Downlink Soft Handoff Mechanisms and Cell Reconfiguration Planning in Mixed-Size CDMA Cellular Networks

• Main Authors: Ching-Yu Liao, 廖青毓
• Other Authors: Chung-Ju Chang
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/85702410159361463026

博士 === 國立交通大學 === 電信工程系所 === 93 === To utilize radio resources efficiently, the cellular system may deploy mixed-size cells in cellular systems when there exists non-uniform traffic loads among cells. This mixed-size cellular architecture raises some challenging and crucial issues about the radio resource management, in which the system design faces the dilemma between system capacity and service coverage, especially in CDMA cellular networks. Because of abundance multimedia traffics in the downlink, the downlink transmission is generally the capacity-limited direction. In this dissertation, we specialize in the downlink soft handoff mechanisms and cell reconfiguration planning in terms of it power balance characteristics to tackle tradeoffs between coverage and capacity in mixed-size CDMA cellular systems. We first investigate impacts of the soft handoff in the CDMA system with mixed-size cells because the soft handoff mechanism directly affects the system capacity and coverage via multi-site transmission. Based on a simple analytic approximation for user capacity in a simplified model of two mixed-size cells, results show that unequal power allocation and maximum link power constraint for each active connection of soft handoff in mixed-size CDMA cellular systems are necessary, otherwise the power exhausting problem may occur in congested microcells, in which the microcell has stringent power budget. To tackle this problem, a downlink power allocation mechanism for soft handoff in mixed-size CDMA cellular systems is proposed. It is based on the concept of power balance by unequal power allocation for active links in proportional to the link qualities, which is link proportional power allocation (LPPA) scheme. A simulation model of mixed-size CDMA cellular environment is adopted, and simulation results show that the LPPA scheme outperforms existing schemes because of its excellent capability of power balance. Besides, it shows that the LPPA scheme offers better resistance to occurrences of measurement errors during active set selection. Next, a soft handoff mechanism in multirate mixed-size WCDMA cellular systems is proposed. Most of previous studies focus on joint power and rate allocation for all users in the homogeneous system with the same-size cells, whereas the possible combinatorial numbers of the solutions are too large to be tractable for optimal allocations. To make system implementation feasible, we emphasize the optimization for multirate soft handoffs by a joint power and rate assignment (JPRA) algorithm to accomplish power balance among cells. The JPRA algorithm contains a LPPA scheme and an evolutionary computing rate assignment (ECRA) method. Compared to existing power allocation schemes with best-effort rate allocation, simulation results show that the JPRA algorithm can reduce the handoff forced termination probability and improve the total throughput, resulting in better cell coverage and higher system capacity. Finally, to balance traffic loads over cells when there are time-varying traffic load distributions among cells, it is crucial for future multimedia cellular networks to be aware of system situations and to configure mixed-size cells dynamically. The problem of dynamic cell configuration is addressed by observing that dynamically adjusting pilot power alone while not changing other radio resource management algorithms can result in performance degradation. We then propose a novel dynamic cell configuration (DCC) scheme with radio resource management for multimedia CDMA networks via reinforcement-learning technologies. The DCC scheme takes into account pilot allocation, maximum link power allocation, call admission control as well as soft handoff mechanisms. Simulation results demonstrate that the DCC scheme is effective in next-generation situation-aware CDMA networks.