Orthogonal Spreading Sequences Research Articles

A reversible data hiding algorithm for audio files based on code division multiplexing

In this paper, a reversible data hiding algorithm for audio files based on code division multiplexing (CDM) is proposed. In the scheme, the orthogonal spreading sequences are employed to carry the embedding data, and the host audio file can also be recovered completely after the secret message has been extracted accurately. At the same time, according the orthogonal character of the spreading sequences, the secret message can be embedded into the audio files repeatedly, and most elements of the spreading sequences are mutually canceled in the process of multi-level data embedding, which maintains the audio file in good auditory even at high data embedding capacity. Moreover, only the receiver who holds the same spreading sequences, as the sender can restore the embedded data, which enhance the security of the proposed scheme. As shown in the experimental results, the CDM based reversible data hiding (RDH) algorithm for audio files can achieve higher data embedding capacity at the same audio distortion compared with those state-of-the-art audio RDH algorithms.

A New Kind of High Capacity and Security Reversible Data Hiding Scheme

A novel high capacity and security reversible data hiding scheme is proposed in this article, in which the secret data is represented by different orthogonal spreading sequences and repeatedly embedded into the cover image without disturbing each other in the light of Code Division Multiple Access (CDMA) technique, and thus the embedding capacity is enlarged. As most elements of orthogonal spreading sequences are mutually canceled in the process of repeated embedding, it keeps the distortion of the embedded image at a low level even with high embedding capacity. Moreover, only the receiver who has the spreading sequence and the embedding gain factor the same as the sender can extract the secret data and achieve the original image exactly, thus the proposed scheme achieves high embedding security than other schemes. The results of the experiment demonstrates that the CDMA based reversible data hiding scheme could achieve higher image quality at moderate-to-high embedding capacity compared with other state-of-the-art schemes.

A Reversible Data Hiding Scheme Based on Code Division Multiplexing

In this paper, a novel code division multiplexing (CDM) algorithm-based reversible data hiding (RDH) scheme is presented. The covert data are denoted by different orthogonal spreading sequences and embedded into the cover image. The original image can be completely recovered after the data have been extracted exactly. The Walsh Hadamard matrix is employed to generate orthogonal spreading sequences, by which the data can be overlappingly embedded without interfering each other, and multilevel data embedding can be utilized to enlarge the embedding capacity. Furthermore, most elements of different spreading sequences are mutually cancelled when they are overlappingly embedded, which maintains the image in good quality even with a high embedding payload. A location-map free method is presented in this paper to save more space for data embedding, and the overflow/underflow problem is solved by shrinking the distribution of the image histogram on both the ends. This would further improve the embedding performance. Experimental results have demonstrated that the CDM-based RDH scheme can achieve the best performance at the moderate-to-high embedding capacity compared with other state-of-the-art schemes.

Orthogonal basis spreading sequence for optimal CDMA

Recently, new spreading sequences have been proposed to increase the capacity of users. In particular, the Weyl spreading sequences have the larger capacity of users than the Gold codes. This paper shows that the Weyl spreading sequences appear in a bit recovering model and they are orthogonal basis vectors. This result shows the reason why they have the large capacity and that any spreading sequence is expressed as the sum of the Weyl spreading sequences.

Turbo receiver for MIMO-CDMA systems employing parity bit selected and permutation spreading

In this paper, a turbo receiver for MIMO-CDMA systems employing parity bit selected and permutation spreading is proposed. In such systems, spreading codes used by transmit antennas are dependent on the transmitted data. In the proposed system, convolutional coding is used as an outer code, while the parity bit selected scheme is used as the inner code. Detection and decoding are performed iteratively for each detected bit. When parity bit selected spreading is used, the parity bits generated by a linear block encoder are used to select a spreading code from a set of orthogonal spreading sequences. The selected spreading code is then used to spread the signals in all transmit antennas. In contrast, in permutation spreading, a permutation of orthogonal spreading sequences is used in each transmit antenna. In the proposed receiver, soft information passes between the detector and the decoder on multiple iterations. Detection is performed using the received signal from all receive antennas in combination with the extrinsic likelihood provided by a SISO decoder. The turbo receiver is further extended to a multiple user system, where the MAI is estimated in each iteration and subtracted out from the received signal. Simulations show a significant improvement in BER when a turbo receiver is used in these systems.

