Abstract:
A method of frequency tracking based on recovered data, for use in an automatic frequency control subsystem at the receiver of a mobile station, is disclosed. The frequency tracking mechanism derives frequency error information from the recovered data to determine the adjustment needed at the mobile station&#39;s local voltage controlled oscillator in order to track the frequency of the base station.

Description:
TECHNICAL FIELD 
   The present invention relates to frequency tracking at a mobile station for use in a wireless communications environment, and more particularly, using information derived from the recovered data at the receiver of the mobile station to tune a reference local voltage controlled oscillator (VCO). 
   BACKGROUND 
   In a wireless communications system, the air interface typically involves a mobile station communicating with a base station over the airwaves. For example, the most common standard for wireless communications in the world is the Global System for Mobile communications (GSM). In one specific implementation, GSM utilizes two bands of 25 MHz, which have been set aside for system use. The 890-915 MHz band is used for mobile station to base station transmissions (reverse link), and the 935-960 MHz band is used for base station to mobile station transmissions (forward link). The GSM protocol uses frequency division duplexing and time division multiple access (TDMA) techniques to provide base stations with simultaneous access to multiple users. Transmissions on both the forward and reverse link are made at a channel data rate of 270.833333 Kbps, using binary Gaussian Minimum Shift Key (GMSK) modulation. Additionally, each link contains traffic channels and control channels. The traffic channels carry the digitized voice or user data. The control channels carry network management or control information such as the frequency correction channel (FCCH). 
   When a mobile station is powered on, it must first perform a power scan across all the control channels, to identify the channel with the strongest signal. The mobile station then tunes into the strongest channel to locate the FCCH. FCCH carries a frequency correction burst, which occupies time slot  0  for the very first GSM frame and is repeated every ten frames within a control channel multiframe. The FCCH burst allows each mobile station to synchronize its reference local oscillator or voltage controlled oscillator (VCO) to the exact frequency of the base station. 
   However, the VCO in a mobile station is usually not as robust as the VCO at the base station. Consequently, the frequency will fluctuate with the temperature of the VCO, in addition to other factors, such as aging, that will also contribute to the fluctuation but in a less significant amount. The frequency fluctuation will accumulate over time resulting in degrading the performance of the mobile station&#39;s receiver. An automatic frequency control (AFC) subsystem is an important component of a receiver for performance stability. For example, the GSM 11.10 specification requires a mobile station to maintain a carrier frequency to within 0.1 parts per million (ppm) of the base station&#39;s reference frequency, in other words, 0.1 ppm compared to the signals received from the base station. Although, the prior art Maximum Likelihood method can be used to obtain reliable frequency tracking, but it requires a long data stream and complex computation. Similarly, the prior art Time Domain Bias method can also be deployed for estimating frequency offset, but it needs a high sampling rate as well as a long data stream. Accordingly, the AFC subsystem in the receiver of a mobile station needs a reliable, simple, and effective frequency tracking technique to minimize frequency error to an acceptable level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic diagram of a baseband transmitter. 
       FIG. 2  is a schematic diagram of an equivalent structure of the baseband transmitter shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram of an Automatic Frequency Control (AFC) subsystem. 
       FIG. 4  is a schematic diagram of an apparatus for frequency tracking based on recovered data. 
       FIG. 5  is a flow diagram of a method of frequency tracking based on recovered data. 
   

   DETAILED DESCRIPTION 
   In the detailed description provided below, numerous specific details are provided to impart a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
   Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     FIG. 1  depicts a baseband transmitter  100 . Prior to sending the outgoing data {a k }  102  to the mobile station, the baseband transmitter  100  modulates the outgoing data {a k }  102  using Gaussian minimum shift keyed (GMSK) modulation. The modulated data passes through an air channel ch(t)  110 . Afterward, a noise component n(t)  112  is added to the data to yield y(t)  114 , which is the signal received at the receiver of mobile station. While in recovering GMSK modulated signal at the mobile receiver, we use the linear approximation of GMSK modulation, that is, we regard GMSK modulation as rotated data p k  pass a filter f(t)  108 . The linear approximation is shown in dashed block in  FIG. 1 . 
   In accordance with the linear approximation of GMSK modulation, p k    106  is mathematically represented as:
 
