)
`
`United States Patent (19)
`Dent
`
`11
`45
`
`USOOS430760A
`
`Patent Number:
`Date of Patent:
`
`5,430,760
`Jul. 4, 1995
`
`(54) RANDOM ACCESS IN MOBILE RADIO
`TELEPHONE SYSTEMS
`
`(75) Inventor:
`
`Paul W. Dent, Stehag, Sweden
`
`FOREIGN PATENT DOCUMENTS
`0288904 11/1988 European Pat. Off. .
`0361299 4/1990 European Pat. Off. .
`WO91/07037 5/1991 WIPO
`OTHER PUBLICATIONS
`Patent Abstracts of Japan, vol. 15, No. 3 (E-1019)7 Jan.
`1990, & JP-A-02 256 331 (Sharp), 17 Oct. 1990.
`Patent Abstracts of Japan, vol. 16, No. 188 (E-1198) 7
`May 1992, & JP-A-04023 623 (NEC Corp), 28 Jan.
`1992.
`Thompson et al., “Automatic Power Control in the
`PTARMIGAN SCRA Sybsystem," IEE Conference
`Related U.S. Application Data
`Publication No. 139, 1976.
`Continuation of Ser. No. 867,149, Apr. 10, 1992.
`63)
`Primary Examiner-Gilberto Barrón, Jr.
`Attorney, Agent, or Firm-Burns, Doane, Swecker &
`51) Int. Cl. ....................... H04J 13/04; H04B 7/216
`Mathis
`52 U.S. C. ................................... 375/200; 455/54.2;
`CT
`ABS
`58) Field of Search
`375/1 4ssisg 57
`led O SeaCl ......................
`y
`- Y -
`The present invention includes a system and method for
`minimizing interference between two radio stations,
`e.g., a mobile radio telephone and a fixed base station, at
`the initiation of a radio communication. A mobile sta
`tion initiates a random access at the lowest power level
`and increases the transmission power level until the base
`station detects the access signal. Once detected, the
`power level of the message is maintained at the detected
`level so that the signal interference is avoided. The
`present invention also provides a mechanism for syn
`chronizing random access communications between
`il
`mobile stations and the base station despite variation in
`distances between the mobile and base stations.
`43 Claims, 6 Drawing Sheets
`O
`
`73) Assignee: Ericsson GE Mobile Communications
`-
`, N.J.
`Inc., Paramus
`(21) Appl. No.: 222,008
`
`22 Filed:
`
`Apr. 4, 1994
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`4,222,115 9/1980 Cooper et al. .......................... 375/1
`455/33
`4,435,840 3/1984 Kojima et al. ......
`... 455/54.2
`4,485,486 11/1984 Webb et al......
`... 455/33
`4,613,990 9/1986 Halpern ...
`4,677,687 6/1987 Matsuo
`... 455/10
`4,694,467 9/1987 Mui.........
`375/1
`4,696,027 9/1987 Bonta .................................... 379/60
`... 455/67.1 X
`4,811,421 3/1989 Havel et al. ............
`4,870,698 9/1989 Katsuy
`et al. ............... 455/67.1
`4,984,247 1/1991 Kaufmann et al. .....................
`is
`5,056,109 10/1991 Gilhousen et al. .....
`... 375/1
`5,151,919 9/1992 Dent ........................................ 375/1
`
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 1 of 18
`
`

`

`US
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`July
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`Sheet 1 of 6
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`Ex. 1005 / Page 2 of 18
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 2 of 18
`
`

`

`U.S. Patent
`
`July 4, 1995
`
`Sheet 2 of 6
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`5,430,760
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`ERICSSONv. UNILOC
`Ex. 1005 / Page 3 of 18
`
`ERICSSON v. UNILOC
`Ex. 1005 / Page 3 of 18
`
`

`

`U.S. Patent
`
`July
`4, 1995
`
`Sheet 3 of 6
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`5,430,760
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 4 of 18
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`

`

`U.S. Patent
`
`July 4, 1995
`
`Sheet 4 of 6
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`5,430,760
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 5 of 18
`
`

