Method and apparatus for eliminating the effects of frequency offsets in a digital communication system

Abstract

The present invention aims at eliminating the effects of frequency offsets between two transceivers by adjusting frequencies used during transmission. In this invention, methods for correcting the carrier frequency and the sampling frequency during transmission are provided, including both digital and analog implementations of such methods. The receiver determines the relative frequency offset between the transmitter and the receiver, and uses this information to correct this offset when the receiver transmits its data to the original transmitter in the return path, so that the signal received by the original transmitter is in sampling and carrier frequency lock with the original transmitter's local frequency reference.

What is claimed is: 
     
         1 . A device in a communication system including a first transceiver unit and a second transceiver unit remote to the first transceiver unit, the second transceiver unit operable to communicate in a bi-directional manner for direct exchange of information with the first transceiver unit, the device comprising:
 means for detecting a sampling frequency offset between a first sampling frequency used by the first transceiver unit to transmit a first signal, and a second sampling frequency used by the second transceiver unit to receive the first signal, wherein detecting the sampling frequency offset comprises comparing a first digital signal produced by digitally sampling the first signal at the second sampling frequency to the received first signal detected by the second transceiver unit; and   means for adjusting sampling points of data to be transmitted by the second transceiver unit in response to the detected sampling frequency offset, whereby an error associated with the sampling frequency offset is reduced.   
     
     
         2 . A device according to  claim 1 , wherein the means for detecting the sampling frequency offset includes means for locking onto the sampling frequency offset by introducing a variable delay to the first digital signal. 
     
     
         3 . A device according to  claim 1 , wherein the means for adjusting the sampling points includes means for digitally interpolating the data to be transmitted. 
     
     
         4 . A device according to  claim 1 , wherein the means for detecting the sampling frequency offset includes:
 means for generating an output frequency corresponding to the second sampling frequency having a desired phase and frequency in accordance with a control signal;   means for comparing the first digital signal and the output frequency to determine a phase difference between the first digital signal and the output frequency; and   means for adjusting the control signal in response to the determined phase difference.   
     
     
         5 . A device according to  claim 4 , wherein the means for generating the output frequency includes means for selecting one of a plurality of phases of the second sampling frequency in response to the control signal. 
     
     
         6 . A device according to  claim 1 , wherein the means for adjusting the sampling points includes:
 means for generating an output frequency corresponding to the second sampling frequency having a desired phase and frequency; and   a digital to analog (D/A) converter that operates in response to the output frequency.   
     
     
         7 . A device according to  claim 4 , wherein the means for adjusting the sampling points includes means for generating a second output frequency corresponding to the second sampling frequency having a second desired phase and frequency. 
     
     
         8 . A method used in a communication system including a first transceiver unit and a second transceiver unit disposed remotely from the first transceiver unit, the second transceiver unit operable to communicate in a bi-direction manner for direct exchange of information with the first transceiver unit, the method comprising:
 detecting a sampling frequency offset between a first sampling frequency used by the first transceiver unit to transmit a first signal and a second sampling frequency used by the second transceiver unit to receive the first signal, by comparing a first digital signal produced by digitally sampling the first signal at the second sampling frequency to the received first signal detected by the second transceiver unit; and   adjusting sampling points of data to be transmitted by the second transceiver unit in response to detecting the sampling frequency offset, wherein an error associated with the sampling frequency offset is reduced.   
     
     
         9 . The method according to  claim 8 , wherein detecting the sampling frequency offset includes locking onto the sampling frequency offset by introducing a variable delay to the first digital signal. 
     
     
         10 . The method according to  claim 8 , wherein adjusting the sampling points comprises digitally interpolating the data to be transmitted. 
     
     
         11 . The method according to  claim 8 , wherein detecting the sampling frequency offset includes:
 generating an output frequency corresponding to the second sampling frequency having a desired phase and frequency in accordance with a control signal;   comparing the first digital signal and the output frequency to determine a phase difference between the first digital signal and the output frequency; and   adjusting the control signal in response to the determined phase difference.   
     
     
         12 . The method according to  claim 11 , wherein generating the output frequency includes selecting one of M phases of the second sampling frequency in response to the control signal. 
     
     
         13 . The method according to  claim 8 , wherein adjusting the sampling points includes:
 generating an output frequency corresponding to the second sampling frequency having a desired phase and frequency; and   operating a digital to analog (D/A) converter in response to the output frequency.   
     
     
         14 . The method according to  claim 11 , wherein adjusting the sampling points includes generating a second output frequency corresponding to the second sampling frequency having a second desired phase and frequency. 
     
     
         15 . A device adapted to be used in a first unit that communicates with a second unit using a common sampling frequency, the device comprising:
 a delay lock loop that is coupled to receive digitally sampled data of a first signal transmitted by the second unit, and digitally detected data of the first signal, the delay lock loop being adapted to detect a sampling frequency offset in the first signal in response to the digitally sampled data of the first signal and the digitally detected data of the first signal, and to produce offset information corresponding thereto; and   a first digital low-pass filter that is coupled to receive the offset information and data to be transmitted by the first unit in a second signal to be received by the second unit, the first digital low-pass filter being adapted to digitally interpolate the data to be transmitted in accordance with the common sampling frequency and the sampling frequency offset wherein an error associated with the sampling frequency offset is reduced.   
     
     
         16 . The device of  claim 15 , further comprising:
 a timing acquisition unit configured to produce the digitally sampled data of the first signal based at least in part on an output of the delay lock loop.   
     
     
         17 . The device of  claim 16 , further comprising:
 a data detection block configured to produce the digitally detected data of the first signal; wherein an output of the data detection block is communicatively coupled with the delay lock loop.   
     
     
         18 . The device of  claim 17 , further comprising:
 a second digital low-pass filter communicatively coupled with an output of the timing acquisition unit and an input of the data detection block.   
     
     
         19 . The device of  claim 15 , further comprising:
 a digital-to-analog converter configured to receive the interpolated data to be transmitted from the first digital low-pass filter.   
     
     
         20 . The device of  claim 19 ; further comprising:
 an analog transmitter front-end; wherein an output of the digital-to-analog converter is communicatively coupled with an input of the analog transmitter front-end.   
     
     
         21 . The device of  claim 15 , further comprising:
 a data modulation block configured to modulate the data to be transmitted;   wherein an output of the data modulation block is communicatively coupled with an input of the first digital low-pass filter.