Bit Loading Algorithms for Adaptive OFDM Wireless Systems

The main goal of next generation wireless systems is to provide multimedia communication with spectrally efficient communication. Most of the up-to-date systems have been standardised to use OFDM modulation which prevents inter-symbol interference and make use of parallel subcarriers. The large number of subcarriers helps to avoid the effects of fast fading by distributing channel coded information over different subcarriers. However, if the transmitter is informed about the channel state (CSI – Channel State Information), more sophisticated adaptive transmission techniques might be applied [4]. CSI can provide information about the channel state (e. g. attenuation on each subcarrier) which can help the transmitter to choose the right constellation size, amplification factor and the state (should we use it or not) of each subcarrier. The method is called adaptive, since transmission parameters are adapted to the channel conditions. In wired communication systems, where OFDM is applied (e. g. on xDSL lines), the adaptive transmission has been widely applied already [1]. However, in a wired environment the conditions are almost constant, thus the transmitter adapts to the channel conditions during the initialisation phase; no adaptation is needed during the communication phase. Wireless communication differs that here continous adaptation is needed to maintain effective links between the mobile node and the access point. Moreover CSI traffic should be as minimal as possible, to avoid the waste of resources. On the other hand multimedia streams consist of different types of flows. Some of them is sensitive to packet losses, others do care of delays. Even if only video frames are transmitted, some information should be protected as much as possible (e. g. intra frames consisting of the complete description of the whole picture), and others might be lost (e. g. frames describing temporal motion information) without noticeable performance degradation. Usually bitstreams with different performance requirements are grouped into traffic classes. Our task is to support different protection mechanisms for different traffic classes. In the literature it is usually referred to as Unequal Error Protection (UEP). UEP is traditionally performed at channel coding level. In this article UEP at the modulation level is considered when OFDM modulation is applied (separate, non-overlapping subcarriers). Basically bit loading is considered (which bit to put to which subcarrier). Only some basic CSI information is assumed to be available at the transmitter. This paper focuses on the method of bit loading, the intelligence behind that. Other parts of the system are beyond the scope. The aim of this paper is to derive clear mathematical equations that define the goal of the bit loading methods. In Section 2 the physical model is illustrated. In Section 3 the mathematical formalisation of the bit loading algorithm is given. Finally, in Section 4 a conclusion closes the document.