Method for receiving an OFDM signal
The method is for receiving an OFDM signal according to which, during a frequency translation phase, there is introduced a phase and gain imbalance between the phase and quadrature components thereof, and a frequency offset relative to a main carrier of the OFDM signal. The method involves a compensation of the effects of the imbalance even in the presence of the frequency offset and provides a reference signal indicative of the effect of the imbalance and according to which a process is carried out to reduce the effect of the imbalance itself.
1. A method for processing an Orthogonal Frequency Division Multiplexing (OFDM) signal, the method comprising:
receiving the OFDM signal and generating a translated signal by translating the OFDM signal in frequency wherein at least one of a phase and gain imbalance is introduced between phase and quadrature components thereof, and a frequency offset relative to a main carrier of the OFDM signal is also introduced;
converting the translated signal into a digital signal;
processing the digital signal according to a compensation coefficient to generate a processed signal comprising a useful signal and an interfering signal depending on the imbalance and offset, the processing comprising reducing a contribution of the interfering signal to the processed signal;
reducing an effect of the frequency offset by combining the processed signal with a compensation signal generated by a numeric control oscillator (NCO); and
further processing at least a portion of the digital signal to obtain a reference signal indicative of an effect of the imbalance, and including combining the reference signal with the compensation signal.
2. The method according to claim 1 , wherein said OFDM signal is associated with the main carrier and includes a plurality of subcarriers each one carrying at least a digital symbol, the method further comprising an OFDM demodulation phase of the processed signal to return a plurality of demodulated signals being obtained by carrying out a discrete Fourier transform and each one being associated to a subcarrier, the demodulated signals in the plurality include a selected signal being associated to a selected k-index subcarrier and a respective digital symbol.
3. The method according to claim 2 , wherein said discrete Fourier transform is carried out with a fast Fourier transform.
4. The method according to claim 1 , further comprising an adaptation phase to modify the compensation coefficient for reduction of the interfering signal upon variations of the received OFDM signal.
5. The method according to claim 2 , wherein said OFDM signal propagates along a transmission channel and the method further comprises:
estimating a transfer function of the transmission channel for at least said selected k-index subcarrier, said selected signal associated to the selected k-index subcarrier depending on the transmission channel;
processing the selected signal to reduce the effect of the transmission channel and to provide an estimated signal corresponding to an estimated symbol associated to said selected k-index subcarrier; and
evaluating an error signal related to a difference between the selected signal and a further reference signal based upon the estimated signal and the transfer function.
6. The method according to claim 5 , further comprising an adaptation phase to modify the compensation coefficient for reduction of the interfering signal based upon variations of the received OFDM signal;
said compensation coefficient depends on said error signal being evaluated such as to be related to a primary signal indicative of said useful signal and said reference signal; and
the adaptation phase evaluating a value of said compensation coefficient such as to reduce the error signal.
7. The method according to claim 6 , wherein said reference signal is obtained with a discrete Fourier transform, relative to the selected k-index subcarrier, of a combination signal obtained from said digital signal and said compensation signal.
8. The method according to claim 7 , wherein said error signal is a mean square error obtained from a difference between the selected signal and a second reference signal based upon the estimated signal and the estimated transfer function.
9. The method according to claim 8 , wherein said compensation coefficient is adapted to each received symbol associated to said received OFDM signal.
10. The method according to claim 9 , wherein said adaptation phase comprises computing a current value of the compensation coefficient associated to a current symbol based on a previous value of the compensation coefficient associated to a previous symbol, the computing being iterative and gradient.
11. The method according to claim 10 , wherein said current value is related to said reference signal and said error signal evaluated for the previous symbol.
12. The method according to claim 2 , wherein the OFDM demodulation phase includes:
a serial-to-parallel transformation phase of a combination signal, from the processed signal and the compensation signal, to return digital samples provided on a plurality of outputs corresponding to each subcarrier; and
a discrete Fourier transform DFT operation of said digital samples providing the plurality of demodulated signals each being associated to a respective subcarrier.
13. The method according to claim 10 , wherein said current value is evaluated according to a Least Mean Square adaptation process.
14. The method according to claim 5 , further comprising:
estimating the transfer function of the transmission channel for several subcarriers in the plurality; and
selecting said selected subcarrier based on the transfer functions.
15. The method according to claim 14 , wherein said selecting is based upon a signal-to-noise ratio of the subcarriers.
16. The method according to claim 15 , wherein said selecting is based upon magnitudes of respective transfer functions associated with the subcarriers.
