Multi-Carrier Spread Spectrum & Related Topics

by Khaled Fazel (Editor), S. Kaiser (Editor)

Overview

Khaled Fazel Stefan Kaiser Digital Microwave Systems German Aerospace Center (DLR) Bosch Telecom GmbH Institute for Communications Technology D-71522 Backnang, Germany D-82234 Wessling, Germany In this last decade of this millennium the technique of multi-carrier transmission for wireless broadband multimedia applications has been receiving wide interests. Its first great success was in 1990 as it was selected in the European Digital Audio Broadcasting (DAB) standard. Its further prominent successes were in 1995 and 1998 as it was selected as modulation scheme in the European Digital Video Broadcasting (DVB-T) and in three broadband wireless indoor standards, namely ETSI-Hiperlan-II, American IEEE-802. 11 and Japanese MMAC, respectively. The benefits and success of multi-carrier (MC) modulation in one side and the flexibility offered by spread spectrum (SS) technique in other hand motivated many researchers to investigate the combination of both techniques, known as multi-carrier spread-spectrum (MC-SS). This combination benefits from the main advantages of both systems and offers high flexibility, high spectral efficiency, simple detection strategies, narrow­ band interference rejection capability, etc. . The basic principle of this combination is straightforward: The spreading is performed as direct SS (DS-SS) but instead of transmitting the chips over a single sequence carrier, several sub-carriers could be employed. As depicted in Figure 1, after spreading with assigned user specific code of processing gain G the frequency mapping and multi-carrier modulation is applied. In the receiver side after multi-carrier demodulation and frequency de-mapping, the corresponding detection algorithm will be performed.

Product Details

ISBN-13:	9781461370109
Publisher:	Springer US
Publication date:	10/12/2012
Edition description:	Softcover reprint of the original 1st ed. 2000
Pages:	360
Product dimensions:	6.30(w) x 9.45(h) x 0.03(d)

About the Author

Stefan Kaiser was the first researcher to address the institutional and legal aspects of the future air navigation systems in a pioneering thesis presented to the Institute of Air and Space Law, McGill University.

Read an Excerpt

On the improvement of HF Multicarrier modem behavior by using Spread Spectrum techniques to combat fading channels

Keywords: HF modem, Multicarrier Spreading, Multistage Interference cancellation

Abstract: This paper deals with the application of spread spectrum techniques to Multicarrier HF modems. It is observed that Multicarrier transmission over fading channels with deep nulls show a quite different performance between carriers: those in the vicinity of deep nulls provide a much higher Bit Error Rate (BER) than others far away of the nulls. Our proposal intends to spread different symbols over all the subcarriers simultaneously in order to homogenize performances. This approach, performed by orthogonal codes, shows an evident link with MC-CDMA techniques. In addition, joint detection strategies as Interference Cancellation (IC) with coherent detection are also applied in order to improve its performance. Several computer simulations support the feasibility of our proposal.

1. Introduction

HF transmission through ionospheric refraction is generally accepted as a very difficult task due to the time variant high dispersive channel characteristic. The ionosphere is a dispersive medium that spreads the signal in both time and frequency. In addition, received radio signals have usually been reflected from more than one ionosphere layer, a phenomenon known as multipath propagation. On the other hand, the ionization characteristic of this atmosphere layer also depends on solar conditions, season, latitude and local weather and time are in fact which the responsible of the time variant characteristic of this medium [1].

This behavior of the physical medium constrains the maximum feasible bandwidth to 3 KHz. Transmission rate over 1.8-2.4 Kbps. requires enhanced techniques with also complexity increase.

Most of the current modems are based on single carrier modulation including very powerful codes and long interleavers to compensate long burst errors [ 1 ]. Probably, the common basis of these modems is the Military Standard MIL-STD-188-110A [2] which specifies a continual transmission system to the maximum of 2400 bps.

