Abstract:
Methods and systems are provided which store frequency settings before switching frequencies, for example, before assignment to the traffic channel when switching between a ground station and another mobile station), thus, making it possible to recall the previous frequency setting and then only perform a second pass of the synchronization process or no synchronization at all. By storing the frequency setting, either as an offset to a given frequency or as an absolute frequency for each system to be monitored, the stored frequency setting can be re-used when subsequently monitoring a system.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless devices and more particularly, to the synchronization to multiple frequencies by the wireless devices. 
     BACKGROUND OF THE INVENTION 
     Digital telecommunication systems typically require that the transmitter and receiver be synchronized precisely to the time and frequency alignment of the other system components. However, such synchronization may present difficulties for a system containing multiple transmitter and/or receiver components. For example, in a system where two digital mobile communications devices in motion are communicating with each other and a ground station through, for example, a satellite, if the ground station&#39;s zero time served as the absolute timing reference, the timing relationship between all components the system would now be known. However, the frequency relationship would be unknown due to the Doppler shift of the frequency caused by the motion between the two terminals. Thus, different frequency offsets would exist between the components of the system. These frequency offsets may present problems when switching communications between the different relative frequencies of the components as insufficient time may be available to compensate for the differing offsets utilizing conventional techniques. 
     Another example of a system which may have difficulties resulting from multiple frequency offsets is a mobile station which monitors a broadcast control channel from one station for control information, communicates with a second station handling voice/data traffic, and reverts back to monitoring the original broadcast control channel from the first station after a call. In such a system the switching between the control station and the voice/data station may result in information loss as a result of the time required for such a switch utilizing conventional techniques. 
     A further example of a system which may have difficulties associated with multiple frequency offsets is a satellite communication system. In satellite-cellular communications systems, such as the ACeS (Asian Cellular Satellite System), an additional frequency shift is applied to the system from the motion of a geosynchronous satellite in an inclined orbit. Ground stations in this type of system communicate with the mobile station (user terminal) via the satellite, which causes a different Doppler shift for each ground station due to the different geographical locations of the ground stations. 
     The user terminal in ACeS will, depending on its mode of operation, experience multiple Doppler frequency shifts. When in Idle mode, which is when the user terminal is monitoring the system for pages, the Network Control Center (NCC) is monitored through the satellite. When a call is initiated between a user terminal and the Public Satellite Telephone Network (PSTN), the user terminal synchronizes to a gateway (GW), which will handle the signaling and voice/data communication with the ground station associated with the PSTN. A user terminal to user terminal connection through the satellite between a first user terminal and a second user terminal requires yet another synchronization process. The call is established initially between each user terminal and the gateway. When the voice channel is allocated each user terminal synchronizes to the other. 
     The voice channel has two logical channels: S-SACCH (Satellite-Slow Associated Control Channel) and S-TCH (Satellite-Traffic Channel). The S-TCH carries voice through the satellite from user terminal to user terminal while the S-SACCH is connected to the gateway so the system can monitor the quality of the call. Thus, the user terminal, typically, must tune to one frequency for the S-TCH, characterized by the Doppler shift caused by the satellite moving relative to the other user terminal; and a second frequency for the S-SACCH characterized by the Doppler shift caused by the satellite moving in relation to the gateway. Also, the user terminal must be able to return to the Network Control Center frequency without delay when the call terminates. 
     Conventional methods for synchronization to a frequency typically involve a time and power consuming synchronization process involving multiple auto-correlations against a given bit-pattern which is performed on each frequency each time it is to be monitored. This synchronization process determines the frequency offset. Such a synchronization process is illustrated in FIG.  1 . As seen in FIG. 1, the conventional synchronization process is a two-pass process with the second pass fine tuning the values from the first pass. The conventional synchronization process may begin by capturing 4 synchronization bursts sampled with a double wide window (i.e. a burst of B width is sampled in a  2 B window). The synchronization bursts include known sequences from the network control center (block  100 ). An autocorrelation is then performed against the fixed pattern across  21  frequency bins with a window size of 30 bits (block  102 ). The timing and frequency are then adjusted (coarse synchronization) based upon the error resulting from the autocorrection determination (block  104 ). After this first pass, a second pass begins by capturing 4 more synchronization bursts utilizing a single width window (i.e. a window of B width) (block  106 ). The autocorrelation is then performed on these 4 synchronization bursts utilizing the fixed pattern across 21 frequency bins with a window size of 4 bits (block  108 ). The final timing and frequency adjustments (fine synchronization) are then made based upon the error resulting from the second autocorrelation process (block  110 ). 
