Method and apparatus for reducing power consumption of a communication device

A method of reducing power consumption in a communication device includes a step of acquiring a signal on a common pilot channel of a radio communication system. A next step includes detecting predetermined bits in the signal on the common pilot channel indicating activity on paging channels of the radio communication system. When no paging channel activity is indicated in the second step, a last step includes powering down portions of the electrical circuitry of the communication device so as to reduce power consumption, and when paging channel activity is indicated in the second step, a next step includes powering up portions of the electrical circuitry of the communication device such that those paging channels indicating activity are monitored by the communication device.

FIELD OF THE INVENTION
 The present invention relates generally to digital communication. More
 particularly, the present invention relates to a method and apparatus for
 reducing power consumption in a spread spectrum communication system such
 as a code division multiple access (CDMA) cellular telephone system.
 BACKGROUND OF THE INVENTION
 Code division multiple access systems such as direct sequence (DS-CDMA)
 communication systems have been proposed for use in cellular telephone
 systems operating at 800 MHz and in the personal communication system
 (PCS) frequency band at 1800 MHz. In a DS-CDMA system, all base stations
 in all cells may use the same radio frequency for communication. Base
 stations are uniquely identified in the system by uniquely-assigned
 spreading codes. Two specified pseudorandom noise (PN) sequences of
 2.sup.15 bits length are used by all the base stations. In a quadrature
 modulated system, one sequence is used for the in-phase (I) channel
 spreading of the I channel symbols and the other is used for the
 quadrature (Q) channel spreading of the Q channel symbols. Mobile stations
 in the system possess the same two 2.sup.15 bits length spreading codes
 and use them for the initial de-spread of the I and Q channels.
 Before the spreading on the I and Q channels, the symbols for transmission
 are spread using a process known as Walsh covering. When in a call, each
 mobile station is assigned a unique Walsh code by the base site to ensure
 that transmission to each mobile station within a given cell is orthogonal
 to transmission to every other mobile station, assuming that a different
 Walsh code is used for each mobile station. In this manner, traffic
 channels are established for two-way communication between a base station
 and a mobile station.
 In addition to traffic channels, each base station broadcasts a pilot
 channel, a synchronization channel, and a paging channel. The pilot
 channel is formed by a constant level signal that is covered by Walsh code
 0, which consists of all the same bits. The pilot channel is commonly
 received by all mobile stations within range and is used by the mobile
 station for: identifying the presence of a CDMA system, initial system
 acquisition, idle mode hand-off, identification of initial and delayed
 rays of communicating and interfering base stations, and for coherent
 demodulation of the synchronization, paging, and traffic channels. At the
 mobile station, the received RF signals include pilot, synchronization,
 paging, and traffic channels from all nearby base stations.
 Referring to FIG. 1, a typical process for a mobile station to receive
 incoming calls is shown. At the start 10, a mobile unit will power up 12
 various circuitry to complete a call. This includes powering up the RF
 portions of the mobile unit, the receiver circuits including a receiver
 search, and a digital signal processor (DSP) including a call processor as
 are known in the art. Once the mobile is powered up, the mobile unit
 proceeds to acquire 14 the pilot channel from the base station. Once the
 pilot channel is acquired the mobile unit will acquire synchronization 16
 which aligns the timing of the mobile unit with the base station. The
 mobile station synchronizes to the base station by correlation to a unique
 Walsh code on the synchronization channel. Typically, mobile stations use
 a correlator as a receiver pilot searching element to serially search for
 the PN phases of the receivable pilots. Knowledge of the correct I and Q
 channel spreading PN phases of the base station(s) with which the mobile
 station communicates allows the coherent detection of all the other code
 channels transmitted by the base station.
 Once the mobile is synchronized with the base station, the mobile will
 monitor the paging channels either continuously or intermittently (slotted
 mode) to see if there is any incoming call activity for the mobile.
