Enhanced Collision Resolution for the IEEE 802.11 Distributed Coordination Function

The IEEE 802.11 standard relies on the Distributed Coordination Function (DCF) as the fundamental medium access control method. DCF uses the Binary Exponential Backoff (BEB) algorithm to regulate channel access. The backoff period determined by BEB depends on a contention window (CW) whose size is doubled if a station suffers a collision and reset to its minimum value after a successful transmission. BEB doubles the CW size upon collision to reduce the collision probability in retransmission. However, this CW increase reduces channel access time because stations will spend more time sensing the channel rather than accessing it. Although resetting the CW to its minimum value increases channel access, it negatively affects fairness because it favours successfully transmitting stations over stations suffering from collisions. Moreover, resetting CW leads to increasing the collision probability and therefore increases the number of collisions. % Quality control editor: Please ensure that the intended meaning has been maintained in the edits of the previous sentence. Since increasing channel access time and reducing the probability of collisions are important factors to improve the DCF performance, and they conflict with each other, improving one will have an adverse effect on the other and consequently will harm the DCF performance. We propose an algorithm, \gls{ECRA}, that solves collisions once they occur without instantly increasing the CW size. Our algorithm reduces the collision probability without affecting channel access time. We also propose an accurate analytical model that allows comparing the theoretical saturation and maximum throughputs of our algorithm with those of benchmark algorithms. Our model uses a collision probability that is dependent on the station transmission history and thus provides a precise estimation of the probability that a station transmits in a random timeslot, which results in a more accurate throughput analysis. We present extensive simulations for fixed and mobile scenarios. The results show that on average, our algorithm outperformed BEB in terms of throughput and fairness. Compared to other benchmark algorithms, our algorithm improved, on average, throughput and delay performance

Medium access control protocol design for wireless communications and networks review

Medium access control (MAC) protocol design plays a crucial role to increase the performance of wireless communications and networks. The channel access mechanism is provided by MAC layer to share the medium by multiple stations. Different types of wireless networks have different design requirements such as throughput, delay, power consumption, fairness, reliability, and network density, therefore, MAC protocol for these networks must satisfy their requirements. In this work, we proposed two multiplexing methods for modern wireless networks: Massive multiple-input-multiple-output (MIMO) and power domain non-orthogonal multiple access (PD-NOMA). The first research method namely Massive MIMO uses a massive number of antenna elements to improve both spectral efficiency and energy efficiency. On the other hand, the second research method (PD-NOMA) allows multiple non-orthogonal signals to share the same orthogonal resources by allocating different power level for each station. PD-NOMA has a better spectral efficiency over the orthogonal multiple access methods. A review of previous works regarding the MAC design for different wireless networks is classified based on different categories. The main contribution of this research work is to show the importance of the MAC design with added optimal functionalities to improve the spectral and energy efficiencies of the wireless networks

Control-theoretic approaches for efficient transmission on IEEE 802.11e wireless networks

With the increasing use of multimedia applications on the wireless network, the functionalities of the IEEE 802.11 WLAN was extended to allow traffic differentiation so that priority traffic gets quicker service time depending on their Quality of Service (QoS) requirements. The extended functionalities contained in the IEEE Medium Access Control (MAC) and Physical Layer (PHY) Specifications, i.e. the IEEE 802.11e specifications, are recommended values for channel access parameters along traffic lines and the channel access parameters are: the Minimum Contention Window CWmin, Maximum Contention Window CWmax, Arbitration inter-frame space number, (AIFSN) and the Transmission Opportunity (TXOP). These default Enhanced Distributed Channel Access (EDCA) contention values used by each traffic type in accessing the wireless medium are only recommended values which could be adjusted or changed based on the condition of number of associated nodes on the network. In particular, we focus on the Contention Window (CW) parameter and it has been shown that when the number of nodes on the network is small, a smaller value of CWmin should be used for channel access in order to avoid underutilization of channel time and when the number of associated nodes is large, a larger value of CWmin should be used in order to avoid large collisions and retransmissions on the network. Fortunately, allowance was made for these default values to be adjusted or changed but the challenge has been in designing an algorithm that constantly and automatically tunes the CWmin value so that the Access Point (AP) gives out the right CWmin value to be used on the WLAN and this value should be derived based on the level of activity experienced on the network or predefined QoS constraints while considering the dynamic nature of the WLAN. In this thesis, we propose the use of feedback based control and we design a controller for wireless medium access. The controller will give an output which will be the EDCA CWmin value to be used by contending stations/nodes in accessing the medium and this value will be based on current WLAN conditions. We propose the use of feedback control due to its established mathematical concepts particularly for single-input-single-output systems and multi-variable systems which are scenarios that apply to the WLAN

Intelligent management and control for Wi-Fi small cells

In order to face the exponential growth of mobile data transmissions, it has been long since the concept of small cells is in the table, which provides high density deployments of small cells so as to provide a high capacity to a large number of users. The SENSEFUL project, being directed by a research team in the I2CAT foundation, studies the use of small cells with Wi-Fi technology, where both the access network and the backhaul share the same radio resource. The deployment of this new paradigm requires a deep study of improvements on the performance of access networks in terms of mobility while, at the same time, trying to improve the behaviour of the backhaul network by means of new techniques to access the shared medium. SENSEFUL has been granted the funding of the WiSHFUL open call, started up by a collective of entities and universities, of which we have mainly worked with the Technische Universität Berlin, due to the use we have made of their testbed, the TWIST. Using new techniques and technologies, such as the Software Defined Networking paradigm, an intelligent network is deployed, which can manage the network resources dynamically according to the requirements of the system. Regarding both of the fronts of SENSEFUL, the performance in the backhaul network and the mobility in the access network, the techniques that were applied are the following: For the backhaul network, an innovative proposal of a shared medium access mechanism has been studied. It is not yet standardized, because there are many research teams trying to achieve a functional system that can be applied to multiple scenarios. In this thesis, the Hybrid TDMA is studied, a Wi-Fi radio medium access protocol that uses a hybrid of carrier sense (CSMA) and time division (TDMA) in order to benefit from both systems. The main advantages that HTDMA brings are a better management of the quality of service in wireless networks, while solving some of the endemic problems of Wi-Fi, such as the hidden node or the exposed node. So as to work in this direction, first of all, a precise synchronization among the devices that will use this medium access mechanism is required; that is why the usual synchronisation mechanisms in Wi-Fi networks is one of the main topics that this thesis deals with. Regarding mobility in the access network, a new technique is used, which, despite being out of the scope of this thesis, it is indeed interesting and innovative. The BigAP unifies several access points under a shared BSSID, providing a seamless handover for the clients by making only a change on the transmission channel. Working in different environments and scenarios, this project studies the best synchronisation mechanisms for this field. Moreover, the HTDMA system is installed in a small test scenario so as to begin with the analysis of the operation of this hybrid mechanism and its performance under different conditions, as compared to the legacy CSMA