U.S. patent application number 10/463188 was filed with the patent office on 2004-06-10 for systems and techniques for channel gain computations.
Invention is credited to Attar, Rashid, Black, Peter J., Challa, Raghu, Sindhushayana, Nagabhushana.
Application Number | 20040110525 10/463188 |
Document ID | / |
Family ID | 26801294 |
Filed Date | 2004-06-10 |
United States Patent
Application |
20040110525 |
Kind Code |
A1 |
Black, Peter J. ; et
al. |
June 10, 2004 |
Systems and techniques for channel gain computations
Abstract
Systems and techniques for controlling transmission power
involve receiving a first to second channel power ratio, receiving
a first to second channel power ratio, adjusting the power ratio if
a combined power of a plurality of channels exceeds a threshold,
the channels including the first and second channels, and computing
gain of the first channel as a function of the power ratio.
Inventors: |
Black, Peter J.; (San Diego,
CA) ; Sindhushayana, Nagabhushana; (San Diego,
CA) ; Challa, Raghu; (San Diego, CA) ; Attar,
Rashid; (San Diego, CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
26801294 |
Appl. No.: |
10/463188 |
Filed: |
June 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10463188 |
Jun 16, 2003 |
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10104208 |
Mar 20, 2002 |
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6594501 |
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60340512 |
Dec 14, 2001 |
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Current U.S.
Class: |
455/522 ;
330/279; 330/284; 370/317; 375/130; 455/561; 455/69 |
Current CPC
Class: |
H04W 52/325 20130101;
H04W 52/16 20130101; H04W 52/10 20130101; H04W 52/36 20130101; H04W
52/08 20130101; H04W 52/26 20130101 |
Class at
Publication: |
455/522 ;
330/279; 330/284; 370/317; 375/130; 455/561; 455/069 |
International
Class: |
H04B 001/00; H04B
007/00; H04K 001/00; H04B 001/69; H04B 001/707; H04B 001/713; H04B
015/00; H04L 027/30; H03G 003/10; H03G 003/18; H03G 003/30; H04B
007/185; H04Q 007/20; H04M 001/00; H04B 001/38 |
Claims
What is claimed is:
1. A method for reducing power ratios of a data rate control (DRC)
channel and an acknowledgment (ACK) channel, comprising: enabling a
DRC power loop, wherein a DRC backoff value is reduced to
incrementally reduce a DRC to pilot channel power ratio if a total
power of traffic channels exceeds a maximum power capability of a
transmitter; and enabling a ACK power loop if the DRC backoff value
is below a minimum threshold, wherein an ACK backoff value is
reduced to incrementally reduce an ACK to pilot channel power ratio
if the total power of traffic channels exceeds the maximum power
capability of the transmitter.
2. Apparatus for reducing power ratios of a data rate control (DRC)
channel and an acknowledgment (ACK) channel, comprising: means for
enabling a DRC power loop, wherein a DRC backoff value is reduced
to incrementally reduce a DRC to pilot channel power ratio if a
total power of traffic channels exceeds a maximum power capability
of a transmitter; and means for enabling a ACK power loop if the
DRC backoff value is below a minimum threshold, wherein an ACK
backoff value is reduced to incrementally reduce an ACK to pilot
channel power ratio if the total power of traffic channels exceeds
the maximum power capability of the transmitter.
3. A method for reducing power ratios of a data rate control (DRC)
channel and an acknowledgment (ACK) channel, comprising: computing
a total power to pilot channel power ratio R; comparing the ratio R
to a maximum allowable total power to pilot channel power ratio
R.sub.M; if R.ltoreq.R.sub.M, then: setting a DRC backoff value and
an ACK backoff value to 1; and refraining from reducing a DRC power
ratio and an ACK power ratio; if R>R.sub.M, then: computing a
new DRC backoff value; determining whether the new DRC backoff
value is positive or negative; if the new DRC backoff value is
positive, then: setting a new ACK backoff value to 1; using the new
DRC backoff value and the new ACK backoff value to reduce the DRC
power ratio; if the new DRC backoff value is negative, then: gating
off the DRC channel; computing a new ACK backoff value; and using
the new ACK backoff value to reduce the ACK power ratio.
4. Apparatus for adjusting power ratios of a data rate control
(DRC) channel and an acknowledgment (ACK) channel, comprising: a
power and gain computation block means for computing a total power
to pilot channel power ratio R; a limiter means for comparing the
ratio R to a maximum allowable total power to pilot channel power
ratio R.sub.M; and a power throttle block means for implementing a
power control algorithm, wherein the algorithm is for determining:
if R.ltoreq.R.sub.M, then: setting a DRC backoff value and an ACK
backoff value to 1; and refraining from reducing a DRC power ratio
and an ACK power ratio; if R>R.sub.M, then: computing a new DRC
backoff value; determining whether the new DRC backoff value is
positive or negative; if the new DRC backoff value is positive,
then: setting a new ACK backoff value to 1; using the new DRC
backoff value and the new ACK backoff value to control a reduction
in the DRC power ratio; if the new DRC backoff value is negative,
then: gating off the DRC channel; computing a new ACK backoff
value; and using the new ACK backoff value to control a reduction
in the ACK power ratio.
