US006006077A
`6,006,077
`(114) Patent Number:
`United States Patent 55
`Shull
`[45] Date of Patent:
`Dec. 21, 1999
`
`
`[54] RECEIVED SIGNAL STRENGTH
`
`OrneMATION METHODS AND
`.
`.
`Inventor: Eric A. Shull, Raleigh, N.C.
`[75]
`.
`,
`.
`[73] Assignee: Ericsson Inc., Research Triangle Park,
`N.C.
`
`[21] Appl. No.: 08/942,645
`[22]
`Filed:
`Oct. 2, 1997
`6
`[S51]
`Tint, Cececeseseeseecnnecneeeneeeneess HO04B 17/00
`[52] U.S. C1 ce eeeseesseneenenteneee 455/226.2; 455/226.4
`[58] Field of Search 0.0. 455/226.2, 226.4;
`375/227, 317
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`10/1984
`4,479,253
`3/1986
`4,578,820
`4/1986
`4,580,287
`4,619,002 10/1986
`5,390,365
`2/1995
`5,408,696
`4/1995
`5,701,601
`12/1997
`5,875,390
`2/1999
`
`Daniel, Jr. oe. eeeeseeseceeeeees 455/226.2
`
`Highton w.cccceseeeseseeeeeee 455/226.2
`eeeeeseceeeeees 455/226.4
`Richards, Jr...
`1ce 375/317
`Enoki et al.
`cc
`ceceseeeceeeeneee 455/553
`Hofverberg ...
`w. 455/226.2
`......
`. 455/226.2
`Tomoeet al.
`
`Brehmeret al. wu... cece 455/226.2
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`505072-A2
`0 601 987
`639901-A2
`0755 133
`0785 640
`405063663
`
`........... 455/226.2
`
`9/1992 European Pat. Off.
`6/1994 European Pat. Off..
`2/1995 European Pat. Off. uo... 455/226.2
`1/1997 European Pat. Off.
`.
`=7/1997 European Pat. Off.
`.
`3/1993 Tapa cecsccssssssssssssssesserseroeee 455/226.2
`
`Primary Examiner—Reinhard J. Eisenzopf
`Assistant Examiner—Eliseo Ramos-Feliciano
`Attorney, Agent, or Firm—Myers Bigel Sibley & Sajovec
`[57]
`ABSTRACT
`Asignalstrength for a receivedsignal such as a radio signal
`transmitted over a communication network is determined.
`The signal strength measurement is compensated for non-
`linear characteristics of the receiver. The compensation is
`provided by taking two signal strength readings with the
`receiver set at two different, known,gain levels. The differ-
`ence between the expected change in the signal strength and
`the change actually measured by the receiver is used to
`generate a compensated signal strength measurementA table
`of compensation factors is generated for each signal strength
`whichis also utilized in generating the compensated signal
`strength measurement. The compensated signal strength
`measurement reading is transmitted to the communication
`network for use in mobile assisted handover.
`
`20 Claims, 4 Drawing Sheets
`
`ANTENNA
`V7
`
`12
`
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`'1
`\
`
`ZL16
`GAIN AND BAND CONTROL
`wees
`-20
`
`IF1=150.84MHz
`
` DUAL-BAND
`
`
`
`FRONT-END
`
`CIRCUIT
`
`
`FIRSTIF
`FILTER
`
`
`
`CONTROL
`PROCESSOR
`
`
`
`OTHER CONTROL
`
`SIGNALS
`36
`
`
`
`
` MODULATION
`
`2nd LO=150.96MHz (629 X 240 KHz)
`bee ee ee ee ee ee ee ewe eee ee
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`195.84MHz{at 800 MHz)
`
`
`N=48 @ 800 MHz
`230.88MHz(at 1900MHz)
`M=1
`7 @1900 MHZ
`OR 26 @ 1900 MHz
`
`
`
`OR 37 @ 800 MHZ
`
`
`
`TX LOOP
`
` 4y’t- -
`FILTER
`
` TRANSMIT CHIP
`
`RECEIVE AND
`TRANSMIT
`SIGNAL
`PROCESSING
`
`Fref = 4.080MHz @ 800MHz
`Fref = 8.880MHz @ 1900MHz
`
`
`
`DIV BY N
`
`DIV BYM
`
`EXHIBIT 1007
`
`1
`
`EXHIBIT 1007
`
`

`

`U.S. Patent
`
`Dec.21, 1999
`
`Sheet 1 of 4
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`U.S. Patent
`
`Dec.21, 1999
`
`Sheet 2 of 4
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`6,006,077
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`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 3 of 4
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`6,006,077
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`OBTAIN FIRST
`MEASUREMENTAT
`FIRST GAIN
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`
`OBTAIN SECOND
`MEASUREMENTAT
`SECOND GAIN
`
`
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`
`
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`DETERMINE
`EXPECTED
`DIFFERENCE
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`OBTAIN PREVIOUS
`COMPENSATION
`FACTOR
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`COMPENSATE
`STRENGTH
`
`MEASUREMENT
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`TRANSMIT
`COMPENSATED
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`MEASUREMENT TO
`NETWORK
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`
`4
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`

`

`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 4 of 4
`
`6,006,077
`
`START
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`SELECT NEXT MAHO
`REQUENCY AND
`TIMESLOT
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`200
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`204
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`NO
