Time Frequency Division Research Articles

Photonic-Based W-Band Integrated Sensing and Communication System With Flexible Time- Frequency Division Multiplexed Waveforms for Fiber-Wireless Network

Photonic-Based W-Band Integrated Sensing and Communication System With Flexible Time- Frequency Division Multiplexed Waveforms for Fiber-Wireless Network

Performance Analysis of Coordinated Interference Mitigation Approach for Automotive Radar

Millimeter automotive radar has great potential in Advanced Driver Assistance Systems (ADAS) to enable safety features such as adaptive cruise control and collision avoidance. However, with widely deployment of millimeter radars on vehicles, the risk of radar mutual interference becomes a major factor limiting the high performance of radar detection. In this paper, we analyze the mutual interference among multiple Frequency Modulated Continuous Wave (FMCW) radars. On one hand, we study the interference in detail by considering co-channel interference and adjacent channel interference simultaneously. Besides, the co-channel interference is analyzed by employing stochastic geometry model while the adjacent channel interference is assessed by the deterministic analysis method. On the other hand, we propose a Time-Frequency Division Multiple Access (TFDMA) scheme to mitigate the interference in a coordinated manner and evaluate it in terms of mitigation delay, the probability of interference, effective detectable density, maximum number of interference-free radar, and control signaling overhead. Finally, we study the power allocation strategy to enable the effectiveness of the coordinated interference mitigation approach based on the interference analysis. Simulation results verify the proposed framework for interference analysis by employing Monte Carlo method, and the performance improvement of the coordinated interference mitigation approach is 3.5 dB.

A high-speed hybrid brain-computer interface with more than 200 targets

Objective. Brain-computer interfaces (BCIs) have recently made significant strides in expanding their instruction set, which has attracted wide attention from researchers. The number of targets and commands is a key indicator of how well BCIs can decode the brain’s intentions. No studies have reported a BCI system with over 200 targets. Approach. This study developed the first high-speed BCI system with up to 216 targets that were encoded by a combination of electroencephalography features, including P300, motion visual evoked potential (mVEP), and steady-state visual evoked potential (SSVEP). Specifically, the hybrid BCI paradigm used the time-frequency division multiple access strategy to elaborately tag targets with P300 and mVEP of different time windows, along with SSVEP of different frequencies. The hybrid features were then decoded by task-discriminant component analysis and linear discriminant analysis. Ten subjects participated in the offline and online cued-guided spelling experiments. Other ten subjects took part in online free-spelling experiments. Main results. The offline results showed that the mVEP and P300 components were prominent in the central, parietal, and occipital regions, while the most distinct SSVEP feature was in the occipital region. The online cued-guided spelling and free-spelling results showed that the proposed BCI system achieved an average accuracy of 85.37% ± 7.49% and 86.00% ± 5.98% for the 216-target classification, resulting in an average information transfer rate (ITR) of 302.83 ± 39.20 bits min−1 and 204.47 ± 37.56 bits min−1, respectively. Notably, the peak ITR could reach up to 367.83 bits min−1. Significance. This study developed the first high-speed BCI system with more than 200 targets, which holds promise for extending BCI’s application scenarios.

Decentralized Federated Reinforcement Learning for User-Centric Dynamic TFDD Control

The explosive growth of dynamic and heterogeneous data traffic brings great challenges for 5G and beyond mobile networks. To enhance the network capacity and reliability, we propose a learning-based dynamic time-frequency division duplexing (D-TFDD) scheme that adaptively allocates the uplink and downlink time-frequency resources of base stations (BSs) to meet the asymmetric and heterogeneous traffic demands while alleviating the inter-cell interference. We formulate the problem as a decentralized partially observable Markov decision process (Dec-POMDP) that maximizes the long-term expected sum rate under the users' packet dropping ratio constraints. In order to jointly optimize the global resources in a decentralized manner, we propose a federated reinforcement learning (RL) algorithm named federated Wolpertinger deep deterministic policy gradient (FWDDPG) algorithm. The BSs decide their local time-frequency configurations through RL algorithms and achieve global training via exchanging local RL models with their neighbors under a decentralized federated learning framework. Specifically, to deal with the large-scale discrete action space of each BS, we adopt a DDPG-based algorithm to generate actions in a continuous space, and then utilize Wolpertinger policy to reduce the mapping errors from continuous action space back to discrete action space. Simulation results demonstrate the superiority of our proposed algorithm to benchmark algorithms with respect to system sum rate.

