Tracking system comprised in an OFDM receiver

#This application claims priority to European Patent application no. 04006933.8 filed Mar. 23, 2004.

#The present invention relates in a first aspect to a tracking system comprised in an Orthogonal Frequency Division Multiplexing (OFDM) receiver.

#According to a second aspect the present invention relates to a method for tracking rapid changes in frequency and phase offset in an OFDM receiver.

#FIG. 1 shows an example of VCO glitch in relation to a 802.11a preamble;

#FIG. 2 is a block diagram of a digital OFDM receiver according to the prior art;

#FIG. 3 is a I-Q-diagram disclosing the effect of signal phase rotation on demodulation;

#In FIG. 6 there is disclosed a block diagram of a tracking system 100 according to the present invention. The tracking system 100 is comprised in an OFDM receiver 300, which only is disclosed diagrammatically in FIG. 6. The tracking system 100 is operable to track rapid changes in frequency and phase offset of the received signal. The tracking system 100 comprises a frequency correction means 102 operable to correct a frequency error of a received OFDM symbol, A, i.e. I/Q Rx samples. A transforming means 104 is connected to said frequency correcting means 102, which transforming means 104 is operable to perform a Fourier transform operation resulting in a number of independently modulated subcarriers. The tracking system 100 also comprises a to said transforming means 104 connected first estimating means 106 operable to perform an initial coarse estimate of the phase of the received OFDM symbol, based on the phase of one or more subcarriers carrying pilot tones. The tracking system 100 also comprises a to said first estimating means 106 connected first phase correction means 108 operable to perform an initial correction of the symbol phase. A second estimating means 110 is connected to said first phase correction means 108, which second estimating means 110 is operable to perform a data-driven phase estimation on the corrected symbol resulting in a refined phase estimate. Phase and frequency tracking means 112 is connected to the second estimating means 110. The reference signs A, B, C and D has the same meaning as in FIGS. 2, 4 and 5.

#In FIG. 7 there is disclosed a more detailed block diagram of the tracking system 100 disclosed in FIG. 6. Some of the means/functional blocks in FIG. 7 corresponds to the same means/functional blocks and these have been allocated the same reference signs in both FIG. 6 and FIG. 7 and will not be described again. The tracking system 100 disclosed in FIG. 7 also comprises a to said first phase correction means 108 connected first demodulating means 114 operable to demodulate said phase corrected OFDM symbol to produce a data stream. A remodulating means 116 is connected to said demodulating means 114, which remodulating means 116 is operable to remodulate said data stream. As is apparent from FIG. 7, said remodulating means 116 is also connected to said second estimating means 110. The tracking system 100 also comprises a to said transforming means 104 connected second demodulating means 118, which in turn is connected to an error correction means 120 resulting in said final estimate of the received data sequence. The tracking system 100 can also comprise a second phase correction means 122, disclosed with a dotted line, connected to said second estimating means 110, to said transforming means 104 and to said second demodulating means 118. The second phase correction means 122 is operable to perform a more precise correction of the received OFDM symbol phase. In the tracking system 100 disclosed in FIG. 7 there is also a first weighting means 124 operable to weight the contribution to the estimate of the OFDM symbol phase from each subcarrier according to the strength of the subcarrier. In FIG. 7 there is also disclosed a second weighting means 126 operable to weight the contribution to the phase estimate from each subcarrier according to a distance between the received I-Q vector and the nearest constellation point for that subcarrier, such that received I-Q vectors with smaller probability of being associated to the nearest constellation point have a smaller contribution to the overall estimate from this subcarrier.

#In a preferred embodiment of the tracking system 100, said first demodulating means 114 and said second demodulating means 118 are provided with an initial estimate of the channel transfer function for each subcarrier.