[19]
`United States Patent
`5,883,899
`[11] Patent Number:
`[45] Date of Patent: Mar. 16, 1999
`Dahlman et al.
`
`
`
`U8005883899A
`
`[54] CODE-RATE INCREASED COMPRESSED
`MODE DS-CDMA SYSTEMS AND METHODS
`
`[75]
`
`Inventors: Erik Dahlman, Bromma; Per Hans P.
`Willars, Stockholm; 0101' E.
`Grimlund, Bromma; Lars-Magnus
`Ewerbring, Stockholm, all of Sweden
`
`[73] Assignee: Telefonaktiebolaget LM Ericsson,
`Stockholm, Sweden
`
`FOREIGN PATENT DOCUMENTS
`
`5/1 995
`652650
`11/1995
`681376
`1/1976
`51—2943
`12/1988
`63—318837
`4/1993
`5—102943
`WO94/29980 12/1994
`WO94/29981
`12/1994
`WO95/08901
`3/1995
`
`.
`.
`
`European Pat. Off.
`European Pat. Ofl'.
`Japan .
`Japan .
`Japan .
`WIPO .
`WIPO .
`WIPO .
`
`OTHER PUBLICATIONS
`
`[21] Appl. No.: 636,646
`
`[22]
`
`Filed:
`
`Apr. 23, 1996
`
`Related US. Application Data
`
`[63] Continuationiinipart of Ser. No. 431,458, May 1, 1995, Pat.
`No. 5,533,014.
`
`[51]
`
`[52] US. Cl.
`
`Int. Cl.6 ................................. H04J 3/16; H04] 3/22,
`H04J 3/06; H04B 7/216
`.......................... 370/468; 370/320, 370/342;
`370/504; 371/432
`[58] Field of Search ............................ 370/203, 206—208,
`370/468, 320, 328, 336, 342, 345, 350,
`504, 375/200, 206; 371/43—46
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`............................ 375/115
`3/1987 Jerrim et al.
`4,653,076
`5/1990 Cripps et al.
`.
`375/1
`4,930,140
`.. 455/33
`8/1991 Dahlin .......
`5,042,082
`
`3/1992 Reed .............
`.. 455/73
`5,095,540
`
`3/1992 Gilliouseii et al.
`....................... 455/33
`5,101,501
`4/1992 Gilhousen et al.
`......................... 375/1
`5,103,459
`4/1992 Uddenfeldt .......
`455/332
`5,109,528
`10/1992 Kitamura et al.
`.
`252/299.61
`5,152,919
`
`..
`.. 370/95.1
`10/1992 Kanai et al.
`5,157,661
`
`11/1992 Schilling
`375/1
`5,166,951
`455/331
`5,175,867 12/1992 Wejke et al.
`
`5,239,557
`8/1993 Dent
`..........
`375/1
`5,274,667 12/1993 Olmstead ..
`375/1
`
`5,373,502 12/1994 Turban .................
`.. 370/18
`
`1/1995 Wheatley, III et al.
`.
`5,383,219
`375/1
`
`.. 370/18
`5,533,014
`7/1996 Willars et al.
`1/1998 Stern ....................................... 375/200
`5,712,868
`
`402
`
`Digital Communications, Second Edition, John G. Proakis,
`pp. 441—443 (1989).
`“Multiple Access Options for Cellular Based Personal Com—
`munications”,
`IEEE Vehicular Technology Conference,
`Hakan Eriksson et al., pp. 1—6, (May 18—20, 1993).
`“On the System Design Aspects of Code Division Multiple
`Access (CDMA) Applied to Digital Cellular and Personal
`Communications Networks”, Allen Salmasi et al., 1991
`IEEE, pp. 57—62.
`“Second Generation Wirelss Information Networks”, David
`J. Goodman, IEEE Transactions on Vehicular Technology,
`vol. 40, No. 2, pp. 366—374 (May, 1991).
`“Trends in Cellular and Cordless Communications”, David
`J. Goodman, Jun. 1991 IEEE Communications Magazine,
`pp. 31—40.
`a
`for
`IIandover
`Provide Seamless
`to
`“Techniques
`DS—CDMA System”, Hakan Persson et al., 5 pages, RACE
`Workshop, Metz (18 Jun. 1993).
