A transmission method for transmitting digital data over N RF frequency channels, N RF being an integer greater than one, the transmission method comprising the steps of: encoding a block of data of a predetermined length with an error correction code; mapping the encoded block of data to a sequence of real-valued symbols a predetermined number of bits at a time; generating first complex symbols each including a real component and an imaginary component by (i) generating a plurality of D-dimensional rotated vectors by multiplying each of a plurality of D-dimensional vectors, each having D of the real-valued symbols as elements, with a D-by-D orthogonal matrix to convert the D-dimensional vector into a D-dimensional rotated vector having D real symbols as elements and (ii) mapping the D real symbols of each of the D-dimensional rotated vectors to D/2 consecutive first complex symbols; writing the real components of the first complex symbols column by column into a first interleaver matrix having N R rows and writing the imaginary components of the first complex symbols column by column into a second interleaver matrix having N R rows; applying, to each column of the second interleaver matrix, a respective cyclic shift in accordance with a predetermined shift pattern; reading the real components from the first interleaver matrix row by row and the imaginary components from the cyclically shifted second interleaver matrix row by row to generate a plurality of second complex symbols; and transmitting the second complex symbols over the frequency channels by grouping consecutive second complex symbols into N R slices of equal length with each slice being mapped to one of the N RF frequency channels, wherein N R is a multiple of N RF greater than N RF, and the shift pattern for the cyclic shifts includes only integers that are not a multiple of N RF. encoding a block of data of a predetermined length with an error correction code; mapping the encoded block of data to a sequence of real-valued symbols a predetermined number of bits at a time; generating first complex symbols each including a real component and an imaginary component by (i) generating a plurality of D-dimensional rotated vectors by multiplying each of a plurality of D-dimensional vectors, each having D of the real-valued symbols as elements, with a D-by-D orthogonal matrix to convert the D-dimensional vector into a D-dimensional rotated vector having D real symbols as elements and (ii) mapping the D real symbols of each of the D-dimensional rotated vectors to D/2 consecutive first complex symbols; writing the real components of the first complex symbols column by column into a first interleaver matrix having N R rows and writing the imaginary components of the first complex symbols column by column into a second interleaver matrix having N R rows; applying, to each column of the second interleaver matrix, a respective cyclic shift in accordance with a predetermined shift pattern; reading the real components from the first interleaver matrix row by row and the imaginary components from the cyclically shifted second interleaver matrix row by row to generate a plurality of second complex symbols; and transmitting the second complex symbols over the frequency channels by grouping consecutive second complex symbols into N R slices of equal length with each slice being mapped to one of the N RF frequency channels, wherein N R is a multiple of N RF greater than N RF, and the shift pattern for the cyclic shifts includes only integers that are not a multiple of N RF.