Byung-Seo Kim

Downlink and uplink resource allocation in IEEE 802.11 wireless LANs

Wireless local area networks (WLANs) based on the IEEE 802.11 standard are becoming increasingly popular and widely deployed. It is likely that WLAN will become an important complementary technology for future cellular systems and will typically be used to provide hotspot coverage. In this paper, the complementary use of WLANs in conjunction with mobile cellular networks is studied. We identify the fairness problem between uplink and downlink traffic flows in the IEEE 802.11 distributed coordination function and then propose an easy solution that can be implemented at the access point (AP) in the MAC layer without modification of the standard for stations (STAs). This solution aims at providing a controllable resource-allocation method between uplink and downlink traffic flows and adapting the parameters according to the dynamic traffic load changes. The proposed solution also enhances the system utilization by reducing the probability of frame collision.

Two-step multipolling MAC protocol for wireless LANs

The IEEE 802.11 standard defines two coordination functions: distributed coordination function (DCF) and point coordination function (PCF). These coordination functions coordinate the shared wireless medium. The PCF uses a centralized polling-based channel access method to support time-bounded services. To design an efficient polling scheme, the point coordinator (PC) needs to obtain information about the current transmission status and channel condition for each station. To reduce overhead caused by polling frames, it is better to poll all stations using one polling frame containing the transmission schedule. In this paper, we propose an efficient polling scheme, referred to as two-step multipolling (TS-MP), for the PCF in wireless local area networks (WLANs). In this new scheme, we propose to use two multipolling frames with different purposes. The first frame is broadcast to collect information such as the numbers of pending frames and the physical-layer transmission rates for the communication links among all stations. The second frame contains a polling sequence for data transmissions designed based on the collected information. This frame is broadcast to all stations. Extensive simulation studies show that TS-MP not only overcomes the aforementioned deficiencies, but also help to implement rate adaptation over time-varying wireless channel.

Throughput enhancement through dynamic fragmentation in wireless LANs

Many rate-adaptive MAC protocols have been proposed in the past for wireless local area networks (LANs) to enhance the throughput based on channel information. Most of these protocols are receiver based and employ the RTS/CTS collision avoidance handshake specified in the IEEE 802.11 standard. However, these protocols have not considered the possibility of bursty transmission of fragments in the corresponding rate adaptation schemes. In this article, a rate-adaptive protocol with dynamic fragmentation is proposed to enhance the throughput based on fragment transmission bursts and channel information. Instead of using one fragmentation threshold in the IEEE 802.11 standard, we propose to use multiple thresholds for different data rates so more data can be transmitted at higher data rates when the channel is good. In our proposed scheme, whenever the rate for the next transmission is chosen based on the channel information from the previous fragment transmission, a new fragment is then generated using the fragment threshold for the new rate. In this way, the channel condition can be more effectively used to squeeze more bits into the medium. We evaluate this scheme under a time-correlated fading channel model and show that the proposed scheme achieves much higher throughput than other rate-adaptive protocols.

OFDMA-Based Reliable Multicasting MAC Protocol for WLANs

Although many wireless communication standards include multicast as well as unicast traffic in layer 2, most parts of the specifications are limited to unicast methods. In addition, although the existing wireless standards provide reliable unicast methods using an automatic repeat request (ARQ) and a retransmission, multicast packets are not reliably transmitted using these standards. To retransmit a multicast packet, the acknowledgements (ACKs) from the receivers need to be received by the sender to determine whether a retransmission is required. However, transmitting an ACK from each group member degrades the network performance due to the overhead induced by multiple ACK packet transmissions and channel access processes for the ACKs. To solve this problem, in this paper, a medium-access control protocol called the orthogonal frequency division multiplex access (OFDMA) based multicast ACK (OMACK), with minimum ACK overhead over wireless local area networks, is proposed. The proposed scheme uses one orthogonal frequency division multiplex (OFDM) symbol for the ACKs from all member stations (STAs), and each member STA indicates its packet reception status by utilizing a subcarrier within the OFDM symbol. The proposed scheme is thoroughly examined by using simulation and theoretical methods, and the results show that it significantly reduces the aforementioned overhead and, as a consequence, improves the performance of wireless networks.

Rate-adaptive MAC protocol in high-rate personal area networks

The specification of high-rate wireless personal area network (HR WPAN) has been standardized by the IEEE 802.15.3 task group for communications of consumer electronics and portable communication devices and a final draft standard has been completed. The physical layer in IEEE 802.15.3 standard is designed to achieve data rates of 11-55Mbps. However, a MAC protocol in IEEE 802.15.3 standard does not specify the method to choose an appropriate data rate. In this paper, we propose a rate-adaptive medium access control (MAC) protocol for HR WPAN. The data rate for the next transmission is selected by channel prediction based on the currently received frame and informs the sender about the changed rate using a rate-adaptive acknowledgement (RA-ACK) frame. By overhearing the RA-ACK frame, a piconet controller can efficiently allocate channel times. In addition, we propose a constant physical layer frame length regardless of a data rate. In this way, the channel can be more effectively utilized by squeezing more bits into one transmission. The proposed scheme is evaluated under a time-correlated fading channel model in terms of the achieved throughput Simulation results show that this scheme achieves a much higher throughput than a non rate-adaptive MAC protocol in HR WPAN does.