Description
CROSS REFERENCE
[0001] This application is a continuation of Utility application
Ser. No. 10/104,208 entitled "SYSTEMS AND TECHNIQUES FOR CHANNEL
GAIN COMPUTATIONS" and filed on Mar. 20, 2002, which claims
priority to Provisional Application No. 60/340,512, filed Dec. 14,
2001, entitled "Systems and Techniques for Channel Gain
Computations" and is assigned to the assignee of the present
application.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates to communications systems, and
more specifically, to systems and techniques for controlling
transmission power in a wireless communications system.
[0004] 2. Background
[0005] Modern communications systems are designed to allow multiple
users to access a common communications medium. Numerous
multiple-access techniques are known in the art, such as time
division multiple-access (TDMA), frequency division multiple-access
(FDMA), space division multiple-access, polarization division
multiple-access, code division multiple-access (CDMA), and other
similar multi-access techniques. The multiple-access concept is a
channel allocation methodology which allows multiple user access to
a common communications link. The channel allocations can take on
various forms depending on the specific multi-access technique. By
way of example, in FDMA systems, the total frequency spectrum is
divided into a number of smaller sub-bands and each user is given
its own sub-band to access the communications link. Alternatively,
in TDMA systems, each user is given the entire frequency spectrum
during periodically recurring time slots. In CDMA systems, each
user is given the entire frequency spectrum for all of the time but
distinguishes its transmission through the use of a code.
[0006] In multi-access communications systems, techniques to reduce
mutual interference between multiple users are often utilized to
increase user capacity. By way of example, power control techniques
can be employed to limit the transmission power of each user to
that necessary to achieve a desired quality of service. This
approach ensures that each user transmits only the minimum power
necessary, but no higher, thereby making the smallest possible
contribution to the total noise seen by other users. These power
control techniques may become more complex in multi-access
communications systems supporting users with multiple channel
capability. In addition to limiting the transmission power of the
user, the allocated power should be balanced between the multiple
channels in a way that optimizes performance.
SUMMARY
[0007] In one aspect of the present invention, a method of
controlling transmission power includes receiving a first to second
channel power ratio, adjusting the power ratio if a combined power
of a plurality of channels exceeds a threshold, the channels
including the first and second channels, and computing gain of the
first channel as a function of the power ratio.
[0008] In another aspect of the present invention, a computer
readable media embodying a method of controlling transmission
receives a first to second power ratio, adjusts the power ratio if
a combined power of a plurality of channels exceeds a threshold,
the channels including the first and second channels, and computes
gain of the first channel as a function of the power ratio.
[0009] In yet another aspect of the invention, an apparatus
including a transmitter gain control configured to receive a first
to second channel power ratio, adjust the power ratio if a combined
power of a plurality of channels exceeds a threshold, the channels
including the first and second channels, compute gain of the first
channel as a function of the power ratio, and a transmitter
configured to apply the computed gain to the first channel, combine
the channels, and apply a second gain to the combined channels.
[0010] In a further aspect of the present invention, an apparatus
includes means for receiving a first to second power ratio, means
for adjusting the power ratio if a combined power of a plurality of
channels exceeds a threshold, the channels including the first and
second channels, and means for computing gain of the first channel
as a function of the power ratio.
[0011] It is understood that other aspects of the present invention
will become readily apparent to those skilled in the art from the
following detailed description, wherein is shown and described only
exemplary embodiments of the invention, simply by way of
illustration. As will be realized, the invention is capable of
other and different embodiments, and its several details are
capable of modifications in various respects, all without departing
from the invention. Accordingly, the drawings and description are
to be regarded as illustrative in nature, and not as
restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Aspects of the present invention are illustrated by way of
example, and not by way of limitation, in the accompanying drawings
in which like reference numerals refer to similar elements:
[0013] FIG. 1 is a simplified functional block diagram of an
exemplary CDMA communications system;
[0014] FIG. 2 is a functional block diagram of an exemplary
subscriber station adapted for operation in a CDMA communications
system;
[0015] FIG. 3 is a functional block diagram an exemplary
transmitter gain control and transmitter from the subscriber
station of FIG. 2;
[0016] FIG. 4 is a flow chart illustrating an exemplary back off
algorithm implemented by the transmitter gain control of FIG. 3;
and
[0017] FIG. 5 is a flow chart illustrating an alternative exemplary
back off algorithm implemented by the transmitter gain control of
FIG. 3.
DETAILED DESCRIPTION
[0018] The detailed description set forth below in connection with
the appended drawings is intended as a description of exemplary
embodiments of the present invention and is not intended to
represent the only embodiments in which the present invention can
be practiced. The term "exemplary" used throughout this description
means "serving as an example, instance, or illustration," and
should not necessarily be construed as preferred or advantageous
over other embodiments. The detailed description includes specific
details for the purpose of providing a thorough understanding of
the present invention. However, it will be apparent to those
skilled in the art that the present invention may be practiced
without these specific details. In some instances, well known
structures and devices are shown in block diagram form in order to
avoid obscuring the concepts of the present invention.
