United States Patent (19)
`Tiedemann, Jr. et al.
`
`54 METHOD AND APPARATUS FOR
`CONTROLLING POWER IN A WARIABLE
`RATE COMMUNICATION SYSTEM
`
`75 Inventors: Edward G. Tiedemann, Jr., San
`Diego, Calif.; Klein S. Gilhousen,
`Bozeman, Mont.; Joseph P.
`Odenwalder, Del Mar, Calif.; Ephraim
`Zehavi; Jeffrey A. Levin, both of San
`Diego, Calif.; Charles E. Wheatley,
`III, Del Mar, Calif.
`73 Assignee: Qualcomm Incorporated, San Diego,
`Calif.
`
`*
`
`Notice:
`
`The term of this patent shall not extend
`beyond the expiration date of Pat. No.
`5,528,593.
`
`21 Appl. No.: 283,308
`22 Filed
`Jul. 29, 1994
`51) Int. Cl. ....................................................... H04Q 7/30
`52)
`370/391; 455/403
`58 Field of Search ..................................... 330/130, 134,
`330/279; 455/69, 116, 38.3,355, 33.1,
`522, 403; 375/205, 285, 224, 259; 379/59,
`60; 370/391
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,925,782 12/1975 Anderl et al. ........................... 343/178
`4,112,257 9/1978 Frost ........................................... 179/2
`4,123,718 10/1978 Lampert et al. ..
`... 325/474
`4,193,031
`3/1980 Cooper et al. ............................ 455/38
`4.222,115 9/1980 Cooper et al. .............................. 375/1
`4,225,976 9/1980 Osborne et al. ..
`... 455/226
`4,495,648
`1/1985 Giger ........................................ 455/73
`
`
`
`BASE STATION
`
`USOO5822.318A
`Patent Number:
`11
`(45) Date of Patent:
`
`5,822,318
`*Oct. 13, 1998
`
`
`
`4,580,262 4/1986 Naylor et al. ............................... 371/5
`4,613,990 9/1986 Halpern .......
`455/33
`4,641,322 2/1987 Hasegawa ................................... 375/1
`4,672,658 6/1987 Kavehrad et al.
`... 379/63
`4,765,753 8/1988 Schmidt ..........
`... 379/60
`4,766,599 8/1988 Miyazaki ........
`... 375/222
`4,777,653 10/1988 Bonnerot et al. ...
`... 455/69
`4,811,421 3/1989 Havel et al. ........
`... 455/69
`4,868,795 9/1989 McDavid et al. ...
`... 367/77
`4870,698 9/1989 Katsuyama et al. ...
`... 455/67
`4,901,307 2/1990 Gilhousen et al. .....
`... 370/18
`5,056,109 10/1991 Gilhousen et al. .
`. 375/1
`5,077,742 12/1991 Tsumura et al. ...
`... 371/32
`5,093,840 3/1992 Schilling .........
`... 375/1
`5,257.283 10/1993 Gilhousen et al.
`... 375/1
`5,425,051 6/1995 Mahany ..........
`... 455/62
`5,450,616 9/1995 Rom ................
`455/54.1
`5,504,773 4/1996 Padovani et al.
`375/200
`5,528,593 6/1996 English et al. ............................ 370/84
`FOREIGN PATENT DOCUMENTS
`0212667 3/1985 European Pat. Off. .......... HO4L 1/08
`9217011 2/1992 WIPO .............................. HO4K 1/OO
`Primary Examiner Wellington Chin
`ASSistant Examiner Jean B. Corrielus
`Attorney, Agent, or Firm-Russell B. Miller; Sean English
`57
`ABSTRACT
`A method and apparatus for controlling transmission power
`in a variable rate communication System is disclosed. The
`method disclosed provides for a closed loop power control
`method. A first remote Station controls the transmission
`power of a Second remote Station by transmitting a rate
`dependent power control signal to the Second remote com
`munication Station. Since only the Second communication
`knows its transmission rate a priori, it must determine its
`course of action in accordance with both the received power
`control Signal and the knowledge of its transmission rate.
