Brian Sung Chul Choi

An opportunistic relay protocol for vehicular road-side access with fading channels

In the drive-thru Internet access systems, vehicles connect to road-side access points (APs) to use IP-based services, such as web-browsing, e-mail, and file download, in addition to the customized vehicular applications. However, the mobility of vehicles and the limited coverage of APs result in the short connectivity duration and low throughput, thus leading to low availability of Internet to vehicle services. Vehicle-to-vehicle (V2V) relay support is an attractive backup solution that can address these limitations by extending the coverage. To fully realize the benefit of V2V relay support, however, the vehicle that gives the best performance must be selected as relay, yet the dynamic wireless channel conditions and the high speed of vehicles render relay selection a challenging problem. In this paper, we evaluate several relay strategies in an analytic framework to compute the resulting overall network capacity with fading channels. We then propose and devise an efficient opportunistic relay protocol that exploits multiuser diversity and effectively copes with the dynamic channel. Through both capacity analysis and Qualnet simulations, we show that the opportunistic relay scheme significantly outperforms others.

CCH: Cognitive Channel Hopping in Vehicular Ad Hoc Networks

In this paper, we consider the use of unlicensed Wi-Fi band for vehicular ad hoc networks. In particular, we focus on exploiting channelization to improve spatial reuse, and avoiding interference that is external to the vehicular network. To this end, we propose Cognitive Channel Hopping (CCH), a decentralized channel hopping protocol where nodes select their channels based on cognitively collected channel quality measurements, in a manner that the networks connectivity is maintained. Our evaluation shows that CCH can take advantage of multiple channels that are available to significantly improve the network performance over a single-channel network, and can effectively tolerate external interference.

Wireless Interrupt: Inter-Device Signaling in Next Generation Wireless Networks

Recent advances in wireless technology have led to the proliferation of wireless devices, ranging from wireless LAN to cellular phones. There has been, and will be, an increase in not only the number of wireless services, but also the number of types of wireless services, and greater demand for mobility, portability, and integrated services. For wireless success to continue, technologies must support a high degree of flexibility and interoperability of wireless devices to scale with such demand. Next Generation Wireless will depend highly upon flexible and interoperable devices in constructing a wireless network that consists of heterogeneous devices that can communicate with each other. Our work investigates the technical issues that must be overcome in bringing such flexibility and interoperability, one of which being device discovery. We propose Wireless Interrupt, an inter- device signaling mechanism that can be employed by software defined radios, such that a device can signal its neighbors of its existence or the services it provides without knowing the protocols or channels used by its neighbors a priori.

Cognitive multicast (CoCast) in vehicular networks using OFDM subchannels and network coding

The deployment of wireless multicast in vehicular networks using the ISM band is challenging due to the interference from residential WiFi users. Recently developed cognitive radio techniques can help overcome such interference, with spectrum sensing and multi-channel assignment. CoCast is a cognitive multi-channel multicast protocol originally designed for the urban environment under the assumption that channels are orthogonal. In fact, Wi-Fi channels in the 2.4-GHz ISM band are partially overlapped. In this paper, we remodel the CoCast protocol to handle the more realistic environment where channels are overlapped in frequency. The main challenge is the adjacent channel interference, the effect of which is mitigated in CoCast by employing two techniques: 1) parallel frame transmission over OFDM subchannels, and 2) network coding. Our evaluation shows that the reliability of multicast communication among vehicles in a dense urban environment can be significantly improved with these protocol extensions.

Channel sensing strategy for channel load estimation

The emergence of multi-channel wireless networks and cognitive radio networks has rendered dynamic channel selection an important task, and many existing channel selection schemes factor in the amount of wireless activities that take place in each channel, or channel load, to achieve load balance and maximize the utilization of wireless resources. In such environments, a monitoring node must sense the channel to estimate the channel load, yet when the node is equipped with a single radio interface, in which case the lone interface must be used for both channel sensing and data communication, there can be only a fixed amount of time allotted to channel sensing. In this paper, we show that a careful scheduling of channel sensing is needed to improve the accuracy of channel load estimation, and based on our findings, we devise a sensing strategy that minimizes the estimation error. Evaluation shows that our scheme can reduce the relative estimation error by as much as 40% in a heavily loaded channel environment.

Cog-Fi: A cognitive Wi-Fi Channel Hopping architecture for urban MANETs

The success of wireless networks has increased the competition for wireless resources in unlicensed bands by a plethora of wireless devices. Mobile devices that operate in the unlicensed band and are not semi-permanently associated with an access point (either residential or business) find it increasingly difficult to compete in the crowded spectrum. An interesting solution is to equip the mobiles with cognitive radios and allow them to identify the set of least occupied channels and hop across them. In our previous work, we have developed the Cognitive Channel Hopping (CCH) protocol, which utilizes a subset of multiple lightly loaded channels, avoiding channels with heavy external interference. While any routing protocol can work with CCH, conventional protocols like DSR and AODV are not well-suited for CCH networks, as they are not designed to work with multiple channels (in the same MANET) and do not take into account other factors that affect the network performance, such as link rates and channel load. In this work, we define a network architecture Cog-Fi, which exploits CCH over multiple channels in MANETs. We devise a routing protocol called CH-LQSR, which further increases the network performance when used in conjunction with CCH. Our evaluation shows that Cog-Fi can exploit channel diversity without explicit coordination. It outperforms conventional approaches in MANET scenarios with multiple flows and heavy external interference.