Secondary Synchronization Signal Research Articles

Reduced Complexity Detection of Narrowband Secondary Synchronization Signal for NB-IoT Communication Systems

Narrowband Internet of Things is one of the most promising technologies to support low cost, massive connection, deep coverage, and low power consumption. In this paper, a computationally efficient narrowband secondary synchronization signal detection method is proposed in the narrowband Internet of Things system. By decoupling the detection of complementary sequence and Zadoff–Chu sequence that make up the synchronization signal sequence, the search space of narrowband secondary synchronization signal hypotheses is reduced. Such a design strategy along with the use of the symmetric property of synchronization signals allows reduced-complexity synchronization signal detection in the narrowband Internet of Things system. Both theoretical and simulation results are provided to verify the usefulness of the proposed detector. It is shown via simulation results that the complexity of the proposed detection method is significantly reduced while producing some performance degradation, compared to the conventional detection method.

Complexity Effective Sequential Detection of Secondary Synchronization Signal for 5G New Radio Communication Systems

In this article, a complexity-effective secondary synchronization signal (SSS) detection scheme is presented for fifth-generation (5G) new radio (NR) communication systems, which uses a gold sequence for SSS generation by multiplying two different m-sequences. By taking advantage of good autocorrelation property between cyclic-shifted versions of m-sequence, the search space of SSS hypothesis testing is reduced by decoupling the detection of two m-sequences. The combination of such a sequential strategy with maximum likelihood detection criterion facilitates low-complexity and reliable SSS detection in the 5G NR system. Both numerical and simulation results are presented to justify the effectiveness of the proposed detection scheme. It is demonstrated from presented results that the proposed SSS detector achieves a considerable complexity reduction while incurring slight performance loss, compared to existing SSS detectors.

Effective Joint Detection of Synchronization Signal, Duplex Mode, and Frequency Offset for LTE Machine-Type Communications

In this article, a complexity effective and robust cell search method is proposed for machine-type communication in a long-term evolution system. The correlation between the primary synchronization signal (PSS) and secondary synchronization signal (SSS) in the frequency domain is exploited to remove the effect of channel fading distortion and symbol timing offset. This design facilitates robust synchronization of the carrier frequency offset, sector ID, and duplex mode without performing hypothesis testing on SSS candidates, followed by the detection of group ID. Both theoretical and experimental results are presented to justify the effectiveness of the proposed cell search scheme. The simulation results show that the proposed method achieves robust and reduced-complexity cell search when there are nonnegligible residual symbol timing error and frequency-selective fading distortion, compared to existing methods.

Robust OFDM-Based Synchronization Against Very High Fractional CFO and Time-Varying Fading

This article investigates the advantages and disadvantages of cell search for LTE and NR. This investigation enables us to design a couple of synchronization signals specified in frequency domain for a whole of cell search. For time/frequency synchronization and partial cell identification, to achieve both low complexity and robustness against very high carrier frequency offset (CFO) and time-varying fading, we propose a primary synchronization signal generated by the centrally symmetric concatenation of a Zadoff-Chu sequence and its modified sequence. Also, for final cell identification, to minimize intercell interference, we propose a secondary synchronization signal constructed by element-wise exclusive-OR operation of two different cyclic-shifted m-sequences. By complexity, intercell interference analysis, and accuracy-optimized searching description, as well as cell-search evaluation, we elucidate that the proposed scheme provides many advantages, such as lower complexity, immunity toward CFO and mobility, and shorter mean cell search time directly related with latency and battery life.

A Robust Joint Cell Identification and Frequency Recovery Scheme for LTE Machine-Type Communications

The main features for future machine-type communication (MTC) cellular networks are to support low-power consumption, low-bandwidth, and long-range communication. One of the most challenging issues to meet these goals in the MTC cellular network is a low-complexity and reliable initial synchronization. To address this issue, in this article an efficient joint detection of integer carrier frequency offset (IFO) and cell identity (CID) is proposed for MTC in a long-term evolution (LTE) system. Unlike the conventional approach, the primary synchronization signal (PSS) and secondary synchronization signal (SSS) transmitted in the LTE-MTC system are used together to remove the effect of channel fading and symbol timing offset (STO). This design facilitates robust detection of the IFO and CID in the presence of residual STO without priori information on the SSS. The grouping of PSS subcarriers is integrated for joint detection, which plays an important role on reducing computational load. Both analytical and simulation results are provided to corroborate the feasibility of the proposed method. It has been demonstrated that the unique characteristics of the PSS and SSS sequences are effectively used for robust cell search in the presence of residual STO, compared to existing methods.

