Spreading Codes Research Articles

GPS L5 Signal Acquisition and Tracking under Unintentional Interference or Jamming

The GPS signal consists of several signals transmitted at the L1, L2, and L5 carrier frequencies. The signal at L1 = 1575.42 MHz and L2 =1227.6 MHz consists of the civilian C/A code signals with pseudorandom sequences of length 1023 and chipping frequency of 1.023 MHz and the P(Y) code signals of length 1 week and chipping frequency of 10.23 MHz. The new L5 signal is QPSKmodulated, centered on 1176.45 MHz. Its two components have each a different spreading code at chipping frequency of 10.23 MHz. The in-phase component carries the navigation message, at 100 symbols per second (50 bits per second with a convolutional encoder) while the quadrature component, called the pilot channel, carries no message at all. Most GPS receivers, whether military or civilian, are designed in a way that the acquisition of the P(Y) code depends on the acquisition and tracking of the C/A code. Moreover, if the acquisition of the P(Y) code is lost, due to reasons that are explained in the paper, then the reacquisition of the P(Y) code will again depend on the acquisition and tracking of the C/A code. Therefore, for most commercially available receivers, the reliable acquisition and tracking of the C/A code is critical for the entire operation of a GPS receiver. However, it is well-known the vulnerability of the GPS C/A code signal, to unintentional interference or jamming. The main theme of this paper is the reexamination of the GPS C/A code signal acquisition and tracking under the situation of the unintentional interference or jamming. A variety of jamming signals are considered such as: (1) periodic, deterministic; (2) aperiodic, deterministic; and (3) noisy type signals or non-deterministic. However, the majority of vendors are still producing correlator type receivers. As shown by the simulation results of this paper and other previous publications a sliding correlator type of receiver used to acquire and track the C/A code or L5 code can be easily jammed by a similar C/A code replica or L5 code replica at power levels 10 dB or higher above noise power. GAMES, GILS, and GIANT are not the most economically viable solutions in the market to handle GPS protection of the civilian receivers. A maximum likelihood GPS receiver which considers all the signals in the environment can be used to acquire and track successfully the C/A and the L5 GPS signals when the jamming signal power are in the range or 10dB or higher above the noise power. In summary, the direction of designing acquisition and tracking receivers for future C/A L1 code or L5 GPS signals should go towards joint signal acquisition and tracking. There are three main challenges with these types of receivers: (1) algorithm complexity, (2) computational power, and (3) test with real data. All these remain to be studied and analyzes further in the future. I think it is going to be a breakthrough in the state of the art of GPS receivers when these algorithms are ultimately implemented and a lot more work is required to arrive at that point which some of it is going to be published in future publications.

Signature Design of Sparsely Spread Code Division Multiple Access Based on Superposed Constellation Distance Analysis

Sparsely spread code division multiple access (SCDMA) is a non-orthogonal superposition coding scheme that allows concurrent communications between a base station and multiple users over a common channel. However, the detection performance of an SCDMA system is mainly determined by its signature matrix, which should be sparse to facilitate the belief propagation (BP) detection. On the other hand, to guarantee good maximum likelihood (ML) detection performance, the minimum Euclidean distance for the equivalent signal constellation after multi-user superposition should be maximized. In this paper, a code distance analysis is proposed for SCDMA systems with a finite number of users and spreading lengths. Based on this analysis, good signature matrices whose factor graphs have very few short cycles and possess large superposed signal constellation distances are designed. The proposed signature matrices have both good BP and ML detection performances. Moreover, their BP detection performances exactly converge to their ML detection performances with few iterations. It is worth pointing out that the proposed signature matrix design could be directly applied to the 5G non-orthogonal multiple access systems.

Amelioration of Acquisition of a Degraded Signal in a GNSS Receiver Embeded in an Observation Satellite by Using Double Bloc Zero Padding Method (DBZP)

In this paper, we study the acquisition of signal in a GPS (Global Positioning System) receiver on board of an observation satellite that receives information on a carrier wave L1 frequency 1575.42 MHz .We simulated GPS signal acquisition. This allowed us to see the behavior of this type of receiver in AWGN channel. Block Zero-Padding (DBZP) acquisition method, known for his efficiency, is deeply analyzed to highlight the acquisition of the modernized GNSS signals can be seriously degraded by the presence of bit sign transitions at each spreading code period. It appears clear that there is a need to use an acquisition method which is bit sign transition insensitive. To tackle this problem an improved and innovative acquisition method, the Double-Block Zero-Padding. Besides this major improvement, other developments are proposed to limit losses on the acquisition performance, in. This paper we will focus on the study of the acquisition of weak signals. The performance study and the results point out the efficiency of this method for the acquisition signals, in comparison with a classical acquisition method which is also data insensitive.

