ct. We will con- sider here the phase noise derived from the frequency fluctuations of the lasers. The phase noise has a complex degrading effect: it reduces the power available for sym- bol estimation on each quadrature, introduces a rotation of the (l1, e2) and (e3, e4) planes in the quadrature space creating crosstalk between the corresponding quadratures, and finally it creates imperfect frequency lock between the LO and the received field carrier. As a matter of fact, Equation 9.34 is rewritten in this case as C1(tj) = c2(tj) (9.67) C4(tj) where PT represents the transmitter phase noise OL is the phase noise of the LO The problem of an accurate evaluation of the impact of phase noise on this class of receiv- ers has been studied extensively; several models have been devised with different levels of accuracy and complexity for all the relevant modulation formats. It is not our inten- tion here to either develop an accurate model or to compare different models in different Next Generation Transmission Systems situations; the interested reader is encouraged to personally analyze the rich technical literature starting from the literature reported in [31]. In practical high performance systems, very high quality tunable lasers are used to reduce cost of spare parts, to ease the LO operation, and to minimize the effect of phase noise via a very narrow linewidth. In general, the linewidth is smaller than a few MHz and in the external cavity, the linewidth is of the order of 500 kHz. In this condition, we have to expect that in general the phase noise impact will be small, which is a necessary condition for our draft design. Thus we will introduce a very sim- plified model for the analysis of the phase noise impact, which holds in the low penalty regime. This model will allow us to evidence all the elements that are important and also to obtain a closed form expression of the sensitivity penalty. In the low penalty condition, Equation 9.67 can be linearized with respect to the phase noise obtaining (9.68) c3(tj) = F