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Ultra Low-power Concurrent Transceiver Architectures for Ubiquitous Networks
H. Hashemi and A. Hajimiri

Abstract. We are proposing a completely new approach to design Ultra-Low Power Concurrent Multiband Transceivers capable of operating at multiple frequency bands simultaneously with minimal overhead to the system resources for a network of sensors.

Motivation. Communication is the essential glue in any functional distributed system. The elements of a network of remote and/or mobile sensors need to be able to communicate effectively and efficiently to be of any practical use. As many sensory networks have to operate as autonomous units in potentially hostile environment, low power dissipation and robustness to the environmental variables while maintaining a high data rate are of highest significance.

In communication networks higher data-rates and more reliability and robustness are usually the major system design issues. Wherever portability is desired, wireless communication becomes very attractive. In today's portable communication systems, the battery is the ultimate bottleneck in determining the physical size, duration of operation and operation range of wireless terminals. Hence, low-power design has become one of the most important constraints in the design of many wireless apparatus. Traditionally, achieving more robustness while maintaining the high data-rate has been realized by data transmission from different communication links, which necessitates multiple transmitting/receiving terminals, one for each link. This solution inherently results in more power consumption and higher cost.

Our newly proposed novel concurrent multiband operation is extremely useful to increase the bandwidth and add diversity to the receivers while allowing for multiple data-streams associated with various applications to be integrated using the same hardware without a significant power consumption penalty.

Simultaneous operation at multiple frequency bands can be used to serve multiple purposes. Since concurrent multiband receivers are capable of operating at multiple frequency bands simultaneously, they automatically have access to more bandwidth. For example, the ISM band at 2.4GHz provides about 85MHz of bandwidth while the three ISM bands at 900MHz, 2.4GHz and 5.8GHz, combined with the 5.2GHz UNII band can provide more than 450MHz of bandwidth that can be used simultaneously. This significant increase in bandwidth is extremely valuable for wideband wireless systems. Furthermore, the extra bandwidth comes via bands at different frequencies. As the wavelengths of the signal at different bands are different, they form multiple parallel channels with different fading properties. Such systems can operate as diversity receivers capable of reliable operation under harsh multipath fading conditions similar to that of in-door wireless applications, big cities or mountainous areas. They are also more robust to strong unintended or intentional jammers. These two features are extremely useful for the reliable operation of any wireless sensory array with high data rate between the sensor cells. Finally, the integrated multi-purpose transceivers are able to operate at different bands, using multiple communication standards for different applications. Mobile and portable systems built using these multiband transceivers will benefit a great deal from the improvement in the power dissipation and size of these concurrent multiband systems.

Research. We have pursued concurrent multi-band operation at different design levels, namely, the architecture level (higher lever) and the building block level (lower level). In the building block level, by observing the inherent wideband trans-conductance of the transistor that can be used to provide "small signal" gain and matching at many frequencies without any penalty in the power dissipation, we have developed a design methodology for concurrent multi-band amplifiers (or more specifically low-noise-amplifiers)

The same observation can be used for other building blocks that work in small-signal region such as mixers, etc. Then at the higher architecture level, after investigating many options, such as wideband receiver, multi-band sub-sampling receiver, etc. we have developed a concurrent dual-band receiver architecture that can be fully integrated. The objective is to devise a receiver that can simultaneously receive signals at two different frequency bands with maximum reuse of power and building blocks.

Achievements. Concurrent dual-band and triple-band LNAs have been designed, fabricated and tested successfully. Figure 1 shows the chip micrograph of a concurrent dual-band CMOS LNA implemented in a 0.35mm BiCMOS technology using only CMOS transistors operating at 2.45GHz and 5.25GHz frequency bands. A concurrent dual-band receiver using a novel image-rejection scheme for two bands and using the concurrent dual-band LNA is designed and fabricated (Figure 2). This chip is currently under measurement.

Figure 1	Figure 2

Future Work. We plan to fabricate newer versions of the receiver chip with more integrated building blocks targeting even lower power consumption with a high performance. Concurrent multi-band operation of large-signal building blocks such as power amplifiers for transmitter chain and oscillators pose many interesting problems to be solved. From a theoretical standpoint we are investigating a more general theory of concurrent operation of multiple (more than two) frequency bands. Practical communication schemes to achieve more diversity using a concurrent receiver is also under study.

Publications/References
Concurrent Dual-Band CMOS Low Noise Amplifiers and Receiver Architectures. H. Hashemi and A. Hajimiri. In: Dig. VLSI Circuits Symp., pp 247-250, June 2001.

Concurrent Multi-Band Low-Noise-Amplifiers: Theory, Design, and Applications. H. Hashemi and A. Hajimiri. To be published in MTT Special issue on Silicon-Based RF and Microwave Integrated Circuits.

Concurrent Multi-Band Low Noise Amplifier Architecture. H. Hashemi and A. Hajimiri. Patent Pending.