Delusional perception is a first rank symptom of schizophrenia. That means presence of delusional perception alone is enough to diagnose schizophrenia.
In Delusional perception, Person attaches an abnormal significance to a real and normal perception. It occurs without any cause that is understandable in rational or emotional terms. It is usually self referent, momentous, urgent and has overwhelming personal significance.
A person suddenly become convinced that he has AIDS, because traffic lights turned to red.
Traffic lights becoming red is a normal phenomenon, however the person attaches abnormal and tremendous personal significance to it. No one can understand the connection between traffic lights becoming red and having himself AIDS.