An Ultra-Low-Power receiver using transmitted-reference and shifted limiters for in-band interference resilience

The coexistence of more and more wireless standards in the ISM bands increases the design difficulty of interference-robust receivers (RX), especially for Wireless Sensor Nodes because of their Ultra-Low-Power (ULP) budget. Superheterodyne receivers [1-3] are difficult to design for low power due to the challenges of generating a clean oscillator signal with low power. The phase noise of the LO leads to reciprocal mixing, and non-linearities and limited low-Q RF filtering make them vulnerable to in-band interferers, even when external passive components are used [2]. To circumvent accurate frequency generation, uncertain IF has been proposed [4], together with envelope detection. Alternatively, envelope detection can be done at RF to avoid frequency downconversion at all. However, selectivity is highly compromised in both cases, as the RX cannot distinguish between the signal and nearby interferers. By transmitting two tones [5], this problem is partly solved, because it detects only signals with the right frequency spacing. However, interferer robustness is still limited because of the high-RFgain amplifiers that process the entire RF band. Strong interferers will either saturate these amplifiers, or the signal will drown in noise. In this paper, we propose a receiver that uses a spread-spectrum Transmitted-Reference (TR) modulation technique to spread the interferer power, combined with the use of Shifted Limiters (SL) to further improve the Signal-to-Interference Ratio (SIR)