A great amount of research has devoted to cognitive radio (CR) in recent years in order to improve spectrum efficiency. In decentralized CR networks, it is not realistic for CR users to sense entire spectrum in practice due to hardware limitations. Consequently, the partially observable Markov decision process (POMDP) can be utilized to provide CR users with sufficient information in partially observable environments. Existing POMDP-based protocols adopt channel aggregation techniques in order to improve spectrum opportunities and system performance. However, the required time for channel sensing is neglected which can result in large sensing time overhead and spectrum opportunity loss in realistic environments. In this paper, based on partially observable channel state with the consideration of sensing overhead, the stochastic multiple channel sensing (SMCS) protocol is proposed to conduct optimal channel selection for maximizing the aggregated throughput of CR users. By adopting the proposed SMCS protocol, CR users can highly accommodate themselves to rapidly varying environment based on the dynamically adjustable channel sensing strategy. Moreover, the channel sensing problem is further extended to imperfect sensing scenario, which can severely degrade system throughput due to packet collision between primary users (PUs) and CR users. Consequently, in addition to channel selection, it is required for CR users to determine the sensing time length in order to address the collision problem. The two-phase SMCS (TSMCS) protocol is proposed to maximize the aggregated throughput of CR users while still fulfilling PUs' quality-of-service (QoS) requirements. Numerical results show that the proposed SMCS and TSMCS protocols can effectively maximize the aggregated throughput for decentralized CR networks.

The IEEE 1609.4 standard has been proposed to provide multi-channel operations in wireless access for vehicular environments (WAVE), where all channels are periodically synchronized into control and service intervals. Communication device in each vehicle will stay at the control channel for negotiation and contention during the control interval, and thereafter switch to one of the service channels for data transmission in the service interval. In this paper, based on the concept of cognitive radio (CR), the vehicles are categorized into primary providers (PPs) that intend to transmit safety-related messages and secondary providers (SPs) with non-safety information to be delivered. The prioritized optimal channel allocation (POCA) approaches are proposed to improve channel utilization of IEEE 1609.4 standard for multi-channel vehicular networks. Prioritized channel access is analyzed in the POCA schemes in order to increase the transmission opportunity of PPs. Moreover, depending on whether the CR network is distributed or centralized, the optimal channel-hopping sequence and optimal channel allocation is assigned for SPs based on dynamic programming and linear programming technique, respectively. These schemes are designed to consider optimal load balance between both channel availability and channel utilization within the throughput constraints of PPs. With the adoption of proposed POCA schemes, simulation results show that maximum throughput of SPs can be achieved with guaranteed quality-of-service requirement for PPs.

In this paper, a novel multi-carrier and multi-antenna for a multi-tier cellular network is presented for Long-Term Evolution Advanced (LTE-A) heterogeneous networks (Het-Nets) with carrier aggregation enhancement. First, we investigate a system model of frequency domain load balancing-based inter-cell interference coordination (ICIC) to minimize the total interference, while satisfying the required quality of service constraints. To achieve the desired system performance, a heuristic solution by joint spectrum and antenna allocation is then proposed, which operates at the base station side to enable efficient interference management. The optimal deployment for component carriers and antennas at the serving base station is updated for specific time periods while passively receiving the system information transmitted from neighboring base stations. Furthermore, system level simulations are carried out to demonstrate how the throughput can be greatly improved by our solution, as compared to conventional methods such as the randomized and static ICIC approaches.

In recent years, femtocell plays an important role in wireless networks not only for its spectrum reuse but also for its low power consumption. However, there exist several critical issues that need to be investigated, especially for the interferences between the macrocell BSs (mBSs) and femtocell BSs (fBSs). The level of interference mainly depends on the access strategies of fBSs. Two major access policies are considered in femtocell network, including the closed access mode and open access mode. The closed access mode only permits authorized subscribers to utilize the fBS; while all users are allowed to connect to the fBS by adopting the open access mode. Closed access will intuitively be advantageous to the femtocell subscribers, however, interference from the fBS to mBS's users can become severe in the closed access mode than in open access mode. System performance of the entire heterogeneous network (HetNet) can be improved if fBS is operated in the open access mode. In order to relax the inflexible access strategies, hybrid access policy is considered in this paper which allows nonsubscribers to possess limited connections to the fBS. Two cell selection games for distinct scenarios are theoretically modeled to formulate the behaviors of nonsubscribers, and the existences of pure strategy Nash equilibria are also proven under feasible utility functions. From the perspectives of subscribers, HetNet system, and operator, numerical results suggest the adoption of hybrid access mode to provide higher flexibility for the performance enhancement.

