【正文】 one or more have to be given up if the others are to do well. Original wireless systems were built to bridge large distances in order to link two parties together. However, recent history of radio shows a clear trend toward improving on the other four attributes at the expense of distance. Cellular telephony is the most obvious example, covering distances of 30 kilometers to as little as 300 meters. Shorter distances allow for spectrum reuse, thereby serving more users, and the systems are practical because they are supported by an underlying wired infrastructure– the telephone network in the case of cellular. In the past few years, even shorter range systems, from 10 to 100 meters, have begun emerging, driven primarily by data applications. Here, the Internet is the underlying wired infrastructure, rather than the telephone network. Many expect the bination of shortrange wireless and wired Internet to bee a fastgrowing plement to next generation cellular systems for data, voice, audio, and video. Four trends are driving shortrange wireless in general and ultrawideband in particular:1. The growing demand for wireless data capability in portable devices at higher bandwidth but lower in cost and power consumption than currently available.2. Crowding in the spectrum that is segmented and licensed by regulatory authorities in traditional ways.3. The growth of highspeed wired access to the Internet in enterprises, homes, and public spaces.4. Shrinking semiconductor cost and power consumption for signal processing. Trends 1 and 2 favor systems that offer not just highpeak bit rates, but high spatial capacity1 as well, where spatial capacity is defined as bits/sec/squaremeter. Just as the telephone network enabled cellular telephony, Trend 3 makes possible highbandwidth, inbuilding service provision to lowpower portable devices using shortrange wireless standards like Bluetooth () and IEEE (). Finally, Trend 4 makes possible the use of signal processing techniques that would have been impractical only a few years ago. It is this final trend that makes UltraWideband (UWB) technology practical. When used as intended, the emerging short and mediumrange wireless standards vary widely in their implicit spatial capacities. For example : has a rated operating range of 100 meters. In the ISM band, there is about 80MHz of useable spectrum. Hence, in a circle with a radius of 100 meters, three 22MHz IEEE systems can operate on a noninterfering basis, each offering a peak overtheair speed of 11Mbps. The total aggregate speed of 33Mbps, divided by the area of the circle, yields a spatial capacity of approximately 1,000 bits/sec/squaremeter. , in its lowpower mode, has a rated 10meter range and a peak overtheair speed of 1Mbps. Studies have shown that approximately 10 Bluetooth “piconets” can operate simultaneously in the same 10meter circle with minimal degradation yielding an aggregate speed of 10Mbps [3]. Dividing this speed by the area of the circle produces a spatial capacity of approximately 30,000 bits/sec/squaremeter. is projected to have an operating range of 50 meters and a peak speed of 54Mbps. Given the 200MHz of available spectrum within the lower part of the 5GHz UNII band, 12 such systems can operate simultaneously within a 50meter circle with minimal degradation, for an aggregate speed of 648Mbps. The projected spatial capacity of this system is therefore approximately 83,000 bits/sec/squaremeter. systems vary widely in their projected capabilities, but one UWB technology developer has measured peak speeds of over 50Mbps at a range of 10 meters and projects that six such systems could operate within the same 10meter radius circle with only minimal degradation. Following the same procedure, the projected spatial capacity for this system would be over 1,000,000 bits/sec/squaremeterCurrent low datarate Wireless Local Area Networks (WLANs) and Wireless Personal Area Networks (WPANs), which have data rates of ~110Mbps, are typically used for applications such as packetswitched data and cordless voice telephony, using Time Division Multiple Access (TDMA) voice circuits. Example technologies supporting these applications are the IEEE (WiFi)*, Bluetooth?, and HomeRF* networking standards. As the IEEE and ETSI BRAN HiperLAN/2* standards (the European equivalent of ) have added physical layer specifications with raw data rates up to 54Mbps, the application space is enlarging to include audio/video applications that are enabled by these higher data rates. These diverse traffic types all have different requirements in terms of the service parameters that quantify the network performance for a user of each of those applications. Thus, for example, voice telephony and video teleconferencing applications place tough demands on the latency and jitter performance. Audio/video applications require large amounts of bandwidth and may need close synchronization (., connecting stereo speakers in a surround sound system). UltraWideband (UWB) systems, with their potential for extremely large data rates over short distances, are naturally going to be used for networking these kinds of highbandwidth/delaycritical data sources and sinks. Hence, it would be natural to look at the approaches to the MAC design undertaken in these other standards when considering the MAC layer design for UWB systems. The most important functions of the MAC layer for a wireless network include controlling channel access, maintaining Quality of Service (QoS), and providing security.。外文文獻(xiàn)原文UltraWideband Technology for Shortor MediumRangeWireless CommunicationsJeff Foerster, Intel Architectur