Low Cross-correlation Research Articles

Design and performance of spectral efficient modified new diagonal code for a spectral amplitude code-optical code division multiple access system

Our study contains performance and design of a spectral amplitude code-optical code division multiple access (SAC-OCDMA) system using proposed modified new diagonal (MND) code. The proposed MND code has advantages as very low cross-correlation value compared to other code sequences. Also the code has a short length for the higher code weight value. Furthermore, due to its enormous code property, it also introduces very low multiple access interference. Thus the MND code-based SAC-OCDMA system permits a higher number of users. The MND code construction method is also explained, which is very easy using diagonal matrices. Based on the theoretical analysis, bit error rate (BER) performance of MND code AND-subtraction scheme compare with conventional codes is better than random diagonal code, Pascal’s triangle matrix code (PTMC), and multidiagonal (MD) code, which makes it spectral efficient. It provides 5 dB lower BER compared to MD code and PTMC for 90 users. Simulation of the MND code-based system for four users with 2 Gbps data rate has been successfully demonstrated using the AND-subtraction scheme and the direct detection scheme.

Generation of Spreading Codes with Minimum Correlation using Sorting Genetic Algorithm-II

In a code division multiple access (CDMA) system, multiple access interference (MAI) and Inter-symbol interference (ISI) appears if generated spreading codes are not maintained orthogonally and the communication channel is taken as multi-path communication channel. When generated spreading codes are multi-path spread and then channel delay occurs, it shows that ortho-gonality of the spreading codes is not maintained. The effect of MAI can be mitigated by maintaining low cross-correlation values as much as low between the large numbers of spreading codes. The code division multiple technique spreading codes must maintain absolutely impulsive autocorrelation at origin and very low cross correlation other than origin to avoid false synchronisation. i.e autocorrelation must be maximum at origin and cross correlation must be minimum at non origin point. In this paper, we propose multi-objective Genetic Algorithm approach –Genetic Algorithm-II (NSGA-II) to reduce the out-of-phase average mean-square aperiodic autocorrelation and average mean-square aperiodic cross-correlation value of randomly initialized binary spreading code set

Polyphase zero correlation zone sequences from generalised bent functions

Sequence families with zero correlation zone (ZCZ) have been extensively studied in recent years due to their important applications in quasi-synchronous code-division multiple-access (QS-CDMA) systems. To accommodate multiuser environments, multiple ZCZ sequence sets with low inter-set cross-correlation are expected. In this paper, we propose a construction of polyphase ZCZ sequences based on generalised bent functions. Moreover, multiple polyphase ZCZ sequence sets with good inter-set cross-correlation are presented. Each generated ZCZ sequence set is optimal with respect to the Tang-Fan-Matsufuji bound.

Novel Three-Hierarchy Multiple-Tag-Recognition Technique for Next Generation RFID Systems

In this paper, we propose a novel hierarchical radio-frequency identification (RFID) tag-recognition method based on blind source separation (BSS), graph-based automatic modulation classification (AMC), and direct-sequence spread-spectrum (DSSS). In our proposed method, RFID tags can be modulated using different modulation schemes according to different scenarios (e.g., different users or different tag devices). For each modulation scheme, the direct-sequence spread-spectrum strategy is employed to allow simultaneous transmissions of multiple commands. In the signal separation phase, BSS is employed to separate different transmitted signals. Then in the first hierarchy of the recognition phase, different modulation types are adopted to distinguish different users, the graph-based AMC is built upon the periodicity of the modulated signals: the cyclic spectrum of the received signal is established; the graph representation is then constructed according to the cyclic spectrum. Ultimately, robust features are extracted from the graph representation. In the second hierarchy of the recognition phase, the DSSS scheme is utilized to differentiate the control or sensed data carried by individual tags; the signature sequence set with low cross-correlations can be generated from Kasami sequences. In the third hierarchy of the recognition phase, the information data are thus spread by these signature sequences. In our proposed new RFID framework, multiple tags can transmit signals simultaneously in the same frequency band where each tag signal can still be separated and identified and its carried information can be recovered. Monte Carlo simulation results demonstrate the promising performance of our proposed new RFID scheme.

