Acquisition dependent random access for connectionless CDMA systems

We consider a secondary user with energy harvesting capability. We design access schemes for the secondary user which incorporate random spectrum sensing and random access, and which make use of the primary automatic repeat request (ARQ) feedback. The sensing and access probabilities are obtained such that the secondary throughput is maximized under the constraints that both the primary and secondary queues are stable and that the primary queueing delay is kept lower than a specified value needed to guarantee a certain quality of service (QoS) for the primary user. We consider spectrum sensing errors and assume multipacket reception (MPR) capabilities. Numerical results are presented to show the enhanced performance of our proposed system over a random access system, and to demonstrate the benefit of leveraging the primary feedback.

This paper presents SECANT (Spectrum Efficient Communications and Advanced Networking Technology) as a novel multiuser communication protocol that integrates advanced physical (PHY) layer techniques for adaptive multiple access and interference management. SECANT supports concurrent multiple access (MA) transmissions to common receivers and parallel peer-to-peer (P2P) transmissions in the network. SECANT combines multiuser multiple-input multiple output (MU-MIMO), multiuser detection (MUD), and interference alignment (IA) methods at the PHY layer such that spectrum efficiency can be improved in both interference and signal spaces. An adaptive medium access control (MAC) mechanism is developed to enable spectrum efficiency gains at the PHY layer by managing concurrent transmissions and interference levels. Numerical results indicate significant spectrum efficiency gains (>14 times for fully loaded, and >56 times for 25% loading) that can be achieved over conventional single antenna systems with static MAC protocols.

Introduction In conventional cellular systems, cochannel interference is a serious issue that degrades system performance. The spectral efficiency of cellular networks is fundamentally limited by the interference between cells and users sharing the same channel for both downlink and uplink. Generally, there are two kinds of cochannel interference: intracell interference and intercell interference. The intracell interference can be resolved by allocating orthogonal frequency resources. To mitigate intercell interference, there are several general approaches such as frequency reuse, cell sectoring, and spread spectrum transmission. The most commonly used technique is to avoid using the same set of frequencies in neighboring cells. This approach leads to the decrease of the number of available channels within each cell. Universal frequency reuse, i.e., reuse factor of 1, is preferred for future broadband wireless communications systems, such as the Third Generation Partnership Project (3GPP) long-term evolution (LTE) and worldwide interoperability for microwave access (WiMAX). In the orthogonal frequency-division multiple access (OFDMA) systems, which do not have processing gain as the code-division multiple access (CDMA) system, how to achieve the goal of both universal frequency reuse and reducing intercell interference is a key challenge. The concept of fractional frequency reuse (FFR) has been suggested to improve spectrum efficiency by applying reuse partition techniques, in which the inner region of the cell is assigned the whole frequency spectrum and the outer region is only assigned a small fraction of the frequency spectrum.

The performance of hybrid random-access systems (HRAS) which use a combination of multiuser codes (MUC) and collision resolution algorithms (CRA) to accommodate the bursty transmissions of many independent users on a single communication channel is analyzed. Besides the computation of the system throughput, another contribution of this paper is the determination of the properties which a MUC must possess such that resulting HRAS performs better than a system which is only based on CRA'S.

This paper considers the collision resolution algorithm when multiple packets transmit simultaneously in each slot in a non-orthogonal multiple access (NOMA) systems. Packets are divided into two groups and two different received powers can be selected. Packets in each group can select one power level as their received power. As more than one packet may transmit in a time slot in each group, collision will happen and throughput may decrease at the same time. We introduce the collision resolution period (CRP) algorithm and the collided packets will transmit in the CRP. By controlling the transmission probability in each group, we can make a big different comparing to the system without a CRP. This paper will show the optimal transmission probability and the maximum throughput. Numerical results show that our proposed algorithm yields the maximum throughput of 0.6489.

