Dongguo Li

称号及び氏名博士（工学）

学位授与の日付

２００７年３月３１日

論文「Studies on Channel Estimation and Equalization for OFDM

名Systems in Mobile Environment (OFDM 移動通信システムを対象としたチャネル推定および等化技術に関する研究)」
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

Since Guglielmo Marconi sent and received his first electromagnetic signal acrossthe sea in 1896, the world has entered into a wireless communication age. Today,
wireless communication has become an indispensable element in our daily lives,and been essentially fully digitalized by the end of last century. In contrast to thepoor information-carrying ability of the initial telecommunication system, thestate-of-the-art
information.

digitalFor

communication

instance,technology

multimedia

makes

information

canhugeamount ofbetransferredinstantaneously. In the past decade, requirements for speed and reliability of
information transmission have increased very quickly and certainly they will keep
growing in the coming future. This ever increasing demand is the major driving
force for the development of the communication technology, which pushes
researchers unceasingly to discover new methods to overcome various technicalobstacles in the communication systems.
Orthogonal frequency division multiplexing (OFDM) is such a technology thatallows a large amount of data to be transmitted quickly and reliably, with aminimum of loss or interference. Comparing with the traditional communication
method of limited capability and weakness in noise, OFDM is considered today to bea reliable choice for high bit-rate transmissions and is now widely adopted andtested in many communication systems. Specifically, OFDM has been chosen for
digital audio and video broadcasting, for high-speed modems over twisted pairs as

xDSL, and, more recently, for broadband wireless local area networks (HIPERLAN/
2, IEEE802.11a/g and UWB standards).
OFDM enables very simple equalization of frequency-selective finite impulse
response (FIR) channels through the implementation of the inverse fast Fouriertransform (IFFT) and the insertion of a guard interval (GI) of length which should
be larger than the channel memory at the transmitter. Presented in each block oftransmitted symbols, the GI consists of redundant symbols circularly replicatedfrom the samples after IFFT implementing. At the receiver end, GI is discarded toavoid inter symbol interference (ISI), and the fast Fourier transform (FFT) of eachtruncated block is then implemented. A combination of IFFT and GI at thetransmitter with the FFT at the receiver converts the frequency-selective channel
into parallel flat-faded subchannels, each corresponding to a different subcarrier.
Unless they are zero, the flat fading can be simply compensated by dividing each
subchannel output with the channel attenuation at the corresponding subcarrier.
Since the channel is usually unknown, the succeeding research is carried out withthe assistance of pilot sequence, or called training sequence, which is known at thereceiver and is transmitted before the information-bearing signal sequence beingsent. It has to be noted that an equivalent problem to channel equalization ischannel identification, since pilot sequence can be used to identify the channel first,
then the equalizer is designed in accordance with the obtained channel information.
Obviously, channel estimator based equalization approaches have the disadvantageof excessive elaboration and costly hardware requirement.
The conventional pilot-aided channel estimation and equalization method cansolve many problems in static communication, however, in a ubiquitous society,people request contact with others through communication network at any time andanywhere. That means, under any conditions, the communication system shouldensure the fluency of data transmission. In this thesis, we focus our interesting onchannel estimation and equalization in mobile wireless OFDM systems. Different
from the stationary communication system, the Doppler shift, especially in highspeed mobile communication systems, becomes the main barrier in improving thesystem transmission performance. As the channel is fading in time domain, eachsubcarrier of the multicarrier signal is convoluted by the channel. Thus thebandwidth of each subcarrier is expanded and its power leaks on others. That is theso called inter carrier interference (ICI) which deteriorates the system performance
significantly. Hence the information of the time-varying channel is necessary.

