Laboratory Report No. 95/20


By: Mark Griffin

October 1995


1. Background

2. Introduction

3. Scope for Infrastructure Sharing

4. Analysis of Infrastructure Sharing Options

5. Current Sharing Arrangements

6. Summary


Telecommunications Industry Division (TID) is undertaking a study to examine infrastructure sharing issues as applied to telecommunications services dealing, in particular, with cellular mobile telephone facility's matters. In order to further develop policy on infrastructure sharing, TID needs to know whether there are technical constraints or technical advantages to infrastructure sharing. TID requested that the Communications Laboratory do a study on the following questions:

This report presents the findings of the study. The majority of the information contained in this report was obtained by consulting with the three mobile carriers. A meeting and site visit were held with Telstra in Melbourne. Optus and Vodafone declined to formally meet and discuss sharing issues. Information from Optus and Vodafone was obtained indirectly through discussions at a sharing meeting and by observation of their facilities. Information was also obtained from books and journals.


Infrastructure sharing is an arrangement whereby one telecommunication carrier shares their cellular mobile telephone infrastructure with one (or more) other carrier(s).

The possible advantages of infrastructure sharing by the three current mobile carriers include:

There are a range of possibilities for infrastructure sharing including:

The first option, co-siting of facilities with no hardware sharing, involves placing multiple facilities at a particular geographical area without sharing towers, antennas, buildings, cables, transmitters etc.

The second option involves carriers partially or fully sharing transmission facilities such as towers, antennas, buildings, cables and transmitters at certain geographical locations.

The third option, inter-carrier roaming, is an arrangement whereby a mobile telephone subscriber to one network has access to the network of other carrier. An example is a mobile phone normally connected to the Telstra GSM network having access to an Optus base station.


The extent to which infrastructure sharing may occur depends on:

Clearly, the extent of infrastructure sharing may be limited in areas where the infrastructure is already in place and is not shared. It is not feasible to re-locate an established non-shared site (or consolidate a number of sites) to a shared site as this requires substantial resources, although it is technically possible to do so. There is more scope for sharing infrastructure in areas where a carrier has not yet provided a service or in areas where anticipated traffic levels do not justify the construction of competing infrastructure. As infrastructure has now been established in most urban or sub-urban areas, these areas will most likely be in rural or low density areas. In these areas, the co-siting and sharing of transmission facilities or alternatively inter-carrier roaming would represent viable options. Also, faster roll-out is possible, as less planning and equipment are required.

If carriers differentiate their services on the basis of coverage then sharing between the carriers may be more difficult. Telstra currently has greater coverage than the other carriers and uses this as a marketing tool. If a competitor had an inter-carrier roaming agreement with Telstra, then it could claim that it had the same coverage as Telstra. Thus Telstra would lose this marketing advantage.

The amount of radio-frequency spectrum available to the carriers is finite and places a limit on the number of users a base station can handle at once. This may reduce the scope for infrastructure sharing in high traffic areas. If a base station site uses all of its available channels to support a particular carriers subscriber during peak periods, then it cannot handle additional traffic from a rival carrier and hence eliminates the possibility of inter-carrier roaming.


4.1 Planning Approaches

The planning approaches used by individual carriers may affect whether infrastructure sharing is possible in a particular area. Different planning approaches will usually result in different cell structures covering different geographical areas and subsequently requiring placement of base stations in different geographical locations (base station sites are usually located in the centre of cells). This may limit the amount of sharing that is possible.

The planning approaches taken by the three carriers are in fact different. The result of this is that the cell structures are, in general, different particularly in built-up areas. Cells are thus of differing size and are centred in different locations. This has restricted the scope for infrastructure sharing. There are numerous reasons for having different cell structures:

4.2 Inter-carrier Roaming

Inter-carrier roaming, is an arrangement in which a mobile telephone has access to a part of the network of another carrier. An example is a mobile phone normally connected to the Optus GSM network having access to a Telstra base station. Inter-carrier roaming is not the same as and should not be confused with international roaming. International roaming is an arrangement between two carriers in which a mobile telephone has full access to the network of another countries' carrier.

Inter-carrier roaming has not been implemented by any of the carriers in this country, and there are no reports of it being used by other GSM carriers overseas. This is because the current Phase 1 release of GSM does not support it. International roaming is, however, supported by Phase 1 and is being increasingly implemented by carriers around the world. The international roaming mechanism could theoretically be used by competing carriers in a given country to allow access to all base stations of a competitor's network in that country. However, it makes no commercial sense to do so.

