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Fast Converging Distributed Pulse-coupled Clock Synchronization for Half-duplex D2D Communications over Multipath Channels
... Based on this algorithm, we then propose a novel fully-distributed pulse-based synchronization protocol for half-duplex D2D communications in 5G networks. This work significantly extends upon our previous contributions in [17], [24], where we focus on a simplified version of the problem by neglecting the presence of clock skew and its influence on data communication. Specifically, our main contributions in this work are summarized as follows: ...
... However, if the devices randomly decide their transceiver mode at each clock tick as in [16], then the set D ν,η j of transmitter devices and path indices seen by the jth receiver device, as defined in (12), will evolve unpredictably over time. In turn, this will result in arbitrary fluctuations in the weighted average TO estimates ∆t j [ν] [24]. ...
In distributed device-to-device (D2D) communications, no common reference time is available and the devices must employ distributed synchronization techniques. In this context, pulse-based synchronization, which can be implemented by distributed phase-locked loops is preferred due to its scalability. Several factors degrade the performance of pulse-based synchronization, such as duplexing scheme, clock skew and propagation delays. Furthermore, in distributed networks, devices should be aware of the synchronization status of others in order to initiate data communications. To address these prevailing issues, we first introduce a half-duplex timing-advance synchronization algorithm wherein each device alternates between being a transmitter and receiver in their exchange of synchronization pulses at each clock period. Based on this algorithm, we propose a novel fully-distributed pulse-based synchronization protocol for half-duplex D2D communications in 5G wireless networks. The protocol allows participating devices to become aware of the global synchronization status, so that they can complete the synchronization process ideally at the same time and proceed to data communication. In simulation experiments over multi-path frequency selective channels, the proposed synchronization protocol is shown to outperform a benchmark approach from the recent literature over a wide range of conditions, e.g., clock skew, number of devices, and network topology.
Device-to-Device (D2D) communication is introduced for cellular networks allowing User Equipments (UEs) to communicate directly in close physical proximity. In traditional cellular networks, time synchronization is performed by the network infrastructure equipment to address the challenges of coordinating independent clocks and tasks of UEs to communicate with each other. These challenges are more critical in D2D as the network has no central point to coordinate UEs. The aim of this paper is twofold: (i) reviewing synchronization standards and techniques for D2D-enabled cellular networks and (ii) discussing different scenarios and contemporary cellular network generations. We review the main signals, channels, and entities used by D2D-enabled cellular networks for synchronization procedures in different scenarios. Moreover, we focus on the synchronization in Vehicle-to-Vehicle (V2V) and Vehicle-to-Everything (V2X). Finally, we conduct a literature study on effective proposed synchronization techniques in D2D and vehicular communications to highlight recent advances and provide future directions.
Supporting ever‐increasing number of mobile users with data‐hungry applications, running on battery‐limited devices, is a daunting challenge for the telecommunication community. Device‐to‐device (D2D) communication, which allows physically proximate mobile users to directly communicate with each other by reusing the spectrum, without going through the base station, holds promise to help us tackle this challenge. In a cellular network, D2D communication offers opportunities for spectrum reuse and spatial diversity that may lead to enhanced coverage, higher throughput, and robust communication in the network. Further, for applications such as weather forecasting and live streaming, which may require the same chunks of data distributed to geographically proximate users, D2D multicasting may provide better utilization of network resources compared to D2D unicast or the base station‐based multicast, such as LTE eMBMS. However, extensive deployment of D2D multicast in a network may introduce various issues, such as severe co‐channel interference due to spectrum reuse, underutilization of spectrum, and rapid battery depletion of the multicasting D2D nodes due to higher transmit power to mitigate co‐channel interference and facilitating data‐relaying. Therefore, this article discusses some of the challenges in supporting D2D multicast communication in cellular networks. It then surveys various existing approaches to address these challenges, which along with some future research directions may help us develop practical D2D multicast protocols for cellular networks.
The increasing number of mobile users has givenimpetus to the demand for high data rate proximity services.The fifth generation (5G) wireless systems promise to improvethe existing technology according to the future demands andprovide a road-map for reliable and resource-efficient solutions.Device-to-device (D2D) communication has been envisioned as anallied technology of 5G wireless systems for providing servicesthat include live data and video sharing. D2D communicationtechnique opens new horizons of device-centric communications,i.e., exploiting direct D2D links instead of relying solely oncellular links. Offloading traffic from traditional network-centricentities to D2D network enables low computational complexity atthe base station besides increasing the network capacity. However,there are several challenges associated with D2D communication.In this article, we present a survey of the existing methodologiesrelated to aspects such as interference management, networkdiscovery, proximity services and network security in D2Dnetworks. We conclude by introducing new dimensions withregards to D2D communication and delineate aspects that require further research.
Dense cooperative network involves communication and coding among multiple uncoordinated nodes. The distributed nature of cooperative networks results in both carrier frequency offset (CFO) and timing offset (TO) that degrade the performance of collaborative communication. Hence, network synchronization is mandatory to guarantee network operations. This paper proposes a novel method to perform joint time and frequency synchronization in presence of large TOs and CFOs by extending the principle of consensus method to synchronization. The estimation is based on assigning to all nodes the same training sequence based on Zadoff-Chu (ZC), that is arranged to decouple CFO from TO in symbol and frame synchronization. The synchronization is based on weighted consensus algorithm to reach synchronization in a connected network by using the superposition of the waveforms from all transmitting nodes as an ensemble of reference timing for synchronization. Time and frequency synchronization is guaranteed in multi-node interference scenario without the need to assign to every node an independent ZC sequence, and thus without any coordination.
