Code Division Multiple Access Signals Research Articles

Estimation of spreading sequences in LC-DS-CDMA signals based on sparse auto-encoder

A method based on a sparse auto-encoder (SAE) network for the estimation of spreading sequences in long-code direct-sequence code-division multiple access (LC-DS-CDMA) signals is proposed. First, a network classification model based on SAE and softmax classifier is established. Next, the effectiveness of the proposed method is verified by estimating Walsh sequences and m sequences. To estimate the spreading sequences, the LC-DS-CDMA signal is divided into fragments. Then, each user’s spreading sequence is separated by the fast independent component analysis (Fast-ICA) algorithm, and the amplitude fuzziness is eliminated by the delay-and-multiply method. Finally, the spreading sequences are estimated by the SAE model. Experimental results showed that the proposed algorithm could effectively estimate the spreading sequences of LC-DS-CDMA signals. Compared to the existing matching algorithm and Fast-ICA algorithm, the estimation time required by the proposed algorithm was shorter, and its estimation performance at low signal-to-noise ratios was superior.

Comparative Analysis of Greedy Pursuits for the Order Reduction of Wideband Digital Predistorters

This paper provides a review of greedy pursuits for optimizing Volterra-based behavioral models structure and estimating its parameters. An experimental comparison of the digital predistortion (DPD) linearization performance achieved by these approaches for model-order reduction, such as compressive sampling matching pursuit (CoSaMP), subspace pursuit (SP), orthogonal matching pursuit (OMP), and the novel doubly OMP (DOMP), is presented. A benchmark of the techniques in the DPD of a commercial class AB power amplifier (PA) and a class J PA operating over a 15-MHz Long-Term Evolution (LTE) signal is presented, giving a clear overview of their pruning characteristics in terms of linearization indicators and regressor selection capabilities. In addition, the benchmark is run in a cross-validation scheme by identifying the DPD with a 30-MHz 5G-new radio (NR) signal and validating with the same signal and a 20-MHz multicarrier wideband code division multiple access (WCDMA) signal. The DOMP is shown to be a promising technique since it achieves an enhanced model-order reduction for a similar linearization performance and precision.

A Novel Detected Peak Samples Combining Technique for PAPR Minimization of MC-CDMA Signal

A new signal scrambling technique is proposed to minimize the peak to average power ratio (PAPR) of multi carrier code division multiple access (MC-CDMA) signal using novel detected peak samples combining technique. This new approach minimizes PAPR by combing the MC-CDMA symbol with a number of copies of the detected peak sample which belong to the same MC-CDMA symbol. The analytical and simulation results show that by the right selection of the number of peak samples; the PAPR for 64 subcarrier system reduces by 4.9 dB at the complementary cumulative distribution function = 10−3. The PAPR can be further reduced by increasing the number of peak samples but it could compromise the data speed. The implementation of this technique is very simple when compared to the existing techniques like selective mapping, partial transmit sequence and tone reservation as it requires lesser number of Inverse Fast Fourier Transform/Fast Fourier Transform blocks, no side information and low processing delay.

Evolution of Digital Iterative Receiver for Multicarrier Code-Division Multiple-Access Signals with Low Peak-to-Mean Envelope Power Ratio Value

Evolution of Digital Iterative Receiver for Multicarrier Code-Division Multiple-Access Signals with Low Peak-to-Mean Envelope Power Ratio Value

Experimental Study on the Impact of Weather Conditions on Wide Code Division Multiple Access Signals in Nigeria

Experimental Study on the Impact of Weather Conditions on Wide Code Division Multiple Access Signals in Nigeria

Experimental Study on the Impact of Weather Conditions on Wide Code Division Multiple Access Signals in Nigeria

