United States Patent
`Gilhousen et al.
`
`[19]
`
`l|||||||||l|||l||Illlllllllllllllllllllllllllllllllllllll|l|||l||l|||l||||l
`US005l01501A
`
`[11] Patent Number:
`
`5,101,501
`
`[45] Date of Patent:
`
`Mar. 31, 1992
`
`[54] METHOD AND SYSTEM FOR PROVIDING A
`SOFT HANDOFF IN COMMUNICATIONS IN
`A CDMA CELLULAR TELEPHONE SYSTEM
`
`Primary Examiner-—-Curtis Kuntz
`Assistant Examiner—-Andrew Faile
`Attorney, Agent, or Firm—Russe1l B. Miller
`
`[57]
`
`ABSTRACT
`
`In a cellular telephone system a system for directing
`communications between a mobile user and cell-sites as
`
`a mobile user changes cell-site service areas. The mobile
`user includes an apparatus for, while in communication
`with another system user via one cell-site, determining
`a transition of the mobile user from the cell-site service
`area to the service area of another cell-site. The system
`includes circuitry responsive to the indication for cou-
`pling communications between the mobile user and the
`other system user via the new cell-site while the mobile
`user also remains in communication with the system
`user via the first cell-site. The system further includes
`apparatus responsive to the coupling of the communica-
`tions between the mobile user and the other system user
`via the new cell-site for terminating the communica-
`tions between the mobile user and another system user
`via cell-site with communications continuing between
`the mobile user and the system user via the new cell-
`site.
`
`23 Claims, 4 Drawing Sheets
`
`[75]
`
`Inventors: Klein S. Gilhousen; Roberto
`Padovnni, both of San Diego; Charles
`E. Wheatley, III, Del Mar, all of
`Calif.
`
`[73] Assignee: Qunlcomm Incorporated, San Diego,
`Calif.
`'
`[21] App]. No.: 433,030
`
`[22] Filed:
`
`Nov. 7, 1989
`
`Int. Cl.5 ..................... .. H040 7/00; H04M 11/00
`[51]
`[52] U.S. Cl. ...................................... .. 455/33; 455/34;
`455/56; 379/60; 375/1
`[58] Field of Search ............................ .. 455/17, 32-34,
`455/54, 56, 67; 379/59, 60, 63; 375/1
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6/1974 Hamrick .......................... 379/60
`9/1978 Frost ............. ..
`455/33
`9/1980 Cooper et al.
`..
`455/33
`8/1983 Hass et al.
`..... ..
`455/33
`8/1986 Goldman
`455/33
`8/1988 Schmidt .............................. .. 455/33
`
`
`
`3,819,872
`4,112,257
`4,222,115
`4,398,063
`4,608,711
`4,765,753
`
`TO/ FROM
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`PST N
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`
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`I4
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`
`24b
`
`200
`
`CLEARWIRE 1008
`
`

`
`U.S. Patent
`
`Mar. 31, 1992
`
`Sheet 1 of 4
`
`5,101,501
`
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`
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`
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`
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`
`

`
`U.S. Patent
`
`Mar. 31, 1992
`
`Sheet 2 of 4
`
`5,101,501
`
`44
`
`30
`
`ANTENNA
`
`32
`
`SEARCHER
`
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`
`FIG. 2
`
`
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`
`
`
`
`I COMBINER
`8 DECODER
`
`
`
`
`
`
`

`
`U.S. Patent
`
`Mar. 31, 1992
`
`Sheet 3 of 4
`
`5,101,501
`
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`
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`
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`
`TO/FROM MTSO
`DIGITAL SWITCH
`
`

`
`U.S. Patent
`
`Mar. 31, 1992
`
`Sheet 4 of 4’
`
`5,101,501
`
`TO/ FROM
`PSTN
`
`TO/FROM OTHER
`CELL-S|TES
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`
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`
`FIG. 4
`
`

`
`1
`
`5,101,501
`
`METHOD AND SYSTEM FOR PROVIDING A
`SOIT I-IANDOFF IN COMMUNICATIONS 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 system for controlling handoff 10
`III communications of cell-site stations with a mobile
`unit in a code division multiple access (CDMA) cellular
`telephone system.
