Hi Mutah,
I understand you would like to know why in a multi-hop wireless network, the BLER increases as number of hops increases.
The reason behind is that the success probability of a transmission decreases as the number of hops increases. For instance, consider a scenario where you have two hops.
Let us say, the probability of success (Ps) for each link is ‘x’,
So, for a two-hop scenario, the probability that the first and second hop transmissions are successfull is < 'x'.
As the probability of success reduces with an increase in number of hops, the subsequent BLER will also increase,
I hope this helps.