Zbynek Fedra

Towards cognitive radio networks: Spectrum utilization measurements in suburb environment

This paper deals with spectrum utilization measurements in the frequency band from 100 MHz up to 3 GHz. The measurement is based on the energy detection principle using wideband logarithmically periodic antenna. The results point out the fact, that the frequency spectrum is not utilized in an optimal manner and that there do exist less or more utilized licensed frequency bands that could be possibly used by cognitive radios in an opportunistic way. Cognitive radio concept for better spectrum utilization is introduced here along with an overall approach regarding spectrum utilization in the next generation wireless networks.

The characteristics of the 2D spreading based communication systems

This paper deals with characteristics and theory of code division multiple access (CDMA), orthogonal frequency division multiplexing (OFDM) and variable spreading factor orthogonal ferquency and code division multiplexing (VSF-OFCDM) systems. As comparison and evaluation of VSF-OFCDM with various 2D spreading factor (SF) as frequency and time dispersive transmission channel according to ETSI standard (typical rural area and warehouse enviroment) is being done.

The new PAPR reduction approach in MC-CDMA

This paper describes a new PAPR (peak to average power ratio) reduction approach in MC-CDMA (multicarrier code division multiple access) system (in so called M-modification). The spreading sequence introduces coherence among subcarriers. This is used for chip position formatting to PAPR minimization. The optimizations methods are used to get best chip interleaving sequence (namely genetic algorithm (GA) and ant colony optimization (ACO)). BER simulation in AWGN (additive white Gaussian noise) channel with clipping is presented to evaluate efficiency of this concept

Reduced Complexity Adaptive Symbol Selection Method for OFDM PAPR Reduction

A lot of methods for peak to average power ratio (PAPR) reduction of OFDM signals have been proposed over the last few years. One of the main drawbacks of many of them is their complexity. In this paper, a new technique for reducing complexity of method based on Adaptive symbol selection principle is proposed and evaluated using simulation

Analysis of spectrum utilization in suburb environment—evaluation of potentials for cognitive radio

This paper deals with spectrum utilization measurements in the frequency band from 100 MHz up to 3 GHz. The measurement method is based on the energy detection principle using wideband log-periodic antenna. Our results show that the radio spectrum is not utilized in an optimal manner and that there do exist sporadically utilized licensed frequency bands that could be possibly used by cognitive radios in an opportunistic way. Cognitive radio concept for better spectrum utilization is introduced here along with an overall approach regarding spectrum utilization in the next generation wireless networks.

Two Dimensional Signal Spreading in UMTS LTE: Exploiting Time-Frequency Diversity to Increase Throughput

This paper proposes a two-dimensional spreading method in UMTS Long Term Evolution (LTE). Such a method brings additional time-frequency diversity which is beneficial in terms of the ability to decode transmitted data at the receiver site under multipath fading. Additional diversity is provided due to the fact that chips of transmitted signals are spread using a Walsh-Hadamard spreading sequence over several subcarriers as well as over several time-slots simultaneously. A comparison with state-of-the-art LTE downlink transmission is provided. To enable reproducibility and to increase credibility of our results, an open source Vienna LTE simulator is utilized. LTE transmissions with the proposed 2D spreading are tested on several channel models. Simulations show that apart from an Additive White Gaussian Noise channel model, this 2D spreading based method outperforms the throughput performance of standard LTE. At high Signal to Noise Ratio (SNR), the throughput increase in most channel models is higher than 10 %.

Indoor channel modeling based on experience with outdoor urban measurement — Multislope modeling

This paper discusses indoor channel modeling for mobile communications. We apply multislope log-distance channel model and simulate indoor path loss distribution. The standard deviation value of normal and log-normal distribution describes various fading effects affecting the signal propagation. We defined the areal signal path loss distribution in a restricted indoor environment. The PSO-based process with the MSE estimation determinates the multi break point model for the tested building. Results of our work are the optimized MSE fitted multislope models.

PAPR and amplitude distribution in MC-CDMA system

This paper analyzes the crash factor (CF) for mul-ticarrier code division multiple access (MC-CDMA) communication system. The CF (or peak to average power ratio - PAPR) is critical in multicarrier orthogonal systems (based on orthogonal frequency division multiplex - OFDM). The amplitude statistics for system are computed and gamma distribution is considered for amplitude distribution modeling. The distribution parameters are estimated for different scenarios (different number of users in system). The usability for PAPR reduction is discussed.

Interleaving Optimization in OFDM PAPR Reduction

One of approaches in the Peak to Average Power Ratio (PAPR) reduction methods for Orthogonal Frequency Division Multiplex (OFDM) is multiple signal representation (MSR) technique. This article presents the improvement of MSR technique where multiple signal representations are produced by different interleavers. The interleavers used in this approach are optimized and integrated with phase rotation (realized as element-by-element multiplying). The simulation for two-branch classical and optimized MSR is introduced in the article. The improvement in complementary cumulative distribution function of(CCDF) PAPR is presented.

The FPGA implementation of dictionary; HW consumption versus latency

This paper presents a study on the possibilities of dictionary implementation for a dictionary compression system in FPGA. The possibilities of creation of the dictionary using the registers and the RAM memory are described. For these implementations, the interdependence of hardware consumption and latency during basic dictionary processes like writing, reading and searching a match is described. With regard to the hardware consumption, RAM dictionary implementation is the most effective. On the contrary, dictionary implementation by the registers is more suitable with regard to the latency during searching the matches in the dictionary. For these reasons, the combination of both implementation approaches was implemented. The core of the dictionary is in RAM memory and the searching part is implemented by the registers. This part is similar to a hash table. It is implemented by the parallel shift registers to minimize the searching latency.