`II. Description of the Related Art
`The use of code division multiple access (CDMA) 15
`modulation techniques is but 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- 10
`ulation schemes such as amplitude companded single
`sideband (ACSSB) are known, CDMA has significant
`advantages over these other modulation techniques.
`The use of CDMA techniques in a multiple access com-
`munication system is disclosed in U.S. patent applica- 25
`tion Ser. No. 06/921,261, filed Oct. 17, 1986, entitled
`“SPREAD SPECTRUM MULTIPLE ACCESS
`COMMUNICATION SYSTEM USING SATEL-
`LITE OR TERRESTRIAL REPEATERS”, now
`US. Pat. No. 4,901,307 assigned to the Assignee of the 30
`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, commu- 35
`nicate through satellite repeaters or terrestrial base sta-
`tions (also known as cell-site stations, or for short, cell-
`sites) using code division multiple access (CDMA)
`spread spectrum communication signals.
`In using
`CDMA communications, the frequency spectrum can 40
`be reused multiple times thus permitting an increase in
`system user capacity. The use of CDMA techniques
`results in a much higher spectral efficiency than can be
`achieved using other multiple access techniques.
`In the conventional cellular telephone systems the 45
`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 50
`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 55
`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 60
`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 65
`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
`
`2
`border. When the signal level falls below the predeter-
`mined threshold value, the cell-site asks system control-
`let to determine whether a neighboring cell-site re-
`ceives 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.
`I-landoff 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 call will be discontinued
`if the handoff to the new cell-site is unsuccessful. There
`are many reasons that a failure in handoff may occur.
`Handoff can fail if there is no idle channel available in
`
`the neighboring cell for communicating the call. Hand-
`off can also fail if another cell-site reports hearing the
`mobile telephone in question, when in fact this cell-site
`actually hears a different mobile unit using the same
`channel in a completely different cell. This reporting
`error will result in the call being switched to a wrong
`cell, typically one in which signal strength is insufficient
`to maintain communications. Furthermore should the
`mobile telephone fail to hear the command to switch
`channels, the handoff will fail. Actual operating experi-
`ence indicates that handoff failures occur frequently
`which questions the reliability of the system.
`Another common problem in the conventional tele-
`phone system occurs when the mobile telephone is near
`the border between two cells. In this situation the signal
`level tends to fluctuate at both cell-sites. This signal
`level fluctuation results in a ping-ponging situation in
`which repeated requests are made to hand the call back
`and forth between the two cell-sites. Such additional
`unnecessary handoff requests increase the possibility of
`the mobile unit incorrectly hearing the channel switch
`command or fail in hearing the command at all. Fur-
`thermore, the ping-ponging situation raises the possibil-
`ity that the call will be discontinued if it is unnecessarily
`transferred to a cell in which all channels are currently
`in use and thus unavailable for accepting the handoff.
`It is therefore an object of the present invention to
`provide in a cellular telephone system improvements in
`call handoff between cell-sites and thus provide greater
`service reliability.
`SUMMARY OF THE INVENTION »
`
`In a CDMA cellular telephone system, the same fre-
`quency band is used for all cells. The CDMA waveform
`properties that provide processing gain are also used to
`discriminate between signals that occupy the sa.me fre-
`quency band. A mobile telephone or unit thus need not
`switch frequencies when handoff of the call is made
`from one cell-site to another. Furthermore, the proba-
`bility that the call will be discontinued if the handoff
`command is received in error is substantially reduced.
`In a CDMA cellular telephone system, each cell-site
`has a plurality of modulator-demodulator units or
`
`

`
`5,101,501
`
`3
`spread spectrum modems. Each modem 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. In the present invention a handoff scheme is em-
`ployed 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.
