data specified in an (N)-
protocol layer and consisting of (N)-protocol control
information and possibly (N)-user data.
[0057] The conversion from rate N(t) to rate M(t) is performed in the
transmitting station 10, 50 in the converter 26, 67 which indicates up-
conversion by
the factor M(t) / N(t). The conversion rate from rate M(t) back to rate N(t)
is
performed in the remote receiving station 30, 70 in the converter 47, 87 which
indicates down-conversion by the factor N(t) / M(t).
[0058] In both systems illustrated in Figures 3 and 4, rate M(t) is shown to
be
higher than rate N(t). This is deliberate. An unintended effect of the upward
rate
conversion, mitigation of which is an object of the invention, occurs only for
case
of up-conversion by repetition in the transmitter, which is described below.
This
effect does not happen if N(t) = M(t) and the effect is different if N(t) >
M(t) which
is not the subject of this invention.
[0059] Up-conversion of a data rate can be implemented by repetition, that
is, repeating selected bits in a rate N block until it contains the same
number of bits
as a block at rate M and to perform down-conversion by numerically combing the
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received repeated "soft" bits. Up-conversion by repetition is illustrated in
an
example shown in Figure 5, where B, is the ith "hard" bit, that is 1, in the
input
sequence, for the simplified case of increasing the data rate from six to
eight bits
per block. In the example, two bits, 2 and 5, are repeated, changing the block
size
from six to eight. In Figure 6, where b, + nj is a "soft" bit, that is, a
digital sample
within the receiver of the transmitted bit B. plus noise component ni at time
j, the
down-conversion process, with input consisting of eight "soft" bits is
illustrated.
Received "soft" bits 2 and 3 are numerically summed to form a scaled version
of the
original bits 2 and 3; similarly, received "soft" bits 6 and 7 are numerically
summed
to form a scaled version of the original bit 5.
[0060] The particular repeated bits used in the example represent uniform
distribution of repeated bits, which, in conjunction with an interleaver, is a
particular scheme used in a 3GPP system. However, the choice of bits to repeat
is
not germane to the invention.
[0061] The above-described method of data rate conversion is a component
of so-called "rate matching" using repetition functions used in the 3GPP TDD
and
FDD systems. It has the advantage, over the simplistic method of sending (two,
in
the example) dummy bits to change the data rate, in that the energy difference
between the original shorter and transmitted longer block can be exploited to
improve signal quality. To illustrate, in the example, received bits 2 and 5
have
twice the energy per bit noise spectrum density ratio (Eb/No) of the other
received
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bits. This results in an overall improvement of bit error and block error
rates of the
received data as compared to what those quality metrics would have been had
the
bits not been repeated and two dummy bits been sent instead. Of course, eight
units
of energy were used to transmit data only requiring six units of energy. There
are
as a result the effect of the unintended but consequential increased
transmission
energy and the effect of improved received data quality. Those effects are
addressed by the present invention.
[0062] The open and closed power control systems shown in Figures 3 and 4
for variable multirate data are virtually the same as those shown in Figures 1
and 2
for single rate data. Figure 3 and Figure 4 represent open and closed power
control
systems for a 3GPP TDD communication system. However, both the open and
closed loop power control systems are less than optimal in addressing the
effects of
rate changes for variable multirate data.
[0063] In the open loop system of Figure 3, with N(t) equal to M(t) in the
steady state and ignoring the variance of a fading channel or any variable
interference, the target SIR will settle at a quiescent point yielding the
desired data
quality. This condition is equivalent to the single rate example of Figure 1.
In a
multiple channel variable rate system, however, at some time, t, N, and/or M
changes. As described above, where this results in an improvement to the
measured
data quality metric, more energy than is actually required is transmitted. The
outer
loop, which operates at a relatively low rate, will eventually detect the
improved
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signal quality and then lower the target SIR for the inner loop to reduce
transmitter
power to compensate for what it perceives as too-high signal quality. In the
meantime, the transmitter 11 will be using more energy than is actually
necessary to
transmit the data (to have it received with the required quality). In the case
of an
open loop power controlled transmit station being a battery powered mobile
unit (as
can be the case in a 3GPP system), unnecessary battery power is expended.
[0064] The invention as it applies to open loop power control for variable
multirate data is illustrated in Figure 7 where corresponding elements are
identified
with the same reference numbers as in Figure 3. As shown in Figure 7, the
transmitting station's converter 27 provides an additional input 29 to the
scale factor
generating processor 15. Though input 29, the converter provides a signal
equivalent to /(N(t)/M(t)) to the processor 15 as a factor in calculating the
transmit
power scale factor. Accordingly, when the modified scale factor is applied to
the
transmitted data, it causes the transmit power to be adjusted by the factor
of:
N(t)/M(t)
to immediately compensate for the rate change in N(t) or M(t).
[0065] This modified scale factor is applied in the same manner as is the
conventional scale factor that sets transmitter power, which is derived from:
PTS SIRTARGET IRS a(L - Lo) + Lo + CONSTANT VALUE Equation 1
where the additive terms represent multiplicative factors expressed in dB. As
a
practical matter, the additional factor used in generating the scale factor
becomes
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simply another term in the above equation, which in the above form becomes:
PTS = SIRTARGET IRS a(L - Lo) + Lo + CONSTANT VALUE + N(t)/M(t)
Equation 2
where:
PTS is the transmitting station's transmission power level in decibels.
