Cell-specific Reference Signals Research Articles

Wireless Interference Identification With Convolutional Neural Networks Based on the FPGA Implementation of the LTE Cell-Specific Reference Signal (CRS)

Wireless Interference Identification With Convolutional Neural Networks Based on the FPGA Implementation of the LTE Cell-Specific Reference Signal (CRS)

In-Band Ambient FSK Backscatter Communications Leveraging LTE Cell-Specific Reference Signals

In-Band Ambient FSK Backscatter Communications Leveraging LTE Cell-Specific Reference Signals

Signal Tracking Algorithm With Adaptive Multipath Mitigation and Experimental Results for LTE Positioning Receivers in Urban Environments

Positioning with cellular signals has been gaining attention in urban and indoor environments, where global navigation satellite system signals have limited availability due to interference, blockage, or multipath. However, accurate and reliable tracking of cellular signals under highly dynamic urban channel conditions remains a challenging task. This article presents a cellular long-term evolution (LTE) signal tracking algorithm implemented by an adaptive multipath estimating delay lock loop (AMEDLL) to achieve carrier phase synchronization and time-of-arrival (TOA) tracking under severe multipath propagation conditions. The analytical expression of the coherently integrated correlation result over multiple slots with the LTE cell-specific reference signal is derived. A multipath estimator along with a simple yet efficient multipath estimation monitoring approach is developed to estimate the parameters of all detected multipath signals. Several heuristic monitoring criteria based on historical multipath parameter estimations are established to enable adaptive adjustment of the estimated path number. Real LTE signals are collected in an urban environment for the signal tracking performance evaluation. This article presents two case studies with varying levels of multipath effects and signal power to illustrate the effectiveness of the developed signal tracking algorithm. Instead of a TOA truth reference, open-loop carrier phase estimations are used to analyze the TOA tracking error. Our analyses demonstrate that the AMEDLL-based tracking algorithm provides improved TOA estimation accuracy over the existing super-resolution-algorithm-based and delay-lock-loop-based tracking schemes.

Multipath Estimating Delay Lock Loop for LTE Signal TOA Estimation in Indoor and Urban Environments

Long-term evolution (LTE) signals are potential signals-of-opportunity for position and navigation, especially in challenging urban and indoor environments. A major challenge is that the LTE signal time-of-arrival (TOA) estimations are susceptible to the multipath propagation effects. In this paper, the multipath estimating delay lock loop (MEDLL), which is originally designed for global positioning system receivers, is applied to LTE signal TOA estimation in multipath environments. We derive the analytical expression of the correlation function for LTE signals and present the procedure for estimating parameters of the detected multipath components. Two initialization methods without and with super-resolution algorithm (SRA) are developed for the MEDLL. Our analyses show that the MEDLL with SRA-based initialization can achieve better multipath resolution, while the one without SRA has less complexity. Extensive simulations involving static multipath scenarios are conducted to examine the statistical TOA estimation performance of the proposed MEDLL with LTE cell-specific reference signal. The simulation results and computational complexity analysis indicate that the proposed MEDLL outperforms the conventional delay lock loop and SRA in term of multipath mitigation performance and computational complexity. Experimental results using real collected LTE signals in urban environments are also provided to demonstrate the effectiveness of the proposed technique for realistic scenarios.

Interference Modeling for Low-Height Air-to-Ground Channels in Live LTE Networks

In this letter, a recently conducted measurement campaign in two suburban scenarios for investigating the low-height air-to-ground channels is presented. A universal-software-radio-peripheral-based channel sounder on-board an unmanned aerial vehicle (UAV) was exploited to record the real-time downlink signals of the live Long Term Evolution networks. Several horizontal flights at different heights were conducted in the measurement campaign. Channel impulse responses (CIRs) of links from the UAV to different live cells were extracted by exploiting the cell-specific reference signals received in the downlink data. By using the empirical CIRs, a maximization likelihood estimator is derived based on the space-alternating generalized expectation-maximization principle to obtain the signal power for individual channels between multiple base stations and the UAV. Interference, number of detected cells, handover rate, etc., are studied, which provide insights to understand interference for low-height air-to-ground channels.

Detection of the First Component of the Received LTE Signal in the OTDoA Method

In a modern world there is a growing demand for localization services of various kinds. Position estimation can be realized via cellular networks, especially in the currently widely deployed LTE (Long Term Evolution) networks. However, it is not an easy task in harsh propagation conditions which often occur in dense urban environments. Recently, time-methods of terminal localization within the network have been the focus of attention, with the OTDoA (Observed Time Difference of Arrival) method in particular. One of the main factors influencing the accuracy of location estimation in the OTDoA method is the nature of the propagation channel that affects the ease of isolating the signal component travelling from the transmitter to the receiver through the shortest path. To obtain the smallest possible localization error, it is necessary to detect the first received component of the useful signal. This aim could be achieved by using a proper algorithm within the receiver. This paper proposes a new algorithm for effective detecting of the first component of the LTE downlink signal in the multipath environment. In a mobile terminal location estimation process, CSRS (Cell Specific Reference Signal) signals were used instead of dedicated PRS (Positioning Reference Signal) signals. New solution was verified during the measurement campaign in a real LTE network.