Scalable recovery of missing blocks in time series with high and low cross-correlations

Missing values are very common in real-world data including time-series data. Failures in power, communication or storage can leave occasional blocks of data missing in multiple series, affecting not only real-time monitoring but also compromising the quality of data analysis. Traditional recovery (imputation) techniques often leverage the correlation across time series to recover missing blocks in multiple series. These recovery techniques, however, assume high correlation and fall short in recovering missing blocks when the series exhibit variations in correlation. In this paper, we introduce a novel approach called CDRec to recover large missing blocks in time series with high and low correlations. CDRec relies on the centroid decomposition (CD) technique to recover multiple time series at a time. We also propose and analyze a new algorithm called Incremental Scalable Sign Vector to efficiently compute CD in long time series. We empirically evaluate the accuracy and the efficiency of our recovery technique on several real-world datasets that represent a broad range of applications. The results show that our recovery is orders of magnitude faster than the most accurate algorithm while producing superior results in terms of recovery.

Analysis of Outage Probability for MC-CDMA Systems Using Different Spread Codes

Modern communication systems demand proper utilisation of bandwidth, high throughput, integration of services and flexibility. To meet these requirements, spread spectrum code-division multiple access (CDMA) techniques have been proposed for various wireless communication systems. Here the individual user is assigned a unique binary code called spreading code used to increase capacity and provide higher robustness to interference. This paper investigates outage probability performance of MIMO multicarrier spread spectrum code-division multiple access (CDMA) with different robust spreading codes namely Walsh-Hadamard (WH), Gold and Kasami codes. Outage probability Poutage is significant performance measure to evaluate the effect of co-channel interference. System’s performance is initially evaluated in terms of outage probability by varying spreading factor (code length L) L= 4, 16, 64,256 for the number of subcarriers (NC) NC =4. In the second case spread factor SF is kept constant at L=64 and Poutage is analysed for varying number of subcarriers NC = 4, 16, 64. Outage probability analysis for system shows that under similar load conditions Kasami spread sequences outperforms Gold codes and WH codes in terms of outage probability for different spreading factor (SF) of codes and for varying number of subcarriers. This is because of its improved peak isolation and low cross-correlation than other.

Post pulse compression and partially adaptive multi‐waveform STAP

The recently developed multi-waveform space-time adaptive processing (µ-STAP) formulation incorporates additional training data into the sample covariance matrix estimate by applying multiple different secondary pulse compression filters to the raw received data, where these filters have a relatively low cross-correlation with the transmitted waveform. The inclusion of this additional training data has been shown to improve robustness to non-homogeneous clutter due to a ‘range smearing’ homogenising effect of the secondary filters. Here, the authors introduce post µ-STAP (Pµ-STAP), a new form of µ-STAP that similarly generates additional training data, albeit after pulse compression has already occurred. In addition, we combine Pµ-STAP with well-known partially adaptive STAP techniques to assess whether the enhanced performance is retained for reduced-dimension operation. Specifically, element-space post-Doppler, beam-space pre-Doppler, and beam-space post-Doppler implementations of Pµ-STAP are evaluated via signal-to-interference-plus-noise ratio analysis and minimum detectable Doppler for different simulated clutter environments.

Optimal Waveform Design Using Frequency-Modulated Pulse Trains for Active Sonar.

Frequency-modulated pulse trains can be applied in active sonar systems to improve the performance of conventional transmitted waveforms. Recently, two pulse trains have been widely researched as the transmitted waveforms for active sonars. The LFM-Costas pulse train was formed by modulating the linear frequency-modulated (LFM) waveform via the Costas sequence to remove the Doppler ambiguity of LFM pulses. The generalized sinusoidal frequency-modulated (GSFM) waveform, another frequency-modulated pulse train, achieved an ideal ambiguity function shape with thumbtack mainlobe. In this paper, we focus on constructing an optimization model to optimize the LFM-Costas and GSFM pulse trains with the genetic algorithm. The pulse trains can be improved on properties of both ambiguity function and correlations between sub-pulses. The optimized pulse trains are proven to have better detection performance than those of the initial pulse trains, including the lower sidelobe levels of ambiguity function, as well as lower cross-correlation property. Moreover, it is affirmed that the reverberation suppression performance of pulse trains has also been improved through the optimization model.

