Method and device for channel estimation, and corresponding computer program product
An embodiment of the time-variant channel estimation in an OFDM transmission system envisages the operations of: a) determining the received signal vector Y j,k for one or more of R receiving antennas, j=1, . . . , R, and at the current time instant k together with the frequency-domain channel-estimate vector Ĥ j,k−1 at a preceding time instant k−1 and for each antenna j of one or more of R receiving antennas; b) producing a first estimate  k of the transmitted digital symbols at the current time instant k on the basis of the aforesaid frequency-domain channel-estimate vectors Ĥ j,k−1 at the preceding time instant k−1; c) computing an updated version of the frequency-domain channel-estimate vectors Ĥ j,k at the current time instant k and for each antenna j of one or more of R receiving antennas on the basis of the received signal vectors Y j,k at the current time instant k and for each antenna j of one or more of R receiving antennas, using the aforesaid estimate  k of the transmitted digital symbols at the current time instant k as set of known transmitted digital symbols; d) using the aforesaid updated version of the frequency-domain channel-estimate vectors Ĥ j,k at the current time instant k and for each antenna j of one or more of R receiving antennas, for equalizing the paths of the transmission channel and decoding the received signal vectors Y j,k at the current time instant k and for each antenna j of one or more of R receiving antennas; and e) storing the updated versions of the channel-estimate vectors for using them in steps a) and b) at the next time instant k+1.
1. A method, comprising:
in an OFDM system wherein digital transmitted symbols modulating OFDM sub-carriers are received via at least one receiving antenna j as a signal vector Y j received over at least one transmission channel represented by a frequency domain channel estimate vector Ĥ j , at a sequence of time instants wherein a current time instant k lies between a previous time instant k−1 and a next time instant k+1;
a) determining said frequency domain channel estimate vector Ĥ j,k−1 at said previous time instant k−1 as well as said received signal vector Y j,k at said current time instant k;
b) producing an estimate  k of the digital transmitted symbols at said current time instant k based on said frequency domain channel estimate vector Ĥ j,k−1 and said received signal vector Y j,k at said current time instant k, each symbol in the estimate  k having a reliability factor;
c) computing an updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k based on said received signal vector Y j,k at said current time instant k using said estimate  k as a set of known digital transmitted symbols based on selecting a threshold number of symbols from estimate  k that include a combination of symbols having a reliability factor that exceeds a threshold reliability and if the threshold number is not reached then supplementing the selection with a balance of symbols having a reliability factor below the reliability threshold;
d) using said updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k for equalizing said at least one transmission channel, in decoding said received signal vector Y j,k at said current time instant k, and
e) storing said updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k for use in steps a) and b) at said next time instant k+1.
2. The method of claim 1 , including, in performing said step b):
attributing respective reliability coefficients to the estimated data symbols in said received signal vector Y j,k , and
selecting, in producing said estimate  k of the digital transmitted symbols, those symbols having an attributed reliability coefficient higher than or equal to a given reliability threshold ψ thr .
3. The method of claim 1 , including extending said estimate  k of the digital transmitted symbols at said current time instant k by supplementing it with known pilot symbols.
4. The method of claim 1 , including producing said estimate  k of the digital transmitted symbols at said current time instant k by means of a hard decision on said received signal vector Y j .
5. The method of claim 2 , including attributing said respective reliability coefficients as a function of the square amplitude of said received data symbols.
6. The method of claim 2 , wherein said digital transmitted symbols modulating OFDM sub-carriers are received via a plurality of receiving antennas J=1 . . . , R, including attributing said respective reliability coefficients as a function of one of:
the sum of the square amplitudes of said received data symbols over said plurality of receiving antennas,
the maximum of the square amplitude of said received data symbols over said plurality of receiving antennas.
7. A method of channel estimation in OFDM transmission systems wherein digital transmitted symbols modulating OFDM sub-carriers are received via at least one receiving antenna j as a signal vector Y j received over at least one transmission channel represented by a frequency domain channel estimate vector Ĥ j ,the method including, at a sequence of time instants wherein a current time instant k lies between a previous time instant k−1 and a next time instant k+1:
a) determining said frequency domain channel estimate vector Ĥ j,k−1 at said previous time instant k−1 as well as said received signal vector Y j,k at said current time instant k;
b) producing an estimate  k of the digital transmitted symbols at said current time instant k based on said frequency domain channel estimate vector Ĥ j,k−1 and said received signal vector Y j,k at said current time instant k;
c) computing an updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k based on said received signal vector Y j,k at said current time instant k using said estimate  k as a set of known digital transmitted symbols;
d) using said updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k for equalizing said at least one transmission channel, in decoding said received signal vector Y j,k at said current time instant k, and
e) storing said updated version of said frequency domain channel estimate vector Ĥ j,k at said current time instant k for use in steps a) and b) at said next time instant k+1;
wherein in performing said step b):
attributing respective reliability coefficients to the estimated data symbols in said received signal vector Y j,k , and
selecting, in producing said estimate  k of the digital transmitted symbols, those symbols having an attributed reliability coefficient higher than or equal to a given reliability threshold ψ thr ; and
further including:
checking if the number of symbols for producing said estimate  k of the digital transmitted symbols is at least equal to a given value L ch , and
if said checking yields a negative outcome, supplementing the symbols for producing said estimate  k of the digital transmitted symbols with further symbols having an attributed reliability coefficient lower than said reliability threshold ψ thr to reach said given value L ch , said further symbols being selected preferably by giving preference to the symbols having higher reliability coefficients.