Recent studies on spectrum sensing for cognitive radio (CR) reveal that sensing based on single CR may not provide reliable information due to the fact that signals degrade as a result of fading and shadowing. Hence, spectrum sensing based on multiple CR has been considered. In this paper we investigate a distributed sensing based on multiple spectrum sensors where the sensors are located outside the CR. Depending on the channel conditions between sensors (sensor- to-sensor channel) and between the sensor and the radio (user-channel), we explore three different modes of sensing where sensors provide the sensing information either cooperatively or independently. Computer simulation results are provided to validate the concepts.