United States Patent (19)
`McGirr et al.
`
`IIII
`5,129,098
`Jul. 7, 1992
`
`USOO5129098A
`Patent Number:
`Date of Patent:
`
`11
`45
`
`73) Assignee:
`
`54) RADIOTELEPHONE USING RECEIVED
`SIGNAL STRENGTH IN CONTROLLING
`TRANSMESSION POWER
`Andrew E. McGirr; Barry J. Cassidy,
`75 Inventors:
`both of Calgary, Canada
`Now Atel Communication Ltd.,
`Calgary, Canada
`21 Appl. No.: 587,004
`22 Filed:
`Sep. 24, 1990
`51) int. C. ............................................... H04B 1/OO
`52 U.S. C. ..................................... 455/69; 455/67.1;
`455/73; 455/126; 455/127
`58 Field of Search ....................... 455/67, 69, 70, 88,
`455/89, 26, 127, 16, 234, 245, 246, 250, 251,
`73; 375/98
`
`56
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,373.206 2/1983 Suzuki et al. ....................... 455A.03
`4,475,010 O/984 Huensch et al. .
`4.495.648 l/1985 Giger .................................... 455/73
`4,556.760 12/1985 Goldman .
`4,608.7
`8A1986 Goldman
`4.760,347 7/1988 L et al.
`4,777,653 0/1988 Bonnerot et al. ..................... 455/69
`4.8 1.421 3/989 Havel et al. ..........
`455/69
`4,870,698 9/1989 Katsuyama et al. .
`455/67
`4,870.699 9/1989 Garner et al. ......................... 45.5/76
`4.885.798 2/1989 Jinich et al. ........................... 455/52
`FOREIGN PATENT DOCUMENTS
`12051.40 5A1986 Canada .
`
`296.38 3/987 Canada .
`22662S 9/1987 Canada
`1226626 9/1987 Canada .
`0318033 1/1988 European Pat. Off. .
`8402043 5/1984 PCT int'l Appl. .
`8701897 3/1987 PCT Int'l Appl. ,
`220425A 1 1/1988 United Kingdom .
`Primary Examiner-Reinhard J. Eisenzopf
`Assistant Examiner-Chi H. Phan
`Attorney, Agent, or Firm-Cesari and Mckenna
`57
`ABSTRACT
`A radio telephone uses a signal indicating the strength
`of received communication signals as a measure of pre
`vailing signal propagation conditions to control the gain
`of a power amplifier in its transmitter. Where conditions
`are favorable, the gain can be reduced without deleteri
`ous effects on transmitted signals, and with a resulting
`savings in power drain on the radio telephone's power
`supply (e.g., batteries). In this way, talktime on a single
`battery charge can be significantly lengthened. Where
`conditions are adverse, e.g., at outlying or fringe areas
`of a cellular telephone system of which the radio tele
`phone is a station, the gain can be increased to produce
`stronger transmitted signals, at times extending the ef
`fective coverage area of such systems. Preferably, the
`output of the power amplifier remains at all times within
`a power range prescribed by applicable standards, and
`is increased or decreased within the power range in
`accordance with prevailing conditions indicated by the
`received signal strength signal.
`
`22 Claims, 6 Drawing Sheets
`
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`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 1 of 14
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`U.S. Patent
`
`July 7, 1992
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`Sheet 1 of 6
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`5,129,098
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`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 2 of 14
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`

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`U.S. Patent
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`July 7, 1992
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`Sheet 2 of 6
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`5,129,098
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`Maxell Ex. No. 2012
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`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 3 of 14
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`U.S. Patent
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`July 7, 1992
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`Sheet 3 of 6
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`5,129,098
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`IPR2020-00203
`Maxell Ex. No. 2012
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`Page 4 of 14
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`U.S. Patent
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`July 7, 1992
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`Sheet 4 of 6
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`5,129,098
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`Maxell Ex. No. 2012
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`Page 5 of 14
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`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 5 of 14
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`

`

`U.S. Patent
`
`July 7, 1992
`
`Sheet 5 of 6
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`5,129,098
`
`12O
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`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 6 of 14
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`

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`U.S. Patent
`
`July 7, 1992
`
`Sheet6 of 6
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`5,129,098
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`Maxell Ex. No. 2012
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`Page 7 of 14
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`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 7 of 14
`
`

`

`RADIO TELEPHONE USING RECEIVED SIGNAL
`STRENGTH IN CONTROLLING TRANSMISSION
`POWER
`
`5
`
`O
`
`5
`
`5,129,098
`2
`output power of the transmitter amplifier, performance
`of the telephone could be improved.