     Typically, capturing four (4) synchronization bursts with the double wide window takes about 1.9 seconds, which is followed by another 120 milliseconds of Digital Signal Processor (DSP) processing. The second process using four (4) bursts with a single width window takes about the same amount of time. The complete synchronization process may take about 4 seconds for burst capture plus processing time. This process generally requires too much time to resynchronize when returning to the control channel from a traffic channel or to synchronize when switching between settings for a ground station versus a base station. 
     In present digital cellular systems, the user terminal tunes its oscillator to each new frequency after. completing each call. Thus, for a user terminal to base station call where the base station is a station other than the Network Control Center station, the user terminal would have to go through a synchronization process when returning to the Network Control Center station. Such a process may result in missed information during the lengthy synchronization process. 
     Furthermore, the synchronization process described above does not facilitate user terminal to user terminal communication because S-TCH and S-SACCH information may be spaced by as little as 900 microseconds while resynchronization between frequencies may require several milliseconds using conventional techniques. Thus, information may be lost during the resynchronization. 
     A further problem may arise as a result of Global System for Mobile Communications (GSM) and ACeS standards which both require that all timing be derived from a single oscillator. Due to frequency-time relationships, it may not possible to readjust the oscillator in a timely manner to account for frequency shifts when communicating with devices at two different locations having slightly different frequency shifts due to the Doppler effect or other frequency shifting mechanism. 
     In light of the above discussion, a need exists for improvements in the synchronization of multiple frequencies in a communication system. 
     SUMMARY OF THE INVENTION 
     In view of the above discussion, it is an object of the present invention to provide systems and methods which allow rapid switching between frequency sources. 
     Still another object of the present invention is to provide for rapid switching between multiple frequencies based on a single oscillator frequency. 
     These and other objects of the present invention are provided by methods and systems which store frequency settings before switching frequencies, for example, before assignment to the traffic channel (or when switching between a ground station and another mobile station), thus, making it possible to recall the previous frequency setting and then only perform a second pass of the synchronization process or no synchronization at all. 
     Because digital communication systems potentially switch between various systems faster than Doppler shifts cause significant frequency change (milliseconds versus seconds), the frequency used for communicating with the system previously is very likely still accurate during subsequent attempts to communicate with the system. By storing the frequency setting, either as an offset to a given frequency or as an absolute frequency for each system to be monitored, the stored frequency setting can be re-used when monitoring a system after an initial synchronization. This saves time and lowers power consumption when monitoring more than one system, for example, where each one has a different Doppler shift. 
     In particular embodiments of the present invention, methods and systems are provided which provide for rapidly synchronizing a wireless device to at least two received signals by determining synchronization parameters associated with a first received signal from a first source device and storing the determined synchronization parameters for the first received signal. The wireless device then also synchronizes to a second received signal from a second source device, where the first source device is remote from the second source device. The first received signal is then re-synchronized to utilizing the stored determined synchronization parameters associated with the first received signal. Thus, utilizing the stored parameters, the speed of the resynchronization process may be increased. 
     In a further embodiment of the present invention, the synchronization parameters are determined by synchronizing to the first received signal. Furthermore, the synchronization to the second received signal may be carried out by determining a frequency offset value associated with the second received signal based on the first received signal. The second received signal is then synchronized to utilizing the frequency offset value. 
     In a particular embodiment of the present invention, the synchronization to the first received signal so as to provide first synchronization parameters synchronizes to the first received signal utilizing a first synchronization process. In such a case then, synchronizing to the first received signal utilizing the stored determined synchronization parameters may be carried out by synchronizing to the first received signal utilizing a second synchronization process, where the second synchronization process utilizes the stored determined synchronization parameters associated with the first received signal. In particular, the second synchronization process may be an abbreviated version of the first synchronization process. 
