Virtually fixed channel assignment in cellular mobile networks with recall and handoffs

A promising approach for implementing channel assignment and control in cellular mobile telephone networks is the virtually fixed channel assignment (VFCA) scheme. In VFCA channels are allocated to cells according to the fixed channel assignment (FCA) scheme, but cells are allowed to borrow channels from one another. As such, VFCA maintains the efficiency of FCA, but adds the flexibility lacking in FCA. One feature of a VFCA network is that, to prevent co-channel interference, it requires several channels to be locked to serve a single call that borrows a channel. This feature raises the concern that VFCA may lead to chain reaction in channel borrowing among cells and cause the network performance to degrade, especially under heavy traffic conditions. In this paper, we propose the virtually fixed channel assignment with recall (VFCAWR) scheme: The network is implemented according to VFCA, but a cell can recall a locked channel to service an arriving handoff call, which occurs when a mobile unit crosses the boundary of its cell. We model the network as a three-dimensional Markov chain and derive its steady-state performance. Through modification of this basic model, we evaluate two dynamic channel assignment strategies, the virtual channel reservation (VCR) strategy and the linear switch-over (LSO) strategy, which exploit the unique borrowing/recall capability of VFCAWR to reduce the weighted cost of blocking fresh and handoff calls by reserving several virtual channels (the channels that may be borrowed from adjacent cells when necessary) for handoff calls. We validate the analytical models by simulation; the simulation test cases show that our models accurately predict the system performance measures of interest. Numerical and simulation results also show that both dynamic strategies outperform conventional channel reservation schemes based on fixed channel assignment and hybrid channel assignment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dynamic Channel Allocation for Real-Time Connections in Highway Macrocellular Networks

The wide deployment of multimedia services in third generation wireless networks will require handoff designs that can simultaneously reduce the blocking probability of handoff requests and decrease the handoff delay. Reducing the handoff blocking probability is needed to prevent frequent call dropping of real-time VBR/VCR connections and decreasing the delay associated with handoff is needed to prevent QoS degradation for multimedia traffic. In this paper, we present a channel assignment/reassignment scheme for highway cellular networks that achieves both requirements. The scheme can be used to deliver real-time data to a large segment of global highways, namely, highways in which the radio channels used in a given cell cannot be simultaneously used in the two neighboring cells to its left and to its right. The scheme possesses the desirable features of real-time algorithms: the execution time per handoff request has a constant time complexity, the number of transmitted messages per request is small, and the space overhead is also O(1). The scheme uses a non-compact initial assignment of nominal channels to neighboring cells and utilizes a set of pointers in each base station to implement an efficient channel assignment and reassignment strategy. The resulting approach greatly simplifies the selection process and avoids the expensive computation and message exchanges typically needed by dynamic channel allocation schemes. The low communication overhead of the scheme can be further reduced via control thresholds. Performance simulation results show that the scheme achieves low blocking probability and is therefore suitable for handling handoffs of real-time connections in highway cellular networks.     

A Distributed Fault-tolerant Resource Planning Scheme for Wireless Networks

Efficient management of wireless channels is critical for the performance of cellular systems. Resource planning represents the allocations of system channels into cells. Accordingly, channel assignment strategies respond for using the allocated channels of cells to provide communication services in cells. However, a cellular system that experiences the varying of traffic distributions and the mobile service stations (MSSs) failing to provide communication services or recovered from failures will lessen the utilization of channels to provide communication services. In this paper, we present a distributed fault-tolerant resource planning scheme that can adaptively allocate channels to cells according to above variations in cellular systems. When the MSS of a cell fails to provide communications, its allocated channels can be reallocated to other non-failed MSSs. Our scheme has the advantages of low message overhead and low time delay. Moreover, freedom from deadlock is ensured. Simulation results, which are observed from reducing the overall average call blocking probability and the message overhead with and without applying our resource planning scheme to various channel assignment strategies, demonstrate that our algorithm is very efficient.     

