U.S. patent application number 11/545955 was filed with the patent office on 2007-02-08 for modified power control for hybrid arq on the reverse link.
Invention is credited to Tao Chen, Sandip Sarkar, Edward G. JR. Tiedemann.
Application Number | 20070030820 11/545955 |
Document ID | / |
Family ID | 32711498 |
Filed Date | 2007-02-08 |
United States Patent
Application |
20070030820 |
Kind Code |
A1 |
Sarkar; Sandip ; et
al. |
February 8, 2007 |
Modified power control for Hybrid ARQ on the reverse link
Abstract
A method for power control in a wireless communication system.
An initial transmission of a data frame in the reverse link is
received, and a first energy level of the data frame is measured.
An energy deficit in the first energy level is then measured if the
first energy level is insufficient to correctly decode the data
frame, so that when the data frame is retransmitted with a second
energy level equal to a difference between the first energy level
and the energy deficit, the data frame can be correctly decoded
with combined energy of the first energy level and the second
energy level.
Inventors: |
Sarkar; Sandip; (San Diego,
CA) ; Chen; Tao; (San Diego, CA) ; Tiedemann;
Edward G. JR.; (Concord, MA) |
Correspondence
Address: |
QUALCOMM INCORPORATED
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Family ID: |
32711498 |
Appl. No.: |
11/545955 |
Filed: |
October 10, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10341319 |
Jan 10, 2003 |
7155249 |
|
|
11545955 |
Oct 10, 2006 |
|
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Current U.S.
Class: |
370/315 |
Current CPC
Class: |
H04W 52/48 20130101;
H04W 52/40 20130101; H04W 52/50 20130101 |
Class at
Publication: |
370/315 |
International
Class: |
H04J 3/08 20060101
H04J003/08 |
Claims
1. A method of operating a remote terminal in a wireless
communication system, the method comprising: transmitting to a base
station an initial transmission on a reverse link, the initial
transmission including a reverse supplemental channel frame and a
reverse pilot channel; receiving a traffic-to-pilot ratio delta and
a negative acknowledgement on a forward channel from the base
station to enable the remote terminal to transmit a sufficient
amount of energy on a retransmission to compensate for an energy
deficit on the initial transmission of the reverse supplemental
channel frame; retransmitting the reverse supplemental channel
frame with the sufficient amount of energy to compensate for the
energy deficit on the initial transmission, wherein the negative
acknowledgement is received by the remote terminal only if the
transmitted initial transmission of the reverse supplemental
channel frame has enough energy such that, if combined with an
energy of the retransmission of the reverse supplemental channel
frame, the combined energy would be sufficient to permit correct
decoding of the reverse supplemental channel frame by the base
station.
2. A remote terminal for a wireless communication system, the
remote terminal comprising: means for transmitting to a base
station an initial transmission on a reverse link, the initial
transmission including a reverse supplemental channel frame and a
reverse pilot channel; means for receiving a traffic-to-pilot ratio
delta and a negative acknowledgement on a forward channel from the
base station to enable the remote terminal to transmit a sufficient
amount of energy on a retransmission to compensate for an energy
deficit on the initial transmission of the reverse supplemental
channel frame; means for retransmitting the reverse supplemental
channel frame with the sufficient amount of energy to compensate
for the energy deficit on the initial transmission, wherein the
negative acknowledgement is received by the remote terminal only if
the transmitted initial transmission of the reverse supplemental
channel frame has enough energy such that, if combined with an
energy of the retransmission of the reverse supplemental channel
frame, the combined energy would be sufficient to permit correct
decoding of the reverse supplemental channel frame by the base
station.
3. A remote terminal for a wireless communication system, the
remote terminal comprising: a transmitter configured to transmit to
a base station an initial transmission on a reverse link, the
initial transmission including a reverse supplemental channel frame
and a reverse pilot channel; a receiver configured to receive a
traffic-to-pilot ratio delta and a negative acknowledgement on a
forward channel from the base station to enable the remote terminal
to transmit a sufficient amount of energy on a retransmission to
compensate for an energy deficit on the initial transmission of the
reverse supplemental channel frame; the transmitter further
configured to retransmit the reverse supplemental channel frame
with the sufficient amount of energy to compensate for the energy
deficit on the initial transmission, wherein the negative
acknowledgement is received by the remote terminal only if the
transmitted initial transmission of the reverse supplemental
channel frame has enough energy such that, if combined with an
energy of the retransmission of the reverse supplemental channel
frame, the combined energy would be sufficient to permit correct
decoding of the reverse supplemental channel frame by the base
station.
