Browsing by Subject "Random access"

Now showing 1 - 7 of 7

Open Access
Advanced asynchronous random access protocols
(2020-08) Akyıldız, Talha
Fifth generation wireless systems and beyond require linking an enormous number of simple machine type devices leading to a new wave of interest in massive machine type communications (mMTC). Diﬀerent from the human-centric communication systems, mMTCs are composed of a large number of devices where each user node generates small data blocks sporadically in an unpredictable manner. In such scenarios, traditional multiple access schemes, e.g., time division multiple access or frequency division multiple access, are not suitable because resource allocation and scheduling based approaches cannot be conveniently adopted due to the required complexity and latency, motivating the use of uncoordinated random access (RA) protocols and making asynchronous ALOHA-like solutions ideal candidates for such applications. In this thesis, we consider the design and analysis of advanced asynchronous RA protocols for diﬀerent settings. We ﬁrst study contention resolution ALOHA (CRA) and irregular repetition ALOHA (IRA) protocols with regular and irregular repetition rates on the collision channel where collisions are resolved through successive interference cancellation. We also propose concatenation of packet replicas with some clean parts with IRA, named irregular repetition ALOHA with replica concatenation (IRARC). Secondly, we introduce energy harvesting (EH) into the framework with the motivation of self-sustainability, and study RA protocols with EH nodes. Finally, we propose a generalization of IRA with packet length diversity to improve the system performance further. We present asymptotic analyses of all the proposed RA protocols, and determine the optimal repetition distributions to maximize the system throughput. We also provide a comprehensive set of numerical results for both asymptotic and practical scenarios to further demonstrate the eﬀectiveness of the proposed approaches.
Open Access
Analysis of Coded Slotted ALOHA with energy harvesting nodes for Perfect and Imperfect Packet Recovery Scenarios
(Institute of Electrical and Electronics Engineers, 2023-03-06) Haghighat, Javad; Duman, Tolga Mete
We analyze the performance of Coded Slotted ALOHA (CSA) protocols in scenarios where users are equipped with limited batteries that are recharged through Energy Harvesting (EH). First, we assume a Perfect Packet Recovery Scenario (PPRS) for which the received packets are decoded with no errors when there is no interference. We introduce Battery Outage Probability (BOP) as an extra performance metric; and, we derive the optimal EH-CSA transmission policies, which offer the maximum attainable traffic load while maintaining an asymptotically negligible Packet Loss Ratio (PLR), under specific rate and BOP constraints. We extend our study to Imperfect Packet Recovery Scenario (IPRS) where impairments at the physical layer, including channel estimation and channel decoding errors, will distort messages being passed through the iterative Successive Interference Cancellation (SIC) process. The distorted messages being passed through the SIC process potentially lead to error propagation. In order to track the error propagation process, we define the concept of Accumulated Noise plus Interference Power (ANIP), and analytically track the evolution of its probability distribution. We employ our results to evaluate the bit error rates for different transmission policies for the case of IPRS. We also demonstrate the advantages of the optimal transmission policies through numerical examples for both PPRS and IPRS. Our results show that the optimal EH-CSA policies outperform the policies optimized for standard CSA without EH considerations, and the schemes that are optimal for PPRS are not necessarily optimal for the IPRS case. Furthermore, the EH-CSA optimal policies strictly outperform standard CRDSA when the system is required to support higher traffic loads.
Open Access
Double-zipper: multiple access with zigzag decoding
(IEEE, 2020) Kazemi, Mohammad; Duman, Tolga M.; Medard, M.
As a building block toward a simple and scalable solution to massive random access, we consider two-user multiple access with ZigZag decoding, with no need for any coordination or codebook differentiation. We derive closed-form bounds on the achievable sum-rates of the original ZigZag and a modified version of it, called double-zipper ZigZag, for both cases of perfect and imperfect channel state information (CSI). We also show that performances of both versions of ZigZag approach that of optimal coordinated time-sharing in the high signal to noise ratio regime, even in the presence of CSI errors.
Open Access
Energy harvesting irregular repetition ALOHA with replica concatenation
(IEEE, 2021) Akyıldız, Talha; Demirhan, U.; Duman, Tolga M.
