Wun-Cheol Jeong

BER analysis of arbitrary rectangular QAM

M-ary quadrature amplitude modulation (M-ary QAM) is an attractive technique to achieve high rate transmission without increasing the bandwidth in wireless communication systems. A great deal of attention has been devoted to the study of bit error rate (BER) performance of M-ary square QAM constellations. Approximate expressions for bit error probabilities of M-ary square QAM have recently been developed by J.J. Lu et al. (1999) and L. Yang and L. Hanzo (2000) based on signal-space concepts and recursive algorithms, respectively. However, it is only recently that an exact generalized closed form expression for the BER of M-ary square QAM constellation has been developed (see Yoon, D. et al., IEEE VTC Fall 2000, vol.5, p.2422-7, 2000). We extend that previous work of ours to an arbitrary I/spl times/J rectangular QAM (R-QAM) scheme. A general closed form expression for the exact BER performance of an arbitrary I/spl times/J R-QAM with Gray code mapping is derived and analyzed under an additive white Gaussian noise (AWGN) channel. The BER performance is also analyzed under fading channels. It is shown that there is a noticeable deviation in the BER performance between the approximation and exact expression over low SNR values.

Performance analysis of IEEE 802.15.4e DSME MAC protocol under WLAN interference

In this paper, we analyze the performance of IEEE 802.15.4e DSME-enabled wireless sensor network (WSN) and IEEE 802.15.4 beacon-enabled WSN employing slotted CSMA/CA in terms of frame error rate and aggregate throughput under the influence of IEEE 802.11b wireless LAN (WLAN) interference. The main purpose of this paper is to provide helpful information in the design of DSME-enabled WSNs in the presence of WLAN interference. Our numerical results show that DSME MAC protocol is resilient to large variance of traffic load conditions (Λwlan) and interference power level of WLAN. It is observed that the dominant interference source is homogeneous WSN devices in IEEE 802.15.4 WSN employing slotted CSMA/CA for small value of Λwlan, while WLAN devices are always the dominant interference sources in DSME-enabled WSNs regardless of Λwlan. Meanwhile, it is observed that DSME-enabled WSN retains high aggregate throughput value for large range of interference power level, while the aggregate throughput of IEEE 802.15.4 WSN employing slotted CSMA/CA degrades severely as the WLAN device approaches to WSN devices.

Design and implementation of an architectural framework for web portals in a ubiquitous pervasive environment.

Web Portals function as a single point of access to information on the World Wide Web (WWW). The web portal always contacts the portal’s gateway for the information flow that causes network traffic over the Internet. Moreover, it provides real time/dynamic access to the stored information, but not access to the real time information. This inherent functionality of web portals limits their role for resource constrained digital devices in the Ubiquitous era (U-era). This paper presents a framework for the web portal in the U-era. We have introduced the concept of Local Regions in the proposed framework, so that the local queries could be solved locally rather than having to route them over the Internet. Moreover, our framework enables one-to-one device communication for real time information flow. To provide an in-depth analysis, firstly, we provide an analytical model for query processing at the servers for our framework-oriented web portal. At the end, we have deployed a testbed, as one of the world’s largest IP based wireless sensor networks testbed, and real time measurements are observed that prove the efficacy and workability of the proposed framework.

A Centralized Hybrid MAC Protocol for Wireless Sensor Networks

A centralized hybrid MAC protocol for wireless sensor networks (WSNs) is proposed, where the design objective is to be able to provide reliable quality of services (QoS) control, high throughput, and scalability to networks by utilizing a priority-based hybrid time-coordinated contention and contention-free medium access control (MAC) protocol. The proposed protocols resource reservation scheme can provide guarantee throughout access for selected users even under extremely emergent high demand situations, and applies a periodic shutdown scheme for low-power consumption.

Enhanced broadband wireless networking through macroscopic diversity combining applications of MIMO technology

Multiple-input multiple-output (MIMO) systems can enable drastic increments in channel capacity by employing array antennas both at the transmitter and the receiver. However, the capacity of MIMO systems degrades severely when spatial correlation among multipath channels is present. In this paper, we show that the channel capacity is dominantly affected by shadowing, rather than multipath correlation. To overcome this adverse shadowing effect, we consider macroscopic selection diversity (MSD) scheme on top of MIMO technology, and provide an analysis of capacity outage probability for MSD-MIMO systems. The numerical results show that significant improvement is observed even with 2 base station (BS) diversity. It is also observed that the effect of spatial correlation due to multipath fading is almost negligible when multiple BSs cooperatively participate in the mobile communication scheme. Combined with the BS selection scheme, the proposed MSD-MIMO system overcomes adverse shadowing phenomena while obtaining spatial multiplexing gain in MIMO channels.