4. A method of operating a remote terminal in a wireless
communication system, the method comprising: transmitting to a base
station an initial transmission on a reverse link, the initial
transmission including a reverse supplemental channel frame; and if
the remote terminal does not receive an acknowledgement signal or a
negative acknowledgement signal from the base station based on the
initial transmission of the reverse supplemental channel frame:
retransmitting the reverse supplemental channel frame with a
default transmission level sufficient to permit correct decoding by
the base station.
5. The method of claim 4, wherein the default transmission level is
predetermined.
6. The method of claim 4, wherein the default transmission level is
dynamically determined in accordance with a transmission condition
of the wireless communication system.
7. A remote terminal for a wireless communication system, the
remote terminal comprising: means for transmitting to a base
station an initial transmission on a reverse link, the initial
transmission including a reverse supplemental channel frame; and
means for retransmitting the reverse supplemental channel frame
with a default transmission level sufficient to permit correct
decoding by the base station if the remote terminal does not
receive an acknowledgement signal or a negative acknowledgement
signal from the base station based on the initial transmission of
the reverse supplemental channel frame.
8. The remote terminal of claim 7, wherein the default transmission
level is predetermined.
9. The remote terminal of claim 7, wherein the default transmission
level is dynamically determined in accordance with a transmission
condition of the wireless communication system.
10. A remote terminal for a wireless communication system, the
remote terminal comprising: a transmitter configured to: transmit
to a base station an initial transmission on a reverse link, the
initial transmission including a reverse supplemental channel
frame; and retransmit the reverse supplemental channel frame with a
default transmission level sufficient to permit correct decoding by
the base station if the remote terminal does not receive an
acknowledgement signal or a negative acknowledgement signal from
the base station based on the initial transmission of the reverse
supplemental channel frame.
11. The remote terminal of claim 10, wherein the default
transmission level is predetermined.
12. The remote terminal of claim 10, wherein the default
transmission level is dynamically determined in accordance with a
transmission condition of the wireless communication system.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present application for Patent is a Continuation of
patent application Ser. No. 10/341,319 entitled "Modified Power
Control for Hybrid ARQ on the Reverse Link" filed on Jan. 10, 2003,
now allowed, and assigned to the assignee hereof and hereby
expressly incorporated by reference herein.
BACKGROUND
[0002] 1. Field
[0003] The disclosed embodiments relate generally to the field of
communications, and more specifically to methods and apparatus for
modified power control for H-ARQ on the reverse link.
[0004] 2. Background
[0005] The field of communications has many applications including,
e.g., paging, wireless local loops (WLL), Internet telephony, and
satellite communication systems. An exemplary application is a
cellular telephone system for mobile subscribers. Modern
communication systems designed to allow multiple users to access a
common communications medium have been developed for such cellular
systems. These communication systems may be based on code division
multiple access (CDMA), time division multiple access (TDMA),
frequency division multiple access (FDMA), or other multiple access
techniques known in the art. These multiple access techniques
decode and demodulate signals received from multiple users, thereby
enabling simultaneous communication among multiple users and
allowing for a relatively large capacity for the communication
systems.
[0006] In the CDMA system, the available spectrum is shared
efficiently among a number of users, and techniques such as soft
handoff are employed to maintain sufficient quality to support
delay-sensitive services (such as voice) without wasting a lot of
power. More recently, systems that enhance the capacity for data
services have also been available. These systems provide data
services by using higher order modulation, faster power control,
faster scheduling, and more efficient scheduling for services that
have more relaxed delay requirements. An example of such a
data-services communication system is the high data rate (HDR)
system that conforms to the Telecommunications Industry
Association/Electronic Industries Alliance (TIA/EIA) cdma2000 High
Data Rate Air Interface Specification IS-856, January 2002 (the
IS-856 standard).
[0007] In a CDMA system, data transmission occurs from a source
device to a destination device. The destination device receives the
data transmission, demodulates the signal, and decodes the data. As
part of the decoding process, the destination device performs the
Cyclic Redundancy Code (CRC) check of the data packet to determine
whether the packet was correctly received. If the packet was
received with an error, the destination device transmits a negative
acknowledgement (NAK) message on its acknowledgement (ACK) channel
to the source device, which responds to the NAK message by
retransmitting the packet that was received with an error.
[0008] Transmission errors may be particularly acute in
applications with a low bit energy-to-noise power spectral density
ratio (E.sub.b/N.sub.o). In this situation, a conventional data
retransmission scheme, such as Automatic Repeat Request (ARQ), may
not meet (or may be designed not to meet) the maximum bit error
rate (BER) required for the system operation. In such a case,
combining the ARQ scheme in conjunction with an error correction
scheme, such as a Forward Error Correction (FEC), is often employed
to enhance performance. This combination of ARQ and FEC is
generally known as Hybrid ARQ (H-ARQ).
