Neighboring Base Stations Research Articles

Enhancing cell-edge performance: a downlink dynamic interference avoidance scheme with inter-cell coordination

Interference management has been a key concept for designing future high data-rate wireless systems that are required to employ dense reuse of spectrum. Static or semi-static interference coordination based schemes provide enhanced cell-edge performance but with severe penalty to the overall cell throughput. Furthermore, static resource planning makes these schemes unsuitable for applications in which frequency planning is difficult, such as femtocell networks. In this paper, we present a novel dynamic interference avoidance scheme that makes use of inter-cell coordination in order to prevent excessive inter-cell interference, especially for cell or sector edge users that are most affected by inter-cell interference, with minimal or no impact on the network throughput. The proposed scheme is comprised of a two-level algorithm - one at the base station level and the other at a central controller to which a group of neighboring base stations are connected. Simulation results show that the proposed scheme outperforms the reference schemes, in which either coordination is not employed (reuse of 1) or employed in a static manner (reuse of 3 and fractional frequency reuse), in terms of cell edge throughput with a minimal impact on the network throughput and with some increase in complexity.

THROUGHPUT IMPROVEMENT UNDER NETWORK LOAD ON FORWARD LINK INTER-CELL INTERFERENCE

A system with more throughput under the impact of network load in the neighboring sectors on the inter-cell interference in a cellular data network is proposed. The signal received by a user over the forward link in such a system contains interference from the neighboring base stations. The proposed scheme provides a more accurate estimate of the user SINR by taking better account of the contribution of inter-cell interference. It builds on top of the current SINR measurement scheme by using a combination of pilot measurement and traffic load measurement. With the proposed scheme a mobile terminal reports an accurate SINR and the same is informed to the base station. Hence the base station increases the data rate and hence the mobile terminal receives a higher throughput. The scheme especially benefits the user, i.e., the users that receive low throughput because they are located far from the base station. In this paper we prove that the data rate is increased for the poor users if the inter cell interference is low and hence the increase in the throughput of individual ‘poor’ mobile users.

Evaluation of Reliable Multicast Delivery in Base Station Diversity Systems

This paper proposes a multicast delivery system using base station diversity for cellular systems. Conventional works utilize single wireless link communication to achieve reliable multicast. In cellular systems, received signal intensity declines in cell edge areas. Therefore, wireless terminals in cell edge areas suffer from many transmission errors due to low received signal intensity. Additionally, multi-path fading also causes dynamic fluctuation of received signal intensity. Wireless terminals also suffer from transmission errors due to the multi-path fading. The proposed system utilizes multiple wireless link communication to improve transmission performance. Each wireless terminal communicates with some neighbor base stations, and combines frame information which arrives from different base stations. Numerical results demonstrate that the proposed system can achieve multicast data delivery with a short transmission period and can reduce consumed wireless resource due to retransmission.

Intercell Interference Coordination through Limited Feedback

We consider the applications of multicell transmission schemes to the downlink of future wireless communication networks. A multicell multiple-input multiple output-(MIMOs) based scheme with limited coordination among neighboring base stations (BSs) is proposed to effectively combat the intercell interference by taking advantage of the degreesoffreedom in the spatial domain. In this scheme, mobile users are required to feedback channel-related information to both serving base station and interfering base station. Furthermore, a chordal distance-based compression scheme is introduced to reduce the feedback overhead. The performance of the proposed scheme is investigated through theoretical analysis as well as system level simulations. Both results suggest that the so-called “intercell interference coordination through limited feedback” scheme is a very good candidate for improving the cell-edge user throughput as well as the average cell throughput of the future wireless communication networks.

Interference mitigation by distributed beam forming optimization

Inter-cell interference is a major issue in OFDMA networks. One approach to reduce the amount of interference is to use beam forming antennas. Further interference mitigation is achieved if neighbor base stations coordinate the directions of their beams. This paper presents a novel distributed interference coordination algorithm using main-lobe steering beam formers. Our algorithm does not require any explicit signaling between base stations. Also, it does not require any additional channel measurements to be signaled to the base stations besides those already performed for basic operation. Our evaluations show that significant performance gains can be achieved even with non-greedy traffic. Index Terms – Interference coordination, beam forming antennas, COMP

Scheduling Constraints and Interference Graph Properties for Graph-based Interference Coordination in Cellular OFDMA Networks

The currently emerging 802.16e (WiMAX) and 3GPP Long Term Evolution (LTE) cellular systems are based on Orthogonal Frequency Division Multiple Access (OFDMA). OFDMA suffers from heavy inter-cell interference if neighboring base stations use the same frequency range. One approach to solve this issue is the application of beamforming antennas in combination with interference coordination (IFCO) mechanisms between base stations. In this paper, we trace the problem of IFCO back to the graph coloring problem and investigate the achievable resource utilization of the interference coordinated system. We develop a heuristic that allows the combination of arbitrary scheduling algorithms with the IFCO mechanism. This allows an efficient utilization of the radio system’s frequency resources while still obeying scheduling constraints, such as Quality of Service requirements. Finally, we study the tradeoff between fairness and the total system throughput.

