| Summary: | 博士 === 國立中正大學 === 資訊工程研究所 === 93 === Supporting multiple classes of service with quality of service (QoS) guarantee for each user is a unique feature of the third generation (3G) wireless communication networks. Among various resource management mechanisms, packet scheduling is the essential technique for providing multiple classes of service. Provided a minimum amount of bandwidth guarantee and a fair shared of residual bandwidth under a certain amount of power are the two major objectives of the packet scheduling in 3G networks. 3G networks differ from local wireless networks in at least following aspects. First, in the 3G wireless standards, UMTS proposes employing the wideband code-division multiple-access (WCDMA) technology. With CDMA, multiple users can transmit their packets simultaneously on the same spectrum. Furthermore, a packet can be transmitted over multiple frames. However, in wired networks, only one user at a time can transmit a packet and that packet must be transmitted without interruption.
The second difference concerns the constraints of UMTS. In the UMTS system, each user is assigned a unique signature orthogonal variable spreading factor (OVSF) code. The OVSF codes can be represented as a binary tree. Those OVSF codes have different spreading factors. The length of the spreading factor in UMTS should be 2K chips, where k is the layer of the code in the OVSF code tree. Thus, spreading factors, and consequently data rates, are constrained to certain values. The orthogonal property is another difference between UMTS and local wireless networks. An OVSF code can be assigned if and only if no other code on the path from the specific code to the root of the tree or in the sub-tree below the specific code is assigned. Due to the orthogonal property, the system may not be able to support a call with a single OVSF code, even though the system has enough capacity to support this requirement. Therefore, how to design a code management strategy that provides the low probability of code blocking and the high code utilization under the orthogonal constraint and the single code constraint is a very important issue.
The limited transmission power on the base station is the fourth difference. The actual capacity depends on users'' channel conditions, which are mobile-dependent and time-dependent. Due to the power control for fading depressive and interference, the amount of power allocated to each channel will fluctuate from frame to frame. Thus, the system capacity is time-varying and user-dependent. This feature is very different to the local wireless network.
We address the above problems, and the goal of this thesis is to provide efficient and integrated solutions for packet scheduling in UMTS. We propose various approaches based on different network environment, evaluate these algorithms by analysis and simulation, and compare them with the existing approaches. The major contributions of this thesis are outline in the following.
We propose two scheduling algorithms, MWF2Q+ and MDRR, for multiple classes of service over the same spectrum in the forward link of UMTS. These scheduling algorithms can allocate bandwidth in proportion to weights of flows sharing the channel under a certain amount of transmission power, and assign OVSF code to backlogged flows on a frame-by-frame basis. We also analyze the efficiency and fairness properties of these two scheduling algorithms. Our simulation results show that these two scheduling algorithms support multiple traffic sources with heterogeneous rate guarantees while fully utilizing the system bandwidth.
We also propose an integrated solution for OVSF code management, assignment, and reassignment in UMTS. The computational complexity of proposed code management structure, code assignment strategy, and code reassignment strategy are analyzed in this thesis. From the simulation results, we show that our proposed solution efficiently utilizes the OVSF codes with the lower computational overhead as compared to previous results.
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