Once an obscure acronym, SDL (Supplemental Downlink) is now an increasingly central element of the rush to add spectrum capacity at affordable cost. The slow pace of bidding in the US 600 MHz incentive auction, and the controversy in India over high reserve prices for its upcoming 700 MHz sale, show that operators are becoming far less willing to fork out huge sums for airwaves. SDL, a standardized mechanism for aggregating downlink-only spectrum with an LTE host network, could be a far more cost effective route. It underpins LTE’s moves into unlicensed spectrum, while Nokia and Qualcomm have just conducted SDL trials in Europe’s fought-over TV bands. The more MNOs’ attentions turn from LTE coverage to high capacity, the more…