All-optical TOAD-based Manchester and Gold code generators

Spread spectrum is a modulation technique where the signal occupies a bandwidth much larger than the minimum necessary to send the information. An all-optical Manchester and Gold code generators are implemented using terahertz optical asymmetric demultiplexer-based D flip-flop and exclusive-OR logic gates in a configuration exactly like the standard electronic setup. The all-optical nonreturnable zero-to-Manchester conversion is implemented in our simulation system, and bit error rate performance (10 Gbit/s) of the format conversion is explained. Transmission performances of the converted Manchester-coded signal are addressed with the transmission length and received optical power. The proposed Manchester and Gold code generators scheme has been theoretically demonstrated in this paper. We are assigned distinguishable unique codes with low cross-correlation values among each other in multiuser environment. The Gold sequences are an important class of sequences which allow construction of long sequences with three-valued cross-correlation function. The Gold sequences are created from pairs of preferred m-sequences of the same length by modulo-2 addition. The performances of those proposed optical designs are evaluated by numerical simulation that confirms its feasibility in terms of the choice of the critical parameters.

Secure storage and retrieval schemes for multiple encrypted digital holograms with orthogonal phase encoding multiplexing.

Recent developments in 3D computational optical imaging such as digital holographic microscopy has ushered in a new era for biological research. Therefore, efficient and secure storage and retrieval of digital holograms is a challenging task for future cloud computing services. In this study, we propose a novel scheme to securely store and retrieve multiple encrypted digital holograms by using phase encoding multiplexing. In the proposed schemes, an encrypted hologram can only be accessed using a binary phase mask, which is the key to retrieve the image. In addition, it is possible to independently store, retrieve, and manage the encrypted digital holograms without affecting other groups of the encrypted holograms multiplexed using different sets of binary phase masks, due to the orthogonality properties of the Hadamard matrices with high autocorrelation and low cross-correlation. The desired encrypted holograms may also be searched for, removed, and added independently of other groups of the encrypted holograms. More and more 3D images or digital holograms can be securely and efficiently stored, retrieved, and managed.

How do ecological resilience metrics relate to community stability and collapse?

The concept of ecological resilience (the amount of disturbance a system can absorb before collapsing and reorganizing) holds potential for predicting community change and collapse—increasingly common issues in the Anthropocene. Yet neither the predictions nor metrics of resilience have received rigorous testing. The cross-scale resilience model, a leading operationalization of resilience, proposes resilience can be quantified by the combination of diversity and redundancy of functions performed by species operating at different scales. Here, we use 48 years of sub-continental avian community data aggregated at multiple spatial scales to calculate resilience metrics derived from the cross-scale resilience model (i.e., cross-scale diversity, cross-scale redundancy, within-scale redundancy, and number of body mass aggregations) and test core predictions inherent to community persistence and change. Specifically, we ask how cross-scale resilience metrics relate community stability and collapse. We found low mean cross-correlation between species richness and cross-scale resilience metrics. Resilience metrics constrained the magnitude of community fluctuations over time (mean species turnover), but resilience metrics but did not influence variability of community fluctuations (variance in turnover). We show shifts in resilience metrics closely predict community collapse: shifts in cross-scale redundancy preceded abrupt changes in community composition, and shifts in cross-scale diversity synchronized with abrupt changes in community composition. However, we found resilience metrics only weakly relate to maintenance of particular species assemblages over time. Our results distinguish ecological resilience from ecological stability and allied concepts such as elasticity and resistance: we show communities may fluctuate widely yet still be resilient. Our findings also differentiate the roles of functional redundancy and diversity as metrics of resilience and reemphasize the importance of considering resilience metrics from a multivariate perspective. Finally, we support the contention that ecological stability is nested within ecological resilience: stability predicts the behavior of systems within an ecological regime, and resilience predicts the maintenance of regimes and behavior of systems collapsing into alternative regimes.

Multiple Complementary Sequence Sets With Low Inter-Set Cross-Correlation Property

Complementary sequences have been applied to multi-carrier code-division multiple-access wireless communication systems. In this letter, multiple complementary sequence sets are constructed. The presented multiple complementary sequence sets have the following properties: first, each set is a complete complementary code (CCC) with parameters $(p,p,p)$ -CCC; second, the maximum inter-set aperiodic cross-correlation amplitude is $p$ ; and finally, an aperiodic quasi-complementary sequence set with asymptotically near optimal correlation amplitude will be obtained by combining these complementary sequence sets together, which is not covered in the literature.

