Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to communications and, more particularly, to code division multiple access techniques. 
   2. Prior Art 
   Referring now to  FIG. 2  there is shown a block diagram of a direct sequence coded spread spectrum system. As shown in the diagram a carrier signal generated by carrier generator  80  is modulated at mixer  98  by data  82 . The data modulation operates at a rate determined by code clock divided by spreading gain G  84 . The modulated signal is further mixed  99  with a code generated by the pseudo-noise (PN) code generator  86  operating at a clock rate determined by code clock  85 . The transmitter  88  transmits the twice modulated signal via antenna  90 . The transmitted signal is received by receiver  94  via antenna  92 . The received signal is correlated with a PN code generated by PN code generator  106  at mixer  96 . A signal passing correlation is then demodulated by mixing the correlated signal with a local carrier recovery  104  in mixer  100 . The transmitted data and clock rate are recovered in data processing  102  and  108 , respectively. The recovered data  110  is passed to the remainder of the system for further processing while the recovered clock is used to drive the PN code generator  106  and the data processing  102  after being reduced by spreading gain G  112 . The spreading gain is determined by the PN code rate Rc divided by the message rate Rb. The time duration of Rc is 1/Rc=Tc and is referred to as a chip. In general, when changing from a low-rate PN code to a higher rate PN code, where Tc low  and Tc high  are the chip times, respectively, the spreading gain is multiplied by the ratio Tc low /Tc high . The average time that is required to complete a search for PN timing is k/Rb per chip of uncertainty, where k is some constant of proportionality based on the search technique. Hence, if we do the initial search using a shorter low-rate PN and then synchronously switch to a higher-rate PN the average search time can be reduced by as much as Tc low /Tc high . 
   SUMMARY OF THE INVENTION 
   In accordance with one method of the present invention, a method for determining a signal code. The method comprising steps of acquiring and correlating a signal with a first code sequence and achieving a timing lock in response to the step of correlating the signal with a first code sequence. Also in response to the correlated signal the receiver transmits an acknowledgement signal to a transmitter of the signal where upon the receiver and transmitter change from the first code sequence to a second code sequence. 
   In accordance with one embodiment of the present invention a signal code acquisition system comprising a first transceiver and a first multi-rate code generator connected to the first transceiver; a second transceiver responsive to the first transceiver; and a second multi-rate code generator connected to the second transceiver. 
   In accordance with another method of the present invention a method of determining a coded signal, the method comprising steps of: transmitting a first coded signal from a transmitter system; receiving the first coded signal on a receiver system; calculating a probability of detection of the first coded signal; and changing the first coded signal to a second coded signal responsive to the probability of detection (PD) of the first coded signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a satellite communications system and a base station communications system to which the satellite communications system is bidirectionally coupled through a wireless RF link and incorporates features of this invention; 
       FIG. 2  is a block diagram of a direct sequence PN coded system; 
       FIG. 3  is a block diagram of multi-rate PN coded system illustrating an embodiment of this invention; 
       FIG. 4  is a flow chart illustrating one method of the present invention; and 
       FIG. 5  is a flow chart illustrating an alternative method of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Although the present invention will be described with reference to the embodiments shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. 
   Reference is first made to  FIG. 1  for illustrating a base station  10 , such as but not limited to satellite communications base station, that is suitable for practicing this invention. The base station  10  includes an antenna  12  for transmitting signals to and for receiving signals from a base orbiting satellite  30 . 
   Referring now to  FIGS. 1 and 3 . The base station  10  and the orbiting satellite  30  include circuitry for transmitting coded signals  51  and circuitry for decoding received signals  61 . The circuitry for transmitting coded signals includes a multi-rate pseudo-noise (PN) code generator  52 , transmitting timing and control circuitry  50 , a transmitter  54 , data source  56 , and an antenna  58 . The circuitry for receiving the coded signal includes a receiver front end  62 , a synchronization detector  68 , receiving timing and control circuitry  66 , a multi-rate PN code generator  64 , data estimator  70 , and an antenna  60  for receiving signals. The coding method for this invention is assumed to be based on CDMA such as is known from or that is similar to air standards IS-95 PCS or W-CDMA, although the teaching of this invention is not intended to be limited only to that particular type of CDMA system. The present invention, providing significant reduction in PN code acquisition time over conventional PN code acquisition time, could be used with any suitable type of radio telephone system or suitable electronic device 
   Referring now to  FIGS. 3 and 4 , there is shown a block diagram of a transmitter and receiver system and a method flow chart incorporating features of the present invention. The transmitter  54  transmits data  56  modulated by a carrier signal and further modulated by multi-rate PN code generator  52  through antenna  58 . The PN code generator  52  is controlled by timing and control circuitry  50 . The receiver  62  receives the twice modulated carrier signal  120  via antenna  60 . The signal is auto-correlated with a PN code  122  supplied by multi-rate PN code generator  64 . If the signal auto-correlation peak is found then the signal is a desired signal and is further demodulated to retrieve data  126  and recover synchronization  68 . The synchronization then tracks the PN timing  128  via the timing and control circuitry  66  and signals the transmitting system and the receiving system to contemporaneously shift to a higher rate PN code  132  via their respective multi-rate PN generators  52 ,  64 . Thus, for purposes of illustration, if the lower rate PN code duration is designated as Tc low , and the higher rate PN code duration is designated as Tc high  the search time, or PN code acquisition time, is reduced by a factor of Tc low /Tc high . For example, in the prior art the search time for a PN code duration is the processing gain times an uncertainty factor. Using the lower code duration Tc low , the processing gain is Tb/Tc low , where Tb is the message bit duration. Without the current invention the processing gain when shifting to a higher rate PN code would be Tb/Tc high . Assuming that Tc high  is some multiple 1/M of Tc low  then the processing gain equation can be rewritten as Tb*M/Tc low . Then, assuming an average search rate of k/Rb per chip of uncertainty, the PN search time is then (M*Tb/Tc low )*k/Rb, where Rb is the message bit rate. Thus, the average search time has been increased by a factor of M when shifting from a low rate PN code to a high rate PN code. By contrast, the current invention uses a narrow bandwidth timing recovery loop to maintain the timing lock achieved during the acquisition of the first or lower rate PN code while both multi-rate generators contemporaneously shift to the higher rate PN code. Since the timing lock is maintained the search time equation is the original gain times the uncertainty factor, which in this example is Tb/Tc low *(k/Rb). Thus, the average search rate has been reduced by a factor M when shifting from the lower rate PN code to the higher rate PN code. 
   Referring now to  FIG. 5  there is shown a flow chart of a second method of the present invention. The transmitter transmits  140  a low rate PN modulated signal. The transmitter calculates a probability of detection(PD)  142  by the receiver  62 . If the PD is greater than a predetermined amount  144  the transmitter will shift  146  the multi-rate PN generator  52  to a higher rate PN code after a predetermined amount of time or event. Meanwhile, the receiver  62  receives  160  the signal and auto-correlates  158  with the low rate PN code generated by the multi-rate PN generator  64 . If the signal auto-correlation peak is found  156  then the data is decoded  154  and the synchronization detector  68  and the timing and control circuitry lock  152  on to the signal timing. The receiver also calculates the PD and if greater than the predetermined amount will shift  150  the multi-rate PN generator  64  to the higher PN code  148  after the predetermined amount of time or event. The processing gain and search times are calculated as before. 
   It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.

Technology Category: 5