Young-June Choi

Flexible Design of Frequency Reuse Factor in OFDMA Cellular Networks

The OFDMA systems are emerging for future cellular networks. Creating multiple data channels, they can support the flexible frequency reuse factor (FRF). Although FRF 1 is the best choice in terms of cell throughput, it causes intercell interference at the cell boundary, thereby being unable to serve the whole cell area. Therefore it was proposed to use the FRF of greater than 3. In this paper, we develop a flexible FRF design mechanism that provides an intermediate value between 1 and 3 while the conventional schemes are dedicated to use some integer numbers only such as 3, 4, or 7. In our design, if the number of shared channels between any neighboring cells is given, we implement it simply according to a difference set. Simulation results show that a FRF of 7/4 achieves better throughput than FRF 3 and overcomes the intercell interference problem of FRF 1, so it can replace the conventional FRF such as 3. We expect that our new FRFs have the advantage in supporting smooth handoff because there are always some common channels between two neighboring cells.

Downlink scheduling with fairness and optimal antenna assignment for MIMO cellular systems

A bandwidth-limited wireless channel can considerably improve its performance by exploiting multiple-input-multiple-output (MIMO) antennas. Combining spatial multiplexing with multiuser diversity, we develop an optimal cross-layer scheduling mechanism that executes fair scheduling at the upper layer and optimal antenna assignment at the physical layer. For fair scheduling, we propose a framework that achieves the objective of maximum capacity and proportional fairness. For optimal antenna assignment, we consider the Hungarian algorithm that maximally utilizes the characteristics of MIMO systems by adopting the graph theoretical approach. Through simulations, we demonstrate the performance of the optimal scheduling.

QoS-aware Selective Feedback and Optimal Channel Allocation in Multiple Shared Channel Environments

It is well known that opportunistic scheduling by using feedback information significantly improves wireless network performance. Most opportunistic scheduling works have focused on the case where a single channel is shared by multiple users. However, emerging wireless technologies (e.g., MIMO, OFDMA, etc.) are characterized by multiple shared channels, which complicates the problem. Moreover, it is necessary for the network to be able to provide various levels of quality of service (QoS). To address these issues, we develop a QoS-aware selective feedback model and a method to do optimal resource allocation. In our feedback model, each user chooses those channel sets that meet its QoS requirements by exploiting user diversity, thus resulting in a significant reduction in the amount of feedback information. Given the feedback channel sets for each user, the base station then distributes channels to each user with the objective of maximizing the number of accommodated users or the sum of users utility values. We use a graph theoretic approach to solve these maximization problems by mapping them to clique searching problems. We develop some interesting theoretical results and properties but show that the complexity of this problem can be exponential in the number of channels. Thus, we also develop two suboptimal algorithms to handle the case when the number of shared channels is large. Finally, we demonstrate the efficacy of our results through an extensive numerical study

Selective channel feedback mechanisms for wireless multichannel scheduling

Opportunistic scheduling can significantly improve wireless network performance by exploiting the feedback information that conveys the underlying channel condition. In emerging multichannel systems, the perchannel feedback induces a substantial amount of feedback overhead and requires high computational complexity. To reduce the feedback overhead, we consider an opportunistic feedback strategy that activates the channel feedback opportunistically according to the channel condition. Then, we combine the opportunistic feedback with the best-n channel feedback scheme where a mobile user chooses the best n channels and transfers this information to the base station. We analyze the throughput and the amount of channel feedback information for proportionally fair opportunistic scheduling under Rayleigh fading i.i.d. channels. The numerical results confirm that our partial feedback schemes achieve a remarkable reduction in the amount of feedback information without a significant throughput degradation, thereby saving the scarce wireless bandwidth and limited battery power

Exploiting Multiuser MIMO in the IEEE 802.11 Wireless LAN Systems

By adopting multiple-input-multiple-output (MIMO) antenna technologies, IEEE 802.11 wireless LANs are evolving into high speed systems. While only one user can transmit at a time in the conventional IEEE 802.11 systems, we investigate the possibility of multiuser transmission by using MIMO antennas, which is now known as multiuser MIMO. The multiuser MIMO technique enables multiple users to receive packets over the downlink simultaneously, but it should be carefully used in the IEEE 802.11 systems for interoperation with non-MIMO legacy terminals. Through analysis and simulation evaluation, we demonstrate that multiuser transmission with a scheduling algorithm and single-user transmission with enhanced spatial multiplexing achieve enhanced performance by exploiting multiuser diversity in the space and time domains. Especially, when the number of stations is large, multiuser transmission shows better performance than enhanced single-user transmission.