`

`U.S. Patent
`
`July 4, 1995
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`Sheet 5 of 6
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`5,430,760
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 6 of 18
`
`

`

`U.S. Patent
`
`July 4, 1995
`
`Sheet 6 of 6
`
`5,430,760
`
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`ERICSSON v. UNILOC
`Ex. 1005 / Page 7 of 18
`
`

`

`1.
`
`RANDOMACCESS IN MOBILE RADIO
`TELEPHONE SYSTEMS
`
`5,430,760
`2
`In other pending applications by the present inventor,
`U.S. patent application Ser. No. 07/628,359, filed Dec.
`17, 1990, and entitled "CDMA Subtractive Demodula
`tion,” now U.S. Pat. No. 5,151,919 and U.S. patent
`application Ser. No. 07/739,446, filed Aug. 2, 1991, and
`entitled "CDMA Subtractive Demodulation,' now
`U.S. Pat. No. 5,218,619 incorporated herein by refer
`ence, a CDMA subtractive demodulation system is
`described in which overlapping, coded signals are de
`coded in the order of strongest to weakest signal
`strength. After each decoding, the decoded signal is
`removed or subtracted from the received, composite
`signal before decoding the next strongest signal. Using
`such a CDMA subtractive demodulation system, signal
`strength differences between mobiles become less in
`portant and capacity is increased. In other words, the
`signals having the greatest potential for causing interfer
`ence, i.e., the strongest signals, are decoded and re
`moved first. In this way, potential sources of interfer
`ence for weaker signals are significantly reduced.
`Even in such a CDMA subtractive demodulation
`system, however, an interference problem still exists
`when mobile stations initiate random access call set-ups.
`Because of the difficulty in gauging an appropriate
`access power level, there is a risk of at least momen
`tarily interference with ongoing conversations.
`Another source of potential interference to ongoing
`conversations during mobile random accesses is time
`misalignment of the mobile random access signals rela
`tive to the base station's frame timing. For mobile sta
`tion signals to be received in a correct timeslot
`(TDMA) or correctly time-aligned to a particular cor
`relating code (CDMA), the mobile station must adjust
`its access signal transmission timing to account for the
`round-trip propagation delay between the base and the
`mobile station. Unfortunately, unless a recent contact
`has been made with a base station, the mobile station
`lacks a mechanism for establishing the correct time
`alignment for a random access.
`These problems undermine the efficient operation of
`current and future cellular systems. Given the fre
`quency with which new calls are placed by mobile
`telephone subscribers, especially in urban and other
`congested areas, it is both desirable and necessary for
`nobiles to make random accesses on the network with
`out generating unnecessary interference. It is also desir
`able to simply and effectively establish a call connection
`from mobile to base station that is synchronized with
`the time-alignment structure of the base station.
`SUMMARY OF THE INVENTION
`The present invention includes a method for minimiz
`ing the interference caused by radio communications
`initiated between at least one of a plurality of first radio
`stations and a second radio station. An access message is
`transmitted from the first radio station at a relatively
`low power level. A determination is made whether or
`not the access message has been detected at the second
`radio station. If the message has not been detected, the
`access message is retransmitted at an increased level
`until the message is eventually detected. When the mes
