Hyeonmok Ko

Fast Optimal Discrete Bit-Loading Algorithms for OFDM-Based Systems

Adaptive bit allocation for Orthogonal Frequency Division Multiplexing (OFDM)-based systems are critical for avoiding performance degradation due to additional bit allocations when channel conditions are bad. In this paper, we present two novel bit-loading algorithms for OFDM-based systems with target bit rate and fixed energy constraints. The proposed algorithms converge to the same bit allocation as the optimal discrete bit-filling and bit-removal algorithms. The proposed algorithms can approach a near-optimal bit allocation with less computational steps by using a water-filling solution at the initial bit allocation step, and then a multiple-bit loading procedure to satisfy the target bit rate. The proposed algorithms were evaluated by comparing with bit-filling and bit-removal algorithms through numerical analysis. Numerical results demonstrate the computational efficiency, fast convergence and optimality of the proposed algorithms.

The Optimal Spectrum Sensing Time for Maximizing Throughput of 802.11-Based MAC Protocol for Cognitive Radio Networks Under Unsaturated Traffic Conditions

Cognitive radio has attracted considerable attention as an enabling technology for addressing the problem of radio frequency shortages. In cognitive radio networks (CRNs), secondary users (SUs) are allowed to opportunistically utilize the licensed spectrum bands of primary users (PUs) when these bands are temporarily unused. Thus, SUs should monitor the licensed spectrum bands to detect any PU signal. According to the sensing outcomes, SUs should vacate the spectrum bands or may use them. Generally, the spectrum sensing accuracy depends on the sensing time which influences the overall throughput of SUs. That is, there is a fundamental tradeoff between the spectrum sensing time and the achievable throughput of SUs. To determine the optimal sensing time and improve the throughput of SUs, considerable efforts have been expended under the saturated traffic and ideal channel assumptions. However, these assumptions are hardly valid in practical CRNs. In this paper, we provide the framework of an 802.11-based medium access control for CRNs, and we analyze this framework to find the optimal spectrum sensing time under the saturated and unsaturated traffic condition. Through simulation, the proposed analytic model is verified and the fundamental problem of the sensing-throughput tradeoff for CRNs is investigated.

A joint approach to bandwidth allocation and AP-client association for WLANs

Recent empirical studies have shown that clients gather together in popular locations, which results in load-imbalance, in a network. One of the key wireless LANs (WLANs) challenges is alleviating the load-imbalance while maximizing the bandwidth utilization. However, the IEEE 802.11-based standard does not provide any formal solution to the load-imbalance problem. Several load-balancing schemes have been proposed. Unfortunately, most of them are of limited-benefit under only certain network conditions. In this paper, we propose a joint approach adopting both dynamic-association and adaptive channel-width to enhance the system performance but significantly reducing computational complexity involved. We show that the proposed approach outperforms any individual approaches without increased computational burden. Furthermore, performance measure obtained using the proposed approach is very close to the best result obtained by using an exhaustive search.

Simple Bit Allocation Algorithms with BER-Constraint for OFDM-Based Systems

Orthogonal Frequency Division Multiplexing (OFDM) has been thoroughly investigated as an enabling technology for future broadband multimedia communication. In this paper, we suggest adaptive bit allocation algorithms that operate in a frequency selective fading channel environment by exploiting channel state information obtained through a feedback channel. The proposed algorithms try to maximize the overall throughput of the system with significantly reduced complexity while guaranteeing that mean bit error rate (BER) of all sub-channels remains below the pre-defined BER threshold. To do that, the first proposed algorithm divides all sub-channels into several groups according to their signal-to-noise ratio (SNR). The grouping criteria are adaptive to the current channel state and BER constraint. The second algorithm tries to find the appropriate constellation size for each sub-channel. The proposed algorithms were compared with existing algorithms through simulation. The simulation results show that the proposed algorithms are close to optimum solution with significantly lower complexity.

Fast Primary User Detection during Ongoing Opportunistic Transmission in OFDM-based Cognitive Radio

Cognitive radio requires that a secondary user (SU) stops its transmission as soon as possible upon the arrival of a primary user (PU). In this paper, we propose a novel approach to quickly detect PUs while an SU transmission is in progress. The proposed method requires an SU transmitter to use OFDM to insert periodic zero-energy intervals in a selected subcarrier and to detect energy during each interval for PU detection. The length and periodicity of these intervals determine the performance of PU detection. We also discuss how to determine the energy detection threshold.

