19 Jun 2017

Cellular Base Station Installation & Maintenance Challenges

Kashif Hussain, CellAdvisor Solutions Marketing at Viavi Solutions looks at some of the challenges facing the installation and maintenance of cellular base stations.

Up until relatively recently, much of the real radio equipment at a mobile or telco network cell tower was safely and conveniently stored in the cabinet on the ground or near to the base of the site. A coaxial cable connected the radio gear to the antennae at the top of the tower.

This meant that the installation crew, and particularly the cell tower riggers, could check the quality of the RF signal being received in the cabinet without the need to climb the tower.

Of course, plenty of the maintenance required at the tower would still involve a climb, especially if the check on the RF signal at ground level showed a fault, but a good number of checks could be carried out on the ground.

Today's maintenance challenges

Fast forward to today, and the situation is changing. Modern cell sites use a distributed architecture in which Baseband units (BBU) are either located at the foot of a tower or located at a remote distance at a BBU hub. In this case it is likely to serve not just the antennae at the top of that tower, but a chain of other towers as well. What’s more, at the top of each of those towers, close to the antenna array is the Remote Radio Head (RRH) which contains the actual radio equipment (RE).

To add to the complexity – but also improve signal quality – instead of traditional coax cable, fibre is frequently used to connect the RE equipment in the RRH to the other radio heads and to the REC gear at the BBU hub, serving the towers.

Today, riggers installing and checking the sites, often need to climb the towers with full pack of advanced technical test equipment, as well as the more physical tools of their trade – such as spanners, wrenches, hammers and multiple screwdrivers.

Recently we spoke to cell tower riggers in both the USA and the UK, looking to get a feeling for how difficult their jobs had become, and what companies like ours could do to make it better. In many ways, the riggers are the unsung heroes of the telecoms world, carrying out highly specialised tasks in often dangerous circumstances and challenging environmental conditions.

Each day, their role can involve a multitude of tasks such as installation, commissioning and decommissioning, dismantling and removal of equipment from great heights be they masts, towers, pylons or rooftops. They are also involved in the earthing, labelling, and testing of feeder cables and optical fiber; as well as the inspection and testing of Outdoor Units (ODUs), Indoor Units (IDUs), Splitters and the CPN testing and auditing of rigging installation. The list is endless.

Small wonder then that more than three-quarters of the riggers we spoke to said that they now needed between 20 and 50 per cent more time at each site to carry out the required maintenance work. What’s more, some 67 per cent of UK riggers believe their job is becoming more and more complex with almost half of the riggers adding that it has become both costly and dangerous to carry out testing at the top of a tower. Hardly surprising either that having lighter, and easier to carry equipment was high on the rigger’s wish list, alongside machines that could perform multiple tasks – preferably from the ground.

Distributed architectures

The move to a distributed architecture with the RF units much closer to the antennae and the use of fibre to link the equipment has undoubtedly boosted signal quality and improved performance. But the riggers could be seen to be the ones paying the price – especially as all the RF functions for each tower are at the RRH meaning that interference analysis needs to be undertaken at the top of the tower. That means higher operational costs and increased security concerns.

The role that standards have to play in tackling these challenges and making life easier for the riggers should not be underestimated. The major network equipment vendors – including Ericsson, Huawei, NEC, and Nokia (which now includes Alcatel-Lucent) – co-operated to define a specification for the communication between the BBU and the RRH. The specification, known as the common public radio interface (CPRI) has quickly become the industry standard for the internal interface between cell towers in a distributed architecture.

The same companies, together with ZTE and Samsung, also worked together on the Open Base Station Architecture Initiative (OBSAI) which defines a set of specifications underpinning the architecture, functions and minimum requirements for integrating common modules in a cellular network base station (BTS). In particular, this governs the interchange of user and signalling data between the BBU and the RRH.

Standards for these types of interfaces might be driven by operators wanting to multi-source equipment but the difference they make to the riggers’ job in terms of maintenance and troubleshooting should not be under-estimated either.

The CPRI interface was also specifically designed to help enable flexible cell site architectures by defining a serialised interface to allow for the different topologies of network – be they simply chained, a tree and branch configuration, or a ring network. The development of a mechanism and process to analyse the RF performance of multiple RRHs from the BBU hub is one significant step forward that can increase maintenance efficiency and reduce the number of tower climbs required.

