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Chapter 1
Introduction to OFDM and MIMO-OFDM
1.1 OFDM History
In recent years Orthogonal Frequency-Division Multiplexing (OFDM) [1–4] has emerged as a successful air-interface technique. In the context of wired environments, OFDM techniques are also known as Discrete Multi-Tone (DMT) [5] transmissions and are employed in the American National Standards Institute’s (ANSI’s) Asymmetric Digital Subscriber Line (ADSL) [6], High-bit-rate Digital Subscriber Line (HDSL) [7], and Very-high-speed Digital Subscriber Line (VDSL) [8] standards as well as in the European Telecommunication Standard Institute’s (ETSI’s) [9] VDSL applications. In wireless scenarios, OFDM has been advocated by many European standards, such as Digital Audio Broadcasting (DAB) [10], Digital Video Broadcasting for Terrestrial television (DVB-T) [11], Digital Video Broadcasting for Handheld terminals (DVB-H) [12], Wireless Local Area Networks (WLANs) [13] and Broadband Radio Access Networks (BRANs) [14]. Furthermore, OFDM has been ratified as a standard or has been considered as a candidate standard by a number of standardization groups of the Institute of Electrical and Electronics Engineers (IEEE), such as the IEEE 802.11 [15] and the IEEE 802.16 [16] standard families.
The concept of parallel transmission of data over dispersive channels was first mentioned as early as 1957 in the pioneering contribution of Doelz et al. [17], while the first OFDM schemes date back to the 1960s, which were proposed by Chang [18] and Saltzberg [19]. In the classic parallel data transmission systems [18, 19], the Frequency-Domain (FD) bandwidth is divided into a number of non-overlapping subchannels, each of which hosts a specific carrier widely referred to as a subcarrier. While each subcarrier is separately modulated by a data symbol, the overall modulation operation across all the subchannels results in a frequency-multiplexed signal. All of the sinc-shaped subchannel spectra exhibit zero crossings at all of the remaining subcarrier frequencies and the individual subchannel spectra are orthogonal to each other. This ensures that the subcarrier signals do not interfere with each other, when communicating over perfectly distortionless channels, as a consequence of their orthogonality [3].
The early OFDM schemes [18–21] required banks of sinusoidal subcarrier generators and demodulators, which imposed a high implementation complexity. This drawback limited the application of OFDM to military systems until 1971, when Weinstein and Ebert [22] suggested that the Discrete Fourier Transform (DFT) can be used for the OFDM modulation and demodulation processes, which significantly reduces the implementation complexity of OFDM. Since then, more practical OFDM research has been carried out. For example, in the early 1980s Peled and Ruiz [23] proposed a simplified FD data transmission method using a cyclic prefix-aided technique and exploited reduced-complexity algorithms for achieving a significantly lower computational complexity than that of classic single-carrier time-domain Quadrature Amplitude Modulation (QAM) [24] modems. Around the same era, Keasler et al. [25] invented a high-speed OFDM modem for employment in switched networks, such as the telephone network. Hirosaki designed a subchannel-based equalizer for an orthogonally multiplexed QAM system in 1980 [26] and later introduced the DFT-based implementation of OFDM systems [27], on the basis of which a so-called groupband data modem was developed [28]. Cimini [29] and Kalet [30] investigated the performance of OFDM modems in mobile communication channels. Furthermore, Alard and Lassalle [31] applied OFDM in digital broadcasting systems, which was the pioneering work of the European DAB standard [10] established in the mid-1990s. More recent advances in OFDM transmission were summarized in the state-of-the-art collection of works edited by Fazel and Fettweis [32]. Other important recent OFDM references include the books by Hanzo et al. [3] and Van Nee et al. [4] as well as a number of overview papers [33–35].
OFDM has some key advantages over other widely used wireless access techniques, such as Time-Division Multiple Access (TDMA) [36], Frequency-Division Multiple Access (FDMA) [36] and Code- Division Multiple Access (CDMA) [37, 38, 40–42]. The main merit of OFDM is the fact that the radio channel is divided into many narrowband, low-rate, frequency-non-selective subchannels or subcarriers, so that multiple symbols can be transmitted in parallel, while maintaining a high spectral efficiency. Each subcarrier may deliver information for a different user, resulting in a simple multiple-access scheme known as Orthogonal Frequency-Division Multiple Access (OFDMA) [43–46]. This enables different media such as video, graphics, speech, text or other data to be transmitted within the same radio link, depending on the specific types of services and their Quality-of-Service (QoS) requirements. Furthermore, in OFDM systems different modulation schemes can be employed for different subcarriers or even for different users. For example, the users close to the Base Station (BS) may have a relatively good channel quality, thus they can use high-order modulation schemes to increase their data rates. By contrast, for those users that are far from the BS or are serviced in highly loaded urban areas, where the subcarriers’ quality is expected to be poor, low-order modulation schemes can be invoked [47].
