A method for processing data for transmission over an optical wireless communications system to optical wireless communication devices (20a-d), the method comprising: determining or receiving channel state information, CSI, relating to a plurality of the optical wireless communication devices; for each of one or more sub-carriers, determining which of the plurality of optical wireless communication devices (20a-d) can communicate using the respective subcarrier according to, or based on, the channel state information or data derived therefrom; allocating optical wireless communication devices (20a-d) to sub-carriers upon which each optical wireless communication device (20a-d) can communicate; and providing data for each of the plurality of optical wireless communication device (20a-d) on the sub-carriers to which the respective optical wireless communication device (20a-d) has been allocated; wherein: the channel state information comprises or is used or usable to determine the variation of signal-to-noise ratio with frequency for each of the plurality of optical wireless communication devices (20a-d); the method comprises determining a cut-off frequency for each optical wireless communication device (20a-d) of the plurality of optical wireless communication devices (20a-d), wherein above the cut-off frequency the signal-to-noise ratio for the respective optical wireless communication device (20a-d) is below a threshold; and the allocating optical wireless communication devices (20a-d) to sub-carriers comprises only allocating optical wireless communication devices (20a-d) to sub-carriers having frequencies or frequency ranges that are equal to or less than the cut-off frequency for the respective optical wireless communication device (20a-d). determining or receiving channel state information, CSI, relating to a plurality of the optical wireless communication devices; for each of one or more sub-carriers, determining which of the plurality of optical wireless communication devices (20a-d) can communicate using the respective subcarrier according to, or based on, the channel state information or data derived therefrom; allocating optical wireless communication devices (20a-d) to sub-carriers upon which each optical wireless communication device (20a-d) can communicate; and providing data for each of the plurality of optical wireless communication device (20a-d) on the sub-carriers to which the respective optical wireless communication device (20a-d) has been allocated; wherein: the channel state information comprises or is used or usable to determine the variation of signal-to-noise ratio with frequency for each of the plurality of optical wireless communication devices (20a-d); the method comprises determining a cut-off frequency for each optical wireless communication device (20a-d) of the plurality of optical wireless communication devices (20a-d), wherein above the cut-off frequency the signal-to-noise ratio for the respective optical wireless communication device (20a-d) is below a threshold; and the allocating optical wireless communication devices (20a-d) to sub-carriers comprises only allocating optical wireless communication devices (20a-d) to sub-carriers having frequencies or frequency ranges that are equal to or less than the cut-off frequency for the respective optical wireless communication device (20a-d). the channel state information comprises or is used or usable to determine the variation of signal-to-noise ratio with frequency for each of the plurality of optical wireless communication devices (20a-d); the method comprises determining a cut-off frequency for each optical wireless communication device (20a-d) of the plurality of optical wireless communication devices (20a-d), wherein above the cut-off frequency the signal-to-noise ratio for the respective optical wireless communication device (20a-d) is below a threshold; and the allocating optical wireless communication devices (20a-d) to sub-carriers comprises only allocating optical wireless communication devices (20a-d) to sub-carriers having frequencies or frequency ranges that are equal to or less than the cut-off frequency for the respective optical wireless communication device (20a-d).