In this paper, we consider an asynchronous random access scheme called irregular repetition ALOHA (IRA) as a generalization of contention resolution ALOHA (CRA) with varying repetitions. We present an asymptotic performance analysis of CRA and IRA on the collision channel for regular and irregular repetition rates. We also propose an improvement by merging the clean parts of packet replicas in partial collisions, and extend our analysis to this scenario as well. Specific designs of repetition distributions based on the new analysis show that the optimized solutions of irregular repetition slotted ALOHA (IRSA) perform well in both IRA and the enhanced scheme, and they considerably outperform the regular repetition distributions. We also introduce energy harvesting (EH) to both schemes as a practical and sustainable adaptation, where users are able to harvest energy and store it in their finite-capacity batteries. We model the battery state by a discrete-time Markov chain and derive an optimal transmission policy to maximize the asymptotic performance of the system. We provide comprehensive numerical results for both practical and asymptotic scenarios to verify the validity of the proposed analyses, and illustrate the benefits of the proposed systems.
Open Access
Irregular repetition slotted ALOHA with energy harvesting nodes
(2017-07) Demirhan, Umut
The importance of wireless networking schemes originating from ALOHA has rapidly risen with the wide-spread use of Internet, advancements in the communications systems and increasing number of wireless devices. Internet-of-Things and machine-to-machine communications concepts have drawn further attention to ALOHA since it is a low-complexity protocol. However, the classical ALOHA is not e cient and cannot handle massive number of users in an e cient manner. Therefore, many improvements have been proposed for over the years. Irregular Repetition Slotted ALOHA (IRSA) is an advanced ALOHA protocol in which each user sends a variable number of copies of their packets in each xed length medium access control (MAC) frame. The collisions may be resolved via successive interference cancellation (SIC) using the copies that are received cleanly. In this way, asymptotic throughputs close to the maximum normalized throughput value of one on the collision channel may be achieved. In this thesis, to reap the bene ts of IRSA for energy harvesting sensor networks, we propose an IRSA based uncoordinated random access scheme for energy harvesting (EH) nodes. Speci cally, we consider the case in which each user has a nite-sized battery which is recharged in a probabilistic manner in each slot with harvested energy from the environment. We analyze this scheme by deriving asymptotic throughput expressions, and obtain optimized probability distributions for the number of packet replicas for each user. We demonstrate that the optimized distributions perform considerably better than those of slotted ALOHA (SA), contention resolution diversity slotted ALOHA (CRDSA) and plain IRSA which do not take into account EH for both asymptotic and nite frame length scenarios.
Open Access
Irregular repetition slotted ALOHA with energy harvesting nodes
(IEEE, 2019-09) Demirhan, Umut; Duman, Tolga M.
We propose an irregular repetition slotted ALOHA (IRSA) based uncoordinated random access scheme for energy harvesting (EH) nodes. Specifically, we consider the case in which each user has a battery that is recharged with harvested energy from the environment in a probabilistic manner. We analyze this scheme starting with a unit-sized battery at the nodes and extend the analysis to the case of a finite-sized battery. For both scenarios, we derive the asymptotic throughput expressions and obtain the optimized probability distributions for the number of packet replicas of the users. We demonstrate that for the case of IRSA with EH nodes, these optimized distributions perform considerably better than the alternatives, including slotted ALOHA (SA), contention resolution diversity slotted ALOHA (CRDSA), and IRSA, which do not take into account the EH process for both asymptotic and finite frame length scenarios.
Open Access
Random access over wireless links: optimal rate and activity probability selection
(2017-07) Karakoç, Nurullah
Due to the rapidly increasing number of devices in wireless networks with the proliferation of applications based on new technologies such as machine to machine communications and Internet of Things, there is a growing interest in the random access schemes as they provide a simple means of channel access. To this end, various schemes have been proposed based on the ALOHA protocol to increase the e ciency of the medium access control layer over the last decade. On the other hand, physical layer aspects of random access networks have received relatively limited attention, and there is a need to consider optimal use of the underlying physical layer properties especially for transmission over wireless channels. In this thesis, we study uncoordinated random access schemes over wireless fading channels where each user independently decides whether to send a packet or not to a common receiver at any given time slot. To characterize the system throughput, i.e., the expected sum-rate, an information theoretic formulation is developed. We consider two scenarios: classical slotted ALOHA, where no multiuser detection (MUD) capability is available and slotted ALOHA with MUD. Our main contribution is that the optimal rates and the channel activity probabilities can be characterized as a function of the user distances to the receiver to maximize the system throughput in each case (more precisely, as a function of the average signal to noise ratios of the users). We use Rayleigh fading as our main channel model, however, we also study the cases where log-normal shadowing is observed along with small scale fading. Our proposed optimal rate selection schemes o er signi cant increase in expected system throughput compared to the same rate approach commonly used in the literature. In addition to the overall throughput optimization, the issue of fairness among users is also investigated and solutions which guarantee a minimum amount of individual throughput are developed. We also design systems with limited individual outage probabilities of the users for increased energy e ciency and reduced delay. All of these analytical works are supported with detailed numerical examples, and the performance of the proposed methods are evaluated.