Frequency-Domain Receiver Design for Doubly Selective Channels

Paulo Montezuma, Fabio Silva, Rui Dinis

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351689359
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Frequency-Domain Receiver Design for Doubly-Selective Channels discusses broadband wireless transmission techniques, which are serious candidates to be implemented in future broadband wireless and cellular systems, aiming at providing high and reliable data transmission and concomitantly high mobility. This book provides an overview of the channel impairments that may affect performance of single carrier and multi-carrier block transmission techniques in mobile environments. Moreover, it also provides a new insight into the new receiver designs able to cope with double selectivity that affects present and future broadband high speed mobile communication systems.

Chapter 1

Introduction

1.1 Motivation and Scope

The tremendous growth of mobile internet and multimedia services, accompanied by the advances in micro-electronic circuits as well as the increasing demands for high data rates and high mobility, motivated the rapid development of broadband wireless systems over the past decade. Future wireless systems are expected to be able to deploy very high data rates of services within high mobility scenarios. As a result, broadband wireless communication is nowadays a fundamental part of the global information and the world’s communication structure.

A major challenge in the design of mobile communications systems is to overcome the mobile radio channel effects, assuring at the same time high power and spectral efficiencies. Since in mobile communications the information data is transmitted across the wireless medium, then the transmitted signal will certainly suffer from adverse effects originated by two different factors: multipath fading and mobility.

Within a multipath propagation environment waves arriving from different paths with different delays combine at the receiver with different attenuations. Multipath propagation leads to the time dispersion of the transmitted symbol resulting in frequency-selective fading.

Besides multipath propagation, time variations within the channel may also arise due to oscillator drifts, as well as due to mobility between transmitter and receiver [31]. The relative motion between the transmitter and the receiver results in Doppler frequency which has a strong negative impact on the performance of mobile radio communication systems since it generates different frequency shifts for each incident plane wave, causing the channel impulse response to vary in time. The channel characteristics change depending on the location of the user, and because of mobility, they also vary in time. Hence, when the relative positions of the different objects in the environment including the transmitter and receiver change with time, the nature of the channel also varies. In mobility scenarios, the rate of variation of the channel response in time is characterized by the Doppler spread. Significant variations of the channel response within the signal duration lead to time-selective fading, and this represents a major issue in wireless communication systems.

Channels whose response is selective in time and frequency are referred to as doubly-selective. As a result of these two phenomena, the equivalent received signal is time varying and may be highly attenuated. This is considered a severe impairment in wireless communication systems, since these effects lead to drastic and unpredictable fluctuations of the envelope of the received signal (deep fades of more than 40 dB below the mean value can occur several times per second).

Block transmission techniques, with cyclic extensions and FDE techniques (frequency-domain equalization) are known to be suitable for high data rate transmission over severely time-dispersive channels due to its reduced complexity and excellent performance, provided that accurate channel estimates are provided. Moreover, since these techniques usually employ large blocks, the channel can even change within the block duration. Fourth generation broadband wireless systems employ CP-assisted (cyclic prefix) block transmission techniques, and although these techniques allow the simplification of the receiver design, the length of the CP should be a small fraction of the overall block length, meaning that long blocks are susceptible to time-varying channels, especially for mobile systems. Hence, the receiver design for doubly-selective channels is of key importance, especially to reduce the relative weight of the CP.

Efficient channel estimation techniques are crucial in achieving reliable communication in wireless communication systems. When the channel changes within the block duration, significant performance degradation occurs. Channel variations lead to two different difficulties: first, the receiver needs continuously accurate channel estimates; second, conventional receiver designs for block transmission techniques are not suitable when there are channel variations within a given block. As with any coherent receiver, accurate channel estimation is mandatory for the good performance of FDE receivers, both for orthogonal frequency-division multiplexing (OFDM) and single carrier with frequency domain equalization (SC-FDE).

The existence of residual carrier frequency offset (CFO) between the transmitter and the receiver’s local oscillators means that the equivalent channel has a phase rotation that changes within the block. It was shown in [1,13,53] that residual CFO leads to simple phase variations that are relatively easy to compensate at the receiver’s side. However, that may not be the case for single frequency broadcast networks. Within single frequency networks (SFN), several receiving zones within the overall coverage location are served by more than one transmitter, meaning that multiple transmitters must broadcast the same signal simultaneously over the network. Hence, each transmission will most likely have an associated frequency offset. This leads to a very difficult scenario where there will be substantial variations on the equivalent channel which cannot be treated as simple phase variations.

For channel variations due to Doppler effects, receiver structures for double-selective channels combining an iterative equalization and compensation of channel variations have already been proposed [44]. These kinds of channel variations can become extremely complex since the Doppler effects are distinct for different multipath components (e.g., when we have different departure/arrival directions relatively to the terminal movement).

It is difficult to ensure stationarity of the channel within the block duration, which is a requirement for conventional OFDM and SC-FDE receivers. Hence, efficient estimation and tracking procedures are required and should be able to cope with channel variations.

This book is dedicated to the study of effective detection of broadband wireless transmission, and it is intended for future broadband wireless and cellular systems which should be able to provide high transmission, together with high mobility (e.g., WiFi/WiMax-type LANs). Contrary to the common approach that assumes that either the channel is fixed or non-dispersive, this book focuses on the problem of digital transmission over severely time-dispersive channels that are also time-varying. Both OFDM and SC-FDE schemes will be considered. Effective detection within channels that are both time dispersive and time varying can be achieved by resorting to receiver designs implemented in the frequency domain, capable of performing channel estimation and compensation, as well as channel tracking techniques. Therefore this book aims to present these techniques for estimating the channel impulse response and track its variations. These techniques should take advantage of reference symbols/block multiplexed with data and/or added to it. Finally are presented receivers analyzed for severely time-dispersive channels that combine the detection/equalization procedures with the channel estimation techniques, while assuring low and moderate signal processing requirements.

