31-03-2011, 04:48 PM
PRESENTED BY:
S.sravya
T.Sandhya
[attachment=11448]
Abstract:
Ultra-Wideband (UMB) technology is loosely defined as any wireless transmission scheme that occupies a bandwidth of more than 25% of a center frequency, or more than 1.5GHz. The Federal Communications Commission is currently working on setting emissions limits that would allow UWB communication systems to be deployed on an unlicensed basis following the Part 15.209 rules for radiated emissions of intentional radiators, the same rules governing the radiated emissions from home computers, for example. This rule change would allow UWB-enabled devices to overlay existing narrowband systems, which is currently not allowed, and result in a much more efficient use of the available spectrum.
A breakdown, of how this paper is organized: The first section looks at UWB technology from the high-level perspective of how this technology compares with other current and future wireless alternatives. Next, we describe the current state of the regulatory process, where UWB transmissions are under consideration for being made legal on an unlicensed basis. Then, some implementation advantages of UMB systems are discussed that distinguish UWB transceiver architectures from more conventional “narrowband” systems. After this, we illustrate the throughput vs. distance characteristics for an example UWB system. Finally, we conclude with a summary of the benefits of UWB and suggest some future challenges that are currently being investigated.
Introduction:
Ultra-Wideband (UWB) technology has been around since the 1980s, but it has been mainly used for radar-based applications until now, because of the wideband nature of the signal that results in very accurate timing information. However, due to recent developments in high-speed switching technology, UWB is becoming more attractive for low-cost consumer communications applications (as detailed in the “Implementation Advantages” section of this paper).
Although the term Ultra-Wideband (UMB) is not very descriptive, it does help to separate this technology from more traditional “narrowband” systems as well as newer “wideband” systems typically referred to in the literature describing the future 3G cellular technology. There are two main differences between UMB and other “narrowband” or “wideband” systems. First, the bandwidth of UMB systems, as defined by the Federal Communications Commission (FCC), is more than 25% of a center frequency or more than 1.5GHz. Clearly, this bandwidth is much greater than bandwidth used by any current technology for communication. Second, UWB is typically implemented in a carrier-less fashion. Conventional “narrowband” and “wideband” systems use Radio Frequency (RF) carriers to move the signal in the frequency domain from baseband to the actual carrier frequency where the system is allowed to operate. Conversely, UWB has a very sharp rise and fall time, thus resulting in a waveform that occupies several GHz of bandwidth. Although there are other methods for generating a UWB waveform (using a chirped signal, for example), in this paper, we focus on the impulse-based UWB waveform -- due to its simplicity.
The high data rates afforded by UWB systems will tend to favor applications such as video distribution and/or video teleconferencing for which Quality of Service (QoS) will be very important. So, in addition to describing the physical layer attributes of UWB systems, it’s important to keep in mind the Medium Access Control (MAC) layer as well. Therefore, we have devoted a section to describing the current mechanisms that exist to support the required QoS for these high-rate applications.
WIRELESS ALTERNATIVES
In order to understand where UWB fits in with the current trends in wireless communications, we need to consider the general problem that communications systems try to solve. Specifically, if wireless were an ideal medium, we could use it to send
1. a lot of data,
2. very far,
3. very fast,
4. for many users,
5. all at once.
Unfortunately, it is impossible to achieve all five attributes simultaneously for systems supporting unique, private, two-way communication streams; 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 page 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 distance 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 combination of short-range wireless and wired Internet to become a fast-growing complement to next-generation cellular systems for data, voice, audio, and video. Four trends are driving short-range wireless in general and ultra-wideband 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 high-speed 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 high-peak bit rates, but high special capacity as well, where spatial capacity is defined as bits/sec/square-meter. Just as the telephone network enabled cellular telephony, Trend 3 makes possible high-bandwidth, in-building service provision to low-power portable devices using short range wireless standards like Bluetooth and IEEE 802.11. 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 Ultra-Wideband (UWB) technology practical.
When used as intended, the emerging short- and medium- range wireless standards vary widely in their implicit spatial capacities. For example:
• IEEE 802.11b has a rated operating range of 100 meters. In the 2.4GHz ISM band, there is about 80MHz of useable spectrum. Hence, in a circle with a radius of 100 meters, three 22MHz IEEE 802.11b systems can operate on a non-interfering basis, each offering a peak over-the-air speed of 11Mbps. The total aggregate speed of 33Mbps, divided by the area of the circle, yields a spatial capacity of approximately 1000 bits/sec/square-meter.
