A method of transmitting from a transmitter to a receiver over a wireless multiple-input-multiple-output channel comprising a series of frequency intervals, the method comprising: based on one or more performance criteria evaluated in relation to a previously received transmission from the transmitter, selecting at the receiver a first precoding matrix out of a predetermined set of precoding matrices for transmitting from the transmitter to the receiver on a first of said frequency intervals; and for each successive frequency interval in the series: determining from said predetermined set of precoding matrices a preferred subset of precoding matrices based on a suitable metric computed with respect to the matrix determined for the previous frequency interval; based on one or more performance criteria evaluated in relation to the previously received transmission, selecting at the receiver a respective precoding matrix from said predetermined set for transmitting from the transmitter to the receiver on said successive frequency interval; and feeding back an index of that respective precoding matrix from the receiver to the transmitter using a differential encoding scheme whereby the respective index is indicated relative to said preferred subset; wherein the method further comprises resetting the differential encoding scheme upon detecting a condition; characterised in that: (a) said condition comprises the condition that the selected precoding matrix is not in the preferred subset; or (b) for each of said successive frequency intervals, the method comprises determining from said predetermined set, separately from the determination of said preferred subset, a subset of most-probable precoding matrices having some conditional probability of being selected for transmission, and said condition comprises the condition that the most-probable subset differs from the preferred subset by a predetermined amount. based on one or more performance criteria evaluated in relation to a previously received transmission from the transmitter, selecting at the receiver a first precoding matrix out of a predetermined set of precoding matrices for transmitting from the transmitter to the receiver on a first of said frequency intervals; and for each successive frequency interval in the series: determining from said predetermined set of precoding matrices a preferred subset of precoding matrices based on a suitable metric computed with respect to the matrix determined for the previous frequency interval; based on one or more performance criteria evaluated in relation to the previously received transmission, selecting at the receiver a respective precoding matrix from said predetermined set for transmitting from the transmitter to the receiver on said successive frequency interval; and feeding back an index of that respective precoding matrix from the receiver to the transmitter using a differential encoding scheme whereby the respective index is indicated relative to said preferred subset; wherein the method further comprises resetting the differential encoding scheme upon detecting a condition; characterised in that: (a) said condition comprises the condition that the selected precoding matrix is not in the preferred subset; or (b) for each of said successive frequency intervals, the method comprises determining from said predetermined set, separately from the determination of said preferred subset, a subset of most-probable precoding matrices having some conditional probability of being selected for transmission, and said condition comprises the condition that the most-probable subset differs from the preferred subset by a predetermined amount. determining from said predetermined set of precoding matrices a preferred subset of precoding matrices based on a suitable metric computed with respect to the matrix determined for the previous frequency interval; based on one or more performance criteria evaluated in relation to the previously received transmission, selecting at the receiver a respective precoding matrix from said predetermined set for transmitting from the transmitter to the receiver on said successive frequency interval; and feeding back an index of that respective precoding matrix from the receiver to the transmitter using a differential encoding scheme whereby the respective index is indicated relative to said preferred subset; wherein the method further comprises resetting the differential encoding scheme upon detecting a condition; characterised in that: (a) said condition comprises the condition that the selected precoding matrix is not in the preferred subset; or (b) for each of said successive frequency intervals, the method comprises determining from said predetermined set, separately from the determination of said preferred subset, a subset of most-probable precoding matrices having some conditional probability of being selected for transmission, and said condition comprises the condition that the most-probable subset differs from the preferred subset by a predetermined amount. (a) said condition comprises the condition that the selected precoding matrix is not in the preferred subset; or (b) for each of said successive frequency intervals, the method comprises determining from said predetermined set, separately from the determination of said preferred subset, a subset of most-probable precoding matrices having some conditional probability of being selected for transmission, and said condition comprises the condition that the most-probable subset differs from the preferred subset by a predetermined amount.