document: CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 60/275,032, entitled “Method and Apparatus for Multipath Signal Detection, Identification, and Monitoring for WCDMA Systems,” filed Mar. 12, 2001, which is incorporated herein by reference in its entirety. 
  
  
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is generally related to wireless communication systems and, more particularly, is related to systems and methods for detection, identification, and monitoring of multipath signals in wideband code division multiple acess (WCDMA) systems. 
     2. Related Art 
     With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone like communication handset. The different modulation and transmission schemes each have advantages and disadvantages. 
     The next generation of wireless communication is referred to as 3G, which stands for third generation. 3G refers to pending improvements in wireless data and voice communications through a variety of proposed standards. One goal of 3G systems is to raise transmission speeds from 9.5 kilobits (Kbits) to 2 megabits (Mbits) per second. 3G also adds a mobile dimension to services that are becoming part of everyday life, such as Internet and intranet access, videoconferencing, and interactive application sharing. This advancement in wireless communication necessitates improvements in the area of signal detection, identification, and monitoring of multipath signals, which are two or more identical signals from the same antenna reaching the receiver at different times due to taking different paths from the antenna to the receiver. 
     SUMMARY 
     The present invention provides a method and system for generating a mobile time reference for a portable transceiver. 
     Briefly described, one embodiment of the system comprises an antenna, a radio frequency subsystem, and a baseband subsystem. The radio frequency subsystem is coupled to the antenna and includes a high frequency oscillator and a low frequency oscillator. The baseband subsystem is coupled to the radio frequency subsystem and includes a free running counter coupled to the high frequency oscillator and the low frequency oscillator. The free running counter provides a mobile time reference to the system and has a wake mode and a sleep mode. During the wake mode the free running counter uses the high frequency oscillator to generate the mobile time reference, and during the sleep mode the free running counter uses the low frequency oscillator to maintain the mobile time reference. 
     The present invention can also be viewed as providing a method of generating a mobile time reference. In this regard, one embodiment of such a method, can be broadly summarized as including the steps of providing a high frequency clock, providing a low frequency clock, generating a mobile time reference using the high frequency clock, maintaining the mobile time reference using the low frequency clock when the high frequency clock is not available, and continuing to generate the mobile time reference using the high frequency clock when the high frequency clock is again available. 
     Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
  
  
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram illustrating one embodiment of a third generation portable transceiver according to the present invention. 
         FIG. 2  is a block diagram of a free running counter in the WCDMA modem of  FIG. 1 . 
         FIG. 3  is a block diagram of the WCDMA modem of  FIG. 1  including the multipath monitor and multipath radio signal recovery circuit. 
         FIG. 4  is a flow diagram of one embodiment of a method of providing a mobile time reference. 
     
  
  
     DETAILED DESCRIPTION 
     Having summarized various aspects of the present invention, reference will now be made in detail to the description of the invention as illustrated in the drawings. While the invention will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed therein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the scope of the invention as defined by the appended claims. 
       FIG. 1  is a block diagram illustrating a simplified 3G portable transceiver  20 . In one embodiment, portable transceiver  20  can be, for example but not limited to, a portable telecommunication handset such as a mobile cellular-type telephone. Portable transceiver  20  includes antenna  22  connected to radio frequency subsystem  24 . RF subsystem  24  includes receiver  26 , receiver baseband analog processor (BAP)  28 , transmitter  30 , transmitter BAP  32 , high frequency oscillator (which may be implemented as a temperature controlled crystal oscillator (TCXO))  34 , low frequency oscillator (which may be a 32 KHz crystal oscillator (CO))  36 , and transmitter/receiver switch  38 . 
     Antenna  22  transmits signals to and receives signals from switch  38  via connection  40 . Switch  38  controls whether a transmit signal on connection  42  from transmitter  30  is transferred to antenna  22  or whether a received signal from antenna  22  is supplied to receiver  26  via connection  44 . Receiver  26  receives and recovers transmitted analog information of a received signal and supplies a signal representing this information via connection  46  to receiver BAP  28 . Receiver BAP  28  converts these analog signals to a digital signal at baseband frequency and transfers it via bus  48  to baseband subsystem  50 . 