A dynamic load balancing strategy for channel assignment using selective borrowing in cellular mobile environment

We propose a dynamic load balancing scheme for the channel assignment problem in a cellular mobile environment. As an underlying approach, we start with a fixed assignment scheme where each cell is initially allocated a set of channels, each to be assigned on demand to a user in the cell. A cell is classified as 'hot', if the degree of coldness of a cell (defined as the ratio of the number of available channels to the total number of channels for that cell), is less than or equal to some threshold value. Otherwise the cell is 'cold'. Our load balancing scheme proposes to migrate unused channels from underloaded cells to an overloaded one. This is achieved through borrowing a fixed number of channels from cold cells to a hot one according to a channel borrowing algorithm. A channel assignment strategy is also proposed based on dividing the users in a cell into three broad types – 'new', 'departing', 'others' – and forming different priority classes of channel demands from these three types of users. Assignment of the local and borrowed channels are performed according to the priority classes. Next, a Markov model for an individual cell is developed, where the state is determined by the number of occupied channels in the cell. The probability for a cell being hot and the call blocking probability in a hot cell are derived, and a method to estimate the value of the threshold is also given. Detailed simulation experiments are carried out in order to evaluate our proposed methodology. The performance of our load balancing scheme is compared with the fixed channel assignment, simple borrowing, and two existing strategies with load balancing (e.g., directed retry and CBWL), and a significant improvement of the system behavior is noted in all cases. This revised version was published online in June 2006 with corrections to the Cover Date.     

An innovative channel management strategy for highly reliable handoff in cellular networks

In this paper, we have resolved the problem of forced call termination that occurs when mobile traffic is concentrated on a hot‐spot cell in cellular networks. Enhanced Channel Management Scheme (ECMS) is an innovative way to increase the flexibility of channel usage over non‐uniform traffic distribution. ECMS exploits mobile hosts initiated or active in overlapping areas between cells. The scheme consists of three phases to monitor the status of channels on each base station and to make a channel reservation using the availability list maintained for the candidate‐MH selection. When the traffic load in a cell is intolerably high, ECMS invokes the load‐balancing procedure to distribute its traffic to adjacent cells. The reserved channels are used to support the safe and fast handoff. From the simulation, we observed that ECMS outperformed other compatible channel assignment schemes such as directed handoff schemes in blocking probability and channel utilization. Copyright © 2002 John Wiley & Sons, Ltd.     

Channel assignment and layer selection in hierarchical cellularsystem with fuzzy control

We consider a hierarchical cellular system, which consists of a macrolayer and a microlayer. The macrocells accommodate fast mobile users. However, if we direct too many mobile users to the macrocells, the system capacity is low. On the other hand, the microcells are designed to increase capacity, but they cause a large number of handoffs. The aim is to maximize the system capacity while keeping the amount of handoff small. We minimize the handoff rate by a fuzzy layer selection algorithm, which makes use of the past cell dwell times of a user and the channel occupancy of the target cell. To maximize the capacity, we propose a distributed channel assignment algorithm to dynamically allocate the channels among the two layers. Exchange of information is allowed between neighboring macrocells. The state of channel assignment in a macrocell and its interfering cells are tabulated in a channel allocation table, which provides all information required in the integrated resource allocation scheme. The performance is evaluated by simulation and compared with the popular layer selection scheme known as the threshold method     

Demand assignment multiple access and dynamic channel allocationstrategies for integrating radio dispatch and telephone services overmobile satellite systems

The authors discuss the performance analyses of a novel demand assignment multiple access (DAMA) scheme addressing the special characteristics of the mobile radio service (MRS), and a new method for dynamically allocating a common pool of channels to both MRS and mobile telephone service (MTS) to improve channel utilization. The new DAMA scheme makes use of call queuing, batch processing, and pipelined signaling to minimize call setup overhead for MRS traffic. MRS call setup delays were analyzed by simulation modeling of a mobile satellite system (MSS) with many mobile voice-dispatch networks operating over a multiple spot beam satellite to investigate the effects of traffic volume, batch size, and batch service disciplines. A reserved channel margin algorithm for dynamic channel allocation was shown to be effective in harmonizing the different call setup performance requirements for MTS and MRS. Numerical results show that dynamic channel allocation applied to a common pool of 40 channels enables a 20-25% increase in the number of mobile terminals compared with a fixed allocation of 20 channels to each of the two services     

Traffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff procedures

A traffic model and analysis for cellular mobile radio telephone systems with handoff are described. Three schemes for call traffic handling are considered. One is nonprioritized and two are priority oriented. Fixed channel assignment is considered. In the nonprioritized scheme the base stations make no distinction between new call attempts and handoff attempts. Attempts which find all channels occupied are cleared. In the first priority scheme considered, a fixed number of channels in each cell are reserved exclusively for handoff calls. The second priority scheme employs a similar channel assignment strategy, but, additionally, the queueing of handoff attempts is allowed. Appropriate analytical models and criteria are developed and used to derive performance characteristics. These show, for example, blocking probability, forced termination probability, and fraction of new calls not completed, as functions of pertinent system parameters. General formulas are given and specific numerical results for nominal system parameters are presented.     

Performance limits for channelized cellular telephone systems

Studies the performance of channel assignment algorithms for “channelized” (e.g., FDMA or TDMA) cellular telephone systems, via mathematical models, each of which is characterized by a pair (H,p), where H is a hypergraph describing the channel reuse restrictions, and p is a probability vector describing the variation of traffic intensity from cell to cell. For a given channel assignment algorithm, the authors define T(r) to be the amount of carried traffic, as a function of the offered traffic, where both r and T(r) are measured in Erlangs per channel. They show that for a given H and p, there exists a function T_H,p(r), which can be computed by linear programming, such that for every channel assignment algorithm, T(r)⩽T_H,p(r). Moreover, they show that there exist channel assignment algorithms whose performance approaches T_H,p (r) arbitrarily closely as the number of channels increases. As a corollary, they show that for a given (H,p) there is a number r₀ , which also can be computed by linear programming, such that if the offered traffic exceeds r₀, then for any channel assignment algorithm, a positive fraction of all call requests must be blocked, whereas if the offered traffic is less than r₀, all call requests can be honored, if the number of channels is sufficiently large. The authors call r₀, whose units are Erlangs per channel, the capacity of the cellular system     

A Network-Based Dynamic Channel Assignment Scheme for TDMA Cellular Systems

Network-based dynamic channel assignment (DCA) schemes can be used to increase the capacity of TDMA cellular systems. In this paper, a new distributed network-based DCA scheme, known as DCA with interference information, DCA-WI, is proposed and its performance is studied. In this scheme, a base station (BS) assigns a channel in such a way as to minimize the effect on the availability of channels for use in its interfering cells. To accomplish this, each BS maintains an interference information table which contains information about the local cell and its interfering cells. DCA-WI does not require system-wide information. Channel reassignment for new and completed calls are used to further reduce the call blocking probability. Simulation results show that DCA-WI provides a lower call blocking probability compared to other existing schemes in both uniform and nonuniform traffic distributions.     

Efficiency comparison of channel allocation schemes for digitalmobile communication networks

This paper provides network designers and operators with simple guidelines on traffic measurements and efficiency evaluation of various channel allocation schemes in digital mobile telecommunications networks. The paper evaluates the efficiency obtained by implementing the following channel allocation schemes: (1) fixed uniform channel allocation (FUCA); (2) fixed nonuniform channel allocation (FNCA); (3) dynamic channel allocation (DCA) where the number of frequency carriers is adaptive and dependent on the load; and (4) dynamic frequency/time channel allocation (DFTCA) (a new scheme which is the most efficient) where the number of channels is adaptive (based on the load), allowing two channels of the same frequency carrier to be used in two neighboring cells. The analysis is based on standard queuing models under the following assumptions: (1) Poisson call arrivals in each cell; (2) exponential call holding time; (3) exponential mobile travel time; and (4) exponential sojourn time of a mobile in a cell. Numerical results are presented to provide insight into the accuracy of the models and efficiency gain by dynamic frequency time channel allocation under different traffic conditions (including conditions related to highway traffic)     

