Slotted ALOHA Medium Access Control Research Articles

A Novel Framed Slotted Aloha Medium Access Control Protocol Based on Capture Effect in Vehicular Ad Hoc Networks

The capture effect is a frequently observed phenomenon in vehicular ad hoc networks (VANETs) communication. When conflicts arise during time slot access, failure to access does not necessarily occur; instead, successful access may still be achieved. The capture effect can enhance the likelihood of multiple access and improve communication efficiency. The security of VANETs communication is undoubtedly the primary concern. One crucial approach to enhance security involves the design of an efficient and reliable medium access control (MAC) protocol. Taking into account both aspects, we propose a novel framed slotted Aloha (FSA) MAC protocol model. Firstly, we derive the closed-form expression for the capture probability in the Rician fading channel in this paper. Subsequently, we analyze how the number of vehicles and time slots influence the success probability of vehicle access channels as well as examine the impact of the capture effect on this success probability. Then, under constraints regarding vehicle access channel success probability, we derive optimal values for slot numbers, access times, and transmission power while proposing a comprehensive implementation method to ensure high access channel success probabilities. We verify both theoretical derivations and proposed methods through simulation experiments.

Design and performance evaluation of successive interference cancellation based Slotted Aloha MAC protocol

As a result of densification, the performance of the wireless networks has become highly interference-limited and energy inefficient. To overcome this problem, interference mitigation techniques such as Successive Interference Cancellation (SIC) can be used to decode multiple packets simultaneously at the receiver. In this context, we analyze a SIC-based Slotted Aloha (SIC-SA) Medium Access Control (MAC) protocol for wireless networks. We derive expressions for packets decoding probability and optimal transmission probability of the nodes of the SIC-SA MAC protocol. Our derivation is based on the order statistics of Independent and Identical/non-Identical exponentially distributed received-signal-powers from the nodes under the Rayleigh channel condition. Throughput, delay, and energy efficiency of the SIC-SA MAC protocol have been derived and validated against simulation. The effect of path loss exponent, SINR threshold, and the number of nodes on the performance of SIC-SA have been studied. The performance of SIC-SA in a network of nodes distributed randomly according to the Poisson Point Process has been analyzed. Extension of our analysis to Power Domain Non-Orthogonal Multiple Access (NOMA) has been demonstrated. We also analyzed the impact of imperfect estimation of channel state information and imperfect SIC at the receiver. Results show an improvement in performance metrics of SIC-SA over the traditional Slotted Aloha.

Joint Routing and Medium Access Control in Fixed Random Access Wireless Multihop Networks

We study cross-layer design in random-access-based fixed wireless multihop networks under a physical interference model. Due to the complexity of the problem, we consider a simple slotted ALOHA medium access control (MAC) protocol for link-layer operation. We formulate a joint routing, access probability, and rate allocation optimization problem to determine the optimal max-min throughput of the flows and the optimal configuration of the routing, access probability, and transmission rate parameters in a slotted ALOHA system. We then also adapt this problem to include an XOR-like network coding without opportunistic listening. Both problems are complex nonlinear and nonconvex. We provide extensive numerical results for both problems for medium-size mesh networks using an iterated optimal search technique. Via numerical and simulation results, we show that: 1) joint design provides a significant throughput gain over a default configuration in slotted-ALOHA-based wireless networks; and 2) the throughput gain obtained by the simple network coding is significant, especially at low transmission power. We also propose simple heuristics to configure slotted-ALOHA-based wireless mesh networks. These heuristics are extensively evaluated via simulation and found to be very efficient.

Achievable Spatial Throughput in Multi-Antenna Cognitive Underlay Networks with Multi-Hop Relaying

In this article, we quantify the achievable spatial throughput of a multi-antenna Poisson cognitive radio network (CRN) collocated with a Poisson multi-antenna primary network. CR users employ Slotted-ALOHA medium access control. The success probability (SP) of a primary link is quantified in the presence of the secondary and primary interferers. It is demonstrated that two fold gains are experienced by employing multiple antennas at primary, i.e., (i) the fixed high desired SP threshold is met; (ii) CRs can also be accommodated without QoS deterioration. Further in this paper, the maximum permissible medium access probability (MAP) for CRN is derived from the link SP and primary users QoS constraint. The impact of the number of antennas and modulation employed at the primary on the permissible MAP of the CRN is also explored. Assuming that CR users employ multi-hop communication, QoS aware relaying with a radian sector forwarding area is studied. The average forward progress (AFP) and isolation probability for a CR user with QoS based connectivity is characterized under the permissible MAP. The spatial throughput for the CRN is quantified by the analysis of the AFP and the permissible MAP. It is shown that there exists an optimal MAP which maximizes the spatial throughput of the CRN. This optimal MAP is coupled with the permissible MAP, density of users, number of antennas and modulation schemes employed in both primary and secondary networks. Lastly, a few important design questions are investigated for multi-hop MIMO underlay CRNs.

