Small Cell Backhaul

Planning for Connected Cities of the future

New York SidewalkWhile we hear a lot of marketing hype about Smart Cities, with various interpretations of what that might mean, several are taking serious steps towards becoming Connected Cities – building out infrastructure to support the data super-highways of the future. This won’t be exclusively wireless or wired but a combination of both.

Urban Densification needs to combine both fibre and wireless

It’s a myth that wireless data services are entirely cable free. Almost every cellular or Wi-Fi data stream will pass through copper or fibre at some point. This might be close by, such as when using Wi-Fi at home or many miles away when using cellular service from a microwave backhauled cell tower.

As we increase the data density within a given area, we’ll need to reuse spectrum as much as possible – hence the need for a layer of short range urban small cells complemented by extensive in-building cellular and Wi-Fi. These would ideally all be directly connected with dedicated fibre to nearby aggregation points, in a similar way to how telephone copper pairs were routed to Central Offices/Exchanges.

Wired or Wireless Backhaul

It’s unrealistic to expect every site or small cell node to be served with its own dedicated dark fibre. Perhaps that might be feasible in a very few specific places, but the cost and timescale generally don’t scale. Those countries with more strict planning/zoning laws will likely adopt wireless backhaul more readily, while those with grid street layouts and easily accessible underground conduits may achieve higher fibre penetration. High speed broadband on copper wire, using G.Fast, may also have a role to play for short distances.

When comparing costs, it’s not just the equipment price that needs to be taken into account. There’s a hefty price for closing off major roads and making good any physical disruption.

Resilience and robustness of backhaul also needs to be considered. Wireless links may be affected by line-of-sight (e.g. foliage growth, Christmas lights etc.). This can be mitigated by using a mesh topology (many nodes with alternate paths) and/or non-line-of-sight technology. Fibre links typically have a single route and could be impacted by a major fault downstream. Again, this could be mitigated with additional redundant fibres.

In practice, there is likely to be a mix of both. Fibre will be preferred but for speed of deployment, total system cost and system resilience we can expect to see considerable use of wireless backhaul.

Aggregation at existing macrocells vs independently

Previously, we’ve seen a popular view that a handful of urban small cells located nearby existing urban macrocell sites would simply use short-range wireless to the nearest one. The existing macrocell backhaul (often fibre today, but multi-gigabit microwave is also quite common) would be uprated to carry this traffic.

More recent thinking seems to be moving away from exclusively using this approach for two reasons:

  • Physically, there may be no direct line-of-sight from the rooftop mast site into the streets nearby. Indeed, additional capacity may be most urgent precisely in those areas that the macrocell is struggling to serve efficiently.
  • The macrocell site becomes a single point of failure. If the power or backhaul to it fails, then the entire area loses service.

This has encouraged wider support for a separate backhaul feed to the urban small cells, typically from one or two fibre points and thereafter relayed using out-of-band wireless.

Dark, Dim or Managed Fibre

Cloud RAN architectures, which move many of the radio processing functions to a centralised datacentre, require very high bandwidth and low latency “fronthaul” links to their remote radio nodes. This almost always means dedicated dark fibre, although eBlink have demonstrated multiple CPRI over relatively short ranges.

The low latency requirements of Cloud RAN also limit the maximum distance between RF nodes and Cloud datacentres – typically 20km fibre run at most. Some of the newer LTE Advanced Pro features could constrain that further.

Standalone Small Cells on the other hand can be connected using IP over Ethernet, making it easier to share and aggregate backhaul capacity. There is also the benefit of wider vendor choice, whereas Cloud RAN will generally mandate being locked in to a single supplier because these systems aren’t designed to be interoperable.

In both cases, there will be demand for a number of aggregation points within the cities and as many fibre breakout connection points as possible.

ZenFi deploy city-wide fibre network throughout New York City

One example of a citywide fibre deployment is in New York City, where Zen-Fi has been investing in a completely new fibre deployment. Their privately funded network will stretch for over 400 miles and offer splice-points at almost every manhole at street intersections throughout Manhattan. It’s not being built purely for small cells, and will connect many Enterprise buildings, residential blocks as well as serving the needs of other telecommunication providers.

They’ve also planned for 15 aggregation points and have already built out eight data centres, including in Brooklyn and Staten Island. These are either co-located in shared space (e.g. a traditional telephone central office) or at their own dedicated premises (e.g. a large warehouse with 24/7 security).

Their architecture uses a different topology than many existing fibre networks in place today, allowing efficient connections almost everywhere without the resulting wastage from “stranded fibre” or requirement to blow new fibres for new customers. This gives ZenFi a strong competitive advantage on cost and capacity.

Current progress with urban Small Cells

I asked Walter Kennet, ZenFi’s VP Business Development and Marketing, for his perspective on the status of Small Cell deployment in the city today. He felt the pace was mostly being set by the network operators rather than technology.

The operators are cautiously testing out new products (both small cells and wireless backhaul) and he can see several initial deployments already in place involving hundreds of radio nodes, which will later grow to match demand. He also notes a move towards Small Cells and Cloud RAN where before outdoor DAS products had been used. I’d expect this is likely also the case in other cities.

Permits are also a major factor – it can take up to six months to gain approval from the municipality – but we are hearing that many applications are in progress. Ideally a fast-track procedure would speed things up.

Equipment sharing would help reduce cost and rollout timescales considerably. Network operators seem to becoming more open to this, although not necessarily regarding spectrum sharing. New York City requires urban small cells to be hidden within a shroud when mounted on light poles. Designs are appearing which accommodate two independent small cells per installation – allowing two different network operators to share. One scenario is that we could see two parallel sets of urban small cell deployments, each shared by two network operators.

Is 5G hampering progress for urban small cells?

Walter has not seen any delay in urban small cell rollout due to 5G. Many are wondering what the implications of proposed new shared and unlicensed spectrum solutions might be.

The focus today is very much on LTE-only rather than multi-mode 3G/LTE and predominantly for capacity.

Given recent negative press comments about the general state of US infrastructure, perhaps this is one aspect where New York is setting a good example of how to invest for the future benefit of all.

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