This paper proposes an adaptive artificial-noise (AN) strategy to enhance the secrecy rate in reconfigurable intelligent surfaces (RIS)-assisted wireless networks with joint active/passive beamforming. Specifically, the AN jams not only the eavesdropper but also the legitimate user. If the AN significantly interferes with the legitimate user, the base station (BS) should adaptively choose not to transmit the AN. Additionally, we compare two RIS-beamforming schemes: the RIS-D and RIS-AN schemes. The RIS-D scheme adjusts the phase shifts of RIS elements to enhance the data signal of the legitimate user. The RIS-AN scheme aims to reflect and increase the interference to the eavesdropper. The adaptive AN strategy will jointly determine whether to transmit the AN and select the proper RIS-beamforming scheme. We introduce a heuristic design that jointly determines the active and passive beamforming vectors through closed-form expressions. We conduct comprehensive simulations to investigate how the AN affects the data rates of both the legitimate user and the eavesdropper under various integrating RIS/AN schemes. Simulation results show that by selecting the proper RIS/AN scheme, the adaptive AN strategy significantly improves the secrecy rate. Furthermore, the results also reveal key principles of adaptive selection based on the locations of the legitimate user and the eavesdropper.

Reconfigurable Intelligent Surface (RIS) is a transformative technology using the passive beamforming capabilities to create the programmable wireless environments. However, the control overhead is a challenging issue. The system needs to deliver the high-precision reflection coefficients to the RIS controller, for adjusting the phase shifts of the numerous RIS elements. Therefore, we propose a Spirograph-based discrete beamdirection strategy, which selects a set of non-uniformly distributed beam directions based on the generic patterns generated by the Spirograph (SG). We evaluate the achievable data rate and the number of controlling bits of SG-based method. We compare the SG-based method, the ideal continuous phase-shift method, and the SISO method. Simulation results show that with the properly-selected discrete beam directions, the SG-based method can achieve a comparable data rate as the ideal method, while having a lower control overhead.

Cooperative transmission is a power-efficient transmission technique, with the advantages of diversity and short-range communications. In this paper, we develop joint transmission power and relay location design for the cooperative networks. We investigate the impacts of transmission (TX) mode, transmission power, and relay location on the capacity and power consumption. Besides, we develop TX-mode adaptation schemes for the users to adaptively select a proper TX mode among the direct, relay, and cooperative TX modes. To achieve the goals of saving power and enhancing capacity, we apply an optimization approach to jointly determine the optimal relay location and transmission power of BS and RS; aiming to maximize energy efficiency under the cell capacity and link reliability requirements. We compare the scheme with joint power and relay location design, and the schemes with only transmission power or relay location design. Simulation results show that the scheme with joint optimal design can significantly enhance the energy efficiency, compared to the other two design schemes. We compare the throughput-oriented and signal-strength-oriented TX-mode selection rules. It is shown that the throughput-oriented TX-mode adaptation schemes outperform the signal-strength-oriented schemes.

The device-to-device (D2D) communications can improve the spectrum efficiency with low power and effectively offload the traffic from the macro-eNB. However, the macro/femto/D2D heterogeneous networks face a complicated three-tier interference issue. The completely centralized resource management is not applicable in such three-tier heterogeneous networks. Therefore, we propose a network-assisted device-decided (NADD) scheme to jointly select the channels and adjust the transmission power for D2D devices. In the NADD method, the macro-eNB broadcasts resource allocation instruction to D2D devices. According to the resource allocation instruction and the device's communication quality, D2D devices autonomously and dynamically select the suitable channels and adjust the transmission power. Simulation results show that NADD scheme can significantly improve the total throughput and ensure the link reliability of macrocell users, comparing to both the fixed power scheme and the network-decided (ND) power control scheme.