Distributed Downlink Beamforming With Cooperative Base Stations

In this paper, we consider multicell processing on the downlink of a cellular network to accomplish ldquomacrodiversityrdquo transmit beamforming. The particular downlink beamformer structure we consider allows a recasting of the downlink beamforming problem as a virtual linear mean square error (LMMSE) estimation problem. We exploit the structure of the channel and develop distributed beamforming algorithms using local message passing between neighboring base stations. For 1-D networks, we use the Kalman smoothing framework to obtain a forward-backward beamforming algorithm. We also propose a limited extent version of this algorithm that shows that the delay need not grow with the size of the network in practice. For 2-D cellular networks, we remodel the network as a factor graph and present a distributed beamforming algorithm based on the sum-product algorithm. Despite the presence of loops in the factor graph, the algorithm produces optimal results if convergence occurs.

Fixed power allocation with nulling for TDD-based cellular uplink

We propose two fixed power allocation schemes with nulling (FPA-N1 and FPA-N2) for time division duplex (TDD) based cellular uplink according to the location of mobile stations (MSs). In the FPA-N1 scheme, MSs located near a base station (BS) do not transmit data when the wireless channels between the MSs and their home cell BS experience deep fading. In the FPA-N2 scheme, MSs located near cell boundaries do not transmit data when the wireless channels between the MSs and neighboring cell BSs cause high interference channel gain because, in this case, their data transmission may induce large interference to neighboring cells. Numerical results show that the proposed power allocation scheme improves the uplink capacity in cellular networks.

A Distributed Topology Adjustment Algorithm for Wireless Networks with Mobile Base Stations

Improper static network topology can reduce the capacity of wide area wireless access networks in which the distribution of subscribers' population often changes. This paper focuses on "fully wireless cellular networks" with mobile base stations, where the network topology can be dynamic due to the mobility of the base stations. In such environment, network topology can be properly controlled by the adjustment of mobile base stations' positions. A novel distributed topology adjustment algorithm is proposed in this paper. The proposed algorithm tracks subscriber's population by moving mobile base stations toward the center of their nearby groups of subscribers. At the same time, mobile base stations are also drawn outward their neighboring base stations in order to solve the "oversupply base station" problem. The balance between 2 movement forces determines the proper position of mobile base stations. The performance evaluation of the proposed algorithm shows that the proposed algorithm performs well, even in rapidly changes of subscribers' population or in the presence of failure of base stations.

The capacity gain from intercell scheduling in multi-antenna systems

The capacity and robustness of cellular MIMO systems is very sensitive to other-cell interference which will in practice necessitate network level interference reduction strategies. As an alternative to traditional static frequency reuse patterns, this paper investigates intercell scheduling among neighboring base stations. We show analytically that cooperatively scheduled transmission, which is well within the capability of present systems, can achieve an expanded multiuser diversity gain in terms of ergodic capacity as well as almost the same amount of interference reduction as conventional frequency reuse. This capacity gain over conventional frequency reuse is <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> square-root of log Ns) for dirty paper coding and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> (min (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> , M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> ) square-root of log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> s) for time division, where N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> is the number of cooperating base stations employing opportunistic scheduling in an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> x <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> MIMO system. From a theoretical standpoint, an interesting aspect of this analysis comes from an altered view of multiuser diversity in the context of a multi-cell system. Previously, multiuser diversity capacity gain has been known to grow as O(log log <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> ), from selecting the maximum of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> exponentially-distributed powers. Because multicell considerations such as the positions of the users, lognormal shadowing, and pathless affect the multiuser diversity gain, we find instead that the gain is <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> (square-root of 2logic <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> ), from selecting the maximum of a compound Iognormal-exponential distribution. Finding the maximum of such a distribution is an additional contribution of the paper.

