Hidden Terminals Research Articles

Learning-Based Autonomous Channel Access in the Presence of Hidden Terminals

We consider the problem of autonomous channel access (AutoCA), where a group of terminals tries to discover a communication strategy with an access point (AP) via a common wireless channel in a distributed fashion. Due to the irregular topology and the limited communication range of terminals, a practical challenge for AutoCA is the hidden terminal problem, which is notorious in wireless networks for deteriorating throughput and delay performances. To meet the challenge, this paper presents a new multi-agent deep reinforcement learning paradigm, dubbed MADRL-HT, tailored for AutoCA in the presence of hidden terminals. MADRL-HT exploits topological insights and transforms the observation space of each terminal into a scalable form independent of the number of terminals. To compensate for the partial observability, we put forth a look-back mechanism such that the terminals can infer behaviors of their hidden terminals from the carrier-sensed channel states as well as feedback from the AP. A window-based global reward function is proposed, whereby the terminals are instructed to maximize the system throughput while balancing the terminals' transmission opportunities over the course of learning. Considering short-packet machine-type communications, extensive numerical experiments verified the superior performance of our solution benchmarked against the legacy carrier-sense multiple access with collision avoidance (CSMA/CA) protocol.

A spectral clustering algorithm for intelligent grouping in Dense Wireless Networks

The density of wireless networks has been increasing with the popularization of mobile devices. Dense wireless networks (DWN) present challenges such as the current spectral scarcity and the growing demand for capacity. The Restricted Access Window (RAW) mechanism was introduced by the IEEE 802.11ah amendment to improve DWN performance. RAW restricts the number of stations that can access the channel by arbitrarily separating them into groups. K-Means clustering has shown potential to find more efficient groups using the geographical coordinates of each station. However, due to the mobile and dynamic nature of such networks, location information is difficult to obtain in practice. In this paper, we consider the use of spectral clustering to increase the performance of DWN with hidden terminals. We discuss how a spectral clustering algorithm that generates RAW groups can be implemented in practice without the geographic location of each node. We also compare the performance of the spectral clustering algorithm with the standard grouping method used in IEEE 802.11ah, with the K-Means clustering (i.e., based on node location information), and with the hidden matrix-based regrouping (HMR) algorithm. Simulation results considering several density levels, different traffic patterns, and different propagation models indicate that spectral clustering significantly outperforms both the standard grouping and HMR in terms of collision rate, throughput, and delay. It also closely approximates – and sometimes surpasses – the performance of the K-Means clustering while being much more practical to implement because it does not require knowledge on nodes’ geographical coordinates.

Hidden terminal aware grouping scheme for IEEE 802.11ah based dense IoT networks

IEEE 802.11ah, a prevalent standard for the Internet of Things, introduces major enhancements to PHY and MAC layer of the legacy standard. One of the significant improvements in IEEE 802.11ah is the restricted access window (RAW) mechanism. The RAW mechanism is a group-based contention technique that limits the contention among the devices by partitioning them into several groups, dividing the channel time into various time slots and assigning each group with a RAW slot. Although the grouping schemes reduce contention among the devices, it eventually leads to hidden terminals in the dense networks. In this article, we propose a hidden terminal aware grouping (HTAG) mechanism to alleviate the hidden terminal problem. The proposed scheme groups the devices based on the SINR and implements a hidden terminal avoidance algorithm to ensure each group with a minimum number of hidden terminals. We present a simple yet accurate analytical model to evaluate the performance of the RAW mechanism with the proposed HTAG scheme in terms of throughput, delay and energy consumption. Results show significant improvement in the performance of the RAW mechanism using the HTAG scheme compared to the state-of-the-art schemes. Finally, all the analytical findings are validated using extensive simulations in ns-3.

Spectrum Sensing for DTMB-A System Using Accumulated Autocorrelation

The digital terrestrial television multimedia broadcasting - advanced (DTMB-A) system demands an accurate and reliable spectrum sensing method for its implementation. In this paper, an effective method based on weighted autocorrelation of the frame headers is proposed for single-node spectrum sensing in DTMB-A. The decision threshold, lower bounds and miss detection rate of the proposed method are researched in detail. Simulation results indicate that the proposed method shows better sensing accuracy and robustness compared with the existing methods under various channels, while the cost of its higher complexity is acceptable. Furthermore, by using piecewise-weighted information fusion, the proposed method is extended into cooperative spectrum sensing to overcome the problems of shading and hidden terminals in networking. Our work ensures the efficiency of spectrum sensing and contributes to the better use of DTMB-A system in future applications.

