SD Drop shouldn’t be linked to AMR, unless perhaps the following situation :
AMR codec is required during TCH assignment, while the MS is sitting on its SDCCH.
The TC and MSC must allocate Ater and A interfaces channels to carry the TCH. And the TC should be able to transcode the AMR codec into the G.xxx codec. Perhaps the TC is not able to serve all the AMR calls in the BSC ? Therefore some calls fail ?
Regarding the intracell HO:
AMR is a great feature that allows rate change during the call (within the same cell). For instance, FR to HR or HR to FR. This is done thx to intracell HO.
Which vendor r u using? Did you check the Power control and HO threshold parameters for AMR ???
Why intracell HO will increase after enabling AMR? I mean if your network have already HR (GSM HR ; not AMR HR) enabled (so in this case intracell ho HR to FR or FR to HR will happen); then why it will increase after enabling AMR HR? I couldn’t get it.
Good reasonning 🙂 But your base assumption is shaky !
With HR and FR (legacy codecs), a MS is not able to “change rate” during the call. If it starts as HR, it’ll stay HR.
The only possibility will be a HO towards another cell. In this case, the TCH allocated for the MS might be with a different channel rate.
But there are no intracell HO to modify the channel rate in case of legacy codecs.