Abstract:
A method of controlling a frequency selection of a PLL used in cooperation with a device of a wireless communication network, such as a Universal Mobile Telecommunication System (UMTS) network, the method comprising:—receiving a first set of signal measurements;—comparing the first set of signal measurements with a first threshold and, selectively switching the PLL frequency from a first value to a second value as a result of the comparison of the first set of signal measurements with the first threshold;—further receiving a second set of signal measurements; and,—comparing the second set of signal measurements with a second threshold, different from the first threshold, and selectively switching the PLL frequency from the second value to the first value as a result of the comparison of the second set of signal measurements with the second threshold.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to phase locked loops (PLL) in wireless communication networks and, more particularly, to a method and a device for controlling PLL frequency selection allowing reduction of spurious frequencies in a serving cell of such a network. 
       BACKGROUND OF THE INVENTION 
       [0002]    The PLL technique has been around for a long time and has a wide variety of applications in wireless communication networks. It is frequently used for implementing operations such as demodulation, decoding, synchronisation, frequency synthesis or sampling. 
         [0003]    In a frequency synthesiser, for instance, the PLL technique may be used for generating one or many accurate signal oscillation frequencies from one or more reference sources. In e.g. a radiofrequency (RF) receiver, a PLL may be associated with a signal mixer. A signal mixer is a circuit capable of translating a signal from a given frequency within the RF band to another, lower frequency known as an intermediate frequency (IF), by combining the RF signal with a signal oscillating at a given frequency generated by the PLL. However, apart from the RF signal, other oscillation frequencies generated by the operation of the PLL may reach the input of the mixer. These unwanted signals, known as spurs, may also be translated within the IF frequency band, thus resulting in a degradation of the receiver&#39;s performance in terms of signal-to-noise ratio (SNR). 
         [0004]    In other part of a receiver, PLL technique may further be used for producing an accurate sampling frequency that can be used as an input for an analog-to-digital converter (ADC). An ADC is a circuit that converts an analog signal at a given frequency into a digital signal. However, apart from the frequency of the analog signal, other frequencies generated by the PLL may reach the analog input of the ADC. These unwanted signals, also known as spurs in that context, may affect the accuracy of the digital output signal resulting in a degradation of the ADC and hence of the receiver&#39;s performance. 
         [0005]    In order to address these problems, some solutions may be considered to switch the default PLL frequency to another, more appropriate frequency. The intended effect is to reject the spurs out of the frequency bandwidth of the analog signal of interest. This technique, known as frequency evasion, however presents some drawbacks because wrong PLL frequency change decisions can be made which may result in a waste of time for re-synchronizing the PLL. In addition, a “ping-pong” effect, whereby the PLL frequency change is done too often, can be experienced. All of this leads to a degradation of the receiver performance. Therefore, it is desirable to improve the control scheme of the PLL frequency for e.g. preventing the degradation of the performances of a receiver of a wireless communication network, while avoiding or at least reducing the above negative effects. 
       SUMMARY OF THE INVENTION 
       [0006]    These problems may be overcome by collecting sets of signal measurements that are compared to signal thresholds. 
         [0007]    Indeed, a first aspect proposes a method of controlling a frequency selection of a PLL used in cooperation with a device of a wireless communication network, such as a Universal Mobile Telecommunication System (UMTS) network, the method comprising the steps of receiving a first set of signal measurements performed at an operation frequency of the device during a sliding window; 
         [0008]    comparing the first set of signal measurements with a first threshold and, selectively switching the PLL frequency from a first value to a second value as a result of the comparison of the first set of signal measurements with the first threshold; after the PLL frequency has changed from the first value to the second value, resetting the sliding window and also receiving a second set of signal measurements performed at the operation frequency of the device during the sliding window; and, comparing the second set of signal measurements with a second threshold, different from the first threshold and selectively switching the PLL frequency from the second value to the first value as a result of the comparison of the second set of signal measurements with the second threshold. 
