Conventional Code Division Multiple Access Research Articles

Multi‐user underwater acoustic communication using binary phase‐coded hyperbolic frequency‐modulated signals

Multi-user communication traditionally uses either code division multiple access (CDMA) or time-division multiple access schemes. The chirp signals resilience to multipath and the Doppler effect makes it suitable for underwater acoustic communication. This paper presents a binary phase-coded hyperbolic frequency-modulated (BPC-HFM) for multi-user underwater acoustic communication. The proposed scheme has the advantage of both linear frequency-modulated signal, which is immune to multipath and Doppler offset, while PN sequence is encoded in such a manner that a waveform possesses either a phase shift of 0 or 180 depending upon the code which results in a unique orthogonal waveform for every user. As a result, it helps to reduce multiple access interference; in addition, the time-reversal mirror technique is used to enhance the performance of the system in the high multipath environment due to the virtue of hydro-acoustic channel. Furthermore, it uses a relatively simple matched filter-based receiver. To substantiate the robustness of the proposed multi-user waveforms three types of hydro-acoustic channels using the different numbers of multiple users are chosen for simulation and the bit error rate (BER) of the system is calculated. Furthermore, the results are compared to binary phase-coded linear frequency-modulated signal and conventional Direct sequence spread spectrum CDMA.

Dual hop relaying using CDMA and reconfigurable intelligent surfaces (RIS)

In this paper, we suggest the use of reconfigurable intelligent surfaces (RIS) in dual hop relaying. There are two hops, the first one uses code division multiple access (CDMA) and the second one uses RIS as transmitter or reflector. We show that the incorporation of RIS allows to increase the throughput of conventional CDMA systems. The throughput of dual hop relaying using a combination of CDMA and RIS is larger than that of dual hop relaying based entirely on CDMA.

Multiuser Chirp Spread Spectrum Transmission in an Underwater Acoustic Channel Applied to an AUV Fleet.

The objective of this paper is to provide a multiuser transmission technique for underwater acoustic communication in the framework of an Autonomous Underwater Vehicle (AUV) fleet. By using a variant of a Hyperbolically Frequency-Modulated (HFM) signal, we describe a new family of transmission techniques called MultiUser Chirp Spread Spectrum (MU-CSS), which allows a very simple matched-filter-based decoding. These techniques are expected to provide good resilience against multiuser interference while keeping good robustness to Underwater Acoustic (UWA) channel impairments like Doppler shift. Their implementation for the UWA scenario is described, and the performance results over a simulated shallow-water UWA channel are analyzed and compared against conventional Code-Division Multiple Access (CDMA) and Time-Division Multiple Access (TDMA) transmission. Finally, the feasibility and robustness of the proposed methods are verified over the underWater AcousTic channEl Replay benchMARK (Watermark), fed by several channel responses from sounding experiments performed in a lake.

Overloaded CDMA Crossbar for Network-On-Chip

On-chip interconnects are the performance bottleneck in modern system-on-chips. Code-division multiple access (CDMA) has been proposed to implement on-chip crossbars due to its fixed latency, reduced arbitration overhead, and higher bandwidth. In CDMA, medium sharing is enabled in the code space by assigning a limited number of ${N}$ -chip length orthogonal spreading codes to the processing elements sharing the interconnect. In this paper, we advance overloaded CDMA interconnect (OCI) to enhance the capacity of CDMA network-on-chip (NoC) crossbars by increasing the number of usable spreading codes. Serial and parallel OCI architecture variants are presented to adhere to different area, delay, and power requirements. Compared with the conventional CDMA crossbar, on a Xilinx Artix-7 AC701 FPGA kit, the serial OCI crossbar achieves 100% higher bandwidth, 31% less resource utilization, and 45% power saving, while the parallel OCI crossbar achieves ${N}$ times higher bandwidth compared with the serial OCI crossbar at the expense of increased area and power consumption. A 65-node OCI-based star NoC is implemented, evaluated, and compared with an equivalent space division multiple access based torus NoC for various synthetic traffic patterns. The evaluation results in terms of the resource utilization and throughput highlight the OCI as a promising technology to implement the physical layer of NoC routers.

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.

