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Use UWB in ultralow-power Zigbee sensor nodes

Posted: 18 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Zigbee? UWB? sensor node? wireless communications?

By Els Parton and Olivier Rousseau

The IEEE 802.15.4 standardization committee who defined the MAC and PHY adopted by Zigbee has proposed an alternative PHY relying on UWB technology. The advantages of this new UWB air interface are mainly increased data rate, extended communication range, lower power consumption and the possibility for use in accurate positioning of transceivers.

In this article, we discuss some of the key challenges associated to the design of UWB transmitters. We further present the first reported transmitter complying with the new 802.15.4a standard, which has been implemented in standard 90 nm CMOS technology and shows a record low-power consumption of 1 mW for a net data rate of 0.85 Mbps.

Wide range of applications
Wireless communication based on UWB signals has attracted much attention from the wireless community both from standardization bodies and chip manufacturers. This air interface promises flexibility, robustness and high-precision ranging capabilities.

For example, wireless sensor nodes for medical applications can not be realized using today's low-power radios such as Bluetooth, ZigBeeor by proprietary radios eitherbecause these can't meet the stringent wireless body-area network power requirements.

Typical chipsets for these radios consume in the order of 10- to 100mW for data rates of 100- to 1,000Kbit/s. This leads to a power efficiency of roughly 100- to 1,000mW/Mbit/s or nJ/bit. For wireless sensor nodes, a radio is needed which is 1 to 2 orders more power efficient. UWB holds this promise.

The Federal Communications Commission has authorized UWB communications between 3.1GHz and 10.6GHz. Although the regulations on UWB radiation define a power spectral density limit of -41dBm/MHz, there are very few regulations on the definition of the time-domain waveform.

The latter can then be tailored for low hardware complexity as well as low system power consumption. In pulse-based UWB, the transmitter only needs to operate during the pulse transmission, producing a strong duty cycle on the radio and the expensive baseline power consumption is minimized.

Moreover, since most of the complexity of UWB communication is in the receiver, it allows the realization of an ultra-low power, very simple transmitter and shift the complexity as much as possible to the receiver in the master.

However, the impact of the type of UWB signal chosen on the communication performance and on the complexity of the radio implementation must be carefully analyzed. The minimum bandwidth of a UWB signal is usually 500MHz.

Indeed, various UWB standard proposals have subdivided the entire UWB spectrum in 500MHz sub-bands as a way to mitigate against strong interferers, to improve the multiple access and to compose with the different regulations on UWB emissions worldwide.

Therefore, in order to comply with these regulations and standards, the generated pulses of UWB impulse-radio (UWB-IR) approaches must fulfill stringent spectral masks that can feature such bandwidths. This poses a serious challenge for the pulse generation of UWB-IR transmitters.

Design challenges
The challenges associates to the design and implementation of UWB transmitters differ significantly from those encountered in classical low-power radios, typically relying on narrowband air interfaces in the 2.5GHz ISM band.

Typical design challenges in that narrowband context are related to the accurate control of the generated frequency references used for modulation of the signal, both in phase and amplitude. In contrast, UWB signals are spread over a relatively large bandwidth (500MHz or more) and the precise control of the carrier is by far less critical than in narrowband radios, which loosens the requirements on the control of the RF frequency reference.

An important challenge in the design of UWB systems stems from the large range of frequencies that must be covered, i.e. from 3GHz to 10GHz. The different elements of the transmitter must be designed to offer constant performance across a spectrum of 7GHz.

Moreover, achieving a maximum absolute frequency of 10GHz with an effective bandwidth of more than 500MHz with low power consumption is not straightforward, especially when low-cost devices relying on standard CMOS technologies are considered. The wide range of frequencies covered by UWB also poses significant challenges when designing UWB antennas.

UWB signals consist in extremely short pulses (about 2 ns), or groups of adjacent pulses, separated by long silence periods. Accurate control of the pulse amplitude and shape, or RF carrier frequency is not of critical importance in UWB. However, the control of the moment at which UWB pulses are generated is a critical aspect of UWB systems as is the control of the phase of the RF carrier.

Finally, an important advantage of UWB resides in the possibility to significantly reduce the power consumption of the radio front-end by switching off the transmitter during the relatively long silence periods between UWB pulses.

In order to exploit this advantage, the front-end circuits must be designed with well-controlled and relatively fast startup behaviors, such that the stringent timing and phase control requirements can be met.

Alternative approach
Recently, the IEEE 802.15.4a standardization committee proposed an alternative physical layer for ZigBee providing positioning on top of low cost, low power and scalable data range using UWB as key technology.

The first digital UWB transmitter chip based on the signal structure depicted in the standard was developed by researchers in Belgium and the Netherlands (IMEC). This signal structure is depicted in Figure 1. The chip consists of a digitally-controlled oscillator, a programmable frequency divider, a unique digital RF modulator and an early-late detector for frequency calibration of the oscillator.

Figure 1: 802.15.4a standard air interface. Bands 4, 7, 11 and 15 (not shown) are optional and have a different bandwidth. Band 3 and 9 are mandatory.

