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Fully Distributed Energy-Efficient Synchronization for Half-duplex D2D Communications
... Pulse-based synchronization is typically implemented by distributed phase locked-loops (DPLL) [16], [17]. The performance of DPLL is affected by the duplexing scheme employed by the devices. ...
... Full-duplex communication significantly decreases the synchronization time compared to half-duplex due to simultaneous signal transmission and reception, and has been considered by several authors [9], [18]. However, implementation of full-duplex technology, especially at the mobile devices poses a number of practical issues in terms of cost, complexity and power consumption [16], [17]. Due to the additional power required for self-interference cancellation mobile devices with a limited battery life cannot currently afford to operate in the full-duplex mode [19]. ...
... Furthermore, the effects of clock phase and propagation delays are entangled within the superimposed timing pulses. In order to achieve synchronization, the aforementioned effects should be estimated from the received pulses and removed by updating the device clocks accordingly [17]. However, this estimation becomes highly challenging as the received pulses are not only altered by these effects, but also by the very mechanisms used to correct them, which could lead to instability. ...
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.
Synchronization is a challenging problem of device-to-device (D2D) communications, particularly when D2D devices are out-of-network coverage. This paper proposes an efficient initial synchronization method for D2D communication in a long term evolution system. The proposed synchronization approach is based on the correlation between both received primary sidelink synchronization signal and secondary sidelink synchronization signal to remove the effects of channel fading and symbol timing offset (STO). This design facilitates reduced-complexity joint detection of carrier frequency offset, sidelink synchronization signal, and cyclic prefix (CP) mode during the initial synchronization procedure. Simulation results show that the proposed synchronization scheme achieves robust and low-complexity detection of carrier frequency offset, sidelink synchronization signal, and CP mode when there is a non-negligible STO or frequency-selective fading distortion.
This article proposes an efficient time, frequency, and sidelink synchronization algorithm for device-to-device communication in a long-term evolution system. To end this, the proposed synchronization scheme uses both the structure of cyclic prefix and sidelink synchronization signals. By performing cyclic prefix mode and primary sidelink synchronization signal detection in the time domain, the performance of the frequency-domain synchronization receiver, including carrier frequency offset and sidelink identity detection, is improved. To assess the usefulness of the proposed synchronization method, the mean square error and the detection probability are theoretically derived. Simulation results show that the proposed synchronization scheme achieves robust detection of carrier frequency offset, symbol timing offset, cyclic prefix mode, and sidelink synchronization signal in a time–frequency doubly selective fading channel.
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.
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.
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.
The past decade has seen many advances in physical layer wireless communication theory and their implementation in wireless systems. This textbook takes a unified view of the fundamentals of wireless communication and explains the web of concepts underpinning these advances at a level accessible to an audience with a basic background in probability and digital communication. Topics covered include MIMO (multi-input, multi-output) communication, space-time coding, opportunistic communication, OFDM and CDMA. The concepts are illustrated using many examples from real wireless systems such as GSM, IS-95 (CDMA), IS-856 (1 x EV-DO), Flash OFDM and UWB (ultra-wideband). Particular emphasis is placed on the interplay between concepts and their implementation in real systems. An abundant supply of exercises and figures reinforce the material in the text. This book is intended for use on graduate courses in electrical and computer engineering and will also be of great interest to practising engineers.
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.