Bandwidth Matters

It perhaps goes without saying that bandwidth matters. In the mobile arena, bandwidth really matters. Devices, for example, might well operate “up to 300 Mbps,” Wi-Fi routers advertise ability to support gigabit speeds.

AT&T Mobility data rates are advertised as ranging from 15 Mbps to 20 Mbps.

Some might note that such divergences in data rates or speeds will confuse and often irritate consumers.

But “speed mismatches” long have been a feature of networking. Traditionally, the highest potential speeds are in the local area network and the wide area backbone, the slowest speeds--and therefore the choke points--in the access network.

That hasn’t changed much too much. There is perhaps a 15-fold difference in the potential speed of many user devices and the potential speed of an access connection, some might well argue.

In the case of the mobile network, 300 Mbps in the access link depends on a number of inputs, but includes the amount of local access spectrum available in any area served by a single macrocell.

To reach 300 Mbps, a mobile service requires at least 40 MHz of usable bandwidth, in any radio sector.

There are other considerations, including distance between the end user device and the radio, radio architecture, radio signal modulation and actual device capabilities.

In AT&T Mobility’s case, their TD-LTE spectrum holdings in the United States support just 15 MHz carrier aggregation, as a rule.

This constrains the peak downstream data rate to about 100 Mbps.  AT&T Mobility has not implemented 1 x 2 (TX x RX) downlink MIMO in their wireless network infrastructure equipment, which further constrains the peak data rate to about 50 Mbps.  

Finally, in typical real-world usage, smartphones utilize 16-QAM, taking the downlink data rate to about 30 Mbps for the most advanced Cat6 models such as the Samsung Galaxy S6 and the upcoming Apple iPhone 6s.  Older Cat4 models such as the iPhone 6 / 6 Plus are typically constrained to about 15 Mbps DL bandwidth.

Bandwidth matters.