Constant False Alarm Rate Detection of Multicarrier Signals With Periodic Power Boosting

Given the prevalence of multicarrier waveforms in wireless systems, the sensing of such signals is of high importance for cognitive radio applications. In this paper, we exploit periodic power boosting of subcarriers in multicarrier signals which is applied, e.g., in LTE to improve the performance of sensing compared to state-of-the-art schemes under practical scenarios where noise uncertainties are present. First, we establish an analytical connection between the subcarrier power distribution and the autocorrelation function of a general multicarrier signal. Second, using this result, we demonstrate that periodic power boosting causes strong peaks in the magnitude of the autocorrelation function. The obtained analytical results match well with over-the-air measurement results corresponding to an orthogonal frequency division multiplexing (OFDM) signal of an LTE transmission. Here, the measurements indicate the presence of a boosting with a period of three subcarriers in the LTE signal which arises due to the cell-specific reference signals. Subsequently, algorithms for both sensing before transmission and simultaneous sensing and reception exploiting the special features of the autocorrelation function resulting from periodic power boosting are developed which outperform energy detection under realistic noise uncertainties as well as several state-of-the-art methods for detecting OFDM signals.

Modeling and analysis of LTE connectivity in a high mobility vehicular environment

Long-Term Evolution (LTE) technology has several features that make it an attractive alternative to be used in vehicle-to-infrastructure communications for intelligent transportation systems. However, before LTE can be widely used in this context, a number of analyses must provide convincing evidence that critical network functions (e.g. resource allocation strategies) yield adequate performance. To this end, in this work, we introduce a Markov-chain based model for LTE downlink channel quality, a prime factor affecting performance. Our model comes from the analysis of a large number of measurements of LTE Cell-Specific Reference Signals that were collected through a crowdsourcing application on a motorway in the UK. The model is intended to be used in performance evaluation studies and we exemplify its use with a case study, where we estimate the downlink transmission capacity of an LTE network. We also discuss other potential applications.

A New Link Adaptation Method to Mitigate SINR Mismatch in Ultra-Dense Small Cell LTE Networks

In this paper, we consider a new interference problem caused by idle small cells, which have no associated user equipment in ultra-dense small cell long-term evolution (LTE) networks. Specifically, we investigate the effect of idle small cells on the signal to interference and noise ratio (SINR) of the cell-specific reference signal (CRS) and the data signals. We confirm that CRS interference from idle small cells produces uneven interference pattern across CRS and data signals and eventually causes an SINR mismatch between CRS and data signals as well as between data signals with and without CRS symbols. In addition, these phenomena become severe with cell densification. In order to solve this mismatch problem, we propose a simple link adaptation framework, which utilizes clustered CRS assignment and hybrid SINR measurement. The numerical results show that the proposed method improves the average sum throughput compared with the conventional approaches. Overall, this paper sheds new light on investigating and coping with the interference problem coming from idle small cells in future ultra-dense small cell LTE networks.

Theoretical Analysis of Interference Effect From Idle Cells in Ultra-Dense Small Cell Networks

One of the key characteristics in ultra-dense small cell networks (UDNs) is the advent of idle cells which have no associated user equipment (UE). Indeed, idle cells always transmit the cell-specific reference signal (CRS), while they do not transmit data signals. In this paper, we focus on providing a theoretical analysis on the effect of idle cells in terms of the aggregate interference and signal-to-interference ratio (SIR). First, we investigate how idle cells affect the CRS interference (i.e., aggregate interference at the CRS) and data interference (i.e., aggregate interference at the data signals). Specifically, we derive the statistical distribution for the ratio of CRS interference to data interference. Second, by using the Kolmogorov–Smirnov statistic, we qualitatively measure the amount of SIR mismatch between CRS SIR and data SIR caused by idle cells in the UDN. In the numerical results, we show that CRS interference becomes larger than data interference with cell densification. We also illustrate that the SIR mismatch between CRS SIR and data SIR becomes severe, as base station (BS)-to-UE density ratio increases (i.e., idle BS ratio increases). Moreover, we can confirm that the SIR mismatch in indoor environment is weaker than outdoor owing to the multiple penetration losses such as outer walls and inner walls.