Joint use of frequency-time division and antinoise coding in radio communication systems with FHSS

In the context of continuous improvement of radio prospecting and active radio jamming technics along with introduction of automated active countermeasures systems (ACS), the frequency-hopping spread spectrum (FHSS) radio communication systems (RCS) are widely used in order to improve reliability and noise immunity of data transmission. The noise immunity of the RCS affected by unintentional or deliberate interference can be significantly perfected by the combined use of frequency-time division and antinoise coding. This paper explores the case when the interference created by an ACS system with a limited transmitter power covers a part of the RCS frequency range. The receiver gets input mix of the wanted signal, the receiver noise, and probably a deliberate interference also considered as a noise. The article analyzes the noise immunity of signals reception with FHSS in the low-speed radio systems with joint use of frequency-time division of information subsymbols and noise combating codes when the deliberate interference destructively impacts a part of the RCS working band. Dependence of the bit error probability on the signal-to-noise ratio is calculated for the joint use of frequency division of information subsymbols and noise combating codes. It is shown that due to effective use of the frequency-energy resource of a radio line, considering the use of correction codes, a quite high noise immunity of RCS under the influence of deliberate interference can be assured. The indicated dependences of the error probability on the signal-to-noise ratio confirm that the reliability of data transmission can be significantly increased by the proper combination of signal spectrum spreading, applying of correction codes, and frequency division of subsymbols followed by their weight processing.

Reconfigurable time-frequency division multiplexing metasurface: giving a helping hand to high signal-to-noise ratio and high data holographic imaging.

A holographic imaging system based on reconfigurable time-frequency division multiplexing metasurface (RTFDMM) has been designed, in an attempt to address the problems of insufficient flexibility, low signal-to-noise ratio (SNR), and low data of traditional metasurface imaging devices. The modified Gerchberg-Saxton (MGS) algorithm in the front end of this system has been proposed to calculate the phase distribution required by the diffraction metasurface at the back end. The 1-bit low loss reconfigurable metasurface has been desiged in X-band. To obtain the best imaging results, the appropriate ranges of imazging distance, pixel size and resolution ratio are given by adjusting the phase distribution of metasurface. Based on the MGS algorithm and the 1-bit metasurface, the high-quality imaging system has been successfully established through the time-frequency division multiplexing (TFDM) technology. The high SNR imaging can be generated with the expansion of the time domain, while the high data imaging can be yielded with the quantization of the frequency domain. The proof-of-concept has demonstrated that the high data, high SNR, and high flexibility of the RTFDMM are critical technologies for the advancement of detection imaging equipment. Therefore, there are great possibilities for RTFDMM holographic imaging system to be applied in the intelligent detection platform in the future.

Interference avoidance and cancellation in automotive OFDM radar networks

With increasing use of millimeter-wave radars in driving safety applications, interference between vehicles becomes a significant issue. Moreover, oscillator imperfections and relative velocity effects induce inter-carrier interference (ICI) owing to frequency offset, leading to degradation of target detection. In this paper, time-frequency resources are divided into several orthogonal logical channels according to the time-frequency division (TFD) scheme. We propose a two-stage interference mitigation method. First, an interference avoidance technique is designed for each piece of radar equipment (RE) to select logical channels with the least ICI. Then, each RE reconstructs and cancels interference according to estimated parameters based on the proposed interference cancellation technique. Computer simulations reveal that the proposed interference avoidance technique can approximately achieve the performance of ground truth, especially when the number of interferers is small. In addition, noise enhancement effects can be effectively mitigated through the proposed cancellation technique.

A Flexible Multicarrier Scheme Based on the Discrete Orthogonal Stockwell Transform

Aiming to reduce the computational complexity of a resource (bit and power) allocation technique, this article introduces an orthogonal time-frequency division multiplexing (OTFDM) scheme. This OTFDM scheme makes use of the discrete orthogonal Stockwell transform (DOST) and exploits temporal and spectral information of power line channels for achieving very low data rate degradation and less computational complexity during the resource allocation process. Also, we present closed-form expression for the tile normalized signal-to-noise ratio (nSNR) for the OTFDM scheme and the existing relationship with its counterpart for the orthogonal frequency division multiplexing (OFDM) scheme. Besides, we demonstrate that OFDM and single carrier-cyclic prefix schemes are particular cases of the OTFDM scheme, showing the flexibility of OTFDM for handling distinct scenarios. Numerical analyses show that OTFDM outperforms OFDM in terms of computational complexity reduction and data rate loss when a resource allocation technique applies. Both schemes attain a similar symbol error ratio under the impulsive noise presence. Finally, OTFDM offers a lower peak-to-average power ratio than OFDM.

