Time-frequency Spreading Research Articles

Joint Activity and Blind Information Detection for UAV-Assisted Massive IoT Access

Grant-free non-coherent index-modulation (NC-IM) has been recently considered as an efficient massive access scheme for enabling cost- and energy-limited Internet-of-Things (IoT) devices that transmit small data packets. This paper investigates the grant-free NC-IM scheme combined with orthogonal frequency division multiplexing for applicant to unmanned aerial vehicle (UAV)-based massive IoT access. Specifically, each device is assigned a unique non-orthogonal signature sequence codebook. Each active device transmits one of its signature sequences in the given time-frequency resources, by modulating the information in the index of the transmitted signature sequence. For small-scale multiple-input multiple-output (MIMO) deployed at the UAV-based aerial base station (BS), by jointly exploiting the space-time-frequency domain device activity, we propose a computationally efficient space-time-frequency joint activity and blind information detection (JABID) algorithm with significantly improved detection performance. Furthermore, for large-scale MIMO deployed at the aerial BS, by leveraging the sparsity of the virtual angular-domain channels, we propose an angular-domain based JABID algorithm for improving the system performance with reduced access latency. In addition, for the case of high mobility IoT devices and/or UAVs, we introduce a time-frequency spread transmission (TFST) strategy for the proposed JABID algorithms to combat doubly-selective fading channels. Finally, extensive simulation results are illustrated to verify the superiority of the proposed algorithms and the TFST strategy over known state-of-the-art algorithms.

Phase Watermarking of Images with Complex-Valued Linear-Phase Subband Filter Banks

A new algorithm is proposed for digital watermarking by applying complex-valued linear-phase filter banks to low frequency-band coefficients of images in the DCT domain. The watermark is conveyed in the phase spectrum of the subband coefficients. The robustness of the algorithm is examined in JPEG encoding with different qualities and compared with the DFT-based approach. Because only low frequency-band DCT coefficients are applied to complex-valued filter banks, the computational load introduced by the complex-valued filter banks is kept low. The watermark decoding is only accessible to users with the key information, i.e., impulse responses of the complex-valued linear-phase filter banks that designed with time-frequency spread property.

Time–Frequency Sparsity by Removing Perceptually Irrelevant Components Using a Simple Model of Simultaneous Masking

We present an algorithm for removing time-frequency components, found by a standard Gabor transform, of a ldquoreal-worldrdquo sound while causing no audible difference to the original sound after resynthesis. Thus, this representation is made sparser. The selection of removable components is based on a simple model of simultaneous masking in the auditory system. Important goals were the applicability to any real-world music and speech sound, integrating mutual masking effects between time-frequency components, coping with the time-frequency spread of such an operation, and computational efficiency. The proposed algorithm first determines an estimation of the masked threshold within an analysis window. The masked threshold function is then shifted in level by an amount determined experimentally, and all components falling below this function (the irrelevance threshold) are removed. This shift gives a conservative way to deal with uncertainty effects resulting from removing time-frequency components and with inaccuracies in the masking model. The removal of components is described as an adaptive Gabor multiplier. Thirty-six normal hearing subjects participated in an experiment to determine the maximum shift value for which they could not discriminate the irrelevance filtered signal from the original signal. On average across the test stimuli, 32 percent of the time-frequency components fell below the irrelevance threshold.

Combined equalisation and code assignment for expanded time-frequency OFDM system

Proposed is an effective approach to suppress noise interference in wireless communications, the expanded time-frequency (ETF-) OFDM system with joint time-frequency spreading. However, in this system, the orthogonality of the spreading code can be destroyed by multipath fading. Developed is a scheme that employs a zero-forcing equaliser at the receiver and assigns spreading codes at the transmitter for this system. Simulation results show that the ETF-OFDM system with the proposed scheme can take advantage of multipath fading to boost performance.

Cooperative subcarrier and power allocation for a two-hop decode-and-forward OFCMD based relay network

In this article, subcarrier and power allocation schemes are proposed and analyzed for different scenarios for a two-hop decode-and-forward OFCDM based relay network. In subcarrier allocation, the effect of considering the channel state information (CSI) of source-base station and source-relay link are evaluated in a cooperative diversity system. Results show that allocation of subcarriers based on source-relay node CSI provides better BER performance at higher E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , and at lower E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , both the source-relay and source-base station links need to be considered. From our numerical simulation, we also noticed that the cross-over E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , point (around which frequency spreading gives better performance than time spreading) moves towards the lower E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , when the subcarrier allocation is done giving more weight to source-base station link rather than the source-relay link which provides additional flexibility in operating environment for OFCDM systems. In power allocation, a cooperative power allocation ratio lambda (=source node power/total power) is defined and BER performance is evaluated for different values of lambda in the relay network. It is found that there exists an optimal power allocation ratio for different operating environment such as source-to-relay channel gains and time-frequency spreading factors. It is reported that: (a) when all three channels (source-to-relay, source-to-destination and relay-to-destination) have equal gains, power ratio is found to be lambda ap 0.8 (i.e., 80% and 20% of the total power is distributed among source and relay node respectively). The performance degrades at much faster rate when lambda increases above the optimal value at higher E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> . On the other hand, the performance remains almost the same when the decrement in lambda is less than the optimal value. (b) For a network with stronger source-to-relay link, the optimal lambda remains almost the same as the case with equal channel gains at higher E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ; however, the optimal power ratio moves toward lower value of lambda of 0.65 at lower <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> . (c) The optimal lambda remains almost the same with different time-frequency spreading factors.