 Monitoring the paging channels includes search a paging channel 18 to see
 if a paging message for the particular mobile unit is present 20. If no
 message is present, the mobile unit will check if it has searched all
 available paging channels 22. If not, the mobile unit will change RF
 frequency to tune to another paging channel 24. The mobile unit will
 search all the available paging channels in this way. If no paging message
 is found the mobile unite will power down 26 its RF portions, the receiver
 circuits including the receiver search, and the digital signal processor
 (DSP) including the call processor and return to an idle or sleep state
 until it is time again to check for paging messages. If a paging message
 is eventually found the mobile and base station will set up and connect to
 a traffic channel 28 for transmitting and receiving the call indicated by
 the paging message to connect to the base station. At this time the mobile
 unit and base station proceeds with the transfer of information (voice,
 data, etc.) until the communication is completed 30. Upon completion, the
 mobile unit powers down its circuitry 26 as before and returns to an idle
 state.
 In accordance with the above procedure, three different channels (pilot,
 synchronizing, paging) are needed to be monitored to see if there is an
 incoming call waiting for the mobile. Unfortunately, acquiring all three
 channels takes time and power, and may not always be successful. In
 addition, the mobile unit must identify all the pilot signals that are
 receivable including the pilot signal from the base station with the
 strongest pilot channel.
 The prior art pilot channel searching method creates further limitations
 for all of the other uses of the pilot channel after initial system
 acquisition. Typical DS-CDMA mobile station receivers utilize a rake
 receiver having three or more independently controlled fingers which are
 time aligned to the correct PN sequence phases using knowledge of the
 pilot channel phases determined by the receiver pilot phase searching
 element. The rake fingers are normally assigned to the strongest rays
 received from all communicating base stations as determined by the
 receiver pilot phase searching element. Ray assignments are updated in a
 maintenance process using the pilot phase searching element information.
 If the pilot phase searching element is slow, resulting in slow
 maintenance of the assignment of the strongest rays to the rake fingers,
 the receiving performance of the mobile station is degraded under fading
 conditions.
 Idle hand-off is the process of attaching to and listening to the paging
 channel of the base station with the strongest pilot as identified by the
 pilot searching element. When the mobile station receives a page or
 accesses the system to place a call, it is important that the mobile
 station is listening to the page from, or tries to access, the base
 station associated with the strongest received pilot. This requires a fast
 pilot phase searching element, particularly when the mobile station is in
 motion.
 A portable station may have to search the possible phase space of as many
 as twenty base stations every time it wakes up. To reliably receive the
 paging slot after waking up, the portable station must be listening to the
 base station which is providing adequate signal strength. When the mobile
 station is in motion, the correct base station to decode can easily change
 from one paging interval to the next paging interval. Therefore it is very
 important to have a fast pilot searching mechanism to identify the correct
 base station pilot before the start of the assigned paging slot.
 For battery powered portable mobile stations it is also very important to
 conserve battery charge when waiting for pages. DS-CDMA provides a slotted
 mode that allows portable stations to power down except for the periods
 when their assigned paging slot information is transmitted by the base
 stations. The paging slot interval can be as short as 1.28 seconds and
 periods of 1.28 seconds multiplied by powers of two for more battery
 savings. During these intervals, the mobile station "sleeps" in a low
 power mode. However, using the prior art pilot searching mechanism
 requires the portable station to wake up well before the paging slot to
 allow sufficient time to sequentially search the PN sequence phase space.
 This negates a substantial part of the potential battery savings afforded
 by slotted mode.
 There has been a proposal (for TIA/EIA interim standard IS-95C) for an
 addition Walsh channel, in addition to the pilot, paging, synchronization,
 and traffic channels, dedicated for paging activity. However, this still
 requires the mobile to acquire multiple channels.
 Accordingly, there is a need for an apparatus and method for a mobile unit
 that avoids the problems associated with acquiring multiple channels to
 see if an incoming call is present. There is also a need to improve on
 battery savings of a mobile while providing simpler operation. It would
 also be an advantage to eliminate some of the problems associated with:
 pilot searching, maintaining system acquisition, soft hand-off, and
 slotted mode timing and operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The present invention provides a method and apparatus for a mobile unit to
 determine if an incoming call is waiting by monitoring only one channel in
 a communication system. The present invention improves battery savings of
 a mobile while providing simpler operation, and eliminates some of the
 problems associated with: pilot searching, maintaining system acquisition,
 soft hand-off, and slotted mode timing and operation.