`SUMMARY OF THE INVENTION
`Briefly, the present invention resides in using a signal
`("RSSI") indicating the strength of received communi
`cation signals as a measure of prevailing transmission
`propagation conditions in controlling the gain of a
`power amplifier at the output of the transmitter of a
`radio telephone.
`More specifically, a digital representation of an RSSI
`is derived from a received communication signal and
`provided to a central processing unit ("CPU") on-board
`the radio telephone, which uses the RSSI data in gener
`ating a digital control signal. The digital values of this
`control signal correspond generally to the amplitude,
`over time, of the RSSI, and, therefore, to prevailing
`propagation parameters. In addition, the digital values
`are selected to produce a transmitter output power level
`tailored to those prevailing propagation parameters. An
`AGC circuit converts this control signal into an appro
`priate analog AGC signal for controlling the gain of the
`RF power amplifier of the transmitter, whereby its
`output power level reflects the prevailing propagation
`parameters indicated by the RSSI.
`The strength (i.e., field strength or energy) of com
`munication signals over particular channels as indicated
`by RSSI varies due to terrain and cultural (e.g., build
`ings) obstructions or impairments to signal propagation,
`and to distance between the radio telephone and the
`base station or cell site with which it is to communicate.
`The prevailing conditions are, of course, variable; for
`instance, as the radio telephone is moved, e.g., as an
`automobile carrying the telephone travels along, the
`conditions experienced by the telephone communica
`tion signal can change. Such prevailing conditions af
`fecting the strength of communication signals as mea
`sured by the RSSI can be called signal propagation
`parameters.
`Basic to the invention is the recognition that such an
`RSSI signal is indicative of signal propagation parame
`ters not only for received signals, but also for transmit
`ted signals. In other words, the transmission propaga
`tion parameters characterizing the path taken by the
`communication signals received by the radio telephone
`will be generally the same as those characterizing the
`path taken by the communication signals sent by the
`radio telephone, provided the communication signals
`are sent and received at about the same time and loca
`tion.
`Where conditions are favorable, the amplifier's gain
`can be reduced without deleterious effects on transmit
`ted signals, and with a resulting reduction in the power
`drain on the radio telephone's power supply (e.g., bat
`teries). In this way, talktime on a single battery charge
`can be significantly lengthened.
`On the other hand, where conditions are adverse,
`e.g., at outlying or fringe areas of a cellular telephone
`system of which the radio telephone is a station, the
`gain can be increased to produce stronger transmitted
`signals, at times extending the effective coverage area of
`such systems.
`Preferably, the output of the power amplifier remains
`at all times within a power range prescribed by applica
`ble standards, and is increased or decreased within that
`power range in accordance with prevailing conditions.
`In this way, for example, the latitude afforded by the 6
`dB range specified by the above-reference standards
`
`FIELD OF THE INVENTION
`The present invention relates to telecommunication
`apparatus, and more particularly to techniques for dy
`namically controlling the amplification of communica
`tion signals to be transmitted by radio telephones in
`response to a measure of prevailing signal propagation
`conditions. The term "radio telephones' is used in its
`broad sense to include wireless two-way communica
`tion devices, including handheld, portable and mobile
`(i.e., vehicularly mounted) units.
`BACKGROUND OF THE INVENTION
`Known radio telephones have variable-gain power
`amplifiers for amplifying modulated radio-frequency
`20
`("RF") signals prior to transmission. The gains of the
`power amplifiers are typically controlled by automatic
`gain control ("AGC") circuits. Conventional AGC
`circuits operate dynamically to maintain the amplifier
`outputs within defined power output tolerances or
`ranges, and usually at or near nominal values (e.g., inter
`mediate points) within those ranges. Various factors,
`including changes in ambient temperature and in power
`supply (e.g., battery) power level, can cause the ampli
`fier output to vary from the targeted output power
`30
`level. It is the job of the AGC circuits to correct for
`such variations.