In this paper, we propose the group-based time control mechanism to improve the network overload and delay performance in the femtocell-based machine type communications (MTC) networks. The MTC network may be congested if numerous MTC devices concurrently deliver messages to the MTC server. Femtocells can solve the radio access network (RAN) congestion, however the core network (CN) congestion becomes more difficult to handle. The proposed group-based time control method can spread the traffic load of MTC devices over the time, and thereby mitigate the RAN overload and CN overload simultaneously. Numerical results show that the proposed approach can significantly improve the network congestion and message delay compared to the existing methods.

In this paper, we propose a low-complexity distributed stable subchannel allocation algorithm combined with a low-cost reconfigurable switched multi-beam directional antenna to improve the capacity and the fairness performance for the orthogonal frequency division multiple access (OFDMA)-based femtocell systems. With the proposed deferred acceptance algorithm, the stable subchannel allocation scheme can achieve a better tradeoff between capacity and fairness. Numerical results show that the proposed stable subchannel allocation scheme with the switched multi-beam antenna can significantly improve femtocell capacity compared to the traditional user-oriented subchannel allocation scheme and ensure the fairness as compared to the traditional channel-oriented subchannel allocation scheme.

It goes without saying that the femtocells will be widely employed in the next generation wireless networks since the femtocells improve indoor capacity and coverage with low power and less cost. However, as the femtocells become popular, the femtocell users suffer from the complicated two-tier interference, including the macrocell-to-femtocell and femtocell-to-femtocell interference. Therefore, the femtocells pose a difficult challenge on managing the interference in a autonomous and distributed manner. In this paper, we investigate how to distributedly select the sub-channels for the OFDMA-based femtocell systems to reduce interference and to improve indoor capacity under a link reliability requirement. We develop the channel-gain oriented and interference-avoidance oriented distributed channel selection schemes. Simulation results show that the interference from the macrocell and other femtocells significantly degrades femtocell link reliability and capacity. However, by properly adjusting the number of used sub-channels, the developed channel selection schemes can improve capacity and ensure the link reliability.

Thanks to silence suppression, the VoIP capacity can be improved. When the voice users are listening or thinking, the VoIP system does not need to allocate resources to them, thereby being able to accommodate more users. The main issue is how to immediately allocate resources to the user, when the user transits from the silence state to the talkspurt state. The IEEE 802.16e/m system provides two typical bandwidth request methods: unicast polling and multicast polling. The unicast-polling method can ensure the bandwidth request delay, but needs more slots for polling. Contrarily, the multicast-polling method needs fewer slots for polling, but the bandwidth request delay cannot be guaranteed. Therefore, we propose the hybrid-polling method, which combines the multicast and unicast polling. Simulation results demonstrate that by properly designing system parameters, the hybrid-polling method can achieve better tradeoff between the bandwidth request failure rate and the resources needed for polling. Moreover, the proposed hybrid-polling method can achieve comparable VoIP capacity as the multicast-polling method, with a guaranteed bandwidth request delay.

Femtocells are low-power, low-cost and user-deployed home base stations that can improve poor indoor coverage and enhance system capacity. However, as femtocells are deployed extensively, the two-tier interference will become serious, including the interference from macrocells and that from femtocells. Thanks to the low side lobe, the E-plane horns based reconfigurable antenna can be used to reduce the interference and enhance the capacity. In this paper, we investigate the effects of applying E-plane horns based reconfigurable directional antenna on link reliability and capacity of the orthogonal frequency-division multiple access (OFDMA) based femtocell systems. We compare our newly designed E-plane horns based reconfigurable antenna,and the sector antenna provided in the IEEE 802.16 m, as well the omni-directional antenna. It is shown that E-plane horns based reconfigurable antenna system has higher femtocell capacity than the IEEE 802.16 m sector antenna system due to the advantages of low side lobe and high main-lobe gain.