 FIG. 2 shows a communication system usable in the present invention. The
 communication system 100 includes a plurality of base stations such as
 base station 102 configured for radio communication with one or more
 mobile stations such as radiotelephone 104. The radiotelephone 104 is
 configured to receive (and transmit) direct sequence code division
 multiple access (DS-CDMA) signals to communicate with the plurality of
 base stations, including base station 102. In the illustrated embodiment,
 the communication system 100 operates according to TIA/EIA Interim
 Standard IS-95, "Mobile Station-Base Station Compatibility Standard for
 Dual-Mode Wideband Spread Spectrum Cellular System," operating at 800 MHz.
 Alternatively, the communication system 100 could operate in accordance
 with other CDMA systems including PCS systems at 1800 MHz.
 The base station 102 transmits spread spectrum signals to the
 radiotelephone 104. During communication the symbols on the traffic
 channels are spread using a pseudorandom noise (PN) Walsh code in a
 process known as Walsh covering, as is known in the art. Each mobile
 station such as the radiotelephone 104 is assigned a unique Walsh code by
 the base station 102 so that the traffic channel transmission to each
 mobile station is orthogonal to traffic channel transmissions to every
 other mobile station. The spread signals are quadrature modulated to form
 in-phase (I) and quadrature-phase (Q) signals. The I and Q signals are
 each spread using two specified PN sequences, typically 2.sup.15 bits in
 length. The same I and Q spreading sequences are used by all base stations
 in the communication system 100.
 In addition to traffic channels, the base station 102 broadcasts a pilot
 channel, a synchronization channel, and a paging channel. As is used in
 the art, the pilot channel is formed by a constant level signal that is
 covered by Walsh code (0), which consists of bits all being the same. No
 decoding is necessary when the pilot channel is encoded using Walsh code
 (0) and a pilot channel decoder may be omitted. However, if another Walsh
 code or another type of coding is used to encode the pilot channel, a
 decoder is necessary. Such a decoder applies a pilot code to the despread
 signal to produce the pilot channel signal. Preferably, the pilot code is
 common to all mobile units and the pilot channel is commonly received by
 all mobile units within range. The pilot channel is used by the
 radiotelephone 104 for identifying the presence of a CDMA system, initial
 system acquisition, idle mode hand-off, identification of initial and
 delayed rays of communicating and interfering base stations, and for
 coherent demodulation of the synchronization, paging, and traffic
 channels. The synchronization channel is used for synchronizing mobile
 station timing to base station timing. The paging channel is used for
 sending paging information from the base station 102 to mobile stations
 including the radiotelephone 104.
 In the present invention, a few bits of paging channel status information
 is punctured on the common pilot channel. These bits give an indication if
 there is any activity on the paging channels and which paging channels are
 active. A mobile unit, such as the radiotelephone 104, periodically polls
 the pilot channel to check these bits. Based on the status of the bits,
 the mobile unit would only do paging channel processing if there were an
 active paging channel, and only on those paging channels indicating
 activity. In this way, mobile unit idle power would be reduced
 proportionally to the average reduction in processing required to monitor
 the paging channels regardless of the number of active paging channels.
 In particular, the present invention provides a method and apparatus for
 reducing power consumption in a communication device having electrical
 circuitry. The invention firstly involves a search for, and lock onto, the
 common pilot channel. Subsequently, the predetermined bits punctured on
 the pilot channel are detected. The bits specify which of any paging
 channels are active. If none of the paging channels are indicated as
 active, the mobile unit will not activate the DSP, not acquire the
 synchronization or paging the pilot channel are detected. The bits specify
 which of any paging channels are active. If none of the paging channels
 are indicated as active, the mobile unit will not activate the DSP, not
 acquire the synchronization or paging channels and instead go directly to
 sleep mode, thus shortening the time in poll mode and eliminating the
 power consumption phases of the poll cycle. Only if the bits specify that
 there are active paging channels will the mobile unit proceed to acquire
 the synchronization channel and read the paging channel. As the number of
 bits punctured on the pilot channel is small (approximately 4-8 bits per
 slot), the degradation in the channel estimate and searcher results that
 are normally used by the radiotelephone during pilot channel acquisition
 is negligible as no I, Q phase distortion is introduced. It is estimated
 that the channel degradation for an 8-bit puncture would be about 0.45 dB
 on the pilot integration of a twenty symbol accumulation.