Dynamic Channel Assignment with Flexible Reuse Partitioning in Cellular Systems

In cellular communications, one of the main research issues is how to achieve optimum system capacity with limited frequency spectrum. For many years, researchers have proposed and studied many dynamic channel assignment (DCA) schemes to increase the capacity of cellular systems. Another proposed technique, Reuse Partitioning (RP), is used to achieve higher capacity by reducing the overall reuse distance. In convention, when RP is exploited in network-based DCA, a portion of channels will be assigned permanently to each partitioned region. However, the number of channels assigned to each region may not be~optimum due to factors like the uneven and time-varying traffics. In this paper, a new network-based DCA scheme is proposed with the flexible use of RP technique, named as flexible dynamic reuse partitioning with interference information (FDRP-WI). In this scheme, channels are open to all incoming calls and no channel pre-allocation for each region is required. As long as the channel assignment satisfies the co-channel interference constraints, any user from any region can use any channel. The scheme aims to minimize the effect of assigned channels on the availability of channels for use in the interfering cells and to reduce overall reuse distance. Both FDRP-WI with stationary users and mobile users are investigated. Simulation results have confirmed the effectiveness of FDRP-WI scheme. In the case with stationary users, FDRP-WI exhibits outstanding performance in improving the system capacity under both uniform and non-uniform traffic distributions. Under the uniform traffic case, the scheme can provide over 100% capacity improvement as compared to conventional fixed channel assignment scheme with 70 system channels at 1% blocking probability. In the case with mobile users, the impact of mobility on the new call probability, P _b, and the call dropping probability, P _d, is evaluated. The effect on system capacity of reserving some channels for handoff calls is first studied. Then, we propose a new handoff scheme, called “Reverse Overflow” (RO), to improve the utilization of channels with smaller reuse distances under mobile environment. Simulation results show that, with RO handoff, the system capacity of FDRP-WI is effectively improved at the expense of higher handoff rates in the cellular system.     

The nonuniform compact pattern allocation algorithm cellular mobilesystems

An algorithm for allocating nominal channels according to traffic distribution is designed. The algorithm attempts to minimize the average blocking probability as nominal channels are allocated one at a time. Simulation results show that the system's traffic-carrying capacity can be increased by about 10% by the use of this algorithm, and that the gain is additional to the improvement obtained from the channel-borrowing strategies. If the effect of shadow blocking is considered in the assignment of channels, only a very small increase in the traffic capacity is observed     

A demand assignment system for mobile users in a community ofinterest

The paper presents a high performance wireless access and switching system for interconnecting mobile users in a community of interest. Radio channel and time slot assignments are made on user demand, while the switch operations are controlled by a scheduling algorithm designed to maximize utilization of system resources and optimize performance. User requests and assignments are carried over a low-capacity control channel, while user information is transmitted over the traffic channels. The proposed system resolves both the multiple access and the switching problems and allows a direct connection between the mobile end users. The system also provides integration of voice and data traffic in both the access link and the switching equipment. The “movable boundary” approach is used to achieve dynamic sharing of the channel capacity between the voice calls and the data packets. Performance analysis based on a discrete time Markov model, carried out for the case of optimum scheduling yields call blocking probabilities and data packet delays. Performance results indicate that data packets may be routed via the exchange node with limited delays, even with heavy load of voice calls. Also the authors have proposed scheduling algorithms that may be used in implementing this system     

Mixed Power Control with Directed Assignment (MPCDA), a Method for Interference Reduction in Channelized Wireless Systems

Power control, often coupled with dynamic channel assignment, has been viewed as a promising answer to the challenge of reducing interference and increasing capacity. Indiscriminate use of power control, however, may exacerbate the near-far-end problem on the down link, and give rise to other complications when users are mobile. We propose a power control policy that can alleviate the near-far-end interference caused by the use of either the same channel or neighbor channels inside the same cell, while at the same time aiding in the reduction of interference from different cells through user matching. We present a heuristic algorithm for user matching, which is distributed and simple to implement. The method can be combined with an array of either fixed or dynamic channel assignment algorithms and applies to both circuit-based and packet-based traffic. It is ideal for fixed or slow circuit-based traffic and for packet-based traffic.A duality relationship is derived for the proposed power control policy between the signal-to-interference ratio of two interfering users experienced in the two communication directions. This relationship enables one to validate channel assignment decisions on both communication directions by analyzing only the decisions for one.     

