Abstract:
Method for acquiring frequency of a desired channel having a carrier frequency F MAIN , for a dynamic receiver frequency F MOBILE , from a starting frequency F START , in the presence of high power adjacent interfering channels, wherein the starting frequency F START  is shifted from F MAIN  by not more than a predetermined frequency gap ΔF, the method includes the steps of determining a first frequency boundary and a second frequency boundary, detecting channels within a filtering bandwidth, selecting a dominant channel from the detected channels, progressing the dynamic receiver frequency F MOBILE  towards the carrier frequency of the dominant channel, detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary, restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary, and repeating from the step of detecting channels.

Description:
This application is a continuation application of U.S. patent application Ser. No. 09/012,361, filed Jan. 23, 1998, now U.S. Pat. No. 6,175,722 issued Jan. 16, 2001, which is hereby incorporated by reference in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to frequency acquisition in general and to frequency acquisition in the presence of high power adjacent channels, in particular. 
     BACKGROUND OF THE INVENTION 
     Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism. 
     FIG. 1B is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG.  1 A. 
     Arrow  14  represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency  10  of a base unit transmitter having a value of F BASE , which is located near by. In the present example the mobile unit further detects a carrier frequency  12  provided by a neighbor transmitter, having a value of F NEIGHBOR . The value of the mobile unit F 0   MOBILE  is located between the values of the base unit frequency F BASE  and the neighbor mobile transmitter frequency F NEIGHBOR . 
     In the present example the mobile unit  14  detects the signals provided by base  10  and the neighbor  12  wherein the received power of the neighbor  12  is higher than the received power of the base unit  10 . 
     According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency  12 . 
     It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc  16 . These techniques often fail when the power of the undesired signal is significantly high. 
     Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency F NEIGHBOR  and starts progressing its frequency  14  towards F NEIGHBOR . Finally the synchronization mechanism allows the frequency of the mobile unit  14  to acquire and synchronize with the frequency of the neighbor unit  12 . This is shown in FIG. 1B by aligning line  12  and arrow  14 . As can be seen, at this stage the frequency  10  of the base transmitter is filtered out by the filter  16 . 
     A conventional synchronization mechanism provides frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that the F NEIGHBOR  can be located outside this range in such a case, F MOBILE , might get stuck at the boundary frequency value which is closest to F NEIGHBOR . 
     It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit  14  is synchronized with the frequency of neighbor unit  12  instead of the frequency of the base unit  10 , is not acceptable. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a novel method for performing accurate initial frequency acquisition in the presence of high power adjacent channels. 
     It is a further object of the present invention to provide a novel device for performing accurate initial frequency acquisition in the presence of high power adjacent channels. 
     In accordance with the present invention there is thus provided a method for acquiring frequency of a desired channel having a carrier frequency F MAIN , for a dynamic receiver frequency F MOBILE , from a starting frequency F START , in the presence of high power adjacent interfering channels. 
     The starting frequency F START  is shifted from F MAIN  by not more than a predetermined frequency gap ΔF. The method includes the steps of: 
     determining a first frequency boundary and a second frequency boundary; 
     detecting channels within a filtering bandwidth; 
     selecting a dominant channel from the detected channels; 
     progressing the dynamic receiver frequency F MOBILE  towards the carrier frequency of the dominant channel; 
     detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary; 
     restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary; and 
     repeating from the step of detecting channels. 
     According to another aspect of the present invention, one of the first frequency boundary and the second frequency boundary is F START −ΔF, while the other is F START +ΔF. 
     The method of the invention can also include the step of determining a frequency advance direction. The frequency advance direction can be fixed at the beginning of each frequency acquisition cycle, wherein the frequency acquisition cycle is determined from the point where F MOBILE  shifts from F START  until the point where F MOBILE  returns to F START . 
     The step of progressing can be performed in a frequency step F STEP . The value of the frequency step F STEP  can be infinitesimal with comparison to the predetermined frequency gap ΔF, or adjustable. Accordingly, the method can further include the step of adjusting the frequency step F STEP  after each step of detecting channels. 
     In accordance with another aspect of the present invention, there is provided a device for acquiring frequency of a desired channel having a carrier frequency F MAIN , for a dynamic receiver frequency F MOBILE , from a starting frequency F START , in the presence of high power adjacent interfering channels. 
