Maarten Lont

Wireless Body Area Networks

The main design challenge for sensor nodes used in a WBAN is a low power consumption. System level aspects play an important role in the overall power consumption of body area networks. In this chapter different sensor network aspects and reported WBAN applications are analyzed and summarized. Additionally, several MAC protocols are compared using the WBAN properties. Since low power consumption is of primary importance, the sensor node energy consumption of the different MAC-layers are compared. With the energy consumption models, the solution space is examined. At the end of the chapter the receiver requirements are obtained.

Ultra-low power FSK Wake-up Receiver front-end for body area networks

In this paper, we present an ultra low-power Wake-up Receiver front-end operating in the 868/915MHz ISM band. It targets short distance body area networks. Its power consumption is only 126uW, including a low-power on-chip ring oscillator. Since the receiver targets small transmission distances, up to 10m, sensitivity is traded against power consumption. This is achieved by removing the LNA and making all the gain at the low IF frequencies. The receiver sensitivity is −65dBm at a BER of 0.1%.

Analytical Models for the Wake-Up Receiver Power Budget for Wireless Sensor Networks

In this paper analytical models of the energy consumption are presented which uses a real world radio model with two different low power modes. This model is used to compare energy consumption of different MAC protocols. The MAC protocols used for the comparison are chosen with sensor networks is mind. The energy consumption of the nodes in a sensor network needs to be minimized to maximize the lifetime of the network. Emphasis is placed on MAC protocols, since they have a big influence on the energy consumption. One of the MAC protocols uses a low power Wake Up Receiver (WURx) which is used to decrease the total energy dissipation. The WURx MAC protocol is compared with two other low power MAC protocols, namely the asynchronous X-MAC and synchronous TDMA protocol. The obtained model is used to derive the WURx power budget. The response time of the nodes is used as the main design requirement and the important application parameters are given that determine the WURx power budget.

9.7 A 0.33nJ/b IEEE802.15.6/proprietary-MICS/ISM-band transceiver with scalable data-rate from 11kb/s to 4.5Mb/s for medical applications

The introduction of the IEEE802.15.6 standard (15.6) for wireless-body-area networks signals the advent of new medical applications, where various wireless nodes in, on or around a human body monitor vital signs. Radio communication often dominates the power consumption in the nodes, thus low-power transceivers are desired. Most state-of-the-art low-power transceivers support only proprietary modes with OOK or FSK modulations, and have poor sensitivity or low data rate [1,2]. In this work, a 15.6-compliant transceiver with enhanced performance is proposed. First, the data-rate is extended to 4.5Mb/s to cover multi-channel EEG applications. Second, while a best-in-class energy efficiency of 0.33nJ/b is achieved in the high-speed mode, a dedicated low-power mode reduces the RX power further in low-data-rate operation. Third, a sensitivity 5 to 10dB better than the 15.6 specification is targeted to accommodate extra path loss due to shadowing effects from human bodies.

A 3µW fully-differential RF envelope detector for ultra-low power receivers

A fully differential envelope detector (ED) operating at 2.4GHz is designed in 90nm CMOS technology. The new design uses the common-gate topology to deal with large common-mode input signals through first-order current cancellation. Thereby, a fully differential ultra-low power super-regenerative front-end is enabled. It has a measured output voltage swing of 2.8–127mV and achieves 19.6dB output SNR at sensitivity input level. The circuit consumes 3µW from a 1.2V power supply.

Requirement driven low-power LC and ring oscillator design

Receiver power consumption should be kept as low as possible in applications such as sensor networks. Zero-IF detection is preferred over the interferer-sensitive and modulation-restricted envelope-detection receiver. However, a disadvantage of the zero-IF topology is that it requires a power consuming oscillator. Therefore, minimal-power oscillator design is desired. This paper shows that the ring type of oscillator becomes more power efficient than the LC type when the maximal tolerated phase noise increases. Which is due to required impedance levels and thus related to technological limitations. This conclusion is contradictory to what is usually assumed, based on fundamental reasoning.

Mixer-First FSK Receiver With Automatic Frequency Control for Body Area Networks

We present a low-power (382 ) body area network receiver operating in the 900 MHz band. The wideband FSK receiver supports bit rates up to 625 kbps. To save power, the power consuming phase-locked-loop (PLL) is replaced by an energy efficient digital automatic frequency control (AFC) loop. The AFC acts as a low-bandwidth frequency-locked-loop (FLL), using the FSK demodulator as frequency detector; the measured frequency offset is fed back to the on-chip digitally controlled oscillator. To further decrease the power consumption, the LNA is removed, the passive mixer being the first circuit in the receiver front-end. The mixer-first topology increases the linearity compared to injectionlocked and envelope detector based receivers. Additionally, analytical passive mixer transducer power gain and noise figure models are presented which are used to obtain an optimal mixer-first design. We achieve a -81 dBm sensitivity at a bit rate of 12.5 kbps.

Ultra-low power FSK receiver for body area networks with automatic frequency control

In this paper we present an ultra-low-power receiver geared towards body area networks (BAN). The presented wideband-FSK receiver consumes only 382.5μW while achieving a BER of 10-3 at -81dBm sensitivity for 12.5kbps. The bit rate is scalable up to 625kbps, enabling a trade-off between sensitivity and bit rate. Taking advantage of the short-range nature of BAN applications, a mixer-first architecture is proposed, leading to a good dynamic range, given the DC power consumption. To further decrease the power consumption a free-running digitally controlled oscillator (DCO), tunable from 782MHz to 932MHz, is implemented, that is controlled by a data-aided automatic frequency control (AFC) loop, making the receiver resilient against DCO frequency variations.

A 3.5mW 315/400MHz IEEE802.15.6/proprietary mode digitally-tunable radio SoC with integrated digital baseband and MAC processor in 40nm CMOS

An energy-efficient, flexible radio SoC with RF front-end (RFFE), digital baseband (DBB) and microcontroller (MCU) for medical/healthcare applications in 315/400 MHz bands is presented. The SoC is fully-compliant with the IEEE 802.15.6 standard in 400MHz bands, and also supports proprietary modes, including high data rate (HDR) modes with x2/4/8 data rates (max 3.6Mb/s) to support applications like EEG, and low-power modes with 1/16 data rate to minimize sensor node power consumption. The total power consumption of 3.5mW (RX, 3.6Mb/s, −77dBm sensitivity) enables best-in-class power efficiency of 1nJ/bit.

Implications of I/Q Imbalance, Phase Noise and Noise Figure for SNR and BER of FSK Receivers

In energy-constrained applications like Body Area Networks (BAN) receiver performance is traded for lower power consumption. The limited receiver performance will deteriorate the output signal-to-noise-ratio (SNR) and bit-error-rate (BER) of the receiver. In this paper we present closed-form output SNR and BER models of a non-ideal receiver front-end with limiter-discriminator demodulator for binary FSK. The presented model is very useful in defining the minimally required receiver specifications. Moreover, the models are useful for gaining insight into the influences of the receiver impairments on its performance. In the models, the gain and phase imbalances between the in-phase and quadrature-phase paths as well as the receiver generated noise and the phase noise produced by the local frequency reference are taken into account. This paper shows that the gain and phase imbalances shift the FM threshold to higher carrier-to-noise-ratios (CNR) and only have small influence on the SNR above threshold. On the other hand, receiver generated noise reduces the output SNR both below and above the FM threshold, and phase noise limits the maximum output SNR. Additionally, a trade-off between the FSK frequency deviation and phase noise robustness is derived.