Method for CDMA handoff in the vicinity of highly sectorized cells

A method for improved sector handoff within a sectorized communication cell utilizing a wireless communication systems. A sector handoff of a mobile radio telephone is performed in accordance with a first set of parameters if the mobile radio telephone is not in close proximity to a base antenna. Sector handoff of the mobile radio telephone is performed in accordance with a second different set of parameters when the mobile radio telephone is in close proximity of the base antenna. The second different set of parameters are utilized in close proximity to the base antenna to avoid drops and adverse handoffs in high interference, low signal strength areas and provide improved sector handoff when the mobile radio telephone encounters distorted conditions, such that improved sector handoff can be achieved.

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates in general to an improved wireless
 communication system and in particular, the present invention relates to
 an improved CDMA cellular telephone system. Still more particularly, the
 present invention relates to a method which allows improved sector handoff
 performance in highly sectorized CDMA cells in the vicinity of a base
 station antenna.
 2. Description of the Related Art
 Mobile radio telephones and mobile telephone systems are well known in the
 prior art. A mobile telephone system generally includes a mobile hand held
 radio telephone transceiver, and a base station connected to a local
 telephone switching system by a landline. Typically, a cellular network
 has an assigned set of landline telephone numbers that allows users of a
 mobile hand held transceiver to place and receive calls within a limited
 range of the base station's antenna.
 Mobile hand held transceivers which are specifically designed for telephone
 communication are often called cellular telephones or mobile radio
 telephones. Cellular telephone systems have developed rapidly since the
 early 1980s. Persons equipped with small mobile communication devices,
 such as mobile radio telephone, can utilize a cellular radio system to
 communicate in the same way as a hard wired household telephone which
 utilizes landline carriers.
 Due to the increase in cellular telephone utilization, digital
 communication is gaining popularity over analog communication. Digital
 communication topologies can simultaneously support many more users than
 analog topologies in a given frequency spectrum. Since a limited number of
 frequencies and channels are available, analog systems can only support a
 very limited number of simultaneous users. A digital radio system can
 handle more than 20 times the capacity of a traditional analog radio
 system in the same frequency spectrum. Digital systems employ methods
 where multiple users share the same frequency. This concept is commonly
 referred to as "spread spectrum communication." Distinct digital channel
 sharing topologies have emerged, such as code division multiple access
 (CDMA), global system for mobile communication (GSM) and time division
 multiple access (TDMA). A digital system has a considerably larger data
 transmission capacity than an analog system and higher capacity translates
 to higher revenues for cellular system owners.
 Typically, a mobile radio telephone system assigns a fixed base transceiver
 to geographic areas. In a typical cellular system, a geographical area is
 divided into small areas, called cells. Coverage is typically measured as
 a radius from the base station antenna. Each cell has a predefined
 coverage radius, for example, large cells commonly referred to as "mega
 cells" can have a coverage of over 20 kilometers (13 miles). Additionally,
 macro cells have a coverage from 1 to 20 kilometers, micro cells have a
 coverage of approximately 1 kilometer, while pico cells have a coverage of
 only 100 meters. Each cell has its own radio transceiver commonly referred
 to as a base station. If necessary, each cell can be further subdivided
 into smaller cells through cell splitting and/or sectorization by steering
 antenna patterns.
 In a typical CDMA system, a honeycomb type pattern of cells is created
 which utilizes the same range of radio frequencies. In many respects, CDMA
 is superior to TDMA and Frequency Division Multiple Access (FDMA) because
 CDMA systems can utilize precisely the same frequency spectrum in all
 sectors without significant interference among sectors. In CDMA the same
 set of frequencies can also be utilized from cell site to cell site. A
 CDMA topology assigns a different binary sequence or code to a transmitted
 signal to identify the message for an individual mobile transceiver. This
 allows a single frequency to serve multiple users.
 The specifications for CDMA operation are outline in the Electronic
 Industries Association/Telecommunications Industry Association (TIA/EIA)
 IS-95-A & TSB74 standards document entitled Mobile Station-Base Station
 Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular
 System or CDMA Principles of Spread Spectrum Communication, by Andrew J.
 Viterbi.
 The correlator, a subcircuit within the mobile transceiver, accepts only
 energy from identified binary sequences or codes and de-spreads across the
 spectrum. The mobile receiver correlates its input with the desired
 carrier and receives the appropriate data. Received signals having codes
 which do not match the receiver codes are not de-spread in bandwidth and
 contribute only to noise. The signal to noise ratio of the desired signal
 is enhanced at the detector of the mobile transceiver by a factor known as
 the processing gain. One advantage to a CDMA system is that the receiver
 is not sensitive to worst case interference, but to the average
 interference.