• Bluetooth, in its low-power mode, has a rated 10-meter range and a peak over-the-air speed of 1Mbps. Studies have shown that approximately 10 Bluetooth “piconets” can operate simultaneously in the same 10-meter circle with minimal degradation yielding an aggregate speed of 10Mbps. Dividing this speed by the area of the circle produces a spatial capacity of approximately 30,000 bits/sec/square-meter.
• IEEE 802.11a 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 U-NII band, 12 such systems can operate simultaneously within a 50-meter circle with minimal degradation, for an aggregate speed of 648Mbps. The projected spatial capacity of this system is therefore approximately 83,000 bits/sec/square-meter.
• UWB systems vary widely in their projected capabilities, but on UWB technology developer has measured peak speed of over 50Mbps at a range of 10 meters and projects that six such systems could operate within the same 10-meter radius circle with only minimal degradation. Following the same procedure, the projected spatial capacity for this system would be over 1000000 bits/sec/square-meter.
As shown in Figure 1, other standards now under development in the Bluetooth Special Interest Group and IEEE 802 working groups would boost the peak speeds and spatial capacities of their respective systems still further, but none appear capable of reaching that of UWB. A plausible reason is that all systems are bound by the channel capacity theorem, as shown in Figure 2. Because the upper bound on the capacity of a channel grows linearly with total available bandwidth, UWB systems, occupying 2GHz or more, have greater room for expansion than systems that are more constrained by bandwidth.
Thus, UWB systems appear to have great potential for support of future high-capacity wireless systems. However, there are still several important challenges ahead for this technology before it can be realized. Not the least of these challenges is finding a way to make the technology legal without causing unacceptable interference to other users that share the same frequency space. This is addressed in the next section.
REGULATORY AND STANDARDS ISSUES:
The Federal Communications Commission (FCC) is in the process of determining the legality of Ultra-Wideband (UWB) transmissions. Due to the wideband nature of UWB emissions, it could potentially interfere with other licensed bands in the frequency domain if left unregulated. It’s a fine line that the FCC must walk in order to satisfy the need for more efficient methods of utilizing the available spectrum, as represented by UWB, while not causing undo interference to those currently occupying the spectrum, as represented by those users owning licenses to certain frequency bands. In general, the FCC is interested in making the most of the available spectrum as well as trying to foster competition among different technologies.
The FCC first initiated a Notice of Inquiry (NOI) in September of 1998, which solicited feedback from the industry regarding the possibility of allowing UWB emissions on an unlicensed basis following power restrictions described in the FCC Part l5 rules. The FCC Part l5 rules place emission limits on intentional and unintentional radiators in unlicensed bands. These emission limits are defined in terms of microvolts per meter (uV/m), which represent the electric field strength of the radiator. In order to express this in terms of radiated power, the following formula can be used. The emitted power from a radiator is given by the following:
Where Eo represents the electric field strength in terms of V/m, R is the radius of the sphere at which the field strength is measured, and η is the characteristic impedance of a vacuum where η = 377 ohms. For example, the FCC Part 15.209 rules limit the emissions for intentional radiators to 500u V/m measured at a distance of 3 meters in a 1MHz bandwidth for frequencies greater than 960MHz. This corresponds to an emitted power spectral density of -41.3dBm/MHz.
In May of 2000, the FCC issued a Notice of Proposed Rule Making (NPRM), which solicited feedback from the industry on specific rule changes that could allow UWB emitters under the Part l5 rules. More than 500 comments have been filed since the first NOI, which shows significant industry interest in this rule-making process. Figure 3 below shows how the current NPRM rules would limit UWB transmitted power spectral density for frequencies greater than 2GHz.
The FCC is considering even lower spectral density limits below 2GHz in order to protect the critical Global Positioning System (GPS) even more, but currently no upper boundary has been defined. Results of a National Telecommunications and Information Administration (NTIA) report analyzing the impact of UWB emissions on GPS, which operate at 1.2 and 1.5GHx, was recently published and suggests that an additional 20-35dB greater attenuation, beyond the power limits described in the FEE Part l5.209, may be needed to protect the GPS band. However, placing proper spectral density emission limits in the bands that may need additional protection wile still allow UWB systems to be deployed in a competitive and useful manner while not causing an unacceptable amount of interference on other useful services sharing the same frequency space. This report, and others, will be carefully considered by the FCC prior to a final ruling.