Twin tree hierarchy: a regularized approach to construction of signature matrices for overloaded CDMA

Overloaded code division multiple access being the only means of the capacity extension for conventional code division multiple access accommodates more number of signatures than the spreading gain. Recently, ternary Signature Matrices with Orthogonal Subsets (SMOS) has been proposed, where the capacity maximization is 200%. The proposed multi-user detector using matched filter exploits the twin tree hierarchy of correlation among the subsets to guarantee the errorless recovery. In this paper, we feature the non-ternary version of SMOS (i.e., 2k-ary SMOS) of same capacity, where the binary alphabets in all the k constituent (orthogonal) subsets are unique. Unlike ternary, the tree hierarchy for 2k-ary SMOS is non-uniform. However, the errorless detection of the multi-user detector remains undeviated. For noisy transmission, simulation results show the error performance of the right child for each subset of 2k-ary to be significantly improved over the left. The optimality of the right child of the largest (Hadamard) subset is also discovered. At higher loading, for larger and smaller subsets the superiority is reported for the 2k-ary and ternary, respectively, and the counter-intuitive deviations observed for the lower loading scenarios are logically explained. For the overall capacity maximization being 150%, superiority is featured by the 2k-ary, but beyond, it becomes a conditional entity. Copyright © 2016 John Wiley & Sons, Ltd.

Noise-Loop Multiple Access

A fundamental problem for signal processing in wireless communications is the separation of multiple signals/users from their mixture. The problem is simpler if they are nonoverlapping in either the time domain or the frequency domain. Two signals may be overlapping in both time and frequency, but they can still be easily separated if they are uncorrelated (orthogonal), as in code-division multiple-access (CDMA) systems. It may be interesting to know that two signals may be overlapping in both the frequency and time domains and not orthogonal to each other, but they can still be separated if each autocorrelation function at the receiver is separable in the time-lag domain. In this paper, we will show how noise-loop (NL) modulation, which was originally thought to produce security at the physical layer, can be used to effectively separate uncoordinated/nonorthogonal users sharing the same time and frequency interval. To show the potential of the NL multiple-access (NLMA) scheme, the theoretical analysis is carried out and compared with simulations. In particular, together with the error probability, a new figure of merit is derived: the multiuser efficiency index. It can be shown that the NLMA scheme is able to distinguish the bit stream of the users, regardless of their number. In addition, the capacity region of the NLMA scheme is derived and compared with the conventional CDMA scheme. All of the aforementioned features, together with the intrinsic security, make the NL an interesting scheme for wireless technology.

Zero intersymbol interference multiuser system: a new architecture utilising m ‐sequence cyclic property

Code division multiple access (CDMA) based on spread spectrum technique has become one of the most important multiple access technologies in contemporary communication systems. The properties of spreading codes largely govern the performance of CDMA systems. Therefore, CDMA can insure interference free operation when code design methodology takes into account of major impairing factors such as intersymbol interference (ISI), asynchronous transmission and random signs in consecutive symbols. In this study, the authors propose a design scheme for CDMA signal reception with zero ISI in the multipath environment. Cyclic shift property of maximal length sequence (m-sequence) is utilised in the design to avoid ISI. The proposed scheme exploits inherent diversity present in the multipath channel and can be seen as antenna diversity. This scheme avoids the use of complex detectors and the performance is compared by simulations with conventional CDMA system using Walsh–Hadamard codes and m-sequence. The impact of the Rician factor on the end-to-end performance is also studied in detail and suitable receiver to be used has been suggested for different fading scenarios. Simulations results presented indicate that the proposed scheme provides superior performance over traditional systems.

Power-Controlled Multicarrier and Multiuser Detection Techniques in Wireless CDMA Systems

This paper describes power-controlled multi carrier and multiuser detection techniques for Code Division Multiple Access (CDMA) systems. In conventional CDMA systems, a RAKE receiver is employed to overcome wireless channel effect due to frequency selective characteristics of wireless CDMA systems. Alternatively, multi carrier technique can also be used to overcome the effect of frequency selective channel. Conventional CDMA systems also employ single-user detection scheemes, in that interference between users, called the multiple access interference (MAI), is considered as interference which contributes to the statistics of detection variables affecting the quality of detection. Now, multiuser detection method is proposed to mitigate the inter user interference in CDMA systems. In multiuser detections, the MAI component can be eliminated or cancelled by the use of all user’s spreading code in the system to be available to each user. Another technique that is commonly employed in CDMA system is power control, to mitigate the effects of near-far and fading channel fluctuations. In this study, the basic concept of multi carrier and multiuser detection technique is described in a simple model and the performance for various number of users is simulated and evaluated. The performance of power-controlled multi-carrier and multiuser detection technique is then evaluated and compared with that of the singleuser detection to show a significant improvement.

Code-division multiple-access multiuser demodulator by using quantum fluctuations.

We examine the average-case performance of a code-division multiple-access (CDMA) multiuser demodulator in which quantum fluctuations are utilized to demodulate the original message within the context of Bayesian inference. The quantum fluctuations are built into the system as a transverse field in the infinite-range Ising spin glass model. We evaluate the performance measurements by using statistical mechanics. We confirm that the CDMA multiuser modulator using quantum fluctuations achieve roughly the same performance as the conventional CDMA multiuser modulator through thermal fluctuations on average. We also find that the relationship between the quality of the original information retrieval and the amplitude of the transverse field is somehow a "universal feature" in typical probabilistic information processing, viz., in image restoration, error-correcting codes, and CDMA multiuser demodulation.

