U.S. patent application number 12/869389 was filed with the patent office on 2010-12-23 for method and apparatus for forward link power control.
This patent application is currently assigned to QUALCOMM Incorporated. Invention is credited to Keith W. Saints, Edward G. Tiedemann, JR..
Application Number | 20100323747 12/869389 |
Document ID | / |
Family ID | 25288778 |
Filed Date | 2010-12-23 |
United States Patent
Application |
20100323747 |
Kind Code |
A1 |
Tiedemann, JR.; Edward G. ;
et al. |
December 23, 2010 |
METHOD AND APPARATUS FOR FORWARD LINK POWER CONTROL
Abstract
A forward link power control mechanism measures the reverse link
power control bits which are transmitted on the forward traffic
channel. At the remote station, the reverse link power control bits
from multiples base stations or multiple signal paths are measured,
combined, and filtered to yield an improved measurement of the
forward link signal quality. The reverse link power control bits
which are deemed unreliable are omitted from use in the power
control loop. The remote station generates a set of forward link
power control bit in accordance with the measurements and transmits
these bits to all base stations in communication with the remote
station. Each base station adjusts its gain of the forward traffic
channel in accordance to its measurement of the forward link power
control bit.
Inventors: |
Tiedemann, JR.; Edward G.;
(San Diego, CA) ; Saints; Keith W.; (San Diego,
CA) |
Correspondence
Address: |
QUALCOMM INCORPORATED
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Assignee: |
QUALCOMM Incorporated
San Diego
CA
|
Family ID: |
25288778 |
Appl. No.: |
12/869389 |
Filed: |
August 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12257342 |
Oct 23, 2008 |
7809044 |
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12869389 |
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|
10877174 |
Jun 25, 2004 |
7680174 |
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12257342 |
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10092749 |
Mar 6, 2002 |
6757320 |
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10877174 |
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08842993 |
Apr 25, 1997 |
6396867 |
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10092749 |
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Current U.S.
Class: |
455/522 |
Current CPC
Class: |
C08L 23/04 20130101;
C08L 23/08 20130101; H04W 52/40 20130101; C08L 23/16 20130101; C08L
23/0853 20130101; H04W 52/08 20130101; C08L 23/0815 20130101; C08L
23/06 20130101; C08L 23/12 20130101; C08L 2205/02 20130101; H04W
52/24 20130101; C08L 23/06 20130101; C08L 2666/04 20130101; C08L
23/08 20130101; C08L 2666/04 20130101; C08L 23/0815 20130101; C08L
2666/04 20130101; C08L 23/12 20130101; C08L 2666/04 20130101; C08L
23/16 20130101; C08L 2666/04 20130101 |
Class at
Publication: |
455/522 |
International
Class: |
H04W 52/04 20090101
H04W052/04 |
Claims
1. A remote station (6) for use in a wireless communication system
comprising one or more base stations (4) and one or more remote
stations (6), characterized in that the remote station (6)
comprises: a receiver (106) for receiving at least one
communication signal including a power control signal transmitted
by a base station (4) in a first transmission channel; a processor
(120) for processing the at least one signal received by the
receiver (106) to derive an attribute of the at least one signal
received by the receiver (1 cm6) from the power control signals;
and a transmitter (136) for transmitting, at a transmission power
determined by the received power control signal, transmission power
control signals for the base station (4) in a second transmission
channel, said signals transmitted in said second transmission
channel representing the attribute of the received communication
signal.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present Application for Patent is a Continuation and
claims priority to patent application Ser. No. 12/257,342 entitled
"Method and Apparatus for Forward Link Power Control" filed Oct.
23, 2008, now allowed, which is a continuation of U.S. Pat. No.
7,680,174 entitled "Method and Apparatus for Forward Link Power
Control" filed Jun. 25, 2004 and issued Mar. 16, 2010, which is a
continuation of U.S. Pat. No. 6,396,867 entitled "Method and
Apparatus for Forward Link Power Control" filed Apr. 25, 1997 and
issued May 28, 2002 all assigned to the assignee hereof and hereby
expressly incorporated by reference herein.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates to data communication. More
particularly, the present invention relates to a novel and improved
method and apparatus for the forward link power control in a
communication system.
[0004] 2. Background
[0005] The use of code division multiple access (CDMA) modulation
techniques is one of several techniques for facilitating
communications in which a large number of system users are present.
Other multiple access communication system techniques, such as time
division multiple access (TDMA) and frequency division multiple
access (FDMA) are known in the art. However, the spread spectrum
modulation techniques of CDMA have significant advantages over
other modulation techniques for multiple access communication
systems. The use of CDMA techniques in a multiple access
communication system is disclosed in U.S. Pat. No. 4,901,307,
entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM
USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee
of the present invention and is incorporated by reference herein.
The use of CDMA techniques in a multiple access communication
system is further disclosed in U.S. Pat. No. 5,103,459, entitled
"SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA
CELLULAR TELEPHONE SYSTEM," also assigned to the assignee of the
present invention and is incorporated by reference herein.
Furthermore, the CDMA system can be designed to conform to the
"TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard
for Dual-Mode Wideband Spread Spectrum Cellular System,"
hereinafter referred to as the IS-95-A standard or
TIA/EIA/IS-95-A.
[0006] CDMA, by its inherent nature of being a wideband signal,
offers a form of frequency diversity by spreading the signal energy
over a wide bandwidth. Therefore, frequency selective fading
affects only a small part of the CDMA signal bandwidth. Space or
path diversity is obtained by providing multiple signal paths
through simultaneous links to a mobile user or remote station
through two or more base stations. Furthermore, path diversity may
be obtained by exploiting the multipath environment through spread
spectrum processing by allowing signals arriving with different
propagation delays to be received and processed separately.
Examples of path diversity are illustrated in U.S. Pat. No.
5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF
IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM," and U.S.
Pat. No. 5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR
TELEPHONE SYSTEM," both assigned to the assignee of the present
invention and incorporated by reference herein.
[0007] The reverse link refers to a transmission from a remote
station to a base station. On the reverse link, each transmitting
remote station acts as an interference to other remote stations in
the network. Therefore, the reverse link capacity is limited by the
total interference due to transmissions from other remote stations.
The CDMA system increases the reverse link capacity by transmitting
fewer bits, thereby using less power and reducing interference,
when the user is not speaking
[0008] To minimize interference and maximize the reverse link
capacity, the transmit power of each remote station is controlled
by three reverse link power control loops. The first power control
loop adjusts the transmit power of the remote station by setting
the transmit power inversely proportional to the received power on
the forward link. In an IS-95-A system, the transmit power is given
by p.sub.out=-73-p.sub.in where p.sub.in is the power received by
the remote station given in dBm, p.sub.out is the transmit power of
the remote station given in dBm, and -73 is a constant. This power
control loop is often called the open loop.
[0009] The second power control loop adjusts the transmission power
of the remote station such that the signal quality, as measured by
the energy-per-bit-to-noise-plus-interference ratio
E.sub.b/I.sub.o, of the reverse link signal received at the base
station is maintained at a predetermined level. This level is
referred to as the E.sub.b/I.sub.o set point. The base station
measures the E.sub.b/I.sub.o of the reverse link signal received at
the base station and transmits a reverse link power control bit to
the remote station on the forward traffic channel in response to
the measured E.sub.b/I.sub.o. The reverse power control bits are
set 16 times per 20 msec frame, or at an 800 bps rate. The forward
traffic channel carries the reverse link power control bits along
with the data from the base station to the remote station. This
second loop is often called the inner closed loop.
[0010] The CDMA communication system typically transmits packets of
data as discrete data frames. Thus, the desired level of
performance is typically measured by the frame-error-rate (FER).
The third power control loop adjusts the E.sub.b/I.sub.o set point
such that the desired level of performance, as measured by the FER,
is maintained. The required E.sub.b/I.sub.o to obtain a given FER
depends upon the propagation conditions. This third loop is often
called the outer closed loop. The power control mechanism for the
reverse link is disclosed in detail in U.S. Pat. No. 5,056,109,
entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER
IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," assigned to the
assignee of the present invention and incorporated by reference
herein.
