As an example of how the 2D spreading on VSF-OFCDM works, if you take the first data symbol, "d"0, and a spreading factor in the time domain, "SF"time, of length 4, and a spreading factor in the frequency domain, "SF"frequency of 2, then the data symbol, "d"0, will be multiplied by the length-2 frequency-domain PN codes and placed on subcarriers 0 and 1, and these values on subcarriers 0 and 1 will then be multiplied by the length-4 time-domain PN code and transmitted on OFDM symbols 0, 1, 2 and 3.