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As non-orthogonal multiple access (NOMA) technology is considered as a new multiple access scheme in cellular networks, a new handover decision algorithm and operational procedure need to be investigated. In this paper, we apply the NOMA technique to handover situations for throughput enhancement in cellular networks. We first derive the condition of NOMA-based handover (NOMA-HO) theoretically to increase the handover user rate while guaranteeing the rate of paired user in the target cell. From this condition, we define a new measurement parameter called Interference to Background Interference-plus-Noise Ratio (IBINR) and design a practical NOMA-HO procedure for the measurement of IBINRs and the handover decision. Results show that the proposed NOMA-HO scheme significantly improves cell-edge user throughput compared with the classical OMA-based handover schemes.

Recently, self-healing has been actively investigated for mitigating an unforeseen network failure. In particular, to enable self-healing operations in indoor wireless communications systems, an autonomous mechanism to resolve unforeseen network failure problems should be considered. Therefore, we here address the issue of autonomous self-healing, in which continuous connectivity can be provided to users by resolving unexpected network failures. To overcome this problem, we propose a low-complexity resource allocation algorithm based on an optimization approach with reduced message passing. In the proposed algorithm, normal base stations perform subchannel and power allocations with a minimum amount of information sharing ( $N_{H}(M-1)$ ) to provide reliable service to users in faulty cells autonomously. We also show that the proposed algorithm converges to a unique fixed point in the low-interference region by using a contraction mapping technique. Through simulation results, we demonstrate that the proposed algorithm achieves good performances with respect to the average cell capacity, user fairness, and outage probability while reducing the message passing overhead and computational complexity.

In opportunistic device-to-device (D2D) networks, the epidemic routing protocol can be used to optimize the message delivery ratio. However, it has the disadvantage that it causes excessive coverage overlaps and wastes energy in message transmissions because devices are more likely to receive duplicates from neighbors. We therefore propose an efficient data dissemination algorithm that can reduce undesired transmission overlap with little performance degradation in the message delivery ratio. The proposed algorithm allows devices further away than the k-th furthest distance from the source device to forward a message to their neighbors. These relay devices are determined by analysis based on a binomial point process (BPP). Using a set of intensive simulations, we present the resulting network performances with respect to the total number of received messages, the forwarding efficiency and the actual number of relays. In particular, we find the optimal number of relays to achieve almost the same message delivery ratio as the epidemic routing protocol for a given network deployment. Furthermore, the proposed algorithm can achieve almost the same message delivery ratio as the epidemic routing protocol while improving the forwarding efficiency by over 103% when k&ge, 10.

To improve the efficiency of carrier sense multiple access (CSMA)-based medium access control (MAC) protocols, CSMA with collision resolution (CSMA/CR) has been proposed. In the CSMA/CR, a transmitting station can detect a collision by employing additional sensing after the start of a data transmission and then resolve the next collision that might occur by broadcasting a jam signal during a collision detection (CD) period. In this paper, we analyze the delay of a CSMA/CR based on a generic p-persistent CSMA model and obtain the minimum achievable delay of the CSMA/CR by finding the optimal length of the CD period according to the number of contending stations. Through this delay analysis, we also investigate the throughput-delay characteristics of the CSMA/CR protocol according to various parameters. Analysis and simulation results show that the CSMA/CR has a considerably lower delay and its throughput-delay characteristic is significantly improved than the conventional CSMA/CA and wireless CSMA/CD protocols.

To manage cell outage problem in indoor wireless communication systems, we should resolve the problem of abrupt network failure quickly. In this paper, we propose a near-optimal cell outage management (NoCOM) scheme to support seamless services to users. In consideration of system throughput, user fairness, and the guarantee of QoS simultaneously, the NoCOM scheme finds the solution of subchannel and power allocations using a non-convex optimization technique and allocates radio resources to users iteratively. Through intensive simulations, we verify the outstanding performances of the proposed NoCOM scheme with respect to the average cell capacity, user fairness, and computational complexity.

In order to support reliable medium access control (MAC) layer multicast services in Broadband Wireless Access (BWA) networks, we here propose spreading codes (Cumulative ACK (CA) code and ARQ Feedback Request (AFR) code) based reliable multicast feedback scheme. The status indications based on the automatic repeat request (ARQ) mechanism are needed in some multicast services. In accordance with various wireless channel environments, we demonstrate the performance excellency of our proposed scheme with respect to required uplink resources compared with the original feedback scheme based on unicast ARQ feedback messages. In addition, we analyze packet error rate (PER) against the various wireless channel environments.