A Theoretical Survey of the Spreading Modulation of the New GPS Signals (L1C, L2C, and L5)

In this paper a survey of the spreading modulation of the new GPS signals (L1C, L2C, and L5) is considered. The new signals seem to offer several improvements which range from higher power, better code selection, and improved modulation schemes which offer receiver designers the opportunity to obtain unmatched performance in many ways such as in the case of a maximum likelihood GPS receiver. For example the Multiplexed Binary Offset Carrier (MBOC) spreading modulation has been recommended by the GPS-GALILEO Working Group on interoperability and compatibility because the MBOC(6,1,1/11) power spectral density is a mixture of BOC(1,1) spectrum and BOC(6,1) spectrum, that would be used by GALILEO for its Open Service (OS) signal at L1 frequency, and also by GPS for its modernized L1 Civil (L1C) signal. It is suggested that a number of different time waveforms can produce the MBOC(6,1,1/11) spectrum, allowing flexibility in implementation and maintaining interoperable waveforms for GALILEO and GPS. On the other hand, the timemultiplexed BOC (TMBOC) implementation interlaces BOC(6,1) and BOC(1,1) spreading symbols in a regular pattern, whereas composite BOC (CBOC) uses multilevel spreading symbols formed from the weighted sum of BOC(1,1) and BOC(6,1) spreading symbols, interplexed to form a constant modulus composite signal. New L1C provides a number of advanced features, which includes 75% of the power in a pilot component for enhanced signal tracking, advanced Weil-based spreading codes, an overlay code on the pilot that provides data message synchronization, support for improved reading of clock and ephemeris by combining message symbols across messages, advanced forward error control coding, and data symbol interleaving to combat fading. The resulting design offers receiver designers the opportunity to obtain a greatly improved performance in many ways. In this paper we perform a theoretical survey of the new signals and suggest a new code spreading modulation scheme which could provide even further improvements to the new GPS III signals.

A Study on Effective Dimming Control Scheme Using Optical Spreading Codes in Visible Light Communication

In this paper, we proposed a new dimming control scheme by using direct sequence spreading coding (DSSC) scheme in visible light communication (VLC). VLC systems using LED lighting can provide illumination functions and communication services at the same time. In order to provide simultaneous illumination and communication services, dimming control should be considered. Dimming control means that LED lighting should maintain a constant brightness in accordance with user requirements while communication services are offered. In IEEE 802.15.7 VLC standard, on-off keying (OOK) and variable pulse position modulation (VPPM) are defined as pulse modulation schemes for dimming control. In particular, 4B/6B, 8B/10B block coding and Manchester coding schemes have been proposed for stable dimming control in OOK. However, in such a case, stable dimming control is possible although the communication performance is degraded. In order to solve those problems, we propose a VLC system based on OOK modulation with DSSC. The proposed system can simultaneously stabilize dimming control and improve communication performance by obtaining spread diversity gain. The simulation results show that the proposed system provides superior dimming and BER performances compared to conventional dimming modulation schemes.

A near‐Nyquist rate transmission: A new minimum‐bandwidth direct‐sequence code division multiple access scheme

SummaryBandlimited direct‐sequence code division multiple access (DS‐CDMA) attracts much attention for its compact spectrum and the ability to suppress inter‐symbol interference. Among the various bandlimited DS‐CDMA systems available, minimum‐bandwidth DS‐CDMA (MB‐DS‐CDMA) is the only realizable Nyquist rate transmission system. But, MB‐DS‐CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS‐CDMA structure which extends the application of MB‐DS‐CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single‐user or multi‐user detection receiver is analyzed and compared with that of the commonly‐used raised‐cosine‐pulsed DS‐CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB‐DS‐CDMA is comparable to that of the raised‐cosine‐pulsed DS‐CDMA scheme; meanwhile, the realizable modified MB‐DS‐CDMA approaches the ultimate transmission rate.