     The device is connected to an antenna via a receiver and to a reference frequency F REFERENCE  source. The device includes controllable frequency generating means for generating an internal frequency F INTERNAL , frequency shift means connected to the controllable frequency generating means, and to the receiver, for shifting received frequency F RECEIVED , of a received channel, according to the internal frequency F INTERNAL . 
     The device also includes a frequency shift detector, connected to the frequency shift means, for detecting a frequency difference between the internal frequency F INTERNAL  and the received frequency F RECEIVED , with respect to the reference frequency F REFERENCE , thereby producing a frequency shift value F SHIFT . 
     The device further includes loop filtering means, connected to the frequency shift detector, for filtering the frequency shift value F SHIFT , thereby producing a filtered frequency shift value F SHIFT-FILTERED , and controlling means, connected to the controllable frequency generating means and to the loop filtering means, for determining a frequency step F STEP  from the filtered frequency shift value F SHIFT-FILTERED . 
     The controlling means provide the frequency shift value F SHIFT  to the controllable frequency generating means. The controllable frequency generating means adjust the internal frequency F INTERNAL  according to the frequency shift value F SHIFT , and the controlling means control the controllable frequency generating means to generate frequency in a range from a first frequency boundary F FIRST  and a second frequency boundary F SECOND . 
     According to one aspect of the invention, the controlling means set the frequency shift value F SHIFT  to be F SECOND −F INTERNAL , when |F INTERNAL −F START |≧|F INTERNAL −F FIRST |, while the controlling means set the frequency shift value F SHIFT  to be F FIRST −F INTERNAL , when |F INTERNAL −F START |≧|F INTERNAL −F SECOND |. 
     According to another aspect of the invention, the device further includes frequency filtering means, connected between the frequency shift detector and frequency shift means. 
     The controlling means reset the loop filtering means when setting the frequency shift value F SHIFT  to be F SECOND −F INTERNAL  or F FIRST −F INTERNAL . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
     FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a conventional initial frequency synchronization procedure; 
     FIG. 1B is a schematic illustration of frequency power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A; 
     FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention; 
     FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention; 
     FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention; 
     FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention; 
     FIG. 2E is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention; 
     FIG. 2F is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention; 
     FIG. 3 is a schematic illustration of a device for synchronizing frequencies, constructed and operative in accordance with another preferred embodiment of the invention; 
     FIG. 4 is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with a further embodiment of the invention; 
     FIG. 5A is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with yet another embodiment of the invention; and 
     FIG. 5B is a schematic illustration in detail of a step of the method of FIG.  5 A. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention overcomes the disadvantages of the prior art by providing a frequency detect and fold mechanism. Accordingly, when the frequency shift exceeds a boundary value, then a predetermined frequency shift is enforced. 
     Reference is now made to FIGS. 2A,  2 B,  2 C and  2 D. FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention. 
     The schematic illustration provided by FIG. 2A describes the frequency  100  of a base station, having a value F BASE , a frequency  104  of a mobile unit, having an initial value F 0   MOBILE  and a frequency  102  of a neighbor transmitter, having the value of F NEIGHBOR , wherein 
      F BASE &lt;F 0   MOBILE &lt;F NEIGHBOR . 
     In conventional communication standards, such as AMPS, NAMPS, JTACS, NTACS, USDC-TDMA and the like, the initial value of F 0   MOBILE  of the mobile unit frequency  104  can be shifted from the value F BASE  of the base station frequency  100 , by no more than a predetermined frequency gap ΔF. Another condition set by these standards is that any neighbor transmitter will transmit in a frequency F NEIGHBOR , which is considerably shifted from F BASE . Accordingly |F BASE −F NEIGHBOR |&gt;2ΔF. 
     The method of the present invention generally searches the received spectrum within a frequency range of [F 0   MOBILE −ΔF, F 0   MOBILE +ΔF], for stabilized frequency values. 
     According to the invention, at the initial stage (i.e., at frequency F 0   MOBILE ) the mobile unit detects all of the signals of transmitters in its vicinity and detects the frequency of the signal with the highest received power, which in the present example is the neighbor transmitted frequency  102 . Accordingly, the mobile unit commences shifting its frequency  104  from the value of F 0   MOBILE , towards the value F NEIGHBOR  of neighbor transmitter frequency  102 . 
     The present invention makes use of the above limitations, of conventional communication standards, which outline that the initial value F 0   MOBILE  of the mobile unit frequency  104  has to be within a frequency gap of ΔF from the value F BASE , of the base transmitter frequency  100 . 
