Brandon HellerAbstract:A number of recent advances in the physical layer are enabling wireless communications devices to reach ever-greater speeds. Thesedevelopments in modulation techniques, coding, and radio architecture enable better spectrum allocation, more power-efficient, error-resistant transmissions, and more flexible architectures for receiving signals. Many of the advances are being included in upcomingstandards, like 802.16 and 802.11n, and will likely improve the already-fast adoption of wireless networking equipment.See Also: A Review of Key Wireless Physical Layer Concepts | Wireless Networking: Issues and TrendsTable of Contents:1. Introduction2. Advanced Modulation Techniques2.1 Ultrawideband2.1.1 IEEE 802.15.3a2.1.2 WiMedia Alliance2.1.3 UWB Forum2.2 Orthogonal Frequency-Division Multiplexing (OFDM)3. Multi-Antenna Techniques3.1 Multiple Input Single Output (MISO)3.2 Multiple Input Multiple Output (MIMO)3.3 Space-time Codes4. Turbo Codes5. Radio ArchitectureSummaryReferencesList of AcronymsKeywordsNetworking, wireless physical layer, gigabit, signal transmission, signal coding, information theory, radio architecture, ultrawideband,UWB, OFDM, MIMO, space-time block codes, STBC, turbo codesDescriptionCovers recent developments in wireless physical layer, including modulation techniques, multi-antenna techniques, coding, and radioarchitecture.1.0 IntroductionRecent advances allow smarter, more complex transmitters and receivers to pack more bits into the same spectrum, by using a widerslice of spectrum (ultrawideband), a more narrow set of frequency bands (orthogonal frequency-division multiplexing), moredirectional use of spectrum (smart antennas), and more intelligent encoding of transmissions (coding). In addition to higher bit rates,more flexible radio architectures are now out there (software-defined radio). These developments provide wireless users a betterexperience, including greater range, better bit rates, greater battery life, and increased reliability. Upcoming standards will integratethese techniques, yielding better wireless devices that are getting closer to gigabit speeds.Section 2 discusses advanced modulation techniques. Section 3 describes different ways to use multiple antennas to achieve additionalThe Quest for Wireless Gigabit: Recent Advances in the Wireless Physica... file:///F:/www/cse574-06/ftp/phy_trends/index.html1 of 10 11/27/2013 2:10 AMthroughput or reduced bit-error rates. Section 4 describes options for more reliably encoding data. Section five discusses recentadvances in radio architecture, specifically software-defined radios.2.0 Advanced Modulation TechniquesModulation techniques concern the way spectrum is used to deliver a wireless signal from receiver to transmitter. Here, we discussultrawideband (UWB), Orthogonal Frequency-Division Multiplexing (OFDM), and the commercial status and implications of thesetechnologies.2.1 UltrawidebandUltrawideband (UWB) is a wireless transmission technology fundamentally different from other radio technologies. With UWB, data isrepresented by impulses, rather than carrier modulation. The Defense Advanced Research Projects Agency (DARPA), a primefinancial supporter of UWB research, defines UWB as any wireless technology where the signal is 25% or more of the frequency used[McCorkle02]. UWB signals can spread over a spectrum range of up to 7 GHz, while carrier-based systems might have a 40 MHz orsmaller bandwidth. Time-modulation pulse-position modulation and pulse polarity-modulation pulse-amplitude modulation are twomodulation techniques used [WikipediaUWB]. UWB applications thus far have focused on extremely high-speed, shorter distancepersonal area networks (PANs). UWB is also called impulse, baseband, zero-carrier or modulated-wavelet technology.UWB proponents note its potential for increased signal quality at reduced transmission power. The pulse duration of a UWBtransmission, often as short as 1 ns, enables the transmitter to be on for a brief period of time, saving battery life in energy-constraineddevices. Obstructions that prevent narrowband signals from reaching their intended targets are inconsequential to UWB, as itswideband nature ensures at least some part of the signal will pass through most obstacles. The extremely wide signal resists fading andjamming, while saving additional power through reduced retransmissions and higher coding density. UWB systems consume very littlepower, around one ten-thousandth of that of cell phones. [Geier03] The simple nature of pulse-based transmissions, combined with thefalling costs of silicon, may lead to highly integrated UWB transceivers that are cheaper than equivalent carrier-based radios, due to thereduced need for complex analog modulation circuitry. These purported cost benefits have yet to be seen, primarily due to a lack ofeconomies of scale.Early UWB implementations encountered significant opposition from military and civil services who were worried about interferenceeffects from UWB. Aviation, fire, police, and rescue services use radios in which the UWB signal overlaps their narrow spectrum. Thisoverlap could create a safety conflict if the extra noise due to UWB were significant enough to those emergency service transmissions.The FCC evaluated the issue, decided it was not a threat, and passed a resolution in Feb 2002 enabling permission for low-power use ofthe 3.1 to 10.6 GHz spectrum area [Paulson03]. All UWB radios with FCC approval right now are considered to operate under thenoise floor of carrier-based radios. In the near future, the FCC is expected to reevaluate their UWB findings for higher-powertransmissions. The lack of FCC approval for high-power and long-range applications, as well as competing industry standards, maydelay the technology's eventual application.UWB has been primarily seen as a high-speed replacement for slow Bluetooth Personal Area Networks (PANs). An initial target is 480Mbps, vs the 1-2 Mbps currently specified for Bluetooth devices. An IEEE task group, 802.15.3a, was created to standardize thisapplication, but was dissolved in January 2006 after failing to unite the two largest industry players [Griffith06].2.1.1 IEEE 802.15.3aTask Group 3a (TG 3a), the IEEE 802.15.3a working group, was created in mid 2003 with the hopes of uniting competing wirelessstandards and producing a high-speed Bluetooth replacement. The working group did have an initial success in reducing the number ofcompeting wireless
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