`For cellular telephony, generally recognized stan
`dards specify the nominal value and a range or toler
`ance for the amplifier output power. For instance, the
`Electronic Industries Association/ Telecommunica
`tions Industry Association Standard "Mobile Station
`Land Station Compatibility Specification", EIA/TIA
`553, September, 1989, Section 2.1.2.2. specifies that the
`power level must be maintained within a 6 dB range
`from +2 dB to - 4 dB of its likewise-specified nominal
`level over the ambient temperature range of -30 de
`grees Celsius to -- 60 degrees Celsius and over the sup
`ply voltage range of 10 percent from the nominal
`45
`value. Thus, according to that specification, the ampli
`fier output power must be controlled by the AGC cir
`cuit so as to fall within a 6 dB range about a nominal
`power level.
`Conventional AGC circuits in such telephones typi
`50
`cally target the nominal power level regardless, e.g., of
`the distance of the station that is to receive the transmit
`ted signal or of other conditions which would otherwise
`suggest the transmission of stronger or weaker signals
`from the telephone.
`In developing transmission signals of that power
`level, the transmitter's power amplifier itself consumes
`most of the dc power required by the radio telephone
`during audio or voice communication (called "talk
`time"). As radio telephones become increasingly self
`powered, e.g., through the use of on-board, recharge
`able storage batteries, its power needs have become an
`increasingly significant design consideration.
`This comes into focus when one considers that, typi
`cally, the greater the power requirements of the power
`65
`amplifier, the shorter the talktime on a single charge of
`the batteries in the telephone. If steps could be taken to
`reduce the power requirements by, e.g., reducing the
`
`25
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`35
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`5,129,098
`3
`4.
`can be more effectively used by radio telephones en
`The transmitter 26 includes a signal processor 32 for
`processing (e.g., filtering and amplifying) audio signals
`bodying the present invention.
`Instead of attempting to maintain the amplifier output
`from the microphone 22 and control signals from the
`CPU 30. The output of the signal processor 32 is called
`power at some nominal level, e.g., at an intermediate
`the "transmit baseband signal." This signal is fed to a
`point within the range, the AGC circuit in accordance 5
`with the invention preferably drives the output power
`modulator and converter 34. There, an RF carrier sig
`nal is, preferably, frequency modulated with the trans
`to the lowest level within the specified range that pro
`vides an adequately strong communication signal in
`mit baseband signal.
`light of prevailing propagation conditions. An "ade
`The transmitter 26 also includes a conventional, vari
`quately strong" communication signal from a radio 10
`able-gain, RF power amplifier 36 for boosting the
`telephone used within a cellular telephone system is one
`power of the modulated RF signal from the modulator
`and converter 34. The power amplifier 36 receives the
`which can be received at the intended base station with
`modulated RF signal at a signal input 36a thereof, and
`sufficient strength so as to produce a received baseband
`produces an amplified version of the signal at its output
`signal with at least about a 14 dB signal-to-noise ratio.
`36b. The amplified signal from the power amplifier 36 is
`Overall, the invention can enhance performance of 15
`radio telephones within the dictates of applicable stan
`fed to the antenna system 16.
`dards, both temporally (due to extended battery life)
`The antenna system 16 includes both an antenna 38
`and geographically (due to improved signal strength in
`and a duplexer 40 for full-duplex two-way conversa
`outlying areas).
`tions, i.e., for permitting the antenna 38 to be used both
`for transmitting the output from the power amplifier 36,
`BRIEF DESCRIPTION OF THE DRAWINGS
`and for receiving communication signals broadcast
`The aforementioned aspects, features and advantages
`from base stations (not shown). The duplexer 40 in
`cludes a filter arrangement, which is not separately
`of the invention, as well as others, are explained in the
`following description taken in connection with the ac
`shown.
