Introduction to the Issue on Visual Signal Processing for Wireless Networks
R ECENT development of mobile computing technology and wireless communications brought into focus a new paradigm of visual signal transmission via wireless channels and networks. With the emergence of smartphones and continued growth of laptops, netbooks, and tablets, there is a huge increase in a number of mobile devices capable of supporting new image and video applications. According to recent estimates, more than two thirds of all communication network traffic is currently consumed by visual signals and this share is still persistently growing with the development of multi-view and 3D video technology. In addition, interactivity of visual content transmission has become an essential ingredient of modern multimedia communication systems, where uplink channel requirements are also augmented to enable real-time connection between users and video encoders. However, conveying visual information to mobile devices over cellular or mobile broadband networks encounters many challenges, such as limited channel bandwidth, constrained computational and energy resources, high required quality and reliability, tight latency requirements, etc. These problems lie at the heart of the ongoing research in wireless video. In this special issue, most manuscripts tackle a range of networking aspects of wireless video transmission, such as streaming, resource and power allocation and adaptive bit rate control. For that reason, Pudlewski et al. analyze state-of-the-art of wireless video communication in their tutorial paper with insight into the multi-layer network architecture. They also look into a cross-layer design perspective and its potential to enhance the video experience perceived by the end user in future wireless video applications and associated devices. Among the other papers, the first group shifts the focus toward considering delivered visual quality to the end user when allocating the wireless network resources across multi-user population. The second group of manuscripts analyzes the resource and power allocation optimization problem while addressing other criteria, such as transmission delay or network utility. In the third group of publications, coding schemes are analyzed and modified to provide multiple description and scalable coding for wireless video transmission to unmanned aerial vehicles or for free viewpoint video applications. Finally, in the fourth group, video processing applications, such as face recognition or video summarization, are developed for wireless sensor network platforms. Quality of service (QoS) and quality of experience (QoE) are often used as primary metrics for measuring the efficiency of transmission performance in multimedia network delivery. In particular, in wireless video networks, QoE can be expressed as objective or subjective metric reflecting the perceived video content either based on mathematical equations (e.g., PSNR or SSIM) or using more sophisticated tools and procedures to quantitatively express the end user’s level of experience. These metrics are then used for optimizing the transmission perfor-