Zorica Nikolic

Advanced frequency hopping modulation for spread spectrum WLAN

A wireless LAN standard developed by IEEE committee P802.11 operates in the unlicensed 2.4 GHz ISM band. This band is a very hostile environment due to many unpredictable interference sources, such as microwave ovens, utilizing the same frequency bands. At the same time the allowed signal power density that can be used is limited in order to minimize the interference to other users in the same band. In order to avoid these interference and keep a low signal power density the standard supports both direct sequence (DS) and frequency hopping (FH) modulation for these applications. The new test beds for multimedia wireless (WLANs) also use FH modulation. We analyze the performance of a modification of the FH multiple frequency shift keying (MFSK) system that includes a multitone multiamplitude MFSK signal, designated as amMFSK modulation. In this case, in order to meet the transmitted power density limits imposed in the ISM band, the signal energy is split into m separate tones. This makes the system more vulnerable to noise and fading, but still the overall flow of useful information will be increased. The results demonstrate that under the large range of the signal, channel and interference parameters this system offers better performance.

Adaptive self-reconfigurable interference suppression schemes for CDMA

In this paper we present a new approach to interference suppression in code-division multiple-access (CDMA) wireless networks. Depending on the interfering signal, these schemes adaptively change not only their parameters but their structures as well. For the relevant types of the interfering signal, suppression is also possible for the wideband interference occupying the same frequency band as the CDMA signal. Probability of error in these schemes is for several orders of magnitude better than in the existing solutions described in the open literature so far. The main applications are the CDMA overlay type wireless network collocated in the same frequency band with a high bit rate microwave communication system, and the multimedia CDMA network where high bit rate signal due to lower processing gain must use a higher level in order to provide the required quality of transmission.

Performance analysis of dual hop relay link in Nakagami-m fading channel with interference at relay

In this paper we consider performance of a dual hop amplify-and-forward system, with co-channel interference at relay. We consider channel state information assisted system in Nakagami-m channel. The outage probability and the average bit error probability of the system are determined. The influence of the interference, as well as the influence of other systems parameters, such as the fading parameter m and outage probability threshold γth on the systems performance is considered.

Multilayer LMS interference suppression algorithms for CDMA wireless networks

The general theory of adaptive self reconfigurable interference suppression schemes is applied to several specific practical problems mainly suppression of m-level m amplitude-shift keying, m phase-shift keying (PSK), and m quadrature amplitude modulation signals. This is a practical situation when a code-division multiple-access (CDMA) network is overlaid with standard microwave systems. Another example is a multirate CDMA network where a limited number of high bit rate CDMA signals are allowed to use much higher power level due to lower processing gain. The algorithm is well suited for a modular software radio concept, which we believe, will be more and more accepted in future wireless communications. Further modifications of the schemes necessary for these applications are described, and numerous results are presented to illustrate performance improvements. A general interpretation of these techniques based on so-called multilayer least mean squares (LMS) algorithm is introduced and discussed. The algorithm is based on estimating fast changing interfering signal parameters by using parallel structures, which are fast but complex. At the same time, estimation of slow-varying signal parameters over a large range is accomplished by using an LMS algorithm that is simple but slower. In this way, suppression of the interference occupying the same bandwidth as the CDMA signal is possible with reasonable implementation complexity. For this case, a BER 0 dB for binary PSK interference, J/S>17 dB for 8-PSK, J/S>27 dB for 32-PSK, and J/S>40 dB for 128-PSK.

Performance enhancement of DSSS systems: two-dimensional interference suppression

We analyze a new interference suppression structure in direct-sequence spread-spectrum (DSSS) systems that is based on two dimensional (frequency- and time-domain) notch filtering. In the first step, the filter is realized in the form of variable-step (function of estimation error-power /spl nu/) least mean square algorithm. This modification will improve the system performance in the transient period by making this period shorter. Unfortunately, in the steady-state regime, the performance will be slightly degraded. This suggests a modification that will include an algorithm with /spl nu/=2 in the transient state and /spl nu/=1 in the steady-state operation. Further modification of the algorithm will use time notching of the code division multiple-access (CDMA) receiver in the period when estimation error is larger than a given threshold. A comprehensive analysis and comparison of these modifications are presented and discussed. These results can be used either for performance enhancement of DSSS systems in the presence of jamming or for CDMA overlay-type mobile communication networks. In the latter case, the interfering signal comes from another user employing narrow-band modulation. The analysis does not include fading so that the results can be used in the case when the narrow-band modulated signal is not significantly distorted by fading. An example is line-of-sight microwave communications, which happens to be one of the most feasible applications.

Performance of the OFDM system

In this paper, the effect of timing offset, and number of subcarriers on the error probability for an orthogonal frequency division multiplexing (OFDM) system is investigated. It is shown that the probability of error increases as the value of the timing offset increases. Timing offset becomes an increasingly important concern as higher data rates are supported.

A correction to Kurt Metzger's algorithm in the paper "On the probability density of intersymbol interference"

Using an operator approach this letter shows that Metzgers Algorithm for pdf computation (see ibid., vol. 35, pp. 396-402, Apr 1987) has an error in formula (15), which defines the procedure of generating the successive series of pdf approximation. A correction is given.

The CoordSS experimentation framework ontologies

The CoordSS experimentation framework aims to provide a solid practical evidence that testbeds and other experimentation infrastructure provided by EU FP7 FLEX project is capable of supporting advanced experimentation with proposed use of LTE Advanced in unlicensed 5 GHz spectrum (LTE-U). By integrating technologies such as spectrum sensing, various coexistence mechanisms between legacy unlicensed technologies (primarily WiFi) and LTE, and a range of coordination protocols, the frameworks primary objective is to enable experimentation with SDR based LTE in unlicensed spectrum. The flexibility of such experimental environment is addressed by adoption of domain and system ontologies for formal representation of semantics of the problem. This paper deals with ontologies that are needed for the CoordSS experimentation framework implementation.

Performance analysis of FFH/FSK system using diversity

We consider a binary orthogonal frequency-shift-keying receiver employing fast frequency hopping waveforms transmitted over a frequency-nonselective slowly fading channel with partial-band interference. The diversity is performed using multiple hops per data bit. It was shown that the system with diversity has better performances in the case of strong fading or strong interference than the system without diversity.

Performance Improvement of MDPSK Signal Reception in the Presence of Carrier Frequency Offset

Performance of mobile systems is significantly influenced by the carrier frequency offset caused by a Doppler shift, which leads to deployment of differential modulation schemes instead of coherent detection-based schemes. In this paper, we propose a novel M-ary differential phase-shift keying (MDPSK) receiver that performs better than the receiver with MDPSK differential detection and is very close to the performance of a coherent MDPSK detector. The symbol error probability of the proposed MDPSK receiver is approximately constant within the practical frequency offset range. The proposed receiver may be used in the systems that work in fading and with high-frequency offset environments.