`
`65 Claims, 4 Drawing Sheets
`
`
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`TRAFFIC
`DATA
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`U.S. Patent
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`Oct. 13, 1998
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`Sheet 1 of 4
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`5,822,318
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`I "?INH
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`U.S. Patent
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`Oct. 13, 1998
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`Sheet 2 of 4
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`5,822,318
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`ENERGY
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`ENERGY
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`PRIOR ART FIG.2a
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`FRAME
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`ENERGY
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`PRIOR ART FIG.2b
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`FRAME
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`ENERGY
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`PRIOR ART
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`FIG. 2C
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`FRAME
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`PRIOR ART FIG.2e
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`ENERGY
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`peer-eeeeeeeeeeee
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`FRAME
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`PRIOR ART FIG.2f
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`Oct. 13, 1998
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`Sheet 3 of 4
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`5,822,318
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`ENERGY
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`ENERGY
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`E
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`FRAME
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`PRIOR ART FIG.2g
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`FRAME
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`ENERGY
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`PRIOR ART FIG. 2h
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`E/2 Piper PIPs per Peppe Parrs Peppe
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`-- FRAME
`ENERGY
`PRIOR ART FIG.2i
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`E/4 PIP2P3 PP1|Pe|Papa PIP2 (P3IP, PIP2 (P3IP.
`PRIOR ART FIG.2
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`FRAME
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`
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`ENERGY
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`E/8 P1 P2P1 P2P1 P2P1 P2P1 P2P1 P2P1 P2P1 P2
`PRIOR ART FIG.2k
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`FRAME
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`Sheet 4 of 4
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`V LVCI
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`QIHHVRIJL
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`RIOSS@HOONHdH
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`TOYHOELNO O
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`1
`METHOD AND APPARATUS FOR
`CONTROLLING POWER IN A WARIABLE
`RATE COMMUNICATION SYSTEM
`BACKGROUND OF THE INVENTION
`I. Field of the Invention
`The present invention relates to communication Systems.
`More particularly, the present invention relates to a novel
`and improved method and apparatus for controlling trans
`mission power in a variable rate communication System.
`II. Description of the Related Art
`The use of code division multiple access (CDMA) modu
`lation 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 tech
`nique of CDMA has significant advantages over these modu
`lation techniques for multiple acceSS communication SyS
`tems. The use of CDMA techniques in a multiple access
`communication system is disclosed in U.S. Pat. No. 4,901,
`307, issued Feb. 13, 1990, entitled “SPREAD SPECTRUM
`MULTIPLE ACCESS COMMUNICATION SYSTEM
`USING SATELLITE OR TERRESTRIAL REPEATERS",
`25
`assigned to the assignee of the present invention, of which
`the disclosure thereof is incorporated by reference herein.
`The use of CDMA techniques in a multiple access commu
`nication system is further disclosed in U.S. Pat. No. 5,103,
`459, issued Apr. 7, 1992, entitled “SYSTEM AND
`METHOD FOR GENERATING SIGNAL WAVEFORMS
`IN A CDMA CELLULAR TELEPHONE SYSTEM”,
`assigned to the assignee of the present invention, of which
`the disclosure thereof is incorporated by reference herein.
`A method for transmission of Speech in digital commu
`35
`nication Systems that offers particular advantages in increas
`ing capacity while maintaining high quality of perceived
`Speech is by the use of variable rate speech encoding. The
`method and apparatus of a particularly useful variable rate
`speech encoder is described in detail in U.S. Pat. No.
`5,414,796, issued May 9, 1995, which is a continuation
`application of now abandoned U.S. patent application Ser.
`No. 07/713,661, filed Jun. 11, 1991, entitled “VARIABLE
`RATE VOCODER', assigned to the assignee of the present
`invention, of which the disclosure thereof is incorporated by
`reference herein.