[0019] In an exemplary embodiment of a communications system, a
power control system can be used to increase the number of users
that can be supported by the system. For those users with
multi-channel capability, gain computation techniques can be
employed to balance the relative transmission power between the
channels. The gain computations can be performed through a power
estimation process which controls the transmission power for one or
more channels. In the event that the total transmission power
exceeds the power limitations of the user, a systematic back off
procedure can be used to reduce the gain of one or more
channels.
[0020] Various aspects of these gain computation techniques will be
described in the context of a CDMA communications system, however,
those skilled in the art will appreciate that the techniques for
gain computation of multiple channels are likewise suitable for use
in various other communications environments. Accordingly, any
reference to a CDMA communications system is intended only to
illustrate the inventive aspects of the present invention, with the
understanding that such inventive aspects have a wide range of
applications.
[0021] CDMA is a modulation and multiple access scheme based on
spread-spectrum communications. In a CDMA communications system, a
large number of signals share the same frequency spectrum and, as a
result, provide an increase in user capacity. This is achieved by
transmitting each signal with a different pseudo-random noise (PN)
code that modulates a carrier, and thereby, spreads the spectrum of
the signal waveform. The transmitted signals are separated in the
receiver by a correlator that uses a corresponding PN code to
despread the desired signal=s spectrum. The undesired signals,
whose PN codes do not match, are not despread in bandwidth and
contribute only to noise.
[0022] FIG. 1 is a simplified functional block diagram of an
exemplary CDMA communications system. A base station controller 102
can be used to provide an interface between a network 104 and all
base stations dispersed throughout a geographic region. For ease of
explanation, only one base station 106 is shown. The geographic
region is generally subdivided into smaller regions known as cells.
Each base station is configured to serve all subscriber stations in
its respective cell. In some high traffic applications, the cell
may be divided into sectors with a base station serving each
sector. In the described exemplary embodiment, three subscriber
stations 108a-c are shown in communication with the base station
106. Each subscriber station 108a-c may access the network, or
communicate with other subscriber stations, through one or base
stations under control of the base station controller 102.
[0023] A power control system can be employed to reduce mutual
interference between the multiple subscriber stations. The power
control system can be used to limit the transmission power over
both the forward and reverse links to that necessary to achieve a
desired quality of service. The forward link refers to
transmissions from the base station to a subscriber station, and
the reverse link refers to transmissions from a subscriber station
to the base station. For the purposes of illustration, the gain
computation techniques will be described with reference to the
reverse link, however, as those skilled in the art will readily
appreciate, these gain computation techniques are equally
applicable to the forward link.
[0024] The reverse link transmission power is typically controlled
with two power control loops. The first power control loop is an
open loop control. The open control loop is designed to control the
reverse link transmission power as a function of path loss, the
effect of base station loading, and environmentally induced
phenomena such as fast fading and shadowing. This open control loop
estimation process is well known in CDMA communications
systems.
[0025] The second power control loop is a closed loop control. The
closed loop control has the function of correcting the open loop
estimate to achieved a desired signal-to-noise ratio (SNR) at the
base station. This can be achieved by measuring the reverse link
transmission power at the base station and providing feedback to
the subscriber station to adjust the reverse link transmission
power. The feedback signal can be in the form of a reverse power
control (RPC) command which is generated by comparing the measured
reverse link transmission power at the base station with a power
control set point. If the measured reverse link transmission power
is below the set point, then an RPC up command is provided to the
subscriber station to increase the reverse link transmission power.
If the measured reverse link transmission power is above the set
point, then an RPC down command is provided to the subscriber
station to decrease the reverse link transmission power.
[0026] The open and closed loop controls may be used to control the
transmission power of various reverse link channel structures. By
way of example, in some CDMA communications systems, the reverse
link waveform includes a traffic channel to carry voice and data
services to the base station and a pilot channel used by the base
station for coherent demodulation of the voice and data. In these
systems, the open and closed loop controls can be used to control
the reverse link power of the pilot channel. In order to optimize
performance, the power of the pilot channel can then be balanced
with the power of the traffic channel. Specifically, each channel
can be spread with a unique orthogonal code generated by using
Walsh functions. A gain can then applied to the traffic channel in
order to maintain an optimal traffic to pilot channel power
ratio.
[0027] This principle can be extended to additional channels in the
reverse link waveform. In CDMA communications systems with a
variable data rate, for example, a data rate control (DRC) channel
containing a DRC message is generally supported by the reverse link
transmission. In the variable data rate mode, the data rate of the
forward link transmission is dictated by the DRC message. The DRC
message is typically based on a carrier-to-interference (C/I)
estimation performed at the subscriber station. This approach
provides a mechanism for the base station to efficiently transmit
the forward link data at the highest possible rate. An exemplary
CDMA communications system supporting a variable data rate request
scheme is a High Data Rate (HDR) communications system. The HDR
communications system is typically designed to conform one or more
standards such as the "cdma2000 High Rate Packet Data Air Interface
Specification," 3GPP2 C.S0024, Version 2, Oct. 27, 2000,
promulgated by a consortium called "3.sup.rd Generation Partnership
Project," the contents of the aforementioned standard being
incorporated by reference herein.