In future enhancements to the GSM system, known as GSM Phase 2, it may be possible to restrict access to a single base station and thus enable a carrier to implement inter-carrier roaming. With inter-carrier roaming, access to the visiting carrier would most likely be restricted to those areas where coverage is insufficient from the subscribers "home" carrier. The carriers have indicated that inter-carrier roaming will be difficult to implement and would involve upgrading software in base stations, switching stations and mobile phones. Also, once a call is handed to another carrier, it would be difficult, if not impossible, for the original carrier to get the call back thus loosing business. A surcharge would probably be levied on the subscriber for this service. Enhanced features offered by one carrier may not be available when operating through another carrier's system.

Whilst Telstra has greater coverage than the other carriers, it could potentially benefit the most from inter-carrier roaming as subscribers would tend to roam to their network in areas not adequately served by other carriers thus increasing the utilisation of their network. Conversely, Telstra would lose its greater coverage marketing advantage and thus may be reluctant to enter into an inter-carrier roaming agreement with another carrier whilst it has this advantage.

A form of inter-carrier roaming can be implemented at the moment. GSM mobiles require a SIM (Subscriber Identification Module), a smart card which holds all the information required to identify a particular subscription to a mobile service. A user can freely purchase a SIM card from another carrier and use it in their mobile phone to access the network of other carriers, simply by swapping the SIM card in their GSM phone. This arrangement allows a user to perform a form of inter-carrier roaming manually.

If inter-carrier roaming is introduced, then the carriers could possibly re-think their plans of establishing additional base stations in areas where it may not be profitable for more than one carrier to compete in, for example in low traffic areas.

4.3 Co-siting of Facilities with No Hardware Sharing

It is possible to co-site facilities without sharing towers, antennas, buildings, cables, transmitters etc. The only technical constraint is that towers need to be spaced by at least 20 metres to avoid interference caused by the proximity of one antenna structure to another. As the size/area of sites would be much larger, the aesthetics of such an arrangement would need to be questioned.

4.4 Hardware/ Site Sharing

It is possible to share hardware such as antennas, towers, buildings at sites.

Antennas have sufficient bandwidth to cover the entire GSM segment without re-tuning. Thus there are no restrictions to sharing due to antenna bandwidth.

However, antenna sharing can only occur when the cell size requirements of each sharing carrier are identical in a particular geographic area. The factors that effect the cell requirements are:

In many cases, particularly in urban areas, the cell requirements are different between carriers. In these cases antenna sharing cannot occur. The cell requirements also place more constraints on site locations. Antenna parameters are often changed at a site to improve coverage or to minimise co-channel interference between neighbouring cells. Telstra apparently adjusts 60 to 70 sites in Victoria alone each year. This fine-tuning is difficult, if not impossible, to perform if an antenna is shared as it would also effect the performance of the shared carrier's service.

As rural services are simpler, usually consisting of a single omnidirectional site near the centre of a town, the above mentioned cell parameters should be identical for all carriers in which case tower and antenna sharing can occur. A similar situation occurs at sites that provide highway coverage due to their relatively simple arrangement.

If antenna sharing cannot occur at a site, then it may be possible to share towers and/or buildings but have separate antenna "head frames" for each carrier as shown below.

(Note: This is a simulated picture of what a site might look like.)

Figure 1. GSM Site With Three Antenna Head Frames

If a tower does not have sufficient strength to handle the increased weight and wind loading subjected by additional antenna head frames or have sufficient space to accommodate additional antenna head frames or microwave link equipment then the tower cannot be shared unless it is strengthened and/or extended thereby increasing it's size. In some cases this may mean that a lattice-style tower would have to be erected instead of a concrete pole tower. A greater tower size may be less aesthetically pleasing. This is less likely to be a problem in rural areas.

Where two or more antenna head frames are placed on a tower, the head frames must be separated by at least 5 metres. This separation is necessary to avoid one antenna structure from interfering with the performance of the other. The tower must be of sufficient height so that the lower antenna head frame has sufficient clearance from nearby objects (such as trees, buildings etc) so that these objects do not adversely effect the base station's coverage. Therefore in this scenario, antenna structures are likely to be higher and more substantial structures to cater for increased loading.

