Abstract: The system in this invention is applicable to bi-directional communication over the electricity network between a head-end and a plurality of users, and permits the synchronization in time and frequency of multiple users in a multi-carrier OFDM communication system. It is characterized in that it consists of the generation of synchronization sequences by means of two identical synchronization symbols, periodically transmitted in the downstream, from the head-end to the users to estimate and correct the sampling frequency and simultaneously estimate the moment in which each OFDM symbol commences.

Abstract: The system enables bidirectional communication between a center and a plurality of users via a power line network and is characterized in that low and high speed communication can be established to provide multiple high-quality services to the users. The system comprises a performing network and frequency time-sharing and/or orthogonal frequency-division multiple access (OFDMA).

Abstract: Where synchronization in time and frequency in the downstream channel has been previously carried out, and determined by the sending of synchronization sequences (3) in a communication between a head-end (1) and user equipment (2); the present invention is characterized because it corrects sampling frequency in the user equipments (2) by starting from the estimation carried out during frequency synchronization in the downstream channel, to synchronize the upstream channel in frequency, removing the need for the head-end to make corrections in reception in the upstream channel. The user equipment (2) estimates (9) the moment the OFDM symbols (7) should be sent to the head-end (1) so that these are received by it at previously fixed moments in time. Furthermore, this invention includes interrogation of the user equipment (2), which allows them to place requests for access to the upstream channel with the head-end then making the distribution between the requests received.

Abstract: The system in this invention is applicable to bi-directional communication over the electricity network between a head-end and a plurality of users, and permits the synchronization in time and frequency of multiple users in a multi-carrier OFDM communication system. It is characterized in that it consists of the generation of synchronisation sequences by means of two identical synchronization symbols, periodically transmitted in the downstream, from the head-end to the users to estimate and correct the sampling frequency and simultaneously estimate the moment in which each OFDM symbol commences.

Abstract: The system enables bidirectional communication between a center and a plurality of users via the electric network and is characterized in that low and high speed communication can be established to provide multiple high-quality services to the users. The system comprises means to perform network and frequency time-sharing and/or orthogonal frequency-division multiple access (OFDMA).