A dynamic CDMA network for multicore systems

Code-division multiple-access (CDMA) is a data transmission method based on the spreading code technology, wherein multiple data streams share the same physical medium with no interference. A novel architecture for on-chip communication networks based on this approach is devised. The proposed design allows sharing coding resources among network׳s users through the use of dynamic assignment of spreading codes. Data transmission latency is reduced by adopting a parallel structure for the coding/decoding circuitry. A 14-node CDMA network based on the proposed architecture is synthesised using 65nm ST technology library. Performance analysis reveals that the proposed approach achieves significantly lower data packet latency compared to both conventional CDMA and packet switched network-on-chip implementations. Large area and power savings compared to existing approaches are also obtained.

Rotational Interleaver for Iterative Interleave-Division Multiple-Access Scheme

In iterative IDMA scheme, the users are distinguished with various chip-level interleaving sequences instead of user-specific signature sequences as in conventional code-division multiple-access (CDMA) scheme. A set of interleavers is considered to be practical if it has zero correlation with each other and also easy to generate as well requires minimal bandwidth during communication process. A properly defined correlation between user-specific interleavers can be utilized to formulate a minimal collision criterion, where zero-correlation (i.e., orthogonality) implies no collision. In this paper, a practical interleaver has been proposed for interleave-division multiple-access (IDMA) systems. Simulation results show that the proposed interleaver performs even better to random interleavers initially reported for iterative IDMA system.

Design and Optimization of a CDMA-Based Multi-Reader Passive UHF RFID System for Dense Scenarios

In dense passive radio frequency identification (RFID) systems, code division multiple access (CDMA) techniques can be used to alleviate severe collisions and thus enhance the system performance. However, conventional CDMA techniques are challenging to implement, especially for passive tags due to cost and power constraints. In this paper, we design a CDMA-based multi-reader passive ultra high frequency (UHF) RFID system in which a reader detects only the strongest tag signal and a tag uses Gold codes only on the preamble and the data bits of RN16 without increasing its clock frequency. We present a new communication procedure based on dynamic framed slotted ALOHA (DFSA). In order to optimize the system, we theoretically analyze the system performance in terms of slot capacity and identification rate, and formally show how the code length and the number of readers affect the identification rate. Furthermore, we propose an effective method for tag estimation and frame size adjustment, and validate it via simulations. Through an example, we demonstrate how the analysis-based technique can be used to optimize the system configurations with respect to the number of readers and the number and length of Gold codes.

Interleave Division Multiple Access for Wireless Communication

[enter Abstract Body]This paper outlines a multiple access technique which exploits the interleaving as only means of user separation instead of by different signatures as in a conventional code division multiple-access (CDMA) scheme. It examines a very simple iterative chip-by-chip multi-user detection strategy for spread spectrum communication systems. The given scheme can achieve near single user performance in situations with very large numbers of users while maintaining very low receiver complexity. This technique shows that high performance can be maintained in multi-path environments. IDMA inherits many advantages from CDMA, such as diversity against fading and mitigation of the worst-case user interference problem. A low-cost iterative chip-by-chip multi-user detection algorithm is also described with complexity independent of the user number and increasing linearly with the path number.

Multi-tone CDMA design for arbitrary frequency offsets using orthogonal code multiplexing at the transmitter and a tunable receiver

The authors propose a new multi-tone (MT) code division multiple access (CDMA) design which has a superior bit error rate (BER) performance than conventional MT CDMA in the presence of frequency offset. The design involves multiplexing of Walsh codes onto the sub-carriers in conjunction with double differential modulation. To exploit the full potential of the design a partial correlation receiver has been proposed. Depending on the signal-to-noise ratio (SNR) and frequency offset it is possible to tune this receiver for the best possible performance. The simulated BER performance of the proposed system has been found to be better than MT CDMA for small as well as large frequency offsets for both single and multi-user systems in additive white Gaussian noise (AWGN) and Rayleigh fading channels.

High user capacity collaborative code-division multiple access

In this study, the authors propose a novel collaborative multi-user transmission and detection scheme for the uplink of code-division multiple access (CDMA) that exploits the differences between users' fading channel signatures to increase the user capacity well beyond the spreading length in multiple access interference (MAI) limited environment. The authors show that it is possible to achieve this increase at a low complexity and high bit error rate (BER) performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading sequence and enable group spreading and despreading operations. The proposed collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low complexity maximum-likelihood detection stage to recover jointly the co-spread users' data using minimum Euclidean distance measure and users' channel gain coefficients. The scheme is investigated analytically and by extensive simulations and comparisons with conventional CDMA, overloaded CDMA using group pseudo-decorrelation (G-PD) and layered space–time (LAST) MUD. It is shown that the total number of full-rate users supported by the scheme is substantially higher than the available sequences. Moreover, it achieves much lower complexity and significantly improved BER for the same user capacity compared with G-PD. It is also shown that, with antenna diversity reception, collaborative CDMA considerably outperforms LAST scheme, particularly when more transmit antennas per group than available receive antennas are employed.

