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EP-4165843-B1 - DEMODULATING MODULATED SIGNALS

EP4165843B1EP 4165843 B1EP4165843 B1EP 4165843B1EP-4165843-B1

Inventors

  • RYAN, DANIEL

Dates

Publication Date
20260506
Application Date
20210616

Claims (15)

  1. An apparatus (15; 17) for demodulating a frequency-modulated signal, the apparatus comprising: a joint frequency-offset and modulation-index estimator (34); and a signal demodulator (37, 39), wherein the joint frequency-offset and modulation-index estimator (34) is configured to: receive data representative of a preamble portion of the frequency-modulated signal, wherein the preamble portion is modulated with predetermined preamble data; and jointly determining a frequency-offset estimate and a modulation-index estimate by using the received data to perform an optimization process, wherein the optimization process minimizes a cost function that is a function of the received data and that is parameterised by a frequency-offset parameter and by a modulation-index parameter, and wherein the signal demodulator (37, 39) is configured to: receive data representative of a message portion of the frequency-modulated signal, wherein the message portion is modulated with message data; and use the frequency-offset estimate to demodulate the message.
  2. The apparatus (15; 17) of claim 1, wherein the signal demodulator (37, 39) is configured to use the modulation-index estimate, as well as the frequency-offset estimate, when demodulating the message portion.
  3. The apparatus (15; 17) of claim 1 or 2, wherein the data representative of the preamble portion comprise a sequence of complex sample values, and wherein the joint frequency-offset and modulation-index estimator (34) is configured to calculate a phase angle for each of the complex sample values, for performing the optimization process in a phase domain.
  4. The apparatus (15; 17) of any preceding claim, configured to store or access stored data relating to the predetermined preamble data, wherein the cost function is a function of both the stored data and the received data, and wherein the joint frequency-offset and modulation-index estimator (34) is configured to use the stored data when performing the optimization process.
  5. The apparatus (15; 17) of claim 4, wherein the stored data represents a sequence, or a function of a sequence, of accumulated phase offsets for a signal modulated with the predetermined preamble data.
  6. The apparatus (15; 17) of claim 4 or 5, wherein the stored data encodes a matrix that is a function of one or more matrices, wherein at least one of the matrices has a row or column containing a series of accumulated phase-offset values that are dependent on the predetermined preamble data.
  7. The apparatus (15; 17) of any preceding claim, wherein the cost function is further parameterised by a phase-unwrapping parameter and wherein the joint frequency-offset and modulation-index estimator (34) is configured to minimize the cost function over the phase-unwrapping parameter.
  8. The apparatus (15; 17) of claim 7, wherein the joint frequency-offset and modulation-index estimator (34) is configured to keep the frequency-offset parameter constant when performing the optimization process, and is further configured to determine the frequency-offset estimate by back-substitution after determining an optimal value for the phase-unwrapping parameter using the optimization process.
  9. The apparatus (15; 17) of claim 7 or 8, wherein the joint frequency-offset and modulation-index estimator (34) is configured to keep the modulation-index parameter constant during the optimization process, and is further configured to determine the modulation-index estimate by back-substitution after determining an optimal value for the phase-unwrapping parameter.
  10. The apparatus (15; 17) of any of claims 1 to 7, wherein the joint frequency-offset and modulation-index estimator (34) is configured to determine the frequency-offset estimate and the modulation-index estimate by performing the optimization process a plurality of times, with the frequency-offset parameter and the modulation-index parameter set to a different pair of constant values each time, and to determine a pair of the pairs of constant values that resulted in a lowest minimum cost for the cost function.
  11. The apparatus (15; 17) of any preceding claim, wherein the joint frequency-offset and modulation-index estimator (34) is configured to minimize the cost function by performing a closest-lattice-point search to identify a closest point in a lattice.
  12. The apparatus (15; 17) of claim 11, wherein the joint frequency-offset and modulation-index estimator (34) is configured to search for a closest point over the lattice A n *, for an integer n, using an algorithm having O(N) complexity.
  13. The apparatus (15; 17) of any preceding claim, wherein the frequency-modulated signal is a radio signal and wherein the apparatus comprises a radio receiver (15) for receiving the modulated radio signal and for generating the data representative of the preamble and message portions.
  14. The apparatus of any preceding claim, wherein the frequency-modulated signal is modulated with constant amplitude.
  15. A method of demodulating a frequency-modulated signal, the method comprising: receiving (301) data representative of a preamble portion of the frequency-modulated signal, wherein the preamble portion is modulated with predetermined preamble data; jointly determining (311) a frequency-offset estimate and a modulation-index estimate by using the received data to perform an optimization process, wherein the optimization process minimizes a cost function that is a function of the received data and that is parameterised by a frequency-offset parameter and by a modulation-index parameter; receiving data representative of a message portion of the frequency-modulated signal, wherein the message portion is modulated with message data; and using (313) the frequency-offset estimate when demodulating the message portion.

