IASA has proposed two practical algorithms for adaptively changing the transmission parameters on a turbo-Coded, Orthogonal Frequency Division Multiplexing (COFDM) communication link, in order to minimize the transmission power while satisfying constraints for a given Quality of Service (QoS) level.
These algorithms use an approximation of the coded system performance in order to minimize the transmission power based on the proper selection of a set of the adaptable parameters (on a frame base). This set of adaptable parameters was the constellation size, the number of sub-carriers used for transmission, and the code rate. The selection of this set was done base on the WF system constraints. The idea of not using the weakest sub-carriers for transmission was called Weak Sub-Carrier Excision (WSCE). It is actually a "hard" (or on-off) bit loading scheme. The first proposed algorithm uses WSCE with a fixed percentage of excised carriers and the second one with variable. The significance of variable WSCE is the ability to choose for the same target rate the best code rate-WSCE percentage option.
The gain in using these algorithms is twofold: First, the transmission power of the system is decreased, and second, the QoS is practically guaranteed per sample channel realization.
These algorithms can be used with minor modifications to apply to a variety of systems that experience channels which are static long enough to necessitate the adaptation of the transmission parameters for a specific channel sample path, but are still fairly dynamic, such that classic bit-loading algorithms (which require high-throughput, error-free feedback channels to work well) would not be applicable. Future work focuses on finding closer approximations to coded performance, for a larger SNR range, when transmitting in the said environment
Throughout this effort IASA has gained knowledge concerning modern adaptive modulation techniques and the designing process, at the algorithmic level, of reconfigurable transceivers. This knowledge will help our group for other related projects and for the research work of PhD and MSc candidates within IASA who focus on the same research topics.