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DS-UWB vs. 802.11n: What's the best connectivity option?

Posted: 12 Sep 2005 ?? ?Print Version ?Bookmark and Share


By Matt Welborn, Freescale Semiconductor

Wireless Net DesignLine

Wireless connectivity for consumer electronics and computing devices is fundamentally changing the way people use these devices. As the penetration of wireless technologies increases, however, the desire is not just to "connect," but to do so at ever higher data rates. The question for designers is, what wireless connectivity approach best meets my application?

Increased memory size in portable devices, larger files sizes for digital pictures and videos, and higher resolution pictures and video for consumer multimedia applications have all been boosting requirements for higher wireless data rates, even at shorter ranges. At the same time, device size and battery life continue to require that portable wireless devices provide low-power operation while supporting increased data rates.

These evolving consumer electronic applications can be clustered into two distinct groups with different device requirements: (1) wireless in-room video distribution (compressed or uncompressed) and (2) high-speed connectivity for low-power handheld devices. Video distribution applications are characterized by a need for relatively high data rates with multiple users, robust performance and a low sensitivity to power requirements (since video sources and displays are often plugged into external power supplies). In contrast, handheld applications impose stringent requirements for low cost and power consumption, with the additional requirement for scalability to extremely high data rates (1 Gbit/s and higher) for high-speed data transfer.

There are essentially two wireless technologies that are available to address these growing application needs: direct sequence ultra-wideband (DS-UWB), which is the leading proposal for UWB standardization by the IEEE, and 802.11 WLAN technology (802.11a/g and extension 802.11n under development). Although each of these technologies is part of a larger, multi-layer communications system, this analysis will focus on handheld applications and the important aspects of the physical layer (PHY) of each approach. In this comparison, we will see that the distinctive differences between DS-UWB and WLAN technologies lead to different solutions and different levels of power efficiency as these two technologies are scaled to meet the requirements for high-data-rate handheld applications.

Understanding the two technologies

DS-UWB is initially being developed for wireless personal area network (WPAN) applications. The DS-UWB proposal under consideration by the IEEE will enable 802.15.3a standard devices to provide both the high performance capabilities as well as the low power/cost scalability required for high-data-rate multimedia and handheld applications.

DS-UWB devices will operate under the UWB rules established by the FCC for the U.S. and under similar rules now under development in other parts of the world. The current FCC rules allow 7.5GHz of spectrum (typical DS-UWB devices will occupy either 1.5GHz or 3GHz of spectrum at any given time), but at very lower transmit power levels (Figure 1). In fact, DS-UWB emission levels will be at or below levels allowed for spurious or out-of-band emissions for other wireless technologies, roughly equivalent to a limit of -41.3dBm per MHz of spectrum. Because of these low emission limits, a typical DS-UWB device will have a total transmit power of about 1/10mW, or about -10dBm, across its entire signal bandwidth.

Figure 1: The relative power spectral densities and bandwidths of various wireless technologies. Notice that UWB technology is limited to emissions of less than -41.3dBm/MHz within the allowable operating band of 3.1GHz to 10.6GHz.

In contrast to DS-UWB, 802.11 WLAN technology has been developed for operation under different FCC rules, and operates in frequency bands specifically set aside for unlicensed wireless devices. Operating bandwidths for 802.11a/g/n are narrower than for DS-UWB, with about 17MHz of occupied spectrum (or about 80 times less bandwidth than DS-UWB, or 40 times less for 11n systems using 34MHz), and transmit power levels are much higher. An 802.11 device has a nominal transmit power of about 50mW. Note that this level of average transmit power is about 500 times the average power emitted by a DS-UWB device, or a difference of about 27dB.

These different specifications for signal bandwidth and transmit power lead to fundamental differences in how effectively these two technologies can scale to meet the higher data rate requirements for the latest power-sensitive handheld applications. In the end, it is the significant difference in bandwidth that will drive many of the fundamental design and performance trade-offs that define these two technologies.

Transmit power and signal bandwidth

So, what accounts for the large difference in transmit power between DS-UWB and WLAN? A first guess might be range capability. One basic rule of thumb is that received signal power falls off inversely with the range squared (at close range). Does this mean that 802.11 technology has v(500) or about 22 times the range for equivalent data rates? Not really; in fact, DS-UWB technology provides 10m or better range in multipath environments for 110Mbps. This is probably about the same range that 802.11a/g technology provides for its top rate of 54Mbps. The 802.11n extensions will also provide 100Mbps or more at roughly these same ranges, but specific range projections depend on antenna and multipath assumptions and are not available at this time.

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