Wooyeol Choi

Coordinating transmit power and carrier phase for wireless networks with multi-packet reception capability

Driven by advances in signal processing and multiuser detection (MUD) technologies, it has become possible for a wireless node to simultaneously receive multiple signals from other transmitters. In order to take full advantage of MUD in multi-packet reception (MPR) capable wireless networks, it is highly desirable to make the compound signals from multiple transmitters more separable on its constellation at the receiver by coordinating both the transmit power level and carrier phase offsets of the transmitters. In this article, we propose a feedback-based transmit power and carrier phase adjustment scheme that estimates the symbol energy and the carrier phase offset for each transmitter’s received signal, computes the optimal received power level and carrier phase shift to maximize the minimum Euclidean distance between the constellation points, and finally feeds the optimal transmit power level and phase shift information back to the transmitters. We then evaluate the performance of the proposed transmit power and carrier phase adjustment scheme and subsequently show that the proposed scheme significantly reduces the error probability in a multiuser communication system having MPR capability.

Power-Controlled Medium Access Control Protocol for Full-Duplex WiFi Networks

Recent advances in signal processing have demonstrated in-band full-duplex capability at WiFi ranges. In addition to simultaneous two-way exchange between two nodes, full-duplex access points can potentially support simultaneous uplink and downlink flows. However, the atomic three-node topology, which allows simultaneous uplink and downlink, leads to inter-client interference. In this paper, we propose a random-access medium access control protocol using distributed power control to manage inter-client interference in wireless networks with full-duplex-capable access points that serve half-duplex clients. Our key contributions are two-fold. First, we identify the regimes in which power control provides sum throughput gains for the three-node atomic topology, with one uplink flow and one downlink flow. Second, we develop and benchmark PoCMAC, a full 802.11-based protocol that allows distributed selection of a three-node topology. The proposed MAC protocol is shown to achieve higher capacity as compared to an equivalent half-duplex counterpart, while maintaining similar fairness characteristics in single contention domain networks. We carried out extensive simulations and software-defined radio-based experiments to evaluate the performance of the proposed MAC protocol, which is shown to achieve a significant improvement over its half-duplex counterpart in terms of throughput performance.

An opportunistic forwarding protocol with relay acknowledgment for vehicular ad hoc networks

Robust and efficient data delivery in vehicular ad hoc networks (VANETs) with high mobility is a challenging issue due to unstable wireless links and dynamic topology changes. In this vehicular environment, the exchanges of topology information and discoveries of routing paths on an end-to-end basis are not desired, because the network topology changes rapidly and the information is out-of-date in short. In this paper, we propose a contention based forwarding mechanism, which locally exploits geographic location information in order to achieve robustness of data delivery for vehicular communications. A node that is closer to the destination node has a higher priority in competing for the wireless channel, and forwards packets as a relay node earlier than other adjacent nodes. When a packet is forwarded by a relay node, its preceding node informs adjacent nodes that it has been already relayed to prevent the duplications of packet delivery. Through ns-2 simulations, we show that the proposed scheme can significantly improve the network performance in terms of the packet delivery ratio and the end-to-end delay in various synthetic and trace-based realistic VANET scenarios with a wide range of vehicle speed and density. Copyright

Power control for multiple access communication systems with multi-packet reception capability

This paper addresses the power control problem in wireless multiple access communication systems with a multi-packet reception (MPR) channel. Driven by advances in signal processing and multiuser detection (MUD) technologies such as UWB and MIMO, it has become possible for a wireless node to simultaneously receive several packets from other transmitters. In this case, the receiver node can determine the set of transmitters from which it wants to receive data and instruct them to adjust their transmit power levels in order to maximize the overall network capacity by exploiting the MPR capability. We first show that the efficiency of the MPR channel utilization can be maximized by equalizing the transmission durations of simultaneously transmitting nodes, and then propose a power control that increases/decreases the transmit power of the nodes to equalize the transmission durations. Under the proposed power control, a node with a long transmission duration is allowed to use a higher transmit power if the next higher transmission rate is achievable with the increased transmit power. Through extensive simulations, we show that the proposed power control significantly improves the overall network capacity in multiple access.

