United States Patent
`
`[191
`
`[1 1]
`
`Patent Number:
`
`5,109,390
`
`Gilhousen et al.
`Date of Patent:
`[45]
`Apr. 28, 1992
`
`|||||||ll|l|llllllllll|||||||l|H|||lIlllllllllllllllllllIlllllllllllllllll
`USOO510939OA
`
`4,736,460 4/1988 Rilling ............................... .. 455/283
`4,752,969
`6/1988 Rilling
`4,765,753
`8/1988 Schmidt
`4,797,950
`1/1989 Rilling ................... ..
`
`
`
`Primary Examiner—Bernarr E. Gregory
`Attorney, Agent, or Firm—Russell B. Miller
`
`[57]
`
`ABSTRACT
`
`DIVERSITY RECEIVER IN A CDMA
`CELLULAR TELEPHONE SYSTEM
`
`Inventors: Klein S. Gilhousen; Roberto
`Padovani, both of San Diego; Charles
`E. Wheatly, III, Del Mar, all of Calif.
`
`Assignee: Qualcomin Incorporated, San Diego,
`Calif.
`
`Appl. No.: 432,552
`
`[541
`
`[75]
`
`[73]
`
`1211
`
`N0V- 7, 1989
`[221 Filed:
`[51]
`Int. Cl.5 ............................................. H04L 27/30
`
`[52] U_5_ CL ._ __ _
`_.___ 375/1; 375/40;
`375/100; 455/10; 455/59; 455/63; 455/70;
`455/332; 455/523; 455/55_1; 370/13
`[58] Field of Search .................... .. 370/18, 50; 455/33,
`455/54, 56, 59, 10, 52, 68, 70; 375/40, 100
`References Cited
`
`[56]
`
`A spread spectrum receiver subsystem for utilization in
`a CDMA cellular telephone having a searcher receiver
`for scanning the time domain so as to use the PN pro-
`Cessing Sal“ ‘md ilme di5°"imi‘_"‘"i°“ P’°P_°"if"5 °f
`spread spectrum coding to determine the location in'the
`time domain and the received signal strength of multiple
`receptions of a pilot signal traveling upon one or more
`physical propagation paths to reception. The searcher
`receiver provides a control signal indicative of the re-
`ceived pilot signals of greatest strength and correspond-
`ing time relationship. A data receiver receives spread
`135- PATENT DOCUMENTS
`spectrum communication signals accompanying each
`3,351,859 11/1967 Groth, Jr. et al.
`.................... .. 375/1
`received Pilot signal and is responsive to the searcher
`4,112.257 9/1978 Froms ............ ..
`. 379/60
`control signal for acquiring and demodulating a spread
`EOOIPET C1 311-
`l spectrum communication signal, concomitant with the
`,
`,
`ap CS 813 .
`’
`'
`‘
`'
`_
`4,630,283 12/1986 Schiff ...........
`375/1
`P'1°t5‘g"a1°fgre.ates.t s’g"a1s."°"gth’.a"dth“S pr°‘”d
`4 669 091
`5/1987 Nossen ___________ N
`_ 375/M
`ing a corresponding information bearing encoded out-
`4,e72:65s
`6/1987 Kavehrad et al.
`. 379/63
`P1" Signal-
`4,694,467
`9/1987 Mul
`................ ..
`375/1
`4,710,944 12/1987 Nossen ................................ .. 375/40
`
`
`
`
`
`30’
`
`ANTENNA
`
`44
`
`SEARCHER
`
`RECEIVER
`
`32
`
`MPLEXER
`
`DIVERSITY
`DIG HAL.
`
`DATA
`I COMBINER
`
`RECEIVER
`a DECODER
`
`18 Claims, 4 Drawing Sheets
`
`
`
`
`
`
`
`
`
`USER
`DIGITAL
`DATABAND
`
`DIGITAL
`DATA
`RECEIVER
`
`
`
`34
`
`ANALOG
`RECEIVER
`
`CONTROL
`
`PROCESSOR
`
`52
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`
`TRANSMIT
`MODULATOR
`
`T RANSMIT
`A TRANSMIT
`POWER
`POWER
`AMPLIFIER
`CONTROL
`
`
`
`
`
`
`
`CLEARWIRE 1007
`
`

`
`U.S. Patent
`
`Apr. 28, 1992
`
`Sheet 1 of 4
`
`‘
`
`5,109,390
`
`TO FROM
`TO FROM OTHER
`PSTN
`'0 CELL-SITES
`
`
`SYSTEM
`
`CONTROLLER
`
`
`
`8: SWITCH
`
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`
`24::
`
`FIG. I
`
`

`
`U.S. Patent
`
`Apr. 28, 1992
`
`Sheet 2 of 4
`
`5,109,390
`
`44
`
`
`
`SEARCHER
`
`RECEIVER
`
`3O
`
`ANTENNA
`
`32
`DIGITAL
` DIVERSITY
`DIPLEXER
`‘ DATA
`I COMBWER.
