(12) United States Patent
`US 6,339,590 B2
`(10) Patent N0.:
`Kim
`*Jan. 15, 2002(45) Date of Patent:
`
`
`US006339590B2
`
`(54) METHOD AND SYSTEM FOR PROVIDING
`INTER-FREQUENCY HANDOFF IN A
`TELEPHONE SYSTEM
`
`(75)
`
`Inventor:
`
`Jong-Youn Kim, Kyungki-do (KR)
`
`(73) Assignee: LG Information & Communications,
`Ltd., Seoul (KR)
`
`(*) Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`154(a)(2).
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/020,040
`
`(22)
`
`Filed:
`
`Feb. 6, 1998
`
`(30)
`
`Foreign Application Priority Data
`
`Nov. 6, 1997
`
`(KR) ............................................ 97/523539
`
`Int. Cl.7 ........................... H04Q 7/00; H04B 7/216
`(51)
`(52) US. Cl.
`....................... 370/331; 370/468; 370/335;
`455/437
`
`(58) Field of Search ................................. 370/331, 332,
`370/334, 335, 328, 329, 468, 342, 441;
`455/436, 437—442
`
`(56)
`
`References Cited
`U.S. PAI'EN'I‘ DOCUMENTS
`
`figfii :
`,
`,
`5,103,459 A
`5,416,797 A
`5,533,104 A
`
`31333 giiflousen 6: ai' """""" 355/33
`rousenea....
`..
`/
`
`4/1992 Gilhousen et a1.
`375/1
`5/1995 Gilhousen et a1.
`375/705
`7/1996 Weiss et a1.
`.................. 379/70
`
`................... 370/331
`7/1997 Lee et a1.
`5,649,000 A *
`........ 370/331
`5,828,661 A * 10/1998 Weaver, Jr. et a1.
`
`5,883,899 A *
`3/1999 Dahlman et a1.
`..... 370/331
`.................... 375/358
`5,937,019 A *
`8/1999 Padovani
`5,999,816 A * 12/1999 Tiedemann, Jr. et al.
`455/437
`6,078,570 A *
`6/2000 Czaja et a1.
`370/331
`
`FOREIGN PATENT DOCUMENTS
`
`GB
`W0
`
`2 297 460 A
`WO 97/40593
`
`7/1996
`10/1997
`
`* cited by examiner
`
`Primary Examiner—Ricky Ngo
`(74) Attorney, Agent, or Firm—Fleshner & Kim, LLP
`
`(57)
`
`ABSTRACT
`
`In the preferred embodiment, the transmission length of the
`non-full rate frame is shortened frame in order for the mobile
`terminal 100 to monitor other frequencies With a single
`receiver. Modulated symbols of non-full rate frame are
`transmitted With reduced repetition maintaining the sum of
`repeated symbol energy. Generally, for 1/n (n=2,4,8) length
`transmission, 1/m (min) rate frame repeated m/n times and
`transmitted With n/m symbol energy of the full rate frame.
`The system may use rate limitation to generate a non-full
`rate frame. A position of the transmitted symbols may be
`randomized by a mobile specific predetermined random
`code to minimize interference between terminals different
`
`from each other. For intensive frequency scanning and
`synchronization signal
`transmission, non—transmitting
`period can be assigned for successive frames of some
`interval. For 1/n (n=2,4,8) length successive transmission,
`the starting position of the transmitted symbols are staggered
`by 1/n frame time in each frame. If there is no part of the
`frame to be staggered, the starting position is the beginning
`of the frame. Such implementation minimizes guard time
`.
`.~.- .
`..
`overhead for frequency sw1tchmg and resynchronization.
