Quasi-synchronous CDMA Research Articles

Novel quadriphase ZCZ sequences for QS-CDMA systems

In this article, a novel method for constructing quadriphase Zero Correlation Zone (ZCZ) sequence sets based on the mutually orthogonal complementary sets (MOCS) matrix and polyphase perfect sequence is presented. Based on the obtained MATLAB results in terms of correlation functions, it can be mentioned that the resultant ZCZ set is suitable for quasi-synchronous Code Division Multiple Access (QS-CDMA) systems. The obtained sequence sets are near-optimal concerning mathematical bound, and their construction is more flexible than other polyphase ZCZ constructions. Furthermore, their parameters can be chosen flexibly.

Construction and Assignment of Orthogonal Sequences and Zero Correlation Zone Sequences Over GF(p)

Orthogonal sequences can be assigned to a regular tessellation of hexagonal cells, typical for synchronised code-division multiple-access (S-CDMA) systems. The sequences within any cell should both be orthogonal in the adjacent cells and require a sufficient number of users in each cell. However, for binary sequences, the capacity of communication system is limited. So, a new family of non-binary orthogonal sequences is constructed, which has more sequences per cell than binary sequences in the network. Next, an efficient assignment of these orthogonal sequence sets to a regular tessellation of hexagonal cells is given, and also the sequence sets have large re-use distance. Finally, based on above the construction, we construct a family of orthogonal sequences with zero correlation zone (ZCZ) property which can reduce the interference among users in a multi-path environment. The constructed ZCZ sequences can be applied to the quasi-synchronous CDMA (QS-CMDA) spread spectrum systems. And the assignment of sequences has the same re-use distance.

Improved LPD Characteristics for QS-DS-CDMA Employing Randomization Techniques

Easily and flexibly deployable ad-hoc communication networks emerging in tactical military or even civilian contexts, frequently suffer from poor synchronization due to lack of coordinating infrastructure. In addition to synchronization issues, and especially in military settings, security from the aspect of detectability is also of crucial importance. Imperfect synchronization can be dealt with by making use of Quasi-Synchronous Code Division Multiple Access (QS-CDMA), relying on Loosely Synchronous Codes to maintain orthogonality in the presence of limited time delays. Security, in terms of low probability of detection (LPD) from the standpoint of a malicious adversary, can be improved (reduced detection) by employing randomization techniques that disrupt the inherent structure of the transmitted QS-CDMA signals. This is based on the fact that QS-CDMA signals are Cyclostationary, having (almost) periodic Auto-Correlation functions (ACF) due to eminent signal periodicities (such as spreading code repetition). In this paper, we propose techniques to disturb the ACF and equivalently the Spectral Correlation function, and reduce the Degree of Cyclostationarity (DCS), our LPD measure. Specifically, we investigate randomization via 1) random per symbol time dithering and 2) random selection of spreading sequences, as well as a hybrid approach combining time dithering and code randomization. In all proposed techniques knowledge of the randomization pattern is not required at the legitimate receiver. We derive the Spectral Correlation function of the QS-CDMA signal under the proposed randomization schemes and compare it to simulations. We show through analysis and extensive numerical simulations that the proposed technique can reduce the DCS by almost two orders of magnitude. We also show that enhanced LPD can be achieved using the proposed techniques while sacrificing a part of the reduced time synchronization requirement. We further analyze the implications of the friendly receiver not knowing the randomization pattern and present results on the resulting communication performance.

New Construction of OVSF-OZCZ Codes in Multi-Rate Quasi-Synchronous CDMA VLC Systems for IoT Applications

Internet of Things (IoT) has become an emerging vision for next-generation network and attracts many researchers' attention. In this paper, we propose a novel quasi-synchronous code division multiple access (QS-CDMA) visible light communication (VLC) system for IoT network. A new construction of optical zero correlation zone (OZCZ) code set with orthogonal variable spreading factor (OVSF) is presented for this system, which can meet the multi-rate requirement and tolerate time delay. After deriving the construction and properties of the proposed code set, the maximum numbers of simultaneous users and multiple access interference (MAI) characteristic are analyzed. Furthermore, the signal noise ratio (SNR) and bit error rate (BER) performance of this system are also investigated while taking into account the effects of phase-induced intensity noise (PIIN), shot noise, and thermal noise. The numerical results indicate that the new construction can eliminate MAI and PIIN, which can achieve high quality transmission in the multi-rate QS-CDMA-VLC system for IoT applications.