 The advantage of the present invention is that the processing time to check
 for pages is reduced as only those paging channels having activity are
 monitored, and the power consumption of the mobile unit is reduced, as a
 preliminary decision to monitor paging channels can be made without
 turning on all of the receiver or DSP circuitry of the mobile unit. Also,
 the present invention takes advantage of the fact that there are
 significant periods of time in which there are no active pages in a
 specific location. When there are no active pages, power is minimized due
 to the fact that the synchronization and paging channels do not require
 monitoring.
 Specifically, when there is no paging channel activity shown by the bits
 punctured on the pilot channel, the present invention allows for the time
 to poll, TPOLL, to be much shorter than the prior art. For example, the
 steps involved in polling for and detecting a paging message in a prior
 art CDMA system includes first having the mobile unit powering up its
 receiver and doing a pilot search. This step involves turning on the
 analog front end 108, ADC 110, receiver searcher 114, the call processor
 of the DSP 116 and takes approximately 80 ms to perform but can vary
 depending on signal conditions and the number of paging channels to be
 searched. Secondly, the mobile unit acquires the synchronization channel,
 which involves turning on the entire receiver (including the receiver
 fingers 122, 124, 126, matched filter 128 and receiver searcher 114) and
 the entire DSP 116 and takes approximately another 80 ms to perform.
 Third, the mobile unit reads the paging channels and processes the
 results. This takes approximately another 80 ms for a total poll time,
 TPOLL, of about 240 ms. and doing a pilot search which would take the same
 amount of time and power as in the prior art. However, the mobile unit
 then only needs to check the punctured bits on the pilot channel to detect
 a paging message, without powering on any more circuits or monitoring the
 synchronization of paging channels. Checking the bits takes a negligible
 amount of time, and results in a total poll time, T.sub.POLL, of about 80
 ms, much less than the 240 ms in the prior art. The present invention
 reduces the poll duration by about 60-70% during periods when there is no
 paging activity. Further, the present invention reduces power consumption
 during polling mode, P.sub.POLL, by about 80% due to the fact that the DSP
 does not have to wake up during a no page situation. The formula for total
 power consumption is given by
 ##EQU1##
 where P.sub.SLEEP is the power consumed by the mobile unit during sleep
 mode and T.sub.SLEEP is the time the mobile unit is powered down between
 paging channel polls. It is observed the acquiring the synchronization
 channel and reading the paging channels are the biggest power consumers
 during a polling cycle in the prior art. These steps are avoided in the
 present invention.
 Referring now to FIG. 3 (and referencing FIG. 2), a flow diagram is shown
 illustrating a method of reducing power consumption in a communication
 device having electrical circuitry, such as a CDMA radiotelephone, in
 accordance with a preferred embodiment of the present invention. The
 method begins at step 200. At step 202, operation of the radiotelephone
 104 is initiated. For example, operating power to the RF section and
 receiver searcher portion of the receiver in the radiotelephone 104 is
 turned on. At this point, the radiotelephone 104 attempts to identify and
 acquire the system. The method includes a step 204 of acquiring a signal
 on a common pilot channel of a radio communication system. This is usually
 done at turn on or after the communication device awakes after sleep mode.
 This step 204 includes a substep of having the radiotelephone 104 tune to
 an RF channel. The analog front end 108 is used for selecting a particular
 RF channel received through the antenna 106. The RF channel may be
 predefined by the communication system 100 according to a system protocol,
 such as IS-95. Alternatively, the RF channel may be located somewhere in
 one or more ranges of frequencies, as is the case in many PCS systems
 operating around 1800 MHz.
