Wireless USB Uses Ultra Wideband (UWB) for High Data Rate

By Gary Breed Editorial Director

n May 24, 2005, –41.3 dBm/MHz A consortium of high-profile the Wireless USB Radiation Limit (FCC) companies is supporting OPromoter Group standards and product announced that its development for wireless Wireless USB specifica- USB port interconnections tion had been completed. With a standard in place, the final development of wireless USB compo-

nents is well underway and consumer prod- Amplitude ucts are expected to appear as early as the 2005 Christmas shopping season. 3432 MHz 3960 MHz 4488 MHz Wireless USB Organizations Each Band is 528 MHz Wide There are three groups involved in the Figure 1 · Wireless USB uses Band Group 1 development of wireless USB: within the UWB spectrum approved by the The Wireless USB Promoter Group—This FCC. Additional UWB Band Groups, which lie group consists of Agere Systems, Hewlett above these frequencies, may be used in Packard, Intel, Microsoft, NEC, Philips the future. Semiconductors and Samsung. Now that they have developed the Wireless USB specification, their role is to encourage companies to Packard, Intel, Kodak, Microsoft, Nokia, adopt the specification in component and Philips, Samsung Electronics, Sony, equipment development. STMicroelectronics, Staccato Communica- USB Implementers Forum, Inc. [1]—The tions, Texas Instruments and Wisair. USB-IF is the governing body established to oversee the Wireless USB specification. This Inside the Specification non-profit organization promotes the new The Wireless USB specification [3] defines specification, providing a logo and compliance a short-range (approximately 10-meter range) program for USB products. wireless replacement for the present wired WiMedia Alliance [2]—This group is the USB interface. With a data rate of 480 Mb/s at developer and promoter of a multiband 3 meters, the wireless system is essentially orthogonal frequency-division multiplexing the same as USB 2.0. At 10 meters, the data (MB-OFDM) radio platform for UWB. This rate is estimated to be 110 Mb/s, subject to transmission method is part of the Wireless local conditions that affect transmission. USB specification. Early this year, the group The UWB radio portion of the specification merged with the MultiBand OFDM Alliance uses the spectrum approved by the FCC in Special Interest Group (MBOA-SIG), the orig- 2002 that includes three 528 MHz wide chan- inator of this UWB transmission method. nels centered at 3.432, 3.960 and 4.488 GHz WiMedia’s board includes Alereon, Hewlett (Figure 1). The FCC limit (indoor) for UWB

58 High Frequency Electronics High Frequency Design SHORT-RANGE WIRELESS emissions is –41.3 dBm/MHz. received or transmitted. Using FFT often connected to signal traces. The architecture of Wireless USB techniques, a single-frequency can be Designers must scrupulously avoid is a classic hub-and-spoke, where the eliminated (“notched”) with minimal creating stubs. For example, bias or host (the user’s PC) communicates effect on the transmission quality. sampling resistors should connect directly with up to 127 USB devices. directly to the trace without using a Communications with each device is Wireless USB Radio Design “tee” junction that introduces a major scheduled to simplify the manage- Most of the Wireless USB radio discontinuity in the signal transmis- ment of communications with multi- will be incorporated into a single sion path. ple devices. CMOS integrated circuit. While this Also, the underlying ground plane The physical layer (PHY)—the will reduce an engineer’s time for should be continuous, since it is part radio portion and its interfaces—is radio design, significant issues still of the transmission line. Gaps and defined by the MB-OFDM specifica- must be addressed, related to UWB’s slots in the ground plane beneath the tion [4]. The PHY supports data rates wide bandwidth and operation at traces introduce resonances and of 53.3, 106.7 and 200 Mb/s (required) microwave frequencies. impedance “bumps” that can greatly and 80, 160, 320, 400 and 480 Mb/s The bandwidth, along with the affect the integrity of the signal. (optional). Wireless USB hosts must time domain nature of the UWB sig- Finally, areas around connectors support the same data rates for both nal, requires careful board layout. or the antenna attachment point are transmit and receive. Even with a reference design from potential trouble spots, since the The PHY defines all hardware the IC manufacturer, there are a mechanical transition can also result interfaces, such as the types of con- number of subtle layout factors that in electrical discontinuity. A common nectors and pin function assign- can be overlooked, especially by engi- problem is separating parallel con- ments, as well as the radio functions, neers who may be more familiar with ductors, creating an open space or including modulation type, frequen- digital design than microwave cir- “loop.” Like other discontinuities, cies of operation, power control, and cuits. these loops can cause reflections, other characteristics that must be Ideally, circuit board traces will be radiation or changes in frequency part of the baseband-TX-propaga- impedance-matched to the devices, response of the circuit. Maintain con- tion-RX-baseband chain. maintaining that impedance with ductor sizes and spacing up to the A typical OFDM transmit chain is consistent manufacturing tolerances connection point, as close as possible. shown in Figure 2. Note that, as a and proper microwave layout. time-domain based transmission Balanced circuits are often used for Summary method, UWB uses inverse-FFT in applications like this, so it is essen- Wireless USB is an excellent fit the transmitter and FFT in the tial to maintain consistent spacing of for UWB technology—short range, receiver. These processes have the the traces and symmetry with high data rate and low cost. But its additional benefit of easily handling respect to the surrounding circuitry. wide bandwidth and operation at narrowband interference—either Bias resistors and test points are microwave frequencies requires knowledgeable design for its imple- mentation. Input Convolutional Bit Scrambler Puncturer Data Encoder Interleaver References 1. USB Implementers Forum, Inc. information is available at www.usb.org. 2. WiMedia Alliance information is available at www.wimedia.org. 3. The Wireless USB specification, IFFT Constellation Insert Pilots & DAC errata and additional information Mapping Guard Intervals can be downloaded at www.usb.org/ wusb. 4. Multiband OFDM Physical exp(j2πf t) c Layer Specification (Revision 1.0), Interleaving Kernel January 14, 2005, WiMedia Alliance. 5. Dave Leeper, Intel Corp., “Overview of MB-OFDM,” WiMedia Figure 2 · A typical UWB MB-OFDM transmit chain for Wireless USB [5]. Alliance, 2005.

60 High Frequency Electronics