 CDMA has often been dismissed as unworkable in the mobile radio environment
 because of signal strength differential, as some users are located near
 the base station and others are located far away. To accommodate the far
 away users, a spreading bandwidth must be thousands of times greater than
 the data rate, making the spectral efficiency intolerable. If a reasonable
 bandwidth is chosen the signal cannot be received from distant users
 because the users near the base station significantly interfere. To
 overcome this inefficiency, the transmitter of each mobile is controlled
 such that the received powers from all users is roughly equal to achieve
 an interference averaging concept of power control. A similar process is
 performed on signals sent to the mobles from the base station(s).
 Computerized switching is essential to the operation of cellular radio
 communication. When a communicating mobile transceiver is switched from
 one cell to another, a transfer of channels must take place without
 interruption, or at most a brief delay. The growth of electronic switching
 systems and the development of microprocessors have made seamless
 communication possible within areas covered by cellular providers. The
 U.S. Federal Communications Commission (FCC) continues to allocate and
 license additional radio frequencies. Due to increasing popularity of
 cellular telephones, in recent years the FCC has awarded additional
 frequency bands to be utilized by cellular telephone technology.
 A cellular telephone system typically includes cellular subscriber units,
 which are portable, and cellular base stations, which are connected to the
 public telephone company via one or more cellular switching networks. Each
 cellular subscriber has an assigned cellular telephone number that allows
 the user to place and receive calls within a widespread range of the
 cellular base stations, such as throughout a metropolitan area.
 Cellular telephone systems are thus based on a structure of associated
 cells. Cells are specified geographic areas that are defined for a
 specific mobile communication system where each cell has its own base
 station(s) and controllers) interconnected with a public telephone
 network.
 Communication between base stations and mobile subscribers is established
 by negotiation protocols upon call origination. As a user passes from cell
 to cell, the cellular service allows calls in progress to be handed over
 without interruption (soft handoff) or minimal interruption (hard handoff)
 to adjacent cells thus providing seamless communication.
 Handoffs of a mobile radio telephone between cells and sectors in CDMA
 ideally occur as soft handoffs. In a CDMA system, mobile radio telephone
 stores a list of active channels (utilized for demodulation purposes)
 which are being received at acceptable levels in an "active set". The
 active set members are sector and/or cell channels that transmit and
 receive identical information with the mobile. A soft handoff occurs when
 the active set contains more than one sector and/or cell. When a
 communication path becomes weak, the mobile radio telephone will remove
 the weak channel from the active list, via protocols defined in IS-95, but
 there is no noticeable disruption in the communication link. Similarly, if
 a new sector and/or cell increases in strength, the mobile and network
 will add this new sector and/or cell to the active set of the mobile via
 IS-95 protocols which are comprised of messages and thresholds, etc.
 A hard handoff typically occurs when communication on a particular
 frequency is dropped and a new channel having a different frequency is
 acquired. For example, a termination of communication on one frequency and
 initiation of a new communication link on another frequency is a hard
 handoff. Hard handoffs typically occur when a mobile moves outside a
 coverage area and the call is switched from one service provider to
 another who utilizes a different frequency. In a CDMA system, typically,
 all cells are owned by a single service provider and operate in the same
 frequency spectrum. Therefore, soft handoffs are the prevalent method of
 handoffs. Hard handoffs can occur when adjacent cells are utilizing the
 same frequency. Typically, hard handoffs are less common than soft
 handoffs in CDMA and a hard handoffs occur according to the service
 providers system parameters and network functionality.
 A typical six sectored CDMA cell contains a twelve element antenna. Each
 antenna element can provide a highly directional radiation and reception
 pattern. As depicted in FIG. 1, a single element radiation and reception
 pattern projecting from antenna mast 6 is commonly referred to as a
 sector. A twelve element antenna provides six sectors in a radial
 configuration placed adjacent to one another separated by imaginary
 partition lines 5. Six patterns each rotated 60 degrees encircle antenna
 mast 6 and provide coverage within cell 34. As the radiation patterns are
 offset by 60 degrees, side lobes 4 also orient in 60 degree offsets. Six
 of the twelve elements are typically utilized for transmission and
 reception, while the other six are utilized for reception only, providing
 dual antenna reception diversity. Therefore, in each sector, one antenna
 is utilized for transmitting and receiving and the other antenna is
 utilized for receiving only. Radially configuring main lobe 2 in each 60
 degree sector creates significant side lobe interference in the area
 depicted by side lobe area 9.
