Video multicast over wireless local area network (WLAN) has been gaining attraction for applications sharing a venue-specific common video with multiple users. However, wireless multicast is limited by a receiver that has the weakest communication link to the source. Collaborative relaying could overcome this challenge by enabling selected receiver nodes to relay the packets from the source to other receivers. We propose EV-CAST, an interference and energy-aware video multicast system using collaborative relays, which entails (i) online topology management based on interference-aware link characterization, (ii) joint selection of relay nodes and transmission parameters, and (iii) polling-based relay protocol. Our proposed algorithm, the core of EV-CAST, judiciously selects the relay nodes and transmission parameters in consideration of interference, battery status, and spatial reuse. Our prototype-based experiment results demonstrate that EV-CAST enhances video multicast delivery under various network scenarios. EV-CAST enables 2x more nodes to achieve a target video packet loss ratio with 0.59x shorter airtime than the state-of-the-art video multicast scheme.

Recently, WiFi has become irreplaceable wireless technology to support increasing mobile applications and data traffic. While today's smartphone users frequently use WiFi on the move, smartphone does not perform WiFi handoff to nearby access points (APs) even if signal quality is severely degraded. Such undesired operation of smartphone is known as sticky client problem. In this work, we focus on resolving sticky client problem through fast WiFi handoff. First, we analyze the causes of sticky client problem based on experiments with commercial Android smartphones by focusing on WiFi scanning and handoff operations. Note that smartphones and state-of-the-art APs are equipped with Bluetooth Low Energy (BLE), we propose a practical solution, called BLEND, utilizing BLE to enable fast WiFi handoff. Smartphone acquires information about nearby APs through BLE advertising packet sent by APs, and then judiciously performs WiFi handoff. We implement BLEND on Android smartphones and demonstrate that BLEND achieves up to 61% and 111% higher throughput and video bitrate, respectively, compared with a commercial Android application. We also validate that BLEND operates on smartphone with negligible energy overhead.

Multicast is a major solution in supporting the explosive growth of the wireless video traffic demand. Also, power saving is a key technology to extend battery life of mobile devices. To meet both purposes, IEEE 802.11 wireless local area network (WLAN) supports power save mode (PSM) for station (STA) while receiving multicast packets. According to recent studies, off-the-shelf chipsets configured to use PSM show un-desired functions, thus resulting in many multicast packet losses. From our extensive measurement, we also verify degradation of multicast performance with widely-used off-the-shelf chipsets using PSM, and present previously-unknown undesired functions. Without modification of the chipsets, STA in PSM cannot enjoy reliable multicast service. Given this, we develop a practical and readily-applicable AP-side solution, called ACT-AP, which avoids multicast packet losses by preventing STA from operating in PSM. Our prototype implementation with off-the-shelf chipsets demonstrates that ACT-AP improves packet delivery ratio by up to 216% with little additional protocol overhead. To our best knowledge, ACT-AP is the first practical effort to support multicast to real devices with undesired functions in PSM.

Multi-rate forward erasure correction (FEC)-applied wireless multicast enables reliable and efficient video multicast with intelligent selection of physical (PHY) layer data rate and FEC rate. The optimal PHY/FEC rates depend on the cause of the packet losses. However, previous approaches select the PHY/FEC rates by considering only channel errors even when interference is also a major source of packet losses. We propose InFRA, an interference-aware PHY/FEC rate adaptation framework that (i) infers the cause of the packet losses based on received signal strength indicator (RSSI) and cyclic redundancy check (CRC) error notifications, and (ii) determines the PHY/FEC rates based on the cause of packet losses. Our prototype implementation with off-the-shelf chipsets demonstrates that InFRA enhances the multicast delivery under various network scenarios. InFRA enables 2.3x and 1.8x more nodes to achieve a target video packet loss rate with a contention interferer and a hidden interferer, respectively, compared with the state-of- the-art PHY/FEC rate adaptation scheme. To our best knowledge, InFRA is the first work to take the impact of interference into account for the PHY/FEC rate adaptation.

In video multicast, it is a challenging task to deliver video at a satisfactory level to multiple receivers with different channel quality and different battery capacities. In order to resolve such receiver heterogeneity, we propose a video multicast system that integrates scalable video coding (SVC) and flexible multicast service (FMS). The system is designed to transmit the base layer (BL) and the enhancement layer (EL) of an SVC-encoded video at different delivery traffic indication maps (DTIMs) such that each receiver can freely select either energy-intensive high quality video or energy-saving baseline quality video. Moreover, we enhance reliability by adding a functionality of intra-flow network coding (NC). As a measure of user satisfaction, we develop a multi-attribute utility model, and then, based on the utility model, we develop a Stakelberg game-based optimization framework to determine modulation and coding scheme (MCS) and the number of NC-generated packets for each layer. Through simulations, we confirm that the proposed scheme achieves higher user satisfaction by exhibiting performance gain of 16.4% to 47% over the legacy 802.11 scheme.

The demand for wireless communication in enterprise network has been increasing with the advent of "smart working environment." In enterprise wireless network, high density of access points (APs) and limited frequency resources raise the significance of radio resource management. In this paper, we propose a centralized algorithm, named JACPoT, for jointly executable transmission power control (TPC) and dynamic channel assignment (DCA). The proposed algorithm first guarantees that the links between AP and client stations are robust to the inter-cell interference via TPC. Next, it minimizes contention between neighboring APs via DCA utilizing a novel metric and annealed Gibbs sampler. Through ns-3 simulations based on measurement in real office environments, we demonstrate that JACPoT reduces uplink frame collisions by 36% and significantly enhances both data and voice traffic delivery.

Various packet-based simulation tools (e.g., NS-3) have been employed for design, validation, and evaluation of new protocols for WiFi networks since they offer cost efficiency, scalability, and reproducibility. These benefits come, however, at the expense of lack of realism compared to live testbed experiments. This is attributed in a major part to the difficulty of capturing detailed characteristics of channel dynamics, bit-level protocol specification (PHY layer), and application/user behaviors in a high-fidelity manner. The performance gap predicted by simulation and live testbed becomes even more pronounced when one considers a wide diversity of device characteristics and the way each device is used by end users. For example, smartphones generally show worse WiFi performance than other WiFi devices (e.g., laptops and tablets) because smartphones suffer from additional signal loss due to hand-grips and the low antenna gains of their embedded antennas. The goal of this study is to significantly close the gap by incorporating survey- and measurement-based smartphone WiFi characteristics and realistic hand-grip models into traditional WiFi network simulators (NS-3 in this study). The enhanced WiFi simulation tool's performance prediction capability is validated through an comparative study between testbed experiments and simulations.