 The receiver searcher 114, under control of the DSP 116, examines the
 stream of received data provided by the ADC 110. The data includes
 detected PN sequences corresponding to spread RF signals received from one
 or more base stations, such as base station 102. The matched filter 128
 compares detected PN sequences and a predetermined PN sequence and
 produces a response. The response may be stored in the memory 130 or the
 memory 132, or elsewhere. The predetermined PN sequence is maintained at
 the radiotelephone 104, for example in the memory 130 or the memory 132.
 The predetermined PN sequence is, for example, 512 chips long. The matched
 filter 128 captures the pilot energies of all receivable base
 transmissions during a predetermined time duration. The preferred
 predetermined time duration for an IS-95 DS-CDMA communication system such
 as communication system 100 is 26-2/3 milliseconds, which is the time
 required for repeat of all phases of the PN sequences used to spread the I
 and Q channels. The matched filter 128 may examine either the I channel or
 the Q channel. Alternatively, the matched filter 128 could include an I
 channel matched filter for the I channel and a Q channel matched filter
 for the Q channel, combining the outputs of the two matched filters for
 improved accuracy.
 The controller DSP 116 examines the response produced by the matched filter
 128. If a DS-CDMA system is present, the response will include a strong
 match indication corresponding to the phase of a base station in the
 vicinity of the radiotelephone 104. The response may include a match set,
 which is two or more closely clustered strong match indications. These
 strong match indications correspond to multiple strong rays received from
 a single base station, delayed in time. Presence of a single strong match
 indication or a match set indicates a DS-CDMA system is present.
 Concurrent with this, the response includes predetermined bits punctuated
 on the pilot channel signal which are passed to the DSP 116.
 A next step 206 includes, the DSP 116 detecting predetermined bits in the
 signal on the common pilot channel indicating activity on paging channels
 of the radio communication system. The predetermined bits indicate if
 there is activity on the paging channels and which of the paging channels
 are active. This is an advantage over the prior art in that it is not
 necessary to subsequently monitor all the paging channels to detect paging
 activity and can be done using the ASIC part of the DSP.
 Preferably, the predetermined bits of the detecting step include n bits
 punctuating a Walsh code of 0 comprising the signal on the pilot channel,
 the n bits indicating which paging channels are active. More preferably,
 the n bits of the detecting step are located in predetermined locations
 within a paging slot of data and punctuate the slot such that degradation
 in a channel estimate and searcher results derived from the pilot channel
 are minimized.
 If at step 206, the DSP determines that no activity is indicated, the call
 processor of the DSP 116 powers down portions of the electrical circuitry
 of the communication device so as to reduce power consumption.
 Specifically, the RF portion, the searcher portion of the receiver, and
 the call processor itself are powered down 208 such that the
 radiotelephone returns to sleep mode. However, if at step 206, the DSP
 determines that activity is indicated in the detecting step, the call
 processor of the DSP 116 powers up 210 further portions of the electrical
 circuitry of the communication device including the rest of the receiver
 circuitry and the rest of the DSP circuitry.
 At step 212, the radiotelephone 104 detects the synchronization channel and
 verifies system synchronization. In response to the synchronization
 channel, timing of the radiotelephone 104 is synchronized to the timing of
 the base station 102 which transmitted the synchronization channel.
 At step 214, the radiotelephone 104 acquires the active paging channel
 broadcast by the base station that had been indicated as active by the
 predetermined bits punctured on the pilot channel. The paging channel
 includes system information, referred to as a System Parameters Message,
 intended for all mobile stations in communication with the base station.
 The paging channel may also include a page or other information directed
 to the radiotelephone 104.
 At step 216, the radiotelephone coordinates with the base station to
 connects to a traffic channel so as to exchange the paging, voice or data
 information.
 Once the communication is completed at step 218, the radiotelephone
 includes a substep 220 (and 208) of powering down the portions of the
 electrical circuitry of the communication device including all of the
 receiver circuitry including the receiver searcher, all of the DSP
 circuitry including the call processor, and the RF circuitry of the
 radiotelephone, and returning to sleep mode at the beginning, step 200.