One of the main civil applications of these modems is the HFDL system (High Frequency Data Link) developed by ICAO (International Civil Aviation Organization). HFDL is a TDMA system dealing with data transmission between aircraft and ground stations providing a data link by HF propagation meanwhile the airplane is far away the coast line [3]. Although this is a burst transmission instead of the continuous stream, the physical layer is almost identical to the single carrier modem of the MIL-STD-188-110A. In the current standard [3], long delays is not a critical point because it is devoted only for data transmission; even, if one slot is lost due to the improper behavior of the equalizer+interleaver+decoder, a request strategy provide the retransmission in probably better conditions. By our own, we are working in this application, but with a much wider purpose: we are focused on the capability of the system to carry out data and digital voice signals in an interactive operation mode. This approach requires a minimum delay also with data transmission rate over 2400 bps. These features do not apply for single carrier techniques but can be obtained by multicarrier modulation (OFDM) due to its more robust behavior against frequency selective fading channels [4, 5,6].

Let us describe briefly our proposal for the next generation of HFDL system in the next section. After that, we will focus on a particular improvement related with the use of multicarrier spreading; in fact, our approach can be formulated as a synchronized MC-CDMA where all the `users' must be demodulated.

2. The MC-HFDL System

This section is devoted to the presentation of the block diagram of the HFDL system. We will like to point out that the multicarrier version MC-HFDL is not a current standard but it has been presented at the ICAO panel on April 1999 at Montreal (Canada) as a potential alternative for the next definition of the standard [5]. Let us also remark the most critical constraints in our design:

- Our application is concerned with interactive communications where longer delays of 50-80 msec. are not acceptable.

- We will incorporate a digital voice coder: of course, the maximum data rate achievable will provide the better voice quality. We expect to guarantee data rate up to 3600 bps even in bad channel conditions.

Figure 1 shows the block diagram of the OFDM data modem where all the main devices are presented both in transmission and reception.

We have labeled it a HF Spread-Spectrum OFDM modem. Due to space reasons, details about channel coding (Reed-Solomon (45, 63)), interleaving (frequency domain), time and frequency estimation schemes, pilot insertion, channel estimation ...must be referred to previous works [5, 6, 7]. On the other hand, spreader/despreader and interference cancellation is the main purpose of the present paper and will be discussed in detail in the next section.

Figure 2 shows an schematic view of the data structure to clarify the previous explanations. In this case, a pilot 16-OFDM symbol is inserted between couples of data 16-OFDM symbols just as an example. Data symbols of 32-OFDM and 64-OFDM are also under analysis. Also, the period insertion of the pilot symbol must be also determined; our experience shows that the channel variability requires channel estimation each 250-300 msec.

3. The Multicarrier Spreading

Let us now discuss about the application of the multicarrier spreading technique to the HF transmission. First, we will address the motivation and intuitive approach that may recommend the spreading. After that, a formulation of our approach as a problem of synchronized multiple access will link both disciplines: HF-OFDM and MC-CDMA. Finally, some computer simulations will confirm the significant improvement of the spreading technique.

Channel estimation can be easily accomplished by known pilots insertion. Several pilot patterns can be proposed: rectangular, hexagonal... which show a satisfactory performance in terms of its density and the final estimation error. In our application, delay is a critical point that unable pilot patterns employing several OFDM symbols. By our experience, the insertion of a pilot patter only in the frequency dimension allows a delay-free estimation with a computational very low cost by using the FFT algorithm. In fact, pilot symbol may use shorter time duration using alternate carriers and the final estimation for all the carriers is performed by interpolation.

However, in spite of the flat fading characteristic, a severe performance degradation can be observed at the subcarriers at the vicinity of deep nulls. Figure 3 shows a schematic view of the estimation procedure remarking the fact that at high SNR subcarrier, proper channel estimation is obtained and therefore an accurate demodulation is provided. On the other hand, at deep nulls, the local SNR decreases and coarse estimation may decrease the system performance dramatically. This is a typical observation in HF multicarrier modems: several carriers provide a low BER meanwhile other ones present a much higher BER which definitely decreases the expected performance...