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.

On the study of a quasi-synchronous CDMA-VLC system with two channels.

In this paper, a new construction of optical zero correlation zone (OZCZ) code set pair is proposed for quasi-synchronous code division multiple access (QS-CDMA) visible light communication (VLC) system. Owing to its excellent correlation properties within zero correlation zone and greater code weight, the multi-user QS-CDMA-VLC system using the proposed code construction can obtain both good performance in illumination and communication. An experimental QS-CDMA-VLC system with two optical channels and two experiment setups is designed to evaluate the performance of the new OZCZ code set pair. When the free-space transmission distance difference is 1 m, we successfully achieve the 2-user and 14-user CDMA-VLC for 32.5 Mb/s and 24.1 Mb/s overall bit rate respectively, under the 7% forward-error-correction (FEC) limit of 3.8×10 -3. The experimental results indicate that, comparing to the other existing optical code sets, the proposed OZCZ construction can effectively reduce the impact of non-perfect synchronous problem, so that the bit error rate (BER) performance is improved. And dimming values and overall bit rates of the system can be significantly improved.

Construction and assignment of orthogonal sequences and zero correlation zone sequences for applications in CDMA systems

Orthogonal sequences can be assigned to a regular tessellation of hexagonal cells, typical for synchronised code-division multiple-access (S-CDMA) systems. In this paper, we first construct a new class of orthogonal sequences with increasing the number of users per cell to be \begin{document}$ 2^{m-2} $\end{document} for even number \begin{document}$ m\geq 4 $\end{document} (where \begin{document}$ 2^m $\end{document} is the length of the sequences). In addition, based on the above construction we construct a family of orthogonal sequences with zero correlation zone property which can be applied to the quasi-synchronous CDMA (QS-CMDA) spread spectrum systems.

Experimental demonstration of quasi-synchronous CDMA-VLC systems employing a new OZCZ code construction.

In this paper, we propose a new construction of optical zero correlation zone (OZCZ) code set for quasi-synchronous code division multiple access (QS-CDMA) visible light communication (VLC) system. The proposed code set includes a pair of unipolar and bipolar code sets, which is more suitable for intensity modulation and direct detection (IM/DD) and maintains dimming values at 50% regardless of user numbers for CDMA-VLC system. A QS-CDMA-VLC system adopting the proposed code set is investigated by numerical simulation and experimental verification. The system bit error rate (BER) performance is evaluated with different user numbers, sample rates, delay user numbers and transmission distances. In a 32-user CDMA-VLC system, the 250 MS/s transmission at a distance of 1.5 m can be achieved successfully with the BER less than the forward-error-correction (FEC) threshold. And the BER varies slightly taking account of small delay time. The results indicate that the new OZCZ code construction can effectively overcome non-perfect synchronous problem when the time delay among users does not exceed the zero correlation zone length.

A Construction of Multiple Optimal ZCZ Sequence Sets With Good Cross Correlation

Zero correlation zone (ZCZ) sequences are a class of spreading sequences having ideal auto-correlation and cross correlation in a zone around the origin. They have been extensively studied in recent years due to their important applications in quasi-synchronous code division multiple access systems. In this paper, a construction of ZCZ sequence sets is proposed based on perfect nonlinear functions. It generates multiple ZCZ sequence sets with the properties: 1) each sequence is perfect in the sense that its out-of-phase auto-correlation is always zero; 2) each ZCZ sequence set is optimal with respect to the Tang–Fan–Matsufuji bound in which all the sequences are pairwise cyclically distinct; and 3) the maximum inter-set cross correlation of multiple sequence sets achieves the well-known Sarwate bound.