     Accordingly, any shift from the initial stage F 0   MOBILE , cannot exceed the value of ΔF. After the frequency  104  of the mobile unit has progressed towards the neighbor transmitter frequency  102  value F NEIGHBOR , by a frequency shift  110 , having a value of ΔF, to the value F 1   MOBILE , then, according to the invention, any further progress in this direction would result in a detection error and hence, should not be pursued. 
     At this stage, the present invention determines a reversed path  112  for frequency  104  (FIG. 2C) for shifting frequency  104  from the value of F 1   MOBILE  to the value of F 2   MOBILE  wherein the shift value of this reverse path  112 , is a frequency gap which is twice the value of ΔF. 
     At the final stage (FIG. 2D) the spectrum is searched, thereby detecting the base frequency  100  as the dominant signal. Accordingly, the mobile unit  104  commences shifting its frequency towards base frequency  100 , from the value of F 2   MOBILE  to F BASE . This shift is shown by path  114 . According to the present example, no direction is enforced for path  114 . 
     It will be noted that applying a filter, such as filter  106 , improves the performance of an initial synchronization process, according to the invention. As illustrated in FIG. 2C, as long as the filter size is less than |F BASE −F NEIGHBOR |×2, (provided that the filter is generally symmetrical), wherein F NEIGHBOR  is not a high power signal, then, F NEIGHBOR  would not be detected as a major signal by the receiver of the mobile unit, in the original direction of progress. 
     Reference is now made to FIG. 3 which is a schematic illustration of a device for synchronizing frequencies, generally referenced  200 , constructed and operative in accordance with another preferred embodiment of the invention. 
     Device  200  includes a frequency shift unit  202 , an inter-mediate frequency (I.F.) filter  204  connected to the frequency shift unit  202 , a frequency shift detector  206  connected to the I.F. filter  204 , a loop filter  208  connected to the frequency shift detector  206 , a non-linear controller  210  connected to the loop filter  208 , and a voltage control oscillator (VCO)  212 , connected to the non-linear controller  210  and to the frequency shift unit  202 . It will be noted that VCO  212  can be replaced with any type of controlled oscillator. 
     The frequency shift unit  202  is further connected to an antenna  220 . The frequency shift detector  206  is further connected to a host  222 . The host  222  provides a reference frequency value to the frequency shift detector  206 . 
     The antenna  220  detects frequency signals of neighbor transmitters wherein one of these detected frequency signals is transmitted by a base station. The antenna  220  provides these received frequency signals to the frequency shift unit  202 . The VCO  212  generates a signal having a frequency and provides it to frequency shift unit  202 . 
     Frequency shift unit  202  shifts frequencies, received from antenna  220 , according to the frequency provided by the VCO and provides the results to the I.F. filter  204 . The I.F. filter  204  filters some of these frequencies and provides the remaining ones to the frequency shift detector  206 . The frequency shift detector  206  attempts to detect the frequency shift of each of these shifted frequencies from the reference frequency value, provided by the host  222 . 
     Accordingly, the frequency shift detector  206  determines a frequency shift value and provides it to the loop filter  208 . The loop filter  208  includes the history of the frequency shifts performed by device  200  and accordingly determines a frequency shift direction and provides it with the frequency shift value to the non-linear controller  210 . 
     The non-linear controller  210  detects if the overall shift, up until this stage has exceeded the value of ΔF. If so, then the non-linear controller  210  provides VCO  212  with the command to generate a reversed frequency shift such as the one according to path  112  (FIG.  2 C). If not, then the non-linear control  210  provides the VCO  212  with a frequency shift value and a frequency shift direction for further shifting the frequency towards the most dominant received frequency. Then the VCO  212  provides a new shift frequency to the frequency shift unit  202  and the process is repeated from the beginning. 
     It will be noted that when using a slow loop filter, such as software implemented loop filter, it would be difficult for such a loop filter to process a considerable shift such as the one defined by path  112 , since such shifts are compared to frequency behavior history contained therein. 
     According to a further aspect of the invention, when the non-linear controller  210  determines a 2ΔF shift, it also sends a clear command back to the loop filter  208 , thereby erasing the frequency history contained in the memory of loop filter  208 . This operation enables the loop filter  208  to further process considerable frequency shifts. 