Besides its implementational flexibility, the low complexity required in transmission and reception as well as the attainable high performance render OFDM a highly attractive candidate for high-data-rate communications over time-varying frequency-selective radio channels. For example, in classic single-carrier systems, complex equalizers have to be employed at the receiver for the sake of mitigating the Inter-Symbol Interference (ISI) introduced by multi-path propagation. By contrast, when using a cyclic prefix [23], OFDM exhibits a high resilience against the ISI. Incorporating channel coding techniques into OFDM systems, which results in Coded OFDM (COFDM) [48, 49], allows us to maintain robustness against frequency-selective fading channels, where busty errors are encountered at specific subcarriers in the FD.
However, besides its significant advantages, OFDM also has a few disadvantages. One problem is the associated increased Peak-to-Average Power Ratio (PAPR) in comparison with single-carrier systems [3], requiring a large linear range for the OFDM transmitter’s output amplifier. In addition, OFDM is sensitive to carrier frequency offset, resulting in Inter-Carrier Interference (ICI) [50].
As a summary of this section, we outline the milestones and the main contributions found in the OFDM literature in Tables 1.1 and 1.2.
Table 1.1: Milestones in the history of OFDM.
| 1957 | The concept of parallel data transmission by Doelz et al. [17] |
| 1966 | First OFDM scheme proposed by Chang [18] for dispersive fading channels |
| 1967 | Saltzberg [19] studied a multi-carrier system employing Orthogonal QAM (O-QAM) of the carriers |
| 1970 | US patent on OFDM issued [21] |
| 1971 | Weinstein and Ebert [22] applied DFT to OFDM modems |
| 1980 | Hirosaki designed a subchannel-based equalizer for an orthogonally multiplexed QAM system [26] Keasler et al. [25] described an OFDM modem for telephone networks |
| 1985 | Cimini [29] investigated the feasibility of OFDM in mobile communications |
| 1987 | Alard and Lasalle [31] employed OFDM for digital broadcasting |
| 1991 | ANSI ADSL standard [6] |
| 1994 | ANSI HDSL standard [7] |
| 1995 | ETSI DAB standard [10]: the first OFDM-based standard for digital broadcasting systems |
| 1996 | ETSI WLAN standard [13] |
| 1997 | ETSI DVB-T standard [11] |
| 1998 | ANSI VDSL and ETSI VDSL standards [8, 9] ETSI BRAN standard [14] |
| 1999 | IEEE 802.11a WLAN standard [51] |
| 2002 | IEEE 802.11g WLAN standard [52] |
| 2003 | Commercial deployment of FLASH-OFDM [53, 54] commenced |
| 2004 | ETSI DVB-H standard [12] IEEE 802.16-2004 WMAN standard [55] IEEE 802.11n draft standard for next generation WLAN [56] |
| 2005 | Mobile cellular standard 3GPP Long-Term Evolution (LTE) [57] downlink |
| 2007 | Multi-user MIMO-OFDM for next-generation wireless [58] Adaptive HSDPA-style OFDM and MC-CDMA transceivers [59] |
Table 1.2: Main contributions on OFDM.
| 1966 | Chang [18] | Proposed the first OFDM scheme |
| 1967 | 1966 Chang [18] Proposed the first OFDM scheme Saltzberg [19] | Studied a multi-carrier system employing O-QAM |
| 1968 | Chang and Gibby [20] | Presented a theoretical analysis of the performance of an orthogonal multiplexing data transmission scheme |
| 1970 | Chang [21] | US patent on OFDM issued |
| 1971 | Weinstein and Ebert [22] | Applied DFT to OFDM modems |
| 1980 | Hirosaki [26] | Designed a subchannel-based equalizer for an orthogonally multiplexed QAM system |
| | Peled and Ruiz [23] | Described a reduced-complexity FD data transmission method together with a cyclic prefix technique |
| 1981 | Hirosaki [27] | Suggested a DFT-based implementation of OFDM systems |
| 1985 | Cimini [29] | Investigated the feasibility of OFDM in mobile communications |
| 1986 | Hirosaki et al. [28] | Developed a groupband data modem using an orthogonally multiplexed QAM technique |
| 1987 | Alard and Lasalle [31] | Employed OFDM for digital broadcasting |
| 1989 | Kalet [30] | Analysed multi-tone QAM modems in linear channels |
| 1990 | Bingham [1] | Discussed various aspects of early OFDM techniques in depth |
| 1991 | Cioffi [6] | Introduced the ANSI ADSL standard |
| 1993–1995 | Warner [60], Moose [50] and Pollet [61] | Conducted studies on time and frequency synchronization in OFDM systems |
| 1994–1996 | Jones [62], Shepherd [63] and Wulich [64, 65] | Explored various coding and post-processing techniques designed for minimizing the peak power of the OFDM signal |
| 1997 | Li and Cimini [66, 67] | Revealed how clipping and filtering affect OFDM systems |
| | Hara and Prasad [68] | Compared various methods of combining CDMA an... |
