Pilot based sampling frequency offset estimation and correction for an OFDM system
View Patent ↗A technique for improving the performance of OFDM (Orthogonal Frequency Division Multiplexing) systems addresses the Sampling Frequency Offset (SFO) estimation and correction problems of conventional techniques and provides improved performance in a wide range of operating conditions. The technique teaches a correction scheme that uses known pilots in each OFDM symbol to estimate slope of phase error and subsequently calculate the correction factor for each of the sub-carriers in the associated OFDM symbol. The present subject matter further teaches an averaging slope estimates over a window of predetermined number of OFDM symbols to average out noise and get a refined correction factor. In addition, the present technique teaches tracking rate of growth of slope across OFDM symbols to estimate SFO, which can be used for the timing error correction in the time domain.
1. A method, comprising:
receiving a sequence of OFDM symbols by a receiver, wherein each OFDM symbol includes multiple pilot symbols;
performing an FFT on a current OFDM symbol to obtain data sub-carriers and associated multiple pilot sub-carriers;
computing a slope of phase error associated with the multiple pilot sub-carriers;
repeating the above steps of receiving, performing, computing the slope of phase error for each subsequent OFDM symbol;
computing an average slope from computed slopes of phase errors associated with a predetermined number of OFDM symbols;
computing a correction factor using the computed average slope;
applying the correction factor to the data sub-carriers associated with one or more OFDM symbols starting from the current OFDM symbol and for each subsequent OFDM symbol;
estimating a rate of growth of computed slopes of phase errors associated with the predetermined number of OFDM symbols;
obtaining an estimate of the sampling frequency offset (SFO) from the estimated rate of growth of the computed slopes of phase errors; and
applying a timing error correction before performing the FFT using the obtained estimate of the SFO to the subsequent OFDM symbols.
2. The method of claim 1 , wherein obtaining the estimate of the SFO comprises:
estimating a rate of growth of the computed average slopes;
obtaining an estimate of the SFO from the estimated rate of growth of the computed average slopes; and
applying a timing error correction before performing the FFT using the estimate of the SFO to the subsequent OFDM symbols.
3. An article of manufacture comprising:
a non-transitory computer-readable storage medium having instructions that, when executed by a computer, causes the computer to perform a method, comprising:
receiving a sequence of OFDM symbols by a receiver, wherein each OFDM symbol includes multiple pilot symbols;
performing an FFT on a current OFDM symbol to obtain data sub-carriers and associated multiple pilot sub-carriers;
computing a slope of phase error associated with the multiple pilot sub-carriers;
repeating the above steps of receiving, performing, computing the slope of phase error for each subsequent OFDM symbol;
computing an average slope from computed slopes of phase errors associated with a predetermined number of OFDM symbols;
computing a correction factor using the computed average slope;
applying the correction factor to the data sub-carriers associated with one or more OFDM symbols starting from the current OFDM symbol and for each subsequent OFDM symbol;
estimating a rate of growth of computed slopes of phase errors associated with the predetermined number of OFDM symbols;
obtaining an estimate of the SFO from the estimated rate of growth of the computed slopes of phase errors; and
applying a timing error correction before performing the FFT using the obtained estimate of the SFO to the subsequent OFDM symbols.
4. The article of manufacture of claim 3 , wherein obtaining the estimate of the SFO comprises:
estimating a rate of growth of the computed average slopes;
obtaining an estimate of the SFO from the estimated rate of growth of the computed average slopes; and
applying a timing error correction before performing the FFT using the estimate of the SFO to the subsequent OFDM symbols.
5. A receiver of an OFDM system, comprising:
an input module for receiving a sequence of OFDM symbols, wherein each OFDM symbol includes multiple pilot symbols;
an FFT module for performing an FFT operation on a current OFDM symbol and on each of the subsequent OFDM symbols to obtain data sub-carriers and associated multiple pilot sub-carriers of each OFDM symbol;
a pilot based SFO estimation module for computing a slope of phase error associated with the multiple pilot sub-carriers of each OFDM symbol, wherein the pilot based SFO estimation module computes an average slope from the estimated slopes of phase errors associated with a predetermined number of OFDM symbols, wherein the pilot based SFO estimation module computes a correction factor using the computed average slope;
a pilot based SFO correction module for applying the correction factor to the data sub-carriers associated with one or more OFDM symbols starting from the current OFDM symbol and for each subsequent OFDM symbol; and
a timing error correction module, wherein the pilot based SFO estimation module to estimate a rate of growth of computed slopes of phase errors associated with predetermined number of OFDM symbols, wherein the pilot based SFO estimation module to obtain an estimate of the SFO from the estimated rate of growth of the computed slopes of phase errors, and wherein the timing error correction module to apply the obtained estimate of the SFO to subsequent OFDM symbols before performing the FFT operation.
6. The receiver of claim 5 , wherein the SFO estimation module estimates a rate of growth of the computed average slopes, wherein the pilot based SFO estimation module obtains an estimate of the SFO from the estimated rate of growth of the computed average slopes, and wherein the timing error correction module applies the estimated SFO to the subsequent OFDM symbols before performing the FFT operation.