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Ultra-Wideband is a high data rate, low power short-range wireless technology, as a high-speed alternative to existing wireless technologies such as IEEE 802.11 WLAN, HomeRF, and HiperLANs.. The FCC liberated the airwaves and laid the regulatory framework for the unlicensed use of approved UWB devices, UWB system is constrained to have a maximum power transmission of -41 dBm over the 3.1 - 10.6 GHz bands. UWB co-exists and does not interfere with the existing narrowband communication systems in the same spectrum. However, due to its low power in the same bandwidth, UWB is affected by narrowband (NB) interference. Technical challenges in developing UWB wireless systems, including UWB channel characterization, transceiver design, coexistence and interworking with other narrowband wireless systems, design of the link and network layers to benefit from UWB transmission characteristics. This paper is to provide an overview of UWB communications.
Keywords-component: UWB, UWB Transmitter, UWB Receiver, LAN, WPAN, Wireless Communication, FCC.
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. UWB does help to separate this technology from more traditional "narrowband" systems as well as newer "wideband" systems. There are two main differences between UWB and other "narrowband" or "wideband" systems. First, the bandwidth of UWB systems, as defined by the Federal Communications Commission (FCC) in, is more than 25% of a center frequency or more than 1.5GHz. Clearly, this bandwidth is much greater than the 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 implementations can directly modulate an "impulse" that 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), A very useful attribute of UWB technology is its ability to perform precision geo-location, established standards like Bluetooth and 802.11a/b/g. In this paper, we look at UWB technology, briefly introduce other related wireless standards such as 802.11 , 802.15.3  Bluetooth , HomeRF  and HIPERLAN  and the regulatory and standards issues in FCC. Some implementation advantages of UWB systems are discussed that distinguish UWB transceiver architectures from more conventional "narrowband" systems.
The growing demands for wireless data capability in portable devices at higher bandwidth but lower in cost and power consumption are requirement of available technologies. Crowding in the spectrum that is segmented and licensed by regulatory authorities in traditional ways. The growth of high-speed wired access to the Internet in enterprises, homes, and public spaces. Shrinking semiconductor cost and power consumption for signal processing in emerging short- and medium range wireless standards vary widely in their implicit spatial capacities.
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 1,000 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. 
HIPERLAN/1 and HIPERLAN/2:
HIPERLAN/1 and HIPERLAN/2 are wireless LAN (WLAN) standards developed by European Telecommunications Standards Institute (ETSI). HIPERLAN/1 is a wireless equivalent of Ethernet while HIPERLAN/2 has architecture based on wireless Asynchronous Transfer Mode (ATM). Both the standards use dedicated frequency spectrum at 5 GHz. HIPERLAN/1 provides a gross data rate of 23.5 Mb/s and net data rate of more than 18 Mb/s while HIPERLAN/2 provides gross data rates of 6/16/36/54 Mb/s and a maximum of 50 Mb/s net data rate. Both standards use 10/100/1000 mW of transmit power and have a maximum range of 50 m. Also, the standards provide isochronous and asynchronous services with support for QoS. However, they have different channel access and modulation schemes. 
The emerging draft standard defines MAC and PHY (2.4 GHz) layer specifications for a Wireless Personal Area Network (WPAN). The standard is based on the concept of a piconet which is a network confined to a 10 m personal operating space (POS) around a person or object. A WPAN consists of one or more collocated pico nets. Each pico net is controlled by a pico net coordinator (PNC) and may consist of devices (DEVs). The 802.15.3 PHY is defined for 2.4 to 2.4835 GHz band and has two defined channel plans. It supports five different data rates (11 to 55 Mb/s). The base uncoded PHY rate is 22 Mb/s.
Home RF working group was formed to develop a standard for wireless data communication between personal computers and consumer electronics in a home environment. The HomeRF standard is technically solid, simple, secure, and is easy to use. HomeRF networks provide a range of up to 150 ft typically enough for home networking. HomeRF uses Shared Wireless Access Protocol (SWAP) to provide efficient delivery of voice and data traffic. SWAP uses a transmit power of up to 100 mW and a gross data rate of 2 Mb/s. It can support a maximum of 127 devices per network. A SWAP-based system can work as an ad-hoc network or as a managed network using a connection point. 
UWB 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 10-meter 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/squaremeter.
As shown in Figure1, 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 Figure2. Because the upper bound on
Figure1 Spatial capacity comparison between IEEE
802.11, Bluetooth, and UWB
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.
What is Ultra Wideband?
