Tomaz Javornik

Comparing the cloud effects on hybrid network using optical wireless and GHz links

The huge bandwidth of optical Wireless link motivates to use it for high data rate demanding communication applications. However, an appreciable high availability of the link is the basic requirement of any communication link. Free Space Optics (FSO) links are highly weather dependent. One of the important attenuating factors is snow that causes reduced the link availability. The use of back up link can provide the solution. Hybrid networks of Free Space Optics (FSO) link and back up link in the GHz frequency range improves the availability besides providing comparable data rates. The back up link should be nearly immune to different attenuation factors including snow for achieving carrier class availability. In this paper effects of dry snow on FSO and GHz frequency range links are studied so that frequencies with best complementary behaviour can be selected as a back up link. The specific attenuation of different FSO lengths has also been analysed using measured results.

Comparison of WiMAX coverage at 450MHz and 3.5GHz

In this paper we calculate, analyze and compare the area covered by radio signal based on WiMAX standard at two carrier frequencies, namely 450 MHz and 3.5 GHz in flat rural, hilly rural and urban environment. The channel model proposed by WiMAX forum has been applied as path loss model at 3.5 GHz for cell coverage prediction, while at 450 MHz the Longley-Rice model for rural areas and Okumura Hata channel models for urban area is used. The cell size prediction strongly suggests to limit the 3.5 GHz frequency band to urban areas, where the higher system capacity is required, while in rural areas the 450 MHz carrier frequency provide good compromise between coverage and system capacity

Virtual multiple input multiple output in multiple high-altitude platform constellations

Multiple-input multiple-output (MIMO) technology, which makes use of the spatial dimension by utilising multiple antennas at the transmitter and receiver, has proved an efficient solution for providing higher data throughput and/or link reliability in wireless systems. In this study, the authors investigate a virtual MIMO (V-MIMO) technique, based on a constellation of multiple high-altitude platforms (HAPs), providing broadband wireless access to high-speed trains under predominantly line-of-sight (LOS) propagation conditions. They analyse the performance of transmit diversity based on space-time block coding (STBC), in particular Alamouti and extended Alamouti schemes, using fixed wide-lobe receive antennas, and compare it to the reference receive diversity scheme based on best HAP selection that requires highly directional and steerable antennas. Simulation results for different diversity schemes are based on the calculation of carrier to interference-plus-noise ratio (CINR) along representative railway lines in a realistic terrain configuration. The CINR levels obtained are mapped onto transmission modes specified by standards developed for two wireless systems operating at distinct frequency bands, the DVB-S2 assumed for operation in mm-wave bands and the IEEE 802.16e (mobile WiMax) for operation at 3.5 GHz. Simulation results show that the use of transmit diversity schemes in a multiple HAP constellation decreases the link outage and also increases the average spectral efficiency, but requires a marked increase in transmit power, especially for the system operating in mm-wave bands.

Rate-compatible punctured DVB-S2 LDPC codes for DVB-SH applications

In this paper, we present and analyze three padding and puncturing schemes for DVB-S2 low-density parity-check (LDPC) codes, in order to obtain coding rates equivalent to DVB-SH specification (2/3, 1/2, 2/5, 1/3, 2/7, 1/4, 2/9 and 1/5). We compare the performance of punctured DVB-S2 LDPC codes against the 3GPP2 turbo codes. Simulation results for bit error rate (BER) and frame error rate (FER) show that the proposed coding schemes exhibit 0.5 dB to 0.75 dB penalty loss compared to 3GPP2 codes. The codes with random and deliberate puncturing show nearly the same performance in AWGN channel, while the optimal puncturing shows 0.2 dB better performance.