         [0009]    A second aspect relates to a receiver comprising a mechanism for controlling a frequency selection of a PLL used in cooperation with a device of a wireless communication network, such as a Universal Mobile Telecommunication System (UMTS) network, the receiver comprising a unit configured for receiving a first set of signal measurements performed at an operation frequency of the device during a sliding window; a unit configured for comparing the first set of signal measurements with a first threshold and, selectively switching the PLL frequency from a first value to a second value as a result of the comparison of the first set of signal measurements with the first threshold; after the PLL frequency has changed from the first value to the second value, a unit configured for resetting the sliding window and also configured for receiving a second set of signal measurements performed at the operation frequency of the device during the sliding window; and a unit configured for comparing the second set of signal measurements with a second threshold, different from the first threshold and selectively switching the PLL frequency from the second value to the first value as a result of the comparison of the second set of signal measurements with the second threshold. 
         [0010]    Another aspect relates to a wireless device such as a smartphone, a tablet or a computer comprising the receiver of the second aspect. 
         [0011]    Thus, in a receiver embodying the principles of such a frequency evasion scheme, a PLL frequency change decision is triggered based on a plurality of signal measurements using two different signal thresholds for alternatively changing the PLL frequency from one given value to at least one different value, thus with an hysteresis effect. Such control scheme for changing the PLL frequency has the advantage of avoiding wrong PLL frequency decisions of changing, and “ping-pong” effect. 
         [0012]    In one embodiment, when all signal measurements of the first set of signal measurements are beyond the first threshold, the PLL frequency change from the first value to the second value is triggered by an event based on the prior comparison. The PLL frequency is set by default to the first value. The second value of the PLL frequency may be chosen from a list of given frequencies. This allows easier implementation using, for instance, a set of discrete control values respectively associated with each of the frequencies, which may be stored in a data register. 
         [0013]    The first threshold may be a signal level below which numerous spurs would ultimately lead to unacceptable degradation of performances of the receiver. This has the advantage of guarantying that the PLL frequency change is made only when the receiver is operated in a range of operation that is the most sensitive to spurs. The likelihood of wrong PLL frequency change can thus be reduced. 
         [0014]    In another embodiment, when at least one third of a second set of signal measurements are beyond the second threshold, the PLL frequency changes from the second value to the first value is triggered by an event based on the prior comparison. The second threshold may be a signal level above which numerous spurs would have negligible impact on the receiver performances. This helps guarantying that the PLL frequency change is made only when the receiver is operated in a range of operation that is considered the least sensitive to spurs. The frequent “ping-pong” switch from one PLL frequency to another PLL frequency can thus be limited. 
         [0015]    In accordance with a feature of the proposed solution, the set of signal measurements is fixed to correspond to an integer N of signal measurements, greater than 1, collected over the sliding window. This helps guarantying that variability of the signal over a sufficient period is considered before triggering the PLL frequency change. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A more complete understanding of the proposed solution may be obtained from an exemplary consideration of the following description in conjunction with the drawings, in which: 
           [0017]      FIGS. 1   a  and  1   b  are block diagrams illustrating conventional art usages of the PLL technique. 
           [0018]      FIGS. 2 and 3  are block diagrams illustrating an embodiment of the device. 
           [0019]      FIG. 4  is a flow diagram illustrating embodiments of the method. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    In the drawings, like reference numbers designate like parts in various Figures. Expressions such as “comprise”, “include”, “incorporate”, “contain”, “is” and “have” are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components that are not explicitly defined also to be present. Reference to the singular is also to be construed in being a reference to the plural, and vice versa. 
         [0021]      FIG. 1   a  is a block diagram illustrating a conventional art usage of the PLL technique associated with a mixer  100 . In this example, a PLL  200  is configured for generating one or many accurate signal oscillation frequencies at the output  103  of the PLL from one or more reference sources that would be combined with the input  101  of the mixer  100 . This would generate a mixed output signal at the output  102  of the mixer  100 . In this example, the oscillation frequencies generated by operation of the PLL  200  at the output  103  of the PLL may reach the input  101  of the mixer  100 . This implementation may correspond to an operation of RF down conversion used for an operation of demodulation of a RF signal where the PLL  100  is used to specify the rate of the sampling of the RF signal. 