`_ RECEIVER
`a DECODER
`
`
`
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`
`34
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`
`DIGITAL
`
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`
`DATA
`RECEIVER
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`DATABAND
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`FIG. 2
`
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`POWER
`AMPLIFIER
`CONTROL
`
`
`
`52
`
`TRANSMIT
`
`MODULATOR
`
`
`
`
`
`

`
`U.S. Patent
`
`Apr. 28, 1992
`
`Sheet 3 of 4
`
`5,109,390
`
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`
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`
`

`
`U.S. Patent ‘
`
`Apr. 28, 1992
`
`Sheet 4 of 4
`
`5,109,390
`
`TO/ FROM
`PSTN
`
`TO/FROM OTHER
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`
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`
`TO OTHER
`DIVERSITY COMBINERS
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`vocooens
`
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`CELL- SITES
`
`FIG. 4
`
`

`
`1
`
`5,109,390
`
`DIVERSITY RECEIVER IN A CDMA CELLULAR
`TELEPHONE SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to cellular telephone
`systems. More specifically, the present invention relates
`to a novel and improved receiver design for enhancing
`the reliability and communications in the cellular tele-
`phone environment.
`11. Description of the Related Art
`The use of code division multiple access (CDMA)
`modulation techniques is one of several techniques for
`facilitating communications in which a large number of
`system users are present. Although other techniques
`such as time division multiple access (TDMA), fre-
`quency division multiple access (FDMA) and AM mod-
`ulation schemes such as amplitude companded single
`sideband (ACSSB) are known, CDMA has significant
`advantages over these other techniques. The use of
`CDMA techniques in a multiple access communication
`system is disclosed in U.S. Patent application Ser. No.
`06/921,261, filed Oct. 17, 1986, entitled “SPREAD
`SPECTRUM MULTIPLE ACCESS COMMUNICA-
`, TION SYSTEM USING SATELLITE OR TERRES-
`TRIAL REPEATERS”, now U.S. Pat. No. 4,901,307
`assigned to the assignee of the present invention, the
`disclosure thereof incorporated by reference.
`In the just mentioned patent, a multiple access tech-
`nique is disclosed where a large number of mobile tele-
`phone system users each having a transceiver communi-
`cate through satellite repeaters or terrestrial base sta-
`tions (also known as cell-sites stations, or for short cell-
`sites) using code division multiple access (CDMA)
`spread spectrum communication signals.
`In using
`CDMA communications, the frequency spectrum can
`be reused multiple times thus permitting an increase in
`system user capacity. The use of CDMA results in a
`much higher spectral efficiency than can be achieved
`using other multiple access techniques. In a CDMA
`system, increases in system capacity may be realized by
`controlling the transmitter power of each mobile user so
`as to reduce interference to other system users.
`In the satellite application of the CDMA communica-
`tion techniques, the mobile unit transceiver measures
`the power level of a signal received via a satellite re-
`peater. Using this power measurement, along with
`knowledge of the satellite transponder downlink trans-
`mit power level and the sensitivity of the mobile unit
`receiver, the mobile unit transceiver can estimate the
`path loss of the channel between the mobile unit and the
`satellite. The mobile unit transceiver then determines
`the appropriate transmitter power to be used for signal
`transmissions between the mobile unit and the satellite,
`taking into account
`the path loss measurement,
`the
`transmitted data rate and the satellite receiver sensitiv-
`ity.
`The signals transmitted by the mobile unit to the
`satellite are relayed by the satellite to a Hub control
`system earth station. The Hub measures the received
`signal power from signals transmitted by each active
`mobile unit transceiver. The Hub then determines the
`deviation in the received power level from that which is
`necessary to maintain the desired communications.