An efficient distributed fault‐tolerant protocol for dynamic channel allocation

AbstractRecent demand for mobile telephone services has been growing rapidly while the electromagnetic spectrum of frequencies allocated for this purpose remains limited. Any solution to the channel assignment problem is subject to this limitation, as well as the interference constraint between adjacent channels in the spectrum. The early research focused on the fixed channel allocation and centralized schemes. Recently, distributed channel allocation schemes have received much attention because of their high reliability and scalability. In these schemes, a base station (BS) has to consult with its neighboring BSs in order to assign a channel to a call. If it cannot communicate with its neighbors, it fails in allocating a channel. However, it is a common phenomenon that a BS fails in communicating with its neighboring BSs due to some reasons, such as heavy traffic load. In this paper, we propose a distributed fault‐tolerant channel allocation schemes which can work well under the mobile host (MH) failures, BS failures, and communication link failures. This algorithm is based upon the mutual exclusion model where the channels are grouped into three equal sized groups and each group of channels cannot be shared concurrently within the same cluster. We prove its correctness. We also report our algorithm's performance with several channel systems using different types of call arrival pattern through comparing with a popular generic distributed algorithm for channel allocation DDRA. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Strategies for fast scanning, ranging and handovers in WiMAX/802.16

In WiMAX/IEEE 802.16 with mobility support, the network entry procedure for a mobile station involves several steps including the scanning for an available channel and performing initial ranging in order to determine power and timing offset requirements. These steps are performed by a mobile station at start up or when about to perform a handover and must be done promptly in order to reduce delays in network access or possible dropped connections. We propose strategies that a mobile station can use to reduce the time required for scanning and ranging operations while attempting to establish network connectivity or perform a handover between neighbouring base stations. We model and simulate an area of WiMAX coverage using real-world mobility trace data and show that there are strategies that reduce the time required for scanning and ranging operations significantly.

Distributed Decoding in a Cellular Multiple-Access Channel

This paper considers the problem of joint detection in the uplink of cellular multiaccess networks with base-station cooperation. Distributed multiuser detection algorithms with local message passing among neighbor base stations are proposed and compared in terms of computational complexity required in the base stations, the amount of serial communications among them, error rate performance, and convergence speed. The algorithms based on the belief propagation algorithm result in complexity and delay per base station which do not grow as the network size increases. In addition, it is observed that these algorithms have near single-user error rate performance for the fading channels considered. Thus it is illustrated that using the belief propagation algorithm, it is possible to have full frequency re-use and achieve near-optimal performance with moderate computational complexity and a limited amount of message passing between base stations of adjacent cells.

PROPOSED METHOD FOR HANDOVER IN MOBILE NETWORK USING FUZZY LOGIC

Processing handoffs is an important task in any cellular radio system. Handoffs must be performed successfully and as infrequently as possible, and be imperceptible to the users. In order to meet these requirements, system designers must specify an optimum signal level at which initiate a handoff. Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station receiver, a slightly stronger signal level is used as a threshold at which a handoff is made. This margin, given by Δ= P r handoff – P r minimum usable, cannot be too large or too small [1]. Therefore, Δ is chosen carefully to meet these conflicting requirements. Handover Controls an essential component of radio resource management that imposes considerable signaling load on the network. The amount of undesirable signaling load depends directly on the speed of the hand set. In this paper, we propose that an handoff can be initiated by measuring parameters like link quality, location of the mobile and signal power w.r.t neighboring base station using Fuzzy Logic. This method will prevent the unnecessary handoff between the base station in turn reducing the burden of the base station & preventing traffic conjunction.

Analysis of noise immunity of OFDMA communication systems operated in the presence of interfering stations

We analyze noise immunity of high-speed data transmission systems operated in a complex communication channel with frequency-selective fadings and interference from the neighboring base stations. This problem is very topical for modern orthogonal frequency-division multiple-access (OFDMA) data transmission standards. Such digital communication systems allow one to measure the noise parameters on individual subcarriers simultaneously with reception useful signals. We perform comparative analysis of different methods for measuring the noise characteristics and using them for demodulating the coded signal. Four processing methods with different complexity and noise immunity degree are proposed for receiver signals. Characteristics of the considered receivers are compared for different channel loadings and algorithms of power control by interfering stations. Practical recommendations for using the considered receivers are given.