The UWB transmitter contains a single digitally-controlled oscillator (DCO) that covers the 3- to 10GHz frequency range. To avoid degrading the SNR of the TX-RX link, the phase of each transmitted chip should be accurate within 15psec at 10GHz which sets a requirement on the jitter of the oscillator and implies a frequency accuracy of 4MHz.

The frequency accuracy and 3 to10GHz frequency range results in the need for 3 different methods to control the DCO. Implementing the full tuning range with a switchable biasing current would entail matching with large transistors for the smallest frequency steps whose capacitances would unbearably slow down the start-up.

Therefore, a 6bit current source is used for coarse tuning such as to benefit from its power scalability. For the medium tuning, a 5bit capacitor bank is used since it does not degrade the start-up time.

However, it is limited in the minimum frequency steps by parasitic capacitances. Therefore, the finest tuning is implemented with an 8bit tuning on the degeneration resistor of the biasing current mirror. A 4bit binary, 4bit thermometer segmentation has been used to ensure a monotonous frequency tuning curve.

In conclusion, although 13 bits would ideally be sufficient to cover the full band, the necessary overlap between the various frequency tunings lead to 19 control bits. The maximum settling time is below 2ns to achieve 4MHz accuracy, which is less than 10 percent of the burst time.

Transistors to supply and ground on the oscillating nodes ensure a predefined initial state. At TX activation these transistors are released at a reference clock edge guaranteeing fast and uniform startup.

The frequency divider (DIV) divides the 3- to 10GHz DCO frequency by an integer value from 7 to 20 to produce the chip rate. The fully dynamic divider is realized in true single-phase clock logic.

It consists of a cascade of 18 half-transparent latches (HTLs) in a loop closed by a precharge unit and clocked by the RF signal. To achieve 10GHz speed, the HTL Clk/Bypass unit has been modified. Any integer division value between 7 and 20 can be obtained by setting the appropriate number of HTLs in bypass-mode.

To produce a burst of binary phase-shift keying modulated chips, the RF local oscillator (LO) must be modulated by a +1/-1 code sequence defined in the standard. Modulation is realized by inverting and shaping the carrier at each code transition. A nice feature of the synchronous divider is the fact that the timing required to shape the pulses is available in its intermediate phases.

Four discrete steps are taken to invert the carrier by using 4 parallel multipliers consecutively activated by properly chosen phases of the divider HTL chain. Each unit multiplies the RF LO with the code value. Thereby, a code transition produces at the sum output an RF LO inversion in 4 discrete steps as a result of the sequential activation of the multipliers.

The DCO RF frequency is calibrated to the desired band prior transmission by using an early-late detector (ELD). The ELD measures the time difference between a burst duration measured at the divide by 16 output and a reference clock period. It then generates a 1bit value specifying whether the RF frequency is too high or too low.

The result is used to increase/decrease the control word setting the DCO frequency via an external FPGA. Three binary-search calibrations are done to set the coarse, medium and fine tuning word, after which the RF frequency is within less than 4MHz of the desired one. Drift of the DCO frequency during operation is monitored by the ELD status at every transmitted burst and the DCO control word is adjusted for the next burst.

The transmitter has been realized in a 90nm digital CMOS process. The measured output power is -10dBm in 50?. The power consumption of the transmitter from 1V supply is 0.65nJ per 16 chips burst (40pJ/pulse) at 3.5GHz and 1.4nJ/burst (87pJ/pulse) at 10GHz which outperforms state of the art low-power narrowband transmitter implementations.

For the mandatory mode, this corresponds to 0.65mW to 1.4mW for 1Mbit/s data rate with 75 percent in the DCO and LO distribution, 5 percent for the DIV and 20 percent for the DMO. A transmitted burst is measured on a high bandwidth scope. The transmitter can operate in any 499.2MHz band of the IEEE 802.15.4a.

The total accumulated jitter is below 6psRMS for the 10GHz carrier where the highest SNR degradation is observed. A 6psRMS jitter at 10GHz degrades SNR by less than 1dB for a 10e-4 BER.

This implementation demonstrates a low-power UWB transmitter compatible with the IEEE 802.15.4a draft standard. It proves that for transmission the standard leads to implementations with power consumptions meeting sensor networks requirements.

UWB transmitter chip for IEEE 802.15.4a outperforming state-of-the-art low-power narrowband transmitter implementations.

Future applications
UWB is a key solution for wireless connectivity characterized by low power consumption and high data rate over a relatively short range. Evidence of its importance is the recent use of UWB in the IEEE 802.15.14a standard.

Research groups worldwide are developing UWB building blocks to optimally serve the wide application domain e.g. transfer of medical data between and from body-worn sensor nodes to the patient's cell phone or doctor's PDA.

The transmitter presented in this article shows the potential of UWB for ultra low power RF transmitters. However, the most recent UWB receivers still consume 10 times more power than the presented transmitter.

Applying the same duty cycling policy as in the transmitter should allow a significant reduction of the power consumption, paving the way to the deployment of UWB devices in mass market applications where power consumption of the radio is an important issue.

About the authors
Els Parton
joined IMEC in 2001 as a scientific editor and is jointly responsible for authoring and editing the research organization's numerous company technical documents and publications.
Olivier Rousseau is a senior researcher and activity leader for the Ultra Low Power Wireless activities of IMEC-NL.

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