Five key challenges to improving LTE network backhaul speed – and how to solve them

Five key challenges to improving LTE network backhaul speed – and how to solve them

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By eeNews Europe

Small cells (also known as femtocells) mounted on lampposts and connected via mesh networks are crucial in enabling operators to roll out the network in line with subscriber numbers, indeed they have been written into the LTE standard, but there are considerable challenges to overcome.

The first problem – how to cost-effectively send data between these small cells – is now effectively solved with the FCC backing the 60 GHz band and allowing an increased permissible power for outdoor 60 GHz operations between fixed points (from +40 dBmi up to a maximum of +82 dBmi with +51 dBi gain antennas) provided a narrow antenna beam (under 0.4o ) is used.

The following text therefore looks at the remaining four problems for network operators and outlines how backhaul small-cell equipment vendors can use the new FCC ruling and deliver backhaul more cost effectively.

2) Using the whole band

60 GHz backhaul equipment already exists, however, these offerings generally use FDD (frequency division duplex). This requires the use of guard frequency bands to separate transmit and receive frequencies, taking up 1-2 GHz (or 29%) of the usable frequency band for a single link.

Switching to TDD (time division duplex) architectures, like those used in WiFi and WiGig, allows the complete band to be used for both send and receive.

This has two additional advantages. Firstly, traffic allocated to up / downlinks can be dynamically adjusted to match the current traffic profile. Secondly, it eliminates the need for

diplexers, which increase the cost (and small cell footprint): they also add significant loss in both the transmit and receive paths, with 2 to 4 dB loss being typical.

3) Optimising the baseband architecture for data rate and operational distance

Whilst WiGig may be used ‘out of the box’ for 2 Gbps links, backhaul applications require the flexibility of trading data rate and operational distance.

The typical backhaul needs of an LTE small-cell base station are less than 1 Gbps; a link using a relatively low-order modulation, such as QPSK, can typically accommodate this.

Doing so is not only robust, it also allows you to future-proof the system, extending the data rate through the deployment of higher order modulation modes in future versions.

One alternative is to increase the radio link budget through the use of reduced channel bandwidth. For example, for each halving of the channel bandwidth the receiver sensitivity is improved by an additional 3 dB.

A flexible baseband architecture allows this scaling of frequency channel bandwidth thus enabling an increase in range, and providing a means of coping with differing operator scenarios.

4) Creating data packets for mesh networks

Data packetisation is controlled via the MAC function and it is not possible to simply use the standard WiGig MAC for LTE backhaul.

Indeed, data packetisation for LTE backhaul is particularly challenging and unlike standard P2P (peer-peer) networks, the wireless mesh networks used to transmit data between points add an extra level of complexity and each small cell needs to know if it is merely relaying – via the backhaul connection to another cell – or transmitting, via the mobile network to the phone.

For the time being there is no fixed standard – at Blu Wireless we are currently co-operating with several equipment vendors and operators to ensure our IP complies universally – but one solution is to use the OpenFlow standard as a MAC framework to define an industry-standard backhaul API.

5) Coping with the elements

LTE backhaul small cells will be positioned on lampposts and other street furniture and are therefore at the mercy of both the elements… and accidents.

The new FCC rules stipulate a narrow antenna beam width (under 0.4 degrees) and, as small cells for LTE backhaul transmit over distances of several hundred metres, even a position change of a fraction of degree will affect the quality of the mobile network.

Therefore, small cells need to not only be easy to install and align, they need to be automatically reconfigurable if moved or disturbed, and self-organising as the operator adds to the network.

Electronic antenna steering using phased array antenna (PAA) technology provides an ideal solution to these problems. Originally designed for military applications, this is now a mature technology and 60 GHz PAA technology is now becoming available at cost points compatible with the commercial constraints for small-cell backhaul. PAA technology also fits well with the emerging use of self-optimizing networks (SON) as SON could utilise PAA to dynamically steer antenna connections and thus re-configure network coverage for very high capacity hotspots.

About the author;

Mark Barrett is Chief Marekting Officer at Blu Wireless,

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