`The use of a variable rate Speech encoder provides for
`data frames of maximum speech data capacity when said
`Speech encoding is providing Speech data at a maximum
`rate. When a variable rate Speech coder is providing speech
`data at a less that maximum rate, there is exceSS capacity in
`the transmission frames. A method for transmitting addi
`tional data in transmission frames of a fixed predetermined
`size, wherein the Source of the data for the data frames is
`providing the data at a variable rate is described in detail in
`U.S. Pat. No. 5,504,773, issued Apr. 2, 1996, which is a
`continuation application of now abandoned U.S. patent
`application Ser. No. 07/822,164, filed Jan. 16, 1992, entitled
`“METHOD AND APPARATUS FOR THE FORMATTING
`OF DATA FOR TRANSMISSION', assigned to the
`assignee of the present invention, of which the disclosure
`thereof is incorporated by reference herein. In the above
`mentioned patent application a method and apparatus is
`disclosed for combining data of differing types from differ
`ent Sources in a data frame for transmission.
`In frames containing leSS data than a predetermined
`capacity, power consumption may be lessened by transmis
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`Siongating a transmission amplifier Such that only parts of
`the frame containing data are transmitted. Furthermore mes
`Sage collisions in a communication System may be reduced
`if the data is placed into frames in accordance with a
`predetermined pseudorandom process. A method and appa
`ratus for gating the transmission and for positioning the data
`in the frames is disclosed in U.S. Pat. No. 5,659,569 issued
`Apr. 19, 1997, which is a continuation application of now
`abandoned U.S. patent application Ser. No. 07/846,312, filed
`Mar. 5, 1992, entitled “DATA BURST RANDOMIZER',
`assigned to the assignee of the present invention, of which
`the disclosure thereof is incorporated by reference herein.
`A useful method of power control of a mobile in a
`communication System is to monitor the power of the
`received signal from the mobile Station at a base Station. The
`base Station in response to the monitored power level
`transmits power control bits to the mobile Station at regular
`intervals. A method and apparatus for controlling transmis
`sion power in this fashion is disclosed in U.S. Pat. No.
`5,056,109, issued Oct. 8, 1991, entitled “METHOD AND
`APPARATUS FOR CONTROLLING TRANSMISSION
`POWER IN A CDMA CELLULAR TELEPHONE
`SYSTEM”, assigned to the assignee of the present
`invention, of which the disclosure thereof is incorporated by
`reference herein.
`In an alternative continuous transmission Strategy, if the
`data rate is less than the predetermined maximum the data is
`repeated within the frame Such that the data occupies the full
`capacity of the data frame. If Such a strategy is employed,
`power consumption and interference to other users may be
`reduced during periods of data transmission at less than the
`predetermined maximum by reducing the power at which
`the frame is transmitted. This reduced transmission power is
`compensated by the redundancy in the data Stream and can
`offer benefits in range for a fixed maximum transmission
`power.
`A problem that is encountered in controlling transmission
`power in the continuous transmission Strategy is that the
`receiver does not know the transmission rate a priori and So
`does not know the power level that should be received. The
`present invention provides a method and apparatus for
`controlling transmission power in a continuous transmission
`communication System.
`SUMMARY OF THE INVENTION
`The present invention is a novel and improved method
`and apparatus for closed loop transmission power control in
`a communication System. It is an object of the present
`invention to provide timely power control that is necessary
`to provide robust communication link quality under fast
`fading conditions. It is noted that the different methods for
`power control can be changed by exchanging of Signaling
`data in the course of the transmission. Such changes in
`power control format may be desirable in response to
`changes in channel characteristics or changes in the Service
`be accommodated.
`Further, it should be noted that power control techniques
`are presented in the exemplary embodiment in a variable
`rate communication System, however the methods presented
`are equally applicable for fixed rate communication Systems
`and for communication Systems where the data rate varies
`with both ends of the communication link aware of the
`transmission rate. In the cases where the transmission rate is
`known, only the information relating to the known rate must
`be transmitted.
`In an exemplary embodiment, the present invention dis
`closes a variable rate communication System where a first
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`communication device is for the transmission of a data
`packet of variable rate data in a data frame of a predeter
`mined data capacity to a Second communication device, and
`when the data packet is less than the data capacity generating
`repeated versions of bits in the data packet and providing
`first version of the data packet bits and the repeated versions
`of the data packet bits in the data frame and wherein the
`transmission power for transmitting the data frame is Scaled
`in accordance with the data rate, a System for controlling the
`transmission power of the first communication device at the
`Second communication device comprising a receiver means
`for receiving the data frame, frame quality determination
`means for determining a frame quality factor from the data
`frame, a comparison means for comparing the frame quality
`factor against at least one threshold value wherein the
`threshold value to provide a quality Signal that is Suitable for
`the data rate, and transmitter means for transmitting the
`quality signal.