[0011] The forward link refers to a transmission from a base
station to a remote station. On the forward link, the transmission
power of the base station is controlled for several reasons. A high
transmission power from the base station can cause excessive
interference with the signals received at other remote stations.
Alternatively, if the transmission power of the base station is too
low, the remote station can receive erroneous data transmissions.
Terrestrial channel fading and other known factors can affect the
quality of the forward link signal as received by the remote
station. As a result, each base station attempts to adjust its
transmission power to maintain the desired level of performance at
the remote station.
[0012] Power control on the forward link is especially important
for data transmissions. Data transmission is typically asymmetric
with the amount of data transmitted on the forward link being
greater than on the reverse link. With an effective power control
mechanism on the forward link, wherein the transmission power is
controlled to maintain the desired level of performance, the
overall forward link capacity can be improved.
[0013] A method and apparatus for controlling the forward link
transmission power is disclosed in U.S. Pat. No. 6,035,209,
entitled "METHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER
CONTROL IN A MOBILE COMMUNICATION SYSTEM," hereinafter the '209
patent, filed Mar. 31, 1995, assigned to the assignee of the
present invention and incorporated by reference herein. In the
method disclosed in the '209 patent, the remote station transmits
an error-indicator-bit (EIB) message to the base station when a
transmitted frame of data is received in error. The EIB can be
either a bit contained in the reverse traffic channel frame or a
separate message sent on the reverse traffic channel. In response
to the EIB message, the base station increases its transmission
power to the remote station.
[0014] One of the disadvantages of this method is the long response
time. The processing delay encompasses the time interval from the
time the base station transmits the frame with inadequate power to
the time the base station adjusts its transmission power in
response to the error message from the remote station. This
processing delay includes the time it takes for (1) the base
station to transmit the data frame with inadequate power, (2) the
remote station to receive the data frame, (3) the remote station to
detect the frame error (e.g. a frame erasure), (4) the remote
station to transmit the error message to the base station, and (5)
the base station to receive the error message and appropriately
adjust its transmission power. The forward traffic channel frame
must be received, demodulated, and decoded before the EIB message
is generated. Then the reverse traffic channel frame carrying the
EIB message must be generated, encoded, transmitted, decoded, and
processed before the bit can be used to adjust the transmit power
of the forward traffic channel.
[0015] Typically, the desired level of performance is one percent
FER. Therefore, on the average, the remote station transmits one
error message indicative of a frame error every 100 frames. In
accordance with the IS-95-A standard, each frame is 20 msec long.
This type of EIB based power control works well to adjust the
forward link transmit power to handle shadowing conditions, but due
to its slow speed is ineffective in fading except in the slowest
fading conditions.
[0016] A second method for controlling the forward link
transmission power utilizes the E.sub.b/I.sub.o of the received
signal at the remote station. Since the FER is dependent on the
E.sub.b/I.sub.o of the received signal, a power control mechanism
can be designed to maintain the E.sub.b/I.sub.o at the desired
level. This design encounters difficulty if data is transmitted on
the forward link at variable rates. On the forward link, the
transmission power is adjusted depending on the data rate of the
data frame. At lower data rates, each data bit is transmitted over
a longer time period by repeating the modulation symbol as
described in TIA/EIA/IS-95-A. The energy-per-bit E.sub.b is the
accumulation of the received power over one bit time period and is
obtained by accumulating the energy in each modulation symbol. For
an equivalent amount of E.sub.b, each data bit can be transmitted
at proportionally less transmission power at the lower data rates.
Typically, the remote station does not know the transmission rate a
priori and cannot compute the received energy-per-bit E.sub.b until
the entire data frame has been demodulated, decoded, and the data
rate of the data frame determined. Thus, the delay of this method
is as described in the aforementioned U.S. Pat. No. 6,035,209, and
the rate is one power control message per frame. This is in
contrast with the reverse link approach in which there can be one
power control message (bit) sixteen times per frame as in
TIA/EIA/IS-95-A.
[0017] Other methods and apparatus for performing fast forward link
power control are described in the aforementioned U.S. Pat. No.
6,035,209, U.S. Pat. No. 6,137,840, entitled "METHOD AND APPARATUS
FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION
SYSTEM," filed Nov. 15, 1995, issued Oct. 24, 2000, to Edward G.
Tiedemann Jr., et al., U.S. Pat. No. 5,903,554, entitled "METHOD
AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM
COMMUNICATION SYSTEM," filed Sep. 27, 1996, U.S. Pat. No.
5,893,035, entitled "CENTRALIZED FORWARD LINK POWER CONTROL," filed
Sep. 16, 1996, and U.S. Pat. No. 6,075,974, entitled "METHOD AND
APPARATUS FOR ADJUSTING THRESHOLDS AND MEASUREMENTS OF RECEIVED
SIGNALS BY ANTICIPATING POWER CONTROL COMMANDS YET TO BE EXECUTED,"
filed Nov. 20, 1996, all are assigned to the assignee of the
present invention and incorporated by reference herein.
[0018] The fundamental difference between the forward link and the
reverse link is that the transmission rate does not need to be
known on the reverse link. As described in the aforementioned U.S.
Pat. No. 5,056,109, at lower rates, the remote station does not
transmit continuously. When the remote station is transmitting, the
remote station transmits at the same power level and the same
waveform structure regardless of the transmission rate. The base
station determines the value of a power control bit and sends this
bit to the remote station 16 times per frame. Since the remote
station knows the transmission rate, the remote station can ignore
power control bits corresponding to times when it was not
transmitting. This permits fast reverse link power control.
However, the effective power control rate varies with the
transmission rate. For TIA/EIA/IS-95-A, the rate is 800 bps for
full rate frames and 100 bps for 1/8 rate frames.
[0019] An alternative reverse link architecture is described in the
U.S. Pat. No. 5,930,230, entitled "HIGH DATA RATE CDMA WIRELESS
COMMUNICATION SYSTEM," hereinafter the '230 patent, filed May, 28,
1996, assigned to the assignee of the present invention and
incorporated by reference herein. In accordance with the '230
patent, an auxiliary pilot is introduced into the reverse link. The
pilot level is independent of the transmission rate on the reverse
link. This permits the base station to measure the pilot level and
to send the reverse link power control bit to the remote station at
a constant rate.
SUMMARY OF THE INVENTION
[0020] The present invention is a novel and improved method and
apparatus for high rate forward link power control. The present
invention improves the response time of the forward link power
control loop and allows for dynamic adjustment of the transmission
power on the forward link by measuring the quality of the reverse
link power control bits which are transmitted on the forward
traffic channel at multiple times within a frame. Measurements over
short time intervals allow the base station to dynamically adjust
the transmission power to minimize interference to other base
stations and maximize the forward link capacity. The improved
response time allows the power control loop to effectively
compensate for slow fading. For fast fading, the block interleaver
in the communication system is effective.
[0021] In accordance with the present invention, the remote station
measures the reverse link power control bits which are transmitted
at a rate of 800 bits per second on the forward traffic channel.
The reverse link power control bits are punctured into the forward
traffic channel data stream. The gain of the power control bits is
adjusted along with the gain of the forward link data bits.
However, unlike the data bits, the transmission level of the power
control bit is not scaled according to the data rate. The measured
signal quality of the power control bits is used to adjust the
transmission power of the base stations.
[0022] It is an object of the present invention to improve the
response time of the forward link power control by the use of the
energy measurements of the reverse link power control bits. The
reverse link power control bits are transmitted at a rate of 800
bps. Thus, the forward link power control mechanism of the present
invention can perform a measurement of the quality of the received
forward traffic channels periodically every 1.25 msec. The
measurements can be transmitted to the base stations for use in
adjustment of the forward link transmission power. The improved
response time allows the base stations to effectively compensate
for slow fades in the channel and improve the performance of the
forward traffic channels.
[0023] It is another object of the present invention to increase
the capacity of forward link by allowing for rapid adjustments in
the transmission power of the base stations. The power control
mechanism of the present invention allows the base stations to
transmit at the minimal transmission power necessary to maintain
the requisite level of performance. Since the total transmission
power of the base stations is fixed, minimal transmission for a
given task results in a saving of transmission power which can be
used for other tasks.