SIRTARGET is determined in the receiving station.
'RS is the measure of the interference power level at the receiving station.
L is the path loss estimate in decibels for the most recent time slot for
which the path loss was estimated.
Lo, the long term average of the path loss in decibels, is the running
average of the pathloss estimate, L.
CONSTANT VALUE is a correction term. The CONSTANT VALUE
corrects for differences in the uplink and downlink channels, such as to
compensate
for differences in uplink and downlink gain. Additionally, the CONSTANT VALUE
may provide correction if the transmit power reference level of the receiving
station
is transmitted, instead of the actual transmit power.
a is a weighting value which is a measure of the quality of the estimated
path loss and is, preferably, based on the number of time slots between the
time slot
of the last path loss estimate and the first time slot of the communication
transmitted
by the transmitting station. The value of a is between zero and one.
Generally, if the
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time difference between the time slots is small, the recent path loss estimate
will be
fairly accurate and a is set at a value close to one. By contrast, if the time
difference
is large, the path loss estimate may not be accurate and the long term average
path loss
measurement is most likely a better estimate for the path loss. Accordingly, a
is set
at a value closer to one. Equations 3 and 4 are equations for determining a.
a = 1 - (D - 1)/(Dmax-1) Equation 3
a = max {1-(D-1)/(Dmax - allowed -1), 01 Equation 4
where the value, D, is the number of time slots between the time slot of the
last path
loss estimate and the first time slot of the transmitted communication which
will be
referred to as the time slot delay. If the delay is one time slot, a is one.
Diriax is the
maximum possible delay. A typical value for a frame having fifteen time slots
is
seven. If the delay is Dmax, a is zero Dmax-allowed is the maximum allowed
time slot
delay for using open loop power control. If the delay exceeds Dmax-allowed,
open loop
power control is effectively turned off by setting a = 0.
[0066] As the data rates N(t) and M(t) change from time-to-time, the
inventive
system of Figure 7 compensates for the change in required power, as opposed to
waiting for a revised target SIR to be determined by the outer loop to
compensate for
the data rate change. Thus, for open loop power control, the invention
virtually
eliminates the period of time when the transmitted signal is sent with excess
power due
to a data rate change.
[0067] With respect to the closed loop system of Figure 4 with N(t) equal to
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M(t) in the steady state, ignoring the variance of a fading channel or any
variable
interference, the target SIR will settle at a quiescent point yielding the
desired data
quality. This is the equivalent of the single rate system of Figure 2. With
variable
multirate, however, at some time t, N and/or M changes. As described above,
where
this results in an improvement to the measured data quality metric, more
energy than
is actually required is transmitted. However, the measured SIR does not change
with
changes in N and M, because the SIR is measured before the down-conversion
with
it concomitant increase in Eb/No (or SIR) per repeated bit. Since the outer
loop
operates at a relatively low rate, in the short term, the power control
commands sent
back to the transmitter will no longer be accurate. However, eventually the
outer loop
will detect the improved signal quality and compute a lower target SIR for the
inner
loop to compensate for what it perceives as too-high signal quality. When that
happens, this too-low target SIR will downward bias the step up/down decisions
and
thus reduce transmitter power. This in turn will result in below-required
signal quality
at the receiver. Eventually, the outer loop will respond to the degraded
signal quality
with a higher target SIR, and in the steady state the system will eventually
converge
to the correct power level. Until then, the received signal will be degraded.
[0068] Figure 8 illustrates the invention as it applies to a closed loop power
control system for variable multirate data where corresponding elements have
the same
reference numerals as in Figure 4. In the transmitter 51 of the transmitting
station 50,
the converter 67 provides an additional input 69 to the scale factor
generating
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processor 55. The converter provides a signal equivalent to [(N(t)/M(t)) so
that the
scale factor output by the processor 55 via output 53 is a function of
N(t)/M(t) as
described above in connection with the open loop system of Figure 7.
[0069] In the receiver, the converter 87 outputs a signal equivalent to
N(t)/M(t)
to a combiner 88, preferably a multiplier. The output of the target SIR
processor 74
is diverted to the combiner 88. The combiner 88 combines the rate change data
from
the converter 87 and the target SIR data from the processor 74 and outputs an
adjusted
target SIR to the combiner 76.
[0070] Through this configuration, the processor 74 effectively outputs a
nominal target SIR. By applying the factor N(t)/M(t) to the nominal target SIR
determined from the measured signal quality, a more rapid response is made to
compensate or adjust for a change received power due to a data rate change.
[0071] As data rates N(t) and M(t) change from time-to-time, the system of
Figure 8 rapidly compensates for the change in required power in the
transmitter and
the changed expected received signal strength in the receiver, as opposed to
waiting
for the outer loop to compensate for the data rate change. Thus, for closed
loop power
control system of Figure 8 the period of time when the received signal is
received
below acceptable quality due to a data rate change is reduced.
[0072] Although various components have been identified separately within the
respective transmitting and receiving stations, those of ordinary skill in the
art will
recognize that various elements can be combined. For example, combiner 88 of
the
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system of Figure 8 can be embodied in a single processor with processor 74.
Other
variations and modifications consistent with the invention will be recognized
by those
of ordinary skill in the art.
