Patent Publication Number: US-7720131-B2

Title: Iterative pilot-aided frequency offset estimation and C/I measurement for TDMA signal with offset larger than nyquist frequency of the reference symbol rate

Description:
FIELD OF THE INVENTION 
   The field of the invention relates to communication devices and more particularly to wireless communication devices. 
   BACKGROUND OF THE INVENTION 
   Time division multiple access (TDMA) communication systems are known. Such systems typically include one or more base stations through which portable wireless devices (e.g., personal digital assistants (PDAs), cell phones, etc.) can access other devices or wireline communication services. 
   In order to gain access through a base station, the portable device may first monitor one or more control channels to detect a signal from a base station. In order to facilitate access, the base station may transmit an identification signal on the control channel. 
   In TDMA wireless systems, it is important for the receiver to acquire timing and frequency synchronization in order to correctly demodulate the received data signal and to accurately measure the carrier to interference ratio. Both of these objectives can be achieved by a transmitting device embedding a known stream of symbols in the transmitted signal. These known symbols are often referred to as reference symbols. 
   The known stream of embedded symbols may include at least some SYNCH and/or PILOT symbols. SYNCH symbols are used primarily for timing estimation and PILOT symbols are used primarily for frequency estimation. 
   The receiver can process the SYNCH symbols to obtain time synchronization by correlating the received SYNCH symbols with known values for the SYNCH symbols within a repeating frame structure. Once time synchronization has been achieved, the receiver may use the PILOT symbols to estimate a frequency offset. 
   The frequency offset is the difference between the received signal carrier frequency and the locally generated reference frequency. The frequency offset may be caused by any of a number of different factors internal to the portable devices (e.g., clock offset, Doppler effects, asynchronous clock drift, etc.). 
   For reference symbols (e.g., pilot symbols) with a repetition frequency of F p , the maximum frequency offset that can be estimated using conventional methods (without introducing aliasing) is F p /2 (i.e., the Nyquist frequency of the pilot symbol repetition rate). If the frequency offset is greater than F p /2, then conventional methods may incorrectly estimate the frequency, and carrier to interference (C/I) measurements will be severely degraded resulting in improper channel selection, demodulation errors and high symbol error rates. Accordingly, a need exists for better methods of calculating C/I values. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a portion of a transceiver in accordance with an illustrated embodiment of the invention; 
       FIG. 2  shows a communication system using the transceiver of  FIG. 1 ; and 
       FIG. 3  shows a comparison of the reliable detection by the transceiver of  FIG. 1  and a conventional transceiver. 
   

   SUMMARY 
   A method and apparatus are provided for decoding a wireless signal from a set of samples with an embedded training sequence. The method includes the steps of determining a first frequency offset from the samples where the first frequency offset is assumed to be less than a Nyquist frequency of the training sequence and calculating a first carrier to interference ratio based upon the first frequency offset. The method further includes the steps of determining a second frequency offset from the samples by subtracting an absolute value of the first frequency offset from an integer multiple of the Nyquist frequency and giving the second frequency offset a sign opposite that of the first frequency offset, calculating a second carrier to interference ratio based upon the second frequency offset and selecting one of the first and second frequency offsets based upon a relative values of the calculated carrier to interference ratios. 
   In another aspect, the method includes selecting one of the first and second frequency offsets further comprising selecting the frequency offset providing a carrier to interference ratio with a greatest relative value. 
   In another aspect, the method includes calculating the carrier to interference ratio (C/I) further comprises determining a carrier energy (E C ) of the wireless signal. 
   In another aspect, the step of determining the carrier energy further includes correlating SYNC and PILOT symbols with a set of reference SYNC and PILOT symbols. 
   In another aspect, the method includes determining a received channel power (I O ). 
   In another aspect, the step of determining the received channel power (I O ) further includes squaring and summing each sample of the set of samples. 
   In another aspect the step of calculating the carrier to interference ratio further comprises solving the equation 
   
     
       
         
           
