Last week, I solved one radio link problem that connects between two places in eastern part of my country. One node is in the mainland while the far end node is in the island.
The problem was significant performance error (BBE and ES errors) that were received by "far-end" node. Actually this problem was reported by our customer 2 weeks earlier and with assistance from our counterpart in Europe, we managed to find the solution within a week.
It happened that the radio unit of the far end was faulty and here is the step-by-step procedures that I had taken to detect the problem:

1) Analyzed list of history alarms for both nodes. As rule of thumb, this is important to detect any problem that is related with telecomm. equipment as it will show you when the problem starts and what cause it to start. In this case we found out that
2) Analyzed G.826 performance report. Just to confirm what you get from test equipment, or in case if you operate from NMC (network management centre)
3) Important step-- do the looping test. This is the simplest, yet very powerful method to detect any fault in transmission area. But you have to understand what you will do from the start.
Normally, for a radio system you will have,

DDF---interface unit---modem unit----radio unit----antenna---

so first you have to start from DDF level, followed by interface unit and so on.. You will know where is your problem area until the test gear/performance start giving error.