Multiple Access Signals Research Articles

A Novel Modified Artificial Bee Colony for DOA Estimation

Aims: We consider the Direction of Arrival (DOA) estimation for code division multiple access (CDMA) signals. Background: Solving this problem requires non-linear optimization and thus the speed of convergence becomes crucial. Objective: A novel Modified Artificial Bee Colony (MABC) has been proposed. We use secondorder Taylor series expansion of the cost function to ameliorate the searchability of artificial bee colony (ABC) for finding the globally optimal solution. Methods: The main idea is to harness the exploration and exploitation features. The optimum point of second-order Taylor expansion of cost function is used as a velocity factor of the ABC algorithm. Results: The proposed technique is used for solving the DOA estimation problem of a CDMA system. Simulation results confirm the performance improvement of our proposed algorithm. Conclusion: The cost function of the DOA estimation usually leads to a non-linear optimization problem. Using evolutionary algorithms can improve convergence rate of such problems.

Blind separation of noncooperative paired carrier multiple access signals based on improved quantum‐inspired evolutionary algorithm and receding horizon optimization

AbstractThe single‐channel blind source separation of paired carrier multiple access (PCMA) signal is a key technology in satellite communications. Due to the high‐order complexity of existing separation methods and the uncertainty of channel parameters, the processing of noncooperative PCMA signals with long memory remains a great challenge. In this article, the blind separation was solved as the joint channel estimation and sequence detection, and a novel blind separation algorithm based on the improved quantum‐inspired evolutionary algorithm (IQEA) and receding horizon optimization was proposed. Considering the practical scenarios, a mixed PCMA signal model with noninteger period sampling was presented in advance. Combined with the proposed signal model, the IQEA was applied to recover the uplink signals from the mixed PCMA signals by its iterative estimation. Meanwhile, to reduce the computational complexity caused by long memory of PCMA signals, the receding horizon optimization was introduced to process the whole signal sequence as a dynamic programming problem. The simulation results verified that the proposed RH‐IQEA outperforms existing state‐of‐the‐art separation algorithms in the blind separation of noncooperative PCMA signals, with higher accuracy, lower‐order complexity and stronger robustness to parameter estimation errors.

NOMA Receiver Design for Delay-Sensitive Systems

Successive interference cancelation (SIC) has been considered widely for the detection of downlink nonorthogonal multiple access (NOMA) signals. However, the sequential detection inherent to SIC process may introduce additional time delay for certain users, making the SIC unsuitable for communication systems with time delay constraints such as wireless networks that utilize unmanned aerial vehicles or low earth orbit satellites. Therefore, this article considers the performance of NOMA systems using a joint multiuser detector (JMuD), which can detect the signals of all users simultaneously and, hence, reduce the detection time requirements. The performance of the JMuD is evaluated in terms of bit error rate (BER), computational complexity, and processing time and compared to the SIC detector (SICD). The exact BER of the JMuD is derived analytically using quadrature phase shift keying modulation where closed-form expressions are derived for the two- and three-user scenarios for the air-to-ground channel, which is modeled as a Rician fading channels with order statistics. The obtained analytical results corroborated by Monte Carlo simulation confirm that the BERs of the JMuD and SICD are identical; however, the processing time of the SICD is 51% more than the JMuD for several cases of interest.

An Application of Spectral-Amplitude-Coding Labels in Optical Signal Buffering Over Optical Packet-Switching Networks

Optical buffering is one of the essential functions in optical packet switching (OPS) network. This letter employs optical code-division multiple-access (OCDMA) signals of spectral-amplitude-coding (SAC) in a packet-buffering architecture. In the proposed scheme, the original OCDMA label of the incoming packet is identified and converted to a new label by a code-switching system. As multiple packets overlap in the time domain in a single device, SAC is adopted for packet encoding to avoid multiple access interference (MAI). The concepts of queuing system and Gaussian approximation are employed to model the buffer architecture. Simulations were conducted to demonstrate the tradeoff between packet dropping probability and the signal variations in the label identification process. The buffering performance in terms of bit-error rate (BER) is examined. The results show SAC labels perform well on packet buffering.