 High capacity cells can employ 8 or 10 sectors for improved performance. An
 ideal antenna pattern has a main lobe 2 which provides clear sectorization
 at a far distance from antenna mast 6, however, near antenna mast 6 there
 is significant overlap of radiation energy due to minor lobes called side
 lobes 4 and back lobes 8. Side lobes 4 and back lobes 8 cause interference
 to other antennae in the array in side lobe area 9. Additionally, each
 antenna transmitting element also has vertical profiles, hence, vertical
 side lobes and back lobes. Therefore, within side lobe area 9 there is no
 single sector providing dominant coverage and all sectors provide weak
 coverage.
 As sectorization within cells increases, the probability of dropping a call
 near and around antenna mast 6 also increases. This is due to the
 attributes of the antenna array's reception and radiation pattern in the
 vicinity of the antenna. An antenna array inherently has undesirable
 interference as a result of overlapping side lobes of individual antenna
 elements near antenna mast 6. Each sector receives a significant amount of
 interference due to leakages, side lobe radiation and back lobe radiation
 due to the inherent electromagnetic characteristics near the antenna. Side
 lobes are most prevalent near antenna mast 6 and signal quality can be
 severely effected in this region. In a CDMA system, all sectors of an
 antenna may be received by the mobile at approximately the same strength
 near or under antenna mast 6.
 Each sector produces a continuously broadcasted pilot signal so any mobile
 radio telephone scanning the spectrum for usable pilots can decide which
 sector will provide the best communication link. When moving towards an
 antenna mast, the change from one dominant pilot to a number of weak
 pilots of equal strength can happen very suddenly. This is due to the lack
 of dominant coverage within side lobe area 9. Less than desirable coverage
 of all sectors occurs due to side and back lobes overlap. Alternatively, a
 change from many weak pilots to a, yet unknown dominant pilot can also
 occur as the mobile radio telephone moves away from antenna mast 6.
 Sudden changes in reception due to a change from a single strong pilot to
 multiple weak pilots can degrade the forward link to such an extent that
 is not possible for the network to instruct the mobile radio telephone to
 add the newly acquired weak pilots, hence, the lone communication link may
 not support the mobile radio telephone. Without newly acquired pilots,
 eventually the call will drop or the frame error rates will increase,
 impacting system capacity and performance and the user may be
 disconnected.
 Additionally, as more sectors per cell are implemented, the Pilot
 Carrier-to-Interference (C/I) ratio of each of the sectors near and around
 antenna mast 6 are relatively equivalent. Pilot C/I ratios are low around
 the base of antenna mast 6 and therefore there is no dominant sector to be
 identified by a mobile radio telephone. Pilot C/I ratios at the base of
 antenna mast 6 are often lower than the threshold level utilized by a
 mobile radio telephone to add carriers and initiate a communication link.
 In other words, the pilots from adjacent sectors may not be acquired by a
 mobile radio telephone near the base of antenna mast 6 because received
 signals are below specified threshold levels and a call in progress can
 drop or degrade drastically without identification of potential alternate
 paths for diversity reception and demodulation.
 Currently, sectored cells function at about 80% forward link loading during
 peak time. Alternately described, each sector on the average utilizes 80%
 of its total high power amplifier power to service users. With typical 16%
 pilot power, the pilot C/I for each sector at or around the antenna mast
 is approximately -14.8 dB. Therefore, in a six sector cell the mobile
 radio telephone may be moving in one of the sectors (its pilot C/I about
 -7 dB when dominant) toward the base station antenna and suddenly, the
 current sector and all other sectors drop to -14.8 dB. Since, the add
 pilot threshold is typically -14.0 dB, a handoff transaction can not occur
 because the additional sectors do not trigger handoff and consequently the
 additional sectors are not acquired by the mobile radio telephone. A
 single sector reception having a pilot of -14.8 dB is insufficient to
 sustain adequate communication and a call may drop or significantly
 degrade under these circumstances. This impacts call quality and also cell
 capacity as the single sector must transmit at very high power in
 attempting to maintain the call with the mobile.