Hybrid Lower-Complexity Wiener Filter for Pilot-Based Channel Estimation for C-RS in LTE-A DL System

This paper presents a pilot-based lower complexity channel estimation for Long Term Evolution Advanced (LTE-A) Cell-specific Reference Signals (C-RS). The proposed system is a hybrid one, based on using 2 × 1-D Wiener filter for noise filtering at pilot locations only with cubic spline interpolation for data symbol locations, with accuracy that almost matches the 2-D Wiener filter and Wiener interpolation under certain channel and noise conditions, and at reduced computational complexity. The performance of the proposed scheme is evaluated in terms of Mean Squared Error (MSE) under various channel conditions. Complexity analysis show that the proposed scheme requires 8.8% of the number of computations needed by 2-D Wiener filtering with Wiener interpolation, while still matching its performance up till specific noise and channel conditions. In addition, 2 × 1-D Wiener filter performance is proven to be sub-optimal when compared with the performance of 2-D Wiener filter. The obtained results show that the best noise filtering method is a combination of both moving average and Wiener filtering, where the lower bound is a function of both Symbol-to-Noise-Ratio (SNR) and the channel statistics. Simulations in real-life LTE-A scenarios confirms the feasibility of the proposed channel estimation scheme. Analytical results wherever possible and, in general, simulation results are presented.

EXTRAPOLATION METHOD FOR MAXIMAL AND 24-H AVERAGE LTE TDD EXPOSURE ESTIMATION.

The Long-Term Evolution (LTE) system represents the evolution of the Universal Mobile Telecommunication System technology. This technology introduces two duplex modes: Frequency Division Duplex and Time Division Duplex (TDD). Despite having experienced a limited expansion in the European countries since the debut of the LTE technology, a renewed commercial interest for LTE TDD technology has recently been shown. Therefore, the development of extrapolation procedures optimised for TDD systems becomes crucial, especially for the regulatory authorities. This article presents an extrapolation method aimed to assess the exposure to LTE TDD sources, based on the detection of the Cell-Specific Reference Signal power level. The method introduces a βTDD parameter intended to quantify the fraction of the LTE TDD frame duration reserved for downlink transmission. The method has been validated by experimental measurements performed on signals generated by both a vector signal generator and a test Base Transceiver Station installed at Linkem S.p.A facility in Rome.

Vehicular Position Tracking Using LTE Signals

This paper proposes and validates, in the field, an approach for position tracking that is based on Long-Term Evolution (LTE) downlink signal measurements. A setup for real data live gathering is used to collect LTE signals while driving a car in the town of Rapperswil, Switzerland. The collected data are then processed to extract the received LTE cell-specific reference signals (CRSs), which are exploited for estimating pseudoranges. More precisely, the pseudoranges are evaluated by using the “ESPRIT and Kalman Filter for Time-of-Arrival Tracking” (EKAT) algorithm and by taking advantage of signal combining in the time, frequency, spatial, and cell ID domains. Finally, the pseudoranges are corrected for base station's clock bias and drift, which are previously estimated, and are used in a positioning filter. The obtained results demonstrate the feasibility of a position tracking system based on the reception of LTE downlink signals.

CRS-aided MMSE algorithm for LTE Downlink

MMSE algorithm has good performance in an ideal situation, but channel information is required. In order to improve the practicality of the MMSE algorithm, in this paper, a method that estimates channel information through CRS (Cell-specific Reference Signal) is proposed in LTE system. In the simulation, MMSE algorithm, ML algorithm and LS algorithm are compared in the LTE system under the same simulation conditions. Simulation results show that CRS-aided MMSE algorithm outperforms ML algorithm and LS algorithm.

Enhanced Signal Detection in 3GPP LTE System with Successive Interference Cancellation

In this paper we introduced interference cancellation (IC) in the cell search and the detection of physical downlink shared channel (PDSCH) in long term evolution (LTE) of 3rd generation partnership project (3GPP). After user equipment (UE) is turned on, it has to first search the target cell using synchronization channel (SCH). After UE gets timing and cell identification (Cell ID) by cell search process, it can start to transmit and receive data through the traffic channel. In most cases, UE experiences the interference from surrounding neighbor cells, which may result in the degradation of the receiver performance. Not only in time division duplex (TDD) but also frequency division duplex network (FDD), the cells must be deployed with the time aligned to support better successful handover and also provide multemedia broadcast and multicast service (MBMS) efficiently in the network. In synchronous network system, since every cell sends his SCH using shared resource allocation, it is inevitable that two or more neighboring cells can interfere each other in some situation. The cell specific reference signal (CRS) is used for channel estimation in downlink reception. The CRS from neighbor cells may overlap with the resource of PDSCH of the target cell. Hence, the collision of SCH and CRS from the neighbor cells can degrade both the performance of cell search and PDSCH reception at the same time. In this paper, we analyzed the improved performance of the cell search and PDSCH reception by using successive interference cancellation. Simulation results show that the proposed scheme can effectively reduce the mutual interference and improve the performance of the signal detection in view of the cell search and PDSCH reception, respectively.