17. The method according to claim 14 , wherein the OFDM demodulation phase includes:
a serial-to-parallel transformation phase of a combination signal, from the processed signal and the compensation signal, to return digital samples provided on a plurality of outputs corresponding to each subcarrier; and
a discrete Fourier transform DFT operation of said digital samples providing the plurality of demodulated signals each being associated to a respective subcarrier;
wherein the method further comprises:
equalization of the demodulated signals based upon the estimated transfer functions; and
obtaining a plurality of the estimated signals, from the equalized signals, including said estimated signal.
18. The method according to claim 2 , further comprising:
an adaptation phase to modify the compensation coefficient for reduction of the interfering signal upon variations of the received OFDM signal;
evaluating an overall signal-to-noise ratio in view of said useful signal, interfering signal, and an additional signal; and
comparing the overall signal-to-noise ratio with a threshold value and modifying the compensation coefficient based thereon.
19. The method according to claim 18 , wherein the overall signal-to-noise ratio is evaluated based on said selected k-index subcarrier.
20. The method according to claim 18 , wherein said threshold value is a function of a ratio of a further useful signal and a further interfering signal evaluated in a case of constant transfer function of the transmission channel and in conditions where effects of the imbalance are the increased.
21. The method according to claim 18 , wherein evaluating the overall signal-to-noise ratio and comparing it with the threshold value are carried out while receiving a first sub-set of the plurality of digital symbols carried by the received OFDM signal.
22. The method according to claim 21 , wherein adapting the compensation coefficient is carried out for a remaining sub-set of the plurality of digital symbols associated with the received OFDM signal.
23. The method according to claim 1 , wherein said received OFDM signal carries symbols being modulated according to a M-QAM modulation.
24. The method according to claim 1 , wherein said received OFDM signal is received over a wireless channel.
25. The method according to claim 1 , wherein said received OFDM signal was transmitted with one of a packet transmission technique and a non-packet transmission technique.
26. A receiver for an OFDM signal associated with a main carrier and including a plurality of subcarriers each carrying at least one digital symbol, the receiver comprising a processor for
receiving the OFDM signal and generating a translated signal by translating the OFDM signal in frequency wherein at least one of a phase and gain imbalance is introduced between phase and quadrature components thereof, and a frequency offset relative to a main carrier of the OFDM signal is also introduced;
converting the translated signal into a digital signal;
processing the digital signal according to a compensation coefficient to generate a processed signal comprising a useful signal and an interfering signal depending on the imbalance and offset, the processing comprising reducing a contribution of the interfering signal to the processed signal;
reducing an effect of the frequency offset by combining the processed signal with a compensation signal generated by a numeric control oscillator (NCO); and
further processing at least a portion of the digital signal to obtain a reference signal indicative of an effect of the imbalance, and including combining the reference signal with the compensation signal.
27. The receiver according to claim 26 , wherein said OFDM signal is associated with the main carrier and includes a plurality of subcarriers each one carrying at least a digital symbol, the processor further performs an OFDM demodulation phase of the processed signal to return a plurality of demodulated signals being obtained by carrying out a discrete Fourier transform and each one being associated to a subcarrier, the demodulated signals in the plurality include a selected signal being associated to a selected k-index subcarrier and a respective digital symbol.
28. A transmission system comprising:
a transmitter to transmit an OFDM signal along a transmission channel, the OFDM signal being associated with a main carrier and including a plurality of subcarriers each carrying at least one digital symbol; and
a receiver to receive the OFDM signal propagated along said transmission channel, the receiver comprising a processor for
receiving the OFDM signal and generating a translated signal by translating the OFDM signal in frequency wherein at least one of a phase and gain imbalance is introduced between phase and quadrature components thereof, and a frequency offset relative to a main carrier of the OFDM signal is also introduced,
converting the translated signal into a digital signal,
processing the digital signal according to a compensation coefficient to generate a processed signal comprising a useful signal and an interfering signal depending on the imbalance and offset, the processing comprising reducing a contribution of the interfering signal to the processed signal,
reducing an effect of the frequency offset by combining the processed signal with a compensation signal generated by a numeric control oscillator (NCO), and
further processing at least a portion of the digital signal to obtain a reference signal indicative of an effect of the imbalance, and including combining the reference signal with the compensation signal.
29. The system according to claim 28 , wherein said OFDM signal is associated with the main carrier and includes a plurality of subcarriers each one carrying at least a digital symbol, the processor further performs an OFDM demodulation phase of the processed signal to return a plurality of demodulated signals being obtained by carrying out a discrete Fourier transform and each one being associated to a subcarrier, the demodulated signals in the plurality include a selected signal being associated to a selected k-index subcarrier and a respective digital symbol.