Frequency Assignment Scheme Research Articles

Heterogeneous phase oscillators encoded by simplexes under coupling strength symmetry breaking: Optimal synchronization and abrupt desynchronization

Recent advances demonstrate that the collective dynamics of heterogeneous phase oscillators ensemble encoded by simplexes play an important role in the storage of information in the human brain. In this letter, we discuss the collective dynamics of heterogeneous phase oscillator networks encoded by simplexes under coupling strength symmetry breaking. Specifically, the strength of the asymmetric coupling between the oscillators is redefined by the degree of the oscillators under 1- and 2-simplex encoding and the heterogeneity parameter. Here, we discuss the effect of different degrees of coupling strength symmetry breaking on the synchronization capability of the system. In particular, we find that proper coupling strength symmetry breaking is beneficial to enhance the synchronization capability of the system. We capture an interesting phenomenon in heterogeneous phase oscillator networks that distinguish symmetric coupling under 1- and 2-simplex encoding, i.e., the system exhibits significant desynchronization behavior under strong symmetry breaking of the coupling strength. Finally, for a fixed heterogeneity parameter and coupling strength assignment probability, we give the optimal intrinsic frequency assignment scheme based on the spectral decomposition of the composite Laplace matrix of the underlying network. Theoretically, these findings may help us to better understand the collective dynamics in heterogeneous phase oscillator networks encoded by simplexes under asymmetric coupling that are prevalent in the real world. In particular, they provide inspiration and guidance for optimizing the synchronization of heterogeneous phase oscillator networks encoded by simplexes under coupling strength symmetry breaking.

Heterogeneous phase oscillators under symmetry breaking of coupling strength: Optimal synchronization and desynchronization under strong gain

Revealing the collective dynamics of heterogeneous oscillator networks under asymmetric coupling is a great challenge. In this study, we discuss optimal synchronization and desynchronization under strong gain in heterogeneous phase oscillator networks with symmetry breaking of coupling strength. Firstly, for a fixed coupling strength, we discuss how the heterogeneity parameter affects the synchronization ability of the system. Secondly, for different values of the heterogeneity parameter, we propose the concepts of gain factor and weakening factor. We find that the synchronization ability of the system is greatly weakened due to the presence of the weakening factor. On the contrary, the presence of the gain factor allows the system to quickly transition to a completely synchronized state even at a small coupling strength. Thirdly, we find an interesting phenomenon that distinguishes the symmetrically coupled heterogeneous phase oscillator networks: the system under symmetry breaking of coupling strength exhibits a significant desynchronization behavior at strong gain. Finally, for a fixed heterogeneity parameter, we give the optimal intrinsic frequency assignment scheme based on the spectral decomposition of the Laplace matrix of the underlying network. Theoretically, these findings may help us to better understand the collective dynamics in heterogeneous phase oscillator networks under asymmetric coupling that are prevalent in the real world. In particular, they provide inspiration and guidance for optimizing the synchronization of heterogeneous phase oscillator networks under symmetry breaking of coupling strength.

Minimizing Interference in Ultra-Dense Femtocell Networks Using Graph-Based Frequency Reuse Technique

This paper investigates the performance of graph colouring schemes for frequency assignment in Long-Term Evolution (LTE) networks with ultra-dense femtocells. The aim of the study is to minimize interference in such networks while ensuring efficient spectrum use for these femtocells. The three schemes investigated are the conventional greedy graph colouring algorithm, the saturation degree algorithm and our proposed graph-based theory (GBT) algorithm. The process of frequency assignment is similar in the last two except that the proposed GBT partitions the femtocells into independent sets for an efficient frequency re-use. The performance of these three schemes was analyzed through extensive simulations to determine the SINR and network capacity that can be obtained with the deployment of these schemes using the ITU-R P1238-7 path loss model. The outcome of this study showed that with the absence of a dynamic frequency assignment scheme, interference level is increased as the number of femtocell users within a particular coverage is increased, leading to a reduction in the capacity of such networks. Simulation results showed that all three algorithms considered have the ability to allocate frequencies to femtocells and minimize interference in a densely deployed environment, thereby increasing network capacity. The proposed GBT assigned the least sub-band thereby ensuring spectral efficiency while minimizing harmful interference. Results show that the greedy algorithm has a disadvantage of inefficiently assigning sub-bands randomly, while the saturation degree assigns more sub-bands when compared with the GBT scheme.Keywords — Femtocell, graph colouring, frequency assignment, LTE.

