Chip Duration Research Articles

Joint synchronization and parameter estimation in OFDM signaling

AbstractChallenges in cognitive radio and tactical communications include recognizing anonymously received signals and estimating parameters in a blind or semiblind manner. In this paper, we examine this issue for orthogonal frequency division multiplexing (OFDM) signaling. There are several parameters in OFDM signaling, and the blind receiver must extract and consider the synchronization issue. We assume that the blind receiver is aware of modulation type, OFDM, and not aware of chip duration and the length of cyclic prefix. First, we present new criteria based on kurtosis to estimate these parameters and compare their performance at different levels of additive white Gaussian noise with methods based on correlation, kurtosis, maximum likelihood, and matched filter. Then, we perform synchronization and estimate the start time based on these criteria and several new criteria in two steps: fine and coarse synchronization. Finally, in a more practical setup, we present the idea of jointly estimating the mentioned parameters and the signal start time as coarse synchronization. We compare different criteria and show that one of the proposed criteria has the highest efficiency.

Design and Analysis of Radio Chaotic Circuit on Communication Network

Data penetration during transmission is one of the key issues with communication technology. In this work, a protected significant communication system combining a variety of chaotic and direct-sequence/spread-spectrum DS/SS approaches is proposed. This is an excellent piece of work. The bit value in this system varies according to chaotic behavior, but it really is a multiple of the process of communication's fixed chip duration. Input signals of varying duration were frequency band by multiplication simultaneously with a pseudo noise (PN) pattern and then manipulated onto harmonic carriers using quadrature phase shift keying (QPSK). The tent map is used by the MATLAB software over a time range of 0 to 1200 seconds. Three sample periods were taken from the time domain duration for each of the four areas S1, S2, S3 and S4. The results of simulations and numerical simulations are presented to demonstrate the system's dependability and viability

Optimized Masking for MIMO Radar Signals

Electronic attack devices can interrupt a radar system by emitting jamming signals. This function typically depends on the support from electronic support measure (ESM) devices, which can capture and identify radar signals in the air. A radar system can avoid being interrupted by confusing adversarial ESM devices. In this article, at the background of colocated multiple-input multiple-output radar, we present an active antijamming method that aims at confusing adversarial ESM devices by masking radar signals with accompanying signals. Masking signals can be transmitted in either the pulsed mode or the periodic continuous-waveform mode. They can be either constant-modulus or with amplitude modulation. The masking signal will cover the radar signal in both time and frequency domains, in order to make adversarial ESM devices find it hard to determine whether an intercepted signal source is a spoofing device or an operating radar. We suppress the impact of masking signals on radar-matched filters, measured by the integrated sidelobe level and peak sidelobe level of their cross-correlation sidelobes, through optimization of masking signals. Masking signals whose chip duration is shorter than that of radar signals are considered for a lower cross-correlation sidelobe level. It is termed as the sequence spread spectrum method, and this method is found critical to obtain good cross-correlation sidelobes, as indicated in numerical results with several kinds of well-designed radar signals. In concerned simulation scenarios, a good masking signal tends to focus its power on frequency bands, where the radar signal power is low.

Direct-Sequence Ultrasonic Wideband Technology for Intra-Body Communications

This letter proposes a direct-sequence ultrasonic wideband (DS-UsWB) for intra-body communications (IBCs). The DS technology superimposed channel coding is proposed to enable simultaneous multi-node communications in body area networks (BANs). We simulate the channel impulse response model in a human arm by $k$ -Wave, and analyze the chip duration and pulse duration requirements to avoid an inter-symbol interference (ISI) and an inter-pulse interference (IPI) for the DS-UsWB. We then derive the closed-form formulations for data rate and bit error rate (BER) for the DS-UsWB system. Simulation results verify our theoretical analysis, and show that the proposed DS-UsWB system performance is better than the existing ultrasonic wideband (UsWB) system.

Analysis of the Code Phase Migration and Doppler Frequency Migration Effects in the Coherent Integration of Direct-Sequence Spread-Spectrum Signals

Coherent integration of direct-sequence spread-spectrum (DSSS) signals is a commonly used technique to improve receiver performance. However, this approach is susceptible to the code phase migration (CPM) and Doppler frequency migration (DFM) effects resulting from the relative motion between transmitter and receiver. In this paper, the CPM and DFM effects are explored and characterized. To evaluate the CPM effect, a simple analytic expression for the coherent integration results under motions of arbitrary orders is developed. In addition, the theoretically derived integration loss and time synchronization error caused by CPM are quantitatively examined. The DFM effect is evaluated under the motions of arbitrary orders by Fresnel integration and numerical fitting. The obtained closed-form expressions are verified by simulation and are shown to be useful for the performance analysis and the DSSS receiver design. The theoretical and the numerical results show that when the amount of CPM is larger than about one code chip duration, the signal-to-noise ratio gain obtained by coherent integration no longer increases because of the integration loss caused by CPM. In addition, the integration loss (in decibel units) caused by DFM is approximately inversely proportional to the square of the time-bandwidth product when the time-bandwidth product is small, and it is inversely proportional to the logarithm of the time-bandwidth product when this product is large.

