Powering the wireless Internet
The technique of carrying information, such as Internet data, via radio waves is known as modulation. In order to minimise the power of the radio transmission while maintaining the quality of data transmitted, a system of adaptive modulation is required. In order for more than one user to use the Internet within the range of a single wireless base station, information is split across different channels transmitted on different frequencies. This is known as Frequency Division Multiplexing (FDM). The variant of FDM used for this research project is known as Coded Orthogonal Frequency Division Multiplexing (COFDM). Orthogonal FDM systems divide the transmitted data among many different frequency channels, with the intervals between frequencies carefully chosen. The ‘C’ stands for coding of the data that is used for error-correction. The receiver builds up information on the reliability from interference of each channel, and adapts its decoding of the information accordingly. A set of adaptive parameters was selected for the algorithm, related to the number of wireless terminals, the number of sub-carrier frequencies and the coding. The transmitter then adaptively excludes the weakest sub-carrier channels (called Weak Sub-Carrier Excision or WSCE), and this adaptive algorithm thus guarantees Quality of Service while reducing the transmission power required. One algorithm functions by suppressing a fixed percentage of the weakest carrier frequencies, while the other uses a variable percentage. These solutions are ripe for further development and can be adapted to a number of other applications.