Due to the lack of a centralized coordinator for wireless resource allocation, the design of medium access control (MAC) protocols is considered crucial for throughput enhancement in the wireless ad hoc networks. The receiver blocking problem, which has not been studied in most of the MAC protocol design, can lead to severe degradation on the throughput performance. In this paper, the multiple receiver transmission (MRT) and the fast NAV truncation (FNT) mechanisms are proposed to alleviate the receiver blocking problem without the adoption of additional control channels. The adaptive receiver transmission (ART) scheme is proposed to further enhance the throughput performance with dynamic adjustment of the selected receivers. Analytical model is also derived to validate the effectiveness of the proposed ART protocol. Simulations are performed to evaluate and compare the proposed three protocols with existing MAC schemes. It can be observed that the proposed ART protocol outperforms the other schemes by both alleviating the receiver blocking problem and enhancing the throughput performance for the wireless multihop ad hoc networks.

The design of wireless local area networks (WLANs) with enhanced throughput performance have attracted significant amounts of attention in recent years. Based on the IEEE 802.11n standard, frame aggregation is considered one of the major factors to improve the system performance of WLANs from the medium access control (MAC) perspective. In order to fulfill the requirements of high throughput performance, feasible design of automatic repeat request (ARQ) mechanisms becomes important for providing reliable data transmission. In this paper, two MAC-defined ARQ protocols are proposed to consider the effect from frame aggregation for the enhancement of network throughput. An aggregated selective repeat ARQ (ASR-ARQ) scheme is proposed which incorporates the selective repeat ARQ scheme with the consideration of frame aggregation. On the other hand, for worse channel quality, the aggregated hybrid ARQ (AH-ARQ) mechanism is proposed to further enhance the throughput performance by adopting the Reed-Solomon (RS) block code as forward error correction (FEC) scheme. Novel analytical models for both the ASR-ARQ and AH-ARQ protocols are established with the consideration of interfering wireless stations. Simulations are conducted to validate and compare the proposed ARQ mechanisms based on the service time distribution and system throughput. Numerical evaluations show that the proposed AH-ARQ protocol can outperform the other schemes under worse channel condition; while the ASR-ARQ scheme is superior to the other mechanisms under better channel condition.

This paper studies the joint resource allocation problem of antenna, subchannel, transmission power, and phase duration for the relay-enhanced bidirectional multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) networks. The goal of resource allocation is to minimize the transmission energy in the networks with multiple relay stations (RSs) under the data rate constraints of user equipment (UE). The challenges of this resource allocation problem arise from the complication of multiple-phase assignments within a subchannel since the RS can provide an additional transmission path from the base station to the UEs. The green resource allocation (GRA) schemes with reduced computational complexity are proposed in this paper to develop the joint resource allocation algorithm for the UEs with the consideration of direct and two-hop communications. Both the separate downlink (DL) and uplink (UL) and mixed DL and UL relaying assignments are adopted to obtain the solutions for proposed GRA schemes. Simulation results show that the GRA schemes can provide comparably better energy conservation with the consideration of quality-of-service (QoS) support.

Relay nodes (RN) have been suggested to be placed near the edge of a cellular network in order to improve link qualities of cell edge user equipments (UEs). Compared to conventional homogeneous networks, system performance for the relay-enhanced network can be significantly degraded if handovers between the RNs and evolved node base-station (eNB) are not correctly and immediately determined by the eNB. Therefore, the UEs may suffer from worse channel conditions which can result in increased energy consumption of entire network. In existing long term evolution advanced (LTE-A) standard, fixed reporting time period from the UEs to eNB is considered inflexible which can be too long to provide immediate handover decisions. Hence, prediction-based handover (PH) scheme is proposed to allow eNBs to make potential handover decisions within the UE's reporting period. The channel qualities for both the direct and relay-enhanced links during this period are predicted based on the partially observable Markov decision process. Moreover, three objectives are designed for the proposed PH scheme including maximizing received signal to interference plus noise ratios of UEs, and minimizing system energy consumption without and with considerations of energy outage of relay nodes. Numerical results show that the proposed PH schemes outperform conventional handover scheme from the perspectives of both system energy consumption and outage probability.