[0028] In the described exemplary HDR communications system, the
reverse link transmission may also support an acknowledgment (ACK)
channel. The ACK channel is used to indicate to the base station
that the subscriber station has successfully decoded a packet
received over the forward link. This can be achieved by sending an
ACK message over the ACK channel.
[0029] In these HDR communications systems, the power of the pilot
channel can also be balanced with the power of the DRC and ACK
channels. This process involves spreading each of the DRC and ACK
channels with a unique orthogonal code generated by using Walsh
functions. A DRC gain can then be applied to the DRC channel to
maintain an optimal DRC to pilot channel power ratio. Similarly, an
ACK gain can also be applied to the ACK channel to maintain an
optimal ACK to pilot channel power ratio.
[0030] A functional block diagram of an exemplary subscriber
station operating in an HDR communications system is shown in FIG.
2. The exemplary subscriber station includes a receiver and a
transmitter both coupled to an antenna 202. The receiver includes
an RF front end 204, a demodulator 206 and a decoder 208. The
transmitter includes an encoder 209, a modulator 210, and shares
the RF front end 204 with the receiver. The transmitter also
includes a transmitter gain control 214 to control the reverse link
transmission power in a manner to be discussed in greater detail
later.
[0031] The RF front end 204 is coupled to the antenna 202. The
receiver portion of the front end 204 downconverts, filters,
amplifies and digitizes a signal received by the antenna 202. The
receiver portion of the RF front end 204 also includes an AGC (not
shown) to maximize the dynamic range of the digitized signal. The
AGC can be utilized by the transmitter gain control 214 to compute
the path loss between the base station and the subscriber station
during the open loop power control estimation. The digitized signal
from the receiver portion of the RF front end 204 can then be
coupled to the demodulator 206 where it is quadrature demodulated
with short PN codes, decovered by Walsh codes, and descrambled
using a long PN code. The demodulated signal can then be provided
to the decoder 208 for forward error correction. The demodulator
206 can also be used to extract the RPC command from the reverse
link transmission and provide it to the transmitter gain control
214 for closed loop power control computations.
[0032] The transmitter includes the encoder 209 which typically
provides convolution coding and interleaving of the reverse link
traffic channel. The encoded traffic channel is provided to the
modulator 210 where it is spread with a Walsh cover and amplified
by a traffic channel gain (G.sub.T) computed by the transmitter
gain control 214. The pilot channel, DRC channel, and ACK channel
are also provided to the modulator 210 where they are each spread
with a different Walsh cover and amplified by respective channel
gains (G.sub.P), (G.sub.D), and (G.sub.A) computed by the
transmitter gain control 214. The channels are then combined,
spread with a long PN code and quadrature modulated with short PN
codes. The quadrature modulated signal is provided to the
transmitter portion of the RF front end 204 where it is
upconverted, filtered, and amplified for over the air forward link
transmission through the antenna 202. The amplification of the
quadrature modulated signal in the transmitter portion of the RF
front end 204 is controlled by an AGC signal from the transmitter
gain control 214.
[0033] A functional block diagram of an exemplary transmitter gain
control 214, modulator 210 and transmitter portion of the RF front
end 204 is shown in FIG. 3. The transmitter gain control 214
includes a power and gain computation block 302 for computing the
gains for the pilot, traffic, DRC, and ACK channels. The gain
computations are based on predetermined power ratios for the
traffic, DRC and ACK channels with respect to the pilot channel. A
feedback loop can be used to reduce the channel gains under power
limiting conditions by "throttling" or "backing off" the
predetermined power ratios for the DRC and ACK channels. The
feedback loop includes a limiter 304 and a power throttle block
306. The limiter 304 determines whether the total reverse link
transmission power resulting from the predetermined power ratios
exceeds the maximum power capability of the transmitter. The
maximum power capability of the transmitter is limited by a
variable gain amplifier (VGA) 308 and a power amplifier (not shown)
in the RF front end 204. In the described exemplary embodiment, the
power and gain computation block 302 also computes the total
reverse link transmission power based on the predetermined power
ratios and the estimated reverse link power for the pilot channel.
If the resultant total reverse link transmission power exceeds the
power capability of the transmitter; the power throttle block 306
is used to back off the power ratios for the DRC and ACK channels
in a manner to be described in greater detail later.
[0034] The transmitter gain control 214 can be implemented with a
variety of technologies including, by way of example, embedded
communications software. The embedded communications software can
be run on a programmable digital signal processor (DSP).
Alternatively, the transmitter gain control 214 can be implemented
with a general purpose processor running a software program, an
application specific integrated circuit (ASIC), a field
programmable gate array (FPGA) or other programmable logic device,
discrete gate or transistor logic, discrete hardware components, or
any combination thereof.
