It is important to cooperatively share perception data among vehicles for the safety of autonomous driving systems. However, since vehicles may experience long data transmission delays, especially in case that the number of vehicles participating in cooperative perception is large, the dissemination of large-sized perception data remains a challenging problem. Considering the high transmission cost of perception data in the overlapping perception area of nearby vehicles, we propose a novel fusion-tree-based perception data dissemination scheme in millimeter-wave (mmWave) vehicular networks. Vehicles first aggregate their perception data at a preselected leader along a dynamically constructed fusion tree, and a merge-and- forward policy is adopted to reduce the amount of redundant data. After the aggregation stage ends, the leader broadcasts the merged data back to all vehicles through multi-hop broadcast links. The performance evaluation results show that the proposed scheme yields the best transmission performance among the three comparison schemes in terms of dissemination delay.

It is very important to detect and prevent bogus messages (here called rumors) in vehicular ad hoc networks (VANETs) because such misbehaviors could cause road safety problems and even casualties. In this paper, we first propose an evolutionary public goods game (EPGG) model to detect bogus messages and stimulate nodes to implement data verification. When a node detects a rumor, it immediately broadcasts an anti-rumor. We here present a susceptible, immune, and neutral (SIN) rumor diffusion model to characterize the spread of rumors and anti-rumors. Moreover, we propose a dynamic, hierarchical public goods game (DHPGG) model to analyze the symbiosis and confrontation of rumors and anti-rumors. Simulation results show these models could detect and suppress rumors effectively in VANETs.

In vehicular ad hoc networks (VANETs), quick and reliable multi-hop broadcasting is important for the dissemination of emergency warning messages. By scheduling multiple nodes to transmit messages concurrently and cooperatively, cooperative transmission based broadcast schemes may yield much better broadcast performance than conventional broadcast schemes. However, a cooperative transmission requires multiple relays to achieve strict synchronization on both time and frequency, which may induce high cost for a cooperative transmission process. In this paper, we analyze the cost and benefit of a cooperative transmission for data broadcasting in vehicular networks, and introduce a new metric called the single hop broadcast efficiency (SBE) to evaluate the overall broadcast performance. We propose an efficient, non-deterministic cooperation mechanism to reduce the cooperation cost. The mechanism maximizes the expected broadcast performance by selecting cooperators with the largest expected SBE value for a lead relay, and initiates cooperative broadcasting process when the expected SBE value is larger than that of a single-relay based broadcasting. Based on the non-deterministic mechanism, we propose an efficient, cooperative transmission based opportunistic broadcast (ECTOB) scheme which further utilizes rebroadcast to improve the reliability of the broadcast scheme. Simulation results show that the proposed scheme outperforms the conventional ones.

In VANETs, it is important to support fast and reliable multi-hop broadcast for safety-related applications. The performance of multi-hop broadcast schemes is greatly affected by relay selection strategies. However, the relationship between the relay selection strategies and the expected broadcast performance has not been fully characterized yet. Furthermore, conventional broadcast schemes usually attempt to minimize the waiting time difference between adjacent relay candidates to reduce the waiting time overhead, which makes the relay selection process vulnerable to internal interference, occurring due to retransmissions from previous forwarders and transmissions from redundant relays. In this paper, we jointly take both of the relay selection and the internal interference mitigation into account and propose a fast, reliable, opportunistic multi-hop broadcast scheme, in which we utilize a novel metric called the expected broadcast speed in relay selection and propose a delayed retransmission mechanism to mitigate the adverse effect of retransmissions from previous forwarders and an expected redundancy probability based mechanism to mitigate the adverse effect of redundant relays. The performance evaluation results show that the proposed scheme yields the best broadcast performance among the four schemes in terms of the broadcast coverage ratio and the end-to-end delivery latency.

In millimeter wave (mmWave) vehicular communications, beam scanning is usually used for beam alignment. However, beam scanning takes a long time, which makes it unsuitable in the mmWave vehicular communications with high-speed nodes. Moreover, the existing research on mmWave vehicular communications is usually based on one-hop transmission. However, due to a limited transmission range of mmWave, in many cases, multi-hop transmissions are required to transmit information from a source node to a destination node. To solve these problems, we propose an opportunistic routing protocol based on position information for beam alignment in mmWave vehicular communications. Beam alignment is performed based on the location information of neighbor nodes instead of beam scanning, which avoids the increase of network load and delay caused by beam scanning. In addition, the interference in concurrent transmissions is considered in order to optimize beamwidth and forwarding priorities. When the opportunistic routing is used, even if one candidate forwarder (CF) fails to forward a packet, the other CFs receiving the packet can still forward it, avoiding an increase in the network load and unicast delay caused by a large number of retransmissions. In an experiment, we extend the existing one-hop communication protocols into multi-hop communication protocols. Compared with these protocols, the proposed protocol yields lower end-to-end delay, higher packet delivery ratio and network throughput.

In this paper, we investigate how to efficiently schedule vehicle-to-vehicle (V2V) requests in millimeter wave (mmWave) vehicular networks. Considering the vulnerability of mmWave signals to blockages and the disadvantages of traditional relay mode, we firstly propose the cooperation of intelligent reflecting surfaces (IRSs) and relays to enable the scheduling of non-line-of-sight (NLOS) requests. To maximize the resource utilization and consequently improve the success ratio of link scheduling, we design a centralized scheduling algorithm based on spatial-time division multiple access (STDMA) and the cooperation scheme. The proposed algorithm can provide more opportunities for scheduling NLOS requests by the cooperation of IRSs and relays, and achieve the goal of collision-free concurrent transmissions by fully leveraging spatial reuse, so that as many V2V requests as possible can be fulfilled. Simulation results demonstrate that the proposed scheme can effectively enhance the success ratio of scheduling V2V requests especially NLOS requests.

Cellular networks have been successfully evolved over the decades. Especially, Long-Term Evolution (LTE) has been exceedingly successful and the security threats against LTE systems have increased rapidly. Particularly,trackingLTE user devices has been shown to be effective as thetemporaryuser identifiers (IDs) are easily extracted and used to locate targeted devices by passive eavesdroppers. We notice that naive approaches, such as frequent updates of temporary user IDs, areinsufficientto mitigate user-tracking attacks since the new and old temporary IDs for the same user device are easilylinkableby adversaries who can measure the wireless channel characteristics between the user device and herself. In this paper, we propose an opportunistic uplink power control scheme to minimize the probability of successful user tracking by an adversary whose location is unknown. We devise the notion of average inference error probability in order to measure the level of users’ location privacy. Moreover, we derive the closed-form expression of the approximated average inference error probability and formulate an optimization problem to maximize the average inference error probability under a constraint of an allowable power budget for each user. Against a passive adversary, our proposed power control scheme effectively degrades an adversary’s inference ability by 50% when 10 users are scheduled in each transmission time slot, which will lead to almost 100% inference error at the adversary over multiple time slots.

Cellular networks have been successfully evolved over the decades. Especially, Long-Term Evolution (LTE) has been exceedingly successful and the security threats against LTE systems have increased rapidly. Particularly, tracking LTE user devices has been shown to be effective as the temporary user identifiers (IDs), which are used in LTE systems to indicate LTE user devices in the system, are easily extracted and used to locate targeted devices by passive eavesdroppers. In this paper, we investigate the impact of uplink power control on the probability of successful user tracking by an adversary whose location is unknown. We devise the notion of average inference error probability in order to measure the level of users’ location privacy. Moreover, we derive the closed-form expression of the approximated average inference error probability and formulate an optimization problem for maximizing the average inference error probability under a constraint of an allowable power budget for each user. For defense, we propose a power control scheme able to effectively degrade an adversary’s inference ability by 50% when 10 users are scheduled in each transmission time slot, which will result in almost 100% inference error at the adversary over multiple time slots.

Abstract Considering different types of base stations (BSs) in future cellular networks are overlapping deployment with the status of dense, multi‐tier and heterogeneous in general, how to optimize the real BS deployment becomes a complicated problem. Based on it, repulsive BS dataset and clustering BS dataset are statically characterized with various types of spatial point processes. It shows that the improvement of coverage probability between 1‐tier and 2‐tier network fluctuates as the signal‐to‐interference ratio (SIR) threshold increases for different datasets. The authors' proposed hybrid model fits well with repulsive BS dataset, while the log‐Gaussian Cox point process (LGCP) and Cauchy model are reasonable models for clustering BS dataset. Besides, in order to dynamically analyze the coverage problem affected by adding new BSs, a cell boundary constructed by an irregular circle is introduced under an equal SIR constraint, and a BS placement scheme is proposed to place new BSs at the points of minimum interference. Numerical results show that the coverage probability may increase after adding BSs in the target area of heterogeneous network by using the proposed scheme. However, as the density of femto BS reaches a certain value, its coverage may remain unchanged even after adding more femto BSs.

Grant-free multiple access (GFMA) is an emerging technology to accommodate a massive number of devices for 6G-enabled Internet of Things (IoT) networks. The main advantages of GFMA are to efficiently reduce control signaling overhead for resource scheduling while improving resource efficiency. In this article, we propose anovelresource-hopping-based GFMA (RH-GFMA) framework with resource hopping schemes for providing massive connectivity in 6G cellular IoT networks, where each IoT device is allowed to access physical radio resources by using a preassigned resource hopping pattern without not only resource request but also grant procedure, which is the so-called “one-shot” noninteractive multiple access. We exploit three types of resource hopping schemes in the proposed RH-GFMA framework: 1) random hopping; 2) resource group hopping; and 3) Latin-square group hopping. We mathematically analyze the RH-GFMA system performance in terms of the hopping pattern collision probability, maximum allowable packet delay, and interference-over-thermal. Finally, we derive an accommodation capacity of the proposed RH-GFMA framework, which is defined as the expected number of IoT devices accommodated in a cell under a maximum allowable packet-delay requirement and an interference-over-thermal constraint. With the proposed GFMA, massive IoT devices are expected to be efficiently accommodated in 6G wireless networks, while satisfying strict latency and reliability requirements.

A novel hybrid model with dual‐slope path loss is proposed to analyse the coverage probability for a given cell, and its coverage probability formula is derived by using a newly dual‐slope path loss expression. A change of base station (BS) density may affect coverage probability obviously, yielding a low bound to simulation result of actual BSs. When the ratio of two path loss factor values is in a higher range, less BSs can achieve the same coverage probability. For the same SINR threshold and BS density, the coverage probability becomes higher under a lower range of path loss factor ratio.

Since the analysis of cell coverage faces complex environments in unmanned aerial vehicle base station (UAV‐BS) systems, general coverage probability in a typical cell is derived to analyse a UAV‐BS multi‐tier network, and this coverage probability is closely related with power difference among UAV‐BSs, two‐dimension (2D) and three‐dimension (3D) UAV‐BS deployment, and general interference fading models. Especially, closed‐form coverage probability expressions are also derived in the form of 2D scenario and 3D scenario, when the interference fading coefficient is assumed to follow a Gamma distribution and an exponential distribution, respectively. It shows that there exists maximum spectral efficiency density (SED) as the BS density increases, and such a BS density achieving the max‐SED is independent of noise power, but is affected by various propagation parameters.

The performance of multi-hop broadcast in vehicular ad hoc networks can be greatly affected by the highly varying link dynamics caused by mobility of vehicles and complex wireless channel environment. The conventional receiver-based and sender-based broadcast protocols usually decide the relay sequences only based on distance, neglecting the influence of links qualities. In this paper, we consider the impact of link qualities and vehicular mobility and investigate how to select broadcast relays to minimize the broadcast delay and maximize the broadcast efficiency. We first propose a novel broadcast scheme based on the prediction of dynamics (BPD), which utilizes the dynamic information to achieve the model-based prediction and combines the sender-based and receiver-based relay selection schemes for multi-hop broadcast. Then, we propose a novel metric called the expected remaining delay (D) and implement it in BDP (BPD-D) for minimizing the broadcast delay. We also propose a novel metric called the expected rebroadcast efficiency (E) and implement it in BDP (BPD-E) for maximizing the broadcast efficiency. The simulation results show that our proposed BPD-D and BPD-E broadcast protocols outperform the conventional protocols, while BPD-D can achieve the least delay and BPD-E has the highest dissemination efficiency.

For opportunistic routing in independent duty-cycled wireless sensor networks (WSNs), a sender dynamically determines a relay candidate set depending on the real-time network conditions. Due to independent and varying duty cycle length, the sender may handle different waking-up orders of the potential forwarders when it tries to forward data packets at different time instants. Conventional opportunistic routing protocols overlook the time-varying property of the waking-up order of the candidate nodes. In this paper, we theoretically analyze how to obtain an optimal candidate set for each node in order to minimize the end-to-end latency. Then, considering the realistic scenarios, we propose an opportunistic routing which jointly considers global and localized optimizations. Based on the relatively stable topology and duty-cycle length information, an original candidate set is constructed. Then, by considering the real-time link and duty cycle information in the local context, a further optimization for the original candidate set can be achieved. Simulation results show that our proposed schemes can significantly improve the end-to-end latency compared with the benchmarks.

There are high demands for cooperation among vehicle nodes to disseminate data in vehicular ad hoc networks (VANETs). Game theory approaches have been applied to analyzing and regulating the behavior of nodes in complex networks. However, game theory-based cooperation in VANETs still poses a research challenge due to continuous changes in the network topology. In this paper, we focus on the issue of the common behavior of cooperation in group vehicular interactions. We first present a dynamic member public goods game (PGG) model for real VANETs in which games are carried out within each group. Then, a dynamic grouping PGG (DGPGG) model is proposed to model VANETs with high-node density. In addition, a greedy neighbor selection strategy is proposed to replace the typical random strategy. Finally, the individual reputation of a vehicle is integrated into the DGPGG model. The simulation results show that the DGPGG model can greatly increase the proportion of cooperative nodes in high-density VANETs no matter whether the network is static or dynamic. When the network topology changes quickly, the greedy neighbor selection strategy can promote cooperation between nodes more effectively than the random strategy.