`
`When mobile unit communications are firmly estab-
`lished with the new cell-site, e.g. the mobile unit is well
`within the new cell, the old cell-site discontinues servic-
`ing the call. The just described handoff techniques can
`be considered as a “soft" handoff in communications
`between cell-sites with the mobile unit. The soft handoff
`is in essence a make-before-break switching function. In
`contrast, conventional cellular telephone systems can
`be considered as providing a break-before-make switch-
`ing function.
`In a CDMA cellular telephone system of the present
`invention, a soft handoff technique is implemented
`which also permits the mobile unit to initiate a handoff.
`The mobile unit is also permitted to determine the best
`new cell-site to which communications are to be trans-
`
`ferred to from an old cell-site. Although it is preferred
`that the mobile unit initiate the handoff request and
`determine the new cell-site, handoff process decisions
`may be made as in the conventional cellular telephone
`system. As discussed previously with respect to conven-
`tional systems, the cell-site determines when a handoff
`may be appropriate and, via the system controller, re-
`quest neighboring cells to search for the mobile unit
`signal. The cell-site receiving the strongest signal as
`determined by the system controller then accepts the
`handoff.
`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-
`
`4
`priate setup channel of that cell-site. The setup channel
`is transmitted by the cell-site using one of a plurality of
`different predetennined 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 signal 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
`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 assisting in the
`assignment of 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 assigned to service the call searches for and
`finds the mobile unit transmitted signal. The cell-site
`modem also begins transmitting an outbound signal to
`the mobile unit. The mobile unit searches for this out-
`bound signal in accordance with the signal and setup
`channel
`information provided by the new cell-site.
`
`5
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`5
`is
`When the new cell-site modern transmitted signal
`acquired, the mobile unit switches over to listening to
`this signal. The mobile unit then transmits a control
`message indicating that handoff is complete. The con-
`trol message is provided by either or both of the old and
`new cell-site modems to the system controller. In re-
`sponse to this control message the system controller
`switches the call over to the new cell-site modem alone
`while discontinuing the call through the old cell-site
`modem. The old cell-site modem then enters a pool of
`idle modems available for reassignment.
`As an additional improvement, the handoff process
`can introduce a second mode of operation. This second
`mode is referred to herein as the cell-site diversity
`mode. The subject matter on the cell-site diversity
`mode is further disclosed in copending U.S. patent ap-
`plication entitled “DIVERSITY RECEIVER IN A
`CDMA CELLULAR TELEPHONE SYSTEM", Ser.
`No. 07/432,552, filed Nov. 7, 1989, by the inventors
`hereof and assigned to the Assignee of the present in-
`vention.
`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. One of the receivers is used for the
`scanning function, while the two other receivers are
`used as a two channel diversity receiver. During opera-
`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 from
`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.
`two
`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 signals on the strongest two paths of the four paths
`available from the original cell-site and from the new
`cell-site. The data demodulation process uses infonna-
`tion from both of these receivers in a diversity combin-
`ing operation. The result of this diversity combining
`operation is a greatly improved resistance to deleterious
`fading that may occur in the multi-paths cellular tele-
`phone environment.
`Although different types of diversity combining tech-
`niques are known in the art, the present invention uses
`diversity combining to significantly advance the quality
`and reliability of communications in a mobile cellular
`telephone system. In the present invention a form of
`maximal ratio combining is utilized. The signal-to-noise
`ratio is determined for both paths being combined with
`the contributions from the two paths weighted accord-
`ingly. Combining is coherent since pilot signal demodu-
`lation 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 for each cell-site receiver. The system
`controller then combines the two versions of the mobile
`
`5
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`5,101,501
`
`6
`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 handoff process in the cell diversity mode is
`initiated as previously discussed. The mobile unit deter-
`mines that a neighboring cell-site transmitted signal is of
`a signal strength great enough to allow good quality
`demodulation of the signal. The mobile unit transmits a
`control message to the current cell-site indicating the
`identity of this new cell-site and a request for the cell
`diversity mode. The cell-site then relays the cell-site
`identity and request to the system controller.
`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 implemented.