         [0022]      FIG. 1   b  is a block diagram illustrating a conventional art usage of the PLL technique associated with an ADC  104 . In this example, a PLL  200  is configured for generating one or many accurate signal oscillation frequencies at the output  103  of the PLL from one or more reference sources that would be combined with the input  105  of the ADC  104 . This would generate a discrete time digital signal at the output  106  of the ADC  106 . In this example, the oscillation frequencies generated by operation of the PLL  200  at the output  103  of the PLL may reach the input  105  of the ADC  104 . This implementation corresponds to an operation of analog-to-digital conversion of a RF signal, where the PLL  100  specifies the rate of the sampling of the RF signal. 
         [0023]    In the rest of the description, we will consider the proposed solution being embodied in a wireless receiver of a wireless communication network such as a Universal Mobile telecommunication System (UMTS) network. This receiver is being located in a serving cell of such network and uses a mixer for down converting and processing RF signals received from the serving cell. However, the following given examples should be understood as being solely an illustration and no way limit the scope of the proposed solution. 
         [0024]      FIG. 2  is a block diagram illustrating a configuration of a wireless receiver according to an embodiment of the proposed solution. It comprises a control unit  400 , a mixer  100 , a PLL  200  and a RF unit  300 . The RF unit  300  consists of a filter  301 , a low noise amplifier  302  and RF signal processor  303 . In this configuration, the RF signals received on the antenna  500  from the serving cell are first filtered by the filter  301  in order to keep only the RF signals within the channel that is considered. Then, the LNA  302  amplifies the filtered RF signals, since received RF signals are usually weak due to propagation in the air. Finally, a processor  303  performs some operations on the resulting RF signals such as the calculation of RF signal measurements parameters. In the case of a UMTS network, such parameters may be the received signal code power (RSCP) measurement parameter or the chip energy to total power (Ec/lo) measurement parameter. The processor  303  connects to both the control unit  400  and the mixer  100 . When the processor  303  connects to the mixer  100 , it sends the RF signals S rf  that have been filtered and amplified in the RF unit  300 . The RF signals S rf  are then combined in the mixer  100  with the oscillation frequency PLL freq  generated by the PLL. This combination results in an intermediate IF signal S if  that is further used in other parts of the wireless receiver. 
         [0025]    When the processor  303  connects to the control unit  400 , it sends RF signal measurements parameters S rf     —     params  such as RSCP or Ec/Io measurement parameters to the control unit  400 . These measurements parameters are usually compulsory in many wireless communication networks and mainly serve for assessing the quality and the strength of received RF signals. For instance, in wireless local area networks (WLAN) the received signal strength indication (RSSI) is equivalent to RSCP and the signal to noise ratio (SNR) is equivalent to Ec/lo. The control unit  400  may thus send a command C PLL  to the PLL  200  to order the frequency change of the PLL  200  based on the signal measurement parameters S rf     —     params  in order to avoid spurs being received at the input  101  of the mixer  100 . 
         [0026]    The generation of the command C PLL  shall now be described with reference to  FIG. 3  that shows an example of implementation of the control unit  400 . The control unit  400  comprises a received signal measurement unit  410 , a switch  420 , comparators  430 , 440 , a decision bloc  470  associated with RF signal measurement thresholds  450 , 460 . The thresholds  450 , 460  may, for instance, be parameters stored in any suitable data registers of the device. 
         [0027]    In this embodiment, the signal measurement parameters S rf     —     params  are received at the signal measurement unit  410  during a given sliding window. The signal measurement parameters S rf     —     params  that are collected may preferably be non-contiguous. As use herein, the term “non-contiguous” refers to signal measurement parameters S rf     —     params  obtained at more than one moment in time where these moments in time are separated by amount of time that is at least two times greater the delay spread of the propagation channel. The non-contiguous requirement helps guarantying that the variability of the RF signal is considered and taken into account (i.e. the fact that RF signal received on the antenna  500  from the serving cell fluctuates over time). Therefore, taking only contiguous signal measurement parameters S rf     —     params  could be misleading in evaluating the real nature of quality and/or strength of the RF signals, thus leading to a wrong PLL frequency change decision characterised by the command C PLL  is. As a result, non-contiguous signal measurement parameters  S   rf     —     params  are a good way of estimating the RF signal behaviour over a sufficient period. 