Many applications in vehicular ad hoc networks (VANETs) are based on broadcast to disseminate information among vehicles. In conventional broadcast protocols, only one vehicle is scheduled to rebroadcast a message at a certain time to avoid collisions among vehicles. In this paper, we derive the maximum temporal displacement required by the concurrent transmissions in VANETs and propose a Concurrent Transmission based Broadcast (CTB) protocol. The CTB includes two parts, broadcast in a street and broadcast at intersections. We divide the transmission range in the broadcast direction into segments and schedule the concurrent transmissions of the forwarders in the same segment. In our protocol, even if some selected forwarders fail to receive the message, other forwarders having received the message can still rebroadcast it, which reduces the broadcast delay and increases the broadcast reliability. Simulation results show that our protocol is faster and more reliable, compared to conventional broadcast protocols.

In cellular Internet-of-Things (IoT) systems, system overload may occur during a random access (RA) procedure under a limited number of preamble resources and physical uplink shared channel (PUSCH) resources especially when there exist massive IoT devices in a cell. In order to resolve the system overload, the commercial system like 3GPP LTE adopted a group paging (GP)-based uplink access technique, but it has been known that the performance of the GP-based technique drastically degrades as the number of devices increases. In this paper, we first propose a dynamic access control (DAC) mechanism for the GP-based cellular massive IoT system, which dynamically adjusts RA-attempting probability by considering not only the number of available preambles but also the number of available PUSCH resources. We also intelligently combine the proposed DAC mechanism with an early preamble collision detection technique to further improve the RA performance of the cellular IoT system. Through extensive computer simulations, we show that the proposed DAC mechanism outperforms the conventional access control mechanisms, which consider only the number of available preamble resources, in terms of GP completion time, PUSCH resource efficiency, transmission efficiency, and energy efficiency.

Per-station fairness is one of the important and challenging issues in wireless local area network based on a contention-based medium access control protocol. The unfairness problem among stations having different channel qualities is caused by the imbalance in access probability and outage probability among them. In this paper, we investigate the per-station fairness by resolving the imbalance in the above two factors through the enhanced distributed coordination function (DCF) adopting hybrid automatic repeat request (HARQ), named the DCF-H. We suggest adopting the HARQ with Chase combining as a solution for the imbalance in the outage probability on top of the loss differentiation mechanism which resolves the imbalance in the access probability. We also analyze the performance of the DCF-H by proposing a new Markov chain model and derive a closed-form expression of system throughput, delay, and outage probability. We compare the performance of the DCF-H with that of the conventional DCF protocols and find out that the DCF-H not only guarantees the near-perfect per-station fairness but also improves the whole system performance, such as throughput, delay, and outage probability.

Currently, over nine billion things are connected in the Internet of Things (IoT). This number is expected to exceed 20 billion in the near future, and the number of things is quickly increasing, indicating that numerous data will be generated. It is necessary to build an infrastructure to manage the connected things. Cloud computing (CC) has become important in terms of analysis and data storage for IoT. In this paper, we consider a cloud broker, which is an intermediary in the infrastructure that manages the connected things in CC. We study an optimization problem for maximizing the profit of the broker while minimizing the response time of the request and the energy consumption. A multiobjective particle swarm optimization (MOPSO) is proposed to solve the problem. The performance of the proposed MOPSO is compared with that of a genetic algorithm and a random search algorithm. The results show that the MOPSO outperforms a well-known genetic algorithm for multiobjective optimization.

Simultaneous wireless information and power transfer (SWIPT) is a promising technique to transmit information and harvest energy at the same time. In this paper, we investigate multiuser scheduling criteria in a multiuser SWIPT system taking into account co-channel interference in a multicell environment, where each transmitter operate independently. We propose an α-adaptive scheduling scheme, which is able to adjust a scheduling criterion between maximizing achievable rate and maximizing harvested energy of the scheduled device by adjusting α factor. Employing a power splitting receiver, we derive closed-form analytical results including cumulative distribution function (CDF) of the harvested energy, CDF of the achievable rate, average harvested energy, and average achievable rate. We also investigate the effect of adjustable α factor in the proposed scheduling scheme and multiuser diversity in terms of the CDF of the harvested energy, the CDF of the achievable rate, and rate-energy tradeoff, compared with two conventional scheduling schemes: random scheduling and max-SNR scheduling. Our analytical results provide insightful information for designing multiuser scheduling criteria considering co-channel interference in the SWIPT systems.

In this paper, we introduce the notions of a user-scaling function and a user-scaling exponent and multiuser diversity (UEMD) pair. Our notions for the UEMD pair could be utilized as a basic tool for analyzing user-scaling laws. We derive a secure user-scaling law in terms of SNR, the number of eavesdroppers, the number of distributed antennas in distributed antenna systems, and design parameter λ. Through analytical and numerical results, we characterize the effect of the number of distributed antennas on the user-scaling behavior in terms of achievable secrecy rate. Compared with the conventional results based on a single antenna system, our results indicate that installing a small number of distributed antennas could provide a significant performance gain in terms of the achievable secrecy rate.

We have proposed a prioritized random access (PRA) scheme, where each node indicates its access priority by differentiating the transmit power of preamble signatures and the eNodeB utilizes multiple detection thresholds to detect the prioritized preamble signatures. In this paper, in order to make the PRA scheme more robust, we enhance our previously proposed PRA scheme by investigating the effect of wireless channels on the detection performance such as false alarms and mis-detections. We find the optimal detection thresholds and the optimal preamble transmit power levels. Moreover, we analyze the energy consumption during the RA procedure by applying the optimal transmit power levels to the PRA scheme. The performance evaluation shows that the PRA scheme enables each node to achieve differentiated collision probability and access delay according to the access priority with reduced energy consumption, compared to the conventional RA scheme.

In urban vehicular ad hoc networks (VANETs), due to the high mobility and uneven distribution of vehicles, how to select an optimal relaying node in an intra-street and how to determine a street selection at the intersection are two challenging issues in designing an efficient routing protocol in complex urban environments. In this paper, we build a link model with a Wiener process to predict the probability of link availability, which considers the stable and unstable vehicle states according to the behavior of vehicles. We introduce a novel concept called the link correlation which represents the influence of different link combinations in network topology to transmit a packet with less network resource consumption and higher goodput. Based on this concept, we design an opportunistic routing metric called the expected transmission cost over a multi-hop path (ETCoP) implemented with our link model as the selection guidance of a relaying node in intra-streets. This metric can also provide assistance for the next street selection at an intersection. Finally, we propose a street-centric opportunistic routing protocol based on ETCoP for VANETs (SRPE). Simulation results show that our proposed SRPE outperforms the conventional protocols in terms of packet delivery ratio, average end-to-end delay, and network yield.

In machine-to-machine (M2M) communications, machine nodes perform a random access (RA) to synchronize with the eNodeB in order to transmit their new packets. However, the conventional RA cannot support prioritized accesses during the RA procedure, even though there are a variety of classes with different quality-of-service (QoS). In this paper, we propose a prioritized random access (PRA) scheme, which enables machine nodes to indicate their access priority during the access phase by differentiating their transmission power level. Moreover, we thoroughly analyze the PRA scheme in terms of collision probability and access delay. The performance evaluation shows that the PRA scheme enables the nodes to indicate the access priority according to their QoS during the RA procedure as well as to achieve much lower collision probabilities and shorter access delay.

Binary-exponential sleep mode (BE-SM) operation is an energy-saving solution of mobile stations for best-effort or non-realtime services. In general, there is a trade-off between energy consumption and delay. Especially, in the conventional BE-SM operations, the standard scheme achieves the lower bound of delay, while the threshold scheme achieves the upper bound of delay. Since the threshold scheme yields the highest energy efficiency at the expense of delay, it does not guarantee given quality-of-service (QoS) requirements. From the viewpoint of various QoS requirements, we propose a relaxed QoS (RQ) scheme with relaxed QoS requirements and a stringent QoS (SQ) scheme with stringent QoS requirements in which we have two policies: additional sleep cycles and buffer-threshold. The results show that the proposed RQ scheme has a trade-off between energy efficiency and user satisfaction ratio at low arrival rate, while the proposed SQ scheme is close to the upper bound of energy efficiency while satisfying the stringent QoS requirements.

Recently, study on designing a new metric to express quality of experience (QoE) has attracted attention since user satisfaction may be different according to the types of served traffic. The utility function for various traffic has been studied for many years and most of the previous work focused on the utility functions with respect to a single performance metric such as throughput. However, multiple performance metrics may affect user satisfaction according to user properties or situations. One of critical factors that affect the sensitivity of user satisfaction is the remaining battery energy. In this paper, we investigate the characteristics of the user satisfaction with respect to throughput and energy efficiency considering the remaining battery energy. Based on the characteristics, we design a multi-variable QoE function based on the unified utility, especially, considering user's remaining battery energy and verify the feasibility of the proposed multi-variable QoE function through numerical results.

Deployment of small cells is one of the most efficient solutions to improve the energy efficiency in mobile communication networks. However, small cell topology has a disadvantage such as frequent handovers of mobile user equipments (UEs). To solve this handover problem for small-cell users in a HetNet environment, we propose a novel handover scheme utilizing a cooperation-based cell clustering scheme for reducing handover overhead in core network (CN) and also signaling overhead among small cells. When a UE in active mode moves randomly, we can significantly reduce the handover ratio through the proposed scheme, compared with that of the conventional schemes. In particular, if the UE speed is 3 km/h, this ratio is approximately 0.07.

In wireless sensor networks (WSNs), the broadcasting is used for remote network configuration, information dissemination, route discovery and so on by propagating messages across the whole network. To reduce the energy consumption of the asynchronous WSNs, sensor nodes usually adopt independent wake/sleep scheduling. However, this may induce unpredictable waiting delay. In some real-time applications, the broadcasting should be accomplished within a limited time. In this paper, we propose a broadcasting protocol to save energy under a constrained end-to-end (E2E) delay. We estimate the average one-hop delay and use an adjusting factor to balance the remaining delay of continuous hops. Then, we set up a sender candidate node set for each node according to its waiting delay constraint, and choose the least-cost energy node to forward the packet under the constrained waiting delay to save energy. The simulation results show that our broadcasting protocol is more energy efficient than conventional protocols under E2E delay constraint.

In heterogeneous networks (HetNets), small cells are overlaid within a large cell in order to increase capacity and coverage beyond the initial deployment of a large cell. Previous studies assumed that small cells are deployed only in the cell edge area of a large cell to support low-SINR users in that area. However, in a future communication environment, we need more small-cells to support much higher user data rates. We propose a spectral-, energy-, and cost-efficient small cell deployment scheme for maximizing the following spectrum, energy, and cost related metrics: spectral efficiency, spectral and energy efficiency, and deployment cost efficiency. We find the optimal values of these metrics for varying the number of small cells and the transmit power of small cells.

In mobile ad hoc networks, node mobility may cause frequent route failures and route rediscoveries, which induce large overhead. Flooding used in a route discovery may cause a large number of unnecessarily redundant forwardings of route request (RREQ) packets, resulting in significant routing overhead. Conventional broadcast schemes, which aim to reduce the overhead in the route discovery by limiting the forwardings of RREQ packets, will also reduce the number of the underlying paths that route discovery can find, thus may miss the stablest path for data packets transmission. In this paper, we propose a novel routing discovery scheme called probabilistic broadcasting algorithm (PBA) based on the connectivity information of predictable rendezvous nodes (PRN) by considering node mobility and link stability in order to predict the possible connection of moving nodes. The PBA can find a stable path while limiting the number of RREQ packets forwardings in the route discovery.

Localization of nodes is a very crucial issue in many wireless sensor network (WSN) applications. In this paper, we consider a WSN initially consisting of a single location-aware mobile anchor nodes and a number of location-unaware, stationary nodes, and propose two algorithms to localize the unknown nodes using the mobile anchor nodes. We can obtain the location information of the unknown nodes through the transmitted location information of the mobile anchor node, even in the presence of obstacles. Received Signal Strength (RSS) is elaborately used in both the two algorithms. The first algorithm controls the trajectory of the mobile anchor node and uses a geometric property to simplify the estimation of the location of the unknown nodes; the second algorithm exploits to use directional information. The proposed algorithms outperform the conventional algorithm based on RSS in terms of localization accuracy. Simulation results show the effectiveness of the proposed algorithms.

In asynchronous wireless sensor networks, finding an optimal candidate set can significantly improve the efficiency of an opportunistic routing protocol by balancing the one-hop cost from a sender to a relay and the remaining cost from the relay to the sink. A larger candidate set indicates smaller one-hop waiting cost to wait for a candidate to wake up, however, a packet is more likely to deviate from the shortest path, which will induce larger remaining cost. Remaining cost of each candidate can be used to construct a coordinate system. Neighbors of a node with high coordinate value is excluded from the candidate set, thus, we can select the next-hop relay by simply taking the first wake-up candidate. In this paper, we propose a distributed algorithm which is similar to the Bellman-Ford algorithm to construct a coordinate system and find an optimal candidate set for each node. Moreover, we propose an algorithm which induces acceptable overhead to maintain the coordinate system and each node's optimal candidate set. We also use simulation results to show that our proposed opportunistic routing protocol is more efficient than conventional strategies in terms of delay, energy efficiency, and miss ratio.

We investigate the maneuvering control of a UAV-based relay for efficient communication with mobile ground nodes. The objective of the maneuvering control is to maximize the minimum ergodic link capacity which is achieved by any ground node. We propose an efficient maneuvering control scheme of a UAV-based relay which does not require any location or direction information of ground nodes. The proposed control scheme exploits the received signal strengths (RSSs) from ground nodes but it dose not need a channel model to estimate the location of ground nodes. Therefore, the proposed control scheme significantly reduces the communication overhead and/or computational complexity while achieving similar performance, compared with that of an optimal scheme which requires the location information of ground nodes.