A Countermeasure against Smart Jamming Attacks on LTE Synchronization Signals

Long-Term Evolution (LTE) is one of the most frequently used wireless communication technology. As every wireless network, LTE is vulnerable to physical layer (PHY) jamming attacks due to the broadcast nature of channels. Since the jammer attacks are getting smarter and energy efficient, they can target a specific region or physical channel instead of entire band. Targeting the physical LTE downlink Synchronization Signals (SS) could be the most dangerous objective. In this paper, we investigate LTE PHY jamming attack against only primary and secondary synchronization signals. Jammer detection is performed by using Neyman-Pearson theorem. Then, a countermeasure method is proposed. Simulation results show that the proposed countermeasure can achieve lower pollution and better correct cell id performances during smart jamming attack against SS.

Improved Joint Estimation of Set Information and Frequency Offset for LTE Device-to-Device Communications

Device-to-device (D2D) communication is a key enabler to facilitate the realization of the Internet of Things in which direct communication between mobile users is allowed without the intervention of the base station. One of the most challenging issues in the D2D cellular network is a low-complexity and reliable initial synchronization. To solve this issue, an efficient joint detection of integer carrier frequency offset (IFO) and set information (SI) is proposed for D2D communication in a long term evolution (LTE) system. Unlike the conventional approach, the primary sidelink synchronization signal (PSSS) and secondary sidelink synchronization signal (SSSS) transmitted in the LTE-D2D system are used together to remove the effect of channel fading and symbol timing offset (STO). In addition, the conjugate property between two PSSS sequences is utilized. This design facilitates robust sequential detection of the IFO and SI without priori information on two synchronization signals when residual STO is present. It has been demonstrated that the inherent properties of the PSSS and SSSS sequences are effectively used for efficient cell search in the presence of residual STO, compared to existing methods.

Cell-search and tracking of residual time and frequency offsets in low power NB-IoT devices

Initial synchronization and cell-search in cellular based systems is the first step of establishing communication link between user equipment (UE) and base station (BS). Under this process UE aligns to the timing and frequency of the BS and acquires critical information like cell-ID, frame number and system bandwidth. Narrow band IoT (NB-IoT) is 3GPP defined cellular based IoT technology in which cell-search is facilitated through a set of two signals namely NB primary synchronization signal (NPSS) and NB secondary synchronization signal (NSSS). Across all BS, the same NPSS sequence is used. However, the NSSS sequences is used to distinguish them using a cell identity. We assume NPSS based initial synchronization is carried out to estimate timing and frequency offsets using existing traditional algorithms. In this paper, we first propose NSSS detection algorithm which is used to detect cell-ID and partial information of frame numbering. Post initial cell-search, UE has to be constantly synchronized to the BS in order to transmit or receive the data. For this purpose of periodic tracking of the timing and frequency offsets we make use of NB reference signals (NRSs). In the second part of the paper, we present an algorithm to track the residual time and frequency offsets using NRS which are distributed across frequency and time grid. We show novel algorithms that achieve significant detection performance even at low SNR values like − 12 dB which is the typical operating range for low power NB-IoT devices. The simulation results prove these claims.

Effective Estimation of Integer Carrier Frequency Offset in LTE Downlink Systems With Symbol Timing Error

An effective integer carrier frequency offset (IFO) detection scheme is proposed in the long-term evolution downlink system without relying on the knowledge of the cell ID (CID) that is carried through the synchronization signal. To enable IFO detection without relying on the CID, the proposed IFO detection method utilizes the symmetric property of the synchronization signal, such as the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). This design avoids the need to retrieve the CID that is carried by the PSS and the SSS at the IFO-matching stage. In addition to investigating the usefulness of the proposed IFO detection method, the probability of detection failure is theoretically calculated. The simulation results demonstrate that both the synchronization signals are effectively exploited for the robust detection of the IFO in the presence of non-negligible symbol timing error, where the conventional approaches show unsatisfactory performance.