CDMA System Design and Capacity Analysis Under Disguised Jamming

This paper considers robust code division multiple access (CDMA) system design and capacity analysis under disguised jamming, where the jammer generates a fake signal using the same spreading code, constellation, and pulse shaping filter as that of the authorized signal. Unlike Gaussian jamming, which is destructive only when jamming is dominant, disguised jamming can be devastating even if the jamming power is comparable to the signal power. In this paper, first, we analyze the performance of the conventional CDMA under disguised jamming, and show that due to the symmetricity between the authorized signal and the jamming interference, the receiver cannot really distinguish the authorized signal from jamming, leading to complete communication failure. Second, we propose to combat disguised jamming using secure scrambling. Instead of using conventional scrambling codes, we apply advanced encryption standard to generate the security-enhanced scrambling codes. Theoretical analysis based on the arbitrarily varying channel model shows that the capacity of conventional CDMA without secure scrambling under disguised jamming is actually zero; however, secure scrambling can break the symmetricity between the authorized signal and the jamming interference, and hence ensures positive channel capacity under disguised jamming. Numerical examples are provided to demonstrate the effectiveness of secure scrambling in combating disguised jamming.

High precision three-dimensional iterative indoor localization algorithm using code division multiple access modulation based on visible light communication

To solve the problem of positioning accuracy affected by mutual interference among multiple reference points in the traditional visible light communication positioning system, an iterative algorithm of received signal strength (RSS) based on code division multiple access (CDMA) coding is proposed. Every light-emitting diode (LED) source broadcasts a unique CDMA modulation identification (ID) code, which is associated with geographic position. The mobile terminal receives a mixed light signal from each LED reference point. By using the orthogonality of the spreading codes, the corresponding ID position information and the intensity attenuation factor of each LED reference point source can be available. According to the ID information and signal attenuation intensity, the location area of each LED and the distance between the receiver end and each LED can be determined. The three-dimensional (3-D) position of the receiver can be obtained by using the iterative algorithm of RSS triangulation. The experimental results show that the proposed algorithm can achieve a positioning accuracy of 5.25 cm in a two-dimensional (2-D) positioning system. And in the 3-D positioning system, the maximum positioning error is 10.27 cm, the minimum positioning error is 0.45 cm, the average positioning error is 3.97 cm, and the proportion of the positioning error exceeding 5 cm is 25 % . With a very good positioning accuracy, this system is simple and does not require synchronization processing. What is more, it can be applied to both the 2-D and 3-D localization systems, which has a broad application prospect.

A Construction of Codebooks Nearly Achieving the Levenstein Bound

Codebooks with small inner-product correlation are preferred in many practical applications such as direct spread code division multiple access communications, coding theory, and compressed sensing. The well-known Welch bound and Levenstein bound are useful benchmarks for the correlation of codebooks. In general, it is very hard to obtain codebooks achieving the Welch bound or the Levenstein bound. The objective of this letter is to present a construction of codebooks based on additive and multiplicative characters of finite fields. It generates codebooks nearly achieving the Levenstein bound.

Generalized Code Index Modulation Technique for High-Data-Rate Communication Systems

In this paper, we propose a generalized code index modulation (CIM) technique for direct-sequence spread spectrum (DSSS) communication. In particular, at the transmitter, the bit stream is divided into blocks in which each block is divided into two subblocks: mapped and modulated subblocks. Thereafter, the bits within the mapped subblock are used to select one of the predefined spreading codes, which is then used to spread the modulated bits of the second subblock. In this design, using the spreading code index as an information-bearing unit increases the overall spectral efficiency of this system. At the receiver side, the spreading code index is first estimated, thus resulting in a direct estimation of mapped subblock bits. Consequently, the corresponding spreading code to this estimated index is used to despread the modulated symbol of the modulated subblock. Subsequently, mathematical expressions for bit error rate (BER), symbol error rate (SER), throughput, energy efficiency, and the system complexity are derived to analyze the system performance. Finally, simulation results show that the proposed modulation scheme can achieve a higher data rate than the conventional DSSS system, with lower energy consumption and complexity.

Run‐length method for determining the segmentation length based on the segmentation long‐code blind estimation

In this study, the blind despreading of long-code direct sequence spread spectrum signals (LC-DSSS) is considered. Qiu et al. presented a heuristic segmentation-based method to estimate the spreading code of LC-DSSS signals, which divided the received signal into far shorter temporal windows than the symbol duration, then spreading code was reconstructed from these short windows by computing the related covariance matrices. However, Qiu et al. did not consider the criterion of determining the segmentation length. By combining the matrix perturbation theory and run length property, the authors propose a run-length method for determining the segmentation length based on the segmentation idea, and this method can also be applied to long-code direct-sequence code division multiple access (LC-DS-CDMA) signals. Monte Carlo simulations for both LC-DSSS signals and LC-DS-CDMA signals are presented to demonstrate the effectiveness of the proposed method.