`25
`companying drawings, wherein:
`The receiver 28 has a conventional front-end con
`FIG. 1 is a block diagram of a radio telephone in
`verter and mixer 44 for converting the RF signal from
`the duplexer 40 into an intermediate-frequency ("IF")
`accordance with a preferred embodiment of the inven
`signal. The receiver 28 also has an FM receiver and
`tion;
`FIG. 2 is a block diagram of the FM receiver and
`RSSI detector 46, which both (i) extracts the received
`baseband signal from the RF signal, and (ii) produces an
`RSSI detector of FIG. ;
`FIG. 3 is a block diagram of the central processing
`RSSI having a voltage amplitude that varies in response
`to the strength of the IF signal, and, thus, of the in-com
`unit of FIG. 1;
`ing RF signal.
`FIG. 4 is a block diagram of the power control circuit
`of FIG. 1;
`The received baseband signal is fed to a conventional
`signal processor 45. The signal processor 45 processes
`FIG. 5 is an algorithm in flow chart form suitable for
`(e.g., filtering and amplifying) the received baseband
`execution by the CPU of FIG. 3 in deriving RSSI-based
`digital control signals; and
`signal, separating it into audio and control signals. The
`FIG. 6 is a graph of the relationship between the
`audio signals are provided to the speaker 24, and the
`power amplifier output power (dB relative to nominal) 40
`control signals go to the CPU 30.
`and RSSI (dBm) for an illustrative application of the
`FIG. 2 shows the FM receiver and RSSI detector 46
`in more detail. The chain of boxes across the top of the
`invention.
`drawing together form an FM receiver 46a. An IF filter
`52 filters the IF signal from the front-end converter and
`mixer 44 so as to reduce broadband noise, and thereby
`improve the signal-to-noise ratio. The IF filter 52 passes
`its output to an IF log amplifier 54. Next, an interstage
`filter 56 further eliminates noise from the IF signal for
`an even better signal-to-noise ratio, and passes its output
`to a limiter amplifier 58. The output of the limiter 58 is
`fed directly to a first input of a conventional quadrature
`detector 62. The output of the limiter amplifier 58 is also
`shifted by 90 degrees in a phase shifter 64, and thence is
`provided to a second input of the quadrature detector
`62. The quadrature detector 62 performs demodulation,
`and its output is the received baseband signal.
`The rest of FIG. 2 constitutes the RSSI detector 46b.
`Current sensors 66, 68 sense the magnitude of the dic
`current drawn respectively by the RF amplifier 54 and
`by the limiter amplifier 58 from the power supply 18.
`The amounts of current drawn by amplifiers 54, 58
`correspond to the degree of amplification performed by
`them in obtaining respective pre-selected outputs, and,
`thus, depend on the strength of the signals received by
`the amplifiers 54, 58. It should be pointed out that the
`amplification performed by the amplifiers 54, 58 does
`not affect the content of the signal, since the signal is
`frequency modulated in the illustrated embodiment.
`
`DESCRIPTION OF PREFERRED EMBODIMENT
`FIG. 1 shows a radio telephone 10 in accordance 4s
`with a preferred embodiment of the invention. The
`radio telephone 10 includes a user unit 12, a transceiver
`14, a conventional antenna system 16, and a conven
`tional regulated power supply, e.g., including batteries
`18.
`The user unit 12 provides a user/telephone interface,
`and includes a microphone 22 for converting sounds,
`e.g., messages spoken by a user or other audio data, into
`an electrical signal representing those sounds, called a
`"transmit audio signal." The user unit 12 also includes a 55
`speaker 24 for converting an electrical signal containing
`audio data, called a "receive audio signal," into sound,
`e.g., messages being communicated to the user. Where
`the user unit 12 is a handset, which is most often the case
`today, a mouthpiece (not shown) of the handset con- 60
`tains the microphone 22 and an earpiece thereof (not
`shown) contains the speaker 24. The user unit 12 typi
`cally includes also a keypad and display (not shown).
`The transceiver 14 has a transmitter 26, a receiver 28,
`and a central processing unit ("CPU") 30 for control- 65
`ling many of the operations of the transmitter 26 and
`receiver 28 (and for receiving and providing informa
`tion to the user unit 12).
`
`50
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`The AGC circuit 100 has a digital-to-analog con
`The RSS detector 46b also has a current summer 72
`verter ("D/A") 102 for converting the digital power
`and an RSSI amplifier 74. The current summer 72 sums
`control signal from the CPU 30 into an analog signal,
`the outputs of the current sensors 66, 68. The resulting
`whose voltage varies in accordance with the digital
`signal has a current whose magnitude corresponds to
`values of the digital power control signal. The AGC
`that of the IF signal. The RSSI amplifier 74 converts
`circuit 100 also has a power control circuit 104.