ABSTRACTThis work focuses on a mobile edge computing (MEC) assisted IoT status update network with multi‐objective sensing, which consists of a wireless sensing network, MEC network, and an information receiver (IR). To simultaneously guarantee information freshness and system throughput, a layer‐superposed non‐orthogonal multiple access (NOMA) HARQ (LS‐NOMA‐HARQ) scheme is proposed. In the proposed LS‐NOMA‐HARQ scheme, an entire status update delivery circle consists of multiple rounds for NOMA symbol feedforward transmission. In each round, the source constructs and transmits a NOMA symbol to the AP that first performs feedforward decoding (FD). Each NOMA symbol includes the newly generated packet, termed the primary packet, and the part of the currently failed packet, termed the secondary one. If the received primary packet can not be correctly recovered by the AP, the NOMA symbol is offloaded to the edge server and stored in the buffer of the edge server. The source continuously generates and sends new NOMA symbols to AP until a successful FD occurs. On the contrary, the decoded result is directly delivered to IR, and backtrack decoding (BD) is triggered at the edge server. Then, the edge server successively decodes the previously stored NOMA symbols by using sophisticated successive interference cancellation (SIC), and delivers the recovered packet to IR. Once SIC‐based BD fails, it is declared that a circle of LS‐NOMA‐HARQ status update delivery completes. Because the primary and secondary packets are independently modulated and superposed in the MAC layer, the proposed LS‐NOMA‐HARQ outperforms the layer‐coded HARQ scheme that is executed in the physical layer. Moreover, this work also considers the two cases of finite and infinite buffer size at the edge server, and truncated HARQ is used for the retransmission of a single NOMA symbol. Under the finite buffer size case, the circle‐shift preemption is used at the buffer edge server. The edge service follows exponentially distributed processes due to the user schedule and can be interrupted by one In‐Out process due to the energy computation at the edge server, which results in a huge data processing delay at the edge server. To characterize this specific issue, this work investigates the age of process information (AoPI). In addition, considering the joint impact of both FD and BD, two modified AoPI metrics, that is, FD‐based AoPI (FD‐AoPI) and BD based AoPI (BD‐AoPI), are proposed. The FD‐AoPI is defined as the elapsed time since the generation of the last successfully feedforward decoded update, but the BD‐AoPI is based on the classical definition of AoI. While the FD‐AoPI simultaneously captures both throughput and information freshness, the BD‐AoPI results in a loss in information timeliness. With the statistical characterization of related statistics, the closed‐form expressions of both FD‐AoPI and BD‐AoPI are derived. The simulated and numerical results give insight into the impact of system parameters on both AoPI and throughput.

Massive number of the uncoordinated machine-type communication (MTC) terminals accessing the satellite hub overloads the CDMA slotted Aloha (SA) random access system with a large capacity, whose performance is drastically reduced, and introduce a long service time for the activated terminals. To improve the performance of the random access scheme, we propose a Least Squares Support Vector Machines (LS-SVM) regression based random access control scheme with a sliding window. By using one step and multistep LS-SVM predicting algorithm, satellite hub predicts the channel loads of the future slots, and computes the access probability, then broadcasts to all MTC devices. Simulation results show that the proposed scheme approaches to the perfect control scheme, and the throughput is more than 2.5 times, and the total service time for all the activated terminals is less than 0.4 times that of the available EKF based control scheme in the satellite network.

A multibeam satellite communications network serving multiple zones with random access uplinks and one concentrating transponder in the downlinks is studied. Two different models are described. By the first model the random access protocol S-ALOHA is used in the uplink, and by the second one - the protocol R-ALOHA. The performance of such a random access system is analysed with the aid of the equilibrium point analysis (EPA). Channel error effects are also included in the analysis to reflect applications in satellite communications using very small aperture terminals (VSAT). The synthesised analytical models allow to design the values of communication satellite system parameters.

The concept of interference alignment has been studied mainly from an information-theoretic and signal processing point of view, focusing on scenarios with a high degree of coordination among nodes often achieved by means of transmission scheduling. However, many real-world networks employ random access policies rather than deterministic scheduling, and it is therefore important to understand how the behavior of the interference alignment algorithms may change in this setting. We investigate this problem by means of two analytical models and we also provide hints on the properties that an effective protocol should have in order to fully exploit the potential of interference alignment in random access systems.

Enhanced mobile broadband (eMBB) is a major scenario in the prospective fifth generation (5G) wireless communications. Achieving high user experienced data rate (UEDR) is one of vital goals of eMBB, which needs to consume massive bandwidth. Considering the limited radio frequency (RF) spectrum resource has got heavily congested, we explore the multi-packet reception (MPR) aided visible light communications (VLCs) system to support eMBB services in this paper. MPR can efficiently alleviate collisions for VLC system and enhance UEDR. However, the implementation of MPR in random access system is challenging because of dynamic and unpredictable transmissions of eMBB terminals. For fully utilizing MPR capability and promoting UEDR maximally, we design a two-period frame structure including the access request period (ARP) and the data transmission period (DTP). UEDR emphasizes not only the data rate measured in physical layer but also the network operation behaviour experienced by users which connects closely with quality of service (QoS). In this paper, effective capacity (EC) is employed to depict UEDRs for eMBB terminals for the first time, which provides an efficient link model to evaluate data rates with consideration of statistical delay QoS. Then, the aggregate UEDR is derived and defined as a system metric, which reflects the transmission features of both two periods. We formulate the hybrid access algorithm for eMBB terminals with heterogeneous QoS requirements as an aggregate UEDR maximization problem and find the optimum access probability (for ARP) and optimal resource allocation strategy (for DTP) via the particle swarm optimization-initialization intervention (PSO-II) algorithm.

