Analog Front End in 130 nm CMOS for UWB Impulse Radio Receivers

This paper presents an integrated ultra-low power analog front-end (AFE) architecture for UWB impulse radio receivers. The receiver is targeted towards applications like wireless sensor networks typically requiring ultra energy-efficient, low data-rate communication over a relative short range. The proposed receiver implements pulse correlation in the analog domain to severely relax the power consumption of the ADCs and digital backend. Furthermore a fully integrated prototype of the analog front-end, containing a PLL, programmable clocking generator, analog pulse correlator, a linear-in-dB variable gain amplifier and a 4-bit ADC, is demonstrated. Several design decisions and techniques, like correlation with a windowed LO instead of with a matched template, exploiting the duty-cycled nature of the system, operation in the sub-1 GHz band as well as careful circuit design are employed to reach ultra-low power consumption. The analog front-end was manufactured in 130 nm CMOS and the active circuit area measures 1000 m 1500 m. A maximum channel conversion gain of 50 dB can be achieved. Two symbol rates, 39.0625 Mpulses per second (Mpps) and 19.531 Mpps are supported. The AFE consumes 2.3 mA from a 1.2 V power supply when operating at 39.0625 Mpps. This corresponds to an energy consumption of 70 pJ/pulse. A wireless link over more than 10 m in an office-like environment has been demonstrated at 19.531 Mpps with a PER under direct LOS conditions.
WIRELESS sensor networks have gained an increasing research interest in the last few years. They provide a huge all-new myriad of application domains in a number of fields (like healthcare, logistics, agriculture, environmental monitoring, etc.) with very specific requirements for the communication. Often only modest data rates over short distances are required. The practical feasibility of such applications depends largely on the power consumption of the sensor nodes, which is typically dominated by the radio. For long-term battery-powered, or even autonomously scavenger-powered devices, ultra energy-efficient radios are required. The FCC defined ultra-wideband (UWB) communication in 2002 [1]. Impulse radio communication is a specific form of
UWB where data is modulated on short pulses in time (less than 2 ns). This form of communication has a high robustness against fading and multipath channels, typically frequency selective effects [2], and allows the use of fairly simple, low-power transmitter [3]–[6] and receiver designs. As such, impulse radio communication becomes increasingly popular for energy-effi- cient low data-rate radios. Due to the recent advances in integrating very potent and complex digital logic, fully digital receivers [7] became attractive. In these receivers, the AFE boils down to adequate amplification after which the signal is digitized. With powerfull digital processing readily available, very good performance can be achieved. However, due to the wide bandwidth signals employed, very fast ADCs at Nyquist rate and digital logic are required which comes at a power penalty. As demonstrated in [7], this type of receivers is well suited for high data-rate applications, but the high power-consumption cannot be tolerated in low-power wireless sensor networks. At the other end of the spectrum, super-regenerative receivers (SRR) typically boast very low power consumptions (e.g., [8] reports a 400 W receiver). The AFE consists of an oscillator core which is periodically activated at the pulserate. If a pulse with the same center frequency as the oscillator core arrives at the same time the core is started, the start-up time will be significantly faster. This difference in start-up time is measured to determine wether a pulse arrived or not. Super-regenerative receivers are thus well suited for OOK. Due to the high gain of the oscillator, these receivers can operate at very low power levels while maintaining a decent sensitivity. A big disadvantage however is the very limited robustness against in-band interferers. Any signal close to the center frequency will increase the start-up time of the oscillator tank. Since UWB is supposed to work overlaying existing and future narrowband users in the same band, this issue cannot be overlooked. Precautionary measures are required adding again to the overall power consumption (e.g., [9] implements 12 narrowband SRRs and combines the output. A wideband pulse will trigger all SRRs, while a narrowband interferer will only corrupt a single one).

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