Massive MIMO is at the heart of many 5G trials (see separate item), but it is starting to become an option for a few extremely capacity-constrained operators, even in LTE networks. The early moves have mainly been Asia-focused – Huawei and ZTE supporting dense hotzone trials or deployments by MNOs like Softbank or China Mobile. But last week a lesser known vendor, Blue Danube Systems, said it had completed the first commercial trials of its Massive MIMO technology in with AT&T (one of its investors) and Virginia-based regional carrier Shentel. The Silicon Valley start-up has its own approach to advanced 3D beamforming, to be used in conjunction with increasingly large MIMO antenna arrays to mitigate interference and boost capacity and…