Novel Radio Architectures for UWB, 60 GHz, and Cognitive Wireless Systems

Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2006, Article ID 17957, Pages 1–18 DOI 10.1155/WCN/2006/17957 Novel Radio Architectures for UWB, 60 GHz, and Cognitive Wireless Systems Danijela Cabric, Mike S. W. Chen, David A. Sobel, Stanley Wang, Jing Yang, and Robert W. Brodersen Berkeley Wireless Research Center, University of California, Berkeley, CA 94704, USA Received 18 October 2005; Revised 17 January 2006; Accepted 19 January 2006 There are several new radio systems which exploit novel strategies being made possible by the regulatory agencies to increase the availability of spectrum for wireless applications. Three of these that will be discussed are ultra-wideband (UWB), 60 GHz, and cognitive radios. The UWB approach attempts to share the spectrum with higher-priority users by transmitting at power levels that are so low that they do not cause interference. On the other hand, cognitive radios attempt to share spectra by introducing a spectrum sensing function, so that they are able to transmit in unused portions at a given time, place, and frequency. Another approach is to exploit the advances in CMOS technology to operate in frequency bands in the millimeter-wave region. 60 GHz operation is particularly attractive because of the 7 GHz of unlicensed spectrum that has been made available there. In this paper, we present an overview of novel radio architecture design approaches and address challenges dealing with high-frequencies, widebandwidths, and large dynamic-range signals encountered in these future wireless systems. Copyright © 2006 Danijela Cabric et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION The demand for wireless connectivity and crowding of unlicensed spectra has pushed the regulatory agencies to be ever more aggressive in providing new ways to use spectra. In the past, the approach for spectrum allocation was based on specific band assignments designated for a particular service, as illustrated by the Federal Communications Commission’s (FCC) frequency allocation chart. This spectrum chart contains overlapping allocations in most frequency bands and seems to indicate a high degree of spectrum scarcity. While spectrum efficiency of some radio systems is improving (e.g., cell phone and WiFi bands), they are faced with increasing interference that limits network capacity and scalability. On the other hand, some bands are poorly utilized. Measurements taken in downtown Berkeley (Figure 1) reveal a typical utilization of roughly 30% below 3 GHz, and 0.5% in the 3–6 GHz frequency band. In order to promote more flexibility in spectrum sharing, the FCC has provided new opportunities for unlicensed spectrum usage with fewer restrictions on radio parameters. Three new opportunities in spectrum access have thus been introduced: (1) an underlay approach with severe restrictions on transmitted power levels with a requirement to operate over “ultra-” wide bandwidths (UWB); (2) an opening of 7 GHz of unlicensed spectrum at millimeter-wave frequencies (around 60 GHz) where oxygen absorption limits long-distance interference; (3) an overlay approach based on avoidance of higher-priority users through the use of spectrum sensing (cognitive radios). The potential opening of these new spectra introduces new opportunities for vastly more wireless connectivity. As indicated in Table 1, these three radio system are (or should be) allowed to operate in 500 MHz or wider spectrum. Therefore, the design of highthroughput radios with 100 Mb/s to even 1 Gb/s data rates is achievable at moderate-to-low spectrum efficiencies. The power limitations and wireless channel propagation characteristics for these bands dictate the range capability which extends from 1 m to 10 km, so that a wide variety of communication modes can be supported with these three new wireless radio technologies. This regulatory shift also has major implications on radio architectures since traditional narrowband radio design techniques are not applicable. Spectrum sharing required in UWB and CR over wide bands implies frequency agility and significant dynamic range improvements of radio front-ends. In addition, new radio functions are required which involve high sensitivity sensing and modulation schemes robust to strong interferers and low signal-to-noise regimes. Interference avoidance through operation at microwave frequencies 2 EURASIP Journal on Wireless Communications and Networking Table 1: Potential system-level specifications consistent with FCC regulations and IEEE standards where they exist. Cognitive radios∗ do not have an allocation at this time. Systems UWB radio (UWB) 60 GHz radio (60 GHz) Cognitive radio∗ (CR) Spectrum access Carrier Bandwidth Data rates Spectrum efficiency Range Underlay [0–1], [3–10] GHz > 500 MHz 100–500 Mb/s ∼ 0.1–1 b/s/Hz 1–10 m Unlicensed [57–64] GHz > 1 GHz > 1 Gb/s ∼ 1 b/s/Hz ∼ 10 m Overlay [0–1], [3–10] GHz > 500 MHz ∼ 10–1000 Mb/s ∼ 0.1–10 b/s/Hz 1 m–10 km architectures, signal processing techniques, and analog circuits. A low-complexity impulse radio architecture, together with its building blocks, will be given as an example of these new opportunities. Measured power (dB) −100 −110 −120 2.1. −130 −140 −150 0 1 2 3 4 Frequency (Hz) 5 ×109 Figure 1: Spectrum utilization measurement (0–6 GHz). introduces challenges in RF circuit implementation to ensure that the eventual solution is cost-effective. In this paper, we present major opportunities and challenges of this new era in CMOS radio design, focusing on the three radio systems outlined in Table 1. Radio architectures which address the unique new requirements of these radios will be discussed including the analog and digital circuit partitioning, and the issues involved in antenna, RF, mixedsignal, and digital circuits. 2. UWB RADIOS In 2002, the FCC released the use of ultra-wideband (UWB) transmission in several frequency bands (0–960 MHz, 3.1– 10.6 GHz, and 22–29 GHz) with an effective isotropic radiation power (EIRP) below −41.3 dBm/MHz and requiring operation at larger than 500 MHz signal bandwidth [1]. The large bandwidth enables short-range high-datarate communication and the possibility to perform highresolution positioning. The new challenge for UWB radio implementation is to fully exploit the wideband nature for lower power and a less costly solution than by increasing the efficiency of narrowband techniques such as occurring in the standard 802.11n. A new opportunity using nonsinusoidal carriers, so-called impulse radios, has allowed designers to take a fundamentally new approach to radio Low-complexity impulse radio architecture The most discussed application of UWB is for short-range, high-speed, indoor communications. Two competing approaches (...truncated)