Hierarchical Cellular Systems Research Articles

Online Steady-State Security Awareness Using Cellular Computation Networks and Fuzzy Techniques

Power system steady-state security relates to its robustness under a normal state as well as to withstanding foreseeable contingencies without interruption to customer service. In this study, a novel cellular computation network (CCN) and hierarchical cellular rule-based fuzzy system (HCRFS) based online situation awareness method regarding steady-state security was proposed. A CCN-based two-layer mechanism was applied for voltage and active power flow prediction. HCRFS block was applied after the CCN prediction block to generate the security level of the power system. The security status of the power system was visualized online through a geographic two-dimensional visualization mechanism for voltage magnitude and load flow. In order to test the performance of the proposed method, three types of neural networks were embedded in CCN cells successively to analyze the characteristics of the proposed methodology under white noise simulated small disturbance and single contingency. Results show that the proposed CCN and HCRFS combined situation awareness method could predict the system security of the power system with high accuracy under both small disturbance and contingencies.

A hierarchical cellular manufacturing system

A hierarchical cellular manufacturing system

A hierarchical cellular manufacturing system

Cellular manufacturing is one of the most common and popular methods in order to take advantage of group technology. Of particular interest, applying cellular manufacturing systems (CMSs) increases the efficiency of the applied transportation system by designing an efficient working floor. Controlling the amount of material handling, results in lower cost and energy required to transport materials/products. Hence, CMS are under consideration in the study at hand. Regarding a hybrid CMS, proper manufacturing systems such as single machines, flow shops, job-shops and open shops are applied between and within cells to harmonise the whole manufacturing procedure. A special case of hybrid CMS, referred to as hierarchical CMS is investigated in this paper. In a HCMS not only machines are placed in several interior cells, but also interior cells themselves must be located in exterior ones. Finally a mixed integer nonlinear programming mathematical model is presented and validated through numerical results.

Joint Antenna Beamforming, Multiuser Scheduling, and Power Allocation for Hierarchical Cellular Systems

Cognitive radio (CR) is concretely embodied in hierarchical cellular systems by deploying an underlying microcellular system to reuse the underutilized spectrum of a macrocellular system. One of the key challenges to the success of hierarchical cellular systems is to manage the intercell interference between the macrocell and the microcell and to maximize the spectrum efficiency. In this paper, antenna beamforming, power allocation, and multiuser scheduling are jointly designed to opportunistically utilize the macrocell's uplink spectrum in serving multiple secondary microcellular users concurrently. The joint design of antenna beamforming, power allocation, and scheduling with the objective of maximizing the sum rate is indeed a mixed-integer nonlinear programming NP-hard problem. The proposed simpler iterative subgradient projection and semidefinite programming approach can obtain better performance than the conventional zero-forcing beamforming. Furthermore, unlike the optimal singular value decomposition (SVD) beamforming that requires all users to have channel knowledge at the receiver for cooperation, the proposed joint design methodology requires no channel knowledge at the receiver and can outperform the SVD beamforming without user scheduling. When considering both implementation complexity and performance enhancement issues, the proposed joint power allocation, multiuser scheduling, and antenna beamforming technique can help provide important insights into the design of interference management techniques for hierarchical CR systems.

On the Design of Delay based Admission Control in Hierarchical Networks

Today, as the hierarchical cellular system is getting more attention than before, some recent studies introduce delay based admission control (AC) scheme which delays the admission to the macro-embedded small cell for a relatively short time to prevent unnecessary handover caused by the short-term visitors of the small cell area. In such delay based ACs, when we use improper delay parameter, the system frequently makes incorrect handover decisions such as where unnecessary handover is allowed due to too short delaying, or where necessary handover is denied due to too long delaying. In order to avoid these undesirable situations as much as possible, we develop a new delay parameter decision method based on probabilistic cell residence time approximations. By the extensive numerical and analytical evaluations, we determine the proper delay parameter which prevents the incorrect handover decision as much as possible. We expect our delay parameter decision method can be useful system administration tips in hierarchical cellular system where delay based AC is adopted.

Performance Analysis of Joint Base-Station Multiantenna Multibeam and Channel Assignment Scheme for Hierarchical Cellular System

Different from the traditional configuration of hierarchical cellular, we introduce the joint base-station multi-antenna multi-beam and channel assignment scheme for hierarchical cellular in this paper. The proposed scheme is based on multi-beam base-station antenna splitting in the elevation-radiating plane, and a dynamic channel allocation scheme by combining the adaptive antenna technologies. Simulation results show that the proposed configuration of the hierarchical cellular can enhance the spectral efficiency remarkably.

