Abstract:
Briefly, one way to minimize time and frequency uncertainties when synchronizing between a transmitter and a receiver is to use a position determining method such as global positioning system (GPS) to determine position and velocity. If position can be determined the delay between the transmitter and receiver can be estimated. Additionally, position information can be used to determine the probability that the signal received will be a reflected signal. If the transmitter and receiver are located in an area with obstructions between them then it is more likely that a signal received will be a reflected signal. If it is determined that it is likely that the signal received will be a reflected signal then the receiver can attempt to acquire the transmitted signal over a wider range of propagation delay possibilities. Additionally, another uncertainty that complicates synchronization is uncertainty regarding frequency. One cause for frequency uncertainty is Doppler shift due to movement of the transmitter or receiver. Using a position determining method such as GPS to determine velocity allows the receiver to estimate the Doppler frequency shift and adjust for it.

Description:
FIELD 
     The present invention relates generally to electronics, and more particularly to wireless communications devices. 
     BACKGROUND 
     Many people use wireless communication systems. In wireless communication systems the information is typically transmitted by electromagnetic waves traveling through the air or other medium. One form of wireless communication is spread-spectrum communication. Spread-spectrum communication systems use a synchronized version of a spreading code to demodulate the received signal. Spreading codes will be discussed below. Typically, the synchronizing process between the locally generated spreading signal and the received spread spectrum signal happens in two parts. The first part is acquisition. During acquisition both signals are coarsely aligned with each other. In other words, the spreading code is rough alignment in time. The spreading code is roughly synchronized. After this coarse alignment the second part, tracking makes fine alignments to maintain the best waveform possible. 
     The acquisition phase involves searching through time and frequency uncertainty to synchronize a received spread-spectrum signal with a spreading code that has been generated locally. Many uncertainties exist during acquisition. Uncertainty in the distance between the transmitter and receiver of a spread-spectrum communications system leads to uncertainty in the amount of delay between the transmitter and receiver. Delay as a signal travels, for example from a transmitter to a receiver, is known as propagation delay. This can happen between any transmitter and receiver. Several examples are given using a base station and a mobile station. 
     An example of a spreading code and uncertainty in the distance between the transmitter and receiver will now be discussed. A spreading code is a pseudo-random sequence. The spreading code would typically be used to modulate a spread-spectrum signal at a base station for transmission to a mobile station. Additionally, the spreading code would typically be used to demodulate signals received from the mobile station. A locally generated spreading code  456  will typically be used to demodulate the received signal from the base station and to modulate signals transmitted to the base station. 
     A locally generated spreading code and the spreading code at the base station are not synchronized initially. One thing that increases the difficulty of synchronizing these two signals, as discussed above, is delay as the signal travels from the base station to the mobile station. Spreading codes will be discussed further with respect to  FIG. 10  below. 
     Additionally, uncertainty regarding a receiver&#39;s velocity relative to a transmitter leads to uncertainty regarding Doppler frequency shift of the transmitted signal. A Doppler shift is the change in apparent frequency of a source of electromagnetic radiation or sound when there is a relative motion between the source, in this case the transmitter, and the observer, in this case a mobile handset that is attempting to synchronize with a spread-spectrum communications network. 
     SUMMARY 
     One problem in the operation of many wireless communication devices such as, for example CDMA mobile handsets is that both the transmitter and the receiver of a communications signal need to be synchronized. In many cases synchronization may take too long. One reason that synchronization may take too long is that time and frequency are uncertain. 
     Many things contribute to the time and frequency uncertainty. The relative position of the transmitter and receiver may be unknown, resulting in an uncertainty in the distance between the transmitter and receiver. This distance uncertainty causes an uncertainty in the amount of time taken for a transmitted electrical signal to travel from the transmitter to the receiver. This is known as propagation delay. Additionally, some areas may be more prone to receiving reflected signals, also leading to uncertainty of propagation delay. Electrical signals can be reflected off of objects, for example buildings. Reflections cause uncertainty regarding the path of travel of a signal. An uncertain path of travel can result in uncertainty regarding propagation delay. 
     In addition to not knowing the position of a transmitter and receiver, velocity may be unknown. Uncertainty regarding velocity translates to uncertainty regarding the apparent change in frequency that is caused when the transmitter and receiver are in motion relative to one another. The apparent change in frequency caused by motion of a transmitter relative to a receiver is known as a Doppler frequency shift. The Doppler frequency shift leads to frequency uncertainty. Doppler frequency shift will be discussed further below. 