Linear‐FM random radar waveform compressed by dechirping method

This research proposes a novel linear frequency modulation (Linear-FM) random radar waveform and introduces a dechirping compression method. The proposed linear-FM random waveforms possess a thumbtack ambiguity function and low cross-correlation properties which are good for fighting against digital radio frequency memory repeat jammer similar to random noise waveforms. The dechirping compression method for the proposed linear-FM random radar waveform works by mixing the random radar echo signal with a local reference linear frequency modulated (LFM) signal to reduce the echo bandwidth and thus allows to use low-speed analogue-to-digital converter similar to the dechirping method for LFM waveform compression. Both numerical simulation and high-resolution circular synthetic aperture radar imaging experiment are conducted to demonstrate the good performance of the proposed waveform and the proposed dechirping compression method.

Packet Switching Strategy and Node Architecture of Extended Spectral-Amplitude-Coding Labels in GMPLS Networks

We present packet switching applications based on extended spectral-amplitude-coding (SAC) labels in generalized multi-protocol label switching (GMPLS) networks. The proposed approach combines the advantages of wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA). The extended SAC labels preserve the orthogonal property to avoid the effect of multiple access interference (MAI) shown at the decoder. We investigate the node architecture of label generation/recognition based on arrayed waveguide grating (AWG). Combining cyclic-shifted maximal length sequence (MLS) codes with the wavelength routed property of AWG simplifies the node structure. The simulation results show that the proposed labels achieve good performances against receiver noise due to the low average cross-correlation values. Under a given bit-error-rate (BER), the switching efficiency of the extended SAC labels outperforms the previous OCDMA schemes, as the network nodes are capable of processing a large number of labels simultaneously.

Constructions of polyphase ZCZ sequence sets with low cross‐correlation property

Zero correlation zone (ZCZ) sequences have received much research attention in many practical applications, such as quasi-synchronous code-division multiple-access systems and MIMO systems. In this study, three constructions of multiple polyphase ZCZ sequence sets with low inter-set cross-correlation property are proposed. The resultant periodic ZCZ sequence sets are optimal with respect to the theoretical bound and have new parameters uncovered in the literature. Consequently, a great deal of flexibility may be provided in the choice of periodic optimal ZCZ sequence sets with good inter-set cross-correlation property for their applications.

Optimization of Orthogonal MSK Waveforms for Active Sonar Using Genetic Algorithm

In order to solve the high peak to average power ratio (PAPR) problem of pseudo random code phase modulation (PRCPM) signals, minimum shift keying (MSK) modulation waveforms with constant envelope were introduced into underwater detection. Genetic algorithm (GA) was proposed to optimize pseudo random binary codes used for MSK waveforms, in order to design sonar waveforms with various performances. After MSK complex envelope signal was obtained by theoretical analysis, the optimizing objective functions for a single waveform and a group of waveforms were presented. The optimized single waveform with low autocorrelation sidelobe values can reduce false alarm number and the difficulty of target decision. When multiple sonar systems work as a team, the optimized group of orthogonal waveforms with low autocorrelation sidelobe values and cross-correlation values can alleviate interferences between each other. In the simulation, the correlation performances of a single waveform and a group of orthogonal waveforms were presented, and ambiguity function showed that the designed waveforms had good velocity and distance resolution, which means that the optimized MSK waveforms are suitable for underwater detection.

Greedy Code Search Based Memetic Algorithm for the Design of Orthogonal Polyphase Code Sets

Multiple-input-multiple-output (MIMO) radar has received significant attention because it offers broad design freedom and superior performance. Polyphase code sets with low autocorrelation sidelobe peaks and low cross-correlation peaks, also known as orthogonal polyphase code sets, can be used as transmission signals by MIMO radar. We present an effective algorithm named the greedy code search-based memetic algorithm (MA-GCS) to design such code sets. MA-GCS is a novel variation of the new evolutionary search, which can be used to design only a single binary sequence with low autocorrelation peaks. MA-GCS integrates an evolutionary search for global searches and a GCS for local searches to improve the accuracy of solutions. Moreover, an accelerating algorithm is designed to reduce the computational complexity of the GCS-based local search. The computational complexity of our algorithm is derived and compared with other algorithms. It is demonstrated by the experimental results that our algorithm is truly effective. On one hand, the code sets designed using our proposed algorithm have better autocorrelation and cross-correlation properties than other methods. On the other hand, the time consumed is acceptable. In addition, the influence of the code set parameters on the performance of the design results is also investigated.