UWB radio is a revolutionary communications mechanism that uses high frequency microwave pulses for transmitting digital data over a wide spectrum of frequency bands with very low power intensity. It can transmit data at very high rates (for wireless local area network applications) and very low rates (for telemetry applications).
Figure2: UWB waveform
Within the power limit allowed under the current FCC regulations, UWB radios can carry large amounts of data over a short distance, at very low power. In addition, it has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and at higher power levels. At higher power levels, UWB signals can travel significantly greater ranges. Instead of transmitting traditional sine wave signals, UWB radio broadcasts digital pulses timed very precisely on a signal across a very wide spectrum. The transmitter and receiver must be coordinated to send and receive pulses with an accuracy of trillionths of a second. Very high-resolution radars and precision (sub-centimeter) radio location systems can also use UWB technology. The effect of transmitting high speed pulses instead of sine waves gives UWB transmissions a degree of immunity to multipath fading. Multipath fading is the constructive and destructive interference created by multiple reflections of the same signal being received simultaneously. This favorable multipath fading capability makes UWB technology well suited for applications in environments that would otherwise suffer from multipath fading with sine wave transmissions. UWB devices work inside the same increasingly crowded radio frequencies that many other systems use. They send out short electromagnetic pulses that last half a billionth of a second, followed by pauses that are perhaps 200 times that length. By spreading the pulses over a wide area of the spectrum (roughly 1 GHz), the devices use extremely low power and little total bandwidth. Many supporters of UWB technology envision applications such as home security and personal-area networks that activate home appliances when the user nears them. Police and fire departments are already trying out devices that can detect people behind walls.
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 15 rules. The FCC Part 15 rules place emission limits on intentional and unintentional radiators in unlicensed bands. These emission limits are defined in terms of microvolt per meter (uV/m), which represent the electric field strength of the radiator. In order to express this in terms of radiated powers (terms that are better understood by communications engineers), the following formula can be used. The emitted power from a radiator is given by the following:
Figure3: Power Spectral density limits in current NPRM
P = E024Ï€R2/Î· (1)
where E0 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 Î· € is the characteristic impedance of a vacuum where Î·€ = 377 ohms. For example, the FCC Part 15.209 rules limit the emissions for intentional radiators to 500uV/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 15 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 1.5GHz, was recently published and suggests that an additional 20-35dB greater attenuation, beyond the power limits described in the FCC Part 15.209, may be needed to protect the GPS band. However, placing proper spectral density emission limits in the bands that may need additional protection will 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. 
The main concern regarding UWB emissions is the potential interference that they could cause to the "incumbents" in the frequency domain as well as to specific critical wireless systems that provide an important public service (for example, GPS). There are many factors which affect how UWB impacts other "narrowband" systems, including the separation between the devices, the channel propagation losses, the modulation technique, the Pulse Repetition Frequency (PRF) employed by the UWB system, and the receiver antenna gain of the "narrowband" receiver in the direction of the UWB transmitter. For example, a UWB system that sends impulses at a constant rate (the PRF) with no modulation causes spikes in the frequency domain that are separated by the PRF. Adding either amplitude modulation or time dithering (i.e., slightly changing the time the impulses are transmitted) results in spreading the spectrum of the UWB emission to look more flat. As a result, the interference caused by a UWB transmitter can be viewed as a wideband interferer, and it has the effect of raising the noise floor of the "narrowband" receiver. 
There are three main points to consider when looking at this type of interference. First, if UWB follows the Part 15 power spectral density requirements, its emissions are no worse than other devices regulated by this same standard, which include computers and other electronic devices. Second, interference studies need to consider "typical usage scenarios" for the interaction between UWB and other devices. Using a "worst case" analysis may result in too great a restriction on UWB and could prevent a promising new technology from becoming viable. Third, FCC restrictions are only a beginning. Further coordination through standards participation may be necessary to come up with coexistence methods for operational scenarios that are important for the industry. For example, if UWB is to be used as a Personal Area Network (PAN) technology in close proximity to an 802.11a Local Area Network (LAN), then the UWB system must be designed in such a manner as to peacefully coexist with the LAN. This can be achieved through industry involvement and standards participation, as well as careful designs. 
Figure 3 illustrates two other important considerations for UWB systems. First, UWB emissions will be allowed only at a much lower transmit power spectral density compared to other "narrowband" services. This low power can be seen as both a limitation and a benefit. It restricts UWB emissions to relatively short distances, but results in a very power-efficient and low-cost implementation, which preserves battery life. Second, Figure 3 also shows that UWB systems will most likely suffer from interference from
other "narrowband" users. For the most flexible solution, these interferers should be suppressed only on an as-needed basis, thus requiring some sort of adaptive interference suppression technique, which is the subject of research currently within the Intel® Architecture Labs (IAL). People familiar with the FCC process suggest that rules governing UWB emissions could be finalized as soon as June or July or as late as December of 2001.
