Carrier Frequency Offset Estimation by Applying Hierarchical Modulations for MIMO-OFDM System in High Mobility Communication
Date Issued
2010
Date
2010
Author(s)
Li, Wen-Ching
Abstract
In this thesis, we address the estimation of carrier frequency offset (CFO) in the orthogonal frequency division multiplexing (OFDM) based on multiple input multiple output (MIMO) communication system. The basic principle of OFDM is to convert a wideband frequency-selective channel into numerous narrow-band flat-fading subchannels. On the other hand, MIMO techniques boost the performance under flat-fading channels for wireless communications significantly. By combining the features of OFDM and MIMO, the MIMO-OFDM techniques can achieve better performance in high data rate wireless communication systems. However, OFDM is sensitive to inter carrier interference (ICI) which destroys the subcarriers’ orthogonality. In addition, the Doppler effect will make ICI more severe in high mobility communication. Therefore, we apply the hierarchical modulation to estimate CFO in MIMO-OFDM system for high mobility communication.
Hierarchical 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), ICI, and Rayleigh fading are different. In the single-input single-output (SISO) system, the error floor will be very close in some range and it is difficult to estimate CFO. On the other hand, the MIMO system can not only separate the error floors of different levels but also can estimate CFO more accurately in high mobility systems.
In this thesis, we proposed the hierarchical 64-QAM to modulate the pilot signals at the transmitter in the MIMO-OFDM system. Based on the factor of different error floor level at different CFO among each level bit, and MIMO gain (diversity), the CFO resulting from the Doppler effect can be estimated more exactly at the receiver. Moreover, we establish two constrains (error floor and error close) in different MIMO domains (antennas), and find more MIMO domains which can estimate CFO more accurately. In the proposed system, we can estimate the CFO by lookup table to the subcarriers’ orthogonality caused by ICI. In addition, considering the tradeoff between the error degree of data signals and different MIMO domains of estimation CFO, we can construct an adaptive system to fulfill quality of service (QoS) of data by utilizing the number of the antennas.
Subjects
MIMO-OFDM
Doppler effect
Hierarchical modulation
ICI
CFO
Type
thesis
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