Composite Interference Mapping Model to Determine Interference-Fault Free Schedule in WSN

The primary focus of most approaches that model interference in WSN is to understand the conditions under which the signals received by a destination node are interfered by the transmissions of other source nodes. While these models support the basic understanding of interference phenomenon between two nodes; models that attempt to map the nodes whose transmissions can potentially interfere with other nodes is highly desirable. To this end, the Composite Interference Mapping (CIM) model presented in this paper, provides a holistic frame-work for determining a comprehensive map of all potential interferers, for each and every node in the WSN. Based on CIM model, we have developed Interference-Fault Free Transmission (IFFT) scheduling algorithms for (i) topology-free, and (ii) tree-structured WSNs. In addition to determining IFFT schedules, the algorithms also address the Exposed Station (ES) and Hidden Terminal (HT) problems. We present the analytical studies developed to support the models developed in the paper. The results of the simulation experiments have shown that the CIM-based IFFT algorithms perform much better compared to some of the existing algorithms in performance metrics such as: number of time slots, packet drop, number of dead nodes, honouring precedence relations, and addressing ES and HT problems.

Interference-Aware Transmission Scheduling for Internet of Vehicles

Next-generation Intelligent Transportation Systems (ITS) use wirelessly connected vehicles for safety and non-safety applications such as autonomous driving, cooperative awareness, route guidance and multimedia transmissions. This network known as the Internet of Vehicles (IoVs) suffers from many challenges such as collisions due to hidden terminals, and interference from simultaneously transmitting vehicles. Moreover, the packet reception ratio of transmissions between vehicles is significantly reduced at high vehicle densities and severe fading scenarios. As safety applications require periodic broadcast of safety messages from each vehicle to all other vehicles in the neighborhood, the development of an efficient medium access technique is a key challenge. In this paper, we propose an interference-aware medium access protocol for vehicular networks that allocate time slots to the vehicles such that interference due to hidden nodes is minimized. We first provide an analytical model to compute the interference range of a vehicle in the presence of simultaneously transmitting vehicles. We also propose a scheduling algorithm that uses vehicle GPS positions and reuse the same time slot only at a distance greater than the interference range. Simulation results show that the proposed technique improves the packet reception ratio by 70% as compared to the standard IEEE 802.11p based CSMA/CA protocol.

Exploiting an Optimal Delay-Collision Tradeoff in CSMA-Based High-Dense Wireless Systems

A novel carrier sense multiple access strategy with collision avoidance (CSMA/CA) balancing contention probability and channel access time is proposed. The approach can be applied to any context where the computational simplicity of the MAC must be preferred to the complexity of the channel access strategy. Our MAC, called Delay-Collision CSMA (DC-CSMA), is a slotted nonpersistent CSMA/CA with nonuniform contention probability distribution, designed to reduce at the same time latency of contenders and preserve a high successful access probability. An utility function aiming at equalizing the effects of these two performance metrics is introduced, and the related theoretical properties and optimal distribution are derived. DC-CSMA is insensitive to the number of contenders and very robust with respect to contention window size, packet length, and impairments such as frame synchronization errors and hidden terminals, and it does not require any adaptive tuning to optimize its performance. Current technologies such as WSN, RFID, IoT devices can benefit from such a simple access technique. The numerical evaluation has been led out considering latency, successful probability and throughput, and DC-CSMA has been compared with other classical strategies such as CSMA with uniformly distributed contention probability, CSMA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p^{\ast}$ </tex-math></inline-formula> and Sift distribution.

Distributed TDMA for Mobile UWB Network Localization

Many applications related to the Internet of Things, such as tracking people or objects, robotics, and monitoring require the localization of large networks of devices in dynamic, GPS-denied environments. Ultrawideband (UWB) technology is a common choice because of its precise ranging capability. However, allowing access and effective use of the shared UWB medium with a constantly changing set of devices faces some particular challenges: high frequency of ranging measurements by the devices to improve system accuracy; network topology changes requiring rapid adaptation; and decentralized operation to avoid single points of failure. In this article, we propose a novel time-division multiple access (TDMA) algorithm that can quickly schedule the use of the UWB medium by a large network of devices without collisions in local network neighborhoods and avoiding conflicts with hidden terminals, all the while maximizing network usage. Using exclusively the UWB radio network, we realize a decentralized system for synchronization, dynamic TDMA scheduling, and precise relative positioning on a multihop network. Our system does not have special nodes (all nodes are equal) and it is sufficiently scalable for real-world applications. Our method can be applied to implement device localization services in large spaces without GPS and complex topologies, such as malls, museums, mines, etc. We demonstrate our method in simulation and on real hardware in an underground parking lot, showing the effectiveness of its TDMA schedule for relative localization.