S.sravya
T.Sandhya
[attachment=11448]
Abstract:
Ultra-Wideband (UMB) technology is loosely defined as any wireless transmission scheme that occupies a bandwidth of more than 25% of a center frequency, or more than 1.5GHz. The Federal Communications Commission is currently working on setting emissions limits that would allow UWB communication systems to be deployed on an unlicensed basis following the Part 15.209 rules for radiated emissions of intentional radiators, the same rules governing the radiated emissions from home computers, for example. This rule change would allow UWB-enabled devices to overlay existing narrowband systems, which is currently not allowed, and result in a much more efficient use of the available spectrum.
A breakdown, of how this paper is organized: The first section looks at UWB technology from the high-level perspective of how this technology compares with other current and future wireless alternatives. Next, we describe the current state of the regulatory process, where UWB transmissions are under consideration for being made legal on an unlicensed basis. Then, some implementation advantages of UMB systems are discussed that distinguish UWB transceiver architectures from more conventional “narrowband” systems. After this, we illustrate the throughput vs. distance characteristics for an example UWB system. Finally, we conclude with a summary of the benefits of UWB and suggest some future challenges that are currently being investigated.
Introduction:
Ultra-Wideband (UWB) technology has been around since the 1980s, but it has been mainly used for radar-based applications until now, because of the wideband nature of the signal that results in very accurate timing information. However, due to recent developments in high-speed switching technology, UWB is becoming more attractive for low-cost consumer communications applications (as detailed in the “Implementation Advantages” section of this paper).
Although the term Ultra-Wideband (UMB) is not very descriptive, it does help to separate this technology from more traditional “narrowband” systems as well as newer “wideband” systems typically referred to in the literature describing the future 3G cellular technology. There are two main differences between UMB and other “narrowband” or “wideband” systems. First, the bandwidth of UMB systems, as defined by the Federal Communications Commission (FCC), is more than 25% of a center frequency or more than 1.5GHz. Clearly, this bandwidth is much greater than bandwidth used by any current technology for communication. Second, UWB is typically implemented in a carrier-less fashion. Conventional “narrowband” and “wideband” systems use Radio Frequency (RF) carriers to move the signal in the frequency domain from baseband to the actual carrier frequency where the system is allowed to operate. Conversely, UWB has a very sharp rise and fall time, thus resulting in a waveform that occupies several GHz of bandwidth. Although there are other methods for generating a UWB waveform (using a chirped signal, for example), in this paper, we focus on the impulse-based UWB waveform -- due to its simplicity.
The high data rates afforded by UWB systems will tend to favor applications such as video distribution and/or video teleconferencing for which Quality of Service (QoS) will be very important. So, in addition to describing the physical layer attributes of UWB systems, it’s important to keep in mind the Medium Access Control (MAC) layer as well. Therefore, we have devoted a section to describing the current mechanisms that exist to support the required QoS for these high-rate applications.
WIRELESS ALTERNATIVES
In order to understand where UWB fits in with the current trends in wireless communications, we need to consider the general problem that communications systems try to solve. Specifically, if wireless were an ideal medium, we could use it to send
1. a lot of data,
2. very far,
3. very fast,
4. for many users,
5. all at once.
Unfortunately, it is impossible to achieve all five attributes simultaneously for systems supporting unique, private, two-way communication streams; 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 page 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 distance 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 combination of short-range wireless and wired Internet to become a fast-growing complement to next-generation cellular systems for data, voice, audio, and video. Four trends are driving short-range wireless in general and ultra-wideband 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 high-speed 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 high-peak bit rates, but high special capacity as well, where spatial capacity is defined as bits/sec/square-meter. Just as the telephone network enabled cellular telephony, Trend 3 makes possible high-bandwidth, in-building service provision to low-power portable devices using short range wireless standards like Bluetooth and IEEE 802.11. 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 Ultra-Wideband (UWB) technology practical.
When used as intended, the emerging short- and medium- range wireless standards vary widely in their implicit spatial capacities. For example:
• IEEE 802.11b has a rated operating range of 100 meters. In the 2.4GHz ISM band, there is about 80MHz of useable spectrum. Hence, in a circle with a radius of 100 meters, three 22MHz IEEE 802.11b systems can operate on a non-interfering basis, each offering a peak over-the-air speed of 11Mbps. The total aggregate speed of 33Mbps, divided by the area of the circle, yields a spatial capacity of approximately 1000 bits/sec/square-meter.
