The width of this phase portrait, as measured along its minor axis, can be interpreted as an approximate measure of the maximum slope of the waveform. (This enables a fixed delay of 25 ps, for example, to be used for both 10- and 40-Gb/s signals.) For short time delays, such as ¼ T, we can interpret the phase portrait as approximating the joint probability density of the amplitude and slope of the waveform with respect to time. We note that while the choice of 1-bit delay Δ t is a useful tool for visualization, it is not critical for pattern recognition. Phase portraits of an NRZ signal showing the effects of small changes in tap delay. This information is absent in eye diagrams that are constructed from samples that are separated by long periods.įigure 7.3. A fundamental difference between the eye diagram and phase portrait is that the latter contains information about the correlation between samples on the time scale of a bit period. We emphasize that the sampling is asynchronous, in that the time between the pairs, T s, is not related to the monitored signal bit rate, and can be many orders of magnitude longer with thousands of bits between sample pairs. The phase portrait is then created by binning the sample pairs into a two-dimensional histogram, as shown in Figure 7.2(a) and b for 1-bit and ¼-bit delays, respectively. To generate the phase portrait, the waveform is sampled in pairs separated by a known delay Δ t, as shown for the NRZ signal in Figure 7.1. 1 This pdf, known as a phase portrait, is sensitive to waveform distortion and noise and contains unique signatures of impairments. Improving the amplifier's NF can increase OSNR, and the improved OSNR can be used to increase the system reach, reducing the channel power in order to suppress the nonlinearities, etc.Īsynchronous delay-tap sampling is an alternative to the eye diagram that uses the joint probability density function (pdf) of a signal x( t), and its delayed version x( t + Δ t) to characterize the signal. The required OSNR should be increased by 6 dB when the data rate is increased by a factor of 4. As an example, a typical OSNR requirement for BER=10 −9 is about 17 dB at 10 Gb/s data rate. The OSNR (in dB) of a signal channel at the end of the system is approximated by: OSNR (dB)=58+ P out− L span−NF−10 log( N amp), when the system has N amp fiber spans, span loss L span (in dB) followed by an optical amplifier with output power P out (in dBm) per channel launched into the span and noise figure (NF) (in dB). OSNR is defined as the ratio of the power of signal channel to the power of ASE in a specified optical bandwidth (conventionally 0.1 nm). In long-haul and undersea transmission system with many EDFA chains, OSNR is an important system parameter to design and characterize the optical transport system. The shape of the eye is generally broadened and distorted (i.e., eye is closed) due to limited bandwidth and fiber impairments, and noise and timing jitter are added onto this broadened and distorted eye shape. Figure 24b shows the eye diagram close to a real situation. Therefore, the eye diagram deviates from the perfect rectangular shape. In reality, the transmitter and receiver have a limited bandwidth with noise and jitter, and the transmission media (i.e., optical fiber) has dispersion and nonlinearites. When a perfect transmitter and receiver (i.e., infinite receiver bandwidth with zero noise characteristics and jitter), and a perfect transmission media (i.e., no dispersion and nonlinearites) are used, the received eye diagram is shaped as a perfect rectangular.
#Noise mapping and asymmetry series
A square bit stream (i.e., series of symbol ‘1’s and ‘0’s) is sliced into sub-bit stream with predetermined eye intervals (i.e., several bit periods), and displayed through bit analyzing equipment (e.g., digital channel analyzer), overlapping the sliced sub-bit stream in order to obtain the eye diagram. An eye diagram provides a simple and useful tool to visualize intersymbol interference between data bits.
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