`The cell-site diversity mode is terminated when the
`mobile unit determines that only one cell-site is provid-
`ing adequate signals for quality demodulation. The mo-
`bile unit then sends a control message indicative of the
`cell-site to remain in communication upon termination
`of the cell-site diversity mode. The cell-site diversity
`mode may also be terminated by the system controller if
`the system were to become overloaded with an insuffi-
`cient number of modems available to support all mobile
`unit requests for this mode of operation. The cell-site
`diversity mode as discussed is implemented by decision
`being made at the mobile unit to operate in the cell-site
`diversity mode. However, it should be understood that
`the cell-site diversity mode can be implemented with
`the decisions for operation in this mode being made at
`the system controller.
`The present invention provides a substantial improve-
`ment over present cellular telephone systems with re-
`spect to mobile unit handoff. The make-before-break
`handoff mechanism of the present invention is a signifi-
`cant
`improvement
`in overall system reliability with
`lower service disruption. The implementation of a cell-
`site diversity mode provides further improvements over
`conventional cellular telephone systems by providing
`additional system reliability and quality in communica-
`tions.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features and advantages of the present invention
`will become more apparent from the detailed descrip-
`tion set forth below when taken in conjunction with the
`drawings in which like reference characters correspond
`throughout and wherein:
`
`

`
`7
`
`5,101,501
`
`FIG. 1 is a schematic overview of an exemplary
`CDMA cellular telephone system in accordance with
`the present invention;
`FIG. 2 is a block diagram of a mobile unit telephone
`configured for CDMA communications in a CDMA
`cellular telephone system;
`FIG. 3 is a block diagram of a cell-site equipment in
`a CDMA cellular telephone system; and
`FIG. 4 is a block diagram of a mobile telephone
`switching office equipment.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`An exemplary telephone system in which the present
`invention is embodied is illustrated in FIG. 1. The sys-
`tem illustrated in FIG. 1 utilizes CDMA modulation
`techniques in communication between the system mo-
`bile units or mobile telephones, and the cell-sites. Cellu-
`lar systems in large cities may have hundreds of cell-site
`stations serving hundreds of thousands of mobile tele-
`phones. The use of CDMA techniques readily facilitates
`increases in user capacity in systems of this size as com-
`pared to conventional FM modulation cellular systems.
`In FIG. 1, system controller and switch 10, also re-
`‘ferred to as mobile telephone switching office (MTSO),
`typically includes interface and processing circuitry for
`providing system control to the cell-sites. Controller 10
`also controls the routing of telephone calls from the
`public switched telephone network (PSTN) to the ap-
`propriate cell-site for transmission to the appropriate
`mobile unit. Controller 10 also controls the routing of
`calls from the mobile units, via at least one cell-site to
`the PSTN. Controller 10 may direct calls between mo-
`bile users via the appropriate cell-site stations since such
`mobile units do not
`typically communicate directly
`with one another.
`
`Controller 10 may be coupled to the cell-sites by
`various means such as dedicated telephone lines, optical
`fiber links or by microwave communication links. In
`FIG. 1, three such exemplary cell-sites, 12, 14 and 16
`along with an exemplary mobile unit 18, which includes
`a cellular telephone, are illustrated. Arrows 20a-20b
`define the possible communication link between cell-
`site 12 and mobile unit 18. Arrows 22a-22b define the
`possible communication link between cell-site 14 and
`mobile unit 18. Similarly, arrows 24a-24b define the
`possible communication link between cell-site 16 and
`mobile unit 18.
`
`The cell-site service areas or cells are designed in
`geographic shapes such that the mobile unit will nor-
`mally be closest to one cell-site. When the mobile unit is
`idle, i.e. no calls in progress, the mobile unit constantly
`monitors the pilot
`signal
`transmissions from each
`nearby cell-site. As illustrated in FIG. 1 the pilot signals
`are respectively transmitted to mobile unit 18 by cell-
`sites 12, 14 and 16 respectively upon communication
`links 20b, 2217 and 24b. The mobile unit then determines
`which cell it is in by comparing pilot signal strength
`transmitted from these particular cell-sites.
