America's second largest carrier, AT&T, will need to use unlicensed spectrum in order for its LTE network to reach a download speed of 1 Gbps. AT&T's Chief Strategy Officer of Technology and Operations, John Donovan, explained that the carrier would be utilizing a number of different network technologies in order to accelerate its download speed to reach the company's stated 1 Gbps target. These include either 4×4 or 8×8 MIMO (multiple input, multiple output), 256-QAM (quadrature amplitude modulation, a means of increasing throughput) and carrier aggregation (that is, combining different parts of the spectrum, which for AT&T will include unlicensed parts). Donovan also explained that the carrier would also be utilizing Licensed Assisted Access (known as LAA) to boost the available spectrum and therefore the increase the speed of the downlink. LAA is a technology designed to improve the co-operation of unlicensed LTE spectrum with Wi-Fi networks, which can operate at similar frequencies.
AT&T's announcement that it would be reliant on unlicensed spectrum would appear to be in contrast with T-Mobile US. Here, T-Mobile explained during the Consumer Electronics Show in early January that to reach the 1 Gbps target download speed, it would only need 50 MHz of spectrum. T-Mobile's Chief Technical Officer, Neville Ray, explained: "So here's the math: 5 megahertz carrier gives you about 35, 38 megabits per second. 256 QAM takes it to 50, 4×4 MIMO takes it to 100 MIPS. Ten of those gives you a gigabit." Ray went on to explain that T-Mobile US has at least 50 MHz of available downlink spectrum available around the country and this, of course, means that America's third largest carrier may not need to use unlicensed spectrum to reach the 1 Gbps target downlink speed. Nevertheless, T-Mobile US is also assessing unlicensed spectrum and LAA technology but with a view to boosting data speeds over the 1 Gbps rate. T-Mobile's network partner in recent high speed LTE tests, Ericsson, has confirmed that the equipment was providing a reliable transfer speed over 900 Mbps during an open air demonstration at T-Mobile's Bellevue lab, in Washington. This test, which used 60 MHz of spectrum, 4×4 MIMO and 256-QAM, peaked at a maximum downlink transfer rate of 979 Mbps. Both Ericsson and T-Mobile US expect the technology to be refined over time.
It is important to remind readers that the promise of a 1 Gbps LTE network – as made by AT&T, Sprint and T-Mobile US – is dependent on a number of factors, including the number of users connected to a given mast (or node). T-Mobile US' Arun Bansal, head of business unit network products, stated that when the carrier is discussing the 1 Gbps data transfer rate, they mean one device connected to one cell and not that every user will be able to receive data at the 1 Gbps rate. However, as the underlying network technologies are refined and improved, the higher the theoretical transfer rate, the higher the number of customers that may be served with faster and faster data transfer rates.