Improving Sensor Network Performance with Directional Antennas: A Cross-layer Optimization

The use of directional antennas for wireless communications brings several benefits, such as increased communication range and reduced interference. One example of directional antennas are electronically switched directional (ESD) antennas that can easily be integrated into Wireless Sensor Networks (WSNs) due to their small size and low cost. However, current literature questions the benefits of using ESD antennas in WSNs due to the increased likelihood of hidden terminals and increased power consumption. This is mainly because earlier studies have used directionality for transmissions but not for reception. In this article, we introduce novel cross-layer optimizations to fully utilize the benefits of using directional antennas. We modify the Medium Access Control (MAC) , routing, and neighbor discovery mechanisms to support directional communication. We focus on convergecast investigating a large number of different network topologies. Our experimental results, both in simulation and with real nodes, show when the traffic is dense, networks with directional antennas can significantly outperform networks with omnidirectional ones in terms of packet delivery rate, energy consumption, and energy per received packet.

Towards the Saturation Throughput Disparity of Flows in Directional CSMA/CA Networks: An Analytical Model

Using directional antennas in wireless Ad hoc networks has many superiorities, including reducing interference, extending transmission range, and increasing space division multiplexing. However, directional transmission introduces two problems: deafness and directional hidden terminals problems. We observe that these problems result in saturation throughput disparity among the competing flows in directional CSMA/CA based Ad hoc networks and bring challenges for modeling the saturation throughput of the flows. In this article, we concentrate on how to model and analyze the saturation throughput disparity of different flows in directional CSMA/CA based Ad hoc networks. We first divide the collisions occurring in the transmission process into directional instantaneous collisions and directional persistent collisions. Then we propose a four-dimensional Markov chain to analyze the transmission state for a specific node. Our model has three different kinds of processes, namely back-off process, transmission process and freezing process. Each process contains a certain amount of continuous time slots which is defined as the basic time unit of the directional CSMA/CA protocols and the time length of each slot is fixed. We characterize the collision probabilities of the node by the one-step transition probability matrix in our Markov chain model. Accordingly, we can finally deduce the saturation throughput for each directional data stream and evaluate saturation throughput disparity for a given network topology. Finally, we verify the accuracy of our model by comparing the deviation of analytical results and simulation results.

An Optimized Routing OLSR Protocol with Low Control Overhead for UAV Ad Hoc Networks

OLSR (Optimized Link State Routing) protocol is a priori Routing protocol applied in MANET network. All nodes in the network have Routing information to other nodes locally and need to send a large number of control messages to maintain the topology information of the whole network. In UAV application scenarios, with the increase of node density, the control overhead will be too high and the problems of hidden terminals will be intensified. Secondly, due to the fast moving speed of UAVs, when the neighboring UAVs have moved out of the communication range of the local node, the local node still retains the routing information of the neighbor node, leading to the problem of packet loss when the link information is updated behind time. Aiming at the above problems, an OLSR protocol with low control overhead and optimal routing is proposed. The protocol uses the incremental HELLO message mechanism to ensure that only incremental information is exchanged when the network topology changes slowly, and the control overhead is greatly reduced. When calculating routing, in the range of n hops, the more stable link is selected under the same number of hops. Simulation results show that the proposed algorithm can significantly improve the success rate of packet transmission and end-to-end delay, and reduce the system control overhead.

Collision Prevention for Duty-Cycle Receiver-Initiation MAC Protocol via Multiple Access Reservation (MAR-RiMAC).

The prevalence of the Internet of Things (IoT) paradigm in more and more applications associated with our daily lives has induced a dense network in which numerous wireless devices, many of which have limited capabilities (e.g., power, memory, computation), need to communicate with the internet. One of the main bottlenecks of this setup is the wireless channel. Numerous medium access control (MAC) protocols have been devised to coordinate between devices that share the wireless channel. One prominent approach that is highly suitable for IoT and wireless sensor networks (WSNs), which rely on duty cycling, is the receiver-initiated approach, in which, rather than the transmitter, the receiver initiates the transaction. The problem with this approach is that when many devices are trying to respond to the receiver’s transmission invitation and transmit simultaneously, a collision occurs. When the network is highly loaded, resolving such collisions is quite tedious. In this paper, we devise an enhancement to the receiver-initiated approach that aims at preventing this inherent collision scenario. Our modification relies on multiple devices sending a short predefined signal, informing their intended receiver of their intention to transmit simultaneously. The data transaction is done via a four-way handshake in which, after all backlogged devices have informed their designated receiver of their desire to transmit simultaneously, the receiver identifies them and polls them one by one, avoiding the collision. We compare the performance of Receiver-Initiated-MAC protocol (RI-MAC), which is one of the prevalent receiver-initiated protocols, with and without the suggested enhancement, and show superior air-time utilization under high traffic loads, especially in the presence of hidden terminals.