`
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`US. Patent
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`Jan. 15, 2002
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`US. Patent
`
`Jan. 15, 2002
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`US. Patent
`
`Jan. 15, 2002
`
`Sheet 3 0f5
`
`US 6,339,590 B2
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`US. Patent
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`Jan. 15, 2002
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`US. Patent
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`Jan. 15, 2002
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`Sheet 5 0f 5
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`US 6,339,590 32
`
`1
`METHOD AND SYSTEM FOR PROVIDING
`INTER-FREQUENCY HANDOFF IN A
`TELEPHONE SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a telephone system, and
`more particularly,
`to a method for controlling an inter-
`freqnency handoff in a code division multiple access
`(CDMA) cellular telephone system.
`2. Background of the Related Art
`Generally, a cellular telephone system includes a service
`area which is divided into a plurality of cells. Each cell
`provides many mobile terminals with a radio communica-
`tion service through a base station. To maintain high quality
`communication when the mobile terminal moves between
`
`10
`
`15
`
`cells, a handoff, which changes a communication link or
`channel with one base station to another base station, must
`be timely established. Therefore, the mobile terminal has to '
`be able to measure the quality of signals from other periph-
`eral base stations during communicatiom, and the base
`station system is provided with the quality of signal infor-
`mation to timely perform the handoff. Such a hand off is
`referred to as a mobile-assisted handoff.
`
`25
`
`In particular, the CDMA system can perform the com-
`munication while using the same frequency and varying an
`offset of a common pseudo-random code that spreads band-
`width of transmitting signal. The CDMA system has a first
`digital receiver, which is referred to a searcher for searching
`a different CDMA signal of the same frequency, and a
`second digital receiver for demodulating the data.
`Typically, the CDMA system uses a plurality of sccond
`digital receivers, which are referred to as “RAKE” receiver,
`to receive the data transmitted via multiple paths.
`Accordingly, the mobile terminal can measure the quality of
`a different base station signal of the same frequency during
`the communication. In the CDMA method, the transmission
`rate of the data can be varied in multilevel, and the overall
`radio capacity within the transmission frequency can be
`increased by reducing an interference between different
`channels, which use the same frequency as the transmission
`rate is reduced. For example, voice data has an information
`amount variable with the time, and half of the communica-
`tion is used for listening, i.e., receiving voice data, and the
`rest is used for speaking, i.e., transmitting voice data. If a
`variable rate transmission is used, a fifty percent or more
`increase in capacity may be produced
`The system capacity in the cellular telephone system may
`be increased by reducing the size of the cell to increase the
`number of the cells within the service area. However, if the
`size of the cell is reduced, the handoff is too often occurred
`generated when moving at a high speed. Hence, the base
`station has difficulty in properly processing the handoff.
`Accordingly, a hierarchical cell structure or a layered cell
`structure is used, where a macro-cell of a larger size is
`placed upon a micro-coll of smallcr size which scrvcs the
`terminal moving at high-speed,
`is placed.
`In this cell
`structure, the handoff between the micro-cell and macro-cell
`according to a moving speed of the terminal, as well as the
`handoff between macro-cells, has to be provided.
`In the hierarchical cell structure, the signal strength of the
`base station of the macro-cell may be much larger than that
`of the base station of the micro-cell, and therefore various
`problems, such as cocktail party effect, egg phenomenon,
`etc., may occur even in the CDMA system. To provide the
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`hierarchical cell structure on different frequencies, in the
`CDMA system has to provide a (mobile-assisted) inter-
`frequency handoff.
`To perform the (mobile-assisted) inter-frequency mobile-
`assisted handoff,
`the mobile terminal has to be able to
`receive the base station signal of different frequency during
`the communication and therefore, it has to be provided with
`an additional radio frequency RF receiver, or alternatively,
`the mobile terminal has to use a single RF receiver in a
`time-division multiplexing (TDM) method for receiving the
`signal of a different frequency.
`The above-mentioned related CDMA cellular telephone
`system continuously transmits a forward signal from the
`base station toward the mobile terminal, the mobile terminal
`may have two or more RF receivers for receiving the base
`station signal of different frequencies during the communi-
`cation. However, this method is undesirable since the cost
`and size of the mobile terminal increase.