[0009] After transmitting a NAK, the destination device receives
the data transmission and retransmission, demodulates the signal,
and separates the received data into the new packet and the
retransmitted packet. The new packet and the retransmitted packet
need not be transmitted simultaneously. The destination device
accumulates the energy of the received retransmitted packet with
the energy already accumulated by the destination device for the
packet received with an error. The destination device then attempts
to decode the accumulated data packet. However, if the packet frame
is initially transmitted with insufficient energy to permit correct
decoding by the destination device, as described above, and is then
retransmitted, the retransmission provides time diversity. As a
result, the total transmit energy of the frame (including
retransmissions) is lower on average. The combined symbol energy
for both the initial transmission and retransmission(s) of the
frame is lower than the energy that would have been required to
transmit the frame initially at full power (i.e., at a power level
that was sufficient on its own to permit correct decoding by the
destination device) on average. Thus, the accumulation of the
additional energy provided by the subsequent retransmissions
improves the probability of a correct decoding. Alternately, the
destination device might be able to decode the retransmitted packet
by itself without combining the two packets. In both cases, the
throughput rate can be improved since the packet received in error
is retransmitted concurrently with the transmission of the new data
packet. Again, it should be noted that the new packet and the
retransmitted packet need not be transmitted simultaneously.
[0010] In the reverse link (i.e., the communication link from the
remote terminal to the base station), the reverse supplemental
channel (R-SCH) is used to transmit user information (e.g., packet
data) from a remote terminal to the base station, and to support
retransmission at the physical layer. The R-SCH may utilize
different coding schemes for the retransmission. For example, a
retransmission may use a code rate of 1/2 for the original
transmission. The same rate 1/2 code symbols may be repeated for
the retransmission. In an alternative case, the underlying code may
be a rate 1/4 code. The original transmission may use 1/2 of the
symbols and the retransmission may use the other half of the
symbols. An example of the reverse link architecture is described
in detail in U.S. Patent Application No. 2002/0154610, entitled
"REVERSE LINK CHANNEL ARCHITECTURE FOR A WIRELESS COMMUNICATION
SYSTEM" assigned to the assignee of the present application.
[0011] In a CDMA communication system, and specifically in a system
adapted for packetized transmissions, congestion and overloading
may reduce the throughput of the system. The congestion is a
measure of the amount of pending and active traffic with respect to
the rated capacity of the system. System overload occurs when the
pending and active traffic exceeds the rated capacity. A system may
implement a target congestion level to maintain traffic conditions
without interruption, i.e., to avoid overloading and underloading
of resources.
[0012] One problem with overloading is the occurrence of delayed
transmission responses. An increase in response time often leads to
application level timeouts, wherein an application requiring the
data waits longer than the application is programmed to allow,
producing a timeout condition. Applications will then needlessly
resend messages on timeouts, causing further congestion. If this
condition continues, the system might reach a condition where it
can service no users. One solution (used in HDR) for this condition
is congestion control. Another solution (used in cdma2000) is
proper scheduling.
[0013] The level of congestion in a system may be determined by
monitoring the data rates of pending and active users, and the
received signal strength required to achieve a desired quality of
service. In a wireless CDMA system, the reverse link capacity is
interference-limited. One measure of the cell congestion is the
total amount of noise over the level of the thermal noise at a base
station (referred to hereafter as the "rise over thermal" (ROT)).
The ROT corresponds to the reverse link loading. A loaded system
attempts to maintain the ROT near a predetermined value. If the ROT
is too high, the range of the cell (i.e., the distance over which
the base station of the cell can communicate) is reduced and the
reverse link is less stable. The range of the cell is reduced
because of an increase in the amount of transmit energy required to
provide a target energy level. A high ROT also causes small changes
in instantaneous loading that result in large excursions in the
output power of the remote terminal. A low ROT can indicate that
the reverse link is not heavily loaded, thus indicating that
available capacity is potentially being wasted.
[0014] However, operating the R-SCH with H-ARQ may require that the
initial transmission of an R-SCH frame not be power controlled very
tightly to meet the ROT constraints. Therefore, the delivered
signal-to-noise ratio (SNR) on the initial transmission of an R-SCH
frame can be below the level sufficient to permit correct decoding
of the received data packet.
[0015] Accordingly, from the discussion above, it should be
apparent that there is a need in the art for an apparatus and
method that enables modified power control for H-ARQ on the reverse
link.
SUMMARY
[0016] Embodiments disclosed herein address the need for an
apparatus and method that enables modified power control for H-ARQ
on the reverse link in a wireless communications system.
