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 .