`
`For the alternate method in the CDMA system, the time
`period where the mobile terminal breaks a part of reception
`signal and receives the signal of the different frequency may
`be used for providing the inter-frequency handoff with a
`single RF receiver. However, such a method may cause the
`degradation of the communication quality and the commu-
`nication may be dropped as the required signaling data is not
`received.
`
`SUMMARY OF THE INVENTION
`
`Accordingly, the present invention is directed to substan-
`tially obviate the problems of the related art.
`An object of the present invention is to provide a method
`for providing inter-frequency handoff in a code division
`multiple access cellular telephone system capable of main-
`taining an excellent communication quality.
`Another object of the present
`invention is to rapidly
`establish an inter-frequency handoff between a mobile ter-
`minal and a peripheral base station as the mobile terminal
`searches a frequency of the other base station or transmits
`searched information.
`
`A further object of the present invention is to allow
`inter—frequency handoff without using dual transceivers.
`To achieve these and other advantages and in accordance
`with the purpose of the present invention, as embodied and
`broadly described,
`a method for providing an inter-
`frequency handoff in a code division multiple access cellular
`telephone system comprises the steps of: forming a non-
`transmitting period in a transmission frame by controlling
`the repeat times of a transmission symbol in a frame and a
`position or form of a transmitting period with a variable rate
`characteristic of a transmission signal; controlling an energy
`transmission level of the signal reconstructed through the
`aforesaid step in inverse proportion to the repeat times to
`transmit it; and searching a base station signal of a different
`frequency through a period where a signal of the frame
`transmitted through the aforesaid step is not transmitted, or
`transmitting a signal to the base station of the different
`frequency, by a mobile terminal.
`The present invention can be achieved in parts or in a
`whole by a method of performing a handoff in a telephone
`system comprising the steps of: modulating data to a pre-
`scribed energy level of a frame unit by controlling repeat
`times of a symbol;
`inserting a searching period into the
`modulated frame to reconstruct the modulated frame as a
`transmission frame; and searching a frequency information
`of a station with the transmission frame to perform a
`handoff.
`
`LGE_0000554
`
`LGE_0000554
`
`

`

`US 6,339,590 32
`
`3
`The present invention can be achieved in parts or in a
`whole by a method for providing a handoif in a telephone
`system comprising the steps of: modulating a band into a
`modulated frame having a repeated frame period by varying
`an energy transmission level based on a full rate frame;
`forming a non-transmitting period to the modulated frame
`and reconstructing a repeat transmission frame by control-
`ling a number of repetition of the energy level; and searching
`frequency information of a peripheral base station with the
`transmission frame to perform a handoff and transmitting the
`searched information to a transmitter of a base station.
`
`The present invention can be achieved in parts or in a
`whole by a method for providing a handoff in a telephone
`system comprising the steps of: forming a non-transmitting
`period in a transmission frame by controlling a number of
`repetitions of a transmission symbol in a frame and at least
`one of a position and form of a transmitting period with a
`variable rate characteristic of the transmission frame; con-
`trolling an energy level of the transmission symbol by an
`inverse proportion of the number of repetitions; and search-
`ing a base station signal of a different frequency during the
`non-transmitting period; or transmitting a signal to the base
`station of the different frequency, by a mobile terminal.
`The present invention can be achieved in parts or in a
`whole by a telephone system comprising: a station that
`modulates data into a transmission frame having an energy
`level based on an inverse proportion of a number of repeti-
`tions of a transmission symbol, and at least one of a position
`and form of a transmitting period with a variable rate
`characteristic of the transmission frame, the station forming
`a non-transmitting period based on the number of repetitions
`as a searching period; and a terminal that searches frequency
`information during the searching period and transmits the
`searched frequency information to the station.
