Earlier this week I noticed an article “Govt Calls for shared Network Towers” in the news. Cambodia’s Minister of Posts and Telecommunications called on the country’s nine mobile phone networks to share their network infrastructure to reduce the number of towers being deployed in the country.

Requests to reduce tower numbers are becoming more common, and the article led me reflect on tower issues I have come across.

1. Over the years many operators have merged. Each company originally had it’s own towers, and mergers have left companies with towers next to each other. The Indonesian government has stepped in and made companies start to remove excess towers.
2. The Brazilian government was concerned about too many towers, so mandated no new towers were allowed in certain areas.
3. As operators load up towers with additional antennas, many are finding the towers are not meeting MTBF specifications due to accelerated rusting or poor construction. Both weight and wind loading can be problems. Many installers I have spoken with mention they can feel the towers move under their own weight.
4. One client was recently considering removing antennas due to excess weight on the tower from the microwave dishes used for back haul. They were using two 2G BTS on each sector, and combining them would have resulted in 3dB loss  in both uplink and downlink. (Additional antennas would have to be added eventually when 3G was installed, thus not really fixing the problem.)
5. Sharing towers can introduce interference and Passive Intermodulation (PIM) problems.

With the race to roll-out 3G, tower issues are an ever increasing headache for operators, with many 3G BTS units in a heavy tower mounted format, which also require the mounting of additional antennas (increasing wind loading problems).

There are three common configurations operators can utilize for rolling out 3G, but each comes with its own advantages and disadvantages.

Option A – GSM and 3G on same feeder and antenna

Used by Telstra in Australia, this system uses existing antenna and feeder infrastructure, which reduces cost, weight and wind loading issues.

A high roll-off filter is used to combine the 2G and 3G systems at the BTS end of the feeder.

For this configuration to work,  particular attention must be paid to the construction quality and Passive Intermodulation in the feeders. The system must have very low PIM, making the system “technology transparent”.

Option B – GSM and 3G on separate feeder and antenna

The 2G and 3G systems utilize different feeders.

Costs are increased by having to run new feeders and mount new antennas, also increasing wind loading significantly.

Passive Intermodulation is less of a problem using separate feeders, but the operators can still have PIM interactions between the systems.

Option C – 3G tower mounted BTS

The 2G utilizes existing feeders, while a tower mounted BTS (normally connected by fiber optic) is used for 3G with an additional set of antennas.

The costs are lower, but both weight and wind loading are significantly increased.

Passive Intermodulation is less of a problem using separate feeders, but the operators can still have PIM interactions between the systems.

Conclusion

Requests to reduce tower numbers will only become more common in the future. As operators add more infrastructure to existing towers, weight and wind loading will begin to be problems.

There are several configurations that can be adopted by operators, but one solution does not fix all the problems. Instead the operator needs to approach each installation individually to determine the best solution.

More articles:-

• Govt Calls for shared Network Towers – 20 Nov 2009
• Rajasthan Bans New Mobile Towers Pending Review – 8 Dec 2009