Page 310 - From GMS to LTE

296 From GSM to LTE-Advanced Pro and 5G

which is dedicated to their own subscribers. While the digital part of the base station
needs to support this approach, mobile devices cannot distinguish such a setup from
two entirely separate base stations as each radio carrier that is on air is dedicated to a
particular network operator.

4.16 From Dipoles to Active Antennas and Gigabit
Backhaul

Over the past two decades, base stations have made a tremendous evolution from sup-
porting overall datarates of a few tens of kilobits per second for voice calls to providing
Internet connectivity with datarates of hundreds of megabits per second. In the early
days of GSM, a base station was relatively simple compared to today and only had to
support a single frequency band, e.g. the 900 MHz band. Especially in rural areas it was
not uncommon to see flagpole antennas (dipoles) being deployed. In cities, 3‐sector
panel antennas were used to increase capacity. From a design point of view, the digital
baseband and the radio part of a base station were located in a voluminous air‐condi-
tioned cabinet usually at ground level or inside a building and coaxial cables conducted
the RF signals to and from the passive antennas on a rooftop or tower.
Complexity of base station deployments somewhat increased with the introduction
of UMTS in 2003, as two frequency bands had to be supported simultaneously, e.g. the
900 MHz band for GSM and the 2100 MHz band for UMTS. Typically, network opera-
tors installed dual‐band antennas at the time so a single antenna panel had two physical
antennas inside. The number of coaxial feeder cables increased but it was still a viable
deployment option.
With LTE first being deployed in 2009, yet another frequency band had to be added
to a base station site. Technology had advanced again at that point so many network
operators used the upgrade cycle to install remote active radio heads close to the anten-
nas, which were connected by an optical cable to the baseband unit further away. In a
typical 3‐sector configuration, separate remote radio heads are necessary for each sec-
tor. This reduced overall cost significantly because the use of expensive coaxial cables
was reduced. Also, efficiency was significantly increased as this removed the power loss
in the long coaxial cable. In this setup, coaxial cables are only used for the short distance
between the remote radio heads and the passive antennas. Also, the size of the digital
baseband unit and backhaul transmission equipment significantly shrank as the digital
processing part of GSM, UMTS and LTE was combined in a single baseband module.
This is also referred to as a ‘Single‐RAN’ base station solution. This meant that the
baseband unit could also move closer to the antenna mast or even onto the mast itself.
In addition, air‐conditioned cabinets for cooling in summer and heating in winter
became unnecessary.
This setup is still used today by many network operators even though in the mean-
time a typical urban base station uses even more frequency bands: for LTE, 800, 1800
and 2600 MHz are often deployed simultaneously and in addition, 900 MHz for GSM
and 2100 MHz for UMTS are required. In the future even more bands will be added. In
the US, a different set of bands is used but their number is comparable. Fortunately,
antenna technology has advanced as well and a single flat antenna casing can now