Spectral Efficiency Comparison Between MC-CDMA Two-Hop Relay Systems With Different Channel Information

The knowledge of channel state information (CSI) is crucial for improving the performance of cooperative communication systems. In multiuser two-hop relay systems, the global CSI between a relay and multiple source-and-destination nodes can be estimated at the relay, and the local CSI between each node and the relay can be estimated at each node. In this paper, we analyze the spectral efficiency of multicarrier code-division multiple-access (MC-CDMA) two-hop relay systems, either with global CSI only at the relay or with local CSI only at every node. We resort to asymptotical analysis with random-matrix theory to derive the average spectral efficiency of decode-and-forward (DF) and amplify-and-forward (AF) two-hop relay systems. We then analyze the impact of spreading sequences, fading channel statistics, and low-complexity transceivers. Analytical and simulation results show that when an orthogonal spreading sequence is used, the relay system with local CSI only at every node is spectrally more efficient than that with global CSI only at the relay. Moreover, an artificially constructed one-tap spreading sequence can achieve a good tradeoff between the performance and the complexity.

Turbo Receiver for Multicarrier Spread Spectrum Systems Employing Parity Bit Selected Spreading Sequences

In this paper a turbo receiver for multicarrier spread spectrum systems employing parity bit selected spreading code (MC-SS-PB) is proposed where detection and decoding are performed iteratively for each detected bit in the receiver. In MC-SS-PB systems, the parity bits generated by a linear block encoder are used to select a spreading code from a set of orthogonal spreading sequences. The selected spreading code is then used to spread the signals in all subcarriers. In the proposed receiver, soft information passes between the detector and the decoder on multiple iterations. Detection is performed by using the received signal in combination with the extrinsic likelihood provided by a soft input soft output decoder. The turbo receiver is further extended to a multiple user system where the multiple access interference is estimated in each iteration and subtracted out from the received signal. Simulations show a significant reduction in bit error rates when a turbo receiver is used in these systems.

On the effect of Multiple Access Interference in a Space Time Spreading Time Hopping PPM UWB system

This study investigates the effect of multiple sources using STS across a Time-Hopping-PPM Modulation that is over an UWB Saleh-Valenzuela channel with each source using two transmit antennas. We model and analyze this system, showing that the presence of such MAI results in a significant degradation in BER performance. Simulation results show that for strong signal strength of interfering MAI signals degradation in BER performance is observed for the cases where we have five and 10 distinct MAI sources present when using Walsh-Hadamard thirty two chip spreading sequences. Using simulation, we investigate the use of other sets of orthogonal spreading sequences with improved cross correlation properties and find that for some sequences a significant improvement is seen compared to Walsh-Hadamard sequences when they are similarly misaligned. We show that MAI degrades the BER performance of such systems and that selection of improved cross correlation spreading code sequence sets improves BER.

Turbo Receiver for DS-SS Systems Employing Parity Bit Selected Spreading Codes

In this paper a turbo receiver for direct sequence spread spectrum (DS-SS) systems employing parity bit selected spreading code (SS-PB) is proposed where detection and decoding are performed iteratively for each detected bit in the receiver. In SS-PB systems, parity bits generated by a linear block encoder are used to select a spreading code from a set of orthogonal spreading sequences. Performance evaluation in both additive white Gaussian noise (AWGN) and fading channels show a significant reduction in bit error rates (BER) when a turbo receiver is used in these systems.

