Applied sciences  
Multi-Channel and Multi-Agent Signal Processing  
[PERSON]  
Dipartimento di Ingegneria dell'Innovazione, University of Salento, Via Monteroni, 73100 Lecce, Italy; [EMAIL_ADDRESS]  
Multi-channel and array signal processing is a well-established field with fundamental applications in wireless communications, radar/sonar, remote sensing, medical imaging, and more [1]. Its focus is on signals from multiple sensors or channels, often involving a pair of entities (transmitter and receiver, possibly in asymmetric configuration, e.g., [2]). Many modern application contexts are, however, networked, that is, interconnection of different devices or agents is today a common setup in many different contexts; this can be exploited to solve problems in a cooperative, possibly distributed way, since cooperation improves the performance and may solve problems that are unsolvable in a non-cooperative setting [3]. Moreover, the distributed (in-network) processing paradigm enables the design of privacy-preserving and robust schemes, while taking advantage of the aggregate processing power of many devices, instead of a single node where all data need to be sent for centralized processing. In doing so, the drawbacks of the latter, more conventional approach, i.e., a single point of failure with very high computational, bandwidth, and energy demands, can be overcome.  
Although research is very active on both multi-channel (single-link) and multi-agent (networked, i.e., multi-link) signal processing, the potential of combining both fields is still underexploited. A positive example is instead the recent trend in antenna array, i.e., multiple-input multiple-output (MIMO) solutions for 5G and beyond cellular networks, in particular using device-to-device (D2D) communications [4]. Important applications in this respect are user localization, intelligent sensing of the environment, coordinated control of autonomous systems, cooperative extended horizon in vehicular networks, smart factory, etc. At the same time, it is of the utmost importance to identify solutions to improve the throughput and reduce interference from multiple radio access, which in certain systems can be massive. Indeed, the latter issue is becoming a bottleneck given the exponential growth of connected devices (smartphones, wearables, smart objects, etc.) as the internet of things (IoT) paradigm spreads out [5].  
This Special Issue was aimed at promoting cross-fertilization between multi-channel/array processing techniques and multi-agent methodologies in order to provide advanced solutions for emerging application contexts. A total of ten papers have been accepted for publication, covering an interesting range of theoretical and practical issues arising in multi-channel, multi-agent contexts. They can be grouped into three, partially overlapping classes: a first one covering multi-channel techniques, in particular regarding MIMO contexts, aimed at improving the performance through the additional degrees of freedom available in the multi-channel setup; a second one focusing on spectrum-sensing techniques for enhanced communications in multi-agent scenarios; and a third one specifically addressing the issue of location-awareness and tracking, which is essential for the practical implementation of multi-agent applications [6]. Particular emphasis is given to multi-channel solutions and radar.