In urban sensor networks, the diversity of propagation conditions can lead to the simultaneous reception of signals having very different power levels. Given the diversity of wireless technologies used in this area, implementing gateways using a Software-Defined Radio (SDR) approach seems to be a very practical solution. Overcoming the large dynamic range may however require a very high resolution Analog-to-Digital Converter (ADC) to digitize the weakest signal with a satisfying precision. One possibility to relax this requirement is to use a companding technique before digitization. This paper describes how to use an additive companding approach to reduce ADC's complexity and proposes two implementations of the compressing law.