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`cin
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`208
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`MEASUREMENT
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`YES
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`REDUCED GAIN
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`LAST TIME
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`SELECT FULL
`GAIN
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`SELECT REDUCED
`GAIN
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`MAKE
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`VALUES
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`COMPENSATE
`MEASUREMENT
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`PROCESS AND
`comer
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`UPDATE MOVING
`AVERAGE
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`5
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`

`

`6,006,077
`
`1
`RECEIVED SIGNAL STRENGTH
`DETERMINATION METHODS AND
`SYSTEMS
`
`FIELD OF THE INVENTION
`
`The invention relates to communication networks and,
`more particularly, to the determination of received signal
`strength in communication networks.
`
`BACKGROUND OF THE INVENTION
`
`Communication networks typically include at least one
`sender and one receiver.
`In either a wired or wireless
`
`transmitted between the sender and
`network, a signal
`receiver must be of sufficient magnitude (or strength) to
`allow the information contained within the signal
`to be
`discriminated from the noise which is generally present in
`the communication network. This may be a greater problem
`with a wireless network, which typically is more susceptible
`to noise from various interference sources.
`
`An example of a wireless networkis a radio network such
`as a cellular network commonly utilized for voice and/or
`data communications between a fixed base station covering
`a geographic region and mobile devices such as cellular
`terminals (or phones) present
`in the covered region. A
`cellular phone typically includes a radio receiver including
`an antennafor receiving signals and an amplifier/detector for
`generating a measure of the strength of received signals or
`noise. Asignal strength measure, commonly knownas Radio
`Signal Strength Indication (RSSI), may be expressed as a
`logarithmic measure of received signal strength and may be
`converted to a digital form by an analogto digital converter.
`It is known in the prior art that radio signal strength
`measurements can be useful
`in determining which base
`station should serve a cellular phone during a call. In the
`U.S. AMPSsystem, the mobile phone would typically use
`such signal strength measurements to determine the stron-
`gest base station to which it should listen for calls during
`standby (idle) mode. Also in the U.S. AMPS system, base
`stations belonging to the cellular network typically listen to
`the signal strengths received from mobile phones that are
`actively transmitting during calls, and the network usesits
`measurements to determine an optimum basestation for
`handling a call in progress. When a call in progress is
`switched from one base station to another, it is commonly
`knownas “handover”or “handoff.” Handoffs enable calls to
`
`be maintained even though the mobile phone may be chang-
`ing location.
`Cellular phones using a Time Division Multiple Access
`method conforming to either the European cellular standard
`known as GSM orany of the American TDMAstandards,for
`example,
`those known respectively as D-AMPS,
`IS54,
`18136 or PCS1900, may use spare time between transmit
`and receive timeslots to change frequency and monitor the
`signal strengths of other base stations. Several measure-
`ments of signal strength may be averaged for the same base
`station. The mobile phone makes measurementsofthe signal
`strengths received from surrounding base stations even
`during the progress of a call. Mobile Assisted Handover
`(MAHO) may be implemented using these measurements.
`The averages are typically reported to the currently serving
`base station, which determines if a handoff should be made
`to another, base station. The mobile typically reports MAHO
`RSSI measurements to the network station using a low-
`bitrate, inband signaling channel called the Slow Associated
`Control Channel or SACCH. The network uses SACCH
`
`measurements to determine the optimum base station to
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`handle a call in progress, preferably the base station that the
`mobile phone is receiving most strongly.
