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Thursday, November 3, 2011

LTE: Backhaul Considerations

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.

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