Distributed Reservation-Based MAC Protocol for UWB WiMedia MAC

UWB WiMedia MAC has been widely used in wireless multimedia networks, but it suffers from inefficient channel utilization. In this paper, we propose a reservation based MAC protocol for multimedia data to increase the channel utilization efficiency. The proposed scheme has two components. First, the protocol lets flows with large amounts of data be reserved early among the nodes. Second, when anode allocates slots, the protocol uses wasted slots. We evaluate the performance of the proposed scheme through simulations that compares channel utilization and drop probability with an existing scheme. The simulation results show that the measured performance metrics are improved.

OSLMAC: One-step look-ahead MAC protocol for concurrent transmission over wireless ad hoc networks

The IEEE 802.11 DCF mechanism is a basic ad hoc MAC protocol. This protocol, however, is prone to the hidden terminal problem. To solve this problem, the incorporation of the request-to-send/clear-to-send (RTS/CTS) mechanism has been suggested; however, this can in turn result in the exposed terminal problem. Concurrent transmission can reduce the exposed terminal problem. The use of concurrent-transmission-based medium access control (MAC) protocols has been considered as a means of improving throughput in wireless ad hoc networks. In this paper, we propose a one-step look-ahead MAC (OSLMAC) protocol that uses a back-off reservation mechanism to increase throughput and utilizes control messages, which are required for concurrent transmission of messages. Currently, where a concurrent transmission state is not possible, redundant overhead comprising exchanged control messages is incurred. However, our proposed mechanism utilizes these wasted control messages to reserve the next data transmission. Thus, this mechanism decreases the number of collisions and reduces back-off waiting times. Evaluation of the performance of our proposed scheme, by means of simulations and comparison of its throughput with those of an existing scheme and the basic IEEE 802.11 DCF MAC, indicate that the measured performance metrics are improved when our proposed scheme is used.

IO---MAC: An Enhancement of IEEE 802.11 DCF Using Implicit Ordering

In this paper, we present a novel medium access control protocol which is based on the IEEE 802.11 distributed coordination function (DCF) and embeds new capability allowing stations to make use of implicit ordering for transmissions. Using them, stations may avoid random access attempts. Implicit ordering is assumed without any control messages and enables contention-free transmissions. We evaluate the performance of the proposed protocol by simulation and show that the proposed protocol outperforms both the legacy DCF and the ideal DCF with respect to the channel utilization.

A directional MAC for switchable directional antennas to enhance throughput in crowded infrastructure wireless networks

In conventional infrastructure wireless networks, access points and nodes use omni-directional antennas. In a crowded situation, users have experienced throughput degradation due to various reasons. In this paper, we propose wireless LANs equipped with switchable directional antennas and a directional MAC protocol designed for them. In the proposed wireless LAN, every node including access point is always engaged in directional transmission and reception, but a node is allowed to receive omni-directionally only for association process. This new system enhances throughput in overlapped multiple basic service sets by reducing interference, increasing spatial reuse, and avoiding transmission loss phenomenon TLP. The simulation results show that the proposed protocol outperforms conventional IEEE 802.11 DCF, in terms of throughput in crowded infrastructure wireless networks.

Distributed Medium Access Scheduling for Wireless LANs

The contention-based Distributed Coordination Function (DCF) scheme performs poorly due to collisions and idle back-off slots. Continuous efforts to improve its performance have rarely been successful because collisions and idle periods are inherent in DCF. This paper proposes a distributed medium access protocol called Distributed Medium Access Scheduling (DMAS), which enables the elimination of most collisions and idle slots in a distributed manner. The stations used for this purpose were divided into blue stations, which have the exclusive right to transmit during their turn, and red stations, which have the opportunity to transmit without overhead. The performances of DMAS, DCF, and the ideal DCF were compared through simulations. The results indicate that DMAS could be used to replace Medium Access Control (MAC) in IEEE standard 802.11 for Wireless LANs with higher throughput.