Patent Publication Number: US-7224754-B2

Title: Frequency offset compensation estimation system and method for a wireless local area network

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a frequency offset compensation estimation system and method for a wireless local area network (WLAN), and particularly to a frequency offset estimation (FOE) device and its compensation method for a wireless local area network which can reduce the operation load of the frequency offset residual phase compensation (FOS RPC) device so as to improve the operation efficiency of the system. 
   2. Description of the Prior Art 
   Recently, the communication on the wireless local area network (WLAN) is greatly and broadly researched, discussed and applied. In the technology of the WLAN based on the orthogonal frequency division multiplex (OFDM), frequency offset compensation at the wireless receiving end is an important task. Generally, the task is accomplished by sending a preamble sequence of the frequency package to a frequency offset estimation device so as to obtain an initial frequency offset signal amount and send it to the frequency offset compensation device for compensation. Next, the compensated signal is sent to a frequency offset residual phase estimation (FOS REP) device so as to figure out the residual frequency offset and send it to the frequency offset residual phase compensation (FOS RPC) device for performing the phase correction and compensation. However, this method will increase the load of the frequency offset residual phase compensation device so as to reduce the operation efficiency of the whole system. 
   In order to resolve the mentioned problem of the prior art frequency offset estimation (FOE) device for the wireless local area network, the present invention provides a frequency offset compensation estimation system and its system for a wireless local area network so as to reduce the operation load of the frequency offset residual phase compensation (FOS RPC) device for improving the operation efficiency of the whole system. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a frequency offset compensation estimation system and its method for a wireless local area network. It mainly employs a residual frequency offset estimation (Residual FOE) device to estimate the residual frequency offset signal of each of the frequency signals compensated by the frequency compensation device and feedback it to the frequency compensation device for compensation. Therefore, the operation load of the frequency offset residual phase compensation (FOS RPC) device can be reduced so as to improve the operation efficiency of the whole system. 
   In order to achieve to the objective, a frequency offset compensation estimation system for a wireless local area network according to the invention is installed at a receiving end of a wireless local area network. The inventive system comprises a frequency offset compensation device for correcting and compensating the inputted offset frequency signal; a frequency offset estimation device for receiving the preamble sequence signal received by the receiving end so as to obtain an initial frequency offset signal amount and send it to the frequency offset compensation device for compensation; a frequency offset residual phase estimation device (FOS RPE) for receiving a frequency domain signal transmitted from a quick Fourier transfer device and estimating it so as to obtain a offset phase amount; a frequency offset residual phase compensation (FOS RPC) device for compensating the offset phase amount; a residual frequency offset estimation (Residual FOE) device for estimating the residual frequency offset signal of each of the frequency signals compensated by the frequency compensation device and feedbacking it to the frequency offset compensation device for compensation. Therefore, the operation load of the frequency offset residual phase compensation (FOS RPC) device will be reduced so as to improve the operation efficiency of the whole system. 
   A frequency offset compensation estimation method for a wireless local area network according to the invention comprises the following steps. First, an initial frequency offset signal amount of a preamble sequence is estimated by a frequency estimation device and sent to a frequency compensation device for compensation. Next, the residual frequency offset signal of each of the frequency offset signals is estimated by a residual frequency offset estimation device. Finally, the residual offset phase amount of the frequency is calculated by a frequency offset residual phase estimation device and sent to the frequency offset residual phase compensation device for compensation so as to accomplish the estimation and compensation for the signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification in which like numerals designate like parts, illustrate preferred embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a block diagram of a frequency offset compensation estimation system installed in a wireless network receiver according to the present invention; 
       FIG. 2  is a block diagram of a frequency offset compensation estimation system  12  according to the present invention; 
       FIG. 3  is a perspective diagram of a transmission package in the wireless local area network according to the present invention; 
       FIG. 4A  is a perspective diagram of the average phase of two long preamble sequence signals; 
       FIG. 4B  is a perspective diagram of the phase of each of the data symbols; and 
       FIG. 5  is a flowchart of a frequency offset compensation estimation method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In a frequency offset compensation estimation system for a wireless local area network according to the present invention, the wireless local area network is a communication technology of a wireless local area network (WLAN) based on the orthogonal frequency division multiplex (OFDM), and the inventive system is installed at the receiving end of the wireless local area network for compensating the frequency offsets when receiving the wireless signals. 
