Asynchronous Access Research Articles

Analysis of an FBMC/OQAM scheme for asynchronous access in wireless communications

Abstract The OFDM/OQAM transceiver belongs to the filter-bank-based multicarrier (FBMC) family and, unlike OFDM schemes, it is particularly able to meet the requirements of the physical layer of cognitive radio networks such as high level of adjacent channel leakage ratio and asynchronous communications. The paper proposes and analyzes a new implementation structure, named frequency spreading, for the OFDM/OQAM transceiver. On flat channels, it is equivalent to the standard one in terms of input-output relations, though more complex. On multipath channels, it offers a crucial advantage in terms of equalization, which is performed in the frequency domain, leading to high performance and no additional delay. With its flexibility and level of performance, the analyzed scheme has the potential to outperform OFDM in the asynchronous access context and in cognitive radio networks.

A coding technique for Q-frequency S-user gaussian channel

The paper addresses the problem of constructing an asynchronous multiple access system for a multiuser Q-frequency channel with additive white gaussian noise (AWGN). To solve the problem we propose a coding scheme for the channel. The major component of the scheme is non-binary low-density parity-check (LDPC) code. To increase the transmission rate we introduce the embedded modulation. The efficiency of the resulting multiple-access system is shown by simulations.

On the Optimum Packet Power Distribution for Spread Aloha Packet Detectors With Iterative Successive Interference Cancelation

Recent work has been stimulated by the need to enhance the satellite networks' Random Access (RA) performance. Results have shown that large performance improvement is possible by exploiting Iterative Successive Interference Cancelation (I-SIC) at the demodulator side. Coded spread spectrum Aloha (SSA) is a particularly suitable access technique for RA satellite networks due to its inherent asynchronous access nature and its high multiple access Achievable Sum Rate (ASR). Moreover, the adoption of I-SIC allows further boosting of the SSA sum rate and its robustness to the packet power unbalance. In this paper, we derive an analytical approach to the optimal packet power distribution that maximizes the performance of SSA RA exploiting I-SIC at the demodulator. The proposed approach takes into account the physical layer coding and modulation together with the number of demodulator iterations and the non-ideal cancelation effects. The analytical findings are successfully compared with the simulation and measurement results. Finally, a practical algorithm that approximates the optimum incoming power packet distribution in a realistic multi-beam satellite broadband network with open-loop power control is proposed and validated in a study case.

Asynchronous Contention Resolution Diversity ALOHA: Making CRDSA Truly Asynchronous

Following the introduction of contention resolution diversity slotted ALOHA (CRDSA), a number of variants of the scheme have been proposed in literature. A major drawback of these slotted random access (RA) schemes is related to the need to keep slot synchronization among all transmitters. The volume of signaling generated to maintain transmitters' slot synchronization is impractical for large networks. In this paper, we describe in detail asynchronous contention resolution diversity ALOHA (ACRDA), which represents the evolution of the CRDSA RA scheme. ACRDA provides better throughput performance with reduced demodulator complexity and lower transmission latency than its predecessor while allowing truly asynchronous access to the shared medium. The performance of the ACRDA protocol is evaluated via mathematical analysis and computer simulations and is compared with that of CRDSA.

Adaptive yielding scheme for link scheduling in OFDM-based synchronous device-to-device (D2D) communication system

Compared to asynchronous contention-based random access, e.g., carrier sensing multiple access, synchronous and distributed link scheduling for orthogonal frequency-division multiplexing (OFDM) systems is a viable solution to improve system throughput for device-to-device (D2D) ad hoc network. In particular, spatial spectral efficiency can be improved by scheduling as many concurrent D2D links necessary to satisfy individual signal-to-interference ratio (SIR) requirements. In this paper, we propose an adaptive yielding mechanism that can further improve the spatial spectral efficiency by allowing for more concurrent D2D links whenever more interference can be accepted, e.g., when the instantaneous bandwidth efficiency requirement is less than the current link capacity. Even if the system throughput varies with the link density, it is shown that the average system throughput can be significantly improved by the proposed yielding mechanism.