Synchronization is considered a particularly difficult task in wireless sensor networks due to its decentralized structure. Interestingly, synchrony has often been observed in networks of biological agents (e.g., synchronously flashing fireflies, or spiking of neurons). In this paper, we propose a bio-inspired network synchronization protocol for large scale sensor networks that emulates the simple strategies adopted by the biological agents. The strategy synchronizes pulsing devices that are led to emit their pulses periodically and simultaneously. The convergence to synchrony of our strategy follows from the theory of Mirollo and Strogatz, 1990, while the scalability is evident from the many examples existing in the natural world. When the nodes are within a single broadcast range, our key observation is that the dependence of the synchronization time on the number of nodes N is subject to a phase transition: for values of N beyond a specific threshold, the synchronization is nearly immediate; while for smaller N, the synchronization time decreases smoothly with respect to N. Interestingly, a tradeoff is observed between the total energy consumption and the time necessary to reach synchrony. We obtain an optimum operating point at the local minimum of the energy consumption curve that is associated to the phase transition phenomenon mentioned before. The proposed synchronization protocol is directly applied to the cooperative reach-back communications problem. The main advantages of the proposed method are its scalability and low complexity.
This paper configures an effective distributed synchronization architecture in a decentralized wireless network which are considered for device-to-device communication standards. The paper proposes a random transmit selection scheme with an adaptive selection factor, which works by allowing a node to choose randomly either transmitting or receiving the clock pulses. Each node updates its own clock time by using clock information from neighbors in a transmit mode, but not by all nodes. With the adaptive selection factor-based method, the proposed scheme achieves performance improvements in both convergence time and synchronization error senses. In all of the contexts, the proposed scheme suggests the simple but strongly effective architecture, and shows enhanced behaviors.
This paper studies the synchronization problem for mobile cellular device-to-device (D2D) networks. Depending on the number of devices that are in coverage of the base station (BS), the D2D environment can be divided into three categories where the partial-coverage and out-of-coverage are challenging scenarios and thus are the focus of this work. Firstly, we discuss five main challenges imposed on the synchronization problem in mobile D2D networks. More specifically, there are different challenges in the two coverage scenarios, since they do not have the exactly same synchronization objectives. Secondly, we propose a low-complexity Adaptive distRibuted nEtwork Synchronization (ARES) algorithm to address the five challenges. The design principles and the theories behind the ARES scheme are also analyzed in detail. Finally, we provide comprehensive simulations to evaluate different synchronization schemes, where the proposed ARES mechanism shows very promising performance.
Direct device-to-device (D2D) communication is considered to be part of fifth generation (5G) networks, while embodying vehicle-to-anything (V2X) communication into the recently defined Long Term Evolution (LTE) sidelink is already on the way. Coexistence of in-band D2D and cellular transmissions requires time synchronization between multiple links to prevent from interference and avoid using longer cyclic prefix or guard bands. This paper proposes a synchronization solution enabling interference-free coexistence of inter-cellular sidelink and existing cellular links, also considering non-synchronized base stations. Multiple synchronization sources including base stations, Global Navigation Satellite System (GNSS) and mobile users are hierarchically combined and network-instructed synchronization avoids any misalignments. Out-of-coverage users can form or join existing mobile networks by deploying the proposed distributed algorithm, which is found to limit the residual average timing error to below 0.5 µs.
We study the problem of distributed time and frequency synchronization for device-to-device communication in LTE [1]. LTE is an OFDMA system that crucially uses tight time and frequency synchronization between UEs and the base station for intracell and intercell interference coordination. For consistency with the cyclic prefix length and tone spacing used in LTE, we target an accuracy of a few microseconds for time synchronization and 150 Hz for frequency synchronization. We examine the problem both from a link and a system-level perspective. Link-level challenges involve designing a synchronization signal for computationally and energy efficient receiver algorithms, and system-level challenges involve achieving consistent timing using a distributed algorithm while leveraging the multiuser aspect of the problem to improve performance.
Orthogonal frequency division multiplexing (OFDM) technique has been widely adopted in wireless systems, but it is known for being sensitive to synchronization errors. In this paper, we present timing and frequency synchronization algorithms for OFDM downlink transmissions using Zadoff-Chu sequences. We show that the current timing synchronization method employing Zadoff-Chu sequences is sensitive to carrier frequency offsets. To reduce this sensitivity, we develop conditions for the selection of appropriate Zadoff-Chu sequences. We then design a training sequence and propose joint signal detection, timing, and carrier frequency offset estimation algorithms for OFDM downlink transmissions. We show that the proposed schemes simplify the overall synchronization architecture and improve the performance compared to the existing schemes in the literature.
This article has explored history, recent advances, and challenges in distributed synchronization for distributed wireless systems. It is focused on synchronization schemes based on exchange of signals at the physical layer and corresponding baseband processing, wherein analysis and design can be performed using known tools from signal processing. Emphasis has also been given on the synergy between distributed synchronization and distributed estimation/detection problems. Finally, we have touched upon synchronization of nonperiodic (chaotic) signals. Overall, we hope to have conveyed the relevance of the subject and to have provided insight on the open issues and available analytical tools that could inspire further research within the signal processing community.
This correspondence describes the construction of complex codes of the form exp i alpha_k whose discrete circular autocorrelations are zero for all nonzero lags. There is no restriction on code lengths.
Network synchronization¨ for mobile Device-to-Device systems
- W Sun
- F Brännström
- E G Sträm