[0024] It is yet another object of the present invention to provide
for a reliable forward link power control mechanism. At the remote
station, the reverse link power control bits from multiple sectors
of a base station or multiple signal paths from the same sector are
combined to yield an improved measurement of the forward link
signal quality. The reverse link power control bits which are
deemed unreliable may be omitted from use in the power control
loop. At the base stations, the forward link power control bits are
received by all base stations in communication with the remote
station. The gains of the forward traffic channels of the base
stations are corrected periodically so that erroneous reception of
the forward link power control bits by the base stations do not
accumulate.
[0025] It is yet another object of this invention to provide a
mechanism to adjust the forward link power to the desired frame
error rate, similar to that done by the outer loop for the reverse
link.
[0026] It is yet another object of this invention to provide a
mechanism to communicate the power control bits between base
stations. The power control bits which control the forward link
transmit power may or may not have been correctly received at
different base stations. The present invention provides base
stations which receive erroneous power control bits with the
information necessary to update their forward link transmit
power.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The features, objects, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly throughout
and wherein:
[0028] FIG. 1 is a diagram of the communication system of the
present invention showing a plurality of base stations in
communication with a remote station;
[0029] FIG. 2 is an exemplary block diagram of the base station and
the remote station;
[0030] FIG. 3 is an exemplary block diagram of the forward traffic
channel;
[0031] FIG. 4 is an exemplary block diagram of the demodulator
within the remote station;
[0032] FIG. 5 is an exemplary block diagram of the decoder within
the remote station;
[0033] FIG. 6 is an exemplary block diagram of the power control
processor within the remote station;
[0034] FIG. 7 is a timing diagram of the forward and reverse link
power control channels; and
[0035] FIG. 8 is a timing diagram of the gain correction mechanism
within the forward link power control loop.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] In the present invention, the base station transmits the
reverse link power control bits along with the data on the forward
traffic channel. The reverse link power control bits are used by
the remote station to control its transmission power so as to
maintain the desired level of performance while minimizing the
interference to other remote stations in the system. The power
control mechanism for the reverse link is disclosed in the
aforementioned U.S. Pat. No. 6,035,209. Because of sensitivity to
processing delays, the reverse link power control bits are not
encoded. In fact, the power control bits are punctured onto the
data (see FIG. 3). In this sense, puncturing is a process by which
one or more code symbols are replaced by the power control
bits.
[0037] In the exemplary embodiment, the reverse link power control
bits are transmitted at a rate of 800 bps, or one power control bit
for every 1.25 msec time slot. The time slot is called a power
control group. Transmitting the power control bits at evenly spaced
intervals can result in the base station sending out power control
bits to multiple remote stations at the same time. This results in
a peak in the amount of transmitted power. As a result, the power
control bits are pseudo-randomly positioned within the 1.25 msec
power control group. This is achieved by partitioning the 1.25 msec
time slot into 24 positions and pseudo-randomly selecting, with a
long PN sequence, the position in which to puncture in the power
control bit. In the exemplary embodiment, only one of the first 16
positions within the power control group is selected as a starting
position and the last 8 positions are not selected.
[0038] The forward traffic channel is a variable rate channel and
the transmission power of the forward traffic channel is dependent
on the data rate. The performance of the forward traffic channel is
measured by the FER which is dependent on the energy-per-bit
E.sub.b of the signal received at the remote station. At the lower
data rates, the same energy-per-bit is spread over a longer time
period, resulting in a lower transmission power level.
[0039] In the exemplary embodiment, transmissions over the forward
link are made in accordance with TIA/EIA/IS-95-A. The IS-95-A
standard provides for transmission using one of two rate sets. Rate
set 1 supports data rates of 9.6 kbps, 4.8 kbps, 2.4 kbps, and 1.2
kbps. The 9.6 kbps data rate is encoded with a rate 1/2
convolutional encoder to yield a 19.2 ksps symbol rate. The encoded
data for the lower data rates are repeated N times to obtain the
19.2 ksps symbol rate. Rate set 2 supports data rates of 14.4 kbps,
7.2 kbps, 3.6 kbps, and 1.8 kbps. The 14.4 kbps data rate is
encoded with a rate 1/2 convolutional encoder punctured to obtain a
rate 3/4. Thus, the symbol rate is also 19.2 ksps for the 14.4 kbps
data rate. The rate set is selected by the base station during the
initiation stage of a call and typically remains in effect for the
duration of the communication, although the rate set can be changed
during the call. In the exemplary embodiment, the duration of the
reverse link power control bit is two symbols wide (104.2 .mu.sec)
for rate set 1 and one symbol wide (52.1 .mu.sec) for rate set
2.
[0040] In this specification, the transmission gain of the forward
traffic channel refers to the energy-per-bit E.sub.b(traffic) of
the transmitted data signal. A frame with a lower data rate
consists of fewer bits transmitted at the specified energy-per-bit,
and therefore is transmitted with less power. In this way, the
power level of the forward link traffic channel scales with the
data rate of the frame currently being transmitted. The
transmission gain of the reverse link power control bits refers to
the energy-per-bit E.sub.b(power control) of the reverse link power
control bits punctured into the data stream. Each reverse link
power control bit has the same duration, and therefore the power
level of these bits does not depend on the data rate of the frame
into which they are punctured. These characteristics of the power
control bits are exploited by the present invention to provide the
improved forward link power control mechanism. The operation of
forward link power control causes the base station to make
adjustments in the traffic channel gain. In the exemplary
embodiment, each adjustment to the traffic channel gain is also
applied to the gain of the reverse link power control bits, so that
the two gains are adjusted together.
[0041] The present invention determines the quality of the forward
link signal, as received by the remote station, by measuring the
amplitude of the reverse link power control bits which are
transmitted on the forward traffic channel. The quality of the data
bits are not measured directly, but rather inferred from the
measured amplitude of the reverse link power control bits. This is
reasonable since the power control bits and the traffic data are
equally affected by changes in the propagation environment.
Therefore, the present invention operates well if the amplitude of
the data bits is maintained at a known ratio to the amplitude of
the power control bits.
[0042] Typically, the reverse link power control bits are
transmitted at a low transmission power level. Furthermore, the
power control bits can be transmitted from multiple base stations
within the communication system. A more accurate measurement of the
amplitude of the power control bits is obtained by receiving the
power control bits, adjusting the phase and amplitude of the power
control bits in accordance with the phase and amplitude of the
pilot signal, and filtering the adjusted amplitude of the power
control bits. The filtered amplitude of the power control bits are
used to control the transmission power of the base station such
that the quality of the forward link signal received at the remote
station is maintained at the desired level.
[0043] The forward link power control mechanism of the present
invention operates two power control loops. The first power control
loop, the closed loop, adjusts the transmission power of the base
station such that the quality of the filtered amplitude of the
reverse link power control bits received at the remote station is
maintained at a target energy level. In most situations, the target
energy level is determinative of the FER of the forward traffic
channel. The remote station requests the base station to adjust the
forward link transmit power by sending forward link power control
bits over the reverse link. Each forward link power control bit
causes the base station to increase or decrease the gain of the
corresponding traffic channel. The second power control loop, the
outer loop, is the mechanism by which the remote station adjusts
the target energy level in order to maintain the desired FER.
[0044] In order to improve the effectiveness of the forward link
power control mechanism, e.g. to combat slow fading in the channel,
the closed loop is designed to operate at a high rate. In the
exemplary embodiment, the reverse link power control bits from
which the quality measurements of the forward link signal are made
are transmitted at 800 bps and the forward link power control bits
are also sent on the reverse traffic channel at 800 bps. Thus, the
transmission power of the base station can be adjusted at rates up
to 800 times per second. However, because the forward power control
bits are sent uncoded and with minimal energy, some forward power
control bits may not be received satisfactorily at the base
station. A base station may elect to ignore any forward power
control bits which it considers to be sufficiently unreliable.