`
`Primary Examiner—Douglas W. Olms
`Assistant Examiner—David R. Vincent
`Attorney, Agent, or Firm—Burns, Doane, Swecker &
`Mathis, L.L.P.
`
`ABSTRACT
`[57]
`Introduction of discontinuous transmission in CDMA com-
`
`munications techniques is achieved by using selectively
`puncturing coded output of a convolutional encoder. By
`temporarily increasing thc coding rate during a framc,
`information only fills an information part of a frame in a
`compressed mode, leaving an idle part of the frame in which
`to perform other functions, such as evaluation of other
`frequencies for use in handover between frequencies.
`
`40 Claims, 6 Drawing Sheets
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`

`5,883,899
`
`1
`CODE-RATE INCREASED COMPRESSED
`MODE DS-CDMA SYSTEMS AND METHODS
`
`RELATED APPLICATIONS
`
`This application is a continuation-in-part of US. Ser. No.
`431,458, now US. Pat. No. 5,533,014 entitled “NON-
`CONTINUOUS TRANSMISSION FOR SEAMLESS
`HANDOVER IN DS-CDMA SYSTEMS" to Willars et al.,
`which application was originally filed on Jun. 14, 1993. The
`disclosure of that application is expressly incorporated here
`by reference. This application is also related to US. patent
`application Ser. No. 08/636,648 entitled “MULTI-CODE
`COMPRESSED MODE DS-CDMA SYSTEMS AND
`
`METHODS” to E. Dahlman et al., which application was
`filed on the same date as the present application and which
`disclosure is also expressly incorporated here by reference.
`BACKGROUND
`
`5
`
`10
`
`15
`
`2
`In a direct sequence (DS) CDMA system the symbol
`stream to be transmitted (i.e., a symbol stream which has
`undergone channel encoding etc.) is impressed upon a much
`higher rate data stream known as a signature sequence.
`Typically, the signature sequence data (commonly referred
`to as “chips”) are binary or quaternary, providing a chip
`stream which is generated at a rate which is commonly
`referred to as the “chip rate”. One way to generate this
`signature sequence is with a pseudo-noise (PN) process that
`appears random, but can be replicated by an authorized
`receiver. The symbol stream and the signature sequence
`stream can be combined by multiplying the two streams
`together. This combination of the signature sequence stream
`with the symbol stream is called spreading the symbol
`stream signal. Each symbol stream or channel is typically
`allocated a unique spreading code. The ratio between the
`chip rate and the symbol rate is called the spreading ratio.
`A plurality of spread signals modulate a radio frequency
`carrier,
`for example by quadrature phase shift keying
`(QPSK), and are jointly received as a composite signal at a
`receiver. Each of the spread signals overlaps all of the other
`spread signals, as well as noise-related signals,
`in both
`frequency and time. If the receiver is authorized, then the
`composite signal is correlated with one of the unique codes,
`and the corresponding signal can be isolated and decoded.
`For future cellular systems, the use of hierarchical cell
`structures will prove valuable in even further increasing
`system capacity. In hierarchical cell structures, smaller cells
`or micro cells exist within a larger cell or macro cell. For
`instance, micro cell base stations can be placed at a lamp
`post level along urban streets to handle the increased traffic
`level in congested areas. Each micro cell might cover several
`blocks of a street or a tunnel, for instance while a macro cell
`might cover a 3—5 Km radius. Even in CDMA systems, the
`different types of cells (macro and micro) will operate at
`different frequencies so as to increase the capacity of the
`overall system. See, H. Eriksson et al., “Multiple Access
`Options For Cellular Based Personal Comm.,” Proc. 43rd
`Vehic. Tech. Soc. Confi, Secaucus, 1993. Reliable handover
`procedures must be supported between the different cell
`types, and thus between different frequencies so that mobile
`stations which move between cells will have continued
`
`support of their connections.
`There are several conventional techniques for determin-
`ing which new frequency and cell should be selected among
`plural handover candidates. For example, the mobile station
`can aid in the determination of the best handover candidate
`
`(and associated new base station) to which communications
`are to be transferred. This process, typically referred to as
`mobile assisted handover (MAHO), involves the mobile
`station periodically (or on demand) making measurements
`on each of several candidate frequencies to help determine
`a best handover candidate based on some predetermined
`selection criteria (e.g., strongest received RSSI, best BER,
`etc.). In TDMA systems, for example, the mobile station can
`be directed to scan a list of andidate frequencies during idle
`time slot(s), so that the system will determine a reliable
`handover candidate if the signal quality on its current link
`degrades beneath a predetermined quality threshold.
