Project Title: Data Access Points in Tiered Wireless Networks

1. Wireless Macrocell Area Data Access Point Data Access Point (small desensitivity factor) (large desensitivity factor) Wireless and Network Engineering Lehigh University 2003 Wireless Day Open House Project Title: Data Access Points in Tiered Wireless Networks PI: Shalinee Kishore, Ph.D. Graduate Assistant: Zhenlei Shen Introduction & Objective • Access method For a given set of users in the DAP, how do we schedule users to time slots? Through analysis and simulation, we conclude one-user-a-time is the best strategy. More than one user will create excessive interference and degrade the DAP performance. Figure 2 also shows the comparison between one-user-a-time method and two-user-a-time method. • Interference suppression methods As the capacity of CDMA network is interference limited, interference suppression is essential to improve the performance. We are exploring space-time filtering or successive interference cancellation methods to reduce cross tier interference. • Delay performance In addition to throughput, QoS is also measured using delay. We have developed analytical methods to quantify the delay as a function of the desensitivity factor. Not surprisingly, the delay values increase as the desensitivity factor (i.e., the number of DAP users) increases. In centralized networks, user terminals communicate directly with access points. The coverage area of an access point depends on various conditions, including system design parameters like allocated transmit power, bandwidth assignment, antenna height and gain, etc. Tiered wireless networks are designed with access points whose coverage areas vary in their order of magnitude. Higher-tier access points which have larger coverage areas offer low speed voice and data service to a large population. On the other hand, lower-tier access points offer exclusively high speed data service to a small group of data users in some specific areas, such as airports, restaurants, etc. In this research project, we study a tiered wireless network in which Macrocells provide wide area low bit-rate coverage while embedded lower-tier Data Access Points (DAPs) give high speed data access to data hotspots. Although our proposed research has been influenced by earlier multi-tier models (e.g. infostations or integrated cellular/WLAN systems), we assume a fundamentally different system characteristic: cross-tier interference. That is, we assume both tiers communicate over the same set of frequencies and thus interfere with each other. One important task in studying such a tiered architecture is to quantify the cross-tier interference pattern and determine its impact on the data throughput of DAP users. Our goal in this research is to understand the throughput and delay enhancements offered by embedded DAPs assuming various system and propagation characteristics. In particular, we wish to study the impact of various tier-selection strategies, multiple access methods, interference suppression techniques, use of multiple transmit and receive antennas, user mobility patterns, multi-user diversity scheduling, multihop access and so on. We aim to develop novel techniques, tradeoffs, and engineering rules enabling the deployment of spectrally-efficient DAPs. Figure 2. DAP performance Figure 1. Conception of Data Access Point Future Directions In our future research, we will incorporate some new directions. • Multiple DAPs So far we have only focused on the throughput/delay characteristics of a single macrocell with a single DAP. Our aim is to extend the system by first considering multiple embedded DAPs and then multiple surrounding macrocells. • Multiple transmit and receive antennas (MIMO) The benefits of multiple transmit and receive antennas have been well established. For our given architecture, we wish to establish when the MIMO link can be exploited for multiplexing gain and when it must be used for diversity gain and determine the capacity/quality improvements. • Mobility pattern A crucial component in studying the benefits of DAPs is to consider the impact of user mobility, i.e., what are the global throughput and delay measures of user terminals as they traverse the large coverage region? • Multi-user diversity With multi-user diversity, the DAP system always operates at its best channel which leads to improved capacity, but the delay will increase. We wish to quantify this tradeoff and develop new fair scheduling algorithms that exploit the fading channel conditions. Current Research • We have been studying throughput and delay characteristics of a CDMA system with one Macrocell and one embedded DAP, when both tiers use the same set of frequencies. We are focusing on the impact of • Tier selection strategy A Desensitivity Factor restricts user access to the DAP. The smaller the Desensitivity Factor is, the fewer users are assigned to the DAP and the higher data rate these users can get. Specifically, the Desensitivity Factor controls the DAP coverage area and the number of DAP users. • Throughput As an important measure of Quality of Service (QoS), throughput is the time-average data rate achieved by individual or all users. The throughput of any one DAP user is the per-user throughput and the throughput of all users is the total DAP throughput. When the Desensitivity Factor increases, more users are assigned to the DAP and they must wait longer to access their assigned time slots. As a result, the per-user throughput decreases. But the DAP BS utilization increases so the total DAP throughput is high when the Desensitivity Factor is high. Figure 2 shows the relationship between the Desensitivity Factor and the two throughput values. Conclusion Our research on Data Access Points is beneficial to both military and commercial service providers. Easy deployment and flexibility of DAPs satisfy the quick and mobile requirements of military service provider. In the commercial scenario, DAPs can be used to give high speed internet access over licensed spectrum.