With the aim to eliminate the ICI, time domain-pilot is thought as a simple andrealizable method as it can compensate the time fading straightly. On the otherhand, the pilot-free method can equalize the channel without bandwidth loss, which
presents its advantage especially in high speed bit rate systems. MIMO has recently
emerged as one of the most significant technical breakthroughs in modern
communications as it is expected to solve the bottleneck of traffic capacity andenhance anti-interference feature of the system. Along these lines, the main
components of this thesis and the major results of the presented work can besummarized as follows:
Chapter 1 introduces the history of the channel estimation and equalization
research, and provides an overview of the thesis.
Chapter 2 introduces the frame form of OFDM signal and establishes amathematical model for a baseband OFDM system, which is the foundation of thisthesis.
Chapter 3 analyzes the multipath channel feature in mobile communication
circumstance. It can be cognized that the frequency spectrum of the channel fadingenvelope (CFE) is limited in the maximum Doppler shift and each of the paths isindependent. That means each path of the CFE is continuous curve in time domain
and the fading degree is in direct proportion to the velocity of the receiver. Thus theCFE can be expanded in Taylor series. Utilizing the mathematical model obtainedin chapter 2, the channel parameters, i.e. the coefficients of the Taylor series, intime domain, is used to establish a system transfer relation with the transmitted
and received data which expressed in frequency domain. Therefore adjusting thevalue of received data after hard decision, the channel estimates can be changed.
This conclusion will help us to obtain the optimal channel estimates in later.
Chapter 4 proposes serious fashions in channel estimation and introduces amethod to decrease the computation complexity. Since the CFE is continuous curve,
if any samples on the CFE are known, the whole CFE can be reconstructed so longas those samples meet the Nyquist theory.
Therefore, we designed a new pilot symbol which inserts between the data
symbols in time domain (TDP) with suitability density. The pilot symbol is set in
the place which keeps the distance with the data symbol to ensure that it not be
polluted. Through this form, the channel impulse response (CIR) can be obtainedcorresponding pilot time slot. Here, we introduce the linear fashion, spline function
and low pass filter (LPF) method to approximate the truth channel with different
velocities to tradeoff the complexity and the estimation accurately. Comparing withthe traditional block-type pilot or comb-type pilot technique, our proposed method
has eliminated the ICI completely, which obtain an accepted performance whateverin the long path delay channel or high velocity receive, while the traditional method
almost cannot work at all. However, even the proposed TDP method is more flexible
and accurate in time varying channel estimates than the conventional pilot-aided
technique, the system transmission efficiency is restricted with the length of thechannel time delay.
Chapter 5 proposes 2 sorts of novel channel estimation and equalization methods
to acquire more efficacy of the bandwidth. One of the methods is founded on therelation expression of the channel parameters in time domain and thetransmitted¥&received data in frequency domain. Assuming the channel during
the pre-symbol period was estimated perfectly, the linear extrapolation was applied
to estimate the transmitted data roughly first. The error was amended through thehard decision. The recovered data from the decision was reused in new estimates of
the channel parameter. Thus the new recovered data was obtained once more.
Applying the algorithm iteratively as this flow, the optimal result could be obtained
through comparing covariance of the input and output in the decision. The otherpilot-free method utilized the GI, which would be cut and thrown before FFT in
traditional receiver, to obtain the CIR during both ends of the symbol. This time, we
just inserted a GI length zero sequence between every two symbols in time domain.
Thus the conjunctive symbol would not overlap each other. We also assumed thechannel during the pre-symbol period was estimated perfectly. Applying the linear
extrapolation of the pre-symbol to recover the samples of the GI in time domain, theestimated samples could be used in estimating the channel which corresponding
with the end GI samples. Thereby, the whole channel could be reconstructed bylinear interpolation simply. Comparing with the pilot-aided technique and the blind
equalizer, our proposed method obtained more effective bandwidth and fewer
computational amount with simpler system structure and practicable. Although theproposed GI based method is not as good as the iterative algorithm method because
the former one needed zero padded which accounts for the bandwidth loss, it wasmore practical due to its lighter computational burden. It can be found that theproposed methods are applicable even when the CFE is varying during one OFDM
symbol without the bandwidth-consuming pilot. Moreover, the statistics of thetransmitted data and channel is not necessary, which simplifies the systemconstruct.

Chapter 6 applies the TDP method in MIMO-OFDM system to acquire higher
bandwidth efficiency introduced. For the MIMO-OFDM system in doubly-selectivechannels, we propose a stagger time domain pilot structure, which eliminates thecross interference in the pilot section and makes the CFE estimation methods in
OFDM system be applied into MIMO-OFDM system straightforwardly. Then, theobtained channel estimates are transferred into frequency domain to help thecascade equalizer to suppress the ICI and cross interference, thus the transmitted
data can be recovered. In contrast to the conventional MMSE equalizer, which deals
with the huge system transfer matrix directly, we propose a downsizing MMSE
equalizer, which first separates the MIMO-OFDM system into several subsystems
by small transfer matrix with optimal size, then set up a MMSE estimator for eachsubsystem and finally combines their results as the equalizer output. Since theproposed equalizer takes the advantage of underlying structure of original systemtransfer matrix sufficiently, it can achieve the satisfied approximation to theconventional MMSE equalizer with significantly reduced computational complexity.
Our simulation results demonstrate the validity of the proposed stagger pilot
structure and the downsizing MMSE equalizer.
Chapter 7 makes a conclusion for the research and gives some topics for futureresearch.

審査結果の要旨
本論文は、OFDM（Orthogonal Frequency Division Multiplexing）移動通信システ
ムにおいて、高速移動時に生じる搬送波間干渉を軽減するための新たなチャネル推定および等化技術を提案したものである。得られた主な結果は、以下の項目に要約できる。
(１) OFDM シンボル内で変化するチャネル特性を推定するため、時間領域で挿入する新たなパイロット構造を提案すると共に、同パイロット信号を用いてチャネル特性を推定し得る LPF 法およびスプライン関数法を提案している。提案手法では、実際のチャネル特性に近いチャネル推定値が得られ，また、良好なビット誤り率特性が得られている。
(２) 伝送効率を高めるため、パイロット信号を利用しないブラインド等化技術が報告されている。しかし、同手法は、大きな処理遅延が生じるため実用化には不向きである。
提案手法では、初期シンボルでのチャネル特性のみを利用したセミブラインド手法を提案し、若干の処理遅延により良好な伝送特性が得られることを、計算機シミュレーションにより検証している。

(３) 送受信側に複数のアンテナ素子を設けた MIMO-OFDM（Multi-Input Multi-Output
OFDM）通信システムを対象モデルに、シンボル内で変化するチャネル特性を推定するための新たなパイロット構造を提案すると共に、チャネル特性を導出する際に必要とする、計算量の軽減化を図ることができる新たに近似手法を提案している。
以上の諸成果は、次世代移動通信システムに求められる高速移動領域で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。