`this signal into an RSSI, that is, into a signal whose
`FIG. 4 is a schematic representation of the power
`voltage level corresponds inversely to the magnitude of
`control circuit 104. An RF power detector 106, e.g., a
`the IF signal, which, in turn, corresponds to the
`diode detector, receives a portion of the output of the
`strength of the received RF signal.
`power amplifier 36 as a power amplifier feedback sig
`In high signal-strength areas, e.g., close to transmit
`nal, and provides a feedback voltage that is proportional
`ting antenna of base stations or cell sites, RSSI values
`to the power level of the power amplifier's output to a
`typically are large (e.g., greater than about-60 dBm),
`power control amplifier 108 at its first input 108a. The
`while in outlying or fringe areas, e.g., far from transmis
`power control signal from the D/A 102 is provided to
`sion sources, RSSI values typically are low (e.g., less
`a second input 108b of the power control amplifier 108.
`than about - 100 dBm).
`Amplifier 108 is adapted with a suitable feedback
`Conventional radio telephones often derive a signal
`capacitor arrangement C to integrate the difference
`indicative of the strength of received communication
`between the signals on its inputs 108a, 108b, with a gain
`signals as well, and this signal is also sometimes referred
`controlled by an RC network 110. The integration thus
`to as a "received signal strength indicator" or "RSSI".
`performed ensures that the AGC circuit 100 exhibits an
`Heretofore, however, the uses to which RSS measure
`appropriate dynamic response, i.e., that the AGC cir
`ments have been put have been unduly limited. RSSI
`cuit 100 is not overly sensitive to transient conditions in
`has long been used to provide the radio telephone user
`the input signals to the power control amplifier 108.
`with a crude indication of received signal strength.
`The output of the differential amplifier 108 is the AGC
`More importantly, radio telephones used in cellular
`signal. A further understanding of the power control
`telephone systems contain RSSI circuitry for purposes
`circuit 104 can be had with reference to U.S. Pat. No.
`of tuning to the strongest available channels. This is
`4,760,347, issued Jul. 26, 1988, and entitled "Controlled
`prescribed by applicable standards, such as the above
`Output Amplifier and Power Detector Therefore."
`mentioned EIA/TIA 553, Sections 2.6.1.1.l., 2.6.1.2.l.,
`It should be apparent from the foregoing discussion
`and 2.6.3.2. The RSSI measurements specified by the 3
`that the power control signal plays a central role in the
`O
`standards provide a general indication of the strength of
`generation of the AGC signal. Accordingly, a further
`received communication signals for each available
`description of the method by which the power control
`channel, and this indication is then used to select the
`signal is derived shall now be given.
`appropriate channel. usually the strongest channel, for
`FIG. 5 shows an algorithm 120 in flow chart form for
`communication. Herein, a novel use for RSSI signals is
`deriving the power control signal based on the RSSI.
`proposed, as will be made clear in the following discus
`When an enable signal is asserted by the CPU 30 over
`line 30a to the power amplifier 36 to indicate that the
`SO.
`With renewed reference to FIG. 1, the RSSI signal
`radio-telephone 10 is to transmit, algorithm 120 starts in
`from the FM receiver and RSSI detector 46 is fed to an
`block 122. In block 124, the algorithm 120 initializes
`analog-to-digital converter ("A/D") 82. The A/D 82
`variables, including RSSI SUM and RSSI AVERAGE.
`converts the RSSI from an analog signal into a digital
`Next, the algorithm 120 enters into a do-loop 126 in
`signal, whose digital values correspond to the amplitude
`which the RSSI is sampled repeatedly over a period of
`of the analog RSSI signal. The A/D 82 passes the digi
`time sufficient to provide an adequate update, e.g., over
`tized RSS to the CPU 30 for processing.
`a period of approximately 2.5 seconds, or sixty-two
`FIG. 3 shows the CPU 30 in more detail. The CPU 30
`iterations of the loop 126.
`5
`4.