Low Complexity Joint Blind Equalization and CFO Estimation for MC-CDMA System

Based on the orthogonality between the used and the unused spreading codes (excess codes) subspaces in one hand and on the restoration of guard interval redundancy of Multicarrier Code Division Multiple Access signal in other hand, we propose joint Carrier Frequency Offset and time domain equalizer estimators. A major difficulty arises from the highly complexity of these methods. Use of an approximation method or an adaptive algorithm has been popular approach to ease problem solving. Our derived methods are found to be good in terms of estimation accuracies, computation complexity, and convergence speed when compared to the block processing algorithms.

Comparative analysis of optical 2D codes using (n, w, λ a , λ c ) optical orthogonal codes for optical CDMA

Hybrid two dimensional (2D) wavelength-hopping time-spreading coding techniques have been focused now days for Optical Code Division Multiple Access (OCDMA) systems to increase the capacity of system. In this paper, design and comparative analysis of five different 2D coding techniques has been performed. These codes use Synchronized Quadratic Congruence sequences (SQC), Prime Code sequences (PC), Synchronized Prime Sequences (SPS) and One-Coincidence Frequency Hopping Code (OCFHC) for wavelength hopping and (n, w, źa, źc) one dimensional optical orthogonal codes for time spreading respectively along with Extended Reed Solomon (E-RS) codes. It has been observed that SQC/OOC code inspite of having larger value of cross correlation than other codes under consideration, out performs due to its ability to support a larger code weight. The comparison on the basis of BER shows that irrespective of the increase in number of hits due to higher code weight, SQC/OOC provides better code performance and hence supports large number of users in the system. The results are reported on the basis of maximum auto- and cross correlation function, cardinality and bit error rate (BER).

Blind estimation of pseudo‐random codes in periodic long code direct sequence spread spectrum signals

An estimation method of pseudo-random (PN) codes in the periodic long code direct sequence spread spectrum signals using a pair of spreading code and scrambling code [i.e. long scrambling code direct sequence spread spectrum (LSC-DSSS)] is investigated in this study. Via the investigation of properties of triple correlation function (TCF) of m-sequences, the existence of common peaks in the TCFs of different m-sequences is proved, and the corresponding relationship between common peaks and primitive polynomials is further investigated. Four theorems are proposed as supplements of triple correlation theory and a novel estimation algorithm of the PN codes in LSC-DSSS signals is put forward on the basis of the theorems. With certain carrier frequency and chip rate of spreading code, this algorithm first eliminates the influence of information codes through delay-and-multiply operation. Then the TCF of signal is calculated, and the two PN codes in signal are successfully estimated finally by searching and using the common peak coordinates in the TCF. Simulation results show that the proposed algorithm exhibits excellent performance in estimating PN codes in LSC-DSSS signals.

Design and analysis of radar waveforms achieving transmit and receive orthogonality

This paper presents the design and analysis of orthogonal, Doppler-tolerant waveforms for waveform agile radar (e.g. multiple-input multiple-output (MIMO) radar) applications. Previous work has given little consideration to the design of radar waveforms that remain orthogonal when they are received. Our research is focused on: 1) developing sets of waveforms that are orthogonal on both transmit and receive, and 2) ensuring that these waveforms are Doppler tolerant when properly processed. Our proposed solution achieves the above-mentioned goals by incorporating direct sequence spread spectrum (DSSS) coding techniques on linear frequency modulated (LFM) signals. We call this spread spectrum coded LFM (SSCL) signaling. Our transmitted LFM waveforms are rendered orthogonal with a unique spread spectrum (SS) code. At the receiver, the echo signal will be decoded using its spreading code. In this manner, transmitted orthogonal waveforms can be match filtered only with the intended received signals. From analytical expressions of the waveforms we have designed and from simulation results, we found that: 1) cross-ambiguity function (CAF) of two LFM SS coded (orthogonal) waveforms is small for all delays and Dopplers (i.e. transmit and receive signals satisfy near orthogonality constraint); 2) the length of the SS code determines the amount of interference suppression (i.e., complete orthogonal or near orthogonal of the received signal); 3) we can process the same received signal in two different ways; one method can provide LFM signal resolution and the other method can provide ultrahigh resolution; 4) biorthogonal codes can be used to reduce bandwidth when code length is large. Our proposed waveforms inherit multiple attributes (e.g. chirp diversity, code diversity, frequency diversity) of diverse waveforms.

Fast acquisition of GPS L5 PRN and NH code using L1 signal for software receivers

As an effort towards the modernization of GPS, the L5 signal has been transmitted from Block IIF satellites since 2010. The spreading code of the L5 signal is 10 times faster than that of the L1 signal, which makes the acquisition processing time of L5 considerably longer than that of L1. To reduce the L5 acquisition time, we propose a fast L5 acquisition method using the L1 signal, and a new NH code phase search method. We compare the acquisition times between the case of using L5 only and that of using the L1 acquisition. We then present the probability of detection for the proposed acquisition method and the new NH code phase detection method using Monte Carlo simulation in order to analyze the performance of the proposed methods.

