Modeling and Simulation of Downlink Subcarrier Allocation Schemes in LTE

The efficient utilization of the air interface in the LTE standard is achieved through a combination of subcarrier allocation schemes, adaptive modulation and coding, and transmission power allotment. The scheduler in the base station has a major role in achieving the required QoS and the overall system performance. The resources for both downlink and uplink transmission need to be assigned such that the capacity, throughput, and cell edge performance are optimized. This paper investigates allocation schemes for downlink transmission based on different criteria and performs evaluation through simulation results. The system for downlink scheduling is modeled using OPNET Modeler. 10-point space Introduction Not only because of the technology but also because it fulfills defined requirements for a pure 4G generation and backward compatibility with its previous generations (GSM, UMTS...), 3GPP Long Term Evolution (LTE) is leading the wireless mobile world. To achieve higher bandwidth, required spectrum deployment, increased spectral efficiency, flexibility and, consequently, a better QoS, the air interface of this all-IP-based network architecture utilizes the SC-FDMA in the uplink and OFDMA in the downlink enhanced by multiple antenna systems, supporting both the time (TDD) and frequency (FDD) division duplex modes. One of the main factors providing for a reduced latency is that LTE uses a relatively simplified network infrastructure consisting of only two nodes: the enhanced NodeB (eNB) and the mobile management entity/serving gateway (MME/S-GW). Thus, protocol processing overhead is reduced, leading to a reduced latency [1]. Due to the all-IP based architecture, the air interface needs to accommodate a mixture of real and non-real time services. In order to support mixture of services with different QoS demands, an end-to-end class based QoS architecture has been defined for LTE. During setup of radio bearers, the eNB needs to assign the necessary QoS class to the radio bearer. Each QoS class is characterized by: resource type (guaranteed and non-guaranteed bit rate), priority, packet delay budged and acceptable packet error loss rate. 10-point space In such a constellation, LTE operates as a scheduled system (on the downlink shared data channel), which means that all traffic, including delay-sensitive services, needs to be scheduled. The main purpose of the LTE scheduling system in the Base Station (BS) is to prioritize and allocate the available frequency-time resources to specific single user equipment (UE). The scheduling is at the MAC (Medium Access Control) layer, it is not standardized and it is an implementation specific mechanism. Therefore, the scheduling is an important issue and the main factor to influence the system performance and reusability of the resources [3]. The design of a downlink scheduling algorithm is a complex procedure and presents a number of design challenges, such as maximization of system capacity and spectral efficiency, bit error performances, fairness approach, etc. 10-point space So far, there has been a lot of research in modeling the scheduler in order to achieve the highest performance while avoiding latency and starvation problems. For the sake of model diversity and testing for our OPNET implementation, we tried to simulate and analyze different scheduling algorithms. In particular, we show that the MAC scheduler needs to be aware not only of channel conditions but also of different QoS classes. Additionally, a buffer-aware based algorithm is proposed, and the results are compared with the maximum capacity algorithm. The algorithms are evaluated on different criteria, such as achieved throughput and delay for each type of service. 10-point space This paper is organized as follows. In section II, we give some general insights on scheduling algorithms present in the literature. In section III, we describe our OPNET implementation of the downlink scheduling framework. In the following section, the traffic source parameters are presented. The fifth and sixth sections deal with the MAC layer implementations at the UEs and eNB. In section VII, the simulation parameters are presented, while the next section provides a discussion of the simulation results. Concluding remarks are offered in the last section. 10-point space Overview of Downlink Allocation Schemes In this section we give a brief overview of downlink allocation schemes based on [2], [4], and [6]. The MAX CQI (Channel Quality Indicator) algorithm exploits multi-user diversity, such that users who have the best channel conditions are prioritized. This scheme increases the system capacity, but fairness and delay requirements for real-time services are not achieved by this algorithm. Users located on the cell edge may not be scheduled for a longer time. The proportional based algorithm provides fairness among users. The algorithm can be designed to provide fairness in terms of throughput, and additionally can consider the delay requirements. 10-point space In the above described scheduling schemes, the CQI feedback is considered by the scheduler. Additionally, the scheduler can be designed to take into consideration the transmission buffer size, which represents the data available for scheduling. Such bufferaware scheduling can significantly improve the performance, as well as reduce resource utilization. 10-point space