Low complexity, low power and long range radio receiver
View Patent ↗A radio receiver for processing digital chirp spread-spectrum modulated signals that comprise a plurality of frequency chirps that are cyclically time-shifted replicas of a base chirp profile, said time-shifts being an encoded representation of a transmitted message. Includes a soft demapping unit that is adapted for working on fully populated as well as on partial modulation sets, and implements a timing error correction loop that acts back both in the time domain and in the frequency domain.
1. A radio receiver for processing digital chirp spread-spectrum modulated signals that comprise a plurality of frequency chirps that are cyclically time-shifted replicas of a base chirp profile, said time-shifts being an encoded representation of a transmitted message, wherein the receiver includes a soft demapping unit arranged to determine, for each received chirp of said plurality of frequency chirps, a plurality of soft values representing probability of possible symbols or bits in the transmitted message., including a sampling unit that provides a series of digital samples of the received frequency chirps, a de-spreading unit that is arranged for multiplying the sampled chirps with a complex conjugate of a local replica of the base chirp, a demodulating unit arranged for performing a Fourier transform of samples provided by the de-spreading unit, and wherein the soft demapping unit is arranged to compute, for each possible symbol or bit, the difference of amplitude at two position in the Fourier-transformed signal.
2. The receiver of claim 1 , wherein the demodulating unit is arranged to partition the received symbols into sub-symbols and compute for each symbol a full Fourier transform and a plurality of partial Fourier transforms on said sub-symbols, and for selecting either a peak value of the full Fourier transform or a peak value of a non-coherent sum of the partial transforms, based on the received symbols' bin index.
3. The receiver of claim 1 , further comprising a timing error estimation unit that is operatively arranged to compare a peak amplitude in the Fourier-transformed signal with amplitudes in neighbouring Fourier bins and extract an amplitude difference therefrom, and derive a timing error by evaluating a non-linear function of the amplitude difference.
4. The receiver of claim 2 , further comprising a timing error estimation unit that is operatively arranged to compare a peak amplitude in the Fourier-transformed signal with amplitudes in neighbouring Fourier bins and extract an amplitude difference therefrom, and derive a timing error by evaluating a non-linear function of the amplitude difference.
5. The receiver of claim 1 , wherein the demodulating unit is arranged to partition the received symbols into sub-symbols and compute for each symbol a full Fourier transform and a plurality of partial Fourier transforms on said sub-symbols, and for selecting either a peak value of the full Fourier transform or a peak value of a non-coherent sum of the partial transforms, based on whether the received symbols' demodulated value falls within a determined boundary.
6. The receiver of claim 2 , wherein the demodulating unit is arranged to partition the received symbols into sub-symbols and compute for each symbol a full Fourier transform and a plurality of partial Fourier transforms on said sub-symbols, and for selecting either a peak value of the full Fourier transform or a peak value of a non-coherent sum of the partial transforms, based on whether the received symbols' demodulated value falls within a determined boundary.
7. The receiver of claim 3 , further arranged to correct the timing error by introducing an offset in the frequency of the local replica of the base chirp based on said timing error estimated by the timing error estimation unit.
8. The receiver of claim 3 , further arranged to correct a sampling drift error by introducing an offset in time/frequency slope of the local replica of the base chirp based on the frequency error estimation.