[0017] In one aspect, a method and apparatus is described for power
control of a reverse link in a wireless communication system. An
initial transmission of a data frame in the reverse link is
received, and a first energy level of the data frame is measured.
If the first energy level is insufficient to correctly decode the
data frame, then an energy deficit in the first energy level is
determined so that when the data frame is retransmitted with a
second energy level equal to a difference between the first energy
level and the energy deficit, the data frame can be correctly
decoded with combined energy of the first energy level and the
second energy level.
[0018] In another aspect, a base station for a wireless
communication system is described. The base station includes an RF
front end configured to receive and appropriately amplify, filter,
and process a reverse link traffic channel data frame from a remote
terminal, and a digital signal processor (DSP) adapted to
demodulate and further process the received data frame. The base
station also includes a power controller having an energy measuring
apparatus and a deficit estimating apparatus. The energy measuring
apparatus is configured to measure a first energy level of the data
frame. The deficit estimating apparatus is configured to estimate
an energy deficit in the first energy level if the first energy
level is insufficient to correctly decode the data frame, so that
when the data frame is retransmitted with a second energy level
equal to a difference between the first energy level and the energy
deficit, the data frame can be correctly decoded with combined
energy of the first energy level and the second energy level.
[0019] Other features and advantages of the present invention
should be apparent from the following descriptions of the exemplary
embodiments, which illustrates, by way of example, the principles
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a diagram of an exemplary wireless communication
system that supports a number of users and is capable of
implementing various aspects of the invention;
[0021] FIG. 2 is a simplified block diagram of an embodiment of a
base station and a remote terminal of the FIG. 1 communication
system;
[0022] FIG. 3 illustrates an exemplary forward link ACK channel
according to the acknowledgement scheme discussed herein;
[0023] FIG. 4 illustrates an exemplary forward link ACK channel
operating in accordance with an assumption that the remote terminal
recognizes which base station is the best base station; and
[0024] FIG. 5 is a flowchart describing an exemplary method for
implementing a modified power control technique operating in
conjunction with an acknowledgement scheme such as that of FIG. 3
or FIG. 4.
DETAILED DESCRIPTION
[0025] 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.
[0026] In recognition of the above-stated need for an apparatus and
method that enables modified power control for Hybrid Automatic
Repeat Request (H-ARQ) on the reverse link, this disclosure
describes exemplary embodiments for efficiently allocating,
utilizing, and controlling the reverse link resources. In
particular, the modified power control provides power control
commands that enable a remote terminal to deliver an appropriate
amount of energy on a retransmission to compensate for an energy
deficit on the initial transmission.
[0027] Although various aspects of the present invention will be
described in the context of a CDMA communications system, those
skilled in the art will appreciate that the techniques for
providing efficient operation of the forward link ACK channel
described herein are likewise suitable for use in various other
communications environments including communications systems based
on TDMA, FDMA, SDMA, PDMA, and other multiple access techniques
known in the art, and communications systems based on AMPS, GSM,
HDR, and various CDMA standards, and other communication standards
known in the art. 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.
[0028] FIG. 1 is a diagram of an exemplary wireless communication
system 100 that supports a number of users and is capable of
implementing various aspects of the invention. The communication
system 100 provides communication for a number of cells, with each
cell being serviced by a corresponding base station (BS) 104.
Various remote terminals 106 are dispersed throughout the system
100. Individual base stations or remote terminals will be
identified by a letter suffix such as 104a or 106c. References to
104 or 106 without a letter suffix will be understood to refer to
the base stations and remote terminals in the general sense.
[0029] Each remote terminal 106 may communicate with one or more
base stations 104 on the forward and reverse links at any
particular moment, depending on whether or not the remote terminal
is active and whether or not it is in soft handoff. The forward
link refers to transmission from a base station 104 to a remote
terminal 106, and the reverse link refers to transmission from a
remote terminal 106 to a base station 104. As shown in FIG. 1, the
base station 104a communicates with the remote terminals 106a,
106b, 106c, and 106d, and the base station 104b communicates with
the remote terminals 106d, 106e, and 106f. The remote terminal 106d
is in a soft handoff condition and concurrently communicates with
both of the base stations 104a and 104b.
[0030] In the wireless communication system 100, a base station
controller (BSC) 102 communicates with the base stations 104 and
may further communicate with a public switched telephone network
(PSTN). The communication with the PSTN is typically achieved via a
mobile switching center (MSC), which is not shown in FIG. 1 for
simplicity. The BSC may also communicate with a packet network,
which is typically achieved via a packet data serving node (PDSN)
that is also not shown in FIG. 1. The BSC 102 provides coordination
and control for the base stations 104. The BSC 102 further controls
the routing of telephone calls among the remote terminals 106, and
between the remote terminals 106 and users communicating with the
PSTN (e.g., conventional telephones) and to the packet network, via
the base stations 104.