Interference management in OFDMA femtocell networks: issues and approaches

One of the effective techniques of improving the coverage and enhancing the capacity and data rate in cellular wireless networks is to reduce the cell size (i.e., cell splitting) and transmission distances. Therefore, the concept of deploying femtocells over macrocell has recently attracted growing interests in academia, industry, and standardization forums. Various technical challenges towards mass deployment of femtocells have been addressed in recent literature. Interference mitigation between neighboring femtocells and between the femtocell and macrocell is considered to be one of the major challenges in femtocell networks because femtocells share the same licensed frequency spectrum with macrocell. Further, the conventional radio resource management techniques for hierarchical cellular system is not suitable for femtocell networks since the positions of the femtocells are random depending on the users' service requirement. In this article, we provide a survey of the different state-of-the-art approaches for interference and resource management in orthogonal frequency-division multiple access (OFDMA)-based femtocell networks. A qualitative comparison among the different approaches is provided. To this end, open challenges in designing interference management schemes for OFDMA femtocell networks are discussed.

Speed-Sensitive Handover Scheme over IEEE 802.16 Multi-Relay Networks

Abstract —Multi-Relay Networks should accommodate mobile users of various speeds. The cellular system should meet the minimum residency time requirements for handover calls while considering an efficient use of available channels. In this paper, we design speed-sensitive handover under dynamic hierarchical cellular systems, in which mobile users are classified according to the mean speed of mobile users and each class has its cellular layer. In order to meet the minimum residency time, the cell size of each cellular layer is dynamically determined depending on the distributions of mean speeds of mobile users. Since the speed-dependent non-preferred cell can provide a secondary resource, overflow and take-back schemes are adopted in the system. We develop analytical models to study the performance of the proposed system, and show that the optimal cell size improves the blocking probability. Keywords —Multi-Relay Networks, Handover 1. I NTRODUCTION The existing IEEE 802.16 is a series of Wireless Broadband standards known as WiMAX as an important technology for providing high-speed Internet access to home and business sub-scribers in a wide area cellular network[1]. Handover is a key factor for the quality of wireless communication services. Due to the geographical problem, significant data loss occurrs, the coverage region for high data rates is decreased, and even dead spot areas in the coverage region occurr. IEEE 802.16j adopts multihop or relay technologies in order to eliminate dead spot areas and extend the coverage region. In IEEE 802.16j networks, the Relay Stations (RSs) relay sig-naling and data messages between the Multihop Relay BS (MR-BS) and the Mobile Station (MS)[2, 3]. Therefore, handover issues for the RS on IEEE 802.16j have been carried out[4]. The increasing demands for mobile communications are proceeding with a strong tendency toward increasing need for high traffic capacity and mobility in the access links within the wire-less links[5, 6]. Since the micro-cell of the micro-cellular systems makes an efficient use of available channels by using low-power transmitters to allow channel reuse at much smaller dis-tances, the micro-cellular systems with an extremely small cell radius can meet the traffic de-mand[7, 8]. However, the micro-cell increases the number of cell boundary crossings for high-mobility users, and the increasing handover rates of ongoing calls may cause an excessive proc-essing load in the network. Furthermore, the handover calls of fast-moving users cannot meet

Mobile Velocity Based Bidirectional Call Overflow Scheme in Hierarchical Cellular System

Mobile Velocity Based Bidirectional Call Overflow Scheme in Hierarchical Cellular System

Uplink analysis for hierarchical CDMA systems with attenuators applied to microcell under imperfect power control

Under the signal-strength-based power control scheme, the effect of various microcell cluster locations on the uplink average signal-to-interference ratio (also called carrier-to-interference ratio, or CIR) is studied for the hierarchical code-division multiple-access cellular system with attenuators applied to each microcell base station. On the basis of the interference statistics analysis, the influence of power control errors on the uplink average bit error rate (BER) is also studied under the multipath fading and shadowing. Results show that both central macrocell and microcell uplink CIR are almost unaffected by the variations of microcell cluster locations, and that the uplink average BER is evidently affected by the imperfection of power control and the shadowing standard deviations.

