There are continuing debates regarding Capex and Opex in mobile networks as well as the particular sensitivity of mobile network systems to the RF connectivity employed.
The latter merits an increased focus as networks grow in complexity and performance requirements, and the cost of operation is under increased scrutiny. It is becoming increasingly apparent that the quality of the RF path between radio and antenna, although a small part of the capital outlay, has a huge impact on the network performance and hence its revenue potential.
Moving into the second decade: general trends
Over the last decade, voice ARPU has fallen by more than 60 per cent. This is accelerated today by the massive buildout activity in developing countries. The market has generally reacted by driving the capex costs down and commoditizing the components used.
More recently there has been an increased focus on the total cost of ownership (TCO) and specifically the costs of operation. The cost and impact of power consumption were regarded as insignificant in the past and are now a top priority for the mobile operator. This is addressed today by reducing the losses incurred by RF feeders, or shortening the RF feeder path by installing active equipment at the top of the tower. Several initiatives are progressing using solar powered BTS sites. This further reduces the need of diesel powering of remote sites where no mains network is available.
Equally, tower loading is becoming an issue of concern. As more and more services are added to existing sites, the mast approaches its maximum loading capacity, or more costly masts need to be dimensioned to accommodate the load. Lighter and leaner solutions are sought for accordingly.
Another increased focus is the cost of maintenance. This can broadly be classed into preventative maintenance and recovery maintenance. In the case of the former, experience in the field dictates what costs are entered into routine procedures. Recovery maintenance is an unpredictable cost. It requires that the cause of a failure or performance degradation is identified and rectified.
Beyond what has been mentioned above, environmental or health-related considerations have migrated from their origins in the power and automotive industry into the telecom industry, playing a significant role today. Examples of these are site-sharing to reduce energy consumption, energy radiation and visual impact on the environment, directives on the return of waste materials after end-of-life and directives concerning the use of hazardous materials, notably lead (RoHS).
Last but not least, as the density and traffic throughput of wireless networks increase, so do the performance criteria of the network. Data ARPU, although still a small fraction of voice ARPU today, is increasing by over 100 per cent per year and will come to dominate revenue streams in the future. Despite the many constraints listed above, the operator must enable and enhance his data ARPU at all costs and thus cannot afford to permit any solutions which compromise network performance.
The critical role of RF connections in the network
For each service offered, a typical cellular site will employ 3 panel antennas and 6 RF paths to connect these antennas to their BTS/Node B interfaces or remote radio head. Each site will therefore employ a minimum of 12 coaxial connectorizations (the connecting of a coaxial feeder to a connector) or, in most cases, multiples thereof.
Using RF feeders with soldered or even simply field-assembled connectors sealed off with tape, the performance and testing criteria of these connectorizations stand in marked contrast to the stringent hi-tech requirements applied to the rest of the system. Typically, the VSWR performance and throughput of the RF path are measured once at the time of installation. Many aspects are neglected which will have future impact. For example, the integrity of the sealing or the passive intermodulation, vital for the overall network performance over time, are generally not tested at all. Technology-wise, the stone age meets the 21st century.
Small percentage statistical performance degradations and failures of the RF connections will have a dominating effect on the overall network quality and availability.
Thus in many cellular infrastructures today, this element of the system represents the weak link.
Exhibit 1: Cellular site showing incidence and role of RF connections
RF path failure mechanisms:
The variety of potential failure modes in the RF path between base station and antenna is manifold.
Typical failures (both detectable and latent) include:
- Increased insertion loss or VSWR (reflection)
- Increased intermodulation
- Moisture ingress
- Change in the electrical parameters due to strain and movement over time
- Cold solder joints/remaining flux inside the cable
These failures result in increased signal attenuation, increased reflection or VSWR and unwanted spurious frequencies, the so-called intermodulation products. In extreme cases the RF path can be completely cut off or short-circuited.
Impact on Opex, TCO and Image
The impacts of each of the above are: an initial alarm or some macro data (e.g. churn) indicating a fault, searching for and identifying the fault, field repairs at bottom and top of tower and associated downtime. This has a significant impact on network health which is a parameter under scrutiny as the traffic demands increase. In addition, intangible damage is caused by customer perceptions of an unreliable network.
Even when customer perception and churn are left out of the equation, the maintenance costs which arise due to hidden connector failures are higher than commonly expected. They are a multiple of several 10’s or even 100’s of times of the cost of the connectors themselves. Some operators have quoted tens or hundreds of thousands of dollars per month for corrective maintenance on faulty connectors. Even more surprising is the extent to which these costs and failures have been tolerated. In many cases, this stems from a division in responsibility for total cost of ownership (TCO).
RF connectors, as a low percentage of the total site costs, have for many years been regarded as commodities with a simple performance specification for which the lowest price is sought. When field failures are finally discovered and addressed, they are typically under a different budget than that which prevailed at the time of sourcing. Maintenance contracts will then also seek the lowest bidder. Had a solution been implemented from the outset which precluded the hidden failures, the scope of work for maintenance and therefore the cost of the same would have been massively reduced.
Exhibit 2: exemplary comparison of capex and opex using standard connectors and compression connectors
Scenario: 5,000 sites, 60,000 field installed connectors, 1 per cent standard connector failure rate per year, EUR 1,500 cost per service call.
Even before the obvious impacts of hidden failures listed above are noticed and actioned, there are also insidious effects. The maximum data throughput rate of 4G systems has been observed to be highly dependent on the PIM performance of the RF path. If this is gradually degraded before the source is identified, the investment in 4G equipment will not achieve the expected benefits.
Technical solutions to overcome failure
PPC have introduced a technology to the wireless market which overcomes the failure mechanisms listed above. Compression technology is based on exerting a very high level of permanent pressure over a large surface area between cable and connector. In addition, the connection process between the two is automated to ensure repeatable results. Finally, the number of steps required to complete the connection assembly is reduced from up to 14 steps down to three.
Exhibit 3: cross-sectional view of compression connector before and after compression
Common faults and failures
In the second half of 2009, PPC conducted a survey amongst 1500 participants from installation and mobile network companies to gain insight into installation habits and typical RF path failure mechanisms in the field.
Survey respondents affirmed that:
- The most common fault identified in the RF path was loose connectors, followed closely by faults due to moisture ingress
- Over 60 per cent of the total failures in the RF path were due to problems associated with connectors. By contrast, failures due to damaged cable and other hardware combined were less than 18 per cent.
It should be noted that over 90 per cent of the damage due to moisture was further ascribed to failure of the sealing tape.
Additionally, information provided by several global carriers revealed:
- The costs for rectification ranged from $800 to over $2,500 depending on troubleshooting duration and ease of accessibility.
- If data was kept at all, the proportion of failed connections to total connectors in service ranged between 2 per cent and 10 per cent.
- Corrective maintenance was ascribed to approximately half of all field activity.
Using the above information it was calculated that the normalized cost of failure per installed connector is approximately $82.50 per connector.
The path to full returns
With RF connectivity hitherto a low priority concern for many network operators, it is now emerging as an area with significant potential for increased reliability and associated cost reduction. Using compression technology and increased automation in the assembly and installation process for connectorizing RF feeders, it is possible to eliminate these failures and their associated costs even if such assemblies are performed on the field, whilst significantly improving the health of the network. In order for an operator to pull the full functionality and value out of his equipment and to gain a full return on their investment, it is essential that they do not neglect the long-term health of the RF connections which “glue the system together”.
Joerg Springer