[0045] In the exemplary embodiment, the second forward link power
control loop, the outer loop, updates the target energy level once
every frame or 50 times per second. The outer loop sets the value
of the target energy level which results in the desired FER
performance. When the propagation environment is not changing, the
outer loop should quickly determine the appropriate value of the
target energy level and keep the target at that level. When there
is a change in the channel characteristic (for example, an increase
in the interference level, a change in velocity of a mobile user,
or the appearance or disappearance of a signal path), it is likely
that a different target energy level will be required in order to
continue operation at the same FER. Therefore the outer loop should
quickly move the target to the new level to adapt to the new
conditions.
[0046] Circuit Description
[0047] Referring to the figures, FIG. 1 represents an exemplary
communication system of the present invention which is composed of
multiple base stations 4 in communication with multiple remote
stations 6 (only one remote station 6 is shown for simplicity).
System controller 2 connects to all base stations 4 in the
communication system and the public switched telephone network
(PSTN) 8. System controller 2 coordinates the communication between
users connected to PSTN 8 and users on remote stations 6. Data
transmission from base station 4 to remote station 6 occurs on the
forward link through signal paths 10 and transmission from remote
station 6 to base station 4 occurs on the reverse link through
signal paths 12. The signal path can be a straight path, such as
signal path 10a, or a reflected path, such as signal path 14.
Reflected path 14 is created when the signal transmitted from base
station 4a is reflected off reflection source 16 and arrives at
remote station 6 through a different path than the line of sight
path. Although illustrated as a block in FIG. 1, reflection source
16 is the results of artifacts in the environment in which remote
station 6 is operating, e.g. a building or other structures.
[0048] An exemplary block diagram of base station 4 and remote
station 6 of the present invention is shown in FIG. 2. Data
transmission on the forward link originates from data source 20
which provides the data to encoder 22. An exemplary block diagram
of encoder 22 is shown in FIG. 3. Within encoder 22, CRC encoder 62
block encodes the data with a CRC polynomial which, in the
exemplary embodiment, conforms to the CRC generator described in
the IS-95-A standard. CRC encoder 62 appends the CRC bits and
inserts a set of code tail bits to the data. The formatted data is
provided to convolutional encoder 64 which convolutionally encodes
the data and provides the encoded data to symbol repeater 66.
Symbol repeater 66 repeats each symbol N.sub.s number of times to
maintain a fixed symbol rate at the output of symbol repeater 66.
The repeated symbols are provided to block interleaver 68. Block
interleaver 68 reorders the symbols and provides the interleaved
data to modulator (MOD) 24. Within modulator 24, the interleaved
data is spread by multiplier 72 with the long PN code which
scrambles the data so that it can be received only by the receiving
remote station 6. The long PN spread data is multiplexed through
MUX 74 and provided to multiplier 76 which covers the data with the
Walsh code corresponding to the traffic channel assigned to remote
station 6. The Walsh covered data is further spread with the short
PNI and PNQ codes by multipliers 78a and 78b, respectively. The
short PN spread data is provided to transmitter (TMTR) 26 (see FIG.
2) which filters, modulates, upconverts, and amplifies the signal.
The modulated signal is routed through duplexer 28 and transmitted
from antenna 30 on the forward link through signal path 10.
Duplexer 28 may not be used in some base station designs.
[0049] MUX 74 is used to puncture the reverse link power control
bits into the data stream. The power control bits are one-bit
messages which command remote station 6 to increase or decrease the
reverse link transmission power. In the exemplary embodiment, one
power control bit is punctured into the data stream in each 1.25
msec power control group. The duration of the reverse link power
control bits is predetermined and can be made dependent on the rate
set used by the system. The location at which the reverse link
power control bit is punctured is determined by the long PN
sequence from long PN generator 70. The output of MUX 74 contains
both data bits and reverse link power control bits.
[0050] Referring to FIG. 2, at remote station 6, the forward link
signal is received by antenna 102, routed through duplexer 104, and
provided to receiver (RCVR) 106. Receiver 106 filters, amplifies,
demodulates, and quantizes the signal to obtain the digitized I and
Q baseband signals. The baseband signals are provided to
demodulator (DEMOD) 108. Demodulator 108 despreads the baseband
signals with the short PNI and PNQ codes, decovers the despread
data with the Walsh code identical to the Walsh code used at base
station 4, despreads the Walsh decovered data with the long PN
code, and provides the demodulated data to decoder 110.
[0051] Within decoder 110 which is shown in FIG. 5, block
de-interleaver 180 reorders the symbols of the demodulated data and
provides the de-interleaved data to Viterbi decoder 182. Viterbi
decoder 182 decodes the convolutionally encoded data and provides
the decoded data to CRC check element 184. CRC check element 184
performs the CRC check and provides the checked data to data sink
112.
[0052] Measurement of the Power Control Bits
[0053] An exemplary block diagram illustrating the circuit for
measuring the energy of the reverse link power control bits is
shown in FIG. 4. The digitized I and Q baseband signals from
receiver 106 are provided to a bank of correlators 160. Each
correlator 160 can be assigned to a different signal path from the
same base station 4 or a different transmission from a different
base station 4. Within each assigned correlator 160, the baseband
signals are despread with the short PNI and PNQ codes by
multipliers 162. The short PNI and PNQ codes within each correlator
160 can have a unique offset in accordance with the base station 4
from which the signal was transmitted and corresponding to the
propagation delay experienced by the signal being demodulated by
that correlator 160. The short PN despread data is decovered by
multipliers 164 with the Walsh code assigned to the traffic channel
being received by the correlator 160. The decovered data is
provided to filters 168 which accumulate the energy of the
decovered data over a symbol time. The filtered data from filters
168 contains both data and power control bits.
[0054] The short PN despread data from multipliers 162 also
contains the pilot signal. At base station 4, the pilot signal is
covered with the all zero sequence corresponding to Walsh code 0.
Thus, no Walsh decovering is necessary to obtain the pilot signal.
The short PN despread data is provided to filters 166 which perform
the lowpass filtering of the despread data to remove the signals
from other orthogonal channels (e.g. the traffic channels, paging
channels, and access channels) transmitted on the forward link by
base station 4.
[0055] The two complex signals (or vectors) corresponding to the
filtered pilot signal and the filtered data and power control bits
are provided to dot product circuit 170 which computes the dot
product of the two vectors in a manner well known in the art. The
exemplary embodiment of dot product circuit 170 is described in
detail in U.S. Pat. No. 5,506,865, entitled "PILOT CARRIER DOT
PRODUCT CIRCUIT," assigned to the assignee of the present invention
and incorporated by reference herein. Dot product circuit 170
projects the vector corresponding to the filtered data onto the
vector corresponding to the filtered pilot signal, multiplies the
amplitude of the vectors, and provides a signed scalar output
s.sub.j(1) to demultiplexer (DEMUX) 172. The notation s.sub.j(m) is
used to denote the output from the m.sup.th correlator 160m, during
the j.sup.th symbol period. Remote station 6 has knowledge of
whether the j.sup.th symbol period of the current frame corresponds
to a data bit or a reverse link power control bit. Accordingly
DEMUX 172 routes the vector of correlator outputs,
s.sub.j=(s.sub.j(1), s.sub.j(2), . . . , s.sub.j(M)), to either
data combiner 174 or power control processor 120. Data combiner 174
sums its vector inputs, despreads the data using the long PN code,
and produces the demodulated data which is presented to decoder 110
shown in FIG. 5.
[0056] The reverse link power control bits are processed by power
control processor 120, shown in detail in FIG. 6. Bit accumulator
190 accumulates one or more symbols s.sub.j(m) over the duration of
a power control bit to form reverse link power control bits
b.sub.i(m). The notation b.sub.i(m) is used to denote the reverse
link power control bit corresponding to the m.sup.th correlator
160m, during the i.sup.th power control group. The vector of power
control bits, b.sub.i=(b.sub.j(1), b.sub.j(2), . . . , b.sub.i(M)),
is presented to identical bit accumulator 192.
