Carrier Frequency Offset and Symbol Timing Recovery for DSP-based Wireless OFDM Systems
Date
2002
Authors
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ORCID
Type
Degree Level
Masters
Abstract
Orthogonal frequency division multiplexing (OFDM) is gaining widespread use in areas such as digital audio broadcasting (DAB), digital video broadcasting (DVB), and wireless networks. One major issue which must be addressed in the design of OFDM systems is the accurate estimation and correction of carrier frequency offsets and symbol timing offsets at the receiver. Carrier frequency offsets within 1% to 2% of the subcarrier frequency spacing may result in as much as a 3 dB degradation in the effective signal to noise ratio due to the increase of interchannel interference. Additionally, large symbol timing offsets may lead to interference from an adjacent OFDM symbol as well as interference between the individual subcarriers, which also results in a degradation in the effective signal to noise ratio.
This thesis investigates a technique for jointly estimating the carrier frequency offsets and symbol timing offsets at the OFDM receiver. The proposed technique is targeted for OFDM systems which transmit information asynchronously (i.e., packet-based or bursty transmissions). This means that synchronization is found as quickly as possible using information extracted from the data symbols. The proposed joint estimator is based on an existing joint maximum-likelihood estimator, where a reference symbol with a lengthened cyclic prefix is used to improve the symbol timing accuracy in dispersive multipath channels. Additional modifications are made to the original symbol timing estimator to ensure that it will not require an automatic gain control at the receiver input for proper operation.
An OFDM communications system utilizing the proposed joint synchronization technique is designed and simulated 'using Simulink. Performance of the modified symbol timing estimator is compared to the original maximum-likelihood timing estimator. It is found that the modified symbol timing estimator performs only slightly better than the original maximum-likelihood timing estimator in single-path channels. However, the performance of the modified joint estimator in dispersive multipath channels showed a more significant improvement over the joint maximum-likelihood estimator. The bit error rate of the OFDM system utilizing the modified joint estimator is also evaluated for a single-path channel, a Rayleigh fading multi-path channel, and two Ricean fading multipath channels.
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Degree
Master of Science (M.Sc.)
Department
Electrical Engineering