`In order for MAHOto operate effectively,it is preferred
`that the RSSI measurementsthat are reported to the network
`using the SACCH are reasonably accurate over a wide range
`of signal strengths that may be encountered from base
`stations. It is known in the prior art to employ logarithmic
`IF amplifiers with progressive saturation and progressive
`detectors in order to produce an RSSI signal approximately
`proportional to the logarithm of the received signal strength.
`See for example U.S. Pat. Nos. 5,048,059 entitled “Logpolar
`Signal Processing” and 5,070,303 entitled “Logarithmic
`Amplifier/Detector Delay Compensation” which are incor-
`porated by referenced herein in their entirety.
`Inaccuracy in RSSI measurements may occur when the
`RSSI value is not exactly proportionalto (linearly related to)
`the received signal strength. Inaccuracy of RSSI measure-
`ments may also occur when measuring strong signals that
`partially saturate amplifing stages prior to the RSST detec-
`tors; a 10 dB increase in received signal level may not then
`be passed through to the RSSI detectors due to the preceding
`amplifiers being unable to deliver a 10 dB increase in output
`signal. The RSSI detectors typically then register a lower
`increase in signal strength than is actually received. These
`inaccuracies may vary between mobile phones or change
`with temperature or other conditions that vary in operation.
`Therefore, there is a need for an improved meansto account
`for such inaccuracies.
`
`SUMMARYOF THE INVENTION
`
`invention to
`therefore, an object of the present
`is,
`It
`provide improved and more accurate methods and systems
`for calculating a received signal strength indication.
`It is a further object of the present invention to provide
`methods and systems which calculate a received signal
`strength indication which is compensated for non-linearity
`in the received signal which might otherwise cause the
`calculated signal strength measurement to not accurately
`represent the actual signal strength.
`These objects are provided according to the present
`invention by taking first and second signal strength readings
`with the receiver set at first and second knowngain levels,
`respectively. The signal strength measurement may then be
`compensated based on the two measurements. In particular,
`the difference between the expected change in the signal
`strength and the change actually measured by the receiver
`may be used, according to an embodiment of the present
`invention, to generate a compensated signal strength mea-
`surement.
`
`According to one embodimentof the present invention,
`the compensated signal strength measurement is compen-
`sated for non-linear characteristics of the receiver. In a
`further embodimentof the present invention the second gain
`level is less than the first gain level. The methods of the
`present invention are particularly beneficial where the the
`first and second signal strength measurements are logarith-
`mic RSSIsignals.
`In a further aspect of the present invention, before obtain-
`ing a first signal strength measurement, an expected strength
`of a next signal strength measurementof the received signal
`is determined. If the expected signal strength is less than a
`predetermined criteria, ie., if no compensation is expected
`to be needed, the second signal strength measurementof the
`received signal is not obtained and the compensated signal
`strength measurementisthe first signal strength.
`In another embodiment of the methods of the present
`invention, operations for generating a compensated signal
`
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`6,006,077
`
`3
`strength measurement include determining an expected dif-
`ference between the first signal strength measurement and
`the second signal strength measurement based on thefirst
`gain level and the second gain level. The actual difference
`between the first signal strength measurement and the sec-
`ond signal strength measurement
`is then generated. The
`actual difference is compared to the expected difference to
`provide a signal strength compensating factor. The compen-
`sated signal strength measurementis generated based on the
`signal strength compensating factor. The compensated sig-
`nal strength measurement may be generated by summingthe
`signal strength compensating factor and the first signal
`strength measurement to provide the compensated signal
`strength measurement. Alternatively, a previously deter-
`mined compensation factor associated with the second sig-
`nal strength measurement may be obtained from a memory
`means. The signal strength compensating factor, the previ-
`ously determined compensation factor associated with the
`second signal strength measurement and the first signal
`strength measurement are then summed to provide the
`compensated signal strength measurement.
`In a further aspect of the present invention, the compen-
`sated signal strength measurement
`is transmitted to the
`network for use in mobile assisted handover. Alternatively,
`the compensated signal strength measurement may be aver-
`aged with an earlier compensated signal strength measure-
`ment to provide an averaged signal strength measurement
`for transmission to the network.