[0057] In TIA/EIA/IS-95-A, when more than one base station 4 is in
communication with the same remote station 6, the base stations 4
can be configured to transmit either identical or non-identical
reverse link power control bits. Base stations 4 are typically
configured to send identical power control bit values when they are
physically located at the same location, such as when they are
different sectors of a cell. Base stations 4 which do not send the
same power control bit values are typically those which are
physically located at different locations. The IS-95-A standard
also specifies a mechanism by which base stations 4 which are
configured to send identical power control bits are identified to
remote station 6. Furthermore, when remote station 6 is receiving
the transmission of a single base station 4 through multiple
propagation paths, the reverse link power control bits received on
these paths are inherently identical. Identical bit accumulator 192
combines the reverse link power control bits b.sub.i(m) which are
known to be identical. The output of bit accumulator 192 is thus a
vector of reverse link power control bits, B.sub.i=(b.sub.i(1),
b.sub.i(2), . . . , b.sub.i(P)), corresponding to the P independent
reverse link power control bit streams.
[0058] The vector of sign bits, sgn(Bi(p)), is presented to reverse
link power control logic 194. The IS-95-A standard specifies that
if any one of the signs is negative, remote station 6 decreases its
transmission power level. If all the sign bits sgn(B.sub.i(p)) are
positive, remote station 6 increases its transmission power level.
Reverse link power control logic 194 processes the vector of sign
bits sgn(B.sub.i(p)) as specified in IS-95-A. The output of reverse
link power control logic 194 is a single bit which indicates
whether remote station 6 should increase or decrease its
transmission gain for the purposes of closed-loop reverse link
power control. This bit is provided to transmitter 136 (see FIG. 2)
which adjusts the gain accordingly.
[0059] The amplitude of the reverse link power control bits, and
not their polarity (e.g. the positive or negative sign), is
indicative of the signal quality as measured by remote station 6.
Non-identical bit accumulator 196 therefore removes the modulated
data and operates on the absolute value of the reverse link power
control bits |B.sub.i(p)| which it combines according to the
formula:
x i = 1 P p = 0 P - 1 B i ( p ) .beta. , ( 1 ) ##EQU00001##
[0060] where the factor .beta. specifies the order of non-linearity
and P is the number of independent reverse link power control bit
streams. In the exemplary embodiment, .beta.=1 corresponds to a
measurement of the absolute value of the amplitude of the power
control bit and .beta.=2 corresponds to measurement of the energy
of the power control bit. Other values for .beta. can be used,
depending on the system design, and are within the scope of the
present invention. The output of non-identical bit accumulator 196
is the value x.sub.i which is indicative of the received
energy-per-bit of the reverse link power control subchannel during
the i.sup.th power control group.
[0061] The reverse link power control bits are not encoded and,
therefore, are especially vulnerable to errors caused by
interference. The fast response time of the closed loop reverse
link power control minimizes the effect of such errors on the
performance of the reverse link power control since these erroneous
adjustments to the transmission gain of remote station 6 can be
compensated for in subsequent power control groups. However, since
the amplitude of the power control bits is used as an indication of
the quality of the forward link signal, filter 198 is used to
provide a more reliable measurement of the amplitude of the power
control bits.
[0062] Filter 198 can be implemented using one of a number of
designs known in the art, such as an analog filter or a digital
filter. For example, filter 198 can be implemented as a finite
impulse response (FIR) filter or an infinite impulse response (IIR)
filter. Using a FIR filter implementation, the filtered power
control bits can be calculated as:
y i = j = 0 N - 1 a j x i - j , ( 2 ) ##EQU00002##
[0063] where x.sub.i is the amplitude of the power control bit
computed by non-identical bit accumulator 196 during the i.sup.th
power control group, a.sub.j is the coefficient of the j.sup.th
filter tap, and y.sub.i is the filtered amplitude of the power
control bit from filter 198. Since delay is sought to be minimized,
the coefficients of the FIR filter taps can be selected such that
the larger coefficients of the FIR filter are those with smaller
indices (e.g. a.sub.0>a.sub.1>a.sub.2> . . . ).
[0064] In the exemplary embodiment described herein, the processing
performed by remote station 6 in order to execute fast forward link
power control has been described in such a way as to share various
components used by other subsystems within remote station 6. For
example, correlator 160a is shared with the data demodulation
subsystem, and accumulators 190 and 192 are shared with the reverse
link power control subsystem. The practice of the present invention
is not dependent on any particular implementation of the other
subsystems of remote station 6. It should be obvious to those
skilled in the art that other implementations to perform the
forward power control processing as described herein can be
contemplated and are, therefore, within the scope of the present
invention.
[0065] Forward Link Power Control Outer Loop
[0066] The filtered amplitude y.sub.i of the reverse link power
control bits from filter 198 is indicative of the quality of the
forward link signal received at remote station 6. Threshold
comparison circuit 202 compares the filtered amplitude y.sub.i
against a target energy level z. In the exemplary embodiment, if
y.sub.i exceeds z, remote station 6 transmits a zero (`0`) bit on
its forward link power control subchannel to indicate that each
base station 4 which is transmitting a forward traffic channel to
remote station 6 should reduce the gain of that traffic channel.
Conversely, if y.sub.i is less than z, remote station 6 transmits a
one (`1`) bit on its forward link power control subchannel to
indicate that each base station 4 should increase the gain on the
forward traffic channel. These zeros (`0`s) and ones (`1`s) are the
forward link power control bit values.
[0067] Although the present invention is described in the context
of one forward link power control bit per power control group, the
present invention is applicable toward the use of more bits for
higher resolution. For example, threshold comparison circuit 202
can quantize the difference between the filtered amplitude y.sub.i
of the reverse link power control bit and the target energy value z
to multiple levels. For example, a two-bit message on the forward
link power control subchannel can be used to indicate any one of
four levels for the quantity (y.sub.i-z). Alternatively, remote
station 6 can transmit the value of the filtered amplitude y.sub.i
over the forward link power control subchannel.
[0068] In the present invention, base station 4 does not have to
adjust its transmission power at each power control group. Due to
the low energy level of the reverse link power control bits, remote
station 6 may receive the bits in error or with a large degradation
due to the noise and interference from other users. Filter 198
improves the accuracy of the measurement but does not totally
alleviate the error. In the exemplary embodiment, remote station 6
can omit the transmission of a forward link power control bit to
base station 4 if it determines that the measurement is unreliable.
For example, remote station 6 can compare the filtered amplitude
y.sub.i against a minimal energy value. If y.sub.i is below the
minimal energy value, remote station 6 can ignore the y.sub.i value
for this power control group and inform base station 4 accordingly
(e.g. by not transmitting a forward link power control bit to base
station 4 or by using one value from a set of forward link power
control values to indicate low received energy). Furthermore, the
forward link power control bits are also transmitted at a low
energy level. Therefore, base station 4 can also compare the
measured forward link power control bit against its own minimal
energy value and not act upon bits which fall below the minimal
energy value.
[0069] In the exemplary embodiment, remote station 6 makes an
absolute determination, based on the output of CRC check element
184 as well as other frame quality metrics such as the Yamamoto
metric, and the number of re-encoded symbol errors, as to whether
the frame has been correctly decoded. This determination is
summarized in the erasure indicator bit (EIB) which is set to `1`
to indicate a frame erasure, and set to `0` otherwise. In the
following, it is assumed that remote station 6 makes use of an EIB
in order to determine if received frames are in error. In the
preferred embodiment, the EIB used for the purposes of controlling
the outer loop of forward link power control is the same as the EIB
actually transmitted over the reverse link. However, an independent
determination of the validity of the received frame for the
specific purpose of controlling the outer loop can also be made and
is within the scope of the present invention.
[0070] In the exemplary embodiment, the outer loop is updated once
per frame, or once in every 16 power control groups. The outer loop
updates the target energy level z in remote station 6. This
mechanism is performed by threshold adjust circuit 200 shown in
FIG. 6. As each frame is decoded, frame quality information
e.sub.i, in the form of an EIB, is provided to threshold adjust
circuit 200 as indicated in FIG. 6. Threshold adjust circuit 200
updates the value of the target energy level z and makes the new
target energy level available to threshold comparison circuit
202.
