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Nonlinear Model of TWT Revisited for OFDM Systems
No full textRecently, OFDM systems have received wide interest, particularly in the field of terrestrial broadcasting of all digital television in the UHF/VHF bands. The use of TWT (Traveling Wave Tubes) is considered for the final amplification of the OFDM signal. The OFDM modulated signal is very close to a Gaussian random process. In this paper we revisit the most popular TWT models and recognize that they belong to a peculiar class of non-linearities, S×FSM, in which the output is the product of the incoming complex signal and a function of its modulus. For S×FSM nonlinearities, driven by a Gaussian input, output statistics can be evaluated in closed forms. We apply this analysis to the TWT non-linearity, deriving the power spectral density of the output signal as well as the probability density function, when the input is an OFDM signa
Implementation of Two-Mode Gaussian States Whose Covariance Matrix Has the Standard Form
Get PDFThis paper deals with the covariance matrix (CM) of two-mode Gaussian states, which, together with the mean vector, fully describes these states. In the two-mode states, the (ordinary) CM is a real symmetric matrix of order 4; therefore, it depends on 10 real variables. However, there is a very efficient representation of the CM called the standard form (SF) that reduces the degrees of freedom to four real variables, while preserving all the relevant information on the state. The SF can be easily evaluated using a set of symplectic invariants. The paper starts from the SF, introducing an architecture that implements with primitive components the given two-mode Gaussian state having the CM with the SF. The architecture consists of a beam splitter, followed by the parallel set of two single-mode real squeezers, followed by another beam splitter. The advantage of this architecture is that it gives a precise non-redundant physical meaning of the generation of the Gaussian state. Essentially, all the relevant information is contained in this simple architecture
Spectral Analysis at the Output of a TWT Driven by an OFDM signal
No full textThe use of high power amplifiers, such as TWTs, at the output of an OFDM modulator is requested when the digital signal is to be transmitted over a broadcasting terrestrial channel. The spectral analysis at the output of a TWT driven by a Gaussian input is carried out and the result is applied to the case of an OFDM signal. Output correlation and spectral density are obtained using the Mahler's expansion of a Gaussian tetravariate and are then applied to a practical OFDM schem
Analysis of Channel Noise in Orthogonally Multiplexed QAM Systems
No full textThe paper deals with the background for the analysis and performance evaluation of orthogonally multiplexed QAM (OQAM) systems in the presence of noise, intersymbol interference and interchannel interference. The main result of the analysis is that the noise at the decision points is always stationary, regardless of the time-varying characteristics of the OQAM system. However, unless the filters are ideal (bandlimited and with a square-root Nyquist-shape) and the carriers are properly chosen, the noises of different branches are correlated even in the presence of an ideal channel. Hence a vector estimate method should be used instead of the classical scalar threshold detectors. Furthermore, the intersymbol interference and interchannel interference impulse responses have been evaluated. Again, the interesting result is that both relationships are time-invariant regardless of the system parameter
Exact performance evaluation of lightwave systems with optical preamplifier
No full textIn this paper the performances of a direct-detection receiver with optical doped fiber preamplifier are investigated. All the sources which contribute to the global noise are taken into account, namely the shot noise, the excess noise due to the randomness of the gain, the amplified spontaneous emission (ASE) noise, and finally the thermal noise generated by the receiver. The approach is based on a probabilistic model of the amplifier that enables us to give the complete statistics of the noises. Comparisons with results previously obtained by different methods are presented. The effects of the amplifier gain, the spontaneous emission factor, the optical bandwidth, and the thermal noise on the error probability are examined and discussed
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