To resolve coverage hole problems caused by indoor base-station (IBS) faults, we propose a new frame structure and a practical algorithm based on an optimization technique. Our main contributions can be described as follows: 1) a frame structure with healing subchannels for overcoming abnormal IBS faults and 2) an efficient suboptimal resource allocation algorithm with opportunistic IBS selection to reduce the complexity of the optimization problem. Through intensive simulations, we evaluate the performance of our proposed algorithm with respect to average cell capacity, user fairness, and outage probability and compare it with the performance of conventional algorithms.

To provide network reliability in indoor wireless communication systems, we should resolve the problem of unexpected network failure rapidly. In this paper, we propose an adaptive resource management (ARM) scheme to support seamless connectivity to users. In consideration of system throughput and user fairness simultaneously, the ARM scheme adaptively determines the set of healing channels, and performs scheduling and power allocation iteratively based on a constrained non-convex optimization technique. Through intensive simulations, we demonstrate the superior performance results of the proposed ARM scheme in terms of the average cell capacity and user fairness.

In this paper, we propose cost-effective device-to-device (D2D) advertisement scenario with properties of proximity and timeliness through the convergence of D2D communications and social commerce service. We numerically analyze advertisement dissemination effect of the proposed scenario according to the number of sectors, and demonstrate the performance of the normalized D2D coverage, the average number of D2D users, and the average D2D coverage per user via intensive simulations. We verify the accuracy of the results for our numerical analysis compared with the simulation results.

Wireless multi-hop sensor networks consist of a wired backbone network, gateway nodes and wireless sensor nodes. To resolve the bottleneck problems in the neighborhood of the gateway node, we propose an effective carrier sensing multiple access algorithm based on a distributed bottleneck alleviation (DiBA) scheme. Using the concept of a relay token, we utilize a linear distribution as the probability density functions for the selection of backoff counter value, rather than using a uniform distribution. From simulation results, we show that system throughput and user fairness are remarkably improved in random topology compared with conventional schemes.

For the purpose of automating network management, self-organizing network (SON) technology is currently being investigated. There are three important issues in SON: self-configuration, self-optimization, and self-healing. This paper focuses on self-healing, in order to resolve the problem of unexpected network faults and improve network throughput simultaneously. To deal with this problem, we propose a healing channel selection and a cooperative beamforming-based iterative resource allocation algorithms. We utilize the cooperative beamforming in the healing channel based on phase pre-adjustment, and this cooperative beamforming can be performed without power cooperation between distributed nodes. Moreover, we derive the sub-optimality and convergence of the proposed algorithm in weak interference condition by using contraction mapping. Finally, simulation results demonstrate that the proposed algorithm improves average cell capacity and user fairness while repairing network failure effectively.

We propose a distributed medium access control (MAC) protocol called carrier sense multiple access with collision resolution (CSMA/CR) by extending the wireless CSMA/CD protocol. The proposed CSMA/CR protocol identifies a collision by additional sensing after data transmission starts. If one station detects a collision, it transmits a jam signal to stop the other stations' transmissions and then retransmits the data without backoff, which resolves the next collision that may occur. The throughput analysis shows that the proposed CSMA/CR protocol outperforms other conventional CSMA-based MAC protocols.

In this letter, we present the outage performance of decode-and-forward based cooperative multicast systems with the best relay selection (CM-RS) over Rayleigh fading channels. Especially, we provide an exact and closed-form expression for the outage probability of CM-RS and its diversity order. Through numerical results, the analysis is verified, and the outage performance of CM-RS is compared with that of a single-hop multicast system.

In wireless device-to-device (D2D) networks, where users can directly communicate each other, the messages should be delivered reliably with low cost and low power consumption. We herein consider a D2D based opportunistic data dissemination algorithm for the advertisement messages in social commerce services when the user distribution is not identical over area. In order to provide reliable connectivity to users and to deliver messages efficiently, target areas where user density is high are partitioned into several clusters, i.e., target groups, using hierarchical and non-hierarchical clustering algorithms. In addition, near-optimal routing paths and a user selection strategy for relaying the messages to their neighboring users are established for effective data dissemination. Through intensive MATLAB simulations, we evaluate the performance of various clustering algorithms in view of the coverage probability and transmission efficiency. We also investigate the optimal way to achieve the maximum coverage probability for given network topology.