`sage has been detected, the power level is fixed at the
`detected level.
`The first station may be a mobile radio telephone
`station and the second station may be a base station. The
`access message itself includes an access code and an
`identification code identifying the first station. Each
`access message is preferably transmitted using spread
`
`55
`
`65
`
`5
`
`10
`
`20
`
`This application is a continuation of application Ser.
`No. 07/867,149, filed Apr. 10, 1992.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to systems for minimiz
`ing interference caused by mobile radio stations initiat
`ing and terminating communication with fixed radio
`stations.
`2. Description of Related Art
`In cellular radio telephone networks, a mobile sub
`15
`scriber may freely choose when and where to initiate a
`telephone call. This procedure is known as a random
`access call set-up. The term random access also applies
`to the mobile station's first transmission in reply to a call
`initiated through the mobile station's fixed home base
`station. In both situations, significant uncertainty exists
`in determining the mobile's transmission power level at
`the time of access.
`Three principal methods enable a radio telephone
`system to support multiple, ongoing conversations in a
`25
`given frequency band. Frequency Division Multiple
`Access (FDMA) is the traditional method, where every
`call connection between a mobile and a base station is
`allocated a unique frequency channel that is occupied
`continuously until the end of the call. At present, mobile
`telephone systems are changing from FDMA to time
`based methods of sharing communications resources. In
`Time Division Multiple Access (TDMA), different
`radio transmitters are allocated short time slots in a
`periodic cycle in which they transmitbursts of informa
`35
`tion. In the third approach, Code Division Multiple
`Access (CDMA), different speech/information signals
`are transmitted with different spread-spectrum codes so
`that the coded signals overlap in both time and fre
`quency. The received CDMA signals are decoded by
`correlation with the code associated with the desired
`speech/information signal.
`In all mobile telephone systems, the physical distance
`between mobile stations and base stations varies signifi
`cantly. The signal propagation loss between a radio
`45
`transmitter and receiver varies as a function of the
`fourth power of their mutual distance. As a result, large
`differences may arise in the strength of signals received
`at the base station from different mobiles. Although
`conventional cellular radio telephone systems employ a
`SO
`number of techniques to avoid interference between
`different signals, interference occurs nonetheless as the
`disparity between the signal strengths from various
`mobiles increases.
`This interference problem is of particular concern in
`CDMA systems where a mobile signal that is twice as
`strong as another mobile signal occupies twice the sys
`tem capacity. Unregulated, it is not uncommon for a
`strong mobile station to transmit signals at thousands of
`times the strength of other mobile transmissions. The
`60
`loss of system capacity to such "strong' mobile stations
`is unacceptable, and thus power regulation is particu
`larly important in CDMA systems. In commonly as
`signed U.S. patent application Ser. No. 07/866,554,
`entitled "Duplex Power Control” filed on Apr. 10,
`1992, the present inventor describes a power regulation
`method and apparatus for a CDMA system. That appli
`cation is incorporated herein by reference.
`
`ERICSSON v. UNILOC
`Ex. 1005 / Page 8 of 18
`
`

