Ultra-low-power radio

The ultra-low-power radio (ULPR) technology program aims at performing low-rate (below 1Mbps) wireless communications in the context of sensor networks at power consumption levels 1 order of magnitude below current solutions. Novel air interfaces relying on impulse radio ultra-wideband (UWB-IR) radio signals have shown a strong potential for ULPR communications in this context. Although very promising, UWB-IR communication systems are still in an early development phase and a number of key challenges must still be addressed to fully demonstrate their high potential in an integrated and reliable communication system. In this context, the study, development and IC implementation of an UWB-IR physical layer that solves the key issues associated to UWB-IR communications and demonstrates reliable communications at ULP consumption has been selected as the primary research goal of the ULPR program in 2006 and 2007.

Two main activities have yielded noticeable results in the UWB research program. Demonstration ICs implementing a proprietary UWB air interface that had been previously developed in the ULPR program have been successfully integrated in a lab demonstration system showing bits-in/bits-out capabilities. Next, a new family of ICs have been designed, focusing on the recently defined IEEE 802.15.4a standard that defines an UWB-IR air interface in line with the strategic goals of the ULPR program. The main result of this second activity is a first-of-a-kind standard-compliant system implemented in 90nm CMOS technology.

First generation UWB-IR lab demonstration system

The ULPR program has previously focused on the realization of first-of-a-kind UWB ICs implementing a first generation of UWB-IR air interface consisting in isolated pulses. These first-generation ICs were realized in 180nm CMOS technology. The transmitter which is shown in figure 1 demonstrated an ULP consumption of approximately 2mW. The corresponding receiver required a slightly higher power consumption with a total of approximately 30mW. This first receiver is depicted in figure 2.

Figure 1

Figure 1: Microphotograph of the first generation UWB Tx chip.

Figure 2

Figure 2: Microphotograph of the first generation UWB Rx chip.

Next to those ICs implementing the analog front-ends of the first-generation UWB-IR air interface, significant effort has been put into the development and implementation of the baseband algorithms that are essential for the successful operation of UWB-IR radios at a reduced level of complexity. Timing acquisition and data demodulation algorithms suitable for first-generation air interface have been developed with a specific focus on reduced implementation complexity in order to enable low-power implementations. Furthermore, ultra-low power implementations of those algorithms have been developed and implemented on prototype field-programmable field arrays (FPGAs). The developed implementations of those baseband algorithms are expected to consume less than 10mW when implemented in 180nm CMOS technology.

Finally, the UWB transmitter (Tx) and receiver (Rx) front-ends of the first generation have been combined with the FPGA implementation of the developed baseband algorithms, and a lab demonstration showing bits-in/bits-out functionalities has been implemented. The lab demonstration setup is illustrated in figure 3 and can be used as a development platform for advanced applications such as ranging or positioning.

Figure 3

Figure 3: Picture of the lab demonstration implementing the first generation of UWB-IR air interface.

First-of-a-kind IEEE 802.15.4a compliant transmitter

In 2006, a digital UWB transmitter has been designed that supports the IEEE 802.15.4a standard. This transmitter is the first ever reported to implement the standard. Furthermore, its power consumption is remarkably low. The key figures of the chip are outlined in this section.

Recently, the IEEE 802.15.4a standardization committee proposed an alternative physical layer that will provide ranging on top of low power low-data-rate communication using UWB as a key technology. The recent draft proposal of the physical layer defines the signal structure depicted in figure 4.

Figure 4

Figure 4: 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 transmitter (figure 5) has been designed according to this standard. It consists of an RF digitally-controlled oscillator (DCO), a programmable divider (DIV), a digital modulator (DMO) and an early-late detector for DCO frequency calibration. The correct RF frequency is not generated by a traditional phased-locked loop (PLL), since the large start-up time would drastically reduce the low-power advantages of low-duty-cycle communication. Instead, a phase-aligned frequency-locked loop is implemented. Phase alignment is done by forcing the startup process of the DCO to be uniform when it is switched on at the beginning of the burst. Thereby, the DCO is realigned to the reference clock at each reactivation. This technique does not only have the advantage of substantially reducing the startup time but also truncates the accumulative jitter process. Since the initial phase of the DCO is uniform for each burst, only the frequency needs to be adjusted. This is done using an early-late (fast-slow) detection mechanism operating on each transmitted burst. The DIV produces the chip rate, used to clock the burst code register, feeding the DMO. This DMO modulates the RF output of the DCO according to the burst code sequence and the data value.

Figure 5

Figure 5: Architecture of the IEEE 802.15.4a-compliant UWB transmitter.

A transmitted UWB burst as measured on a high bandwidth scope is shown in figure 6.

Figure 6

Figure 6: Time-domain measurement of a burst of UWB pulses.

The measured output power is -10dBm in a 50Ohm load. The power consumption of the transmitter from a 1V supply is from 0.65nJ per 16 chips burst at 3.5GHz to 1.4nJ/burst at 10GHz. This outperforms state-of-the-art low-power narrowband transmitter implementations. For the mandatory mode, this corresponds to 0.65mW to 1.4mW for 1Mb/s data rate, with 75% in the DCO and local oscillator (LO) distribution, 5% for the DIV and 20% for the DMO. The transmitter can operate in any of the bands (499.2MHz bandwidth) defined in the IEEE 802.15.4a standard. The total accumulated jitter is below 6psRMS for the 10GHz carrier where the highest signal-to-noise (SNR) degradation is observed. A 6psRMS jitter at 10GHz degrades SNR by less than 1dB for a 10e-4 big error rate (BER).

This implementation demonstrates an ULP UWB transmitter compatible with the IEEE 802.15.4a draft standard. On a larger scale, it shows that as far as transmission is concerned the standard leads to implementations with power consumptions meeting sensor network requirements.

The transmitter has been realized in a 90nm digital CMOS process with the chip microphotograph shown in figure 7.

Figure 7

Figure 7: Microphotograph of the UWB Tx chip. The chip area is 1x1.4mm2.

The R&D for the Wireless Autonomous Transducer Solutions (WATS) program is carried out by IMEC's sister company IMEC-NL at Holst Centre.