[0035] In the described exemplary embodiment, the power ratios for
the traffic, DRC and ACK channels with respect to the pilot channel
can be used to compute the channel gains. Accordingly,
computational complexity can be reduced if the appropriate power
ratios are first determined using the feedback loop before the gain
computations are made. As explained above, the feedback loop is
used to "throttle" or "back off" the predetermined power ratios of
the DRC and ACK channels with respect to the pilot channel if the
total reverse link transmission power exceeds the maximum power
capability of the transmitter. The total reverse link transmission
power can be computed by solving the following equation:
Total Power=Pilot Channel Power+10
log.sub.10(1+P.sub.T+.beta..sub.D.quadr- ature.P.sub.D+.beta.A
.quadrature.P.sub.A) (1)
[0036] where: P.sub.T is the traffic to pilot channel power
ratio;
[0037] P.sub.D is the DRC to pilot channel power ratio;
[0038] .beta..sub.D is a value used to back off the DRC power
ratio;
[0039] P.sub.A is the ACK to pilot channel power ratio; and
[0040] .beta..sub.A is a value used to back off the ACK power
ratio.
[0041] The "Pilot Channel Power" is estimated by two power control
loops. An open loop control 310 generates an estimate of the
required transmission power on the pilot channel based on the
average value of the AGC from the receiver. The open loop estimate
can then be computed by means well known in the art for nominal
base station loading and Effective Radiated Power (ERP).
Information about variations from the nominal base station loading
and ERP can be communicated from the base station to the subscriber
station and used to adjust the open loop estimate by means well
known in the art.
[0042] A closed loop control 312 can be used to increment or
decrement the current closed loop estimate based on the RPC
commands recovered from the demodulator. The resultant closed loop
estimate is summed with the open loop estimate by a summer 314. The
sum of the open and closed loop estimates yields the total reverse
link power for the pilot channel. This sum is provided to the power
and gain computation block 302 where it is used as the "Pilot
Channel Power" in equation (1).
[0043] The traffic to pilot channel power ratio P.sub.T can be
computed in a variety of fashions. In at least one embodiment, the
traffic to pilot channel power ratio can be predetermined for each
data rate supported by the reverse link transmission either by
empirical analysis, simulation, experimentation, or any other means
to achieve a desired quality of service. By way of example, in at
least one simulation it has been shown that for a 9.6 k data rate,
the traffic to pilot channel ratio should be between -2.25 db and 9
dB. If the data rate is increased to 38.4 k, the traffic to pilot
channel power ratio should be between 3.75 dB and 15 dB. Those
skilled in the art will readily be able to determine the
appropriate traffic to pilot channel power ratio values for all
data rates supported by their particular application. These
predetermined power ratio values can be stored at the base station
and transmitted to each subscriber station in its respective cell
or sector over a forward link control channel. Alternatively, the
predetermined power ratio values can be stored or computed at the
subscriber station.
[0044] The DRC to pilot channel power ratio P.sub.D and the ACK to
pilot channel ratio P.sub.A can also be computed in a variety of
fashions. Similar to the traffic to pilot channel power ratio, the
DRC and ACK power ratios can be predetermined either by empirical
analysis, simulation, experimentation, or any other means to
achieve a desired quality of service. In at least one embodiment of
the exemplary HDR communications system, the DRC to pilot channel
power ratio can take on values between -9 dB and 6 dB in 1 dB
increments and the ACK to pilot channel power ratio can take on
values between -3 dB and 6 dB also in 1 dB increments. These
predetermined power ratio values can be stored at the base station
and transmitted to each subscriber station in its respective cell
or sector over the control channel. Alternatively, the
predetermined power ratio values can be stored or computed at the
subscriber station.
[0045] To compute the total reverse link transmission power from
equation (1), the channel power ratio values can be converted into
the linear domain as follows:
P.sub.T=10.sup.(Traffic Power Ratio Value/10) (2)
P.sub.D=10.sup.(DRC Power Ratio Value/10) (3)
P.sub.A=10.sup.(ACK Power Ratio Value/10) (4)
[0046] The power throttle block 306 is used to reduce the power
ratios for the DRC and ACK channels under power limiting
conditions. The power throttle block 306 can accomplish this by
generating values .beta..sub.D and .beta..sub.A and feeding them
back to the power and gain computation block 302 for scaling the
DRC and ACK power ratios, respectively. The DRC and ACK values can
be expressed in the linear domain respectively as:
.beta..sub.D=10.sup.(-DRCbackoff/10) (5)
.beta..sub.A=10.sup.(-ACKbackoff/10) (6)
[0047] From the above equations, one skilled in the art will
readily appreciate that a 1 dB reduction in the DRC to pilot
channel power ratio can be achieved by setting "DRCbackoff" in
equation (5) to 1. A 2 dB reduction in the ACK to pilot channel
power ratio can be achieved by setting "ACKbackoff" in equation (6)
to 2. Thus, any incremental reduction scheme can be implemented
depending on the particular design parameters and the specific
communications application.