A Novel Hybrid Access Protocol Based on Traffic Priority in Space-Based Network

The space-based network is made up of aircrafts, spacecrafts, and all kinds of satellites in different orbits, which is hierarchical and tridimensional. In this special hierarchical network, a suitable multiple access protocol is required if the satellites with high velocity sharing the limited channel resources reasonably. There are majority kinds of services and traffic required during the disaster or emergent times. Thus, a novel hybrid access protocol based on traffic priority in the space-based network is proposed, where the frequency-time division multiple access (F-TDMA) is introduced into the space-based network first, and some improvements by jointing the time-domain of F-TDMA to meet the specific requirements of the satellite users are in this paper. Moreover, different traffics in the space-based network are considered as well. The analysis and simulation results show that the hybrid access protocol can obtain a higher throughput and a lower collision probability with a lower average delay comparing with the traditional collision avoidance carrier sense multiple access (CSMA/CA).

Physical Layer Framing for ATSC 3.0

An ATSC 3.0 physical layer frame consists of a bootstrap immediately followed by a preamble containing layer 1 control signaling and then one or more subframes carrying payload data. Subframes within a particular frame may be of the same or different types, with different subframe types being configured for different classes of receivers (e.g., mobile versus fixed). This paper focuses on the subframe structure, configuration, and contents, including subframe boundary symbols which carry additional pilots to assist with channel estimation at the receiver and cell multiplexing which maps physical layer pipe (PLP) payload data to physical layer resources within each subframe. Various methods for cell multiplexing multiple PLPs within a subframe are discussed and described, including time division multiplexing, frequency division multiplexing, time-frequency division multiplexing, and layered division multiplexing. Frequency interleaving, pilot insertion, and guard interval insertion are also covered, including a method of distributing extra samples to guard intervals in order to obtain frame lengths equal to an integer number of milliseconds.

Primary users ON/OFF activities over MF-TDMA based Cognitive Radio networks

Spectrum underutilizations are attributed to the primary users (PUs) ON/OFF behavior, thus giving rise to spectrum holes. However, cognitive radio technology (CRT) has being a research paradigm to the identification and exploitation of TV white space (TVWS) across frequency and time. Moreover, several research focus on Orthogonal Frequency Division Multiple Access (OFDMA) as a perfect match for CRT and little or none on Multiple Frequency Time Division Multiple Access (MF-TDMA). This paper focuses on investigating the primary/incumbent users ON/OFF markov activities over MF-TDMA platform in various channel environments (rural, urban and open space) and show the amount of available resources ( OFF mini-slots) in these areas which are very useful for cognitive users (CUs). Simulation was carried out in various scenarios (environments) to see the availability of resources (IDLE/OFF mini-slots) which are of utmost importance to the secondary users. The transition from ON to OFF was adopted from a measured data from theCommunication Research Centre, Harbin Institute of Technology. The experiment shows the state probability matrix and the state transition matrix which depicts a typical ON/OFF behaviour of primary users in the three environments. The simulation of this behavior shows that there are more available resources found in the open space with , closely followed byremote/rural environment with and lastly, by urban areas with . Thus reason being that, urban areas are jam packed with users of all kind. Keywords: cognitive radio network (CRNs), wireless channel, MF-TDMA, Primary users, ON/OFF

Two Dimension Spectrum Allocation for Cognitive Radio Networks

In this paper, we develop a truthful and efficient combinatorial auction scheme under a novel spectrum allocation model that can achieve a worst-case approximation ratio \sqrt{m} in social welfare. We propose to tackle the dynamic spectrum access problem in cognitive radio (CR) networks with time-frequency flexibility requirements. We model the spectrum opportunity in a time-frequency division manner and the spectrum allocation as a combinatorial auction. Then we design an auction mechanism to reach the upper bound in polynomial time and propose a combined approach to improve the bound in the cost of increasing computational complexity. A truthful payment that gives incentive to the SUs for revealing the truthful valuation of the desirable bundle of slots is presented. In order to reduce the complexity, we simplify the general model to a modified model that only allows frequency flexibility, and then present a truthful, optimal and computationally efficient auction mechanism. Extensive simulation results of the social welfare and spectrum ratio show that the performance of the combined approximation algorithm is better than the sorting based greedy algorithm.