`Preferably the desired power level is a minimum power
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`level necessary to maintain quality communications so
`as to result in a reduction in system interference.
`The Hub then transmits a power control command
`signal to each mobile user so as to adjust or “fine tune"
`the transmit power of the mobile unit. This command
`signal is used by the mobile unit to change the transmit
`power level closer to a minimum level required to main-
`tain the desired communications. As channel conditions
`change, typically due to motion of the mobile unit, both
`the mobile unit receiver power measurement and the
`power control feedback from the Hub continually read-
`just the transmit power level so as to maintain a proper
`power level. The power control feedback from the Hub
`is generally quite slow due to round trip delays through
`the satellite requiring approximately Q of a second of
`propagation time.
`—
`One important difference between satellite or terres-
`trial base stations systems are the relative distances
`separating the mobile units and the satellite or cell-site.
`Another important different in the satellite versus the
`terrestrial system is the type of fading that occurs in
`these channels. Thus, these differences require various
`refinements in the approach to system power control
`for the terrestrial system.
`In the satellite/mobile unit channel, i.e. the satellite
`channel, the satellite repeaters are normally located in a
`geosynchronous earth orbit. As such, the mobile units
`are all at approximately the same distance from the
`satellite repeaters and therefore experience nearly the
`same propagation loss. Furthermore, the satellite chan-
`nel has a propagation loss characteristic that follows
`approximately the inverse square law, i.e. the propaga-
`tion loss is inversely proportional to the square of the
`distance between the mobile unit and the satellite re-
`peater in use. Accordingly, in the satellite channel the
`variation in path loss due to distance variation is typi-
`cally on the order of only 1-2 dB.
`In contrast to the satellite channel, the terrestrial/mo-
`bile unit channel, i.e. the terrestrial channel, the distance
`between the mobile units and the cell sites can vary ‘
`considerably. For example, one mobile unit may be
`located at a distance of five miles from the cell site while
`another mobile unit may be located only a few feet
`away. The variation in distance may exceed a factor of
`one hundred to one. The terrestrial channel experiences
`a propagation loss characteristic as did the satellite
`channel. However, in the terrestrial channel the propa-
`gation loss characteristic corresponds to an ‘inverse
`fourth-power law, i.e. the path loss is proportional to
`the inverse of the path distance raised to the fourth
`power. Accordingly, path loss variations may be en-
`countered which are on the order of over 80 dB in a cell
`having a radius of five miles.
`The satellite channel typically experiences fading that
`is characterized as Rician. Accordingly the received
`signal consists of a direct component summed with a
`multiply reflected component having Rayleigh fading
`statistics. The power ratio between the direct and re-
`flected component is typically on the order of 6-10 dB,
`depending upon the characteristics of the mobile unit
`antenna and the environment about the mobile unit.
`Contrasting the satellite channel with the terrestrial
`channel, the terrestrial channel experiences signal fad-
`ing that typically consists of the Rayleigh faded compo-
`nent without a direct component. Thus, the terrestrial
`channel presents a more severe fading environment
`than the satellite channel where Rician fading is the
`dominant fading characteristic.
`
`

`
`3
`The Rayleigh fading characteristics in the terrestrial
`channel signal is caused by the signal being reflected
`from many different features of the physical environ-
`ment. As a result, a signal arrives almost simultaneously
`at a mobile unit receiver from many directions with
`different transmission delays. At the UHF frequency
`bands usually employed for mobile radio communica-
`tions, including those of cellular mobile telephone sys-
`tems, significant phase differences in signals traveling
`on different paths may occur. The possibility for de-
`structive summation of the signals may result, with on
`occasion deep fades occurring.
`Terrestrial channel fading is a very strong function of
`the physical position of the mobile unit. A small change
`in position of the mobile unit changes the physical de-
`lays of all the signal propagation paths, which further
`results in a different phase for each path. Thus, the
`motion of the mobile unit through the environment can
`result in a quite rapid fading process. For example, in
`the 850 MHz cellular radio frequency band, this fading
`can typically be as fast as one fade per second per mile
`per hour of vehicle speed. Fading on this order can be
`extremely disruptive to signals in the terrestrial channel
`resulting in poor communication quality. ‘However,
`additional transmitter power can be used to overcome
`the problem of fading.