[0048] In the described exemplary embodiment, the "Total Power" in
equation (1) is initially computed by the power and gain
computation block 302 with the values .beta..sub.D and .beta..sub.A
set to 1 so that the power ratios for each channel are set to their
predetermined, optimal or desired values. The computed "Total
Power" is provided to the limiter 304. The limiter 304 compares the
computed "Total Power" to the maximum power capability of the
transmitter. If the computed "Total Power" exceeds the power
limitations of the transmitter, then the reverse link data rate can
be lowered to reduce the total reverse link transmission power. In
response to the reduced data rate, the power and gain computation
block 302 selects a new predetermined traffic to pilot channel
power ratio corresponding to the reduced data rate. The "Total
Power" can then be recomputed by the power and gain computation
block 302 and provided to the limiter 304 for comparison with the
maximum power capability of the transmitter. This procedure
continues until the computed "Total Power" comes within the power
capability of the transmitter or the reverse link data rate is
reduced to the lowest data rate supported by the communications
system.
[0049] In the event that the "Total Power" computed by the power
and gain computation block 302 exceeds the maximum power capability
of the transmitter at the lowest data rate supported by the
communications system, the limiter 304 can be used to ratchet the
closed loop control 312 such that RPC up commands are ignored. This
can be achieved by holding the current closed loop estimate
constant in response to an RPC up command and reducing the current
closed loop estimate in response to an RPC down command. In some
embodiments, ratcheting can be supported at both ends of the
transmitter power level such that RPC down commands are ignored if
the reverse link transmission power is below a minimum operating
threshold.
[0050] The limiter 304 also enables the power throttle block 306 to
implement a "back off" algorithm to adjust the values .beta..sub.D
and .beta..sub.A to systematically reduce the power ratios for one
or both of the DRC and ACK channels until the "Total Power"
computed by the power and gain computation block is within the
maximum power capability of the transmitter. The manner in which
the values .beta..sub.D and .beta..sub.A are reduced and the
resultant incremental reduction in the power ratios of the channels
may vary depending on the system application and the overall design
constraints.
[0051] A flow chart illustrating an exemplary back off algorithm is
shown in FIG. 4. Initially, the limiter is used to enable the back
off algorithm in step 402. Once the back off algorithm is enabled,
a DRC power loop 404 is entered into. In the DRC power loop 404,
the DRC to pilot channel power ratio can backed off by 1 dB in step
406. This can be achieved by recomputing the DRC value .beta..sub.D
with the "DRCbackoff" in equation (5) at 1. Alternatively, the
"DRCbackoff" in equation (5) can be set to any value to achieve a
desired reduction in the DRC to pilot channel power ratio. In any
event, the recomputed DRC value .beta..sub.D can be fed back to the
to the power and gain computation block. The "Total Power" can then
be recomputed and provided to the limiter to determine whether the
total reverse link transmission power is within the maximum power
capability of the transmitter.
[0052] In step 408, the results from the limiter are provided to
the gain throttle block. If the recomputed "Total Power" is within
the maximum power capability of the transmitter, the power throttle
block is disabled in step 410. Conversely, if the recomputed "Total
Power" still exceeds the maximum power capability of the
transmitter, then the power throttle block determines whether the
DRC channel remains in the "on" state in step 412. The DRC channel
is determined to be in the "on" state if the DRC .beta..sub.D is
greater than 0, or some other minimum threshold value. If the DRC
channel remains in the "on" state, then the algorithm loops back to
step 406 and reduces the DRC to pilot channel power ratio another
dB by setting the "DRCbackoff" in equation (5) to 2 which will
result in a 2 dB reduction in the DRC to pilot channel ratio. The
back off algorithm remains in the DRC power loop 404 until either
the power throttle block is disabled in step 410 or the DRC value
.beta..sub.D is reduced below the minimum threshold. Should the DRC
value .beta..sub.D be reduced below the minimum threshold, then the
back off algorithm exits the DRC power gain loop 404 and enters an
ACK power loop 416.
[0053] In the ACK power loop 416, the ACK to pilot channel power
ratio can backed off by 1 dB in step 418. This can be achieved by
recomputing the ACK value PA with the "ACKbackoff" in equation (6)
at 1. Alternatively, the "ACKbackoff" in equation (6) can be set to
any value to achieve a desired reduction in the ACK to pilot
channel power ratio. In any event, the recomputed ACK value
.beta..sub.A can be fed back to the to the power and gain
computation block. The "Total Power" can then be recomputed and
provided to the limiter 304 to determine whether the total reverse
link transmission power is within the maximum power capability of
the transmitter.