An asymmetric ZCZ sequence set with inter-subset uncorrelated property and flexible ZCZ length

In this paper, we propose a novel method for constructing new uncorrelated asymmetric zero correlation zone (UA-ZCZ) sequence sets by interleaving perfect sequences. As a type of ZCZ sequence set, an A-ZCZ sequence set consists of multiple sequence subsets. Different subsets are correlated in conventional A-ZCZ sequence set but uncorrelated in our scheme. In other words, the cross-correlation function (CCF) between two arbitrary sequences which belong to different subsets has quite a large zero cross-correlation zone (ZCCZ). Analytical results demonstrate that the UA-ZCZ sequence set proposed herein is optimal with respect to the upper bound of ZCZ sequence set. Specifically, our scheme enables the flexible selection of ZCZ length, which makes it extremely valuable for designing spreading sequences for quasi-synchronous code-division multiple-access (QS-CDMA) systems.

Families of asymmetric sequence pair set with zero‐correlation zone via interleaved technique

An asymmetric zero-correlation zone (A-ZCZ) sequence pair set is a ZCZ sequence pair set consisting of multiple ZCZ sequence pair subsets. Two arbitrary sequence pairs which belong to different subsets should have a large zero cross-correlation zone (ZCCZ) in the A-ZCZ sequence pair set. Additionally, each subset is a typical ZCZ sequence pair set. The proposed A-ZCZ sequence pair sets can be generated based on interleaved technique from perfect sequence or perfect sequence pair of length P = Nmk with N ≥ 1, m ≥ 1, k > 1. The new A-ZCZ sequence pair sets are applied to quasi-synchronous code-division multiple-access (QS-CDMA) systems and it can hypothetically achieve larger ZCCZ than typical ZCZ sequence pair sets, as well as eliminating both multiple access and multipath interference in the QS-CDMA system.

New constructions of quaternary low correlation zone sequence sets

Sequence sets with low correlation have very important applications in modern communication systems. As in quasi-synchronous code-division multiple access (QS-CDMA) system, sequence sets with low correlation zone (LCZ) perform better than other well-known sequence sets. Furthermore, binary or quaternary sequence sets are preferred because of their easy implementation. In this paper, based on the inverse Gray mapping and special binary sequence pairs, new quaternary LCZ sequence sets were constructed. In the LCZ, the maximum of the nontrivial autocorrelation and crosscorrelation values is 1 which show that the QS-CDMA system used the new sequences sets can control the interference in a very low level.

New design of optical zero correlation zone codes in quasi-synchronous VLC CDMA systems

Direct-sequence optical code division multiple access technology is one efficient and simple way to reduce the multiple access interferences that existed in visible light communication system due to its ease implementation by simply turning the light-emitting diodes (LEDs) on and off. To overcome the multi-path effect and non-perfect clock impairment, an optical zero correlation zone code set is proposed in visible light communication quasi-synchronous optical code division multiple access system. Bit error rate performance is evaluated while taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise. Based on the proposed optical zero correlation zone codes, the multiple access interference and phase-induced intensity noise can be eliminated finally; therefore, a better bit error rate can be obtained.

Sequence Design for Cognitive CDMA Communications under Arbitrary Spectrum Hole Constraint

To support interference-free quasi-synchronous code-division multiple-access (QS-CDMA) communication with low spectral density profile in a cognitive radio (CR) network, it is desirable to design a set of CDMA spreading sequences with zero-correlation zone (ZCZ) property. However, traditional ZCZ sequences (which assume the availability of the entire spectral band) cannot be used because their orthogonality will be destroyed by the spectrum hole constraint in a CR channel. To date, analytical construction of ZCZ CR sequences remains open. Taking advantage of the Kronecker sequence property, a novel family of sequences (called "quasi-ZCZ" CR sequences) which displays zero cross-correlation and near-zero auto-correlation zone property under arbitrary spectrum hole constraint is presented in this paper. Furthermore, a novel algorithm is proposed to jointly optimize the peak-to-average power ratio (PAPR) and the periodic auto-correlations of the proposed quasi-ZCZ CR sequences. Simulations show that they give rise to single-user bit-error-rate performance in CR-CDMA systems which outperform traditional non-contiguous multicarrier CDMA and transform domain communication systems; they also lead to CR-CDMA systems which are more resilient than non-contiguous OFDM systems to spectrum sensing mismatch, due to the wideband spreading.