     In still another embodiment of the present invention, the determination of synchronization parameters may involve determining a phase offset between the first received signal and a reference frequency so as to provide a first received signal phase offset. A phase offset between the second received signal and the reference frequency may also be determined so as to provide a second received signal phase offset. The storage of the synchronization parameters may then involve storing the first received signal phase offset to allow rapid return to synchronization to the first received signal. Furthermore, synchronizing to the first received signal and to the second received signal may be carried out by applying respective ones of the phase offsets to the reference frequency. 
     In a further embodiment of the present invention, the reference frequency comprises the first received signal. In such a case, the determination of a phase offset between the first received signal and the reference frequency may comprise setting the first received signal phase offset to zero. 
     In another embodiment of the present invention, a local oscillator of the wireless device is synchronized to a reference frequency transmitted by a third device. In a specific embodiment, the first device and the third device are located at the same geographic location and may comprise the same device. 
     As will be appreciated by those of skill in the art, the present invention may be provided as systems or methods, including radiotelephones or base stations which incorporate such systems or methods. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart of a conventional synchronization process; 
     FIG. 2 is a block diagram of a wireless device suitable for use with the present invention; 
     FIG. 3 is a diagram of an operating environment having a wireless device utilizing one embodiment of the present invention communicating between a network control center and a traffic/data broadcast; 
     FIG. 4 is a flow chart illustrating operations of one embodiment of the present invention in the operating environment of FIG. 3; 
     FIG. 5 is a diagram of an operating environment having a wireless device utilizing one embodiment of the present invention communicating between a network control center, a gateway and a second wireless device; and 
     FIG. 6 is a flow chart illustrating operations of one embodiment of the present invention in the operating environment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. As will be appreciated by one of skill in the art, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. 
     An embodiment of a wireless device  10 , such as a mobile user terminal, radiotelephone or other such wireless device, which incorporates the frequency offset determinations and synchronization procedures according to the present invention is illustrated in FIG.  2 . As seen in FIG. 2, the wireless device  10  includes a voltage controlled oscillator (VCO)  12 . The frequency of the output of the VCO  12  is dependent on an input voltage. The input voltage in the wireless device  10  illustrated in FIG. 2 is provided by digital to analog converter (DAC)  14 . DAC  14  receives a digital input value from control processor  30  and converts this digital value to an analog voltage level which is provided to VCO  12  to control the output frequency of VCO  12 . 
     As is further illustrated in FIG. 2, the VCO output is provided to a transceiver  22 . The antenna structure  26  of wireless device  10  may be used to both entitled “Method of Rapidly Controlling the Frequency of a Coherent Radio Receiver and Apparatus for Carrying Out the Method,” the disclosures of which are incorporated by reference as if set forth fully herein. Using any such method, the determined frequency error of the received signal relative to the receiver&#39;s own reference oscillator is periodically supplied to processor  30  to control the voltage controlled oscillator  12 . 
     FIG. 2 also includes storage  20 , which may be random access memory (RAM), non-volatile memory or other suitable storage media for storing synchronization parameters. Storage  20  is illustrated as associated with control processor  30 , however, as will be appreciated by those of skill in the art, storage  20  may be associated with other functions, components or circuits, such as signal processing circuit  18 . All that is needed for purposes of the present invention is that storage be available to the function or functions which maintain synchronization with the two or more received frequencies to which the wireless device  10  is to be synchronized. Thus, if the synchronization to the received signals is performed by signal processing, such as for example, by the application of a phase ramp to the reference oscillator, then such storage should be accessible to signal processing circuit  18  as illustrated in FIG.  2 . 
     In any event, while FIG. 2 provides an example of a single oscillator mobile terminal  10  suitable for use with ACeS or GSM, the present invention should not be construed as limited to any particular hardware configuration. Other suitable hardware configurations known to those of skill in the art may also be utilized and need not be described further herein. Thus, the configuration of FIG. 2 is provided for exemplary purposes only and is not intended to limit the scope of the present invention. 
