Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Ultra Wideband Systems 121to a single-carrier TDMA system. Furthermore, a frequency synchronous system wouldsimplify the MC-TDMA receiver synchronization tasks.Combining OFDMA and MC-TDMA achieves a flexible multi-user system with highthroughput [9].3.4 Ultra Wideband SystemsThe technique for generating an ultra wideband (UWB) signal has existed for more thanthree decades [27], which is better known to the radar community as a baseband carrierless short pulse [1]. A classical way to generate a UWB signal is to spread the datawith a code with a very large processing gain, i.e. 50 to 60 dB, resulting in a transmittedbandwidth of several GHz. Multiple access can be realized by classical CDMA, wherefor each user a given spreading code is assigned. However, the main problem of such atechnique is its implementation complexity.As the power spectral density of the UWB signal is extremely low, the transmittedsignal appears as a negligible white noise for other systems. In the increasingly crowdedspectrum, the transmission of the data as a noise-like signal can be considered a mainadvantage for the UWB systems. However, its drawbacks are the small coverage and thelow data rate for each userIn References [25] and [37] an alternative approach compared to classical CDMA isproposed for generating a UWB signal that does require sine-wave generation. It is basedon time-hopping spread spectrum. The key advantages of this method are the ability toresolve multi-paths and the low complexity technology availability for its implementation.3.4.1 Pseudo-Random PPM UWB Signal GenerationThe idea of generating a UWB signal by transmitting ultra-short Gaussian monocycleswith controlled pulse-to-pulse intervals can be found in Reference [25]. The monocycleis a wideband signal with center frequency and bandwidth dependent on the monocycleduration. In the time domain, a Gaussian monocycle is derived by the first derivative ofthe Gaussian function, given by√ eπs(t) = 6a3( )t t 2τ e−6π τ , (3.22)where a is the peak amplitude of the monocycle and τ is the monocycle duration. In thefrequency domain, the monocycle spectrum is given by√S(f) =−j 2fτ2 eπ π3 2 e− 6 (f τ)2 , (3.23)with center frequency and bandwidth approximately equal to 1/τ.In Figure 3-10, a Gaussian monocycle with τ = 0.5 ns duration is illustrated. This monocyclewill result in a center frequency of 2 GHz with 3 dB bandwidth of approximately2 GHz (from 1 to 3.16 GHz). For data transmission, pulse position modulation (PPM) canbe used, which varies the precise timing of transmission of a monocycle about the nominalposition. By shifting each monocycle’s actual transmission time over a large time frame