Code Division Multiple Access Technique Research Articles

Multiple-watermarking scheme of the European Article Number Barcode using similar code division multiple access technique

The code division multiple access technique is widely used in digital communication systems. In this paper, a mix codec, hamming codec, and spread spectrum technique construct a similar code division multiple access (SCDMA) structure. It is used to configure a multiple-watermarking scheme for still images. Meanwhile, another type of multiple-watermarking scheme, a non-code division multiple access (NCDMA) watermarking structure, is used to demonstrate that the SCDMA structure is superior. In the embedding process, three watermark bit-streams are modulated to the bit-stream of the SCDMA code. Successively, the SCDMA code stream is concealed into three coefficient pairs of each DFT block by an adaptive phase shift keying (APSK) modulation. The barcode has several advantages: it is not as easily worn down as the magnetic card; it has a very low rate of error; and it is very cheap and easy to be fabricate. Therefore, we use the barcode pattern to generate the watermarks to enhance the effect of the digital rights management. Compared to other multiple-watermarking schemes, our proposed method has two advantages: (1) the secret image has very high security by using SCDMA and APSK as in digital communication; and (2) the extracted watermarks have the same quality even under all kinds of attacks, which is better than other schemes with different extracted watermark quality.

CDMA Transmission with Complex OFDM/OQAM

We propose an alternative to the well-known multicarrier code-division multiple access (MC-CDMA) technique for downlink transmission by replacing the conventional cyclic-prefix orthogonal frequency division multiplexing (OFDM) modulation by an advanced filterbank-based multicarrier system (OFDM/OQAM). Indeed, on one hand, MC-CDMA has already proved its ability to fight against frequency-selective channels thanks to the use of the OFDM modulation and its high flexibility in multiple access thanks to the CDMA component. On the other hand, OFDM/OQAM modulation confers a theoretically optimal spectral efficiency as it operates without guard interval. However, its orthogonality is limited to the real field. In this paper, we propose an orthogonally multiplex quadrature amplitude modulation (OQAM-) CDMA combination that permits a perfect reconstruction of the complex symbols transmitted over a distortion-free channel. The validity and efficiency of our theoretical scheme are illustrated by means of a comparison, using realistic channel models, with conventional MC-CDMA and also with an OQAM-CDMA combination conveying real symbols.

Can RF Help CMOS Processors? [Topics in Circuits for Communications

Digital circuits implemented in CMOS technology have been the workhorses of high performance computer processors for more than a decade, following Moore's law with exponentially increasing integration and performance. Driven by lower cost, increasing performance, and mixed-signal benefits, CMOS technology also has found increasing use in analog, and more recently, RF applications. Now, with transistor performance still improving, wires are becoming the limiting factor for speed and performance by imposing limits on communication bandwidth and latency between processing cores and memories, both off- and on-chip. Communication and circuit techniques, developed mainly for narrow band-wireless RF communication can help increase the wired communication speed in digital systems. This approach, dubbed RF Interconnect (RF-I), picks up speed for on-board and on-chip applications, changing the communication paradigm from the old parallel unidirectional time-shared bus to new transmission lines enabling reconfigurable communication using both frequency and code division multiple access techniques.

WDM-Compatible Channel Scrambling for Secure High-Data-Rate Optical Transmissions

We propose a noninvasive optical encryption technique, taking advantage of the specificities of the physical layer of a backbone transmission network, to secure optical transmissions at high data rates ( >10 Gb/s). Our goal is to secure data transmission without impacting the physical layer by guaranteeing the transparency of the encryption technique with respect to the transmission equipment. The proposed method belongs to the optical code-division multiple-access technique using spectral-phase encoding, based on pulse-overlapping sequence. The goal is to scale the protection levels to different threat and attack scenarios and to make them resilient to intrusions and robust to fiber-propagation parameters. In this paper, linear and nonlinear transmission effects (where self- and cross-phase modulations become important) are considered to maintain the quality of transmission. Finally, particular attention is paid to the ciphering key transmission for which we propose an adapted solution.

Performance Analysis of Spectral-Phase-Encoded Optical CDMA System Using Two-Photon-Absorption Receiver Structure for Asynchronous and Slot-Level Synchronous Transmitters

In this paper, we analyze the performance of a nonlinear two-photon-absorption (TPA) receiver and compare its performance with that of a single-photon-absorption (SPA) receiver in the context of spectral-phase-encoded optical code-division multiple access (CDMA) technique. The performances for the above systems are evaluated for two different transmission scenarios, namely, asynchronous and slot-level synchronous transmitters. Performance evaluation includes different sources of degradation such as multiple-access interference, noise due to optical amplification, shot noise, and thermal noise. In obtaining the performance, the mean and variance of the received signal in each of the above techniques are derived, and bit error rate is obtained using Gaussian approximation. In general, it is shown that TPA receivers are superior in performance with respect to SPA receivers when the receiver employs a much slower photodetector in comparison with the laser's transmitted pulse duration. This, indeed, is the reason behind the choice of nonlinear receivers, such as TPA, in most spectral-phase-encoded optical CDMA systems.