`In an exemplary embodiment, the present invention fur
`ther discloses a first communication device for the trans
`mission of a data packet of variable rate data in a data frame
`of a predetermined data capacity to a Second communication
`device wherein when the data packet is less than the data
`capacity generating repeated versions of bits in the data
`packet and providing first version of the data packet bits and
`the repeated versions of the data packet bits in the data frame
`and wherein in the transmission power for transmitting the
`data frame is Scaled in accordance with the data rate, a
`System for controlling transmission power at the first com
`munication device responsive to a power control Signal
`comprising a receiver means for receiving the power control
`Signal and a control processor means for determining in
`accordance with the power control signal and the data rate
`a transmission control Signal.
`
`4
`messages between mobile Station 6 and base Station 4 is
`referred to as the reverse link and the transmission link for
`communication of messages between base Station 4 and
`mobile station 6 is referred to as the forward link.
`In the exemplary embodiment, the present invention is
`used to control the transmission power of mobile Station 6.
`However, the methods of power control of the present
`invention are equally applicable to controlling the transmis
`sion power of base station 4. Referring to FIG. 3, base
`station 30 and mobile station 50 are illustrated in block
`diagram form showing the apparatus for providing control of
`the transmission power of mobile station 50 of the present
`invention.
`In conventional reverse link implementations, frames of
`variable rate data are transmitted from a mobile Station to a
`base Station using transmission gating when the data of
`transmission frames is less than a predetermined maximum.
`FIGS. 2a-g illustrates an exemplary frame Structure for a
`transmission gated communication link. FIG.2a illustrates a
`frame of full rate data comprised of 16 unique power control
`groups (P1-P9) of transmission data.
`FIGS. 2b-c illustrate a transmission frame of half rate
`data. Half rate data only requires half of the capacity of the
`data frame. The data is then provided in duplicate as
`illustrated in FIG.2b, with each unique power control group
`(P1-P) provided twice in the frame. This repetition frame is
`provided to a gating means that gates out half of the power
`control groups. So that only one unique version of each power
`control group is transmitted as illustrated in the transmission
`frame of FIG. 2C.
`FIGS. 2d e illustrate a transmission frame of quarter rate
`data. Quarter rate data only requires one quarter of the
`capacity of the data frame. The data is replicated four times
`as illustrated in FIG. 2d, with each unique power control
`group (P-P) provided four times in the frame. This rep
`etition frame is provided to a gating means that gates out
`three fourths of the power control groups. So that only one
`unique version of each power control group is transmitted as
`illustrated in the transmission frame of FIG. 2e.
`FIGS. 2f g illustrate a transmission frame of eighth rate
`data. Eighth rate data only requires one eighth of the
`capacity of the data frame. The data is provided as eight
`duplicates as illustrated in FIG. 2f, with each unique power
`control group (P1-P) provided eight times in the frame.
`This repetition frame is provided to a gating means that gates
`out Seven eighths of the power control groups, So that only
`one version of each unique power control group is trans
`mitted as illustrated in the transmission frame of FIG.2g.
`Power control, in Systems where frames are transmitted as
`illustrated in FIGS. 2a-g, is provided by comparing the
`received power of each power control group against a
`predetermined power threshold and transmitting a single bit
`in return indicative of the power received being too high or
`too low. Since the mobile station is aware of which of the
`power control groups were gated out, it ignores the power
`control messages Sent for gated out power control groups.