 Further, a substep can be included of maintaining the power off of the
 portions of the electrical circuitry of the communication device until a
 next slot of the pilot signal following the first slot, wherein at the
 next slot continuing at step 202. For example, the radiotelephone is
 maintained in a low-power mode (referred to as sleep mode or slotted mode
 battery savings), periodically interrupted by an active mode. Sleep mode
 is a low-power mode for reducing battery consumption, thereby extending
 battery life. In sleep mode, high-power circuit elements such as the
 analog front end 108, the ADC 110, and the rake receiver 112 are powered
 down. The radiotelephone enters sleep mode for a predetermined time. In
 accordance with IS-95, sleep mode continues for a duration of 1.28
 seconds, or powers of two multiples thereof. The controller identifies a
 strongest DS-CDMA pilot signal based on the response upon entering the
 active mode.
 The present invention can provide further benefit by adding steps for
 receiving a second set of predetermined bits so as to provide an estimate
 of channel gain.
 In all of the above cases, the radio communication system of the detecting
 step is at least one of a personal digital cellular (PDC) system, a Japan
 digital cellular (JDC) system, a code division multiple access (CDMA)
 system, a direct sequence code division multiple access (DS-CDMA) system,
 a groupe speciale mobile (GSM) system, and a slotted paging mode groupe
 speciale mobile (GSM-DRX) system. Other possible systems include AMPS
 (Advanced Mobile Phone Service) systems, GSM (Global System for Mobile
 communication) systems, TDMA (time division multiple access) systems such
 as the North American Digital Cellular, satellite systems, such as the
 Iridium system proposed by Iridium, LLC, or cordless systems such as DECT
 (Digital Extended Cordless Telephone) or PHS (Personal Handyphone System).
 In operation, the above method provides a method of reducing power
 consumption in a communication device having analog receiver circuitry,
 digital receiver circuitry including searcher circuitry in a portion
 thereof, and a digital signal processor including call processor circuitry
 in a portion thereof. The method includes a first step of providing analog
 receiver circuitry coupled to digital receiver circuitry including
 searcher circuitry in a portion thereof and a digital signal processor
 including call processor circuitry in a portion thereof, in a powered off
 sleep mode. A second step includes powering on the analog receiver
 circuitry, the searcher circuitry, and the call processor circuitry such
 that the analog receiver circuitry is operable to receive signals from a
 radio communication system. A third step includes sensing a signal of a
 common pilot channel through the analog receiver circuitry and coupled to
 the searcher circuitry. A fourth step includes processing the signal from
 the sensing step by the call processor circuitry so as to retrieve
 predetermined bits indicate activity on paging channels of the radio
 communication system. When no activity is indicated in the processing
 step, a last step includes powering down the analog receiver circuitry,
 searcher circuitry, and call processor circuitry of the communication
 device so as to reduce power consumption. When activity is indicated in
 the processing step, a fifth step includes powering up the digital
 receiver circuitry and the digital signal processor such that those paging
 channels indicating activity are monitored by the communication device.
 The present invention also incorporates an apparatus for reducing power
 consumption in a communication device. The apparatus includes: analog
 receiver circuitry to receive signals from a communication system and
 digital receiver circuitry including searcher circuitry in a portion
 thereof. The digital receiver circuitry provides a baseband demodulated
 data stream from the incoming signals coupled from the analog receiver
 circuitry. The searcher circuitry detects a common pilot channel signal in
 the data stream. A digital signal processor includes call processor
 circuitry in a portion thereof. The digital signal processor decodes
 messages in the data stream, and the call processor retrieves
 predetermined bits indicating activity on paging channels of the radio
 communication system.