     One way to minimize time and frequency uncertainties when synchronizing between a transmitter and a receiver is to use a position determining method such as global positioning system (GPS) to determine position and velocity. If position can be determined the delay between the transmitter and receiver can be estimated. Additionally, position information can be used to determine the probability that the signal received will be a reflected signal. If the transmitter and receiver are located in an area with obstructions between them then it is more likely that a signal received will be a reflected signal. If it is determined that it is likely that the signal received will be a reflected signal then the receiver can attempt to acquire the transmitted signal over a wider range of propagation delay possibilities. 
     As stated above, another uncertainty that complicates synchronization is uncertainty regarding frequency. One cause for frequency uncertainty is Doppler shift due to movement of the transmitter or receiver. Using a position determining method such as GPS to determine velocity allows the receiver to estimate the Doppler frequency shift and adjust for it. 
     Incorporating position and velocity information into the acquisition of a spread spectrum communication system such as, for example, a CDMA network has many advantages. The synchronization and acquisition steps may typically be completed more efficiently. In some cases synchronization and acquisition may be faster. As the antenna beam becomes more narrow less energy is transmitted for a given receive energy at a receive antenna. 
     A CDMA network is used as an example above. Other air-interfaces are possible. For example, Inter-system acquisition is possible. For example, a CDMA mobile station could be used to acquire an 802.11 network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, tables and attachments, in which: 
         FIG. 1  shows a diagram of a mobile station operating in a communication network. 
         FIG. 2  shows a diagram of estimating chip delay. 
         FIG. 3  shows a mobile station in an urban environment with many sources of signal reflection. 
         FIG. 4  shows a mobile station in motion toward a base station. 
         FIG. 5  is a diagram that shows a representation of a Doppler shift due to the motion of a mobile handset. 
         FIG. 6  is a flowchart that shows adjusting signal acquisition based on relative position. 
         FIG. 7  is a flowchart that shows adjusting signal acquisition based on an estimated chip delay. 
         FIG. 8  is a flowchart that shows adjusting signal acquisition based an environment type. 
         FIG. 9  is a flowchart that shows adjusting signal acquisition based on an estimated Doppler shift. 
         FIG. 10  is a diagram that shows a chip delay between a spreading code and a locally generated spreading code. 
         FIG. 11  is a diagram of a mobile station in the form of a mobile handset. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1  a mobile station in the form of a mobile handset  54  is shown operating in a communication network. The network includes a base station  52 . The base station  52  transmits and receives signals  63  from the mobile handset  54 . The mobile handset is some distance  57  from the base station. The distance can be determined if the location of the base station  52  is known and the location of the mobile handset  54  is determined using global positioning system satellites  67 . It will be understood by those of skill in the art that other location determination methods are possible. The communication network environment includes open areas and areas that contain many sources of reflection. For example, an urban area  65 , would typically tend to be an area that contains many sources of refection. The mobile handset  54  is moving as shown by an arrow indicating velocity  60 . Location and motion can inhibit the acquisition of a communication network by complicating the synchronization process between the mobile handset  54  and the base station  52 . Several examples of how location and motion complicate the synchronization process will be discussed with respect to  FIGS. 2 ,  3 ,  4 , and  5 . An advantage to using relative position or relative velocity to help in the synchronization process is that typically, synchronization can occur more efficiently. 
     Referring now to  FIG. 2  a diagram  75  is shown. The diagram  75  shows how a chip delay between a mobile station in the form of the mobile handset  54  and a base station  52  is estimated. One example of how location and motion complicate the synchronization process is caused by chip delay. 
     Simply stated, the estimated chip delay is the time it takes a signal to travel from the base station  52  to the mobile handset  54 , or the distance  57  divided by the speed of light. Chip delay was discussed with respect to  FIG. 10 . With an estimate of chip delay the spreading code and the locally generated spreading code can typically be synchronized more efficiently. 
       FIG. 3  shows a diagram  150  with the mobile handset  54  operating in the urban environment  65 . Operating in the urban environment  65  is another example of how location can make signal acquisition more difficult. An urban environment  65  is an example of a environment that typically has many causes of signal reflection. For example, large buildings would tend to reflect transmitted signals. It will be understood by those of skill in the art that many other environments also have sources of reflection. The term urban environment is typically used herein to describe these possible areas. However, other possible signal reflective areas are possible, for example, mountainous areas. It will be understood by those of skill in the art that examples given that refer to urban areas will typically also apply to other areas. 
