Umut Yunus

称号及び氏名博士（工学）

学位授与の日付

２０１０年３月３１日

論文

名「Analog Impairments Compensation in OFDM

Direct-Conversion

Receiver (ダイレクトコンバージョンOFDM 受信機におけるアナログ損失補償)」
論文審査委員

主査

山下勝己

副査

勝山豊

副査

大橋正治

論文要旨
The history of wireless communications began in 1886, when Hertz generated
electromagnetic waves. Based on this, the radio communication by Morse code acrossthe Atlantic Ocean had been established in 1901. More than one century has passed,
since then. Today, telecommunication has become so indispensable that we almost
cannot imagine our lives without it.

From satellite transmission, radio and television

broadcasting to the now ubiquitous mobile telephone, wireless communications have
revolutionized human society. In contrast to the poor information-carrying ability of theearly telecommunication system, instantaneous transferring of a large number ofinformation, such as multimedia information, becomes possible.
Under the increasing demand to high-speed, high-spectral efficiency of radiocommunication, the digital-communication employing orthogonal frequency divisionmultiplexing (OFDM) had emerged. The origin of OFDM started in 1966, when thestructure of OFDM was proposed, where the concept of using orthogonal overlapping
multi-tone signals was given. At that time, although the theory of OFDM was welldeveloped, the implementation of OFDM systems still had some difficulties due to thehardware. In 1971, the idea of using a discrete Fourier transform (DFT) to perform
baseband modulation and demodulation of the signals was introduced. This presented
an opportunity of an easy implementation of OFDM, especially with the use of fast
Fourier transform (FFT). In 1980, to combat mutipath fading, the cyclic prefix (CP) wasintroduced. Then, communication of OFDM had become possible according to thedevelopment of digital signal processor (DSP) circuit. As a result, in 1987, OFDM wasused in the European digital audio broadcasting (DAB) standard. Nowadays, OFDM iswell-known modulation scheme, which has been adopted in many wirelesscommunication systems such as DAB, digital video broadcasting (DVB), and IEEE
802.11 wireless local area network (WLAN).
OFDM is well-known by its high spectral efficiency, high-data rate, and robustness
against multi-path fading channel. The main drawback of OFDM systems is it's
sensitivity to carrier frequency offset (CFO). Orthogonality among subcarriers is thefundamental of OFDM systems. CFO, mainly caused by frequency mismatch between
the transmitter and receiver local oscillators (LOs). While CFO occurs, the spectrum of
the received signal will be shifted. This will destroy the required orthogonality andresult in severe performance degradation. Therefore, the estimation/compensation ofCFO is very crucial in OFDM systems. Conventionally, the CFO can be estimated easily
from the autocorrelation of periodic pilot (PP).
On the other hand, according to the receiver architecture, the receivers can beclassified to the superheterodyne receiver and direct-conversion receiver (DCR). The
superheterodyne principle was introduced in 1918 by the U.S. Army major Edwin
Armstrong in France during World War I, which is generally thought to be the receiverof choice owing to its high selectivity and sensitivity. About 98% of radio receivers use
this architecture. In a superheterodyne receiver, the input signal is first converted by an
offset-frequency local oscillator to a lower intermediate frequency (IF), andsubstantially amplified in a tuned IF. Direct-conversion receiver, also known as
homodyne, synchrodyne, or zero-IF receiver was developed in 1932 by a team of British
scientists searching for a method to surpass the superheterodyne. DCR is a radio
receiver that demodulates the incoming signal by a local oscillator signal synchronizedin frequency to the carrier of the wanted signal. Therefore, the wanted modulation
signal is obtained immediately by low-pass filtering, without further detection. Thus a
direct-conversion receiver requires only a single stage of detection and filtering. In
recent years, direct-conversion receiver (DCR) has attracted a lot of attention, for its
smaller size and lower cost over the traditional superheterodyne receivers. However, theprice is the additional disturbances, such as DC offset (DCO), and I/Q (in-phase andquadrature-phase) imbalance.
DCO is induced by the self-mixing associated with the imperfect isolation and is
known as the most serious problem of DCR. The I/Q imbalance is caused by themismatched components between the in-phase (I) and quadrature-phase (Q) branches,i.e., is basically any mismatch between the I and Q branches from the ideal case.

Therefore, the estimation/compensation of CFO in the presence of I/Q imbalance andthe DCO is a critical problem in an OFDM DCR. In the absence of DCO, the CFO can be
estimated easily from the autocorrelation of periodic pilot (PP). Considering DCO,
several joint compensation schemes have been proposed. Some works discussed theCFO estimators in the presence of DCO and of I/Q imbalance. However, all of theseworks treated the DCO as time-invariant. In practice, automatic gain control (AGC) isusually used to keep the received signal amplitude proper fixed level. Also, a high passfilter (HPF) is often employed in the DCR to reduce DCO. Therefore, the gain shift in
the low noise amplifier (LNA) will cause a TV-DCO, whose high frequency components
may pass through the HPF. As a result, the ordinary CFO estimation will be corrupted
by the residual TV-DCO. Until now, there is only one CFO estimator taking the residualTV-DCO into account. In this scheme, a differential filter is used to eliminate theresidualTV-DCO,