`In conventional CDMA systems, however,
`the mobile
`station is continuously occupied with receiving information
`from the network. In fact, CDMA mobile stations normally
`continuously receive and transmit in both uplink and down-
`link directions. Unlike TDMA, there are no idle time slots
`available to switch to other carrier frequencies, which cre-
`ates a problem when considering how to determine whether
`handover to a given base station on a given frequency is
`
`The present invention relates to the use of Code Division
`Multiple Access (CDMA) communications techniques in ,
`cellular radio telephone communication systems, and more
`particularly, to a method and system related to handover of
`connections between frequencies using non-continuous
`Direct Sequence—Code Division Multiple Access (DS—
`CDMA) transmissions.
`DS-CDMA is one type of spread spectrum communica-
`tion. Spread spectrum communications have been in exist-
`ence since the days of World War II. Early applications were
`predominantly military oriented. However, today there has
`been an increasing interest in using spread spectrum systems
`in commercial applications. Some examples include digital
`cellular
`radio,
`land mobile radio, satellite systems and
`indoor and outdoor personal communication networks
`referred to herein collectively as cellular systems.
`Currently, channel access in cellular systems is achieved
`using Frequency Division Multiple Access (FDMA) and
`Time Division Multiple Access (TDMA) methods.
`In
`FDMA, a communication channel is a single radio fre-
`quency band into which a signal’s transmission power is
`concentrated. Interference with adjacent channels is limited
`by the use of band pass filters which pass substantial signal
`energy only within the specified frequency band. Thus, with
`each channel being assigned a different frequency band,
`system capacity is limited by the number of available
`frequency bands as well as by limitations imposed by
`frequency reuse.
`In TDMA systems which do not employ frequency
`hopping, a channel consists of a time slot in a periodic train
`of time intervals over the same frequency band. Each period
`of time slots is called a frame. A given signal’s energy is
`confined to one of these time slots. Adjacent channel inter-
`ference is limited by the use of a time gate or other
`synchronization element that passes signal energy received
`at the proper time. Thus, the problem of interference from
`dilferent relative signal strength levels is reduced.
`With FDMAor TDMA systems (or hybrid FDMA/TDMA
`systems), one goal is to insure that two potentially interfer—
`ing signals do not occupy the same frequency at the same
`time. In contrast, Code Division Multiple Access (CDMA)
`is an access technique which uses spread spectrum modu—
`lation to allow signals to overlap in both time and frequency.
`There are a number of potential advantages associated with
`CDMA communication techniques. The capacity limits of
`CDMA-based cellular systems are projected to be higher
`than that of existing analog technology as a result of the
`properties of wideband CDMA systems, such as improved
`interference diversity and voice activity gating.
`
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`3
`appropriate at a particular instant. Since the mobile station
`cannot provide any inter-frequency measurements to a han-
`dover evaluation algorithm operating either in the network
`or the mobile station, the handover decision will be made
`without full knowledge of the interference situation experi-
`enced by the mobile station, and therefore can be unreliable.
`One possible solution to this problem is the provision of
`an additional receiver in the mobile unit which can be used
`to take measurements on candidate frequencies. Another
`possibility is to use a wideband receiver which is capable of
`simultaneously receiving and demodulating several carrier
`frequencies. However, these solutions add complexity and
`expense to the mobile unit.
`this
`In the parent patent application to Willars et al.,
`problem is addressed by introducing discontinuous trans—
`mission into CDMA communications techniques. For
`example, a compressed transmission mode is provided using
`a lower spreading ratio (i.e., by decreasing the number of
`chips per symbol) such that with a fixed chip rate the spread
`information only fills a part of a frame. This leaves part of
`each frame, referred to therein as an idle part, during which '
`the receiver can perform other functions, such as the evalu-
`ation of candidate cells at other frequencies for purposes of
`handover.
