Hierarchical Multi-User Detection for Highly Overloaded Uplink Grant-Free NOMA

Grant-Free Non Orthogonal Multiple Access (NOMA) offers promising solutions to realize uplink (UL) massive Machine Type Communication (mMTC) using limited spectrum resources, while reducing signalling overhead. Because of the sparse, sporadic activities exhibited by the user equipments (UE), the active user detection (AUD) problem is often formulated as a compressive sensing problem. In line of that, greedy sparse recovery algorithms are spearheading the development of compressed sensing based multi-user detectors (CS-MUD). However, for a given number of resources, the performance of CS-MUD algorithms are fundamentally limited at higher overloading of NOMA. To circumvent this issue, in this work, we propose a two-stage hierarchical multi-user detection framework, where the UEs are randomly assigned to some pre-defined clusters. The active UEs split their data transmission frame into two phases. In the first phase an UE uses the sinusoidal spreading sequence (SS) of its cluster. In the second phase the UE uses its own unique random SS. At phase 1 of detection, the active clusters are detected, and a reduced sensing matrix is constructed. This matrix is used in Phase 2 to recover the active UE indices using some sparse recovery algorithm. Numerical investigations validate the efficacy of the proposed algorithm in highly overloaded scenarios.

Hierarchical Multi-User Detection for Highly Overloaded Uplink Grant-Free NOMA

Grant-Free Non Orthogonal Multiple Access (NOMA) offers promising solutions to realize uplink (UL) massive Machine Type Communication (mMTC) using limited spectrum resources, while reducing signalling overhead. Because of the sparse, sporadic activities exhibited by the user equipments (UE), the active user detection (AUD) problem is often formulated as a compressive sensing problem. In line of that, greedy sparse recovery algorithms are spearheading the development of compressed sensing based multi-user detectors (CS-MUD). However, for a given number of resources, the performance of CS-MUD algorithms are fundamentally limited at higher overloading of NOMA. To circumvent this issue, in this work, we propose a two-stage hierarchical multi-user detection framework, where the UEs are randomly assigned to some pre-defined clusters. The active UEs split their data transmission frame into two phases. In the first phase an UE uses the sinusoidal spreading sequence (SS) of its cluster. In the second phase the UE uses its own unique random SS. At phase 1 of detection, the active clusters are detected, and a reduced sensing matrix is constructed. This matrix is used in Phase 2 to recover the active UE indices using some sparse recovery algorithm. Numerical investigations validate the efficacy of the proposed algorithm in highly overloaded scenarios.

Study of the Performance of DSSS UAC System Depending on the System Bandwidth and the Spreading Sequence

A signal transmitted in an Underwater Acoustic Communication (UAC) system operating in a shallow-water channel suffers from strong time dispersion due to multipath propagation. This causes the Inter-Symbol Interference (ISI) observed in the received signal, which significantly limits the communication system’s reliability and transmission rate. In such propagation conditions, the Direct-Sequence Spread Spectrum (DSSS) method is one of the solutions that make reliable data transmission possible. In systems with one-to-one communication, it ensures communication with a satisfactory Bit Error Rate (BER). Additionally, it makes it possible to implement the Code-Division Multiple Access (CDMA) protocol in underwater acoustic networks. This paper presents the results of simulation and experimental communication tests on a DSSS-based UAC system using three types of spreading sequence, namely m-sequences, Kasami codes and Gold codes, and occupying different bandwidths from 1 kHz to 8 kHz around a carrier frequency equal to 30 kHz. The UAC channel was simulated by impulse responses calculated by the virtual sources method and the UAC chanel models available in the Watermark simulator. The experimental tests were conducted in a model pool. Based on the obtained results, a transmission rate was estimated, which is possible to achieve in strong multipath propagation conditions, assuming reliability expressed as BER less than 0.001.

Synchronization in DSSS system

Direct sequence spread spectrum (DSSS) communication systems offer huge performance focal points in perspective on their low probability of block, improved performance in multipath fading situations and their capacity to stay away from interference by spreading the signal over a wide bandwidth subsequently conveying the power. For the transmitted sequence to be effectively received and demodulated, the spreading sequence utilized at the receiver ought to be like that utilized in the transmitter. This paper uses MATLAB Simulink to show a technique for synchronizing the code clock at the receiver with the code clock at the transmitter. This fine arrangement procedure is known as code tracking.

Quadrature MC-DCSK Scheme for Chaos-Based Cognitive Radio

In this study, we investigate an enhanced architecture of multicarrier differential chaos-shift keying (MC-DCSK) scheme using quadrature modulation (QM), namely QMC-DCSK. The use of quadrature modulation aims at doubling the data rate over a defined bandwidth and hence improve bandwidth efficiency of the system. In the proposed scheme, the chaotic spreading sequence is transmitted on a predefined frequency while each of the remaining frequencies is phase-shifted at a [Formula: see text] angle in order to produce two quadrature subcarriers located at the same frequency. These subcarriers are modulated by the product of the chaotic spreading sequence and the corresponding bit substreams in parallel. Noncoherent demodulation is carried out in the receiver in order to recover the original data based on the sign of correlation values. Architecture and operation of the conventional and proposed schemes are described. The BER performance over a wireless channel is theoretically derived and then numerically verified. The bit rate, energy and bandwidth efficiencies are evaluated in comparison with those of MC-DCSK. In particular, the application of the proposed scheme to cognitive radio (CR) in the scenario of using multicarrier modulation and chaotic spread-spectrum is discussed and evaluated. The obtained results prove that QMC-DCSK is a fit technique for CR communications.