[0031] FIG. 2 is a simplified block diagram of an embodiment of a
base station 104 and a remote terminal 106, which are capable of
implementing various aspects of the invention. For a particular
communication, voice data, packet data, and/or messages may be
exchanged between the base station 104 and the remote terminal 106.
Various types of messages may be transmitted such as messages used
to establish a communication session between the base station and
the remote terminal and messages used to control a data
transmission (e.g., power control, data rate information,
acknowledgment, and so on). Some of these message types are
described below. In particular, the implementation of the reverse
link data acknowledgement using the forward link ACK channel is
described in detail.
[0032] For the reverse link, at the remote terminal 106, voice
and/or packet data (e.g., from a data source 210) and messages
(e.g., from a controller 230) are provided to a transmit (TX) data
processor 212, which formats and encodes the data and messages with
one or more coding schemes to generate coded data. Each coding
scheme may include any combination of cyclic redundancy check
(CRC), convolutional, Turbo, block, and other coding, or no coding
at all. Typically, voice data, packet data, and messages are coded
using different schemes, and different types of message may also be
coded differently.
[0033] The coded data is then provided to a modulator (MOD) 214 and
further processed (e.g., covered, spread with short PN sequences,
and scrambled with a long PN sequence assigned to the user
terminal). The modulated data is then provided to a transmitter
unit (TMTR) 216 and conditioned (e.g., converted to one or more
analog signals, amplified, filtered, and quadrature modulated) to
generate a reverse link signal. The reverse link signal is routed
through a duplexer (D) 218 and transmitted via an antenna 220 to
the base station 104.
[0034] At the base station 104, the reverse link signal is received
by an antenna 250, routed through a duplexer 252, and provided to a
receiver unit (RCVR) 254. The receiver unit 254 conditions (e.g.,
filters, amplifies, downconverts, and digitizes) the received
signal and provides samples. A demodulator (DEMOD) 256 receives and
processes (e.g., despreads, decovers, and pilot demodulates) the
samples to provide recovered symbols. The demodulator 256 may
implement a rake receiver that processes multiple instances of the
received signal and generates combined symbols. A receive (RX) data
processor 258 then decodes the symbols to recover the data and
messages transmitted on the reverse link. The recovered
voice/packet data is provided to a data sink 260 and the recovered
messages may be provided to a controller 270. The processing by the
demodulator 256 and the RX data processor 258 are complementary to
that performed at the remote terminal 106. The demodulator 256 and
the RX data processor 258 may further be operated to process
multiple transmissions received via multiple channels, e.g., a
reverse fundamental channel (R-FCH) and a reverse supplemental
channel (R-SCH). Also, transmissions may be received simultaneously
from multiple remote terminals, each of which may be transmitting
on a reverse fundamental channel, a reverse supplemental channel,
or both.
[0035] On the forward link, at the base station 104, voice and/or
packet data (e.g., from a data source 262) and messages (e.g., from
the controller 270) are processed (e.g., formatted and encoded) by
a transmit (TX) data processor 264, further processed (e.g.,
covered and spread) by a modulator (MOD) 266, and conditioned
(e.g., converted to analog signals, amplified, filtered, and
quadrature modulated) by a transmitter unit (TMTR) 268 to generate
a forward link signal. The forward link signal is routed through
the duplexer 252 and transmitted via the antenna 250 to the remote
terminal 106.
[0036] If the messages from the controller 270 in the forward link
include power control commands, then the controller 270 will act as
a power controller that computes a traffic-to-pilot ratio (T/P) by
measuring the energy level of a reverse traffic channel (e.g., the
R-SCH) relative to the energy level of the reverse pilot channel.
This measured T/P value is compared to the total T/P value
sufficient to permit correct decoding of the R-SCH frame by the
base station, to generate a T/P delta value, which is transmitted
to the remote terminal to enable the remote terminal to deliver an
appropriate amount of energy on a retransmission to compensate for
the energy deficit on the initial transmission.
[0037] At the remote terminal 106, the forward link signal is
received by the antenna 220, routed through the duplexer 218, and
provided to a receiver unit 222. The receiver unit 222 conditions
(e.g., downconverts, filters, amplifies, quadrature demodulates,
and digitizes) the received signal and provides samples. The
samples are processed (e.g., despreaded, decovered, and pilot
demodulated) by a demodulator 224 to provide symbols, and the
symbols are further processed (e.g., decoded and checked) by a
receive data processor 226 to recover the data and messages
transmitted on the forward link. The recovered data is provided to
a data sink 228, and the recovered messages may be provided to the
controller 230.