In this paper, we propose an opportunistic user selection criterion to obtain a multiuser diversity gain for secure communication. At the same time, we also propose a strategy for utilizing a small base station in heterogeneous networks as a helper or a jammer adaptively. Our approach is to maximize the secrecy achievable rate based on an analytically derived lower bound instead of the original objective function due to its complexity. Based on a simple proposed user selection criterion considering both macro and small base stations, we propose two jamming strategies according to knowledge of eavesdropper's channel: adaptive jamming and probabilistic jamming strategies. Through simulations in two scenarios, it is shown that our proposed user selection and jamming schemes provide a good performance gain in terms of secrecy achievable rate.

Due to spectrum shortage in the conventional microwave bands, millimeter wave (mmWave) bands have been attracting great attention as an additional spectrum band for 5G cellular networks. In this paper, we show the feasibility of using a mmWave-based small cell with directional antennas. We investigate the effect of the number of antennas, nant, on the capacity and coverage probability, and propose an antenna clustering scheme (ACS) to utilize the antennas more efficiently. For a given nant, we also investigate the effect of the number of clusters, ncluster, on the capacity and coverage probability. In addition, we propose two design criteria: a capacity-maximization criterion (CapMC) and a coverage-maximization criterion (CovMC), in the mmWave-based small cell. Each design criterion is formulated as a joint optimization problem to find the optimal values of nantand nclusterto maximize each objective function while satisfying system requirements such as capacity and coverage probability. Performance evaluation shows that CapMC can achieve the highest capacity while satisfying a given coverage probability requirement and CovMC can achieve the highest coverage probability while satisfying a given capacity requirement.

In order to reduce the power consumption of cellular base stations (BSs), the following BS architectures have been developed: micro cell BSs, and remote radio head (RRH)-based BSs. In this paper, we propose a novel BS power consumption model for comparing the power consumption and energy efficiency of above three different BS architectures. The RRH-based BS architecture outperforms the conventional macro BS architecture with approximately 30% reduction in power consumption and 7 times increase in energy efficiency.

The hierarchical cooperation (HC) scheme is known to exhibit linear scaling from the viewpoint of information theory under the assumption of no control and data frame overhead. However, if we consider the control and data frame overheads of the HC scheme in a more practical ad-hoc network environment, the scaling law of the aggregate throughput is expected to be far from linear scaling. In this paper, we observe the effect of the control and data frame overheads on the capacity scaling of the HC scheme. From the results of performance evaluation, we find that the data frame overhead only affects the degradation of the aggregate throughput in proportion to the data frame overhead ratio. Moreover, the control overhead affects the trend of the aggregate throughput. The slope of the aggregate throughput converges to near zero as the total number of nodes, N, increases, and the aggregate throughput becomes saturated for any given number of stages, h. The significant performance degradation in the aggregate throughput is inevitable due to the control and data frame overheads. Therefore, it is not always good to use as many stages as possible, and, thus, there exists an optimal number of stages according to the given system parameters.

In emergency situations such as a disaster and a blackout, machine nodes should report emergency alarms, including the disaster type and disaster area to an M2M server. When many related machine nodes attempt a random access (RA) to report emergency alarms, and this may cause a physical random access channel (PRACH) overload. In this case, the machine nodes hardly access the network within a given delay requirement of emergency alarms. In this paper, we propose a novel random access scheme based on a special preamble for emergency indication in an emergency situation and adaptively reserve more PRACH resources in proportion to the number of machine nodes using the special preamble. The proposed RA scheme outperforms the conventional RA scheme in terms of access delay and can accommodate much more machine nodes in emergency situations, compared with the conventional RA scheme.

When multiple users are communicating with an access point based on a random access, the stability region is known as all possible combinations of the mean packet arrival rates to keep their queue lengths bounded. This paper investigates the stability region of p-persistent carrier sense multiple access (CSMA) systems, where two users have different mean packet arrival rates and (re)transmission probabilities. We then extend our results to p-persistent CSMA systems with N users and discuss applicability of our results to IEEE 802.11 systems with the basic and request-to-send/clear-to-send access mechanisms. In numerical studies, we show the stability region by varying system parameters such as packet arrival rates, (re)transmission probabilities, and radio channel errors, and how the stability region of p-persistent CSMA systems gets close to that of a time-sharing system.

We evaluate a hybrid automatic repeat request (HARQ) scheme adopting incremental redundancy (IR) type under a finite-length codeword condition in AWGN channels. In the IR-type HARQ scheme, a long codeword is divided into L blocks, so that L becomes the maximum number of HARQ rounds. Although a finite-length codeword has a transmission rate loss from the channel capacity, IR-type HARQ schemes can significantly reduce the loss due to an early-termination effect. We find the sub-optimal coding rate of a codeword for given parameters such as signal-to-noise ratio (SNR), error probability constraint, and L. In addition, we scale the gap between the channel capacity and the average transmission rate of the IR-type HARQ and show that the gap decreases in the order of 1/L when a specific condition is satisfied while the gap of the non-HARQ case decreases in the order of 1/√L.

In this paper, we investigate both link adaptation and user scheduling for an HARQ-based multi-user system in downlink multi-cell environments with inter-cell interference (ICI). First, we analyze two approximation methods on the ICI terms for optimal rate selection. One is a conventional well-known Gaussian approximation (GA) and the other is our proposed identical path-loss approximation (IPLA). Thereafter, we propose a baseline procedure for both the link adaptation and user scheduling. Then, we introduce three conventional policies and our proposed IPLA-based policy. Finally, through system-level simulations, the proposed IPLA-based policy is evaluated in terms of system throughput. Numerical results show that the traditional GA is not accurate any more in the HARQ-based multi-user system with some dominant ICIs. Instead of the GA, our proposed IPLA-based policy achieves more accurate rate selection and higher system performance.

Machine-to-machine (M2M) communications exhibit quite different characteristics, compared with human-to-human (H2H) communications. Since the number of M2M devices is much larger than that of users for H2H communications, it may cause a shortage problem of radio resources if we do not consider an efficient radio resource management scheme. In this paper, we propose an efficient resource allocation scheme to support a large number of M2M devices in the network. First, we investigate the tradeoff relationship between a non-orthogonal resource allocation scheme based on user-pairing and an orthogonal resource allocation scheme by considering pilot overhead in resource usage. Second, we set up an optimization problem for minimizing the amount of allocated resources for a large number of devices in the network. Finally, we propose a user-pairing based resource allocation algorithm. When the number of devices is 500 and the average SNR is set to 10 dB, our proposed suboptimal minimum resource allocation (SMRA) scheme reduces the total required number of resource blocks by 29% and 28%, compared with the orthogonal resource allocation (ORA) scheme and the random user-pairing based resource allocation (RUPRA) scheme, respectively.

Small-cell networks are expected as one of key solutions for high system capacity. However, self-organizing and self-healing mechanisms are necessarily required to deploy and manage an increasing number of small-cell networks. In this paper, we consider a future small-cell network as an intelligent distributed antenna system such as an adaptive array antenna. We propose a self-organizing mechanism and a self-healing mechanism for small-cell networks through cooperative clusters. We evaluate the system performance in terms of resource utilization in both normal and failure cases of a small-cell network. The results show that proposed mechanisms outperform the conventional mechanisms.

In order to alleviate a mobile data explosion problem, WiFi offloading has been proposed. The key issue is how many access points (APs) are needed to accommodate a proper number of users per WiFi AP without severe performance degradation. Although WiFi networks can provide higher throughput as more and more APs are deployed, it may not be a reasonable solution if we consider an increase in the corresponding capital and operational expenditure (CAPEX/OPEX). Therefore, it is important to investigate the minimum required number of WiFi APs which achieve a certain level of performance improvement. In this paper, we focus on the minimum required number of WiFi APs for efficient WiFi offloading. We first set the target average per-user throughput when a WiFi network can play a role as an offloading network of a given cellular network. Based on this criterion, we find the minimum required number of WiFi APs in an overlay network through mathematical analysis.

In this paper, we consider wireless random access networks (RANs) with both single packet reception (SPR) and multi-packet reception (MPR) techniques in fading channels, and compare their performance in terms of sum rate and energy efficiency. We adopt a simple power consumption model including transmission power and idle power. With MPR techniques, the transmission rate selection at each user is very important for achieving higher throughput in the uplink of RANs, but the optimal transmission rate at each user may not be feasible in a distributed manner. Thus, we propose a suboptimal rate selection technique in which each user chooses an appropriate transmission rate and, thus, it operates in a distributed manner for practical RANs. It is shown that the MPR yields higher sum rate than the SPR through extensive computer simulations. As for the energy efficiency, the MPR also significantly outperforms the SPR especially when the transmission power is comparable with the idle power.

The conventional hierarchical cooperation (HC) scheme proposed by Ozgur et al. [1] is a novel communication scheme that can achieve linear capacity scaling, O(N) in wireless ad-hoc network from the viewpoint of information theory without considering control and data overhead. In this paper, we investigate the effects of practical parameters such as packet arrival rate, control overhead and data overhead on the HC scheme. We formulate an aggregate throughput optimization problem in terms of cluster size m and solve it by well-known algorithms. Through the performance evaluations, we observe three main effects of the practical parameters. First, large data overhead ratio degrades the aggregate throughput. 31% performance degradation occurs when we consider data overhead ratio α = 0:45. Second, large control overhead also degrades the aggregate throughput and affects to determine the cluster size m for maximum aggregate throughput. Third, the packet arrival rate λ plays a key factor for system stability and we obtain a sufficient condition for system stability.

Carrier sensing schemes have been recognized as a key knob for improving network performance. The distributed coordination function (DCF) and its modifications focus on the feature of the ongoing transmission links to enhance link throughput, however, a lack of coordinated adjustment of the surrounding nodes may reduce the aggregate throughput of entire network. In this paper, we are concerned with the delayed transmissions from exposed terminals, and interference and collisions from hidden terminals. We classify the neighboring areas for a given transmission link into three areas: a hidden area, an exposed area and an overlapped area, and then propose a coordinated dynamic physical carrier sense (CDPCS) scheme in which the carrier sensing threshold (CST) values of the neighbors are adjusted at the same time to achieve local optimization by utilizing the exchanged information and states of neighboring nodes. Simulation results show that the proposed schemes can work effectively.

Interference is one of key factors of performance degradation in wireless communications. Various interference models have been proposed in the literature. However, most of them did not take node mobility into account. In fact, as an interferer approaches a carrier sensing range due to node mobility, the transmitted frame may also encounter a collision. Due to the interfering source, it is necessary to make an effective interference model to characterize the interference and signal-to-interference ratio(SIR). In this paper, we propose a novel interference model through interference and SIR distribution functions obtained from mobile nodes. In particular, we focus on a random direction (RD) model. Based on the probability distribution function of distance between any node pairs, we theoretically estimate the distribution of the accumulated interference contributed by concurrent transmissions and the corresponding SIR values. Through this result, we investigate the probability of successful transmissions and the probability of carrier sensing failures.

Due to high mobility of nodes in mobile ad hoc networks (MANETs), there exist frequent link breakages which lead to frequent path failures and route discoveries. The overhead of a route discovery cannot be neglected. In a route discovery, broadcasting is a fundamental and effective data dissemination mechanism, where a mobile node blindly rebroadcasts the first received route request packets unless it has a route to the destination, and thus it causes the broadcast storm problem. In this paper, we propose a neighbor coverage-based probabilistic rebroadcast protocol for reducing routing overhead in MANETs. In order to effectively exploit the neighbor coverage knowledge, we propose a novel rebroadcast delay to determine the rebroadcast order, and then we can obtain the more accurate additional coverage ratio by sensing neighbor coverage knowledge. We also define a connectivity factor to provide the node density adaptation. By combining the additional coverage ratio and connectivity factor, we set a reasonable rebroadcast probability. Our approach combines the advantages of the neighbor coverage knowledge and the probabilistic mechanism, which can significantly decrease the number of retransmissions so as to reduce the routing overhead, and can also improve the routing performance.

Power saving mechanisms with sleep-mode operations have been used in IEEE 802.16e in order to extend the battery lifetime of mobile devices. The standard sleep-mode operation needs to meet a given delay constraint. However, it may incur unnecessary energy consumption due to frequent state transitions. In this paper, we propose a new energy-efficient sleep-mode operation while meeting a given delay requirement. We apply an M/G/1 queue model with multiple vacations for downlink traffic in an error-free environment in order to evaluate the energy efficiency performance of the proposed sleep-mode operation in terms of packets/Joule. We compare the proposed sleep-mode operation with the standard sleep-mode operation. The results show that our proposed sleep-mode operation improves the total energy efficiency consumption per unit frame time by 35% and energy efficiency by 21% while meeting a given delay requirement.

Recent standardization efforts in the 3rd Generation Partnership Project (3GPP) have made the Long Term Evolution (LTE) system a very attractive radio access technology for various types of devices and applications. However, the system has not yet been optimized to support diverse traffic profiles. In this paper, we propose a group-based communication scheme to alleviate inefficiencies in the radio access network. The proposed scheme groups multiple connections triggered by different User Equipments (UEs). For the efficient grouping of the UEs, the location information of the Machine-Type Communication (MTC) devices is utilized to identify each group. A four-phase system model is presented to analyze each segment of the connection establishment procedure in the access network. We found that the random access procedure is the main variable factor for the communication delay, and we, therefore, carefully design the procedure with details. Simulations are conducted on our LTE testbeds, and the results show that the proposed scheme is very effective in ensuring the system performance of legacy services and in utilizing radio resources within a given delay bound.

Topology control is a powerful solution to reduce power consumption and the number of collisions by minimizing the transmission range of each node by maintaining a certain level of network connectivity. Although many topology control algorithms have been developed for static networks, e.g., sensor and ad hoc networks with less topology change, where nodes are fixed and the network topology never changes, the topology control technologies can also be adopted for dynamic networks such as Mobile Ad hoc NETworks (MANETs) with an aim to efficiently construct reliable networks. However, in order to apply topology control technologies into MANETs, it is essential to address the issue of performance degradation due to node mobility. Topology information in each node needs to be frequently and appropriately updated according to its moving speed so as to maintain the connectivity with neighbors. In our proposed mechanism, each node determines an appropriate value of topology control update interval according to the mobility information of its neighbors. The proposed mechanism with an adopted topology control technique, based on a localized algorithm, can maintain local connectivity which results in keeping global network connectivity although the network is dynamic. This is a significant advantage of our approach. Simulation results demonstrate that our scheme can ensure a certain level of network connectivity even in MANETs.

