Method for improving spectral efficiency in Wi-Fi OFDM systems
View Patent ↗Wireless networks that use orthogonal frequency division multiplexing require a receiving device to accurately acquire and maintain synchronization with a transmitting device with respect to carrier and sampling frequency for coherent demodulation. Described herein are techniques for enabling such synchronization using pilot signals with reduced transmission overhead.
1. A device for receiving a transmitted OFDM (orthogonal frequency division multiplexing) signal, comprising:
a radio transceiver including a mixer for downconverting a received carrier modulated with OFDM symbols;
a processing circuitry connected to the radio transceiver to:
demodulate the OFDM symbols to extract a plurality of frequency-domain symbols from each OFDM symbol;
estimate phase errors for the OFDM symbols based upon pilot signals contained in non-consecutively received OFDM symbols and not based upon information contained in other received OFDM symbols;
estimate the phase error for an OFDM symbol not containing a pilot signal as a function of the phase error estimated for a previous OFDM symbol containing a pilot signal; and,
rotate the phases of frequency domain data symbols extracted from a particular OFDM symbol to compensate for the phase error estimated for that particular OFDM symbol.
2. The device of claim 1 wherein the processing circuitry is further to perform initial timing and frequency synchronization based upon training fields contained in a preamble of a packet containing a series of OFDM symbols.
3. The device of claim 1 wherein the processing circuitry is further to estimate the phase error based upon pilot signals contained in M out of every N received OFDM symbols, where M and N are integers and M<N.
4. The device of claim 1 wherein the processing circuitry is further to compute an estimated phase error Φ i for the ith OFDM symbol as:
Φ i =Φ i−1 +βφ i
where, if one or more pilot signals are contained in the ith OFDM symbol, Φ i corresponds to measured phase deviations of the one or more contained pilot signals and β is a weighting function.
5. The device of claim 4 wherein the processing circuitry is configured such that, if one or more pilot signals are contained in the ith OFDM symbol, φ i corresponds to measured phase deviations of the one or more contained pilot signals but, if no pilot signals are contained in the ith OFDM symbol, φ i equals φ i−1 .
6. The device of claim 1 wherein the processing circuitry is further configured such that the weighting function β is computed as:
if φ i corresponds to a new measurement of phase deviations of pilot signals contained in the ith OFDM symbol, then
β=C
where C is a specified constant, but, if no pilot signals are contained in the ith OFDM symbol so that φ i equals then φ i−1 , then
β=αC
where α is a specified constant.
7. The device of claim 6 wherein the processing circuitry is further configured such that the constants C and α are both numbers between 0 and 1.
8. The device of claim 1 wherein the processing circuitry is further configured such that the measured phase deviation φ i of the one or more pilot signals contained in the ith OFDM symbol is computed as a weighted average of individual phase deviations measured for each of the one or more pilot signals.
9. The device of claim 1 wherein the processing circuitry is to operate as a client device in an 802.11-type network.
10. The device of claim 1 wherein the processing circuitry is to operate as an access point device in an 802.11-type network.
11. A method for receiving a transmitted OFDM (orthogonal frequency division multiplexing) signal, comprising:
receiving a carrier waveform modulated with a series of OFDM symbols;
downconverting the received carrier waveform and demodulating the OFDM symbols to extract a plurality of frequency-domain symbols from each OFDM symbol;
estimating phase errors for OFDM symbols in the series based upon pilot signals contained in non-consecutively received OFDM symbols and not based upon information contained in other received OFDM symbols;
estimate the phase error for an OFDM symbol not containing a pilot signal as a function of the phase error estimated for a previous OFDM symbol containing a pilot signal; and,
compensating for the phase error estimated for a particular OFDM symbol by rotating the phases of frequency domain data symbols extracted from that particular OFDM symbol.
12. The method of claim 11 further comprising estimating the phase error based upon pilot signals contained in M out of every N received OFDM symbols, where M and N are integers and M<N.
13. The method of claim 11 further comprising computing an estimated phase error Φ i for the ith OFDM symbol as:
Φ i =Φ i−1 +βφ i
where, if one or more pilot signals are contained in the ith OFDM symbol, φ i corresponds to measured phase deviations of the one or more contained pilot signals and β is a weighting function.
14. The method of claim 13 wherein, if one or more pilot signals are contained in the ith OFDM symbol, φ i corresponds to measured phase deviations of the one or more contained pilot signals but, if no pilot signals are contained in the ith OFDM symbol, φ i equals φ i−1 .
15. The method of claim 11 wherein the weighting function β is computed as:
if φ i corresponds to a new measurement of phase deviations of pilot signals contained in the ith OFDM symbol, then
β=C
where C is a specified constant, but, if no pilot signals are contained in the ith OFDM symbol so that φ i equals φ i−1 , then
β=αC
where α is a specified constant.
16. The method of claim 15 wherein the constants C and α are both numbers between 0 and 1.
17. The method of claim 11 wherein the measured phase deviation φ i of the one or more pilot signals contained in the ith OFDM symbol is computed as an average of individual phase deviations measured for each of the one or more pilot signals.
18. The method of claim 11 wherein the series of OFDM symbols are contained in a packet of specified length that includes a preamble at the beginning of the packet and further comprising performing initial time and frequency synchronization based upon training fields in the preamble.
19. A machine-readable non-transitory medium containing instructions that, when executed, cause a machine to carry out functions performed by a processing circuitry as recited by claim 11 .