Today it is a challenge to get a proper solution in terms of linguistics. Significant contribution to the development of this problem has made A. A. Shakhmatov

The train has come. It suddenly has become noisily. (Agatha Christie)
In each of these sentences there is one sentence each: "arrival of the train" (the physical action of the subject-in this case, the subject) and "noisy" (the state, the subject of which is not represented). Complicated The sentences is polypropositive : each of its components includes a proposition:
It was one o'clock when I had lunch. (Salinger J. D.)
Two predicative units - two propositions related to super predicate "when" (the ratio of the simultaneity of situations, the second of which is basic, and the first - its temporary landmark). Nevertheless, in a semantic sense, simple and complex sentences are not opposed.
On the other hand, a complex sentence can contain only one proposition, if one of the predicative units has a non-propositional, and modus value. This can be observed in complex explanatory sentences:
I think that we can meet. (Agatha Christie)
The main part has the value of authorization with respect to the subordinate part.
Consequently, there is a difference between simple and complex and sentences in the semantic aspect, but there is no opposition.
Consider the features of a complex sentence from the point of view of the communicative aspect.
In this aspect, a complex and simple sentence is also not opposed. Both are representations, formed with intonation as a single whole.
Have you consulted any other member of my profession?
Can you give any reason why it should be this particular man?
(Agatha Christie)
These units are grammatically different (in the first sentence there is one grammatical basis, in the second sentence there are two grammatical bases). In the communicative plan, there is no difference. The so-called intonation completeness characterizes a complex sentence as a whole: the non-lasting part (the predicative unit) of a complex sentence is formed according to intonational types that cannot form a separate utterance (intonation incompleteness).
From the point of view of the actual division between simple and complex and sentences, there is a similarity and a difference. Similarity manifests itself in such structures of a complex sentence, when its components represent one theme and one bump. In other cases, in a complex sentence, each component has its actual division.
So, between the simple and complex sentence there are differences in all aspects, but they are contrasted only in the formally syntactical, constructive aspect.
Retaining the term "complex sentence" and systematized the concept of the term, in this paper, it refers to a compound sentence syntactic unity, as a phenomenon of a higher level of linguistic communication, containing the two or more predicate unit.