[0038] The reverse link has some characteristics that are very
different from those of the forward link. In particular, the data
transmission characteristics, soft handoff behaviors, and fading
phenomenon are typically very different between the forward and
reverse links. For example, the base station typically does not
know a priori which remote terminals have packet data to transmit,
or how much data to transmit. Thus, the base station may allocate
resources to the remote terminals whenever requested and as
available. Because of uncertainty in user demands, the usage on the
reverse link may fluctuate widely.
[0039] Apparatus and methods are provided to efficiently allocate,
utilize, and control the reverse link resources in accordance with
exemplary embodiments of the invention. The reverse link resources
may be assigned via a supplemental channel (e.g., R-SCH) that is
used for packet data transmission. In particular, a reliable
acknowledgment scheme and an efficient retransmission scheme are
provided. The efficient retransmission also involves modified power
control to enable the remote terminal to deliver an appropriate
amount of energy on the retransmission to compensate for the energy
deficit on the initial transmission.
[0040] A reliable acknowledgement scheme and an efficient
retransmission scheme should consider several factors that control
communication between base stations and a remote terminal. One of
the factors to consider include the fact that the base stations
with path losses that are about a few dB larger than a base station
with the smallest path loss to the remote terminal (e.g., the base
station that is closest to the remote terminal), but are in the
Active Set of the remote terminal, have relatively little chance of
correctly receiving reverse supplemental channel (R-SCH)
frames.
[0041] In order for the soft handoff to work and the overall remote
terminal transmit power to be reduced, the remote terminal needs to
receive indications for these missed or bad R-SCH frames. Since the
remote terminal is going to receive significantly more negative
acknowledgements than positive acknowledgements, an exemplary
acknowledgement scheme is configured (see FIG. 3) so that the base
station (BS) sends a remote terminal (RT) an acknowledgement (ACK)
for a good frame and a negative acknowledgement (NAK) for a bad
frame only if the received bad R-SCH frame has enough energy such
that, if the energy of the bad frame is combined with energy from
the retransmission of the R-SCH frame, the combined energy would be
sufficient to permit correct decoding of the frame by the base
station. The base station will not produce a NAK signal in response
to bad frames having insufficient energy (even when combined with
retransmission energy) to permit correct decoding of the frame by
the base station. Thus, if the remote terminal does not receive an
ACK or NAK signal, the remote terminal will assume that the bad
frame received at the base station did not have sufficient energy
to permit correct decoding of the frame, even with combining. In
this case, the remote terminal will need to retransmit the frame
with a default transmission level sufficient to permit correct
decoding. In one embodiment, this default transmission level may be
predetermined to enable correct decoding by the base station. In
another embodiment, this default transmission level may be
dynamically determined in accordance with a transmission condition
of the wireless CDMA system.
[0042] FIG. 3 illustrates operation of an exemplary forward link
ACK channel according to the acknowledgement scheme discussed above
for the devices of FIG. 2. In the illustrated embodiment, the
remote terminal sends an R-SCH frame to the base station(s). The
base station receives the R-SCH frame and sends an ACK signal if
the received R-SCH frame is recognized as being a "good" frame.
[0043] In one embodiment, the recognition of the quality of the
received R-SCH frame (i.e., as being "good" or "bad") can be made
by observing the reverse link pilot signal, or, equivalently, based
on the power control bits sent from the remote terminal. Therefore,
if the reverse link pilot signal includes sufficient energy to
permit correct decoding of the frame by the base station, the frame
is considered to be "good". Otherwise, if the reverse link pilot
signal includes insufficient energy to permit correct decoding of
the frame by the base station, the frame is considered to be
"bad".
[0044] The exemplary forward link ACK channel of the base station
sends a NAK signal with a traffic-to-pilot ratio (T/P) delta if the
received R-SCH frame is recognized as being a "bad" frame but has
enough energy to combine with retransmission. This condition occurs
when the received bad R-SCH frame has enough energy such that if
combined with energy from the retransmission of the R-SCH frame, it
would be sufficient to permit correct decoding of the frame by the
base station.
[0045] As described above, the traffic-to-pilot ratio (T/P) can be
computed by measuring the ratio between the energy level of the
reverse traffic channel (e.g., the R-SCH) and the reverse pilot
channel. Thus, in this embodiment, this ratio is used for power
control of the R-SCH and is compared to the total energy level
sufficient to permit correct decoding of the R-SCH frame by the
base station. The difference between the T/P value of the initial
transmission and the total energy level sufficient to permit
correct decoding of the R-SCH frame provides a parameter referred
to as a T/P delta. In general, the total energy level is the energy
level required to maintain a certain quality of service (QoS),
which depends on speed, channel condition, and other parameters
related to QoS.