 At lower loading, for example 60% loading, the pilot C/I of all the sectors
 are approximately -13.5 dB, hence a message to add communication links to
 the mobile radio telephone would be sent by the mobile radio to the base
 station. However, a sharp decline of signal strength at the base of
 antenna mast 6 causes protocol problems. For example, the movement of a
 mobile radio telephone producing a change from a single pilot C/I ratio
 from -5.7 dB to all pilots at -13.5 dB may also cause a call to drop. As
 soon as the mobile radio telephone detects that pilots exist which are all
 above the add pilot threshold, the mobile attempts to send an add signal
 to the network, via the first original sector. The communication
 instructing the mobile radio telephone to add other sectors may not be
 received by the mobile radio telephone because of the significant
 interference from the other sectors. The add message is re-attempted a
 number of times (IS-95 protocol) but after a number of attempts the system
 drops the call. Otherwise, the add sector message may get through on
 subsequent re-sends, but the frame error rate and transmit power
 requirements of the base station will increase impacting, capacity, and
 performance until additional sectors are acquired.
 At present, the only solution available is to data-fill the add pilot
 threshold and drop pilot thresholds to -16.0 and -18.0 dB respectively.
 However, this extends the cells handoff boundary outwards as the mobile
 radio telephone moves away from a base antenna to other cells. Changing
 pilot thresholds redefine cell boundaries and creates cell to cell handoff
 problems. Lower handoff thresholds compromises resource capacity, such as
 channel elements and RF capacity as additional sectors in handoff with a
 mobile radio telephone must transmit to the mobile radio telephone
 although their signal is never utilized for demodulation purposes.
 The disadvantage of lowering thresholds is that excessive handoff may
 result, which compromises capacity. Additional software development has
 been attempted to improve handoff performance, however, this is costly. It
 should therefore be apparent that an improved method for handoff
 performance utilizing CDMA technology in highly sectorized cells in the
 vicinity of the base antenna is highly desirable.
 SUMMARY OF THE INVENTION
 It is therefore one object of the present invention to provide an improved
 wireless communication system.
 It is another object of the present invention to provide an improved CDMA
 cellular telephone system.
 It is yet another object of the present invention to provide improved
 sector handoff performance in highly sectorized CDMA cells in the vicinity
 of a base station antenna.
 The foregoing objects are achieved as is now described. A method for
 improved sector handoff within a sectorized communication cell utilizing a
 wireless communication systems is provided. A sector handoff of a mobile
 radio telephone is performed in accordance with a first set of parameters
 if the mobile radio telephone is not in close proximity to a base antenna.
 Sector handoff of the mobile radio telephone is performed in accordance
 with a second different set of parameters when the mobile radio telephone
 is in close proximity of the base antenna. The second different set of
 parameters are utilized in close proximity to the base antenna to avoid
 drops and adverse handoffs in high interference, low signal strength areas
 and provide improved sector handoff when the mobile radio telephone
 encounters distorted conditions, such that improved sector handoff can be
 achieved.
 The above as well as additional objects, features, and advantages of the
 present invention will become apparent in the following detailed written
 description.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT
 With reference now to the figures and in particular with reference to FIG.
 2, there is depicted a block diagram of a cellular telephone system 10 in
 accordance with the present invention. Cellular telephone system 106
 includes a telephone company phone system (TELCO) 14, connected by
 telephone landlines to control terminal 16 which in turn is coupled, also
 by telephone landlines, to cellular base station 18 that is located near
 the center of a cell within a mobile cellular telephone system.
 Mobile radio telephone 12 communicates with cellular base station 18 via
 mobile rx/tx antenna 20, base tx/rx antenna 22 and base rx antenna 24.
 Typically, in a CDMA cellular system base tx/rx antenna 22 and base rx
 antenna 24 are part of a multi-element antenna atop of a tower or
 building. A multi-element antenna creates antenna patterns having defined
 sectors. Implementing two antennae per sector for receive purposes is
 common to improve system performance. A twelve element antenna can create
 six sectors having two antenna elements per sector. A high degree of
 sectorization is desirable to accommodate heavy communication density.
 A wide variety of other antennae can also be utilized in conjunction with a
 wireless communication system such as cellular telephone system 10. Also,
 landlines utilized in association with cellular telephone system 10 are
 lines that can be placed in areas on land or inland waterways, and can
 include twisted pair lines, coaxial cables, and fiber optic cables
 utilized in overhead, direct buried and underground applications.
 Additional cellular base stations may be located throughout a geographic
 area to provide telephone service to mobile radio telephone 12. Mobile
 radio telephone 12 may be installed in a vehicle, or a transportable unit
 which has a battery installed in a carrying case, or mobile radio
 telephone could be a self contained hand held unit. Mobile radio telephone
 12 includes mobile rx/tx antenna 20 to transmit and receive on cellular
 radio channels. Currently in the United States, the cellular radio
 channels are in the frequency band from 824-894 MHz. More particularly, in
 the United States, a total bandwidth of 50 MHz is allocated for cellular
 mobile service, the 50 MHz is distributed between 824 MHz and 849 MHz, and
 also between 869 MHz and 894 MHz.
