156 results about "Residual frequency offset" patented technology

The invention relates to a novel methodology and apparatus for clock-offset compensation and common-phase offset correction in Frequency Division Multiplixing based wireless local area network (WLAN) environment, such as an Orthogonal Frequency Division Multiplexing (OFDM) environment. A curve fit, such as a threshold-based, least mean squares (LMS) fit of phase of the pilot sub-carriers in each OFDM symbol is used to estimate and counteract the rotation of the data sub-carriers due to residual frequency offset, low frequency phase noise, and clock offset. The invention is particularly well suited to wireless channels with multipath where pilots typically undergo frequency-selective fading. The thresholding LMS is implemented in a hardware-efficient manner, offering cost advantages over a weighted-LMS alternative. Additionally, the invention uses a unique phase-feedback architecture to eliminate the effects of phase wrapping, and avoid the need to refine channel estimates during packet reception.

An apparatus for compensating for a frequency offset and a channel variation, includes: a frequency offset compensation unit estimating the frequency offset of receptions signals received via reception ends based on a final metric value of the reception signals, and a compensator compensating for the frequency offset of the receptions signals based on the estimated frequency offset; Fast Fourier Transformers (FFTs) converting reception signals having the compensated frequency offset into frequency domain reception signals; and a frequency offset and channel variation compensation unit estimating channel coefficients of signals output from the FFTs by sub carriers, compensating for a residual frequency offset and a channel variation of the reception signals from the FFTs based on pilot signals and the estimated channel coefficients, and detecting signals transmitted from transmission ends based on the reception signals having the compensated residual frequency offset and channel variation and the estimated channel coefficients.

A method and apparatus for processing a data packet at a receiver. The method includes receiving a data packet having been previously transmitted by a transmitter over a channel, in which the data packet includes (i) a preamble portion having a plurality of orthogonal frequency division multiplexing (OFDM) training symbols, wherein each OFDM training symbol comprises a plurality of subcarriers, and (ii) a data portion. The method further includes, for each subcarrier, determining an estimate of the channel based on the plurality of OFDM training symbols in the preamble portion; compensating each estimate of the channel with a compensation factor having a first term and a second term, wherein (i) the first term varies linearly with respect to an index of the plurality of subcarriers and (ii) the second term is a constant term; and processing the data portion based on the compensated estimates of the channel.

The invention provides an OFDM-IM system frequency offset estimation method. The method comprises the steps of S1, using a two-step method to perform preliminary frequency offset compensation on a receiving signal that is subjected to non-uniform frequency offset: 1, resampling and down-converting; and 2, performing unified compensation on the residual frequency offset (epsilon), wherein the sum of energy for using null sub-carriers used in the sub-step 2 is used as a cost function, a preliminary estimated value of the epsilon is acquired via one-dimensional search, and preliminary compensation is performed; S2, estimating a non-activated sub-carrier position in an OFDM-IM system by using the preliminarily compensated signal; and S3, endowing each estimated sub-carrier with a certain weight, adding the energy of the estimated sub-carriers into the cost function according to the different weights, performing the one-dimensional search on the epsilon to acquire a final estimated value ofthe epsilon, and performing compensation once more. According to the method provided by the invention, the sub-carriers using the OFDM-IM are partially activated. It means that the energy of the non-activated sub-carriers can still be used besides the preliminarily set null sub-carriers. Therefore, the estimation of Doppler frequency shift is optimized by using the non-activated sub-carriers.

Improved performance, particularly gain in signal-to-noise ratio (SNR), in high-bandwidth Orthogonal Frequency Division Multiplexing (OFDM) receivers, software and systems is achieved by compensating channel estimates not only for carrier frequency offset and phase noise, but also for interference caused by sample phase jitter. At high bandwidths, e.g., using 256 quadrature amplitude modulation (QAM) symbol constellation, significant SNR improvement is gained by determining and applying a corresponding compensation to each data sub-carrier channel, the compensation preferably including a term that increases linearly with the sub-carrier index.

A method of determining a residual frequency offset between a transmitter and a receiver in a transmission of data via a communication channel, is described, wherein the message is transmitted from the transmitter to the receiver via the communication channel and the message comprises at least one short preamble (201), at least one long preamble (202) and user data (203). The at least one long preamble (202) comprises residual frequency offset determination information based on which the residual frequency offset is determined.