Adaptive modulation for wireless communication systems
Multi-antenna systems for both transmitter and receiver are currently being intensively investigated due to their ability to provide higher throughput with improved bit error performance over single-input and single-output systems. Orthogonal frequency division multiplexing (OFDM) may further be employed in the antenna arrays to increase diversity gain on time-variant and frequency selective wireless channels. OFDM has already proven its applicability in various standards for broadcast and point-to-point communications by allowing for easy adaptation to the channels' conditions due to its frequency granularity. Within the European project FLOWS, an innovative OFDM transmission scheme for multiple-input and multiple-output (MIMO) systems, adaptive to the current channel situation at the transmitter side was introduced. Information on the current channel situation at the receiver side is provided in practice by means of channel estimation and prediction techniques. On the other hand, channel knowledge at the transmitter site can only be guaranteed if there is a backward channel and therefore, such transmission was more of a theoretical interest. If the MIMO transmission channel was quasi-static all necessary information on channel parameters can be re-transmitted back to the transmitter using a backward channel. With the available signal power optimally distributed over sub-channels, this Singular value decomposition (SVD) approach yields the best possible gain factors and importantly, the overall performance is maximised. In situations where the user starts to move during the time of transmission and the transmission channel fades rapidly, the most important or no information is re-transmitted. By always selecting the strongest signal among the detected ones, the newly proposed adaptive transmission scheme corresponds to a V-BLAST scheme. The price that has to be paid for the increased flexibility in dealing with transmission channel fluctuations is additional implementation complexity. Extensive simulations showed that while increasing the number of re-transmitted parameters enhances the transmission channel's overall performance, further antennas and sub-carriers would lead to a compromise between performance and complexity.
