Small Cell Backhaul

KUMU aim to bypass urban small cell backhaul

KUMU NetworksHow do you introduce really innovative new wireless technology into a well-established ecosystem without needing to rip-and-replace billions of dollars of infrastructure? KUMU Networks reckon they have come up not only a radical method to double network capacity, but also a novel approach on how to introduce it efficiently.



Doubling network capacity by sending and receiving at the same time

I’ve met with KUMU Networks at several conference events over the past year, and each time they seem to have progressed further along their planned path. Their founders are a team of Stanford University Professors and Ph.D.’s who have succeeded in translating academic theory into practical reality. Last year, many were sceptical about their claims to be able to achieve full duplex simultaneous transmit/receive. Today’s wireless systems are all separated either by frequency (FDD) or time (TDD). With their technology, there is no “DD” – it’s full speed in both directions simultaneously at the same frequency. You can imagine this as a powerful echo-cancellation scheme on steroids.

They have conducted trials with a number of Tier 1 wireless operators and are now publicly endorsed by SK Telecom and Telefonica who both state the technology really does work and has significant commercial potential especially as part of 5G.

Radio technology agnostic

Joel Brand, VP Product Management, tells me this is not specific to any particular RF technology and can support 2G, 3G and 4G even including future LTE-Advanced features yet to be deployed. There’s no reason it shouldn’t work with Wi-Fi or Microwave too.

[Picture below outlines the concept taken at Mobile World Congress]

KUMU Full Duplex

Replacing the duplexer

In the medium term, KUMU propose to replace the existing RF duplexers that are found on every basestation. These passive devices take separate RF connections from the basestation’s transmit and receive circuitry and combine them into a common antenna signal. There is naturally some RF signal loss of a few dB through these devices.

This would be swapped out for a unit with exactly the same inputs and outputs, but which can cope with simultaneous send and receive.  The RF loss through the KUMU box would be the same or less than that through the duplexer it replaced. The same cabling connections and form factor are used, except that a power source is added.

Since today there are no devices able to cope with the signal at the receiving end, a proprietary device would be needed. Prototypes have been demonstrated by replacing the duplexer on a standard Qualcomm smartphone with a KUMU design. For point-to-multipoint operation, the basestation itself would need to be software enhanced to make use of that capability, which today is not supported by 3GPP standards.

Once fully implemented, full duplex operation has the potential to double the capacity of the system within the same spectrum bands.

The diagram below illustrates the concept, and was shown as a live demo at Mobile World Congress.

KUMU Full Duplex Demo

Self-Backhauled Small Cells

Their long term vision would be that this IP (Intellectual Property) is embedded in every wireless device, both the basestations/access points and smartphones/tablets. That isn’t realistic in the short or medium term because it would be incompatible with existing infrastructure and devices in use today.

So they’ve come up with a faster route to market that sounds potentially easy to deploy as an alternative for both wired and wireless small cell backhaul.

LTE standards already define an LTE relay node, which use LTE for both access and backhaul but at different frequency bands or modes. These can be useful to extend coverage at the cell edge, but don’t necessarily add capacity.

Kumu proposes to build a relay device that combines a standard LTE small cell and a standard phone, reusing the same LTE spectrum for both ‘access’ and ‘backhaul’ technologies. It wouldn’t require any change to the macrocell, smartphones or the small cell. Their full duplex system would allow the small cell to operate simultaneously with the backhaul, hence the term “Self Backhauled Small Cell”.

The end-to-end network architecture is similar to that used for residential Femtocells and shown below. In this initial configuration, the serving macrocell would not treat the backhaul data from the small cell any differently to data sessions from smartphones. This would be routed back to an LTE gateway or directly to the core network, where the small cell would be seen as just another cellsite.

KUMU Self Backhaul Small Cell Architecture

Using standard LTE and/or bands for urban backhaul isn’t a new concept

The 3GPP standards already specify a LTE Relay Node which does exactly that.

Release 11 further enhances these with additional features related to LTE-Advanced.

As with standard LTE Relay Nodes, the Kumu solution benefits from being able to use a separate high gain directional antenna, MIMO and the fact they remain static (unlike users moving around quickly) which all improve the quality of the link. This enables use of higher order modulation rates, squeezing much greater throughput from precious macrocell radio resources. Relays only require suitable site and power, avoiding the need for dedicated backhaul.

KUMU’s solution then takes this a step further by maximising spectrum reuse.

Ultimately, the capacity of any sector would be limited by the peak rate of an LTE carrier at the macrocell. Higher modulation rates effectively increase the utilisation of the basestation equipment, squeezing out more throughput from the capital investment. Increased backhaul capacity to the macrocell site may be required to take full advantage and avoid bottlenecks.

Tighter integration

KUMU argue that in addition to the benefit of 2x spectrum efficiency by using full duplex, the end-to-end delay budget would be less than using a standard LTE relay, leading to faster end-to-end throughput.

KUMU claim their technology goes one step further. Leveraging the operators own spectrum for  direct, low latency communication between small cells and macrocells should allow LTE-Advanced features such as eICIC and CoMP to be implemented.

This would require tighter integration with the macrocell using the defined X.2 interface.

Other applications

Joel told me he thought that such full-duplex relays could also be used for connected cars and other hard to serve use cases where small cells would be mobile.  The thermally efficient materials used to build cars these days can be just as hostile to RF conductivity as modern buildings, making it difficult to serve any high speed devices inside.

In the longer term, the technology would then be embedded within devices and the system make more general use of it. Perhaps this will appear as part of 5G using dedicated frequency bands rather than becoming retrofitted to 4G.

This is definitely a technology to watch. Whether it becomes fully mainstream as KUMU Networks would like, only time will tell. I’d still expect there to be a strong market for standalone wireless small cell backhaul products, with the most direct competition for this technology being the use cases involving NLoS (Non-Line-of-Sight) backhaul for coverage not-spots.

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