`Additional features and advantages of the invention will
`be set forth in the description which follows, and in part will
`be apparent from the description, or may be learned by
`practice of the invention. The objectives and other advan-
`tages of the invention will be realized and attained by the
`structure particularly pointed out in the written description
`and claims hereof as well as the appended drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will be described in detail with reference to
`
`the following drawings in which like reference numerals
`refer to like elements wherein:
`
`FIGS. 1A to ID are transmission timing diagrams applied
`to a forward link or channel of a variable rate CDMA
`system;
`FIGS. 2A to 2D are transmission timing diagrams applied
`to a reverse link or channel of a variable rate CDMA system;
`FIG. 3 is a block diagram showing a preferred embodi-
`ment of the CDMA cellular telephone system;
`FIGS. 4A to 4E are transmission timing diagrams applied
`to a method for providing an inter-frequency handoff in
`accordance with a preferred embodiment of the present
`invention; and
`FIGS. 5A to 5G are timing diagrams showing a continu-
`ous frequency searching period upon a control of the inter-
`frequency handoif in accordance with one of the preferred
`embodiment.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`FIGS. 1A to 1D and FIGS. 2A to 2D, respectively, show
`a timing diagram in accordance with a signal transmission of
`
`10
`
`15
`
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`
`25
`
`30
`
`35
`
`40
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`45
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`50
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`55
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`60
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`65
`
`4
`a variable rate used in a code division multiple access
`CDMA cellular telephone system of “IS-95” style (see US.
`Pat. No. 5,416,797). FIGS. 1Ato ID are applied to a method
`of varying transmission energy, and FIGS. 2A to 2D is
`applied to a method varying the transmission time.
`In other words, FIGS. 1A to ID are signal transmission
`timing diagrams of a forward link (voice channel) transmit-
`ted from a base station to a mobile terminal, and FIGS. 2A
`to 2D are signal transmission timing diagrams of a reverse
`link (voice channel) transmitted from the mobile terminal to
`the base station.
`A transmission frame of the voice channel transmitted
`from the base station is divided into sixteen subframes, and
`is transmitted to the mobile terminal as a variable rate of four
`
`levels, such as a full rate (FIG. 1A), a half (1/2) rate (FIG.
`1B), a quarter (1A) rate (FIG. 1C) and an eighth (Vs) rate
`(FIG. 1D). Then, if the data rate is reduced to 1/n (herein, n
`i
`2, 4 and 8),
`the number of bit per the frame of the
`transmission data is accordingly reduced and therefore,
`transmission data may be transmitted by a repetition of n
`times to keep transmission data rate constant.
`If a transmission party repeatedly transmits each symbol
`of transmission data, a reception party combines the
`repeated symbols to achieve time diversity. The symbol is
`repeatedly transmitted at the different time and thereby, the
`transmission symbol can be correctly demodulated although
`parts of the repeated symbols are corrupted. The repetition
`is performed after a sequence of transmission data is all
`transmitted so that each repeated symbol is transmitted with
`enough time distance for maximizing the effect of the time
`diversity.
`With reference to FIGS. 2A to 2D, the transmission frame
`of voice information transmitted from the base station is
`distributed into set
`random patterns in a transmission/
`reception period to be transmitted. For example, in case of
`FIG. 2A, all frames are transmitted. In case of 1/2 rate shown
`in FIG. 2B, any one of two subframes is transmitted, and in
`case of 1A: rate of FIG. 2C, any one of four subframes is
`transmitted. In case of V3 rate of FIG. 2D, any one of eight
`subframes is selected to be transmitted to the receiver of the
`
`terminal in the random pattern.
`FIG. 3 is a block diagram showing a transceiver applied
`to the method of controlling the inter-frequency handoff in
`the CDMA cellular telephone system in accordance with the
`present
`invention. With reference to FIG. 3, an inter-
`frequency handoff transceiver in the CDMA cellular tele-
`phone system is comprised of a receiving portion 100 of the
`mobile terminal and a transmitting portion 200 of a base
`station.
`
`The receiving portion 100 of the mobile terminal includes
`an RF receiver 102 receiving a radio signal selected in the
`radio signals transmitted from the transmitting portion 200
`of the base station; a digital demodulator 101 demodulating
`the radio signal
`to an original signal; and a frequency
`synthesizer 103 generating frequency so that the RF receiver
`102 may selectively receive any one of the radio signals.