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   DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT 
     FIG. 1  is a portion of a radio transceiver  10  shown generally in accordance with an illustrated embodiment of the invention. The radio transceiver  10  uses a novel frequency offset estimation method for carrier-to-interference measurement. The radio transceiver  10  may be used in a narrow band time division multiple access (TDMA) cellular radio communication system  100  such as that shown in  FIG. 2 . 
   In TDMA wireless receiver systems  100 , a frequency offset between a received signal  104  and a locally generated reference signal is frequently encountered due to clock drift, Doppler Effect and asynchronous timing reference systems. The frequency offset estimation is an essential step for carrier-to-interference (C/I) measurement. Prior pilot-aided frequency offset estimation systems had taught that an upper bound on the maximum frequency offset that can be estimated is the Nyquist frequency of the reference symbol repetition rate. When the frequency offset is larger than the Nyquist frequency, conventional frequency offset estimation methods cannot provide correct frequency offset values. This results in severely degraded carrier-to-interference measurement and complete loss of frequency synchronization in some scenarios. To overcome this issue, an iterative pilot-aided frequency offset method is provided that can accurately estimate frequency offset values that are larger than the Nyquist frequency of the pilot repetition rate. The iterative pilot-aided frequency offset method described below integrates frequency offset estimation and carrier-to-interference calculation algorithms together, and provides accurate frequency offset in an iterative manner. It is shown to provide a dramatic improvement on carrier-to-interference measurement for TDMA based system with relatively large frequency offsets between transmitter and the receiver. 
   The radio transceiver  10  accurately measures the C/I ratio and frequency offset within the TDMA based system  100 , when the frequency offset in the received signal is significantly more than the Nyquist frequency of the reference symbol repetition rate, F p . This scenario can occur when a receiver has a single stage frequency estimation/correction block or the coarse frequency estimation block is not able to bring the frequency offset below the Nyquist frequency of the reference symbol repetition rate. Note that the approach described herein has substantial advantages when the data collection is in short bursts, i.e., the channel data is collected only for a short period of time. In this case, other conventional estimation techniques such as phase-lock-loop and direct frequency estimator, cannot effectively facilitate frequency synchronization. 
   Turning now to  FIG. 1 , the radio transceiver  10  may include a processing platform  12 , a reference receiver  16  and a signal receiver  14 . The reference receiver  16  may receive a Global Positioning System (GPS) signal through an antenna  18  and provide the received signal to a GPS receiver  20  where a GPS Time signal is recovered from the signal. The GPS Time signal is used within a local timing generator  26  to generate a local clock signal. 
   The signal receiver  14  may receive a signal  104  from a local base station  102  of the communication system  100 . The signal  104  may be reduced to base band within a radio subsystem  24  and sampled and stored within an I/O sample buffer  27  to provide a set of I and Q values for each sampling interval under control of the local clock. 
   The sampled signal from the buffer  27  may be provided as an input to a correlation processor  28  where the sampled signal is correlated with a known SYNC sequence. The known SYNC sequence may be retrieved from a memory or generated within a reference PILOT and SYNC symbol generator processor  30 . 
   Within the SYNC processor  28 , the signal samples are continuously compared with the known SYNC sequence until a match is detected. Detection of a match provides the radio transceiver  10  with frame synchronization. 
   The signal samples are then passed to a pilot channel symbol based frequency estimation processor  32 . Within the estimation processor  32 , the I and Q values of the PILOT symbols from the sampled signal are compared with a reference set of symbols from the generator  30 . In this case, frame synchronization identifies the location of PILOT symbols within the frame or multiframe. The estimation processor  32  functions to determine a phase of a PILOT symbol in a PILOT location and then an overall difference of the phase at a previous PILOT location and a current PILOT location. This phase difference is divided by the PILOT period (M*T, where M represents the number of symbols between successive PILOT symbol locations and T is the symbol period) to obtain an estimate, F, of the residual frequency error at the output of the estimation processor  32 . 
   With a reference symbol repetition rate of F p  Hz for the signal  104 , conventional frequency estimation methods can only provide a value of frequency offset F=F O  (Hz) in the frequency range of from −F p /2 to F p /2. This estimated value, F O , would be correct if the original frequency offset is less than F p /2. Otherwise, it is incorrect due to aliasing. For conventional frequency estimate methods, it may be observed that there is an ambiguity in the estimation for the frequency offset greater than F p /2. In fact, for a given estimate F O , the actual frequency offset could be:
 