In recent years, channel-hopping-based medium access control protocols have been proposed to improve the capacity in a decentralized multichannel cognitive radio (CR) network without using extra control channels. Each CR user has to stochastically follow a default channel-hopping sequence in order to locate a channel and conduct its frame transmission. In this paper, theoretical analysis is conducted on the probability of channel availability and the average frame delay for primary users (PUs) by considering the impact caused by imperfect sensing of CR users and imperfect synchronization between the primary and CR networks. According to the proposed analytical model with realistic considerations, an optimal channel-hopping sequence (OCS) approach is designed for the CR users based on a dynamic programming technique. It is designed by exploiting the optimal load balance between channel availability and channel utilization within the delay constraints of PUs. By adopting the OCS approach, maximum aggregate throughput of CR users can be achieved while considering PU's quality-of-service (QoS) requirements. Moreover, in addition to the paired CR networks, the logical partition problem that occurs in generalized CR networks will also be addressed. This problem can severely degrade the aggregate throughput due to the decreased probability of connectivity between CR users, especially in a CR network with heavy traffic. Therefore, both wake-up successive contention (WSC) and wake-up counter-reset successive contention (WCSC) algorithms are proposed to increase the number of negotiations by both exploring the blind spot of imperfect sensing and amending the contention mechanisms between CR users. Compared to conventional channel-hopping sequences, numerical results illustrate that the proposed approaches can effectively maximize aggregate throughput for CR users under the QoS requirements of PUs.

In recent years, the femtocell plays an important role in wireless networks not only for its spectrum reuse but also for its low power consumption. However, there exists several critical issues that need to be investigated, especially for the interferences between the macrocells and the femtocells. Two major access policies are considered in the femtocell network, including the closed access mode and the open access mode. The closed access mode only permits authorized subscribers to utilize the femtocells; while all users are allowed to connect to the femtocell by adopting the open access mode. The closed access will intuitively be advantageous to the femtocell subscribers, however, it has shown that interference from the macrocell to the femtocell can be mitigated by using the open access mode. In this paper, a cell selection game is theoretically modeled to formulate the behaviors of the nonsubscribers who have the opportunities to connect to the femtocells. The distinct connection manners of nonsubscribers to access the macrocells and the femtocells are modeled as the primary users and cognitive users, respectively. Considering the channel capacity of femtocell as the utility function of this game, the existence of a pure strategy Nash equilibrium is illustrated to provide the win-win situation between the subscribers and nonsubscribers.

The IEEE 1609.4 standard has been proposed to provide multi-channel operations in wireless access for vehicular environments (WAVE), where all the channels are periodically synchronized into control and service intervals. In this paper, based on the concept of cognitive radio, the vehicles are categorized into primary providers (PPs) that intend to transmit safety-related messages and secondary providers (SPs) with non-safety information to be delivered. The cognitive radio-enabled optimal channel-hopping sequence (CROCS) approach is proposed to improve the channel utilization of IEEE 1609.4 standard for multi-channel vehicular networks. Prioritized channel access is analyzed in the CROCS scheme in order to increase the transmission opportunity of the PPs. Moreover, optimal channel hopping sequence is assigned for the SPs based on the dynamic programming technique. It is designed to consider the optimal load balance between both the channel availability and channel utilization within the throughput constraints of PPs. With the adoption of proposed CROCS approach, simulation results show that maximum throughput of SPs can be achieved with guaranteed quality-of-service requirement for the PPs.

This paper studies the subchannel and power allocation problem in the relay-enhanced downlink orthogonal frequency-division multiple access (OFDMA) systems. The challenges of this resource allocation problem arise from the complication of two-phase assignments within a subchannel since the relay station (RS) can provide an additional two-hop signal path from the base station to the user equipments (UEs). Existing research work does not fully consider all the influential factors to achieve feasible resource allocation for the relay-based networks. In this paper, the QoS-based resource allocation (QRA) schemes are proposed to design the subchannel and power allocations for the UEs with the consideration of direct and two-hop communications. Both the selective and fixed phase assignments for the UEs are addressed to obtain the suboptimal solution for the proposed QRA schemes. Different weights are designed for the UEs to exploit the fairness and maximization of system throughput for the relay-enhanced networks. Simulation results show that the proposed QRA schemes with selective phase assignment can provide comparably higher network throughput with QoS consideration.