`The transmitting portion 200 of the base station includes
`a digital modulator 202 forming the transmission frame of
`data in a frame unit and converting each bit to a modulation
`symbol; a gain control element 203 controlling an energy
`level of the modulated transmission symbol by n/m; and an
`RF transmitter 201 varying and amplifying a frequency of
`the gain controlled radio signal to transmit it to an outside
`through an antenna.
`In the preferred embodiment, the transmission length of
`the non-full rate frame is shortened in order for the mobile
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`US 6,339,590 32
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`5
`terminal 100 to monitor other frequencies with a single
`receiver. Modulated symbols of non-full rate frame are
`transmitted with reduced repetition maintaining the sum of
`repeated symbol energy. Generally, for 1/n (n=2,4,8) length
`transmission, 1/m (min) rate frame is repeated m/n times
`and transmitted with n/m symbol energy of the full rate
`frame. A frame of other channels except voice channel to a
`mobile terminal is not transmitted during the frame having
`a non-transmitting period. The system may use rate limita-
`tion to gcncratc a non-full ratc framc.
`FIG. 4A illustrates a full (n=1) transmission at 1/2 (m=2)
`rate frame with two (m/n=2/1) time repetition. FIGS. 4B—4D
`illustrate 1/2 transmission (n=2) for 1/2 (m=2) rate frame, 1A
`(m=4) rate frame and 1/8 (m=8) rate frame. As shown in FIG.
`4B,
`the 1/2
`rate frame is transmitted without repetition
`(m/n=1) with the same symbol energy of the full rate frame.
`In FIG. 4C, the 1A: rate frame is repeated only two (m/n=4/
`2=2) times and transmitted with 1/2 (n/m=2/4) times symbol
`energy X of the full rate frame. The 1/8 rate frame is repeated
`only four (_m/n=8/2) times and transmitted with 1A1 (_n/m=2/8)
`times symbol energy X (FIG. 4D). FIG. 4E illustrates 1A
`(n=4) transmission at 1%: (m=4) rate frame without repetition
`(m/n=4/4) with the symbol energy of the full rate frame.
`As shown in FIGS. 1B and 1C illustrating respective
`transmission timing diagrams for the half rate and quarter
`rate in the conventional method, data of the half rate and the
`quarter rate are, respectively, repeated in two subframe units
`and in four subframe units, and the transmission symbol
`energy is controlled by dividing the full rate symbol energy
`by the repeat times. As shown in FIGS. 4C and 4E, the repeat
`times of the subframe is not set to a result obtained by
`dividing full rate into sub rate, and the symbol energy of the
`transmitting period is set to a result obtained by dividing the
`full rate symbol energy into the subframe repeat times, while
`reducing or removing the repeat times.
`As shown in FIG. 1C, the transmission is made through
`the four times repetition with symbol energy of x/4 in the
`conventional method.
`In the preferred embodiment,
`the
`transmission at
`1A:
`rate is made through the two times
`repetition with symbol energy of x/2 (FIG. 4C), or is made
`without repetition with symbol energy X (FIG. 4E) by the
`digital modulator 202. If the transmission is made without
`reduction of the repeat times of the subframe or without the
`repetition as mentioned above, a period where the signal is
`not transmitted can be made within the frame.
`
`The gain control element 203 reconstructs the modulated
`frame as the transmission frame by inserting the non-
`transmitting period including a terminal searching period
`into the modulated frame. The gain control element 203 can
`be located after the digital modulator 202 as shown in FIG.
`3, or into the digital modulator 202 e.g., before baseband
`filtering of the digital modulator.