[0071] In the first embodiment, threshold adjust circuit 200
updates the value of z according to the equation:
z k = { z k - 1 + .gamma. e k - 1 = 1 z k - 1 - .delta. e k - 1 = 0
, ( 3 ) ##EQU00003##
[0072] where z.sub.k is the target energy level at the k.sup.th
frame, e.sub.k-1 is the frame error at the (k-1).sup.th frame,
.gamma. is the size of an upward step to be applied to the target
energy level, and .delta. is a size of a downward to be applied to
the target energy level. In the exemplary embodiment, e.sub.k-1 is
set equal to 1 if there was a frame error for the (k-1).sup.th data
frame and 0 otherwise. The values for .gamma. and .delta. are
selected to provide a desired level for the FER. Typically, .gamma.
is large and .delta. is small. This selection creates a
sawtooth-like pattern for z.sub.k. When a frame error occurs,
z.sub.k increases substantially to minimize the probability of
another frame error. When there is no frame error, z.sub.k slowly
decays to minimize the transmission power. In the exemplary
embodiment, the values for z.sub.k, .gamma. and .delta. are in dB
scale, although a linear scale for these variables can also be
used.
[0073] In the second embodiment, the stepsizes .gamma. and .delta.
can be made functions of the current target energy level z.sub.k-1
so that the correction to z.sub.k is dependent on the current
target energy level. Thus, equation (3) can be modified as:
z k = { z k - 1 + .gamma. ( z k - 1 ) e k - 1 = 1 z k - 1 - .delta.
( z k - 1 ) e k - 1 = 0. ( 4 ) ##EQU00004##
[0074] In the exemplary embodiment, remote station 6 completes
demodulation of the data frame and updates the target energy level
z.sub.k during the middle of the succeeding frame. If the
(k-1).sup.th data frame is received in error, the probability of a
frame error for the k.sup.th data frame is greater. This is because
any adjustment to the target energy level will not have an
immediate impact on the FER performance until the system has had
sufficient time to make a transition to the new operating point.
Therefore, the second of two consecutive frame errors should not be
interpreted as indicative of the performance of the target energy
level value which was just updated as a result of the first frame
error.
[0075] In the preferred embodiment, base station 4 increases the
gain of the traffic channel fully after the first frame error, then
ignores a second frame error if it occurs in the following frame.
Applying this concept to the second embodiment described above,
equation (4) becomes:
z k = { z k - 1 + .gamma. ( z k - 1 ) e k - 1 = 1 , e k - 2 = 0 z k
- 1 e k - 1 = 1 , e k - 2 = 1 z k - 1 - .delta. ( z k - 1 ) e k - 1
= 0. ( 5 ) ##EQU00005##
[0076] In the exemplary embodiment, the outer loop power control
mechanism is standardized across all remote stations 6 to ensure
conformance by all remote stations 6. The values of .gamma. and
.delta. can be transmitted to each remote station 6 by base station
4 during the initiation stage of a call. New values for these
parameters can also be specified by base station 4 during the
course of the call.
[0077] In a communication system which conforms to the IS-95-A
standard, the gains of the forward traffic channels are typically
decreased when remote station 6 enters soft handoff. This is done
without any degradation in the FER performance since the data bits
received at remote station 6 from the base stations 4 are combined
to yield a larger composite signal before decoding. However, the
reverse link power control loop within remote station 6 does not
combine the reverse link power control bits received from different
base stations 4 since these bits are independent. The decrease in
the gain on the forward traffic channel can increase the bit error
rate of the power control bit stream transmitted on the forward
traffic channel, and therefore degrade the reverse link power
control mechanism. To remedy this situation, the gain of the power
control bits is typically boosted when remote station 6 enters soft
handoff. This results in the gain of the reverse link power control
bits being slightly higher than the gain of the data bits whenever
remote station 6 is in soft handoff.
[0078] In the present invention, the absolute values of the power
control bits from different base stations 4 are combined according
to equation (2). Thus, the boost in the gain of the power control
bits results in larger values for y.sub.i relative to the data
bits. The larger y.sub.i values cause remote station 6 to request
an inappropriate decrease in transmission power from base station 4
which can result in one or more frame errors on the forward traffic
channel. In this case, the target energy value z set by the outer
loop automatically increases. After a while, the outer loop then
adjusts the target energy value z to the new nominal value. To
combat these effects, y.sub.i can be scaled before comparison with
the target energy level z. Alternatively, the target energy level z
can be slightly increased when remote station 6 enters soft
handoff. This can reduce the likelihood of these errors.
[0079] In the present invention, the comparison of the filtered
amplitude y.sub.i to the target energy level z is performed within
power control processor 120 (see FIG. 2). Furthermore, the update
of the target energy level in accordance with equation (3), (4) or
(5) is also performed within power control processor 120.
Controller processor 120 can be implemented in a microcontroller, a
microprocessor, a digital signal processing (DSP) chip, or an ASIC
programmed to perform the function as described herein.
[0080] Transmission of the Forward Link Power Control Bits
[0081] The forward link power control bits can be transmitted to
base station 4 by one of several methods. In the exemplary
embodiment, each remote station 6 has a forward link power control
channel on the reverse link which is dedicated for the transmission
of the forward link power control bits. In the alternative
embodiment, wherein the dedicated power control channel is not
available, the forward link power control bits can be punctured or
multiplexed onto the reverse link data bit stream in a manner
similar to that done on the forward traffic channel.
[0082] In the exemplary embodiment, the forward link power control
bits are transmitted to base station 4 on a dedicated forward link
power control channel. A method and apparatus for providing a
dedicated forward link power control channel is described in detail
in the aforementioned U.S. Pat. No. 5,930,230. Timing diagrams of
the transmission of the forward and reverse link power control bits
are shown is FIG. 7. In each power control group, delineated by the
heavy hashmarks on the timelines, a reverse link power control bit
is transmitted on the forward traffic channel, as depicted in the
top diagram of FIG. 7. In the exemplary embodiment, one reverse
link power control bit is transmitted in each 1.25 msec power
control group and each reverse link power control bit is two
symbols in duration for rate set 1. Furthermore, each reverse link
power control bit can start from one of 16 positions within the
power control group, depending on the long PN sequence.
[0083] Remote station 6 processes the reverse link power control
bit and transmits a forward link power control bit on the reverse
power control channel to base station 4 as a pulse. In the
exemplary embodiment, the pulse is sent with positive polarity to
indicate a forward link power control bit with value zero (`0`) and
with negative polarity to indicate a the value one (`1`). The
timing and duration of the pulses are design parameters which are
described in the following embodiments. Other choices for these
parameters can be contemplated and are within the scope of the
present invention.
[0084] In the first embodiment, the forward link power control bits
are transmitted as pulses of length 1.25 msec, beginning at 0.625
msec after the last possible (i.e. the 16.sup.th) power control bit
position on the forward traffic channel. This configuration is
illustrated in the middle diagram of FIG. 7, wherein the parameter
"delay1" is set to 0.625 msec. A delay of 0.625 msec allows some
time for remote station 6 to deskew the paths of the forward link
signal in a worst-case scenario. The deskewing properly aligns the
signals from different signal paths before combining and ensures
that the reverse link power control bit from the previous power
control group are processed by the time the forward link power
control bit is transmitted. However, the actual delay from the
reception of the reverse link power control bit to the transmittal
of the forward link power control bit can be as large as 1.45 msec
when the reverse link power control bit is transmitted in the
earliest possible bit position.
[0085] In the second embodiment, the forward link power control
bits are transmitted as pulses of length 1.25 msec, beginning at
approximately 0.050 msec after the latest possible (i.e. the
16.sup.th) power control bit position on the forward traffic
channel. This configuration is identical to the first embodiment,
except that the parameter "delay1" is set to 0.050 msec. In the
worst-case scenario, the reverse link power control bit from the
previous power control group will not have been processed, due to
deskewing delays, by the time the next forward link power control
bit is scheduled to be transmitted. In this situation, remote
station 6 can be configured to repeat the latest forward link power
control bit. However, deskewing delays are typically in the tens of
.mu.sec so, in the majority of cases, the forward link power
control bit will still be able to take into account the processing
of the most recent reverse link power control bit. It should be
evident that the parameter "delay1" can be chosen to optimize the
performance of the system.
[0086] In a third embodiment, shown in the bottom diagram of FIG.
