Niranjan Sudhir Joshi

Downlink scheduling in CDMA data networks

Packet data is expected to dominate third generation wireless networks, unlike current generation voice networks. This opens up new and interesting problems. Physical and link layer issues have been studied extensively, while resource allocation and scheduling issues have not been addressed satisfactorily. In this work, we address resource management on the downlink of CDMA packet data networks. Network performance (for example, capacity) has been addressed, but user centric performance has not received much attention. Recently, various non-traditional scheduling schemes based on new metrics have been proposed, and target user performance (mostly without reference to wireless). We adapt these metrics to the CDMA context, and establish some new results for the offline scheduling problem. In addition, we modify a large class of online algorithms to work in our setup and conduct a wide range of experiments. Based on detailed simulations, we infer that:Algorithms which exploit “request sizes” seem to outperform those that do not. Among these, algorithms that also exploit channel conditions provide significantly higher network throughput. Depending on continuous or discretized bandwidth conditions, either pure time multiplexing or a combination of time and code multiplexing strikes an excellent balance between user satisfaction and network performance. Discrete bandwidth conditions can lead to degraded user level performance without much impact on network performance. We argue that the discretization needs to be fine tuned to address this shortcoming.

Design and analysis of an IEEE 802.16e-based OFDMA communication system

IEEE 802.16e has emerged as a strong candidate standard for future wireless systems primarily because it offers the potential for high spectral efficiency, flexible spectrum options (e.g., 2–6 GHz), scalable carrier bandwidth options (e.g., from 1.25 MHz to 20 MHz), multiple duplexing options (time and frequency division duplex), various subchannelization options, and, unlike its IEEE 802.16 predecessors, mobility. Because of the recent emergence of IEEE 802.16e and the complexity it poses in system analysis, there is little published work in the literature regarding the actual system capacity/throughput performance of IEEE 802.16e for high data rate services. In this paper, we investigate the link and system level performance on the downlink of an IEEE 802.16e-based orthogonal frequency division multiple access (OFDMA) communication system and provide recommendations on high performance IEEE 802.16e system design and deployment configurations. We also propose dynamic resource allocation methods that may be used in OFDMA systems and investigate their performance.

On transmit diversity and scheduling in wireless packet data

Downlink time division multiplexing is considered for cellular packet-switched systems, in conjunction with scheduling algorithms for fast packet switching across the served users. Additionally, open and closed loop transmit diversity techniques at the base station are considered to improve the spectral efficiency of the resulting system. A study is conducted on the effect of candidate transmit diversity techniques for packet data systems on the overall network data throughput in the presence of scheduling. Contrary to the circuit-switching paradigm, the presented results highlight the conflicting objectives of specific transmit diversity schemes and efficient scheduling and illustrate the impact of the medium access control layer functions on the physical layer performance. The open loop space time spreading technique is shown to offer inferior performance to no transmit diversity under greedy packet scheduling. An appropriate transmit diversity scheme is proposed for the downlink packet-data multiple access.

Zone Based Resource Allocation for OFDMA Systems

This paper investigates the performance benefits of different resource allocation strategies that take advantage of permutation zones specified in IEEE 802.16e-based OFDMA systems. In particular, we propose and investigate the benefits of a sub-zone scheduling approach where each permutation zone is partitioned into sub-zones that are used for scheduling subsets of eligible users according to their prevailing link quality. In the scenarios considered, the proposed sub-zone scheduling approach allows improvements in sector throughputs as high as 80% relative to a baseline IEEE 802.16e-based OFDMA system while maintaining the system outage at acceptable levels.