APR 2009 . ISSUE 48

Solution

By Liu Wang

Difficulties in site acquisition

he first thing to be considered in WiMAX deployment is the costs for wireless coverage, which usual ly account for

over 70% of all total investments. Among them, the most typical costs come from site leasing, construction, and management and maintenance. Take for example, Hong Kong SAR. Hong Kong has a mixture of complex and typical conditions as it relates to network building, such as, flat areas, bays and dense commercial areas. Needless to say, the conditions for acquiring base station sites are quite onerous. For instance, the monthly rental for the roof top area of a five star hotel is about 40,000 USD, and another hotel asked for an admission fee of 210,000 USD in additional to rental!

Besides high rental fees, operators usually spend much time negotiating with owners of property about leasing fees for prospective sites, thus many operators may miss good opportunities in the market. However, if an operator already has sufficient base station sites, then the TCO can be reduced by up to 70%, compared with operators who have no existing sites. In addition, the operator already has a base of mobile service subscribers and rich experiences in building wireless networks. Hence, all these factors may help the operator achieve success in the market.

Other reasons contributing to the difficulty in site acquisition would include: environmental protection conditions and urban landscape design. Therefore, a practical solution would entail sharing base station sites with 2G networks.

Potential of base station

When building hybrid WiMAX/2G

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networks, sharing of base station sites helps operators not only obtain economic benefits, but also secure windows of opportunity.

WiMAX/2G shared base station includes such features as, a shared equipment room, transmission equipment, power supply, roof top resources (such as towers, poles and outdoor wire channels) and other auxiliary facilities (such as monitoring equipment and air conditioning). However, because existing equipment rooms already have GSM equipment, switching power supply, transmission cabinets and floor air conditioners, the equipment rooms intended to accommodate new WiMAX equipment should have plenty of free space. It is preferred that spaces set aside for new cabinets should be reserved when operators decide to expand their networks.

In addition, when considering the room between equipment sets, more extra spaces should be made available. In regard to sharing of equipment rooms, the weight bearing conditions of the rooms should also be examined. Lastly, it is necessary to plan in advance for the arrangement of the main equipment and power supply (particularly, capacity expansion of batteries), examine and reinforce equipment rooms (particularly, leased ones).

Is it feasible?

Any feasibility study done on sharing of base station sites should cover the following four aspects:

First, there are differences in planning strategies among different networks. For instance, 2G and WiMAX networks are oriented to the provision of voice and broadband services, respectively, and the initial goals for building the two types of networks are different. Moreover, 2G networks initially focus on coverage, and as the user base begins to increase, the coverage radius of a base station will become smaller and smaller. As

Site resource sharing of WiMAXa result, more base stations will be deployed. WiMAX networks initially focus on block by block coverage, and gradually shift to wide and in-depth coverage, because adopting this type of optimum networking policy provides better possibilities for sharing of base station sites. Further, coverage of the 2.5GHz band, 64Kbps and 2T2R indoor customer premises equipment (CPE) in dense urban/urban/suburb, urban/rural scenarios is equivalent to that of GSM900. Considering that WiMAX networks require urban coverage in the initial stage and that the capacity of GSM is limited in urban areas, then the actual coverage radius of a GSM base station is about 500 meters in urban areas, and can even be 200-400 meters in dense urban areas. Therefore, WiMAX and GSM 900MHz/1800MHz are able to share most of the base station sites.

Second, different systems usually have various kinds of interference between them. Interference is inevitable, especially when different systems coexist. The major types of interference between mobile telecommunication systems are generally spurious interference, barrage jamming, and intermodulation interference. Spurious interference is when out-of-band emissions occur and the receiving devices are not able to overcome this type of interference. Barrage jamming relates to the out-of-pass-band suppression of receiving devices, and involves carrier transmitting power and the characteristics of filters in receiving devices. The receiving devices will usually be aversely affected and may fail to work due to over saturation. Intermodulation interference relates to the nonlinearity of the transmitting devices and receiving devices. Hence, system interference problems can be resolved by employing specific technologies and measures that will allow sharing of base station sites. Huawei reduces interference through the addition of bandwidth separators and other technologies.

The distance between the upper and lower platform of a tower is usually 3–5 meters, so

Site resource sharing of WiMAX

Huawei Communicate

APR 2009 . ISSUE 48

it is relatively easy to meet the requirement in regard to vertical isolation. Poles used for height increase and masts can also easily satisfy any necessary requirement. However, how the requirement for horizontal isolation is met depends largely upon individual roof top areas. In order to reduce the need for isolation and also minimize interference between systems, the following engineering measures may be used in specific designs: adjustment of angles of declination of antennas or bearing angles of main lobes, a reduction in the transmitting power of WiMAX base stations, addition of high performance filters, and so on.