The Primary Synchronization Signal of 5G NR

Initial access is the process that a user equipment (UE) establishes initial connection to the cellular network, and the first step is to detect the presence of primary synchronization signal (PSS) and secondary synchronization signal (SSS) to conduct cell-search and downlink synchronization. This paper focus on the PSS waveform of 5G New Radio (NR). We present two waveforms suitable for NR-PSS: m sequence and Zadoff-Chu (ZC) sequence, then discuss their properties of autocorrelation and cross-correlation when adopted for NR-PSS. In addition, we discuss their feasibility since frequency allocation for 5G NR includes millimeterwave (mmWave) bands that may bring large frequency offset. On the basis of matched-filter detection with hypothesis, we further evaluate the frequency offset estimation performance of these two sequence.

A Joint Low-Power Cell Search and Frequency Tracking Scheme in NB-IoT Systems for Green Internet of Things.

As a dedicated communication protocol for Internet-of-Things, narrowband internet of things (NB-IoT) needs to establish the communication link rapidly and reduce retransmissions as much as possible to achieve low power consumption and stable performance. To achieve these targets, the low-power scheme of the initial cell search and frequency tracking is investigated in this paper. The cell search process can be subdivided into narrowband primary synchronization signal (NPSS) detection and narrowband secondary synchronization signal (NSSS) detection. We present an NPSS detection method whose timing metric is composed of symbol-wise autocorrelation and a dedicated normalization factor. After the detection of NPSS, the symbol timing and fractional frequency offset estimation is implemented in a resource-efficient way. NSSS detection is conducted in the frequency domain with a calculation-reduced algorithm based on the features of NSSS sequences. To compensate the accumulated frequency offset during uplink transmission, a pilot-aided rapid frequency tracking algorithm is proposed. The simulation results of the proposed cell search scheme are outstanding in both normal coverage and extended coverage NB-IoT scenarios, and the accumulated frequency offset can be estimated with high efficiency.

A complete cell search and synchronization in LTE

The initial process of identifying any available base station (BS) by a user equipment (UE) that wants to communicate is termed as cell search. To ensure a reliable communication, any UE has to be synchronized with the BS both in time and frequency domains. Cell search process is said to be complete once cell ID associated with long-term evolution (LTE) BS is decoded successfully. This paper presents a series of cell search and synchronization algorithms, which efficiently estimated time and frequency offsets as well as cell ID. The synchronization signals present in LTE, namely, primary synchronization signal (PSS) and secondary synchronization signal, that carry cell ID are critically exploited in the algorithms. The aforementioned algorithms are classified into two modules, namely, module I and module II, based on their computation complexity.In module I, a cyclic prefix (CP)-based maximum likelihood (ML) estimator is employed to obtain a coarse estimate of time and fractional frequency offset; however, the estimates are refined using synchronization signals. A joint estimation of timing, integer frequency offset (IFO), and PSS ID (sector ID) is carried out in module II. Both the modules operate on the cross-correlation approach of PSS with the received signal for obtaining timing and sector ID. IFO as a part of module II is detected from a finite hypotheses set using synchronization signals. Extensive simulations are carried out on a time-varying frequency-selective channel to analyze the performance of the algorithms.

