Full duplex radio: key to backhaul

Geoff Carey
Full duplex technology for cellular backhaul
As backhaul seeks to increase capacity with the expected data explosion that is happening, full duplex seems to offer a useful increase in capacity.

Wireless traffic continues to increase exponentially, and backhaul is now struggling to keep up with the required capacity. This is already a problem for 4G operators, but with the explosion in traffic expected when 5G begins to be deployed, the magnitude of this problem will only get worse.

Evolutionary developments in modulation technology – using higher orders of quadrature amplitude modulation (QAM), data compression engines, and similar – have yielded only incremental increases in overall throughput capacity, so it is clear that a more radical step forward in wireless transport technology is required.

Recent advances in full duplex radio technology research and development could provide the answer, so it is worth us looking in more detail at this technology and the benefits this technique can bring. Deploying full duplex radio provides an immediate doubling of capacity and spectral efficiency compared with conventional frequency division duplex links, and this has recently been demonstrated for the first time in a live mobile network in Italy.

Radio frequency spectrum is a highly regulated and often costly resource, both in the access layer (the handset to base station connectivity) of mobile telecommunications networks and the transport or backhaul layer, which typically provides the connectivity between the base station and the core network. The regulatory framework has historically mandated relatively narrow frequency channel allocations, which have typically been restricted to 28 MHz and lower channelization – and multiples thereof – in the microwave frequency bands between 7 GHz and 42 GHz. Up until now this has effectively limited the usable throughput of the channels in these frequency bands.

Full duplex radio is an innovative technology that can relieve this bottleneck. By using the same frequency spectrum allocation to provide both the forward and reverse directions of communication simultaneously, it immediately gives both improved spectral efficiency and enhanced throughput capacity. Traditional radio methods require the use of either frequency division duplexing (FDD) or time division duplexing (TDD) to achieve bi-directional communication. With traditional FDD techniques, twice the spectrum is required compared with that for full duplex radio to achieve the same throughput. With TDD, the channel is time-multiplexed between send and receive transmissions, limiting the overall throughput to half that for full duplex radio.

The realization of full duplex radio has been made possible with recent advances in digital signal processing techniques and improvements in RF and microwave device fabrication and performance. While much research and development in full duplex radio has been focused on solutions for the access layer, MIMOtech has taken a different approach by developing a solution to double the capacity of traditional microwave backhaul. Its Air Division Duplexing (ADD) technique applies a combination of spatial multiplexing, microwave engineering and digital signal processing to achieve a full duplex radio implementation in the frequency bands between 15 GHz and 42 GHz, for high capacity wireless backhaul transport.

MIMOtech has implemented the ADD technique in its latest Janus AirDuplexTM Radio, to achieve a net doubling of throughput capacity when compared to traditional microwave FDD solutions. With this approach, mobile operators are now able to backhaul with transport capacities of up 1 Gigabit per second in a 28 MHz RF channel allocation. The technology is scalable, giving multi-gigabit throughput solutions in channel bandwidths of 56 MHz and above in the traditional microwave bands, as well as in the larger channel bandwidths available in the millimetre-wave frequency bands, allowing true fibre-like capacities up to 10 Gigabit per second to be transported over wireless backhaul links.

A major advantage of ADD to mobile network operators and telcos is that it extends the usable life of the extensive microwave spectrum investments that they already hold in the bands between 15 GHz and 42 GHz. These operators now have the option to replace legacy equipment with higher capacity microwave radios instead of migrating the backhaul network to millimetre-wave spectrum in the quest for higher transport capacities.

Shortage of backhaul capacity is a huge problem that needs a game-changing solution. Full duplex radio will be a key component of this solution, and that it will play a crucial role in the transport networks of the future.

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