| 摘要 |
<p>Utilisation of frequency divided OFDM (FD OFDM) in the uplink of a multi-access system requires that all mobile stations meet the requirements for orthogonality between sub-carriers in the time and frequency domain. Alternatively, another access technique can be used with OFDM, which can tolerate some relaxation in orthogonality requirements, e.g. code division OFDM (CD OFDM, or COFDM). The use of FD OFDM thus requires a random access technique that does not disturb the orthogonality between sub-carriers. Because time synchronisation is essential if orthogonality is to be preserved between sub-carriers, it is vital that delay difference between a mobile station and a base station be estimated during a random access protocol. In a random access protocol, a mobile station transmits a known signal sequence to a base station. The base station can lock onto the known sequence, detect it and estimate the time delay. According to the present invention, the random access sequence can be repeated cyclically and may comprise a m-sequence, a Gold sequence, or a four phase sequence with good cross-correlation properties, which is modulated onto the sub-carriers in the frequency domain. The random access sequence is transmitted without guard spaces. The random access sequence may be modulated onto all sub-carriers, or alternatively onto selected sub-carriers only. Alternatively, the random access sequence may be modulated onto all sub-carriers, but some sub-carriers may be transmitted at higher power than others. <IMAGE></p> |
