Jeonghoon Mo

Comparison of Multichannel MAC Protocols

This paper compares, through analysis and simulation, a number of multichannel MAC protocols. We first classify these protocols into four categories based on their principles of operation: dedicated control channel, common hopping, split phase, and parallel rendezvous protocols. We then examine the effects of the number of channels and devices, channel switching times, and traffic patterns on the throughput and delay of the protocols. Here are some of the conclusions of our study: (1) parallel rendezvous protocols generally perform better than single rendezvous protocols, (2) the dedicated control channel protocol can be a good approach with its simplicity when the number of channels is high and the packets are long, and (3) the split phase protocol is very sensitive to the durations of the control and data phases. Our study focuses on a single collision domain.

Analysis and comparison of TCP Reno and Vegas

We propose some improvements of TCP Vegas and compare its performance characteristics with TCP Reno. We argue through analysis that TCP Vegas, with its better bandwidth estimation scheme, uses the network resources more efficiently and fairly than TCP Reno. Simulation results are given that support the results of the analysis.

Comparison of multi-channel MAC protocols

This paper compares, through analysis and simulation, a number of multichannel MAC protocols for wireless networks. We first classify these protocols into 4 categories based on their principles of operation. We then examine the effects of the number of channels and devices, channel switching times, and traffic patterns on throughput and delay. Our study focuses on a single collision domain.

McMAC: A Parallel Rendezvous Multi-Channel MAC Protocol

Many multiple channel MAC protocols for wireless networks have been proposed to make efficient use of multiple channels where each node has a single radio which allows it to send or receive on one channel at a time. However, most of the proposed protocols are single rendezvous protocols that are subject to the congestion of the control channel. The paper proposed a new parallel rendezvous protocol, McMAC, to avoid control channel congestion so that it can scale to use a large number of channels efficiently. The authors validate the protocol design using simulation and implementation.

Resource allocation for multicast services in multicarrier wireless communications

We consider a multicast resource allocation problem for the downlink in OFDM-based wireless cellular network systems. In a conventional multicast system, to accommodate users with bad channel conditions, the transmission is based on the worst case user. We show that such a multicast system saturates the capacity when the number of users increases in fading environments. We exploit the multicarrier nature of OFDM and advances in coding techniques such as MDC (multiple description coding), in which arbitrary combinations of layers can be decoded at the receiver. Different MDC layers are carried over different subcarriers and users with good channels receive data from more subcarriers than users with poor channel conditions. We present an optimal subcarrier/bit allocation method requiring full search of possible candidates. To reduce the complexity, we propose a two-step suboptimum algorithm by separating subcarrier allocation and bit loading. Numerical results show that the proposed heuristics significantly outperform the conventional multicast transmission scheme. The difference between optimum and heuristic solutions is less than 5%.

Resource Allocation for Multicast Services in Multicarrier Wireless Communications

We consider a multicast resource allocation problem for the downlink in OFDM-based wireless cellular network systems. In a conventional multicast system, to accommodate users with bad channel conditions, the transmission is based on the worst case user. We show that such a multicast system saturates the capacity when the number of users increases in fading environments. We exploit the multicarrier nature of OFDM and advances in coding techniques such as MDC (multiple description coding), in which arbitrary combinations of layers can be decoded at the receiver. Different MDC layers are carried over different subcarriers and users with good channels receive data from more subcarriers than users with poor channel conditions. We present an optimal subcarrier/bit allocation method requiring full search of possible candidates. To reduce the complexity, we propose a two-step suboptimum algorithm by separating subcarrier allocation and bit loading. Numerical results show that the proposed heuristics significantly outperform the conventional multicast transmission scheme. The difference between optimum and heuristic solutions is less than 5%.

The Economic Effects of Sharing Femtocells

Femtocells are a promising technology for handling exponentially increasing wireless data traffic. Although extensive attention has been paid to resource control mechanisms, for example, power control and load balancing in femtocell networks, their success largely depends on whether operators and users accept this technology or not. In this paper, we study the economic aspects of femtocell services for the case of monopoly market, and aim to answer questions on operators revenue, user surplus, and social welfare by considering practical service types and pricing strategies. We consider three user subscription services, that is, users can access only macro BSs (mobile-only), or deploy femto BSs in their house and open / exclusively use their femto BSs (open- / closed-femto). For pricing strategies, flat pricing and partial volume pricing are exploited. The main messages include the following: 1) open-femto service is beneficial to both users and providers; 2) in flat pricing, the impact on operator revenue of allowing or blocking the access of mobile-only users to open femto BSs is minor; and 3) compared with partial volume pricing, flat pricing is advantageous to the operator when users are sensitive to price.

Open or close: On the sharing of femtocells

The femtocell is an enabling technology to handle exponentially increasing wireless data traffic. Despite extensive attentions paid to resource control, e.g., power control and load balancing in femtocell networks, the success largely depends on whether operators and users accept this technology or not. In this paper, we study the economic aspects of femtocell services with game theoretic models between providers and/or users. We consider three services: users can access only macro BSs (mobile-only), or open/exclusively use their femto BS (open or closed-femto). The main messages include: 1) it is better off for the operator to provide just the open-femto service than a mix of closed and open-femto services; 2) two polices of allowing or blocking the access of mobile-only users to open femto BS are not significantly differentiated in the revenue.

Joint network-wide opportunistic scheduling and power control in multi-cell networks

We present a unified analytical framework that maximizes generalized utilities of a wireless network by network-wide opportunistic scheduling and power control. That is, base stations in the network jointly decide mobile stations to be served at the same time as the transmission powers of base stations are coordinated to mitigate the mutually interfering effect. Although the maximization at the first glance appears to be a mixed, twofold and nonlinear optimization requiring excessive computational complexity, we show that the maximization can be transformed into a pure binary optimization with much lower complexity. To be exact, it is proven that binary power control of base stations is necessary and sufficient for maximizing the network-wide utilities under a physical layer regime where the channel capacity is linear in the signal-to-interference-noise ratio. To further reduce the complexity of the problem, a distributed heuristic algorithm is proposed that performs much better than existing opportunistic algorithms. Through extensive simulations, it becomes clear that network-wide opportunistic scheduling and power control is most suitable for fairness-oriented networks and under loaded networks. We believe that our work will serve as a cornerstone for network-wide scheduling approaches from theoretical and practical standpoints.

Investigation of handoffs for IEEE 802.11 networks in vehicular environment

Users in vehicles who are interested in accessing Internet services can be achieved by connecting to IEEE 802.11 WLAN APs along the roadside. However, due to the small coverage range of 802.11 APs and high mobility of vehicles, handoffs frequently occur as vehicles migrate across a series of adjacent access points which can be a major source of interruption to seamless connections and throughput reduction. In this paper, we investigate the well-known handoff schemes that have worked to reduce the 802.11 handoff latency and address the possible issues that arise when applied to vehicular scenarios. Our main objective is to provide groundwork for future research on the enhancement of fast handoffs for vehicular scenarios and emphasize that smart and optimized fast handoffs are a critical requirement for seamless connectivity. To meet these requirements, we also suggest that trajectory information (on-board navigation systems) along with neighbor AP information (802.11k) have the potential to provide smart and fast handoffs to support seamless communication in a vehicular scenario.