`

`5,430,760
`3
`spectrum signal coding including orthogonal block
`error-correction codes. Moreover, each access message
`is scrambled before transmission using a scrambling
`code selected from a reserved group of scrambling
`codes. When the base station has received the access
`message it acknowledges the receipt and commands the
`mobile station to discontinue power level increases. The
`acknowledgment may also include time alignment in
`formation that is used by the mobile station in conjunc
`10
`tion with the access message transmission power level
`to determine the time when subsequent mobile commu
`nication transmissions should occur.
`In one aspect of the invention, a mobile radio station
`for communicating with at least one other radio station
`15
`includes means for transmitting an access message to
`another radio station at a relatively low power level. A
`detecting means determines if a reply message has been
`received from the other radio station. The mobile sta
`tion has means for retransmitting the access message at
`20
`an increased power level if the reply message is not
`detected. The retransmitting means increases the power
`level of the random access transmission in accordance
`with a ramp function. The mobile station also has means
`for selecting a scrambling code from a list of available
`25
`scrambling codes broadcast from the other radio station
`to generate the random access message. The mobile
`station additionally has means for adjusting the time of
`transmission of the access message based on the in
`creased power level.
`30
`In another aspect of the invention, a communication
`system including plural mobile radio telephone stations
`and at least one fixed base station is disclosed in which
`each mobile radio station has means for transmitting an
`access message initially at a relatively low power level;
`35
`means for regulating the power level of said transmit
`ting means; and control means for controlling said regu
`lating means depending on whether said access message
`has been detected. The base station includes: means for
`receiving a composite of signals from said mobile sta
`40
`tions; means for detecting mobile access messages;
`means for decoding detected access messages; and
`means for transmitting a reply message to the mobile
`station corresponding to detected access message.
`45
`The base station further includes means for ordering
`received signals that include access messages according
`to signal strength; means for selectively decoding the
`strongest signals; and means for removing the decoded
`signal from the received composite signal. The mobile
`station includes means for encoding scrambled access
`messages using bi-orthogonal block codes and means
`for scrambling access messages using scrambling codes.
`The base station transmitting means broadcasts a list of
`reserve scrambling codes separate from scrambling
`55
`codes used for other radio communications.
`The mobile station includes means for adjusting the
`time of transmission of the access message based on
`regulated power level and means for detecting time
`alignment information in the reply message. The base
`60
`station includes means for determining the difference
`between the signal strength of the random access mes
`sage detected in the base station and a predetermined
`signal strength, and means for determining a time differ
`ence between the times the random access was detected
`65
`and a predetermined time. Finally, the base station de
`tecting means searches for particular access messages at
`staggered time intervals.
`
`4.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The features and advantages of the invention will
`become apparent from reading the following detailed
`description in conjunction with the drawings, in which:
`FIG. 1 is a functional schematic of the transmitter of
`a mobile station according to the present invention;
`FIG. 2 is a functional schematic of a transceiver por
`tion of a base station according to the present invention;
`FIG. 3A is an exemplary signal format for a random
`access message from a mobile station;
`FIG. 3B illustrates positional variation of random
`access message startcode sequences;
`FIGS. 4A-4E illustrate the underlying operation of a
`RAKE receiver;
`FIG. 5 is a functional schematic of multi-stage power
`amplifier in accordance with the present invention; and
`FIG. 6 is a functional schematic of a balanced modu
`lator in accordance with the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT(S)
`To facilitate an understanding of the invention, an
`exemplary embodiment in the context of the CDMA
`subtractive demodulation system such as disclosed in
`the above-incorporated U.S. patent application Ser. No.
`07/628,359, now U.S. Pat. No. 5,151,919 is described.
`Those skilled in the art will recognize that the present
`invention may be applied to any radio communications
`system, including all cellular radio telephone systems, in
`which it is desirable to minimize interference caused by
`a random access call attempt between two radio com
`munications devices.
`Briefly summarized, in the CDMA subtractive de
`modulation system, information between plural mobile
`radio stations and the base station is transmitted in
`blocks of codewords, e.g., forty-two codewords per
`block. A convenient signal transmission format is se
`quences of 128-bit codewords transmitted serially over
`a radio communications channel. A radio receiver am
`plifies, filters, samples, and converts the received com
`posite signal, which consists of overlapping communi
`cations signals, into digital form for processing. The
`digitized composite signal is descrambled with a unique
`scrambling code corresponding to the information sig
`nal having the greatest signal strength. The descram
`bled signal is correlated with "spreading' codes known
`as orthogonal (or bi-orthogonal) block codes that are
`associated with the information signals. The 128-bit
`signal samples are decoded by bi-orthogonal block de
`coder, by determining which block code has the best
`correlation to the sample signal, to produce an 8-bit
`information signal. The decoded information signal, i.e.,
`eight bits, identifies which one of the 128-bit bi-ortho
`gonal codes was transmitted, the latter then being sub
`tracted from the composite signal before attempting to
`decode the next strongest, coded information signal.
`In an exemplary embodiment of the present invention
`illustrated in FIGS. 1 and 2, a mobile station transmitter
`10 includes a radio frequency (RF) power amplifier 100
`coupled to a duplex antenna 102. A frequency synthe
`sizer 104 generates the transmission carrier waveform
`that is modulated with an information signal, e.g.,
`speech, by a quadrature modulator 106. The quadrature
`modulator 106 may implement a modulation technique
`such as impulse-excited Quadrature Amplitude Modula
`tion (QAM) in which information bits are modulated
`alternately on the in-phase (I) channel and the quadra
`
`ERICSSON v. UNILOC
`Ex. 1005 / Page 9 of 18
`
`