     Baseband subsystem  50  includes WCDMA modem  52 , microprocessor  54 , memory  56 , digital signal processor (DSP)  58 , and peripheral interface  60  in communication via bus  62 . Bus  62 , although shown as a single bus, may be implemented using multiple busses connected as necessary among the subsystems within baseband system  50 . WCDMA modem  52 , microprocessor  54 , memory  56 , and DSP  58  provide the signal timing, processing, and storage functions for portable transceiver  20 . Memory  56  may include dual port random access memory (RAM) shared by microprocessor  54  and DSP  58 . 
     Peripheral interface  60  provides connection to baseband subsystem  50  for a variety of items. These items may include, but are not limited to, devices that are physically part of portable transceiver  20 , such as speaker  62 , display  64 , keyboard  66 , and microphone  68 , and devices that would be externally connected to portable transceiver  20 , such as personal computer (PC)  70 , test system  72 , and host system  74 . Speaker  62  and display  64  receive signals from baseband subsystem  50  via connections  76  and  78 , respectively, as known to those skilled in the art. Similarly, keyboard  66  and microphone  68  supply signals to baseband subsystem  50  via connections  80  and  82 , respectively. PC  70 , test system  72 , and host system  74  all receive signals from and transmit signals to baseband subsystem  50  via connections  84 ,  86 , and  88 , respectively. 
     Baseband subsystem  50  provides control signals to RF subsystem  24  via connection  90 . Although shown as a single connection  90 , the control signals may originate from WCDMA modem  52 , microprocessor  54 , or DSP  58 , and are supplied to a variety of points within RF subsystem  24 . These points include, but are not limited to, receiver  26 , receiver BAP  28 , transmitter  30 , transmitter BAP  32 , TCXO  34 , and switch  38 . 
     WCDMA modem  52  receives the digital signal from receiver BAP  28  on bus  48  and provides a digital signal to transmitter BAP  32  on bus  92 . Transmitter BAP  32  converts this digital signal to an analog signal at radio frequency for transmission on connector  94  to receiver  30 . Receiver  30  generates the transmit signal which is provided to antenna  22  via connectors  40 ,  42  and switch  38 . The operation of switch  38  is controlled by a control signal from baseband subsystem  50  via connection  90 . 
     In accordance with an embodiment of the invention, TCXO  34  provides a clock to receiver  26 , transmitter  30 , and WCDMA modem  52  via connectors  96 ,  98 , and  100 , respectively, and CO  36  provides on connector  102  a 32 KHz clock to WCDMA modem  52 . These two clocks are used by WCDMA modem  52  to create a mobile time reference. This mobile time reference is constantly running and has an accuracy of approximately 32 nanoseconds. 
     Referring now to  FIG. 2 , a portion of WCDMA modem  52  is shown illustrating free running counter (FRC)  104  which generates the mobile time reference for use by portable transceiver  20 . FRC  104  is provided with a clock signal from the TCXO on line  100  and a clock signal from the CO on line  102 . The clock signal from the TCXO on line  100  can be a 30.72 MHz, and the clock signal from the CO on line  102  can be 32 KHz. FRC  104  includes TCXO circuit  106 , phase locked loop (PLL)  108 , counter  110 , drift estimator  112 , and correction circuit  114 . TXCO circuit  106  using the 30.72 MHz clock generates the mobile time reference. The 32 kHz clock is phase locked to the 30.72 MHz clock for improved performance using PLL  108 . Counter  110  counts the cycles of the 32 KHz clock. Drift estimator  112  provides an estimate of the drift of the 32 KHz clock for use by correction circuit  114 . The estimate of drift includes both the drift and bias of the clock as provided by a Kalman estimation as known to those having ordinary skill in the art. FRC  104  operates in two time domains, 30.72 MHz or 32 KHz, depending on whether the portable transceiver  20  is in active mode or idle mode, respectively. 
     In active mode the portable transceiver is actively transmitting, receiving, processing, or looking for signals. During idle mode the portable transceiver powers down most of its circuits to conserve power. The CO  36  is always on providing a continuous 32 KHz clock signal to FRC  104 , but the TCXO  34  is turned off during idle mode. 