Low-powered and light-weight sensor nodes will be deployed for e-health services in wireless body area networks (WBANs). Recently, the Institute of Electrical and Electronic Engineering (IEEE) introduced a new standard, IEEE 802.15.6 for wireless body area communications. The goal of this standard is to specify several physical layers (PHY) and medium access control (MAC) layer protocols for variety of applications with various QoS requirements. In this paper, we evaluate the performance of the IEEE 802.15.6 WBAN MAC protocol in terms of throughput, power consumption, and energy efficiency under unsaturated conditions. We develop a discrete-time Markov chain based analytical model to evaluate the performance of CSMA/CA based WBAN MAC protocol during contention access phases. To verify the numerical results obtained from analytical model, we performed simulations and compared the results.

Energy efficiency in wireless communications is one of important research issues. Previous studies on reducing the energy consumption of IEEE 802.11 WLAN systems mainly focused on the energy consumption of a single-link transmission only. However, due to its carrier sensing property, all the WLAN stations in the network receive the transmitted frames. Hence, this single-link transmission not only consumes the energy of the transmitter and receiver of the link, but also consumes the energy of the other stations. Moreover, most of the previous work on energy efficiency optimization problems were considered in MAC perspectives. In this paper, we show that the energy efficiency of all the stations in WLAN network can be optimized by our proposed adaptive transmission power control and rate selection scheme, with more accurate analysis of the energy consumption in IEEE 802.11 MAC and IEEE 802.11n PHY. The proposed adaptive transmission power control and rate selection scheme achieves an increase in an average energy efficiency of 34% at the cost of 5% throughput degradation for 800bits payload size and 20 users, for varying the distance from 0m to 90m.

An inter-cell interference (ICI) problem between co-channel BSs has been one of main challenging research issues in downlink cellular networks. Fractional frequency reuse (FFR) and BS cooperation are useful techniques to manage the ICI problem. By combining the BS cooperation and FFR techniques, we propose to determine an efficient cooperation mode and the type of subchannels, i.e., reusable subchannels or exclusively used subchannels. The optimization problem to minimize the total power consumption in a cooperative cluster is formulated while satisfying a given constraint in terms of the number of subchannels. When the number of users is 20, our proposed energy-efficient subchannel allocation (EESA) scheme consumes 21%, 25%, 43%, and 43% less power than the conventional resource minimization (RM), fractional frequency reuse (FFR), exclusive (EX), and full cooperation (FCO) scheme respectively. Moreover, our proposed EESA scheme outperforms the other four schemes in terms of outage probability.

IEEE 802.11ac standard has newly adopted a downlink multi-user multiple-input and multiple-output (DL-MU-MIMO) scheme. For user multiplexing in downlink WLAN, we can also use a frame aggregation scheme for multiplexing multiple users' data with space-time block coding (STBC) for achieving spatial diversity. We compare the performance of the two downlink user multiplexing schemes: multi-user MIMO and frame aggregation in IEEE 802.11ac. If each user's encoded data stream has a similar length, the multi-user MIMO scheme yields better average throughput than the frame aggregation scheme. On the other hand, if each user's encoded data stream has a different length, the frame aggregation scheme outperforms the multi-user MIMO scheme in terms of average throughput. In a fast-varying channel, the multi-user MIMO scheme yields worse throughput due to the channel feedback overhead, compared to that with the frame aggregation scheme. We also observe that the multi-user frame aggregation scheme with STBC always outperforms a single-user transmission scheme with STBC in terms of average throughput due to enhanced MAC layer efficiency through frame aggregation.

In Rayleigh block fading channels which represent fast-varying channels, long-term rate adaptation is required instead of instantaneous rate adaptation because the channel information fed back may be outdated. We maximize the long-term average transmission rate (LATR) in a two-hop relay network which adopts Chase combining (CC) type Hybrid Automatic-Repeat-reQuest (HARQ). The round transmission rate, i.e. the transmission rate of each HARQ round in each hop, is optimally selected based on the channel statistics of two hops. Two constraints are considered: the outage probability and the maximum number of HARQ rounds, L. In an infinite L case, we show that the optimal round transmission rate of one hop is determined only by the channel statistics of that hop, and can be expressed as a Lambert W function. In a finite L case, we propose a numerical search algorithm to find the optimal round transmission rate. If HARQ is not adopted, the LATR performance becomes very poor. As L increases in the two-hop relay with CC-based HARQ, the LATR performance becomes close to the LATR performance in the infinite L case. We also show the benefits of the proposed rate selection method compared to a non-optimal rate selection method in terms of the LATR.

The authors propose a new power saving scheme based on a buffer-threshold and timer in wireless packet networks. In a standard sleep mode operation with a binary exponential algorithm, a mobile station (MS) can begin a wake period if it is aware of the existence of its own downlink packets during a listening interval. In a light traffic environment, this conventional standard operation consumes a considerable amount of power to switch modes (sleep/listen/wake). To solve this problem, a new power saving scheme based on joint control of buffer-threshold and timer (JCBT) is proposed. To see the performance behaviour of the JCBT scheme, the authors consider the total power consumption per unit time and the average packet transfer delay as performance measures. Based on the performance measures, the authors obtain the optimal solution for the proposed JCBT scheme that minimises the total power consumption per unit time with a given delay requirement.

Most studies on hybrid automatic repeat request (HARQ) with respect to rate adaptation have been focused on throughput maximization. However, for real-time traffic, resource minimization is a more efficient rate adaptation method in OFDMA-based systems. Moreover, most previous work have dealt with either slow or fast fading channel although practical wireless channels typically exhibit a time-correlated property. In this paper, we analyze the outage probability and average resource usage of HARQ with rate adaptation in a time-correlated channel model. Then, we propose a rate adaptation scheme to minimize the average resource usage and evaluate the performance of the proposed scheme under delay and packet loss constraints of real-time traffic. Our analytical and numerical results show that the proposed rate adaptation scheme is more efficient than the conventional schemes.

In this paper, we propose a mobile grouping procedure to cooperate, and propose three different cost metrics for grouping: the remaining battery energy, WLAN physical (PHY) data-rate, and the number of MSs in the expected cooperation group. Moreover, we propose three cooperative MS-grouping schemes based on the above three metrics in order to extend the battery lifetime of MSs with both WLAN and Bluetooth interfaces and to increase energy efficiency. When the data rate is low, the proposed cooperative MS-grouping scheme based on the number of MSs in the expected cooperative group can maximize the average battery lifetime and energy efficiency. However, when the data rate is high, the proposed cooperative MS-grouping scheme based on the available PHY data-rate in WLAN channel outperforms the other two cost metrics based schemes. Furthermore, we obtain a threshold which determines whether the MS should cooperate or not. The threshold is the number of cooperative MSs in the WLAN access point (AP) coverage. If the number of cooperative MSs in the WLAN AP coverage is larger than the threshold, the cooperative grouping schemes become more energy-efficient. Otherwise, the non-cooperation mode is more energy-efficient. Moreover, these cooperation schemes are more energy-efficient in low mobility regime.

We propose a block scheduling (BLS) scheme which can significantly reduce signaling overhead to accommodate more users with low-rate, real-time traffic. The basic concept of the BLS scheme is to allocate the resource to each user by using resource blocks which are a basic unit of resource allocation. Since the performance of the BLS scheme depends on traffic models, we consider two types of traffic: 1) VoIP traffic - nearly constant packet size; 2) game traffic - variable packet size. We present an optimization problem to select the size of resource blocks to minimize the average resource usage per user. Simulation results show that our proposed scheme outperforms the dynamic scheduling (DS).

We propose two scheduling protocols which are applicable to two-hop relay networks based on mobile WiMAX system: the first protocol adopts a persistent scheduling (PS) scheme in both hops, called the PS-PS protocol, while the second protocol adopts the PS scheme only in the first hop and the dynamic scheduling (DS) scheme in the second hop, called the PS-DS protocol. We optimize the above two protocols by minimizing their average resource usage (ARU) through appropriate selection of the modulation and coding scheme (MCS) level for given delay and packet error requirements. Taking into account different characteristics in both a source-relay (SR) link and a relay-destination (RD) link, resource usage due to hybrid automatic repeat request (HARQ) retransmissions, and the accompanying signaling overhead, we compare the performance of the proposed optimization methods with that of the conventional method in terms of ARU. The PS-PS protocol outperforms the PS-DS protocol and the conventional method considered in most channel case, while the PS-DS protocol outperforms other schemes in a harsh channel condition.

Orthogonal frequency division multiple access (OFDMA) systems, such as IEEE 802.16e/m and 3GPP Long Term Evolution (LTE), require significantly large signaling overhead for delivering small-sized delay-sensitive traffic such as voice over IP (VoIP) service. It may result in a significant decrease in the spectral efficiency. In order to overcome this drawback, recently, a persistent scheduling scheme has been standardized in standard bodies. In order to reduce the signaling overhead, the modulation and coding scheme (MCS) is set to be fixed during a burst period for the persistent scheduling procedure and an incomplete transmission, if it occurs, is recovered by a hybrid automatic repeat request (HARQ) scheme. In this paper, considering the HARQ retransmissions, we propose a rate selection scheme for the persistent scheduling satisfying given quality of service (QoS) requirements. In our proposed rate selection scheme, we consider two important factors which were not considered in previous HARQ-related work: time-correlations of the wireless channel in the HARQ retransmissions and the resource usage of signaling overhead. Numerical results show that the proposed rate selection scheme can efficiently enhance the utilization of radio resources compared to the conventional schemes.

Due to high spectral-efficiency of multiple-input multiple-output (MIMO) transmission techniques, IEEE 802.11n WLAN system adopted a single-user MIMO (SU-MIMO) scheme in which multiple symbol streams are transmitted from a single station (STA) to enhance the system performance. On the other hand, recently, adoption of a multi-user MIMO (MU-MIMO) scheme for multi-packet reception (MPR) in uplink WLAN has also attracted attention. The SU-MIMO scheme achieves a MIMO multiplexing gain at physical (PHY) layer while the MU-MIMO scheme achieves a MIMO multiplexing gain at medium access control (MAC) layer. Thus, there is a fundamental question which scheme is a better solution for uplink WLANs and, in this paper, we analyze and compare these two schemes with random STA distribution scenarios. Moreover, with the adaptation of MAC layer parameters, we also analyze and compare the maximum throughput performance of both the SU- and MU-MIMO schemes in uplink WLANs and we find a proper decision criterion to select the MIMO mode in uplink WLANs.

Most analytical studies on hybrid automatic repeat request (HARQ) have been carried out in either slow fading or fast fading assumptions. However, since practical wireless channels typically exhibit a time-correlated property, we need to take into account the effects of time correlation in the design of HARQ. In this paper, we analyze the outage probability and delay-limited throughput of HARQ with rate adaptation in a time-correlated channel model. Then, we propose a rate adaptation scheme to maximize the delay-limited throughput and evaluate the performance of the proposed scheme under a delay constraint. Our analytical and numerical results show that the proposed rate adaptation scheme outperforms the conventional rate adaptation schemes. The proposed rate adaptation scheme is further simplified by using a Gaussian approximation on the effective channel power gain. The rate adaptation based on the proposed Gaussian approximation reduces the complexity and achieves a throughput gain comparable to that with the exact channel distributions.

Data aggregation significantly reduces signaling overhead and delay by encapsulating multiple packets in queue into an aggregated packet in uplink two-hop relaying networks. In addition, hybrid automatic repeat request (HARQ) operation increases the system throughput by retransmitting the entire or part of the packet transmitted at the initial transmission. We combine data aggregation and HARQ schemes in uplink multi-hop relaying. We discuss a problem about how to combine data aggregation and HARQ at an uplink relay station(RS). We propose two data aggregation schemes : a fine classification mode (FCM) and a coarse classification mode (CCM), in which packets in queue are aggregated based on their quality of service(QoS) with different criteria. We compare the throughput performance of the conventional scheme and our two proposed schemes in case of a fixed RS and a moving RS. The CCM scheme is more efficient in the fixed RS, while the FCM scheme is more efficient in the moving RS.

This paper investigates power allocation algorithms for OFDM-based cognitive radio systems, where the intra-system channel state information (CSI) of the secondary user (SU) is perfectly known. However, due to loose cooperation between the SU and the primary user (PU), the inter-system CSI is only partially available to the SU transmitter. Two types of PUs are considered to have different capabilities. One is a dumb (Peak Interference-Power tolerable) system that can tolerate a certain amount of peak interference at each subchannel. The other is a more sophisticated (Average Interference-Power tolerable) system that can tolerate the interference from the SU as long as the average interference over all subchannels is within a certain threshold. Accordingly, we introduce an interference power outage constraint, with which the outage is maintained within a target level. The outage is here defined as the probability that peak or average interference power to the PU is greater than a given threshold. With both this interference-power outage constraint along with a transmit-power constraint, we propose optimal and suboptimal algorithms to maximize the capacity of the SU. We evaluate the spectral efficiency through extensive simulations and show that the SU can achieve higher performance (up to two times) with the more sophisticated PU than with the dumb PU.

In order to reduce MAP overhead for periodic small-sized services such as voice over IP (VoIP) service, a persistent scheduling scheme has been proposed in IEEE 802.16e/m. Although this persistent scheduling scheme is designed for periodic services, we propose to adopt the persistent scheduling for non-periodic real-time services with small-sized packets by considering the advantage of MAP overhead reduction. Assuming Poisson packet arrivals which show a typical non-periodic property, we mathematically analyze the average delay performance and derive the optimal resource allocation period of the persistent scheduling with a given delay requirement. In this paper, we also compare the average occupied resources per frame between the persistent scheduling and the conventional scheduling both in downlink and uplink. Analysis and numerical results show that for the non-periodic services, the persistent scheduling shows more efficient resource usage than that of the conventional scheduling with a smaller packet size and higher modulation and coding scheme.