UWB communications systems
Figure4: Block Diagram of Basic Communication System
Figure 4 shows a general model of a single-link communication system. It includes three major blocks of communication, viz., a transmitter, the channel, and a receiver. 
The input data to the transmitter is the message to be sent from the source to the destination. The main function of
the transmitter is to send out the message to the channel, which is done with the help of an antenna. An antenna is a means for radiating (transmitting antenna) or receiving (receiving antenna) radio waves. Data modulation is the systematic variation of some properties of the carrier signal such as amplitude, phase, or frequency according to the message signal. There are several reasons for modulating a message using a carrier: a) ease of radiation, b) to reduce noise and interference, and c) for transmission of several messages over a single channel. Besides these, other functions of a transmitter are mixing, filtering and amplification [refer to Figure 5(a)]. 
An ideal mixer produces an output consisting of the sum and difference frequencies of its two input signals (one signal from the modulated signal and the other from the local oscillator). In the transmitter, filter after the mixer
Figure 5 (a): Block Diagram of Typical Narrow Band Transmitter
filter out (will not allow) the lower frequency components of the two frequencies signals also known as frequency up-
conversion. As we know, the higher the power of the signal, farther destinations it can reach. The signal is amplified using a power amplifier before sending it out to the channel from the transmitting antenna. The channel is the medium through which the transmitted data reaches the destination or receiver. [refer to Figure 5(b)] 
Figure 5 (b): Block Diagram of UWB Transmitter
When the transmitted data passes through the channel, other unwanted effects also come into picture. For instance, it picks up noise from the surrounding area and due to reflection and refraction of the signal from various obstacles on the way of the signal, the transmitted signal from the transmitter will be received at different times in different versions at the receiver, which is also known as multipath. Sometimes the signal from the transmitter could be completely blocked by the obstacles or black out. 
The duty of the receiver is to extract the desired message from the received signal from the receiving antenna. The receiver signal may be extremely weak, so it needs to be amplified first with the help of a lownoise noise amplifier (LNA) as illustrated in Figure. 6(a).
Figure 6 (a): Block diagram of typical narrowband receivers
The use of employing LNA is to amplify the desired signal and not amplify the noise component of the signal. If we use a power amplifier in the receiver, both the noise and signal will be amplified equally. We don't want this to happen, since noise is an unwanted component of the signal. The filter next to LNA also filters out the noisy components of the received signal. Then the signal is passed through the mixer, which will give us two frequencies of the signal as described before in the transmitter. But the filter after the mixer will filter out (will block) the higher frequency component and this process is known as down-conversion. The main function of the receiver is to demodulate the signal to get the message sent by the transmitter. After data demodulation, we can get the message sent from the source.
Figure 6 (b): Block diagram of UWB Receiver
UWB communications are fundamentally different from all other communication techniques because it employs extremely narrow radio frequency (RF) pulses, which are generated from the UWB pulse generator, to communicate between transmitters and receivers. Utilizing short-duration pulses as the building blocks for communications directly generates a very wide bandwidth, as we know that a very short signal in time domain produces a very wide spectrum signal in frequency domain from Fourier analysis. A significant difference between traditional radio transmissions and UWB radio transmissions is that traditional transmissions transmit information by varying the power/frequency/and/or phase of a sinusoidal wave also known as modulation. 
UWB transmissions can transmit information by generating radio energy at specific time instants and occupying large bandwidth thus enabling a pulse position or time-modulation. Information can also be imparted (modulated) on UWB signals (pulses) by encoding the polarity of the pulse, the amplitude of the pulse, and/or also by using orthogonal pulses. UWB pulses can be sent sporadically at relatively low pulse rates to support time/position modulation, but can also be sent at rates up to the inverse of the UWB pulse bandwidth. 
Pulse-UWB systems have been demonstrated at channel pulse rates in excess of 1.3 giga-pulses per second, supporting forward error correction encoded data rates in excess of 675 Mbit/s. Such a pulse-based UWB method using bursts of pulses is the basis of the IEEE 802.15.4a draft standard and working group, which has proposed UWB as an alternative physical layer. UWB transmission is carrier-less, meaning that data is not modulated on a continuous waveform with a specific carrier frequency, as in narrowband and wideband technologies. Carrier-less transmission requires fewer RF components than carrier based transmission as shown in Figure 5(b). The transmission of low-powered pulses eliminates the need for a power amplifier (PA) in UWB transmitters. Also, because UWB transmission is carrier less, there is no need for mixers and local oscillators to translate the carrier frequency to the required frequency band; consequently there is no need for a carrier recovery stage at the receiver end. 