`
`10
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`
`This solution is readily applicable to CDMA systems
`wherein non-orthogonal code words are used to spread the
`information data sequence. In these types of systems, com-
`monly referred to as “long code” systems, one signature
`sequence is much longer than one symbol (often billions of
`symbols long). Since these codes are non-orthogonal to
`begin with, temporarily changing the spreading ratio of one
`or several channels to provide compressed mode transmis-
`sions does not create extra inter-code interference.
`
`The solution proposed in the parent application becomes
`problematic, however,
`for DS-CDMA systems where
`orthogonal code words are used to spread data streams. In
`so-called “short” code systems, a short code set (e.g.,
`including 128 codes of length 128 chips) is chosen so that all
`codes are orthogonal to each other over one symbol interval,
`i.e., over the length of the code. Consequently, the number
`of chips per symbol,
`i.e.
`the spreading ratio, cannot be
`changed on one or several channels.
`Accordingly,
`it would be desirable to provide a
`DS-CDMA system in which transmission and reception was
`discontinuous but which did not rely on a reduction in the
`spreading ratio to provide idle time for the receiver to
`measure on different frequencies.
`SUMMARY
`Introduction of discontinuous transmission in CDMA
`
`communications techniques is achieved by, for example,
`using selectively punctured coded output of a convolutional
`encoder. Ry temporarily increasing the coding rate during a
`frame, the coded information only fills an information part
`of a frame in a compressed mode, leaving an idle part of the
`frame in which to perform other functions, such as evalua-
`tion of other frequencies for use in handover between
`frequencies. A mode control device can, for example, switch
`an encoded signal stream output from a convolutional
`encoder between a first signal processing branch associated
`with a normal mode of transmission and a second signal
`processing branch associated with a compressed mode of
`transmission, the latter of which includes a code puncturing
`unit.
`
`BRIEF DESCRIPTION OF TIIE DRAWINGS
`
`The foregoing, and other, features, objects and advantages
`of the present invention will become apparent from the
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`detailed description set forth below when read in conj unc-
`tion with the drawings, in which:
`FIG. 1 is a schematic illustration of a cellular radio
`
`communications system;
`FIG. 2A is a schematic illustration of a downlink traffic
`
`information processor in accordance with the present inven-
`tion;
`FIG. 2B is a schematic illustration of a short-code modu-
`lator in accordance with one embodiment of the present
`invention;
`FIG. 2C is a schematic illustration of a base station
`transmitter in accordance with one exemplary embodiment
`of the present invention;
`FIGS. 3A and 3B are examples of a normal mode trans-
`mission and a compressed mode transmission, respectively,
`during four frames; and
`FIG. 4 is a block diagram of alternate signal processing
`branches for providing normal mode and compressed mode
`transmissions.
`
`DETAILED DESCRIPTION
`
`In the following description, for purposes of explanation
`and not limitation, specific details are set forth, such as
`particular circuits, circuit components, techniques, etc. in
`order to provide a thorough understanding of the invention.
`For example, various details are provided relating to exem-
`plary modulation and transmitting techniques. However it
`will be apparent to one skilled in the art that the present
`invention may be practiced in other embodiments that depart
`from these specific details.
`In other instances, detailed
`descriptions of well-known methods, devices, and circuits
`are omitted so as not to obscure the description of the present
`invention with unnecessary detail.
`An exemplary cellular radio communication system 100
`is illustrated in FIG. 1. As shown in FIG. 1, a geographic
`region served by the system is subdivided into a number, n,
`of smaller regions of radio coverage known as cells 110a—n,
`each cell having associated with it a respective radio base
`station l70a—n. Each radio base station l70a—n has associ-
`
`ated with it a plurality of transmit and receive radio antennas
`130a—n. Note that the use of hexagonal-shaped cells 110a—n
`is employed as a graphically convenient way of illustrating
`areas of radio coverage associated with a particular base
`station 170a—n. In actuality, cells 110a—n may be irregularly
`shaped, overlapping, and not necessarily contiguous. Each
`cell 110a—n may be further subdivided into sectors accord—
`ing to known methods. Distributed within cells 110a—n are
`a plurality, m, of mobile stations 120a—m. In practical
`systems the number, m, of mobile stations is much greater
`than the number, 11, of cells. Base stations 170a—n comprise
`inter alia a plurality of base station transmitters and base
`station receivers (not shown) which provide two-way radio
`communication with mobile stations 120a—m located within
`
`their respective calls. As illustrated in FIG. 1, base stations
`170a—n are coupled to the mobile telephone switching office
`(MTSO) 150 which provides inter alia a connection to the
`public switched telephone network (PSTN) 160 and hence-
`forth to communication devices 180a—c. The cellular con-
`
`cept is known to those skilled in the art and, therefore, is not
`further described here.
