Why do we need Ultra-wideband? (1)

 

Abstract

Since the release by the Federal Communications Commission (FCC) of a bandwidth of 7.5GHz (from 3.1GHz to 10.6GHz) for Ultra-wideband (UWB) wireless communications, UWB is rapidly advancing as a short range high-speed high data rate wireless communication technology. A new technology has been employed into our daily lives with minimal interference known as the ultra-wideband or UWB. This technology is an unlicensed service that can be used anywhere, anytime, by anyone. UWB which is well known for its use in ground penetrating radar (GPR) has shown interest in communications and radar applications. Unlike traditional systems, this can only operate over a specific range of frequencies. UWB devices operate by employing a series of very short electrical pulses (billionths of a second long) that result in very wideband transmission bandwidths. In addition, UWB signals can run at high speed and low power levels. All these unique features of UWB technology make it suitable for many different applications such as positioning, geo-location, localization (accurate positioning, high multipath environments and obscured environments), radar/sensor applications (vehicular, marine, GPR, imaging, wall-imaging, sense-through-the-wall (STTW), surveillance systems), communications (high multipath environments, short range communications, high data rates).

State of the Art

 

Current Trends in RF and Microwave Integrated Circuits Research (1)

Why do we need Ultra-wideband?

UWB Technology

Currently, there is an increased interest in Ultra-wideband (UWB) technology for use in several present and future applications. UWB technology received a major boost especially in 2002 since the US Federal Communication Commission (FCC) permitted the authorization of using the unlicensed frequency band starting from 3.1 to 10.6 GHz for commercial communication applications [1]. Although existing third-generation (3G) communication technology can provide us with many wide services such as fast internet access, video telephony, enhanced video/music download as well as digital voice services, UWB –as a new technology– is very promising for many reasons.

Large Bandwidth

The FCC allocated an absolute bandwidth more than 500 MHz up to 7.5 GHz which is about 20% up to 110% fractional bandwidth of the center frequency. This large bandwidth spectrum is available for high data rate communications as well as radar and safety applications to operate in. Fig. 1 shows the comparison between conventional narrowband (NB) versus UWB communications in both time- and frequency-domains. The conventional NB radio systems use NB signals which are sinusoidal waveforms with a very narrow frequency spectrum in both transmission and reception. Unlike a NB system, an Ultra-wideband radio system can transmit and receive very short duration pulses. These pulses are considered UWB signals because they have very narrow time duration with very large instantaneous bandwidth [2].

Very Short Duration Pulses

A typical received UWB pulse shape which is known as a Gaussian doublet is shown in Fig. 2(a). This pulse is often used in UWB systems because its shape is easily generated. Ultra-wideband pulses are typically of nanoseconds or picoseconds order. This is the origin of the name Gaussian pulse, monocycle or doublet. Transmitting the pulses directly to the antennas results in the pulses being filtered due to the properties of the antennas. This filtering operation can be modeled as a derivative operation. The same effect occurs at the receive antenna. The spectrum of the Gaussian doublet is shown in Fig. 2(b). Due to using UWB systems those very short duration pulses, they are often characterized as multipath immune or multipath resistant.


Fig. 1 Time- and frequency-domain behaviors for narrowband versus UWB communications [3].



Fig. 2 (a) Idealized received UWB pulse shape and (b) idealized spectrum of a single received UWB pulse [4].

 

