Opportunistic Scheduling in Wireless Networks: An Overview of Issues and Design Considerations

The exponential growth of wireless internet systems in recent years has created strong demand for new high data rate wireless data services. Such systems suffer from the limited resources due to the limited bandwidth. Therefore, it is important to develop techniques that utilize the available bandwidth efficiently. One way of utilizing the wireless resources efficiently is by adopting a good scheduling scheme. Good scheduling schemes in wireless networks should opportunistically seek to exploit channel conditions to achieve higher network performance. By transmitting to the best user in terms of channel condition we are opportunistically scheduling users. However, transmitting every time to the best user introduces an important tradeoff between wireless resource efficiency and level of satisfaction among different users. For example, allowing only users close to the base station to transmit at high transmission power may result in very high throughput, but sacrifices the transmissions of other users. Providing quality of service (QoS), in particular meeting the data rate and packet delay constraints of real-time data users, is one of the requirements in emerging highspeed data networks. In opportunistic scheduling other considerations must be taken into account, e.g., throughput, fairness, queue delay, etc. In this tutorial, we present an overview of multiuser opportunistic scheduling. The tutorial begins by an overview of the constraints of a wireless shared medium and the relevant performance measures in designing any scheduling scheme. The tutorial then introduces various kinds of opportunistic scheduling schemes (fair, semi-fair, and non-fair). The tutorial then proceeds to opportunist scheduling algorithms that make use of multiple antennas. The tutorial also covers practical considerations in opportunistic scheduling such as the effect of parameter estimation and quantization, limited feedback, and channel correlation (in time, frequency, and space). The tutorial ends with some consideration of the opportunistic scheduling in current wireless standards such as CDMA/HDR and other IEEE 802 standards.