Ultra-wideband Technology 

The trend of modern day electronic communication is going wireless. One of the advantages of free-space communication is the dispersion free propagation even at higher frequencies. Consequently, UWB (Ultra-wideband) technology utilizes short pulses to send data over short distances. There are several advantages in using short pulses for communication.

Firstly, it is easy to resolve multi-path components resulted by the reflections in the propagation environments. Secondly, the wide frequency spectrum of short pulses can re-use the valuable spectrum resources. However, it is important to keep the power level of the pulses as low as possible. This is when regulation comes to the picture. The Federal Communications Commission (FCC) of USA, which regulates the spectrum in USA, has made 3.1-10.6 GHz band available for license-free UWB. This has raised many questions from current wireless system operators. Interference between narrowband fixed/mobile wireless systems and the UWB systems has been widely discussed and resolved. Thirdly, the UWB technology can be used to make wireless systems with location awareness. Of course, there are dedicated systems based on UWB technology for location detection.

The main attraction of a UWB system for customers is the high data rates capable of handling video applications. As shown in the above figure, it is possible to interconnect computer peripherals using this technology. Another example is connecting video applications such as LCD TVs, Data Projectors and DVD players. Most of these applications currently use Fire wire or USB, which can be turned into wireless. Physical layer of wireless USB is indeed UWB.

UWB Standards

There are two standards that has been proposed and implemented to use UWB pulses for communication. The difference is in the type of pulse modulation used. The Multiband Frequency Division Multiplexing (MB-OFDM) is the technology used by the members of WiMAX forum to implement UWB. They use multiple narrowband pulses, as small as 500 MHz, within the frequency range allocated by the FCC. In contrast, the members of UWB forum are using short pulses covering large portions of the allocated range, with the modulation scheme known as Direct Sequence Spread Spectrum Technique (DS-UWB). Neither is accepted as the formal IEEE802.15 standard as of today. Figure 1 shows the difference between pulses used in two standards.

In an OFDM system the pulses are relatively long and the spectrum is narrow. As a result, it is possible to employ well known narrowband techniques in these systems. The power spectral density of these pulses is generally bounded by the limit of -41.3dBm/MHz as regulated by FCC. Other countries and regions may adopt different limits. Pulse repetition rates and out-of-band interference is also considered in regulation. As a result, at some frequencies the power spectral density limits are less than -41.3dBm/MHz.

The DS-UWB systems have also split the spectrum into two, leaving out a frequency band in the middle to prevent interference with other wireless systems. The lower band is from 3.1-4.9 GHz and the upper band is 6.2-9.7 GHz. Note that 5.4 GHz ISM (Industrial, Scientific and Medical) band and IEEE802.11a band are excluded in this range. However, the regulation requirements in European regions are much more restricted, demanding for more exclusion of bands. It is highly likely that these demands are met with Detect and Avoid (DAA) technologies.

UWB Antennas

As communication devices, computer peripherals and multimedia devices become UWB enabled, each of those has to be fitted with an antenna capable of radiating pulses efficiently. These antennas are different to those used in WiFi or mobile phone systems. UWB antennas need to have: 

• Large impedance matched bandwidth, as big as 2-3 GHz. 
• Stable radiation patterns within that frequency band, or direction independent distortion of pulses. 
• Low dispersion. 
• Low-profile, low-cost and high efficient.

Some of these features already exist in antennas used in mobile phones and WiFi enabled, cards or laptops. However, the high bandwidth and pattern stability with respect to frequency is unique to UWB antennas.
Some potential UWB antennas are: 

• Printed monopole antennas fed by a microstrip line. 
• Printed wide slot antenna. 
• Vivaldi antennas. (Figure 3) 
• Tapered slot antenna. 
• Planar monopole antenna fed by a probe and coax combination. (Figure 2) 

Figure 2 Probe Fed Planar Circular monopole antenna

Figure 3 Vivaldi Antenna

 

Characterizing UWB antennas

UWB antennas are first characterized using input impedance. Usually the return loss is expected to be higher then 10 dB within the operating frequency range. Typical reflection coefficient versus frequency plot of a UWB antenna may look like as follows:

Figure 4 Typical reflection coefficient vs frequency plot of a UWB antenna, this has 10dB return loss bandwidth from 4-10 GHz

Then the radiation pattern stability is calculated using Pattern Stability Factor. It gives an idea of how stable the antenna patterns with frequency. The best of UWB antennas have stable patterns and as a result the frequency response of those antennas is direction independent. It is convenient to optimize these antennas in multiple directions than those having less pattern stability. Remember, the antenna may not have enough pattern stability over the entire impedance-matched bandwidth. Therefore, it is better to consider a frequency band which has both sufficient pattern stability and return loss as the operating bandwidth of an antenna.

Integration of UWB Antennas

Modern UWB antennas used in communication systems is low profile, low cost and highly efficient. Preserving these properties when they are connected to devices is a challenge. UWB antennas are operating over a large range of frequencies. There is a high probability that the antenna is coupled to nearby structures at least in one of the frequencies. Metallic edges, dielectric superstrates and other components can easily modify the radiation performance of the antenna. Therefore special consideration must be given, when antenna is integrated to such devices.