Performance Analysis of Multi-user Multi Input Multi Output- Interleave-Division Multiple-Access System Employing Turbo Coding with Multi-User Detection over Frequency-Selective Wireless Communication Channel

Problem statement: This study presents the performance analysis of multi-user Multi Input Multi Output (MIMO) assisted interleave based multiple-access system. In IDMA, different interleavers are used to distinguish users as against different signature sequence in a conventional code-Division Multiple-Access (CDMA) scheme. Approach: The basic principle of IDMA is that the interleaver is unique for the users. Results: In this study, we consider that Interleavers are generated independently and randomly. Also the IDMA technique is extended to multi user MIMO IDMA with multi-user detection. At the receiver, OSIC detector is realized using ZF for frequency fading channel to combat MAI and MUI problem. The performance of the system is analyzed for different channel conditions using extensive simulation runs based on Monte Carlo simulation trials. Conclusion: It is shown that the IDMA scheme can achieve near single user performance in situations with very large numbers of users while maintaining very low receiver complexity. It is discerned from the computer simulation results that IDMA outperforms CDMA in frequency selective channel for high load conditions.

Quasi-Synchronous CDMA Using Properly Scrambled Walsh Codes as User-Spreading Sequences

An orthogonal spreading code family based on scrambled Walsh-Hadamard (WH) sequences is proposed for quasi-synchronous (QS) code-division multiple-access (CDMA) systems. With 1 chip offset, either the top or the bottom half rows of the WH matrix remain orthogonal if they are scrambled using the proposed scrambling patterns. With quadriphase scrambling applied on a quarter row of the WH matrix, the zero-correlation zone (ZCZ) is extended to 3 chips. The proposed spreading codes satisfy the theoretical limit of code size for a given code length and ZCZ. Compared with existing spreading codes for QS-CDMA, the proposed spreading codes have several advantages: 1) A special procedure for code family construction is not needed; it only requires a simple modification to the scrambling sequence of the conventional CDMA, which employs WH sequences as user signatures. 2) Flexible code family size; the size can easily be extended for multicell applications without having to consider existing codes used. Moreover, if synchronization is guaranteed to be within 1 chip, the code size can be doubled by allocating another quarter member of the WH sequence set. 3) The benefits of long code scrambling from the conventional cellular CDMA systems are inherited, such as robustness against interception and multiaccess interference randomization.

MIMO Multi Carrier Interleave Division Multiple Access System with Multiuser Detection: Performance Results

This paper provides the performance analysis of multi input multi output (MIMO) Multi carrier (MC) interleaver based multiple-access system with multiuser detection. Interleave-division multiple access (IDMA) is a scheme in which users are separated by employing different interleavers instead of different signatures as in a conventional code-division multiple-access (CDMA) scheme. The basic principle of IDMA is that the interleaver is different for different users. Interleavers are generated independently and randomly. At the receiver, ZF, LLSE, VBLAST/ZF/MAP and VBLAST/LLSE/MAP detectors are employed. The performance of the system is analyzed for different channel conditions using extensive simulation runs based on Monte Carlo simulation trials. Simulation results divulge that VBLAST/LLSE/MAP detector results in significant performance amelioration compared to other detectors as applied to MC/IDMA system.

Novel Low-Density Signature for Synchronous CDMA Systems Over AWGN Channel

Novel low-density signature (LDS) structure is proposed for transmission and detection of symbol-synchronous communication over memoryless Gaussian channel. Given N as the processing gain, under this new arrangement, users' symbols are spread over N chips but virtually only dv < N chips that contain nonzero-values. The spread symbol is then so uniquely interleaved as the sampled, at chip rate, received signal contains the contribution from only dc < K number of users, where K denotes the total number of users in the system. Furthermore, a near-optimum chip-level iterative soft-in-soft-out (SISO) multiuser decoding (MUD), which is based on message passing algorithm (MPA) technique, is proposed to approximate optimum detection by efficiently exploiting the LDS structure. Given beta = K/N as the system loading, our simulation suggested that the proposed system alongside the proposed detection technique, in AWGN channel, can achieve an overall performance that is close to single-user performance, even when the system has 200% loading, i.e., when beta = 2. Its robustness against near-far effect and its performance behavior that is very similar to optimum detection are demonstrated in this paper. In addition, the complexity required for detection is now exponential to dc instead of K as in conventional code division multiple access (CDMA) structure employing optimum multiuser detector.