Cranio-caudal and medio-lateral navicular translation are representative surrogate measures of foot function in asymptomatic adults during walking

IntroductionThe translation of the navicular bone is thought to be a representative surrogate measure to assess foot pronation and hence foot function; however, it is not known how it is related to multi-segment foot kinematics.MethodsCranio-caudal (NCC) and medio-lateral (NML) navicular translation and multi-segment foot kinematics from the Oxford Foot Model (OFM) were simultaneously assessed during the stance phase of walking in 20 healthy adults. Relationships to forefoot to hindfoot (FFtoHF), hindfoot to tibia (HFtoTBA) and global hindfoot (HFL) motion were explored by cross-correlations at zero phase shift.ResultsFFtoHF sagittal, transversal and frontal plane angles showed median cross correlations of -0.95, 0.82 and 0.53 with NCC and of 0.78, -0.81 and -0.90 with NML. HFtoTBA transversal and frontal plane angles had correlations of 0.15 and 0.74 with NCC and of -0.38 and -0.83 with NML. The HFL frontal plane angle showed correlations of 0.41 and -0.44 with NCC and NML, respectively.DiscussionThe strongest relationships were found between FFtoHF sagittal plane angles and NCC and between FFtoHF frontal plane angles and NML. However, cranio-caudal and medio-lateral navicular translation seem to be reasonable surrogates for the triplanar motion between the fore- and hindfoot. The medial longitudinal arch dropped and bulged medially, while the forefoot dorsiflexed, abducted and everted with respect to the hindfoot and vice-versa. The lower cross-correlation coefficients between the rear foot parameters and NCC/NML indicated no distinct relationships between rearfoot frontal plane and midfoot kinematics. The validity of rearfoot parameters, like Achilles tendon or Calcaneal angle, to assess midfoot function must be therefore questioned. The study could also not confirm a systematic relationship between midfoot kinematics and the internal/external rotation between the hindfoot and the tibia. The measurement of navicular translation is suggested as an alternative to more complex multi-segment foot models to assess foot function.

On the design of full duplex wireless system with chaotic sequences

This paper proposes a novel approach for full duplex using chaotic sequences which is known as the asynchronous code-division duplex (Async-CDD) system. The Async-CDD system can transmit and receive signals at the same time and in the same frequency channel without time slot synchronization. The data rate of the Async-CDD system is 8 times higher than the conventional CDD system and is the same as a non-spreading system. The property of low block cross-correlation of the chaotic sequence allows the Async-CDD system achieve duplex interference suppression at any duplex delay. And the huge number of available code words/blocks of the chaotic sequence allows the Async-CDD system increase the data rate by increasing the number of multiplexed sub-channels. When both of the code length of the orthogonal chaotic code and the number of multiplexed sub-channels are 128, the orthogonal chaotic code provides 30.40 dBc self-interference suppression in average, which is 6.99 dB better than the orthogonal Gold code.

The Dodge City Tornadoes on 24 May 2016: Damage Survey, Photogrammetric Analysis Combined with Mobile Polarimetric Radar Data

AbstractPolarimetric measurements recorded by a mobile X-band radar are combined with photographs of the Dodge City, Kansas, tornado to quantitatively document the evolving debris cloud. An inner annulus or tube of high radar reflectivity encircled the tornado at low levels. A column of low cross-correlation coefficient ρhv was centered on the funnel cloud during the early stage of the tornado’s life cycle. In addition, two areas of low ρhv were located near the inner annulus of high radar reflectivity and were hypothesized to be regions of high debris loading that have been reproduced in simulations of lofted debris. Another column of low ρhv was a result of strong wind speeds that were progressively lofting small debris and dust as inflow rotated around and within the weak echo notch of the hook echo. A column of negative differential reflectivity ZDR was also centered on the tornado and was hypothesized to result from common debris alignment. The polarimetric structure undergoes a dramatic transition when the debris cloud was prominent and enveloped most of the funnel cloud. The weak echo column (WEC) began to fill at lower levels as large amounts of debris were lofted into the circulation. The axis of minimum ρhv shifted to a radius just beyond the funnel cloud. A column of positive ZDR was collocated with the funnel surrounded by negative ZDR. The negative ZDR and low ρhv within the debris cloud were likely the result of some common debris alignment from wheat stems. The positive ZDR within the funnel signified the presence of a few hydrometeors.