Joint packet scheduling and dynamic base station assignment for CDMA data networks

AbstractIn current code division multiple access (CDMA) based wireless systems, a base station (BS) schedules packets independently of its neighbours, which may lead to resource wastage and the degradation of the system's performance. In wireless networks, in order to achieve an efficient packet scheduling, there are two conflicting performance metrics that have to be optimized: throughput and fairness. Their maximization is a key goal, particularly in next‐generation wireless networks. This paper proposes joint packet scheduling and BS assignment schemes for a cluster of interdependent neighbouring BSs in CDMA‐based wireless networks, in order to enhance the system performance through dynamic load balancing. The proposed schemes are based on sector subdivision in terms of average required resource per mobile station and utility function approach. The fairness is achieved by minimizing the variance of the delay for the remaining head‐of‐queue packets. Inter‐cell and intra‐cell interferences from scheduled packets are also minimized in order to increase the system capacity and performance. The simulation results show that our proposed schemes perform better than existing schemes available in the open literature. Copyright © 2007 John Wiley &amp; Sons, Ltd.

A Relay-Assisted Handover Technique with Network Coding over Multihop Cellular Networks

A relay-assisted handover technique exploiting network coding for multihop cellular networks is described. In the scheme, each MS (mobile station) in the outer region receives data related to the home cell from the home BS (base station) as well as a coded version of data related to a neighboring BS from a relay node located in the cell boundary via the use of network coding. Thus, MSs can achieve more reliable macro diversity with the assistance of the relay-based handover technique. In a simulation, it is demonstrated that this technique is very effective in maintaining QoS (quality of service) in terms of outage probability over a multi-cell environment

Laser-free inter-networking hybrid base stations towards convergence of wireless and wired access networks

A laser-free base station scheme that can be integrated with an optical network unit for overlaying the transport of wireless signals in an optical access network is proposed and experimentally demonstrated. It is shown that this scheme has the potential to establish inter-communication among neighbouring base stations to provide simultaneous access to wireless and wired services.

Impact of Network Load on Forward Link Inter-Cell Interference in Cellular Data Networks

We study the impact of network load in the neighboring sectors on the inter-cell interference in a cellular data network. The signal received by a user over the forward link in such a system contains interference from the neighboring base stations. We note that the strength of this interference is a function of the network load in the neighboring cells. We obtain an expression for the received SINR (signal to interference and noise ratio) as a function of the traffic load in the interfering cells. Using this result, we propose an improvement to the conservative pilot-based SINR estimation scheme that is implemented in the current cellular data networks. The proposed scheme provides a more accurate estimate of the user SINR by taking better account of the contribution of inter-cell interference. It builds on top of the current SINR measurement scheme by using a combination of pilot measurement and traffic load measurement. With the proposed scheme, a terminal reports a less conservative data rate, and hence it receives a higher throughput. The scheme especially benefits the "poor" users, i.e., the users that receive low throughput because they are located far from the base station. For example, when the network load in the neighboring sectors is 0.5, with the proposed scheme, the throughput received by a single vehicular user located about three-fourth of the way between the serving base station and the cell boundary, is about 35% higher than the throughput obtained using the scheme that is used in current practice

Effectiveness of Bootstrap Bias Correction in the Context of Clock Offset Estimators

Estimating and correcting the offset between two or more clocks is an important problem in data communication networks. For example, Internet telephony depends on network routers having a common notion of time, and cellular networks provide a higher quality of service by using transmission protocols that depend on neighboring base stations knowing the offset that exists between their local clocks. In previous work it was shown that bootstrap bias correction of Paxson's well-known estimator of clock offset produces an estimator with improved bias and mean squared error (MSE) properties. In addition, the ordered-BLUE (o-BLUE) under an exponential distribution for network delays was derived, and its bootstrap bias-corrected form was shown to have lower bias and MSE than the bootstrap bias-corrected form of Paxson's estimator when network delays follow lognormal, gamma, and Weibull distributions. The inferred robustness of the bias-corrected o-BLUE to the assumed distribution of network delays is an attractive property, because no single family of distributions can consistently characterize network delays. Recent Internet traffic modeling research has suggested that the Pareto distribution is an applicable distribution for network delays. That finding motivates the work in this article concerned with clock offset estimation and the effectiveness of bootstrap bias-corrected estimators in the context of heavy-tailed network delays. An important result is the demonstrated robustness of the bias-corrected form of Paxson's estimator in the presence of heavy-tailed network delays. An additional and somewhat surprising result is that if network delays follow a Pareto distribution, then bootstrap bias correction of the exponential o-BLUE fails in the sense that the absolute bias increases. The same finding is observed in an alternative class of heavy-tailed distributions, where the success or failure of bootstrap bias correction is traceable to a parameter that reflects the weight of the tail. Although it is well known that bias correction can increase MSE, to the best of our knowledge no practical application in which it increases the absolute bias has been identified up to this point.