Patent ID: 7423959

Claim:
A process for synchronization in the downstream of multiple users in a point to multipoint system with OFDM modulation, applicable to two-way communication over the electricity network between a head-end and various user equipments that includes the means to add and extract a cyclic prefix to the OFDM symbols, including interpolators and decimators in the transmission and reception systems of the equipments and digital band translation, and which comprises the sending of synchronization sequences in the information that is sent via the downstream channel which runs from the head-end to the user equipments, to provide synchronization in frequency and time in reception, and wherein the process comprises: generating synchronization sequences by means of two identical synchronization symbols; transmitting periodically said sequences from the head-end equipment by the downstream channel to all the user equipments; detecting said synchronization sequences in reception by the user equipments; estimating the beginning of the OFDM symbol in order to synchronize in time by using maximum likelihood criteria so that time synchronization begins from the calculation of the maximum correlation of the samples of two received symbols, and this maximum becomes the mid point in the flat zone for the correlation peak, whose size in number of samples is equal to the number of samples of the cyclic prefix without intersymbol interference, ISI; and simultaneously estimating the sampling frequency error and caring out the frequency synchonization, by means of calculating the angle of this correction in the moment determined as maximum correlation, and selectively performing at least one of the following steps in order to synchronize in frequency; correcting the sampling frequency in the analog/digital converters that are included in the receptors in the user equipments; or re-sampling the digital signal.