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

	Editorial Introduction	ix
	Acknowledgement	xiii
I	General Issues
	A Review of Multicarrier CDMA	3
	Time Division Duplex-CDMA	13
	Timing Synchronization for the Uplink of an OFDMA System	23
	Does OFDM Really Prefer Frequency Selective Fading Channels?	35
II	Applications
	An Adaptive Orthogonal Multicarrier Multiuser CDMA Technique for a Broadband Mobile Communication System	45
	Performance of Low Complex Antenna Diversity Techniques for Mobile OFDM Systems	53
	Common Architectures for TD-CDMA and OFDM Based Mobile Radio Systems Without the Necessity of a Cyclic Prefix	65
	Multi-Carrier CDMA for Air Traffic Control Air/Ground Communication	77
	Development and Implementation of an Advanced Airport Data Link Based on Multi-Carrier Communications	85
III	Coding and Modulation
	Potential and Benefits of the Spatial Dimension in Case of MC-CDMA	99
	Variable Spreading Factor Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM)	113
	Spreading Sequences Selection for Uplink and Downlink MC-CDMA Systems	123
	Serial Code Concatenation with Complex Valued Walsh-Hadamard Codes Applied to OFDM-CDMA	131
	A Study on Down-link Capacity of FD-MC/CDMA for Channels with Frequency Selective Fading	139
	On the Use of DMT and FMT for MC-CDMA Systems	147
	A Delayed-Multiplexing Multi-Carrier CDMA System	155
IV	Detection, Multiplexing, Interference Cancellation
	Comparison of MC-CDMA and COFDM/TDMA Techniques Taking into Account the System Load for Wireless Indoor Communications	165
	Decorrelation MUD for MC-CDMA in an Uplink Transmission Mode	173
	A Downlink SS-OFDM-F/TA Packet Data System Employing Multi-User Diversity	181
	Joint Detection in Multi-Antenna and Multi-User OFDM Systems	191
	Rate-Adaptive Coded Multiuser OFDM for Downlink Wireless Systems	199
	Adaptive Subspace Multiuser Detection for Multicarrier DS-CDMA	209
	Optimization of Capacity Assignment in MC-CDMA Transmission Systems	217
	Optimum Coefficient Matrix of Multi-Dimensional Lattice Filter for OFDM Transmission	225
	Capacity Enhancement of DS-CDMA Downlink Through Multicarrier with Frequency Overlapping	233
	Combating Near Far Effects of Linear MMSE Multiuser Detection in Coded OFDM-CDMA	241
	Downlink Performance of Randomized OFDM Access	249
	Implementation of a SIC Based MC-CDMA Base Station Receiver	257
	Investigation on Using Channel Information of MC/CDMA for Pre-Rake Diversity Combining in TDD/CDMA System	265
	Multicarrier CDMA System with Parallel Interference Cancellation in Fading Channel	273
V	Synchronization and Channel Estimation
	Iterative Blind Channel Estimation for OFDM Receivers	283
	Non-Data Aided EM-Based Channel Estimation for OFDM Systems with Time-Varying Fading Channels	293
	Joint Directions of Arrival and Delay Channel Characteristics Estimation Using Circular Array Antenna for MC/CDMA	303
	Channel Estimation for Time-Hopping Impulse Radio	313
	Comparison of Channel Estimation Techniques for a Single Carrier System Using Frequency Domain Equalization and HIPERLAN/2 Like OFDM Systems	325
VI	Realization and Implementation
	Array Antenna Assisted Doppler Spread Compensator for OFDM	337
	Filter Design Methodology Controlling the Impact on Bit Error Rate Performances in WLAN-OFDM Transceivers	345
	Proposal of ISM Band High Data Rate Wireless LAN System Using SS-OFDM Technique	353
	Joint Technique of PTS and Adaptive Clipping Level Control with Filtering for OFDM Peak Power Reduction	361
	Radio Link Availability Enhancement by Applying Spread-Spectrum Technique	369