 p   k   =a   k   ·j   k 
 
   where a k  is the outgoing data and a k  is either 1 or −1. k is the data index. j k  is the modulation phase shift. 
     FIG. 2  illustrates an equivalent channel h(t)  204 , which is a concatenation of the filter f(t)  108  and the air channel ch(t)  110  of  FIG. 1 . h(t)  204  is mathematically represented as:
   h ( t )= f ( t )* ch ( t ) 
   where f(t) is filter f(t)  108  and ch(t) is air channel ch(t)  110 . 
   When incoming data y(t)  114  arrives at the receiver, it is first demodulated and then a frequency offset is added to the demodulated data. The resulting received data is r(k). The frequency offset here does not take into account any frequency offset that may be caused by Doppler shift because Doppler shift cannot be tracked in mobile communications and it is not required by the GSM specification. The received data r(k) is mathematically represented as:
 
 r ( k )= y ( k )·e jωk 
 
   where e jωk  is the frequency offset, ω is the angle of the frequency offset and k is the data index. 
   A key component of a mobile station receiver is the AFC subsystem. The AFC contains a frequency tracking mechanism to ensure that the frequency of the mobile station&#39;s local VCO tracks the frequency of the base station. 
     FIG. 3  is a schematic diagram of an AFC subsystem. The received data r(k)  304  is one of the inputs into a frequency tracking block  322  used to determine the frequency error within the incoming data bursts. The received data r(c)  304  is also used as an input to a channel estimation block  306  to obtain the estimated channel tap ĥ(k)  308 . Additionally, the received data r(k)  304  is input to a Viterbi equalizer  310  to obtain the hard decision Viterbi Equalizer output â(k)  312 . 
   Prior to frequency tracking, initial frequency estimation is performed on the received data r(k)  304  to obtain the estimated channel tap ĥ(k)  308 . The channel is estimated by a training sequence in p(k). The estimated channel tap ĥ(k)  308  is mathematically represented as: 
   
     
       
         
           
             
               h 
               ^ 
             
             ⁡ 
             
               ( 
               k 
               ) 
             
           
           = 
           
             
               1 
               16 
             
             ⁢ 
             
               
                 ∑ 
                 
                   l 
                   = 
                   5 
                 
                 20 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   r 
                   ⁡ 
                   
                     ( 
                     
                       k 
                       + 
                       l 
                     
                     ) 
                   
                 
                 ⁢ 
                 
                   
                     p 
                     * 
                   
                   ⁡ 
                   
                     ( 
                     l 
                     ) 
                   
                 
               
             
           
         
       
     
   
