Multicarrier Spread Spectrum Research Articles

Digital multi-carrier spread spectrum versus direct sequence spread spectrum for resistance to jamming and multipath

We compare single user digital multi-carrier spread spectrum (MC-SS) modulation with direct sequence (DS) SS (with a modified implementation) in the presence of narrowband interference (NBI) and multipath fading. We derive closed-form expressions for the symbol error probability for both the linear MMSE receiver as well as the conventional matched-filter receiver under different scenarios: additive white Gaussian noise (AWGN) channel with NBI, multipath channel with or without NBI. We show that DS-SS can achieve the same performance as MC-SS if the spreading code is carefully designed to have perfect periodic autocorrelation function (PACF). On the other hand, MC-SS is more robust to narrowband interference and multipath fading than is DS-SS with the widely used spreading codes that do not possess perfect PACE. Our analysis reveals that the performance improvement of MC-SS is precisely due to the implicit construction of an equivalent spreading code having nonconstant amplitude but possessing perfect periodic autocorrelation.

Multicarrier CDMA for future generation mobile communication

Future generations of wireless communication systems will be designed with the aim of making the best possible use of the limited radio spectrum in order to further increase throughput as well as user-capacity. In this paper, the application of multicarrier CDMA (code division multiple access) within mobile communication systems is discussed. An overview of the main types of multicarrier spread-spectrum systems is given. The multicarrier CDMA system is described in detail, including its relationship with OFDM (orthogonal frequency division multiplexing), its general performance in comparison with direct sequence CDMA and some specific implementation problems. The paper also presents some results comparing the performance of a selection of algorithms that can be used to isolate the individual signal of each user. Potential capacity increases achievable with adaptive loading in time and frequency slots is briefly demonstrated. By allocating users multiple codes, very high data rates can be achieved, and the paper gives a comparison of multicode, multicarrier CDMA with OFDM-based wireless LAN systems.

A multicarrier DS/SSMA system with reduced multiple access interference and higher data rate in Rician fading channels

We propose to use multiple signature sequences (instead of one) per user in a multicarrier direct-sequence spread spectrum multiple access system in Rician fading channels. Every user has a distinct set of spreading sequences, with a different spreading sequence for each carrier in each user's set. By analysis and computer simulation, we show that when these sets of sequences are chosen to be mutually orthogonal complementary sets of sequences, multiple access interference (MAI) is reduced in Rician fading channels. We also show that we can pack symbols more closely together, resulting in a higher data rate. The proposed system with a distinct set of deterministic spreading sequences per user has lower MAI and higher data rate than another system with one distinct random spreading sequences per user in Rician fading channels.

Multicarrier orthogonal-CDMA for fixed wireless access applications

In this paper, we suggest applying multicarrier spread spectrum (MC-SS) techniques to fixed wireless access applications. Due to the fixed nature of subscribers in these applications, cross-polarization discrimination and directional subscriber antennas can be used to reduce interference in a multicellular system. Power control schemes are applied on both uplink and downlink. The suggested system has a high capacity and uses a simple multiaccess layer. It is robust against multipath effects and can provide service coverage not only to line-of-sight (LOS) but also to nearly LOS subscribers. Only a single carrier frequency is used in the entire network.

Performance analysis of coded multicarrier spread-spectrum systems in the presence of multipath fading and nonlinearities

In this paper, we analyze the effects of a nonlinear amplifier on the performance of convolutionally coded multicarrier spread-spectrum systems in the presence of multipath fading. Two performance measures, bit-error rate (BER) and adjacent channel power ratio (ACPR) are considered to assess the effects of in-band interference and out-of-band interference (spectral regrowth) from the nonlinear amplifier, respectively. A memoryless polynomial model is used to represent the bandpass amplifier amplitude nonlinearities (AM/AM) and a slow frequency-nonselective Rayleigh fading channel is assumed for each modulated carrier. Analytical results for the BER for uncoded systems and upper bounds on the BER for coded systems are presented. The optimum amplifier output power backoffs for both systems, which yield the smallest system power consumption, are also determined. The obtained ACPR shows its slow (inverse linear) falloff with the amplifier output backoff.

Comparison of Time and Frequency Domain Processing of a Multi-Carrier Spread-Spectrum System

Multi-carrier spread-spectrum (MC-SS) modulation is a new kind of spread-spectrum modulation[1],[2]. The spreading code is designed in the frequency domain. Hence, the system is dual to a direct-sequence spread-system (DS-SS) where the spreading code is given in the time domain. There exist two different views: the OFDM view and the DS-SS view. Many publications deal with the frequency domain processing. Transmitter and receiver structures are similar to OFDM. Therefore all these implementations use a guard interval to avoid ISI. The disadvantage of this guard interval is the higher hardware complexity and the waste of symbol energy. In the following paper the effect of ISI on a MC-SS system is investigated. Both types of implementation (time or frequency domain) are compared and the dependencies of the performance from the spreading code are demonstrated. It is shown that with proper code selection and a time domain implementation without guard interval (RAKE receiver) it is possible to outperform equivalent frequency domain receivers with guard interval (MRC).

