Fatih Ulupinar

Deriving and Validating User Experience Model for DASH Video Streaming

Ever since video compression and streaming techniques have been introduced, measurement of perceived video quality has been a non-trivial task. Dynamic adaptive streaming (DASH) over hypertext transfer protocol, is a new worldwide standard for adaptive streaming of video. DASH has introduced an additional level of complexity for measuring perceived video quality, as it varies the video bit rate and quality. In this paper, we study the perceived video quality using DASH. We investigate three factors which impact user perceived video quality: 1) initial delay; 2) stall (frame freezing); and 3) bit rate (frame quality) fluctuation. For each factor, we explore multiple dimensions that can have different effects on perceived quality. For example, in the case of the factor stall, while most previous research have studied how stall duration correlates with user experience, we also consider how the stalls are distributed together with the amount of motion in the video content, since we believe they may also impact user perceived quality. We conduct extensive subjective tests in which a group of subjects provide subjective evaluation while watching DASH videos with one or more artifacts occurring. Based on the subjective tests, we first derive impairment functions which can quantitatively measure the impairment of each factor, and then combine these impairment functions together to formulate an overall user experience model for any DASH video. We validate with high accuracy the user experience model, and demonstrate its applicability to long videos.

LTE Relay Architecture and Its Upper Layer Solutions

The relay technology is an essential piece of LTE-Advanced in its pursuit of cost effective capacity enhancement and coverage extension. Simple RF repeaters have been widely used in cellular technologies to offer coverage extension. However, they cannot distinguish desired signals from interference and noise and are unable to operate with sophisticated mechanisms, such as power control, beam-forming, and adaptive modulating/coding. As a result, simple RF repeaters do not provide capacity enhancement nor do they reach the capacity potential of extended coverage areas. The use of more sophisticated relay in cellular technologies is relatively new. With its great benefits in QoS differentiation, capacity enhancement, and coverage extension, the development of LTE relay technology faces challenges in providing backward compatibility, minimizing complexity, providing sufficient QoS differentiation, reducing over the air overhead, and ensuring security. This paper addresses these challenges with solutions in the aspect of relay architecture and upper layer designs.