In cellular network activities, before a site is integrated it is expected that each cell of the site meets the Nigerian Communication Commission (NCC) standard of ≥98% for both service accessibility and call completion rate which in turn depicts a ≤2% in both blocked call rate (BCR) and dropped call rate (DCR). It is suggested that weather conditions have a very strong negative effect on the performance of wideband code division multiple access (WCDMA) network as it could lead to signal attenuation or change the polarization. In this paper, we study the impact of weather conditions on WCDMA network in Nigeria. To achieve this, network samples (log-files) were collected weekly during a driving test in Enugu State Nigeria for a period of five years for both rainy and dry seasons, in which blocked and dropped calls were extracted. Results show that during adverse weather conditions, BCR and DCR rise greater than 8% and 4% respectively. Although with a slight relationship between the weather conditions, the weather condition during the dry season has a better-blocked call rate of 8.76% in comparison with the rainy season with 12.89%. Calls tend to drop more during the dry season. From the outcome of the experiment, a model was developed for predicting an unknown network call statistics variables.

Hysteresis Current Control for Multilevel Converter in Parallel-Form Switch-Linear Hybrid Envelope Tracking Power Supply

Parallel-form switch-linear hybrid (SLH) envelope tracking (ET) power supply is constituted by a linear amplifier and a switched-mode converter connected in parallel. The switched-mode converter is usually implemented by a buck converter with hysteresis current control. However, the slew rates of the load current and the inductor current of the buck converter cannot match well when the slew rate of the load current has a wide variation range. The unmatched slew rates will lead to a high switching frequency of the buck converter or a large loss of the linear amplifier, which degrades the overall efficiency. In this paper, a multilevel converter is adopted to replace the buck converter in the parallel-form SLH ET power supply, and the hysteresis current control scheme is proposed for the multilevel converter to dynamically adjust its inductor current slew rate to approach that of the load current to improve the overall efficiency. The optimal design for the efficiency-related parameters including the inductor, the current hysteresis, and the number and values of the voltage levels in the multilevel converter is given. A prototype of the parallel-form SLH ET power supply with hysteresis current controlled multilevel converter is fabricated and tested in the lab. The measured overall efficiency is 79.4% when tracking the envelope of wideband code division multiple access signal with 5 MHz bandwidth and outputs 18 W average power, and a 3.8% efficiency improvement is achieved over that with the hysteresis current controlled buck converter.

A Novel Multi-Band Reduced Sampling Rate and I/Q Compensation Technique for RF Power Amplifiers

In this paper, a novel multi-band reduced sampling rate technique is proposed for power amplifier (PA) linearization system. It is used to enhance performance and reduce hardware costs in spectrum aggregation discontinuous broadband systems and compensate in-phase/quadrature (I/Q) imbalance in the multi-band case. During the feedback loop process of sampling rate, a band-limited filter is introduced, and the frequency band extrapolation is carried out so that the spectrum outside the limited band is compensated effectively. Theoretical analysis is performed to compare the proposed method and the conventional methods. To evaluate the performance of the proposed method, a broadband PA is excited by a tri-band signal composed of a 20 MHz long-term evolution (LTE) signal at 1.31 GHz, a 20 MHz 4-carrier wideband code division multiple access (WCDMA) signal at 2.49 GHz, and a 20 MHz 2-carrier WiMAX signal at 2.58 GHz respectively. The experimental results demonstrate that the normalized mean square error (NMSE) can get high accuracy less than −41 dB, with peak-to-average power ratio (PAPR) equaling 9 dB accompanied by a large reduction in sampling rate. Error vector magnitude (EVM) is 1% better than band-limited memory polynomial (MP) model and 2.3% better than conventional MP model.

A Memory Term Reduction Approach for Digital Pre-Distortion Using the Attention Mechanism