     FIG. 3 illustrates one operating environment where the present invention may be utilized. As seen in FIG. 3, a wireless device  10  communicates with a first station  50 , which may act as a network control center, through satellite  60  and a receives voice and/or data traffic from a second station  52  through satellite  60 . The wireless device  10  communicates with the first station  50  over communication channels  62  and  54  and with the second station over communication channels  62  and  56 . The wireless device  10  may utilize differing frequencies for communication with the first station  50  and the second station  52 . Furthermore, the frequency of the communications over channels  62 ,  54  and channel  56  may be affected differently by the motion of wireless device  10 , thus, requiring wireless device  10  to operate at one frequency when communicating with network control center  50  and a different frequency when communicating with the voice/data traffic station  52 . As described with respect to FIG. 4, the present invention may accommodate such differing frequencies without requiring extensive resynchronization procedures as would conventionally be carried out. 
     FIG. 4 illustrates operations of a wireless device  10  according to the present invention. As seen in FIG. 4, the wireless device  10  determines if its synchronization with the network control center (station  50 ) is the initial synchronization to the network control center (block  300 ). If the synchronization is the initial synchronization, then the wireless device  10  performs a complete synchronization to the network control center frequency (block  302 ), such as by performing the conventional two step correlation procedure of FIG.  1 . Such a synchronization may be carried out by tuning VCO  12  to the reference frequency of station  50  as described above. 
     The wireless device  10  then stores the synchronization parameters resulting from this initial synchronization (block  304 ), for example, in storage  20  of FIG.  2 . The synchronization parameters stored may depend on the synchronization process performed but should include sufficient information to allow an abbreviated synchronization procedure for future synchronization or no synchronization procedure at all with the first station  50 . For example, the synchronization parameters stored may include VCO input voltage, phase differences, error values or the like. The particular parameters to be stored would be implementation specific and would be readily ascertainable to those of skill in the art in light of the present disclosure. Accordingly, the present invention should not be construed as limited to storing any particular parameters or combination of parameters. However, for systems where the differences in frequency are small, it is preferred that phase rotation and de-rotation (or phase ramp) be utilized with the present invention as is described below. 
     After synchronization and storage of the synchronization parameters, the wireless device  10  then monitors the network control center to wait for a call, page or other communication requiring switching to a second station (block  310 ). When a call is received by the wireless device  10 , the wireless device  10  determines the frequency offset between the second station  52  and the first station  50  and synchronizes to the second station  52  (block  312 ). Such a frequency offset determination may be made by conventional determination methods known to those of skill in the art. For example, in the ACeS system, the frequency offset may be determined by utilization of the autocorrelation process of FIG. 1 to determine the frequency offset of the receive synch bursts from the nominal carrier frequency obtained during the immediate assign phase of call start up. 
     The wireless device  10  utilizes the frequency offset to the second station  52  to synchronize to the second frequency to carry out communications with the second station  52  at the second stations frequency, and thus, takes into account the relative frequency differences resulting from motion in differing directions relative to the two stations. Synchronization to the second frequency may be accomplished by adjusting the frequency of the VCO  12  or by compensating the VCO output to adjust the frequency. 
     As is further illustrated in FIG. 4, the wireless device may optionally continue compensation for drift in the frequencies from the first station  50  and the second station  52  (block  314 ) during the call and store these compensations (block  316 ). This compensation and storing process may continue during the call (block  318 ). The drift may be tracked using conventional tracking procedures known to those of skill in the art and, therefore, will not be described further herein. While drift tracking is described herein with respect to both stations, drift tracking for the first station  50  may be optionally carried out if, for example, insufficient processing capabilities are available to carry out tracking both frequencies. In such a case, the stored synchronization parameters of block  304  would be utilized for subsequent re-synchronization to the first station. 
     When the call is completed, the wireless device  10  returns to operation at the network control center frequency. Because it is not the initial synchronization to the network control center of the first station  50  (block  300 ), the wireless device  10 , when returning to the network control center, may recall the synchronization parameters previously stored (block  306 ) and utilize those synchronization parameters in an abbreviated synchronization procedure to resynchronize to the network control channel frequency of the first station  50  (block  308 ). As described above, these parameters may be the parameters stored at block  304  if drift tracking of the NCC frequency is not performed or may be the parameters stored at block  316  if drift tracking of the NCC frequency is performed. 
