Pulsed UWB Technology

1.2.1 Introduction: What is UWB ?

1.2.1.1 Definition

Ultra wide-band (UWB) is a generic term used to represent a radio access technique which has been studied under different names. These include the terms impulse ra-dio (IR), carrier-free rara-dio, baseband rara-dio, time domain rara-dio, non-sinusoid rara-dio, orthogonal function radio and relative bandwidth radio etc. [2]. The fractional band-width of a communication system is defined as:

B_f = 2f_H −f_L

f_H +f_L (1.1)

wheref_H and f_L are respectively the higher and the lower frequency boundaries at a threshold (e.g. 3 dB) below the strongest radiated emission. A UWB signal was first defined as a signal having a 3 dB fractional bandwidth (B_f3dB) greater than 25 %.

In 2002, the FCC defined a UWB signal as any signal having a −10 dB fractional bandwidth larger than 20 % and/or an absolute bandwidth of more than 500 M Hz [3]. Typically, the bandwidth of UWB signals ranges from 500M Hzto several GHz.

Fig. 1.1 compares the power spectral density (PSD) of conventional narrow band systems which generally modulate the narrow band signals on a frequency carrier, spread spectrum (UMTS) system which has lower PSD and a bandwidth of 5M Hz, and UWB system having very low PSD.

1.2.2 Historical Developments

In 1901, Guglielmo Marconi employed the very first transmission system based on the UWB technology to transmit Morse code across the Atlantic Ocean. Approximately half a century after, modern pulsed base systems started intrusion in the military applications. The study of electromagnetism in the time domain started in early six-ties. Early research was focused on radar applications due to the nature of broadband

G Hz

M Hz K Hz

Spread Spectrum Systems

-41 dBm/M Hz Conventional

narrow band systems

Bandwidth (Hz) Power spectral density (dBm/M Hz)

UWB

Figure 1.1: Comparison of the spectrum allocation for different wireless radio systems

signals, which provide a strong temporal resolution. A comprehensive survey of early research in this field was presented by Bennett and Ross [4], while Taylor [5] presents the foundations of the technology applied to UWB radar. Regular research advances have been made since the mid-60s, as revealed by the historical study of Barrett [2].

However, the use of UWB signals in the field of radio communication was not envis-aged before the end of the 20^th century. In 1990, the department of defense of United States of America (USA) government published the results of its evaluation of the UWB technology, which focused exclusively on the radar, since no other application of UWB communication systems were then proposed [6]. More recently, research has been focused on UWB signals for radio communication, [7, 8], developing the main characteristics of this technique: A temporal resolution in the order of nanoseconds due to the frequency bandwidth, low duty cycle allowing multiple access without a transmission carrier, which simplifies the architecture of radio systems [9]. Since 1998, the FCC launched a first study on UWB. In February 2002, a first regulatory report is published, allowing wireless communications in particular in the band of 3.1 GHz - 10.6 GHz with strong constraints on the power spectral density [3].

UWB technology is opening the doors for a range of new applications as well as complementing existing wireless systems. One of the important applications, which is not always a low power application is the radar. UWB radars are used for mar-itime and air navigation and as remote speedometers. There are many commercial applications of UWB radar and imaging, such as intrusion-detection radars, ground-penetrating radar (GPR) and precision geo-location systems. Some new applications include automotive security and rescue operations. In these applications one active device transmits narrow pulses and analyzes the echoes from the target, which is

usu-ally passive and unaware of the signal. Among the low power applications for the UWB are the ranging and short range communication applications. The communica-tion system can either utilize entire bandwidth and achieve very high data rate over one link or can have multiple low data rate links (e.g. sensor networks) with very high aggregate data rate.

1.2.2.1 Strengths and Weaknesses of UWB

One of the biggest strengths of the UWB technology is derived by its ultra large bandwidth. Owning to such a large bandwidth, UWB systems can attain very high data rates. The information theory tells us that it is possible to transmit data at a bit error rate (BER) below an arbitrarily low threshold, if the data rate is below the capacity of the transmission channel. The channel capacityC is thus an indication of maximum data rate which is theoretically possible for a given channel. In the presence of additive white Gaussian noise (AWGN), it can be calculated by the famous theorem of Shannon [10]:

C =B log₂(1 + S

N) (1.2)

whereC is the capacity in bits per second (bps), B is the bandwidth in Hz and _N^S is the signal to noise ratio (SNR). Note that the channel capacity increases quasi linearly with the bandwidth and logarithmically with SNR.Fig. 1.2shows the channel capac-ity as a function of SNR with different bandwidths. In a context of growing demand for high data rate wireless communication, systems working with large frequency bands are more likely to achieve adequate data rate. Thus, UWB with frequency bandwidth of up to several GHz, is more adapted to the increasing consumer’s demands [9].

Because of their wide bandwidth, UWB signals have very high temporal resolution, typically in the order of nanosecond. A first implication of this involves localization:

with an ability to detect the delay of a signal with precision in the order of 0.1 to 1 ns, it is possible to obtain information about the location of the transmitter with an accuracy of 3 to 30 cm. Temporal resolution of UWB radio signal also provides robustness against fast fading of the propagation channels caused by the multi-paths.

As narrow UWB waveforms detect multiple reflections of the radio channel separately, therefore destructive recombination of the multi-paths is no longer experienced at the receiver.

From the implementation point of view, the conventional radio systems are gener-ally heterodyne in design: the baseband data signals are modulated by using a carrier of higher frequency. UWB communication system allows the transmission of baseband pulse directly over the radio channel without any carrier modulation. The carrier free transmission simplifies the architecture of the radio systems. One of the weaknesses of UWB signals is their low PSD. This property is not intrinsic to the UWB signal as we defined above (1.1), but is imposed by the regulatory authorities of the radio spectrum radio. In fact, the vast spectral range of UWB signals occupies frequencies already allocated to other radio systems. To allow the peaceful co-existence of UWB with other radio narrow band technologies, the federal communication commission

−10 0 10 20 30 40 2

4 6 8 10 12

SNR (dB)

C (bps)

500 Mbps UWB

Bandwidths

NB

1 MHz 10 MHz 20 MHz 30 MHz 40 MHz 50 MHz 60 MHz 70 MHz 80 MHz 90 MHz 100 MHz 250 MHz 500 MHz 1 GHz

Figure 1.2: Channel capacity as a function of SNR for different bandwidths

(FCC) has limited the PSD of signals UWB to −41 dBm/M Hz, which corresponds to the limit of PSD allowed for unintentional radio emission. This low PSD results in a secure communication as transmitted signals becomes harder to detect. An-other consequence of this peculiarity concerns the distance of propagation, which is limited to few meters. Therefore, UWB applications are limited to short-range and high data rate telecommunication systems, and are therefore particularly suited to the development of ad-hoc networks.