`If the transmission signal processed in the above-
`mentioned method is transmitted by the RF transmitter 201,
`the RF receiver 102 of the receiving portion 100 receives the
`radio signal. Thereafter,
`the digital demodulator 101
`dcmodulatcs thc radio signal into thc signal, as shown in
`FIGS. 4C and 4E. The mobile terminal varies a frequency of
`the frequency synthesizer 103 during the period where the
`signal is not transmitted in the frame (hereinafter, which is
`referred to a frequency searching period), and then searches
`the base station signal of the different frequency, thereby
`being capable of performing the inter—frequency mobile—
`assisted handoff.
`
`However, the mobile terminal does not know the time and
`period of a frequency searching period. This problem can be
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`solved by sending information indicating the time when the
`frequency searching period is generated from the base
`station to the mobile terminal. The frequency searching
`period may periodically be generated in a discontinuous
`form, and/or may be periodically or continuously during a
`given period.
`On the other hand, in case of FIG. 4A, since the frequency
`searching period can not be made for the full rate,
`the
`frequency searching period can be inserted by limiting the
`encoding rate of a source such as a vocoder, if needed.
`Substantially, the CDMA cellular telephone system applies
`a rate limitation which properly limits the encoding rate of
`the vocoder for inserting signaling data into the voice frame
`during the communication, and the degradation of the voice
`quality due to this is negligible.
`As the above-mentioned transmitting method is applied to
`the reverse link transmitted from the terminal to the base
`station, the terminal may use an empty period to transmit the
`synchronization signal of the different
`frequency.
`Accordingly, the base station of the different frequency can
`previously be synchronized with the mobile terminal for the
`handoff.
`
`A starting position of the transmitted symbols may be
`randomized by a mobile specific predetermined random
`code,
`i.e., user
`long code in 18-95 CDMA system,
`to
`minimize interference between terminals different from each
`
`other. For intensive frequency scanning and synchronization
`signal transmission, non-transmitting period can be assigned
`for successive frames of some interval. For 1/n (n=2,4,8)
`length successive transmission, the starting position of the
`transmitted symbols are staggered by 1/n frame time in each
`frame, for instance 20 frame. If there is no part of the frame
`to be staggered, the starting position is the beginning of the
`frame. Such implementation minimizes guard time overhead
`for frequency switching and resynchronization.
`As shown in FIGS. 5A to 511 illustrating the position
`assignment of successive frequency searching period, the
`frequency searching period is enlarged as the length of the
`frame by modularly incrcasing thc starting point of thc
`frequency searching period in the successive frame in a
`length unit of the transmitting period. For example, in case
`of the quarter rate without repetition (FIGS. 5E and SF), the
`starting point of the frequency searching period of the
`present frame is the first period in four transmitting periods,
`is allocated to the second transmitting period in the next
`frame. Further, the starting point of the frequency searching
`period is allocated differently and randomly for respective
`mobile terminals, thereby being capable of statistically dis-
`tributing the interference between the mobile terminals.
`As shown in FIGS. 5A to 5G illustrating other position
`assignmcnt, cach starting position of transmission symbols
`during successive frequency searching period is alternately
`changed to the front part and the rear part of the frame to
`maximize frequency searching period. This assignment is
`suitable for the CDMA system that uses various offsets for
`the frame boundary for each mobile terminals.
`As discussed previously, a method for providing interfre-
`quency handoff in the CDMA cellular telephone system of
`the present invention has an effect capable of maintaining
`high communication quality by rapidly establishing an inter-
`frequency handoff as the mobile terminal searches a fre-
`quency of the different base station.
`The method in accordance with the preferred embodiment
`fully enhances inter-frequency handoff capability of TIA/
`EIA/IS-95 compatible system with minimum changes of the
`physical layer, e.g., there is no need for channel structure
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`US 6,339,590 32
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`7
`modifications. There is no perceptible voice quality degra-
`dation even in the worst case scenario that requires intensive
`frequency scanning. Moreover, power control and time
`tracking are well maintained, compared to discarding of an
`entire frame for inter-frequency handoff.
