Estimation and Compensation of Front-end Nonidealities in OFDM Systems
Date Issued
2009
Date
2009
Author(s)
Chung, Yuan-Hwui
Abstract
Recently, the direct-conversion transceiver for OFDM systems have received a lot of attention because of its low cost implementation and low power consumption. Nonetheless, the direct-conversion receiver may suffer serious performance loss because of the front-end mismatches, such as in-phase and quadrature phase (I/Q) imbalances, carrierrequency offset (CFO), etc. [5]. The I/Q imbalances are due to the amplitude and phase mismatches of the local oscillator, the mismatched low pass ?lters (LPFs) and the mismatched digital-to-analog (D/C) converters or analog-to-digital (C/D) converters. CFO is due to the mismatch of carrier frequency at the transmitter and receiver. It is shown [6][7] the I/Q imbalances and CFO destroy the subcarrier orthogonality of OFDM systems, and hence may degrade the performance of OFDM systems seriously. It is an important issue to estimate and compensate these mismatches using training sequences. In this dissertation, we study the joint estimation of transmitter, receiver I/Q imbalances, CFO and channel responses using training sequences for OFDM systems. The estimation problems are divided into two types by categorizing the types of the I/Qmbalances as below:i) Joint estimation of frequency independent I/Q, CFO and channel responses.ii) Joint estimation of frequency selective I/Q, CFO and channel responses.o solve the two joint estimation problems, we introduce a new concept called channel residual energy (CRE). By minimizing the CRE, we can jointly estimate the I/Q and CFO parameters without knowing the channel response. Once the I/Q and CFO parameters are obtained, the channel response can be obtained by simple substitution. The proposed method needs only OFDM block for training. Moreover there is no restriction on the training data. Any modulation symbols with constant modulus can be used for training. When the I/Q imbalance is frequency independent, the problem can be solved using a non-iterative method. If two repeated blocks are available for training, we propose a low complexity two-step approach to solve the joint estimation problem. Both the proposed methods can provide a good performance close to the Cramer-Rao bound. When the I/Q imbalance is frequency selective, we propose an iterative method to solve the joint estimation problem. For both the frequency independent and frequency selective I/Q imbalances, the proposed methods can provide a BER performance close to the ideal case where all the I/Q parameters, CFO and channel responses are known at the receiver. In addition, we also extend the proposed methods to the case of multipleinput multiple-output (MIMO) OFDM systems where there are multiple antennas at the transmitter and receiver.
Subjects
OFDM
I/Q Imbalance
CFO
Channel Response
Type
thesis
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