Power-Amplifier Stabilization Through Out-of-Band Feedback

This letter presents a power-amplifier (PA)-stabilization technique based on the low-frequency out-of-band feedback. The proposed structure replaces a conventional stability network to ensure unconditional stability at the frequencies up to the radio-frequency (RF) operating band, with the in- and above-band frequencies stabilized through the matching network design. The approach offers an alternative to inserting the lossy components in the RF signal path, which can significantly degrade the realizable in-band gain of the amplifier. The 900-MHz proof-of-concept PA based on a 10-W gallium-nitride (GaN) device is characterized under the continuous-wave (CW) and Wideband Code Division Multiple Access signal (W-CDMA) excitations and realizes 25.6-dB small-signal gain and 73% power added efficiency (PAE) at P3dB.

Load‐modulation technique without using quarter‐wavelength transmission line

A proposed method for achieving active load-modulation technique without using a quarter-wavelength transmission line is discussed and evaluated. The theoretical analysis shows that the active load-modulation can be achieved without using a quarter-wavelength line, where the main amplifier sees a low impedance when the input signal level is low, and this impedance increases in proportion to the amount of current contributed from the peaking amplifier. The peaking amplifier sees an impedance decreasing from infinity to the normalized impedance. To validate the method, a circuit was designed, simulated and fabricated using two symmetrical gallium nitride (GaN) transistors (6 W) to achieve a peak power of 12 W and 6 dB output back-off efficiency. The design operates with 400 MHz bandwidth at 3.6 GHz and showed an average efficiency of 50% at 6 dB back-off and an efficiency of 75% at peak power. The designed circuit was tested with CW and modulated signals, the amplifier showed an Adjacent Channel Power Ratio (ACPR) of 31–35.5 dB when tested with a wideband code division multiple access signal of 6 dB peak-average-power ratio (PAPR) at 35.5 dBm average power. Additional 20 dB of linearity improvement was achieved after adding a lineariser.

Multiple‐access ultrawideband communications using frequency modulation of quasi‐orthogonal impulse‐like waveforms

This paper presents a detailed study of multiple access ultrawideband communications based on M-ary frequency-shift keying modulation of impulse-like carrier waveforms. The proposed signal model is described using expressions of duty cycle, energy, and correlation coefficient analytically derived and analyzed in terms of signal parameters. The frequency sets for coherent as well as non coherent detection utilized in narrow band communications are extended to impulse communications, and the resultant signals' correlation properties are investigated. Analytic expression of the frequency spectrum for continuous transmission is obtained, and a parametric study of the spectrum is carried out to analyze its properties. System performance analysis in terms of error probability curves is given for different signal model parameters. For single user communications, error probability curves for M-ary system are plotted using the union bound, while for multiple access communications, analytic expression of binary error probability in Rayleigh fading channel is given and plotted. The plots demonstrate the advantages of multiple access coding and signal design in improving system performance. Comparison between the bit error performance of impulse frequency shift keying, impulse phase shift keying, pulse position, and binary phase shift keying modulation schemes is presented too.

Positioning bias of different frequency observations in double-differenced relative positioning