     It will be noted that the terms base, mobile and neighbor are presented as a matter of convenience only. The present invention is applicable for any type of initial frequency acquisition in the presence of a high power adjacent channels, wherein the base of the above example is assigned to a main transmitter, the mobile of the above example is assigned to a receiver and the neighbor of the above example is assigned to an adjacent interfering transmitter. 
     It will be noted that each of the main transmitter, the adjacent transmitter and the receiver may be implemented for a mobile unit, a base unit and the like. 
     Reference is now made to FIG. 4 which is a schematic illustration of a method for operating the device  200  of FIG. 3, operative in accordance with a further embodiment of the invention. 
     In step  300 , the device  200  stores the value F 0  of the internal initial frequency F. F 0  is used to determine, later on, the total amount of shift from the initial frequency. It will be noted that for this purpose, the device  200  can store and accumulate the values of the later frequency shifts, instead. 
     In step  302 , the device  200  detects incoming frequency signals. 
     In step  304 , the device  200  filters the incoming frequency signals, thereby obtaining selected frequencies. 
     In step  306 , the device  200  determines a target frequency value F TARGET , from the selected frequencies. In the present example (FIG.  2 A), the device  200  (FIG. 3) selects the right side signal  102  (F NEIGHBOR ), as the target frequency F TARGET . 
     In step  308 , the device  200  progresses the internal frequency F towards the target frequency F TARGET  by a predetermined frequency step F STEP . It will be noted that F STEP  can be determined using a range of considerations, such as speed, accuracy and the like. In general, F STEP  is determined to be significantly smaller than ΔF, thereby yielding higher accuracy. It will further be noted that F STEP  can be infinitesimal thereby yielding an analog like behavior. 
     In step  310 , the device  200  detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device  200  proceeds to step  312 . Otherwise, the device  200  proceeds to step  314 . 
     In step  312 , the device  200  reverses F by 2ΔF. In the present example (FIG.  2 C), reverse path  112 , describes such a reverse shift, from the value of F 1   MOBILE  to the value of F 2   MOBILE . Then, the device  200  repeats the steps of the above method, from step  302 . 
     It will be noted that at this stage, signal  102  appears to be outside of the filtering bandwidth of filter  106 , thereby leaving the base station frequency signal  100 , the strongest, at the output of filter  106 . Accordingly, the device  200  determines F BASE  as F TARGET . 
     In step  314 , the device  200  detects if the internal frequency F is synchronized with the target frequency F TARGET . If so, then the device  200  has completed the initial frequency acquisition procedure and accordingly, locks the frequency F (step  316 ). Otherwise, the device  200  repeats the steps of the above method, from step  302 . 
     The method of FIG. 4 overcomes a situation where there exists interfering neighbor frequencies such as F NEIGHBOR  (reference numeral  102 ) on one side of the spectrum. 
     In a situation where there exist interfering neighbor frequencies on both sides of the base frequency F BASE , the present invention provides a slightly different solution, as will be disclosed hereinbelow. 
     Reference is now made to FIGS. 2E and 2F. FIG. 2E is a schematic illustration of frequency versus power, describing a stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention. FIG. 2F is a schematic illustration of frequency versus power, describing a final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention. 
     According to the present example, there exists an additional neighbor frequency  120  having a value of F* NEIGHBOR , on the left side of the base frequency  100  F BASE . When the mobile frequency completes the 2ΔF frequency shift  112 , additional neighbor frequency  120  falls within the filtering bandwidth of filter  106 , together with base frequency  100 . 
     It will be noted that if, at the output of filter  106 , the signal of the additional neighbor frequency  120  appears to be stronger than the signal of the base frequency  100 , then, according to the method of FIG. 3, the mobile frequency  104  would be drawn towards the additional neighbor frequency  120 . 
     According to another aspect of the present invention, the initial direction set forth in the second stage (i.e., the direction of frequency shift  110 , (FIG.  2 B)), is stored. In the present example, this direction is from left to right. 
     Then, after the mobile frequency completes the 2ΔF frequency shift  112 , the acquisition mechanism continues searching in that initial direction, only. It will be noted that such forced search direction provides an accurate acquisition of the desired base frequency, in one or less search cycle. 
     In a more detailed form, at the final stage (FIG. 2F) the spectrum is searched again in the direction set forth in the initial stage (i.e., the direction of shift  110 ), thereby detecting the base frequency  100  as the dominant signal. Accordingly, a path  122  is set towards base frequency  100 , for shifting mobile frequency  104  from the value of F 2   MOBILE  to F BASE . 