`In the communication link of the present invention, rep
`etition of the data in power control groups is provided as
`described in relation to FIGS. 2b, 2d and 2f. It should be
`noted that the ordering of the power control groups of the
`frames in FIGS. 2b, 2d and 2f is for exemplary purposes and
`that the present invention applies equally to any power
`control group ordering. In the present invention, gating of
`the redundant data is not performed, rather the entire rep
`etition frame is transmitted, but with the transmission power
`decreased proportionally to the amount of redundancy exist
`ing in the transmission frames.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`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:
`FIG. 1 is an illustration of an exemplary mobile telephone
`System;
`FIGS. 2a-kare illustrations of frame formats of the prior
`art reverse link and of the frame formats for frames of the
`present invention; and
`FIG. 3 is an illustration of the apparatus of the present
`invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`Referring to FIG. 1, information may be provided to and
`from a public Switching telephone network (PSTN) to
`System controller and Switch 2, or may be provided to and
`from controller and Switch 2 by another base station if the
`call is a mobile Station to mobile Station communication.
`System controller and Switch 2, in turn, provides data to and
`receives data from base Station 4. Base Station 4 transmits
`data to and receives data from mobile Station 6.
`In the exemplary embodiment the Signals transmitted
`between base Station 4 and mobile Station 6 are spread
`Spectrum communication Signals, the generation of the
`waveforms of which are described in detail in the above
`mentioned U.S. Pat. No. 4,901,307 and U.S. Pat. No.
`5,103,459. The transmission link for communication of
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`The exemplary transmission frames of the present inven
`tion are illustrated in FIGS. 2h-2k. It should be noted that
`the present invention applies equally to any ordering of the
`power control groups in transmission frames. In FIG. 2h, a
`full rate frame is illustrated. There are sixteen unique power
`control groups of data which occupy the entire capacity of
`the data frame and which are transmitted at a highest
`transmission power level. In FIG. 2i a half rate frame is
`illustrated. There are eight unique power control groups of
`data with a repetition rate of two and which are transmitted
`at approximately one half of the highest transmission power
`level. In FIG.2i, a quarter rate frame is illustrated. There are
`four unique power control groups with a repetition rate of
`four and which are transmitted at approximately one quarter
`of the highest transmission power level. In FIG. 2k, an
`eighth rate frame is illustrated. There are two unique power
`control groups with a repetition rate of eight which are
`transmitted at approximately one eighth of the maximum
`transmission power level.
`The transmission power can be reduced without degrad
`ing the link quality in the transmission of repetition frames
`by taking advantage of the redundancy by coherently or
`noncoherently combining the repeated Signals and by taking
`advantage of forward error correction techniques available
`in the correction of data frames containing redundancy,
`which are well known in the art.
`In this transmission Scheme, power control is significantly
`more complicated if the receiver does not know the rate a
`priori at which the data is being transmitted. AS illustrated in
`FIGS. 2h-2k, the adequacy of the received power depends
`entirely on the rate at which the data was transmitted,
`information which the receiver does not know a priori. The
`following describes methods by which power control can be
`implemented in a communication System of this type.
`If a communication link degrades, then the link quality
`can be improved by reducing the data rate of transmissions
`on the link and introducing redundancy for error correction
`purposes into the traffic Stream or by increasing the trans
`mission power of the transmitting device. In the exemplary
`embodiment of controlling transmission power of the mobile
`station 50, some of the methods for determining that the
`transmission power of mobile station 50 should be increased
`or that the data rate of mobile station should be decreased
`include:
`(a) base station detection of high frame error rate on
`reverse link,
`(b) mobile Station detects its power is at a maximum for
`the reverse link,
`(c) base station detects that received power is low on
`reverse link,
`(d) base station to mobile Station range is large; and
`(e) mobile Station location is poor.
`Conversely, Some of the methods for determining that the
`transmission power of mobile station 50 should be decreased
`or that the data rate of mobile Station may be increased
`include:
`(a) base Station detection of low frame error rate on
`reverse link,
`(b) mobile station detects its power is lower than a
`threshold for the reverse link;
`(c) base Station detects that received power is high on
`reverse link,
`(d) base station to mobile Station range is low; and
`(e) mobile Station location is good.
`It is often desirable to reduce the data transmission rate,
`in order to Strengthen a data link instead of increasing the
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`transmission power on that link. There are three reasons to
`reduce the data rate in order to improve link connections.