Description

BACKGROUND OF THE INVENTION This invention relates to methods and apparatus for demodulating modulated signals. Transmissions systems are known for transmitting signals from a transmitter to a receiver. In some systems, digital message data is modulated on a frequency- or phase-modulated carrier signal. Examples include Bluetooth™ and Bluetooth Low Energy (BLE)™, which use Gaussian FSK (GFSK) modulation. In binary frequency-shift key (FSK) modulation, data is encoded by the transmitter transmitting one of two different frequencies, offset either side of a carrier frequency. The modulation may be applied so that the phase of the transmitted signal is continuous using continuous-phase FSK (CP-FSK). GFSK is a type of CP-FSK in which spectral leakage is reduced by Gaussian shaping of the modulating data signal. Receivers can use coherent detection, in which the receiver uses knowledge of the phase of the carrier to demodulate the signals, or non-coherent detection in which a phase reference is not used. A known approach for non-coherently demodulating a signal is to use a matched filter bank (MFB), which cross-correlates the incoming sampled signal with a bank of predetermined filter sequences, representing respective waveforms corresponding to different modulated data, in order to determine the most likely symbol sequence that is communicated by a particular portion of the incoming signal. If a reference clock used by the receiver is not locked to a reference clock of the transmitter, there is the potential for a frequency mismatch, or offset, to be present. Such a frequency offset can result in the receiver inaccurately demodulating modulated message data, such as frequency- or phase-modulated data. It is known to provide a predetermined, known preamble or training sequence in an incoming data packet (typically located at or towards the beginning of the packet), which the receiver can process to determine a frequency-offset estimate. The receiver can then use this frequency-offset estimate to apply appropriate compensation to later-received samples-e.g. by rotating the complex samples through a suitable phase angle-to enable more accurate demodulation of unknown message data from the data packet. US 5,930,243 discloses a method and apparatus for estimating the parameters of a wireless communication system, the parameters being one or more of: the time alignment, the frequency offset, and the weight vector for spatial processing. Various approaches to estimating the carrier frequency are known. For example, the applicant's earlier patent application WO 2014/167318 describes using a correlator to determine both timing information and a carrier frequency offset, while the paper "Frequency Estimation by Phase Unwrapping" by McKilliam et al., IEEE Transactions on Signal Processing, vol. 58, no. 6, pp. 2953-2963, 2010, proposes performing frequency estimation of a single carrier frequency using phase unwrapping and a nearest-point lattice search. However, the applicant has determined that the accuracy of known approaches can suffer in some situations. The present invention therefore seeks to provide an alternative approach to demodulating a modulated signal. SUMMARY OF THE INVENTION From a first aspect, the invention provides a method of demodulating a modulated signal, the method comprising: receiving data representative of a preamble portion of the modulated signal, wherein the preamble portion is modulated with predetermined preamble data;jointly determining a frequency-offset estimate and a modulation-index estimate by using the received data to perform an optimization process, wherein the optimization process minimizes a cost function that is a function of the received data and that is parameterised by a frequency-offset parameter and by a modulation-index parameter;receiving data representative of a message portion of the modulated signal, wherein the message portion is modulated with message data; andusing the frequency-offset estimate when demodulating the message portion. From a second aspect, the invention provides an apparatus for demodulating a modulated signal, the apparatus comprising: a joint frequency-offset and modulation-index estimator; anda signal demodulator, wherein the joint frequency-offset and modulation-index estimator is configured to: receive data representative of a preamble portion of the modulated signal, wherein the preamble portion is modulated with predetermined preamble data; andjointly determine a frequency-offset estimate and a modulation-index estimate by using the received data to perform an optimization process, wherein the optimization process minimizes a cost function that is a function of the received data and that is parameterised by a frequency-offset parameter and by a modulation-index parameter,and wherein the signal demodulator is configured to: receive data representative of a message portion of the modulated signal, wherein the message portion is modulated with mess