unrequited love, google returned: 1. 单恋, 2. 单相思
单恋, it is 单(相)思, without 相, it is the same as 单思
单相思 may be two are in love by assumption, but only one is missing the other.
Added:
"The beloved may or may not be aware of the admirer's deep affections." It is 单恋 or 单思.