Third, weight bearing conditions and safety aspects should be examined before sharing procedures are initiated. Furthermore, towers which are not able to bear necessary weights and ensure safety should be redesigned or reinforced.

Fourth, certain pipelines may be shared. Usually, the cost for laying a new 500-meter pipeline is equivalent to that of building a new tower, so pipelines sharing is a practice that can dramatically reduce repeated construction efforts. Technically speaking, this kind of sharing is completely possible without any modification.

How to achieve?

Distributed base stations

Use of distributed base stations has proven to be an effective way to resolve difficulties in obtaining base station sites.

Huawei was the earliest supplier to introduce distributed architecture and put it into large scale commercial use in wireless applications. WiMAX DBS3900 base stations feature compactness, light weight and flexible mounting, and their BBUs are only 2U in height. Additionally, if the space in equipment rooms is limited, then BBUs can be mounted in GSM, APM or transmission cabinets, and RRUs of 15 kg per unit can be mounted outdoors, together with antennas. In scenarios where there is no indoor space available for use, BBUs can be mounted in outdoor auxiliary cabinets (in which power supply, batteries, transmission and other equipment are provided), and RRUs can be mounted outdoors, together with antennas,

or mounted inside outdoor cabinets. Moreover, outdoor auxiliary cabinets and cabinets containing RRUs can be placed on the ground, roofs of buildings, and other outdoor environments, thus no indoor equipment room is required.

When Globe’s WiMAX commercial network in the Philippines was built, remote distributed base stations represented the majority of base stations, and these sites shared transmission resources with Globe’s existing GSM/UMTS equipment.

Sharing of transmission resources

WiMAX networks and 2G networks are oriented to deliver different types of services and have different requirements in terms of transmission resources. For instance, a 2M E1 line which can support 32 simultaneous calls can only support one service stream for broadband services. Therefore, existing 2G transmission resources prove to be insufficient for meeting the requirements of WiMAX. Generally speaking, WiMAX should have dedicated transmission systems, but it is possible to share transmission racks. The WiMAX base stations supplied by Huawei are able to support various physical ports (FE/GE/E1), thus, allowing transmission sharing, if sufficient transmission bandwidths are available.

Sharing of auxiliary resources

Sharing of power supply: The power supply may be shared directly if it has sufficient capacity. If not, it should be expanded. If an equipment room has enough free space, then a new power supply unit may be provided for dedicated use, including batteries and switching power supply. Generally, a WiMAX and GSM/UMTS base station site requires at minimum, a 63A AC distribution box, 300A switching power supply, and 400–600AH batteries. However, if it is difficult to expand the capacity of the power supply (for example, the switching power supply in early days was wall mounted), then the existing power supply may need to be replaced.

Sharing of auxiliary equipment: Equipment to be shared mainly includes: digital distribution frames (DDFs), optical distribution frames (ODFs), air conditioners, incoming lines of grid electricity and fire

fighting devices. Generally, sharing of such equipment is relatively easy to achieve.

Sharing of towers, masts and roof tops: Towers and masts to be shared would include: towers on roofs and on the ground, and single pipe towers and poles for height increases. It may be necessary to renovate existing towers and masts and justify necessary costs, in order to be able to meet the special mounting and engineering requirements of WiMAX antenna and feeder systems. However, roof tops tend to be rare resources. So, if towers and masts (such as masts, poles for height increases, wiring towers) can not be shared, the same roof tops can be shared. Then it might be necessary to consider requirements in regards to isolation.

Unified maintenance and monitoring

Besides sharing of s i te resources, environmental monitoring and alarm equipment may likewise be shared, in order to achieve unified maintenance and management of different networks. Huawei’s WiMAX DBS3900 base stations provide standard ports for receiving signals from environment monitoring equipment and outputting alarms, in accordance with preset threshold values.

In the WiMAX network that Huawei recently built for Vodacom in South Africa, 90% of WiMAX/UMTS base stations use shared sites. 70% of WiMAX base stations are distributed ones and share towers with UMTS base stations. As for microwave expansion, transmission facilities are shared by UMTS and WiMAX. All of the above measures help to effectively reduce rent for equipment rooms, roof tops and towers, and allow unified operation and management, and reduce operating costs. The network was quickly put into commercial use.

As the commercialization process of WiMAX is speeding up worldwide, sharing of sites and resources between WiMAX and 2G networks is now becoming a very attractive concept. In summary, WiMAX can better use existing resources, protect the investments of operators, and allows operators to provide wireless broadband networks more quickly.Editor: Chen Yuhong chyhong@huawei.com

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