Incidence Coloring-Assisted Frequency Assignment in Two-Hop OFDMA-based Cellular Networks

In this paper, we consider frequency assignment for orthogonal frequency division multiple access-based cellular networks. We develop a new framework for the mathematical modeling of the frequency assignment problem, which aims at suppressing multi-links interference and enhancing spectral efficiency. The concept of incidence coloring from modern graph theory is utilized to recast the original frequency assignment problem and guide the solution from graph theory perspective. In addition, optimal position of relay stations in a hierarchical cluster based two-hop cellular networks is investigated, and proposes an efficient frequency assignment scheme based on the incidence coloring. System-level simulation results demonstrate that the frequency assignment scheme can effectively mitigate inter-cell interference and improve signal-to-interference-plus-noise ratio. Compared with existing frequency assignment schemes, better coverage performance is obtained and throughput of cell-edge mobile stations is greatly improved.

Resource allocation with interference mitigation in femtocellular networks

ABSTRACTThe introduction of femtocells enabled high data rates and better indoor coverage without the need for expanding or increasing the density of cellular networks deployment or upgrading the cellular network infrastructure. Despite this direct advantage, the introduction of femtocells challenges traditional interference mechanisms because of the ad hoc and dense nature of femtocell deployment. In this paper, we propose an optimal downlink frequency assignment scheme. The proposed algorithm is modeled as a transportation problem to optimally allocate frequency subcarriers with the objective of maximizing the system capacity. The scheme can be used as a benchmark for assessing the suitability of interference mitigation schemes in femtocellular networks.Copyright © 2012 John Wiley & Sons, Ltd.

System-wide energy optimization for multiple DVS components and real-time tasks

Most dynamic voltage and frequency scaling (DVS) techniques adjust only CPU parameters; however, recent embedded systems provide multiple adjustable clocks which can be independently tuned. When considering multiple components, energy optimal frequencies depend on task set characteristics such as the number of CPU and memory access cycles. In this work, we propose a realistic energy model considering multiple components with individually adjustable frequencies such as CPUs, system bus and memory, and related task set characteristics. The model is validated on a real platform and shows less than 2% relative error compared to measured values. Based on the proposed energy model, we present an optimal static frequency assignment scheme for multiple DVS components to schedule a set of periodic real-time tasks. We simulate the energy gain of the proposed scheme compared to other DVS schemes for various task and system configurations, showing up to a 20% energy reduction. We also experimentally verify energy savings of the proposed scheme on a real hardware platform.

Complexity Analysis of a Heuristic Method for Fixed-Frequency Assignment Including Adjacent Channel Interference

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we extend an existing heuristic algorithm for fixed-frequency assignment in cellular communication networks in order to account for the effects of adjacent channel interference (ACI). Our algorithm utilizes estimations of carrier-to-interference ratio instead of binary channel separation constraints. The extended algorithm avoids overoptimistic frequency assignments that do not guarantee interference-free operation environments. Its cost is the increased computational complexity. We estimate the upper bound of the algorithm's complexity when ACI is taken into account. Simulations demonstrate that despite the theoretical high increase in complexity, the algorithm is suitable even for a quasi-dynamic frequency assignment scheme in practical situations. The algorithm enhances the spectrum utilization in cellular networks while reducing harmful interference. </para>

