Abstract:
The invention relates to an apparatus supporting an acquisition of a received code modulated signal by determining the correlation between the received code modulated signal and an available replica code sequence at different code phases. It is proposed that the apparatus comprises a first acquisition engine for selecting code phases which are good candidates for being the code phase at which a received code modulated signal and an available replica code sequence have the highest correlation. The apparatus further comprises a second acquisition engine for performing a refined comparison between a received code modulated signal and an available replica code sequence for each code phase selected by the first acquisition engine. The invention relates equally to a corresponding method and to a system comprising such an apparatus and a network.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus supporting an acquisition of a received code modulated signal by determining the correlation between the received code modulated signal and an available replica code sequence at different code phases relative to each other. The invention relates equally to a system and to a method supporting such an acquisition of a received code modulated signal. 
     BACKGROUND OF THE INVENTION 
     A code modulated signal has to be acquired for example in CDMA (Code Division Multiple Access) spread spectrum communications. 
     For a spread spectrum communication in its basic form, a data sequence is used by a transmitting unit to modulate a sinusoidal carrier and then the bandwidth of the resulting signal is spread to a much larger value. For spreading the bandwidth, the single-frequency carrier can be multiplied for example by a high-rate binary pseudo-random noise (PRN) code sequence comprising values of −1 and 1, which code sequence is known to a receiver. Thus, the signal that is transmitted includes a data component, a PRN component, and a sinusoidal carrier component. A PRN code period comprises typically 1023 chips, the term chips being used to designate the bits of the code conveyed by the transmitted signal, as opposed to the bits of the data sequence. 
     A well known system which is based on the evaluation of such code modulated signals is GPS (Global Positioning System). In GPS, code modulated signals are transmitted by several satellites that orbit the earth and received by GPS receivers of which the current position is to be determined. Each of the satellites transmits two microwave carrier signals. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier signal is modulated by each satellite with a different C/A (Coarse Acquisition) Code known at the receivers. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 chips, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with the navigation information at a bit rate of 50 bit/s. The navigation information, which constitutes a data sequence, can be evaluated for example for determining the position of the respective receiver. 
     A receiver receiving a code modulated signal has to have access to a synchronized replica of the employed modulation code, in order to be able to de-spread the data sequence of the signal. More specifically, a synchronization has to be performed between the received code modulated signal and an available replica code sequence. Usually, an initial synchronization called acquisition is followed by a fine synchronization called tracking. In both synchronization scenarios, a correlator is used to find the best match between the replica code sequence and the received signal and thus to find their relative shift called code phase. The match can be determined for example with chip accuracy. If an accuracy of a fraction of a chip is needed, the chip can be presented by several samples after an analog-to-digital conversion. 
     During the acquisition, the phase of the received signal relative to the available replica code sequence can have any possible value due to uncertainties in the position of the satellite and the time of transmission of the received signal. 
     Moreover, an additional frequency modulation of the received signal may occur, which can be as large as +/−6 kHz, for example due to a Doppler effect and/or a receiver clock inaccuracy. The search of the code phase is therefore usually performed with different assumptions on an additional frequency modulation. For a sensitivity increase, a receiver normally uses long integrations that require the frequency uncertainty to be as small as a few Hz. Therefore, even with the aligned code, a large number of frequency assumptions should be checked. 
     The initial acquisition is thus a two-dimensional search in code phase and frequency. To meet the real time processing and weak signal sensitivity requirements, usually, a massive correlator bank which is able to check in parallel hundreds and thousands of options is employed for implementing the acquisition stage of a receiver. 
     Each correlator of such a massive correlator bank checks simultaneously another option defined by a specific code phase and a specific frequency of modulation. To this end, each correlator multiplies a received code modulated signal to a predetermined compensating sinusoidal signal, aligns the compensated code modulated signal with the replica code sequence at a predetermined code-phase, multiplies the samples of the compensated code modulated signal and the samples of the replica code sequence element by element and integrates the multiplication results. The integration can be either purely coherent or include a non-coherent stage. In a non-coherent stage, consecutive coherent integration results for a certain number of multiplication results, respectively, are further integrated by summing the absolute or the squared values of these integration results. 
     If the assumptions on the code-phase and the frequency modulation belonging to one option are correct for the received code modulated signal, then the correlation results in a larger integration value than in the case of a misalignment or an inappropriate compensation of a frequency modulation. Thus, detecting the correlation peak and comparing it with a certain threshold allows to find the correct code phase and the correct frequency of modulation. 
