OFDM tutorial, part 1: PAPR, ECC, and DAR

Part two shows how to enhance OFDM with MIMO and other techniques. The many papers noted in this series are listed in the references section.

2.1 Introduction
This chapter covers specific wireless standards and air interfaces. For wireless systems currently actively under development there are two important classes: those based on OFDM, including WLAN, standards and proposed for future high speed mobile systems, and those based on CDMA, especially 3rd generation mobile standards. For this reason the chapter is divided into two sections: Section 2.2 covers OFDM-based systems, while Section 2.3 covers CDMA. Multi Carrier–CDMA (MC-CDMA) systems, which can be regarded as a combination of the two, are discussed in Subsection 2.3.4.

2.2 OFDM systems
2.2.1 Introduction
During the past 15 years, Orthogonal Frequency Division Multiplexing (OFDM) has been gaining year after year a well-deserved reputation, demonstrating its high data rate and robustness to wireless environments capabilities. In the multipath environment, broadband communications will suffer from frequency selective fading. In such a situation, the above technologies do not work optimally and a transport scheme better suited for the environment is needed. For this reason, OFDM became very popular recently [Enge02].

OFDM is an attractive modulation scheme used in broadband wireless systems that encounter large delay spreads. The complexity of Maximum Likelihood (ML) detection or even sub-optimal equalization schemes needed for single carrier modulation grows exponentially with the bandwidth delay spread product. OFDM avoids temporal equalization altogether, using a cyclic prefix technique with a small penalty in channel capacity [PaNG03].

Where Line-of-Sight (LoS) cannot be achieved, there is likely to be significant multipath dispersion, which could limit the maximum data rate. Technologies like OFDM are probably best placed to overcome these, allowing nearly arbitrary data rates on dispersive channels. OFDM, in particular for broadband systems in dispersive environments, is a technology that could have a place in the 4G concept.

Although the principle of OFDM communication has been around for several decades, it was only in the last decade that it started to be used in commercial systems. The most important wireless applications that make use of OFDM are Digital Audio Broadcast (DAB), DVB, WLAN and more recently Wireless Local Loop (WLL) [Enge02].

The DAB system is seen as the future of radio as it makes more efficient use of crowded airwaves and provides CD quality sound that is noticeably better than an FM analogue broadcast. DAB makes use of an OFDM transmission scheme with differential Quaternary PSK (QPSK) modulation. The DVB system is very similar to DAB standard, but is intended for broadcasting of digital television signals. Due to the high data rates, the DVB system uses an 8 MHz bandwidth. The subcarriers in the OFDM signal are modulated with a higher order Quadrature Amplitude Modulation (QAM) constellation, with up to 64 points.

The third generation of WLAN systems is intended to offer high data rates in the 5 GHz frequency band and more recently in the 2.5 GHz band. The communication is based on OFDM in the 20 MHz bandwidth. Per subcarrier, the modulation schemes are Binary Phase Shift Keying (BPSK), QPSK, 16-QAM and 64-QAM. Together with a variable error coding rate, this allows the data rates to be adapted from 6 Mbit/s to 54 Mbit/s depending on the propagation environment. Future WLAN standards are being studied to overcome that range.

Wireless Local Loops provide high speed Internet access and multimedia services to fixed users. WLL is a competitive technology to Very-High-Rate Digital Subscriber Line (VDSL) and cable modems. OFDM is one of the supported transport schemes for WLL technologies [Enge02].

The simplification of equalization and the high bandwidth efficiency and flexibility are the main motivations for using OFDM, which is always a preferred alternative if a high data rate is to be transmitted over a multipath channel with large maximum delay [Corr01].

Channel coding plays an important role in OFDM systems. Due to the narrowband subcarriers and the appropriate cyclic prefix, OFDM systems suffer from flat fading. In this situation, efficient channel coding leads to a very high coding gain, especially if soft decision decoding is applied. For this reason OFDM systems will always have to make use of channel coding [AlLa87]. Furthermore, OFDM allows multiple access techniques to certain time and/or frequency regions of the channel in a very simple way [RoGr97].

Finally, OFDM signals have a large peak-to-mean power ratio due to the superposition of all subcarrier signals, therefore in each Transceiver (TRX), the power amplifier will limit the OFDM signal by its maximal output power. This also disturbs the orthogonality between subcarriers, leading to both intercarrier and out-of-band interferences, which is unacceptable [Corr01].