A Novel Wireless Local Positioning System via a Merger of DS-CDMA and Beamforming: Probability-of-Detection Performance Analysis Under Array Perturbations

This paper investigates the probability-of-detection (POD) performance of a novel wireless local positioning system (WLPS) realized via direct sequence code division multiple access (DS-CDMA) and beamforming techniques. The proposed WLPS has unique signaling schemes that discriminate it from the traditional wireless systems and allows the WLPS to have many civilian and military applications. The WLPS consists of two main parts: 1) the detecting unit, a base station carried by a mobile unit defined as dynamic base station (DBS), and 2) the being detected unit, a transponder (TRX) that is mounted on the targets, each assigned a unique identification code. Each DBS should be capable of detecting and locating all available TRXs in its coverage area. As a result, the main complexity of this system is focused at the DBS receiver. In this paper, we introduce WLPS structure, and both theoretically and numerically compute, and compare the POD performance of the DBS receiver realized by a merger of DS-CDMA and antenna arrays. We also analyze and simulate the performance of this system under perturbations in the DBS antenna array vector. The simulations are particularly performed for vehicular collision avoidance applications

Advanced sensorial system for an acoustic LPS

This work presents the development and implementation of a Local Positioning System (LPS) based on the transmission of ultrasonic signals, proposed for indoor positioning of mobile robots. The LPS consists of several ultrasonic beacons located in the environment, all of them emitting periodically and simultaneously. A portable receiver collects all the transmissions and autonomously computes its position. Neither infrared nor radiofrequency are required for synchronization in the localization process, compared to other related works. Direct Sequence Code Division Multiple Access (DS-CDMA) techniques are used to allow simultaneous emission from several beacons; each one transmitting a different 255-bit Kasami code. The portable receiver can detect the time of arrival for the different signals coming from the beacons, by carrying out all the correlations with the emitted codes. Absolute position of the receiver is determined by hyperbolic trilateration, using the Differences in Times-of-Arrival (DTOA) between a reference beacon and the other ones. The main advantage of this proposal is to permit the coexistence of an unlimited number of portable receivers, what is useful in applications like multirobot cooperation.

Packet scheduling in third-generation mobile systems with UTRA-TDD air interface

UTRA-TDD is one of the adopted air interfaces for third-generation mobile communication systems (UMTS). In UTRA-TDD, information packets are transmitted organized into radio frames. A radio frame is divided into a fixed number of time slots and different packets can be sent on the same time slot by means of the Code Division Multiple Access technique. Packets belong to different traffic classes and have different formats and Quality of Service (QoS) requirements in terms of delay, transmission error probability and priority level. In this paper, we address the problem of scheduling packets for downlink transmissions in the time slots of a frame, in such a way that QoS requirements are fulfilled. In particular, exact pseudo-polynomial and heuristic scheduling algorithms are compared in terms of typical performance parameters. Computational results for three traffic classes show that the proposed algorithms are suitable for UTRA-TDD implementation, both for solution quality and computational time.

Code-division multiple-access techniques in optical fiber networks - part III: optical AND logic gate receiver structure with generalized optical orthogonal codes

Code-division multiple-access techniques in optical fiber networks - part III: optical AND logic gate receiver structure with generalized optical orthogonal codes

Multiple Casting in a Wired LAN Using CDMA Technique

In this paper, we discuss the details of a Code Division Multiple Access (CDMA)-based multiplecasting system in a wired Local Area Network (LAN). This technique enables multiple users to transmit and receive data simultaneously among themselves using a single serial wire link at any given time, hence, called multiplecasting. This is unlike the method followed with the Ethernet in a wired LAN. With security being the prime demand of today’s world, allowing multiple users to access the channel simultaneously may be critical; but in this respect, the CDMA technology proposed by us, since it uses unique code for each user, also provides a solution to the problem. In this paper we aimed at simulating this technique. The simulation was done using Mat lab. We studied the effectiveness of the scheme, and the results are discussed in this paper.

Multiple-watermarking scheme for still images using the discrete cosine transform and modified code division multiple-access techniques

A modified code division multiple-access (MCDMA) technique is constructed by the spread-spectrum technique and a mix coder, and then it is used to configure a multiple-watermarking scheme for still images. Another type of multiple-watermarking scheme, non-code-division multiple-access watermarking (NCMW), is used to demonstrate that the MCDMA scheme is superior. In the embedding process, three watermark bits are modulated to the five-bit MCDMA code. Then the 15 MCDMA bits are concealed in the coefficients of a group that consists of four DCT blocks. Empirical results show that our proposed scheme can sustain attacks like JPEG lossy compression, rotation, resizing, cropping, painting, noising, and blurring. Compared with other multiple-watermarking schemes, our proposed method has two advantages: (1) The secret image has very high security owing to the use of MCDMA for digital communication. (2) The extracted watermarks have the same quality under all kinds of attacks, whereas other schemes yield different extracted-watermark quality.