Open-Loop Spatial Multiplexing and Diversity Communications in Ad Hoc Networks

This paper investigates the performance of open-loop multi-antenna point-to-point links in ad hoc networks with slotted ALOHA medium access control (MAC). We consider spatial multiplexing transmission with linear maximum ratio combining and zero forcing receivers, as well as orthogonal space time block coded transmission. New closed-form expressions are derived for the outage probability, throughput and transmission capacity. Our results demonstrate that both the best performing scheme and the optimum number of transmit antennas depend on different network parameters, such as the node intensity and the signal-to-interference-and-noise ratio operating value. We then compare the performance to a network consisting of single-antenna devices and an idealized fully centrally coordinated MAC. These results show that multi-antenna schemes with a simple decentralized slotted ALOHA MAC can outperform even idealized single-antenna networks in various practical scenarios.

Cross-Layer Design of MIMO-Enabled WLANs With Network Utility Maximization

Wireless local area networks (WLANs) have become a ubiquitous high-speed data-access technology. The recent IEEE 802.11e standard provides quality-of-service (QoS) support, and the pending 802.11n standard further increases the transmission rate by using the multiple-input-multiple-output (MIMO) technique. Multiple antennas can be used to improve the performance gain by either increasing the transmission reliability through spatial diversity or increasing the transmission rate through spatial multiplexing. This new characteristic at the wireless physical (PHY) layer requires the corresponding adaptation at the medium access control (MAC) layer to reach the best performance gain. In this paper, we propose cross-layer design schemes for WLANs under two different MAC protocols: the carrier sense multiple access with collision avoidance (CSMA/CA)-based 802.11e MAC and the slotted Aloha MAC. For the 802.11e MAC, two different contention window (CW) size adaptation schemes, namely, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U-MAC</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D-MAC</i> , are proposed, which facilitate the MAC protocol to jointly adapt its CW size with the PHY layer's MIMO operating parameters. For the slotted Aloha MAC, a cross-layer optimization framework <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NUM-O</i> is proposed to jointly optimize the MIMO configuration at the PHY layer and the persistent probabilities for different classes of multimedia traffic at the MAC layer. A distributed algorithm <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NUM-D</i> based on dual decomposition and a simplified version <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NUM-S</i> are also proposed. Simulation results are presented to show the effectiveness of the proposed methods.

Performance Analysis of Wireless Networks With Directional Antennas

In this paper, we consider wireless networks with directional antennas using slotted ALOHA medium access control. The channel model incorporates the effects of propagation loss and shadowing with Rayleigh fading. By deriving the cumulative distribution function of the signal-to-interference-and-noise ratio (SINR) of a certain link in the network, the outage probability of the link is analyzed. In particular, the analysis works for the arbitrary beam patterns of the directional antennas used in the networks. Moreover, we propose a new parameter, i.e., the array interference factor, which can characterize the average performance of an arbitrary beam pattern in random wireless networks. This provides an efficient way for designers to evaluate the system performance under different beam patterns. The network throughput and the transport capacity of a random wireless network with directional antennas are also analyzed. It is shown that the network transport capacity is of the same order as the square root of the node density, which conforms with the well-known capacity of wireless networks. The analysis is useful for designers to optimize the system performance of wireless networks with directional antennas.

A wireless local area network employing distributed radio bridges

This paper presents a novel distributed wireless local area network (WLAN) architecture, where each wireless terminal (WT) accesses a backbone local area network (LAN) segment via multiple radio bridges (RB's). We introduce a self-learning routing algorithm for the RB's, which automatically adapts to changes in terminal locations, and prevents multiple copies of each data frame from being forwarded over the backbone LAN segment. The distributed WLAN architecture eases the management of network topological changes and terminal mobility, compared to centralized cellular architectures. We consider the use of direct-sequence spread-spectrum (DS/SS) signaling and the slotted Aloha medium access control (MAC) protocol over the wireless links, with multiple uplink receivers and downlink transmitters at the RB's for each MAC frame. Simulation results for the uplink show that the multi-receiver site diversity and the capture effect of DS/SS signaling effectively combat multipath and multiaccess interference, resulting in high throughput capacity and stable operation for the channel. Under overload traffic, the system is able to maintain a high level of throughput with bounded delays. It is shown that the use of multiple receiver reduces the access fairness problem for WT's at different locations caused by the near-far effect.