Evaluation of diversity gain and system capacity increase in a multiple HAP system

High altitude platform (HAP) system offers establishment of communication links with predominantly line-of-sight (LOS) conditions due to high elevation angles. However, in mobile scenario, the transmitted signal is still subject to blocking due to obstacles between HAP and the user, which increases the probability of link outage. In the light of this, multiple HAP constellations with overlapped coverage areas can be exploited for platform diversity to improve the system reliability and availability or for the increase of the system capacity through the use of highly directional user antennas. In this paper we evaluate a potential diversity gain and capacity increase for a mobile scenario based on a simulation taking into account a digital relief model (DRM) of Slovenia and two main railway tracks, providing results in terms of the timeshare of different visibility events and the CINR values

Experimental analysis of wireless temporary networks deployed by low altitude platforms

The design of opportunistic wireless networks for disaster relief operations requires a good understanding of communication demands and technological capabilities. Theoretically determined or simulation based parameters such as data rate and communication range need to be experimentally confirmed in practice, requiring measurement campaigns in realistic environment. This paper reports the results from three experiments, performed with a hot-air balloon and a personal car, equipped with TETRA, WiFi and WiMAX communication equipment. The results for signal measurements show the strengths and weaknesses of the three technologies used for low altitude platform communications and reveal some practical aspects, which are important for the network designers and operators.

Radio environment map (REM): An approach for provision wireless communications in disaster areas

In this paper we propose radio environment map (REM) as a mean for setting up and maintaining wireless communications in disaster areas providing services to public protection disaster relief (PPDR) forces and disaster victims. The wireless communication infrastructure is expected to be partially or fully damaged in the disaster area, resulting in an unknown radio environment. In our approach, the initial base station parameters are estimated by applying the knowledge of the radio environment prior to the disaster and by an initial spectrum sensing, while during the operation phase the base station parameters are updated based on the REM, which relies on distributed spectrum sensing. The REM implementation using the open source geographical information system GRASS and its add-on RaPlaT is described.

N-MSK adaptive modulation techniques in a slow Rayleigh fading channel

Various N-MSK adaptive modulation techniques are described. Two of them, the 2-MSK and 4-MSK adaptive modulation schemes, are analysed in a slow Rayleigh fading channel. Adaptive rate communication systems can track the channel variations. Therefore, the average system throughput is increased at given BER. The proposed modulation techniques are especially suitable for communication systems, where power sources are limited.

Target BER Driven Adaptive Coding and Modulation in HAP Based DVB-S2 System

In this paper we are concerned with the provision of broadband wireless access to mobile users via high altitude platform (HAP) system, which is characterized by a challenging propagation environment in the allocated frequency bands, requiring the development of efficient radio interface. The latter has to offer the ability to adapt to rapid channel variations, for which the low propagation delay is the essential enabling characteristic. Assuming the forward link based on the DVB-S2 standard, adapted for the operation over HAPs, we focus in this paper on the design and performance evaluation of an adaptive coding and modulation (ACM) scheme, which is used to maximize the link reliability and/or throughput. The scheme considers a subset of transmission modes (MODCODs) defined in the standard and exploits two parameters in making the MODCOD switching decisions, the estimated signal-to-noise ratio (SNR) and the estimated bit error rate (BER), which take effect on different time scales. For the SNR estimation SNORE algorithm was adopted, while for the BER estimation we propose a new algorithm exploiting the properties of LDPC decoder and compare its performance to the traditional approach based on cyclic-redundancy check (CRC) computation. The performance evaluation results are provided in terms of suitable internal parameters of the algorithm and as the trade-off analysis between BER and spectral efficiency.

Effects of nonlinear high power amplifier on the area covered by WiMAX signal

In this paper we analyze the effects of nonlinear high power (HPA) amplifier on the area covered by WiMAX signal and nonlinearity impact on the capacity of the WiMAX system assuming the uniform spatial distribution of users. The link degradation due to nonlinear amplifier in line of sight (LOS) channel and two channel models, SUI 4 and SUI 5, for multipath propagation is estimated first. The results obtained are the input for further analysis of the nonlinearity effects on the area covered by the WiMAX signal and the system capacity. It was found out that the coverage area is not seriously reduced due low sensitivity of BPSK and QPSK signals on the nonlinearity, while the significant decrease in the system capacity is observed, because of the rise in BER of higher level modulation schemes caused by nonlinearity.