Hello Fatin,
It is mentioned there that besides the typical global simulation parameters of OptiSystem, such as Bit rate, Samples per bit and Sequence length, there are two additional parameters that are very important when designing bidirectional systems: Iterations & Initial delay
Iterations force a component to calculate again until it generates, at the output ports, the Iterations number of signals.Initial delay forces a component to generate a null signal for each output port. Because the majority of OptiSystem components will not calculate if a signal is not available at the input ports, the Iterations and Initial delay parameter will help the user to avoid a situation in which the simulation will not perform because a component cannot find signals at the input port – and the system will be deadlocked. The concept of multiple iterations and delays is very confusing, because it does not have a physical meaning. It is a simulation technique and requires an understanding of the calculation scheduler of OptiSystem.
Please visit the link below. I hope it will help you.
Lesson 5: Bidirectional Simulation — Working with Multiple Iterations
Regards