`The terrestrial cellular mobile telephone system typi-
`cally requires a full-duplex channel to be provided in
`order to allow both directions of the telephone conver-
`sation to be simultaneously active such as provided by
`_the conventional wired telephone system. This full-
`duplex radio channel is normally provided by using one
`frequency band for the outbound link, i.e. transmissions
`from the cell-site transmitter to the mobile unit receiv-
`ers. A different frequency band is utilized for the in-
`bound link, i.e. transmissions from the mobile unit trans-
`mitters to the cell-site receivers. According, this fre-
`quency band separation allows a mobile unit transmitter
`and receiver to be active simultaneously without feed-
`back or interference from the transmitter into the re-
`ceiver.
`
`In the conventional cellular telephone system the
`available frequency band is divided into channels typi-
`cally 30 KHz in bandwidth while analog FM modula-
`tion techniques are used. The system service area is
`divided geographically into cells of varying size. The
`available frequency channels are divided into sets with
`each set usually containing an equal number of chan-
`nels. The frequency sets are assigned to cells in such a
`way as to minimize the possibility of co-channel inter-
`ference. For example, consider a system in which there
`are seven frequency sets and the cells are equal size
`hexagons. A frequency set used in one cell will not be
`used in the six nearest or surrounding neighbors of that
`cell. Furthermore, the frequency set in one cell will not
`be used in the twelve next nearest neighbors of that cell.
`In the conventional cellular telephone system, the
`handoff scheme implemented is intended to allow a call
`to continue when a mobile telephone crosses the bound-
`ary between two cells. The handoff from one cell to
`another is initiated when the cell-site receiver handling
`the call notices that the received signal strength from
`the mobile telephone falls below a predetermined
`threshold value. A low signal strength indication im-
`plies that the mobile telephone must be near the cell
`border. When the signal level falls below the predeter-
`mined threshold value,
`the cell-site asks the system
`controller to determine whether a neighboring cell-site
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`4
`receives the mobile telephone signal with better signal
`strength than the current cell-site.
`The system controller in response to the current cell-
`site inquiry sends messages to the neighboring cell-sites
`with a handoff request. The cell-site neighboring the
`current cell-site employs special scanning receivers
`which look for the signal from the mobile unit on the
`specified channel. Should one of the neighboring cell-
`sites report an adequate signal level to the system con-
`troller, then a handoff will be attempted.
`Handoff is then initiated when an idle channel from
`the channel set used in the new cell-site is selected. A
`control message is sent to the mobile telephone com-
`manding it to switch from the current channel to the
`new channel. At the same time, the system controller
`switches the call from the first cell-site to the second
`cell-site.
`In the conventional system a break-before-
`make scheme is utilized such that no diversity reception
`is possible in overcoming fades.
`.
`_
`Furthermore should the mobile telephone fail to hear
`the command to switch channels, the handoff will fail.
`Actual operating experience indicates that handoff fail-
`ures occur frequently which questions the reliability of
`the system.
`In the conventional cellular telephone system, path
`fading deleteriously affects communications and can
`cause disruption in call service. It is therefore an object
`of the present invention to provide, in a cellular tele-
`phone system, receiver a design which facilitates recep-
`tion and processing of the strongest signals transmitted
`from one or more cell-sites, these signals being multi-
`path signals from a single cell-site or signals transmitted
`by multiple cell-sites.
`SUMMARY OF THE INVENTION
`
`In a CDMA cellular telephone system, the same fre-
`quency band is used for communication in all cells. The
`CDMA waveform properties that provide processing
`gain are also used to discriminate between signals that
`occupy the same frequency band. Furthermore the high
`speed pseudonoise (PN) modulation allows many differ-
`ent production paths to be separated, provided the dif-
`ference in path propagation delays exceed the pN chip
`duration, or one/bandwidth. If a PN chip rate of 1 MHz
`is employed in a CDMA system, the full spread spec-
`trum processing gain, equal to the ratio of the spread
`bandwidth to system data rate, can be employed against
`paths that differ by more than one microsecond in path
`delay from the desired path. A one microsecond path
`delay differential corresponds to differential path dis-
`tance of 1,000 feet. The urban environment typically
`provides differential path delays in excess of one micro-
`second, and up to 10-20 microseconds are reported in
`some areas.