Collaborative space-time spreading scheme for higher than unity rate CDMA downlink

A novel scheme referred to as collaborative space-time spreading is proposed to provide a higher than unity rate with spatial diversity gain for the downlink of a CDMA system operating in a fading channel. This is achieved by performing collaborative coding and spreading over groups of T users, each of which is assigned a single orthogonal spreading sequence and followed by space-time encoding on the total composite signal of all groups. A user terminal simply despreads the incoming signal to recover the composite T-user group data and then extracts its intended data after space-time and collaborative decoding. Extensive comparisons with other relevant schemes are provided to show the superiority of this approach.

Orthogonally-spread block transmissions for ultra-wideband impulse radios

Differential, transmitted reference (TR) and energy detection (ED) based ultra-wideband impulse radios (UWBIR) can collect the rich multipath energy offered by UWB channels with a low-complexity receiver. However, they perform satisfactorily only when the channel induced inter-pulse interference (IPI) is negligible. This can be achieved by appending a guard interval with duration greater than or equal to the channel's delay spread to each frame an operation limiting the maximum achievable data rate. As a remedy, this Letter advocates block transmissions in conjunction with orthogonal spreading sequences to remove the introduced IPI. The resultant scheme requires no channel knowledge besides timing offset and incurs slightly more complexity than non-block alternatives, while it increases the data rate at no cost in error performance. Given a fixed data rate of 25 Mbps, the novel block scheme exhibits about 1.8 dB gain relative to its non-block counterpart in single-user simulated tests.

Multi-user Detection of Nonlinearly Distorted MC-CDMA Symbols by Microstatistic Filtering

Multi-carrier code division multiple access is a powerful modulation technique that is being considered in many emerging broadband communication systems. In a downlink scenario orthogonal spreading sequences are used since they reduce multiple access interference compared to non-orthogonal. However, the nonlinear amplification of the transmitted signal destroys the orthogonality and, thus, reduces the system performance. In order to avoid performance degradation without requiring large back-offs in the transmitter amplifier, it becomes necessary to use multi-user detection techniques at the receiver side. Conventional multi-user detectors (MUD) are designed for linear environments and, as a result, might not exhibit enough performance improvement. In this paper a new MUD based on microstatistic filtering is proposed. The presented MUD uses piece-wise linear filtering in conjunction with threshold decomposition of the input signal, which introduces a nonlinear effect, to improve performance when a nonlinearity is present. Maximum performance improvement compared to conventional MUD is achieved for low spreading factors and user loads no greater than 50%.

Throughput performance of Hybrid ARQ schemes with packet and code combining for DS‐CDMA downlink systems using orthogonal spreading sequences

AbstractIn this paper, we evaluate the throughput performance of Automatic Repeat reQuest (ARQ) and HARQ I, with packet combining, and HARQ II, with code combining, using convolutional codes for DS‐CDMA downlink systems employing a Rake receiver. The different user packets are code multiplexed using a set of orthogonal variable spreading factor sequences. The analysis is carried out in two extreme scenarios, with either a constant or a packet‐by‐packet independent multipath fading channel during the different transmissions. The obtained results are valid for any multipath intensity profile of the channel and take simultaneously into account AWGN noise, path gains estimation noise, intersymbol interference and multiuser interference. Copyright © 2008 John Wiley & Sons, Ltd.

Improved BLAST for wireless communications

Abstract Bell layered space-time architecture (BLAST) is a multi-antenna communication structure with high spectrum efficiency, and it has found wide applications in LANs and WLANs. However, its performance is much poorer than those of other space-time coding approaches. In order to improve its performance, an improved BLAST based on RAKE receiving is investigated. The new system introduces orthogonal spreading sequences (OSS) into the transmitter while retains the basic structure of BLAST. The proposed receiver suppresses interferences from other antennas by the orthogonality contained in the received signals, and extracts information from each receiving antenna by using RAKE receiving principle to construct efficient statistic decision. Simulation results show that the improved system performs well over both frequency-flat and frequency-selective fading channels.