The wireless mesh network (WMN) is a promising solution to support wireless broadband applications. However, mesh networks face the power unfairness issue. Compared to the users far away from the gateway, the users near the gateway have to relay more traffic and consume more power. This paper proposes a scalable ring-based WMN that can ensure power fairness among users by adjusting its ring widths. On top of the ring-based cell structure, frequency planning is suggested to reduce the contending users, thereby making the system more scalable to accommodate more users. To investigate the overall tradeoffs among power fairness, capacity, and coverage, we develop an analytical model to evaluate the throughput and power consumption of the ring-based WMN using carrier sense multiple access (CSMA) medium access control (MAC) protocol in the unsaturated situation. Then, the optimization approach is applied to determine the best number of rings and the optimal ring widths, aiming to maximize cell capacity and coverage subject to the requirement of power fairness. Numerical results show that compared to the uniform ring-width strategy, the variable ring-width design criterion can improve cell capacity and coverage.

Relay stations (RSs) are usually used to enhance the signal strength for the users close to the cell boundary. However, transmission through a relay station needs two transmission phases, i.e., one is from the base station to the relay station and the other is from relay station to mobile stations. Thus, relay may also decrease system capacity if two-phase transmission time is considered. As a result, whether or not data are transmitted by one-hop or two-hop transmission should be determined based on both signal strength and throughput. In this paper, we investigate the optimal relay location aiming to maximize system capacity. We consider two relay selection rules for determining whether a two-hop transmission is necessary: signal strength-oriented and throughput-oriented. We find that the signal strength-oriented two-hop transmission may yield even lower system capacity than the one-hop transmission. Based on the throughput-oriented rule, we find that the throughput in the two-hop transmission can be higher than that in the one-hop transmission at some locations. We also identify the optimal relay location that can achieve the highest system capacity.

With the goal to provide ubiquitous wireless Internet access, large-scale deployment of outdoor public access networks will continue at a rapid pace in the next years. In outdoor environment, connecting access points (APs) via cables is costly and requires a lot of cabling engineering work. Therefore, the multihop-augmented network architecture has become an option of upsurging interest. This paper investigates how to deploy APs in an outdoor public access network to improve throughput and quality-of-service (QoS) while extending the coverage by multihop communications. Frequency planning is suggested to improve user throughput with QoS provisioning, and to make the multihop network more scalable to facilitate coverage extension. To investigate the overall tradeoffs among QoS, throughput, and coverage, we develop an analytical throughput and delay model for this multihop network using the carrier sense multiple access (CSMA) medium access control (MAC) protocol. This model also considers bidirectional asymmetric traffic for users operating in the unsaturated situation. Then, we apply an optimization approach to determine the optimal number of APs in a cluster and the best separation distances between APs, aiming to maximize the capacity and coverage of a cluster of APs subject to the delay requirement.

The wireless mesh network (WMN) is an essential low-power solution to support ubiquitous broadband services. However, mesh networks face the power unfairness and throughput bottleneck issues. Compared to the users far away from the gateway, the users near the gateway need to relay more traffic and consume more power. This paper proposes a scalable ring-based WMN that can ensure power fairness among users by adjusting the ring widths. On top of the ring-based WMN, frequency planning is suggested to overcome the throughput bottleneck issue for the inner-ring users near the gateway, thereby making the system more scalable to accommodate more users and facilitating coverage extension. We also investigate the overall tradeoffs of the ring-based WMN in terms of power fairness, capacity, and coverage. An analytical model is developed to evaluate the throughput and power consumption of the ring- based WMN using carrier sense multiple access (CSMA) MAC protocol in the unsaturated situation. Then, a mixed-integer nonlinear optimization problem aiming to maximize cell capacity and coverage subject to the constraint of power fairness is formulated. Applying this optimization approach, we obtain the optimal number of rings and the associated ring widths of the ring-based WMN.