7, the forward link power control bit is transmitted as a short
pulse of approximately 0.41 msec in duration at a predetermined
amount of time ("delay2" in FIG. 7) after reception of the reverse
link power control bit on the forward traffic channel. The duration
of the forward link power control bit is chosen to be small enough
so that it will be finished by the time the following forward link
power control bit is to be sent, even in the worst case when the
latest possible time slot is used in the current power control
group and the earliest possible time slot is to be used in the
following power control group. In the exemplary embodiment, the
amount of delay is set to 0.050 msec (delay2=0.050 msec). As
illustrated in FIG. 7, this embodiment entails higher transmission
power for the duration of the pulse in order to transmit the same
amount of energy over a shorter pulse duration. One drawback of
this method is that the transmission of large amounts of energy
within short pulses at 800 Hz can potentially cause interference in
the audio band to persons with hearing aids. However, since remote
station 6 transmits the forward link power control bits a fixed
amount of time after the reverse link power control bits and the
reverse link power control bits are randomly positioned, the
forward link power control bits are also randomly positioned.
Randomizing the starting position of the power control bits
spectrally distributes the energy at 800 Hz and minimizes the audio
interference. Furthermore, the forward link power control channel
sent on the reverse link from remote station 6 is one of many data
streams transmitted on the reverse link. Since the power in the bit
is low, the net variation in the output power of remote station 6
due to the power control bits is small.
[0087] Finally, in a fourth embodiment, the forward link power
control bit is transmitted after a fixed amount of time,
delay2=0.050 msec, following the reception of a reverse link power
control bit. In this embodiment, however, the duration of the
forward link power control bit is variable, and transmission of the
current forward link power control bit is continued until the next
forward link power control bit is scheduled. Remote station 6 can
send each forward link power control bit with the same gain or it
can adjust the transmit gain based on the duration of the bit in
order to send each bit with the same amount of energy.
[0088] Referring to FIG. 2, the forward link power control bits are
processed by power control processor 120 within remote station 6.
Power control processor 120 computes the forward link power control
bits which are sent on the reverse link and sends the bits to
modulator (MOD) 134. Modulator 134 covers the bits with the Walsh
code corresponding to the reverse power control channel, spreads
the covered data with the long and short PN codes, and provides the
spread data to transmitter (TMTR) 136. Transmitter 136 can be
implemented as described in the aforementioned U.S. Pat. No.
5,930,230. Transmitter 136 filters, modulates, and amplifies the
signal. The modulated signal is routed through duplexer 104 and
transmitted from antenna 102 on the reverse link through signal
path 12.
[0089] At base station 4, the reverse link signal is received by
antenna 30, routed through duplexer 28, and provided to receiver
(RCVR) 50. Receiver 50 filters, amplifies, and downconverts the
signal to obtain the baseband signals. The baseband signals are
provided to demodulator (DEMOD) 52. Demodulator 52 despreads the
baseband signals with the short PN codes, decovers the despread
data with the Walsh code identical to the Walsh code used at remote
station 6, and provides the demodulated data to controller 40. The
demodulated data includes the forward link power control bits.
Controller 40 can adjust the gain of the forward traffic channel
and/or the transmission power of base station 4 as indicated by the
forward link power control bits.
[0090] Base Station Response
[0091] In the present invention, base station 4 receives the
forward link power control bits which are transmitted on the
reverse power control channel and controls the gain of the forward
traffic channel. In the exemplary embodiment, upon receipt of a one
(`1`) for the forward link power control bit, base station 4
increases the gain of the forward traffic channel. Upon receipt of
a zero (`0`), base station 4 decreases the gain. The amount of
increase or decrease in the gain is dependent on the implementation
and system considerations. In the exemplary embodiment, the
increase or decrease in gain can be in steps of 0.5 dB to 1.0 dB,
although other step sizes can be utilized. The step size for the
gain increase can be the same or different from the step size for
the gain decrease. Furthermore, the step size in gain can be made
dependent on the gains of other forward traffic channels at base
station 4. The present invention is applicable to all step sizes in
gain adjustment.
[0092] Base station 4 can also adjust the increase in gain, the
decrease in gain, or both as a function of the velocity and the
fading conditions of remote station 6. Base station 4 does this
since the optimal step size is a function of the fading conditions
and the velocity of remote station 6. For example, at very high
velocities, smaller step sizes may work better since the rate of
the power control bit is not fast enough to follow the rapid
fading. Since the forward link interleaver averages the fading,
large power control step sizes just tend to add amplitude jitter to
the forward link waveform. However, fast power control is needed to
dynamically adjust the average waveform to the correct level.
Demodulator 52 within base station 4 can estimate the fading
conditions and the velocity of remote station 6. Searcher elements
in demodulator 52 can determine the number of multipath components
currently being received and compute their profile. These searcher
elements are described in U.S. Pat. No. 5,109,390, entitled
"DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM" and U.S.
patent application Ser. No. 08/316,177, entitled "MULTIPATH SEARCH
PROCESSOR FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION
SYSTEM," filed Sep. 30, 1994, both are assigned to the assignee of
the present invention and incorporated by reference herein.
[0093] Demodulator 52 can also estimate the velocity of remote
station 6 by estimating the reverse link frequency error using
demodulation techniques which are well known in the art. The
frequency error is approximately 2f.sub.cv/c+.epsilon. where
f.sub.c is the operating frequency, v is the velocity of remote
station 6, c is the speed of light, and .epsilon. is the residual
frequency error of remote station 6. In conformance with
TIA/EIA/IS-95-A, remote station 6 measures the frequency which is
received on the forward link and uses this to set the transmit
frequency on the reverse link. A discussion of the setting the
transmit frequency based on the measured received frequency is
disclosed in U.S. Pat. No. 5,822,318, entitled "METHOD AND
APPARATUS FOR CONTROLLING POWER IN A VARIABLE RATE COMMUNICATION
SYSTEM," filed Jul. 29, 1994, assigned to the assignee of the
present invention and incorporated by reference herein. Remote
station 6 does this to remove the error from its own oscillator.
This process results in a doubling of the Doppler frequency error
of the signal received at base station 4 as there is a frequency
error of f.sub.cv/c on the forward link and a frequency error of
f.sub.cv/c on the reverse link. The error in setting the transmit
frequency in remote station 6 from the received frequency is
.epsilon.. For a high velocity mobile, the error .epsilon. is
relatively small. Thus, demodulator 52 can provide velocity and
multipath estimates to controller 40 which then uses these
information to determine the gain increase and decrease and the
step sizes.
[0094] Base station 4 has a maximum transmission power that is
determined by the system design constraints and FCC regulations.
Inevitably, base station 4 will experience a situation in which it
does not have enough available power when remote station 6 requests
a gain increase. If base station 4 limits or ignores the gain
increase command because of inadequate transmission power, the FER
for the forward traffic channel can increase. When this occurs, the
target energy level at remote station 6 can increase substantially
and quickly. This is due to the fact that the upward step .gamma.
in equation (5) is typically large relative to the downward step
.delta.. If the poor channel condition disappears or base station 4
is able to transmit additional power to remote station 6, the time
it takes the target energy level z to settle to the proper range
can be long since the downward step .delta. is typically small. In
the preferred embodiment, base station 4 transmits new values for
the upward step .gamma. and the downward step .delta. during the
time when the FER on the forward link is higher than nominal
[0095] In the present invention, the FER performance of the forward
traffic channel is related to the target energy level z. Thus, base
station 4 can directly adjust the target energy level z to obtain
the desired FER. For example, if base station 4 realizes that the
system is highly loaded and one or more remote stations 6 need to
operate at higher FERs, base station 4 can alter the target energy
levels of these remote stations 6 by transmitting the new target
energy levels z to the remote stations 6. Alternatively, base
station 4 can manipulate the target energy levels by commanding
these remote stations 6 to use the new upward steps .gamma. and
downward steps .delta.. In the exemplary embodiment, whenever base
station 4 is not able to respond to the power control command from
remote station 6, base station 4 adjusts the target energy level,
or upward and downward steps, to prevent the power control loop
from hitting the maximum transmission power and operating in the
non-linear region.