A Low Complexity Architecture for OFCDM Downlink Transmitter Using Joint Time-Frequency Spreading and IFFT

In this letter, a low complexity transmitter is proposed for the downlinks of orthogonal frequency code division multiplexing (OFCDM) systems. The principle is based on a joint time-frequency spreading and inverse fast Fourier transform (TFS-IFFT), which combines the frequency spreading with partial stages of IFFT, so as to simplify the real-time processing. Compared with the conventional one, the proposed OFCDM transmitter is of lower real-time computational complexity, especially for those with large spreading factor or low modulation level. Furthermore, the proposed TFS-IFFT can also be applied to other frequency spreading systems, such as MC-CDMA, for complexity reduction.

Performance Analysis for OFDM-CDMA With Joint Frequency-Time Spreading

In this paper, a novel OFDM-CDMA system with joint time-frequency spreading methods is proposed. The average bit error probability of the proposed system using maximum-ratio combining (MRC) is derived for a frequency-selective fading channel and that of conventional MC-CDMA is also presented for comparison. Numerical analysis and simulation results indicate that the proposed system outperforms MC-CDMA system.

Frame-theory-based approach for determining the time–frequency distribution characterizing a dense group of reflecting objects

An inverse-scattering problem concerning the determination of a time-frequency spreading function is addressed. Such a function characterizes a dense group of reflecting objects at different ranges and moving with different velocities. The problem, arising in radar and other remote-sensing techniques, is a classical inverse problem. The aim is to reconstruct a function of two variables by means of signals (of one variable) reflected from the environment being observed. The proposed approach is developed by recourse to the frame theory in order to provide a reconstruction formula that asymptotically converges to a unique spreading function. The realistic situation with respect to the transmission of a finite number of signals is further considered. In this case the reconstruction formula is shown to yield the orthogonal projection of the spreading function onto a subspace generated by the outgoing signals.

Algorithm for the design of transmultiplexers with time-frequency spread criteria

A new class of transmultiplexers are designed with the filter responses spread in both the time and frequency domains. In contrast to design algorithms with passband flatness and stopband attenuation criteria, the new algorithm includes a time/frequency property. Transmultiplexers with five users and linear-phase property are experimentally constructed.

Communication in time-frequency spread media using adaptive equalization

In high-speed communication through a medium with time-frequency spread (such as in HF ionospheric, sonic underwater, and voice-quality telephone line transmission), the transmission speed and system errors are determined by an overall system variance (or equivalent noise). This overall variance comprises: 1) the intersymbol interference variance from time spread (or dispersion in the unit impulse response of the medium); 2) the variance from frequency spread (or dispersion it the CW response of the medium); and 3) the variance from channel noise at the receiver. In order to mitigate intersymbol variance from time spread, after discussing some early and limited attempts, a review is made of the general synthesis of the infinite and finite Wiener optimal networks or equalizers using delay lines with feedforward and/or feedback taps (or their shift register digital network counterpart). Also considered are optimal finite feedforward delay line approximations to the Wiener networks, synthesized nonsequentially and sequentially using a steepest descent evolutionary network synthesis, resulting in a monotonically decreasing and convergent overall variance. Applications of these networks are shown in the transmission of speeded-up analog facsimile pictures over unconditioned voice-quality telephone lines. It is shown that the received smeared picture can be refocused or de-smeared, or its time spread mitigated, to allow overall satisfactory high-speed picture transmission. Moreover, in over-the-horizon HF ionospheric analog and/or data transmission, time spread occurs in the form of discrete multi-path reception, such as the familiar ghosts observed in television. A particularly stringent multipath comprises two equal or nearly equal magnitude paths, a situation which is actually observed in HF ionospheric communication between two ships at sea. For this case it is shown that serial data transmission with pulses thin enough to resolve the multipath achieves, with optimal equalization, an irreducible bit error ratio (BER) which is a number of orders of magnitude less than the irreducible BER of the parallel data transmission method used in all contemporary HF modems (modulators and demodulators). Design curves are given showing the intersymbol interference variance from time spread as a function of the number of taps it the delay line correction networks, along with the variance arising from channel noise, and the optimal allocation of both variances for minimal overall BER with a given number of taps for the finite realizable correction networks. Because of frequency spread, the ever-changing unit or impulse response of the medium (e.g., HF ionospheric) causes the correction or equalization networks to become aged, giving rise to frequency spread variance. This is formulated both for determinate and for random changes of the medium unit response. For transmission media having simultaneous time spread and frequency spread, the equivalent overall variance is a simple function of the time-frequency spread product of the medium. These formulas are applied to find the overall minimal BER as a function of the time-frequency product for a contemporary parallel data modem and for a new serial adaptive data transmission system (ADAPTICOM) which periodically in real time and with digital techniques rejuvenates the time spread digital correction networks. It is shown that the transmission limit of communication of the parallel data modem is for a time-frequency spread product of about 1/2000, while that for the new serial adaptive data modem is about ten times larger, or 1/200. As a result, new communication channels are opened up for serial adaptive transmission, such as the HF spectrum below the maximum usable frequency (MUF).