The throughput of conventional wireless multicast systems is limited by the multicast user with the lowest channel quality, which leads to a very low throughput especially if the number of users is large. In this paper, we show that hybrid-automatic repeat request with Chase combining (HARQ-CC) is a promising technique to overcome this problem and optimize the transmission rate of wireless multicast systems with HARQ-CC. We analyze the throughput of wireless multicast systems with HARQ-CC and derive a closed-form approximation for the optimal rate. To further improve performance, we propose a dynamic rate allocation scheme which combines the advantages of conventional multicast and HARQ-aided multicast.

Most wireless networks adopt power saving mechanisms in order to extend the battery lifetime of mobile devices. In this paper, we investigate the performance of a power saving mechanism in IEEE 802.16m by using a simulator which models the sleep mode operation with a flexible length of listening window specified in IEEE 802.16m standard. Through simulations, we evaluate the proportion of listening window in a given sleep operation period as a performance measure for power consumption instead of direct estimation of power consumption and measure the power efficiency in various environments. The sleep mode operation in traffic indication messages can reduce the proportion of the listening window by approximately 10% than that without traffic indication messages.

Future mobile communication systems demand much higher data rates which may require significantly more energy consumption, compared with the current 3G systems. Recently, green radio has attracted worldwide attention. Energy efficiency is considered as a very important factor for designing future communication systems in this green radio environment because reducing energy consumption of base stations (BSs) and mobile stations (MSs) can directly contribute to reducing CO2emission. We investigate the effects of cell size on energy saving and system capacity and then show the effectiveness of small-cell based future mobile communication systems in terms of energy efficiency. If the path-loss exponent increases and cell-radius ratio becomes smaller, the per-energy capacity ratio becomes larger. For example, if the path-loss exponent is 4 and the per-energy capacity of a macro-cell is normalized to 1, the per-energy capacities of a micro, pico-, and femto-cell are 16,104, and 108in downlink and 64,106, and 1012in uplink, respectively. The amount of CO2emission of one BS transmitter is approximately 181[kg] in a year, if the cell radius is 1[km]. If the cell radius is 500[m], 100[m], and 10[m], the amount of CO2emission in a year is reduced to 45.25[kg], 1.81[kg], and 18.1[g], respectively. Thus, small-cell based mobile communication systems can be a very effective solution to accommodate high data rates with low energy consumption in a future green radio environment.

Generalized processor sharing (GPS) service policy is a scheduling algorithm to allocate the bandwidth of a queueing system with multi-class input traffic. Simulating the GPS system in realistic traffic environments requires a large amount of time. Thus, fluid simulation is useful because it requires much less time. We analyze the bandwidth allocation for fluid simulation in GPS servers, in which the traffic into the server is treated as fluid. Three properties which characterize GPS servers with fluid input are discussed. We show that there exists a unique bandwidth allocation with the properties. It is shown that our previously proposed algorithm gives the unique bandwidth allocation and it is equivalent to the well-known Newton-Rapson method. In numerical study, the performance of finding the unique bandwidth allocation based on other known root finding methods is compared with that of our previous algorithm. We also investigate the impact of the length of the unit time on the accuracy of the performance measures.

In this paper, we propose an adaptive random beamforming scheme with beam selection. In order to recover the orthogonality among beams, we adopt a minimum mean-square error (MMSE) interference suppression receiver for the orthogonal random beamforming (ORBF) scheme and find efficient power allocation strategies. Based on a proportional-fairness criterion, we propose a sub-optimal beam selection scheme and show that the random beam selection (or power randomization) scheme can outperform the conventional equal power allocation scheme, mitigating the burden of optimal beam selection. Therefore, the proposed ORBF/MMSE with beam selection scheme can improve the degraded performance in the interference-limited environment considering the feedback overhead and time selectivity of the wireless channel for practical cellular systems.

Scheduling schemes achieving multiuser diversity gain are suitable to support high rate bursty data services, while orthogonal resource hopping schemes achieving statistical multiplexing gain are suitable to accommodate low/medium rate applications and real-time traffic. Hence, a hybrid mode system has been proposed to support both the scheduling and hopping schemes. We propose a distributed radio resource allocation scheme for the hybrid mode system and compare the system performance of the proposed distributed allocation scheme with that of a localized resource allocation scheme for varying the number of users. In addition, scheduling criterion threshold values for the proposed distributed allocation scheme are derived to maintain the proportion of scheduling to hopping modes. The proposed scheme allocates radio resources adaptively according to users' channel states between scheduling and hopping modes and, thus, it significantly increases system capacity as the scheduling criterion threshold value increases.

The IEEE 802.11 standard supports multiple PHY rates. However, the IEEE 802.11 DCF in a multiple-rate environment may cause a performance anomaly. There have been many studies about the performance analysis and improvement of single- and multi-rate WLAN systems. However, there were a few studies about a generalized analysis on throughput and channel utilization for successful transmissions in multi-rate WLAN systems with different frame parameters and channel errors. In particular, the performance anomaly problem of the multi-rate WLANs still needs to be solved. We propose a more generalized mathematical model for each station with a different data transmission time and mathematically analyze the throughput and channel utilization for successful transmission in error-prone multi-rate WLANs. Moreover, we propose a contention window size adjustment scheme and a payload adjustment scheme to resolve the well-known performance anomaly problem in multi-rate WLANs by achieving temporal fairness. Numerical results show that the proposed scheme is very effective in achieving temporal fairness.

A new flexible multiple access control (MAC) scheme embedding channelization in multi-band carrier sense multiple access / collision avoidance (CSMA/CA) systems is proposed to provide different priority classes among users, and its performance is investigated. In the proposed scheme, two priority classes of users, primary and secondary users, are considered; the classical CSMA/CA protocol is modified to multiband operation for secondary users, whereas channelization is provided for primary users to ensure the quality of service (QoS). The performance of the proposed MAC scheme is analyzed using a new multi-band CSMA/CA model based on a Markov chain capturing the primary user channel activity and the number of bands. The throughput of secondary users is obtained as a function of the primary user activity as well as other CSMA/CA parameters. It is shown that this mixed MAC scheme can guarantee the throughput of primary users certainly with minimal performance degradation to secondary users, especially when the number of available bands is large or the maximum contention window (CW) size for the CSMA/CA secondary users is small compared with the number of secondary users.

In IEEE 802.16e OFDMA system, the system capacity for accommodating VoIP traffic with small-sized PDUs is severely reduced due to a large amount of signalling overhead. In order to mitigate this problem, the IEEE 802.16 maintenance group has recently proposed a persistent allocation scheme to increase the voice over IP (VoIP) system capacity. However, it is difficult to use the conventional modulation and coding scheme (MCS) selection algorithms, which are based on instantaneous SINR, for the persistent allocation scheme. In this paper, we propose MCS selection algorithms which are effective for using radio resources considering a hybrid ARQ retransmission mechanisms while satisfying given QoS requirements for the persistent allocation scheme. Numerical results show that the proposed algorithms are effective for utilizing radio resources efficiently while satisfying given QoS requirements.

Unmanned aerial vehicle (UAV) network is a network consisting of lying UAVs with communication capabilities. There are many technical challenges and problems due to a highly dynamic and mobile UAV network environment. In this paper, we focus on an inter-node interference problem. To overcome the interference problem and guarantee the communication quality, we propose an adaptive beamforming antenna scheme to meet a given outage constraint and to mitigate the interference. We analyze the outage probability of the communication link, and find the optimal beamwidth of the directional beam. Numerical results show that an adaptive beamforming antenna scheme is the most efficient among fixed beamwidth schemes and an omnidirectional antenna scheme in terms of outage and the amount of interference.

Multi-hop relays will be deployed as one of the promising technologies for future cellular networks to enhance the performance of cell boundary users and to reduce deployment and operating cost. However, they have many challenges which include signaling overhead and latency in order to be practically deployed due to a hierarchical multi-hop structure, and the situation is much severer in uplink case. To overcome these problems, we propose a novel hybrid radio resource management scheme, which combines a hybrid multiple access scheme and a window-based virtual bandwidth multiple access scheme, in an uplink OFDMA system based on IEEE 802.16j. The system-level simulation results show that the proposed scheme significantly reduces both the delay and signaling overhead, compared with the conventional schemes.

Collision mitigation is one of classical research issues for wireless local area networks (WLANs). Recently, multiple-input multiple-output (MIMO) transmission techniques have been widely deployed in wireless systems, while a multi-user MIMO-based collision mitigation scheme in uplink WLANs was proposed by authors, and we showed the scheme is very efficient for the uplink performance. However, for an infrastructure-based WLAN, we observe a significant performance unbalance problem between uplink and downlink, compared to the conventional WLANs. Moreover, access point (AP) yields lower throughput performance than each contending station(STA). In order to solve this unbalance problem between uplink and downlink, we adopt a modified minimum contention window (CWmin) adjustment scheme and a random piggyback scheme to the multi-user MIMO-based WLANs. We also develop an analytical model to evaluate the performance of multi-user MIMO-based WLANs in a saturated traffic environment. The result shows that the random piggyback scheme performs more efficiently for the multi-user MIMO-based WLANs.

Underlay transmission in cognitive radio enables a secondary (unlicensed) system to utilize a frequency band of primary (licensed) system as long as the unlicensee interferes less than a certain threshold with the licensee. The secondary system needs to carefully consider not only its own channel to achieve a capacity gain by this sharing spectrum in multi- carrier systems, but also the interference channel to reduce interference at the primary receiver. In this paper, we formulate a capacity maximization problem of the secondary system under an interference-power constraint as well as a conventional transmit- power constraint, and propose an optimal power allocation policy in which we exploit a two-dimensional frequency-selectivity on both channels. Through extensive simulations, we compare the performance of optimal power allocation policy with that of equal power allocation policy and further investigate the effect of the primary's power allocation policy on the performance of the secondary system. Numerical results show that the optimal power allocation policy can achieve a higher capacity in more frequency- selective channels, compared to an equal power allocation policy. Interestingly, a water-filling policy for the primary system also gives additional opportunities to the secondary system than the equal power allocation policy.

We propose a decode-and-forward (DF) based cooperative phase steering scheme and analyze its outage probability. The cooperative phase steering scheme is to make the received signals from multiple relay nodes co-phased at a destination node by pre-adjusting the phase differences.With a reasonable amount of feedback information from a destination node, the cooperative phase steering scheme circumvents the drawbacks of conventional cooperative diversity techniques such as maximal ratio combining (MRC) reception, maximal ratio transmission (MRT), and opportunistic relay selection schemes. Our analytical and simulation results show that the cooperative phase steering scheme outperforms the opportunistic relay selection scheme and approaches the MRT scheme known as a theoretically optimal cooperative diversity technique. It is also shown that cooperative phase steering has sufficient robustness to phase incoherence.

We investigate the effects of multi-user diversity in a spectrum sharing system where secondary users restrictively utilize a spectrum licensed to primary users only if interference perceived at primary users is regulated below a predetermined level. This interference regulation affects the characteristics of multiuser diversity gains previously known in non-spectrum sharing systems. Our numerical and analytical results show that the multiuser diversity gain in a spectrum sharing system increases differently according to conditions given by the transmit power of secondary users, P, and a predetermined interference temperature, Q - if P is sufficiently larger than Q, the multiuser diversity gain in terms of capacity scales like log2(W (Ns)) similarly to a previously known scaling law in the non-spectrum sharing systems, where W(middot) and Nsdenote a Lambert W function and the number of secondary transmitters, respectively. However, the scaling law of multiuser diversity gain becomes log2(Ns) as P becomes sufficiently larger such that P Gt QNs.

This paper investigates the capacity and energy efficiency of spectrum sharing systems with opportunistic user selection where a secondary network utilizes spectrum bands licensed to a primary network under interference regulation. In spectrum sharing systems, secondary users consume a fraction of their resources in sensing the channels to the primary users to comply with the interference constraints. Although more resources for sensing improve reliability and performance, the throughput loss due to time overhead and energy loss due to power overhead should be properly incorporated in performance evaluation. In this context, we define and derive a new metric-average capacity normalized by the total energy consumption-reflecting time and power overhead for spectrum sensing. Based on the developed framework, the optimal normalizedcapacity is investigated. We also propose a simple and practical suboptimal best-n scheme motivated by the infeasibility and high computational complexity of the optimal strategy, where n denotes the number of sensing secondary users. Our analytical and simulation results show that the proposed best-1 scheme is an energy-efficient technique with near optimality in terms of the capacity normalized by the energy consumption.

Hidden nodes may severely degrade the performance of wireless LANs. Focusing on the hidden node effect, we analyze goodput in a single cell WLAN environment with hidden nodes under a non-saturated condition. Uplink and downlink goodputs are analytically derived using an approximation technique and are compared with simulation results.

We propose an orthogonal code hopping multiple access (OCHMA) scheme in order to improve the capacity of an uplink-synchronized code division multiple access (CDMA) systems. When orthogonal codes (OCs) are used for channelization in uplink-synchronized CDMA systems, a finite set of OCs tends to severely limit the capacity gain of the uplink-synchronized CDMA systems. The OCHMA system allows each user to use a different OC for each symbol according to an allocated hopping pattern (HP). It also allows multiple users to use the same OC at a specific symbol time, which is called an HP collision. Thus, the proposed OCHMA scheme can accommodate more users than the number of available OCs. We analyze the capacity of the OCHMA scheme and compare the performance of the OCHMA with that of conventional schemes including the system using multi-scrambling codes (MSC) which have also been proposed to overcome a code-limited situation.

We propose a hybrid scheduling scheme which can efficiently support various types of traffic in a cooperative relay system of downlink cellular networks. The hybrid scheduling scheme harmonizes the shared-channel resource allocation scheme and dedicated-channel resource allocation scheme to accommodate various traffic in heterogeneous traffic environments. We show that the proposed scheme significantly improves the system performance in terms of system throughput in heterogeneous traffic environments through the analysis of the system capacity and throughput.

