In this section, the high-level perspective on the potential of UWB technology in indoor wireless communications is presented. According to the FCC definition [5], Any wireless transmission scheme occupying a fractional bandwidth, η, greater than 25% of a centre frequency, or more than 500 MHz, which of the two is lower, is known as Ultra-wideband (UWB) technology. The frequency limits of the emission bandwidth using the formula influences the value of the fractional bandwidth η [49] [50] [51]:
H L
L H f f f f 2 (2.2)Where fH and fL are the upper and lower frequency of the -10 dB emission point, the centre frequency is defined as the average of fH and fL, i.e.
2 L H C f f f (2.3) The narrowband (NB) and UWB spectrums with the defined fC, fH and fL are shown in Figure 2-9.Figure 2-9 Comparison of the Fractional Bandwidth η of a Narrowband (NB) and Ultra- Wideband (UWB) Spectrums
Frequency, f, Hz P owe r spe ctra l de nsit y, P S D, dB
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The examples of two different common forms of UWB radio systems are provided by IEEE 802.15.3a task group i.e. (i) very short duration pulses are transmitted for conveying information, known as impulse-UWB (I-UWB), (ii) multiple simultaneous carriers are transmitted for conveying information, known as multi-carrier UWB (MC-UWB). One of these two approaches has not been standardised, leaving it on the disposal of physical descriptions of a radio to pick the more suitable approach. The same radio regulatory definitions of the FCC are employed again, a flexibility advantage of UWB standard implementation. Both approaches have their pros and cons. The basic differences between the two approaches on the basis of spectrum management are shown in Figure 2-10 [25]. The orthogonal frequency division multiplexing (OFDM) is the most popular form of multi-carrier modulation, which has become the leading modulation technique for high data rate systems.
Contrary to classic communications pure impulse radio does not use a modulated sinusoidal carrier for transmittance of information. The baseband pulses are used for this purpose. The pulses are extremely short and last for nanoseconds or less, consequently, the transmit signal bandwidth is on the order of Giga Hertz [25].
The use of multi-carrier communications started between the 1950s and 1960s to achieve higher data rate transmittance in military communications. The densely spaced sub-carriers and overlapping spectra are used by the OFDM, which is special case of multi-carrier modulations. OFDM was patented in the US in 1970 [25]. And is in application in Asymmetric Digital Subscriber Line (ADSL) services, Digital Audio Broadcast (DAB), Integrated Services Digital Broadcasting (ISDB) in Japan, IEEE 802.11a/g, 802.16a, and Power Line Networking (HomePlug) today. OFDM also being considered for the fourth generation (4G) wireless services, as it is suitable for high data rate systems IEEE 802.11n (high speed 802.11) and IEEE 802.20 (MAN) [25].
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Figure 2-10 Comparison of impulse and multi-carrier UWB spectrums, [25]
The balance between advantages and disadvantages of I-UWB and MC-UWB is complicated to find and the standards bodies have debated over them for a long time. In a UWB system, it is most important to minimise interference in transmitted and received signals. MC-UWB is takes care of this issue by avoiding interference with its precisely
Frequency, f, GHz R elative Powe r, P , dB
(a) Spectrum of a Gaussian monocycle-based I-UWB signal. Frequency, f, GHz R elative Powe r, P , dB
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selected carrier frequencies to dodge the narrowband interference to or from narrowband systems. Moreover, the flexibility and scalability provided by MC-UWB is also relatively better; however, an additional layer of control in the physical layer is required. The impact of interference on the UWB system can be reduced by application of spread spectrum techniques in both forms of UWB [25].
Fast switching times for the transmitter and receiver are required by I-UWB. While designing the radio and antenna the transient properties must be considered. The interference to UWB systems can be resolved with the help of high instantaneous power during the brief interval of the pulse; however, doing so increases the probability of interference from UWB to narrowband systems. The RF front-end of an I-UWB system appears like a digital circuit, therefore, most problems associated with mixed-signal integrated circuits can be avoided [25]. Simple I-UWB systems are rather cost effective. In contrast, implementation of a MC-UWB front-end is complex because of the continuous variations in power over a very wide bandwidth. The job of the power amplifier becomes difficult because of this. High-speed Fast Fourier Transform (FFT) processing having significant processing power is required for OFDM.
The general detection theory assumption that the system operates in an additive white Gaussian noise (AWGN) environment poses as an additional limitation in the implementation of a UWB system. Even though under real circumstances this does not always apply to communication system and particularly to UWB systems [25]. Other signals may also exist within the UWB pass band having Gaussian noise statistics. The operation of the system is performed at higher transmit power due to these narrowband signals or the in-band interference needs to be removed.
I-UWB is the main focus of this thesis, which is more popular but is a poorly understood form of UWB. We will explore different types of modulation techniques operating on this system. Mainly focusing on pulse position modulation systems (PPM) operating with time hopping spread spectrum (THSS) as a spreading approach and a multiple access technique.
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