CRS interference cancellation algorithm for heterogeneous network

Heterogeneous network is introduced to improve the network capacity in LTE Release 9 and system beyond. The potential traffic congestion due to increased users can be alleviated by the cooperation between macro-cell and pico-cell. However, the inter-cell interference caused by RF signal from macro-cell will reduce the performance severely. Enhanced inter-cell interference coordination is proposed in Rel. 10 to solve this problem using almost blank subframe (ABS). Yet, the cell specific reference signal in ABS can still cause interference to the data resource element (RE) from the pico-cell inevitably for non-colliding scenario. In this Letter, a novel interference cancellation (IC) algorithm is proposed to mitigate the interference. First, the timing and carrier frequency offset of interference signal is estimated and compensated. Second, the interfering channel response is estimated by utilising the channel statistics. Third, the interference signal is reconstructed based on the channel estimation and cancelled in the received signal in time domain. The experiment results show that the performance of proposed IC algorithm is robust.

Implementation of an LTE-Based Channel Measurement Method for High-Speed Railway Scenarios

Channel measurements are the prerequisite condition for the design of the next-generation high-speed railway (HSR) communication system. Because of the measurement restriction and measurement efficiency issues of applying conventional channel sounders in HSR scenarios, railway-network-based channel measurements are becoming quite attractive. In this paper, a method that employs the cell-specific reference signal in long-term-evolution (LTE) railway networks as the excitation signal to perform HSR channel measurements is presented. This method is implemented by a novel channel measurement system composed of existing LTE railway networks and a dedicated LTE receiver, which can efficiently collect the time-frequency–space channel data and is able to measure different communication links simultaneously. Based on this system, field measurements that consider both direct coverage and relay coverage schemes are conducted on the Beijing-to-Tianjin HSR in China. Statistical results, involving path loss, Ricean $K$ -factor, root-mean-square delay spread, single-link spatial correlation (SC), and multilink SC, are derived, which not only confirm the viability of the proposed method but also provide realistic HSR channel characteristics for the research on future HSR communications.

Downlink training scheme for massive MIMO systems

A novel downlink training scheme is proposed for time-division duplex massive MIMO systems. Different from conventional solutions, including the cell-specific reference signal in LTE, the UE-specific reference signal in LTE-A and the commonly used method without the downlink training sequence, the cell-specific demodulation reference signal is proposed by utilising the inherent asymptotic orthogonality, which is shared by all users within one cell and used for the estimation of effective channel coefficients. The training overhead is reduced and the downlink spectral efficiency can be improved considerably.

Macro-assisted data-only carrier for 5G green cellular systems

As the commercial operation of 4G systems is speeding up worldwide to meet the increasingly explosive growth of mobile Internet in the 2020 era, a great many R&D efforts targeting the next generation wireless systems (5G) have been launched. With densely deployed small cells in a heterogeneous network, the excessive signaling overhead of conventional carrier design greatly degrades system performance. A new carrier designed for small cells in the next generation system will be highly beneficial toward a green communication network. In this article, we propose a macro-assisted data-only carrier for future 5G networks from a green prospective. With the help of macro base stations, control channel overhead, and cell-specific reference signals for small cells can be minimized to achieve a pure-data carrier. Under this architecture, key procedures are designed including small cell identification and access, synchronization, hand over, and small cell sleeping. Furthermore, to evaluate the potential of our proposed scheme, a complete system and link-level simulation platform according to the current 3GPP LTE standard is built. The simulation results show that our proposed systems, achieving significant performance enhancement of signaling overhead, can lead to more than 17 percent throughput improvement and 90 percent energy efficiency gain over the current LTE HetNet with on-off strategy implemented. The proposed scheme appears to be highly attractive as part of the future 5G green mobile networks.

Reference Signals Design and Evaluation on Radio-Interface Based Synchronization of LTE Base Station System

This paper discusses the reference signals used in synchronization for LTE system. In general, primary synchronization signal (PSS) correlation based acquisition and reference signals based tracking loop are applied for time and frequency synchronization but it is unclear which legacy reference signals (RS) will be used as the listening RS for radio-interface based synchronization. Besides the legacy cell-specific reference signals (CRS) utilized as the listening RS, the other potential candidate could be either CSI reference signals (CSI-RS) or position reference signals (PRS). Upon performance evaluation, we compare the pros and cons of CRS, CSI-RS and PRS based synchronization and present our views.