Table of Contents

I General Issues of Multi-Carrier Spread-Spectrum & Related Topics.- Some Novel Concepts in Multiplexing and Multiple Access.- Wideband Multimedia Solution using Hybrid CDMA/OFDMA/SFH Techniques.- Generalised Multicarrier CDMA for Wireless MUI/ISI-Resilient Uplink Transmissions Irrespective of Frequency Selective Multipath.- Performance Tradeoffs in Asynchronous Spread Spectrum Multi-Carrier Multiple-Access.- II Applications of Multi-Carrier Spread-Spectrum & Related Topics.- Performance Tradeoffs in Asynchronous Spread Spectrum Multi-Carrier Multiple-Access.- A Multi-Carrier Spread-Spectrum Approach to Broadband Underwater Acoustic Communications.- A New Combined OFDM-CDMA Approach for Cellular Mobile Communications.- A Digital Microwave Point-to-Multi-Point (PMP) System Based on Multi-Carrier FDMA-Transmission.- A Hybrid TDMA/CDMA System Based on Filtered Multitone Modulation for Upstream Transmission in HFC Networks.- III Coding and Modulation.- On Coding and Spreading for MC-CDMA.- MCM-DSSS with DPSK Modulation and Equal Gain Combining in Delay and Doppler-Spread Rician Fading.- Influence of Code Selection on the Performance of a Multi-Carrier Spread-Spectrum System.- Complex Valued Block Codes for OFDM-CDMA Application.- Two-Dimensional Differential Demodulation of OFDM Frames.- Block Turbo Coding in OFDM-based HIPERLAN/2 Systems.- The Performance Analysis of Multi-Carrier CDMA Systems Using Different Spreading Codes in Frequency Selective Fading Environments.- IV Detection and Multiplexing.- Each Carrier Transmission Power Control with Antenna-Carrier Diversity for OFDM-DS-CDMA.- A Wavelet-based Multicarrier Spread Spectrum System with Constant Power.- A Subspace Method for Blind Channel Identification in Multi-Carrier CDMA Systems.- Multiuser Detection with Iterated Channel Estimation.- MUD Improvement by using Hexagonal Pilot Distributions for Channel Acquisition and Tracking in MC-CDMA Systems.- Adaptive Multi-Shot Multiuser Detection for Asynchronous MC-CDMA Using Bootstrap Algorithm.- V Interfrence Cancellation.- Blind Adaptive Interference Cancellation for Multicarrier Modulated Systems.- Performance of Downlink MC-CDMA with Simple Interference Cancellation.- Wideband and Narrowband Interference Cancellation for Asynchronous MC-CDMA.- Multiple Access Interference Cancellation in Multicarrier CDMA Systems Using a Subspace Projection Technique.- Interference Cancellation for a Multi-Carrier Spread-Spectrum FCDMA System.- Narrowband Interference Mitigation in OFDM-based WLANs.- Multi-Carrier CDMA using Interference Cancellation.- VI Synchronization and Channel Estimation.- An Overview of MC-CDMA Synchronization Sensitivity.- Blind Frequency Offset/Symbol Timing/Symbol Period Estimation and Subcarrier Recovery for OFDM Signals in Fading Channels.- Doppler Spread Analysis and Simulation for Multi-Carrier Mobile Radio and Broadcast Systems.- Reduced State Joint Channel Estimation and Equalization.- Channel Estimation with Superimposed Pilot Sequence Applied to Multi-Carrier Systems.- A Comparison of Channel Estimation Techniques for OFDM-CDMA and MC/JD-CDMA Systems.- Burst Synchronization for OFDM Systems with Turbo Codes.- Acquisition of Synchronisation Parameters for OFDM Using a Single OFDM Training Symbol.- VII Realization and Implementation.- Implementation of Multicarrier Systems with Polyphase Filterbanks.- On the Improvement of HF Multicarrier Modem Behavior by Using Spread Spectrum Techniques to Combat Fading Channels.- An Improved MMSE based Block Decision Feedback Equalizer.- MC-CDMA vs. DS-CDMA in the Presence of Nonlinear Distortions.