Research on Estimator and Testing Acquisition Schemes in Direct Sequence Spectrum Spreading Systems

Serveral methods for the pararrel acquisition of a PN sequence in a baseband direct sequence spread spectrum system are investigated. Four different kinds of schemes are considered: the optimal estimation scheme, the locally optimal estimation scheme, the optimal testing searches and the locally optimal testing scheme. In the four kinds of parallel acquisition schemes, the expressions for the probability of error are given and compared with the actual error probabilities obtained via Monte Carlo simulation. We also outline a technique that can be suboptimal because of a large amount of hardware and computation when using the parallel acquisition schemes.

Optical encryption scheme based on spread spectrum ghost imaging

An optical encryption(OE) scheme based on the spread spectrum ghost imaging(SSGI), named as SSGI-OE, is proposed to obtain a high security with a smaller key. In the scheme, the randomly selected row number of a Hadamard matrix of order N is used as the secure key, and shared with the authorized user, Bob, through a private channel. Each corresponding row vector of the order-N Hadamard matrix is then used as the direct sequence code to modulate a speckle pattern for the ghost imaging system, and an image is encrypted with the help of the SSGI. The measurement results from the bucket detector, named as ciphertext, are then transmitted to Bob through a public channel. The illuminating speckle patterns are also shared with Bob by the public channel. With the correct secure key, Bob could reconstruct the image with the aid of the SSGI system, whereas the unauthorized user, Eve, could not obtain any useful information of the encrypted image. The numerical simulations and experimental results show that the proposed scheme is feasible with a higher security and a smaller key. For the 32 × 32 pixels image, the number of bits sent from Alice to Bob by using SSGIOE(M = 1024, N = 2048) scheme is only 0.0107 times over a computational ghost imaging optical encryption scheme.When the eavesdropping ratio(ER) is less than 40%, the eavesdropper cannot acquire any information of the encrypted image. The extreme circumstance for the proposed SSGI-OE scheme is also discussed, where the eavesdropper begins to extract the information when ER is up to 15%.

A SCHEME OF M-ARY MULTI-CARRIER SPREAD SPECTRUM BASED ON WAVELET PACKET

A novel scheme of M-ary multi-carrier spread spectrum based on wavelet packet is proposed. Its performance is investigated for a multipath, slow Rayteigh fading channel. The performance advantages of the system over that based on DFT are demonstrated by both analytical and simulation methods.

High-Sensitivity Acquisition of Ultrahigh Dynamic Direct Sequence Spread Spectrum Signals in Space Communications

According to the requirements of the high-sensitivity acquisition of Direct Sequence Spread Spectrum(DSSS) signals under ultrahigh dynamic environments in space communications, a three-dimensional joint search of the phase of Pseudo-Noise-code(PN-code),Doppler frequency and its rate-of-change is presented to achieve high sensitivity in sensing high-frequency dynamics. By eliminating the correlation peak loss caused by ultrahigh Doppler frequency and its rate-of-change offset,the proposed method improves the acquisition sensitivity by increasing the non-coherent accumulation time. The validity of the algorithm is proved by theoretical analysis and simulation results. It is shown that signals with a carrier- to-noise ratio as low as 39 dBHz can be captured with high performance when the Doppler frequency is up to ±1 MHz and its rate-of-change is up to ±200 kHz/s.

New Acquisition Algorithm for Direct Sequence Spread Spectrum Signals

Aiming at improving acquisition performance for direct sequence spread spectrum （DSSS） signals, a new acquisition algorithm based on fast Fourier transform （FFT） is proposed. The code phases are parallel searched throughout the Doppler frequency domain. Since the influence of Doppler frequency errors on peak-to- average ratios of correlation power shows a pattern similar to the sinc function, it is our belief that by judging on peak-to-average ratios for different Doppler frequency errors false alarm probabilities should be lowered. Analyses have supported the underlying principle of this new algorithm. Simulation results have also indicated that the new algorithm can effectively improve the acquisition performance for DSSS signals.

Asymptotic Analysis of Transmission Capacities for Overlaid Spread-Spectrum Wireless Networks

We study the transmission capacities of two coexisting spread-spectrum wireless networks (a primary network vs. a secondary network) that operate in the same geographic region and share the same spectrum. We defi ne transmission capacity as the product among the density of transmissions, the transmission rate, and the successful transmission probability. The primary (PR) network has a higher priority to access the spectrum without particular considerations for the secondary (SR) network, while the SR network limits its interference to the PR network by carefully controlling the density ofits transmitters. Considering two types of spread-spectrum transmission schemes (FH-CDMA and DS-CDMA) and the channel inversion power control mechanism, we quantify the transmission capacities for these two networks based on asymptotic analysis. Our results show that if the PR network permits a small increase ofits outage probability, the sum transmission capacities of the two networks (i.e., the overall spectrumefficiency per unit area) will be boosted significantly over that of a single network.