 Recently, the FCC has allocated bandwidths in the 1.9 GHz frequency range
 for cellular communication. All of these frequency allocations can be
 utilized in accordance with a preferred embodiment of the present
 invention. Cellular telephone system 10 as described herein is presented
 for illustrative purposes only and should not be considered as limiting
 the scope of the present invention.
 FIG. 3 is a block diagram of a mobile radio telephone 12 which may be
 utilized in accordance with the present invention. However, the mobile
 radio telephone depicted and described should not be construed to limit
 the scope of the present invention.
 A typical mobile radio telephone 12 includes rx/tx antenna 20, cellular
 telephone transceiver 120, microcomputer 130, keypad 140, display 180,
 audio switch 150, and audio I/O 160, including speaker 162 and microphone
 164. Microcomputer 130 is a computer contained within a single chip
 microprocessor. Less powerful than mini-computers and mainframe computers,
 microcomputer 130 is nevertheless capable of complex tasks involving the
 processing of logical operations, such as controlling radio frequency
 circuitry. Microcomputer 130 includes a central processing unit (CPU)
 (i.e., not shown), which is the computational and control unit of
 microcomputer 130, and which interprets and executes instructions for
 mobile radio telephone 12. Cellular telephone transceiver is comprised of
 cellular receiver 122, cellular transmitter 124 and audio circuitry 126.
 FIG. 4 depicts a pictorial representation illustrative of a cellular
 telephone system 30 in accordance with the present invention. Cellular
 telephone system 30 is analogous to cellular telephone system 10 depicted
 in FIG. 2, and may incorporate utilization of communications devices such
 as mobile radio telephone 12 depicted in FIG. 3. Cellular telephone system
 30 is composed of a variety of cells 34. Each cell encompasses a specified
 geographic area. Each geographic area has its own base station 36
 comprised of an antenna and base station controller 33. Base station
 controller 33 is interconnected with a public telephone network (not
 shown). Ideally, cells 34 are arranged adjacent to another cell to create
 a honeycomb pattern of cells. Cells 34 can cover a large metropolitan
 area. Actual cell coverage is highly irregular and the neat hexagons
 coverage depicted is idealistic. The actual coverage depends upon the
 strength of each base station signal, elevation of the base station
 antenna and any obstructions to the propagation of the signal.
 Specific cell radii are not necessary features of the present invention.
 Cells can have ranges from several hundred feet from the tower up to a
 radius of approximately 25 miles from the tower. The specific numbers
 described herein are for demonstrative purposes only and are not necessary
 features of the present invention. In FIG. 4, each cell 34 is subdivided
 into sectors. CDMA cells are typically divided into three sectors.
 However, six sectors coverage provides improved performance over three
 sector coverage. Illustrated is a CDMA system having six sectors per cell
 distinguished by sector lines 38. In a CDMA system, the honeycomb pattern
 of cells 34 can repeatedly utilize the same range of radio frequencies by
 controlling interference.
 A street or road 32, (not to scale) such as a metropolitan highway, is
 depicted as extending through cells 34 contained within cellular telephone
 system 30. Thus, a user can travel along road 32 through cells and while
 travelling, perform cellular mobile telephone operations. Cellular
 telephone system 30 further includes Base Station Controller (BSC) 33 at
 each cell. A Mobile Telephone Switching Office (MTSO) 37 is a central
 office connected to base station controller 33 to control switching in
 cellular telephone systems.
 MTSO 37 is comprised of a fielded monitoring and relay stations (not shown)
 for switching calls from cell sites to wire line central offices typically
 present at (TELCO) 14 depicted in FIG. 2. The relay station may be coupled
 to a public switched telephone network (PSTN), made up of local networks,
 exchange area networks, and long distance carrier networks that
 interconnect telephones and other communication devices on a worldwide
 basis.
 MTSO 37 can control system operations in a digital cellular network. For
 example, MTSO 37 can control calls, track billing information, and locate
 cellular subscribers. MTSO 37 is a switch that provides services and
 coordination between mobile radio telephone users in a network, such as
 cellular telephone system 30, and external networks (not shown).
 In FIG. 5, six sector antenna tower 41 within cell 34 typically includes
 twelve or more directional antennae 40 atop antenna tower 41. Antenna
 tower 41 is normally centrally located in each cell 34. A twelve element
 antenna utilizing six antenna elements for radiating can effectively
 radiate into six sectors offset by sixty degrees. In FIG. 5, sector 47 is
 highlighted. Area 62 at the base of antenna tower 41 projecting outward to
 a radius r illustrates how the beam of sector 47 projects to the ground at
 predetermined distance r from the base of antenna tower 41 and provides
 dominant coverage in this area to mobile radio telephone 12.
