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 0.4 o ) 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