Scattered pilot pattern and channel estimation method for MIMO-OFDM systems
View Patent ↗Methods and apparatus are provided for inserting data symbols and pilot symbols in an OFDM (orthogonal frequency division multiplexing) transmission resource utilizing frequency hopping patterns for the data symbols and/or the pilot symbols. Data symbols and pilot symbols are allocated for down link (base station to mobile station) and up link (mobile station to bases station) transmission resources in a two-dimensional time-frequency pattern. For each antenna of a MIMO-OFDM (multiple input multiple output OFDM) communication system, pilot symbols are inserted in a scattered pattern in time-frequency and data symbols are inserted in an identical frequency-hopping pattern in time-frequency as that of other antennas.
1. An OFDM receiver, comprising:
at least one receive antenna operable to receive OFDM symbols transmitted from a plurality of transmit antennas, the OFDM symbols transmitted from each transmit antenna comprising pilot symbols in a respective pattern in time-frequency and data symbols in an identical frequency-hopping pattern in time-frequency, the pilot symbols for each respective pattern in time-frequency inserted such that pilot symbols from different transmit antennas do not occupy the same location in time-frequency; and
a channel estimator operable to compare the received pilot symbols to pilot symbol values known to be transmitted by a transmitter to estimate a channel between the transmitter and the receiver.
2. The OFDM receiver as defined in claim 1 , wherein the channel estimator is operable for each transmit antenna/receive antenna combination:
to estimate a channel response for each point in a respective pattern using the received pilot symbols;
to estimate a channel response for each of a plurality of points not in the respective pattern by performing a two-dimensional (time direction, frequency direction) interpolation of channel responses determined for points in the respective pattern; and
to perform an interpolation in the frequency direction to estimate channel responses corresponding to remaining OFDM sub-carriers within each OFDM symbol.
3. The OFDM receiver as defined in claim 2 , wherein the channel estimator is operable to filter the channel responses prior to interpolating in the frequency direction to estimate the channel responses corresponding to remaining OFDM sub-carriers within each OFDM symbol.
4. The OFDM receiver as defined in claim 3 , wherein the channel estimator is operable to filter the channel responses by performing a three-point smoothing operation.
5. The OFDM receiver as defined in claim 2 wherein the channel estimator is operable to estimate the channel response of a plurality of points not in the respective pattern by performing a two-dimensional interpolation of channel responses for each of a plurality of points in the respective pattern by, for each sub-carrier to be estimated, averaging:
a channel response of the given channel estimation period of a sub-carrier before the sub-carrier to be estimated in frequency (when present);
a channel response of the given channel estimation period of a sub-carrier after the sub-carrier to be estimated in frequency (when present);
a channel response for a previous estimation period (when present); and
a channel response for a following estimation period (when present).
6. The OFDM receiver as defined in claim 2 , wherein the channel estimator is operable to perform an interpolation in the frequency direction by:
performing a linear interpolation for sub-carriers at a lowest useful frequency or a highest useful frequency within the OFDM symbol; and
performing a cubic Lagrange interpolation for sub-carriers not at the lowest or highest useful frequency.
7. The OFDM receiver as defined in claim 1 , wherein the OFDM symbols transmitted from each antenna comprise information identifying a particular form of pre-processing used to encode the received pilot symbols from at least one source, the OFDM receiver further comprising differentiating pilot logic operable to process the information identifying the particular form of pre-processing used to encode the received pilot symbols to differentiate between received pilot symbols from different sources.
8. The OFDM receiver as defined in claim 7 , wherein the differentiating pilot logic is operable to process the information identifying the particular form of pre-processing used to encode the received pilot symbols to differentiate between received pilot symbols from different sources occurring at a same time-frequency location.
9. A method of processing OFDM symbols transmitted from a plurality of transmit antennas, the OFDM symbols transmitted from each transmit antenna comprising pilot symbols in a respective pattern in time-frequency and data symbols in an identical frequency-hopping pattern in time-frequency, the pilot symbols for each respective pattern in time-frequency inserted such that pilot symbols from different transmit antennas do not occupy the same location in time-frequency, the method comprising:
receiving the OFDM symbols on at least one receive antenna; and
comparing the received pilot symbols to pilot symbol values known to be transmitted by a transmitter to estimate a channel between the transmitter and the receiver.
10. The method as defined in claim 9 , further comprising, for each transmit antenna/receive antenna combination:
estimating a channel response for each point in a respective pattern using the received pilot symbols;
estimating a channel response for each of a plurality of points not in the respective pattern by performing a two-dimensional (time direction, frequency direction) interpolation of channel responses determined for points in the respective pattern; and
performing an interpolation in the frequency direction to estimate channel responses corresponding to remaining OFDM sub-carriers within each OFDM symbol.
11. The method as defined in claim 10 , further comprising filtering the channel responses prior to interpolating in the frequency direction to estimate the channel responses corresponding to remaining OFDM sub-carriers within each OFDM symbol.
12. The method as defined in claim 11 , wherein the step of filtering the channel responses comprises performing a three-point smoothing operation.
13. The method as defined in claim 10 , wherein the step of estimating the channel response of a plurality of points not in the respective pattern by performing a two-dimensional interpolation of channel responses for each of a plurality of points in the respective pattern comprises, for each sub-carrier to be estimated, averaging:
a channel response of the given channel estimation period of a sub-carrier before the sub-carrier to be estimated in frequency (when present); a channel response of the given channel estimation period of a sub-carrier after the sub-carrier to be estimated in frequency (when present);
a channel response for a previous estimation period (when present); and
a channel response for a following estimation period (when present).
14. The method as defined in claim 10 , wherein the step of performing an interpolation in the frequency direction comprises:
performing a linear interpolation for sub-carriers at a lowest useful frequency or a highest useful frequency within the OFDM symbol; and
performing a cubic Lagrange interpolation for sub-carriers not at the lowest or highest useful frequency.
15. The method as defined in claim 9 , wherein the OFDM symbols further comprise information identifying a particular form of pre-processing used to encode the received pilot symbols from at least one source, the method further comprising processing the information identifying the particular form of pre-processing used to encode the received pilot symbols to differentiate between received pilot symbols from different sources.
16. The method as defined in claim 15 , wherein the step of processing the information identifying the particular form of pre-processing used to encode the received pilot symbols to differentiate between received pilot symbols from different sources differentiates between received pilot symbols from different sources occurring at a same time-frequency location.