Ji-Hoon Yun

Novel collision detection scheme and its applications for IEEE 802.11 wireless LANs

Many control schemes proposed for IEEE 802.11 wireless LANs (WLANs) behave adaptively to transmission failures, which occur mostly by two causes: collision and channel noise. However, in generic 802.11 WLANs, a station cannot know the cause of a transmission failure, and thus the current adaptive schemes assume that all transmission failures occur by one of the causes, which may lead to erroneous behavior in the real world. In this paper, we propose a novel scheme to detect collisions, which can help to differentiate the causes of transmission failures. The proposed scheme conducts accurate collision detection basically in two phases: failure notification (FN) and collision notification (CN). In the FN phase, a station disseminates the information about a failed transmission, i.e., transmission time or RF energy time on the channel, and the rest of the stations judge the cause by checking the received information against their own transmission history. If a station detects a collision through the FN phase, it starts the CN phase by disseminating the collision information so that the rest of the collision-involved stations self-detect the collision. In addition, the proposed scheme can detect the occurrence of the capture effect as well as the existence of hidden stations. To demonstrate the effectiveness of the proposed scheme, we present four applications and verify the performance improvement in each of these applications through comprehensive computer simulation, i.e., throughput enhancement of ARF (Automatic Rate Fallback) rate adaptation algorithm in various channel environments, efficiency improvement of the backoff mechanism with a few contending stations, fairness improvement against the capture effect and the improvement of the system throughput as well as fairness by the adaptive usage of the Request-to-Send/Clear-to-Send (RTS/CTS) exchange.

Interplay Between TVWS and DSRC: Optimal Strategy for Safety Message Dissemination in VANET

In vehicular safety systems, two types of safety messages are required: Emergency Safety Message (ESM) and Periodic Beacon Message (PBM). The ESM has to be disseminated within a specified area with stringent delay and delivery ratio requirements, while the PBM does not need to meet these requirements. For exchanging the safety messages in Vehicular Ad-hoc NETwork (VANET), Inter-Vehicle Communication (IVC) is necessary whose de facto standard is Dedicated Short-Range Communications (DSRC). However, the effective transmission range in the DSRC-based IVC is short since a signal can be attenuated due to blocking by obstacles. In order to cover a large dissemination area in the DSRC-based IVC, multi-hop dissemination is required, which however causes channel collision and network congestion. Moreover, the coexistence with PBMs aggravates the collision and the congestion, which make it hard to satisfy the requirements of the ESM dissemination. To overcome the limitation of the DSRC, we utilize an extra TV White Space (TVWS) band that has a large communication range for ESM disseminations, and exploit a DSRC band for 1) the exchange of control data and 2) the compensation of ESM reception errors. In this paper, we propose and analyze a distributed channel usage framework that exploits advantages of DSRC and TVWS bands for ESM dissemination under the existence of PBMs. Our scheme employs TVWS Channel Rendezvous Algorithm (TCRA), ensuring that vehicles within a dissemination area select the same channel with the ESM sender. To compensate ESM reception failures in a TVWS band, our scheme adopts Two-Way Recovery Algorithm (TWRA) that uses DSRC and TVWS bands for ESM retransmission. Further, we establish an analytical delivery ratio model that considers a delay bound of an ESM for optimal parameter selections. To the best of our knowledge, this is the first attempt to propose a distributed channel usage scheme that leverages the strengths of TVWS and DSRC bands for safety message dissemination. Through an in-depth simulation study, we show that the proposed scheme satisfies ESM requirements for latency and packet delivery ratio, and outperforms previous approaches in various vehicular scenarios.