         [0028]    The set of signal measurement parameters S rf     —     params  that have been collected are thus sent to a switch  420  that decides which comparator  430 , 440  may be used for comparison to the associated threshold  450 , 460 . Each comparator  450 , 460  may have different criteria while comparing the set of signal measurement parameters S rf     —     params  to the associated threshold  450 ,  460 . Each threshold  450 , 460  may be a signal quality threshold such as Ec/lo thresholds or SNR thresholds and may be a signal strength threshold such RSCP thresholds or RSSI thresholds. The behaviour of the switch  420  is controlled by the decision bloc  470 . 
         [0029]    By default, the decision bloc  470  order the switch  420  to select the comparator  450 , 460  associated to the less stringent threshold  450 , 460 . The less stringent threshold being the one that makes it less difficult to change the PLL frequency based on the of signal measurement parameters S rf     —     params . Another ability of the decision bloc  470  is to request another set of signal measurement parameters S rf     —     params  to the received signal measurement unit  410  when the result of the comparators  430 , 440  associated with the thresholds  450 , 460  are not successful. This means that no PLL frequency change is required on the PLL  200 . 
         [0030]    Another ability of the decision bloc is to send a command C PLL  to the PLL  200  to order the frequency change of the PLL  200  based on the result of the comparators  430 , 440  associated with the thresholds  450 , 460 . Hereafter is described the generation of the C PLL  command. We will consider as an example that threshold  450  is the first threshold and threshold  460  is the second threshold. However, it is important to remember that it could the converse without departing from the scope of the proposed solution. 
         [0031]    When the PLL  200  is operating at a frequency set to the first value and when all the set of a first RF signal measurement parameters S rf     —     params  are beyond the first threshold  450 , the decision bloc  470  sends a command C PLL  that order the PLL  200  to switch the PLL frequency from a first value to a second value. If the preposition “beyond” from the previous statement is understood as the preposition “below” then the first given signal threshold would preferably be a signal level below which numerous spurs would ultimately lead to unacceptable receiver degradation. On the other end, if the preposition “beyond” from the previous statement is understood as meaning the same as the preposition “above” then the first given signal threshold would preferably be a signal level above which numerous spurs would ultimately lead to unacceptable receiver degradation. In some variants, one may interpret the preposition “beyond” as corresponding to the preposition “below”, depending on the definition of the measurements that is considered. In both case, this helps guarantying that the PLL frequency change is made only when the receiver is considered the most sensitive to spurs. The likelihood of wrong PLL frequency change can thus be reduced. 
         [0032]    When the PLL  200  is operating at a frequency set to the first value and when not all the set of a first RF signal measurement parameters S rf     —     params  are beyond a first given threshold  450 , the decision bloc  470  sends a command to the received signal measurement unit  410  ordering the reception of a new set of RF signal measurement parameters S rf     —     params . 
         [0033]    When the PLL  200  is operating at a frequency set to the second value and when at least one third of the set of a second RF signal measurement parameters S rf     —     params  are beyond a second given threshold  460 , the decision bloc  470  sends a command C PLL  that order the PLL  200  to switch the PLL frequency from the second value to the first value. If the preposition “beyond” from the previous statement is understood as the preposition “above” then the second given signal threshold would preferably be a signal level above which numerous spurs would have negligible impact on the receiver degradation. On the other end, if the preposition “beyond” from the previous statement is understood as the preposition “below” then the second given signal threshold would preferably be a signal level below which numerous spurs would have negligible impact on the receiver degradation. An embodiment would consider the preposition “beyond” being considered as the preposition “above”. In both case, this helps guarantying that the PLL frequency change is made only when the receiver is considered the least sensitive to spurs. The frequent “ping-pong” switch to another PLL frequency change can thus be limited. 