Parameter estimation of MC-CDMA signals based on modified cyclic autocorrelation

An algorithm, based on frequency domain accumulation for the cyclic autocorrelation of MC-CDMA signals, is proposed to estimate the signal parameters: useful data duration, symbol duration, and chip duration in multipath fading channel. Firstly the cycle autocorrelation of received MC-CDMA signals is computed, then frequency domain accumulation is used to reduce the influence of noise. At last, through interval measurement between the peak pulses in different slices, parameters mentioned above can be estimated. Meanwhile, for the estimation of symbol duration, a novel accumulative average method is developed: by averaging amplitudes of the spectral lines in each column parallel to the time-delay axis in the three-dimensional diagram, the symbol duration can be obtained. The cyclic autocorrelation expression of MC-CDMA signal has been derived, and simulation results show that the proposed algorithm is effective and works well under the condition of low signal-to-noise ratio (SNR).

Optical Code-Division Multiplexing (OCDM) Networks Adopting Code-Shift Keying/Overlapping PPM Signaling: Proposal and Performance Analysis

Optical code-division multiplexing (OCDM) systems use short pulses compared with bit-duration to achieve high transmission rate. The use of short pulses poses several problems as a result of group velocity dispersion (GVD), intersymbol interference (ISI) (due to avalanche photodiode (APD) buildup time), and receivers limited bandwidth. In this paper, an OCDM system employing code-shift keying (CSK) and overlapping pulse-position modulation (OPPM) signaling is proposed and theoretically investigated. By using CSK while maintaining same data rate, the chip duration can be increased to counteract the GVD effect in 2D OCDM systems. Moreover, by increasing the chip duration, the chip rate is decreased and the stringent requirement on receiver bandwidth is relaxed. In addition, using overlapping property in OPPM allows for further chip duration increase. We consider using correlation receivers with hard-limiters and APDs at the receiver side. The bit error probability (BEP) of the proposed system is derived taking into account the impacts of APD noise, thermal noise, GVD, ISI, and multiple-access interference (MAI). A performance comparison between OOK-, PPM-, OPPM-OCDM and the proposed system is carried out. Our results reveal that the use of CSK/OPPM-OCDM with data rate constraint allows the reduction of MAI, GVD and ISI effects with improved spectral efficiency.

Design and analysis of a spread‐spectrum communication system with chaos‐based variation of both phase‐coded carrier and spreading factor

This study designs and analyses a new phase-coded spread-spectrum communication system where both phase-coded carrier and spreading factor are varied based on a chaotic behaviour in the communication process. This design aims to reduce the probability of interception of the considered system. Discrete values generated by a chaotic map are exploited to create a non-return-to-zero (NRZ)-chaos sequence and simultaneously make bit duration variable. The NRZ-chaos sequence is then modulated by binary phase-shift keying technique to produce the phased-coded carrier. Owing to chip duration being constant, the variation of bit duration also leads to the variation of spreading factor. Spectrum spreading in the transmitter is performed by multiplying directly the variable-duration bits with the phase-coded carrier. A coherent receiver relying on a direct correlator is used for recovering the data. Design of the transmitter and receiver as well as analysis of bit error probability for the proposed system in cases of single-user and multi-user under additive white Gaussian noise channel is presented. Simulation results are shown to confirm the operation of the designed structures and the obtained analytical performance.

Comparative Performance Analysis of G - RAKE Receivers with Suboptimal Finger Placement

Generalized RAKE (G-RAKE) reception reduces the total amount of interference and provides enhanced diversity by comprising extra fingers to collect information about interference and further using channel and impairment correlation estimates for fingers allocation. However, the hardware complexity and the excessive computational requirements of GRAKE receivers may restrict their application in real systems; thus, suboptimal solutions are commonly used. In this paper, we propose and evaluate three maximum likelihood G-RAKE structures for colored noise with suboptimal finger placement. In all implementations, the fingers are optimally distributed within a time window that spans from several chip periods before the first arriving multipath to several chip periods after the latest one. The first receiver has its fingers at integer multiples of the chip period while in the rest two structures the search window is segmented in halves and tenths of the chip duration. This work also extends earlier studies by thoroughly investigating the impact of fractionally spaced finger placement on system performance. Our analysis shows that a suboptimal finger allocation reduces hardware complexity with negligible performance loss. The impact of channel delay spread and processing gain on system performance is also investigated and gives interesting results.