`
`4
`assisted handover, as more accurate data may be reported
`from the mobile to the cellular system on the relative
`strength of the signal from various available base stations.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
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`FIG. 1 is a block diagram illustrating a signal receiver
`apparatus according to an embodimentof the present inven-
`tion.
`
`FIG. 2 is a graphical illustration of non-linear signal
`strength detection characteristics.
`FIG. 3 is a flowchart illustrating operations according to
`an embodiment of the present invention.
`FIG. 4 is a flowchart illustrating operations according to
`an embodiment of the present invention.
`
`DETAILED DESCRIPTION OF ILLUSTRATED
`EMBODIMENTS
`
`The present invention now will be described more fully
`hereinafter with reference to accompanying drawings, in
`which preferred embodiments of the invention are shown.
`This invention may, however, be embodied in manydifferent
`forms and should not be construedas limited to the embodi-
`
`ments set forth herein; rather, these embodiments are pro-
`vided so that this disclosure will be thorough and complete,
`and will fully convey the scope of the invention to those
`skilled in the art.
`
`Referring now to FIG. 1, an embodiment of a single
`receiver apparatus 10 according to the present invention will
`now bedescribed. FIG. 1 illustrates a receiver circuit block
`diagram for a wireless device such a mobile phone. Receiver
`10 includes an antenna 12 or other means for receiving
`signals over a communications medium. While it is to be
`understood that the present invention may be applied to
`signals over wired communication links, the benefits of the
`present
`invention are particularly applicable in wireless
`communications environments such as radio frequency
`transmissions over air and cellular telephone communica-
`tion networks.
`
`In another aspect of the present invention, a table of
`compensation factors may be dynamically maintained.
`According to this aspect of the methods of the present
`invention, a previously determined compensation factor
`associated with the second signal strength measurementis
`obtained. The previously determined compensation factor
`associated with the second signal strength measurementis
`adjusted based on the calculated signal strength compensat-
`ing factor to provide an updated compensation factor asso-
`ciated with the second signal strength measurement.
`Asystem for measuring a strength of a received signal is
`also provided according to the present invention. The system
`Signals received by antenna 12 are processed through
`includes a receiver capable of operating at a first gain and a
`transmit/receive duplexer 14. Duplexer 14 provides a swit-
`chable connection between antenna 12 and receive circuit 16
`second gain. Also included is a received signal strength
`indication generating circuit electrically connected to the
`and transmit circuit 18. A receive signal
`is routed by
`receiver which provides a signal strength indication corre-
`duplexer 14 to dual band front end circuit 20. This signal is
`sponding to a strength of a signal received by the receiver.
`provided to front end amplifier 22. In the illustrated embodi-
`A compensated signal strength measurement generating
`ment of FIG. 1, duplexer 14 may be a duplexingfilter for
`circuit which characterizes the strength of the received
`simultaneous transmit and receive in an AMPS modeor,
`signal based onafirst signal strength measurement at the
`alternatively, a transmit/receive switch for operating in a
`50
`first gain and a second signal strength measurementat the
`time-duplex mode such as the D-AMPS mode, or,
`in a
`second gain is operatively connected to the received signal
`dual-band telephone, may comprise a duplexingfilter at 800
`strength indication generating circuit. In one embodimentof
`MHz (the AMPS band) and a transmit/receive switch at
`the systems of the present invention,
`the system further
`1900 MHz (the PCS band) in which only the D-AMPS
`includes meansfor causing the receiver to operate at one of
`time-duplex mode is used. In a dual-band telephone oper-
`the first gain or the second gain.
`ating at both 800 MHz and 1900 MHz,the front-end radio
`While the present invention has been summarized above
`frequency amplifier 22 and first super heterodyne downcon-
`primarily with respect
`to the methods of the present
`verter 23 comprise circuits adapted to both frequency bands,
`invention,it is to be understood that the present invention is
`with means to select the operating frequency band under
`also directed to systems for carrying out
`the operations
`control of the control processor 24.
`described above with respect to the method aspects of the
`As illustrated in FIG. 1, the down conversion takes place
`present invention as will be described more fully herein.
`in dual-band front end circuit 20 for with processor 24
`The present invention is particularly beneficial when the
`providing band selecting control information to front end
`received signal, is a signal received over a communications
`circuit 20. Received signals are down converted in front end
`medium at a receiver station from a sender station. More
`circuit 20 to a suitable first intermediate frequency (first IF).