     A massive correlator bank has the advantage that it is much faster than a sequential search correlator bank, in which the number of correlators is restricted and in which each correlator searches only one candidate at a time. It is a disadvantage of a massive correlator bank, however, that its complexity is significant, if a correlation value is determined for all desired options. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to reduce the complexity of massive acquisition engines used for the acquisition of code modulated signals, for example of massive correlator banks. 
     An apparatus supporting an acquisition of a received code modulated signal by determining the correlation between the received code modulated signal and an available replica code sequence at different code phases relative to each other is proposed. The proposed apparatus comprises a first acquisition engine for selecting code phases which are good candidates for being the code phase at which a received code modulated signal and an available replica code sequence have the highest correlation, and for outputting information on each selected code phase. The proposed apparatus further comprises a second acquisition engine for receiving information on selected code phases from the first acquisition engine and for performing a refined comparison between a received code modulated signal and an available replica code sequence for each selected code phase on which information is received. The acquisition engines can be in particular, though not exclusively, correlator banks. 
     Further, a system is proposed which comprises the proposed apparatus and in addition a network. The apparatus and the network are able to exchange data between each other. The exchanged data can be used for supporting the acquisition in various ways. 
     Finally, a method for supporting an acquisition of a received code modulated signal by determining the correlation between the received code modulated signal and an available replica code sequence at different code phases relative to each other is proposed. The proposed method comprises a first step of selecting code phases which are good candidates for being the code phase at which a received code modulated signal and an available replica code sequence have the highest correlation. The proposed method comprises as a second step performing a refined comparison between the received code modulated signal and the available replica code sequence for each of the selected code phases. 
     The invention proceeds from the consideration that for a large fraction of search options, a decision can be made with little effort, for example after a short integration length in a cross-correlation. A conventional massive correlator bank will continue nevertheless checking all search options with the entire operation cycle, thus performing unnecessary computations. It is therefore proposed to distribute the calculation resources required for the acquisition of a code modulated signal to two separate acquisition engines. 
     A first acquisition engine carries out only a preliminary search for all possible options. It does not make a firm decision at each operation cycle but uses for instance a reduced operation cycle for sorting out a large amount of improbable options and for providing several remaining possible options. The duration of such a reduced operation cycle can be fixed or be defined by the operational conditions. The preliminary search can be stopped for example, as soon as a decision can be made for most of the search options. The first acquisition engine can be for example a modified massive correlator bank performing many preliminary searches in parallel. 
     The first acquisition engine then transfers the task to check the remaining search options in more detail to a second acquisition engine. The second acquisition engine can be a small engine using pipe-lining, for example a modified tracking unit. Thereupon, the first acquisition engine is free to check other possible options in a preliminary way. 
     It is an advantage of the invention that it allows to reduce the complexity of massive acquisition engines used for the acquisition of code modulated signal, and thereby for example the gate count and the area of a massive correlator bank used conventionally as acquisition engine. 
     Alternatively, the invention allows to accelerate the processing without increasing the complexity of the acquisition engine and to reduce, for example, the delays in position calculations. For instance, the time to first fix (TTFF) of a position can be reduced to one half or less. 
     It is further an advantage of the invention that due to the structural separation, the two acquisition engines may use different algorithms and different integration length in order to optimize the processing at each stage. 
     In some cases, in which assistance is available, for example from some network, it is also possible to use only the second acquisition engine and to keep the first acquisition engine off. Thereby, the power consumption can be reduced in certain assistance applications. 
     The acquisition engines can be implemented in particular in hardware, while any supplementary processing can be implemented in hardware and/or software. Such supplementary processing can be performed for example by a digital signal processor (DSP) or some other processing unit. 
     The invention can be employed in particular, though not exclusively, for CDMA spread spectrum transmissions, for instance for a receiver of a positioning system like GPS or Galileo. The proposed apparatus can be for example such a receiver or a device comprising such a receiver, for instance a mobile terminal. In the latter case, part of the processing can be carried out outside of the receiver, for example in the mobile terminal or in a network to which the mobile terminal transmits the required information. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not drawn to scale and that they are merely intended to conceptually illustrate the structures and procedures described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  schematically shows a system in which the invention can be employed; 
         FIG. 2  is a schematic block diagram of a combination of a massive correlation bank and a supplementary correlation bank employed in an embodiment of the invention; 
         FIG. 3  is a flow chart illustrating the operation of the correlation banks of  FIG. 2 ; 
         FIG. 4  is a schematic block diagram of a first embodiment of the supplementary correlation bank employed in the structure of  FIG. 2 ; 
         FIG. 5  is a flow chart illustrating the operation of the supplementary correlation bank of  FIG. 4 ; 
         FIG. 6  is a schematic block diagram of a second embodiment of the supplementary correlation bank employed in the structure of  FIG. 2 ; and 
         FIG. 7  is a flow chart illustrating the operation of the supplementary correlation bank of  FIG. 6 ; 
         FIG. 8  is a schematic block diagram of a third embodiment of the supplementary correlation bank employed in the structure of  FIG. 2 , which is cooperating with a DSP; and 
         FIG. 9  is a flow chart illustrating the operation of the supplementary correlation bank of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically presents a system  10  in which the invention can be implemented. 
     The system comprises a mobile terminal  11  including a GPS receiver  12  and a mobile communication network  16 . The GPS receiver  12  includes a receiving portion  13  for receiving code modulated signals from GPS satellites  19 , an acquisition portion  14  for acquiring a received code modulated signal, and a digital signal processor (DSP)  15  supporting the acquisition. The mobile communication network  16  may provide assistance data to the acquisition portion  14  using a regular radio-based communication between the mobile terminal  11  and the mobile communication network  16 . Alternatively or in addition, the mobile communication network  16  may perform computations for supporting the acquisition of a code modulated signal received by the GPS receiver  12  using a regular radio-based communication between the mobile terminal  11  and the mobile communication network  16 . 
       FIG. 2  presents the general structure of the acquisition portion  14  of the GPS receiver  12  of  FIG. 1 . 
     The acquisition portion  14  comprises a massive correlator bank MCB  21  and a supplementary correlator bank SCB  22 . The receiving portion  13  of the GPS receiver  12  is connected to both correlator banks  21 ,  22 . The massive correlator bank  21  is further connected with several lines to the supplementary correlator bank  22 . 
       FIG. 3  is a flow-chart illustrating the operation of the acquisition portion  14  of  FIGS. 1 and 2 . 
     The massive correlator bank  21  receives from the receiving portion  13  samples of a received code modulated signal and performs a cross-correlation between the received code modulated signal and a replica code sequence with a reduced integration length. 
     To this end, a correlator of the massive correlator bank  21  multiplies the input samples to a sinusoidal signal for compensating a possible modulation with a selected frequency. The massive correlator bank  21  then aligns the resulting samples with an available replica code sequence at a selected code phase and multiplies predetermined ones of the frequency compensated samples, for example the first ones of the samples, element by element with the respectively aligned sample of the replica code sequence. The number of the predetermined samples is significantly smaller than the number of all samples of the received code modulated signal which are overlapping with the samples of the aligned replica code sequence. It is to be noted that the order of the two different multiplication operations can also be reversed. The multiplication results are integrated coherently, the integration result constituting a first indication of the amount of correlation. 
     Alternatively, a non-coherent integration could be used. In this case, the multiplication results originating from sections of the received code modulated signal of equal size are integrated separately for each section in a coherent integration. The results of these subcorrelations are multiplied with a shifted, conjugated version of themselves. In a final step, the results of these second multiplications are integrated in a non-coherent integration. Thereby, residual sinusoidal modulations in the raw data, in particular from a Doppler frequency, are reduced. 
     In either case, the correlation is based on a shorter operation cycle than in correlators of a conventional massive correlator bank. 
     The massive correlator bank  21  then selects a new set of a frequency and a code-phase, and the correlator continues with the correlation based on this new set. 
     At the same time, the massive correlator bank  21  checks whether the last integration result indicates that the last assumed frequency modulation and the last employed code-phase could be the correct set of frequency and code-phase and constitutes thus a search option for a refined search. The checking may comprise for example comparing the integration result with a threshold value. 
     If the set of frequency and code-phase constitute a search option, an indication of the associated frequency f i  and the associated code-phase τ i  is provided to the supplementary correlator bank  22  using one of the connecting lines. In addition, the associated integration result S i  is either equally provided to the supplementary correlator bank  22  or to a processing unit, for example to the DSP  15  of the GPS receiver  12 . The latter alternative is indicated in  FIG. 2  by an arrow with a dashed line. The index i is used for identifying the respective option. 
     The supplementary correlator bank  22  assigns one of its correlators to continue the processing with the received parameters f i , τ i  and possibly the parameter S i , in order to allow a determination of the code phase and the compensation frequency resulting in the best match between the received code modulated signal and the replica code sequence. The final determination is carried out in a processing unit, for example the DSP  15  of the GPS receiver  12 . 
     The number of the correlators in the supplementary correlator bank  22  can be sufficiently large for processing all search options which may be output by the massive correlator bank  21  in parallel. Alternatively, a quality indication may be assigned to each search option. In case none of the correlators of the supplementary correlator bank  22  is free when a new search option is output by the massive correlator bank  21 , the quality indication associated to the current search option is compared to a quality indication which was associated to previous search options now occupying the correlators of the supplementary correlator bank  22 . In case a higher quality grade was associated to the new search option than to one of the search options currently processed in one of the correlators of the supplementary correlator bank  22 , then the corresponding correlator of the supplementary correlator bank  22  will stop processing the previously assigned search option and start processing the new search option. 
     The supplementary correlator bank  22  can be implemented in various forms, three of which will be presented by way of example in the following with reference to  FIGS. 4 to 9 . 
     It is to be noted that the exact structure of the massive correlator bank  21  is not of importance. The focus lies on the distribution of tasks between the massive correlator bank  21  and the supplementary correlator bank  22  and the structure of the supplementary correlator bank  22 . Also different parallel solutions, concerning the temporal and spatial dimensions, can be used to implement the correlators in the massive correlator bank  21  and the supplementary correlator bank  22 . 
       FIG. 4  is a schematic block diagram of a first embodiment of a supplementary correlator bank  22  in the structure of  FIG. 2 . 
     The supplementary correlator bank  22  of  FIG. 4  comprises a plurality of correlators  41 . In each correlator  41 , a sample input is connected via two subsequent multiplication elements  42 ,  43  and a multiplexer  44  to an integrating portion  45 . Moreover, a code-phase indication input is connected to an input of a code generator  46 . The output of the code generator  46  is connected to the first multiplication element  42 . In addition, a frequency indication input is connected to an input of a carrier generator  47 . The output of the carrier generator  47  is connected to the second multiplication element  43 . Finally, an integration result input is connected to the multiplexer  44 . 
     The operation of the supplementary correlator bank  22  of  FIG. 4  is illustrated in the flow chart of  FIG. 5 . 
     The supplementary correlator bank  22  assigns each search option received from the massive correlator bank  21  to a specific one of its correlators  41 . The search option comprises an indication of a specific code-phase τ i , which is fed to the code-phase indication input of the respective correlator  41 , and a specific frequency f i , which is fed to the frequency indication input of the respective correlator  41 . The result s i  of a coherent integration associated to the search option is provided to the integration result input of the correlator  41 . The integration result S i  is the result of a coherent integration at the massive correlator bank  21 . 
     The code-phase indication τ i  is fed within the correlator  41  to the code generator  46 , which generates a replica code sequence and aligns it according to the indicated code-phase. The received input samples are then multiplied by the first multiplication element  42  element-wise with the respectively aligned samples of the replica code sequence, as far as they have not been used already in the massive correlator bank  21 . 
     The frequency indication f i  is fed within the correlator  41  to the carrier generator  47 , which generates a corresponding sinusoidal signal. The second multiplication element  43  multiplies the samples output by the first multiplication element  42  element-wise with the sinusoidal signal generated by the carrier generator  47 . 
     It is to be noted that the order of the two different multiplication operations by the first multiplication element  42  and the second multiplication element  43  can also be reversed. 
     The output of the second multiplication element  43  and the result of the coherent integration S i  are provided via the multiplexer  44  to the integrating portion  45 . 
     The integrating portion  45  integrates the multiplication results and includes in the integration as well the integration result S i  provided by the massive correlator bank  21 . The integration may consist in a coherent accumulation, but it may include as well a non-coherent accumulation, as described above as second alternative for the integration in the massive correlator bank  21 . If the final integration result lies below a predetermined threshold, the result is dumped. Otherwise, the final integration result is provided to some processing means, for instance to the DSP  15 , for determining the best correlation result for all search options. Then, the correlator  41  is released for a refined correlation based on the next search option. 
       FIG. 6  is a schematic block diagram of a second embodiment of a supplementary correlator bank  22  in the structure of  FIG. 2 . 
     The supplementary correlator bank  21  of  FIG. 6  comprises again a plurality of correlators  61 , of which only one is shown. In each correlator  61 , a sample input is connected via two subsequent multiplication elements  62 ,  63  to an integrating portion  64 . The integrating portion  64  is further connected via a portion  65  forming absolute or square values of input values and via a multiplexer  66  to a second integrating portion  67 . Further, a code-phase indication input is connected to an input of a code generator  68 . The output of the code generator  68  is connected to the first multiplication element  62 . A frequency indication input is connected to an input of a carrier generator  69 . The output of the carrier generator  69  is connected to the second multiplication element  63 . Finally, an integration result input is connected to the multiplexer  66 . 
     The operation of the supplementary correlator bank  22  of  FIG. 6  is illustrated in the flow chart of  FIG. 7 . 
     The supplementary correlator bank  22  assigns each search option received from the massive correlator bank  21  to a specific one of its correlators  61 . The search option comprises an indication of a specific code-phase τ i , which is fed to the code-phase indication input of the respective correlator  61 , and a specific frequency f i , which is fed to the frequency indication input of the respective correlator  61 . The result s i  of a coherent integration associated to the search option is provided to the integration result input of the respective correlator  61 . The integration result S i  is the result of a non-coherent integration at the massive correlator bank  21 . 
     The code-phase indication τ i  and the frequency indication f i  are made use of by the code generator  68  and the carrier generator  69  as described with reference to  FIG. 4  for code generator  46  and the carrier generator  47 , respectively. Also the output of the code generator and of the carrier generator is made use of in the multiplication elements  62 ,  63  as described with reference to  FIG. 4  for multiplication elements  42 ,  43 , respectively. 
     The output of the second multiplication element  63  is provided to the integrating portion  64 . The integrating portion  64  integrates subsequent groups of multiplication results provided by the second multiplication element  63  separately. Portion  65  determines the square value or the absolute value of each integration result. 
     The square values or the absolute values, respectively, and the non-coherent integration result S i  provided by the massive correlator bank  21  are provided via the multiplexer  66  to the second integrating portion  67 . In the second integrating portion  67 , the square values or the absolute values, respectively, are integrated in a non-coherent integration, the non-coherent integration result S i  of the massive correlator bank  21  being included in this second integration. 
     If the final non-coherent integration result lies below a predetermined threshold, the result is dumped. Otherwise, the final non-coherent integration result is provided to some processing means, for instance to the DSP  15 , for determining the best correlation result for all search options. Then, the correlator  61  is released for a refined correlation based on the next search option. 
       FIG. 8  is a schematic block diagram of a third embodiment of a supplementary correlator bank in the structure of  FIG. 2 . 
     The supplementary correlator bank  21  of  FIG. 8  comprises again a plurality of correlators  81 , of which only one is shown. In each correlator  81 , a sample input is connected via two subsequent multiplication elements  82 ,  83  to an integrating portion  84 . The integrating portion  84  is further connected via a portion  85  forming absolute or square values of input values to a second integrating portion  86 . Further, a code-phase indication input is connected to an input of a code generator  87 . The output of the code generator  87  is connected to the first multiplication element  82 . Moreover, a frequency indication input is connected to an input of a carrier generator  88 . The output of the carrier generator  88  is connected to the second multiplication element  83 . 
     The second integrating portion  86  is connected via an output of the correlator  81  to the DSP  15  of the GPS receiver  12 . Also the massive correlator bank  21  is connected to the DSP  15 . 
     The operation of the supplementary correlator bank  22  of  FIG. 8  is illustrated in the flow chart of  FIG. 9 . 
     The operation of the correlator  81  of the supplementary correlator bank  22  of  FIG. 8  is the same as the operation of the correlator  61  of the supplementary correlator bank  22  of  FIG. 6 , except that the second integrating portion  86  does not include any integration results from the massive correlator bank  21  in the non-coherent integration. 
     Non-coherent integration results S i  of the massive correlator bank for each search option are rather provided directly to the DSP  15 . 
     The DSP  15  uses the non-coherent integration results S i  from the massive correlator bank  21  and from the supplementary correlator bank  22  in a multistage acquisition algorithm for the final signal acquisition. Such an algorithm has been described for example by Kaplan. 
     Alternatively, the DSP  15  shown in  FIG. 8  and in  FIG. 1  could also be part of the mobile terminal  11  outside of the GPS receiver  12 , or be implemented in the mobile communication network  16 . In the latter case, the integration results of the supplementary correlator bank  22  and, in the case of  FIG. 8 , of the massive correlator bank  21  are transmitted to the mobile communication network  16  making use of the regular communication abilities of the mobile terminal  11 . 
     In cases in which the mobile communication network  16  provides assistance data to the mobile terminal  11 , the search options may already be limited due to this assistance data, so that the entire acquisition may be performed by the supplementary correlator bank  22 . Such assistance data may comprise for example information on the positions of the GPS satellites  19  and on a rough position of the mobile terminal  11 , which limits the possible code phases. The massive correlator bank  21  can then be switched off in order to reduce the power consumption. 
     While there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.