Carrier Frequency Offset Detection by Applying Hierarchical Modulations for OFDM Communications
Date Issued
2009
Date
2009
Author(s)
Hsiao, Yu-Lin
Abstract
ABSTRACTs a result of its high spectral efficiency and multipath immunity, orthogonal frequency-division multiplexing(OFDM ) has considered one of the major modulation schemes for high-speed communication systems. OFDM systems are expected to operate at high carrier frequencies, high speed, and high throughput for mobile users. However, it is well known that OFDM systems are very sensitive to local oscillator carrier-frequency offset, local oscillator phase noise, time synchronization error, and Doppler frequency spread, since these factors can result in the loss of the orthogonality between sub-carriers and create inter-carrier interference (ICI). In this essay, we exploit hierarchical 64-QAM modulations for acquiring the information of carrier frequency offset.ierarchical constellations offer a different degree of protection to the transmitted messages according to their relative importance. In this thesis, it is observed that the bit error rate and error floor of each level caused by additive white Gaussian noise (AWGN) and inter carrier interference (ICI) are different. Therefore, we estimate the degree of Doppler effect by applying the hierarchical modulations.n this thesis, we proposed two novel designs for the carrier frequency offset (CFO) estimation. In the first design, we exploit the hierarchical 64-QAM to modulate the pilot signals at transmitter in the OFDM system. Based on the factor of different error floor level at different carrier frequency offset among each level bit, the carrier frequency offset resulting from the Doppler effect can be estimated at the receiver. The simulation results show that the BER of each level is higher than the BER of theoretical analysis. This result makes the estimation of CFO is higher than accurate CFO. In the second design, we use the hierarchical 64-QAM modulation to modulate data signals at the transmitter. It is because data signals can estimate CFO more promptly than pilot signals. Considering the tradeoff between quality of service(QOS) of data signals and information of carrier frequency offset, we design four hierarchical 64-QAM modulations in accordance with the different channel qualities. We expect that this design not only can estimate carrier frequency offset (CFO) when the channel is bad, but satisfies the quality of service.
Subjects
OFDM
Doppler effect
Hierarchical modulation
ICI
CFO
Type
thesis
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