RF over CPRI

The technique is called RF over CPRI (RFoCPRI) interference analysis. RF interference affects both the uplink and downlink but the affect is more pronounced on the uplink due to the limited transmission power of the mobiles. The interference might come from external sources or from within the downlink in the cell site. A new generation of base station analysers with RFoCPRI technology can perform this interference testing without disrupting customer service by allowing RF maintenance and troubleshooting activities to be performed via the fibre interfaces at the BBU.

RFoCPRI technology verifies the CPRI control signals and the RF data transmitted between the BBU and each RRH, permitting the monitoring and analysis of the interference of both the uplink and downlink signals across multiple sites in a distributed architecture from a single point on the ground. The spectrum analysis can be undertaken on all RF signals transmitted to and from the BBU, including multiple signals of the same carrier and multiple signals transmitted on different frequencies.

Interference in the uplink can be generated internally by the cell site’s infrastructure where conductivity is impaired by loose connections, bent cables or corrosion. Being able to identify which tower being served by the BBU has one of these potential problems is a major breakthrough that can help ensure tower climbs are targeted at those sites with likely issues at the RRH level.

Next generation analyzer for base station maintenance

The level of detail these next generation analysers can extract makes them powerful tools in the rigger’s tool belt. At ground level, the tests can extract downlink RF information and demodulate it to obtain the power and modulation performance of control signals such as pilot channels, cell site identifiers and data channels.

In addition, the RFoCPRI technology can perform Layer 1 measurements of CPRI such as optical wavelengths and transmission rates as well as Layer 2 maintenance tests as specified in the CPRI standard. These include the four key performance tests - loss of frame (LOF), loss of signal (LOS) remote alarm indication (RAI) and SAP defect indication (SDI). Running all these tests gives the rigger the capability to undertake a comprehensive assessment of a CPRI’s control plane and user plane and create a full picture covering the health of a distributed network using one piece of equipment from a single point on the ground.

In short, these modern analysers remove the guess work from fibre testing and cell tower site installation and maintenance. With an increasing number of active transmitters on the RF spectrum, the potential for radio interference in cellular networks is becoming more and more prevalent – especially when coupled with the trend to network densification. As well as the growth of licensed RF services such as mobile networks, digital broadcasting and local radio systems, unlicensed services and malfunctioning devices can also generate interference.

For the army of riggers sent out into the field to install and test the network, the growth of the distributed network approach and the removal of the core RF component from the base of the tower to its new place in the RRH at the top of the mast presented a challenge that would be difficult, dangerous, time consuming, and costly.

When we spoke to the riggers they wanted kit that can carry out multiple functions; they wanted equipment that would enable good, sensible workflow; and they wanted easy to use test equipment that generated simple to understand pass/fail reports. Above everything else, they wanted to spend less time climbing up and down towers carrying out tests in difficult surroundings.

This latest generation of RFoCPRI analysers are an answer to the rigger’s prayers. More RF tests can be carried out at the base, more faults can be found on the ground, and the number of tower climbs can be reduced. What’s more a far greater percentage of tower climbs are now targeted at known faults.

Riggers are a vital part of keeping our networks running. And RFoCPRI analysers are making it easier and more efficient for them to do their job.

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About the author

Kashif Hussain is the CellAdvisor Solutions Marketing at Viavi Solutions (formerly JDSU) for the wireless business unit. He has more than 20 years of wireless industry experience. Kashif's expertise in RF, DAS, HetNets, and LTE comes from developing, managing, supporting, marketing and consulting on major mobile communications projects. Kashif's industry experience also includes various senior roles at MobileNet, Tektronix Communications, Ericsson and Nortel. He has also authored patent for wireless products.

Viavi is a global provider of network test, monitoring and assurance solutions to communications service providers, enterprises and their ecosystems, supported by a worldwide channel community including Viavi Velocity Solution Partners. The company delivers end-to-end visibility across physical, virtual and hybrid networks, enabling customers to optimize connectivity, quality of experience and profitability. Viavi is also a leader in high performance thin film optical coatings, providing light management solutions to anti-counterfeiting, consumer electronics, automotive, defense and instrumentation markets.

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