Adaptive Multirate Auto Rate Fallback Protocol for IEEE 802.11 WLANS

An adaptive multirate auto rate fallback (AMARF) protocol is proposed for IEEE 802.11 WLANs. The key idea is to assign each data rate a unique success threshold, which is a criterion to switch one rate to the next higher rate, and the success thresholds can be changed dynamically in an adaptive manner according to the running conditions, such as packet length and channel parameters. Moreover, the proposed protocol can be implemented without any change to the current IEEE 802.11 standards. Our in-depth simulation shows that AMARE yields significantly higher throughput than other existing schemes including auto rate fallback (ARF) scheme and its variants, in various running conditions

An Efficient MAC Protocol for Improving the Network Throughput for Cognitive Radio Networks

The limitation of spectrum has become a major bottleneck of the development of next generation radio system. One of the key challenges of the emerging opportunistic cognitive radio is how to utilize unused spectrum holes efficiently without interference to incumbent system. Assuming that channel status is known, in this paper, we propose a decentralized adaptive medium access control (AMAC) protocol that has no dedicated global common control channel (CCC) and can utilize available resources efficiently. In AMAC, each cognitive radio maintains channel status table and indexes them frequently. The channel that has more probability to be stable is ranked channel one. The most reliable common channel between communicating pair is selected as a CCC. We proposed two data communication mode according to the channel condition. The simulation results show that the proposed decentralized adaptive medium access control (AMAC) protocol significantly increases cognitive radio networks (CRNs) connectivity and data throughput.

Dynamic fragmentation scheme for rate-adaptive wireless LANs

Many rate-adaptive MAC protocols have been proposed in the past for wireless LANs over time-varying media to provide efficient channel usage and throughput enhancement. Most of these protocols are receiver-based and employ RTS/CTS collision avoidance handshakes specified in the distributed coordination function (DCF) of the IEEE802.11 MAC. However, these protocols do not consider the possible bursty transmission of fragments in a rate-adaptive fashion. In this paper, a rate-adaptive protocol with a dynamic fragmentation scheme is proposed in order to enhance the throughput by employing bursty fragment transmissions based on the channel condition. Instead of using one fragmentation threshold, multiple thresholds for different data rates are proposed so that more data can be transmitted at higher data rates when the channel is good. In the fragmentation scheme of IEEE802.11, all fragments are generated with equal size at the same time, and all fragments will not change until they are transmitted. In our proposed scheme, whenever the rate for the next transmission is chosen based on the channel condition estimated from the previous fragment transmission, a new fragment is generated using the fragment threshold for the new rate. In this way, the channel can be more effectively utilized to squeeze in more bits. This scheme is evaluated under a time-correlated fading channel model in terms of the achieved throughput. Simulation results show that this scheme achieves a much higher throughput than other rate-adaptive protocols do.

QoS-Aware and Heterogeneously Clustered Routing Protocol for Wireless Sensor Networks

Wireless sensor networks (WSNs) have gained much attention in today’s research domain for supporting a wide variety of applications including the multimedia applications. Multimedia applications that are regarded as the quality-of-service (QoS)-aware, delay sensitive, and bandwidth hungry applications require enough energy and communication resources. WSNs being the energy-scarce network have now been designed in such a way that they can support these delay-sensitive and time-critical applications. In this paper, we propose an energy-efficient routing protocol for heterogeneous WSNs to support the delay sensitive, bandwidth hungry, time-critical, and QoS-aware applications. The proposed QoS-aware and heterogeneously clustered routing (QHCR) protocol not only conserves the energy in the network, but also provides the dedicated paths for the real-time and delay sensitive applications. The inclusion of different energy-levels for the heterogeneous WSNs also provides the stability in the networks while minimizing the delay for the delay-sensitive applications. Extensive simulations have been performed to validate the effectiveness of our proposed scheme. Our proposed routing scheme outperforms other state-of-the-art schemes in terms of the delay performances.

NDN-CRAHNs: Named Data Networking for Cognitive Radio Ad Hoc Networks

Named data networking (NDN) is a newly proposed paradigm for future Internet, in which communication among nodes is based on data names, decoupling from their locations. In dynamic and self-organized cognitive radio ad hoc networks (CRAHNs), it is difficult to maintain end-to-end connectivity between ad hoc nodes especially in the presence of licensed users and intermittent wireless channels. Moreover, IP-based CRAHNs have several issues like scalability, inefficient-mapping, poor resource utilization, and location dependence. By leveraging the advantages of NDN, in this paper, we propose a new cross layer fine-grained architecture called named data networking for cognitive radio ad hoc networks (NDN-CRAHNs). The proposed architecture provides distinct features such as in-networking caching, security, scalability, and multipath routing. The performances of the proposed scheme are evaluated comparing to IP-based scheme in terms of average end-to-end delay and packet delivery ratio. Simulation results show that the proposed scheme is effective in terms of average contents download time and packet delivery ratios comparing to conventional cognitive radio ad hoc networks.