ALL-IP 4G NETWORK ARCHITECTURE FOR EFFICIENT MOBILITY AND RESOURCE MANAGEMENT

In this article, we investigate 4G network architecture and consider two underlying layers: PHY and MAC. We compare two models of wireless access network: pure all-IP and subnet based networks. The pure all-IP model is simple and cost-efficient but causes implementation issues of mobility management and resource coordination. In contrast, the subnet based network enables layer 2 and layer 3 handoffs to be executed independently, deploying several access points under an access router. Further, to handle various cases efficiently according to traffic class and mobility, we present an advanced model of a hierarchical cellular system that combines multiple access techniques of OFDMA and FH-OFDMA with microcells and macrocells. Finally, as an integrated approach to support diverse QoS requirements, we consider an IP-triggered resource allocation strategy (ITRAS) that exploits IntServ and DiffServ of the network layer to interwork with channel allocation and multiple access of MAC and PHY layers, respectively. These cross layer approaches shed light on designing a QoS support model in a 4G network that cannot be handled properly by a single layer based approach

Soft Handoff Analysis of Hierarchical CDMA Cellular Systems

A new soft handoff analysis for hierarchical code-division multiple-access (CDMA) cellular systems is presented. Hierarchical cellular architectures have been proposed to increase cellular system capacity and flexibility. In order to extend such architectures to CDMA-based systems, the performance of soft handoff in hierarchical architectures must be considered. We first develop an analytical method for studying the interference in hierarchical CDMA cellular systems. We then apply the obtained results to a soft handoff analysis model to study the performance of soft handoff in hierarchical architectures. It is observed that dynamic handoff parameter assignment, where parameters are dynamically adjusted according to given interference conditions, offers a more efficient handoff mechanism than fixed handoff parameter assignment.

Handoff counting in hierarchical cellular system with overflow scheme

In mobile computing scenario, the number of handoff (called as handoff counting) is very important for the signalling network design and traffic load analysis. In this paper, we study the handoff counting characteristics in terms of probability mass function (pmf) and statistical moments in the hierarchical cellular system applying overflow scheme under the general cell residence time and general call holding time distribution. The analytical result and the algorithm for the evaluation of handoff counting are, respectively, presented for the cases whether or not the Laplace–Stieltjes Transform of call holding time exists. The numerical results reveal that as the call holding time is heavy-tailed, the handoff counting follows the heavy-tailed tendency. In addition, the handoff counting pmf as well as the average value shows a significant different pattern with the utilization of different call holding time distribution.

A neural fuzzy resource manager for hierarchical cellular systems supporting multimedia services

Using intelligent techniques to perform radio resource management is an effective method. The paper proposes neural fuzzy control for radio resource management in hierarchical cellular systems supporting multimedia services. A neural fuzzy resource manager (NFRM) is designed, which mainly contains a neural fuzzy channel allocation processor (NFCAP). The NFCAP has a two-layer architecture: a fuzzy cell selector (FCS) in the first layer and a neural fuzzy call-admission and rate controller (NFCRC) in the second layer. The FCS chooses not only the handoff failure probabilities and the resource availabilities in both microcell and macrocell, but also the user mobility, as input linguistic variables. The NFCRC takes the handoff failure probability and the resource availability of the selected cell as input variables to perform call admission control and rate control for the call. Simulation results show that the NFRM can always guarantee the quality of service (QoS) requirement for handoff failure probability for all traffic loads. Also, the NFRM improves the system utilization by 31.1% while increasing the handoff rate by 2% over the overflow channel allocation (OCA) scheme; it enhances the system utilization by 6.3% and 1.4%, and still reduces the handoff rate by 14.9% and 6.8%, as compared to the combined channel allocation (CCA) and fuzzy channel allocation control (FCAC) schemes, respectively, under a predefined QoS constraint.

Dynamic guard channel assignment for bidirectional call-overflow scheme in Hierarchical Cellular System

In this paper, a Hierarchical Cellular System (HCS) supporting two classes of users with high and low velocity is considered. Based on the analytical model of the bidirectional call-overflow scheme, a new dynamic guard channel assignment method is proposed. In this method, the number of guard channels in both macrocells and microcells varies in realtime according to the different call traffic load in the HCS, and as a result, the lowest dissatisfaction grade of users is achieved.

