Interference to digital broadband communications and spread spectrum communications

Wireless transmission systems were initially noise limited and as they matured became interference limited. Now wireless systems are in transition from analog to digital. There are various classes of digital wireless systems: Those which require the signal-to-noise ratio (SNR) to be above a positive SNR threshold, for example 3G and 4G (LTE) and Digital TV Systems. The other systems employ spread spectrum technology, using Direct Digital Synthesis (DDS) which operate with negative SNR, for example the Global Positioning System (USA), and similar systems: Galileo (Europe), Glonass (Russia), and Compass in China. Interference between 3G or 4G (LTE) digital signals to DDS based signals and interference from DTV signals to 3G or 4G signals is the subject of this paper. The measurement techniques that proved useful for analog transmission systems such as two-tone testing are not suitable for testing of broadband systems and their active components. These inadequacies will be discussed and alternatives proposed.