Fourth-generation (4G) cellular communication systems must provide different quality-of-service (QoS) to users according to the service type, membership class, and usage case of each user. For the purpose of user's QoS differentiation, we propose an enhanced uplink random access scheme for bandwidth request. More specifically, we divide a bandwidth request channel into two parts: One is a primary region and the other is a secondary region. Then, each region has a unique minimum access class that allows only specific users to perform random access through that region. By doing so, we can reduce collision probability and increase the success probability of bandwidth request. From the perspective of standardization, we presented this scheme in the IEEE 802.16m Session #66 held on March 2010. As a result, the concept of the proposed scheme and required messages were defined in the 802.16m standard.

We herein analyze the capacity of voice over IP (VoIP) and propose a new method for finding the minimum detection and false-alarm probabilities to ensure the quality-of-service (QoS) requirement of VoIP users in a cognitive radio system. We propose a model for the system supporting the VoIP service as a 2-D discrete-time Markov chain (DTMC). The VoIP traffic and wireless channels in the cognitive radio system are described as a Markov-modulated Poisson process (MMPP) model and a Markov channel model, respectively. In addition, we introduce a simple spectrum-sensing model based on energy detection. By means of the DTMC approach, we demonstrate various analytical and simulation results under the constraint of imperfect spectrum sensing, such as the packet dropping probability, average throughput, and VoIP capacity.

A new algorithm is proposed for wireless voice over IP (VoIP) codecs that allocates radio resources in consideration of the variations in packet size and packet-generation period. This algorithm allows resources to be utilized more efficiently in mobile-WiMAX systems. The outstanding performance of our proposed algorithm with respect to average throughput and average packet dropping probability is demonstrated through intensive numerical analysis and simulations. VoIP capacity is compared with respect to the application of different VoIP codecs (G.729B, adaptive multirate (AMR)) and VoIP header compression schemes (payload header suppression (PHS) and robust header compression (ROHC)).

In social commerce services, if the purchase condition for the minimum number of ordered users is satisfied, customers can get a great deal of discount for the corresponding products. The convergence of D2D communications and the social commerce services may create a synergistic effect because the D2D user can spontaneously relay the advertisement messages to their neighbors. Accordingly, we here propose Wi-Fi Direct based voluntary advertisement dissemination scenario and algorithm (VADA) for social commerce services. By using our proposed VADA algorithm, the small business owners are able to transmit advertisement messages to many local D2D users cost-effectively. Through intensive simulations, we evaluate the performance excellency of our proposed algorithm with respect to total number of successfully received users, average number of relay users, and transmission efficiency compared with conventional algorithms and optimal algorithm based on exhaustive search.

We here propose geographical proximity based target-group formation algorithm to efficiently distribute advertisement messages of social-commerce services with device-to-device (D2D) communications. The convergence of D2D communications and the social-commerce services is able to invoke synergistic effects because the D2D users can voluntarily relay the messages to their neighbor users to obtain a great deal of discounts for the corresponding products. By using both the angular distances among target-areas and the physical distances between the D2D access point and the target-areas, we make target-groups for efficient D2D advertisement dissemination. Through intensive simulations, we evaluate the performances of our algorithm with respect to the total number of successfully received users, the average number of relay users, and transmission efficiency compared with the conventional algorithm with cell sectorization based target-group formation.

Internet of Things is a system where people and things with unique identifiers are connected and able to interact with each other over a network without external intervention. To connect things anytime, network reliability is one of the main requirements for the Internet of Things, in which continuous service can be provided to nodes even in the event of a sudden network failure. To serve consistent quality of service for nodes in any case, we propose a radio resource sharing (R2S) method which consists of scheduling and power allocation using an optimization method. By evaluating performances with respect to average cell capacity and outage probability in simulations, we show that the R2S algorithm can solve a coverage hole problem efficiently in Internet of Things.

In this letter, we analyze a voice over IP (VoIP) capacity in a cognitive radio system. We formulate the system as a two-dimensional discrete time Markov chain (DTMC). The VoIP traffic and wireless channel in the cognitive radio system are described as a Markov modulated Poisson process (MMPP) model and a Markov channel model, respectively. We demonstrate various numerical and simulation results, such as packet dropping probability and VoIP capacity.

An efficient dedicated retransmission scheme for reliable multicast services is proposed to improve performance with respect to throughput and resource utilization in OFDMA systems. Through analysis and simulation, we show that the downlink cell throughput of the proposed scheme is much greater than that of the conventional scheme. In addition, the proposed scheme saves a much greater amount of downlink resources than the conventional scheme.