 The region defined by r has side lobes/back lobes of other sectors in the
 cell which interfere with sector 47 near the base of the antenna. Briefly
 referring back to FIG. 1, side lobes 4 for a single sector are depicted.
 Side lobes from six sectors each rotated 60 degrees in relation to one
 another illustrates the interference encountered in side lobe area 9.
 Referring back to FIG. 5, certain areas within area 62 may have six pilot
 signals with equivalent strength as determined by mobile radio telephone
 12. With heavy loading conditions on the base station all six pilot
 signals can be below acceptable levels and disrupt sector to sector
 handoff triggering in area 62.
 Base station 36 is connected to the public telephone company (i.e. TELCO)
 via one or more switching networks (not shown). Each cellular subscriber
 or mobile radio telephone 12 has an assigned cellular telephone number
 that allows the user to place and receive calls within a widespread range
 of each base station 36. During a cellular telephone call, a communication
 link is established. When a mobile radio telephone moves from one cell to
 another or from sector to sector within a cell, the communication link
 must be handed off in order to maintain the communication link. In a CDMA
 system each cell utilizes the same frequency. However, the pilot channels
 are effectively different because the codes utilized for spreading are
 un-correlated from pilot channel to pilot channel. The specifications for
 CDMA operation are outline in the Electronic Industries
 Association/Telecommunications Industry Association (EIA/TIA) IS-95
 standards document entitled U.S. CDMA Cellular Systems.
 As a mobile unit utilizing CDMA communication moves within a region of CDMA
 coverage, the mobile will soft handoff from one sector to another to
 maintain a continuous communication link. As the mobile demodulates
 received information and transmits modulated information, it also searches
 for other useful unique pilot signals transmitted by adjacent sectors. The
 active set (list of currently demodulated channels) and the candidates set
 (potential active set members) contained within the mobile radio telephone
 are constrained in size by the IS-95 standard. A maximum of six sectors
 are allowed in a mobile radio telephones' active set, thus, only six
 channels or sectors can be received by a mobile radio telephone in
 compliance with the IS-95 specification. Therefore, a maximum handoff
 scenario is a six way soft handoff.
 If a mobile radio telephone detects a new pilot whose pilot strength
 (carrier to total interference ratio) is above an upper threshold (T_ADD),
 the mobile radio telephone will place the sector associated with the new
 pilot into its candidate set and send a Pilot Strength Measurement Message
 (PSMM) to base station controller (BSC) 33 via the current active
 sector(s). If the active set contains more than one sector, then the
 mobile is in a soft handoff mode.
 In accordance with the present invention, mobile radio telephone 12 sends a
 PSMM request for entry of candidates into the mobile's active set. Base
 station controller 33 instructs mobile radio telephone 12 to add the new
 pilot via an Extended Handoff Direction Message (EHDM) transmitted by all
 the sectors in the current active set. The mobile radio telephone on
 receiving the message, adds the candidate to the active set and
 acknowledges the EHDM and reception from a new sector is acquired. After a
 soft handoff is accomplished, a Handoff Completion Message (HCM) is
 transmitted by mobile radio telephone 12 to base station controller 33.
 If mobile radio telephone 12 detects that the current sector active pilot
 signal strength has dropped below a lower threshold (T_DROP) and remains
 consistently below this threshold level for a predetermined amount of time
 (T_TDROP), then a PSMM is transmitted to the network by mobile radio
 telephone 12, requesting that such a sector be dropped from the active
 set. The HDM and HCM communications follow in order as explained above to
 complete the soft handoff by dropping sectors having low signal strength.
 The T_ADD, T_DROP, T_TDROP and other related soft handoff parameters are
 typically stored in a database of base station controller 33. Each
 sector/cell may have a different value for these parameters. Therefore,
 when mobile radio telephone 12 enters into soft handoff with these
 parameters, base station controller 33 must decide what value of T_ADD,
 T_DROP, T_TDROP, etc. to send to mobile radio telephone 12. The decision
 guidelines utilized for parameters are typically, the minimum of the
 various T_ADD values in dB, the minimum of the T_TDROP values in dB and
 the maximum of the T_DROP values in seconds. Therefore, if the sectors
 involved in handoff with mobile radio telephone 12 have different minimum
 and maximum threshold values, the decision logic is such that parameters
 sent to mobile radio telephone 12 via the EHDM will change the defined
 handoff regions. Typical values for T_ADD, T_DROP and T_DROP are -14.0 dB,
 -16.0 dB and 4 seconds respectively. The location for storage of the
 parameters and which subsystem initiate the handoff should not be
 construed to limit the present invention, various embodiments can be
 efficiently utilized by the present invention.