Channelization for dynamic multi-frequency, multi-hop wireless cellular networks

Multi-hop relaying in cellular networks can greatly increase capacity and performance by exploiting the best available links to a base station. We envision an environment in which relay networks are dynamically formed when performance on the radio access network is degraded and then dissolved when the performance improves or the radio spectrum on which the relay network is operating is reclaimed. Each relay network operates on a different frequency band. Likewise, a relay network may channelize its frequency band to offer non-interfering links among the mobile nodes within a single relay network. We propose a set of algorithms used to form such relay networks on-demand. Each algorithm provides a simple and distributed frequency assignment scheme. We also propose two enhancements to improve network throughput of resulting relay networks. We evaluate these algorithms in terms of the overhead of the relay network formation. The evaluation results show that having nodes outmost from the BS initiate route discovery first is the best approach for reducing the formation overhead. The results also show that there is a large increase in throughput when using multiple frequencies in a relay network. Further, the performance of the network using multiple frequencies based on our simple frequency assignment is very close to that of a network using optimal frequency assignment.

Window function and interpolation algorithm for ofdm frequency-offset correction

In orthogonal frequency-division multiplexing (OFDM) digital communication systems, an uncorrected frequency offset can lead to a severe degradation of the system performance. The paper describes a technique to mitigate the influence of frequency offset on system performance by applying a weighted discrete Fourier transform (DFT) to a novel OFDM frequency-assignment scheme in which null carriers are regularly inserted among modulated data carriers. This frequency-assignment scheme makes possible the application of a window function at the demodulator to broaden the signal spectrum and renders the demodulated signal more immune to the influence of frequency offset. In addition, a variant of this frequency-assignment scheme also leads to a DFT-based measurement technique for easy and accurate determination of the frequency offset. Interpolation algorithms and frequency-assignment schemes suitable for both initial frequency acquisition and subsequent frequency tracking are described in detail. The Rife and Vincent (Class-I) windows (see Rife, D.C. and Vincent, G.A., Bell Syst. Tech. J., vol.49, p.197-228, 1976) are examined and their estimation results are obtained for an additive white Gaussian noise (AWGN) and a multipath fading channel.

On the use of optical spreading codes to design DWDM systems with reduced FWM crosstalk

Four-wave mixing (FWM) is a fiber nonlinear effect that limits the maximum reach and minimum channel spacing of a dense wavelength-division multiplexing (DWDM) communication system. We propose an alternative frequency assignment scheme to reduce FWM crosstalk by allocating frequencies to a set of unevenly spaced channels determined by an optical spreading code. This assignment scheme provides several advantages over previously proposed frequency assignment schemes, these include, a reduced optical bandwidth requirements, ease of computing, unlimited channel counts and the complete avoidance of FWM intrachannel crosstalk. Further analysis of the spreading code-based scheme carried out on a 50 GHz spaced 16×2.5 Gbit/s DWDM system, indicates a consistent improvement in Q-factor by over 2 dB compared to similar equal spaced channel DWDM systems.

Efficient frequency assignment scheme for intermodulationdistortion reduction in fibre-optic microcellular systems

An efficient and effective frequency assignment scheme for fibre-optic microcellular systems is presented. The scheme is suitable for intermodulation distortion reduction in multichannel transmission systems. The computation time reduction is quite substantial with good intermodulation distortion reduction.

A spectrum-efficient interference-free frequency allocation scheme for a cellular radio system

A large volume of work has been devoted to searching for a better frequency assignment scheme to handle varying traffic loads in a cellular radio system. However, very little has been accomplished in designing a spectrum-efficient interference-free frequency allocation scheme for this type of system. It is attempted to describe a novel spectrum-efficient frequency allocation scheme that provides an operation which is free from co-channel interference as well as intracell adjacent channel and two-signal third-order intermodulation interference. The proposed scheme is fundamentally different from the existing cellular frequency assignment schemes in which traffic loading is considered without taking interference into account. In the design, we assume that the traffic is uniform in all the cells of the cellular system and there are the same number of channels in each cell.