Code-Division Multiple-Access Techniques in Optical Fiber Networks—Part III: Optical AND Gate Receiver Structure With Generalized Optical Orthogonal Codes

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (CDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$lambda = 1$</tex> were considered, we consider generalized OOCs with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1 leq lambda leq w$</tex> , where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$w$</tex> is the weight of the corresponding codes. To enhance the performance of such systems, we propose that use of optical AND gate receiver, which, in an ideal case, e.g., in the absence of any noise source except the optical multiple-access noise, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$lambda = 1,2,3,ldots,w$</tex> , with optical AND gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$lambdageq 1$</tex> . In particular, we can obtain some simple relations between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ P_ e min$</tex> (minimum error rate), <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L_min$</tex> (minimum required OOC length), and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N_max$</tex> (maximum number of interfering users to be supported), which are the most desired parameters for any optical CDMA system design. Furthermore, we show that in most practical cases, OOCs with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$lambda = 2$</tex> or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3$</tex> perform better than OOCs with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$lambda = 1$</tex> , while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$sqrt2lambda L$</tex> where L is the length of the OOCs used in systems.

Rejection of interlabel crosstalk in a digital lightpath labeling system with low-cost all-wavelength receivers

Theoretical and experimental results on the code design and bit error rate (BER) performance of a digitally encoded lightpath labeling system for multiwavelength photonic networks are reported. For low cost, the shared label receivers operate without wavelength filters, relying on electrical code-division multiple access (CDMA) techniques to resolve the labels from various wavelengths. Code designs capable of operating in noise-limited networks with up to 80 wavelengths are presented, and the tradeoff between complexity and coding overhead is explored. Multiwavelength laboratory experiments show a good agreement with theory. The BER performance of a decorrelating decoder is shown to exceed that of a conventional decoder for systems with large wavelength counts

A Joint MAC-PHY Approach for Medium Access Control in VBR MC-CDMA Broadband Indoor Connections

Multicarrier Code Division Multiple Access (MC-CDMA) techniques were originally proposed at mid of 90's for wideband multi-user communications in wireless environments characterized by hostile propagation characteristics. In this work, the design of a MC-CDMA-based infrastructure is considered for VBR broadband indoor connections with real-time asynchronous multiple access. At the present time, Broadband Fixed Wireless Access (BFWA) standards like IEEE 802.16 and HIPERMAN can bring broadband services inside buildings, but indoor access should be conveniently provided by a local area connection. The capability of MC-CDMA of supporting asynchronous multi-user variable-bit-rate (VBR) transmission is exploited jointly with an efficient and real-time Medium Access Control (MAC) strategy in order to allow a significant number of indoor VBR users to transmit information in CDMA modality with different quality of service (QoS) profiles. Different classes of users are defined at the MAC level. The available radio resources (i.e. the orthogonal subchannels) are selectively attributed to transmitting users depending on their performance achieved at MAC level and measured by an "intelligent" gateway. When the quality level is not satisfactory for one or more users, the AP issues a decrease of the data rate for such users while providing them with an increased number of subcarriers, guaranteeing a slower transmission fostered against frequency-selective channel distortions. The paper presents an overview of the system and tests its performance through extensive simulations. The proposed joint MAC-PHY approach demonstrates good performance in terms of achieved throughput and high flexibility in radio resource management.

Performance analysis of the combination of LDPC and space–time codes on multi-carrier CDMA systems over fading channels

The authors present the asymptotic bit error rate (BER) performance analysis of a system, over a Rayleigh fading channel that is based on the combination of low density parity check (LDPC)-based space–time (ST) codes and multi-carrier code division multiple access (MC-CDMA) techniques. MC-CDMA systems have the benefits of both orthogonal frequency division multiplexing (OFDM) and CDMA techniques in fading channels. Additionally, the LDPC-based ST codes not only increase the capacity of the system by using MIMO techniques, but also increase the coding gain owing to the use of powerful LDPC codes. First, the downlink of the MC-CDMA system with MIMO channel is analysed, and then the LDPC is applied to the results to complete the analysis. The simulation results are also compared with those previously obtained using turbo codes instead of the LDPC codes.

