This study describes a method of implementing a fully integrated ultra-low-power (ULP) radio for wireless sensor networks (WSNs). This is achieved using an ad hoc modulation scheme (impulse radio), with a bandwidth of 17.7 MHz in the 2.4 GHz-ISM band and a specific medium access control (MAC) protocol, based on a duty-cycled wake-up radio and a crystal-less clock generator. It is shown that the total average power consumption is expected to be less than 100 ¿W with a clock generator inaccuracy of only 1%.