02-05-2011, 09:56 AM
Presentation
Dogan Hakan Caner
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ULTRA WIDEBAND(UWB) TECHNOLOGY
Basic Concepts
As the name implies UWB, ultra wide band technology, is a form of transmission that occupies a very wide bandwidth. Typically this will be many Gigahertz, and it is this aspect that enables it to carry data rates of Gigabits per second.
The fact that UWB transmissions have such a wide bandwidth means that they will cross the boundaries of many of the currently licensed carrier based transmissions. As such one of the fears is that UWB transmission may cause interference.
However the very high bandwidth used also allows the power spectral density to be very low, and the power limits on UWB are being strictly limited by the regulatory bodies.
In many instances they are lower than the spurious emissions from electronic apparatus that has been certified. In view of this it is anticipated that they will cause no noticeable interference to other carrier based licensed users. Nevertheless regulatory bodies are moving forward cautiously so that users who already have spectrum allocations are not affected.
This technology has gained much interest during the last few years as a potential candidate for future wireless short-range data communication.
Characteristics of UWB
UWB has a number of features which make it attractive for consumer communications applications. In particular, UWB systems
have potentially low complexity and low cost;
have a noise-like signal spectrum;
are resistant to severe multipath and jamming;
have very good time-domain resolution allowing for location and tracking applications.
Two UWB, ultra wideband technologies
Despite the single named used for the ultra wideband (UWB) transmissions, there are two very different technologies being developed.
Carrier free direct sequence ultra wideband technology(DS-UWB) or impulse radio(IR UWB): This form of ultra-wideband technology transmits a series of impulses. In view of the very short duration of the pulses, the spectrum of the signal occupies a very wide bandwidth.
MBOFDM, Multi-Band OFDM ultra wideband technology: This form of ultra wideband technology uses a wide band or multiband orthogonal frequency division multiplex (MBOFDM) signal that is effectively a 500 MHz wide OFDM signal. This 500 MHz signal is then hopped in frequency to enable it to occupy a sufficiently high bandwidth.
To achieve these requirements the FCC(Federal Communications Commission) has mandated that UWB, ultra wideband transmissions can legally operate in the range 3.1 GHz up to 10.6 GHz, at a limited transmit power of -41dBm/MHz.
Additionally the transmissions must occupy a bandwidth of at least 500 MHz, as well as having a bandwidth of at least 20% of the centre frequency.
To achieve this last requirement, a transmission with a centre frequency of 6 GHz, for example, must have a bandwidth of at least 1.2 GHz. Consequently, UWB provides dramatic channel capacity at short range that limits interference.
The fact that very low power density levels are transmitted means that the interference to other services will be reduced to limits that are not noticeable to traditional transmissions.
Additionally the lowest frequencies for UWB, ultra wideband have been set above 3 GHz to ensure they do not cut across bands currently used for GPS, cellular and many other services.
UWB Spectrum
UWB Technology DS-UWB
DS UWB, direct sequence format for ultra wideband is often referred to as an impulse, baseband or zero carrier technology. It operates by sending low power Gaussian shaped pulses which are coherently received at the receiver.
In view of the fact that the system operates using pulses, the transmissions spread out over a wide bandwidth, typically many hundreds of Megahertz or even several Gigahertz. This means that it will overlay the bands and transmissions used by more traditional channel based transmissions.
Each of the DS UWB pulses has an extremely short duration. This is typically between 10 and 1000 picoseconds, and as a result it is shorter than the duration of a single bit of the data to be transmitted.
The short pulse duration means that multipath effects can usually be ignored, giving rise to a large degree of resilience in ultra wideband UWB transmissions when the signal path is within buildings.
Energy Density of DS-UWB
In view of the wide bandwidth over which the DS UWB transmissions are spread, the actual energy density is exceedingly low.
Typically a DS UWB transmitter might transmit less than 75 nanowatts per Megahertz. When integrated over the total bandwidth of the transmission, it means that transmissions may only be around 0.25 milliwatts.
This is very small when compared to 802.11 transmissions that may be between 25 and 100 mW, or Bluetooth that may be anywhere between 1 mW and 1 W.
This very low spectral density means that the DS UWB transmissions do not cause harmful interference to other radio transmissions using traditional carrier based techniques and operating in the existing bands
DS-UWB Modulation
The most common two DS-UWB modulation techniques are Pulse Position Modulation and Bi-Phase Modulation(or BPSK)
PPM : This technique uses time shift of regularly timed pulses for two modulation states. Any two distinct modulation states can encode binary information. More than two states can be used.
BPM : is modulation of the pulse polarity. BPM can not define more than two states but it has some advantages. BPM is antipodal modulation method, whereas PPM, when separated by one pulse width delay for each pulse position, is an orthogonal modulation method. Therefore, BPM has theoretically the 3dB gain in power efficiency. If PPM delays by one pulse width, then BPM can send twice number of pulses and, twice the information.