`has a processor 84, a read-only memory ("ROM") 86
`In each iteration, the algorithm 120, a block 128, takes
`for storing, e.g., a telephone operating program, a non
`in plural (e.g., two) samplings of the RSS signal from
`volatile memory ("NVM') 88, for storing, e.g., various
`the A/D 82, preferably at fixed time intervals (e.g., at 20
`databases (described below) a universal asynchronous
`msec. apart). The reason for taking the double sampling
`receiver/transmitter ("UART") interface 92 for con
`of the RSSI is to avoid an aberrationally low reading
`5
`O
`munication with the keyboard and display unit of the
`resulting, e.g., from temporary fading of the RF signal
`user unit 12, input port 94 for receiving the RSSI from
`during its propagation to the radio telephone 10. Ac
`the A/D 82, output port 96, and a control bus 98 inter
`cordingly, the higher of the readings is selected for
`connecting all of the other CPU components, among
`further processing, and, in block 130, is added to a run
`other conventional components (not shown). The CPU
`ning total, called RSSI SUM. Then, as indicated by
`5
`s
`30 stores various data concerning the RSSI, and com
`block 132, the loop 126 is repeated (starting with block
`putes a digitized power control signal using the digi
`128) until sixty-two iterations have been completed, at
`tized RSS from the A/D 82, as will be described
`which time the loop 126 is exited.
`shortly.
`Next, in block 134, the algorithm 120 calculates a
`Again with reference to FIG. 1, the radio telephone
`time-averaged value, called RSSI AVERAGE, by di
`10 also has an automatic gain control circuit 100, as
`viding RSSI SUM by the number of iterations of the
`mentioned above, for controlling the gain of the power
`loop 26 (in the example, by 62). This further eliminates
`amplifier 36. In accordance with the invention, the
`any false or short term fluctuations in the RSSI mea
`AGC circuit 100 processes the digitized power control
`Stee.
`signal obtained from the output port 96 of the CPU 30
`Then, also in block 134, an RSSI calibration factor is
`to derive an AGC signal. This AGC signal is applied to
`added (or, e.g., otherwise applied) to the calculated
`a control input 36c of the power amplifier 36 to regulate
`RSSI AVERAGE to yield a calibrated or absolute
`the gain of the power amplifier 36.
`RSSI value. The RSSI calibration factor is preferably
`
`3
`5
`
`
`
`2 5
`
`
`
`6 5
`
`Apple v. Maxell
`IPR2020-00203
`Maxell Ex. No. 2012
`
`Page 10 of 14
`
`

`

`5,129,098
`7
`8
`stored in a calibration look-up table ("LUT") 88a in the
`In addition, for RSS values below the second thresh
`old, the power amplifier can be controlled so as to ren
`NVM 88 of the CPU 30. The LUT 88a is a database in
`der its output power greater than the nominal value,
`which calibration factors are stored in locations corre
`thereby improving transmission strength when (and
`sponding to measured RSSI values, and the channels or
`only when) such improvement is most needed, e.g., in
`frequencies to which the receiver 28 (FIG. 1) can be 5
`fringe or outlying areas.
`tuned. Thus, the calibration factor is the entry corre
`sponding to the particular measured RSSI value, and to
`With reference again to FIG. 5, in block 138, the
`algorithm 120 obtains the power control voltage that
`the particular channel over which the communication
`would produce the desired output power (computed in
`signal that produced that value was received.
`block 136), e.g., again through the use of a suitable,
`Calibration of the measured RSSI is required for 10
`stored look-up table 88c. The entries of the power-con
`various reasons. First, the RSS value provided to the
`trol-voltage/amplifier output-voltage look-up table 88c
`CPU 30 can have a transient component due to non
`are preferably empirically derived to account for, and
`linearities in the frequency characteristics of the du
`plexer 40 (FIG. 1) and other components of the radio
`substantially eliminate, unit-to-unit variations in the
`responses of power amplifiers to control signals. In
`telephone 10. Thus, the measured RSSI's can vary from 15
`other words, each entry of the LUT 88c is the precise
`one channel to the next, despite identical strengths of
`control signal needed to produce a specified output
`the received signals on the various channels. Second,
`power in the power amplifier 36.