In multicarrier-based UWB systems, usually orthogonal frequency- division multiplexing (OFDM) is employed. In multiband UWB systems, instead of using the whole UWB spectrum of 7.5 GHz as a single band, it can be divided into 15 subbands of 500 MHz or more. Then, we can use OFDM techniques to transmit the information using orthogonal carriers in each subband. One obvious advantage of a multiband scheme is avoid sending signals in those frequency regions where a radio communication device is present. Disadvantages of a multiband scheme is that it requires sophisticated signal processing techniques 
APPLICATION AREAS FOR UWB
Imaging and Printing
In terms of data transfer in computing applications, UWB has an opportunity to supplant wired USB 2.0 for bandwidth-intensive imaging and printing applications.
Radar in Automotive Industry
A compelling application for UWB is radar in the automotive industry. It is ideally suited for collision avoidance, detecting the movement and location of objects near a vehicle, improving airbag activation and suspension settings. UWB devices will have to support a wide range of automotive operating temperature and failure rate. 
Applications such as ground penetrating radar (GPR), through-wall surveillance, appear attractive given today's focus on detection, but are best handled by established systems companies. 
Applications involving the tracking of children, personnel, equipment and inventory, to an accuracy of less than one inch, are attractive, especially as UWB can work indoors (factories, shopping malls), unlike GPS. 
In sensor networks, where data rates are low (< 1 Mbit/s), data transmission with a very high rate (more than 100 Mbit/s). Example: HDTV (high de¬nition television)
Another promising application is biological imaging, e.g., for cancer detection .
Wireless Home Networks
Typically, a wireless home network should provide connection among various electronic consumer devices such as PC, MP3 player, digital camera, printer, scanner, High-Definition TV (HDTV) and video game console. popular usage of home networking is sharing data from PC to PC and from PCs to peripherals. With increased customer demand for home control, multiplayer gaming, and video distributions, significant efforts are being invested in building solutions around UWB enabled home networks.
Future generations of communication systems will require high mobility, flexibility, and very high data-rate. In this context, broadband wireless digital communications is inevitable. As the capacity of the channel is directly related to its bandwidth, UWB technologies are advantageous for use in a number of wireless communication applications, especially in short range high data rate networking. Impulse radio, a form of UWB spread-spectrum signaling, has properties that make it a viable candidate for short-range communications in dense multipath environments. There are many advantages of UWB communication systems over conventional communication systems including:
High data rate and very low power:
Very low power secure communications:
Very low power operation, low-cost, and minimal hardware:
Multiple access communications: Due to large bandwidth, UWB-based radio multiple access communication system can accommodate many users.CDMA is a widely accepted multiple access communication system and the two most popular methods of CDMA are: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS).
Resolvable multipath components of UWB signals: so most signal reflections do not overlap the original pulse, and thus the traditional multipath fading of narrow band signals does not exist.
Since multipath reflections of UWB signals are resolvable, there is a potential for obtaining diversity gain by combining them.
Localization of radio signals indoors is difficult because of the presence of shadowing and multipath reflections from walls and objects. The fine time resolution required by UWB signals makes them ideal for high resolution positioning applications.
Such systems have some inherent problems due to the huge spectrum (7.5 GHz bandwidth from 3.1-10.6 GHz) allotted by the FCC. Some of the disadvantages of this promising technology include:
Interference: Since UWB communication devices occupy a large frequency spectrum, interference mitigation or avoidance with coexisting users is one of the key issues of UWB technology. 
Complex signal processing: For narrowband systems that use carrier frequency, frequency-division multiplexing is very straightforward but for carrier-less transmission and reception, every narrowband signal in the vicinity is a potential interferer and also every other carrier-less system has to rely on relatively complex and sophisticated signal processing techniques to recover data from the noisy environment.
Transmitter and receiver to achieve bit synchronization can be as high as a few milliseconds. So, the channel acquisition time is very high, which can significantly affect performance.
Conclusion and Future Scope
UWB include potential low-cost implementations, low-power consumption due to limits on transmit power spectral density, high throughput afforded by the wide occupied bandwidth, accurate position location that can be combined with communications capabilities, and favorable multi-path fading robustness due to the nature of the short impulse.
However, there are still challenges in making this technology live up to its full potential.