An efficient uplink scheduling algorithm that can be used in IEEE 802.16d/e system for VoIP services is proposed to support as many voice users as possible. Our proposed algorithm requires no modification of the framework of IEEE 802.16d/e system. In this letter, we analyze the system throughput and the access delay of our proposed algorithm, and show, via OPNET simulation, that the throughput of our algorithm is superior to that of conventional algorithms in IEEE 802.16d/e system.

This paper proposes a distributed medium access control (MAC) protocol called carrier sensing multiple access with collision resolution (CSMA/CR) based on a fully connected single hop network considering next-generation wireless LAN environments with a high density of stations. The proposed CSMA/CR follows the typical CSMA/CA protocol and identifies a collision by additional sensing after data transmission starts without a ready-to-send (RTS) and clear-to-send (CTS) exchange. If one station detects a collision, it transmits a jam signal to stop the other stations' transmissions and has access priority for the retransmission without backoff, which prevents the next collision that may occur. The simulation results show that the proposed CSMA/CR has a low overhead due to the detection of a collision without an RTS/CTS exchange and always achieves the best throughput regardless of the number of access stations by using the collision resolution scheme.

The medium access control protocol plays an important role to decrease access collisions in dense wireless sensor networks where multiple sensors in the same vicinity attempt to transmit a packet simultaneously. In this article, we propose a distributed medium access control protocol that uses successive multiple collision detection phases for dense wireless sensor network environments by enhancing the typical carrier sense multiple access with collision resolution protocol that uses only a single collision detection phase. In the proposed medium access control protocol, colliding stations are filtered in each collision detection phase and only surviving stations compete again in the next collision detection phase. Therefore, the collision detection probability becomes higher as the collision detection phases proceed. Utilizing the successive multiple collision detection phases, we analyze the throughput numerically and find optimal operating parameters—such as the number of collision detection phases and the number of collision detection slots per phase—that maximize the throughput. Analysis and simulation results show that the proposed medium access control protocol using the successive collision detection technique significantly outperforms the conventional carrier sense multiple access with collision resolution protocol.

The main objectives of a self-organizing network (SON) technology are autonomous network deployment, network performance optimization and real-time adaptation to environmental changes. There are three functionalities in SON, such as self-configuration, self-optimization and self-healing. In this paper, we focus on the self-healing issue with respect to abrupt network faults. In order to solve this problem, we design a healing channel (HC) and propose a collaborative resource allocation (CRA) algorithm based on a modified iterative water-filling (MIWF) algorithm. Through intensive simulations, we show that CRA efficiently supports users in disabled femtocell base stations (FBS) with the little degradation in system capacity.

In this paper, we investigate how femtocells can make conventional cellular networks greener. We present an energy consumption modeling framework to evaluate total energy consumption in a cellular network with femtocells. Using the proposed framework, we investigate the energy consumption and performance of the cellular network with femtocells. We consider various network environments, including indoor propagation environment, user distribution near the femtocells, and a femtocell access policy, which have effect on the performance of the cellular network. Simulation results in the various environments show that femtocell is, in general, a greener technology that reduces the total energy consumption in a cellular network. We obtain system performance and energy consumption for three different scenarios: different femtocell penetration rate, different open access rate, and different cell coverage. We have finally provided a guideline that helps system operators deploy and manage a cellular network with femtocells in more greener way.

We review previous resource allocation algorithms for voice over IP (VoIP) services in Mobile WiMAX based on the IEEE 802.16e-2005 standard, and propose a new algorithm that allocates radio resources in the consideration of packet-size and packet-generation period variations of wireless VoIP codecs; hence, we can improve the efficiency of resource utilization in Mobile WiMAX. Through simulations, we demonstrate the outstanding performance of our proposed algorithm with respect to packet dropping probability and VoIP capacity for various VoIP codecs, such as adaptive multi-rate wideband (AMR-WB) codec, enhanced variable rate codec (EVRC), and G.729B codec. In addition, we compare VoIP capacity of each algorithm according to the application of VoIP header compression schemes, such as payload header suppression (PHS) and robust header compression (ROHC).

Cognitive radio is a promising and challengeable technology for maximizing radio resource utilization in a future wireless communication system. In this paper, we describe a problem caused by primary users of a cognitive radio system and propose underlay-overlay switching and underlay-relay methods to solve the problem in the cognitive radio system. Through performance analysis and simulation results, we show that our proposed method has outstanding performance in view of outage probability compared with a conventional method.