1.2 Book Structure

Chapter 2 is devoted to a review of the mathematical models representing the physical channels and introduces time-varying frequency selective channels. Having in mind the high data rate requirements while dealing with severely time-dispersive channel effects, this chapter includes an overview of the state-of-the-art of the equalization techniques at the receiver side that become necessary to compensate the signal distortion and guarantee good performance. Channel characterization covers both frequency and time selective channels. So, besides multipath propagation resulting in frequency-selective fading, channels with time variations within the channel may also arise due to oscillator drifts, as well as due to motion between transmitter and receiver. This characterization also covers channels whose response is selective in time and frequency. These are referred to as doubly-selective and represent a severe impairment in wireless communication systems, since the multipath propagation combined with the Doppler effects due to mobility can lead to drastic and unpredictable fluctuations of the envelope of the received signal.

Chapter 3 starts with a brief introduction of OFDM and SC-FDE block transmission techniques that are especially adequate for severely time-dispersive channels. It includes several aspects such as the analytical characterization of each modulation type and some relevant properties of each modulation. For both modulations special attention is given to the characterization of the transmission and receiving structures, with particular emphasis on transmitter and receiver performance. MC modulations and their relations with SC modulations are analyzed. Section 3.4.1 describes the OFDM modulation. Section 3.5 characterizes the basic aspects of the SC-FDE modulation including the linear and iterative FDE receivers. A promising FDE technique for single carrier modulation, the iterative block-decision feedback equalizer (IB-DFE), is also analyzed and an explanation of the feedforward and the feedback operations is given. It is shown that this iterative FDE receiver offers much better performance than the non-iterative methods, with performance near to the MFB as will be shown in Chapter 4. Finally, in Section 3.6, the performance of OFDM and SC-FDE for severely time-dispersive channels is compared.

In Chapter 4, the impact of the number of multipath components and the diversity order on the asymptotic performance of OFDM and SC-FDE for different channel coding schemes is analyzed. It is shown that the number of relevant separable multipath components is a fundamental element that influences the performance of both schemes and, in the IB-DFE’s case, the iteration gains. A set of results is presented that demonstrates that SC-FDE has an overall performance advantage over OFDM, especially when employing the IB-DFE, in the presence of a high number of separable multipath components, because it allows a performance very close to the matched filter bound (MFB), even without diversity. With diversity, the performance approaches MFB faster, even for a small number of separable multipath components.

Chapter 5 is devoted to OFDM-based broadcasting systems with SFN operation and presents an efficient channel estimation method which takes advantage of the sparse nature of the equivalent channel impulse response (CIR). For this purpose, low-power training sequences are used in order to obtain an initial coarse channel estimate, and an iterative receiver with joint detection and channel estimation is designed. The results achieved by this receiver show very good performance, close to the perfect channel estimation case, even with resort to low-power training blocks as well as for the case where the receiver does not know the location of the different clusters that constitute the overall CIR.

Chapter 6 is dedicated to the joint CFO estimation and compensation over the severe time-distortion effects inherent in SFN systems. Most conventional broadband broadcast wireless systems employ OFDM schemes in order to cope with severely time-dispersive channels. As shown in Chapter 3, the high peak-to-average power ratio (PAPR) of OFDM signals leads to amplification difficulties. Moreover, the presence of a carrier frequency offset compromises the orthogonality between the OFDM subcarriers. Thus, this chapter explores the possibility of using SC-FDE schemes in broadcasting systems with SFN operation. An efficient method for estimating the channel frequency response and CFO associated with each transmitter is presented, along with receiver structures able to compensate the equivalent channel variations due to different CFO for different transmitters. Subsequently, an efficient technique is also presented for estimating the channel associated with the transmission between each transmitter and the receiver, as well as the corresponding CFOs. This technique has been shown to be sufficient for obtaining the evolution of the equivalent channel along a given frame. Closing this chapter, it is analyzed a set of iterative FDE receivers able to compensate the impact of the different CFOs between the local oscillators at each transmitter is analyzed.

Finally, Chapter 7 focuses on the problem of the use of SC-FDE transmission in channels with strong Doppler effects. For this purpose, new iterative frequency-domain receivers able to attenuate the impact of strong Doppler effects, at the cost of a slight increase in complexity when compared with the IB-DFE, are defined. The first step is to do a channel characterization appropriate to model short-term channel variations, modeled as almost pure Doppler shifts which were different for each multipath component. Then, this model will be used to design the frequency-domain receivers able to deal with strong Doppler effects. These receivers can be considered as modified turbo equalizers implemented in the frequency-domain, which are able to compensate the Doppler effects associated with different groups of multipath components while performing the equalization operation, which makes them suitable for SC-FDE scheme based broadband transmission in the presence of fast-varying channels.

Chapter 2

Fading

In order to enable communication over wireless channels it is necessary to characterize the propagation models. However, trying to make an analysis of the mobile communication under such harsh propagation conditions might seem a very hard task to accomplish. Nevertheless, starting from a model based on the multipath propagation we will see that many of the properties of the transmission can be successfully predicted by applying powerful techniques of statis...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Table of Contents
• Authors
• List of Abbreviations
• List of Symbols
• 1. Introduction
• 2. Fading
• 3. Block Transmission Techniques
• 4. Approaching the Matched Filter Bound
• 5. Efficient Channel Estimation for Single Frequency Networks
• 6. Asynchronous Single Frequency Networks
• 7. Multipath Channels with Strong Doppler Effects
• A. Important Statistical Parameters
• B. Complex Baseband Representation
• C. Minimum Error Variance
• References
• Index