`
`According to the present invention radio communications
`between the base stations and the mobile stations are
`elfected using direct sequence code division multiple access
`(DS-CDMA). In the following, the term downlink, or for-
`ward channel, refers to the radio transmission of information
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`5,883,899
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`5
`bearing signals from base stations 170a—n to mobile stations
`120a—m. Similarly,
`the term uplink, or reverse channel,
`refers to the radio transmission of information bearing
`signals from mobile stations 120a—m to base stations
`170a—n.
`
`Today, radio communication systems are being used for
`an ever increasing array of applications. Traditional voice
`communications now coexist with the radio transmission of
`images, and a mix of other medium and high speed data
`applications. Such applications require a radio channel
`capable of conveying a variable mix of low, medium, and
`high bit rate information signals with a low transmission
`delay. To make efficient use of the radio spectrum, only that
`bandwidth which is needed for a particular application
`should be allocated. This is know as “bandwidth on
`demand.” Accordingly,
`the following exemplary systems
`describe a multi-rate, DS-CDMA system.
`Downlink
`
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`6
`Similarly, higher received variable rate bitstream produce a
`greater number of information frames per predetermined
`time interval. Each information frame resulting from high
`bitrate information data is separately coupled to a separate
`short-code modulator resulting in a plurality of so-called
`parallel short-code channels.
`Arranging the information data bitstream into a sequence
`of information frames allows the information data to be
`processed conveniently in short-code modulators 210a—M.
`Referring now to FIG. 2B, a schematic illustration of the
`short-code modulators 210a—M, is generally shown as 210.
`Prior to channel coding in convolutional encoder 230, the
`first overhead bits (X1) comprising, for example, a portion of
`the cyclic redundancy check (CRC) bits are added to the
`information frame in time multiplexer 220. The frame
`comprising the information bits and the first overhead bits is
`coupled to convolutional encoder 230 and subjected to
`channel coding using, for example, a rate 1/3 convolutional
`encoder which adds redundancy to the frame. The encoded
`frame is then coupled to bit
`interleaver 240 where the
`encoded frame is subjected to block-wise bit interleaving.
`After interleaving, the second overhead bits X2 are added to
`the encoded and interleaved frame in time multiplexer 250.
`Downlink power control bits are also added to the encoded/
`interleaved frame in time multiplexer 260. The downlink
`power control bits instruct the mobile station to increase or
`decrease its transmitted power level. After the insertion of
`the power control bits, each frame is coupled to quadrature
`phase shift keying (QPSK) modulator 270. Those skilled in
`the art will appreciate that modulations other than QPSK
`modulation could also be used. QPSK modulator 280 maps
`the input bits, or symbols,
`into a sequence of complex
`symbols. The output of QPSK modulator is a complex
`sequence of symbols represented by, for example, Cartesian
`coordinates in the usual form I+jQ. Spreading of the output
`of the QPSK modulator is performed using so-called short-
`codes. Other encoding, interleaving, and modulation com-
`binations are possible.
`Short-Codes
`Referring back to FIG. 1, each radio base station 170a—n
`transmits a unique downlink signal to enable mobile termi-
`nals 120a—m to separate the signals broadcast in adjacent
`cells or adjacent sectors (i.e., inter-cell signals) from the
`downlink signals received in the cell where the mobile
`terminal is located. Further, signals transmitted to individual
`mobile terminals in a particular cell, are orthogonal to one
`another to separate the signals of multiple mobile stations
`120a—m operating in the same cell (i.e., intra-cell signals).
`According to the present invention, downlink transmissions
`to multiple risers in the same cell, or same sector, are
`separated by spreading the modulated signal with different
`orthogonal short-codes.
`Parallel short-code channels representing a high bitrate
`signal are separated from each other in the same way
`downlink traffic signals to mobile terminals operating in the
`same cell are separated, namely by assigning different short
`codes SM(real) to each parallel CDMA channel.
