Small Cell Sites + NLOS Wireless Backhaul – The Specs

The plethora of new small cell sites are known by many names: picocells, microcells, Compact BTSs, outdoor WiFi access points, outdoor femtocells and metro femtocells. Independent of the name they all share a few key characteristics:

• They are deployed mainly outdoors on poles or walls of buildings.

• They usually weigh less than 15 kg (37 pounds).

• They are usually less than 20 L (5 gallons) in volume

• They are ideally deployed in traffic hotspots at the cell edge or in coverage holes throughout the cell site.

• They can be placed near a building to provide inbuilding coverage from the outside.

• They are low capacity - 1 carrier and 1 sector – for now.

• They have low transmit power – Up to 5 watts per transmitter - sometimes as low as 200 mW per transmitter

Some the leading market analysts, such as ABI and InStat, predict that these small cell sites will be deployed in the hundreds of thousands by 2013. However, there is one big issue: backhaul. Small cell sites cannot be deployed unless a low cost, flexible backhaul can be found.

So what are the perfect specs for a small cell site / picocell backhaul system? Here is our view of a few of the most fundamental requirements:

• Non Line Of Site (NLOS) - When an operator mounts there picocell on a lamppost or a utility pole, the small cell site will be Below Roof Top (BRT) i.e. in the RF clutter. This deployment usually creates an NLOS environment for the backhaul link. A NLOS technology requires long wavelengths and as a result the backhaul system must operate below 4 GHz.

• Throughput - More is always better but, a backhaul system must handle the realistic peak traffic from the picocells: 20 Mbps DL + UL for an HSPA+ picocell and 60 Mbps (DL + UL) for an LTE picocell.  This should suffice for several years. The use of licensed spectrum for backhaul is also important.  If unlicensed spectrum is used, backhaul throughputs cannot be assured since random, uncontrolled unlicensed users will force the spectrum to be shared and backhaul throughput to decrease.

• Environmental - Outdoor equipment designed to meet weather extremes is a must.

• Point-to-Multipoint (PTMP) - Traffic from 2 to 6 or more picocells should be aggregated at a single wired managed Ethernet location. The Taqua NLOS backhaul architecture shown in the diagram below illustrates the value and flexibility of a point-to-multipoint solution.

• Size - Smaller is better but the trade off with range and data rates must be considered –a variety of antenna sizes is ideal, with larger antennas for suburban and rural deployments and smaller antennas for urban deployments.

• Range - Any distance from 200 m (dense urban) to 4 km (rural) could be required. The key initial deployment scenario will be to deploy the small cell site at the macrocell cell edge. Since most cell sites have radius greater than 500 m, it is important to have system with a range of > 500 m in an NLOS environment. The following diagram shows that NLOS range is very important:

• Powering - AC power is essential at the utility pole. Fortunately, as the diagrams below from Alcatel-Lucent and Solis Energy show, the muni WiFi industry has solved the lamppost powering challenge with AC Power Tap Adapters:
  
  
  

At the hub site, AC or DC may be available. -48VDC will be the standard powering for hubs located at a macrocell site.

• Element Management System (EMS) - Some carriers will want to have the FCAPs of the NLOS backhaul system integrated via SNMP into their existing NSS / EMS systems. Other carriers will want a standalone NLOS Backhaul EMS that provides detailed performance and fault data with logical and geographic views. The key is to have a system that is flexible and easy to monitor and manage.

• Ease of Installation - The NLOS backhaul will usually be installed at the same time that a small cell site is installed. The approaches used to mount and point the remote backhaul module must be simple and fast to implement.

• Timing / Synchronization - Accurate timing is critical for the backhaul system itself and the small cell site. GPS timing at the hub is an important option. Initially, most small cell sites will use NTPv4 for timing so the requirements for the backhaul system are minimal. However, over time the system may have to support IEEE1588v2 or SyncE for future small cell sites and to coordinate with the carriers overall sync/timing strategy.

Non-Line-of-Sight Backhaul is a product category that makes use of 4G technology (OFDM / MIMO) optimized for a backhaul environment. The specifications will evolve over time but over the next several years the focus should be on a cost effective, simple to install, simple to operate, high capacity implementation.