`The signal is then filtered using first IF filter 26. This signal
`is down converted once more in second mixercircuit 30 to
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`particularly, the present invention is beneficial for use where
`the communications medium is a wireless communications
`
`medium such as a cellular system which utilizes mobile
`
`a second IF. Further filtering and amplification takes place at
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`6,006,077
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`5
`second IF filter 32 and amplifier 34 to yield a hard limited
`second IF signal from which amplitude modulation has been
`removed in addition to an RSSI signal approximately pro-
`portional to the logarithm of the receive signal strength
`containing the removed amplitude information. The hard
`limited second IF signal containing phase information plus
`the RSSI signal containing amplitude information are passed
`to the receive/transmit signal processor circuit 36 where they
`are processed, for example, using the method of U'S. Pat.
`No. 5,048,059 which has been previously incorporated
`herein by reference.
`Receive signal processing by receive/transmit processing
`circuit 36 includes, among other things, analog to digital
`conversion of the RSSI signal. The digitized RSSI value is
`provided to control processor 24 for use in constructing a
`SACCH message to report received signal strength to the
`communication network for mobile assisted handover
`(MAHO)purposes.
`Transmit signal processing by circuit 36 converts SACCH
`messages of voice or user data signals such as FAX signals
`to I, Q modulation for modulating the transmitter using I, Q
`modulator circuit 40. I, Q modulator circuit 40, as illustrated
`in FIG. 1, is part of dual-band circuit 18 which provides for
`a transmit carrier frequency which is separated from the
`selected receive frequency by a specified amounttypically
`referred to as the duplexing spacing. Duplex spacing is
`typically 45 MHz for 800 MHz band operation and 8.04
`MHzfor 1800 MHz bandoperation according to the D-AMP
`standard IS 136. While additional details of receiver 10 are
`illustrated in the embodiment of FIG. 1, they will not be
`discussed further herein as they are not required to under-
`stand the benefits and operations of the present invention
`and further as they are knownto those of ordinary skill in the
`art.
`
`Control processor 24, in addition to providing means for
`controlling the band select of front end circuit 20, further
`includes meansfor controlling whetherfirst RF amplifier 22,
`located in front end circuit 20 prior to the first down
`converter, operates at a first, fill gain or a second, reduced
`gain. According to one embodimentof the present invention,
`a reduced gain in the order of 20 dBis obtained by switching
`off the RF amplifier 22 currentor, alternatively, by reducing
`its current under the control of a control signal from the
`control processor 24. The control processor 24 further
`provides means to detect when RSSI measurement values
`fall in a region where non-linearity of the RSSI detector
`characteristic or compression in the first RF amplifier 22 or
`second down converter 30 may reduce the measurement
`accuracy. This would normally only occur for strong signals
`that can be successfully received even with a 20 dB gain
`reduction in RF amplifier 22. Consequently,
`the control
`processor 24, upon detecting RSSI measurements of suffi-
`cient magnitude to be in the range or distortion could be
`introduced, causes a controlled reduction in the gain of RF
`amplifier 22 and then initiates a second RSSI measurement
`with the gain reduced.
`The difference in the RSS] measurements with and with-
`
`out the gain reduction should correspond to the controlled
`gain reduction itself, assuming linearity. Therefore, if the
`gain reduction measured does not correspond to the con-
`trolled or expected gain reduction, this is indicative of a
`non-linearity in the RSSI detection characteristics of
`receiver 10. The difference in RSSI values less the expected
`difference corresponding to the selected gain reduction is
`then indicative of the amount of non-linearity and may be
`used, according to the present invention, by control proces-
`sor 24 to compute a corrected RSSI value which has been
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`compensated for this non-linearity. The corrected RSSI
`values are then used for reporting MAHO measurements to
`the network using the SACCH. Corrected RSSI values may
`also, optionally, be used for improved logpolar signal pro-
`cessing.
`Operations for an embodimentof the present invention for
`a particular non-linear received signal will now be explained
`more fully with reference to FIG. 2. FIG. 2 shows a
`non-linear RSSI detection characteristic caused by compres-
`sion of amplifier stages prior to the detectors which may,
`typically, be encountered at high signal levels. As illustrated
`in FIG. 2, when the RF amplifier gain is reduced by, for
`example, 20 dB, the RSSI change actually measured will
`likewise be 20 dB at small signal levels. Thisis illustrated
`by the region of FIG. 2 below -70 dBm on the X axis where
`the actual detected signal strength, as illustrated by the solid
`line, substantially tracks the linear approximationillustrated
`by the dotted line.