     Now referring to  FIG. 3 , a block diagram of the WCDMA modem  52  is shown. When the portable transceiver is in active mode, FRC  104  provides the mobile time reference including clock-phase, chip-counter, and slot-counter on bus  150  to primary sync searcher  116 , secondary sync searcher  118 , gold code searcher  120 , and single-path processor (SPP) controller  122  as shown in  FIG. 3 . A 10 millisecond radio frame is divided into 15 slots (slot-counter  0 – 14 ). Each slot includes 2,560 chips (chip counter  0 – 2 , 559 ). Each chip contains 8 ticks (clock-phase  0 – 7 ). FRC  104  also generates a frame counter ( 0 – 511 ) for the mobile time reference by counting the frames that occur within a 5.12 second period. Also, the drift estimate is continually updated when the portable transceiver is in active mode. 
     When transitioning into the idle mode, a sleep/awake control signal on line  124  from the microprocessor to FRC  104  transitions to a low state. Counter  110  is reset and begins counting the rising edges of the 32 KHz clock signal. At the next rising edge of the 32 KHz clock after the sleep/awake control signal transitions to a low state, the current mobile time reference and drift estimate from TXCO circuit  106  is provided to correction circuit  114 . At this time the portable transceiver goes into idle mode. During each subsequent count, correction circuit  114  updates the mobile time reference using the count and the drift estimate. Thus, the mobile time reference and drift estimate is maintained during the idle mode. 
     When transitioning to the active mode, the sleep/awake control signal transitions to a high state. At the next rising edge of the 32 KHz clock, the updated mobile time reference maintained in correction circuit  114  is provided to TCXO circuit  106  and FRC  104  begins providing the mobile time reference for the portable transceiver using the 30.72 MHz clock and the updated mobile time reference as a starting point. 
     The idle time may extend into a number of seconds, and the active time with no paging detected could be as long as 5 milliseconds. Maintaining the mobile time reference during idle mode allows the portable transmitter to quickly transition to an active state, which translates into a shorter duration in the active state, thus reducing power consumption and extending battery life. Maintaining the mobile time reference to a 32 nanosecond accuracy improves the efficiency of detecting, identifying, and monitoring the incoming multipath signals. 
     The FRC provides a timing reference for the portable transceiver system and for acquiring the parameters required to recover the multipath signals and operate a nultipath signal receiver. 
     WCDMA modem  52  includes FRC  104 , receiver equalizer  126 , multipath monitor  128 , and multipath radio signal recovery circuit  130 . The mobile time reference from FRC  104  is provided to both multipath monitor  128 , such as a code acquisition system, and multipath radio signal recovery circuit  130 , such as a RAKE receiver as known to those having ordinary skill in the art. The digital signal from the receiver BAP  28  is provided to receiver equalizer  126  and equalized prior to being provided on bus  144  to multipath monitor  128  and multipath radio signal recovery circuit  130 . 
     Multipath monitor  128  includes primary sync searcher  116 , secondary sync searcher  118 , and gold code searcher  120 , and provides information regarding these searches to the microprocessor. 
     In one embodiment, multipath radio signal recovery circuit  130  includes SPP controller  122 , twelve SPPs  132 , twelve first-in first-out (FIFO) circuits  134 , twelve phase correctors  136 , deskewing and timing controller (DTC)  138 , four maximal rate combiners (MRC)  140 , and four demodulation units  142 . 
     SPP controller  122  maps up to twelve multipath signals to SPPs  132  and provides a start command on bus  146  to each of the SPPs  132 . Each SPP recovers and tracks incoming clock information relative to a basestation, provides the clock information to DTC  138 , and provides phase estimation for both single and multiple basestation antennas to the corresponding phase corrector  136 . The clock information provided to DTC  138  is in the same form as the mobile time reference having a clock-phase, a chip-counter, and a slot-counter. The mapped equalized signal is passed through each SPP  132  to the corresponding FIFO  134 . Each FIFO  134  has a subperiod of 512 chips. Each radio frame includes 38,400 chips or seventy-five subperiods of 512 chips. 