`The first reason is that the transmission System may already
`be at its maximum transmission power. The Second reason is
`that the transmission System may be operating off of Stored
`battery power and as Such increased transmission power
`would reduce operating time. The third reason is that, in the
`exemplary embodiment of a CDMA System, a users trans
`missions are noise to other users transmitting to the base
`Station and it is desirable to limit this interference.
`When mobile station 50 detects a need to modify the
`transmission rate, control processor 58 in mobile station 50
`Sends a signal Specifying a modified rate Set to variable rate
`dataSource 60. The modified rate set is a set of rates at which
`data Source 60 is permitted to Source data. In response to the
`modified rate Signal, variable rate data Source 60 provides all
`data for transmissions within the modified rate Set. Data
`Source 60 may Source modem, facsimile or Speech data.
`Data source 60 may be a variable rate source that varies its
`transmission rate on a frame to frame basis throughout the
`transmission or it may be able to vary rates only upon
`command. A variable rate Speech Source is described in
`detail in the aforementioned U.S. Pat. No. 5,414,796.
`A need for modification of the data rate Set may be
`indicated by one of the conditions enumerated above. If the
`method by which it is determined that the data rate set
`should be modified is a position related effect Such as range
`or mobile Station location, then an external Signal is pro
`vided to control processor 58 indicative of the location
`condition. This location condition may be detected by
`mobile station 50 or by base station 30 and transmitted to
`mobile station 50. In response, control processor 58 provides
`a signal indicative of a modified rate set at which mobile
`station 50 may transmit.
`Alternatively if the method by which a need for rate
`modification is detected is due to a transmission power
`condition (e.g., the transmission power of the mobile is a
`maximum or below a threshold), then a signal is provided
`from transmitter (XMTR) 64 to control processor 58 indica
`tive of the transmission power. Control processor 58 com
`pares the transmission power to predetermined thresholds
`and in accordance with this comparison may provide a rate
`set indication to variable rate data Source 60.
`In a closed loop power control implementation, power
`control signals are provided from base station 30 to mobile
`station 50. The method by which base station 30 determines
`the power control Signal depends upon the link characteristic
`that base station 30 uses as the determination of link quality.
`For example, base station 30 may determine the power
`control Signal in accordance with received power or alter
`natively in accordance with frame error rate. The present
`invention is equally applicable to any link quality factors.
`If the link quality factor used is received power, then the
`signal from mobile station 50 received at base station 30 by
`antenna 40 is provided to receiver (RCVR) 42 which pro
`vides an indication of the received power to control proces
`Sor 46. If the link quality factor used is frame error rate, then
`receiver 42 downconverts and demodulates the Signal and
`provides the demodulated signal to decoder 44. Decoder 44
`determines an indication of error rate and provides a signal
`indicative of the error rate to control processor 46.
`Control processor 46 compares the link quality factor
`provided against a threshold or Set of thresholds which may
`be static or varying. Control processor 46 then provides the
`power control information to either encoder 34 or power
`control encoder (P.C. ENC.) 47. If the power control infor
`mation is to be encoded into the data frame, then the power
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`45
`
`50
`
`55
`
`60
`
`7
`control data is provided to encoder 34. This method requires
`that an entire frame of data be processed before transmitting
`the power control data. Then encoded traffic data and power
`control data frames are then provided to transmitter (XMTR)
`36. The power control data may simply overwrite portions of
`the data frame or may be placed in predetermined vacant
`positions in the transmission frame. If the power control data
`overwrites traffic data, then this may be corrected by forward
`error correction techniques at mobile station 50.
`In implementations that process a full frame of data
`before providing the power control data, delay is caused
`which is undesirable in fast fade conditions. An alternative
`is to provide the power control data directly to transmitter 36
`where it may be punctured into the outgoing data Stream. If
`the power control data is transmitted without error correc
`tion coding then power control encoder 47 Simply passes the
`power control data to transmitter 36. If error correction
`coding is desired for the power control data, without incur
`ring the delay of waiting for a full frame of data to be
`processed, then power control encoder 47 provides an
`encoding of the power control data without regard to the
`outgoing traffic data. Transmitter 36 upconverts, modulates
`the Signal and provides it to antenna 38 for transmission.