     It will be noted that the present invention provides a search shift step which can be calibrated at each search stage. For example, on the one hand, in the presence of a powerful additional neighbor  120 , frequency shift  122  may include a large number of infinitesimal frequency shift steps. Otherwise, frequency shift  122  may include a small number of larger frequency shift steps. 
     Reference is now made to FIGS. 5A and 5B. FIG. 5A is a schematic illustration of a method for operating the device  200  of FIG. 3, operative in accordance with yet another embodiment of the invention. FIG. 5B is a schematic illustration in detail of step  406  of the method of FIG.  5 A. 
     In step  400 , the device  200  stores the value F 0  of the internal initial frequency F. 
     In step  402 , the device  200  detects incoming frequency signals. 
     In step  404 , the device  200  filters the incoming frequency signals, thereby obtaining selected frequencies. 
     In step  406 , the device  200  determines frequency step F STEP  and a frequency advance direction, in a way which is described in detail in FIG.  5 B. 
     In step  418 , if the detection performed according to step  402  is the first detection in the current acquisition cycle, then the device  200  proceeds to step  420 . Otherwise, the device  200  proceeds to step  408 . 
     In step  420 , the device  200  determines an initial advance direction which will be constant during the present acquisition cycle, and proceeds to step  408 . 
     In step  408 , the device  200  progresses the internal frequency F by frequency step F STEP , in the advance direction. 
     In step  410 , the device  200  detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device  200  proceeds to step  412 . Otherwise, the device  200  proceeds to step  414 . 
     In step  412 , the device  200  reverses F by 2ΔF. In the present example (FIG.  2 E), reverse path  112 , describes such a reverse shift, from the value of F 1   MOBILE  to F 2   MOBILE . Then, the device  200  repeats the steps of the above method, from step  402 . 
     It will be noted that at this stage, additional neighbor frequency signal  120  falls within the filtering bandwidth of filter  106 , which poses a problem if additional neighbor frequency signal  120  appears stronger than the base station signal  100 , at the output of filter  106 . 
     Referring now to FIG. 5B, the device  200  determines a target frequency value F TARGET  from the selected frequencies (step  430 ). In the present example, when the mobile frequency is at a value of F 0   MOBILE  (FIG.  2 A), the device  200  (FIG. 3) selects the right side signal  102  (F NEIGHBOR ), as the target frequency F TARGET . Alternatively, when the mobile frequency is at a value of F 2   MOBILE  (FIG.  2 E), the device  200  (FIG. 3) selects the left side signal  120  (F* NEIGHBOR ), as the target frequency F TARGET . 
     In step  432 , if the detection performed according to step  402  is the first detection in the current acquisition cycle, then, the device  200  proceeds to step  440 . Otherwise, the device  200  proceeds to step  434 . 
     In step  434 , the device  200  determines an advance direction from the mobile frequency value F and the target frequency value F TARGET . 
     In step  436 , if the advance direction determined in step  434  is equal to the initial advance direction, determined in step  420 , then the device  200  proceeds to step  440 . Otherwise, the device  200  proceeds to step  438 . It will be noted that a situation where these directions are not equal occurs, for example, when a neighbor signal, such as the one of additional neighbor frequency  120 , appears to be stronger than the signal of the base frequency  100 , at the output of the filter  106 . 
     In step  440 , the device  200  determines the frequency step F STEP  according to the position of F and F TARGET . In the present example, F STEP ≦|F−F TARGET |. 
     In step  438 , the device  200  determines the advance direction to be the initial advance direction. 
     In step  442 , the device  200  determines the frequency step F STEP  relatively small. It will be noted that, according to the present example, the size of F STEP  is smaller, compared to the size of ΔF. 
     Referring back to FIG. 5A, wherein if the device  200  detects if the internal frequency F is synchronized with the target frequency F TARGET  (step  414 ), then the device  200  proceeds to step  416  and locks F. Otherwise, the device  200  repeats the steps of the above method, from step  402 . 
     Hence, the method of FIGS. 5A and 5B overcomes a situation where there exist interfering neighbor frequencies such as F NEIGHBOR  (reference numeral  102 ) and F* NEIGHBOR  (reference numeral  120 ) on either side of the F BASE . 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the claims which follow.