`
`In narrow band modulation systems such as the ana-
`log FM modulation employed by conventional
`tele-
`phone systems, the existence of multiple paths results in
`severe multipath fading. With wideband CDMA modu-
`lation, however, the different paths may be discrimi-
`nated against in the demodulation process. This discrim-
`ination greatly reduces the severity of multipath fading.
`Multipath fading is not
`totally eliminated in using
`CDMA discrimination techniques because there will
`occasionally exist paths with delay differentials of less
`than the minimum path delay for the particular system.
`Signals having path delays on this order cannot be dis-
`criminated against in the demodulator. It
`is therefor
`
`

`
`5
`desirable that the system should provide diversity to
`further reduce the effects of fading.
`The deleterious effects of fading can be controlled
`somewhat by controlling transmitter power in the
`CDMA system. A system for cell-site and mobile unit
`power control is disclosed in copending U.S. Patent
`Application entitled “METHOD AND APPARATUS
`FOR CONTROLLING TRANSMISSION POWER
`IN A CDMA CELLULAR MOBILE TELEPHONE
`SYSTEM”, Ser. No. 07/433,031, filed Nov. 7, 1989, by
`the inventors hereof and assigned to the Assignee of the
`present invention. Furthermore the effect of multipath
`fading can be reduced in the handoff mode when the
`mobile unit
`is transitioning between cell-site service
`area with the mobile unit communicating cell-sites dur-
`ing the handoff process. The handoff scheme is dis-
`closed in copending U.S. Patent Application entitled
`“SOFT HANDOFF IN A CDMA CELLULAR
`TELEPHONE SYSTEM”, Ser. No. 07/433,030, filed
`Nov. 7, 1989, by the inventors hereof and assigned to
`the Assignee of the present invention.
`The existence of multipaths can provide path diver-
`sity to a wideband PN CDMA system. If two or more
`paths are available with greater than one microsecond
`differential path delay, two or more PN receivers can
`be employed to separately receive these signals. Since
`these signals will typically exhibit independence in mul-
`tipath fading, i.e., they usually do not fade together, the
`outputs of the two receivers can be diversity combined.
`Therefore a loss in performance only occurs when both
`receivers experience fades at the same time. Hence, one
`aspect of the present invention is the provision of two or
`more PN receivers in combination with a diversity
`combiner.
`Another aspect of the present invention is that as a
`mobile unit moves through the physical environment,
`the number of multiple paths and their signals strengths
`constantly vary. The present invention therefore uti-
`lizes a special receiver, called a searcher receiver,
`which constantly scans the time domain of the channel
`to determine the existence, the location in the time do-
`main, and the relative signal strengths of signals in the
`multiple path environment. The searcher receiver pro-
`vides control over the data receivers in tracking the best
`signals available on differing paths.
`In a CDMA cellular telephone system, each cell-site
`has a plurality of modulator-demodulator units or
`spread spectrum modems. Each modern consists of a
`digital spread spectrum transmit modulator, at least one
`digital spread spectrum data receiver and a searcher
`receiver. Each modem at the cell-site is assigned to a
`mobile unit as needed to facilitate communications with
`the assigned mobile unit. Therefore in many instances
`many modems are available for use while other ones
`may be active in communicating with respective mobile
`units. A soft handoff scheme is employed for a CDMA
`cellular telephone system in which a new cell-site
`modem is assigned to a mobile unit while the old cell-
`site continues to service the call. When the mobile unit
`is located in the transition region between the two cell-
`sites, the call can be switched back and forth between
`cell-sites as signal strength dictates. Since the mobile
`unit is always communicating through at least one cell-
`site, no disrupting effects to the mobile unit or in service
`will occur. The present invention utilizes multiple re-
`ceivers at the mobile unit which are also used in a diver-
`sity function when in the handoff process or firmly in a
`single cell.
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`6
`In the CDMA cellular telephone system, each cell-
`site transmits a “pilot carrier“ signal. This pilot signal is
`used by the mobile units to obtain initial system syn-
`chronization and to provide robust time, frequency and
`phase tracking of the cell-site transmitted signals.
