Methods and apparatus for calibrating the VCO element of a phase-locked loop to correct for non-linearities are disclosed. The emails are periodically downloaded without attachments in response to a poll of the server and if a sufficient high-speed connection to the server is available, and then the downloaded email is locally stored in a local cache of the portable computer. The system also includes an ICI measuring module coupled to the receiver to detect the N*N test symbols received in the receiver and to arrange the test symbols as a receiving matrix in the same way as the orthogonal matrix in the transmitter.