`It will be apparent to those skilled in the art that various
`modifications and variations can be made in a method and
`system for providing an inter-frequency handoff in a code
`division multiple access cellular telephone system of the
`present invention without departing from the spirit or scope
`of the invention. Thus, it is intended that the present inven-
`tion cover the modifications and variations of this invention
`provided they come within the scope of the appended claims
`and their equivalents.
`The foregoing embodiments are merely exemplary and
`are not to be construed as limiting the present invention. The
`present teaching can be readily applied to other types of
`apparatuses. The description of the present
`invention is
`intended to be illustrative, and not to limit the scope of the
`claims. Many alternatives, modifications, and variations will
`be apparent to those skilled in the art.
`What is claimed is:
`1. A method of performing a handoff in a telephone
`system, comprising:
`modulating data of a non-fiill rate frame having at least
`one repeated symbol by a first base station at a pre-
`scribed energy transmission level;
`reducing the number of repeated symbols while maintain-
`ing the prescribed energy transmission level by the first
`base station to generate a searching period; and
`searching frequency information of at least one second
`base station by a mobile terminal during the searching
`period to perform a handoff.
`2. The method of claim 1, wherein said modulating step
`further comprises controlling an energy per modulation
`symbol.
`3. The method of claim 2, wherein the modulating step
`further comprises controlling a position of the symbol.
`4. The method of claim 3, further comprising controlling
`a form of a transmitting period with a variable rate charac-
`teristic of the transmission frame.
`5. The method of claim 4, wherein said modulating step
`further comprises the step of controlling an energy per
`information bit of the symbol.
`6. The method of claim 1, wherein said modulating step
`further comprises controlling a position of the symbol.
`7. The method of claim 1, further comprising controlling
`a form of a transmitting period with a variable rate charac-
`teristic of the transmission frame.
`
`8. The method of claim 1, wherein the repeat time is an
`integer number of repetitions.
`9. The method of claim 1, wherein the transmission frame
`has the same energy level as the frame of the first length.
`10. The method of claim 1, wherein the modulated data is
`non—compressed.
`11. The method of claim 1, wherein shortening a trans-
`mission length of the frame is shortened from a first length
`by reducing the number of symbol repetitions and wherein
`the inserted search period reconstructs the frame to the first
`length.
`12. The method of claim 1, wherein the prescribed energy
`transmission level
`is maintained by adjusting a symbol
`energy of the remaining symbols.
`13. The method of claim 12, wherein the symbol energy
`is adjusted in proportion to a number of remaining symbols.
`14. The method of claim 1, wherein the at
`least one
`repeated symbol is identical to at least one other transmitted
`symbol.
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`15. A method for providing a handoff in a telephone
`system, comprising:
`modulating data by a base station into a non-full rate
`frame having at least one repeated symbol period by
`varying an energy transmission level based on a full
`rate frame;
`forming a non—transmitting period within the modulated
`frame at
`the base station by reducing a number of
`repeated symbols while maintaining the energy level of
`the frame; and
`searching frequency information of a peripheral base
`station during the non-transmitting period to perform a
`handoff and transmitting the searched information to a
`transmitter of the base station.
`16. The method of claim 15, wherein a non-transmitting
`period is inserted into the transmission frame by controlling
`the number of repeated symbols based on the transmission
`symbol energy.
`17. The method of claim 15, wherein said non-
`transmitting period of said transmission frame is inserted in
`a random pattern through a variable rate limitation method.
`18. The method of claim 17, wherein said random pattern
`is any one of a half rate, a quarter rate, and an eighth rate.
`19. The method of claim 15, wherein said non—
`transmitting period of said transmission frame is a frequency
`searching period.
`20. The method of claim 19, wherein a starting point of
`said frequency searching period is modularly increased in a
`length unit of a transmitting period.
`21. The method of claim 19, wherein a starting point of
`said frequency searching period is alternately changed to a
`front part and a rear part of a frame to maximize the
`frequency searching period.
`22. The method of claim 15, wherein the non-transmitting
`period is formed based on a form of a transmitt