A complex sentence is a sentence that has one independent clause and one or more dependent clauses. And also, the use of subordinating conjunction also signifies a complex sentence.
In the given sentence, the subordinating conjunction is "since". This makes the first part of the sentence the dependent clause. And the second part of the sentence is an independent clause as it can be taken as a full sentence on its own. And with that, the whole sentence contains a dependent clause with a subordinating conjunction and an independent clause, which makes it a complex sentence.
