Ultra-Wideband (UWB) radio, originally blessed by the FCC in 2002, has since been implemented in several different forms. This wide-bandwidth wireless technology uses low power to transmit highspeed data to 480 Mbits/s over short distances in the 3.1- to 10.6-GHz range. The most common implementation uses the WiMedia Alliance’s Multiband Orthogonal Frequency- Division Multiplexing (MB-OFDM) standard.
Commercial chips and products have been available for more than a year and are becoming more widely adopted. All of them operate in the 3.1- to 5-GHz range. With new applications emerging, companies now look to use those bands beyond 6 GHz. The challenge, though, lies in the semiconductor processes used. While standard CMOS designs show promise, a better technique may be a CMOS-SiGe (silicon-germanium) combination.
UWB Background in review
The MB-OFDM radio developed by WiMedia uses proven concepts from other OFDM systems, such as 802.11a, 802.11g, 802.16e, and DSL, but applies them at far higher bandwidth and data rates than ever before. As defined in the WiMedia physical-layer (PHY) specification, which details the MB-OFDM architecture, the WiMedia system has an instantaneous bandwidth of 528 MHz and can switch between three separate bands in the 3.1- to 10.6-GHz range at 3.2 MHz (312.5 ns) (Figures 1 and 2).
The OFDM system uses a 128-point fast Fourier transform (FFT) spanning the 528-MHz channel, for a basic resolution of 4.125 MHz per tone. The FCC requires a minimum of 500 MHz for a UWB system, which must operate in the band from 3.1 to 10.6 GHz. Also, the power is constrained to a maximum of −41.3 dBm/MHz when averaged over a 1-ms span, with an instantaneous peak of no more than 0 dBm, per the recent waiver granted by the FCC.
Thus, the WiMedia radio can operate at approximately PT-FFI = −41.3 + 10Log10 (528 × 106) = −14.1 dBm if it’s not performing band switching (called fixed frequency interleaved mode, or FFI) but, because of the averaging rules over the three bands, can increase the output power to PT-FFI = 14.1+10Log10 (3) = −9.3 dBm when switching between bands as shown in Figure 2.
Note: The actual figures are slightly different in the specification because the signal isn’t transmitted during the entire symbol period. There’s an interval of 60.6 ns, shown as a cyclic prefix in Figure 2, that’s actually a zero prefix in the final specification. This "quiet time" of 60.6 ns actually reduces the power on air by about 1 dB, but removes spectral lines introduced by the cyclic prefix.
Due to the nature of UWB, received signals don’t require a high signal-to-noise ratio (SNR). The WiMedia PHY specification only requires a maximum of 4.9 dB of SNR, which occurs at the highest data rate of 480 Mbits/s. The corresponding sensitivity requirement, assuming a modest 6.6-dB noise figure (NF) and 3-dB implementation loss, is −72.7 dBm. Dynamic range, expressed as signal-toquantization noise (SQNR) required in the baseband, is a minimum of 24 dB (4 bits) for data rates from 53.3 to 200 Mbits/s, and 30 dB (5 bits) for data rates beyond 200 Mbits/s up to 480 Mbits/s. Interference protection, analogto- digital converter (ADC) limitations, and other impairments may force a higher number of bits in the ADC.
So, the WiMedia radio needs to operate at low SNR, over a very wide bandwidth, at very high speeds, at low power, and at a low cost to gain wide acceptance in consumer products. What’s the right design approach? This article will discuss whether it’s single-chip CMOS, or perhaps a CMOS chip and SiGe chip in an MCM or SiP.
Regulatory push to higher frequencies
As previously mentioned, most of today’s WiMedia UWB systems operate in the 3- to 5-GHz band. Obviously, this was the lowest-risk and fastest time-tomarket path for UWB. However, the regulatory framework outside the United States has changed the game since development of these first-generation systems.
It’s a different playing field because UWB is an “underlay”—a wireless technology operating at such a low level that it can function in the proximity of other radios without causing harmful interference, thus reusing the spectrum. While this concept was acceptable to the FCC, it’s been difficult to convince incumbents in other geographies because there’s no common agreement on how to quantify “harmful interference.” The same debate rages over the use of “white spaces” in the 700-MHz cellular bands.
There are 14 channels defined for the WiMedia PHY in the U.S., but the usable channels vary according to the regulatory domain (Fig. 3). As of mid-2008, only one channel (band 3) is legal worldwide in band group 1, which limits its usefulness. China has not announced final regulations as of July 2008, although an announcement is expected by the end of the year. The bands shown in Figure 3 are based on a draft that was circulated in late 2006.
Still, even band 3 will be forced to use a technique called “Detect and Avoid” (DAA) by 2010. DAA does what it says: It listens and avoids the channel if it’s already occupied.
Bands above 6 GHz are less restricted, and there are more of them, so the overall capacity of UWB is greatest in the upper bands. In addition, fewer incumbent systems exist in those bands, so regulators have had less opposition from those incumbents as they’ve crafted the rules. Lastly, UWB systems that will be used to implement Bluetooth profiles can only operate above 6 GHz, according to the Bluetooth SIG.
Therefore, for a variety of reasons, UWB systems from now on will have to support bands above 6 GHz. This puts a big burden on UWB chip designers, because Wireless USB and UWB are targeted for consumer devices that demand high-volume, low-cost solutions.
Continued on page 2.
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