Gang Uk Hwang

Enhanced Markov Chain Model and Throughput Analysis of the Slotted CSMA/CA for IEEE 802.15.4 Under Unsaturated Traffic Conditions

In this paper, we propose an analytical Markov chain model of the slotted carrier-sense multiple-access/collision-avoidance (CSMA/CA) protocol for IEEE 802.15.4 under unsaturated traffic conditions. Our proposed Markov chain model reflects the characteristics of the IEEE 802.15.4 medium-access control (MAC) protocol, such as a superframe structure, acknowledgements, and retransmissions with and without limit. We evaluate the throughput performance of the slotted CSMA/CA and verify the analytical model using simulation results.

On the Exact Analysis of a Discrete-Time Queueing System with Autoregressive Inputs

In this paper, we provide an exact analysis of a discrete-time queueing system driven by a discrete autoregressive model of order 1 (DAR(1)) characterized by an arbitrary marginal batch size distribution and a correlation coefficient. Closed-form expressions for the probability generating function and mean queue length are derived. It is shown that the system performance is quite sensitive to the correlation of the arrival process. In addition, a comparison with traditional Markovian processes shows that arrival processes of DAR(1) type exhibit larger queue length as compared with the traditional Markovian processes when the marginal densities and correlation coefficients are matched.

Design of a fair scheduler exploiting multiuser diversity with feedback information reduction

In this letter, we propose a new downlink fair scheduling scheme exploiting the multiuser diversity to enhance the transmission capacity. In the proposed scheme, only the MSs (mobile stations) whose normalized SNR (signal-to-noise ratio) values are larger than a given threshold feedback one-bit information to the BS (base station). As a result, while achieving the strict fairness, the proposed scheme can efficiently utilize the spectrum by reducing the considerable amount of the feedback information, compared to the proportional fair scheduling scheme where all the MSs feed back the normalized SNR values to the BS. Numerical studies show that the transmission capacity in the proposed scheme with a suitable value of the threshold is very close to that in the proportional fair scheduling scheme.

Mathematical modeling of rayleigh fading channels based on finite state markov chains

To mathematically model the Rayleigh fading channel, we propose a new method to partition the received Signalto-Noise ratio (SNR) range into a finite number of states and construct a finite state Markov chain (FSMC). Our method is based on the assumption that the transitions of the FSMC occur only between adjacent states. We provide numerical and simulation results to verify that our method constructs an accurate FSMC and our method outperforms existing methods.

Design and Analysis of Optimal Random Access Policies in Cognitive Radio Networks

In this paper, we consider a time slotted cognitive radio network where multiple secondary users (SUs) contend to access wireless channels. We propose and analyze a channel access policy where each secondary user stochastically determines whether to access a wireless channel (or not) based on a given access probability. In our analysis we consider two extreme cases - where all SUs a) possess full information and b) have no information regarding idle channels. We propose to obtain the access probabilities that minimize the steady-state queue length tail probabilities in the two extreme cases based on Effective Bandwidth theory. Our analysis provides an insight on how to design optimal channel access policies in the two extreme cases. We also show how the optimal channel access policies of two extreme cases are related with each other. Some numerical examples are provided to validate our analysis and investigate the performance behaviors of the optimal channel access policies.

Cross-Layer Design and Analysis of Wireless Networks Using the Effective Bandwidth Function

In this paper, we propose a useful framework for the cross-layer design and analysis of wireless networks where ARQ (automatic repeat reQuest) and AMC (adaptive modulation and coding) schemes are employed. To capture the joint effect of the packet transmission error rate at the PHY layer and the packet loss probability at the MAC layer, we introduce the effective bandwidth function of the packet service process. Base on queueing analysis with this effective bandwidth function, our cross-layer design tries to satisfy the required packet loss probability by each user and minimize the average packet transmission error rate. Numerical examples are provided to show the usefulness and characteristics of our framework.

SUPPLEMENTARY VARIABLE METHOD APPLIED TO THE MAP/G/1 QUEUEING SYSTEM

In this paper we consider the MAP/G/1 queueing system with infinite capacity. In analysis, we use the supplementary variable method to derive the double transform of the queue length and the remaining service time of the customer in service (if any) in the steady state. As will be shown in this paper, our method is very simple and elegant. As a one-dimensional marginal transform of the double transform, we obtain the generating function of the queue length in the system for the MAP/G/1 queue, which is consistent with the known result.

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.

The analysis of a multiserver queue fed by a discrete autoregressive process of order 1

Based on matrix analytic methods and the theory of Markov regenerative processes, we obtain the stationary distributions of the system size and the waiting time in a multiserver queue into which packets arrive according to a discrete autoregressive process of order 1.

Delay Analysis of OFDMA-Aloha

OFDMA is the basis of future broadband access, due to its many inherent advantages such as scalability and fine granularity for multi-user access. OFDMA-Aloha combines the flexibility of OFDMA with basic Alohas collision resolution mechanism over sub-carriers, in an attempt to reduce packet collisions and achieve faster retransmission. However, this comes at the expense of a larger slot size, due to lower channel rates per subcarrier. The above gives rise to a fundamental question: whether to use a single wide-band Aloha channel and retransmit via random back-off in next K time slots, or to retransmit immediately in one of K narrow-band sub-channels which are each 1/K slower (OFDMA-Aloha)? We answer this question, by analyzing the two protocols: Aloha and OFDMA-Aloha under the same total bandwidth and load conditions. We first derive the exact distribution of the packet access delay of OFDMA-Aloha in the saturated case. Then, we extend the analysis to the unsaturated case and derive the mean queue length and packet delay by decomposing the system of interfering queues into multiple independent queues utilizing the symmetry in our system. Our results show that if the network is already saturated, channelization does not bring substantial reduction in the collision rate to the point where it outweighs the effect of expanded slot size. In this case the single channel Aloha performs better than OFDMA-Aloha especially when the gap between the number of channels and the number users is large. On other hand, when the network is lightly loaded, OFDMA-Aloha enjoys smaller packet delays, but not for long as it saturates faster than the single channel Aloha.