andthentheCFO

isestimatedbytheconventional

autocorrelation-based method. Since the differential filter increases the noise varianceand the residual TV-DCO cannot be eliminated completely, this method will cause
performance loss.
On the other hand, the one reason of that OFDM has been selected for high-speed
communication systems, is its good performance in multipath channels. In order to fightmultipath, the cyclic prefix (CP), which is also referred to as guard interval (GI), isinserted between symbols. While the length of GI is longer than channel delay spread, it
can protect received signal from inter-symbol interference (ISI). However, the price isthe transmission speed. In order to increase the transmission speed, several attempts
have been made to the cancellation of ISI and ICI (inter-carrier interference) in OFDM
systems with insufficient GI. Since GI insertion reduces transmission efficiency, someworks discussed OFDM systems without GI. In the absence of GI, eliminating ISI
between adjacent symbols is a crucial problem. A method was introduced to cancel
interference in OFDM systems without GI. In this work, it was proposed to compensate
ISI distortion by using null subcarriers, i.e., inactive subcarriers, which is usually being
placed at the edges of the frequency band and the DC to avoid aliasing and easetransmit filtering. In another work, an iteration method was proposed to eliminate theISI and ICI. However, the iteration increases the complexity of computing and also
results in error propagation, specially for high-degree constellation.
The main components of this thesis and the major results of the presented work canbe summarized as follows:
Chapter 1 introduces the history of wireless communication, briefly explains theimpairments of OFDM systems with direct-conversion receiver, and gives an overview

of this thesis.
Chapter 2 describes the mathematical model of baseband OFDM systems, thenexplains the reason of ISI and the impairments such as CFO, DCO, and I/Q imbalance,
where also gives their mathematical model by matrix form.
Chapter 3 investigates CFO estimation in OFDM-DCR in the presence oftime-varying DCO. Until now, there is only one CFO estimator taking the residual
TV-DCO into account. In this scheme, a differential filter is used to eliminate theresidualTV-DCO,

andthentheCFO

isestimatedbytheconventional

autocorrelation-based method. Since the differential filter increases the noise varianceand the residual TV-DCO cannot be eliminated completely, this method will cause
performance loss. We develop a novel CFO estimation method in the presence ofTV-DCO. It was shown the residual DCO after high-pass filtering varies in a linearfashion. Based on this observation, we model the residual DCO using a linear function.
Then, from the periodicity of the training sequence, we derive a CFO estimator inclosed-form.
Chapter 4 focuses on the joint estimator of CFO and I/Q imbalance in the presence ofTV-DCO. To the best of our knowledge, until now, only a few works considered thescenario of the coexistence of CFO, I/Q imbalance, and TV-DCO. The idea is to employ adifferential filter to ease the effect of the TV-DCO, and then estimate the CFO by theconventional autocorrelation-based method. However, the differential filter not only
fails to completely eliminate the TV-DCO, but also enhances the noise. Also, the CFO
estimation is severely biased by the remaining uncompensated I/Q imbalance. Moreover,in this method, only a CFO estimator was proposed and how to estimate/compensate theI/Q imbalance in this situation was not mentioned. We develop a novel joint estimation
method for CFO and I/Q imbalance in the presence of TV-DCO, where similarly weapproximate TV-DCO by linear function. From the periodicity of the pilot, we derive alow-complexity estimator, which can obtain the necessary estimates in closed-form.
Chapter 5 develops a novel ISI cancellation method for OFDM systems without GI.
Since GI insertion reduces transmission efficiency, beside the common GI insertion,some studies discussed the OFDM systems without GI. In these methods, an iteration
method was proposed to eliminate the ISI and ICI. However, the iteration increases thecomplexity of computing and also results in error propagation, specially for high-degree
constellation. To obtain a good performance with the low-complexity, a novel ISIcancellation method is developed. While the channel is minimum phase, we derive anovel ISI cancellation method, by using the channel information and signal achieved at
the tail part of OFDM symbols after frequency domain equalization (FDE). For thechannel which is not minimum-phase, the proposed method is also applicable afterconverting the channel into minimum-phase by using a proper filter.
Chapter 6 concludes this thesis and gives some topics for future research.

審査結果の要旨
本論文は、ダイレクトコンバージョン OFDM 受信機(OFDM-DCR)における時変直流オフセット(TV-DCO)下でのアナログ損失補償、すなわち、搬送波周波数オフセットおよびI/Q 不均衡を補正するための新たな技術を提案すると共に、高速通信を目指したガードインターバルを用いない OFDM システムの新たな技術を提案するものであり、以下の成果を得ている。
(１) OFDM-DCR において TV-DCO が生じた際に、搬送波周波数オフセットを推定・補正するため、時間領域での TV-DCO を線形近似し、搬送波周波数オフセットに対する解析解を導出している。
(２) OFDM-DCR において TV-DCO が生じた際に、搬送波周波数オフセットおよび I/Q不均衡を補償するため、TV-DCO の線形特性を利用した搬送波周波数オフセットおよび I/Q 不均衡補正の新たなジョイント法を提案し、その有効性を検証している。
(３) OFDM システムでは、シンボル間干渉を除去するためガードインターバルを各シンボル毎に挿入しているが、ガードインターバルを利用せず、かつシンボル間干渉を引き起こさない OFDM システムの新たな技術を提案し、伝送効率の改善を図っている。
以上の諸成果は、次世代移動通信システムに求められる伝送特性および伝送効率改善の実用化に向けた基礎的な知見や基盤を与えるものであり、この分野の技術の発展に貢献するところ大である。また、申請者が自立して研究活動を行うに十分な能力と学識を有することを証したものである。学位論文審査委員会は、本論文の審査ならびに最終試験の結果から、博士（工学）の学位を授与することを適当と認める。