US 9,553,752 B1Granted

Method and apparatus for frequency offset detection in OFDM systems with frequency reuse

AssigneeMbit Wireless, Inc.

InventorsBhaskar Patel, Arumugam Govindswamy

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Abstract

Estimating and compensating the frequency offset between a transmitter and a receiver is essential for reliable communication in wireless communication systems. Before a client terminal synchronizes with a base station, the initial frequency offset may be high. The frequency offset estimation may need to be performed even without having time synchronization with a base station. In case of cellular systems multiple base stations may be using the same RF channel to transmit their signals. The signals from different base stations may be superimposed at the receiver of the client terminal. A method and apparatus are disclosed for frequency offset estimation in presence of the signals from multiple base stations and with unknown timing of the base station.

Claims20

Dependent claims

A method for frequency offset detection in a wireless communication system using a plurality of Orthogonal Frequency Division Multiplexing (OFDM) signals received at a wireless communication device, in which each OFDM signal includes a cyclic prefix (CP) and a symbol, the symbol of each OFDM signal has a same length and the OFDM signals are of a predetermined periodic interval, the method comprising:

controlling, by a processing device, for each of the OFDM signals received at the wireless communication device, determining a CP correlation value indicating an amount of phase rotation accumulated between the CP and a tail portion of the symbol over a duration of the symbol using a predetermined shortest CP length of the wireless communication system;
and
controlling, by the processing device:
determining an average power of the OFDM signals;
coherently combining the CP correlation values for the OFDM signals with stored second CP correlation values respectively for a plurality of second OFDM signals, which are offset from the OFDM signals modulo the predetermined periodic interval, to obtain combined CP correlation values;
non-coherently combining the average power of the OFDM signals with a stored second average power of the second plurality of OFDM signals to obtain a combined average power;
normalizing the combined CP correlation values with the combined average power to obtain normalized CP correlation values;
and
determining a fractional frequency offset from a final CP correlation value, in which the final CP correlation value is determined from coherently combining a maximum of an absolute value of the normalized CP correlation values and each additional peak of the normalized CP correlation values determined to satisfy first and second threshold conditions for the normalized CP correlation values.

6 more elements

An apparatus for frequency offset detection in a wireless communication system using a plurality of Orthogonal Frequency Division Multiplexing (OFDM) signals received at a wireless communication device, in which each OFDM signal includes a cyclic prefix (CP) and a symbol, the symbol of each OFDM signal has a same length and the OFDM signals are of a predetermined periodic interval, the apparatus comprising:

circuitry configured to, for each of the OFDM signals received at the wireless communication device, determine a CP correlation value indicating an amount of phase rotation accumulated between the CP and a tail portion of the symbol over a duration of the symbol using a predetermined shortest CP length of the wireless communication system;
and
wherein the circuitry is further configured to:
determine an average power of the OFDM signals;
coherently combine the CP correlation values for the OFDM signals with stored second CP correlation values respectively for a plurality of second OFDM signals, which are offset from the OFDM signals modulo the predetermined periodic interval, to obtain combined CP correlation values;
non-coherently combine the average power of the OFDM signals with a stored second average power of the second plurality of OFDM signals to obtain a combined average power;
normalize the combined CP correlation values with the combined average power to obtain normalized CP correlation values;
and
determine a fractional frequency offset from a final CP correlation value, in which the final CP correlation value is determined from coherently combining a maximum of an absolute value of the normalized CP correlation values and each additional peak of the normalized CP correlation values determined to satisfy first and second threshold conditions for the normalized CP correlation values.

6 more elements

A wireless communication device for detecting frequency offset in a wireless communication system, the device comprising:

a receiver to receive a plurality of Orthogonal Frequency Division Multiplexing (OFDM) signals, in which each OFDM signal includes a cyclic prefix (CP) and a symbol, the symbol of each OFDM signal has a same length and the OFDM signals are of a predetermined periodic interval;
and
circuitry configured to, for each of the OFDM signals, determine a CP correlation value indicating an amount of phase rotation accumulated between the CP and a tail portion of the symbol over a duration of the symbol using a predetermined shortest CP length of the wireless communication system,
wherein the circuitry is further configured to:
determine an average power of the OFDM signals;
coherently combine the CP correlation values for the OFDM signals with stored second CP correlation values respectively for a plurality of second OFDM signals, which are offset from the OFDM signals modulo the predetermined periodic interval, to obtain combined CP correlation values;
non-coherently combine the average power of the OFDM signals with a stored second average power of the second plurality of OFDM signals to obtain a combined average power;
normalize the combined CP correlation values with the combined average power to obtain normalized CP correlation values;
and
determine a fractional frequency offset from a final CP correlation value, in which the final CP correlation value is determined from coherently combining a maximum of an absolute value of the normalized CP correlation values and each additional peak of the normalized CP correlation values determined to satisfy first and second threshold conditions for the normalized CP correlation values.

7 more elements

Dependencies are inferred from the claim text.