Backhaul Capacity Issue

Hello Experts,

With 28 MHz Bandwidth and QPSK Modulation, maximum capacity will be 28 x 2 = 56 Mbps.
But backhaul is getting chocked at around 40 Mbps, any reasons for that?

It’s hard to know what is the exactly problem…too many issues to be considered on the whole transmission path. Must be assured that no rate limited was set, adaptive modulation, and so on. What is the hardware model?