Performance analysis of some timing offset equalizers for FBMC/OQAM systems

The requirements of future wireless networks lead to reconsidering the physical layer and, particularly the respective merits of the major multicarrier transmission techniques, OFDM and FBMC. In comparison, equalization has been a sticking issue for filter bank multicarrier systems, because of performance and implementation aspects. In contrast, equalization is straightforward to implement in OFDM systems and it is easy to figure out the performance achieved, thanks to the presence of the cyclic prefix. Here, OFDM and FBMC/OQAM systems are compared for timing offset equalization, which is representative of global channel equalization and appears as such in some use cases, e.g. asynchronous access. The single tap equalizer and the multitap time-domain equalizer are analyzed for a specific, yet representative, FBMC scheme. It is shown that multitap time-domain timing-offset sub-channel equalization brings improvements over single-tap equalization but its discrete time nature limits its performance. There is no such limitation in the frequency domain and a recently proposed implementation of the FBMC concept, named frequency spreading, is analyzed. Equipped with this receiver, an FBMC system can withstand any timing offset and, then, unlike OFDM, it is able, in a multicarrier context, to support asynchronous users while keeping spectral efficiency.

Construction and performance analysis of variable-weight optical orthogonal codes for asynchronous OCDMA systems

A construction scheme of variable-weight optical orthogonal codes (VW-OOCs) for asynchronous optical code division multiple access (OCDMA) system is proposed. According to the actual situation, the code family can be obtained by programming in Matlab with the given code weight and corresponding capacity. The formula of bit error rate (BER) is derived by taking account of the effects of shot noise, avalanche photodiode (APD) bulk, thermal noise and surface leakage currents. The OCDMA system with the VW-OOCs is designed and improved. The study shows that the VW-OOCs have excellent performance of BER. Despite of coming from the same code family or not, the codes with larger weight have lower BER compared with the other codes in the same conditions. By taking simulation, the conclusion is consistent with the analysis of BER in theory. And the ideal eye diagrams are obtained by the optical hard limiter.

Optimal power allocation and relay selection for multiple code division multiple access peer‐to‐peer communication

The authors consider a half-duplex asynchronous code division multiple access cooperative network with N source–destination (SD) pairs and a number of relay nodes where the nodes of all pairs have to exchange data in two hops via assistance of one of the available relays. In this study, they minimise the total transmit power and derive the closed-form solution for choosing the best relay, the best relay gain and the transmit powers of all sources where some predefined signal-to-interference plus noise-ratios (SINRs) are guaranteed. Interestingly, the feasibility condition of the problem depends only on the required SINRs, the number of SD pairs and the maximum cross-correlation of users’ codes. They suggest two control procedures for admitting or dropping of users to the network to satisfy the feasibility condition. For a reciprocal environment, the best relay and its gain are proved to remain unchanged for reversing the communication directions. In addition, the authors’ power control algorithm can be directly applied to the case of two-hop two-way relaying. Computer simulations are used to demonstrate the system performance.

The Comparative Analysis of Low Power, High Speed Asynchronous Static Random Access Memory Architecture

The Comparative Analysis of Low Power, High Speed Asynchronous Static Random Access Memory Architecture

The combined automatic repeat request and rate control mechanism in S‐band mobile interactive multimedia asynchronous return link—design background, parameter dimensioning and first experimental results

SUMMARYIn the S‐band mobile interactive multimedia specification, an asynchronous access using Enhanced Spread Spectrum Aloha random access has been defined for occasional messaging applications. This access scheme is complemented by a combined automatic repeat request and terminal rate control mechanism that is described in this paper. Furthermore, a queuing theory‐based model is developed to describe the behaviour of the automatic repeat request mechanism, and performance results from the first proof‐of‐concept implementation are presented. Copyright © 2014 John Wiley & Sons, Ltd.