Multi-Carrier Spread Spectrum

European Transactions on TelecommunicationsVolume 11, Issue 6 p. 523-526 Guest Editorial Multi-Carrier Spread Spectrum Prof. Khaled Fazel, Prof. Khaled Fazel khaled.fazel@marconi.com Marconi Communications, Wireless Access Systems, D-71522 Backnang, GermanySearch for more papers by this authorProf. Stefan Kaiser, Prof. Stefan Kaiser Stefan.kaiser@dlr.de German Aerospace Center (DLR), Institute for Communications and Navigation, D-82234 Wessling, GermanySearch for more papers by this author Prof. Khaled Fazel, Prof. Khaled Fazel khaled.fazel@marconi.com Marconi Communications, Wireless Access Systems, D-71522 Backnang, GermanySearch for more papers by this authorProf. Stefan Kaiser, Prof. Stefan Kaiser Stefan.kaiser@dlr.de German Aerospace Center (DLR), Institute for Communications and Navigation, D-82234 Wessling, GermanySearch for more papers by this author First published: 12 September 2008 https://doi.org/10.1002/ett.4460110602AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume11, Issue6November–December 2000Pages 523-526 RelatedInformation

A demonstrator for a low-cost, cordless, multi-carrier spread-spectrum system

Because of the need for spectrally efficient systems for wireless communication, many research activities have been carried out in the area of spread-spectrum techniques. Multi-carrier spread-spectrum (MC-SS) is a new modulation technique with better spectral properties than direct-sequence spread-spectrum (DS-SS). In this paper, a new MC-SS system is introduced. A customized surface acoustic wave (SAW) filter has been designed as a fast analog correlator. A demonstrator testbed has been developed for the 2.4-GHz industrial, scientific, and medical (ISM) band. Experimental measurements of the intermediate frequency (IF) and baseband correlation are presented.

Equal-gain combining of multichannel DPSK in Doppler-spread Ricean fading

A method for computing the bit error probability (BEP) of multichannel differentially coherent phase-shift keying (DPSK) with equal gain combining in Doppler spread Ricean fading is given. It is based on Gauss-Chebyshev quadrature integration of the moment generating function of the decision statistic at the combiner output. The method allows nonuniform power delay profiles for the received multipath rays. Applications to multicarrier spread spectrum modulation and spread spectrum with RAKE reception systems are discussed. It is found that for moderate values of the Ricean K-factor (ratio of specular to diffuse fading signal power) an optimum order of diversity exists whereas, for large K-factors, diversity does not improve performance.

Optimum power control over fading channels

We study optimal constant-rate coding schemes for a block-fading channel with strict transmission delay constraint, under the assumption that both the transmitter and the receiver have perfect channel-state information. We show that the information outage probability is minimized by concatenating a standard Gaussian code with an optimal power controller, which allocates the transmitted power dynamically to the transmitted symbols. We solve the minimum outage probability problem under different constraints on the transmitted power and we derive the corresponding power-allocation strategies. In addition, we propose an algorithm that approaches the optimal power allocation when the fading statistics are not known. Numerical examples for different fading channels are provided, and some applications discussed. In particular, we show that minimum outage probability and delay-limited capacity are closely related quantities, and we find a closed-form expression for the delay-limited capacity of the Rayleigh block-fading channel with transmission over two independent blocks. We also discuss repetition diversity and its relation with direct-sequence or multicarrier spread-spectrum transmission. The optimal power-allocation strategy in this case corresponds to selection diversity at the transmitter. From the single-user point of view considered in this paper, there exists an optimal repetition diversity order (or spreading factor) that minimizes the information outage probability for given rate, power, and fading statistics.

Spreading sequences for multicarrier CDMA systems

The paper contains an analysis of the basic criteria for the selection of spreading sequences for the multicarrier CDMA (MC-CDMA) systems with spectrum spreading in the frequency domain. It is shown that the time-domain crosscorrelation function between the spreading sequences is not a proper interference measure for the asynchronous MC-CDMA users. Therefore, the spectral correlation function is introduced and, together with the crest factor and the dynamic range of the corresponding multicarrier waveforms, is used for the evaluation of MC-CDMA sequences. Some well-known classes of sequences, such as Walsh, Gold, orthogonal Gold, and Zadoff-Chu sequences, as well as Legandre and Golay complementary sequences, are evaluated with respect to the aforementioned basic criteria. It is also shown that the crest factors of the multicarrier spread spectrum waveforms based on the multilevel Huffman sequences are very close to or even lower than the crest factor of a single sine wave.