WIRELESS NETWORKS OF DEVELOPING COUNTRIES -

EVOLUTION AND PLANNING PROBLEMS

R. Passerini - ITU, Switzerland, I. Stanev - ITC, Bulgaria,

R.Goetz, W.Schutz - LS telcom, Germany

1. IntroductionMobile communications has advanced beyond imagination in all areas including

coverage, services, technology, handsets and regulation. Perhaps the most revolutionary change is that in the space of around 20 years, mobile subscribers surpassed fixed lines in 2002 to become the predominant method of voice communications - Figure 1 [1].

Fig 1: Number of worldwide fixed and mobile telephone subscribers, 1982-2003

Table 1 shows statistics for the compound annual network growth

of the telephone lines and the mobile subscribers for 2000-2001.

Table 1: Statistics for Network growth (compound annual growth rate in %)

New telephone lines added 2000-2001

New mobile subscribers added 2000-2001

Low Income 8,3 72,4Lower Middle Income 17,2 70,5Upper Middle Income 7,4 27,8High Income 0,3 14,8Africa 7,6 51,0Americas 2,1 21,2Asia 12,4 38,4Europe 2,4 20,0Oceania 0,2 26,7

ITU&ITC Workshop for Developing Countries at ITC18, Berlin, Germany, 2 - 4 September 2003 4.5 - 1

WORLD 6,0 26,7

It is seen that even the highly developed countries experience considerable growth of the mobile subscribers ( 14,8% ), especially in comparison to the growth of the telephone lines ( 0,3% ) [2].

2. Evolution of wireless networks First generation mobile cellular networks, launched in the early 1980s, employed

analogue technology. Developments in digital technology led to second-generation (2G) systems. These networks, developed by the end of the 1980s, provided better quality, greater capacity and additional functionality. By the end of 2002, the world had almost completed the transition to digital cellular networks with analogue users accounting for a mere 2.7 per cent of total mobile subscribers. There are four digital cellular radio technologies in use around the world (Figure 2):

Global System for Mobile Communications (GSM). This is the predominant technology in Europe as well as many nations in Africa, Asia and some in the Americas. At December 2002, there were 787.5 million GSM subscribers on 467 networks in 169 countries.

Time Division Multiple Access (TDMA). This is the leading technology in the Americas with 109.2 million subscribers at December 2002.

Code Division Multiple Access (CDMA). At December 2002, there were 146.7 million CDMA subscribers with 61 per cent in the Americas, 37 per cent in the Asia-Pacific region and less than two per cent in Europe, the Middle East and Africa.

Personal Digital Cellular (PDC). This propriety system is deployed only in Japan with 60 million subscribers at December 2002.

Note: The Personal Handyphone System (PHS) is a digital system launched in Japan in 1995. As a low-cost alternative to cellular, it has limited coverage area. There were 5.6 million PHS subscribers, all in Japan, at December 2002.

Fig 2: Distribution of worldwide mobile digital subscribers by technology, 2002

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A significant development with 2G systems has been the increasing utilization of data like services. For example, Short Message Service (SMS) allows text messages to be sent between mobile handsets. Some 360 billion SMS were sent over GSM networks in 2002. Mobile handsets can also be used to access the Internet. This has become successful in some countries such as Japan where 80 per cent of cellular users subscribe to a mobile Internet Service Provider.

The growing use of mobile data has led to demand for faster speeds since the normal transmission rate is quite low (e.g., only 9.6 kbps for GSM). This is being accomplished through the launching of so-called 2.5G networks such as General Packet Radio Services (GPRS) for GSM and cdmaOne IS-95B for CDMA. These technologies offer speeds that are equivalent to or even faster than conventional dial-up (GPRS provides a data transmission rate of 115 kbps in ideal situations whereas cdmaOne IS-95B has speeds of up to 64 kbps).