Barker coded excitation with linear frequency modulated carrier for ultrasonic imaging

A new Barker coded excitation using linear frequency modulated (LFM) carrier (called LFM-Barker) is proposed for improving ultrasound imaging quality in terms of axial resolution and signal-to-noise ratio (SNR). The LFM-Barker coded excitation has two independent parameters: one is the bandwidth of LFM carrier, and the other is the chip duration of Barker code. To improve the axial resolution, increase the bandwidth of LFM carrier; and to improve the SNR, increase the chip duration of Barker code. In this study, a LFM pulse with proper (<5.5) time–bandwidth product is considered as the carrier in order to avoid sidelobes inside the mainlobe of matched filtered output. A pulse compression scheme for the LFM-Barker coded excitation is developed, and it consists of the LFM matched filter and Barker code mismatched filter. In the simulations, the impulse response of transducer can be approximated by a Gaussian shaped sinusoid with 5MHz central frequency of 60% −6dB fractional bandwidth. The pulse compression filter is performed to suppress sidelobes below −40dB roughly, which is acceptable in medical imaging. Simulation results show that in comparison with conventional Barker coded excitation using sinusoid carrier (called Sinusoid-Barker), the axial resolution of the LFM-Barker coded excitation system can be doubled, and the SNR can be improved by about 3dB. Simulation of B-mode images with the Field II program demonstrates that the axial resolution is improved from 0.7mm to 0.4mm. In addition, the LFM-Barker coded excitation is robust for frequency dependent attenuation of tissues.

Impact of polarization mode dispersion on the performance of optical CDMA transmission system

The impact of fiber polarization mode dispersion (PMD) on the bit error rate (BER) performance of a direct sequence optical code-division multiple-access system is analyzed by the split-step Fourier method based on the coupled nonlinear Schrödinger equation, the three-dimensional Poincare sphere theory and the Jones matrix method. When the incident pulse's width is bigness than the chip duration, which causes spreading and overlapping of chips and degrades system performance due to increased interchip interference and reduced received optical power conduce cannot decoding or error decoding. When the incident pulse's width is less than the chip duration, good encoding/decoding performance can be achieved.

A Chaos-Based Secure Direct-Sequence/Spread-Spectrum Communication System

This paper proposes a chaos-based secure direct-sequence/spread-spectrum (DS/SS) communication system which is based on a novel combination of the conventional DS/SS and chaos techniques. In the proposed system, bit duration is varied according to a chaotic behavior but is always equal to a multiple of the fixed chip duration in the communication process. Data bits with variable duration are spectrum-spread by multiplying directly with a pseudonoise (PN) sequence and then modulated onto a sinusoidal carrier by means of binary phase-shift keying (BPSK). To recover exactly the data bits, the receiver needs an identical regeneration of not only the PN sequence but also the chaotic behavior, and hence data security is improved significantly. Structure and operation of the proposed system are analyzed in detail. Theoretical evaluation of bit-error rate (BER) performance in presence of additive white Gaussian noise (AWGN) is provided. Parameter choice for different cases of simulation is also considered. Simulation and theoretical results are shown to verify the reliability and feasibility of the proposed system. Security of the proposed system is also discussed.

On the Blind Estimation of Chip Time of Time-Hopping Signals Through Minimization of a Multimodal Cost Function

In this correspondence, we focus on blind estimation of the chip duration of time-hopping signals by introducing a cost function based on time-of-arrival (ToA) folding over multiple observation sets. An optimization algorithm that takes advantage of the highly oscillatory behavior in the nearby of the global minimum is proposed and a performance bound for the chip time estimate is derived. The pertinence of our approach is shown through numerical results, considering alternative methods like periodogram and separable least squares line search. The proposed technique enables a good tradeoff between statistical accuracy and computational complexity.

Largely reconfigurable impulse radio UWB transceiver

This letter presents a new and efficient digital baseband architecture for impulse radio ultra wideband (IR-UWB) transceivers with reconfigurable capacities for wireless sensor network (WSN) applications. An implementation is done on FPGA for validation by measurements. It achieves the highest data rate and the largest number of operating modes for an IR-UWB reconfigurable transceiver, to the best of the author's knowledge. Measurements demonstrate reconfigurability in data rate (from 4 to 125 MBits/s), chip duration (from 8 to 32 ns), pulse duration (from 1 to 4 ns), bit duration (from 8 to 256 ns), processing gain (from 1 to 8 pulses/bit), occupied bandwidth, radio range, BER performance, synchronization accuracy (from 0.5 to 4 ns), pulse amplitude, transmitted power density, duty cycle (from 3 to 50 %), power consumption, spectrum occupation, and maximum supported UWB channel delay spread (from 4 to 31 ns).

