A method of transmitting digital information via a total number of Orthogonal Frequency Division Multiplexing (OFDM) subcarriers over a nonlinear and dispersive optical channel, the method comprising the steps of: predetermining a plural number of frequency sub-bands, wherein each sub-band comprises a selected two or more of the total number of OFDM subcarriers; receiving data comprising at least a portion of the digital information; generating from the data a plurality of frequency domain symbols; assigning each symbol to one of the predetermined plural number of frequency sub-bands; generating an optical signal comprising the total number of OFDM subcarriers having a uniform frequency spacing, wherein the plural number of sub-bands are combined adjacently in the frequency domain; and transmitting the optical signal via said nonlinear and dispersive optical channel, characterised in that the plural number of frequency sub-bands is predetermined so as to reduce nonlinear optical distortion of an optical signal within the nonlinear and dispersive optical channel relative to a corresponding single frequency band signal having the same total number of OFDM subcarriers by steps comprising: transmitting a plurality of training signals via the nonlinear optical channel, each training signal comprising predetermined data values and having a different number of sub-bands; detecting each training signal after transmission via the nonlinear optical channel; processing each detected signal to recover received data; determining a bit-error rate of each detected training signal by comparing received data values with the predetermined data values of the training signal; and determining the predetermined plural number of sub-bands using the determined bit-error rates. predetermining a plural number of frequency sub-bands, wherein each sub-band comprises a selected two or more of the total number of OFDM subcarriers; receiving data comprising at least a portion of the digital information; generating from the data a plurality of frequency domain symbols; assigning each symbol to one of the predetermined plural number of frequency sub-bands; generating an optical signal comprising the total number of OFDM subcarriers having a uniform frequency spacing, wherein the plural number of sub-bands are combined adjacently in the frequency domain; and transmitting the optical signal via said nonlinear and dispersive optical channel, characterised in that the plural number of frequency sub-bands is predetermined so as to reduce nonlinear optical distortion of an optical signal within the nonlinear and dispersive optical channel relative to a corresponding single frequency band signal having the same total number of OFDM subcarriers by steps comprising: transmitting a plurality of training signals via the nonlinear optical channel, each training signal comprising predetermined data values and having a different number of sub-bands; detecting each training signal after transmission via the nonlinear optical channel; processing each detected signal to recover received data; determining a bit-error rate of each detected training signal by comparing received data values with the predetermined data values of the training signal; and determining the predetermined plural number of sub-bands using the determined bit-error rates. transmitting a plurality of training signals via the nonlinear optical channel, each training signal comprising predetermined data values and having a different number of sub-bands; detecting each training signal after transmission via the nonlinear optical channel; processing each detected signal to recover received data; determining a bit-error rate of each detected training signal by comparing received data values with the predetermined data values of the training signal; and determining the predetermined plural number of sub-bands using the determined bit-error rates.