     Due to the urban environment  65  a signal  154  transmitted from the base station  52  to the mobile handset  54  is reflected. Reflected signals can be difficult to synchronize with. This difficulty can be, for example, due to the fact that the delay from transmission at the base station to reception at the mobile station is difficult to estimate since the distance traveled is difficult to estimate. If the mobile handset is located in an urban environment, or any environment that has many possible sources of reflection signal acquisition can be adjusted accordingly. Such as, for example, when the mobile handset is attempting to synchronize the locally generated spreading code with the spreading code from the base station the range of possible chip delays considered can be increased in a highly communication signal reflective environment, such as an urban area  65 . 
     Referring now to  FIG. 4  a diagram  200  will be discussed. The diagram  200  shows another possible examples of how location and motion complicate the synchronization process. Specifically, the diagram  200  shows a mobile station  54  traveling at a velocity  60  towards a base station  52 . The velocity  60  leads to a Doppler frequency shift. Doppler frequency shift will be discussed in more detail with respect to  FIG. 5 . Additionally, due to the velocity  60  the distance  57  is constantly changing. The change in distance leads to a constant change in chip delay. One advantage to using velocity to help improve signal acquisition is that chip delay estimations can be updated as the position of the mobile handset changes. 
     As stated above, the velocity  60  leads to a Doppler frequency shift. Doppler frequency shift will now be discussed in more detail with respect to  FIG. 5 .  FIG. 5  shows a diagram  230 . The diagram  230  shows the mobile handset  54  moving at velocity  60 . As the mobile handset moves, it can be seen that the electromagnetic waves are closer together in the direction of travel and farther apart behind the mobile handset  54 . The higher frequency  237  electromagnetic waves are shown in front of the mobile handset  54 . Lower frequency  235  electromagnetic waves are shown behind the mobile handset  54 . Many people have experienced this phenomenon with respect to sound. One example is the horn on a train that seems to change frequency as the train travels by. One advantage to using a relative velocity estimate to estimate Doppler frequency shift is that the frequency that the mobile handset should use for signal acquisition can be more accurately estimated. 
     Referring now to  FIG. 6  a flowchart  300  will be discussed. The flowchart shows a high level overview of using position as part of signal acquisition. The flowchart  300  begins at  302 . In step  304  the position of the mobile station is determined. Position can be determined, as stated with respect to  FIG. 1  using the global positioning system, however, other positioning systems are possible. The flowchart  300  continues with step  307 . In step  307  the relative position of the mobile is determined. This may, for example, be determined using a known position of the base station and comparing it to the position determined in step  304 . The flowchart  300  continues at step  310 . At step  310  signal acquisition is adjusted based on the relative position. The flowchart  300  is general. Specific examples of how signal acquisition is adjusted have been discussed with respect to  FIGS. 2 ,  3 , and  4 . Note that  FIG. 5  discussed changing signal acquisition based on velocity, change in position. These examples will be discussed further with respect to  FIGS. 7 ,  8 , and  9 . 
       FIG. 7  is similar to  FIG. 6 . In  FIG. 7  the adjustment to signal acquisition is an adjustment based on chip delay.  FIG. 7  is a flowchart  325 . The flowchart begins at  327 . Step  330  of  FIG. 7  is similar to step  304  of  FIG. 6 . In step  330  the position of a mobile station is determined. In step  332  the relative position of the mobile station is determined. Step  332  is similar to step  307  of  FIG. 6 . The flowchart continues with step  335 . In step  335  chip delay is estimated based on the relative position estimated in step  332  and signal acquisition is adjusted based on the chip delay in step  338 . Step  338  is similar to step  310  of  FIG. 6 . An advantage to adjusting signal acquisition based on chip delay is that in some cases the signal acquisition may be performed more quickly. 
       FIG. 8  is similar to  FIGS. 6 and 7 . In  FIG. 8  the adjustment to signal acquisition is an adjustment based on environment type.  FIG. 8  is a flowchart  350 . The flowchart begins at  355 . Step  359  of  FIG. 7  is similar to step  304  of  FIG. 6  and step  330  of  FIG. 7 . In step  359  the position of a mobile station is determined. In step  364  the relative position of the mobile station is determined. Step  364  is similar to step  307  of  FIG. 6  and step  332  of  FIG. 7 . The flowchart continues with step  369 . In step  369  environment type is determined based on the relative position estimated in step  364  and signal acquisition is adjusted based on the environment type in step  375 . Step  375  is similar to step  310  of  FIG. 6  and step  338  of  FIG. 7 . An advantage to adjusting signal acquisition based on environment type is that the mobile handset, as stated above with respect to  FIG. 3  can use a greater range of possible chip delays due to the uncertainty of the distance the transmitted signal has traveled and therefore the delay time. 
