Uplink Reference Signals Research Articles

Bayesian Filtering for Joint Multi-User Positioning, Synchronization and Anchor State Calibration

Millimeter (mmWave) positioning can go beyond classical localization, allowing to extract more complete situational awareness in terms of, e.g, clock offsets, antenna orientations or landmark locations. In this article, we formulate an extended Kalman filtering (EKF)-based framework called MU-PoSAC ( <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</u> ulti- <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</u> ser <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Po</u> sitioning, <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</u> ynchronization and <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</u> nchor State <underline xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</u> alibration), that allows to jointly estimate and track the locations and clock offsets of multiple users together with the unknown locations and orientation offsets of the anchors, building on angle-of-arrival (AoA) and time-of-arrival (ToA) measurements. We provide an extensive set of numerical results in the context of mmWave 5 G New Radio (NR) deployment in an industrial facility with moving robots and other industrial vehicles, incorporating full-scale ray-tracing for accurate propagation modeling as well as actual uplink reference signal based AoA and ToA estimators. Our numerical results show that estimating and tracking the overall system state is feasible, and that a single reference anchor can further enhance the estimation accuracy. In addition, more users are shown to lead to better performance, due to the beneficial coupling of the anchor state. Therefore, our study demonstrates that in order to maximize the estimation performance, it is desirable to have at least one anchor state precisely known, and to have multiple users in the system. Finally, the important practical aspect of Line-of-Sight (LoS) blockage is addressed. It is shown that in the considered industrial use case, the proposed MU-PoSAC framework can offer robustness against intermittent LoS blockage.

Combining Reciprocity and CSI Feedback in MIMO Systems

Reciprocity-based time-division duplex (TDD) Massive MIMO (multiple-input multiple-output) systems utilize channel estimates obtained in the uplink to perform precoding in the downlink. However, this method has been criticized of breaking down, in the sense that the channel estimates are not good enough to spatially separate multiple user terminals, at low uplink reference signal signal-to-noise ratios, due to insufficient channel estimation quality. Instead, codebook-based downlink precoding has been advocated for as an alternative solution in order to bypass this problem. We analyze this problem by considering a "grid-of-beams world" with a finite number of possible downlink channel realizations. Assuming that the terminal accurately can detect the downlink channel, we show that in the case where reciprocity holds, carefully designing a mapping between the downlink channel and the uplink reference signals will perform better than both the conventional TDD Massive MIMO and frequency-division duplex (FDD) Massive MIMO approach. We derive elegant metrics for designing this mapping, and further, we propose algorithms that find good sequence mappings.

A dynamic handover scheme based on bidirectional VLC channel in multi-user attocell networks

Light fidelity (LiFi) would be a potential optical wireless communications technology for future mixed-spectrum 5G/6G indoor attocell network, which uses light-emitting diodes based as an optical antenna and offers Gbits capacity. For this scenario, a dynamic handover scheme based on bidirectional visible light communications (VLC) channel is proposed, which uses the uplink light sources to characterize terminal’s movement status for the mobility and make the handover decision in contrast to the unidirectional links. The uplink reference signal received power and the movement state are examined, and the created algorithm is executed in the bidirectional VLC test bed. The experimental results show that the proposed method will reduce the terminal’s handover rate by up to 45% and increase the optical network throughput by up to 26% at the experimental data rate compared with the skipping handover scheme for indoor LiFi network.

An integrated approach for energy efficient handover and key distribution protocol for secure NC-enabled small cells

Future wireless networks must serve dense mobile networks with high data rates, keeping energy requirements to a possible minimum. The small cell-based network architecture and device-to-device (D2D) communication are already being considered part of 5G networks and beyond. In such environments, network coding (NC) can be employed to achieve both higher throughput and energy efficiency. However, NC-enabled systems need to address security challenges specific to NC, such as pollution attacks. All integrity schemes against pollution attacks generally require proper key distribution and management to ensure security in a mobile environment. Additionally, the mobility requirements in small cell environments are more challenging and demanding in terms of signaling overhead. This paper proposes a blockchain-assisted key distribution protocol tailored for MAC-based integrity schemes, which combined with an uplink reference signal (UL RS) handover mechanism, enables energy efficient secure NC. The performance analysis of the protocol during handover scenarios indicates its suitability for ensuring high level of security against pollution attacks in dense small cell environments with multiple adversaries being present. Furthermore, the proposed scheme achieves lower bandwidth and signaling overhead during handover compared to legacy schemes and the signaling cost reduces significantly as the communication progresses, thus enhancing the network’s cumulative energy efficiency.