         [0034]    When the PLL  200  is operating at a frequency set to the second value and when not all the first RF signal measurement parameters S rf     —     params  are beyond the second given threshold  460 , the decision bloc  470  sends a command to the received signal measurement unit  410  ordering the reception of a new set of RF signal measurement parameters S rf     —     params . 
         [0035]    Referring to  FIG. 4 , in step S 500 , the default PLL frequency is set to PLL_f 1 . In step S 510 , the sliding window is reset such that it contains no signal measurement parameters S rf     —     params  such as RSCP measurement parameters. The sliding window, after being reset is further used in step S 520  to collect signal measurement parameters S rf     —     params  such as RSCP measurement parameters. In step S 530 , a test is performed to verify whether at least N signal measurement parameters S rf     —     params  such as RSCP measurement parameters have been gathered in the sliding window. 
         [0036]    If it is not the case, the collection of signal measurement parameters S rf     —     params  such as RSCP measurement parameters in step S 520  will continue. On the contrary, if at least N signal measurement parameters S rf     —     params  such as RSCP measurement parameters have been gathered in the sliding window during step S 520  then the algorithm proceeds to step S 540 . In step S 540 , all signal measurement parameters S rf     —     params  such as RSCP measurement parameters of the sliding window are compared to a first threshold TH 1 . 
         [0037]    If at least one signal measurement parameter S rf     —     params  such as RSCP measurement parameter of the sliding window is greater than threshold TH 1 , the collection of signal measurement parameters S rf     —     params  such as RSCP measurement parameters in step S 520  will continue. On the contrary, if all signal measurement parameters S rf     —     params  such as RSCP measurement parameter of the sliding window are lower than threshold TH 1  then the algorithm proceeds to step S 550 . In step S 550 , PLL frequency is changed from PLL_f 1  to PLL_f 2 . 
         [0038]    In step S 560 , the sliding window is reset such that it contains no signal measurement parameters S rf     —     params  such as RSCP measurement parameters. The sliding window, after being reset is further used in step S 570  to collect signal measurement parameters S rf     —     params  such as RSCP measurement parameters. In step S 580 , a test is performed to verify whether at least N/3 of signal measurement parameters S rf     —     params  such as RSCP measurement parameters have been gathered in the sliding window where N is the number of signal measurement parameters S rf     —     params . 
         [0039]    If it is not the case, the collection of signal measurement parameters S rf     —     params  such as RSCP measurement parameters in step S 570  will continue. On the contrary, if at least N/3 of signal measurement parameters S rf     —     params  such as RSCP measurement parameters have been gathered in the sliding window during step S 570  then the algorithm proceeds to step S 580 . In step S 580 , all signal measurement parameters S rf     —     params  such as RSCP measurement parameters of the sliding window are compared to a second threshold TH 2 . 
         [0040]    If at least one signal measurement parameter S rf     —     params  such as RSCP measurement parameter of the sliding window is lower than threshold TH 2  then the collection of signal measurement parameters S rf     —     params  such as RSCP measurement parameters in step S 570  will continue. On the contrary, if all signal measurement parameters S rf     —     params  such as RSCP measurement parameter of the sliding window are greater than threshold TH 2 , the algorithm proceeds back to step S 510  where PLL frequency is changed back from PLL_f 2  to PLL_f 1 . 
         [0041]    The proposed solution is applicable not only to PLL used in mixers, but also to ADC, For instance, and more generally to all type of PLL within a wireless device wherein spurs resulting from the operation of the PLL may affect the efficiency of the device. 
         [0042]    While the proposed solution has been illustrated and described in details in the drawings and foregoing description, it is to be understood that such the above-described illustration and description are to be considered illustrative and exemplary only, the proposed solution being not restricted to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed proposed solution, from a study of the drawings, the disclosure and the appended claims. It is therefore intended that such variations be included within the scope of the Claims. 
         [0043]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the proposed solution.