A great amount of research has devoted to cognitive radio (CR) in recent years in order to improve spectrum efficiency. In decentralized CR networks, the CR users are expected to be capable of dynamically and opportunistically accessing unused spectrums in primary networks. However, since the spectrum of primary networks is comparatively wide, it is not realistic for the CR users to sense the entire spectrum in practice. Consequently, the partially observable Markov decision process (POMDP) is utilized to provide the CR users with sufficient information in partially observable environments. Moreover, existing POMDP-based protocols exploit techniques of channel aggregation in order to improve the spectrum opportunities and system performance. However, the required time for channel sensing is neglected, which is considered inevitable to result in large sensing overhead and spectrum opportunity loss in realistic environments with increased number of channels. Therefore, in this paper, the stochastic multiple channel sensing (SMCS) protocol is proposed to conduct optimal decision-making based on partially observable channel state information under the consideration of sensing overhead. By adopting the proposed SMCS protocol, the CR user can highly accommodate itself to the rapidly varying environment since the optimal decision-making on multiple channel sensing is dynamically adjusted. Furthermore, the steady-state based SMCS (SMCS-S) scheme with simplified decision-making process is proposed in consideration of implementation complexity. Numerical results illustrate that the proposed SMCS protocol can effectively maximize the aggregated throughput for decentralized CR networks.

In the conventional IEEE 802.11 medium access control protocol, the distributed coordination function is designed for the wireless stations (WSs) to perform channel contention within the wireless local area networks (WLANs). Research work has been conducted to modify the random backoff mechanism in order to alleviate the packet collision problem while the WSs are contending for channel access. However, most of the existing work can only provide limited throughput enhancement under specific number of WSs within the network. In this paper, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed to both improve packet collision and reduce the backoff delays from the random access scheme. With its adaptable reservation period, the contention-based channel access can be adaptively transformed into a reservation-based system if there are pending packets required to be transmitted between the WSs and the access point. Moreover, in order to support quality-of-service requirements, the enhanced-ARCR (E-ARCR) protocol is further proposed to provide adaptation for multiple prioritized traffic in the WLAN. Analytical models are derived for both proposed schemes to evaluate their throughput performance. It can be observed from both analytical and simulation results that the proposed protocols outperform existing schemes with enhanced channel utilization and network throughput.

In recent years, channel-hopping based medium access control (MAC) protocols are proposed to improve the capacity in a decentralized multi-channel cognitive radio (CR) networks without extra usage of a control channel. Each CR user has to stochastically follow a default channel-hopping sequence in order to sense a channel and to conduct its frame transmission. In this paper, based on the channel-hopping protocol, an analysis is conducted on both the probability of channel availability and the average frame delay for the primary queueing networks. The analytical model is proposed by considering the impact caused by imperfect sensing of the CR users and the imperfect synchronization between the primary and CR networks. According to the proposed model with more realistic considerations, an optimal channel-hopping sequence (OCS) approach is designed for the CR users based on dynamic programming technique. It is designed by exploiting the optimal load balance between both the channel availability and channel utilization within the delay constraints of primary users (PUs). By adopting the OCS approach, the maximum aggregate throughput of CR users and the quality of service (QoS) requirement of PUs can both be achieved. Numerical results illustrate that the proposed OCS scheme can effectively maximize the aggregate throughput compared to conventional channel-hopping sequences, and as well guarantee the QoS requirement of the PUs.

The next generation wireless local area networks (WLANs) with enhanced throughput performance have attracted significant amounts of attention in recent years. Based on the IEEE 802.11n standard, frame aggregation is considered one of the major factors to improve the system performance of WLANs from the medium access control perspective. In order to fulfill the requirements of the high throughput performance, feasible design of automatic repeat request (ARQ) mechanisms is considered important for providing reliable data transmission. In this paper, an aggregated selective repeat ARQ (ASR-ARQ) algorithm is proposed, which incorporate the conventional selective repeat ARQ scheme with the consideration of frame aggregation. A novel analytical model based on the signal flow graph is established in order to realize the behaviors of ASR-ARQ algorithm. Simulations are also conducted to validate the effectiveness of proposed ASR-ARQ mechanism.

In conventional IEEE 802.11 medium access control protocol, the distributed coordination function is designed for the wireless stations (WSs) to perform channel contention within the wireless local area networks (WLANs). Packet collision is considered one of the major issues within this type of contention-based scheme, which can severely degrade the network performance for the WLANs. Research work has been conducted to modify the random backoff mechanism in order to alleviate the packet collision problem while the WSs are contending for channel access. However, most of the existing work can only provide limited throughput enhancement under specific number of WSs within the network. In this paper, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed to improve the packet collision from the random access schemes. With its adaptable reservation period, the contention-based channel access can be adaptively transformed into a reservation-based system while there are pending packets required to be transmitted from the WSs. Furthermore, according to the designed reservation table within the access point, the fairness for channel access between the WSs is addressed within the ARCR protocol. Numerical results indicate that the proposed ARCR scheme can outperform the existing schemes with enhanced channel utilization and network throughput.