     Thus, for example, when returning to the network control channel, wireless device  10  may recall the previously stored synchronization parameters for the network control center and then perform a single pass correlation process, such as is described in blocks  106  through  110  of FIG.  1 . Alternatively, the stored parameters may simply be utilized to set the frequency of the wireless device  10 . Thus, the speed with which the resynchronization to the network control frequency is achieved may be increased and the delay resulting from switching between two frequency sources may be reduced. The wireless device  10  would then wait for the next call which would require switching to the traffic/data station  52  or some other second station. 
     FIG. 5 illustrates a second operating environment in which an embodiment of the present invention may be advantageously utilized. As seen in FIG. 5, a wireless device  10  communicates with a first station (not shown), which may act as a network control center, and a second station  52  through a communication path such as a satellite  60 . The wireless device  10  communicates with the second station  52  over communication channels  62  and  54  and with the second wireless device  10 ′ over communication channels  62  and  62 ′. As is further seen in FIG. 5, a second wireless device  10 ′ may also communicate with the second station  52  over channels  62 ′ and  54  and the first wireless device  10  over channels  62  and  62 ′. The first wireless device  10  may be in motion in the direction of arrow  58  and the second wireless device  10 ′ may also be in motion in the direction of arrow  58 ′. The satellite  60  acts as a conduit between the channels and translates the transmit and receive frequencies between the wireless devices  10  and  10 ′. Thus, the relative motions of the wireless devices  10  and  10 ′ may result in differing frequencies required for communications on the different channels. Furthermore, the satellite  60  may also be in motion which may also add to the differing frequencies. 
     Typically, for wireless device  10  and wireless device  10 ′ to communicate directly through satellite  60  they will each tune to the network control center of station (not shown), and then to the gateway of station  52  and with each other over channels  62  and  62 ′ and then back to the network control center when a call is complete. Each device should, therefore, be capable of rapidly adjusting for the differences in frequency associated with differing relative motion of the wireless devices  10  and  10 ′ or other causes which may result in deviation in frequency of operation. A further complication may also be present if, as with GSM or ACeS, the wireless devices are constrained to have a single local reference oscillator such as illustrated in FIG.  2 . 
     As described with respect to FIG. 6, the present invention may accommodate such differing frequencies in a single local reference frequency device without requiring extensive resynchronization procedures as would conventionally be carried out. As discussed above, these conventional techniques may be too time consuming to allow proper operation in an environment such as is illustrated in FIG.  5 . 
     FIG. 6 illustrates operations of wireless devices  10  and  10 ′ according to the present invention. For clarity, however, the operations of FIG. 6 will be described with respect to wireless device  10 . However, as will be appreciated in light of the present invention, preferably these operations are carried out by both wireless device  10  and wireless device  10 ′. As seen in FIG. 6, the wireless device  10  determines if synchronization with the network control center is the initial synchronization to the network control center (block  400 ). If the synchronization is the initial synchronization, then the wireless device  10  performs a complete synchronization to the network control center frequency (block  402 ), such as by performing the conventional two step correlation procedure of FIG.  1 . The wireless device then stores the synchronization parameters resulting from this initial synchronization (block  404 ), for example, in storage  20  of FIG.  2 . 
     After synchronization and storage of the synchronization parameters, the wireless device  10  then monitors the network control center to wait for a call, page or other communication requiring switching to a second station (block  410 ). When a call is received by the wireless device  10 , the wireless device  10  determines the frequency offset between the gateway of the second station  52  and the network control station and synchronizes to the gateway station  52  which allocates communication channels  62  and  62 ′ for communication between wireless device  10  and wireless device  10 ′ (block  412 ). The synchronization to the gateway station  52  may be carried out as described above with respect to second station. The gateway offset is also stored (block  414 ), for example, in storage  20 . 
     As an example, the gateway allocates a Satellite-Slow Associated Control Channel (S-SACCH) and a Satellite Traffic Channel (S-TCH) for the call. The S-TCH is a channel which carries voice from wireless device  10  to and from wireless device  10 ′ and the S-SACCH is for maintaining communication between the gateway station  52  and each wireless device  10  and  10 ′. Thus, the S-TCH and the S-SACCH would correspond to differing frequencies in channels  62  and  62 ′ in FIG.  5 . 