`Each cell-site also transmits a “setup” channel com-
`prised of spread spectrum modulated information, such
`as cell-site identification, system timing. mobile paging
`information and various other control signals. The pilot
`signal transmitted by each cell-site is of the same spread-
`ing code but with a different code phase offset. Phase
`offset allows the pilot signals to be distinguished from
`one another resulting in distinguishment between cell-
`sites from which they originate. Use of the same pilot
`signal code allows the mobile unit to find system timing
`synchronization by a single search through all pilot
`signal code phases. The strongest pilot signal, as deter-
`mined by a correlation process for each code phase, is
`readily identifiable. The identified pilot signal corre-
`sponds to the pilot signal transmitted by the nearest
`cell-site.
`
`i.e.
`Upon acquisition of the strongest pilot signal,
`initial synchronization of the mobile unit with the stron-
`gest pilot signal, the mobile unit searches for the appro-
`priate setup channel of that cell-site. The setup channel
`is transmitted by the cell-site using one of a plurality of
`different predetermined spread spectrum codes. In an
`exemplary embodiment of
`the present
`invention,
`twenty-one different codes are used. However,
`it
`should be understood that more or less codes could be
`
`used in the setup channel as determined by system pa-
`rameters. The mobile unit then begins a search through
`all of the different codes used in the setup channel.
`When the mobile unit identifies the appropriate setup
`code for that cell-site, system information is received
`and processed. The mobile unit further monitors the
`setup channel for control messages. One such control
`message would indicate a call is waiting for transfer to
`this mobile unit.
`The mobile unit continues to scan the received pilot
`carrier signal code at the code offsets corresponding to
`neighboring cell-site transmitted pilot signals. This
`scanning is done in order to determine if the pilot signal
`emanating from neighboring cells is becoming stronger
`than the pilot signal first determined to be strongest. If,
`while in this call inactive mode, a neighbor cell-site
`pilot signal becomes stronger than that of the initial
`cell-site transmitted pilot signal, the mobile unit will
`acquire the stronger pilot signals and corresponding
`setup channel of the new cell-site.
`When a call is initiated, a pseudonoise (PN) code
`address is determined for use during the course of this
`call. The code address may be either assigned by the
`cell-site or be determined by prearrangement based
`upon the identity of the mobile unit. After a call is initi-
`ated the mobile unit continues to scan the pilot signal
`transmitted by cell-sites located in neighboring cells.
`Pilot signal scanning continues in order to determine if
`one of the neighboring cell-site transmitted pilot signals
`becomes stronger than the pilot signal transmitted by
`the cell-site the mobile unit is in communication with.
`When the pilot signal transmitted by a cell-site located
`in a neighboring cell becomes stronger than the pilot
`signal transmitted by a cell-site in the current cell, it is -
`an indication to the mobile unit that a new cell has been
`entered and that a handoff should be initiated. In re-
`sponse to this pilot signal strength determination, the
`mobile unit generates and transmits a control message to
`
`

`
`7
`the cell-site presently servicing the call. This control
`message, indicative that a new cell-site transmitted pilot
`signal is now stronger than the current cell-site trans-
`mitted pilot signal, is provided to the system controller.
`The control message further contains information iden-
`tifying the new cell-site and PN code. The control mes-
`sage as relayed to the system controller is interpreted
`that a handoff in mobile unit communications to the
`identified new cell-site is to begin.
`The system controller now begins the handoff pro-
`cess. It should be understood that during handoff the
`PN code address of the particular mobile unit which is
`to undergo the handoff process need not change. The
`system controller begins the handoff by assigning a
`modem located in the new cell-site to the call. This
`modem is given the PN address associated with the call
`in communications between the mobile unit and the
`current cell-site modem. The new cell-site modem as-
`signed to service the call searches for and finds the
`mobile unit transmitted signal. The cell-site modern also
`begins transmitting an outbound signal to the mobile
`unit. The mobile unit searches for this outbound signals
`in accordance with the signal and setup channel infor-
`mation provided by the new cell-site. When the new
`cell-site modem transmitted signal is acquired, the mo-
`bile unit switches over to listening to this signal. The
`mobile unit then transmits a control message indicating
`that handoff is complete. The control message is pro-
`vided by either or both of the old and new cell-site
`modems to the system controller. In response to this
`control message the system controller switches the call
`over to the new cell-site modem alone while discontin-
`uing the call through the old cell-site modem. The old
`cell-site modem then enters a pool of idle modems avail-
`able for reassignment.
