Yunbae Kim

Design and analysis of medium access protocol: throughput and short-term fairness perspective

We consider a simple MAC protocol, called the renewal access protocol (RAP), that adopts all of the legacy 802.11 standard but the backoff stage feature. To meet two objectives in the design of the RAP-optimal throughput and high short-term fairness-we develop a mathematical model of the RAP and rigorously analyze the performance of the RAP. First, we show that the throughput performance of the RAP depends only on the expectation of the selection distribution where the backoff counter is selected, provided that the number of terminals is fixed, which is in accordance with a well-known result. Second, with the help of renewal and reliability theories, we analyze the short-term fairness of the RAP. We also show that if the RAP has a selection distribution of the New Better than Used in Expectation (NBUE) type, the RAP can guarantee high short-term fairness. Third, we construct a special binomial distribution that is obviously of the NBUE type that can achieve high short-term fairness as well as optimal throughput when used as the selection distribution of the RAP. Furthermore, by the Poisson approximation for binomial distributions, we propose to use in practice a Poisson distribution corresponding to the special binomial distribution. Numerical and simulation results are provided to validate our analysis.

Delay analysis and optimality of the renewal access protocol

For many years, the IEEE 802.11 distributed coordination function (DCF) has been widely used as a dominant medium access control (MAC) protocol in wireless networks and a large number of works have been done for analyzing and improving its performance. In our earlier work, as a substitute of the IEEE 802.11 DCF, a simple MAC protocol, called the renewal access protocol (RAP), is proposed. The RAP adopts all of the legacy 802.11 standard but the backoff stage feature. Each terminal selects its backoff counter value from a fixed sized window according to a priori given selection distribution in the RAP, regardless of the packet transmission result. It is shown that, if a Poisson distribution is used as the selection distribution, then the resulting RAP achieves high short-term fairness as well as optimal throughput. In this work, we analyze the relation between delay performance and the selection distribution of the RAP. With the help of effective bandwidth theory, we derive the conditions for the selection distribution of the RAP that optimizes the queue overflow probability. We also construct the delay optimal selection distribution satisfying the optimal conditions for throughput and delay. However, we show that the use of the delay optimal selection distribution results in an extremely slow convergence to steady state compared with that of the Poisson selection distribution. Moreover, we show that the Poisson selection distribution provides near-optimal delay performance. Therefore, we conclude that the use of a Poisson selection distribution is still recommended even from the delay perspective.

Throughput Performance Optimization of Super Dense Wireless Networks With the Renewal Access Protocol

As a promising solution for handling super dense wireless networks, wireless local area networks (WLANs) have been intensively considered due to their wide availability. However, the contention-based MAC protocol in WLANs should be modified because of its inefficiency. To this end, we consider a recently proposed novel MAC protocol called the renewal access protocol (RAP). With the RAP, we analyze two strategies for resolving collisions efficiently and achieving optimal throughput performance in super dense WLANs: strategies without and with grouping. First, we analyze the asymptotic behavior of the RAP itself (i.e., without grouping) as the number of terminals goes to infinity. We show that the RAP can achieve optimal throughput even in super dense WLANs and the relevant optimal access probability can be derived in a closed form. Second, we propose a grouping strategy in the RAP called the grouped RAP (G-RAP). While a grouping strategy in the IEEE 802.11ah standard is based on time division, our G-RAP is based on transmission attempts. So the G-RAP does not waste channel resources. We show that the G-RAP achieves the optimal network throughput for any group structure if terminals use the optimal access probability that we derive. Our analytical results are validated by simulation.

Optimal throughput analysis of a super dense wireless network with the Renewal Access Protocol

As Wireless Local Area Networks (WLANs) become denser and denser recently, the contention-based MAC protocol such as the IEEE 802.11 DCF, the de facto standard for the WLAN, should be modified to handle such dense WLANs. To this end, we consider a recently proposed novel MAC protocol called the Renewal Access Protocol (RAP) in this paper. With the RAP, we consider two strategies for resolving collisions efficiently and achieving high throughput performance in a super dense WLAN: strategies without and with grouping. First, we analyze the asymptotic behavior of the RAP itself (i.e., without grouping) as the number of terminals goes to infinity. We show that the RAP can achieve optimal throughput even in a super dense WLAN and the related optimal access probability of the RAP can be derived in a closed-form from the analysis. Second, we propose a new grouping strategy in the RAP and call it the grouped RAP (G-RAP). While a grouping strategy in the IEEE 802.11ah standard is based on time division, which can cause a waste of channel, our grouping strategy is based on transmission attempts, which does not waste channel resources. From the analysis we show that the G-RAP easily achieves the optimal network throughput performance for any group structure (i.e., unform group size and arbitrary group size) if terminals use the optimal access probability that we derive. Our analytical results are validated by simulation.

A new contention based adaptive MAC protocol based on the renewal access protocol

Efficient sharing of the network spectrum is required to meet the exponentially growing demand of wireless communications, and the role of the MAC protocols has been becoming more important. In this regard, a novel MAC protocol called the Renewal Access Protocol (RAP) was recently proposed. It was shown that the RAP achieves optimal throughput, high short-term fairness, and near-optimal delay performances when the number of nodes in the network is known to all nodes. However, it is not easy for a node to know the number of nodes in the network and the number of nodes is time varying. So we propose an adaptive version of the RAP called the Adaptive Renewal Access Protocol (A-RAP) in this paper. A node equipped with the A-RAP estimates the number of nodes utilizing two dimensional Backoff Selection State (BSS), the Estimated Number of Nodes (ENN) and phase, based on its transmission results. For an efficient design of the A-RAP, we analyze the performance of the A-RAP and carefully determine two key parameters, the adjustment probabilities and the numbers of phases. Numerical and simulation results are provided to verify that the proposed A-RAP achieves good throughput and short-term fairness as the RAP. The results also show that the A-RAP achieves better short-term fairness than other MAC protocols proposed in the open literature.

The supplementary data transmission scheme for OFDM-based broadcasting systems

In the cyclic delay diversity (CDD), the concept of cyclic delay is used for increasing system performance of OFDM systems under the flat-fading channel. In contrast with CDD, this paper suggests a supplementary data transmission scheme using the concept of cyclic delay (STCD) to transmit additional data bits without the additional system BW or antennas.