A Frequency－Diversity Spread－Spectrum Multiple Access System with Random Signature Sequences

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SINGLE TONE JAMMING ELIMINATION SCHEME OF MULTICARRIER SPREAD SPECTRUM SYSTEM

The proposed scheme is based on Discrete Fourier Transform (DFT) domain processing. The key technology of this scheme is jamming parameters' accurate estimation and jamming reconstruction. Compared with the 'threshold exciser' scheme the proposed scheme can eliminate more jamming energy on the whole frequency band with the minimum loss of useful signal energy. As shown in the research and simulation, the proposed scheme is much better than the 'threshold exciser' scheme, especially in the case of high power jamming whereas the 'threshold exciser' scheme might be invalid.

3D attitude measurement by means of spread spectrum modulated ultrasonic wave

Local position and attitude measurement methods are necessary for the navigation of autonomous robots and the other applications. In this paper, a 3D attitude measurement method is proposed. This method uses the spread spectrum modulated ultrasonic wave for the measurement of the pseudo-distance of each pathway between transmitter and receiver elements that are arranged in an array. The relative attitude between transmitter and receiver unit could be calculated from the relationship between these pseudo-distances. The use of spread spectrum modulation enables the high accuracy, noise tolerance, and coexistence of multiple transmitter devices by code division multiple access (CDMA) between transmitter and receiver elements. A pair of transmitter and receiver prototype for proposed method was manufactured, and the fundamental performance was examined in experiments. As a result, accuracy of σ=0.78 mm in the pseudo-distance measurement of each pathway between transmitter and receiver elements was confirmed, and about 0.01 rad resolution in the attitude measurement was also confirmed.

Narrow-band interference excision in spread-spectrum systems using new N-sigma algorithm based on fuzzy threshold

The key to narrow-band interference excision in frequency domain is to determine the excision thresh- old in direct-sequence spread-spectrum （DS-SS） systems. The excision threshold is a non-linear function related to the number and the power of interference, and attempting to get the exact relation of threshold related to the number and the power of interference is almost impossible. The N-sigma algorithm determines the excision threshold using subsection function; however, the excision threshold determined by this method is not exact. A new method to determine the threshold of N-sigma algorithm is proposed. The new method modifies the scale factor N by use of the membership function. The threshold determined by this method is consecutive and smooth, and it is closer to the fact than that of the initial N-sigma algorithm. The GPS signal and single-tone （CW） interference （that is, typical narrow-band interference） are implemented in the simulation, and the results are presented to demonstrate the validity of the new algorithm.

Comparative analysis between chaotic sequence and PN sequence in spread spectrum system

This paper introduces two kinds of sequences used in spread spectrum communication system： pseudo-noise （PN） se-quence generated by linear feedback shift register（LFSR）, such as m-sequence and Gold sequence, and the chaotic sequence generated by the determination of the nonlinear system. The characteristics of PN sequence and chaotic sequence are analyzed by simulation, and the advantages and disadvantages of different sequences in spread spectrum communication system are ob-tained. The advantages of sequence can be used to improve signal transmission in a spread spectrum communication system.

Radar-Communication Integration Based on MSK-LFM Spread Spectrum Signal

At present, with the progress and innovation of technology, the radar communication dual function wave has become a hot research at home and abroad. Excellent integrated waveform can make full use resources of combat platform, reduce the size of equipment, and realize the actual functionality of the reality of the battle-field without affecting the radar communication func-tion. The MSK-LFM dual-function wave is a typical representative;it is based on LFM, through the MSK modulation to achieve the integration function. This paper proposes a scheme of combining the spread spectrum technology with the MSK-LFM waveform based on the previous literature. The simulation results show that the waveform envelope is more stable and the energy is more concentrated. With the introduction of spread-spectrum technology, the new waveform ambiguity function graph is much closer to the thumbtack than the traditional MSK-LFM waveform.