`In one embodiment, the short orthogonal codes are real-
`valued orthogonal Gold codes with a length of one symbol
`interval. For example, with a 120 kbps total bit rate (60 kbps
`on each quadrature branch) and a chip rate of 7.68 Mcps, the
`code length is 128 chips. Orthogonal Gold codes are ordi-
`nary Gold codes of length 2m—1, where a zero (or one) is
`added to the end of all code words producing 2’” orthogonal
`code words, each of length 2’”. Gold codes are known to
`those of skilled in the art. Referring again to FIG. 2A, the
`output of each short-code modulator 210a—M is coupled to
`
`FIG. 2A illustrates a schematic block diagram of a down-
`link traffic information processor 200. Downlink traffic ,
`information processor 200 is part of the base station trans-
`mitter. Each downlink connection requires the resources of
`at least one downlink traffic information processor 200. A
`base station which is dimensioned to supply a number K of
`simultaneous downlink connections should have at least an
`equal number K of downlink traffic information processors
`200. Referring to FIG. 2A, variable rate downlink traffic
`information data 205 such as, for example, speech or image
`information originating from an information source (not
`shown), is received by framing buffer 220 in the form of a
`variable rate digital bitstream. The information source may
`be, for example, an ordinary telephone 180a, a computer
`180b, a Video camera 180C, or any other suitable information
`source which is linked via PSTN 160 to MTSO 150, or to
`MTSO 150 directly, and henceforth coupled to base stations
`170a—n according to known methods.
`The bitrate (i.e., number of kilobits per second (kbps)) of
`the variable rate bitstream received by framing buffer 220 is
`dependent upon the type or amount of information to be
`transmitted to mobile stations 120a—m. The bitrate may be
`defined by a Basic Bitrate and multiples thereof, i.e.:
`
`25
`
`30
`
`35
`
`40
`
`. N
`.
`Bitrate=(Basic Bitrate) *k; k=0,1,2, .
`where (Basic Bitrate) * N is the maximum bitrate.
`In an exemplary embodiment having a Basic Bitrate of 32
`kbps and an information frame time interval of 10 ms, each
`information frame comprises 320 bits. For bitrates higher
`than 32 kbps, more than one information frame per 10 ms
`time interval is produced. As an example, suppose that the
`bitrate is 128 kbps. Then, four information frames, each
`comprising 320 bits, are produced for each 10 ms time
`interval. In general, the number M of information frames is
`the same as the number k of multiples of the Basic Bitrate.
`Referring again to FIG. 2A, each information frame is
`coupled to one of a plurality of so-called short-code modu-
`lators 210a—M for subsequent processing. The number M of
`short—code modulators 210a7M is equal to the number N of
`possible multiples of the Basic Bitrate. According to the first
`exemplary embodiment of the present invention, when the
`received information data bitrate is the Basic Bitrate (e.g., 32
`kbps) only one information frame is produced for each 10
`ms time interval which is coupled to short-code modulator
`210a. When the received variable rate bitstream is two times
`
`the Basic Bitrate (i.e., 64 kbps) two information frames are
`produced for each 10 ms time interval: one information
`frame is coupled to short-code modulator 210a and the other
`information frame is coupled to short-code modulator 210]).
`
`45
`
`50
`
`55
`
`60
`
`65
`
`LGE_0000436
`
`LGE_0000436
`
`

`

`5,883,899
`
`
`
`7
`adder 215 where the individually spread signals of each
`information frame are formed into a single composite signal.
`Long-Codes
`Referring now to FIG. 2C, the composite signals from
`each downlink trafiic information processor 200A—K, are
`coupled to base station transmitter 150. The signals from
`each downlink traffic information processor are added in
`block 290. In order to separate downlink signals transmitted
`from different base stations, each base station 170a—n is
`assigned a unique long code. In one embodiment of the
`present invention the long code may be complex-valued: for
`example, an ordinary Gold code of length 241—1 chips. After
`scrambling (at blocks 300 and 302) the composite signa
`with the long-code generated by the long code generator
`285,
`the signal
`is filtered (blocks 304, 306), convertec
`(blocks 308, 310), summed (block 312), amplified anc
`transmitted according to known techniques.
