Scheduling and Link Adaptation for Uplink SC-FDMA Systems - A LTE Case Study

Long Term Evolution (LTE) is a beyond 3G system conceived with the objective of providing substantial advances in terms of data rates, Quality of Service (QoS) provisioning and cost reduction for users and operators with respect to currently available 3G systems. This project focuses on a subset of Radio Resource Management (RRM) functionalities within the context of LTE uplink and specifically on Power Control (PC), Adaptive Transmission Bandwidth (ATB) and Packet Scheduler (PS). PC and ATB are link adaptation techniques whose role is to adapt the power and transmission bandwidth to the time-varying nature of the channel. The PS entity, instead, multiplexes users in time and frequency domain based on a large variety of parameters like channel conditions and QoS requirements. In a LTE system the orthogonality of users within the same cell, realized via the Orthogonal Frequency Division Multiplexing (OFDM) scheme, eliminates the intra-cell interference leaving the PC functionality with the task of limiting the inter-cell interference due to a frequency reuse factor of one and the terminal power consumption. A PC algorithm therefore is proposed which, compared to the open-loop form of the standardized PC formula, introduces the dependency from the user generated interference when setting the transmitting power. The results show a considerable performance gain that can be used to improve the outage user throughput (by more than 50%) or the cell capacity (by more than 15%) by proper parameter tuning. Such performance boost is preserved only in a full (infinite) buffer traffic whereas a finite buffer traffic scenario considerably reduces the gain due to the different distribution of users in the cell and consequently the different interference patterns. The main contribution of this research project pertains the design of the PS entity. In LTE uplink the adoption of the Single Carrier Frequency Domain Multiple Access (SC-FDMA) scheme limits the flexibility of the allocation schemes. The problem is initially bypassed by designing a simple scheduler which assumes a Fixed Transmission Bandwidth (FTB). This gives the possibility to quantify the frequency selectivity and multi-user diversity gains obtained from a channel-aware approach compared to a channel-blind one. In a second phase a scheduling algorithm (novel to the knowledge of the author) is designed to integrate the flexibility of ATB into the scheduler. Such algorithm is shown throughout the thesis to provide flexibility in terms of inbuilt adaptation to cell load, user power limitations and QoS requirements when driven by appropriate scheduling metrics. Moreover the comparison of FTBand ATB-based schedulers reveals interesting viewpoints regarding the principles of multi-user diversity gain. In the last part of the thesis the design of the scheduling framework is completed by the addition of a time domain unit motivated by control channel limitations, computational complexity and QoS requirements like Guaranteed Bit Rate (GBR). The focus is therefore shifted on the design of time and frequency domain metrics which are modified as to take into account additional requirements, e.g. the GBR, and different traffic types, e.g. the mix of Constant Bit Rate (CBR) and Best Effort (BE) traffics. The proposed metrics are shown to effectively prioritize the users based on their traffic type and GBR requirement while keeping a similar cell throughput performance.