Combinatory Spread－Spectrum Multiple－Access Based on Residue Number System：System and Performance

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Performance Improvement of Wireless Communications Using Frequency Hopping Spread Spectrum

To improve the performance of short-range wireless communications, channel quality must be improved by avoiding interference and multi-path fading. Frequency hopping spread spectrum(FHSS) is a transmission technique where the carrier hops from frequency to frequency. For frequency hopping a mechanism must be designed so that the data can be transmitted in a clear channel and avoid congested channels. Adaptive frequency hopping is a system which is used to improve immunity toward frequency interference by avoiding using congested frequency channels in hopping sequence. In this paper mathematical modelling is used to simulate and analyze the performance improvement by using FHSS with popular modulation schemes, and also the hopping channel situations are investigated.

Several Modulation in Direct-Sequence Spectrum-Spread Systems

Performances of BPSK, balanced quaternary modulation, dual quaternary modulation and complex quaternary modulation in direct sequence spectrum spread (DS SS) system are discussed in this paper. Based on the criterion the power of original signal is the same, it is shown that complex quaternary modulation has the best performance, and dual quaternary modulation degrades 2dB. Moreover, the frequency efficient of the two modulations is 2bit/Hz. Balanced quaternary has the same performance as BPSK with frequency efficient 1bit/Hz.

Performance Analysis of Wireless Mobile Radio Hybrid(DS/FH) Spread Spectrum Network

By the flexible redefinition of frequency-occupation and frequency-collision event, the frequency-(collision) probability of hybrid(DS/FH) spread spectrum network is analyzed. This probability is based on the simultaneous transmission number threshold and is discussed in synchronous and asynchronous circumstances respectively. And then, the network throughput based on the packet correct reception probability is analyzed. Two models which have finite and infinite population respectively is discussed. At last, the numerical results are given.

Frequency Hopping Spread Spectrum Security Improvement with Encrypted Spreading Codes in a Partial Band Noise Jamming Environment

Frequency Hopping Spread Spectrum (FHSS) system is often deployed to protect wireless communication from jamming or to preclude undesired reception of the signal. Such themes can only be achieved if the jammer or undesired receiver does not have the knowledge of the spreading code. For this reason, unencrypted M-sequences are a deficient choice for the spreading code when a high level of security is required. The primary objective of this paper is to analyze vulnerability of linear feedback shift register (LFSRs) codes. Then, a new method based on encryption algorithm applied over spreading codes, named hidden frequency hopping is proposed to improve the security of FHSS. The proposed encryption security algorithm is highly reliable, and can be applied to all existing data communication systems based on spread spectrum techniques. Since the multi-user detection is an inherent characteristic for FHSS, the multi-user interference must be studied carefully. Hence, a new method called optimum pair “key-input” selection is proposed which reduces interference below the desired constant threshold.

Spread Spectrum Watermark Based on Wavelet Transform for Still Digital Image

The proposed method inserts a watermark into the spectral components of the data using techniques analogous to spread spectrum communications, hiding a narrow band signal in a wideband channel. The watermark is difficult for an attacker to remove. It is also robust to common signal and geometric distortion such as JPEG compression, cropping and scaling. In addition, the watermark can be extracted accurately without source host image at the receiver. Key words digital watermark - spread spectrum techniques - wavelet transform CLC number TN 911.73 - X 913. 2 Foundation item: Supported by Wuhan University Innovation Foundation (904270072)Biography: Shao Hai-mei (1975-), female, lecturer, research direction: wavelet application, image process, digital watermark.

Microwave and IF Cancellation of Leakage Signal for Spread Spectrum Radar

This paper discusses the requirements for the cancellation of leakage signal, analyzes in detail the principals of cancelling leakage signal in microwave and IF band, and proposes the schemes to realize adaptive cancellation. Computer analog has shown that the proposed scheme has a very high convergence speed and can reach a high cancellation ratio.

Non-Supervised Learning for Spread Spectrum Signal Pseudo-Noise Sequence Acquisition

An idea of estimating the direct sequence spread spectrum（DSSS） signal pseudo-noise（PN） sequence is presented. Without the apriority knowledge about the DSSS signal in the non-cooperation condition, we propose a self-organizing feature map（SOFM） neural network algorithm to detect and identify the PN sequence. A non-supervised learning algorithm is proposed according the Kohonen rule in SOFM. The blind algorithm can also estimate the PN sequence in a low signal-to-noise（SNR） and computer simulation demonstrates that the algorithm is effective. Compared with the traditional correlation algorithm based on slip-correlation, the proposed algorithm＇s bit error rate（BER） and complexity are lower.