`Discontinuous Transmission
`Normally in CDMA systems, information is transmittec
`in a structure of frames with fixed length, e.g., 5—20 ms.
`Information to be transmitted within a frame is coded anc
`spread together. This information is spread over each frame,
`resulting in continuous transmission during the whole frame
`at a constant power level, as shown for example in FIG. 3A.
`This type of full frame, continuous transmission is denotec
`herein as “normal mode transmission”.
`As described above,
`the present
`invention introduces
`discontinuous transmission into CDMA systems for, e.g.,
`reliable handover candidate evaluation. According to the
`present invention, this is achieved by temporarily increasing
`the rate of the channel coder by deleting bits from the coded
`bit stream (i.c., puncturing the code). This results in coded
`information which is compressed into a portion of a frame,
`leaving a residual,
`idle interval
`in which no power is
`transmitted, as shown in FIG. 3B. This is referred to herein
`as “compressed mode transmission”. An illustrative example
`will serve to further explain how idle intervals can be created
`according to the present invention.
`Punctured convolutional coding techniques in digital
`communication systems are, per se, known as shown by the
`teachings of the following documents each of which are
`incorporated herein by reference: U.S. Pat. No. 5,029,331,
`issued on Jul. 2, 1991,
`to Heichler et al.; U.S. Pat. No.
`4,908,827, issued on Mar. 13, 1990, to Gates; U.S. Pat. No.
`4,462,101, issued on Jul. 24, 1984, to Yasuda et al.; Punc-
`tured Convolutional Codes of Rate (n—1)/n and Simplified
`Maximum Likelihood Decoding, by J. Bibb Cain, George C.
`Clark, Jr., and John M. Geist,
`in IEEE Transactions on
`Information Theory, Vol.
`IT—25, No. 1, January 1979, pp.
`97—100; and High Rate Punctured Convolutional Code for
`Soft Decision Vitcrbi Decoding, by Yutaka Yasuda, Kan-
`shiro Kashiki, and Yasuo Hirata, in IEEE Transactions on
`Communications, Vol. COM-32, No. 3, March 1984, pp.
`3157319.
`
`In general, communication systems using punctured con-
`volutional coding include a coder for coding a digital input
`to be transmitted from a transmitter and a decoder for
`
`decoding the coded input received at the receiver. The coder
`includes a convolutional coding circuit which receives the
`digital input and outputs a convolutional coded output. The
`digital input is coded by the convolutional coding circuit so
`that for every k-bits inputted into the convolutional coding
`circuit, a corresponding n-bits, where n>k, is outputted. The
`k-bits inputted and the corresponding n-bits outputted are
`referred to as k-tuples and n-tuples, respectively. A convo-
`lutional coding rate for the convolutional coding circuit is
`defined as the ratio of the number of k-bits inputted to the
`
`8
`number of n-bits outputted, and can be expressed as k/n. For
`example, the coding rate is 1/2 when for each bit inputted into
`the convolutional coding circuit there is a corresponding two
`bits outputted.
`the
`In order to increase the code rate of the coder,
`convolutional coded output is passed through a puncturing
`circuit which includes a transmission mask circuit and
`
`deleting pattern memory for transmitting only selected bits
`of the convolutionally coded output. The puncturing circuit
`outputs a punctured output having a punctured code rate of
`z/q. Apunctured code rate of z/q means that for every z input
`bits inputted into the convolutional coding circuit q bits are
`outputted from the puncturing circuit.
`The desired punctured code rate is achieved by passing a
`convolutional coded output through the transmission mask
`circuit and puncturing the convolutional coded output on a
`block-by-block basis. Each block to be punctured is formed
`from a plurality of n-tuples and is referred to as a puncturing
`block. The number of n-tuples used to form each puncturing
`block is currently determined by recognizing that to provide
`a punctured code rate of z/q, where z=yk, for a convolutional
`coded output of rate k/n, at least y convolutionally coded
`n-tuples must be grouped and punctured as a puncturing
`block to achieve the desired punctured code rate.
`Accordingly, the bit length of each puncturing block is equal
`to y convolutionally coded n-tuples multiplied by the num-
`ber of bits in each n-tuple. The bit length of the puncturing
`block can be expressed as L=yn.
`The puncturing blocks are punctured according to a