We analyze the capacity of an opportunistic scheduling system in a spectrum sharing environment where multiple secondary users can share a frequency spectrum with multiple primary users as long as secondary users do not cause interference power exceeding a given threshold to the primary users. We consider three different power control schemes of secondary users: fixed transmit power, adaptive transmit power, and infinite transmit power schemes. Our numerical and simulation results show that the capacity of the adaptive transmit power scheme is similar to that of the fixed transmit power scheme in the low transmit power region, while the capacity of the adaptive transmit power scheme is close to that of the infinite transmit power scheme in the high transmit power region and is saturated beyond a certain point.

In this paper, the performance of the carrier sense multiple access/ collision avoidance (CSMA/CA) protocol under the presence of carrier sensing error is analyzed. Based on our previous work [1], we extend the results to n-user case (n>2), and analyze the sensitivity of the throughput with respect to the key physical-layer parameter, the sensing threshold, via a cross-layer approach. The result provide guidelines about how to operate the CSMA/CA considering imperfect sensing at physical layer. It is shown that the throughput sensitivity highly depends on the ratio of the contention window size W to the frame length L, and the throughput is sensitive to the design of the sensing threshold when W/L is either small or large.

In emerging ubiquitous wireless environments, mobile devices can have multiple communication modules such as WLAN and ZigBee. Since IEEE802.15.4 ZigBee devices and IEEE802.11b WLAN devices share the same 2.4 GHz ISM band, a ZigBee network operating in a low power environment can be severely interfered by overlapped WLAN networks with much larger bandwidth due to higher transmission power. To overcome this inter-system interference problem and guarantee the ZigBee communications, we propose an interference mediation scheme in an overlaid network environment of WLAN and ZigBee devices by using an interference mediator, and evaluate the performance of ZigBee and WLAN networks for three different resource allocation schemes in terms of throughput and channel occupancy time.

In this paper, we compare the performance of wireless local area networks (WLANs) with single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) in terms of collision probability, average throughput, and delay. In the SU-MIMO scheme, multiple antennas are used for transmitting multiple data streams of a single user and this MIMO technique increases link capacity at physical (PHY) layer. In the MU-MIMO scheme, however, multiple antennas at different users are used for transmitting data streams of multiple users. The MU-MIMO scheme reduces the collision probability at medium access control (MAC) layer and increases the link capacity. Both MIMO schemes yield different collision, throughput, and delay performance at the MAC layer of WLANs. Numerical results show that the MU-MIMO scheme yields lower collision probability and shorter delay performance than the SU-MIMO scheme. Furthermore, the SU-MIMO scheme yields better throughput performance for high SNR values and a small number of contending stations. In other cases, the MU-MIMO scheme yields better throughput performance.

The IEEE 802.11n standard aims at providing a data transmission rate of up to 600 Mbps. The frame aggregation techniques of IEEE 802.11n MAC layer play an important role in the enhancement of channel utilization. In this paper, we investigate the effect of IEEE 802.11n frame aggregation for two aggregation mechanisms: MAC protocol data unit aggregation (A-MPDU) and MAC service data unit aggregation (A-MSDU). We propose an analytical model based on an enhanced discrete time Markov chain (DTMC) model in order to describe the post- backoff behavior due to frame aggregation, and then evaluate the throughput performance. Analytical results based on the mathematical model are verified through simulation. The throughput performance of the A-MSDU mechanism outperforms the A-MPDU mechanism as the frame aggregation size becomes larger under an error-free environment. In addition, the throughput performance is significantly improved as the frame aggregation size increases.

We propose an optimal modulation and coding scheme (MCS) selection criterion for maximizing user throughput in cellular networks. The proposed criterion adopts both the Chase combining and incremental redundancy based hybrid automatic repeat request (HARQ) mechanisms and it selects an MCS level that maximizes the expected throughput which is estimated by considering both the number of transmissions and successful decoding probability in HARQ operation. We also prove that the conventional MCS selection rule is not optimized with mathematical analysis. Through link-level and system-level simulations, we show that the proposed MCS selection criterion yields higher average cell throughput than the conventional MCS selection schemes for slowly varying channels.

Deployment of relay stations (RSs) has been proposed to support both high data rates and quality of service (QoS) for 4G wireless communication system. It is expected that the relay systems enhance system throughput and extend service coverage. In this paper, the capacity of a cellular-based two-hop relay system is evaluated in terms of transmit power ratio between base station (BS) and RS, cell radius, and resource utilization values of backhaul links. The cellular-based relay system has an advantage in system throughput as the cell radius increases. Furthermore, the cellular-based relay system can increase the system throughput through spatial resource reuse and reduction of radio resource utilization values of backhaul links.

In this paper, we analyze the operation of ZigBee devices under the presence of Bluetooth interference and formulate a mathematical model for evaluating the performance of ZigBee devices. Our proposed mathematical model for ZigBee devices is based on a Markov chain concept. We here consider Bluetooth devices which adopt class 1 or class 2 Bluetooth chips as an interference source and the transmission power of Bluetooth devices is set several to hundred times higher than that of ZigBee devices. Thus, we assume Bluetooth devices interfere with ZigBee devices, but the ZigBee devices do not. The throughput and energy consumption of ZigBee devices in a ZigBee network are derived for varying the number of actively interfering Bluetooth devices. The result shows that analytical results obtained from the mathematical model agree well with simulation results.

We propose two relay selection in cooperative relay communications. In a fixed scheme, M multiple relays that have strong signal strength are selected out of K relays and forward their received data from a source node to a destination node. As an alternative approach, a threshold-based adaptive relay selection scheme is also proposed to minimize the number of forwarding relays while satisfying a given outage requirement because if the number of forwarding relays increases, then the number of interfering sources also increases. The minimum number of relays that can prevent an outage event are selected to forward data to a destination. The performance of both schemes are evaluated through numerical analysis and Monte-Carlo simulations in terms of end-to-end outage probability and the number of forwarding relays. The result presents a bound that the fixed and adaptive relay selection schemes can achieve information-theoretically. Furthermore, the outage performance of the adaptive relay selection scheme is identical to that of the fixed relay selection scheme with M = K, while the number of forwarding relays is much less than that of the fixed relay selection scheme with M = K.

As a demand for high speed uplink packet services increases, the WCDMA enhanced uplink, also known as high speed uplink packet access (HSUPA), has been specified in release 6 by 3GPR The performance of the HSUPA has been improved using fast scheduling, hybrid ARQ, and shorter TTI. This HSUPA will provide various types of multimedia services, such as realtime video streaming, gaming, VoIP, and FTP. Generally, the performance of HSUPA is dominated by scheduling policy. Therefore, it is required to design a scheduling algorithm considering the traffic characteristics to provide QoS guaranteed services in various traffic environments. In this paper, we propose a scheduling algorithm considering the traffic characteristics to guarantee QoS in a mixed traffic environment. Finally, the performance of the proposed scheduling algorithm is evaluated in terms of average packet delay, packet delay jitter, and system throughput using a system level simulator.

In this paper, the capacity of a TH-PPM UWB system with an outage constraint in multi-user environments is mathematically analyzed and evaluated. To mitigate a near-far problem, a near-interference erasure scheme is adopted and the capacity of the TH-PPM UWB system using the near-interference erasure scheme is evaluated through numerical analysis and Gaussian approximation. The numerical results show that the capacity of the TH-PPM UWB system using the near-interference erasure scheme is significantly improved and there exists an optimum threshold for the near-interference erasure scheme which maximizes the capacity. The capacity which is mathematically derived is very similar to that of numerical results in the region near the optimum threshold. Furthermore, the results are more accurate as the number of bit repetitions increases.

In this paper, we propose an adaptive sub-band nulling technique in order to improve the performance of orthogonal frequency division multiplexing (OFDM)-based wireless communication systems. It excludes some sub-bands experiencing deep-fading and the transmission power for the excluded sub-bands is reallocated for the remaining sub-bands. We compute the optimal number of nulled sub-bands in order to maximize the capacity. Water-filling is one well-known optimal resource allocation scheme. However, it has tremendous complexity at the transmitter and requires full channel state information from the receiver. We compare the performance of the proposed scheme with that of the water-filling scheme and the result shows that the performance of the proposed scheme is similar to that of the water-filling in a wide range of signal-to-noise ratio (SNR) values. Furthermore, the proposed adaptive sub-band nulling can be used as an enhanced distributed transmission mode in future OFDM-based wireless communication systems after a small modification in the frame structure.

In this paper, we present a joint power control, link scheduling and rate control (PSR) algorithm for wireless ad hoc networks by using the convex optimization theory. This algorithm practically considers the power control problem in the interference-based link scheduling process, and provides a congestion control on the transport layer. Both our theoretical analysis and simulation results prove that the PSR algorithm can converge quickly and is possible for distributed implementation in the wireless ad hoc networks.

Beyond-3G wireless communication systems should support packet traffic with both low/medium rate and high rate services. Scheduling-based resource allocation schemes may cause a problem in accommodating low/medium rate services due to a rate mismatch and a large signaling overhead. Dedicated resource allocation/multiplexing schemes are also inadequate for accommodating packet-type traffic that is characterized by low channel activity and high burstiness. Therefore, we propose a new orthogonal frequency and code hopping multiplexing (OFCHM) scheme for downlink multiplexing to provide efficient support of packet traffic with low/medium data rates in a spread OFDM environment. The performance of the proposed scheme is evaluated in terms of the statistical multiplexing gain, the block error rate, and the overall capacity by using both an analytical approach and simulation. The proposed scheme yields a larger overall downlink capacity than a dedicated resource allocation/multiplexing scheme. A comparison study with the variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) system through system-level simulations is also performed. The comparison results show that the OFCHM system yields better performance than the VSF-OFCDM system in packet delay, packet delay variance, and fairness, and has a system throughput advantage with low/medium rate services.

We analyze the user capacity of downlink CDMA systems with quasi-orthogonal sequences (QOSs) considering various system parameters, such as user activity, spreading factor, the amount of transmission symbol energy allocated to common control channels, the amount of outer-cell interference, and sectorization factor. We also consider a power control mechanism with geometric factors such as propagation loss and shadowing effect. This user capacity analysis gives us the pole capacity of the system, which is the user capacity when all BSs transmit signals with unlimited power. Numerical examples show that the introduction of QOSs makes it possible to overcome the code limitation. We discuss how much the user capacity is increased according to various system parameters. For example, in an omni-cell environment with minimum outer-cell interference, if the mean channel activity and the required (Eb/I0) value are set to 0.2 and 2.0 [dB] respectively, the user capacity is 194 which is much higher than the code limitation.

We compare the performance of the downlink capacity improvement schemes based on orthogonal code hopping multiplexing (OCHM) and multiple scrambling codes (MSC). Both OCHM and MSC have been proposed in order to overcome a code-limitation problem in CDMA downlink. However, both schemes exhibit different characteristics. In order to increase user capacity in CDMA downlink, OCHM allocates a random hopping pattern using a set of orthogonal codes instead of orthogonal code for each connection and MSC allocates a non-orthogonal codeword to each connection. In the OCHM scheme, hopping pattern (HP) collisions may degrade the system performance. The HP collisions in OCHM systems differ from the hits in frequency- hopping (FH) systems because it can be effectively controlled through synergy and perforation techniques. In OCHM, the receiver requires additional energy for a given frame error rate (FER) due to the HP collision. In the MSC scheme, inner-cell interference is induced by non-orthogonal codewords and it degrades the performance. Numerical examples show that the OCHM-based system is more effective than the MSC-based system.

In orthogonal code hopping multiplexing (OCHM) systems, hopping pattern (HP) collisions may degrade the system performance. Previous studies on the effect of HP collisions in OCHM systems were mainly based on computer simulations and there was no rigorous mathematical analysis of bit error rate (BER) performance. The HP collisions in OCHM systems differ from the hits in frequency-hopping (FH) systems or intracell interference in DS-CDMA systems because it can be effectively controlled through synergy and perforation techniques. In this paper, we introduce a received signal model for OCHM systems and analyze the BER performance for OCHM systems. Through the analysis of the BER performance, OCHM systems can be characterized more clearly and the allocated power at base station can be estimated. Furthermore, the user capacity is analyzed for a given channel coding scheme.

In HSDPA, mobile terminals measure their own radio channel status and report the quality to base station using a channel quality indicator (CQI). The main purpose of the CQI feedback is to utilize this information for scheduling and adaptive modulation and coding (AMC) in a base station. However, since CQI is another version of signal-to-interference and noise ratio (SINR), we can extract more information about radio channel states of mobile terminals. In this paper, we propose an effective two-dimensional radio channel estimation scheme from the current and past CQI information for each mobile terminal. We show the effectiveness of the proposed scheme by evaluating the performance through mathematical analysis and simulations.

We consider a bandwidth optimization problem in a Generalized Processor Sharing servers with multiple class queues to minimize the total bandwidth such that QoS requirements for each class queue are satisfied. In our previous paper [13], since it is difficult to obtain the performance such as the delay distribution mathematically with self-similar input traffic in a GPS server, we obtained an optimized bandwidth vector using a simulation-based optimization technique. However, the previous optimization algorithm requires rather long simulation time to solve the problem by using exterior penalty function methods. We propose a new bandwidth optimization algorithm based on bandwidth ratio adjustment without complex calculations. In numerical results, the required time to find the optimal bandwidth allocation in GPS servers is greatly reduced.