Uplink Reference Signals for Power-Efficient Handover in Cellular Networks With Mobile Relays

When a vehicle moves from one base station (BS) to another, a large number of on-board user equipments (UE) may simultaneously and individually perform a handover (HO) procedure, resulting in increased HO overheads. A mobile relay node (MRN), connected via a wireless backhaul to a donor base station (DBS), is deployed on the rooftop of a bus to improve the link quality and reduce the associated HO overhead via group mobility. However, at moderate to high speeds, the on-board UEs can still suffer from frequent HOs due to the MRN failing to HO to a new DBS using the legacy downlink measurement-based HO (DL-HO) method. As a consequence, the connection towards all associated mobile users will be lost which poses tight reliability requirements on the backhaul link to avoid becoming a single point of failure (SPoF). In order to improve the reliability during group handover, in this work, we propose an uplink reference signal (UL RS) based HO procedure (coined as UL-HO) for the MRN which relies on the existing sounding reference signal in long term evolution (LTE) /new radio (NR). In the proposed scheme, and unlike the legacy DL-HO procedure in LTE/NR, the measurement report (MeasReport) transmission is not required between MRN and the DBS, therefore the HO delay can be reduced, decreasing the SPoF chances and thus, uninterrupted services can be provided to on-board UEs. We investigate the gain in terms of HO rate, HO failure rate, ping-pong rate and power consumption (both at the UE and the BS). Performance evaluations demonstrate that the proposed UL-HO scheme outperforms the legacy DL-HO scheme in current cellular networks.

Wake-up Radio-Based 5G Mobile Access: Methods, Benefits, and Challenges

Future mobile networks will enable new use cases, requiring further enhanced data rates, latency, coverage, capacity, and reliability. In this article, to emphasize sustainable energy consumption and improved device battery lifetime, the concept, benefits, and challenges of utilizing wake-up radio-based access in 5G networks are reviewed and discussed. To this end, the operating principle and associated wake-up signal structures are first reviewed, together with the corresponding power consumption and buffering delay trade-offs. Then the applicability of wake-up methods at mmWave bands and beamforming systems is addressed and highlighted. Additionally, an energy-efficient mobility management procedure for wake-up radio-based devices is described and demonstrated, utilizing narrowband uplink reference signals. Overall, the article provides an overview of wake-up-based access in 5G systems as a promising power-saving mechanism, and discusses the associated prospects, benefits, and challenges.

Scheduling and Decoding of Downlink Control Channel in 3GPP Narrowband-IoT

Narrowband Internet of Things (NB-IoT) is a low power wide area network technology introduced by the $3^{rd}$ Generation Partnership Project (3GPP). It is a derivative of the existing 3GPP Long Term Evolution (LTE) that will enable cellular service to a massive number of IoT devices. In comparison with LTE and 5G New Radio, the NB-IoT devices will be of low cost, low throughput, and delay-tolerant. The reduction in available bandwidth and introduction of repetitions for achieving wider coverage requires modified Narrowband Physical Downlink Control Channel (NPDCCH) search space design and decoding as compared to the LTE. Hence, in this paper, we first explain the NPDCCH physical layer procedures, along with the search space decoding. Unlike LTE, there is no channel feedback mechanism in NB-IoT. Therefore, we propose a novel resource mapping scheme for NPDCCH based on the uplink reference signals. We perform system-level simulations and analyze the impact of the proposed mapping for varying operating frequencies and channel conditions. Further, the NB-IoT devices have limitations on the battery power, and hence, the existing control channel schedulers cannot be reused for the NB-IoT scenario. Thus, we propose a novel scheduler for NPDCCH. We have also modified the current state-of-the-art algorithms to meet the NPDCCH constraints and compared them against the proposed scheduler. We derive bounds for such scheduling algorithms and show that the proposed scheduler additionally conserves up to 25% of the IoT device battery power. Through Monte Carlo simulations, we show that the proposed scheduler better achieves the various trade-offs between power consumption, search space utilization, and fairness as compared to the existing schedulers.

Massive MIMO With Massive Connectivity for Industrial Internet of Things

In this paper, we investigate the feasibility of using massive multi-in multi-out (MIMO) to provide connectivity to Industrial Internet of Things (IIoT) wirelessly. Single-cell massive MIMO is used to support a large number of low-power IIoT devices in the uplink simultaneously. To support a large number of devices simultaneously, orthogonal uplink reference signals are heavily reused, which severely compromises the quality of the channel estimation for all devices. We show that the attempt by zero-forcing (ZF) decoding to zero out the interference from a large number of devices results in very poor performance, whereas maximum ratio (MR) decoding outperforms ZF decoding by a large margin. We present theoretical closed-form equations of spectral efficiency for both MR and ZF processings with massive connectivity. Simulation results validate our theoretical analysis. Furthermore, we provide analyses that are useful for designing low-power massive IIoT networks using massive MIMO.