`the measured RSSI will depend on the normally-other
`wise-acceptable manufacturing tolerances of these com
`The entry of LUT 88c identified by the target output
`power is applied to D/A 102 as the power control volt
`ponents that cause their characteristics to vary from 20
`age, and continues to be applied thereto until such time
`unit to unit. Third, the measured RSSI will depend on
`as a different power-control voltage is determined in
`the selection of the output level of the IF and limiter
`block 138. Then, after step 138, the algorithm 120 re
`amplifiers 54, 58 (FIG. 2), since that level dictates the
`turns to its starting block 122.
`level of currents drawn by amplifiers 54, 58 (FIG. 2),
`A suitable methodology for deriving the calibration
`which currents are detected in deriving the RSSI. For 25
`look-up table 88a for RSSI values is as follows: An RF
`all these reasons, RSSI calibration is appropriate.
`signal generator provides RF signals to an antenna port
`After obtaining a calibrated RSSI AVERAGE, algo
`40b (FIG. 1) of the duplexer 40 (FIG. 1). These signals
`rithm 120 calculates, in block 136, the power amplifier
`output desired for the prevailing conditions indicated
`are of known power levels, and of known and tunable
`frequencies. The radio telephone 10 (FIG. 1) treats each
`by the absolute RSSI just calculated. This, too, can be 30
`achieved expeditiously using an appropriate look-up
`signal from the signal generator as it would a received
`communication signal, and derives the RSSI AVER
`table 88b stored in NVM 88 (FIG. 2) in the CPU 30
`(FIG. 1). The entries in the look-up table 88b establish
`AGE value for that signal, as described hereinabove.
`an RSSI/output-power relationship.
`This derived RSSIAVERAGE value is compared with
`FIG. 6 depicts graphically the inverse relationship 35
`a known value corresponding to that RF signal, and the
`difference is stored as an entry in the look-up table 88a
`between calibrated RSSI and the power amplifier out
`put power for an illustrative cellular-telephone applica
`at a location corresponding to the frequency of the
`applied signal. Typically, it is necessary to sample RSSI
`tion. As can readily be seen, for this application, output
`at only a single received level provided by the RF sig
`power increases in steps as RSSI decreases.
`nal generator for each of a plurality of frequency seg
`Between a first threshold, e.g., about -100 dBm, and 40
`ments or bands. Preferably, RSSI calibration is per
`a second threshold, e.g., about - 90 dBm, nominal out
`formed at the radio telephone manufacturing facility.
`put power can be used. For lower RSSI values, i.e.,
`The operation of radio telephone 10 will now be
`below the first threshold, higher output power can be
`used advantageously, that is, the output power can be
`described. In accordance with the invention, when a
`raised above the nominal value. As depicted, for exam- 45
`communication signal is received over a voice channel,
`ple, output power can be increased by 1 dB for each 10
`the FM receiver and RSSI detector 46 produces an
`RSSI signal to indicate the prevailing signal propaga
`dBm decrease in RSSI below the first threshold. For
`tion conditions encountered by the received signal. A
`higher values of RSSI, i.e., above the second threshold,
`the output power can be lowered from nominal power.
`digitized version of the RSSI is supplied to the CPU 30
`The graph shows output power being lowered by 1 dB 50
`by receiver 28.
`The CPU 30 samples this RSSI signal and derives an
`for each 10 dBm increase in RSSI. Of course, the rates
`average value for the signal over a reasonably long
`of increase in output power per drop in RSSI values
`period of time (e.g., 2.5 seconds). This average value is
`below the first threshold and of decrease in output
`then calibrated to provide a truer indication of prevail
`power per rise in RSSI values above the second thresh
`ing signal propagation conditions. To accomplish such
`old can be any desired amounts, and certainly need not 55
`be equal.
`calibration, the average RSSI value and the frequency
`of the received signal are used as pointers into the cali
`In other words, for RSSI values above the first
`threshold, the power amplifier can be controlled so as
`bration table 88a. The resulting calibrated absolute
`to render its output power lower than the nominal out
`RSSI value is used in deriving a desirable transmission
`put power established by applicable standards and typi- 60
`power for the signal propagation conditions indicated
`cally targeted by conventional AGC circuits. Conse
`by the RSSI. To do so, the absolute RSSI i