`In the example illustrated in FIG. 1, mobile unit 18
`may be considered closest to cell-site 16. When mobile
`unit 18 initiates a call, a control message is transmitted
`to the nearest cell-site, cell-site 16. Cell-site 16 upon
`receiving the call request message, signals system con-
`troller 10 and transfers the call number. System control-
`ler 10 then connects the call through the PSTN to the
`intended recipient.
`
`l0
`
`15
`
`20
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`25
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`30
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`35
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`40
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`45
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`50
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`65
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`8
`Should a call be initiated within the PSTN, controller
`10 transmits the call information to all the cell-sites in
`the area. The cell-sites in return transmit a paging mes-
`sage to the intended recipient mobile unit. When the
`mobile unit hears a page message,
`it responds with a
`control message that is transmitted to the nearest cell-
`site. This control message signals the system controller
`that this particular cell-site is in communication with
`the mobile unit. Controller 10 then routes the call
`through this cell-site to the mobile unit.
`Should mobile unit 18 move out of the coverage area
`of the initial cell-site, cell-site 16, an attempt is made to
`continue the call by routing the call through another
`cell-site. In the handoff process there are two different
`methods of initiating the handoff of the call or routing
`through another cell-site.
`The first method, called the cell-site initiated handoff,
`is similar to the handoff method employed in the origi-
`nal first generation analog cellular telephone systems
`currently in use.
`In the cell-site initiated handoff
`method, the initial cell-site, cell-site 16, notices that the
`signal transmitted by mobile unit 18 has fallen below a
`certain threshold level. Cell-site 16 then transmits a
`handoff request to system controller 10. Controller 10
`relays the request to all neighboring cell-sites, 14, 12 of
`cell-site 16. The controller transmitted request includes
`information relating to the channel, including the PN
`code sequence used by mobile unit 18. Cell-sites 12 and
`14 tune a receiver to the channel being used by the
`mobile unit and measure the signal strength, typically
`using digital techniques. If one of cell-sites 12 and 14
`receivers report a stronger signal than the initial cell-site
`reported signal strength, then a handoff is made to this
`cell-site.
`
`The second method of initiating a handoff is called
`the mobile initiated handoff. The mobile unit
`is
`equipped with a search receiver which is used to scan
`the pilot signal transmission of neighboring cell-sites 12
`and 14, in addition to performing other functions. If a
`pilot signal of cell-sites 12 and 14 is found to be stronger
`than the pilot signal of cell-site 16, mobile unit 18 trans-
`mits a control message to the current cell-site, cell-site
`16. This control message contains information identify-
`ing the cell-site of greater signal strength in addition to
`information requesting a handoff to this cell-site. Cell-
`site 16 transfers this control message to controller 10.
`The mobile initiated handoff method has various
`advantages over the cell-site initiated handoff method.
`The mobile unit becomes aware of changes in paths
`between itself and the various neighboring cell-sites
`much sooner and with less effort than the cell-sites are
`capable of doing. However, to perform a mobile initi-
`ated handoff, each mobile unit must be provided with a
`searching receiver to perform the scanning function.
`However,
`in the exemplary embodiment described
`herein of a mobile unit CDMA communications capa-
`bility,
`the search receiver has additional
`functions
`which require its presence.
`FIG. 2 illustrates in block diagram form an exemplary
`mobile unit. The mobile unit includes an antenna 30
`which is coupled through diplexer 32 to analog receiver
`34 and transmit power amplifier 36. Antenna 30 and
`diplexer 32 are of standard design and permit simulta-
`neous transmission and reception through a single an-
`tenna. Antenna 30 collects transmitted signals and pro-
`vides them through diplexer 32 to analog receiver 34.
`Receiver 34 receives the RF frequency signals from
`diplexer 32 which are typically in the 850 MHz fre-
`
`

`
`9
`quency band for amplification and frequency downcon~
`version to an IF frequency. This frequency translation
`process is accomplished using a frequency synthesizer
`of standard design which permits the receiver to be
`tuned to any of the frequencies within the receive fre-
`quency band of the overall cellular telephone