2. UWB Technology and Applications

UWB differs substantially from conventional narrowband radio frequency (RF) and spread spectrum technologies(SS), such as Bluetooth Technology and 802.11a/g. UWB uses an extremely wide band of RF spectrum to transmit more data in a given period of time than the more traditional technologies.

Figure1. Comparison of narrowband (NB) ,Spread spectrum (SS), and Ultra-wideband(UWB)

UWB is a unique and relatively new usage of a recently legalized frequency spectrum. UWB radios can use frequencies from 3.1 GHz to 10.6 GHz - a band more than 7 GHz wide. Each radio channel can have a bandwidth of more than 500MHz , depending on its center frequency. To allow for such a large signal bandwidth, FCC( USA s , Federal Communications Commission) put in place severe broadcast power restrictions. By doing so, UWB devices can make use of an extremely wide frequency band while not emitting enough energy to be noticed by narrower band devices nearby , such as 802.11a/g radios. This sharing of data throughput, but they must be with in close proximity.
Strict power limits mean the radios themselves must be low-power consumers.Because of this low power requirements, it is feasible to develop cost-effective CMOS implementations of UWB radios.With the characteristics of low power, low cost, and very high data rates at limited range, UWB is positioned to address the market for a high-speed WPAN. UWB Technology also allows spectrum reuse due to its short range property.
The advantages of UWB can be summarized as:

• Promising high data rates at very low cost
• Very high spatial capacity
• Extremely difficult to intercept or interfere with
• Best for severe multi-path environments
• Low power consumption and small size
• Sharing the bandwidth with other systems
  
• High precition positioning and radar

2.1 Two Ultra Wideband Technologies
Despite the single name used for ultra wideband (UWB) transmissions, there are two different technologies being developed:

• Carrier free direct sequence ultra wideband technology
• Multi-band OFDM ultra wideband Technology
  

These two technologies are pushed by two different industry groups. These groups are, at this time, trying to bring their technologies to market. But neither could communicate with the other. The detail situation of standardizing UWB technology is presented in the next section (section 3). In order to make things more suitable for this assignment, I will give a brief descriptions of the characteristics of each of the above two technologies.

a. Carrier free direct sequence ultra wideband technology:
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.
This technology is known with the following characteristics:

• Based on Direct sequencing technology (discussed next)
  
• Divides the spectrum in to two bands
• Upper band: 6.2- 9.7 GHz
• Lower band: 3.1 - 4.85 GHz
• Omit U-NII band portion which is 5.15-5.35 GHz and 5.75- 5.825GHz
• 6 channels for each band
• Supported data rates is determined by mixture of spreading code length, convolution code rate and modulation scheme
• The Standard calls for BPSK or 4-ary bi-orthogonal keying modulation

The DS-UWB Data Rates











Direct-sequence modulators-process a narrowband signal to spread it over a much wider bandwidth. With this approach , each user terminal is assigned a unique spreading signature that makes each user's communication approximately orthogonal to those of others . This is similar to the way in which distnict carrier frequencies and time slots make users' transmissions approximately orthogonal in FDMA and TDMA , respectively. Spreading the signal de-sensitizes the original narrowband signal to some potential channel degradetions and to interference, this property becomes advantageous as the demand for spectrum reuse increases. The transmitted energy remains the same, but due to the much larger bandwidth, the signal spectrum is often below the noise floor of receivers. the signal looks like noise to any receiver that does not know the signal's structure. This makes the signal difficult to detect, even if one is looking for it.


b. 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 500Mhz signal is then hopped in frequency to enable it to occupy a sufficient high bandwidth.
This technology is known with the following characteristics:

• It is combination of frequency hopping (discussed next) and OFDM technology
• Spectrum Divided in to 14 groups each 528MHz band
• 4 groups of 3 sub-bands and 1 group of 2 sub-band
• Frequency-domain spreading, time-domain spreading, and forward error coding are used to vary the data rates
• In each OFDM period the transmitted signal hops over 3 different sub-bands of one of the four groups
• Time frequency codes used to vary FH (frequency hopping) pattern
  

The MB-OFDM Data Rates:












Frequency- hopped (FH) modulators - process the narrowband signal and change the carrier frequency every few symbols. A pseudo-random hopping pattern that is known by the receiver is used by the transmitter. To an outside observer, the signal appears to be transmitting on randomly selected frequencies, although the hop time on each frequency is usually constant. Multiple FH transmitters share the same frequency band by using different hopping patterns. If the transmitters are synchronized, then the hopping patterns can be selected so that there are few or no collisions ( both transmitters communicating on the same frequency and at the samre time.)

2.2 Application of UWB Technology:
There is a wide number of applications that UWB technology can be used for. They range from data and voice communications through to radar and tagging. With the growing number of way in which wireless technology can be used, the list is likely to grow.
Although much of the hype about ultra wideband UWB has been associated with commercial applications, the technology is equally suited to military applications. One of the advantages is that with the pulses being spread over a wide spectrum they can be difficult to detect. This makes them ideal for covert communications.

Commercial:

• High speed WPAN/WLAN
  
• Avoidance radar
• Altimeter (aviation)
• Tags for intelligent transport systems
• Intrusion detection
• Geolocation
  

Military:

• Radar
• Covert communications
• Intrusion detection
• Precision geo-location
• data links

Of all the above applications, appliacation of UWB technology for WPAN is gaining much attention. Specially, wireless connectivity for digital home.

Some of the WPAN applications of UWB technology includes:

• Replacing IEEE1394 cables between portable multimedia CE (consumer electronics) devices, such as camcorders, digital cameras, and portable MP3 players, with wireless connectivity
• Enabling high-speed wireless universal serial bus (WUSB) connectivity for PCs and PC peripherals, including printers scanners, and external storage devices
• Replacing cables in next-generation Bluetooth Technology devices, such as 3G cell phones, as well as IP/U PnP-based connectivity for the next generation of IP-based PC/CE/ Mbile devices
• Creating ad-hoc high-bit- rate wireless connectivity for CE,PC and mobile Devices

More explanations about the above applications can be found here