UWB simplifies portable design
UWB simplifies portable design
By John McCorkle, CTO, Xtreme Spectrum, Vienna, Va., EE Times
November 8, 2002 (12:16 p.m. EST)
Because ultrawideband (UWB) communications rely on an unusual wireless technology and suit a wide range of products, designing UWB into a system raises a number of questions. With a 7.5-GHz-wide transmission spectrum, is interference an issue? What sort of antenna is appropriate? Since UWB targets consumer products, what sort of interface makes sense on a UWB media access controller (MAC)?
Unlike conventional amplitude- or frequency-modulated radios, UWB relies on extremely short pulses (from 10 to 1,000 picoseconds) that encode information by varying the pulse timing, amplitude or shape. The relatively simple digital techniques used to generate these pulses eliminate the need for RF/IF conversion stages and expensive surface acoustic wave (SAW) filters. The pulses are transmitted at a power level so minuscule (less than 75 nano-watts/MHz of bandwidth) that the signal looks like low-level background noise to other wireless technologies.
Despite the low power, UWB's data-carrying capacity is enormous. Today's products offer data rates as high as 100 Mbits/second, and next-generation products will scale higher than 500 Mbits/s-ideal for multimedia-rich applications. In fact, UWB is expected to be used primarily in consumer-oriented products, including many types that have never before utilized wireless communications-for example, digital cameras, DVD players, a variety of display products and set-top boxes. Typical applications will include interfacing a set-top box or residential gateway to multiple displays or interfacing multiple video players (DVD, DVR and home theater equipment) to a single display.
Unlike computer products such as laptops, consumer products have widely varying sizes, shapes, power supplies (battery or line power), price points and interface requirements. Some designers will thus be dealing with unfamiliar RF issues, while designers familiar with RF designs will find UWB unusually simple. For both groups of desig ners, this article addresses design-in challenges, including appropriate applications and security; hardware issues, such as the MAC implementation and interfaces; and RF issues, such as antenna configurations.
One of the most important concepts to remember when applying ultrawideband is that it is a short-range communications technology. With a range of approximately 10 meters at 100 Mbits/s, it serves well as a way to connect products within a single room.
Ultrawideband technology could be pushed to expand the radio's range, but the signal processing required would increase complexity and power consumption. In its current form, UWB has about the same range, cost and power consumption as Bluetooth but offers 100 times the data rate.
In addition to the higher throughput, the IEEE 802.15.3 draft MAC standard specifically aims to provide the ease of use, quality of service (QoS) and security required by streaming-media applications. These provisions are cr ucial for satisfying consumers. While data networks tolerate occasional dropped packets, consumers will not put up with a system that sporadically blanks the TV screen for even a fraction of a second. Extreme multipath effects, the nemesis of in-building communications systems, are no excuse, even when people are moving about the room reflecting radio waves in every direction.
The 802.15.3 MAC overcomes this design-in challenge with features that handle streaming data in a highly deterministic way. The features include guaranteed time slots and jitter limits. Additionally, because multipath resolution improves as RF bandwidth increases, UWB deals with the multipath challenge by resolving multiple reception paths to within a few inches, allowing the MAC to choose the best of a multitude of paths.
Security is another MAC issue that concerns both consumers and intellectual property owners, so security issues include both device authentication and data protection. 802.15.3 authentication uses pub lic keys for encryption/decryption, where each device has a public/private key pair. The standard supports three public key suites (RSA-OAEP, NTRUEncrypt and ECMQV), each with different characteristics that will suit particular applications.
While the MAC issues discussed so far must be addressed within the MAC, systems designers must also deal with the MAC interface. Most wireless networking chips have been used in computing environments where the MAC interface has been obvious: the Peripheral Component Interconnect (PCI) bus. but in the consumer product world where UWB will be applied, the choice is less clear. Depending on the product, you might want to use USB 2.0, Fast Ethernet, 1394, PCI or a proprietary interface.
The UWB MAC must either directly support your chosen interface or provide a versatile interface that you can adapt to different needs. Otherwise, you will have to choose a different interface. UWB chip vendors will undoubtedly support a wide range of standard interfaces for t heir MACs, so the choices will expand as time goes on.
One of the most obvious challenges to designing-in a communications link with a 7.5-GHz-wide spectrum is dealing with RF interference. The question looms particularly large if your design also includes a technology such as 802.11a, which transmits its highest power range squarely in the middle of UWB's frequency allocation. FCC rules limit UWB transmissions to a profile that resembles the unintentional emissions of a PC, thus eliminating the potential for an unlicensed UWB transmitter to interfere with other radio products.
From the point of view of a UWB transceiver, emissions from other RF sources are long compared with the extremely short UWB pulse bursts. The long-duration signals from other sources are thus fairly easy to distinguish, even when the desired signals are of relatively low amplitude. The UWB radio can use adaptive methods (as well as non-adaptive methods) to avoid being interfered with. Careful RF-driven layout rules can allow a single product such as a PDA to include UWB along with other wireless devices (GPS, PCS, 802.11a).
Antenna design challenge
Another RF challenge is the antenna design. While wide-bandwidth types of antennas are well-understood in other applications -notably, TV-UWB devices require an antenna with a flat group delay, so that the high- and low-frequency signal components arrive at the receiver simultaneously. Also, the UWB antenna must be small enough to fit products such as digital cameras and camcorders. An appropriate antenna configuration should be part of a UWB chip set's reference design.
Anytime a wireless link is considered, power consumption, cost, size and design complexity become potential design-in challenges. A UWB transceiver needs no external passive components and no digital signal processing (DSP). Eliminating the DSP results in major cost and power savings. As a result, UWB suits a wide variety of price-sensitive, battery-powered products. A complete first -generation UWB subsystem fits easily into a CompactFlash format.
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