`

`O
`
`15
`
`5,430,760
`S
`6
`ture (Q) channel using the waveforms generated by two
`dently controllable 20-dB fixed attenuators may be se
`lectively switched into the output, thereby achieving
`low-pass (LP) filters 108, 110. A complex modulator
`112 calculates impulse response waveforms that corre
`the same 60-db control range. An example of a multi
`spond to the polarities of received digital information
`stage power amplifier is shown in FIG. 5.
`signals and converts those waveforms into analog form.
`FIG. 5 is a functional schematic of a multi-stage
`The LP filters 108, 110 principally remove the digital
`power amplifier 50 in which modulating in-phase (I)
`to-analog conversion sampling frequency components.
`and quadrature (Q) signals are input to a quadrature
`Alternatively, the information signal may be mixed
`modulator 106. The quadrature modulator 106 includes
`initially to a convenient intermediate frequency, and
`level-switchable balanced medulators controlled by
`then converted to the higher carrier transmission fre
`level control bits B1-B4 to provide a first 0-20 dB con
`quency by heterodyne mixing the modulated intermedi
`trol. A first frequency F providing the carrier fre
`ate frequency waveform with an offset frequency.
`quency is also input to the quadrature modulator 106
`The digital information signals received by the com
`from the transmitter frequency synthesizer 104. The
`plex modulator 112 are produced by either a block
`output of the quadrature modulator 106 is input to an
`codeword generator and scrambler 114 or a speech
`upconverter 504, which is provided with a second fre
`encoder 122. When the mobile station 10 is transmitting
`quency F2 from the transmitter frequency synthesizer
`a random access message, i.e., before speech communi
`104. The upconverter 504 heterodyne mixes the modu
`cation begins, the message is generated in the control
`lated signal (which is at a lower frequency for technical
`processing unit 166 and input to the block codeword
`convenience) with the higher, fixed second frequency
`generator and scrambler 114 eight bits at a time. How
`F2 to translate it to a higher output frequency. Con
`ever, when speech transmission commences, the eight
`versely, a downconverter, or super heterodyne mixer, is
`bit inputs to block codeword generator and scrambler
`generally employed in a receiver to convert a high
`114 come from a speech digitizer and encoder 122. The
`frequency signal received on the antenna to a lower,
`speech encoder 122 receives a microphone signal from
`fixed intermediate frequency at which amplification is
`a microphone 124 and outputs eight-bit words. A
`25
`more conveniently achieved. In either case, it is advan
`switch 126 controlled by the control processing unit
`tageous to modulate or demodulate a signal at a lower,
`116. For input to the block codeword generator and
`fixed frequency and to change the oscillator that drives
`scrambler 114, the control processing unit 116 selects
`the mixer to vary the frequency at the antenna.
`either itself for transmission of random access messages
`The output of the converter 504 is input to abandpass
`or the speech encoder 122 for transmitting conversa
`30
`filter 506 and fed to a driver 507. The gain on the driver
`tion. Even after the random access procedure is com
`57 is controlled between 0-20 dB by power level con
`pleted, the control processing unit 116 can operate the
`troller 118. The output of the driver 507 is input to a
`switch 126 from time to time to select message transmis
`power amplifier 508, the gain of which is controlled
`sion and interrupt speech transmission. This is done, for
`between 0–20 dB by gain control of the power level