[0054] In step 420, the results from the limiter are provided to
the gain throttle block. If the recomputed "Total Power" is within
the maximum power capability of the transmitter, the power throttle
block is disabled in step 424. Conversely, if the recomputed "Total
Power" still exceeds the maximum power capability of the
transmitter, then the power throttle block determines whether the
ACK channel remains in the "on" state. The ACK channel is
determined to be in the "on" state if the ACK value .beta..sub.A is
greater than 0, or some other minimum threshold value. If the ACK
channel remains in the "on" state, then the algorithm loops back to
step 418 and reduces the ACK to pilot channel power ratio another
dB by setting the "ACKbackoff" in equation (6) to 2 which will
result in a 2 dB reduction in the ACK to pilot channel ratio. The
back off algorithm remains in the ACK power loop 416 until either
the power throttle block is disabled by the limiter in step 424 or
the ACK value .beta..sub.A is reduced below the minimum threshold.
Should the ACK value .beta..sub.A be reduced below the minimum
threshold, then the back off algorithm is disabled by exiting the
ACK power loop in step 424. In that event, other power reduction
techniques can be employed to bring the total reverse link
transmission power within the maximum power capability of the
transmitter.
[0055] The exemplary embodiment of the back off algorithm described
in connection with FIG. 4 may be computationally intensive
depending on the predetermined power ratios for the DRC and ACK
channels. By way of example, if the predetermined power ratios for
the DRC and ACK channels are each set to the 6 dB maximum, there is
a possibility that 16 passes through the DRC power loop and 10
passes through the ACK power loop might be required to back off the
DRC and ACK power ratios. To reduce the potential computational
complexity, an alternative back off algorithm may be implemented
that computes the DRC value .beta..sub.D in a single step, and if
necessary, computes the ACK value .beta..sub.A in a single step.
This can be achieved in a variety of ways. By way of example,
equation (1) can be manipulated to solve for the value of interest
by defining a total power to pilot channel power ratio and setting
it to a value relating to the maximum power capability of the
transmitter. This can be achieved by rewriting equation (1) in the
linear domain as:
R=1+P.sub.T+P.sub.D+P.sub.A (7)
R.sub.M.gtoreq.1+P.sub.T+.beta..sub.D.quadrature.P.sub.D+.beta..sub.A.quad-
rature.P.sub.A (8)
[0056] where R represents the total power to pilot channel power
ratio before enabling the back off algorithm, and R.sub.M is the
maximum allowed value of R after computing the values .beta..sub.D
and .beta..sub.A.
[0057] A flow chart illustrating an exemplary algorithm utilizing
equations (7) and (8) is shown in FIG. 5. In step 502, the total
power to pilot channel power ratio R is computed. The computed
power ratio is then compared to the maximum allowable total power
to pilot channel power ratio R.sub.M in step 504. If
R.ltoreq.R.sub.M, then the DRC and ACK power ratios do not require
back off. In that event, the values .beta..sub.D and .beta..sub.A
are set to 1 by the power throttle block and fed back to the power
and gain computation block in step 506. Conversely, if R is greater
than R.sub.M, the DRC value .beta..sub.D can then be computed in
step 508.
[0058] The DRC value .beta..sub.D can be computed by setting the
ACK value .beta..sub.A to 1 and solving for the DRC value
.beta..sub.D in equation (8). With the ACK value .beta..sub.A set
to 1, equation (8) can be rewritten as:
.beta..sub.D=(R.sub.M-1-P.sub.T-P.sub.A)/P.sub.D (9).
[0059] In step 510, the resultant DRC value computation is examined
to determine whether it is positive. If .beta..sub.D.gtoreq.0, then
the computed DRC value .beta..sub.D will reduce the DRC to pilot
channel power ratio to a level that will result in a "Total Power"
computation within the maximum power capability of the transmitter.
In that event, the ACK value .beta..sub.A is set to land fed back
along with the computed DRC value .beta..sub.D to the power and
gain computation block in step 512. Conversely, if the DRC value
.beta..sub.D is negative, then the DRC channel is gated off by
setting the DRC value .beta..sub.D to 0 in step 514.
[0060] Once the DRC channel is gated off, the ACK valve
.beta..sub.A value can be computed in step 516. The ACK value
.beta..sub.A can be computed by setting the DRC value .beta..sub.D
to 0 and solving for the ACK value .beta..sub.A in equation (8).
With the DRC value .beta..sub.D set to 0, equation (8) can be
rewritten as:
.beta..sub.A=(R.sub.M-1-P.sub.T)/P.sub.A (10).
[0061] In step 518, the resultant ACK value computation is examined
to determine whether it is positive. If .beta..sub.A.gtoreq.0, then
the computed ACK value .beta..sub.A will reduce the ACK to pilot
channel power ratio to a level that will result in a "Total Power"
computation by the power and gain computation block within the
maximum power capability of the transmitter. In that event, the DRC
value .beta..sub.D is set to 0 and fed back along with the computed
ACK value .beta..sub.A to the power and gain computation block in
step 520. Conversely, if the ACK value .beta..sub.A is negative,
then the ACK channel is gated off by setting the ACK value
.beta..sub.A to 0 in step 522.
