On implementation of link adaption in OFDM wireless networks

The popularity of wireless networks has soared in recent years. The wireless channel, along with its many benefits, poses a host of challenges for system designers, due to its time-varying and frequency selective behavior. Orthogonal frequency division multiplexing (OFDM) addresses some of these challenges at the physical layer. In this paper, we discuss various issues in a practical implementation of an OFDM based system which supports adaptive modulation. A novel blind scheme for detecting inactive carriers, resulting from water-filling or other adaptive modulation, is proposed and investigated. Extensive simulation of the detection scheme, in an IEEE 802.11a framework, shows it to be robust against changes in code rate, data modulation and water-level. Backward compatibility of the new scheme with existing devices is also discussed and detection of new devices at the system level is suggested and described. Overall, the work in this paper forms the basis for a cross-layer approach to the optimization of wireless networks, in which the physical, link and network layers are adapted to system parameters such as buffer state, traffic statistics, and channel conditions