 Mobile radio telephone 12 can demodulate a number of received signals from
 sectors in the assigned cell and sectors from adjacent cells to accomplish
 multipath reception. Multipath reception must be coordinated within mobile
 radio telephone 12. Multipath coordination is accomplished by RAKE fingers
 within mobile radio telephone 12. Typically, mobile radio telephone 12 has
 three RAKE fingers as part of its signal demodulating hardware. Mobile
 radio telephone 12 will attempt to demodulate the highest quality paths
 from any of the links, at any given time, by a method referred to as
 maximal ratio combining.
 For additional information regarding CDMA communication refer to the
 Electronic Industries Association/Telecommunications Industry Association
 (TIA/EIA) IS-95-A & TSB74 Standards document entitled Mobile Station-Base
 Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum
 Cellular System or CDMA Principles of Spread Spectrum Communication, by
 Andrew J. Viterbi.
 Hence, excessive handoffs sacrifice channel capacity as sectors utilize
 power to transmit to mobile radio telephone 12 even though mobile radio
 telephone 12 may not be utilizing the transmission because it is receiving
 other sectors. As the number of fingers in a mobile radio telephone RAKE
 receiver are increased, sector power utilization is improved.
 Optimal handoff between sectors is required to maintain frame error rates
 and dropped call probabilities below acceptable target levels. Further,
 satisfactory handoff and diversity of signal paths must be provided to
 mobile radio telephone 12. Efficient sector handoff is especially
 beneficial in regions of low speeds and low multipath, as the effects of
 fading more seriously affect the forward link transmit requirements.
 Without handoff, and at low speeds, the energy per bit to total noise
 power spectral density or (Eb/No) requirements for successful forward link
 demodulation are very high. A high Eb/No requirement mandates higher base
 station transmit powers and impacts system capacity. Hence, sector handoff
 provides diversity against fading because each multipath fades
 independently and the Eb/No requirement for a given grade of service is
 much lower.
 If mobile radio telephone 12 detects strong pilots along its drive route
 which are above T_ADD, typically, mobile radio telephone 12 will add these
 strong pilots to its candidate list and notify base station controller 33.
 It is critical to add candidates to the active set because a RAKE finger
 could lock onto a stronger multipath reception which would improve
 performance. Additionally, if sectors with strong pilots are not added to
 the active set, the sectors produce significant interference and are not
 utilized for signal demodulation. Therefore, improved future performance
 by switching to a sector having a stronger pilot can not be accomplished
 without mobile radio telephone 12 identifying the stronger sector as a
 candidate.
 Additionally, each sector contributes to the multiple access interference
 to RAKE fingers which are locked onto other sectors. Therefore, by
 utilizing the principle of maximal ratio combining, the fingers can lock
 onto various sectors/paths and optimize communication performance.
 It is also important to add candidates into the active set because pilots
 that are comparable in strength often become immediately stronger, and
 significantly detract from the forward link performance by interfering.
 Therefore, if these sectors are added into the active set, a finger can
 quickly lock onto stronger paths as soon as they become the dominant
 sectors and the user does not receive degraded service.
 The pilot carrier to interference ratio (C/I) can be represented by the
 following equation:
 ##EQU1##
 Where, sector I is the sector of interest, total pilot power
 recvd.sub.sector i is the total pilot power received by the mobile in
 sector i, N.sub.o is the total noise power spectral density, W is the
 bandwidth, N Figure.sub.mobile is the noise figure of the mobile receiver
 and received.sub.sectork is the total power received by mobile radio
 telephone 12 from sector k.
 Referring to FIG. 6, cell 34 is divided into sectors 39 labeled 1 through
 N. RTD.sub.in 62 and RTD.sub.out 66 are also illustrated. Proximity of
 mobile radio telephone 12 to base antenna 80 can be determined utilizing
 Round Trip Delay (RTD). RTD is twice the propagation delay interval of the
 radio wave between base antenna 80 and mobile radio telephone 12.
 Therefore, the mobile radio telephone's distance from base antenna 80 can
 be estimated by utilizing the product of half the propagation delay and
 the velocity of radio waves in air.