`However, when the mobile unit is within a single cell
`service area, in which the cell-site signals are multipath
`signals, the corresponding cell-site transmitted signals
`are stronger than any other cell-site transmitted signals
`which may be received at the mobile unit. In the single
`cell mode of operation, the searcher receiver monitors
`the multipath signals and identifies the strongest as re-
`ceived on the various multipaths. The searcher receiver
`provides this information to the mobile unit control
`processor which instructs the data receivers to track the
`signals upon these strongest paths. The signals are then
`output from the data receivers where they are provided
`to a diversity combiner.
`During call handoff, mobile unit communications
`with the various cell-sites is subject to path diversity.
`These communications are also processed by the multi-
`ple receivers at the mobile unit for diversity combina-
`tion. Furthermore the signals transmitted through the
`various cell-sites are combined in a diversity combiner
`at the system controller. The present invention further
`permits what is referred to herein as the cell-site diver-
`sity mode at times other than a handoff. In this mode the
`mobile unit is permitted to communicate with various
`cell-sites on an ongoing basis.
`In the cell-site diversity mode the call is allowed to
`linger in the in-between state as described above with
`reference to the call being processed by two cell-sites.
`In the exemplary embodiment described herein with
`reference to the mobile telephone of the present inven-
`tion, a total of three demodulator processors or receiv-
`ers are utilized.'0ne of the receivers is used for the
`scanning function, while the two other receivers are
`used as a two channel diversity receiver. During opera-
`
`l0
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`15
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`20
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`25
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`30
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`35
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`45
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`50
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`55
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`65
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`5,109,390
`
`8
`tion in a single cell, the scanning receiver attempts to
`find the cell-site transmitted signal travelling upon mul-
`tiple paths to the mobile unit. These multipath signals
`are typically caused by reflections of the signals by the
`terrain buildings and other signals obstructions. When
`two or more such reflections are found, the two receiv-
`ers are assigned to the two strongest paths. The scan-
`ning receiver continues to evaluate the multiple paths to
`keep the two receivers synchronized with signals on the
`two strongest paths as path conditions change.
`In the cell-site diversity mode, the strongest paths
`from each cell-site is determined by the search receiver.
`The two receivers are assigned to demodulate the sig-
`nals on the strongest two paths of the paths available
`from the original cell-site and from the new cell-site.
`The data demodulation process uses information from
`both of these receivers in a diversity combining opera-
`tion. The result of this diversity combining operation is
`a greatly improved resistance to deleterious fading that
`may occur in the multipath cellular telephone environ-
`ment.
`
`The present invention uses diversity combining to
`significantly advance the quality and reliability of com-
`munications in a mobile cellular telephone system. In
`the present invention a form of maximal ratio combin-
`ing is utilized. The signal-to-noise ratio is determined
`for both paths being combined with the contributions
`from the two paths weighted accordingly. Combining is
`coherent since pilot signal demodulation allows the
`phase of each path to be determined.
`In the path from the mobile unit to the two cell-sites,
`path diversity reception is also obtained by having both
`cell-sites demodulate the mobile unit transmitted sig-
`nals. Both cell-sites forward their demodulated data
`signals to the system controller along with an indication
`of signal quality in the cell-sites receiver. The system
`controller then combines the two versions of the mobile
`
`unit signal and selects the signal with the best quality
`indication. It should be understood that it is possible to
`transmit
`the undecoded or even the undemodulated
`signals to the system controller in order to allow a bet-
`ter diversity combining process to be utilized.
`The system controller responds by connecting the
`call to a modem in the new cell-site. The system con-
`troller then performs diversity combining of the signals
`received by the two cell-sites while the mobile unit
`performs diversity combining of the signals received
`from the two cell-sites. The cell diversity mode contin-
`ues as long as signals received from both cell-sites are of
`a level sufficient to permit good quality demodulation.
`The mobile unit continues to search for signals trans-
`mitted from other cell-sites. If a third cell-site transmit-
`ted signal becomes stronger than one of the original two
`cell-site signals, the control message is then transmitted
`by the mobile unit via at least one current cell-site to the
`system controller. The control message indicates the
`identity of this cell-site and a request for handoff. The
`system controller then discontinues the call being com-
`municated via the weakest cell-site signal of the three
`while providing the call
`through the two strongest
`cell-sites. Should the mobile units be equipped with
`additional receivers, such as three receivers, a triple
`cell-site diversity mode may be