Documentation

Analog Channel Loss in SerDes System

One of the important limiting factors in high-speed data transmission is the loss in the channel due to cross talk, attenuation, and reflection noise. The Analog Channel block and `serdes.ChannelLoss` System object™ parameterize a channel model that represents a lossy transmission line typical in high-speed SerDes application. The loss model is constructed either from a channel loss metric or from an impulse response from another source.

Loss Model from Channel Loss Metric

A discrete time, band-limited analog impulse response characterizes the `serdes.ChannelLoss` System object. It represents the response of a system to an impulse response vector with an impulse of $\frac{1}{dt}$, where dt is the sample interval.

To calculate the impulse response, `serdes.ChannelLoss` first calculates the S-parameter component S21 according to channel loss at frequencies ranging from 0 to fmax, maximum frequency of interest, where ${f}_{\mathrm{max}}=\frac{1}{dt}$. This is done by determining the loss at the target frequency, and then linearly extrapolating required channel length to achieve target channel loss. Then transmitter and receiver termination S-parameter are then calculated according to the equations 93A-17 and 93A-18 from the IEEE 802.3bj-2014 specifications [1].

After calculating S21, the System object adds the negative frequency data points so that the real components of S21 have even symmetry and the imaginary components of S21 have odd symmetry about 0 Hz. The impulse response is calculated from the inverse Fourier transform of S21. Finally, the impulse response is resampled so that the sample interval is dt.

Loss Model from Impulse Response

To construct a loss model from an impulse response vector, input the impulse response vector from another source. You can also define the impulse sample interval. Changing the symbol time and number of samples per symbol changes the data rate of the SerDes system. If you change the data rate you have to define the impulse sample interval for the specified data rate.

Introducing Cross Talk

You can include the effects of cross talk in the time domain simulation using a custom impulse response from the SerDes Designer app. The impulse sample interval, which defines when the cross talk impulse response is sampled, must match the simulation time step.

To introduce cross talk in Simulink®, first place the cross talk impulse responses as input to the FIR filters. Then add the cross talk stimulus blocks with a pattern and phase offset to combine with the through path. If necessary, manually resample the cross talk impulse response to match simulation step time.

References

[1] IEEE 802.3bj-2014. "IEEE Standard for Ethernet Amendment 2: Physical Layer Specifications and Management Parameters for 100 Gb/s Operation Over Backplanes and Copper Cables." https://standards.ieee.org/standard/802_3bj-2014.html.