Other modulation techniques which are be used in DS-UWB modulation. These are PAM, OOK and OPM;
PAM(Pulse Amplitude Modulation) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. PAM is not the preferred modulation method for most short-range communication. AM signal which has smaller amplitude is more susceptible to noise than that with larger amplitude.
OOK (On Off Keying) is a modulation technique where the presence or absence of a pulse represents pair of modulation states. It makes difficult to determine the absence of a pulse.
OPM(Orthogonal Pulse Modulation): is special modulation method for UWB systems. OPM is not only the data modulation method, but it is also the multiple access method. OPM is based on the set of pulse shapes, which are orthogonal to each other. Each pulse shape corresponds to one of the modulation states.
UWB Technology MB-OFDM UWB
MB-OFDM UWB transmits data simultaneously over multiple carriers spaced apart at precise frequencies. Fast Fourier Transform algorithms provide nearly 100% efficiency in capturing energy in a multi-path environment, while only slightly increasing transmitter complexity.
Although a wide band of frequencies could be used from a theoretical viewpoint, certain practical considerations limit the frequencies that are normally used for MB-OFDM UWB. Based on existing CMOS technology geometries, use of the spectrum from 3.1GHz to 4.8GHz is considered optimal for initial deployments.
Bandplan for MB-OFDM UWB
Multiband OFDM
• Multiband OFDM: leading proposal for high-rate UWB
• First-generation: use three 528 MHz bands in 3.1–4.8 GHz
• Data rates between 55 and 480 Mbps
• Frequency hopping (simultaneously operating piconets)
• WLAN band can be avoided
Transmitter and Receiver Architectur
Block diagram of the UWB impulse radio concept by time domain corporation.
UWB Technology-Antennas
There are different types of UWB antennas. They are categorized into the following classes according to form and function:
Frequency dependent antennas: The log-periodic antenna is an example of this type of antennas where the smaller scale geometry of antenna contri-butes to higher frequencies and the larger scale part contributes to the lower frequencies.
Small-element antennas: These are small, omni-directional antennas for commercial applications. Examples of this type of antennas are bow-tie or diamond dipole antennas.
Horn antennas: Horn antennas are electromagnetic funnels that concentrate energy in a specific direction. These antennas have large gains and narrow beams. The Horn antennas are bulkier than small-element antennas.
Reflector antenna: These antennas are high gain antennas that radiate energy in a particular direction. They are relatively large but easy to adjust by manipulating the antenna feed. Hertz’s parabolic cylinder reflector is an example of this type of antennas.
Performance Analysis and Comparison
UWB shows significant throughput potential at short range
UWB against BLUETOOTH, ZIGBEE and WI-FI PROTOCOLS
Emission Limits for Indoor Communication and Measurement Applications
Equipment must be designed to ensure that operation can only occur indoors or it must consist of hand- held devices that may be employed for such activities as peer- to-peer operation.
Operate in 3.1 – 10.6 GHz band
Emission Limits for Outdoor Communication and Measurement Applications
Equipment must be hand-held
Operate in 3.1 – 10.6 GHz band
UWB standardization in the IEEE
IEEE 802.15 : Wireless Personal Area Network (WPAN)
IEEE 802.15.1 : Bluetooth, 1Mbps
IEEE 802.15.3 : WPAN/high rate, 50Mbps
IEEE 802.15.3a: WPAN/Higher rate, 200Mbps, UWB
IEEE 802.15.4 : WPAN/low-rate, low-power, mW level, 200kbps
UWB Applications : WPAN
UWB Applications
Positioning, Geolocation, Localization
High Multipath Environments
Obscured Environments
Communications
High Multipath Environments
Short Range High Data Rate
Low Probability of Intercept/ Interference
Radar/Sensor : MIR (motion detector, range-finder, etc.)
Military and Commercial: Asset Protection
Anti-Terrorist/Law Enforcement
Rescue Applications
With the growing level of wireless communications, ultra wide band UWB offers significant advantages in many areas. One of the main attractions for WAN / LAN applications is the very high data rates that can be supported. With computer technology requiring ever increasing amounts of data to be transported, it is likely that standards such as 802.11 and others may not be able to support the data speeds required in some applications. It is in overcoming this problem where UWB may well become a major technology of the future.
Conclusion
The key benefits of UWB include:
High data rates
Low power consumption
Multipath immunity
Low costs
Simultaneous ranging and communication
However, there are still challenges in making this technology live up to its full potential. The regulatory process is still in motion. Several companies in the wireless market are involved in helping the FCC identify emission limits favorable to UWB systems. These limits allow the systems to be competitive within the marketplace, while, at the same time, prohibiting them from causing an unacceptable level of interference for other wireless services sharing the same frequency band.
FCC regulators are in the beginning phase of assessing the usability of the UWB technology, and it is anticipated that standardization will be needed in the future to help make this technology widely available in the consumer market.