There are several scheduling algorithms for Voice over IP (VoIP) services in IEEE 802.16e systems, such as unsolicited grant service (UGS), real-time polling service (rtPS), and extended real-time polling service (ertPS). The ertPS is a new scheduling algorithm for VoIP services with variable data rates and silence suppression, and this algorithm is recently proposed and accepted in the IEEE 802.16e standard. In this paper, we analyze and discuss the performance of the scheduling algorithms recommended in IEEE 802.16e systems including the ertPS algorithm. Through the analysis of resource utilization efficiency and VoIP capacity, we show that the UGS and rtPS algorithms have some problems, which are the waste of uplink resources in the UGS algorithm, and additional access delay and MAC overhead due to bandwidth request process in the rtPS algorithm, to support the VoIP services. In addition, for analysis of VoIP capacity, we utilize OPNET simulation, and show that the ertPS algorithm can support more 21% and 35% voice users compared with the UGS and rtPS algorithms, respectively.

Handover has been the most important thing for supporting the mobility and has been researched in various wireless communication systems. Handover schemes in systems with multiple antennas, the technology highlighted for 4G wireless communications to increase the capacity, have frequently been made and considered to be very important. However, handover decision method using the number of multiple antennas has not been researched. In this paper, we propose a new handover decision method using the number of detected antennas and list requirements and distinctive features for this scheme. Simulation results show that our proposed scheme and conventional scheme trigger handovers at the almost same point. Since our proposed scheme is simpler than the conventional scheme, it has the better performance in view of feedback quantity.

In OFDMA systems, multicast data packets are always transmitted with control messages that identify mutli- cast data packets. In case of reliable multicast services, if a multicast data packet is destroyed, the BS must retransmit this packet with a control message. In this case, a lot of downlink resources are wasted, because the control message uses a very robust modulation and coding scheme (MCS) level. Therefore, to solve this problem, we propose a new retransmission scheme for reliable multicast services in OFDMA systems. Using our proposed scheme, we can save a lot of downlink resources because of the reduction of control messages for multicast data packet retransmissions. In this paper, we analyze the downlink cell throughput and the required resources of our proposed scheme for reliable multicast services, and show, via OPNET simulation, that the throughput of our proposed scheme is superior to that of the original scheme in OFDMA systems.

There are several scheduling algorithms for Voice over IP (VoIP) services in IEEE 802.16 systems, such as unsolicited grant service (UGS), real-time polling service (rtPS), UGS with Activity Detection (UGS-AD), and Lee's algorithm using Grant-Me bit of the generic MAC header. However, these algorithms have some problems of a waste of uplink resources, additional access delay, and MAC overhead for supporting VoIP services with variable data rates and silence suppression. To solve these problems, we propose a novel uplink scheduling algorithm (Extended-rtPS) for the VoIP services in IEEE 802.16 systems. Through the performance analysis and simulation results of resource utilization, VoIP capacity, total throughput, and packet transmission delay, we show that our proposed algorithm can solve the problems of the conventional algorithms, and has the best performance among these algorithms. In addition, with simulation results of packet transmission delay, we prove that our proposed algorithm can support more 74%, 24%, and 9% voice users compared with the UGS, rtPS, and UGS-AD (Lee's) algorithms, respectively.

IEEE 802.16 OFDMA (orthogonal frequency division multiple access) system does not support reliable medium access control (MAC) layer multicast services. However, reliable multicast services must be supported in this system, because the status indications based on the ARQ mechanism of the subscriber stations (SSs) are required in some multicast services. Therefore, we propose a new scheme to support reliable MAC layer multicast services using cumulative ACK (CA) code and ARQ feedback request (AFR) code. In this paper, we analyze packet error rate (PER), and required uplink resources in case of our proposed scheme compared with the original scheme using unicast ARQ feedback messages against various wireless channel environments. Through the performance analysis, we can show that our scheme can save a lot of uplink resources compared with the original scheme. Using our proposed scheme, we can support reliable multicast services without a waste of a lot of uplink resources.

An efficient uplink scheduling algorithm for VoIP services is very important for supporting a large number of users with various levels of quality of service (QoS). However, conventional uplink scheduling algorithms for VoIP services in IEEE 802.16 broadband wireless access (BWA) systems cause some problems, such as waste of uplink resources, MAC overhead and unnecessary access delay. We propose an efficient uplink scheduling algorithm for VoIP services in these systems. The base station (BS) assigns uplink resources to the subscriber stations (SS) by considering voice state transitions of the SS. These voice state transitions can be informed to the BS by utilizing one reserved bit in the generic MAC header of IEEE 802.16 BWA systems. We analyze the system throughputs and the access delays of conventional algorithms and of our proposed algorithm, and show, via OPNET simulation, that, in terms of throughput, our proposed algorithm is superior to conventional algorithms in these systems.