Chenxi Zhu

Proportional fair scheduling algorithm in OFDMA-based wireless systems with QoS constraints

In this work we consider the problem of downlink resource allocation for proportional fairness of long term received rates of data users and quality of service for real time sessions in an OFDMA-based wireless system. The base station allocates available power and subchannels to individual users based on long term average received rates, quality of service (QoS) based rate constraints and channel conditions. We formulate and solve a joint bandwidth and power optimization problem, solving which provides a performance improvement with respect to existing resource allocation algorithms. We propose schemes for flat as well as frequency selective fading cases. Numerical evaluation results show that the proposed method provides better QoS to voice and video sessions while providing more and fair rates to data users in comparison with existing schemes.

Improving smartphone battery life utilizing device-to-device cooperative relays underlaying LTE networks

The utility of smartphones has been limited to a great extent by their short battery life. In this work, we propose a new approach to prolonging smartphone battery life. We introduce the notions of valueless and valued battery, as being the available battery when the user does or does not have access to a power source, respectively. We propose a cooperative system where users with high battery level help carry the traffic of users with low battery level. Our scheme helps increase the amount of valued battery in the network, thus it reduces the chance of users running out of battery early. Our system can be realized in the form of a proximity service (ProSe) which utilizes a device-to-device (D2D) communication architecture underlaying LTE. We show through simulations that our system reduces the probability of cellular users running out of battery before their target usage time (probability of outage). Our simulator source code is made available to the public.

Optimal radio resource management in multihop relay networks

Multihop relay networks (MR) use relay stations (RS) to extend or enhance the coverage of a base station (BS) in a cellular network. The base station is attached to a wired backhaul. Relay stations use wireless transmission to connect to the base station and to the mobile stations (MS), while direct BS-MS connection is also possible. Low cost relay technologies are recently proposed for OFDM based broadband wireless technologies. Especially for low-cost relay stations that have single radio interface how to share the channels and allocate proper amount of bandwidth/power to the base station (BS), relay stations (RSs) and the mobile stations (MSs) is an important issue. This work develops methods for radio resource management in MR networks supporting heterogeneous traffic. We propose a scheduling method, where the transmission of base station to each relay station and transmissions of each relay station to its respective mobile stations (i.e. composite links) are scheduled in a TDMA fashion and the resource allocation in those microcells are performed in an OFDMA manner. We numerically evaluate the performance of our proposed scheme with respect to a number of parameters.

Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple Receivers

When directional antennas are used in 802.11 based wireless LANs, higher network capacity is often accompanied with more collisions. This is due to the enhanced hidden node problem and the deafness problem that arise in the directional transmission/reception scenario. These problems are caused by an inconsistent view of the medium status by neighboring nodes. We have developed a new variation to the 802.11 protocol called sectorized MAC (S-MAC), which employs a novel architecture consisting of multiple directional antennas and multiple receivers. With a new self-interference cancellation scheme, an S-MAC node can continuously monitor the channel status in all directions in order to avoid the aforementioned problems. It is fully compatible with the standard 802.11 protocol and can inter-operate with omni-antenna-based 802.11 nodes. It is applicable to both infrastructure mode and ad hoc mode. Simulation studies show that it achieves significant capacity gains, even if only used in part of the network.

Practical Resource Allocation Algorithms for QoS in OFDMA-Based Wireless Systems

In this work we propose an efficient resource allocation algorithm for OFDMA based wireless systems supporting heterogeneous traffic. The proposed algorithm provides proportionally fairness to data users and short term rate guarantees to real-time users. Based on the QoS requirements, buffer occupancy and channel conditions, we propose a scheme for rate requirement determination for delay constrained sessions. Then we formulate and solve the proportional fair rate allocation problem subject to those rate requirements and power/bandwidth constraints. Simulations results show that the proposed algorithm provides significant improvement with respect to the benchmark algorithm.

On Spatial Fairness of the 802.11 DCF Protocol and the Role of Directional Antenna

The IEEE 802.11 DCF protocol suffers from the spatial fairness problem where different nodes achieve different throughput due to their topological distributions. In this paper we extend Bianchis Markov model of the DCF protocol to multihop WLANs with some simple topologies and study the behavior of the binary exponential backoff (BEB) algorithm under heavy load conditions. Our study is focused on the interaction between the BEB processes of nodes at different locations. We identified several courses of the fairness problem, and found the BEB processes of some nodes have significant advantage over some other nodes. The BEB algorithm provides an undesired positive feedback and worsens the fairness problem. We further study directional antenna as a means to improve network fairness, and find that fairness can be improved significantly with directional antenna without changing the BEB algorithm.