List of Tables
1.1 Background and motivation
The wireless (mobile) communication system took time to evolve to where it is today, changed drastically since Guglielmo Marconi first demonstrated radio’s ability to provide continuous contact with ships sailing the English channel. Nowadays, in a world of high mobility, transmission system speed and capacity are essential components to maintain communication and connectivity among remote people all around the world. The early digital communication systems were based on single carrier modulation. Generally, by reducing the symbol duration, we can increase the bit rate of a transmission system.
However, the presence of a multipath channel introduces intersymbol interference (ISI) requiring complex equalization. Multicarrier modulation (MCM) is a good alternative in order to counteract the multipath fading effects. In multicarrier communications the data is transmitted over many frequencies instead of a single carrier, dividing the wideband frequency selective communication channel into several subbands with mildly selective fading. The subcarriers are generally selected to be orthogonal, in such a way that their spectra overlap but without causing interference to the other subcarriers. The bandwidth of each subchannel is smaller than coherence bandwidth of the propagation channel. It means, the maximum delay spread of the channel is much lower than the
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Chapter 1. Introduction 2 symbol duration. Consequently, each subchannel experiences a flat fading and the ISI in each subchannel becomes very small.
Till date, Orthogonal Frequency Division Multiplexing (OFDM) is the most widespread MCM scheme. Nevertheless, OFDM suffers from loss of spectral efficiency due to cyclic prefix insertion. Moreover, it is highly sensitive to residual frequency and timing offsets that can be generated by a defective synchronization as well as the Doppler effect [1,2].
The use of a rectangular impulse response in OFDM causes large sidelobes at each sub-carrier. Thus, the subchannels at the edge of the transmission bandwidth could be a source of interference for other neighboring systems [33]. The severity of high peak-to-average power ratio (PAPR) in OFDM can be seen from the 3rdgeneration partnership project (3GPP) specification for 4th generation (4G) long term evolution (LTE); where in the physical layer uplink (UL), the single carrier-frequency division multiple access (SC-FDMA) has been chosen instead of OFDM, owing to the high PAPR of the later.
These drawbacks have motivated researchers to develop alternative solutions and some enhanced physical layers based on filter bank processing, have been proposed. Filter-bank based multi-carrier (FBMC) have a long history in telecommunication research.
FBMC uses a frequency well-localized pulse shaping for each subcarrier, which reduces significantly the (OOB) out-of-band leakage, thus a higher number of subcarriers can be used within the allocated frequency band. There are mainly three FBMC techniques that have been studied in the literature: filtered multitone (FMT) [34–36], offset quadra-ture amplitude modulation (QAM) [3] (also called staggered multitone (SMT) [37]), and cosine modulated multitone (CMT) [38,39]. This dissertation focuses on the Saltzberg’s scheme [3] called FBMC-OQAM (also called FBMC/OQAM, OFDM/OQAM or SMT).
An efficient discrete Fourier transform (DFT) implementation of this modulation method has been proposed by Hirosaki [40]. Fig. 1.1 shows the frequency response comparison between OFDM and FBMC-OQAM system comprising of prototype filter designed by Bellanger [41]. Saltzberg showed, in [3], that by introducing a shift of half the symbol period between the in-phase and quadrature components of QAM symbols, it is possible to achieve a baud-rate spacing between adjacent subcarrier channels and still recover the information symbols free of ISI and intercarrier interference (ICI). Thus, each sub-carrier is modulated with an offset QAM (OQAM) and the orthogonality conditions are considered only to the real field [7]. Indeed, the data at the receiver side is carried only by the real (or imaginary) components of the signal, and the imaginary (or real) parts appear as interference terms.
Chapter 1. Introduction 3
−4 −3 −2 −1 0 1 2 3 4
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized Frequency
Frequency response
FBMC−OQAM OFDM
Figure 1.1: Illustration of frequency responses of OFDM and FBMC-OQAM.
Nowadays, energy crisis and global warming problems are hanging over everyone’s head, urging much research work on energy saving. It is coming to a consensus that the infor-mation and communication technologies (ICT) industry has emerged as one of the major contributors to the CO2 emission and world power consumption. According to the In-ternational Telecommunications Union (ITU), carbon emission related to ICT is about 2% of the worldwide emissions, which is comparable to the emissions by all commercial airplanes or one quarter of the emissions by all vehicles around the world. But, telecom-munications usage is expected to expand rapidly over the coming decade; especially in developing countries. If nothing is done, the ICT contribution to global greenhouse gas emissions is projected to nearly double, i.e. about 4% by 2020. Putting an increasing emphasis on green communications, one of the main agendas of the ITU by 2020, is to cut short the carbon emissions per device by 30% [4].
Also, energy consumption has become a key challenge in the last few years. Accord-ing to several studies [4, 5], the ICT sector alone is responsible of a percentage which varies widely from 2% to 10% of the worldwide energy consumption and several projects, both from industries and universities, are trying to reduce the power consumption of electronic devices. The power consumption helps to define the battery life for mobile
Chapter 1. Introduction 4 communications systems [42].
Power amplifiers are one of the most expensive and most power-consuming devices in the communication systems. Power amplifier (PA) represents more than 60% of the total power consumption at transmitter [6]. Thus, it is imperative to improve the amplifier efficiency for reducing the energy consumption in the communication systems. For this purpose, the contemporary modern communication systems need to operate PA at near saturation level. Unfortunately, PA is an analog component and is inherently non-linear (NL). So, it is important to opt for techniques to diminish the NL effects. These tech-niques are broadly classified into two categories. The former one aim at PAPR reduction and the later one tries to linearize the PA.
Being a MCM technique, FBMC-OQAM suffer from high PAPR and the reduction of this remains to be one of the most crucial issues that need to be solved effectively with a reasonable complexity. The main objective of this thesis is to analyze the impact of PA non-linearities on FBMC-OQAM systems and propose mitigating techniques in this regard, with a prime focus on PAPR reduction.