Probe/PreAck: A Joint Solution for Mitigating Hidden and Exposed Node Problems and Enhancing Spatial Reuse in Dense WLANs

The IEEE 802.11ax standard is the latest innovation in WiFi, also referred to as WiFi 6 (sixth generation). It focuses on improving the performance in dense Wireless Local Area Networks (WLAN). It proposes various features to improve the same. OFDMA, spatial-reuse, and Downlink and Uplink MUMIMO are the most important features introduced. In this paper, we focus on the spatial-reuse feature. The standard proposes OBSS (Overlapping Basic Service Set) colouring to increase spatial reuse in dense scenarios. We evaluate the expected performance benefits offered by the spatial reuse technique. We have used ns-3 to simulate a variety of real networks and examine the performance of the proposed reuse technique. We show that spatial-reuse capabilities help achieve significant performance benefits for various network types under certain configurations of Clear Channel Assesment (CCA) threshold, inter-AP distance and power. These results shall act as a reference point for future testing to take place before eventual deployment.

Queueing systems to study the energy consumption of a campus WLAN

This paper proposes a decentralized scheme to improve the energy efficiency (EE) in dense wireless local area networks (WLANs), based on inversely proportional setting (IPS) of carrier sense threshold (CST) and transmit power. Jointly adjusting the CST and transmit power in an inversely proportional manner, i.e., keeping their product to be a constant, is a promising technique to increase transmit opportunities in dense WLANs, without generating unfairness issue of asymmetric carrier sensing. The proposed scheme is formulated in the non-cooperative game framework. Two utility functions are designed, where the first utility function is an approximation of the EE. To guarantee the existence of the equilibrium, and to ensure the convergence of the proposed scheme, we further incorporate potential games, based on which the second utility function is de- signed. The second utility function can be considered as assigning a cost on the first utility function. Simulation results show that the proposed scheme provides considerable improvement in EE and average throughput under various node densities. The second utility function outperforms the first utility function, while the first one requires only local information to be calculated.

In dense Wireless Local Area Networks (WLANs), high-density Access Points (APs) bring severe interference that seriously affects the experience of users, resulting in lower throughput and poor connection quality. Due to the heavy computation workload raised by the sizable networking systems and the difficulty in estimating instantaneous Channel State Information (CSI), existing works are hard to solve interference problem. In this paper, we propose a Joint Power control and Channel allocation based on Reinforcement Learning (JPCRL) algorithm combining with statistical CSI to reduce interference adaptively. Firstly, we analyze the correlation between transmit power and channel, and formulate the interference optimization as a Mixed Integer Nonlinear Programming (MINLP) problem. Secondly, we use the statistical CSI method to take the power and channel state as the state and action space, the overall throughput increment as the reward function of Q-learning, and obtain the optimal joint optimization strategy through off-line training. Moreover, for the periodic reinforcement learning process leading to resource consumption, we design an event-driven mechanism of Q-learning, which triggers online learning to refresh the optimal policy by event-driven condition and the consumption of computing resources can be reduced. The evaluation results show that the proposed algorithm can effectively improve the throughput compared with the existing scheme.

A potential game based distributed channel selection scheme is proposed in this paper to mitigate the flow-in-the-middle (FIM) throughput starvation problem that frequently occurs in dense wireless local area networks (WLANs). The FIM throughput starvation occurs when neighbors of a given node are not within the carrier sense ranges of each other. Since they spatially reuse the channel and at least one of them transmits with a high probability, the node in the middle would detect the channel being occupied for a prolonged time and therefore experience extremely low throughput. The basic idea of the proposed scheme is to let each access point (AP) select the channel that reduces the number of three-node chain topologies on its two-hop neighborhood contention graph. The proposed scheme is proved to be a potential game, i.e., the proposed scheme is guaranteed to converge. Graph-based simulation shows that starvation occurs on 20% of nodes when nodes randomly select their frequency channels. The proposed scheme significantly reduces the number of starved nodes along with iterations, outperforming the compared traditional potential game based scheme.