Performance Analysis of a Spectrally Phase-Encoded Optical Code Division Multiple Access Packet Network

We analyze the performance of a spectrally phase-encoded optical code division multiple access slotted packet network based on a simple protocol. The steady-state throughput and average packet delay are derived as two measures to assess the performance of the network. First, only multiple access interference is considered and other sources of noise are neglected. In this context, comparing different systems with the fixed bit rate and chip duration leads us to conclude that increasing the code length improves the performance of networks with small average activity; but in highly active networks, decreasing the code length results in a significant improvement in the throughput and average packet delay. Next, Gaussian approximation is used in our performance analysis to consider both shot noise and thermal noise as well as multiple access interference. The packet success probability is derived as a function of transmitted power. It is shown that in a fixed bandwidth and with a fixed bit rate, increasing the code length can lead to better performance in high average powers. But, in the low-power regime, decreasing code length leads to better performance due to the lower level of activity.

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d2TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.

On chip pulse shape design for precise synchronization of DS‐CDMA systems

AbstractIn this work, a methodology is established to design optimum chip pulse shapes which are absolutely bandlimited, their energy mainly concentrated in the chip duration, and which minimize the variance of the time‐delay estimation error of an unbiased estimator. The low‐complexity optimization method is based on the prolate spheroidal wave functions. Thus, the variational problem at hand can be transformed into a tractable parametric optimization problem. It is shown that the designed optimum chip pulse shapes significantly improve the synchronization performance of direct sequence code‐division multiple access systems by minimizing the Cramer–Rao lower bound (CRLB). The time‐delay estimation bias under the presence of multipath is compared with the conventionally used bandlimited rectangular chip pulse shape. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Gating Functions for Multipath Mitigation in GNSS BOC Signals

A new multipath mitigation technique is proposed for binary offset carrier (BOC) signals in global navigation satellite systems (GNSS) using the concept of gating function originally conceived for the GPS coarse-acquisition (C/A) code. Specially-tailored pulses are utilized to diminish the number of false-lock points of the code discriminator response and to improve the multipath mitigation capability. The code loop includes only four real correlators (two extra correlators are required for the simplified bump-jumping algorithm with BOC(n,n) signals). Results obtained with BOC(n,n) and BOC(2n,n) signals show that this technique eliminates the multipath ranging errors for reflected rays with relative delays typically above twenty percent of the spreading code chip duration, thus comparing favorably with the conventional receiver correlation techniques.

Enhancement of tolerance to MAIs by the synergistic effect between M-ary PAM and the chip-level receiver for optical CDMA systems

In this paper, an M-ary pulse-amplitude modulation (PAM) coded optical code division multiple access (CDMA) system applying the chip-level receiver with M-level threshold detection is proposed. First, in order to increase the number of transmitted bits per pulse, the PAM coded system is considered. However, when the correlator is applied in the receiver, the multiple access interferences (MAIs) with high intensities deteriorate the system performance significantly even if the number of MAIs is small. Consequently, the chip-level receiver with M-level threshold detection instead of the correlator in the M-ary PAM-CDMA system is proposed. The proposed system can obtain the synergistic effect between the PAM and the chip-level receiver, in which the chip-level receiver reduces more MAIs compared with the correlator and the PAM has the larger number of transmitted bits per frame than ON-OFF keying (OOK). As a result, the proposed system can increase the code length and the number of weights, and achieves higher tolerance to MAIs than the OOK-CDMA system with the chip-level receiver under the condition in which the chip duration, the bit rate, and the number of subscribers are kept constant. It is shown that the proposed M-ary PAM-CDMA system with the chip-level receiver achieves better bit error probability (BEP) than the OOK-CDMA system with the chip-level receiver.

Demonstration of 10 Gbit/s-Based Time-Spreading and Wavelength-Hopping Optical-Code-Division-Multiplexing Using Fiber-Bragg-Grating En/Decoder

We studied 10Gbit/s-based time-spreading and wavelength-hopping (TS-WH) optical code division multiplexing (OCDM) using fiber Bragg gratings (FBGs). To apply it to such the high bit rate system more than ten gigabit, two techniques are adopted. One is encoding with the maximum spreading time of 400 ps, which is four times as data bit duration, to encode without shortening chip duration. Another is encoder design. The apodized refractive index profile to the unit-gratings composing the encoder is designed to encode the pulses with 10-20ps width at 10Gbit/s rate. Using these techniques, 2 × 10Gbit/s OCDM is demonstrated successfully. In this scheme, transmission distance is limited due to dispersion effect because the signal has wide bandwidth to assign a wavelength-hopping pattern. We use no additional devices to compensate the dispersion, in order to construct simple and cost-effective system. Novel FBG encoder is designed to incorporate both encoding and compensating of group delay among chip pulses within one device. We confirm the extension of transmission distance in the TS-WH OCDM from the demonstration over 40 km-long single mode fiber.