Narrow band receiver

A method of identifying frequency data for use in adjusting a frequency offset of a receiver, the method comprising:

• receiving a signal at the receiver, the signal comprising a set of slots, at least one of said slots comprising a preamble portion and being transmitted at a predetermined transmission frequency, wherein said predetermined frequency is within a known range of frequencies;
• dividing the range of frequencies into a plurality of frequency bands;
• for each data item contained within the preamble portion:
• identifying a magnitude of signal received within each said frequency band;
• identifying a plurality of pairs of frequency bands, each of said pairs of frequency bands comprising a first frequency band and a second frequency band,
• for at least some of the plurality of pairs of frequency bands, subtracting a first signal magnitude identified for said first frequency band from a second signal magnitude identified for said second frequency band, whereby to demodulate the signal within at least part of said preamble portion;
• combining the subtracted signal magnitudes corresponding to respective pairs of frequency bands with output from an oscillator tuned to a fundamental of a period associated with said preamble portion;
• and
• for each pair of first and second said frequency bands, accumulating the combined signal magnitudes over the preamble portion, whereby to identify frequency data for use in adjusting a frequency offset of the receiver.

A signal processor for use in a narrow band receiver, the signal processor being for decoding data contained within a signal, the signal comprising a set of slots, at least one of said slots comprising a preamble portion and a payload and/or synchronisation word portion and being transmitted at a predetermined transmission frequency, the preamble portion comprising at least a first sequence of data and a second sequence of data, the second sequence being the inverse of the first sequence, the signal processor comprising a first demodulating part and a second demodulating part, the first demodulating part being arranged to:

• perform a first process to derive timing data and to identify a transition between said first and second sequences of data from the preamble portion;
• and the second demodulating part being arranged to:
• perform a second process to extract information from the payload and/or synchronisation word portion using said timing data, the second process being triggered on the basis of the identification of the transition by the first process,
• in which, when executing the first process, the first demodulating part is arranged to:
• perform an accumulation on a filtered component of the preamble portion;
• identify the transition point between said first sequence and second sequence of data on the basis of the accumulation;
• and
• derive said timing data on the basis of a phase associated with demodulation of at least part of said preamble portion.

A receiver for identifying frequency data for use in adjusting a frequency offset of said receiver, the receiver comprising:

• an antenna for receiving a signal, the signal comprising a set of slots, at least one of said slots comprising a preamble portion and being transmitted at a predetermined transmission frequency, wherein said predetermined frequency is within a known range of frequencies;
• a digital filter arranged to divide the range of frequencies into a plurality of frequency bands, wherein, for each data item contained within the preamble portion, the filter is arranged to identify a magnitude of signal received within each said frequency band and identify a plurality of pairs of frequency bands, each said pair of frequency bands comprising a first frequency band and a second frequency band, the filter being arranged, for at least some of the plurality of pairs of frequency bands, to subtract a first signal magnitude identified for said first frequency band from a second signal magnitude identified for said second frequency band, whereby to demodulate the signal within at least part of said preamble portion;
• and
• a detector element arranged to combine the subtracted signal magnitudes corresponding to respective pairs of frequency bands with output from an oscillator tuned to a fundamental of a period associated with said preamble portion, wherein, for each pair of first and second said frequency bands, the detector element is arranged to accumulate the combined signal magnitudes over the preamble portion, whereby to identify frequency data for use in adjusting a frequency offset of the receiver.