A Maximum Likelihood Approach for SSS Detection in LTE Systems

Before establishing a communication link with the serving base station (eNodeB), user equipment (UE) operating in a long-term evolution (LTE) multi-cellular network must acquire some specific information, including the sector identity and cell group identity. For this purpose, two training sequences called primary synchronization signal (PSS) and secondary synchronization signal (SSS) are periodically transmitted in the downlink to convey such information. In this work, we present a novel maximum likelihood (ML) approach for SSS detection assuming that the PSS has been successfully identified at an earlier stage. As we shall see, the resulting scheme turns out to be too complex for practical implementation as it requires perfect knowledge of the channel covariance matrix. Therefore, we look for simpler solutions and propose two reduced-search methods that operate in a mismatched mode. The first scheme exploits channel state information emerging from both the primary and secondary synchronization signals, while the second scheme operates using only the secondary synchronization signal. Numerical analysis indicates that the proposed methods outperform existing alternatives and can be successfully applied even in a severe propagation scenario. The price for such an advantage is a certain increase of the processing requirement.

Synchronization method for long-term evolution-based machine-type communication in low-power cellular Internet of Things

The significant growth of machine-to-machine applications for low-power cellular Internet of Things has compelled 3rd Generation Partnership Project to ensure that the future release of long-term evolution can support massive transfer of small, infrequent packets using ultra-low-power and low-cost devices. The 3rd Generation Partnership Project version of machine-to-machine, called “machine-type communication,” is currently being standardized for low-cost machine-type communication operations. In this article, a complete synchronization and cell search procedure is described for machine-type communication devices in long-term evolution systems. Low-complexity algorithms for primary synchronization signal and secondary synchronization signal detection, which requires the highest computational complexity in synchronization and cell search period, are also proposed for low-power machine-type communication devices. Through simulation under long-term evolution-based machine-type communication environments, we show that the proposed methods for primary synchronization signal and secondary synchronization signal detection require six and five times less computational complexity than the conventional methods, respectively, while their performance is similar. The proposed algorithms allow machine-type communication devices in a discontinuous reception cycle to resynchronize quickly with less power when synchronization is lost during a deep sleep period.

A Robust Maximum Likelihood Scheme for PSS Detection and Integer Frequency Offset Recovery in LTE Systems

Before establishing a communication link in a cellular network, the user terminal must activate a synchronization procedure called initial cell search in order to acquire specific information about the serving base station. To accomplish this task, the primary synchronization signal (PSS) and secondary synchronization signal (SSS) are periodically transmitted in the downlink of a Long Term Evolution (LTE) network. Since SSS detection can be performed only after successful identification of the primary signal, in this work we present a robust scheme for joint PSS detection, sector index identification and integer frequency offset (IFO) recovery in an LTE system. The proposed algorithm relies on the maximum likelihood (ML) estimation criterion and exploits a suitable reduced-rank representation of the channel frequency response to take multipath distortions into account. We show that some PSS detection methods that were originally introduced through heuristic reasoning can be derived from our ML framework by selecting an appropriate model for the channel gains over the PSS subcarriers. Numerical simulations indicate that the proposed scheme can be effectively applied in the presence of severe multipath propagation, where existing alternatives provide unsatisfactory performance.

Design and Implementation of Time and Frequency Synchronization in LTE

A novel architecture for efficient time and frequency synchronization, applied to the long-term evolution (LTE) standard, is proposed. For symbol timing, we propose applying a symbol-folding method on top of the sign-bit reduction technique, leading to a novel algorithm for the cyclic prefix-type recognition in LTE. Following the symbol timing, the fractional carrier frequency offset is estimated and compensated using an adaptive gain loop, which allows for a high-accuracy compensation in a short interval. In the frequency domain, for cell search, we propose a sign-bit reduction technique on top of the matched filter method for the primary synchronization signal detection. In addition, we propose the sign-bit maximum-likelihood sequence detection algorithm for the secondary synchronization signal analysis. These methods result in >90% hardware reduction in the cell search compared with the state-of-the-art design. Moreover, possible structures for the frequency tracking in LTE are investigated leading to an experimental comparison, used to choose the best hardware-efficient structure. The proposed architecture is fabricated in a 130-nm CMOS technology occupying 0.68 $\mathrm{mm}^{2}$ of silicon area. At 25 °C and 1.2 V supply voltage, the fabricated chip consumes 34.4 mW at 42 MHz in the time domain and 34.6 mW at 188 MHz in the frequency domain. The fabricated and tested synchronizer core proves to have an outstanding performance for all defined communication modes in LTE.