The Construction and Performance of a Novel Intergroup Complementary Code

On the basis of the analyses for intergroup complementary (IGC) code and zero correlation zone complementary code, a novel IGC code has been proposed to adapt M-ary orthogonal code spreading spectrum system or quasi-synchronous CDMA system. The definition and construction methods of the new IGC codes are presented and an applied example is given in this paper. Theoretical research and simulation results show that the main advantages of the novel IGC code are as following: The code sets of the novel IGC code is more than IGC code under the same code length. The zero correlation zone length is longer than the intergroup IGC code, but shorter than the intergroup IGC code. Under the same code length, the auto-correlation performance of the novel IGC code is better than that of the IGC code, and both are of similar cross-correlation performance.

DS-UWB indoor positioning system implementation based on FPGAs

In Ultra-Wideband (UWB) Local Positioning Systems (LPS), Inter-Symbol Interference (ISI), Multiple-Access Interference (MAI) and near-far effect are significant issues that can limit system performance. This paper presents a DS-UWB (Direct Sequence UWB) LPS based on FPGAs (Field-Programmable Gate Array) and on commercially available Radio-Frequency (RF) modules. In contrast to other DS-UWB LPS, which uses TDMA (Time Division Multiple Access) and base-band pulses with low-repetition frequency rates, the proposal uses QS-CDMA (Quasi-Synchronous Code Division Multiple Access) techniques with Loosely Synchronized (LS) spreading sequences. This allows to deal with ISI, MAI and near-far effect, as well as to use very large spreading sequences to increase the processing gain.

A generic construction of low correlation zone sequences based on interleaved technique

Low correlation zone (LCZ) sequences are useful in quasi-synchronous code-division multiple access (QS-CDMA) communication systems. In this paper, a generic construction of LCZ sequences based on interleaved technique is investigated. Firstly, the shift sequence is shown to correspond to two-tuple balanced d -form function essentially, which results in new shift sequence. Secondly, an optimal design of p 2 -ary sequences over the integer residue class ring Z p 2 is proposed, which improves the previous construction when p is an odd prime.

Construction of Shift Distinct Sequence Sets with Zero or Low Correlation Zone

A construction of shift sequence sets is proposed. Multiple distinct shift sequence sets are obtained by changing the parameters of the shift sequences. The shift sequences satisfy the conditions that P|L and P ≥ 2, where P is the length of the shift sequences, L is the length of the zero-correlation zone or low-correlation zone (ZCZ/LCZ). Then based on these shift sequence sets, many shift distinct ZCZ/LCZ sequence sets are constructed by using interleaving technique and complex Hadamard matrices. Furthermore, the new construction is optimal under the conditions proposed in this paper. Compared with previous constructions, the proposed construction extends the number of shift distinct ZCZ/LCZ sequence sets, so that more sequence sets are obtained for multi-cell quasi-synchronous code-division multiple access (QS-CDMA) systems.

The Construction and Performance of a Novel Intergroup Complementary Code

Based on the analyses of intergroup complementary (IGC) codes and zero correlation zone complementary codes, novel IGC codes were proposed to adapt M-ary orthogonal code spreading spectrum system or quasi-synchronous CDMA system. The theoretical research and simulation results show that the main advantages of novel IGC are that under the same code length, the number of the novel IGC code sets are more than that of the intra-group IGC codes; the length of the zero correlation zone is longer than that of the intergroup IGC codes, but shorter than that of the intergroup IGC codes between groups; and under the same code length, the auto-correlation performance of the novel IGC codes is better than that of the IGC codes and both codes are of similar cross-correlation performance.

Zero Correlation Zone Sequence Sets over the 8-QAM+ Constellation

In this letter, constructions of zero-correlation zone (ZCZ) sequence sets over the 8-QAM+ constellation are presented. The resultant ZCZ sequence sets have flexible parameters and are optimal with respect to the Tang-Fan-Matsufuji bound in some cases. These ZCZ sequences over the 8-QAM+ constellation can be used in quasi-synchronous CDMA (QS-CDMA) communication systems to remove the multiple access interference (MAI) and multi-path interference (MPI).