A Modified RR-ALOHA Protocol for Safety Message Broadcast in VANETs

MAC protocol design of VANETs is a real challenging task due to the high mobility and mobile hidden terminals (MHTs) issues. Slotted ALOHA based MAC protocols is regarded as one of the most promising solution in which hidden terminals issues are solved by broadcasting frame information (FI). However, when network topology changes rapidly, the vehicles using the same slot comes close to each other, merging collisions take place and a new round of slot reservation is required. This paper proposes a modified Reliable Reserved ALOHA (RR-ALOHA) protocol in which merging collisions are detected and an election process is employed to reduce collisions directly after one merging collision. Simulation results show that our proposal achieves better packet delivery ratio than RR-ALOHA in most scenarios.

On Wireless Blind Spots in the C-V2X Sidelink

The third generation partnership project (3GPP) has issued specifications for the autonomous assignment of radio resources by vehicles on the sidelink of the cellular-vehicle-to-everything (C-V2X) technology. It is based on a sensing mechanism for resource selection and a semi-persistent scheduling for resource reservation to periodic safety messages. Imperfect sensing due to hidden terminals and to half-duplex on board transceivers may result in the selection of interfered resources for successive message transmissions. As a consequence of the lost packets, involved vehicles may become blind to the presence of other vehicles in their vicinity even for many seconds, with threats to the road safety. In this paper, we define these events as wireless blind spot s (WBSs) and characterize their probability to occur. We propose an enhancement to the autonomous mode in order to reduce the WBS duration and demonstrate the benefits of the proposal against the legacy mode, both analytically in a simplified scenario and through simulations in a highway environment.

Optimal optical camera communication-ALOHA random access algorithm aided visible light communication system

We propose an optical camera communication-based ALOHA-like (OCC-ALOHA) random access algorithm in which the hidden terminal issue is avoided. We use the spatial resolution of the image sensor of the OCC link to enable the access point (AP) to obtain multipacket reception capability. Then, we use the spot estimation algorithm to estimate the number of active terminals, the distance between the AP and terminal, and the emission angle of the terminal light-emitting diode. Based on the above estimation information, the double iteration algorithm is used to calculate the access probability of each terminal to achieve the optimization of the access fairness and system throughput rate. The results show that the throughput rate of the proposed scheme is better than that of carrier sense multiple access with collision avoidance with hidden terminals.

Application-Level Performance of IEEE 802.11p in Safety-Related V2X Field Trials

In this article, we study the application-level performance of safety-related communications over IEEE 802.11p, presenting the first outcomes resulting from several short- and medium-range field tests. These have been performed with low-cost off-the-shelf equipment operating on the IEEE 801.11p PHY/MAC stack, with devices configured or modified, when necessary, with open-source software to ensure repeatability. In particular, this article discusses the relation that transmission power, antenna quality, distance, and different degrees of congestion, alone and combined, have on round-trip times, TCP Goodput, UDP Goodput, Jitter, and Datagrams or Packets lost. It also details an investigation on the hidden terminal problem, by presenting the results of field tests where destructive interference due to hidden terminals has been artificially introduced at the host premises. Finally, this article presents the metrics obtained by performing initial mobility tests in urban and suburban scenarios. The results are encouraging and attest that, with the appropriate transmission power, it is possible to provide satisfying safety guarantees under the tested conditions.

Minimization of IEEE 802.11p Packet Collision Interference through Transmission Time Shifting

V2I communications are characterized by the presence of network nodes in vehicles and in the infrastructures that these vehicles use, as well as by the wireless interactions among them. Safety-related applications demand stringent requirements in terms of latency and packet delivery probability, especially when safety messages have to be delivered to vehicles by the infrastructure. Interference issues stem from the typical characteristics of wireless communications, i.e., the noise of the wireless medium, the limited communication range of the wireless entities, and the receiver passivity of all the conventional wireless transceivers during transmissions. This paper presents a synchronization mechanism to artificially replicate at a host premises destructive interference due to hidden terminals, together with an application-level technique to minimize that interference by shifting the packet transmission time, similarly to the MAC TDMA channel access method. As both have been field-tested, the paper also analyzes the results of these tests, all performed with real hardware on IEEE 802.11p over different frequencies and transmission powers, and with repeatability in mind. The resulting figures attest that interference effects due to hidden terminals may indeed take place on real IEEE 802.11p networks, and that carefully designed time-shifting mechanisms can actively mitigate them.