Performance analysis of asynchronous optical code division multiple access with spectral‐amplitude‐coding

In this study, the performance of a spectral-amplitude-coding optical code division multiple access (SAC-OCDMA) system in the asynchronous regime is evaluated using a Gaussian approximation of the decision variable for codes with fixed cross-correlation used in SAC-OCDMA systems. The authors consider the effect of phase-induced intensity noise (PIIN), thermal noise and shot noise. Moreover, the validity of the Gaussian approximation is confirmed by a Kolmogorov–Smirnov fitness test. For sake of comparison, the bit error rate (BER) of the asynchronous SAC-OCDMA system is also plotted numerically in comparison with the BER of the synchronous SAC-OCDMA. They show that a SAC-OCDMA system without any time management for the users, that is, the asynchronous regime, has a better performance than the synchronous SAC-OCDMA when PIIN effect exists.

Performance of RAKE-LMMSE Receivers in Wideband Communication Systems

High speed multimedia services with flexible data rate are main cause of popularity of 3 G WCDMA. WCDMA air interface of UMTS has the bandwidth of 5 MHz and chip rate of 3.84 Mcps. The flexibility in data rate is achieved by varying the length of its spreading code. WCDMA supports seamless connections of dynamic data rate ranging from 15 Kbps to 1 Mbps and respective spreading factors are 256 and 4 respectively for indoor and outdoor applications. The conventional RAKE receiver is near far and interference limited. Its performance at high data is limited due to short spreading factor of a code. A simple MIMO receiver structure of LMMSE in downlink asynchronous direct-sequence code division multiple access is proposed in this paper. This paper investigates RAKE receiver characteristics and does the analysis of zero forcing and LMMSE equalizer for MIMO channels.

Penalised and doubly‐penalised parallel/successive interference cancellation multi‐user detectors for asynchronous upstream optical code division multiple access passive optical network

Optical code division multiple access (OCDMA) is one of the emerging technologies for next generation passive optical networks that can offer customers a Gb/s-class experience in the upstream. However, OCDMA suffers from multiple access interference (MAI) that significantly reduces the capacity of the system. To mitigate MAI, three novel interference cancellation (IC) techniques have been proposed for incoherent direct sequence OCDMA systems that make use of some prior information to enhance their performance. Some of the IC structures exploit the non-negativity of the solution, while others exploit in addition to the non-negativity information the noise variance information. The proposed IC detectors outperform the conventional correlation receiver, the decorrelator detector, the linear parallel/successive interference cancellation detectors and even the linear minimum mean square error detector.

An optical CDMA system based on chaotic sequences

In this paper, a coherent asynchronous optical code division multiple access (OCDMA) system is proposed, whose encoder/decoder is an all-optical generator. This all-optical generator can generate analog and bipolar chaotic sequences satisfying the logistic maps. The formula of bit error rate (BER) is derived, and the relationship of BER and the number of simultaneous transmissions is analyzed. Due to the good property of correlation, this coherent OCDMA system based on these bipolar chaotic sequences can support a large number of simultaneous users, which shows that these chaotic sequences are suitable for asynchronous OCDMA system.

An information theory criteria based blind method for enumerating active users in DS-CDMA system

In this paper, a new and blind algorithm for active user enumeration in asynchronous direct sequence code division multiple access (DS-CDMA) in multipath channel scenario is proposed. The proposed method is based on information theory criteria. There are two main categories of information criteria which are widely used in active user enumeration, Akaike Information Criterion (AIC) and Minimum Description Length (MDL) information theory criteria. The main difference between these two criteria is their penalty functions. Due to this difference, MDL is a consistent enumerator which has better performance in higher signal-to-noise ratios (SNR) but AIC is preferred in lower SNRs. In sequel, we propose a SNR compliance method based on subspace and training genetic algorithm to have the performance of both of them. Moreover, our method uses only a single antenna, in difference to the previous methods which decrease hardware complexity. Simulation results show that the proposed method is capable of estimating the number of active users without any prior knowledge and the efficiency of the method.