In this study, the double-differenced positioning bias of different frequency observations (PBDFO) in a code division multiple access (CDMA) signal system is analyzed based on real-time kinematic (RTK) positioning using raw observations at different frequencies. To test and verify the existence of the DD PBDFO, a set of ultra-short baseline (25 m) GNSS observation data were collected. The stations NT03 and NT04 were located in Xi’an City, Shaanxi Province. The observation time ranged from UTC 11:00 to 17:00 for 10 consecutive days from DOY 012 to DOY 021 in 2017. The results showed that there are mm–cm differences in the RTK solution results for the raw observations at different frequencies, the mean bias and the standard deviation (STD) of the positioning results of GPS L1 and L2 in the East, North, and Up components are (−0.595 mm, 1.35 mm, −0.133 mm) and (3.11 mm, 3.32 mm, 6.44 mm), respectively, but for BDS B1 and B2, the corresponding values are (2.16 mm, −4.24 mm, 9.48 mm) and (3.35 mm, 4.85 mm, 7.41 mm), respectively. The PBDFO between GPS L1 and L2 frequencies is lower in magnitude and more stable than the PBDFO between BDS B1 and B2 frequencies, the mean and the STD of the PBDFO between GPS L1 and L2 obtained from the parameter estimation method in the East, North, and Up components are (−0.606 mm, 1.03 mm, −0.217 mm) and (0.162 mm, 0.169 mm, 0.276 mm), respectively; but for BDS B1 and B2, the corresponding values are (3.00 mm, −3.83 mm, 8.59 mm) and (0.375 mm, 1.14 mm, 2.47 mm), respectively. Both PBDFO correction model and parameter estimation approach can achieve consistency in terms of the RTK positioning results at different frequencies. The double-differenced (DD) PBDFO obtained from the direct difference method and parameter estimation approach are consistent and relatively stable for the same period of continuous days.

IDFT-VFDM for Supplementary Uplink and LTE-NR Co-Existence

During the first phase of the deployment of new radio (NR) networks, base station (BS) and user equipment (UE) are expected to operate alongside the pre-existing long-term evolution (LTE) networks. This paper proposes a solution to be adopted by NR UE to perform an uplink transmission towards NR BS over the same time and frequency resources as the legacy LTE system, which would extend the uplink coverage when operating at the supplementary uplink band. In order to guarantee the absence of interference towards LTE BS, we design a new waveform termed as inverse discrete Fourier transform-based Vandermonde-subspace frequency division multiplexing (IDFT-VFDM) with the following important features. On one hand, an NR UE communicating with an NR BS through IDFT-VFDM would not generate any interference at the legacy LTE BS, regardless of the number of antennas present at both ends of the interference channel. On the other hand, an IDFT-VFDM signal can always be designed to occupy the same signal bandwidth as a legacy single-carrier frequency division multiple access signal for LTE uplink transmission. Numerical results demonstrate the merit of IDFT-VFDM and confirm its potential as a candidate solution to achieve LTE-NR co-existence in multi-antenna multi-user scenarios.

On companding techniques to mitigate PAPR in SC-FDMA systems

Single Carrier Frequency Division Multiple Access (SC-FDMA) is being used successfully in the uplink communications of Long Term Evolution (LTE) standard. Owing to smaller envelope changes in SC-FDMA signal, it has a lower Peak to Average Power Ratio (PAPR) than Orthogonal Frequency Division Multiple Access (OFDMA) signal. But for higher order modulations, PAPR is still high and there is a requirement to mitigate PAPR in localised SC-FDMA systems. This paper discusses error function, exponential, rooting and logarithmic companding techniques that can be utilised to mitigate PAPR in SC-FDMA systems. Simulation results show that exponential companding technique provides better PAPR and Bit Error Rate (BER) performances at the cost of higher side lobes, and error function companding transform provides good PAPR and Power Spectral Density (PSD) performances at the cost of higher BER without increasing average signal power.

A 20.5 dBm Outphasing Class-F PA with Chireix Architecture at 3.5 GHz for RF Transmitter Front-end

An outphasing class-F power amplifier (PA) with Chireix architecture is presented in Global Foundry (GF) 0.13µm Complementary Metal Oxide Semiconductor (CMOS) technology. The proposed circuit is composed of two branch class-F PA and Chireix power combiner with a floating load. The designed outphasing amplifier can provide 20.5 dBm output power from 1.8 V power supply at 3.5 GHz with 52.9 % power added efficiency (PAE). Wideband Code Division Multiple Access (W-CDMA) signal is implemented as input signal to simulate ACPR and the adjacent channel leakage ratios (ACLRs) at ±5 MHz are -20.5 dBc/-20.7dBc. After DPD (Digital Pre-Distortion) ACLRs can achieve -44.5 dBc/-44.7dBc at ±5 MHz. Furthermore, the chip area including testing pads is 1.98*1.62 mm2.