 −F   p   +F   O , for  F   O &gt;0 and  F   O   &lt;|F   p /2|  (1)
 
 F   p   +F   O , for  F   O &lt;0 and  F   O   &lt;|F   p /2|  (2)
 
Therefore, using frequency offset estimation F O  obtained via conventional methods, the radio transceiver  10  will perform the following additional steps to resolve the ambiguity. As a first step, a frequency processor  42  within the frequency estimation processor  32  calculates another frequency F′ using either equation (1) or (2) based on the sign of the original estimate F. For example, if F p  were to be equal to some frequency value (e.g., 50 Hz), then Fp/2=25 Hz. If F were to be estimated to be equal to 10 Hz, then (using equation (1)) F′=−40 Hz. However, F′ is outside of the Nyquist frequency.
 
   The frequency estimation processor  32  may pass the two values (F and F′) to a pilot channel symbol based phase estimation processor  34 . Within the phase estimation processor  34 , the received PILOT symbols from the sample stream are compared to determine a phase change with regard to a local time base. 
   A carrier energy processor  44  within the frequency estimation processor  34  may calculate a carrier energy E C . The carrier energy processor  44  may obtain the carrier energy E C  by comparing the magnitude of the correlation between the received SYNC and PILOT symbols at the respective offset frequencies F and F′ to their known values. 
   A channel power processor  46  within the frequency estimation processor  34  may also determine a received channel power I O . The channel power processor  46  may determine the received channel power by summing the squares of the magnitude of each received sample at F and F′. 
   The values of the carrier energy E C  and the channel power I O  may be transferred to a C/I measurement and decision control block  36 . Within the control block, a respective C/I processor  48 ,  50  may first determine a ratio of E C /I O  for F and F′. Finally, a respective C/I ratio for F and F′ is determined within the respective processor  48 ,  50  by solving the equation 
   
     
       
         
           
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   As a final step, the CIP processors  48 ,  50  transfers the C/I values to a comparator  52  that selects the frequency offset F or F′ which provided the higher C/I ratio. In this step, the radio transceiver  10  has combined the frequency offset estimation and C/I calculation in order to increase the upper bound of frequency offset that can be handled by the conventional methods. 
   The above method may be used in any of a number of situations and usually iteratively. For example, the method may be used for selection of the nearest base station  102  in the system  100 . Alternatively, the method may be used in an iterative manner to detect and track changes in a communication channel. 
   It should be noted that, while the assumption has been made that frequency offset is between −Fp and Fp in this discussion, the above approach can be extended to any window of frequency offset (integer multiple of the Nyquist value) by using the aliasing analogy under the Nyquist sampling theorem discussed above. At the expense of processing power and memory storage requirement, the proposed method can be extended to estimate arbitrarily large frequency offsets given that appropriate channel filters are utilized. 
   A representative simulation result is shown in  FIG. 3 . Here, a digital color code detection rate based on conventional frequency offset method and the described method are compared. The digital color code is the information that is modulated on the transmitted signal by the base station. This information is used by the receivers to distinguish between various base stations. The resulting graph shows the digital color code demodulation performance under various offset conditions. 
   It can be seen in  FIG. 3  that the described method provided excellent performance when conventional methods fail. As shown in  FIG. 3 , with a C/I ratio of 10 dB, the digital color code is detected by the radio transceiver  10  approximately 100% of the time while conventional receiver detects the digital color code 0% of the time. Moreover, with a C/I ratio of 6 dB, the receiver detects the digital color code is detected by the radio transceiver  10  approximately 87% of the time versus 0% for a conventional system. 
   A specific embodiment of method and apparatus for determining an offset frequency has been described for the purpose of illustrating the manner in which the invention is made and used. It should be understood that the implementation of other variations and modifications of the invention and its various aspects will be apparent to one skilled in the art, and that the invention is not limited by the specific embodiments described. Therefore, it is contemplated to cover the present invention and any and all modifications, variations, or equivalents that fall within the true spirit and scope of the basic underlying principles disclosed and claimed herein.