MIMO Frequency Hopping Spread Spectrum Multi-Carrier Multiple Access: A Novel Uplink System for B3G Cellular Networks

In this paper, we propose and give the performance of a novel uplink system based on the combination of multi-carrier (MC), code division multiple access (CDMA) and multiple input multiple output (MIMO) techniques. First, we describe the interests of spread-spectrum multi-carrier multiple access (SS-MC-MA) scheme for uplink, especially compared to MC-CDMA. Classically, with SS-MC-MA, each user spreads its data symbols on a specific subset of adjacent or multiplexed subcarriers, to facilitate the channel estimation and reduce complexity at the reception. In order to compensate for the lack of frequency diversity of SS-MC-MA with adjacent subcarriers, we first combine it with an orthogonal space-time block code (STBC) and demonstrate the resulting spatial diversity gain. Then, we propose to allocate the subsets to the different users by applying a frequency hopping pattern (FH). In that case, each user benefits from the frequency diversity linked to the total bandwidth as with the multiplexed subcarriers solution, while keeping the advantages of the adjacent subcarriers solution. The gain provided by the use of the frequency hopping is stressed on. Finally, the performance of this scheme is evaluated over realistic MIMO channel with channel turbo coding for systems offering asymptotic spectrum efficiency of 1, 2, 3 and 4.5 bit/s/Hz. Thus, the efficiency of the novel proposed STBC FH SS-MC-MA system as a very promising multiple access and modulation scheme for the uplink of the future wideband wireless networks is successfully demonstrated.

An integrated CDMA code assignment model for voice and data traffic in wireless communications

Wireless personal communication requires a provision of integrated services of multimedia traffic, such as voice and data, over the radio link. The multiple access protocols of code-division multiple-access (CDMA) techniques have been widely investigated in the recent literature. This paper presents an innovative multiple access protocol for CDMA-based wireless communication systems by fully utilizing the characteristics of voice and data traffic. In other words, a voice terminal can reserve a spreading code to transmit packets in multiple talk spurts, while a data terminal can transmit packets by either using the unassigned codes or borrowing the codes from the voice terminals during their silent periods. We build mathematical models for voice and data subsystems, respectively. Two performance parameters, the average dropping probability for voice packets and the average transmission delay for data packets, are derived based on the equilibrium point analysis. The effects of the two performance parameters on the system performance are discussed by varying the code reservation intervals of the voice terminals.

Comparison of DS-CDMA and MC-CDMA techniques for dual-dispersive fading acoustic communication networks

Recently, code division multiple access (CDMA) techniques have been utilised to achieve reliable multiuser communication in asynchronous shallow-water acoustic networks. Two different communication techniques, are considered. Both have their origins in spread spectrum principles and can offer significant benefits in such underwater environments, which are characterised by limited transmission bandwidth and are required to operate asynchronously with low signal-to-noise ratios. In direct-sequence (DS)-CDMA, spread data are transmitted at a single carrier frequency. In contrast, in multi-carrier (MC)-CDMA a set of carrier frequencies is employed to achieve frequency diversity. The proposed adaptive receiver architectures, based on the minimisation of the mean square error (MSE), integrate the functions of multi-access interference cancellation, equalisation and phase-carrier tracking. The performance of DS and MC-CDMA has been evaluated and compared under different simulation scenarios with respect to the multipath delay, Doppler broadening and number of users, indicating the effectiveness of each system in an underwater environment. Finally, analytical derivation of the BER performance of the MMSE multicarrier receiver in a Rayleigh-fading environment is presented.

Robust adaptive MMSE/DFE multiuser detection in multipath fading channel with impulse noise

The direct-sequence code-division multiple-access (DS/CDMA) technique is extensively used for cellular, personal, and mobile radio communication systems. However, the conventional channel estimators cannot provide accurate channel information for multiuser detection in the presence of impulsive noise and Doppler spread, especially in the time-varying fading channel. We propose a linear-trend tracking method for estimating the impulse response of the time-varying channel via a robust Kalman filtering with a scheme that can automatically adjust the variance of the driving noise. The proposed method need not identify the parameters of the channel model and is insensitive to the speed of channel variation. Following channel estimation, a robust multiuser detection algorithm is developed via a Kalman-based decision feedback equalization (DFE). A nonlinear limitation function is incorporated to mitigate the effects of channel estimation error and impulsive noise and to prevent severe error propagation. Simulation results conclude that the performance of the DS/CDMA system is improved by using the proposed receiver.

Dynamic resource allocation for handoff using MC-CDMA in wireless ATM networks

Focusing on the characteristics of WATM networks based on multicarrier code-division multiple access technique, an improved optimised cell-scheduling algorithm and a dynamic resource allocation scheme with a resource compensation method to improve performance are proposed. Performance measurements are obtained by an analytical model and simulation experiments and compared for different schemes such as handoff call-dropping probability and new call-blocking probability. The results obtained by the proposed algorithms perform better than other algorithms.