With OFDM systems the frequency domain data is the Fourier transform of the received time domain OFDM frames. The time domain frames must be sampled, at the receiver, in synchronism with the transmitter, so that each received frame contains data from only a single transmitted frame. It is vital for this synchronism to be maintained in order to maintain the orthogonality of the frames. A typical multi-carrier system, of the OFDM type, which uses a cyclic prefix permits orthogonality to be maintained when there is a small deviation from exact frame synchronisation. Because the signalling interval includes both an entire frame and the cyclic prefix, which is a repetition of part of the frame, a frame sampled within the signalling interval will contain data from only one frame. Since the signalling interval is greater than the frame period, this gives some leeway in frame alignment. In a multi-carrier system of the OFDM type, an adaptive channel equalizer, operating in the frequency domain, is often used. The internal parameters in such an equalizer contain, in addition to information about the characteristics of the channel, information which can be interpreted as the time deviation between the sampling clocks of the transmitter and the receiver. The present invention utilizes this information to control the sampling clock of the receiver in a more robust way than has previously been possible with known techniques. The present invention is particularly suitable for use in ADSL and VDSL modems which can be used to give broadband access over copper networks. The invention is also of relevance to broadband transmission in mobile and semi-mobile systems for transmission over the radio channels.
A digital subscriber line (DSL) receiver includes a discrete multitone (DMT) receiver circuit operative to receive a DSL signal and to generate signal samples corresponding to respective frequency division multiplexed tones of the DSL signal. The receiver further includes a synchronization circuit operative to determine phase information for a crosstalk source indication signal, e.g., an integrated services digital network (ISDN) timing reference indication signal, carried by one of the plurality of frequency-division multiplexed tones from the signal samples generated by the DMT receiver circuit and to determine frame timing for the DSL signal responsive to the determined phase information. The DMT receiver circuit may include a buffer operative to receive and store samples of the DSL signal, and a fast Fourier transformer (EFT) operative to generate the signal samples for the respective ones of the plurality of frequency-division multiplexed tones from the stored samples of the DSL signal. The synchronization circuit may be operative to control transfer of stored samples of the DSL signal from the buffer to the FFT responsive to the determined phase information.
In a multi-carrier system employing OFDM, for example DMT, an adaptive channel equalizer is normally used, operating in the frequency domain. The sampling clock is controlled so that the time delay between the transmitter and the receiver is effectively eliminated. If the information used to control the sampling clock is received from the equalized data stream, it will introduce an ambiguity between the operation of the channel equalizer and the mechanism used to control the sampling clock. Operation of the equalizer can mask an increasing time difference, between transmitter and receiver, which the sample clock controller should be tracking. The present invention eliminates the ambiguities in the operation of the equalizer and sample clock controller by preventing the equalizer accepting time differences which should be corrected by operation of the sample clock controller.
A method that includes wirelessly receiving a signal, and detecting a start of packet (SOP) for a packet that conforms to a wireless networking standard. The detecting from the received signal uses at least one SOP detection criterion. In the case an SOP is detected, the method further includes determining a plurality of metrics from the received signal, and using at least two of the plurality of metrics to determine an initial timing for a received packet. Different versions combine the metrics in different ways to determine the initial timing. Also an apparatus that includes a radio receiver to receive a signal and output a received signal, and an SOP detector coupled to the radio receiver to detect a start of packet (SOP) from a received signal using at least one SOP detection criterion for a packet that conforms to a wireless networking standard. The apparatus further includes a processing circuit coupled to the radio receiver to determine a plurality of metrics from the received signal, and a initial time determining circuit coupled to the SOP detector and the processing circuit. In the case an SOP is detected, the initial time determining circuit uses at least two of the plurality of metrics to determine an initial timing for a received packet.