• Bluetooth, in its low-power mode, has a rated 10-meter range and a peak over-the-air speed of 1Mbps. Studies have shown that approximately 10 Bluetooth “piconets” can operate simultaneously in the same 10-meter circle with minimal degradation yielding an aggregate speed of 10Mbps. Dividing this speed by the area of the circle produces a spatial capacity of approximately 30,000 bits/sec/square-meter.
• IEEE 802.11a 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 U-NII band, 12 such systems can operate simultaneously within a 50-meter circle with minimal degradation, for an aggregate speed of 648Mbps. The projected spatial capacity of this system is therefore approximately 83,000 bits/sec/square-meter.
• UWB systems vary widely in their projected capabilities, but on UWB technology developer has measured peak speed of over 50Mbps at a range of 10 meters and projects that six such systems could operate within the same 10-meter radius circle with only minimal degradation. Following the same procedure, the projected spatial capacity for this system would be over 1000000 bits/sec/square-meter.
As shown in Figure 1, other standards now under development in the Bluetooth Special Interest Group and IEEE 802 working groups would boost the peak speeds and spatial capacities of their respective systems still further, but none appear capable of reaching that of UWB. A plausible reason is that all systems are bound by the channel capacity theorem, as shown in Figure 2. Because the upper bound on the capacity of a channel grows linearly with total available bandwidth, UWB systems, occupying 2GHz or more, have greater room for expansion than systems that are more constrained by bandwidth.
Thus, UWB systems appear to have great potential for support of future high-capacity wireless systems. However, there are still several important challenges ahead for this technology before it can be realized. Not the least of these challenges is finding a way to make the technology legal without causing unacceptable interference to other users that share the same frequency space. This is addressed in the next section.
REGULATORY AND STANDARDS ISSUES:
The Federal Communications Commission (FCC) is in the process of determining the legality of Ultra-Wideband (UWB) transmissions. Due to the wideband nature of UWB emissions, it could potentially interfere with other licensed bands in the frequency domain if left unregulated. It’s a fine line that the FCC must walk in order to satisfy the need for more efficient methods of utilizing the available spectrum, as represented by UWB, while not causing undo interference to those currently occupying the spectrum, as represented by those users owning licenses to certain frequency bands. In general, the FCC is interested in making the most of the available spectrum as well as trying to foster competition among different technologies.
The FCC first initiated a Notice of Inquiry (NOI) in September of 1998, which solicited feedback from the industry regarding the possibility of allowing UWB emissions on an unlicensed basis following power restrictions described in the FCC Part l5 rules. The FCC Part l5 rules place emission limits on intentional and unintentional radiators in unlicensed bands. These emission limits are defined in terms of microvolts per meter (uV/m), which represent the electric field strength of the radiator. In order to express this in terms of radiated power, the following formula can be used. The emitted power from a radiator is given by the following:
Where Eo represents the electric field strength in terms of V/m, R is the radius of the sphere at which the field strength is measured, and η is the characteristic impedance of a vacuum where η = 377 ohms. For example, the FCC Part 15.209 rules limit the emissions for intentional radiators to 500u V/m measured at a distance of 3 meters in a 1MHz bandwidth for frequencies greater than 960MHz. This corresponds to an emitted power spectral density of -41.3dBm/MHz.
In May of 2000, the FCC issued a Notice of Proposed Rule Making (NPRM), which solicited feedback from the industry on specific rule changes that could allow UWB emitters under the Part l5 rules. More than 500 comments have been filed since the first NOI, which shows significant industry interest in this rule-making process. Figure 3 below shows how the current NPRM rules would limit UWB transmitted power spectral density for frequencies greater than 2GHz.
The FCC is considering even lower spectral density limits below 2GHz in order to protect the critical Global Positioning System (GPS) even more, but currently no upper boundary has been defined. Results of a National Telecommunications and Information Administration (NTIA) report analyzing the impact of UWB emissions on GPS, which operate at 1.2 and 1.5GHx, was recently published and suggests that an additional 20-35dB greater attenuation, beyond the power limits described in the FEE Part l5.209, may be needed to protect the GPS band. However, placing proper spectral density emission limits in the bands that may need additional protection wile still allow UWB systems to be deployed in a competitive and useful manner while not causing an unacceptable amount of interference on other useful services sharing the same frequency space. This report, and others, will be carefully considered by the FCC prior to a final ruling.