Intra and Inter-Cell Resource Management in Full-Duplex Heterogeneous Cellular Networks

Full-duplex communication is drawing high attention as a means of enhancing wireless capacity considerably by enabling simultaneous transmission and reception on the same frequency spectrum. While it had been considered infeasible for a long time due to strong self-interference, recent researches have made substantial progress on addressing this challenge. This paper focuses on designing full-duplex cellular networks with co-channel femtocells. Due to relatively high transmit power, cancellation of self-interference by existing techniques could still be imperfect for cellular systems and thus residual interference may impact performance significantly. To overcome this in OFDMA systems, a new radio resource management scheme assigning downlink and uplink transmissions jointly while considering the gain of self-interference cancellation is developed. Three available transmission modes of a frequency resource block and crossover points between their achievable capacities are identified for the mode selection of each resource block. Users are then assigned resource blocks and transmit power levels are determined such that the total utility sum is maximized. To handle new femtocell interference scenarios, the transmit powers of femtocells and their connected users are adjusted by a coordination algorithm such that both data transmission and mode selection of an underlying macrocell are protected.

Collision detection based on transmission time information in IEEE 802.11 wireless LAN

The standard exponential backoff mechanism and many proposed schemes for 802.11 wireless LAN behave adaptively to transmission failures, which occur by two causes: collision and channel noise. However, in 802.11 wireless LAN, a station cannot know the cause of a transmission failure, thus the adaptive schemes assume the ideal situation in which all transmission failures occur by one cause. Therefore, they may behave erroneously in the real world where transmission failures occur by both causes. In this paper, we propose a novel scheme to detect collision, which can help 802.11 and many adaptive schemes to differentiate the causes of transmission failures. In the proposed scheme, stations exchange transmission time information when transmission failures occur. The proposed scheme can detect collision accurately with low bandwidth overhead and it can be implemented without modifying the physical layer of existing wireless LAN devices. In addition, we model the bandwidth overhead of the proposed scheme mathematically. In the simulation, we show that the performance of the automatic rate fallback link adaptation algorithm can be improved by more than 400% with the proposed scheme when the number of stations in a basic service set is 10

WODEM: wormhole attack defense mechanism in wireless sensor networks

The wormhole attack, which is accomplished by selectively relaying packets between two adversaries, can ruin routing and communication of the network without compromising any legitimate nodes. There have been a few countermeasures against the wormhole attack in generic ad hoc networks, but they are not appropriate for sensor networks since they require special devices (e.g. directional antenna) or put much overhead on each node. In this paper, we propose a new countermeasure against the wormhole attack for sensor networks, named WODEM. In WODEM, a few detector nodes equipped with location-aware devices and longer-lasting batteries detect wormholes, and normal sensor nodes are only required to forward control packets from the detector nodes. Therefore, WODEM is efficient in cost and energy. From the simulation results, we show that 10 detector nodes can detect a wormhole within the accuracy of 90% in a densely deployed sensor network.

Throughput analysis of IEEE 802.11 WLANs with Automatic Rate Fallback in a lossy channel

This paper presents a new two-step mathematical model which analyzes the throughput of the IEEE 802.11 distributed coordination function (DCF) with the automatic rate fallback (ARF) rate adaptation algorithm. First, the ARF algorithm is modeled as a discrete-time Markov chain and, from the Markov chain, the station distribution among physical rates is obtained. Then, it is fed to the DCF throughput model which takes the backoff mechanism into account. This two-step approach simplifies the overall model by separating the ARF algorithm and the backoff mechanism in modeling.

Collision chain mitigation and hidden device-aware grouping in large-scale IEEE 802.11ah networks

