Kyoung-Hak Jung

Energy efficient Wifi tethering on a smartphone

While numerous efforts have been made to save energy of “client” devices but it has not been addressed for access points (APs) as they are assumed to be supported by AC power. This paper proposes E-MAP, which is an energy saving algorithm for a tethering smartphone that plays a role of mobile AP (MAP) temporarily. It saves MAPs energy by introducing the sleep cycle as in power save mode (PSM) in 802.11 but successfully keeps clients from transmitting while it sleeps. One important design goal of E-MAP is backward compatibility, i.e., it requires no modification on the client side and supports PSM and adaptive PSM (A-PSM) as well as normal constant awake mode (CAM) clients. Experiments show that E-MAP reduces the energy consumption of a Wifi tethering smartphone by up to 54% with a little impact on packet delay under various traffic patterns derived from real-life traces.

An adaptive collision resolution scheme for energy efficient communication in IEEE 802.15.4 networks

IEEE 802.15.4 provides a widely accepted solution for low-cost and low-power wireless communications, and it is known to be applicable to many types of WSN application scenarios. To enhance energy efficiency and throughput of sensor nodes, many CSMA/CA schemes based on IEEE 802.15.4 have been proposed. However, they still suffer from unnecessary waste of energy and bandwidth due to inefficient backoff management. In order to overcome these problems, we propose an enhancement to existing schemes, called adaptive collision resolution (ACR). ACR appropriately adapts backoff exponent (BE) to the current network contention level based on physical and link-layer measurements. In addition, it adjusts backoff periods (BP) to avoid meaningless backoffs and additional clear channel assessments (CCAs) by using the estimated time remaining until the channel becomes idle. To verify the feasibility and applicability of our scheme, we studied mathematical analysis and tested it on the Matlab and USRP/GNU Radio testbed. We also performed simulation study using OPNET, and our simulation results indicate that ACR shows improved energy efficiency and throughput performance over existing schemes.

A link contact duration-based routing protocol in delay-tolerant networks

Delay Tolerant Networks (DTNs) provide message delivery services to users via intermittently connected nodes. In DTNs, routing is one of the most challenging issues since end-to-end connectivity between nodes may not be available most of the time. Although many routing protocols for DTNs have been proposed, they do not achieve satisfactory performance, since they exploit only some of the network characteristics. In this paper, we present a new DTN routing protocol, called the Link Contact Duration-based Routing Protocol (LCD). Like existing protocols, LCD uses the disconnect duration of a link between two nodes to find the routing path with the shortest end-to-end delay. In addition, LCD uses the contact duration of a link and the number of buffered messages to deliver as many messages as possible in a short time. Our simulation results show that LCD has better performance than existing DTN routing protocols.

Guided Tour of Handoff Steering for Bandwidth in Indoor Venues

With the advancement and development for more than two decades, WiFi has become an indispensable infrastructure in most of indoor venues. Indoor localization using this pre-existing infrastructure has been studied in the literature and is proven practically feasible, which mostly relies on WiFi fingerprints and the corresponding radiomap database. Since the radiomap essentially tells which APs are available and how strong their signals are at positions in the venue, this information can usefully utilized to “steer” to a better AP for the purpose of increasing the user throughput, which we call Guided Tour of Handoff Steering (GUT) in this paper. This steering is not much different from a conventional handoff, but it does not need an expensive scanning process owing to the radiomap, thus some additional handoffs are allowed than conventional handoffs. This leads to higher throughput, because a client can switch to a better AP whenever available. Based on an extensive experiment in a large-scale indoor venue of 119,685m2 with 1,734 APs, it is observed that GUT achieves as much as 84% higher throughput, but the number of handoffs increases only a few. This result supports that the handoff steering is not as expensive as the conventional handoff in terms of latency and traffic.

Joint rate and voltage adaptation to save energy of software radios in underutilized WLAN

This paper proposes an energy-saving bit-rate adaptation algorithm for Software-defined Radio (SDR)-based wireless systems, called Joint Rate and Voltage Fallback (JRF). It exploits the unused network time in underutilized networks by using a lower bit-rate than what an underlying rate adaptation algorithm such as Auto-rate Fallback (ARF) or Robust Rate Adaptation Algorithm (RRAA) dictates. While slow or low bitrate communication potentially consumes more energy due to its extended communication time, it saves energy in SDR due to the possibility of reducing the speed and the voltage of the microprocessor that runs the SDR software. The net result favors a lower rate communication in terms of energy performance. To analyze the tradeoff quantitatively, this paper evaluates the computational complexity of SDR communication software by using BBN 802.11b implementation in GNU Radio. Moreover, for an extensive evaluation, we conducted the simulation study based on OPNET, which shows that JRF improves energy cost by as much as 78.0% compared to No Rate and Voltage Scaling (NoRVS) and as much as 51.3% compared to Independent Rate and Voltage Scaling (IRVS).

Improving IEEE 802.11 PSM for smartphones by exploiting Hidden Tail-time

WiFi interface is a significant contributor to battery draining on mobile device such as WiFi smartphone. Since distinguishing between unwanted packet and active packet is not feasible at MAC layer, the power management mechanism is equally affected by unwanted packets (packets with unreachable destination port). In this paper, we propose a probing and discarding scheme. It forces unwanted packet to be delivered through the “Hidden Tail-time” that is an additional awake time immediately before smartphone switches from awake-state to sleep-state. Thus, unwanted packets never affect the PSM mechanism. Our proposed scheme improves power saving by up to 67% under mixed setting of unwanted packet and active packet.

A contention/collision resolution scheme for large-scale sensor networks based on IEEE 802.15.4

IEEE 802.15.4 has prominent advantages that are the low cost and low energy consumption. It is suitable to several energy critical applications suffered from limited resources. For this reason, many studies proposed energy-efficient solutions, however, they may waste energy caused by the frequent packet collisions since they do not consider the contentions in multi-coordinator environments. In response, we proposed a new contention resolution scheme to enhance the energy efficiency by reducing the contention among nearby coordinators. In the proposed scheme, the coordinators exchange schedules based on the collected information. It leads that the coordinators isolate the superframe duration by staggering own beacon transmission. This can mitigate significant contentions among burst packets at the beginning of the superframe to reduce energy consumption. Simulation results show that the proposed scheme outperforms existing solutions.