`The illustrated example of FIG. 2 reflects the RSSI
`measurementvalue on the Y axis showing a range of zero to
`240 approximating the value range which would typically be
`encountered for an 8-bit analog to digital conversion of an
`RSSI signal for processing by control processor 24. For
`purposes of the illustrated embodiments of the present
`invention, such an 8-bit analog to digital conversion will be
`assumed although it is to be understood that the benefits of
`the present invention may be obtained with other resolutions
`and are not limited to 8-bit analog to digital converters.
`As is further illustrated in the example of FIG. 2, at
`progressively higher signal levels, the measured values of
`the solid line deviate from the linear approximation. The
`amount by which the RSSI changes, as illustrated, is less
`than 20 dB, indicating the amount of compression taking
`place over a 20 dB range of signal level. This can be best
`understood with reference to points A and B on FIG. 2. As
`illustrated in FIG. 2, the actual received signal level for point
`Ais -50 dBm. The RSSI measurementis made with an 8-bit
`
`AtoD converter yielding an energy value between zero and
`255 with the least significant bit (at small signal levels) of
`0.5 dB perbit and the zero to 255 energy range corresponds
`nominally to the signal level range -127.5 dBm to 0 dBm.
`For this arrangement,
`the expected reading based on the
`nominally linear RSSI detection curve for a signal level of
`-50 dBm is, therefore, 155. In general, the expected reading
`for a signal level of X dBm is given by the formula 255 -2X.
`As can be seen by the readingsat point Aof FIG. 2, however,
`the actual reading at -50 dBmis not 155, but is instead 151.
`This indicates an error of 4 counts or, approximately, 2 dB
`(at 0.5 dB per bit). The apparent signal level is therefore,
`detected at -52 dBm as compared with the true value of -50
`dBm.
`
`According to the methods and systems of the present
`invention, a second set of readings is taken at point B to
`provide information for use in compensating the RSSI
`measurement. Control processor 24 reducesthe gain of front
`end amplifier 22 by a controlled amount, for example, 20 dB
`as illustrated in FIG. 2. Therefore, the actual signal level
`passed through to the RSSI detectors at point B is expected
`to be -70 dBm,i.e., 20 dB lowerthan the previous -50 dBm.
`An RSSIreading is taken again at point B with the expected
`reading as 115. However, as can be seen from FIG. 2, some
`non-linearity is still present and the actual reading obtained
`with the 20 dB program gain reduction is not 115, but
`instead is 114. This reflects an error of 0.5 dB at the -70
`dBm signal level.
`The change in the signal readings from 151 to 114, a
`decrease of 37, compares with the expected change of
`
`8
`
`8
`
`

`

`6,006,077
`
`7
`reading of 40 bits based on 0.5 dB perleast significant bit
`(LSB). This indicates that the compression at a reading of
`151 is 3 units (or bits) more than the compression at a
`reading of 114. The control processor 24, according to the
`illustrated embodiment of the present invention, therefore
`adds a compensating amountof3 units to the reading of 151.
`Further accuracy may be provided by also adding to the
`RSSI reading a compensating amount previously deter-
`mined and stored for the reading of 114. For example,
`assuming that a similar RSSI compensating measurement
`sequence had previously taken place and determined a
`compensating amount of one unit per reading of 114, then
`the reading of 151 illustrated in FIG. 2 would be compen-
`sated by adding three units plus an additional one unit to
`obtain a compensated RSSI value of 155. This would
`compensate the RSSI value andset it equal to the expected
`value, thereby correcting the 2 dB error in the reading and
`providing a more accurate value for the signal strength
`measurement.