Over recent years, the memory effect compensated digital pre-distortion (DPD) has been widely used for linearizing wideband radio frequency power amplifiers (RF-PAs). However, with the increase of memory depth, the DPD model becomes more accurate, but the computational complexity of the DPD is significantly increased. In this paper, an approach using the attention mechanism is proposed, which can be used for locating and eliminating the memory terms with a small contribution to the performance of the DPD model so that the complexity of the DPD model is significantly reduced. The attention mechanism is employed to obtain the weighted correlation coefficient matrix of the memory effect. And the memory terms in the DPD model will be retained only if the contributions are high, which are evaluated by ensemble averaging over each diagonal of the weighted correlation coefficient matrix. To verify the applicability of the approach, a three-carrier wideband code-division multiple access signals with a bandwidth of 15 MHz and a single carrier long-term evaluation signal with a bandwidth of 20 MHz are employed for testing two Doherty RF-PAs with an operating frequency of 460 and 1900 MHz. Moreover, the generalized memory polynomial model is used to verify the effectiveness of the proposed approach. The simulation and experimental results show that the modeling accuracy and the DPD linearization performance of the DPD model with and without the memory term reduction are all almost the same, which indicates the effectiveness of the proposed approach.

Signal power calibration of complex urban kinematic navigation system simulator based on C/N0 estimation

For a complex urban kinematic navigation system, which can realize the rapid deployment of the fire scene environment and provide an effective means for indoor positioning, it adopts Time Division Multiple Access and Code Division Multiple Access (TDMA + CDMA) signal structure to solve the problems of “near-far effect” of the ground-based navigation system and the weak indoor positioning signal. A simulator of the kinematic navigation system is a very important equipment for signal validation, receiver design and performance test. The simulator signal power precision is very essential for the validation of receiver signal power range and sensitivity. As a result, the simulator itself needs calibration, especially for the weak signal power. This paper deals with the time division weak signal power calibration of the simulator, which is based on the Carrier to Noise power density ratio (C/N0) estimation. Firstly, the kinematic navigation system simulator with its signal structure and power design is analyzed. Secondly, the architecture of the measurement system, where the calibration is carried out, is designed. Thirdly, a test strategy is designed to solve the problem of time division weak signal calibration, and then a description and comparison of three known C/N0 estimators is performed, which shows a clear gain in favour of the method “the correlator comparison method with varying noise”. Finally, a performance comparison between the estimation obtained from the receiver and the method proposed in this paper is done for two values of C/N0. From these results, it is concluded that the proposed method is precise and effective in time division weak signal power calibration.

A Sequential Approach to Direct Multi-Target Localization in MIMO Radar Networks

This paper presents a direct multi-target localization algorithm for multiple-input multiple-output radar networks employing code division multiple access signals. The proposed algorithm localizes targets one by one whose contributions remain dominant in the received signal after removing the contributions of the localized targets. The sequence of isolating the contributions improves the accuracy of multi-target localization and avoids the computational inefficiency of the maximum likelihood estimator (MLE). Simulation results demonstrate the superiority of the proposed algorithm in comparison with the MLE.

CDMA RoFSO Links With Nonzero Boresight Pointing Errors Over M Turbulence Channels

Radio-on-Free-Space Optical systems constitute a very attractive option for interconnecting central base stations with remote antenna units. Nonetheless, the optical signal propagation through the atmosphere is tremendously affected by turbulence- and misalignment-induced fading. In the present study, we investigate the transmission of code division multiple access signals over a free-space optical (FSO) link. The atmospheric turbulence is modeled by the M (Malaga) distribution, which acts as a unifying statistical model encompassing all the well-known distributions that have been proposed in the FSO area. In addition, nonzero boresight pointing errors (PEs) are employed, which are described by an accurate approximation of the Beckmann distribution. Novel mathematical expressions are extracted regarding the average bit error rate and the outage probability of the forward and the reverse link. Moreover, appropriate numerical results are depicted for various turbulence and PE scenarios, along with their associated corroborative outcomes.

Hiding Stealth Optical CDMA Signals in Public BPSK Channels for Optical Wireless Communication

A new optical steganography scheme is proposed that transmits a stealth optical code-division multiple-access (OCDMA) signal through a public binary phase-shift keying (BPSK) channel. Polarization beam splitters and arrayed waveguide gratings are used to implement a spectral-polarization coding (SPC) system with an incoherent optical source. We employ a Walsh–Hadamard code as the signature code of the user who wants to transmit stealth information using the system. A free space optical link applied to this system maintains the polarization states of light during propagation. The secret data are extracted using correlation detection and balanced subtraction in the OCDMA decoder of the intended receiver, and the other signal from the public channel is reduced by the OCDMA decoder. At the demodulator of the public channel, BPSK demodulation eliminates the stealth signal so that the public channel is not affected by the stealth signal. The two signals cannot interfere with each other. The results of this study show that our proposed optical steganography system is highly secure. The stealth signal can be favorably hidden in the public channel when the average source power of the stealth signal, public noise, and public signal are −5, −3, and 0 dBm, respectively.