An Enhanced Random Access With Preamble-Assisted Short-Packet Transmissions for Cellular IoT Communications

We propose an enhanced random access (RA) with preamble-assisted short-packet transmissions to support cellular Internet-of-Things (IoT) communications. A key feature of the proposed scheme is that the base station (e.g., eNodeB in LTE networks) utilizes the RA preambles as uplink reference signals to estimate uplink channel state information of IoT devices, which subsequently provides additional opportunities to transmit short packets during the RA procedure without extra signaling (e.g., connection and scheduling requests). Through simulations, we evaluate the performance of the proposed scheme in terms of a channel estimation mean squared error, a bit error rate, a preamble collision probability, and a success probability of short-packet transmissions. Results show that the proposed scheme can support reliable and low-latency featured short-packet transmissions during the RA procedure by efficiently incorporating both multiple antenna and detection techniques.

Improved Energy Efficiency of Massive MIMO-OFDM in Battery-Limited IoT Networks

We investigate the feasibility of improving the energy efficiency (EE) of massive multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems applied to a battery-limited Internet of Things (IoT) networks. Improving EE is especially important for battery limited IoT devices. We observe the uplink and downlink aspects of massive MIMO-OFDM-based IoT networks and categorize some of the effective methods to consider. As uplink aspect, we consider the uplink reference signal (RS) power control. Reducing uplink RS power could induce the battery saving of IoT devices but could cause an increase in channel estimation error. As downlink aspect, we consider the peak-to-average power ratio reduction of the OFDM signal and downlink transmitter power control. These techniques are well-known as effective EE improvement methods, but there is little work showing the actual EE gain in system perspective. In addition, we also consider the utilization of radio frequency energy transfer using unmanned aerial vehicles to extend the operating time of battery-limited IoT devices. We derive the theoretical closed-form approximations of spectral efficiency and EE when applying these methods and provide EE gains for various scenarios. Numerical results show that the theoretical analysis is in good agreement with the simulation results and thus can be used as useful tools to improve EE.

Joint Device Positioning and Clock Synchronization in 5G Ultra-Dense Networks

In this paper, we address the prospects and key enabling technologies for highly efficient and accurate device positioning and tracking in fifth generation (5G) radio access networks. Building on the premises of ultra-dense networks as well as on the adoption of multicarrier waveforms and antenna arrays in the access nodes (ANs), we first formulate extended Kalman filter (EKF)-based solutions for computationally efficient joint estimation and tracking of the time of arrival (ToA) and direction of arrival (DoA) of the user nodes (UNs) using uplink reference signals. Then, a second EKF stage is proposed in order to fuse the individual DoA and ToA estimates from one or several ANs into a UN position estimate. Since all the processing takes place at the network side, the computing complexity and energy consumption at the UN side are kept to a minimum. The cascaded EKFs proposed in this article also take into account the unavoidable relative clock offsets between UNs and ANs, such that reliable clock synchronization of the access-link is obtained as a valuable by-product. The proposed cascaded EKF scheme is then revised and extended to more general and challenging scenarios where not only the UNs have clock offsets against the network time, but also the ANs themselves are not mutually synchronized in time. Finally, comprehensive performance evaluations of the proposed solutions on a realistic 5G network setup, building on the METIS project based outdoor Madrid map model together with complete ray tracing based propagation modeling, are provided. The obtained results clearly demonstrate that by using the developed methods, sub-meter scale positioning and tracking accuracy of moving devices is indeed technically feasible in future 5G radio access networks operating at sub-6 GHz frequencies, despite the realistic assumptions related to clock offsets and potentially even under unsynchronized network elements.

Design and Improving Techniques for Uplink Reference Signal in LTE Compatible Mobile Satellite Communication System

Reference Signal (RS) is used for channel estimation and sounding in LTE uplink of terrestrial system to accomplish data demodulation. However, the situation in multiple-beam mobile satellite communication system is different. The Uplink Reference Signal (UL-RS) designed in terrestrial LTE doesn’t fit the satellite environment and may result to system performance degradation. In this paper, the UL-RS pattern used in GEO multiple-beam mobile satellite communication system is proposed, as well as two enhancement techniques, i.e., full bandwidth sounding, retransmit and combined receive. System performance index calculation and simulation results prove the effectiveness of the proposed design.