Can millimeter waves solve the small cell backhaul problem?

The mobile industry is counting on future wireless networks being heterogeneous: Rather than the honeycomb grids of big cells we have today, we’ll see complex multi-layered networks of overlapping big and small cells, supplying our smartphones, tablets and cars with enormous amounts of cheap bandwidth. But there are many problems the industry has to solve before hetnet becomes a reality – planning systems with over 100,000 nodes being at the top of the list – but one of the biggest obstacles is figuring out how to link all of those small cells back to the network.

LTE and HSPA+ require fat backhaul pipes. Operators have managed to handle those demands by laying fiber to their towers, but that option won’t be available in a hetnet world. Those small cells will be mounted on street poles, building walls and every manner of urban fixture where access to fiber isn’t ready available and the cost of laying it is prohibitive.

But there’s an emerging group of wireless radio vendors such as Siklu, BridgeWave, and, soon, E-band Communications that think millimeter wave technologies will provide both the reach and capacity at the right prices to backhaul the hetnet. E-Band this week said it raised new funding – though it didn’t reveal how much and from whom – to develop millimeter wave small mesh radios.

Millimeter waves technically encompass a huge swath of spectrum from 30 to 300 GHz, but for wireless backhaul purposes, regulators have designated three big blocks of licensed spectrum in the 70-95 GHz range for point-to-point high-bandwidth radio links. According to E-Band co-founder and CEO Sam Smookler those frequencies are ideal for small cell backhaul for two reasons:

It’s relatively easy to get licenses for big blocks of millimeter wave spectrum, which would allow carriers to deploy large backhaul pipes over 1 Gbps in size. While a single small cell may not need that much capacity, the complexity of hetnets will require daisy-chaining many small cells together, each cell passing its load down the line. The final backhaul link in such a mesh or chain winds up handling dozens of cells worth of traffic before it can dump it onto a fiber network, Smookler said.
Small cell backhaul makes the best use of millimeter waves’ high frequency characteristics. The higher the frequency the shorter distance a wave propagates unless it gets a serious power boost. But the hetnet by definition will be composed of densely packed cells in urban environments, meaning no millimeter wave will have to travel far between hops, Smookler said.

Plenty of alternatives

E-Band and other millimeter vendors aren’t the only ones trying to solve this small cell backhaul problem. Microwave is the traditional powerhouse of wireless backhaul technologies, using its longer range, lower frequency links to connect far flung towers back to the network proper. Microwave equipment vendors like Ericsson, NEC, Aviat Networks, DragonWave, and Ceregon Networks are retooling their long-haul radio designs for small cell deployments.

The lightRadio Cube, Alcatel-Lucent’s vision for the small cell.

Like the millimeter folks, the microwave vendors are focusing on higher frequencies, where spectrum is plentiful enough to support ultra-fat links. The problem is those 60 GHz airwaves are all unlicensed, meaning no single operator will have proprietary use of them in any given area. That means there is the potential for interference between radios in areas where multiple carriers are running multiple hetnets of small cells.

Interference is also the knock on probably the hottest technology in small cells today: Wi-Fi. Metro Wi-Fi equipment makers Ruckus Wireless and BelAir Networks – just acquired by Ericsson – are not only building small cells that pair Wi-Fi and LTE together as access technologies, they’re also using Wi-Fi mesh architectures to backhaul those cells.

The big advantage of such an approach is its cheap: the 802.11n gear may be souped-up for carrier networks, but it’s certainly not the highly specialized equipment used in microwave systems. The problem is that everyone and their dogs, cats and hamsters are using the unlicensed airwaves of Wi-Fi. Operators will need to fight interference not just from one another, but home and business wireless networks.

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