There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, utilise OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than a coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols.
To facilitate detection and recovery of the data at a receiver, some communications systems which utilise OFDM include a signalling OFDM symbol to provide signalling information and also to provide a facility for detecting a timing of a time divided frame, which is subsequently repeated. The time divided frame may include the signalling OFDM symbol and data bearing OFDM symbols at predetermined positions within the frame, which is then repeated in the next and subsequent frames. Accordingly, by detecting the signalling OFDM symbol, a receiver can synchronise with the repeating pattern of the OFDM symbols within the frames and therefore correctly identify data bearing OFDM symbols at predetermined positions in the frame.
Signalling OFDM symbols are designed to facilitate detection of the signalling OFDM symbols by a receiver. For the example of DVB-T2, as described in Digital Video Broadcasting (DVB); Frame structure, channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, Draft ETSI EN 302 755, Version 1.1.1-0.2, October 2008, a P1 signalling OFDM symbol is arranged to include a pre-amble guard interval and a post-amble guard interval, the pre-amble and the post-amble guard intervals being formed by copying samples from a useful part of the signalling OFDM symbol. A recommended technique for detecting the P1 OFDM symbol is disclosed in “Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVB-T2)”, Draft ETSI TR 102 831, Version 0.7.6, September 2008.
Although designed to facilitate detection at receiver, detecting a signalling OFDM symbol at a receiver in the presence of typical transmission disturbances such as multi-path, additive white Gaussian noise and analogue interference (modelled by the continuous wave interference) can present a technical problem.