A new IEEE standard for large-scale wireless connectivity in IoT and M2M applications, called IEEE 802.11ah, has recently been introduced. A single access point (AP) of 802.11ah can provide connectivity to a large number of devices (up to 8192) with the communication range of up to 1źkm. Due to such a large coverage area with a large number of connected devices, however, the hidden node problem of 802.11ah networks is severer than typical Wi-Fi networks. Especially, we observe that frequent occurrences of a collision chain results in significant deterioration of network performance even with the group-based access restriction mechanism of 802.11ah. To solve this problem, we propose a collision chain mitigation scheme that detects and interrupts a collision chain, lets a smaller number of devices contend thereafter and also provide the information from which a carrier-sensitivity table is constructed by AP. Although the proposed scheme mitigates performance deterioration due to a collision chain when occurred, collision chains still occur. So, we propose a grouping algorithm which can perform both initial grouping and regrouping from existing groups based on the carrier-sensitivity table constructed by the mitigation scheme so that only a negligible number of hidden devices remain in each group and the root cause of collision chain is obviated. Our simulation study shows that the mitigation scheme alone makes network throughput comparable to the case of no hidden devices and its combination with the grouping algorithm improves throughput performance over the 8-group case of the conventional mechanism by up to 146%.

Revisiting overlapped channels: Efficient broadcast in multi-channel wireless networks

In wireless networks, broadcasting is a fundamental communication primitive for network management and information sharing. However, in multi-channel networks, the broadcast efficiency is very poor as devices are distributed across various channels. Thus, a sender tries all channels for broadcasting a single message, which causes large overhead. In this paper, we propose a novel scheme for efficient broadcast in multichannel networks. Our scheme leverages the overlapped band, which is the frequency range that partially overlapped channels (i.e., adjacent channels) share within their channel boundaries. Specifically, a sender advertises the rendezvous channel through the overlapped band of adjacent channels; the message sharing via broadcast is done on the rendezvous channel. Our scheme employs Signaling via Overlapped Band (SOB), which defines a new signal processing mechanism for communication via the overlapped band. SOB is integrated with MAC layer mechanisms: 1) Reserve Idle Spectrum Fragment (RISF) to reduce waiting time, 2) Reinforce Switch Notification (RSN) to reduce the residing time at a wrong channel, and 3) Multi-sender Agreement on Rendezvous CHannel (MARCH) to support multisender broadcasts. We implemented our scheme on the SORA platform. Experiment results validated communication through the overlapped band. Intensive simulation studies showed that our scheme drastically outperformed previous approach.

Performance analysis of IEEE 802.11 WLANs with rate adaptation in time-varying fading channels

The IEEE 802.11 supports multiple transmission bit rates by using different modulation and coding schemes. Due to different bit error characteristics and transmission efficiencies of the rates, stations may benefit from an adaptive use of them for a varying channel condition, called rate adaptation. The accuracy of rate adaptation is expected to be highly affected by a time varying nature of typical radio channels due to multipath fading. This paper presents an analytic model of the IEEE distributed coordination function (DCF) with the automatic rate fallback (ARF) rate adaptation algorithm, which is the most widely used one in the 802.11 market, under time-correlated Rayleigh fading. The key idea behind the approach is to exploit the first-order Markovian approximation of Rayleigh fading channels, based on which transmission failure probabilities are obtained depending on the current and previous transmission status. By using those probabilities, the ARF process of a station is modeled as a Markov chain, then, the rate distribution obtained by solving the Markov chain is fed to a DCF model. The proposed DCF model is described in a per-station manner, thus enables the analysis of heterogeneous channel conditions and medium access control (MAC) configurations among stations.

Cross-layer analysis of the random access mechanism in Universal Terrestrial Radio Access

In 3GPP Universal Terrestrial Radio Access, a user equipment uses the Random Access Channel (RACH) for uplink transmission if it does not have a pre-established dedicated channel. RACH is a contention-based transport channel in which access control and open-loop power control functionalities are provided under the interactions between Medium Access Control (MAC) and physical layers. This paper develops a Markov chain model to analyze complex cross-layer interactions in RACH procedures, which is crucial to understanding the RACH performance. Besides, the Markov chain accurately describes the time-synchronized RACH behavior in slot level. The presentation of this time-synchronized behavior is simplified by merging multiple sub-states, each of which corresponds to a slot delay, into a single state. Finally, the proposed model analytically evaluates the RACH performance in Rayleigh fading channels in terms of throughput and delay.