Cognitive Radio Systems

Cognitive radio is a hot research area for future wireless communications in the recent years. In order to increase the spectrum utilization, cognitive radio makes it possible for unlicensed users to access the spectrum unoccupied by licensed users. Cognitive radio let the equipments more intelligent to communicate with each other in a spectrum-aware manner and provide a new approach for the co-existence of multiple wireless systems. The goal of this book is to provide highlights of the current research topics in the field of cognitive radio systems. The book consists of 17 chapters, addressing various problems in cognitive radio systems

D13.2 Techniques and performance analysis on energy- and bandwidth-efficient communications and networking

Deliverable D13.2 del projecte europeu NEWCOM#The report presents the status of the research work of the various Joint Research Activities (JRA) in WP1.3 and the results that were developed up to the second year of the project. For each activity there is a description, an illustration of the adherence to and relevance with the identified fundamental open issues, a short presentation of the main results, and a roadmap for the future joint research. In the Annex, for each JRA, the main technical details on specific scientific activities are described in detail.Peer ReviewedPostprint (published version

Delivery Time Minimization in Edge Caching: Synergistic Benefits of Subspace Alignment and Zero Forcing

An emerging trend of next generation communication systems is to provide network edges with additional capabilities such as additional storage resources in the form of caches to reduce file delivery latency. To investigate this aspect, we study the fundamental limits of a cache-aided wireless network consisting of one central base station, $M$ transceivers and $K$ receivers from a latency-centric perspective. We use the normalized delivery time (NDT) to capture the per-bit latency for the worst-case file request pattern at high signal-to-noise ratios (SNR), normalized with respect to a reference interference-free system with unlimited transceiver cache capabilities. For various special cases with $M=\{1,2\}$ and $K=\{1,2,3\}$ that satisfy $M+K\leq 4$, we establish the optimal tradeoff between cache storage and latency. This is facilitated through establishing a novel converse (for arbitrary $M$ and $K$) and an achievability scheme on the NDT. Our achievability scheme is a synergistic combination of multicasting, zero-forcing beamforming and interference alignment.Comment: submitted to ICC 2018; fixed some typo

MIMO signal processing in offset-QAM based filter bank multicarrier systems

Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.Peer ReviewedPostprint (author's final draft

On rate capacity and signature sequence adaptation in downlink of MC-CDMA system

This dissertation addresses two topics in the MC-CDMA system: rate capacity and adaptation of users\u27 signature sequences. Both of them are studied for the downlink communication scenario with multi-code scheme. The purpose of studying rate capacity is to understand the potential of applying MC-CDMA technique for high speed wireless data communications. It is shown that, to maintain high speed data transmission with multi-code scheme, each mobile should cooperatively decode its desired user\u27s encoded data symbols which are spread with different signature sequences simultaneously. Higher data rate can be achieved by implementing dirty paper coding (DPC) to cooperatively encode all users\u27 data symbols at the base station. However, the complexity of realizing DPC is prohibitively high. Moreover, it is found that the resource allocation policy has profound impact on the rate capacity that can be maintained in the system. Nevertheless, the widely adopted proportional resource allocation policy is only suitable for the communication scenario in which the disparity of users\u27 channel qualities is small. When the difference between users\u27 channel qualities is large, one must resort to non-proportional assignment of power and signature sequences. Both centralized and distributed schemes are proposed to adapt users\u27 signature sequences in the downlink of MC-CDMA system. With the former, the base station collects complete channel state information and iteratively adapts all users\u27 signature sequences to optimize an overall system performance objective function, e.g. the weighted total mean square error (WTMSE). Since the proposed centralized scheme is designed such that each iteration of signature sequence adaptation decreases the WTMSE which is lower bounded, the convergence of the proposed centralized scheme is guaranteed. With the distributed signature sequence adaptation, each user\u27s signature sequences are independently adapted to optimize the associated user\u27s individual performance objective function with no regard to the performance of other users in the system. Two distributed adaptation schemes are developed. In one scheme, each user adapts its signature sequences under a pre-assigned power constraint which remains unchanged during the process of adaptation. In the other scheme, pricing methodology is applied so that the transmission power at the base station is properly distributed among users when users\u27 signature sequences are adapted. The stability issue of these distributed adaptation schemes is analyzed using game theory frame work. It is proven that there always exists a set of signature sequences at which no user can unilaterally adapt its signature sequences to further improve its individual performance, given the signature sequences chosen by other users in the system