Third Eye: Context-Aware Detection for Hidden Terminal Emulation Attacks in Cognitive Radio-Enabled IoT Networks

Recently, the Internet of Things (IoT) technology has been drawing increasing attention because it has a great potential to positively impact human life in a broad range of applications. Nonetheless, the dense deployment of multiple co-located IoT networks that may follow different wireless protocols will essentially bring new network vulnerabilities. In this paper, we introduce a novel attack scenario in co-located cognitive radio (CR) enabled IoT networks, where a reactive attacker can emulate the radiation pattern of a hidden terminal (the attacker is from a different network) and can interfere with the transmissions from its hidden counterparts, namely the hidden terminal emulation (HTE) attack. As the dense deployment of IoT nodes-from different networks and technologies-will naturally create such hidden terminal scenarios among IoT devices of different networks, it provides the HTE attacker plausible deniability to reactively interfere with its hidden counterparts; hence, the state-of-the-art reactive attack detection techniques are infeasible in this scenario where benign hidden terminals could be flagged as reactive attackers. In this paper, we capture the behavior of a benign hidden terminal and an HTE attacker via parsimonious Markov models and propose a context-aware detection solution using the Markov chain hypothesis testing, namely the Third Eye. Though there has been extensive research on malicious interference detection, to the best of our knowledge, this work is the first that considers hidden terminals as benign interference sources, foresees this unique attack scenario, and leverages the existing carrier sensing technique as a natural and effective way to detect HTE attacks.

Modelling link failure in the presence of extended hidden terminals over multi-hop wireless ad hoc networks

IEEE 802.11 uses RTS/CTS (request to send/clear to send) reservation mechanism in multi-hop ad hoc networks to prevent collision drops caused by the hidden terminals. Extended hidden nodes impose great challenges to RTS/CTS mechanism by creating additional interferences and route failures. We introduce a Markov model to compute the probability of collision due to hidden and extended hidden nodes present in multi-hop ad hoc network under saturated traffic loads. This model is derived from the well-known Bianchi Markov chain model of IEEE 802.11 Distributed coordination function by introducing additional states in every back-off stage. Our model differentiates the collisions that could happen to RTS/CTS and data packets. We also compute the probability of link failure that occurs when the maximum retry limit is exceeded. The model is tested for varying network sizes with number of nodes 10, 50, 100, 150 and 200. The model validates the throughput, collision probability and link failure probability against the Bianchi model and ns-2.35 simulation result.

An analytical framework for reliability evaluation of d-dimensional IEEE 802.11 broadcast wireless networks

In this paper, we validate that the deterministic distance-based analytical model can be used to estimate the reliability of one-dimensional (1-D) 802.11 broadcast wireless networks compared with the interference-based analytical model. Therefore, we propose a deterministic distance-based reliability analytical framework for such networks in d-dimensional (d-D, $$d \ge 1$$ ) scenarios. This framework takes into account the fading channel and hidden terminal problem and makes three commonly used reliability metrics able to be resolved, including point-to-point packet reception probability (NRP), packet delivery ratio (PDR), and packet reception ratio (PRR). There are two key factors involved in deducing the effect of hidden terminals. One is to measure the hidden terminal transmission probability during the vulnerable period, which can be calculated based on the approximate solution of the semi-Markov process model capturing the channel contention and the back-off behavior. Another is the challenge to determine the size of the area to which the hidden terminals belong. First, we give a general mathematical expression on the size of the hidden terminal coverage affecting NRP which is an important part of the closed-form solution of NRP/PRR. Second, we adopt the Monte-Carlo method to solve the size of general hidden terminal coverage affecting PDR, making it possible to approximate PDR, as well as control the efficiency and accuracy by constraining the relative error. Finally, we adopt a multi-parameter optimization scheme to find the optimum settings for the network to ensure the quality of service and maximize channel utilization. A series of experimental results show that the framework can be used to access the reliability of 802.11 based d-D broadcast wireless network and pave the way for further optimization.