[0046] In one embodiment, for a given target QoS (e.g., a target
frame error rate (FER)), the traffic-to-pilot ratio (T/P) varies
with the velocity of the remote terminal. The TIP for a pilot
energy level required for a given FER is calculated for three
different possible remote terminal velocities (high (e.g., 120
km/hr.), low (e.g., 30 km/hr.), and static (e.g., additive white
Gaussian noise (AWGN) at 0 km/hr)) and averaged. The resultant
average value is stored in a gain table such as illustrated in
"3GPP2 Physical Layer Standard for cdma2000 Spread Spectrum
Systems," Document No. C.P0002-A, TIA/EIAIS-2000-2-A, Nov. 19,
1999.
[0047] For example, to estimate the T/P ratio for the total energy
level required for a remote terminal moving at 120 km/hr (i.e., in
high velocity and fast fading), the T/P value in the gain table is
compared to the T/P value of the AWGN (i.e., no fading). The
difference can be approximately 2 dB. Thus, this value is used as
the estimate of the total energy level sufficient to permit correct
decoding of the R-SCH frame by the base station in the above
calculation of the T/P delta. The T/P ratio for the total energy
level can be estimated differently using different QoS parameters,
as long as the estimated value conforms to the ROT requirement for
congestion control.
[0048] Accordingly, the T/P delta provides a differential energy
value that must be delivered by the remote terminal on the
retransmission to compensate for the energy deficit on the initial
transmission, and allow the base station to correctly decode the
R-SCH frame on the reverse link. The calculated T/P delta can be
transmitted to the remote terminal on the forward ACK channel along
with acknowledgement signals. In the case where there are two or
more base stations in the Active Set of the remote terminal, and
both base stations send NAK signals with different T/P deltas in
response to bad R-SCH frames, the remote terminal chooses the one
with the lower T/P delta so at least one base station sufficient
energy to correctly decode the packet.
[0049] Errors in the T/P delta bits sent to the remote terminal to
control the T/P for congestion power control might cause the T/P
value to be other than that desired. However, the base station
typically monitors the level of the reverse pilot channel for
reverse power control or for channel estimation. The base station
can also monitor the energy level of the received R-SCH frame. By
taking the ratio of the R-SCH energy level to the reverse pilot
channel energy level, the base station can estimate the T/P in use
by the remote terminal. If the T/P is not that which is desired,
then the base station sets the bit that controls the T/P to correct
for the discrepancy. Thus, there is a self-correction for bit
errors in the T/P delta.
[0050] The base station will not send a NAK signal (i.e., NULL
data) when the received bad R-SCH frame, combined with
retransmission energy, has insufficient energy to permit correct
decoding of the frame by the base station. The remote terminal
recognizes this "NULL" condition as a signal from the base station
to the remote terminal to retransmit the R-SCH frame with a default
transmission level sufficient to permit correct decoding.
[0051] The acknowledgement scheme illustrated in FIG. 3 can be
further optimized if the remote terminal can detect or determine
which base station has the smallest path loss to the remote
terminal (i.e., the best base station). In one embodiment, the base
station can measure the energy deficit of the actually received
frame relative to the power control target (as is done in the
closed-loop power control) to determine which base station has the
smallest path loss to the remote terminal. By averaging the energy
deficit over many frames, the base station can determine whether it
is the best base station or not. This information can be
transmitted to the remote terminal. In an alternative embodiment,
the best base station can be readily determined if the remote
terminal is operating in a data/voice (DV) mode of a 1.times.Ev-DV
system. In this mode, both the base station and the remote terminal
must know which base station is the best base station.
[0052] FIG. 4 illustrates an exemplary forward link ACK channel
operating in accordance with an assumption that the remote terminal
recognizes which base station is the best base station. Hence, in
the illustrated embodiment, the remote terminal sends R-SCH frames
to the best base station and the secondary base station(s). Since
the best base station will typically be receiving many more "good"
frames than "bad" frames, the acknowledgement scheme from the best
base station is biased toward not sending ACK signals for "good"
frames but sending NAK signals for "bad" frames. The secondary base
station will be biased in reverse since it will be receiving a lot
more "bad" frames than "good" frames. Thus, the acknowledgement
scheme from the secondary base station is biased toward sending ACK
signals for "good" frames but not sending NAK signals for "bad"
frames.