Scheduling is a widely used technique in recent wireless communication systems to utilize radio resource efficiently and adaptively according to varying radio channel conditions. In this paper, scheduling-based systems in Rayleigh fading channels are analyzed mathematically. First, a scheduling-based system and signal models are described. Then, the signal-to-noise ratio (SNR) distribution is formulated and the joint distribution of SNR, distance, and selected user is also derived. Using the derived joint distribution, we evaluate the selection probability and the distribution of selected users' SNR values. Finally, system capacity is evaluated from the derived formulation. The analysis results and evaluated performance show that a multiuser diversity gain increases the system capacity and the scheduling-based systems take advantage of radio fading channel variation efficiently

As the demand for real-time (RT) services increases, service providers need to provide their end-to-end quality of service (QoS). Hence, a hierarchical packet scheduling algorithm is proposed to guarantee a strict QoS requirement of RT traffic in wireless communication systems. The proposed scheduling algorithm gives priority to urgent head-of-line (HOL) RT packets and meets a QoS requirement of RT traffic. Through system-level simulation, the performance of the proposed scheduling algorithm is evaluated in terms of packet drop ratio, packet delay, utilization, service throughput, and over-the-air (OTA) throughput. It is confirmed that the proposed scheduling algorithm yields excellent performance in terms of RT packet drop ratio, RT packet delay, and RT traffic throughput. The proposed hierarchical scheduling algorithm can be utilized to guarantee the strict QoS requirements of delay-sensitive traffic and extended to support the traffic with various delay requirements

In the HSDPA system, code multiplexing in one TTI is required to transmit packets for multiple VoIP users. This paper proposes an adaptive power allocation (APA) scheme for code multiplexing. The proposed APA scheme adaptively allocates the transmit power to each user in downlink according to the user's link status. The user capacity in the HSDPA system based on the proposed APA scheme is compared with that of the conventional equal power allocation (EPA) scheme through simulation. The simulation results show that the user capacity of HSDPA based on the proposed APA scheme is 136, compared to 75 of the conventional EPA scheme

A preplanned path-protection scheme with sufficient spare bandwidth is appropriate for real-time fault restoration in multiprotocol label switching (MPLS) networks. In this case, however, it is important to reduce the amount of spare bandwidth to prevent degradation of network efficiency. A distributed label switched path (D-LSP) scheme is proposed to reduce the amount of spare bandwidth required for protecting against network faults in MPLS networks. The main idea of the proposed D-LSP scheme is to partition traffic into multiple LSPs, each of which is established on a distinct link-disjoint route between each pair of end nodes. The D-LSP scheme is evaluated in terms of the reduction ratio of total network cost in comparison with the conventional LSP scheme. Traffic partitioning in the D-LSP scheme can decrease the statistical multiplexing gain (SMG) obtained by aggregating IP packet flows into an LSP. The tradeoff between spare bandwidth reduction and degradation of SMG due to traffic partitioning is also investigated. The numerical results show that the proposed D-LSP scheme yields the network cost-reduction ratio (NCRR) of at least 29%, 27%, and 15% for the networks where average node degrees are 4.6, 4.4, and 3.2, respectively. The D-LSP scheme shows the similar performance of NCRR in both Markovian traffic and self-similar traffic environments.

A new frame level orthogonal code hopping multiplexing (FrOCHM) scheme is proposed to statistically multiplex bursty data user traffic in downlink. The FrOCHM scheme solves the channel inefficiency problem arising in conventional CDMA systems. Moreover, combining both a scheduling-based multiplexing scheme and our previously proposed orthogonal code hopping multiplexing (OCHM) scheme, the FrOCHM mitigates the complexity of the scheduling-based multiplexing scheme and does not need additional E/sub b//N/sub 0/ required in the OCHM scheme. Simulation results show that the FrOCHM can support 251 mobile stations, while the scheduling-based multiplexing scheme supports 136 mobile stations when the user activity is 0.05 and the data rate is 64 kbps.

Since there are various packet-type services with different QoS's , a scheduling algorithm that can support multiple QoS classes is needed to efficiently provide data services in the high speed downlink packet access (HSDPA). In this paper, we investigate the capability of HSDPA to support multiple QoS classes and propose an improved scheduling algorithm to serve multiple QoS classes.

This paper analytically compares three packet scheduling schemes: round robin (RR), proportional fair (PF), and generalized processor sharing (GPS), under a per-user minimum throughput assurance requirement in the HSDPA system. The results show that the PF scheme yields the highest system throughput and the worst case user throughput is also bigger than for other schemes. Moreover, we also investigate the acceptable number of users when the minimum throughput requirement for all users in cell is fixed.

We have recently proposed an orthogonal code hopping multiplexing (OCHM) scheme which is based on a statistical multiplexing scheme for orthogonal downlink in direct sequence spread spectrum systems. This is a feasible candidate for accommodating a large number of bursty packet-based users with good backward compatibility. In OCHM, code-collisions which degrade channel coding performance and result in an increase in the required E/sub b//N/sub 0/ are inevitable for obtaining a statistical multiplexing gain. In this paper, we lower code-collision probability by discarding or delaying excessive frames whose number is larger than a threshold value. It reduces the required E/sub b//N/sub 0/ at the transmitter and saves system power. However, there is an increase in BLER (block error rate) or delay. Therefore, there exist trade-offs between the required E/sub b//N/sub 0/ and BLER (or delay). We can determine an operating point which can reduce the required E/sub b//N/sub 0/ by considering how much BLER or delay the system requires as a target value.

Bandwidth optimization is considered when several classes of Internet traffic are served in generalized processor sharing (GPS) servers. Internet traffic shows self-similar patterns that make it difficult to obtain analytical performance in GPS. Thus, for performance estimation of different classes of traffic, we use fluid simulation techniques that can reduce the simulation complexity, compared to packet-level simulation. Using the relationship between the guaranteed bandwidth vector and the corresponding performance, we propose a bandwidth optimization problem to minimize the total bandwidth such that performance requirements are satisfied. We use an exterior penalty function method to solve the optimization problem. However, a penalized objective function may have local minimum which is not a global minimum. Thus, we propose a new methodology to circumvent the limitation of the exterior penalty function method.

Spare bandwidth is required for recovering the network service from network faults. However, it degrades the efficiency of network utilization. Spare bandwidth demand can be reduced significantly by letting spare bandwidth be shared among several network services. Spare bandwidth reserved on a network element can be shared by a set of network services for a network fault if they are not simultaneously affected by the network fault. A new, and more practical spare bandwidth sharing scheme, which is based on the network reliability, is proposed in this paper. In the proposed scheme, multiple link failures are allowed with a given link failure rate, and a reasonable restoration level of near 100%; while in the conventional scheme, only a single link failure, and 100% restoration level are considered. To develop the spare bandwidth sharing scheme, we first investigate the framework for evaluating the reliability of path-based network services, and then we explain the proposed spare bandwidth sharing scheme with decision parameters such as lifetime of the path, restoration level, and the maximum number of working paths which can be protected by a backup link. Simulation results show that the proposed spare bandwidth sharing scheme requires a smaller amount of spare bandwidth than the conventional scheme.

The paper is concerned with estimating the available bandwidth of a network path. We first develop a theory to estimate the available bandwidth of a queueing system. In order to estimate the available bandwidth, we propose a probing method, called a minimal backlogging method, and a statistic based on the service rate of minimally backlogging probing traffic. We show that the available bandwidth of a queueing system can be estimated by the statistic if probing packets are sent to the queueing system by the minimal backlogging method. For a network path consisting of multiple hops, we extend the approach for a single server by introducing a simplified path model. Since the proposed mechanism can estimate the available bandwidth quickly and track it adaptively, a reasonable range of available bandwidth for a short time interval can be obtained using the mean and variance of the estimated available bandwidth. The performance of the proposed available bandwidth estimation mechanism is evaluated by simulation in a multiple hop network topology.

Mobile communication networks can provide various types of services, such as voice, data, video, and multimedia as they evolve toward 3G networks. To increase the efficiency of limited resources in the UTRAN Iub interface, AAL2/ATM link transmission schemes with multiplexing have been introduced. As an all-IP based network technology develops, the AAL2/ATM link transmission schemes will be replaced by IP based link transmission schemes in future to reduce large protocol overheads. Accordingly, CIP, LIPE and PPPmux have been proposed in 3GPP standards. In this paper, we describe a link transmission model in the UTRAN Iub interface and evaluate the performance of ATM based and IP based link transmission schemes in terms of link efficiency and average link transmission delay. Simulation results show that the IP based transmission schemes perform better than the ATM based transmission scheme as the number of users increases.

Rapid growth of Internet traffic causes a new challenge in the design of high-speed switches. One of main design issues for high-speed switches is a scalability problem. This paper proposes a scalable crossbar matrix (SCM) switch architecture, which consists of multiple crossbar switch units (XSU) with virtual output queues (VOQs) at the inputs and single-cell scheduling decomposition buffers (SDBs) at the outputs. We propose a distributed scheduling algorithm for the proposed scalable crossbar switch architecture, which consists of a credit based SLIP (C-SLIP) scheduling for crossbar switch units and backlog weighted round robin (BWRR) scheduling for switching fabric output ports. In this paper, we show that large-scale switch fabric can be built up by small-size crossbar switch units with improved delay performance and enough arbitration time margins. The simulation results show that the proposed switch architecture and distributed scheduling algorithm can provide 100% throughput under i.i.d. uniform traffic with a single iteration arbitration in a time slot.

Recently, mobile communication systems have supported various services including voice, data, video, and multimedia services as they evolve toward 3G. An AAL2 multiplexing scheme is introduced for transmission link efficiency in the UTRAN (UMTS terrestrial radio access network). However, limited radio resources and various quality of services (QoS) make it difficult to guarantee QoS requirements at radio access networks. The conventional AAL2/ATM multiplexing systems focused on link efficiency in multiplexing real-time voice and nonreal-time data traffic. In this paper, we classify various services into four classes according to QoS requirements and propose AAL2/ATM multiplexing schemes using statistical multiplexing and scheduling to guarantee the given requirements. Simulation results show that the proposed schemes perform better than the conventional schemes in guaranteeing QoS requirements of various services.

A multisector antenna based access point (AP) network architecture with omni-directional mobile nodes (MNs) is proposed. We investigate the effect of hidden nodes and power control on the system performance, and compare the performance of the proposed network architecture with that of an omni-directional AP/MNs network architecture in terms of throughput, delay, probability of collision, and power consumption per throughput. The data throughput in the multisector antenna based AP network architecture with power control (PC) is approximately 5.65 and 6.59 times higher than for the conventional omni-directional AP network architecture with PC and without PC, respectively, at an aggregate offered load of 1.0 (2.0 Mbps). The peak data throughput of the proposed network architecture with PC is approximately 1.9 (3.8 Mbps) for an aggregate offered load of about 2.1 (4.2 Mbps). Furthermore, the power consumption of MNs per throughput in the proposed network architecture with PC is only 1.02% and 2.25% of those of the omni-directional AP network architecture without PC and with PC, respectively, at an aggregate offered load of 1.0 (2.0 Mbps). The proposed network architecture with PC yields a significant improvement in system throughput and power saving in comparison with the conventional omni-directional AP/MNs network architecture.

We proposed an Orthogonal Code Hopping Multiplexing (OCHM) scheme as a new statistical multiplexing scheme in synchronous downlink. OCHM enables a large number of users to share a limited number of code channels through statistical multiplexing. In this paper, we obtain the optimal code rate in several traffic load environments by simulation and summarize the appropriate traffic load region for each code rate as the optimal code rate. An adaptive code rate control scheme is proposed and the base station adaptively changes the code rate according to traffic environments in order to save power.

For data communication services gradually dominating in future wireless networks, one crucial problem is to save battery power of data terminals usually requiring much power. A mobility management scheme in IP-based wireless networks is proposed to reduce battery power consumption of mobile hosts. The proposed scheme manages six states: communicating, attention/cell-connected, attention/PA-connected, idle, off/attached, and detached states for efficient management of battery power, radio resources, and network load. Steady state probabilities for mobile host (MH) state transitions are derived using a semi-Markov process approach, and the power-saving effect of the proposed scheme is analyzed. The effects of various input parameters on the steady state probabilities and power consumption are also investigated. The proposed mobility management scheme yields significant power saving in comparison with the current mobility management schemes for Mobile IP-based wireless networks.

We propose a new label switched path (LSP) mechanism, called the distributed LSP (D-LSP) mechanism, for multiprotocol label switching (MPLS) networks. In the proposed D-LSP mechanism, traffic belonging to a forwarding equivalent class (FEC) is partitioned into several sub-classes and is distributed to the corresponding sub-LSPs, each of which is set up on a different node-disjoint route connecting the traffic source and destination node pair. To protect service, a D-LSP reserves the spare bandwidth of which the amount is equal to the amount of the working bandwidth of one sub-LSP and thus the total amount of spare bandwidth in a network can be reduced. The traffic partitioning of the D-LSP mechanism may decrease the statistical multiplexing gain obtained by aggregating IP packet flows into an LSP, as compared to that of the conventional MPLS networks. Sub-LSP and backup LSP assignment algorithms are proposed and the effect of D-LSP mechanism on the required amount of spare bandwidth is evaluated by numeric analysis and is compared with a conventional LSP mechanism. The results show that there is a trade-off between the reduction of spare bandwidth and the degradation of statistical multiplexing gain.

General Packet Radio Service (GPRS) is a system for efficient data packet radio transmission services which is used in conjunction with the Global System for Mobile Communications (GSM) system. The three mobile station (MS) states idle, ready, and standby are defined for efficient management of radio resources and signaling networks. The MS state transition behavior is modeled and the steady state probability of the MS states is derived using a semi-Markov process approach. The effect of various input parameters on the steady state probability is investigated. These results can be used in analyzing the effect of the input parameters on the tradeoff between location update and paging signaling, and thus, providing guidelines for proper selection of GPRS system parameters.

Universal Mobile Telecommunication System (UMTS) is a third generation system for efficient data packet radio transmission services. The three mobility management (MM) states PMM detached, PMM connected, and PMM idle are defined for efficient management of radio resources and signaling networks. In addition, two radio resource control (RRC) states idle and connected are introduced for the management of more detailed location information. The connected state is further divided into cell connected and URA connected states. These MM state models and RRC state models are combined together. The user equipment (UE) state transition behavior is modeled and the steady state probability of the UE states is derived using a semi-Markov process approach. The effect of various input parameters on the steady state probability is investigated.