Fluorescence spectrum-based biofouling prediction method for RO membrane systems

Abstract Monitoring reverse osmosis (RO) membrane conditions is an important task because it helps reduce the operation and maintenance cost in the RO membrane desalination systems by achieving long membrane lifetime and energy saving. As biological interactions between the membrane itself and microorganism cause the rapid degradation of membrane performance, it is crucial to identify and quantify potential biofoulants that are sensitive to each specific RO membrane. This study proposed a biofouling prediction method that indirectly quantifies the degree of biofouling by comparing the fluorescence excitation-emission matrix (EEM) of foulants sampled on the fully fouled RO membrane and those of brine samples from currently operating RO system. The experiment showed that the similarity distance measured from the comparison between the two fluorescence EEMs tends to increase when brine samples were secured from relatively clean RO membranes.

A forwarding protocol with relay acknowledgement for vehicular ad-hoc networks

In vehicular ad-hoc networks (VANETs), mobility is one of key characteristics that have a significant effect on routing performance because it causes dynamic changes of network topology. In this paper, we propose a contention based forwarding protocol that exploits geographic or virtual location information. A node that is closer to the destination node has a higher priority in competing for the wireless channel, and forwards packets as a relay node earlier than other adjacent nodes. When a packet is forwarded by a relay node, its preceding node informs adjacent nodes that it has been already relayed to prevent duplication of packet delivery. Through ns-2 simulations, we show that the proposed scheme can significantly improve the network performance in terms of the packet delivery ratio and the end-to-end delay in various VANET scenarios.

Immediate acknowledgement for single-channel full-duplex wireless networks

Driven by advances in the signal processing and antenna technology, it has become possible for wireless nodes to simultaneously transmit and receive a packet through self-interference cancellation using multiple antennas. This is known as a full-duplex communication. In this paper, we propose a MAC protocol with immediate acknowledgement (ACK) for single-channel full-duplex wireless networks. The proposed scheme in a full-duplex communication can solve the hidden terminal problem by using the immediate ACK. That is, the proposed scheme ensures that the transmission of a packet that is being correctly received is continued, and prevents the transmission of an already failed packet. To evaluate the performance of the proposed scheme, we carried out extensive simulations and subsequently showed that the proposed scheme significantly improves the network throughput.

Antenna allocation scheme for full-duplex communication in IEEE 802.11ac WLAN

We proposed an antenna allocation scheme for a full-duplex communication in IEEE 802.11ac WLAN. In the proposed scheme, an access point (AP) with multiple antennas finds the optimal number of the antennas for transmitting and receiving signals to maximize the sum capacity of the uplink and downlink transmissions. We carry out a simulation and show that the proposed scheme can improve the sum capacity in the full-duplex wireless networks.

Admission control for wireless LANs with multipacket reception capability

In a multipacket reception MPR system, a wireless node can successfully receive multiple packets from simultaneous transmitters. The increase in the number of simultaneous transmissions leads to higher throughput as long as the transmissions can be successfully decoded by the receiver. The maximum number of simultaneous transmissions that the receiver can successfully decode should be determined and used as the criterion of admission control in order to maximise the multipacket reception system performance. In this paper, we propose an admission control scheme that derives the maximum number of simultaneous transmissions and regulates the number of simultaneous transmissions based on the derived number. To evaluate the performance of our proposed scheme, we carry out extensive simulations and show that our proposed scheme significantly improves the network throughput. Copyright

Fluorescence imaging for biofoulants detection and monitoring of biofouled strength in reverse osmosis membrane

Biofouling is a crucial issue, and it causes seawater reverse osmosis membrane to deteriorate the performance of desalination. In this study, excitation emission matrix (EEM) fluorescence spectroscopy and parallel factor analysis (PARAFAC) were used to monitor the strength of biofouling on the fouled membrane which was obtained from real plant. Based on EEM and PARAFAC results of raw seawater, feed water, permeate, brine and fouled membrane, three components were identified as the major peaks: (1) microbial product-like materials at Ex/Em = 280/370 nm, (2) humic-like substances at Ex/Em = 330/420 nm, and (3) aromatic proteins at Ex/Em = 240/320 nm. Using the fluorescence intensity changes, the effects of replacing fouled RO membranes were found to be most significant at one of the components (Ex/Em = 270–300/350–380 nm) which could be considered the substances desorbed from fouled RO membrane. Compared to the data for salt rejection, this component monitoring of the brine EEM image is shown to be more sensitive than conductivity monitoring for predicting the biofouling strength during the desalination process.