One would have to assess whether the visual impact of such a structure (with two or three antennas) is less than having two or three separate towers in the same vicinity.

Buildings may be shared where there is sufficient space, electrical power capacity and air-conditioner capacity. Otherwise, the base station equipment must be housed in a separate building. The carriers prefer to have their own separate building on site as this gives them full control of the building (security, expansion etc).

Microwave link equipment may be shared provided that there is adequate capacity to handle the additional traffic and the link can be connected by some means to the shared carrier's network. Towers need to be stiffer to accommodate microwave link equipment to avoid antenna de-pointing due to high winds.


The Government has sought rapid roll-out of mobile network infrastructure in order to encourage competition. Optus and Vodafone have, as licence conditions, specified network roll-out requirements to achieve a certain percentage coverage by a given time. As the carriers will not share network roll-out plans, site establishment has been performed on an ad-hoc basis with little coordination between the carriers. In areas where none of the carriers have established a service, so called "greenfields" sites, the first carrier in would establish a site without consulting the other carriers. If one carrier met with another for example in the title's office, then they would usually get together and try to arrange a common site.

The carriers are now obliged to share facilities under the Telecommunications National Code 1994. The Code governs the activities of telecommunications carriers in carrying out the construction of specified facilities, including towers for mobile telephone facilities. The Code includes provisions governing environmental and heritage matters, and establishes a consultation process that carriers must undertake with relevant Government authorities and local councils.

The Code specifies circumstances where carriers are automatically required to notify the Department of the Environment, Sport and Territories (DEST) of their intention to establish a site. Objections raised by DEST must be resolved before the carrier can proceed. Local councils are to receive full details of the carriers' proposed activities so that effected communities can be consulted. If, after consultation, there is still disagreement between a carrier and a local government authority about the environmental significance of a site, that proposal must be referred to DEST if the council so requests. The carrier cannot proceed until any objections by DEST are resolved.

Under the Code, carriers are required to cooperate with other mobile carriers to co-locate facilities on the same sites where it is technically feasible, compatible with network configuration, and will minimise any adverse effects on the environment.

The code is somewhat limited in that it does not encourage sharing of non-carrier facilities such as those used for television and radio broadcasting, local council services, police, fire, taxi etc. It also does not specifically state that buildings should be shared where possible.

In order to try and satisfy the requirements of the Telecommunications National Code, Optus and Vodafone have jointly developed a Code of Practice for the sharing of public mobile telecommunications facilities. The Code of Practice sets out flexible cooperative arrangements to share sites, buildings, and towers based on reciprocity of benefits and contributions by each mobile carrier to these sharing arrangements. The Code Of Practice came into effect in June 1995. The document may be considered by Austel with the view of making it mandatory for all carriers.

Some site sharing between the carriers exists at present. To date, Optus and Vodafone have established more than 110 shared sites out of an estimated 2000 GSM base stations in Australia with only one site that is shared among all three carriers, located in the Adelaide Hills. There are few sites shared with Telstra. The reasons for this is are discussed in Section 4.2 of this report. Telstra have a policy not to establish more new analogue cellular (AMPS) sites.

Most Optus and Vodafone sites are leased from councils. Very few sites are owned by these carriers. At shared Optus/Vodafone sites, only one carrier owns the tower. Separate buildings are constructed on separate leases. Vodafone designs its sites for two carriers. The majority of Optus and Vodafone sites use fixed (microwave) links to communicate with the Mobile Switching Centre. In some rural or remote sites, microwave is not possible so they are forced to use Telstra's fixed network at a higher cost. The majority of Telstra's sites are connected via fibre-optic cable.


It is possible, from a technical point of view, to share GSM infrastructure. As noted in this report, the extent of sharing can be restricted by a number of technical and other factors. The greatest scope for sharing, whether it is inter-carrier roaming or hardware sharing, is in areas that have not yet been served by a GSM mobile telephone service or areas where traffic levels are low. These areas will most likely be in rural and and low density areas.

The planning approaches and roll-out plans of the three carriers have been different. This has resulted in network cell structures and roll-out being different particularly in built-up areas. This has severely restricted the scope for site sharing in built-up areas.

The views presented in this report are consistent with the views of the three mobile carriers.