       FIG. 9  is similar to  FIGS. 6 ,  7 , and  8 . For brevity,  FIG. 9  will be compared to  FIG. 6 . In  FIG. 9  velocity is determined and used to estimate a Doppler frequency shift.  FIG. 9  is a flowchart  400 . The flowchart begins at  402 . Step  405  of  FIG. 9  is similar to step  304  of  FIG. 6 . However, velocity is determined instead of location. In step  408  a Doppler frequency shift is estimated. The Doppler frequency shift is used to adjust signal acquisition in step  411 . Step  411  in  FIG. 9  is similar to step  310  in  FIG. 6 . Both are a signal acquisition adjustment step. As stated with respect to  FIG. 5 , an advantage of using a relative velocity estimate to estimate Doppler frequency shift is that the frequency that the mobile handset should use for signal acquisition can be more accurately estimated. 
     Referring now to  FIG. 10 , a diagram  450  is shown. The diagram  450  depicts two versions of a pseudo-random sequence. The first pseudo-random sequence  453  could be, for example, the sequence used by a base station. The second pseudo-random sequence  456  could be, for example, the sequence used by a mobile. In order for the base station and the mobile station to communicate, the sequence used to transmit at the base station, must be synchronized to the sequence used to receive at the mobile station and the sequence used to transmit at the base station must be synchronized to the sequence used to receive at the mobile station. Typically, the same sequence is used; however, the sequences will be offset due to the distance between the base station and the mobile station. 
     As shown on  FIG. 10  the second pseudo-random sequence  456  is offset from the first  453  by some amount of time  459 . Each sequence contains the same synchronization word. In order to synchronize the base station to the mobile station the sequences are correlated. The goal is to align the synchronization words in time. Aligning the synchronization words in time results in aligning the symbols of the synchronization words. When they are aligned the base station and the mobile station are synchronized and can communicate with each other. 
     This process can be sped up by making a better initial approximation of how far off the synchronization word in the mobile station is from the synchronization word in the base station. 
     By estimating the amount of time  459  as described with respect to  FIG. 2 , that described estimated chip delay,  57  of  FIG. 2 , the two pseudo-random sequences,  453  and  456  may be more quickly aligned. 
     Referring now to  FIG. 11 , a mobile station in the form of a mobile handset  600  is shown. The mobile handset  600  includes an antenna  603 . The antenna is connected to a position-determining device  611 . The position-determining device  611  may be a device that uses global positioning satellites to determine position. The position-determining device  611  is connected to a processor. The processor is capable of implementing the methods described with respect to the flowcharts  FIGS. 6 ,  7 ,  8 , and  9 . The processor  608  may be a stand-alone processor, or multiple processors. Additionally, it may be include stand-alone digital logic. The processor is coupled to a memory  614 . The memory is used to store information needed by the processor. Additionally, the processor  608  is coupled to a transceiver  622 . The transceiver  622  is connected to a second antenna  620  and used to send and receive signals from a communication network, typically a base station. A mobile power source in the form of the battery  617  is coupled to the processor  608  and supplies power to the processor. It should be noted that the mobile power source could be a fuel cell, or other type of transportable power source. Additionally, the mobile power source could include a combination of power sources, such as, for example a battery and a fuel cell. Additionally, the processor  608  and typically most other components are enclosed in a case  605  that protects the components. 
     Global positioning satellites are not the only way position can be determined. Position information can be stored at a base station and transmitted to a mobile station. These methods can also be applied to mobile to mobile communication. In this case for example, a base station could transmit the location of a second mobile station to facilitate communication between a first mobile station and the second mobile station. 
     Several examples have been discussed using mobile stations and base stations. However, as was stated above any transmitter and receiver can be used. For example, mobile station to mobile station communication is possible. Additionally, for example, acquiring signals from another air-interface is possible. Usually, in CDMA systems each mobile station has a list of known PN-codes for neighboring base stations. If propagation delay and velocity is known in advance this acquisition could be faster. Additionally, the mobile station could acquire any other spread spectrum system. For example, 802.11 systems using a similar method. As stated above, inter-system acquisition is possible.