[0053] Accordingly, in response to the receipt of the R-SCH frame
from the remote terminal, the exemplary forward link ACK channel of
the best base station does not send an ACK signal (i.e., NULL data)
if the received R-SCH frame is recognized as being a "good" frame.
The remote terminal recognizes this "NULL" condition as a signal
from the best base station that the transmitted R-SCH frame was
received with sufficient energy to permit correct decoding and that
there is no need for retransmission of the frame. If the received
R-SCH frame is recognized as being a "bad" frame but has enough
energy to combine with retransmission, the best base station sends
a NAK signal with a T/P delta. This condition occurs when the
received bad R-SCH frame has enough energy such that if combined
with energy from the retransmission of the R-SCH frame, it would be
sufficient to permit correct decoding of the frame by the best base
station. The best base station sends a NAK signal without a T/P
delta if the received bad R-SCH frame, combined with retransmission
energy, has insufficient energy to permit correct decoding of the
frame by the best base station. Thus, the remote terminal
retransmits the R-SCH frame with a default transmission level
sufficient to permit correct decoding.
[0054] The exemplary forward link ACK channel of the secondary base
station, in response to the receipt of the R-SCH frame from the
remote terminal, sends an ACK signal if the received R-SCH frame is
recognized as being a "good" frame. If the received R-SCH frame is
recognized as being a "bad" frame but has enough energy to combine
with retransmission, the secondary base station sends a NAK signal
with a T/P delta. This condition occurs when the received bad R-SCH
frame has enough energy such that if combined with energy from the
retransmission of the R-SCH frame, it would be sufficient to permit
correct decoding of the frame by the secondary base station. In
contrast to the best base station, the secondary base station does
not send a NAK signal (i.e., NULL data) when the received bad R-SCH
frame, combined with retransmission energy, has insufficient energy
to permit correct decoding of the frame by the base station. The
remote terminal recognizes this "NULL" condition as a signal from
the secondary base station to the remote terminal to retransmit the
R-SCH frame with a default transmission level sufficient to permit
correct decoding.
[0055] An exemplary method for implementing an above-described
modified power control operating in conjunction with an
acknowledgement scheme (of FIG. 3 or FIG. 4) is illustrated in a
flowchart shown in FIG. 5. In the first operation, a first energy
level of the reverse traffic channel is measured, at box 500. In
one embodiment, this energy level is the energy level measured on
the R-SCH. At box 502, a second energy level is measured on the
reverse pilot channel. The ratio (T/P) is then computed, at box
504, between the first energy level and the second energy level.
The total energy level sufficient to permit correct decoding of the
R-SCH frame is estimated at box 506. The T/P delta is computed, at
box 508, by taking the difference between the total energy level
and the T/P. Finally, the T/P delta is appropriately transmitted,
at box 510, on the forward ACK channel to enable the remote
terminal to deliver an appropriate amount of energy on the
retransmission to compensate for the energy deficit on the initial
transmission. The condition under which the T/P delta can be
transmitted on the forward ACK channel is specified according to
the procedures described for an acknowledgement scheme of FIG. 3 or
FIG. 4.
[0056] As described above, the modified power control scheme can
improve the utilization of the reverse link, and may also allow
data frames to be transmitted at lower transmit power. For example,
without retransmission, a data frame needs to be transmitted at a
higher power level (P.sub.1) required to achieve one percent frame
error rate (1% FER). If retransmission is used and is reliable, a
data frame may be transmitted at a lower power level (P.sub.2)
required to achieve 10% FER. The 10% erased frames may be
retransmitted to achieve an overall 1% FER for the transmission
(i.e., 10%.times.10%=1%). Moreover, retransmission provides time
diversity, which may improve performance. The retransmitted frame
may also be combined with the initial transmission of the frame at
the base station, and the combined power from the two transmissions
may also improve performance. The recombining may allow an erased
frame to be retransmitted at a lower energy level.
[0057] Those of skill in the art will understand that method steps
can be interchanged without departing from the scope of the
invention. Those of skill in the art will also understand that
information and signals might be represented using any of a variety
of different technologies and techniques. For example, data,
instructions, commands, information, signals, bits, symbols, and
chips that may be referenced throughout the above description may
be represented by voltages, currents, electromagnetic waves,
magnetic fields or particles, optical fields or particles, or any
combination thereof.
[0058] Those of skill will further appreciate that the various
illustrative logical blocks, modules, circuits, and steps of a
technique 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, and steps 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.
[0059] The various illustrative logical blocks and modules
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.
[0060] The steps of a method or technique 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
subscriber station. In the alternative, the processor and the
storage medium may reside as discrete components in a subscriber
station.
[0061] 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 and 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.
* * * * *