Signal Design for Reduced Complexity and Accurate Cell Search/Synchronization in OFDM-Based Cellular Systems

This paper proposes a variant of the frequency-domain synchronization structure specified in the long-term evolution (LTE) standard. In the proposed scheme, the primary synchronization signal used in step-1 cell search is the concatenation of a Zadoff-Chu (ZC) sequence and its conjugate (as opposed to only the ZC sequence in LTE). For step-2 cell search, we propose a complex scrambling sequence requiring no descrambling and a new remapped short secondary synchronization signal that randomizes the intercell interference (as opposed to the first/second scrambling sequence and swapped short signals in LTE). Through a combination of analysis and simulation, we demonstrate that the proposed synchronization signals lead to lower searcher complexity than LTE, a lower detection error rate, a shorter mean cell search time, and immunity toward a frequency offset.

Low-Complexity Cell Search Algorithm for Interleaved Concatenation ML-Sequences in 3GPP-LTE Systems

In this paper, we present a low-complexity cell-search algorithm and architecture for secondary synchronization signal in 3GPP-LTE downlink systems. Differential correlations are computed to remove common channel effects so that two sets of synchronization signals can be combined to obtain enhanced detection quality. To achieve low complexity, we further adopt one-bit quantization and exploit the properties of limited offset in the indices of the two maximal-length sequences. The arithmetic complexity of the proposed algorithm is significantly reduced while attaining similar detection performance comparable to the conventional schemes. Finally, hardware architecture of the proposed algorithm is presented to demonstrate feasibility and efficiency in the implementation.

Low-Complexity Cell Search With Fast PSS Identification in LTE

Cell search and synchronization in the Third-Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system is performed in each user equipment (UE) by using the primary synchronization signal (PSS) and secondary synchronization signal (SSS). The overall synchronization performance is heavily dominated by robust PSS detection, which can be achieved in the conventional noncoherent detector by exploiting the essential near-perfect autocorrelation and cross-correlation properties of Zadoff-Chu (ZC) sequences. However, a relatively high computational complexity is observed in conventional algorithms. As compared with them, two new detectors, i.e., almost-half-complexity (AHC) and central-self-correlation (CSC) detectors, are proposed in this paper to achieve reliable PSS detection with much lower complexity by exploiting the central-symmetric property of ZC sequences. The complexity of the proposed CSC detector is only 50% that of the AHC detector, which achieves exactly the same PSS detection accuracy as that of the conventional detector with one half of complexity being saved. An improvement of CSC, i.e., CSC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ins</sub> , is also proposed in this paper to combat the large-frequency offset. The performance of the CSC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ins</sub> detector is independent of the frequency offset, and numerical results show that the 90% PSS acquisition time of CSC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ins</sub> can be well within an 80-ms duration, even in a heavy intercell-interference environment with a signal-to-noise ratio (SNR) of -10 dB.

In situ LTE exposure of the general public: Characterization and extrapolation

In situ radiofrequency (RF) exposure of the different RF sources is characterized in Reading, United Kingdom, and an extrapolation method to estimate worst-case long-term evolution (LTE) exposure is proposed. All electric field levels satisfy the International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference levels with a maximal total electric field value of 4.5 V/m. The total values are dominated by frequency modulation (FM). Exposure levels for LTE of 0.2 V/m on average and 0.5 V/m maximally are obtained. Contributions of LTE to the total exposure are limited to 0.4% on average. Exposure ratios from 0.8% (LTE) to 12.5% (FM) are obtained. An extrapolation method is proposed and validated to assess the worst-case LTE exposure. For this method, the reference signal (RS) and secondary synchronization signal (S-SYNC) are measured and extrapolated to the worst-case value using an extrapolation factor. The influence of the traffic load and output power of the base station on in situ RS and S-SYNC signals are lower than 1 dB for all power and traffic load settings, showing that these signals can be used for the extrapolation method. The maximal extrapolated field value for LTE exposure equals 1.9 V/m, which is 32 times below the ICNIRP reference levels for electric fields.