[0062] Regardless of the back off algorithm implemented by the
power throttle block 306, the power and gain computation block 302
will compute the gains for the traffic, DRC, ACK and pilot channels
once the limiter 304 determines that the total reverse link
transmission power is within the maximum power capability of the
transmitter. Since the gains will be applied to their respective
channels in the digital domain, it is advantageous to scale the
gains to prevent an increase in the number of bits as the gain
adjusted channels are added together in the modulator. This can be
accomplished by setting the gains such that the sum of their
squares equals 1 as follows:
G.sub.P.sup.2+G.sub.T.sup.2+G.sub.D.sup.2+G.sub.A.sup.2=1 (10)
[0063] Equation (10) can be resolved as follows for each channel
gain:
G.sub.p=1/{square root}{square root over
(1+P.sub.T+.beta..sub.D.multidot.-
P.sub.D+.beta..sub.A.multidot.P.sub.A)} (11)
G.sub.T={square root}{square root over (P.sub.T)}/{square
root}{square root over
(1+P.sub.T+.beta..sub.D.multidot.P.sub.D+.beta..sub.A.multidot.-
P.sub.A)} (12)
G.sub.D={square root}{square root over
(.beta..sub.D.multidot.P.sub.D)}/{s- quare root}{square root over
(1+P.sub.T+.beta..sub.D.multidot.P.sub.D+.bet- a..sub.AP.sub.A)}
(13)
G.sub.A={square root}{square root over
(.beta..sub.A.multidot.P.sub.A)}/{s- quare root}{square root over
(1+P.sub.T+.beta..sub.D.multidot.P.sub.D+.bet- a..sub.AP.sub.A)}
(14)
[0064] Referring back to FIG. 3, the channel gains computed by the
power and gain computation block 302 can be coupled to the
modulator 210. The modulator 210 includes a mixer 316b which is
used to spread the encoded traffic channel from the encoder with a
Walsh function. The pilot, DRC and ACK channels are also provided
to mixers 316a, 316c, and 316d, respectively, where they are each
spread with a different Walsh cover. The Walsh covered traffic,
pilot, DRC and ACK channels are provided to gain elements 318a-d,
respectively, where their respective gains computed by the power
and gain computation block 306 are applied. The output of the gain
elements 318a-d are provided to a summer 320 where they are
combined with the pilot channel. The combined channels are then
coupled to a mixer 322 where they are spread using the long PN
code. The spread channels are then split into a complex signal
having an in-phase (I) component and a quadrature phase (Q)
component. The complex signal is quadrature modulated with the
short PN codes by mixers 324a and 324b before being output to the
transmitter portion of the RF front end 204.
[0065] A complex baseband filter 326 is positioned at the input to
the RF front end 204 to reject out of band components of the
quadrature modulated signal. The filtered complex signal is
provided to quadrature mixers 328a and 328b where it is modulated
onto a carrier waveform before being combined by a summer 330. The
combined signal is then provided to the VGA 308 to control the
power of the reverse link transmission through the antenna. An AGC
signal from the power and gain computation block 302 is used to set
the gain of the of the VGA 308. The AGC signal is based on the
"Total Power" computed by the power and gain computation block 302
from equation (1).
[0066] Those skilled in the art will appreciate that the various
illustrative logical blocks, modules, circuits, and algorithms
described in connection with the embodiments disclosed herein may
be implemented as electronic hardware, computer software, or
combinations of both. To clearly illustrate this interchangeability
of hardware and software, various illustrative components, blocks,
modules, circuits, and algorithms have been described above
generally in terms of their functionality. Whether such
functionality is implemented as hardware or software depends upon
the particular application and design constraints imposed on the
overall system. Skilled artisans may implement the described
functionality in varying ways for each particular application, but
such implementation decisions should not be interpreted as causing
a departure from the scope of the present invention.
[0067] The various illustrative logical blocks, modules, and
circuits described in connection with the embodiments disclosed
herein may be implemented or performed with a general purpose
processor, a digital signal processor (DSP), an application
specific integrated circuit (ASIC), a field programmable gate array
(FPGA) or other programmable logic device, discrete gate or
transistor logic, discrete hardware components, or any combination
thereof designed to perform the functions described herein. A
general purpose processor may be a microprocessor, but in the
alternative, the processor may be any conventional processor,
controller, microcontroller, or state machine. A processor may also
be implemented as a combination of computing devices, e.g., a
combination of a DSP and a microprocessor, a plurality of
microprocessors, one or more microprocessors in conjunction with a
DSP core, or any other such configuration.
[0068] The methods or algorithms described in connection with the
embodiments disclosed herein may be embodied directly in hardware,
in a software module executed by a processor, or in a combination
of the two. A software module may reside in RAM memory, flash
memory, ROM memory, EPROM memory, EEPROM memory, registers, hard
disk, a removable disk, a CD-ROM, or any other form of storage
medium known in the art. An exemplary storage medium is coupled to
the processor such the processor can read information from, and
write information to, the storage medium. In the alternative, the
storage medium may be integral to the processor. The processor and
the storage medium may reside in an ASIC. The ASIC may reside in a
user terminal. In the alternative, the processor and the storage
medium may reside as discrete components in a user terminal.
[0069] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *