Abstract:
A method for controlling integration for a CDMA signal and a receiver implementing the same. In accordance with the present invention, non-coherent and even coherent integration periods for a received signal are dynamically and adaptively controlled depending upon the condition of the received signal. The integration period can be very short when signal strength is strong and no blocking exists; while it can be extended to be longer when the signal strength is weak or there is a blocking. Therefore, it is possible to keep locking even under bad circumstances. In addition, the fix time can be shortened when the signal strength is very strong.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to GNSS navigation, more particularly, to control of coherent and non-coherent integrations of a receiver for CDMA signals. 
     BACKGROUND OF THE INVENTION 
     Code Division Multiple Access (CDMA) has been widely used in satellite communication field such as GNSS. Civic GPS (Global Positioning System) application is rapidly developed. To achieve excellent GPS navigation, sufficiently available satellite number and qualified signal measurement accuracy are basic requirements to be satisfied. However, such conditions are hardly satisfied in deep urban circumstances. 
     A GPS receiver suffers problem when receiving signals in urban. For example, signal power level may abruptly and rapidly change since the receiver, which may be set up in a vehicle, is moving, or the signal is blocked by buildings. 
     The receiver uses coherent integration and non-coherent integration to obtain sufficient signal power. Conventionally, a coherent integration period is fixed and is ranged from 1 ms to 20 ms, in general. In addition, a non-coherent number is also set in advance. If the coherent integration period is predetermined to be 20 ms, the receiver integrates the receiver power for 20 ms. The 20 ms coherent integration is stored, and another 20 ms coherent integration is calculated. The 20 ms coherent integrations are accumulated. The non-coherent number of coherent integrations are accumulated. This is called non-coherent integration. A non-coherent integration period is predetermined since the non-coherent number is fixed. For example, the non-coherent integration period is predetermined as 2 seconds. Then, the 20 ms coherent integrations are accumulated until a 2-second non-coherent integration is obtained. The non-coherent integrated signal power is checked to see if it is sufficiently high. However, in bad circumstances, such as urban, the signal may be sometimes very weak or even be blocked. It will take a significantly long time to obtain sufficient signal power level by using the prior art scheme. For example, when a receiver receives a satellite signal in urban environment, the power of the satellite signal may be changed dramatically. The strong signal power may suddenly become weak. Under such a circumstance, the receiver is likely to lose lock for the satellite if the integration time is not long enough. Therefore, it is necessary to use a long integration interval. However, the weak signal power may also suddenly become strong. If the long integration time interval is still used, the response of the receiver to the satellite signal will become slow and blunt. To overcome such a problem, the integration period needs to be more effectively and adaptively controlled. 
     SUMMARY OF THE INVENTION 
     The present invention is to provide a method for controlling integration for a CDMA signal. By using the method of the present invention, integration period for the signal can be effectively and adaptively controlled. Therefore, it is possible to keep locking the signal even when the signal strength is weak or there is blocking. In addition, the fix time can be effectively shortened. 
     The method comprises steps of executing coherent integration for a coherent integration period and storing the coherent integration result into a first memory; calculating a signal power from the coherent integration result; accumulating the signal power of each coherent integration into a second memory as a non-coherent integration comparing the accumulated signal power with a threshold; and repeating the above steps if the accumulated signal power has not exceeded the threshold while stopping accumulating if the accumulated signal power exceeds the threshold. 
     In the case that the coherent period is set to be long, the coherent period can be divided into a plurality of sub-units. The coherent integration result is calculated to obtain an updated signal power by every sub-unit, and the signal power is compared with the threshold, the signal power is used in further processing if it exceeds the threshold. 
     The present invention is further to provide a receiver for receiving CDMA signals. The integration period for the signal received by the receiver can be effectively and adaptively controlled. Therefore, it is possible to keep locking the signal even when the signal strength is weak or there is blocking. In addition, the fix time can be effectively shortened. 
     The receiver comprises an antenna for receiving a signal; a correlator for executing correlation to the signal; a coherent integration block executing a coherent integration to the signal for a coherent integration period; a magnitude calculator calculating a signal power from a result of the coherent integration of the coherent integration block; a non-coherent integration block accumulating the signal power of each coherent integration as a non-coherent integration; a comparator comparing the signal power with a threshold whenever signal power of a coherent integration is accumulated to the non-coherent block; and a controller stopping the non-coherent integration and passing the signal to further process when the signal power exceeds the threshold. 
     In the case that the coherent period is set to be long, the coherent integration period is divided into a plurality of sub-units, the magnitude calculator calculates the coherent integration result and updates the signal power by each sub-unit for the first coherent integration period, and the comparator compares the signal power with the threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be further described in details in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic block diagram showing a general structure of a receiver in accordance with the present invention; 
         FIG. 2  is a flow chart generally illustrating a process of an embodiment in accordance with the present invention; 
         FIG. 3  shows relationships among signal strength, integration period and fix state according to prior art; 
         FIG. 4  shows relationships among signal strength, integration period and fix state according to the present invention; 
         FIG. 5  is a flow chart generally illustrating a process of another embodiment in accordance with the present invention; 
         FIG. 6  is a schematic diagram showing prediction for Doppler frequency; 
         FIG. 7(A)  shows a curve of Doppler frequency of conventional receiver, and  FIG. 7(B)  shows a curve of Doppler frequency of the present invention; and 
         FIG. 8  is a schematic block diagram showing a Doppler frequency processing device in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic block diagram showing a general structure of a receiver in accordance with the present invention. The receiver is used for GNSS (e.g. GPS). The receiver includes an antenna  10  for receiving RF signals such as satellite signals of CDMA form, an IF down converter  14  for converting the RF signals into IF signals, and a correlation unit for conducting correlation to the IF signals. The receiver further includes a coherent integration block  20 , which has an adder  22  and a coherent integration RAM (or memory of another type)  24 , a magnitude calculator  30 , a non-coherent integration block  40 , which has an adder  32  and a non-coherent integration RAM (or memory of another type), a controller and a comparator  60 . The details will be further described later. 
     An embodiment of the present invention will be described in conjunction with  FIG. 2 , which is flow chart illustrating a process in accordance with the present invention. In the present embodiment, coherent integration period (simply referred to as “coherent period”) is set as 1 ms. The process starts from step S 21 , the receiver receives GPS signals, for example. In step S 23 , coherent integration is conducted to the received signals. When a coherent period ends (step S 25 ), the comparator  60  compares the signal power obtained from the coherent integration with a threshold (step S 27 ). It is noted that the signal power is calculated by the magnitude calculator  30 , which may calculate the signal power from I and Q components of the signal or simply I component stored in the coherent integration memory  24 , depending on the condition. If the signal strength is very strong, it is possible to obtain sufficient signal power within one ms. If the signal power has exceeded the threshold, the comparator  60  informs the controller  50  (step S 33 ), and the controller  50  clears up the coherent integration memory  24  accordingly (step S 35 ). A new coherent integration can be started then. If the signal power has not exceeded the threshold, the calculated coherent integration (i.e. coherent signal power) is sent to the non-coherent block  40  and stored in the non-coherent integration memory  34  to be accumulated, this is called non-coherent integration (step S 29 ). In the present embodiment, before a predetermined non-coherent maximum period is reached (step S 31 ), whenever a coherent integration is completed, the signal power is accumulated to the non-coherent integration memory  34 . The accumulated signal power is compared with the threshold by the comparator  60 . Once the threshold is achieved, the controller passes the signal power to be used for navigation. In other words, the non-coherent integration period is dynamically variable rather than fixed. 
     In a condition that the signal strength is strong, the non-coherent integration period can be very short. However, in a condition that the signal strength is weak or signal is blocked, the non-coherent integration period can be extended. It is noted that the threshold is also adjusted by the controller  50  as the non-coherent integration period is varied. When the non-coherent integration period is long, the effect of noises will be more significant, therefore the threshold should be higher. The threshold is determined depending on the coherent integration period and the non-coherent integration period. The advantage of the present invention can be easily observed by comparing  FIG. 3  and  FIG. 4 , which will be described as follows. 
       FIG. 3  shows relationships among signal strength, integration period and fix state according to prior art. In this example, the normal signal strength is −130 dBm. The signal strength drops below −145 dBm when the receiver, which may be carried in a vehicle, passes through some blockings such as buildings or trees. When the blockings have been passed, the signal strength recovers to be −130 dBm. In prior art, the non-coherent integration period is fixed, and therefore the integrated signal power is abruptly drops as shown. The fix state of 3D or 2D fix mode for the receiver is very poor.  FIG. 4  shows relationships among signal strength, integration period and fix state according to the present invention. As can be seen, when the signal strength drops to the low level, the non-coherent integration period is extended, so that the signal power can be maintained sufficiently high. Therefore, the fix state of 3D or 2D fix mode for the receiver can be maintained to be excellent even when the signal is temporarily blocked. 
     As mentioned, the coherent integration period is usually ranged from 1 ms to 20 ms for GPS in practice. If the coherent integration period is selected to be long, (e.g. 20 ms), the coherent integration period is also controlled in accordance with the present invention. Assuming the coherent integration period is set as 20 ms, conventionally, the I and Q components of a received signal is stored and accumulated in the coherent integration memory. The magnitude calculator calculates the signal power every 20 ms and the coherent integration memory is cleared for the next coherent integration. In the present embodiment of the present invention, the I and Q components of the signal are accumulated to the coherent integration memory  24  within 20 ms the same as before. However, the magnitude calculator  30  calculates the signal power every 1 ms, and the comparator  60  compares the calculated signal power. When the signal power exceeds the threshold provided by the controller  50 , the controller stops the coherent integration and sends the signal power to successive processing. If sufficiently high signal power is failed to be obtained within the first coherent integration period, then the process will be the same as that described with reference to FIG . 2 . 
     As described, the non-coherent integration period is not lengthened without limits A maximum period limit is set in advance. If sufficient signal power fails to be obtained after the non-coherent integration has been executed for a long period, it may mean that the signal is lost. The present invention also provides a solution to such a problem. The method will be described with reference to  FIG. 5 . Most steps of the process indicated by the flow chart of  FIG. 5  are generally similar to that of  FIG. 2 , but are described more specifically. The main difference is that in the present embodiment, the solution for solving the problem that the signal is failed to be obtained even the non-coherent integration period is extended to the utmost is provided. The process starts from step S 51 . In step S 52 , the method of the present invention, which can be referred to as an auto AGC (automatic gain control) function, is activated and the maximum non-coherent count, by which the allowable non-coherent integration period is directly derived, MaxInc, is set. In step S 53 , hardware integrates 20 ms coherent data , since the coherent integration period is set as 20 ms in this embodiment, and the coherent data is added to the non-coherent (integration) memory  34 . In step S 54 , the controller  50  sets a threshold according to a current non-coherent count, Noncoh_Cnt. When no coherent integration data is sent to the non-coherent memory  34  yet, Noncoh Cnt =0. After the first coherent integration data is stored into the non-coherent memory  34 , Noncoh Cnt =1. The rest can be deduced accordingly. In step S 55 , the comparator  60  compares the value of the signal power stored in the non-coherent memory  34  with a threshold determined by the controller  50 . If the signal power has exceeded the threshold, the process goes to step S 56 . In step S 56 , further processing is executed, such as calculating Doppler frequency and code phase of the signal. In step S 57 , the non-coherent RAM  34  is cleared, and the non-coherent count Noncoh_Cnt is zeroed. A new correlation process can be executed. If the value of the signal power stored in the non-coherent RAM  34  does not exceed the threshold, then the non-coherent count Noncoh_Cnt is added by 1 in step S 60 . The controller  50  checks if Noncoh_Cnt exceeds the upper limt MaxInc in step S 61 . If not, the integration can be continued. However, if Noncoh_Cnt exceeds MaxInc, it means that the non-coherent integration period has been over a predetermined allowable period. The non-coherent integration should be abandoned. According to the present embodiment, the receiver uses Doppler frequency and/or code phase prediction scheme to estimate the signal. Taking the Doppler frequency as an example, the receiver obtains a Doppler frequency curve from the signal having been received. Then a slope of the Doppler frequency curve is calculated. The receiver then predicts the curve trend based on the slope. 
       FIG. 6  is a schematic diagram showing prediction for Doppler frequency. In the drawing, f d +f d′  is the Doppler frequency due to the movements of the satellite and the receiver, wherein fd results from satellite movement and fd′ results from receiver movement. The term fd′ can be obtained by calculating the following equation: 
               f   d   ′     =         f   c     c     ⁢     v   ⁡     (   t   )       ⁢   cos   ⁢           ⁢   θ           
wherein fc is carrier frequency (e.g. fc of L1 band is 1575.42 MHz); c is the light speed (3×10 8  m/s), θ is the elevation angle of the satellite. As can be seen, the variation of Doppler frequency can be estimated by observing the receiver speed v(t) and the satellite elevation angle θ. For example, if acceleration of the receiver is 1 m/s 2 , and θ=0, then the variation rate of Doppler frequency can be calculated as 5 Hz/sec.
 
       FIGS. 7(A)  and (B) respectively show the Doppler frequency curves without and with use of prediction scheme when the receiver loses lock for the signal. As can been seen, by considering the slope of the previous Doppler frequency curve to predict, the section of the curve where the signal is lost lock can be estimated. 
     Another estimation scheme for Doppler frequency curve is free running.  FIG. 8  shows a phase lock loop (PLL), which is a Doppler frequency processing device utilized in the receiver. The PLL includes a phase detector  72  detecting phase of the signal, and a loop filter  74  feeding the detected phase back to the phase detector  72  via a numeral controlled oscillator (NCO)  76 . If a value controlling the NCO  76  is maintained unchanged regardless of the output of the loop filter  74 , the phase curve will keep going with the same slope. It is called free running. When the signal is lost lock, the value controlling the NCO  76  is fixed to the previous value, and so that the phase curve will be generated continuously with the previous slope. By doing so, the signal can be simulated when the receiver loses lock for the signal. 
     While the preferred embodiment of the present invention has been illustrated and described in details, various modifications and alterations can be made by persons skilled in this art. The embodiment of the present invention is therefore described in an illustrative but not in a restrictive sense. It is intended that the present invention should not be limited to the particular forms as illustrated, and that all modifications and alterations which maintain the spirit and realm of the present invention are within the scope as defined in the appended claims.