ADAPTIVE CODED MODULATION FOR WIRELESS OFDM CHANNELS: UPPER BOUNDS ON AVERAGE SPECTRAL EFFICIENCY AND INFLUENCE OF IMPERFECT CHANNEL KNOWLEDGE

An adaptive coded modulation (ACM) scheme for a frequency-flat fading channel utilizes a set of channel codes with different spectral efficiencies. A feedback channel between the decoder and encoder makes it possible for the encoder to switch adaptively between these codes based on channel state information (CSI) fed back from the decoder. An orthogonal frequency division multiplexing (OFDM) system may utilize ACM on its subchannels to obtain a large average spectral efficiency (ASE), measured in transmitted information bits per second per unit bandwidth. In this paper we present a new upper bound on the ASE of OFDM/ACM systems. We also discuss how CSI estimation errors will influence the performance of an ACM system, and limit its possible region of operation with regard to channel signal-to-noise ratios and terminal velocities. 1