The need for faster speeds and multimedia services has led to the development of third generation (3G) systems. In an effort to consolidate the existing incompatible mobile environments into a seamless global network, the ITU at the Radiocommunication Assembly and World Radiocommunications Conference held in May and June 2000, adopted a set of 3G standards. The new global standard, realized after years of collaborative work between the ITU and the global cellular community, is known as International Mobile Telecommunications-2000 (IMT-2000). In addition, the conference provided guidelines for the licensing and manufacturing of IMT-2000 equipment. IMT-2000 is based on Code Division Multiple Access (CDMA) and consists of five different radio access technologies (Figure 3):

IMT-DS Direct Spread, referred to as W-CDMA, UTRA, UMTS-FDD IMT-MC Multi-Carrier, referred to as CDMA2000, includes CDMA2000 1X

and CDMA 2000 1X EV-DO/DV IMT-TC Time-Code, referred to as UTRAN-TDD and TD-SCDMA IMT-SC Single Carrier, referred to as UWC-136/EDGE IMT-FT Frequency Time, referred to as DECT

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Fig 3: IMT-2000 Terrestrial Radio InterfacesWCDMA (Wideband CDMA) represents the next step in the technology chain,

delivering the full potential of 3G through its high data rate capabilities. WCDMA, which is used for UMTS systems, can handle simultaneously access to a variety of voice, data and video services [3].

UMTS will build successfully on today’s investments in second generation infrastructure equipment and systems, by re-using their resources, investments and experience. GSM is the world standard that underpins the global evolution to 3G. The success factor of GSM need to be maintained and build upon, going forward to the ‘third generation’.

3. Modeling and planning tools

The process of planning wireless networks always encounters a set of parameters that must be optimised simultaneously. Not surprisingly, some of these “global” parameters interfere with each other. For example a good coverage of the network can be achieved with a large number of base stations, but also with a smaller number of sites –for the price of a resulting higher average interference and smaller capacity on the air interface: These “global” goals must thus be guided by some “local” planning goals. As this little example demonstrates, there does not exist a single “optimal” planning solution but a set of “equally optimal” solutions.

Fortunately, the global conditions could be divided into several independent steps; this forms the classical approach of network planning for 2G and 2.5G networks as done in the last 20 years with manual and PC based planning systems.

A classical site is a base station with one or more antennas, where normally, each antenna corresponds to one cell of the network. Several antenna configurations have shown up in the past, while antenna space diversity is a popular configuration in GSM and also in 3G CDMA networks. However, for the planning task itself the antenna configuration is not that interesting while the antenna characteristics (antenna patterns, azimuth, mechanical/ electrical downtilt and gain) are important parameters for controlling coverage and interference [4].

The classical approach to place sites is still the manual one, based on a large level of expertise and skill by the planner. A “green field” layout starts by using a regular hexagonal grid based on flat earth propagation assumptions. Figure 4 shows an example of a city after placing such a regular hexagonal site grid without optimisation as an initial network scenario.

ITU&ITC Workshop for Developing Countries at ITC18, Berlin, Germany, 2 - 4 September 2003 4.5 - 4

Fig 4: Initial network scenario with regular grid layout

As hexagonal grids allow to achieve good results in homogenous areas but don’t match the requirements for more real environments a refinement in order to account for additional conditions like street orientation and high traffic areas is done in the next step. The result of such a refinement based on traffic and clutter data is given in Figure 5.

Fig 5: Optimised site locations for morphological structure and traffic density

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The underlying principle is to create small cells in areas where high traffic is expected (typical in urban areas) and to extend base station distances to it’s maximum in areas where capacity is not an issue (e.g. rural and suburban areas), thus minimizing the overall network costs. The resulting cellular structure for the used example is given in Figure 6

Modern tools [5] support the user in doing this with high efficient user interfaces and automatically take terrain and clutter information of the environment into account. Graphical interaction with the GIS system and the database enables the user by easy click and drop mechanisms to define the sites. The user can select from a template of sites and place them on the geographical location, move, copy, rearrange, create or delete sites and/or cells and quickly evaluate these scenarios.

