In most mobile communication systems of today, there are specific requirements regarding synchronization of a base station and a mobile terminal in order to secure a correct data transmission. Examples of such systems are the Universal Terrestrial Radio Access (UTRA) and Evolved UTRA.
In Evolved UTRA, Single-Carrier Frequency Division Multiple Access (SC-FDMA) may be used as multiple access scheme for the uplink communication. The transmission scheme of SC-FDMA is the so-called Discrete Fourier Transform-spread Orthogonal Frequency Domain Multiplexing (DFT-spread OFDM), which can be seen as OFDM with pre-coding. Whereas OFDM, which produces a multi-carrier signal, has a high peak-to-average ratio (PAPR), the DFT pre-coding gives a single-carrier signal with lower PAPR. The low PAPR serves to extend the coverage and to reduce the battery drain in the mobile.
In DFT-spread OFDM, cyclic prefix is used to achieve equalization in the frequency domain. However, a requirement for successful equalization in DFT-spread OFDM, as well as in OFDM, is that the signals transmitted from all mobile terminals in the cell are synchronized in such a manner that the delay spread of the signal plus the spread in the time of arrival is less than the duration of the cyclic prefix. Therefore, it is required that each transmitting mobile terminal is synchronized to within a fraction of the duration of the cyclic prefix before it can transmit data.
In Evolved UTRA, synchronization is performed both in uplink and downlink. In one step of the synchronization, downlink synchronization, the mobile terminal synchronizes (or locks) to the carrier frequency and the frame timing of the base station. This synchronization, however, is not sufficient to ensure that the base station can properly receive the signals from the mobile terminal, since mobile terminals may be located at various distances relative to the base station.
Consequently, further synchronization, uplink synchronization, is needed since the distance between a base station and a mobile terminal, and hence the round trip time, is in general unknown.
In Evolved UTRA, a random access channel (RACH) supports uplink synchronization of the mobile terminals. RACH in Evolved UTRA is contention-based, i.e. any mobile terminal within the cell may transmit on the resource allocated to RACH. Consequently, several mobile terminals may attempt to transmit synchronization signals simultaneously, and in order to reduce the risk that the base station fails to distinguish signals from different mobile terminals, a set of signature sequences is provided, wherein each mobile terminal randomly selects one signature sequence.
In UTRA and Evolved UTRA a binary pseudo-random sequence generated by a shift register is modulated by 16-bit Hadamard sequences to produce these signature sequences. Even though these signature sequences in many instances provide good correlation properties, there still exists a need for enhanced detection capabilities to detect a specific signature in presence of other simultaneous signatures, especially at low SIR values.