A comparison between uplink and downlink MC-DS-CDMA sensitivity to static timing and clock frequency offsets

We study the effect of fixed timing offsets and clock frequency offsets on the performance of multicarrier direct-sequence CDMA (MC-DS-CDMA) for both uplink and downlink communication, assuming orthogonal spreading sequences and a slowly varying multipath channel. We point out that a constant timing offset does not give rise to performance degradation for neither uplink nor downlink MC-DS-CDMA. We derive simple but accurate approximate expressions that allow us to easily quantify the effect of clock frequency offset and the influence of the different system parameters on the receiver performance in practical situations. Further, we show that for both uplink and downlink MC-DS-CDMA, the performance in the presence of a clock frequency offset rapidly degrades with an increasing number of carriers. It turns out that this degradation is larger in the uplink than in the downlink because the former suffers from a higher level of multiuser interference. For a given maximum relative clock frequency offset, enlarging the spreading factor in a fully loaded system does not affect the downlink degradation but strongly increases the uplink degradation.

Parity bit selected spreading sequences: a block coding approach to spread spectrum

Parity bits are used to select the spreading sequence from a set of orthogonal spreading sequences, The information bits are then spread by this spreading sequence. At the receiver, the spreading code employed is determined from the outputs of the filters that are matched to each spreading sequence. The information bits are then determined from the output of the specific matched filter. Error correction is then performed by assuming that the parity bits are correct.

The Effect of Timing Jitter on MC-DS-CDMA

In this paper, we consider the effect of timing jitter on the performance of a multicarrier direct-sequence code-division multiple-access system for both uplink and downlink transmission, assuming orthogonal spreading sequences. Theoretical expressions are derived for the performance degradation caused by the timing jitter, in the presence of a multipath channel. Assuming an additive white Gaussian noise channel, perfect power control, and full load, it is shown that the performance degradation for the downlink transmission is independent of the number of subcarriers, of the spreading factor, and of the spectral contents of the jitter at the receiving mobile station, but only depends on the jitter variance. Under the same assumptions, we point out that, if the jitter spectra of all transmitting mobile stations are the same, the degradation on the uplink is the same as the degradation on the downlink.

MMSE equalization of downlink CDMA channel utilizing unused orthogonal spreading sequences

For the code division multiple access (CDMA) downlink channel, the chip-level equalization has been considered in this paper. There is no interference after despreading if all spreading codes are orthogonal, as in IS-95. However, it cannot be true for a frequency-selective fading channel. In this case, the chip-level equalization can be applied to restore the orthogonality. We investigate the chip-level equalization using finite impulse response (FIR) equalizers for the mobile station with multiple receive antennas. A blind approach and the minimum mean square error (MMSE) approach with code-multiplexed pilot are considered. A generalized MMSE equalization, which combines the MMSE and blind approaches together, is also investigated. It is shown that the generalized MMSE equalizer can effectively increase the number of samples to track the variation of channel and thereby performs better when the coherence time is small. In addition, we derive closed-form solutions of the blind, MMSE, and generalized MMSE equalizers for given channels.

Convolutional coded DS/CDMA with two sets of orthogonal spreading sequences and iterative detection

In this letter a convolutional coded Direct-Sequence Code-Division Multiple Access(DS/CDMA) concept is introduced. The number of the users it can accommodate is greater than the spreading factor N. Each of the available orthogonal spreading sequences of length N is assigned to one of the first N users which employ a common Pseudo-Noise (PN) scrambling sequence. When the number of users K exceeds N, say K=N+M with M N. The proposed technique thus accommodates N users without any mutual interference and a number of additional users almost without signal-to-noise ratio penalty.

An exact error floor for downlink MC-CDMA in correlated Rayleigh fading channels

Downlink multicarrier code-division multiple access (MC-CDMA) with maximal ratio combining (MRC) in correlated Rayleigh fading channels is considered. An exact error floor in a closed-form expression is provided based on the characteristic function (CF) and residue calculation method. Numerical results show that orthogonal spreading sequences (Walsh and orthogonal Gold sequences) perform better than nonorthogonal sequences (Gold sequences) and Walsh codes have the best performance. In addition, downlink MC-CDMA is found to benefit from the correlation between subcarriers.