The throughput characteristics of contention-based random-access systems (RAS's) which use <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -ary tree algorithms (where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q \geq 2</tex> is the number of groups into which contending users are split) of the Capetanakis-Tsybakov-Mikhailov-Vvedenskaya type are analyzed for an infinite population of identical users generating packets according to a Poisson process. Both free and blocked channel-access protocols are considered in combination with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -ary collision resolution algorithms that exploit either binary ("collision/no collision") or ternary ("collision/ success / idle") feedback. For the resulting RAS's, functional equations for transformed generating functions of the first two moments of the collision resolution interval length are obtained and solved. The maximum stable throughput as a function of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> is given. The results of a packet-delay analysis are also given, and the analyzed RAS's are compared among themselves and with the slotted ALOHA system in terms of both system throughput and packet delay. It is concluded that the "practical optimum" RAS (in terms of ease of implementation combined with good performance) uses free (i.e., immediate) channel access and ternary splitting (i.e., <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q = 3</tex> ) with binary feedback.

In this paper we propose and evaluate several protocols for a random access channel. The channel is divided into a control subchannel which is used to inform all users of the approximate times of arrival of new packets and a data subchannel which is used to transmit the data. Packets arriving at different users at approximately the same time may cause a collision on the control subchannel. Such collisions are resolved on the data subchannel by a collision resolution algorithm. Eight protocols that differ in the feedback available on both channels and the time that the feedback is available are described and evaluated. The expected delay of each protocol is determined by modeling the system as a slotted concentrator. Comparisons are made among the protocols described and analyzed in this paper and between these protocols and other protocols that have been studied previously.

In CDMA cellular system, it is important to reduce interchannel interference in order to enlarge the channel capacity. For this purpose, CDMA using the orthogonal code is applied to the downlink. It is difficult, however, to apply the method to the uplink multiple access, since interchannel interference appears when the synchronization is lost. Regarding this point, this paper considers CDMA system using the orthogonal code without synchronization, where the interchannel interference does not appear even if the synchronization is lost, and evaluates its effectiveness by computer simulation. The following results are verified. In the uplink multiple access without the transmitter power control for the fading, the BER characteristic is degraded by interchannel interference in CDMA system using M sequence code, as well as in CDMA system using orthogonal Walsh code with synchronization. The BER characteristic in CDMA system using the orthogonal code without synchronization is the same as in the single‐station case (i.e., without interchannel interference). © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(4): 19–34, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10077

A new channel coding approach was proposed in [1] for random multiple access communication over the discrete-time memoryless channel. The coding approach allows users to choose their communication rates independently without sharing the rate information among each other or with the receiver. The receiver will either decode the message or report a collision depending on whether reliable message recovery is possible. It was shown that, asymptotically as the codeword length goes to infinity, the set of communication rates supporting reliable message recovery can be characterized by an achievable region which equals Shannon's information rate region possibly without a convex hull operation. In this paper, we derive achievable bounds on error probabilities, including the decoding error probability and the collision miss detection probability, of random multiple access systems with a finite codeword length. Achievable error exponents are obtained by taking the codeword length to infinity.

For slotted random access systems, the slotted ALOHA protocol provides the maximum throughput of 0.368 (packets/slot) while in the category of splitting (or tree) algorithms, the maximum achievable throughput can reach up to 0.487 with the first-come first-serve (FCFS) algorithm. It has been so far demonstrated that the FCFS algorithm can achieve this maximum throughput only for Poisson traffic. This may limit its application in practical systems, where packet arrivals may not be Poissonian. In this paper, we propose a novel online transmission control framework that introduces random splitting upon collisions and controls the transmission probabilities optimally at each slot by estimating the number of active users in the system. The proposed algorithm is said to be online as it estimates the number of active users slot by slot recursively, and thus can adapt to network dynamics. We first show that the splitting algorithm of our interest can achieve the throughput of 0.532 if the number of users involved in a collision could be known, which serves as a guideline for the upper limit for the random access systems with splitting. Then, when the information on the number of collided users is not available, we show that the proposed algorithm can achieve the maximum throughput of 0.487 for Poisson arrivals while achieving shorter access delay than FCFS. When more bursty traffic than Poisson process is applied, the proposed algorithm shows much better throughput and delay performance than FCFS.