Method and apparatus for timing recovery in an OFDM system
A method ( 500 ) for timing recovery in an orthogonal frequency division multiplexing (OFDM) system includes the steps of detecting ( 502 ) a lack of a synchronization symbol and determining ( 504 ) a timing offset from calculating the average group delay across the subcarriers over multiple OFDM symbols. The timing offset is fed back ( 506 ) to a demodulator and the symbol timing is adjusted ( 508 ) based on the Average Group Delay phasor fed back to the demodulator.
1. A method for timing recovery in an orthogonal frequency division multiplexing (OFDM) system, comprising the steps of:
detecting a lack of a synchronization symbol;
determining a timing offset from calculating an average Group delay over a set of OFDM symbols by using a phasor to estimate an average delay of a multi-carrier modulation symbol, wherein the determining step uses the phasor to estimate the average delay of the multi-carrier modulation symbol by computing a differential phasor between each pair of adjacent OFDM subcarriers, removing QPSK data by rotating the differential phasor to a first quadrant, and computing an average phasor angle;
feeding back the timing offset to a demodulator; and
adjusting a symbol timing based on the Average Group Delay fed back to the demodulator.
2. The method of claim 1 , wherein the step of determining the timing offset further comprises the step of determining a phase offset directly from the OFDM symbols using a discriminator in a feedback loop.
3. The method of claim 1 , wherein the step of adjusting the symbol comprises the step of adjusting the symbol timing towards a target phase rotation.
4. The method of claim 1 , wherein the method further comprises the step of maintaining symbol synchronization without ever detecting the synchronization symbol.
5. The method of claim 1 , wherein the angle of the phasor is an estimate of the Average Group Delay and is directly proportional to the timing offset.
6. A digital receiver unit, comprising:
a receiver;
an orthogonal frequency division multiplexing demodulator; and
a processor coupled to the receiver and the demodulator, wherein the processor is programmed to:
detect a lack of a synchronization symbol;
determine a phase offset from a set of OFDM symbols using a phasor to estimate an average delay of a multi-carrier modulation symbol by computing a differential phasor between each pair of adjacent OFDM subcarriers, removing QPSK data by rotating the differential phasor to a first quadrant, and computing an average phasor angle;
feed back the phase offset to the demodulator; and
adjust a symbol timing based on the phase offset fed back to the demodulator.
7. A digital receiver unit of claim 6 , wherein the digital receiver unit further comprises a phase detector coupled to the processor, wherein the phase detector detect the phase offset.
8. The digital receiver unit of claim 6 , wherein the processor is further programmed to determine the phase offset directly from the OFDM symbols using a discriminator in a feedback loop.
9. The digital receiver unit of claim 6 , wherein the processor is further programmed to adjusting the symbol timing towards a target phase rotation.
10. The digital receiver unit of claim 6 , wherein the processor is further programmed to maintain symbol synchronization without ever detecting the synchronization symbol and only using the phase offset.
11. The digital receiver unit of claim 6 , wherein the angle of the phasor is an estimate of the Average Group Delay and is directly proportional to the timing offset.