In this paper, we investigate maximum likelihood (ML) time delay (TD) and carrier frequency offset (CFO) synchronization in multi-node decode-and-forward (DF) cooperative relaying systems operating over time-varying channels (TVCs). This new synchronization scheme is embedded into a distributed multiple input multiple output (MIMO)-relay beamforming transceiver structure to avoid the drawbacks of multidimensional ML estimation at the destination and to minimize the overhead cost. The new technique can be jointly implemented with any Doppler spread estimator in an iterative scheme using a time-constant channel (TCC) based synchronization method at the initialization step. The resulting TD and CFO estimates along with the channel estimates are then fed into a distributed MIMO-relay beamforming transceiver of K single-antenna nodes, for pre-compensation at each node of the transmitted signals, to ensure constructive maximum ratio combining (MRC) at the destination. Simulation results show significant synchronization accuracy improvement over previous distributed multi-node synchronization techniques assuming TCCs. The latter translates into noticeable gains in terms of useful link-level throughput, more so at higher Doppler or with more relaying nodes.

In this paper, we investigate maximum likelihood (ML) time delay (TD) and carrier frequency offset (CFO) synchronization in multi-node decode-and-forward cooperative relaying systems operating over time-varying channels. This new synchronization scheme is embedded into a distributed multiple input multiple output (MIMO)-relay beamforming transceiver structure to avoid the drawbacks of multidimensional ML estimation at the destination and to minimize the overhead cost. By accounting for a perfect Doppler spread value, the new synchronization solution delivers accurate TD and CFO estimates. For real-world operation, however, this new technique can be jointly implemented with any Doppler spread estimator in a new iterative scheme using a time-constant channel (TCC)-based synchronization method at the initialization step. The resulting TD and CFO estimates along with the channel estimates are then fed into a distributed MIMO-relay beamforming transceiver of K single-antenna nodes, for pre-compensation at each node of the transmitted signals, to ensure constructive maximum ratio combining (MRC) at the destination. Simulation results show significant synchronization accuracy improvement over previous distributed multi-node synchronization techniques assuming TCCs. The latter translates into noticeable gains in terms of useful link-level throughput, more so at higher Doppler or with more relaying nodes.

User-centric wireless access virtualization (WAV) allows each user to be served by a set of carefully selected transmission points (TPs) forming a user-specific virtual base station (uVBS) adapted to its environment and quality-of-service (QoS) requirement. In this way, this new concept breaks away from the conventional cell-centric architecture to provide boundaryless communications in future fifth-generation (5G) networks. This fundamental structural 5G evolution and the ultra-dense multi-tier heterogeneous context foreseen in such networks require an inevitable rethinking of efficient scalable TP clustering. As such, this paper proposes three innovative low-cost clustering approaches that enable the user-centric WAV and provide dynamic, adaptive, and overlapping TP clusters while requiring not only negligible overhead cost but also minimum signaling changes at both network and user sides. Contrary to existing clustering techniques, the new ones we propose better leverage the 5G features such as extreme densification and massive connectivity as well as new concepts such as millimeter wave (mmWave) spectrum and massive multiple-input-multiple-output (MIMO). The simulations show that they may achieve until 154% and 282% of throughput and coverage gains, respectively. Furthermore, these approaches are flexible enough to be adapted to different network dimensions (i.e., space and time), thereby paving the way for achieving the dramatic performance improvements required by the 5G networks to cope with the upcoming mobile data deluge.

In this paper, we investigate maximum likelihood (ML) time delay (TD) and carrier frequency offset (CFO) synchronization (i.e., estimation and pre- compensation) in decode-and-forward (DF) cooperative systems operating over time-varying channels (TVCs). The new technique is embedded at each relay node in order to avoid the drawbacks of multidimensional ML estimation at the destination and to minimize the overhead cost. By accounting for a perfect Doppler spread value, the new synchronization solution delivers accurate TD and CFO estimates at each relay. The resulting TD and CFO estimates along with the channel estimates are then exploited by the MIMO relay for precompensation at each node of the distributed transmit beamforming signals to ensure constructive maximum ratio combining (MRC) at the destination. Simulation results show significant synchronization accuracy improvement over previous distributed multi-node synchronization techniques assuming time-constant channels (TCCs). The latter translates into noticeable gains in terms of useful (i.e., after accounting for incurred overhead) link-level throughput, more so at higher Doppler.

This paper develops an innovative scalable and low-cost QoS-based user equipment (UE) virtualization (UEV) scheme that capitalizes on the massive connectivity and the enhanced resources of the new UE generation in terms of connectivity, computing, battery/power, etc. Exploiting the need/excess duality and the heterogeneity in such resources at the UE plan, we form virtual UEs (VUE)s, dynamically, owing to a carefully-designed time-adjusting scheme for the selection of the proper cooperative UE sets. The new UEV scheme is able to adapt to each target UE (TUE) environment, meet its demands, and scale with its needs, offers a reliable and efficient yet low-cost inter-UE cooperation, reduces the overhead and power consumption with respect to conventional approaches, and substantially reduces the number of communication links and, hence, incurs much less interference. System-level simulation results show that the proposed QoS-based UEV scheme largely outperforms the “dummy UEs” approach.

This paper aims to evaluate the system level performance of the newly proposed non-orthogonal multiple access scheme sparse code multiple access (SCMA) with uplink grant-free transmissions for 5G massive machine-type communication (mMTC) scenario. The challenge lies in the modeling and abstraction of physical layer actions as the link-to-system interface, including the features of overloading, grant-free transmission, as well as non-linear receiver. Specifically, we propose to use single bound to simplify the maximum-likelihood (ML) receiver modelling with only linear computational complexity, while still guaranteeing sufficient accuracy requirement. Extensive simulations verify the correctness of the method under a series of practical assumptions. Evaluation results show that non-orthogonal multiple access schemes such as SCMA with grant-free can provide significant system capacity gain over orthogonal multiple access such as grant-free OFDMA (about 3.5 times as shown in the evaluated scenario) in terms of packet arrival rate at given system outage (defined as 1% packet drop rate). It is noted that the proposed method has been just agreed in 3gpp RAN1 86 meeting and included in the technical report.

5G wireless networks are expected to support new applications demanding ultra high reliability and ultra low latency, such as self-driving cars, industrial control, and real-time gaming. Such services are known as ultra-reliable and low-latency communications (URLLC) targeting at 99.999% transmission correctness within 1 ms delay bound. Current Long Term Evolution (LTE) system is far from supporting such services, especially for uplink. Even with around 10 times shortened transmission interval, due to the waste of time in requesting and receiving the scheduling grant, only one shot uplink transmission can be supported within 1 ms, which is very hard to meet the reliability requirement. Grant-Free transmission is thus proposed to effectively save the time of requesting/waiting for grant and thus relax the transmission interval design in frame structure. This paper presents the design of grant-free transmission for URLLC services and also the evaluation results. We shall show that the saved time can be used for more Hybrid Automatic Repeat reQuest (HARQ) retransmissions and thus further enhance the reliability. Moreover, contention based grant-free transmission with proper HARQ design can well meet the reliability requirement. Finally, the performance can be further enhanced with non-orthogonal multiple access schemes such as Sparse Code Multiple Access (SCMA) and advanced receivers such as Message Passing Algorithm (MPA) or Expectation Propagation Algorithm (EPA) receiver.