`example, for high priority signalling message exchange
`35
`controller 118. The output of the final amplifier 508 is
`between the base station 20 and the mobile station 10,
`input to the antenna 102 for broadcast. This circuit
`such as Fast Associated Control Channel (FACCH)
`permits a total transmission power control range of
`messages.
`0-60 dB. It will be appreciated that variable attenuators
`In the block codeword generator and scrambler 114,
`may also be used. Switchable and variable attenuators
`eight bits of information may be spread using a suitable
`are commercially available from number of sources,
`bi-orthogonal block code to a 128-bit codeword, for
`such as Avantek, Inc., Santa Clara, Calif.
`example. The 128-bit codeword may then be scrambled
`A suitable balanced modulator for the quadrature
`by modulo-2 adding a unique scrambling code to the
`modulator 106 for controlling gain may be provided as
`codeword. The information bits and the scrambling
`shown in FG. 6. The circuit of FIG. 6 includes twin
`code originate from a control processing unit 116 that
`45
`balanced modulators 602a and 602b. Each modulator
`also selects the carrier frequency to be generated by the
`602 comprises twin paired transistors 603a, 603b; 605a,
`frequency synthesizer 104 and transmits a power level
`605b, the emitters of which are coupled together within
`command signal to a power level controller 118.
`each pair. The bases of symmetrically opposing transis
`The power level controller 118 advantageously con
`tors from 603a, 605b, 603b, 605a of each pair are cou
`prises a combination of attenuators and components for
`50
`pled together to form the bridge wherein the local oscil
`controlling the bias of the power amplifier 100 to
`lator (transmitter frequency synthesizer 104) supplying
`achieve the commanded power level when transmitting
`the carrier frequency F1 is connected in parallel to the
`each codeword. A combination of attenuators and an
`bridged circuit. The collectors of the transistors 603,
`plifier bias control is useful in achieving a suitably wide
`605 are cross-linked and form the output of the quadra
`transmission power level control range, e.g., 60 dB, and
`55
`ture modulator 106. The coupled emitters of each bal
`it will be appreciated that a wide variety of combina
`anced modulator 602 are coupled to the collector of
`tions are known and the following techniques can be
`multiple emitter transistors 604, the bases of which are
`used as desired in the present invention.
`controlled by the respective modulation inputs. Each of
`Because the power amplifier's final stage might be
`the emitters of each of the multiple emitter transistors
`controllable within only a 20-dB power range, a wide
`604 are connected together by respective series con
`transmission power control range is difficult to achieve
`by controlling the bias of only one stage of the power
`nected resistor pairs R1-R4. The resistor pairs R1-R4
`amplifier. Accordingly, for a two-stage power amplifier
`are center tapped with switchable current generators
`100, controlling both stages would yield a 40-dB control
`606 (I, I2, I3, 4), which are controlled by control bits
`range and providing a fixed 20-ds attenuator selectively
`B1-B. The values of unbypassed emitter resistors
`switched into the output of the power amplifier can
`R-R4 in each balanced modulator 602 can be selected
`by selectively energizing the tail current sources 606
`achieve the desired 60-dB range. Of course, if bias con
`trol of a single amplifier stage is preferred, two indepen
`associated with each center-tapped emitter resistor
`
`ERICSSON v. UNILOC
`Ex. 1005 / Page 10 of 18
`
`