`The transmitted Signal is received at antenna 52 and
`provided to receiver (RCVR) 54 where it is downconverted
`and demodulated. If the power control data is encoded with
`a full frame of traffic data then the traffic and power control
`data is provided to decoder 56. Decoder 56 decodes the
`Signal and Separates the power control Signal from the traffic
`data.
`If, on the other hand the power control data is not encoded
`with a full frame of data but rather punctured into the
`transmission Stream of data, then receiver 54 extracts the
`power control data from the incoming data stream and
`provides the encoded data to power control decoder (P.C.
`DEC.) 55. If the power control data is encoded, then power
`control decoder 55 decodes the power control data and
`provides the decoded power control data to control proces
`Sor 58. If the power control data is not encoded then the data
`is provided directly from receiver 54 to control processor 58.
`The power control Signal is provided to control processor
`58, which in accordance with the power control signal
`provides a signal to variable rate data Source 60 indicative of
`an appropriate rate Set or a transmission Signal to transmitter
`64 indicative of a modified transmission power level.
`Base station 30 does not know the data rate of the
`transmitted frame a priori, So in the implementation wherein
`power varies in accordance with the redundancy of the data
`in the frame or data rate of the frame, then the determination
`of a power control Signal from received link quality char
`acteristics is rate dependent. In one implementation, mobile
`Station 30 may include bits representing the data rate at the
`beginning of a frame in an uncoded fashion. This may be
`acceptable if the frames contain enough bits of information
`that the impact on capacity is not large.
`In an alternative implementation, base Station 30 may
`estimate the rate of the frame from the first part of the frame.
`For example, a preamble could be added at the Start of each
`frame and the base Station could estimate the rate to be the
`one that provides the best preamble correlation.
`In another exemplary implementation of providing a rate
`dependent power control Signal, multiple bits of power
`control information may be provided from base station 30 to
`mobile station 50. In a first multiple bit power control signal
`implementation received power is used as the link quality
`factor. Receiver 42 provides a received power measurement
`65
`Signal to control processor 46. Control processor 46 com
`pares the received power value against a set of thresholds.
`
`8
`In the exemplary first multiple bit power control Signal
`implementation embodiment, there is one threshold indica
`tive of a nominal received power for each rate hypothesis.
`Control processor 46 provides a Signal indicative of where
`within a range of power quantization levels the received
`power lies. The required power for the full-rate mode will be
`highest, and the required power for the /s-rate mode will be
`lowest. For example, the following 5 levels can be defined:
`Level 4-received power larger than the nominal full-rate
`power
`Level 3-received power between the /3-rate and full-rate
`nominal powers
`Level 2-received power between the 4-rate and /3-rate
`nominal powers
`Level 1-received power between the /s-rate and 4-rate
`nominal powers
`Level 0-received power less than the /s-rate nominal
`power.
`BitS indicating the received power level are then combined
`with traffic data in encoder 34 and transmitted back to
`mobile station 50 as previously described. Mobile station 50
`knows the data rate at which it transmitted and So power
`adjustments could be based on the knowledge of the trans
`mission rate for the power control group corresponding to
`the feedback information as shown in Table 1. Table 1
`illustrates a benefit to the multiple level implementation,
`which is that if the measured quality is much different than
`the desired level (possibly due to a Sudden deep fade), a
`larger power adjustment can be made. In this
`implementation, 3 bits are needed to send back this 1-of-5
`information. This increases the overhead on the feedback
`link.
`
`TABLE 1.
`
`Power Adiustments with Five Quality Levels
`
`Received
`
`Power Adiustment in dB
`
`Quality Level
`
`Full Rate
`
`A Rate
`
`A Rate
`
`vs. Rate
`
`4
`3
`2
`1.
`O
`
`-1
`+1
`+2
`+2
`+2
`
`-2
`-1
`+1
`+2
`+2
`
`-2
`-2
`-1
`+1
`+2
`
`-2
`-2
`-2
`-1
`+1
`
`One way of reducing the amount of overhead necessary
`for power control in a variable rate system would be to
`restrict the number of transmission rates to a number that is
`of the form 2'-1 where