Even when the cellular structure will be adjusted to the offered (or expected) traffic, the planning procedure is more based on coverage targets than on QoS requirements. Of course, coverage is a prerequisite for QoS, but typically, QoS is first respected during frequency planning. Planning goal during frequency assignment is to achieve an interference level that does not exceed a quality criteria defined by the service. (Or technically speaking, the carrier to interference ratio C/I has to exceed a defined threshold). For 2G networks this interference level has been adjusted to the voice transmission. (See Figure 7 for a C/I calculation before and after frequency optimisation)

The overall network quality may be calculated by combining the results from coverage and interference calculation for the network. This will result in a print out giving for each pixel inside the coverage area the probability that a successful call can be done (Figure 8).

Fig 6: Resulting cellular structure after refinement

ITU&ITC Workshop for Developing Countries at ITC18, Berlin, Germany, 2 - 4 September 2003 4.5 - 6

Fig 7: QoS: C/I before and after frequency planning

Fig 8: Probability for successful call as combination of interference and coverage

As different services have different requirements regarding field strength and admissible interference level, service areas for different applications would be different and therefore have to be planned (and verified) separately. This is not a big issue for 2G networks with their focus on voice service, but will be crucial for 2.5G and 3G networks coming up with a service mix.

In UMTS for example a given “service” offered to any user may be achieved by the allocation of spreading sequences or a number of sequences depending on orthogonal variable spreading factor (OVSF) sequence availability. The network may achieve the target bit error rate (BER) through higher transmit power or higher SIR target setting, or through use of more powerful FEC coding depending on the delay constraints requested

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for the service [6]. Unfortunately, these settings cannot be optimised per service but only per service mix as a UMTS base station must serve all users and their specific services within its cell simultaneously. These all interact which makes an “all-in-one”-planning process necessary.

In 2.5G systems the relation between service requirement and the need for individual service planning is better to understand as the technology is not as complex as in 3G systems like UMTS. In GPRS systems, for example, possible data rates offered depend on the number of packet switched users in the cell and on different modulation types, so called coding schemes (CS). Depending on the hardware suppliers different QoS ratings are necessary to enable a specific data rate (Table 2).

Table 2: Coding schemes and data rates for GPRS

Higher coding schemes like CS-4 are offering higher data rates but need a better network in terms of field strength and interference. These effects have to be taken into account when the site locations are planned to ensure seamless availability of service in all areas where it is required from the customer. Modern tools therefore provide the possibility to calculate different parameter like available coding schemes and the resulting throughput for the network (Figure 9, 10).

Fig 9: Possible coding schemes for GPRS

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Fig 10: Throughput in kBits/s

4. Conclusion

Wireless systems such as IMT-2000 may be cost effective and flexible for operators that want to expand their network as demand for voice/data services increases: less expensive, faster deployment, handling of both fixed and mobile traffic, voice and data services providing high speed connectivity to be used by clinics, schools, libraries, governments, telecenters and others.

For developing countries, “mobile” networks are becoming de-facto unique networks as it is cheaper to install (less cost due to wireless access).

Appropriate planning tools will become more and more necessary requirement to solve in a cost effective way all complex network issues.

References:[1] ITU, World Telecommunication Indicators Database, 7th Edition 2003

[2] I.Stanev, Application of ICT Indicators for Network Planning, World Telecommunication/ICT Indicators Meeting, Geneva, 15-17 Jan, 2003

[3] UMTS Forum, Evolution to 3G/UMTS Services, White paper # 1, August 20002

[4] Bureau de Développement des Télécommuni-cations, “Manual on Mobile Communication Development”, ITU-Geneva 1997

[5] xG_Planner “Computer based planning system for 2G , 2.5G and 3G cellular networks”, LS telcom AG, 77839 Lichtenau, Germany

[6] Harri Holma, Antti Toskala, WCDMA for UMTS, Artech House

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