     FIG. 1  is a block diagram of a frequency offset compensation estimation system  23  installed in a wireless network receiver  1  according to the present invention. After the wireless local area network receiver  1  receives a signal, an analog/digital converter (A/D converter)  10  will convert the analog signal into a digital signal. The converted digital signal will be separately outputted to a quick Fourier transfer device  11  for transferring a time domain signal into a frequency domain signal, transmitted to the inventive frequency offset compensation estimation system  23  for estimating the frequency offset, and transmitted to a signal detection automatic gaining control and power measure device  13  for gaining and controlling. The outputted frequency domain signal will be feedback to the frequency offset compensation estimation system  23  for estimating and compensating the residual frequency offset and compensating the phase rotation due to residual frequency offset. Then, the signal is transmitted to a channel correction device  14  for correcting the effect of the compensation channel, and sequentially connected to an inverse constellation mapper  15  for performing de-constellation mapping. A de-interleaver  16  is used for de-interleaving the signal, and then the signal is outputted to a Viterbi decoder  17  for decoding the signal. The decoded signal will be transmitted to a de-scrambler  18  for decoding the signal scrambled in the original emission device. Next, the signal is connected to a media access controller (MAC)  19 . As shown in  FIG. 1 , it is a circuit block diagram of the inventive frequency offset compensation estimation system  23  installed in a wireless network receiver  1 . 
     FIG. 2  is a block diagram of a frequency offset compensation estimation system  23  according to the present invention. The system  23  comprises a frequency offset estimation device  120 , a frequency offset compensation device (FOC)  121 , a frequency offset residual phase estimation (FOS RPE) device  122 , a frequency offset residual phase compensation (FOS RPC) device  123 , and a residual frequency offset estimation (Residual FOE) device  124 . The frequency offset estimation device  120  is used for receiving the preamble sequence signal outputted by the A/D converter  10  and figuring out an initial frequency offset signal amount so as to send it to the frequency offset compensation device for compensation. The frequency offset compensation device (FOC)  121  is used for correcting and compensating the inputted offset frequency signal. The frequency offset residual phase estimation (FOS RPE) device  122  is used for receiving the frequency domain signal transmitted form a quick Fourier transfer device  11  and estimating the offset phase rotation amount. The frequency offset residual phase compensation (FOS RPC) device  123  is used for compensating the offset phase amount. The residual frequency offset estimation (Residual FOE) device  124  is used for estimating the residual frequency offset signal of each of the frequency signals compensated by the frequency offset compensation device  121  and feedbacking it to the frequency offset compensation device  121  for compensation. This will reduce the operation load of the frequency offset residual phase compensation (FOS RPC) device  123  so as to improve the operation efficiency of the system. 
     FIG. 3  is a perspective diagram of a transmission package in the wireless local area network according to the present invention. The package comprises two long preamble sequence signal packages  30 , a signal package  31  having the data of the transmission rate, the length and the like, and several data packages  32 . 
   In the present invention, the compensation method is described as follows. 
     FIG. 4A  is a perspective diagram of the average phase of two long preamble sequence signals. The X axle represents the tone, and the Y axle represents the phase. In the embodiment, four pilot tone phases are sampled and the phase ε L  calculation formula is as the following:
 phase ε L =(θ −21 +θ −7 +θ +7 +θ +21 )  (1) 
     FIG. 4B  is a perspective diagram of the phase of each of the data symbols. The X axle represents the tone, and the Y axle represents the phase. In this embodiment, four pilot tones are sampled and the phase ε L  calculation formula is as the following:
 phase ε i =(θ −21 +θ −7 +θ +7 +θ +21 )  (2) 
   The calculation formula of the phase rotation of each data is as the following:
 
Phase rotation=ε i −ε L   (3)
 
   If the calculated window length of the residual offset frequency is L, then the average phase rotation ε avg  is: 
   
     
       
         
           
             
               
                 
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     FIG. 5  is a flowchart of a frequency offset compensation estimation method according to the present invention. The method comprises the following steps. 
   First, in the step  20 , an initial frequency offset signal amount of a preamble sequence is estimated by a frequency estimation device and sent to a frequency compensation device for compensation. Next, in the step  21 , the residual frequency offset signal of each of the frequency offset signals is estimated by a residual frequency offset estimation device. The estimated step size feedback to  20 . Finally, in the step  22 , the offset phase amount of the frequency is calculated by a frequency offset residual phase estimation device and sent to the frequency offset residual phase compensation device for compensation so as to accomplish the estimation and compensation for the signals. 
   The above is the detailed description of the frequency offset compensation estimation system and its method for the wireless local area network according to the present invention, the invention mainly employs a residual frequency offset estimation device (Residual FOE device) to estimate the residual frequency offset signal of each of the frequency signals compensated by the frequency compensation device and feedback it to the frequency compensation device for compensation. Therefore, the operation load of the frequency offset residual phase compensation (FOS RPC) device will be reduced so as to improve the operation efficiency of the system. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.