Scheduling for VoIP service in cdma2000 1x EV-DO

Recently cdma2000 1x EV-DO (HDR) system has begun to be deployed in some countries to support high data rate services in cellular networks. The system is originally designed to support data services, but now is expected to serve some real-time traffic including VoIP. For VoIP service with delay hound and low loss requirements, we propose a frame structure considering delay bound and a scheduling algorithm reflecting channel conditions. To schedule VoIP, we adopt the maximal rate algorithm and the proportionally fair algorithm. The proportionally fair algorithm (PF) was known to be appropriate for elastic-traffic, however, from simulation results, we conclude that the PF algorithm with the channel test is an appropriate scheduling scheme to provide QoS of VoIP. When the required slot portion of VoIP is 75%, the loss rate is about 1% on the average and 3% in the worse case. On the other hand, the maximal rate algorithm shows twice of the loss rate for the same delay bound and load. Additionally we propose a simple admission control scheme for VoIP service that controls the average portion of slots occupied by VoIP packets.

QoS scheduling for multimedia traffic in packet data cellular networks

CDMA data networks such as cdma2000 1x EVDO are proposed in the midst of evolving to the 3rd generation wireless networks. Basically they use time division multiplexing and rate control that need a downlink scheduling to increase the system capacity, thereby being able to support high speed data rates. As the systems will eventually support multimedia and data traffic together, we need to have a propose criterion for scheduling that can count various service requirements such as delay and loss. Therefore, we visit the concept of utility and opportunity cost considering these together. The opportunity cost is defined as the maximum utility lost among the other users by giving the current turn to a particular user. We design an algorithm to select a job for transmission with the maximum profit that is obtained by subtracting the opportunity cost form its expected utility. The simulation results show that it can support various QoS levels in terms of delay and loss for various traffic scenarios.

Multichannel wireless scheduling under limited terminal capability

Emerging systems like OFDMA and MIMO systems require multichannel scheduling over a wireless link. In this paper, we focus on the case that the number of channels to be assigned to a mobile terminal is limited, which we call limited matching. This case often occurs over a MIMO downlink when the number of receive antennas at a mobile user is smaller than that of transmit antennas at the base station. When the system exploits channel-aware opportunistic scheduling based on the channel feedback, finding the sum of channel gains by the limited matching is an NP-complete problem. To solve it easily, we can use the Hungarian algorithm, but its complexity is still too high and not amenable to performance analysis. Hence, we develop a heuristic algorithm, and analyze its cell throughput. Also, we investigate its performance when the channel feedback information is partially available. For analytic simplicity, we consider proportionally fair (PF) scheduling that is widely accepted as an opportunistic scheduler. Numerical results demonstrate that our heuristic limited-matching scheduling algorithm works well with partial channel feedback.

Optimal antenna assignment considering QoS under MIMO environments

Exploiting multiple-input-multiple-output (MIMO) diversity, systems with error-prone and bandwidth-limited wireless channels can easily support reliable transmission. We develop optimal cross-layer scheduling that consists of QoS scheduling at the upper layer and optimal antenna selection at the physical layer. For QoS scheduling, we design a framework for optimal scheduling to meet users QoS requirements. To solve this optimization problem, we consider a clique-searching algorithm for antenna selection that maximally utilizes the characteristics of MIMO systems by adopting the graph theoretical approach. As the clique searching problem becomes NP complete with the increase of transmit antennas in number, we propose a suboptimal antenna selection algorithm to deal with a large number of transmit antennas. We derive some theorems and properties for our approach and, through simulations, we demonstrate the performance of QoS scheduling which is effective to handle real-time traffic.

Upper-level scheduling supporting multimedia traffic in cellular data networks

Wireless data networks such as cdma2000 1x EV-DO and UMTS HSDPA use downlink scheduling that exploits channel fading to increase the system throughput. As future wireless networks will eventually support multimedia and data traffic together, we need a proper criterion for scheduling that can count various service requirements such as delay and packet loss. Although some previous approaches proposed opportunistic schedulers at the lower layer, it has not been investigated well whether they are able to meet explicit QoS defined at the upper layer. Hence, in this paper, we develop a hierarchical scheduling model that considers QoS provisioning and the time-varying channel feature separately. We focus on the upper-level QoS scheduling that supports various traffic classes in a unified manner. Supposing that a user gets some satisfaction or utility when served, we introduce a novel concept of opportunity cost, which is defined as the maximum utility loss among users incurred by serving a particular user at the current turn. We obtain each users net profit by subtracting the opportunity cost from its expected utility, and then select a user with the maximum profit for service. Simulation results reveal that our scheme supports various QoS classes well that are represented by delay and packet loss under various traffic loadings.