Ad Hoc Network Duplexing, Multiplexing, and Multiple Access: Canonical Results for Two Limiting Topologies

Wireless ad-hoc networks have seen much attention in recent years, for the numerous benefits they offer. There are still a number of open research questions regarding these networks, and this paper addresses the question of which duplexing, multiplexing, and multiple access (D/M/MA) techniques are preferable in ad hoc networks. These techniques have substantial impact on network performance, yet this particular topic has seen surprisingly little attention. In this paper, we investigate D/M/MA techniques in ad-hoc networks and specifically address how to allocate time/frequency resources to improve network performance. We provide a comparison of time, frequency, and time-frequency schemes in terms of a number of features, including throughput, relative range and capacity. To keep the analyses tractable, we conduct our study using two limiting or "extreme" topologies: full mesh and pure relay networks. Our results show that for a peak power constraint, in terms of data rate, range, or capacity, continuous single-carrier waveforms are superior to bursted multi-carrier waveforms, and these schemes are attained with appropriate application of "hybrid" time-frequency division. Latency and throughput simulation results are provided for mesh networks, and analytical signal-to-noise-plus-interference ratio and simulation results for relay networks are also provided, to support our theoretically-based claims.

Distributed Time-Frequency Division Multiple Access Protocol for Wireless Sensor Networks

It is well known that biology-inspired self-maintaining algorithms in wireless sensor nodes achieve near optimum time division multiple access (TDMA) characteristics in a decentralized manner and with very low complexity. We extend such distributed TDMA approaches to multiple channels (frequencies). This is achieved by extending the concept of collaborative reactive listening in order to balance the number of nodes in all available channels. We prove the stability of the new protocol and estimate the delay until the balanced system state is reached. Our approach is benchmarked against single-channel distributed TDMA and channel hopping approaches using TinyOS imote2 wireless sensors.

MF-TDMA DAMA 방식 위성 망에서 효율적인 자원할당 시스템 설계

본 논문에서는 MF-TDMA (Multi Frequency-Time Division Multiple Access) DAMA (Demand-Assigned Multiple Access) 방식을 사용하는 위성 망에서 지상 단말이 네트워크 제어기에게 자원을 요청할 때, 자신에게 유입되어지는 트래픽에 대한 이전 정보(History) 즉, 트래픽 발생 패턴이나 트래픽 종류 등을 바탕으로 자원을 요청하고, 네트워크 제어기는 지상 단말들이 이전에 요청했던 정보(History)를 바탕으로 보다 효율적으로 위성 자원을 할당하여 위성 망 자원 할당 시간의 지연으로 인한 다량의 데이터 폐기를 최대한 억제하기 위한 MF-TDMA DAMA 방식 위성 시스템을 설계하였다. 미군의 전략적 군사 정보망 (Warfighter Information Network-Tactical, WIN-T) 커뮤니티에서는 L-3 Linkabit 다중 주파수-시간 분할 다중 접속 (MF-TDMA) 요청에 의해 할당되어지는 다중 접속 (DAMA) 네트워크 중심 파형 (Network Centric Waveform, NCW)을 full-mesh IP over SHF 위성 통신을 위한 네트워크 표준으로 선택하였다. MF-TDMA 위성 망에서는 네트워크 제어기(Network Controller, NC)가 최대 255개의 지상 단말들 (Earth Terminals, ETs)의 주기적인 자원요청에 대하여 동적으로 위성 망 자원을 할당하여 단말들 간의 통신을 가능하도록 한다. In this paper, I designed the satellite system, using the MF-TDMA (Multi Frequency-Time Division Multiple Access) DAMA (Demand-Assigned Multiple Access) method, that allocates the satellite resources more effectively to prevent a large quantity data discard when Earth Terminals (ETs) request their satellite network resource to Network Controller (NC) by using their history information, such as traffic pattern or traffic class which have been receiving, and the network controller allocates the satellite network resource and send to earth terminals by using his history information that earth terminals have been requested already. The U.S. Military Warfighter Information Network-Tactical (WIN-T) community has selected the L-3 Linkabit MF-TDMA DAMA Network Centric Waveform (NCW) as the networking standard for full-mesh IP over SHF satellite communications. In the MF-TDMA DAMA satellite network, network controller allocates the satellite network resources and enables maximum 255 earth terminals to communicate each other for periodic satellite network resource requests of earth terminals.