In the past few decades, the rapid development of wireless local area networks (WLANs) provides great convenience to human lives. However, as the number of users and the complexity of deployment scenarios have increase, the next-generation WLANs face the unprecedented challenge of quality of service (QoS) and quality of experience (QoE) of cell-edge users. In response to this challenge, access point (AP) cooperation is supposed to be a promising solution. In this study, we propose a joint transmission medium access control (MAC) protocol based on channel reservation (CRJT). This protocol can make full use of the wireless channels to complete channel reservations and support joint transmissions effectively between APs. The simulation results show that this protocol provides a robust communication guarantee for edge users.

This paper presents a novel mechanism, called distributed cycle stealing (DCS), for improving the performance of DCF (distributed coordination function) operations of IEEE 802.11 wireless LAN (WLAN). DCS is an extension of the existing IEEE 802.11 DCF media access protocol (MAC) by applying the scheme of power control and spatial reuse. Research results show that a power control mechanism could be applied on IEEE 802.11 MAC. Thus, a station could adjust its transmission range by controlling the transmission power level. Using spatial reuse, a newer communication is allowed if there is no conflict between the newer and the existing communication. However, spatial reuse would partition a single hop WLAN into a multihop one. DCS solves the issue by reusing the space without separating the single hop WLAN into a multihop one. We illustrate the advantages of DCS via extensive simulations performed over ad hoc networks containing 50 nodes under variant traffic load. The results show that up to an 11% enhancement of the packet throughput can be obtained. Packet loss ratio and packet mean delay time are also significantly reduced.

Wireless Local Area Network (WLAN) has been greatly developed for the last twenty years. Quality of Service (QoS) and Quality of Experience (QoE) in high-dense deployment scenario greatly challenges the next-generation WLAN [1]. To address this challenge, researchers proposed channel reservation mechanism for WLAN, but the premise of channel reservation mechanism is to know the next packets exact arrival time. This assumption is impractical since the next packets arrival time is a stochastic process. In order to solve the above problem, this paper proposes an arrival prediction based channel reservation media access control (MAC) for the next generation WLAN. Simulations show that the protocol can reduce the network collisions generated by concurrency to a certain extent and improve network throughput.

Wireless local area network (WLAN) technology has become mature in the past few decades. However, with the development of social application forms, the situation of high-density deployment of cells has become more frequent in recent years, so the next-generation WLANs face the unprecedented challenge of quality of service (QoS) and quality of experience (QoE) of high-density cell users. In response to this challenge, The Overlapping Basic Service Set Power Detection (OBSS PD) mechanism and Spatial Reuse Parameter (SRP) mechanism are proposed in IEEE 802.11ax (Institute of Electrical and Electronics Engineers) to improve spatial multiplexing capability and certain throughput capabilities. But both of them possess inherent drawbacks. Based on this, this paper proposes an enhanced spatial reuse mechanism, named ESR (Enhanced Spatial Reuse), by limiting the value of the CCA threshold in the SRP mechanism to improve the quality of concurrent links, which further improves the average throughput. Compared to traditional SRP, simulation results show that ESR can increase throughput by 10% and reduce packet loss by 10%.

In future's high density Wireless Local Area Networks (WLANs), to improve the overall throughput, we envision the optimization of the MAC protocols and mechanisms. Traditionally, it's clear in the literature that Transmit Power Control (TPC) is chosen as one of the most powerful tools for optimizing MAC protocols. However, the TPC is not always possible to implement due to hardware and licensing limitations on one hand, and on the other, its unbalanced use results in starvation situations. In the same context, the Clear Channel Assessment (CCA) threshold adaptation is proving its effectiveness. This adaptation is preferable to TPC that behaves aggressively towards transmitters having lower transmit power. In this paper we show that CCA adaptation mechanism may be used effectively to optimize the spatial reuse in dense WLAN deployments. For a dense IEEE 802.11n network topology, our simulations show a global gain of 190% in aggregate throughput compared to the current bound assumed by the present MAC protocol.