Performance analysis of macroscopic diversity combining of MIMO signals in mobile communications

In this paper, the system model and performance analysis of macroscopic diversity combining (MDC) of multiple-input multiple-output (MIMO) systems are presented for mobile cellular communications. For point-to-point dual antenna array (DAA) systems applying directional beam forming, if the antenna spacing is insufficient or the scattering environment does not provide completely uncorrelated channels then the MIMO system capacity deteriorates. In addition, the shadowing component of the directional signal is a common factor among the scattered channels resulting in significant reductions in obtainable channel capacity. Therefore, in this paper, a MDC topology is proposed to enable the mobile terminals (MTs) and base stations (BSs) to adaptively select the most uncorrelated MIMO channels to obtain higher data rates and lower signal outage. The channel capacity as well as its upper and lower bounds are derived for the MIMO-based MDC system. Compared to a single communicating MIMO systems pair, the results show that the macroscopic diversity MIMO communication topology enables a larger numbers of uncorrelated shadowed and scattered channels to exist, and therefore, significant improvements of enhanced channel capacity and reduced outage can be obtained.

Jitter analysis of homogeneous traffic in wireless differentiated services networks

A critical challenge for wireless networking vendors and cellular/mobile phone carrier companies is to be able to accurately estimate the quality of service (QoS) that can be provided based on the network architecture, router/switch topology, and protocol applied. In addition, due to the development of technologies like multiprotocol label switching (MPLS) and generalized MPLS (GMPLS), which can deploy differentiated services (DiffServ), the variation in QoS performance based on the priority assignment is of significant importance. This paper provides a call admission control (CAC) scheme and a theoretical analysis of delay jitter of homogeneous traffic based on a nonpreemptive head-of-the-line (HOL) priority scheme in wireless networks that apply automatic retransmission request (ARQ) to recover the packet errors. The CAC scheme regulates the lower class traffic to provide the required levels of delay jitter for the higher priority classes.

A multi-channel timeslot scheduling algorithm for link recovery in wireless multi-hop sensor networks

In this paper, we propose a TDMA scheduling algorithm to provide on-time packet delivery in wireless multi-hop sensor networks. In multi-hop environments, scheduling of timeslot is optimized on end-to-end basis. However, the end-to-end transmission is unavailable if a scheduled link on the communication path is failed by wireless interferences and/or device functional errors. We exploit substitute communication paths to avoid the link failures. Our proposed algorithm schedules communication links on the substitute path by using dedicated timeslots in the frame. The max-min optimization is used to maximize the number of dedicated timeslots to establish substitute paths. We evaluate the performance of our proposed algorithm using QualNet network simulator. The simulation results show that our proposed algorithm achieves higher path survival ratio while satisfying end-to-end delay bound. Especially, our algorithm recovers communication paths eleven times more than a previous work when all communication links in the network are failed.

Spectral efficiency of time division duplex fixed wireless cellular system for dynamic traffic

We analyze the spectral efficiency of a dynamic time division duplex system in a fixed wireless cellular network. Conventionally, spectral efficiency has been analyzed under static and fully-loaded conditions. We investigate how asymmetric and dynamic traffic affects TDD system spectral efficiency. Dynamic TDD (D-TDD) is an efficient duplex scheme for high-speed data communications; it exploits dynamic and asymmetric data traffic. However, cochannel interference can be present, so the benefits of dynamic TDD may not be justifiable, unless a proper countermeasure is employed. To suppress the effects of strong cochannel interference, we employ a time slot allocation (TSA) strategy along with a sector antenna layout (Jeong, W.-C. and Kavehrad, M., IEEE Trans. Commun., vol.50, no.10, p.1627-36, 2002). By employing the TSA strategy, 9 times higher spectral efficiency is obtained in D-TDD systems. We also evaluate the spectral efficiency of a D-TDD system employing adaptive modulation, assuming traffic is delay tolerant. By employing adaptive modulation, 26 times higher spectral efficiency can be obtained. The effect of frame loading condition is also evaluated. When the TSA algorithm is employed in a D-TDD system employing a conventional fixed modulation scheme, statistical multiplexing gain can be obtained for a fairly wide range of frame loading, compared to a static TDD system.

Multi-channel TDMA link scheduling for wireless multi-hop sensor networks

In this paper, we propose the conflict-free TDMA link scheduling algorithm to minimize slot length in multi-channel operation wireless multi-hop sensor networks. We exploit conflict graph model to schedule conflict-free transmissions for all links in the network. The min-max optimization is used to find minimum slot length including all link transmissions. Our proposed algorithm also minimizes the end-to-end delay by ordering of link transmissions. We evaluate the performance of our proposed algorithm in terms of required slot length and energy consumption in IEEE802.15.4e multi-superframe structure. Our numerical results show that the improvement of performance is noticeable compare with previous works while consumes least energy to deliver the frame on the multi-hop communication in large networks.