OFDM timing and frequency recovery system
AssigneeTelecommunications Research Laboratories
InventorGrant McGibney
Life of this patent
Estimating the term…
Abstract
A synchronizing apparatus for a differential OFDM receiver that simultaneously adjust the radio frequency and sample clock frequency using a voltage controlled crystal oscillator to generate a common reference frequency. Timing errors are found by constellation rotation. Subcarrier signals are weighted by using complex multiplication to find the phase differentials and then the timing errors. The reference oscillator is adjusted using the timing errors. Slow frequency drift may be compensated using an integral of the timing error. Frequency offset is found using the time required for the timing offset to drift from one value to another.
Claims16
A method of processing an OFDM signal comprised of plural sub-carriers, in which information is carried in phase differentials between sub-carriers, the phase differentials having a pre-determined distribution in the complex domain, the OFDM signal being received by a receiver having a reference oscillator, the method comprising the steps of:
- repeatedly detecting real and imaginary values of plural subcarriers wherein the real and imaginary values form a constellation of real and imaginary values of the plural sub-carriers;
- computing successive timing errors by repeatedly estimating the rotation required to rotate the constellations to coincide with the pre-determined distribution;
- and
- adjusting the reference oscillator according to the timing errors.
A wireless terminal, comprising:
- RF means for detecting RF signals wherein information is carried as phase differentials between subcarriers;
- a sampler connected to the RF means for receiving downconverted signals from the RF:
- means and producing digitized RF signal blocks of the downconverted signals;
- a differential OFDM receiver comprising means for computing a timing error estimate for the digitized RF signal blocks based on rotation of a constellation of real and imaginary values of the digitized RF signal blocks, the OFDM receiver having as output a timing error estimate;
- a voltage controlled reference oscillator having as input a voltage related to the timing error estimate and as output a signal having a reference frequency;
- an RF signal generator connected to receive the reference frequency signal from the voltage controlled reference oscillator and to provide an RF signal to the RF means;
- and
- a digital clock generator connected to receive the reference frequency signal from the voltage controlled reference oscillator and to provide a digital clock to the sampler.
Dependencies are inferred from the claim text.