Performance evaluation of a hierarchical cellular system with mobile velocity-based bidirectional call-overflow scheme

With the increase of teletraffic demands in mobile cellular system, hierarchical cellular systems (HCSs) have been adopted extensively for more efficient channel utilization and better GoS (Grade of Services). A practical issue related to HCS is to design a scheme for controlling and allocating call traffic to different layers. There are several strategies to deal with this problem, such as no call-overflow scheme, unidirectional call-overflow scheme and bidirectional call-overflow scheme. The objective of this paper is to investigate a bidirectional call-overflow scheme, based on the velocity of the mobile making the calls. To ensure that hand off calls are given higher priorities, it is assumed that guard channels are assigned in both macrocells and microcells. In order to evaluate the performance of the new scheme and compare the performance of several related schemes, two now models based on a one-dimensional Markov process are developed and analytical results are derived. Theoretical analysis and numerical evaluation show that the proposed scheme outperforms others in terms of average hew call blocking and hand off failure probability of the system. In addition, when the teletraffic to the HCS reaches a certain grade, the GoS is insensitive to the maximum velocity and the velocity threshold which is used to assign calls to different layers in our scheme.

Modeling and analysis of hierarchical cellular networks with general distributions of call and cell residence times

We present an analytic model for the performance evaluation of hierarchical cellular systems, which can provide multiple routes for calls through overflow from one cell layer to another. Our model allows the case where both the call time and the cell residence time are generally distributed. Based on the characterization of the call time by a hyper-Erlang distribution, the Laplace transform of channel occupancy time distribution for each call type (new call, handoff call, and overflow call) is derived as a function of the Laplace transform of cell residence time. In particular, overflow calls are modeled by using a renewal process. Performance measures are derived based on the product form solution of a loss system with capacity limitation. Numerical results show that the distribution type of call time and/or cell residence time has influence on the performance measure and that the exponential case may underestimate the system performance.

Call admission control in the microcell/macrocell overlaying system

Call admission control can significantly affect the performance of a cellular system by adding additional bandwidth to a wireless network. In this paper, we show how call admission control can be used to optimize the performance of a hierarchical cellular system. The hierarchical system we study is based on a novel frequency planning scheme, whereby both the micro and macro layers can share the same spectrum and a hard partition of the frequency spectrum is not needed. The original analysis of the hierarchical system showed that a significant capacity gain can be achieved by the scheme. However, this capacity is gained at the expense of the carrier-to-interference ratio (C/I) performance of the macrocells. In this paper, we show that call admission control can be used in hierarchical cellular systems to achieve a capacity gain without sacrificing the C/I performance of the macrocells.

Performance analysis of flexible hierarchical cellular systems with a bandwidth-efficient handoff scheme

In this paper, a bandwidth-efficient handoff strategy is proposed and analyzed for hierarchical cellular systems. Mobile subscribers are divided into two classes, i.e., low- and high-mobility subscribers. In our bandwidth-efficient handoff strategy, each of the originating and handoff calls of both slow and fast mobile subscribers first tries to get a channel in a microcell. Macrocells are overlaid over the microcells to handle overflowed calls. A call overflowed into a macrocell requests a take-back to the new microcell at each border crossing of the microcells. The request will be accommodated by the target microcell if there is any idle traffic channel in the cell. An analytical model is developed, and the most important performance measures such as the blocking probability of originating calls and the forced termination probability of calls are evaluated. It is shown through extensive comparisons with other candidate handoff strategies that if the total traffic load of the system is not very heavy, our scheme has the best bandwidth efficiency and can provide better quality of service for mobile subscribers without bringing too much processing expense to the system.

Mobile speed estimation for TDMA-based hierarchical cellular systems

A new algorithm is presented for estimating mobile speed for handoff in hierarchical cellular systems. The proposed algorithm is based on normalized autocorrelation values of received signals to estimate mobile speed; it contains six steps. First, the instantaneous power of the received baseband signal is calculated to remove the offset and data/speech information-bearing signals, while keeping the Doppler information. Second, the calculated power signal is filtered using a low-pass linear phase finite impulse response filter to suppress interference and noise. Third, the filtered power signal is decimated to ease the computational burden, while the decimation factor is properly chosen to avoid aliasing. Fourth, autocorrelation values of the decimated filtered power signals are calculated on shifting by to suppress the slot burst frequency interference. Fifth, the calculated autocorrelation values are normalized to suppress the power fluctuation of the received signals. Finally, the normalized autocorrelation values are compared with thresholds to estimate mobile speed. The simulation results indicate that the new algorithm works very well for both nondispersive channels and dispersive channels to distinguish fast and slow moving mobiles. The method has very low latency, with results being available typically within 1 s after communication is established, and it can report estimation result every second or less. The algorithm has been implemented by software code into Nortel's base-station radios and tested in Nortel's wireless communications labs.