 The RTD is the electrical distance between base antenna 80 and mobile radio
 telephone 12, not the physical distance. Hence, due to multipath
 considerations, the physical distance and the electrical distance are not
 exactly the same. It is preferred that RTD is computed utilizing the
 earliest arriving multipath. However, many methods are available to
 determine distance in the present invention and it can be appreciated by
 those having skill in the art that the method described should not limit
 the scope of the present invention.
 RTD can be calculated by the switching network utilizing synchronization
 techniques. The base stations are coordinated or synchronized to "true"
 time utilizing a Global Position System (GPS). Signals are sent from base
 antenna 80 and received by mobile radio telephone 12 offset by one way
 delay, (OWD). When received, mobile radio telephone 12 synchronizes the
 clock of the transmitter to received data. When mobile radio telephone 12
 transmits, it is offset or delayed in time by OWD. After receiving a
 transmission from a mobile radio telephone 12, base station 36 can compute
 the RTD between mobile radio telephone 12 and base station 36. RTD is
 utilized by the present invention to modify handoff threshold levels
 between sectors in the vicinity of base antenna 80.
 In accordance with a preferred embodiment of the present invention, two
 sets of handoff parameters, T_ADD, T_DROP, T_TDROP and others are
 maintained by base station controller 33. The first set (upper set) of
 handoff parameters contain higher values of T_ADD, T_DROP etc., and mobile
 radio telephone 12 utilizes the upper set when mobile radio telephone 12
 is not in the vicinity of base antenna 80. The second set (lower set) of
 handoff parameters contains lower values of T_ADD, T_DROP, etc. Mobile
 radio telephone 12 is instructed to utilize the lower set via an EHDM
 message from base station controller 33 when mobile radio telephone 12 is
 in the vicinity of base antenna 80.
 In a preferred embodiment, RTD is determined by the drive test disclosed
 above. However, alternate methods of proximity to the antenna could be
 utilized without departing from the scope of the present invention. The
 RTD value is monitored by base stations in the active set and is stored in
 base station controller 33. When RTD exceeds the determined value, a
 different set of parameters are utilized for the soft handoff procedure.
 Referring to FIG. 7, a high level flow chart in accordance with the present
 invention is depicted. The process starts at block 50 and proceeds to
 block 52. The mobile telephone continuously or selectively monitors the
 RTD. The value of RTD is compared to preselected parameters of RTD in and
 RTD.sub.out. If RTD&lt;RTD.sub.in, the method queries the base station to
 determine if the lower set of handoff parameters are being utilized as in
 block 54. If the lower set are being utilized, the method returns to
 monitor RTD. If the lower set of parameters are not being utilized, an
 EHDM is sent by the base station to the mobile station with the lower set
 of handoff parameters in accordance with block 58. When the conditions are
 satisfied a soft handoff will occur pursuant to the lower set of
 parameters.
 If a mobile radio telephone is initially powered up and attempts to
 initiate a call when it is in close vicinity to the antenna mast, the
 upper set of parameters may prevent initialization. It is preferred that
 on power up, a mobile radio telephone loads the lower set of parameters
 supplied by the network or the paging channel via the "system parameters
 message," referred to IS-95. After a communication link is established,
 the proper parameters will be loaded in accordance with the method of FIG.
 7. Alternately, if during power up, initialization can not be achieved,
 the mobile can automatically load lower parameters and re-attempt
 initialization. Many embodiment could be utilized on power up and the
 method disclosed should not be utilized to limit the scope of the present
 invention.
 Referring back to block 52, if RTD&gt;RTD.sub.out, the method determines if
 the upper set of handoff parameters are being utilized as in block 56. If
 the upper set of parameters are being utilized the method returns to again
 monitor RTD as in block 52. If the upper set of parameters are not being
 utilized, the base station sends an EHDM message containing the upper set
 of parameters in accordance with block 66. When communication conditions
 dictate, a soft handoff occurs pursuant to the upper set of parameters.
 In a preferred embodiment, the lower set of handoff parameters is sent to
 the mobile radio telephone before it gets in close proximity to the
 antenna. This enables the mobile radio telephone to search for pilot C/I
 signals utilizing the lower level parameters and the mobile radio
 telephone can add marginal channels and avoid being dropped when the
 mobile radio telephone enters a high interference area with high network
 loading.
 Once the mobile radio telephone moves out of the high interference area or
 RTD.sub.in, the upper set of parameters are sent by the base station
 controller to the mobile radio telephone. Therefore, cell boundaries are
 not affected in cell to cell handoff procedures.
 While the invention has been particularly shown and described with
 reference to a preferred embodiment, it will be understood by those
 skilled in the art that various changes in form and detail may be made
 therein without departing from the spirit and scope of the invention.