Quasi-Interleaved Current Control for Switch-Linear Hybrid Envelope-Tracking Power Supply

An envelope-tracking (ET) technique can greatly enhance the efficiency of the power amplifier in wireless communication system. Among the structures of the ET power supply, the switch-linear hybrid (SLH) structure can obtain both high-tracking bandwidth and high efficiency. An implementation of series–parallel-form SLH ET power supply is proposed in this paper, which includes one multilevel converter, one linear amplifier, and two buck converters. The two buck converters adopt current sensor-less prediction control and are operated at critical conduction mode (CRM). In order to ensure the operation of CRM and provide as much load current as possible, a quasi-interleaved current control scheme is proposed for the two buck converters. Moreover, a peak current restriction method is proposed to prevent the output current of the two interleaved buck converters from excessively exceeding the load current, avoiding the increase of the power loss in the linear amplifier. Finally, a prototype is fabricated with the specifications as 5-MHz tracking bandwidth, 9.6–26.4-V output voltage, and 20-W average output power. The overall measured efficiency is 81.0%, and the normalized-mean-square error is 0.28% when tracking the envelope of wideband code division multiple access signal.

Navigation With Cellular CDMA Signals—Part II: Performance Analysis and Experimental Results

A framework for navigation using cellular code division multiple access (CDMA) signals is studied in this paper. Theoretical lower bounds on the navigation performance using pseudorange measurements drawn from the cellular CDMA base transceiver stations (BTSs) are derived. Moreover, the navigation performance for a mapper/navigator framework is studied in the presence of timing discrepancies between the mapper and navigator. In this framework, a mapping receiver (mapper) estimates the stochastic dynamic clock biases of the BTSs and shares these estimates with a navigating receiver (navigator). The optimal navigation performance of the mapper/navigator framework in the presence of timing discrepancies is analyzed, and a practical upper bound on the resulting position error is derived. Experimental results for a ground vehicle and unmanned aerial vehicles (UAVs) are presented. The ground vehicle results show a mean distance difference of 5.51 m between the cellular CDMA-only navigation solution and a GPS navigation solution in the absence of clock bias discrepancies. The UAV results show an improvement of 10.57 m in the root-mean-square error of the cellular CDMA navigation solution, when the sector clock bias discrepancies are accounted for utilizing the statistical model relating observed clock biases from different sectors of the same BTS cell.

Navigation With Cellular CDMA Signals—Part I: Signal Modeling and Software-Defined Receiver Design

A software-defined receiver (SDR) for navigation using cellular code-division multiple access (CDMA) signals is presented. The cellular forward link signal structure is described, and models for the transmitted and received signals are developed. Particular attention is paid to relevant information that could be extracted and subsequently exploited for positioning and timing purposes. The pseudorange from the proposed receiver is modeled and the pseudorange error is studied in an additive white Gaussian channel. Experimental results with aerial and ground vehicles utilizing the proposed SDR are presented demonstrating a close match between the variation in pseudoranges and the variation in true ranges between the receiver and two cellular CDMA base transceiver stations (BTSs). Moreover, the dynamics of the discrepancy between the observed clock biases of different sectors of the same BTS cell is modeled and validated experimentally. The consistency of the obtained model is analyzed through experimental tests in different locations, at different times, and for different cellular providers.