Coding Versus Spreading for Narrowband Interference Suppression

The use of active narrowband interference (NBI) suppression in direct-sequence spread-spectrum (DS-SS) communications has been extensively studied. In this paper, we address the problem of optimum coding–spreading tradeoff for NBI suppression. With maximum-likelihood (ML) decoding, we first derive upper bounds on the error probability of coded systems in the presence of a special class of NBI, namely, multitone interference with orthogonal signatures. By employing the well-developed bounding techniques, we show that there is no advantage in spreading, and hence, a low-rate full-coding approach is always preferred. Then, we propose a practical low-rate turbo-Hadamard coding approach, in which NBI suppression is naturally achieved through iterative decoding. The proposed turbo-Hadamard coding approach employs a kind of coded spread-spectrum signaling with time-varying spreading sequences, which is sharply compared with the code-aided DS-SS approach. With a spreading sequence of length 32 and a fixed bandwidth allocated for both approaches, it is shown through extensive simulations that the proposed turbo-Hadamard coding approach outperforms the code-aided DS-SS approach for three types of NBI, even when its transmission information rate is about five times higher than that of the code-aided DS-SS approach. The use of the proposed turbo-Hadmard coding approach in multipath fading channels is also discussed.

Photonic generation of bipolar direct-sequence UWB signals based on optical spectral shaping and incoherent frequency-to-time conversion

A novel technology to obtain binary phase-coded ultrawideband (UWB) signals for direct-sequence spread-spectrum communication systems is investigated by using a cost-effective incoherent source. The bipolar encoding is performed based on an all-fiber spectrum shaper composed of two FBG arrays to tailor the optical spectrum, and a section of single-mode fiber to achieve incoherent frequency-to-time conversion. We demonstrate a 1.325-Gb/s UWB encoding system by the use of binary spreading codes of 4-chip length via computer simulations. The proposed bipolar UWB encoding technology can be applied to high-speed UWB-over-fiber communication systems.

Vhdl Implementation Of Ds Ss-cdma Transmitter And Receiver For AD Hoc Network

In past few years, lot of research is performed in both industries and academics into the development of CDMA. In DS-SS CDMA multiple signal channels occupy the same frequency band being distinguished by the use of different spreading codes. Digital cellular telephone system and personal communication system uses CDMA communication. In this project direct sequence spread spectrum principle based code division multiple access (CDMA) transmitter and receiver is implemented in VHDL for FPGA. The transmitter module mainly consists of data generator, programmable chip sequence generator (PN sequence generator), direct digital frequency synthesizer (DDFS), BPSK modulator blocks. The receiver modular mainly consists of BPSK demodulator, programmable chip sequence generator (PN sequence generator), matched filters, threshold detector blocks. Modelsim Altera 13.1 tool will be used for functional and logic verification at each block. The Xilinx synthesis technology of Xilinx ISE 9.2i tool will be used for synthesis of transmitter and receiver on FPGA Spartan 3E. A transmitter and Receiver components have been designed individually using Bottom-up approach. The designs then are combined and defined by component declaration and port mapping. This project concentrates on application of VHDL simulation and FPGA compiler to Wireless Data components..

A New Walsh-Like Near Orthogonal (WNO) Sequence for Asynchronous CDMA System

Present generation mobile communication system employs one of the most popular wireless access technologies called code division multiple access (CDMA). Design of CDMA spreading codes has drawn significant attention amongst the researchers over the last few decades. CDMA code family is generally categorized into purely orthogonal and non-orthogonal (near-orthogonal) members which have established their application in synchronous (downlink) and asynchronous (uplink) CDMA system respectively. Walsh code has been regarded as the most useful spreading code to be used in synchronous link because of its orthogonality property. However, the performance of Walsh code is significantly inferior in asynchronous surroundings. A number of codes have consequently been proposed with an aim to mitigate the shortcomings of Walsh code. This paper makes an innovative attempt to enhance the correlation properties of existing Walsh code through one simple yet powerful algorithm. Proposed code of length `N' has been generated from code sets of length `N/4' and thus makes the code generation algorithm recursive in nature. Performance of the proposed code has subsequently been compared with some existing orthogonal and semi orthogonal codes in terms of various performance metrics and finally the supremacy of our proposition has been established.