We previously proposed the orthogonal code hopping multiplexing (OCHM) scheme as a new statistical multiplexing scheme in synchronous downlink and applied the OCHM scheme to IS-95 and wideband CDMA. OCRM enables a large number of users to share the limited number of code channels through statistical multiplexing. In this paper, we compare OCHM with HDR through link-level simulation. OCHM outperforms HDR in medium and high mobility conditions, while HDR operates better in a low mobility condition.

This paper presents a comparative evaluation of a modified version (A-protocol) of the distributed-queueing request update multiple access (DQRUMA)/multi-code code division multiple access (MC-CDMA) protocol and an enhanced version (P-protocol) of the DQRUMA/MC-CDMA protocol with a lattice pool for request accesses (LPRA) scheme in a packet-based voice traffic environment. The analytical results agree with the simulation ones and show that the P-protocol outperforms the A-protocol in terms of the packet loss rate for voice traffic.

First, we propose a new modeling method for superposed ATM traffic by the MMPP(2), which is a special case of the MAP(2). In this new method, we measure the mean and autocorrelation of cell interarrival times, and the histogram of the number of arrivals during measurement windows of fixed size. The MMPP(2) has interarrival times with a second-order hyper-exponential distribution with coefficient of variation c/sub /spl nu// > 1. However, superposed traffic is often observed to have c/sub /spl nu// < 1. To cover this situation, we extend the MMPP(2) to a MAP(3) by adding a new state with inter-state transition accompanied by an arrival. For the MAP(3) model, we take into account the second moment of the interarrival times. From numerical examples, we observe that both the proposed MMPP(2) and MAP(3) yields very good estimation of the cell loss ratio (CLR) for usual superpositions of voice and/or VBR video sources. However, when we have superpositions from CBR video sources together with other VBR sources, c/sub /spl nu//. is much less than 1, and the MAP(3) outperform the MMPP(2), as expected. The proposed MAP(3) well characterizes the cell scale component as well as the burst scale component of superposed traffic streams.

We consider a common-memory (CM) type N /spl times/ N ATM switch, where CM block consists of K (K /spl ges/ N) separated submemories. We propose an address assignment algorithm to avoid input/output contentions so that we can have the read/write speed of submemories as low as the interface (input/output) port speed. Taking a replication-at-sending approach to multicast, we pursue memory efficiency and maximum throughput. We develop an analytical model to evaluate the system in terms of cell loss ratio and average delay time. In the analysis, we take into account two loss factors causing losses of incoming cells: (1) the failure of scheduling to avoid the input/output contentions and (2) overflow in the CM block. The first factor is dominating and can be significantly reduced by increasing K. From our analytical results compared with simulations, it is observed that we can take K /spl ap/ 3N as a guide of system design.

A high-performance variable-length packet scheduling algorithm is proposed for efficiently accommodating IP traffic in input. queued crossbar switches. It uses a rotating priority(round-robin) arbitration and a masking operation for variable-length packets. This algorithm achieves 100% throughput with a single iteration for uniform IP traffic and performs better in terms of packet latency and similar cell latency, compared with the well-known iSLIP cell-level scheduling algorithm.

Conventional input switches usually employ a single crossbar switch fabric to transfer cells from input buffers to output ports. This type of switch suffers from input and output cell contention problems which cause lower performance than for output buffer switches. However, dividing one crossbar fabric into several smaller crossbar fabrics, we can decrease the input and output contention probabilities. Based on this principle, we propose a new decomposed crossbar switch architecture. Since a decrease in input and output contention probabilities causes an increase in the grant probability for the cells at input buffers, the proposed decomposed crossbar switches yield better performance than conventional input switches. We derive the grant probability for a simple arbitration algorithm and evaluate the performance of the proposed switch architecture in terms of the average cell latency through simulation.

Two mobile terminal (MT) power on/off state management schemes are introduced based on the level of location registers (LRs) for managing the state information of MTs. The cost of the two schemes is obtained based on modeling of the state transition behavior of an MT. Based on the observation that there is a tradeoff between the two schemes, we propose an adaptive scheme that incorporates the two schemes. The results show that the adaptive scheme selects an appropriate scheme in real time.

Code collisions and packet corruption due to multiple access interference (MAI) are important problems in a receiver-oriented slotted code division multiple access (CDMA) system. Herein, we propose an enhanced distributed-queueing request update multiple access (DQRUMA)/multi-code(MC)-CDMA protocol with a variable request pool (VRP) as a new slotted MC-CDMA protocol designed to reduce request collisions and average total delays, and to improve the efficiency of receiver utilization in base stations.

This paper describes a comparison of system performance using two different chip waveforms of spreading sequences in multiple-chip-rate (MCR) direct-sequence (DS)/code-division multiple-access (CDMA) systems. The chip pulses used in this study are closely related to the characteristics of output filter employed at transmitter. In general, the chip waveform is an important factor to determine the link performance. The raised cosine chip pulse with a roll-off factor of /spl alpha/ will be adopted for IMT-2000 systems in order to reduce both the intersymbol effect and the spectral width of the modulated signal. However, due to the complexity of obtaining quantitative results on the performance of MCR-DS/CDMA systems, rectangular chip pulses are mainly utilized in performance analysis. Therefore, it is necessary to investigate the effect of the chip pulses used, i.e., a rectangular and a raised cosine chip pulses on system performance in order to evaluate MCR-DS/CDMA systems accurately. Thus, the effect of the chip pulses used on the performance in MCR-DS/CDMA systems is investigated in terms of the system capacity and blocking probability. It is shown that the system using a raised cosine chip pulse (i.e., RC system) supports at least 80% more capacity and 57% more traffic than that using a rectangular chip pulse (i.e., R system).

Many techniques for calculating bit-error probabilities (BEPs) of direct-sequence spread-spectrum multiple-access systems (DS-SSMA) have been reported. Among them are the following three techniques: (1) the standard Gaussian approximation; (2) the improved Gaussian approximation; (3) and the simplified improved Gaussian approximation. We extend these techniques to derive the BEPs of multicode DS-SSMA systems. We assess the three techniques by comparing their results with the result of computer simulation.

Real-time traffic requires strict delay constraints in terms of maximum cell transfer delay (CTD) and cell delay variation (CDV). In order to support the required quality of service (QoS) of each connection in ATM networks, it is important to estimate the current status of networks by using local information such as cell delay and delay variation in switches. We develop a framework for obtaining delay distribution in a single switch module by monitoring the internal buffer. We also propose a new delay estimation mechanism utilizing a fast convolution approximation scheme in order to reduce the number of operations and the required amount of the data for estimating end-to-end delay. The feasibility of the proposed delay estimation mechanism is verified by simulation for various types of CBR and VBR traffic loads.

When a universal personal telecommunications (UPT) user visits a mobile network, both terminal mobility and personal mobility are involved and it may take a long time for UPT call delivery to the UPT user because it is required to retrieve both the terminal mobility related data in mobile network and the UPT user related data in the UPT database. If the UPT database also manages location and status information of the MT on which the UPT user has registered, incoming call delivery to the UPT user can be improved. Mobility management schemes for UPT users roaming in a mobile network are analyzed. The performance is evaluated in terms of signaling cost and the number of node accesses during an InCall registration.

For the superposition of on/off sources, we propose a real-time computable two-state Markov modulated Poisson process [MMPP(2)] modeling method characterizing the aggregate cell arrival rate and the variance-time curve for cell counts. Numerical examples show that the proposed method yields a good estimation of the cell loss ratio (CLR) in asynchronous transfer mode (ATM) multiplexers. Then, we propose an approximation method for calculating the workload for the MMPP(2)/D/1 system. By considering the workload as an upper bound of CLR of ATM multiplexers, we propose a connection admission control (CAC) algorithm utilizing peak cell rate, sustainable cell rate, and maximum burst size as user traffic descriptors. Using the proposed CAC algorithm, an acceptance decision is made with computational complexities of less than 3000 floating point operations. It is also observed from numerical evaluations that the proposed CAC algorithm yields a high multiplexing gain, compared with other methods in the literature.

A general expansion architecture is proposed that can be used in building large-scale switches using any type of asynchronous transfer mode (ATM) switch. The proposed universal multistage interconnection network (UniMIN) switch is composed of a buffered distribution network (DN) and a column of output switch modules (OSMs), which can be any type of ATM switch. ATM cells are routed to their destination using a two-level routing strategy. The DN provides each incoming cell with a self-routing path to the destined OSM, which is the switch module containing the destination output port. Further routing to the destined output port is performed by the destination OSM. Use of the channel grouping technique yields excellent delay/throughput performance in the DN, and the virtual FIFO concept is used for implementing the output buffers of the distribution module without internal speedup. We also propose a "fair virtual FIFO" to provide fairness between input links while preserving cell sequence. The distribution network is composed of one kind of distribution module which has the same size as the OSM, regardless of the overall switch size N. This gives good modular scalability in the UniMIN switch. Performance analysis for uniform traffic and hot-spot traffic shows that a negligible delay and cell loss ratio in the DN can be achieved with a small buffer size, and that DN yields robust performance even with hot-spot traffic. In addition, a fairness property of the proposed fair virtual FIFO is shown by a simulation study.

A new cell re-sequence mechanism is proposed to restore cell sequence in multipath ATM switches. Since the proposed mechanism uses per-VC logical queues which store only the cells belonging to the same VC, the mechanism can reduce processing time compared to conventional re-sequence mechanisms. The mechanism also has no limitation on the peak rate of the VCs and no arbitration functions to select an output cell. The mechanism can be implemented using a RAM buffer, a CAM/RAM table, a controller, etc.

An intelligent network architecture with fixed local anchor (FLA) is proposed to support Universal Personal Telecommunication (UPT) services. The performance of the proposed IN architecture is compared with the architecture based on IS-41 considering UPT user's personal mobility in terms of call delivery cost and location update cost.

We propose an approximation method for sojourn time distributions in telecommunication networks consisting of general queuing networks. The queuing network is of the open network type, and has n single server queuing systems with arbitrary interconnections. Customers may enter the network at any queuing system. The interarrival times of the external arrival processes and the service times at the queuing systems are generally distributed. The analysis is based on the methods of decomposition, two-moment approximation, waiting time approximation and semi-Markov chain construction. All related processes are approximated by the first two moments only. Numerical results are compared with simulation ones in order to validate the proposed method.

We propose a hybrid channel assignment (HCA) scheme in DS-CDMA systems for accommodating integrated voice/data traffic and the required power levels of voice and data traffic are derived. These levels can be used to maintain the required link qualities of all calls. In the proposed scheme, delay-sensitive voice traffic is accommodated in a circuit mode and delay-nonsensitive data traffic is accommodated in a packet mode. The proposed scheme can achieve a high link efficiency with the reduced control overhead by statistically multiplexing voice and data traffic.

Since mobile user mobility behaviors can vary according to time periods, e.g. office-going/closing and office hours, the location management traffic load for tracking user positions is also a function of time. The transient characteristics of location management traffic vary according to users' initial starting positions. In this paper, we propose a mobile user mobility model considering office-going/closing and office hours for evaluating the effect of user mobility on the performance of mobile networks. We analyze the transient characteristics of a mobile network in terms of the mean location update rate.

We are concerned with a real-time quality of service (QOS) estimation method for ATM statistical multiplexers with heterogeneous ON/OFF sources. We propose a real-time modeling method for the superposed traffic by a two-state MMPP by approximating the density function of aggregate cell arrival rate as a Bellows-like Poisson process. We also investigate a real-time calculation of the waiting time distribution for MMPP(2)/D/1 system to consider an upper bound of the cell loss ratio (CLR) for the finite-buffer system. Numerical examples show that the proposed QOS estimation method gives a good estimation of CLR.

We calculate the capacity of DS-CDMA systems supporting multi-class services with different transmission rates and bit error rates. Two techniques, a single-code scheme using a variable processing gain and a multi-code scheme, are considered to provide multi-rate services. The capacity is expressed by an inequality to which the number of accepted calls of each class should conform. The inequality is the necessary and sufficient condition satisfying the requirement of the bit energy-to-interference power spectral density ratio. The optimal received power is derived to cause the least interference to other signals while maintaining the acceptable bit energy-to-interference power spectral density ratio. Finally, the capacity is also derived in a dynamic multiple cell environment.

To accommodate multimedia and broadband mobile services, future mobile communication networks are likely to be integrated with broadband fixed networks. Previous MAN-based architectures may be rather complex to provide various isochronous/non-isochronous services due to complicated slot generation processings. We propose a broadband personal communication network (BPCN) architecture based on B-ISDN/ATM. We employ simple cell based add-drop multiplexers (ADMs) instead of rather complex medium access control systems. We also suggest a ring management scheme. After describing call processing procedures, we calculate and compare the signaling processing loads at the various nodes and the signaling loads on the various links for six types of configurations according to the locations of basestation controller (BSC), visitor location register (VLR), and mobile switching center (MSC). These results may be utilized in designing BPCNs in the given conditions, such as PCN subscriber environments and the capabilities of network entities. A 150 Mbps-dual ring can support the maximum carried traffic load of about 5,500 on-off voice traffic in the case that 300 connections are supported in the ring node (RN). This ring capacity corresponds to covering approximately 180 cells, if we assume 30 connections per cell.

Advanced satellite on-board baseband switching processors have time-space-time (T-S-T) structures which are similar to the terrestrial switching networks (Sw-Nw). Generally, the satellite systems require higher reliability than ground equipment because of more severe environment and lack of repair. This paper proposes fault-tolerant satellite on-board T-S-T Sw-Nw with multiple separated space switches instead of a single spare switch. The authors analyze the mean time to unreliable operation (MTUO), as a performance and reliability index for the T-S-T systems with multiple separated space switches as well as conventional T-S-T systems. The MTUO varies depending on the threshold level of blocking-probability and the offered traffic. In general, T-S-T Sw-Nw with multiple space switches have better performance and reliability than those with the single space switch, and their performabilities are appreciably improved, especially for one additional spare space switch. This study can be extended to analyze the performance and reliability of entire satellite communication systems including the on-board baseband Sw-Nw, Rx interfaces, and Tx interfaces.