Page 311 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 297

contain three antennas inside, one for 700–900 MHz, one for 1800–2100 MHz and one
for 2600 MHz. There are also designs that include only two antennas for the same num-
ber of bands as a single antenna combines in the 1800–2600 MHz range [30].
The next step in the base station and antenna design evolution are active antennas
where the previously separate remote radio heads move to the back side of the antennas
and become an integrated part. In addition, a single antenna has been split into several
smaller parts which are individually controlled by separate integrated remote radio
heads. This way it is possible to control tilt electronically to increase or decrease the cell
size and to add 4 × 4 MIMO capability via two separate cross‐polarized antenna strings.
As a consequence, the antenna panel width somewhat increases. Thickness and weight
of the antenna panel also increases due to the integrated remote radio heads. Depending
on the structure to which the antenna panels are attached a structural reinforcement
might be required during an upgrade from previously lighter passive antenna panels to
cope with the additional weight and the resulting additional wind forces on the struc-
ture. In return, there is only a single fiber cable between the baseband unit and an active
antenna panel and a second cable for power. One of the first network operators to
announce the use of such base station equipment to enable 4 × 4 MIMO was T‐Mobile
in the US in 2016 [31].
Like the base station equipment, backhaul transmission equipment has also seen a
significant evolution over the years. High‐speed backhaul links are essential today to
ensure that the capabilities of the LTE air interface can be fully utilized. A 3‐sector
eNode‐B with a channel bandwidth of 20 MHz in each sector can easily achieve peak
datarates beyond 300 Mbit/s in total. In addition, many network operators now deploy
several 20 MHz channels per sector (carrier aggregation), which again significantly
increases the overall bandwidth requirements. In practice, LTE eNode‐Bs are usually
collocated with UMTS and GSM equipment which add additional demands on the total
required backhaul bandwidth.
Today, two backhaul technologies are suitable for such high datarates. Traditionally,
copper‐based twisted pair cables had been used to connect base station sites to the
network. UMTS networks initially used 2 Mbit/s E‐1 links and for some time, the aggre-
gation of several links was sufficient to provide the necessary backhaul bandwidth. For
LTE, this was not a sustainable option since peak datarates far surpass the capabilities
of this backhaul technology.
For higher datarates, copper‐based cables had to be replaced with optical fibers.
While the datarates that can be achieved over fibers match the requirements of a mul-
tiradio base station, it is costly to deploy, as in many cases new fiber cable deployments
are required for buildings and often also alongside roads. Network operators that own
both fixed‐line and wireless networks can deploy and use a common fiber backhaul
infrastructure to offer fixed‐line VDSL and fiber connectivity to private and business
customers and use the same network for wireless backhaul. This significantly improves
the cost‐effectiveness of the overall network deployment.
Wireless network operators that do not have fixed‐line assets have two possibilities
for connecting their base stations to a fast backhaul link. The first option is to rent
backhaul capacity from a fixed‐line network operator. The second option is to use high‐
speed Ethernet‐based microwave solutions that offer backhaul capabilities of several
hundred megabits per second. The latest generation of microwave equipment is capable
of speeds beyond one gigabit per second.