Consistent lower GSM signal

I bought a second hand iPhone 5S and i have one problem with it;

My phone barely gets 5 bars of 2G, whereas every other device around me has 5, i only have 4. But it doesn't stop there. When i compared the signal strength between two 5s' using the same carrier, i can see a consistent "one bar less" on my phone and they are both using a 2G/3G network. I tried putting my sim in my friends phone and he receives 4 bars on it while i only get 3, so it's not the simcard. It's like my antenna is way more sensitive than his.

Could it be the gsm antenna? The phone probably fell on the edge where the GSM antenna is, considering that edge has some damage.

What else could it be? I'd like to know what's wrong before actually visiting a service center.