[0096] To ensure that the forward link power control mechanism
works properly and that no remote station 6 requests more or less
transmission power than necessary for the requisite level of
performance, base station 4 can monitor the FER of the forward
traffic channel. In the exemplary embodiment, remote station 6
transmits an error message to base station 4 whenever a data frame
is received in error. This error message can be the erasure
indicator bit (EIB) described previously. Base station 4 can
monitor the error messages from remote station 6, calculate the
FER, and manipulate the target energy level z of remote station 6
by assigning remote station 6 the proper values for the upward step
.gamma. and the downward step .delta..
[0097] Gain Correction Mechanism
[0098] The forward link power control mechanism of the present
invention performs better when delays are minimized. In order to
compensate for fading of the forward traffic channel, base station
4 should apply the increase or decrease in transmission power, as
requested by remote station 6, as soon as possible. When remote
station 6 is not in soft handoff, the forward link power control
bits are received by a single base station 4 which adjusts the gain
of the forward traffic channel in response to the forward link
power control bit. A remote station 6 in softer handoff
communicates with multiple sectors simultaneously. In the exemplary
embodiment, a single channel element in one base station 4 is used
to control the communication between remote station 6 and all
sectors in softer handoff. Therefore, base station 4 can quickly
adjust the transmission power of all sectors upon reception of the
forward link power control bit from remote station 6.
[0099] A remote station 6 in soft handoff can communicate with
multiple base stations 4 simultaneously. The method and apparatus
for performing distributed forward link power control is described
in detail in the aforementioned U.S. Pat. No. 5,893,035. Some base
stations 4 may not receive the forward link power control bit
stream or may not receive the power control bit stream with
sufficient reliability. In the present invention, a forward link
power control correction mechanism is used to ensure that the gains
of the forward traffic channels of all base stations 4 in the
active member set of remote station 6 are set properly and that
erroneous reception of forward link power control bits by the base
stations 4 do not accumulate. In the exemplary embodiment, when
remote station 6 is in soft handoff, the gain of the forward
traffic channel of the base station 4 which receives the reverse
link signal the strongest is used by all base stations 4 in the
active member set. The power control correction mechanism can be
accomplished by the following embodiments.
[0100] In the first embodiment, to ensure that gains of the forward
traffic channels are approximately equal for all base stations 4 in
communication with remote station 4, the selected forward link
power control bit stream is provided to all base stations 4. For
each frame, all base stations 4 in the active member set send the
forward link power control bits which were received by the base
stations 4 to a selector within system controller 2. The selector
selects the power control bits from the base station 4 which
receives the reverse link signal the strongest. The selected power
control bits from this base station 4 is then provided to all base
stations 4 in the active member set. Each base station 4 receives
the selected forward link power control bits from the selector,
compares the selected bits with the bits that it actually received
and processed, and readjusts the gains on the forward traffic
channels to conform with the selected forward link power control
bits.
[0101] Base stations 4 can send the power control bits to the
selector within controller 40 in backhaul frames. The backhaul
frame selection can be done in accordance with existing procedures
used in TIA/EIA/IS-95-A systems. After processing, the selector can
send the selected forward link power control bits to all base
stations 4 in backhaul frames carrying user traffic for
transmission to remote station 6.
[0102] In the second embodiment, each base station 4 sends the gain
of the forward traffic channel to the selector at every frame. The
selector selects the gain corresponding to the base station 4 which
received the reverse link signal the strongest. The selector sends
the selected gain to all base stations 4 in the active member set
and the base stations 4 update their gains accordingly. The
selected gain is just the gain value sent from the selector to base
stations 4 in existing TIA/EIA/IS-95-A systems. This gain value is
carried on backhaul formats which are sent on interface A3 as
specified in the TIA/EIA/IS-634-A standard which is incorporated by
reference herein.
[0103] Due to processing delays, the update of the gains of the
forward traffic channels requires some care. In the exemplary
embodiment, each base station 4 can adjust the gain of its forward
traffic channel based on its measurement of the forward link power
control bits from remote station 6. However, the selector may
determine that the power control bits received by another base
station 4 should be used. This decision is usually not made until a
predetermined amount of time after the base stations 4 have applied
their own measurements of the forward link power control bits.
Therefore, the base stations 4 need to adjust the gains of their
forward traffic channels in accordance with the power control bits
the base stations 4 actually received and the selected power
control bits from the selector. The base stations 4 also need to
account for the delay between the original gain adjustments and the
receipt of the selected power control bits from the selector.
[0104] In the exemplary embodiment, each base station 4 stores the
gains that were used by that base station 4 at each update period.
The selector sends the selected power control bit (or the selected
gain) of the base station 4 which was determined to be the most
likely to have received the power control bit correctly. Each base
station 4 then compares the gains that were stored at the update
period to that which was received from the selector and updates the
gain at the current time slot by the difference. The gain G.sub.i
for the i.sup.th power control bit is thus:
G.sub.i=G.sub.i-1+.upsilon.(2b.sub.i-1)+(G.sub.M|(i-M)/M|+p-H.sub.M|i/M|-
).delta..sub.(M.left brkt-bot.i/M.right brkt-bot.+q)i, (6)
[0105] where G.sub.i is the gain during the i.sup.th time slot,
b.sub.i is the value (one or zero) of the i.sup.th power control
bit, .upsilon. is the gain step size, M is the number of power
control bits per frame, p is the offset in time slots from the
start of a frame to the time the power control bits are sent from
base station 4 to the selector (0=p=M-1), H.sub.k is gain of the
forward traffic channel specified by the selector during the
k.sup.th frame where k=.left brkt-bot.i/M.right brkt-bot., q is the
offset in time slots from the start of a frame to the time the
updated gain is received at base station 4 from the selector
(0=q=M-1), and S.sub.ij equals 1 if i=j and 0 otherwise. In the
exemplary embodiment, M is equal to 16 although other values of M
can be used and are within the scope of the present invention.
[0106] An exemplary timing diagram of the forward link power
control correction mechanism is shown in FIG. 8. Forward traffic
channel frames and reverse link data frames are almost exactly
aligned in time, skewed only by the over-the-air propagation delay.
Frames (of duration of 20 msec) are indexed as k, k+1, k+2, and
k+3, and delineated by thick hashmarks in FIG. 8. Frame k of the
reverse link data stream is received at base station 4 and, after
some processing delay, is decoded some time during frame k+1, as
indicated by block 210. Meanwhile, base station 4 is also
processing forward link power control commands with considerably
less processing delay. Thus the shaded forward link power control
bits in the lower timeline of FIG. 8 depict the 20 msec block of
forward link power control bits which is sent to the selector in
the same backhaul frame along with frame k of the reverse link data
stream. During frame k+2, the selector selects the forward link
power control bits from the base station 4 which received the
strongest reverse link signal and sends these selected power
control bits to all base stations 4 in the active member set of
remote station 6, at block 212. Typically, the selected power
control bits are sent in a backhaul frame. Shortly thereafter, also
within frame k+2, the base stations 4 receive the selected power
control bits from the selector and correct the gains of the forward
traffic channels in accordance with the selected power control bits
in the manner described above, at block 214. By the beginning of
frame k+3, the base stations 4 transmit with the updated gains, as
indicated by block 216.
[0107] The above example shows three frames of processing delay
from the time remote station 6 transmits the forward link power
control bits to the time the base stations 4 corrects the gains of
the forward traffic channels. However, in the exemplary embodiment,
each base station 4 can adjust the gain of its forward traffic
channel in response to its measurement of the forward link power
control bit. In this way, each base station 4 can rapidly adjust
the gain of its forward traffic channel on its own and the
processing delay is minimized. The forward link power control
correction mechanism, wherein the power control bits from the base
station 4 which measures the reverse link signal the strongest are
used to correct the gains of the forward traffic channels of other
base stations 4 in the active member set, ensures that the
erroneous reception of power control bits by the base stations 4 do
not accumulate. Other embodiments to ensure correct operations of
the forward link power control mechanism by all base stations 4 can
be contemplated and are within the scope of the present
invention.
[0108] Although the present invention is described in terms of the
forward link power control mechanism, the inventive concept
disclosed herein is also applicable for the reverse link power
control.
[0109] The previous description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the
present invention. The 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 the use of the inventive faculty. 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.
* * * * *