     DTC  138  using the clock information from each of SPPs  132  provides a read address and read strobe signal from bus  148  to each SPP, which time aligns the outputs of FIFOs  134  relative to one another. The operation of DTC  138  is a complex PLL operation. The output of each FIFO  134  is provided to the corresponding phase corrector  136  which corrects the phase of the signal using the phase estimation provided by the corresponding SPP  132 . The outputs of each phase corrector  136  is mapped to one of the four MRCs  140 . Each MRC  140  combines the signals mapped to it to increase the strength of the signal. The strengthened signal from each MRC  140  is provided to the corresponding demodulation unit  142 . Demodulation unit  142  recovers information from the signal on up to eight channels. Thirty-two different channels are provided for information recovery. 
     One of the benefits of the above recovery system is that the information is not recovered from the signals until the signals are time aligned improving the efficiency of the recovery. Another benefit is that signals from either or both of the antennas from a basestation can be utilized and mapped to an SPP. Still another benefit is the ability to align the signals from asynchronous basestations, i.e. basestations operating using different clocks. 
     The present invention provides a wideband spread spectrum multipath signal detector, identification mechanism, and multipath monitoring technique that monitors signal strength from a plethora of asynchronous transmitters within a network in the presence of a moving time base, slotted operations, and effects of the mobile radio channel. The time base is moving with movement of the portable transceiver. Slotted operations are utilized to spread the required processing out over multiple active periods. The system performs matched filtering of the primary synchronization code and creates a non-coherent energy measurement that is transformed into the log2-domain to reduce memory requirements for multiple hypotheses of slot level timing. Hypotheses are low pass filtered to enhance the probability of detection of signals that are candidates for further processing. The system recovers the transmitted code groups and frame timing for the received signal components. A parallel set of correlators performs the final identification of the despreading code over all possible codes in parallel. Timing is maintained in the system through use of Kalman estimation that recovers clock drift and bias when utilizing low cost crystals for standby, low power consumption, and slotted operations. 
     A fast wakeup mechanism recovers multiple hypotheses in parallel to arrive quickly at a time base that can initiate subsequent demodulation of the spread spectrum signal, thus reducing power consumption and minimizing duty cycle of the slotted mode operations. A method for recovery of network timing in the presence of frequency uncertainty, that uses energy measurements form the primary synchronization code, allows user equipment to detect, identify, and perform subsequent demodulation of the downlink signal. 
     Referring now to  FIG. 4 , one embodiment of a method of providing a mobile time reference for the portable transceiver of  FIG. 1  is shown. Initially, a high frequency clock (30.72 MHz) is provided by a temperature controlled crystal oscillator as shown in block  200 . In block  202 , a low frequency clock is provided by a crystal oscillator operating at 32 KHz. A mobile time reference is generated using the high frequency clock in block  204 . The mobile time reference includes a clock-phase signal, a chip-counter, a slot-counter, and a frame-counter. As shown in block  206 , the mobile time reference is maintained using the low frequency clock, when the high frequency clock is not available. Finally, when the high frequency clock is again available, the mobile time reference continues to be generated with the high frequency clock and the maintained mobile time reference as a starting point in block  208 . 
     The low frequency clock is phase locked to the high frequency clock, and an estimate of the drift and bias of the low frequency clock is made using a Kalman estimation. During the step of maintaining the mobile time reference, the mobile time reference is updated using the cycles of the low frequency clock counted when the high frequency clock is not available and the estimated drift/bias of the low frequency clock. 
     Although an exemplary embodiment of the present invention has been shown and described, it will be apparent to those of ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described may be made, none of which depart from the scope of the present invention. All such changes, modifications, and alterations should therefore be seen as within the scope of the present invention.
summary: Methods to generate a mobile time reference are provided. A representative method includes providing a high frequency clock, providing a low frequency clock, generating a mobile time reference using the high frequency clock, maintaining the mobile time reference using the low frequency clock when the high frequency clock is turned off, and continuing to generate the mobile time reference using the high frequency clock when the high frequency clock has been turned back on. Systems and other methods are also provided.