On companding techniques to mitigate PAPR in SC-FDMA systems

Single Carrier Frequency Division Multiple Access (SC-FDMA) is being used successfully in the uplink communications of Long Term Evolution (LTE) standard. Owing to smaller envelope changes in SC-FDMA signal, it has a lower Peak to Average Power Ratio (PAPR) than Orthogonal Frequency Division Multiple Access (OFDMA) signal. But for higher order modulations, PAPR is still high and there is a requirement to mitigate PAPR in localised SC-FDMA systems. This paper discusses error function, exponential, rooting and logarithmic companding techniques that can be utilised to mitigate PAPR in SC-FDMA systems. Simulation results show that exponential companding technique provides better PAPR and Bit Error Rate (BER) performances at the cost of higher side lobes, and error function companding transform provides good PAPR and Power Spectral Density (PSD) performances at the cost of higher BER without increasing average signal power.

Multiple-Access Interference and Multipath Influence Mitigation for Multicarrier Code-Division Multiple-Access Signals

Sidelobe raising, caused by multipath and multiple-access interference (MAI), hinders the signal detection and time-delay estimation of multicarrier code-division multiple-access (MC-CDMA) signals, which are crucial for localization and tracking applications as well as the sampling-instant configuration of demodulators. Therefore, we propose a method based on reiterative minimum mean-square error (RMMSE) filters to mitigate the MAI and multipath influence. The RMMSE filter is modified to adapt to the multicarrier modulation and symbol involvement characteristics of MC-CDMA signals. The superiority of the proposed method in terms of sidelobes suppression, compared with existing matched filter, least square estimator, least mean square, and single-carrier RMMSE methods, is verified through simulations in the presence of MAIs and short-time-delay multipath. The results show the advantages of the proposed method in terms of high temporal resolution and robustness to MAI influence. This study could contribute toward multipath detection and aid multipath mitigation techniques in strong MAI scenarios.

A Statistical Evaluation of Detection Response of an Electric Field Probe Loaded With Nonlinear Diodes for Modulated Signals

To improve the calibration uncertainty of an electric field probe loaded with diodes to modulated signals, an approximate method for evaluating the detection response using the probability density function (pdf) of the envelope amplitude of an actually transmitted modulated signal waveform is proposed. In this article, we focus on the high-strength modulated signals and evaluate the effect of the nonlinearity of the transmission system on the probe gain. First, the equivalent circuit parameters of an electric field probe are estimated using an unmodulated signal and then used to simulate the probe gain for the amplitude-modulation (AM) signal. The probe gain for the modulated signal deviates considerably from that for the unmodulated signal, depending on the modulating frequency and modulation index of the AM signal, especially when a large incident voltage is applied to the equivalent circuit simulating the probe. Moreover, we propose two approximate expressions using the pdf to obtain the fluctuation range of the detection response of the modulated signal. One is an approximate expression that considers the equivalent circuit parameters of the probe. The other one is an approximate expression for the modulated signal with slow envelope variation. These two expressions are found to give a good approximation of the asymptotic value of the probe gain for an AM signal with fast and slow envelope variations, respectively. By applying these approximate expressions, the upper and lower limits of the gain variation of the probe for the AM signal are obtained. As an application to actual communication signals, integrated services digital broadcasting terrestrial (ISDB-T) signals and single-carrier frequency-division multiple access (SC-FDMA) signals are demonstrated as examples. It is shown that the probe gain in the saturation region increases as the signal bandwidth or peak-to-average-power ratio (PAPR) increases, which can be understood similar to the probe response to AM signals.