Wireless mesh network (WMN) is a viable solution to support ubiquitous broadband services with low transmission power. This paper investigates the tradeoffs among delay, throughput, and coverage in a multi-channel ring-based wireless mesh network. In the proposed WMN, a simple ring-based frequency planning is employed to effectively utilize available channels. To evaluate the delay and throughput in the proposed WMN, we develop a physical (PHY)/medium access control (MAC) cross-layer analytical framework, which incorporates the carrier sense multiple access (CSMA) MAC protocol and a physical-layer distance-based rate adaptation with multi-hop connections. On top of the analytical model, the mixed-integer nonlinear programming (MINLP) optimization approach is applied to analytically determine the optimal number of rings and the associated ring widths, aiming at maximizing the coverage of a mesh cell subject to the requirements of throughput and delay.

Adaptively adjusting transmission rate and power to concurrently enhance goodput and save energy is an important issue in the wireless local area network (WLAN). However, goodput enhancement and energy saving are two contradictory goals, since high data rate usually requires high transmission power to meet the bit-error-rate (BER) requirement. To achieve the tradeoff between energy efficiency and goodput, therefore, we suggest a fast channel-driven rate and power adaptation (CDRPA) algorithm. Furthermore, we develop a physical (PHY)/medium access control (MAC) cross-layer analytical method to evaluate goodput and energy efficiency, incorporating the impacts of Nakagami fading channel and the carrier sense multiple access (CSMA) MAC protocol. This paper compares two adaptation

The wireless mesh network (WMN) is an economical solution for disseminating broadband wireless information in the intelligent transportation systems (ITS). This paper investigates the issue of deploying access points (APs) in an ITS wireless mesh network, where several adjacent APs form a cluster. Each AP in a cluster operates as a wireless relay to forward neighboring AP's traffic to the central access point connected to the Internet through cables. In general, access points are placed for maximizing cell coverage. However, larger coverage of an AP leads to lower throughput and longer delay in the access link as well as in the relay link. To find the optimal tradeoffs among delay, capacity, and coverage, we develop a physical (PHY)/medium access control (MAC) cross-layer analytical model to evaluate the throughput and delay of the considered ITS wireless mesh network. We consider the carrier sense multiple access (CSMA) protocol and the impact of hop distance on the data rate in the physical layer. Then, we apply the mixed-integer nonlinear programming (MINLP) optimization approach to determine the optimal number of APs in a cluster and the best cell radius of each AP, aiming at maximizing the capacity and coverage of a cluster of APs subject to the delay and fairness requirements.

The wireless mesh network (WMN) is an economical solution to support ubiquitous broadband services. This paper investigates the tradeoffs among quality-of-service (QoS), capacity, and coverage in a scalable multichannel ring-based WMN. We suggest a simple frequency planning in the proposed ring-based WMN to improve the capacity with QoS support, and to make the system more scalable in terms of coverage. We develop a physical (PHY)/medium access control (MAC) cross-layer analytical model to evaluate the delay, jitter, and throughput of the proposed WMN, by taking account of the carrier sense multiple-access (CSMA) MAC protocol, and the impact of hop distance on transmission rate in the physical layer. Furthermore, the mixed-integer nonlinear programming optimization approach is applied to determine the optimal number of rings and the associated ring widths, aiming at maximizing the capacity and coverage of a mesh cell subject to the delay requirement

A resource-allocation strategy in an integrated voice/data cellular CDMA network is presented and analyzed. The delay-sensitive voice service is considered to have higher priority to use the channel resource, while the error-sensitive data traffic has higher priority to use the power resource. The concept of guard channel is used to improve service reliability. Three power resource allocation schemes are adopted and compared: equal power scheme, equal service-reliability scheme, and robust data scheme. Moreover, the effect of the data packet length is also investigated.

The code-division multiple-access (CDMA) scheme has been considered as one possible choice of the future standards for cellular networks because of its various advantages. Since there can be only one carrier frequency being used in CDMA systems, a handoff scheme with diversity, a so-called "soft handoff", was proposed for higher communication quality and capacity. A mathematical model is developed to analyze the soft handoff process. Markov's concept is applied to describe the system's steady state statistical behavior. System performance such as blocking probability, handoff refused probability, and channel efficiency are also determined. It is concluded that the larger the area the soft handoff region is, the better users in the cellular network will feel.