A new class of block coordinate algorithms for the joint eigenvalue decomposition of complex matrices

Several signal processing problems can be written as the joint eigenvalue decomposition (JEVD) of a set of noisy matrices. JEVD notably occurs in source separation problems and for the canonical polyadic decomposition of tensors. Most of the existing JEVD algorithms are based on a block coordinate procedure and require significant modifications to deal with complex-valued matrices. These modifications decrease algorithms performances either in terms of estimation accuracy of the eigenvectors or in terms of computational cost. Therefore, we propose a class of algorithms working equally with real- or complex-valued matrices. These algorithms are still based on a block coordinate procedure and multiplicative updates. The originality of the proposed approach lies in the structure of the updating matrix and in the way the optimization problem is solved in CN×N. That structure is parametrized and allows to define up to five different JEVD algorithms. Thanks to numerical simulations, we show that, with respect to the more accurate algorithms of the literature, this approach improves the estimation of the eigenvectors and has a computational cost significantly lower. Finally, as an application example, one of the proposed algorithm is successfully applied to the blind source separation of Direct-Sequence Code Division Multiple Access signals.

A new modulator for digital RF power amplifiers utilizing a wavetable approach

This paper presents for the first time a wavetable-based coding scheme to generate a high-speed binary input signal for digital RF power amplifiers. The approach maximizes the utilization of the time domain handling capabilities of the pulse forming circuit. Key features are a greatly improved output spectrum purity in comparison with common digital modulators, the ability to adjust the modulator to any given pulse forming hardware and a built-in signal correction option that comes without additional computational cost. To give a first impression on the modulators behavior and its possibilities to adapt to different hardware constraints, simulations are carried out for different parameter variations and for different baseband bandwidths. Furthermore, the proposed concept is emulated with an arbitrary waveform generator to gather additional measurement data. For a 5 MHz wideband code division multiple access (WCDMA) signal (6.5 dB peak-to-average power ratio) at 900 MHz, an error vector magnitude (EVM) of 0.26% and better than >58 dB adjacent channel leakage ratio (ACLR) were recorded. To the authors’ knowledge, these are the best values achieved so far for a single-bit coding scheme without digital predistortion that still maintains maximum power coding efficiency.

Application of principal component analysis based effective digital predistortion technique for low-cost FPGA implementation

This article investigates the issue of low-cost digital predistortion (DPD) implementation in fixed-point field programmable gate array (FPGA) by considering the bit-resolution along with lower number of coefficients. The impact of principle component analysis (PCA) on bit-resolution of DPD solution is proposed within the context of established DPD models. Unlike previously proposed PCA based solutions, it is established by simulation and measurement that the numerical stability problem associated with popular models such as memory polynomial (MP) can be alleviated when PCA is applied to the observation data matrix. It is reported with measurement results that PCA based model provides better linearization performance with the least memory size requirement and number of LUTs in 16-bit fixed-point FPGA operation than MP, orthogonal memory polynomial (OMP), and generalized memory polynomial (GMP) models. The performance of the proposed model, is evaluated in terms of normalized mean square error, adjacent channel error power ratio, matrix condition number, and dispersion coefficient for continuous Class-AB and ZX60-V63+ power amplifiers using wide code-division multiple access signal (WCDMA) and long term evolution (LTE) signal with peak-to-average-power ratio (PAPR) around 9.895 and 11.92 dB, respectively.

Analysis and Design of CMOS Doherty Power Amplifier Based on Voltage Combining Method

The impedance at the each input terminal of paper presents a voltage combining Doherty power amplifier in a standard 180-nm CMOS process. This Doherty PA uses a series combining transformer (SCT) to combine the output power and realize the load modulation, which is different from the conventional current combining method. The series combining transformer is analyzed for impedance modulation behavior, and we have provided the design method. The proposed Doherty PA achieves a maximum output power of 27.6 dBm at 1.75 GHz with a peak power added efficiency (PAE) of 35.2% at 3.4 V supply voltage. The PAE at 6 dB back-off is still high, about 29.2%. The PA has 24.2 dBm output power with 30.2% PAE at −37 dBc ACLR (5 MHz offset) and 25.2 dBm output power with 32% PAE at −33 dBc ACLR (5 MHz offset) at 1.75 GHz under a wideband code division multiple access signal with 3.3-dB PAPR and 3.84-MHz BW.