High Data Rates with Fast Speed

The huge bandwidths for UWB systems −compared to other conventional NB systems− can show a number of important advantages. There is an increasing demand for high speed and high data rate applications in communication systems [5]. One of those advantages of UWB transmission for communications is the ability of UWB system to achieve high data rates in future wireless communications which requires increasing the bandwidth of the communication system. While current chipsets are continually being improved, most UWB communication applications are targeting the range of 100 Mbps to 500 Mbps [6]. Table 1 shows the spatial and spectral capacity for different communication systems such as UWB, wireless local area network (WLAN) and Bluetooth. The speeds of data transmission for different communication systems are summarized in Table 2. Using UWB technology will enable us to achieve higher data rates with higher spatial capacity compared to other existing systems. In addition, UWB technology achieves very high speed for data transmission. Another advantage of UWB systems is the ability to effectively reduce fading and interference problems in different wireless propagation channel environments because of the limited transmitted power of UWB systems [7]. This is in addition to exploiting multipath or frequency diversity because of the huge bandwidth of UWB systems [8]. The signal-to-noise ratio (SNR) of the UWB system can be increased using some techniques such as antenna diversity and beamforming which in turn will provide range extension and boost the capacity of worldwide interoperability for microwave access (WiMAX) for wireless metropolitan area networks (WMAN), and wireless fidelity (Wi-Fi) for wireless local area networks (WLAN) [9].

Table 1 Spatial and spectral capacity for different communication systems

System

Maximum data rate [Mbps]

Transmission distance [m]

Spatial capacity [kbps/m2]

Spectral capacity [bps/Hz]

UWB

100

10

318.3

0.013

WLAN 802.11a

54

50

6.9

2.7

Bluetooth

1

10

3.2

0.012

WLAN 802.11b

11

100

0.35

0.1317

Table 2 Speed of data transmission for different communication systems

Standard

UWB, USB 2.0

UWB (4m min.)

UWB (10m min.)

Fast Ethernet

WLAN

Ethernet

Bluetooth

802.11a

802.11g

802.11b

Speed [Mbits/s]

480

200

110

90

54

20

11

10

1

 

Some Basic Definitions:








 

Low Power Consumption

The UWB technology has another advantage from the power consumption point of view. Due to spreading the energy of the UWB signals over a large frequency band, the maximum power available to the antenna –as part of UWB system– will be as small as in order of 0.5mW according to the FCC spectral mask shown in Fig. 3.This power is considered to be a small value and it is actually very close to the noise floor compared to what is currently used in different radio communication systems [2]. Table 3 shows the power spectral density (PSD) of some wireless broadcast and communication systems such as UWB, radio, television, 2G cellular and WLAN. The PSD is defined as the total transmitted power over the operating bandwidth.

According to the definition in [10], one important feature of a radar and communications transmitter is called the effective isotropic radiated power (EIRP) which can be defined as the product of its gain and input power. Fig. 4 shows the FCC spectral mask of the indoor UWB EIRP emission level. It can be seen that the maximum signal power is limited to−41.3 dBm per MHz throughout the whole UWB frequency range from 3.1 to 10.6 GHz. All the UWB systems and devices must work within this spectral mask for legal operation in order to comply with the FCC standards and regulations.


Fig. 3 UWB versus other radio communication systems [3].

Table 3 PSD of some wireless broadcast and communication systems

System

Transmission power [W]

Bandwidth [Hz]

Power spectral density [W/MHz]

Classification

Radio

50 kW

75 kHz

666,600

Narrowband

Television

100 kW

6 MHz

16,700

Narrowband

2G Cellular

10 mW

8.33 kHz

1,2

Narrowband

WLAN 802.11a

1 W

20 MHz

0.05

Wideband

UWB

1 mW

7.5 GHz

0.013

Ultra wideband


Fig. 4 FCC spectral mask for indoor UWB systems [1]

Small Size and Low Cost

The small size of UWB transmitters is a requirement for inclusion in today’s consumer electronics. The main arguments for the small size of UWB transmitters and receivers are due to the reduction of passive components. However, antenna size and shape is another factor that needs to be considered. Ultra-wideband antennas are considered in the next article. Among the most important advantages of UWB technology are those of low system complexity and low cost. Ultra-wideband systems can be made nearly “all-digital”, with minimal radio frequency (RF) or microwave electronics. The low component count leads to reduced cost, and smaller chip sizes invariably lead to low-cost systems. The simplest UWB transmitter could be assumed to be a pulse generator, a timing circuit, and an antenna.

 

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