In this study, the authors investigate average capacity of free space optics communication over Málaga atmospheric turbulence channel with pointing errors and path loss, for intensity modulated/direct detection (IM/DD) and heterodyne detection. Various algorithms which use adaptive transmission with both types of detection are considered, such as: optimal rate adaption (ORA), optimal power and rate adaption (OPRA), channel inversion with fixed rate (CIFR) and truncated channel inversion with fixed rate (TIFR). Analytical closed‐form expressions for channel capacities of ORA, OPRA and TIFR adaptive transmission are presented, and the authors prove that CIFR transmission is not feasible in the strict sense for the conditions considered. Obtained analytical results are numerically evaluated and graphically presented for different strengths of atmospheric turbulence (in weak, moderate and strong turbulence regime) for both types of detection (IM/DD and heterodyne), and for considered algorithms of adaptive transmission (ORA, OPRA and TIFR). The authors have developed expressions suitable for approximating high signal‐to‐noise ratio channel capacity, and they graphically present and compare the asymptotic approximations with the obtained analytical results for different strengths of turbulence for both types of detection. Also, obtained analytical results were confirmed by Monte‐Carlo simulations, and graphically compared for different strengths of turbulence regimes.

The performance of a time hopping combined with pulse position modulation ultra wide band system with a nonlinear receiver using an adaptive filter and a soft-limiter in the presence of narrowband interference has been considered. This system brings a significant performance improvement compared with the system that uses an adaptive filter and a linear receiver, particularly in the case of high-interference power where its absence leads to the reception loss. Also, the proposed receiver has much better interference rejection performance compared with minimal mean-square error interference rejection, in case of high-interference power (interference to signal ratios from 30 to 65 dB).

In this paper we analyze capacity losses in advanced code division multiple access (CDMA) network due to imperfections in the operation of the system components. In addition to the standard WCDMA technology both, base stations and mobile units use antenna beam forming and self steering to track the incoming (and transmitted) signal direction. By using high directivity antennas and antenna pointer tracking the level of multiple access interference (MAI) and the required transmitted power are reduced. In order to exploit the available propagation diversity signals arriving from different directions (azimuth psi elevation phi) and delay tau, are combined in 3D (psi phi, tau) RAKE receiver. This is expected to significantly improve the system performance. The main result of this work is a systematic mathematical framework for capacity evaluation of such CDMA network in the presence of implementation imperfections and fading channel. The theory is general and some examples of practical set of channel and system parameters are used as illustration. As an example, it was shown that in the case of voice applications and 2D (4 antennastimes4 multipaths) RAKE receive, up to 90% of the system capacity can be lost due to the system imperfections. Further elaboration of these results, including extensive numerical analysis based on the offered analytical framework, would provide enough background for understanding of possible evolution of advanced W-CDMA and MC-CDMA towards the fourth generation of mobile cellular communication networks

We introduce a new concept of multiple access called /spl tau/-CDMA. The abbreviation stands for code division multiple access with delay (or) modulation and hopping. The concept is based on a modification of direct sequence spread spectrum (DSSS) system where transmitted waveform includes multiple amplitude and delay replicas of DSSS signal. In general notation amM/spl tau/-DSDH is used for the DS signal that includes m delayed (/spl tau/) replicas of different amplitudes (a), sent in a limited delay window of M chip intervals. The position of the delay window is hopped (delay hopping-DH) in the range of the code length N. This provides resistance to near far effect without the need for complex multiuser detectors. The variable impacts of the near far effect, for different positions of the delay window and fading, are simultaneously reduced by interleaving. If the signal energy is split to m>1 separate components, making it more vulnerable to noise and fading, the overall flow of useful information will be still increased. The results demonstrate that under the large range of the signal, channel and interference parameters this system offers better performance. This system combines the good characteristics of DSSS and FHSS systems. The near far effect is mitigated without need for complicated multiuser detectors and at the same time the simplicity of the DSSS system is preserved. There is no need for a frequency synthesizer. The coherency for coherent RAKE reception is maintained in a much simpler way than in the FH system. Higher suppression of the multipath component is achieved than in a standard DSSS system. For the above signal formats this paper offers a number of receiver configurations ranging from very simple to more complex ones providing better and better performance. The application is energy aware software radio where a configuration control algorithm would chose the simplest configuration in order to provide the required QoS for the given environment with the minimum processing power resulting in minimum energy consumption.

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.

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 BER0 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.

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.

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.

In this paper we analyze the performance of wireless ad hoc networks based on mMFSK frequency hopping modulation. This modulation is a modification of the FH/MFSK system that includes a multitone MFSK signal which is designated as mMFSK modulation. In this case the signal energy is split to m separate tones making it more vulnerable to noise and fading but still the overall flow of useful information is increased. The results demonstrate that under the large range of the signal, channel and interference parameters this system offers better performance than the standard MFSK/FH system.

Using an operator approach this letter shows that Metzger's 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.