   where k=−5, −4, . . . , 0, 1, . . . , 5. The frequency offset has a negligible affect on the channel estimation in the range of −500 Hz to 500 Hz. Thus, the weak taps of the estimated channel taps can be masked. 
   In one embodiment as illustrated in  FIG. 3 , the frequency tracking mechanism  322  utilizes the received data r(k)  304 , the estimated channel tap ĥ(k)  308 , and the hard decision Viterbi output â(k)  312  to determine the direction of the phase rotation, either in the positive direction or negative direction. For each data burst the direction of the phase rotation is ascertained based on the frequency error. Within a given number of data bursts, the number of positive direction rotations and the number of negative direction rotations are added. If the number of positive direction rotations is greater than a predetermined threshold, then the local VCO is tuned one frequency step in the negative direction. If the number of negative direction rotations is greater than a predetermined threshold, then the local VCO is tuned one frequency step in the position direction. 
   In another embodiment also depicted by  FIG. 3 , the frequency tracking  322  uses the received data r(k)  304 , estimated channel tap ĥ(k)  308 , and the recovered convolutionally coded output â′(k)  320  (shown in dash lines) to determine the direction to tune the VCO of the mobile station. Here, the hard (or soft) decision Viterbi equalizer output â(k)  312  is input into the Viterbi decoder  316  and the resulting data is input into the convolutional encoder  318  to produce the resulting recovered convolutionally coded output â′(k)  320 . 
   For each data burst the direction of the phase rotation is determined based on the frequency error. Within a given number of data bursts, the number of positive direction rotations and the number of negative direction rotations are calculated. If the number of positive direction rotations within the given number of data bursts is greater than a predetermined threshold, then the local VCO of the mobile station is tuned one frequency step in the negative direction. If the number of negative direction rotations within the given number of data bursts is greater than a predetermined threshold, then the local VCO of the mobile station is tuned one frequency step in the position direction. 
   Turning first to  FIG. 5 , which illustrates method  500  for frequency tracking based on the recovered data, at the start of frequency tracking, step  506  correlates the recovered data {circumflex over (r)}(k) with the received data r(k). The recovered data {circumflex over (r)}(k) is the rotated estimated input {circumflex over (p)}(k) passing through the estimated channel tap ĥ(k). The rotated estimated input {circumflex over (p)}(k) comprises either the hard decision Viterbi equalizer output â′(k) or the recovered convolutionally coded output â′(k). 
   Next, in step  508  the result of the first correlated data is delayed. In step  510 , a second correlation is performed on the first correlated data of step  506  with the delayed first correlated data of step  508 . The second correlation provides the rotation angle caused by the frequency offset. The second correlated data is added to the imaginary part of the second correlated data in step  512 . If the result of the sum in step  512  is positive, then the positive register is increased by one. If the result of the sum in step  512  is negative, then the negative register is increased by one. Steps  502  through step  518  are then repeated for N data bursts. 
   After N data bursts are reached in step  520 , if the positive register is greater than the predetermined threshold, then the VCO is tuned one step in the negative direction in step  524 . Otherwise, if the negative register is greater than the predetermined threshold, then the VCO is tuned one step in the positive direction in step  526 . Finally, in step  530  the positive register and the negative register are cleared. 
     FIG. 4  provides a schematic diagram of a frequency tracking system. In one embodiment, the inputs to a data recover  402  mechanism are the estimated channel tap ĥ(k)  308  and the hard decision Viterbi equalizer output â(k)  312 . In another embodiment, the inputs to the data recover  402  mechanism are the estimated channel tap ĥ(k)  308  and the recovered convolutionally coded output â′(k)  320 . The input that is either the hard decision Viterbi equalizer output â(k)  312  or the recovered convolutionally coded output â′(k)  320  can be estimated as:
   {circumflex over (p)}   k   =â   k   ·j   k   
   The rotated estimated input {circumflex over (p)}(k) passes through the estimated channel tap ĥ(k)  308  to get the recovered data {circumflex over (r)}(k)  404 . The recovered data {circumflex over (r)}(k)  404  is mathematically represented as:
 
 {circumflex over (r)} ( k )= {circumflex over (p)} ( k )* ĥ ( k )= ŷ ( k )
 
   A first correlation  406  is performed on the received data r(k)  304  and the recovered data {circumflex over (r)}(k)  404  to obtain the first correlated data z(k)  410 . The first correlated data z(k)  410  is mathematically represented as:
 
 z ( k )= r ( k )·conj( {circumflex over (r)} ( k ))=( y ( k )· e   jωk   +n ( k ))·conj( ŷ ( k ))≈| y ( k )| 2   ·e   jωk   +n ′( k )
 
   where ω is the angle of the frequency offset, k is the data index, and ·n′(k)=n(k)·conj(ŷ(k)). 
   A second correlation  414  is performed on the first correlated data z(k)  410  and the delayed first correlated data z(k+L)  412 , where L is the delayed sample, which is set to adjust the rotation angle in a range to obtain a more precise estimation. 
   The resulting second correlated data s′(k)  416  is mathematically represented as:
 
 s ′( k )= z ( k+L )·conj( z ( k ))
 
   The mathematical representation of the imaginary part of s′(k) is:
 
 s ( k )=imag{ z ( k+L )·conj( z ( k ))}
 
   Add the imaginary part s(k) to the second correlated data s′(k)  416  to obtain d  420 . The mathematical representation of d  420  is: 
   
     
       
         
           d 
           = 
           
             sign 
             ( 
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 M 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 s 
                 ⁡ 
                 
                   ( 
                   k 
                   ) 
                 
               
             
             ) 
           
         
       
     
   
   where M is the length of s(k). 
   The decision  422  mechanism counts the number of d&gt;0 and the number of d&lt;0 within every N data bursts. If the number of d&gt;0 is greater than a predetermined threshold, the VCO is tuned one frequency step in the negative direction. If the number of d&lt;0 is greater than a predetermined threshold, the VCO is tune one frequency step in the position direction. 
   The parameters for a mobile station in a GSM communications system based on the GSM 11.10 requirements in one example may be as follows: L=100; M=40; N can be 100 or 200 received data bursts; and predetermined threshold can be larger than half of N. 
   While specific embodiments of the invention have been illustrated and described herein, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.