A transmission queuing scheme is described that increases downlink throughput on wireless local area networks (WLANs) while also increasing the total throughput. When the amount of uplink traffic increases on a WLAN, the carrier sense multiple access with collision avoidance (CSMA/CA) protocol, which is the prescribed scheme for IEEE 802.11 WLAN channel access, may substantially reduce the rate of downlink data frame transmission. This results in severe throughput degradation for mobile stations with downlink traffic. The proposed scheme comprises a transmission control function based on consecutive transmission, as described in the IEEE 802.11e standard, and a dynamic queue prioritization algorithm. Simulation results demonstrate that the proposed scheme increases the maximum total throughput for uplink and downlink traffic by 17% compared with the conventional distributed coordination function (DCF) scheme and that it reduces the difference between uplink and downlink throughput. In an environment where transmission errors occur, the difference in throughput is reduced by about 50% compared with the conventional schemes.

Chapter 4 - Wireless Local Area Networks

The first giga-bit class wireless local area network (wireless LAN) below 6 GHz frequency (the IEEE 802.11ac standard) is attracting attention due to its capability of handling much more of the rapidly growing data traffic. The increased density of wireless LAN access points (APs), however, has induced inter-cell interference that severely degrades system performance. This problem seems to be getting worse with the increase of 802.11n- and 802.11ac-based APs, which establish basic service sets (BSSs) with wider channel bandwidths. In order to mitigate this problem, we propose an efficient radio resource allocation scheme called RATOP that can be applied to a managed wireless LAN system with a central coordinator. On the basis of channel monitoring results, capability, and data traffic information obtained from APs, the central coordinator computes the quasioptimal frequency channel and channel bandwidth of each AP in such a way that the given utility function is maximized. Numerous simulations with UDP traffic flows on the coexistence scenarios of 802.11a/n/ac show that the RATOP works well and reduces the number of overlapping BSSs by 65%-89%. As a result, the RATOP improves the minimum throughput and fairness among throughput of APs even when there exist foreign APs of other network domains that cannot be controlled by the coordinator.

The next generation wireless local area network (WLAN) needs to significantly improve the area throughput in high-dense deployment scenario. Orthogonal frequency division multiple access (OFDMA), considered as the key technology of the next generation WLAN, has been adopted by IEEE 802.11ax. However, most existing studies have one tricky problem: interference extension problem, i.e., the stations (STAs) placed at dispersive locations from one basic service set (BSS) access channel and transmit data simultaneously through OFDMA, thereby interfering a large area and suppressing the potential transmissions around this BSS. Unfortunately, high-dense deployment scenario exacerbates interference extension problem. This article proposes a spatial clustering group division-based OFDMA (SCGD-OFDMA) protocol, which enables the geographically close STAs to form spatial clustering groups, named SCGs. Each SCG has a leader STA and several member STAs. Each SCG’s leader STA contends for channel resources. After that, the leader STAs that successfully contending channel are scheduled by the access point one by one to trigger its member STAs to transmit uplink data by using OFDMA. Therefore, the geographical interference area is reduced and the area throughput is improved since the concurrent STAs in one SCG are located in limited area. This article theoretically analyzes the optimal SCG establishment, and throughput and area throughput of SCGD-OFDMA. Theoretical analysis is consistent with simulation results. The simulation results also show that when the number of STAs is 200 and all resource units are used for random access, SCGD-OFDMA outperforms IEEE 802.11ax and OMAX in area throughput by 56.13% and 190.97%, respectively.

Roaming across multiple basic service sets (BSS) using access points (AP) in wireless LAN (WLAN) always poses considerable challenges, as it will cause heavy packet loss error. Especially for real-time video streaming, it will damage receiving video quality. In this paper, we combine the benefits of multiple description coding (MDC) and multipath routing to maintain a best constant video quality despite packet loss when roaming in WLAN. We incorporate a channel status detection mechanism to decide which channel will be selected or multiple channels will be used to take advantage of path diversity to deliver the streaming video content. The loss-rate and round-trip time are used to determine the channel status by using active probing. Simulation results indicate that the proposed method can significantly improve the quality of streamed video when a mobile client roams in WLAN.