Maximum packing channel assignment in cellular networks

We study the performance of the maximum packing channel assignment algorithm (MPA) in channelized cellular networks. MPA is a greedy algorithm, which rejects a call only when it is forced to do so, even if this involves rearrangement of channels assigned to the ongoing calls, without dropping any of them. We ignore handoffs and model the channel reuse constraints in the cellular network by a hypergraph. As the traffic and the number of channels are scaled together, we get a limiting regime where the blocking probability in the cells can be computed by solving a nonlinear optimization problem. The carried traffic in this limiting case is an upper bound on the performance of MPA for practical finite-channel systems. We show that the performance of MPA in a finite-channel cellular system can be closely approximated by considering a simple fixed-routing circuit-switched network. Thus, the finite-channel performance of MPA can be studied using methods well known in the area of circuit-switched networks. We compare the performance of MPA with other asymptotically optimal algorithms and demonstrate its optimality for low and moderate offered traffic. We envisage MPA as a practical channel assignment algorithm, for moderate size systems, and suggest approximations to reduce its complexity     

Some Effects on Channel Occupancy of Limiting the Number of Available Servers in Small Cell Mobile Radio Systems Using Dynamic Channel Assignment

The performance of large scale multicell mobile radio systems using dynamic channel assignment and having limitations on the number of servers available in each coverage cell was investigated by computer simulation. At low system blocking the traffic carried is determined solely by the server limitations while at higher blocking the traffic carried is determined by channel limitations.     

Nodes organization for channel assignment with topology preservation in multi-radio wireless mesh networks

The wireless mesh network is a new emerging broadband technology providing the last-mile Internet access for mobile users by exploiting the advantage of multiple radios and multiple channels. The throughput improvement of the network relies heavily on the utilizing the orthogonal channels. However, an improper channel assignment scheme may lead to network partition or links failure. In this paper we consider the assignment strategy with topology preservation by organizing the mesh nodes with available channels, and aim at minimizing the co-channel interference in the network. The channel assignment with the topology preservation is proved to be NP-hard and to find the optimized solution in polynomial time is impossible. We have formulated a channel assignment algorithm named as DPSO-CA which is based on the discrete particle swarm optimization and can be used to find the approximate optimized solution. We have shown that our algorithm can be easily extended to the case with uneven traffic load in the network. The impact of radio utilization during the channel assignment process is discussed too. Extensive simulation results have demonstrated that our algorithm has good performance in both dense and sparse networks compared with related works.     

Transitions from DCA to FCA behavior in a self-organizing cellularradio network

This paper studies self-organizing in a cellular radio network with dynamic channel assignment (DCA). Conceptually, channel assignment is split into two subsystems, one performing real-time assignment and one adjusting assignment probabilities, aiming to prefer selection of channels less affected by interference in the past. The latter subsystem is modeled by a differential equation. The resulting “learning” network is able to perform phase transitions from a fully dynamic state in which all channels are used with equal probability to a state in which each cell prefers channels which are less frequently chosen by interfering cells. Both theoretical considerations and numerical simulations show that the simple strategy to avoid interference is generally unable to provide optimum channel reuse distances. The situation can be improved by refined learning strategies. Further numerical simulation of inhomogeneous networks indicates that channel reuse performance achieved by this strategy is reasonable, but still inferior to that of dedicated channel assignment algorithms     

Optimal channel assignment strategies for forced channel hopping in CDPD systems

To provide an acceptable call blocking probability in circuit-switched cellular networks, such as the Advanced Mobile Phone System (AMPS) networks, a significant fraction of the channel capacity in each cell is normally unused. This “free” capacity can be effectively used for packet data transmissions that yield to voice traffic when necessary. Cellular Digital Packet Data (CDPD) is a packet-switched data service which may share radio channels with the AMPS service on a secondary basis to tap this “free” capacity. The length of time that a CDPD stream can occupy a channel is greatly influenced by the channel assignment strategies of both the AMPS and the CDPD systems. This paper investigates these channel assignment strategies and their effects on the CDPD channel holding times, in comparison with the optimal channel assignment strategy that interrupts the CDPD service only when a new AMPS call finds no other idle channels in the cell site. One such optimal strategy is the cooperative strategy in which the CDPD and AMPS networks actively communicate with each other. It is shown that other optimal strategies exist without the need of communications between the two systems. The effects of AMPS traffic levels, number of channels, and number of CDPD streams at the cell site on the CDPD channel holding time and channel utilization are also considered. This revised version was published online in June 2006 with corrections to the Cover Date.