博 士 論 文 概 要

早稲田大学大学院国際情報通信研究科

博 士 論 文 概 要

論 文 題 目

Studies on Four Single Sideband Modulation and Demodulation Scheme

in OFDM Wireless Systems

OFDM

4SSB

申 請 者

江 奕

JIANG Yi

国際情報通信学専攻 ワイヤレス システム研究Ⅱ

2013 年 06 月

Studies on Four Single Sideband Modulation and Demdulation Scheme in OFDM Wireless Systems

by

Yi JIANG

The worldwide steep increase in the recent years of the usage of smartphones, WiFi-enabled laptops and other connected devices has pushed ups the demand for higher wireless data throughput. However, all developed wireless technologies so far have proved unable to satisfy the resulting high traffic demand, thus leading to the need to replace frequently current infrastructures, may it be cellular systems or others, by more powerful ones and to the rapid successive developments of several wireless standards (IEEE 802.11, 3GPP, etc.). However, frequent replacements of infrastructures as well as protocol updates are costly. Consequently, several research projects in companies and academic activities have been focusing on designing and building new wireless technologies able to cope better with the throughput requirements expected in the coming twenty years, while reusing as much as possible current infrastructures. Also, given that the frequency spectrum is already congested for the portion which concerns wireless mobile applications, new technology developments have focused for increasing the spectral efficiency of wireless systems, that is, improving the achievable throughput on a given fixed bandwidth.

One such important breakthrough was the invention of practical Orthogonal Frequency Division Multiplexing (OFDM) transceivers using the Fast-Fourier Transform (FFT) algorithm. This enabled to divide by almost two the bandwidth necessary to transmit a given signal and also dramatically simplified the otherwise complex task of estimating and equalizing the effects of multi-path (that is, frequency-selective) channels. In parallel, many other new techniques have appeared in the research literature and are slowly making their way into practical wireless system implementations. Among them, the class of turbo techniques for channel equalization, originally derived from the turbo-coding technique, has proven to be extremely effective at recovering highly distorted signals. However, note that most of the time, significant innovations have actually come from giving a fresh look at old ideas, as was the case regarding the development of Low-Density Parity Check (LDPC) codes ¥cite{LDPC}.

Indeed, it is critical to not only explore totally new concepts in order to build new technology, but also constantly explore old, sometimes forgotten ideas which were deemed impractical in the past.

In this dissertation, we revisit an old concept, namely, Single-Sideband Modulation, in the light of all the new powerful transmission techniques developed until now, in order to design new, more powerful, transmission techniques.

Single Sideband Modulation, as its name indicates, suppresses one side of the spectrum of a signal, without altering the information it contains, hence its attractiveness with regard to spectral efficiency.

A single sideband modulated signal is obtained through the application of a specific filter to any conventional signal. In the continuous time case, this is equivalent to a convolution with a certain function. The key component of the process is the so-called Hilbert Transform (which performs a convolution on the input signal), invented by David Hilbert in the beginning of the 20th century.

The main problem of single sideband modulation is that, when applied to complex signals, strong Inter-Symbol-Interference renders very challenging the demodulation process of the original signal.

Hence the needs for powerful decoding techniques enabling to harness the spectral efficiency gain of single sideband modulation.

This is where powerful turbo-equalization techniques could possibly be used to enable such demodulation. By enabling iterative extrinsic information exchange between equalizer and decoder, turbo-equalization techniques provide a significant performance gain with regard to the demodulation of signals going through an Inter-Symbol Interference channel. Extrinsic information refers to the notion that each component of the receiver exploits its own knowledge of the signal structure to improve the decoded output signal of the other receiver component. Iterating this process in a loop creates the turbo effect giving its name to the technique.

In this dissertation, we explore the applications opened by the use of recent wireless techniques, notably turbo-equalization, when applied to the demodulation of Single Sideband modulated signals.

The first contribution we make in this dissertation is to develop a demodulation technique for the spectrally efficient modulation "Four elements Single Sideband Modulation" (4-SSB). 4-SSB modulation was proposed in previous works but no viable demodulation technique had been proposed until now. We also propose a design of 4-SSB modulation for transmission over OFDM, which was not made previously in the literature. Computer simulation results show that 4-SSB modulation over OFDM can perform as well as conventional transmission techniques in terms of Bit-Error Rate, while having twice higher throughput over the same bandwidth. Our second contribution is the proposal of a new transmission technique called "Pilot-Single Sideband Modulation" (P-SSB modulation), which achieves the feat of rendering the signal transmitted extremely resilient to multi-path channel effects.

These two techniques offer a trade-off between spectral efficiency and transmission robustness against channel impairments, hence providing a new set of tools for the design of future wireless systems.

Moreover, both techniques are applied to the popular OFDM transmission technique, which make them readily practical.

In more details, 4-SSB modulation consists in transmitting over the same carrier two single-sideband signals. The contributions of this dissertation are as follows regarding 4-SSB modulation. Firstly, we analyze the feasibility of such modulation from a technical viewpoint by studying the influence of digital finite-impulse response Hilbert Transform filters on performances.

Then, we develop a transceiver architecture based on turbo-equalization techniques enabling to

demodulate 4-SSB signals. Finally, we enhanced the proposed architecture by incorporating a special inter-symbol-interference cancelling filter into the turbo-structure of the receiver.

Regarding P-SSB modulation, it consists in transmitting simultaneously a pilot-signal and a data signal using the previously developed 4-SSB modulation. The pilot signal is used to estimate the channel. By co-locating pilot and data signals, the need for specific pilot-dedicated symbols is removed, hence the potential for greatly improving channel estimation accuracy in any channel conditions. In this dissertation, we present the transceiver architecture and analyze the performances of P-SSB modulation over OFDM compared to conventional transmission techniques. We analyze the influence of different environment parameters such as the Doppler spread and the root-mean-square delay spread on throughput performances. Simulation results show that P-SSB modulation exhibits unique characteristics making it an attractive technique for transmission over highly distorting time-varying channels, where other transmission techniques usually fail.

In the remainder, we first introduce the concepts necessary to the understanding of wireless transmissions over OFDM (that is, channel modeling, OFDM basics and channel estimation) and previous works on Single Sideband Modulation (Chapter 1 and Chapter 2), then we present our main contribution, that is, 4-SSB signal demodulation, technical analysis of 4-SSB modulation over OFDM and enhanced receiver design (Chapter 3). To this, follows the treatment of P-SSB modulation in which we provide a transceiver structure and thorough performances comparisons with conventional transmission schemes (Chapter 4). Finally, a conclusion summarizes this dissertation and explores future research directions.