Asynchronous Data Access and Transaction Decomposition in Distributed Databases

A RDBMS is usually accessed using blocking drivers like JDBC/ODBC which require clients to block and wait for the result of each query they issue. An asynchronous database access mechanism would eliminate the need for such blocking and greatly improve client performance. Thread-Per-Connection and Thread Pooling are two methods currently being used to provide this asynchrony. These methods require the use of multiple threads in the database server, which provides the clients with access to the database. This is inefficient since a lot of memory and computing power is spent in creating, scheduling and switching multiple threads. In this paper, we show how asynchronous database access can be achieved with a single thread using the Fork-Join mechanism which employs Future objects in Java. We also show how Asynchronous Transaction Decomposition in distributed databases can help improve client performance. Keywords: Asynchronous, Transaction Decomposition, Future objects, Fork-Join

Asynchronous multiple access using flexible bandwidth allocation scheme in SCM-based 32/64QAM-OFDM VLC system

This paper reports a novel asynchronous multiple access using flexible bandwidth allocation scheme in sub-channels-multiplexing-based visible light communication system, adopting quadrature amplitude modulation orthogonal frequency division multiplexing modulation. In this demonstration, an ordinary phosphorescent light-emitting diode with relatively low modulation bandwidth is used. Four sub-channels without any channel spacing are adopted to simultaneously support four users. By using this scheme combined with frequency domain equalization, the data rate of each user can be dynamically reconfigured from 10 to 120 Mb/s, and the overall throughput is above 125 Mb/s at a distance of 120 cm. Furthermore, a data rate of 225 Mb/s by adopting two sub-channels at a distance of 66 cm within the FEC limit of $$3.8\times 10^{-3}$$ 3.8 × 10 - 3 has been also achieved. This scheme has good scalability for supporting more terminals, and high spectrum efficiency due to the dynamic traffic reconfiguration, which shows that it has great potential advantages in future access network.

Multiuser analogue network coding with code division multiple access

An analogue network coding technique combined with asynchronous code division multiple access is proposed. A scenario is considered where there are multiple pairs of nodes with a single relay and where each pair of nodes exchange information asynchronously. With the assumption of asynchronous transmission, the analysis and the simulations show a trade-off between the number of nodes and the inter-user interference because the spreading codes are not orthogonal to each other. The analysis also shows that analogue network coding improves throughput compared with the conventional routing protocol with a large E b / N 0 , a low number of users and a large spreading factor.

Receiver-initiated asynchronous MAC protocol for wireless sensor networks

Based on a Receiver-Initiated Medium Access Control(RI-MAC) protocol,a new receiver-initiated asynchronous Medium Access Control(MAC) protocol called Receiver-Predictive MAC(RP-MAC) protocol was proposed for the purpose of reducing the energy consumption caused by idle listening in wireless sensor networks.The wakeup time of the receiving node could be predicted by RP-MAC.In the proposed MAC protocol,a random number was used as a wakeup interval,the next wakeup time was forecasted by the previous two wakeup intervals,and the retransmission mechanism of RI-MAC was improved as well,thereby the idle listening and network conflicts were reduced.The simulation on NS2 shows that the duty cycle of the node work is significantly reduced by the improved MAC protocol in high load network,and the delivery rate of packet is successfully improved,and the network energy consumption is reduced at the same time.

Extended Reed-Solomon Codes for Optical CDMA

In this paper, the extended Reed-Solomon codes are modified to construct a new family of 2-D codes for asynchronous optical code-division multiple access (O-CDMA). In addition of having expanded and asymptotically optimal cardinality, these 2-D asynchronous optical codes can be partitioned into multiple tree structures of code subsets, in which code cardinality is a function of the (periodic) cross-correlation value assigned to the subset. The performance of these 2-D optical codes is analyzed and compared with that of the multilevel prime codes. Our results show that the unique partition property of the new optical codes supports a trade-off between code cardinality and performance for meeting different system requirements, such as user capacity and throughput. In addition, the multiple tree structures of the new codes potentially support applications that require rapid switching of many codewords, such as in O-CDMA-network gateway or in strategic environments where code obscurity is essential.