`

`5
`
`10
`
`20
`
`5,430,760
`8
`7
`The power level of the initial random access message
`R-R4, thus resulting in a balanced modulator circuit 60
`from the mobile station 10 is set by the power controller
`with a binary programmed output level.
`118 at a low power level. The power controller 118
`Controlling the transmitted power level can also be
`gradually increases the transmission power in small
`achieved by numerically scaling the digital I, Q values
`increments, e.g., 0.1 dB, after each successive codeword
`generated in the complex modulator 112 before they are
`is transmitted. These successive power increases con
`converted to analog form for the quadrature modulator
`tinue for the total number of codewords making up the
`106. The control range is somewhat limited, but the gain
`random access message. Preferably, the power level
`can be easily and precisely selected, for example in
`increases according to a ramp function with the ramp
`0.1-dB steps.
`slope determining the magnitude of the power increase
`FIG. 2 shows an exemplary base station receiver/-
`increment. A ramp function is readily implemented and
`transmitter 20 for detecting mobile random accesses in a
`simplifies the signal strength prediction process of the
`communications environment of overlapping, ongoing
`base station signal tracker and sorter 210. Each succes
`radio traffic signals. An antenna 200 receives a compos
`sive transmission of the random access message by the
`ite signal which is amplified by a low-noise, RF ampli
`mobile station 10 occurs at a power level greater than
`15
`fier 202. The amplified signal is spectrally shaped by a
`the preceding access message transmission until the base
`filter 204, and a dual analog-to-digital converter 206
`station 20 detects the access message and transmits a
`converts the filtered analog signal into a stream of com
`reply message to the mobile station 10. Once that reply
`plex digitized signals having real or in-phase parts (I)
`message is received and the base station 20 indicates
`and imaginary or quadrature parts (Q). Alternatively,
`that the mobile power level is in the desired signal
`an intermediate frequency mixing stage may precede
`strength range, the mobile station 10 fixes its transmitted
`the amplifier 202 so that amplification and filtering
`power at a level specified by the base station 20 in the
`occur at a lower intermediate frequency.
`reply message.
`After the frequency demodulation process, the com
`It takes approximately 0.5 milliseconds to serially
`plex, digitized composite signal is processed by a
`transmit a single 128-bit codeword. If the mobile in
`25
`CDMA subtractive signal processor 208. Because the
`creases its transmitted power level by only 0.1 dB per
`individual signals to be demodulated are each scram
`codeword, the power level's rate of change is 200 dB
`bled with a unique scrambling code generated by the
`per second, which will be recognized as a high rate of
`mobile station's control processing unit 116, the CDMA
`change. Since the typical received power level range
`signal processor 208 sequentially descrambles the com
`for traffic or access signals is on the order of 60 dB,
`30
`posite signal with each scrambling code in order of
`fewer than six hundred codewords need be transmitted
`greatest to weakest signal strength. The descrambled
`before the base station detects the access message. Thus,
`signal is decoded by correlation with all of the bi-ortho
`the worst-case delay in access detection is only about
`gonal codes possibly used for encoding to extract eight
`300 milliseconds. In this way, the present invention
`bits of information for each 128-bit bi-orthogonal code
`allows mobiles to randomly access the cellular network
`35
`word. Correlation may be carried out by using for ex
`without noticeable delay to the mobile subscriber and
`ample the Fast Walsh-Hadamard transform processor
`without interference to other communications signals.
`described in the present inventor's pending U.S. patent
`To simplify the base-station's task of detecting ran
`application Ser. No. 07/735,805 filed Jul. 25, 1991 and
`dom access messages, the first two codewords out of
`entitled 'Fast Walsh Transform Processor'. The de
`every 42-codeword message are fixed at a first value, A.
`coded information bits are transmitted to a base station
`Thus, a 42-codeword message begins "AA . . . ' For
`control processor 212 for further speech/data process
`simplicity, message lengths are chosen to be 42 code
`words because that is also the length of speech coder
`1ng.
`By selecting the scrambling codes corresponding to
`frames in the present system. When transmitting speech
`the signals having the greatest signal strength, the base
`traffic, the first two codewords also take on the value
`station CDMA processor 208 demodulates the various
`"AA. . . ' to indicate when speech frames are transmit
`overlapping signals in order of predicted signal strength
`ted in full, but may also take on a second value "BB . .
`from strongest to weakest. A signal strength tracker and
`..' when it is desired to indicate that the remaining 40
`sorter 210 predicts the signal strengths from past obser
`codewords in the following speech frame are not going
`vations and orders them. Recognizing that power levels
`to be transmitted because the speaker was momentarily
`change over time, the signal strength tracker and sorter
`silent. This so-called discontinuous transmission or
`210 freely reorders the signal decoding sequence to
`"DTX' is described in commonly assigned U.S. patent
`accommodate relative power level changes. Expected
`application Ser. No. 07/866,555 entitled "Discontinu
`signal strength levels may be predicted based on a his
`ous CDMA Reception” filed on Apr. 10, 1992 now U.S.
`tory of past power levels by extrapolating a next power
`Pat. No. 5239,557.
`55
`level using an estimated change of power level.
`The base station receiver's capability to search for
`A random access by a mobile station 10 using low
`occurrences "AA. . .” or "BB ...' is therefore, in the
`preferred system, used for dual purposes of identifying
`power levels is detected only after stronger signals are
`DTXed speech frames as well as random access at
`decoded. In addition to decoding information or traffic
`signals, the base station processor 208 searches for and
`tempts.
`The staggering