Uniquely Decodable Codes for Physical-Layer Network Coding in Wireless Cooperative Communications

Physical-layer network coding (PNC) has received much attention, both from academia and industry lately, since it takes advantages of multiple-access signals. Because of a reduced number of time slots (TSs), PNC is capable of improving throughput significantly, especially for a typical 3-node network consisting of 2 user terminals and 1 relay node. However, a network with $M$ nodes ( $M>3$ ) and without adopting additional techniques needs $(2M-2)$ TSs to realize information exchange among all the nodes, since network coding is unable to distinguish between users at the relay node. This paper proposes uniquely decodable codes (UDCs) for PNC in wireless cooperative communications. The proposed UDC requires only two TSs for information exchange irrespective of the number of nodes in the network. This paper also investigates the amplify-and-forward and decode-and-forward modes for UDC-based PNC. The bit error rate and throughput of the proposed schemes are analyzed in this paper. Using simulation results, we demonstrate that our theoretical results align well with the simulation results.

Spatial assessment of array antenna based joint SDMA-TDMA architecture for DS-CDMA signals

Fifth Generation (5G) technology is looking forward to coagulation of Multiple Access (MA) techniques for integrating individual benefits. Equidistant Direct-Sequence Code Division Multiple Access (DS-CDMA) signals with no relative separation by Propagation Delay suffer from severity of Multiple Access Interference (MAI). Joint implementation of Space Division Multiple Access (SDMA) and Time Division Multiple Access (TDMA) is proposed here in overcoming the limitations for equidistant sources. Spatial assessment toward effectiveness of DS-CDMA signals with appropriate beamforming in SDMA-TDMA architecture is explored here. Quadriphase Shift Keying (QPSK) modulated equidistant DS-CDMA sources having non-overlapping time deployment are transmitted through Additive White Gaussian Noise (AWGN) channel in occurrence of Near-Far Effect. Direction of Arrival (DOA) estimate and spatial behaviour of DS-CDMA sources are exemplified utilizing MUSIC (Multiple Signal Classification), Root-MUSIC and gold-MUSIC algorithms for Uniform Linear Array (ULA) antenna.

Feasibility analysis of non‐orthogonal waveforms for multiple input multiple output passive radar application

New non-orthogonal multiple access technology is proposed with the development of 5G, which is expected to support higher spectrum efficiency and massive terminal access, it enables the communication system to achieve better bit error rate performance and alleviate the spectrum congestion problem effectively under limited resources. With the design of spectrum sharing for radar and communication getting more attention, 5G non-orthogonal signal has become potential passive radar illumination source. In this article, an orthogonal multiple input multiple output (MIMO) radar waveform design metric by minimising the integration sidelobe level, is introduced firstly. Then based on sparse code multiple access (SCMA) technology, the non-orthogonal waveform is designed as the passive radar illumination sources. The distance and Doppler resolution of non-orthogonal waveform are evaluated by a more generalised MIMO ambiguity function. Simulation results demonstrate that non-orthogonal SCMA signals have competitive performance with orthogonal code division multiple access signals.

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.

Spatial Modulation-Aided Cooperative NOMA: Performance Analysis and Comparative Study

The combination of non-orthogonal multiple access (NOMA) and multiple-input multiple-output (MIMO) signaling is capable of providing significantly improved spectral efficiency and effective multiple access. However, conventional MIMO-NOMA entails significantly increased power consumption and implementation complexity. Inspired by the essence of spatial modulation (SM), in this paper, a novel three-node cooperative relaying system (CRS) using SM-aided NOMA, termed CRS-SM-NOMA, is proposed, in which the messages for two users are conveyed by two different information-bearing units of SM. As the reference scheme, conventional CRS-NOMA is further re-investigated with multiple receive antennas. Besides the achievable rate, which is adopted by most of the existing NOMA works as the performance metric, the bit error rate (BER) of CRS-SM-NOMA and CRS-NOMA is analyzed assuming maximum-likelihood detection. Approximate bit error probability expressions are derived in closed form for both users. Simulation results verify the analysis and show that the proposed CRS-SM-NOMA outperforms conventional CRS-NOMA and SM-OMA in terms of the BER as well as ergodic sum rate.