Owing to the high peak and average speeds
of LTE, high-speed backhaul links are essential to ensure that the capabilities
of the LTE air interface can be fully utilized. A three sector eNode-B with a channel
bandwidth of 20MHz in each sector can easily achieve peak datarates that are
three times 100 Mbit/s, that is, 300 Mbit/s in total. As eNode-Bs are usually
colocated with UMTS and GSM base stations, the required combined backhaul
bandwidth could hence be even higher. Today, three backhaul technologies are
suitable for such high datarates.
Traditionally, copper-based twisted pair
cables have 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 aggregation
of several links was sufficient for providing the necessary backhaul bandwidth.
For LTE, this is not an option since peak datarates far surpass the
capabilities of this backhaul technology. An alternative is very high speed DSL
lines (VDSL) that can deliver datarates of the order of 100 Mbit/s. This might not
be enough to cover the peak datarates required for a cell site but is a much
better alternative compared to E-1 link bundling.
For higher datarates, copper-based cables
have to be replaced with optical fibers. While the datarates that can be
achieved over fibers match the requirements of a multiradio 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 to connect 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. Once high-speed backhaul connectivity is
available at a base station site, it can potentially be used by all equipment
at that site. In many cases, this will be GSM, UMTS and LTE.
As LTE is purely based on IP technology, the
backhaul link should preferably offer native IP connectivity. UMTS base stations
often require ATM connectivity, which can be simulated over IP. The current
generation of UMTS digital modules in base stations is also capable of natively
connecting Node-Bs over an IP connection to the RNC. GSM technology continues
to be based on an E-1 timeslot-based architecture on the Abis backhaul link.
Here, virtualization of E-1 connections can help to transparently tunnel the backhaul
link over the installed IP connection. This way, all three base stations can be
backhauled over a single link. In the future, GSM, UMTS and LTE multimode base
stations might only contain a single digital backhaul module and thus the
different traffic types can be transparently routed over a single IP
connection.