Algorithms for construction of orthogonal well-localized bases

The paper considers an algorithm of the construction of orthogonal Weyl-Heisenberg bases which is based on the singular decomposition of the Gabor basis’s matrix. The obtained basis has a good time-frequency localization, because its initializing function is close to the ideally localized Gaussian function. The sequence of the orthogonality conditions of the bases allows the formulation of a new computationally efficient algorithm of orthogonalization based on the fast Fourier transform. The modeling results confirm the identity of the initializing functions obtained as a result of these two algorithms and the good basis localization. The developed fast algorithm makes it possible to considerably expand the field of possible practical application of such bases, including the telecommunication devices based on the principle of orthogonal frequency-time division multiplexing (OFTDM).

Overlapped frequency-time division multiplexing

A technique named overlapped frequency-time division multiplexing (OVFTDM)) is proposed in this article. The technique is derived from Nyquist system and frequency-time division multiplexing system. When the signals are compactly overlapped without the orthogonality in time domain, the technique is named overlapped time division multiplexing (OVTDM), whereas when signals are compactly overlapped without the orthogonality in frequency domain, the technique is called overlapped frequency division multiplexing (OVFDM). To further improve spectral efficiency, the OVFTDM in which signals are overlapped both in frequency domain and in time domain is explored. OVFTDM does not depend on orthogonality whatever in time domain or in frequency domain like Nyquist system or OFDM system, but on the convolutional constraint relationship among signals. Therefore, not only the spectral efficiency but also the reliability is improved. The simulations verify the validity of this theory.

A Practical Method for Improving Delay Performance of Time Slot Allocation in DVB-RCS

In digital video broadcasting and return channel via satellite (DVB-RCS) systems, the time slot location assigned to a given traffic in multiple frequency-time division multiple access (MF-TDMA) frame has significant effects upon the traffic delay performance. This article proposes models to analyze the relationships among frame length, bandwidth assignment (assigned time slot count), time slot location in frame, and traffic delay performance for traffics of constant bit rate (CBR) and variable bit rate (VBR). From the proposed models, it is found that the time slot location plays a leading role in affecting the traffic delay performance, which ameliorates more when the time slot location is more uniform in frame. Compared with the traditional mode of consecutive time slot allocation, the mode of uniform time slot allocation can effectually reduce the average delay and jitter, which will not vary with the frame length. Furthermore, with the mode of uniform time slot allocation increasing bandwidth assignment could promote the delay performance more effectually. This article also puts forward a practical time slot uniform allocation method (TUAM), which is based on the DVB-RCS standard and compatible with it. The computer simulation, apart from testifying to the validity of the model, shows that it leads to efficacious reduction of the mean traffic delay and jitter. By adopting the method, the real-time traffic-like voice over IP (VoIP) traffic's packets will not suffer from dropping due to an overlong end-to-end delay, thus improving quality of service (QoS) provision for real-time traffics in DVB-RCS systems.

Semidefinite programming detection for OvHDM signal

Non-orthogonal time-frequency division multiplexing (NTFDM) transmission scheme has been proposed to further improve the bandwidth efficiency and overcome the drawbacks of the conventional orthogonal frequency division multiplexing (OFDM) method. Based on such approach, the fast signal detection algorithm, semidefinite programming (SDP) detection, has been studied. As the coefficient matrix tends to be ill conditioned, the modified SDP algorithm combined with successive interference cancellation (SIC) has been developed. The improved algorithm is a good tradeoff between performance and detection complexity. Simulation results show that the proposed algorithm can achieve better performance than cutting plane aided SDP method.

On the optimization of fully distributed power control techniques in cellular radio systems

In this paper, we evaluate the performance of fully distributed power control algorithms (PCAs) for cellular mobile radio systems based on frequency-time division multiple access; the goal is to show that the optimization of the PCA requires a suitable definition of the performance figure and proper consideration of user mobility and the implemented handover algorithm (HA). Signal-level-based and carrier-to-interference-ratio (CIR) balancing schemes are investigated. Both uplink and downlink are taken into account. The optimization is carried out by observing both the outage probability and the drop-out probability, which is related to the duration of outage events. It is shown that the difference between the performance evaluated through the two metrics is related to the presence of suitable HAs. The average number of handovers per call is also considered, as a measure of network stability when using the different PCAs.