To reduce the number of subcarriers used for the transmission of reference symbols by base stations in an OFDM based cellular telecommunications network, the level of data for transmission from the base station to at least one subscriber station is determined and, when traffic levels are low and/or where the mobile station is unable to make efficient use of the full bandwidth, reference symbol data is transmitted from the base station using a number of the subcarriers less than the total number of subcarriers available. Thus the reference symbol frequency spread—the bandwidth between the lowest frequency subcarrier and the highest frequency subcarrier being used to transmit reference symbols—is made dependent upon the determined level of data for transmission.