LTE: Backhaul Considerations

Owing to the high peak and average speedsof LTE, high-speed backhaul links are essential to ensure that the capabilitiesof the LTE air interface can be fully utilized. A three sector eNode-B with a channelbandwidth of 20MHz in each sector can easily achieve peak datarates that arethree times 100 Mbit/s, that is, 300 Mbit/s in total.

As eNode-Bs are usuallycolocated with UMTS and GSM base stations, the required combined backhaulbandwidth could hence be even higher. Today, three backhaul technologies aresuitable for such high datarates. Traditionally, copper-based twisted paircables have been used to connect base station sites to the network. UMTSnetworks initially used 2 Mbit/s E-1 links and for some time, the aggregationof several links was sufficient for providing the necessary backhaul bandwidth.For LTE, this is not an option since peak datarates far surpass thecapabilities of this backhaul technology.

An alternative is very high speed DSLlines (VDSL) that can deliver datarates of the order of 100 Mbit/s. This might notbe enough to cover the peak datarates required for a cell site but is a muchbetter alternative compared to E-1 link bundling. For higher datarates, copper-based cableshave to be replaced with optical fibers. While the datarates that can beachieved over fibers match the requirements of a multiradio base station, it iscostly to deploy as in many cases new fiber cable deployments are required forbuildings and often also alongside roads.

Network operators that own bothfixed-line and wireless networks can deploy and use a common fiber backhaulinfrastructure to offer fixed-line VDSL and fiber connectivity to private andbusiness customers and use the same network for wireless backhaul. Thissignificantly improves the cost-effectiveness of the overall networkdeployment. Wireless network operators that do not havefixed-line assets have two possibilities to connect their base stations to afast backhaul link.

The first option is to rent backhaul capacity from a fixed-linenetwork operator. The second option is to use high-speed Ethernet-basedmicrowave solutions that offer backhaul capabilities of several hundred megabitsper second. The latest generation of microwave equipment is capable of speedsbeyond one gigabit per second. Once high-speed backhaul connectivity isavailable at a base station site, it can potentially be used by all equipmentat that site.

In many cases, this will be GSM, UMTS and LTE. As LTE is purely based on IP technology, thebackhaul link should preferably offer native IP connectivity. UMTS base stationsoften require ATM connectivity, which can be simulated over IP. The currentgeneration of UMTS digital modules in base stations is also capable of nativelyconnecting Node-Bs over an IP connection to the RNC. GSM technology continuesto 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 backhaullink over the installed IP connection.

This way, all three base stations can bebackhauled over a single link. In the future, GSM, UMTS and LTE multimode basestations might only contain a single digital backhaul module and thus thedifferent traffic types can be transparently routed over a single IPconnection.

Find out more at the LTE Conference as part of Telecoms Tech World on 4-5 June.

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