     After assignment of the S-TCH, the wireless device  10  determines the frequency offset to the second wireless device  10 ′ (block  416 ). The wireless device  10  then applies the gateway offset to the S-SACCH (block  418 ) and the frequency offset of the second wireless device  10 ′ to the S-TCH (block  424 ). Each of these offsets may be compensated for drift (blocks  420  and  426 ) and the compensation stored (block  422 ). However, the determination of the drift of the S-TCH may be optional. Thus, in an alternative embodiment, block  426  may be skipped. These operations would continue until the call ends (block  428 ). 
     Because it is often the case that a single oscillator must be utilized in the wireless device  10  and  10 ′, it is preferred that each channel, S-SACCH and S-TCH, be synchronized to by adjusting the local reference frequency with a phase offset from the local frequency. The local reference frequency is preferably synchronized to the most stable reference frequency transmitted. The phase rotation and de-rotation of the other frequencies to which the wireless device  10  or  10 ′ are to be synchronized to may then be applied to the reference frequency to synchronize to the other frequencies. 
     Such a system is suitable because phase change is linear with respect to time and therefore, frequency may be expressed as frequency =Δ/Δt where Δ is a phase offset and Δt is a change in time. Thus, if the frequency differences between the S-TCH and the S-SACCH are not too great, such as would be the case with respect to Doppler shift of the same frequency from two different sources, then the phase offset between the two frequencies may be used to adjust the reference frequency. Such a phase offset approach may be referred to as phase rotation or de-rotation (or “phase ramp” if the rotation and/or de-rotation is linear over time) and may be utilized to adjust the reference frequency to the new offset by adjusting the phase of the reference frequency by the phase offset, which varies over time, so as to adjust the apparent frequency of the reference frequency. Thus, for example, the phase of the reference frequency at any particular instant may be varied over a period of the reference frequency to delay the reference frequency so as to appear as a lower frequency. Similarly, the phase of the reference frequency at any particular instant may be varied over a period of the reference frequency to advance the reference frequency so as to appear as a higher frequency. As discussed above, because these “instantaneous” variations in phase vary linearly over the period of the reference frequency, this phase rotation or de-rotation process is often referred to as phase ramp. Such phase rotation and de-rotation techniques are known to those of skill in the art and, therefore, need not be described further herein. 
     Accordingly, the present invention allows for a user device to synchronize to multiple frequencies utilizing a single synchronized reference frequency through the utilization of phase rotation or de-rotation associated with each channel so that each channel may be synchronized without requiring resynchronization of the reference frequency. Furthermore, in such a system, a phase offset may be determined based on the difference in frequencies between the signal to be synchronized to and the reference frequency. A predefined phase ramp may then be applied to the reference frequency to provide the second frequency. Alternatively, a single or multiple phase offsets may be utilized in combination with one or more predefined phase ramps to generate the second frequency. In such a case, one or more phase offsets may be stored as well as the phase ramps which generate the second frequency. Thus, in addition to the frequency synchronization parameters described above, the present invention may store frequency synchronization parameters as frequency offsets, phase offsets, phase ramps or combinations of any of such frequency specifications. 
     When the call is completed, the wireless device  10  returns to operation at the network control center frequency. Because it is not the initial synchronization to the network control center (block  400 ), the wireless device  10  when returning to the network control center may recall the synchronization parameters previously stored (clock  406 ) and utilize those synchronization parameters in an abbreviated synchronization procedure to resynchronize to the network control channel frequency (block  408 ) as described above with respect to FIG.  4 . Alternatively, if the network control center frequency is the reference frequency, no synchronization process may be needed at all. The wireless device  10  and  10 ′ would simply resume monitoring and tracking the reference frequency from the network control center. 
     Various aspects and embodiments of the present invention have been described with respect to FIGS. 1,  4  and  6  which are flowchart illustrations. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These program instructions may be provided to a processor or signal processor, such as control processor  30  or signal processing  18 , to produce a machine, such that the instructions which execute on the processor create means for implementing the functions specified in the flowchart block or blocks. The computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions specified in the flowchart block or blocks. 
     Accordingly, blocks of the flowchart illustrations support combinations of means for performing the specified functions and combinations of steps for performing the specified functions. It will also be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.