 The analog receiver circuitry, the searcher circuitry, and the call
 processor circuitry are powered on from an initial sleep mode such that
 call processor can retrieve predetermined bits indicate activity on paging
 channels of the radio communication system. Where no activity is indicated
 by the predetermined bits, the analog receiver circuitry, the searcher
 circuitry, and the call processor circuitry are powered down so as to
 reduce power consumption. Where activity is indicated by the predetermined
 bits, the digital receiver circuitry and the digital signal processor are
 powered up such that those paging channels indicating activity are
 monitored by the communication device. After those paging channels
 indicating activity have been monitored by the communication device and
 any incoming calls received by the radio communication device have been
 completed, the analog receiver circuitry, the digital receiver circuitry
 and the digital signal processor can be powered down. by the communication
 device and any incoming calls received by the radio communication device
 have been completed, the analog receiver circuitry, the digital receiver
 circuitry and the digital signal processor can be powered down.
 FIG. 4 shows the timing of detecting punctured bits in pilot symbols in a
 paging slot. In a DS-CDMA system, a paging channel slot has a duration of
 80 ms: three times the duration of a rollover period of the CDMA system
 time pseudonoise (PN) generator (26.667 ms) and four times the duration of
 one frame (20 ms). Eighteen pilot symbols (Pilot j) are contained within
 64 pilot channel groups (PCG k) for each paging channel slot. In a typical
 slotted paging mode CDMA system the radiotelephone will wake up (power up)
 before Slot N to acquire the pilot channels, synchronization, and poll the
 paging channels before receiving the paging message in Slot N.
 In contrast, the present invention identifies active paging channels that
 occur in Slot N through n predetermined bits punctured on the pilot
 symbols of Slot N-1. The radiotelephone will wake up before Slot N to
 acquire the pilot channels and read the punctured bits identifying active
 paging channels, and can decide to power down where there are no paging
 messages indicated. This results in a total up time for a decision of much
 less than one slot time of 80 ms. In addition, computer simulations show
 that power consumption in this mode is less than half that of conventional
 CDMA paging polling.
 Preferably, the predetermined punctured bits include at least three bits
 punctuating a Walsh code of 0 comprising the pilot symbols. The at least
 three bits indicate paging activity and which of the paging channels are
 active. More preferably, the punctured bits include four bits at
 predetermined locations within the paging symbols of a slot. These
 locations can change from slot to slot. The predetermined locations can be
 preprogrammed into the radiotelephone or downloaded form the base station.
 In this way, the radiotelephone will know where the punctured bits are
 located within any particular slot. The punctured bits can be located in
 either or both of the I and Q channels of the pilot signal. It is
 preferred to puncture the bits on both the I and Q channels to maintain
 signal integrity. It is also preferred to repeat the punctured bits to
 improve decision integrity. Therefore, in a preferred embodiment, separate
 control of the bits on the PCG and Pilot symbols can yield one of 144
 unique combinations (144=64*18/8 bits, (4 bits repeated)), or control the
 eight possible pagings per slot.
 Referring back to FIG. 2, the radiotelephone 104 comprises an antenna 106,
 an analog front end 108, a receive path including an analog to the
 vicinity. Some of the received RF signals are directly transmitted, line
 of sight rays transmitted by the base station. Other received RF signals
 are reflected rays and are delayed in time.
 Received RF signals are converted to electrical signals by the antenna 106
 and provided to the analog front end 108. The analog front end 108 filters
 the signals and provides conversion to baseband I and Q signals. The
 analog baseband I and Q signals are provided to the ADC 110, which
 converts them to streams of I and Q digital data for further processing.
 The call processor of the DSP 116 controls the functions of the
 radiotelephone 104. The call processor operates in response to stored
 programs of instructions and includes a memory 132 for storing these
 instructions and other data. The call processor has a clock input 134 for
 receiving a clock signal. The clock 134 controls timing of the
 radiotelephone 104. For example, the clock 134 establishes a chip clock
 signal to control timing of the processing of received PN sequences
 throughout the radiotelephone 104. The call processor receives from the
 base station 102 the interval on which the radiotelephone must look for
 pages. Over this interval, the radiotelephone monitors the pilot channel
 for up to 160 ms and can sleep the remainder of the time. The call
 processor coordinates the events in the radiotelephone required for entry
 into and exit from sleep mode. Such events include keeping track of system
 time, advancing LSG states, restarting the oscillator, enabling power to
 the RF portion of the radiotelephone, and restarting the timer clock. The
 call processor is coupled to other elements of the radiotelephone 104.