`
`In the illustrated embodiment, control processor 24 may
`also estimate the compensation factor to be added at other
`signal strengths between -70 dBm and -50 dBm by assum-
`ing that the 3 units of compression over this 20 dB range
`would be proportionally less over a smaller range. For
`example, at -60 dBm,
`the control processor 24 would
`anticipate 1.5 units of compression, as -60 dBm is half way
`between -—70 and -—50 dBm,therefore half the number of
`units of compression is anticipated for a linear model. This
`is added to the amount of compression already estimated on
`a previous occasion for -70 dBm (assumed to be 1 unit
`above) to obtain a total of 2.5 units for -60 dBm. In this way,
`the control processor 24 may compute the expected com-
`pression for every signal level (RSSI reading) overthe range
`-50 dBm to -70 dBm (corresponding to uncorrected RSSI
`readings of 114 to 151) and store them in a memory means
`such as a look-up table in Electrically Erasable and Repro-
`grammable Memory (EEPROM)or other, preferably non-
`volatile, storage means. By repeating the procedure on other
`occasions while receiving other signal strengths, the table
`maybefilled with learned correction values for each reading
`level (0-255).
`Oncethe table is filled, future computations will result in
`new values of compensation for a signal level reading that
`may or may notbe identical to a previously estimated value.
`Whenthis occurs, the new estimated values can be substi-
`tuted or alternatively averaged with the previous values. For
`example, the previous value can be adjusted a fraction of the
`way towards the new value, for example, “6” or other
`reciprocal power of two maybe chosento facilitate division
`by a nght-shift of the difference between the old and new
`values in simple microprocessors.
`As will be appreciated by those of skill in the art, the
`above described aspects of the present invention in FIGS. 1,
`and 2 may be provided by hardware, software, or a combi-
`nation of the above. While the various components of the
`systemsof the present invention have beenillustrated in part
`as discrete elements in FIG. 1, they may, in practice, be
`implemented by a microcontroller including input and out-
`put ports and running software code, by custom or hybrid
`chips, by discrete components or by a combination of the
`above.
`
`FIG. 3 showsa flow chart furtherillustrating operations of
`an embodimentof the present invention. At block 100,a first
`signal strength measurement of the received signal
`is
`obtained with receiver 10 set at a first gain level. A second
`signal strength measurement of the received signal
`is
`obtained with receiver 10 set at a second gain level at block
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`102. At block 104, the expected difference betweenthefirst
`signal strength measurement and the second signal strength
`measurement is determined based onthe first gain level and
`the second gain level. For example, if the gain level is
`dropped by 20 dB, the expected difference between the two
`readings would be 20 dB. The actual difference between the
`first signal strength measurement and the second signal
`strength measurement is also generated at block 104 by
`subtracting the two measurementvalues.
`At block 106, a previously determined compensation
`factor associated with the second signal strength measure-
`mentis obtained, preferably from a memory storage means
`operatively associated with receiver 10. Also at block 106,
`the actual difference is compared to the expected difference
`to provide a signal strength compensating factor. A com-
`pensated signal strength measurementis generated at block
`108 based on the signal strength compensating factor and the
`previously determined compensation factor associated with
`the second signal strength measurement. The compensated
`signal strength measurement,
`in one embodiment of the
`present
`invention,
`is generated by summing the signal
`strength compensating factor,
`the previously determined
`compensation factor associated with the second signal
`strength measurementandthe first signal strength measure-
`ment. The compensated signal strength measurement
`is
`transmitted to the communication network for use in mobile
`assisted handover at block 110.
`
`The above-described procedure functions both for RSSI
`curves that give too low a reading and for RSSI curvesthat
`sometimesyield too high a reading,that is for both concave
`and convex curves as well as S-shaped curves. Optionally,
`an initial table of compensation values can be determined in
`the factory during production testing and programmed into
`the EPROM or EEPROMofthe receiver 10. Subsequently,
`after delivery to a customer, the table can be read from such
`memory into Random Access Memory (RAM)for updating
`based on RSSI measurements made during further use.
`Values updated in RAM may berewritten to EEPROM ifit
`is desired to remember the updated values for future use.
`Alternatively,
`the factory-programmed values may be
`recalled every time receiver 10 is switched on and updated
`values produced only for that period of use.
`In a TDMAsystem such as the GSM/PCS1900 system or
`in the DAMPS/IS54/IS136 system, spare time between
`transmit and receive timeslots is used by the mobile phone
`receiver 10 during active conversation to make RSSI mea-
`surements on other base station’s signals. The network will
`generally previously have downloaded to the mobile phone
`a list of the channel numbersof surrounding basestations on
`which measurements should be made. In GSM,roughly 220
`measurements per second are typically made and in
`D-AMPS 50 measurements are made on typically 6 sur-
`rounding base stations. Thus, repeat measurements on th