 Such connections are not shown so as to not unduly complicate the drawing
 figure.
 The rake receiver 112 has a plurality of receiver fingers 122, 124, 126. In
 the illustrated embodiment, the rake receiver 112 includes three receiver
 fingers. However, any suitable number of receiver fingers could be used.
 The receiver fingers are of conventional design. In a manner to be
 described below, the receiver fingers of the rake receiver 112 are
 controlled by the controller DSP 116.
 The receiver searcher 114 detects pilot signals received by the
 radiotelephone 104 from the plurality of base stations including the base
 station 102. In accordance with the invention, the receiver searcher 114
 includes a matched filter 128 and a memory 130. The matched filter 128
 compares a detected I and Q PN sequence received from the ADC 110 and
 predetermined PN sequences stored in the memory and produces a response.
 In the illustrated embodiment, the predetermined PN sequences are stored
 in the memory 130.
 The matched filter 128 receives the I and Q streams of data from the ADC
 110. The data correspond to the spread, quadrature modulated signal
 received from the base station 102, including directly received or initial
 rays and reflected rays having a time delay. In addition, the data
 correspond to spread, quadrature modulated signals, direct and reflected,
 received from other base stations in the communication system 100. The
 data includes the PN sequences used for spreading the I and Q channels at
 the base station 102 and at all other base stations.
 The matched filter 128 compares the detected I and Q PN sequences with
 predetermined PN sequences. The predetermined PN sequences correspond to a
 portion of the 2.sup.15 element short PN sequences used to spread the I
 and Q channels at all base stations. The radiotelephone 104 includes a
 storage element such as the memory 130 or the memory 132 which stores a
 fixed pattern of PN values. The predetermined PN sequence includes the
 fixed pattern that comprises a predetermined number of chips of a PN
 sequence, for example the 512 last chips of a PN sequence such as the
 short PN sequence. The storage element also contains a table corresponding
 to paging channels which are identified by the predetermined bits
 punctured on the pilot channel.
 In the illustrated embodiment, the pilot signals are quadrature modulated,
 with each of the pilot signals including in-phase (I) symbols and
 quadrature-phase (Q) symbols. The I symbols are spread using an I PN
 sequence and the Q symbols are spread using a Q PN sequence. The matched
 filter 128 accordingly includes an I filter 140 for comparing a detected I
 PN sequence and a stored I PN sequence and a Q filter 142 for comparing a
 detected Q PN sequence and a stored Q PN sequence and producing the
 response. Either or both of the I filter 140 and Q filter 142 responses
 may be used as the response of the matched filter 128. The response is
 passed to the call processor to detect the predetermined bits of the pilot
 signal. Use of both responses improves the quality of the response of the
 matched filter 128. In the illustrated embodiment, summing element 144
 combines the response of the I filter 140 and the response of the Q filter
 142 to produce the response of the matched filter 128. A comparator 146
 suppresses the response when the response does not exceed a predetermined
 threshold. For example, the matched filter will continuously produce the
 response, even if no CDMA system is present or if only noise is present.
 The threshold is set to a predetermined value to prevent storage of the
 response in the memory 130 when is no meaningful input symbols are
 received.
 The matched filter 128 provides a response to the comparison of the
 detected PN sequence and the predetermined PN sequence. The response in
 stored, for example in memory 130 or memory 132. In the illustrated
 embodiment, the response is double buffered. That is, the matched filter
 114 stores the response in a first set of memory locations (such as memory
 130) as the response is determined. However, it should be recognized that
 a single memory could be used. The controller DSP 116 reads the response
 and compares it to the look-up table stored in one of the memory locations
 (such as memory 132).
 Although the invention has been described and illustrated in the above
 description and drawings, it is understood that this description is by way
 of example only and that numerous changes and modifications can be made by
 those skilled in the art without departing from the broad scope of the
 invention. For example, although the present invention finds particular
 application in portable cellular radiotelephones, the invention could be
 applied to any communication device, including pagers, electronic
 organizers, or computers.