Most WLL networks have envisaged using fixed subscriber units mounted on the outside wall of subscribers’ premises with a lead connecting to a phone socket inside the house. Radio, however, can provide a capability that copper cannot: mobility. Recently, a number of manufacturers and operators started to examine whether mobility should be provided in a WLL system. The reasons for considering mobility are varied. The fact that those involved in WLL typically have been involved in cellular radio certainly has had an impact. Manufacturers, particularly those using cordless technology, have stressed the added bonus of mobility in their WLL product literature. Literature about early WLL systems recounts stories about how the system performed well as a mobile system during some disaster. This section sets out the issues.
First, it is important to define mobility:
■ Full mobility is the mobility offered by cellular radio, in that the user can expect to roam to most parts of the country and make and
receive calls from a handset. Implicit in this definition is the handover of calls.
■ Limited mobility is the user being able to move within a small area around the home, perhaps a radius of 1 to 2 km, and still make and receive calls. Call handover is not expected.
■ No mobility is defined as a phone fixed by a wire into a base unit.
The provision of mobility has the potential to add significantly to the cost of a WLL system, due to the cost of enhanced coverage and the need for additional elements in the network.
A significant enhancement in coverage is required above that for a standard WLL deployment if mobility is to be offered. Compared to mobile systems, WLL deployments achieve a gain in link budget of up to 20 dB through the use of directional antennas mounted at roof-top level. To provide mobility in the home, a building penetration loss must also be built into the calculations; measurements have shown that for a reasonably reliable house penetration a further 20-dB loss must be allowed. The requirement for up to an additional 40 dB on the link budget has significant implications for the size of the cells. Use of propagation models applicable to WLL suggests that a 40-dB difference would reduce a cell size of 5 km without mobility to 400m with mobility. Experience with those deploying DECT systems has shown an even more severe coverage restriction, from 5 km to 200m when mobility is required. In the case of DECT, 600 times more cells are required if a mobility service is to be offered over the same coverage area.
For full mobility, a number of additional network components are required, including home- and visitor-location registers and gateway switches. Functionality, such as idle mode cell selection and handover, is required in the mobile, and the network and the cell planning must provide sufficient overlap to allow handover to occur. For some of the WLL technologies that have been derived from cellular, that functionality will be mostly in place. For others, it will not be possible to achieve the functionality.
For limited mobility, very little in the way of additional components is required, but the network planning may need to take care of propaga-
228 Introduction to Wireless Local Loop
tion phenomena such as multipath, which would not be so severe in a deployment where there is no mobility.
The major issue, though, is the coverage. For in-building coverage, between 150 and 600 times as many cells will be required. Even if mobility is restricted to outside the house, allowing the range to increase to perhaps 1.4 km, 12 to 15 times as many cells will be required. They will add significantly to the network cost, which, as a crude approxima- tion, varies linearly with the number of cell sites.
The communications marketplace is a competitive one, where the PTO, cellular operators, and, in a few cities, cordless operators increas- ingly compete for traffic. The WLL operators will be entering into that competitive environment and need to ensure that their offerings are targeted appropriately against the different forms of service that already exist. Table 15.1 summarizes the competitive environment.
WLL, as a replacement for fixed-loop telephony, needs to offer equally good services, including excellent voice quality (requiring a low BER channel), potentially ISDN, and services such as call-back, call forwarding, and follow-me. To what extent it needs to provide mobility and what price it can charge is the subject of this section.
It is clear from Table 15.1 that there is a strong relationship between price and mobility. Users are prepared to pay more and tolerate a lower grade of service to obtain mobility. The WLL operator potentially can
Table 15.1
The Current Competitive Environment
Operator Mobility Quality Price Services Market
PTO None Excellent Low Wide
ranging
All homes
Cellular Full Variable High Limited All
individuals Cordless Limited Good to
excellent
Medium Limited City
workers and dwellers
WLL ? Needs to be
excellent
? Needs to be
wide ranging
All homes in coverage area
charge more for a service with mobility but cannot reduce the quality of the service offered below that of wireline. If prices increase while quality decreases, users will retain their wireline phones and the WLL operator will become a mobile operator using inappropriate equipment in an inappropriate frequency band.
This analysis allows the case for mobility to be stated quite simply.
An operator needs to judge whether the additional cost it will incur in providing a service able to offer mobility can be offset by the greater call charges it can achieve, while still providing a service of equivalent quality to wireline.
A first pass at some of these figures would be as follows. The increase in network cost was calculated to be at least a factor-of-12 increase in the number of base stations, probably requiring a tenfold increase in the network cost. A comparison of cellular tariffs to PTO tariffs reveals that monthly rental is about 50% higher and call charges perhaps 100% higher (depending on a complex mix of time of day and bundling package). It is unlikely that the WLL operator will be able to achieve cellular revenue.
Thus, the case for full mobility in the local loop simply can be discounted.
Indeed, a moment’s lateral thinking helps validate those figures.
Were it not expensive to provide indoor coverage, the cellular operators would be offering a WLL service on the back of their existing infrastruc- ture. However, as One-2-One discovered in the United Kingdom, such a service is too expensive to provide and the revenues too low to justify the expense, even when the premium for full mobility was included.
There are, however, a number of options for offering different types of mobility that are considerably less expensive and likely to be of interest to customers.
■ Provide mobility only in areas close to the base station. Even if a base station covers a radius of 5 km, those users within, say, 300m of the base station could have a mobile service around their houses and gardens. For no additional cost, limited mobility could be offered to a subset of subscribers, maximizing revenue. The diffi- culty in such an approach will be in determining which subscribers can receive the service and then marketing a range of different packages to different subscribers, depending on location.
230 Introduction to Wireless Local Loop
■ Let users install cordless phones. An alternative would be to install a WLL line without mobility (which could be any technology) and then sell users DECT cordless home base stations. Such a phone and base station costs around $200 and will provide the user with mobility within a range of 200m from the house. The WLL opera- tor could then choose to deploy DECT base stations in a few key locations, such as shopping malls, and then market a sort of cordless service.
■ Use dual-mode phones. Dual-mode DECT/GSM phones are being used on a trial basis and may become available during 1998. Some predict that by 1999, the price of dual-mode phones is expected to be only slightly greater than the price of a GSM phone, although others feel that the additional costs over a GSM phone will be much greater and that GSM itself will be adapted to provide cordless capabilities. Whatever, by selling the customer a WLL line to the house, a DECT or GSM home base station, and either a dual-mode DECT/GSM handset or an enhanced GSM handset in a bundled package, full mobility could be provided at no cost to the WLL operator. A user with such a phone could roam throughout the world. On returning home, the phone would automatically switch to the home base station when within range, routing calls across the WLL system. (Users could, of course, put together such a package for themselves, but with appropriate marketing the WLL operator could appear to be providing something different.) The disadvantage of the third approach is that the WLL operator will not carry the traffic when the subscriber is out of range of the home base station and so will lose revenue. But, as shown earlier, capturing that revenue requires a disproportionate expenditure.
Table 15.2 shows the positioning that a WLL operator must adopt to be able to operate profitably.
The WLL operator needs to take advantage of the fact that wireless connections to a building generally are less expensive than wired connec- tions to be able to operate from a lower cost base than the PTO. That allows WLL operators to attract customers with a combination of lower prices and differentiated services. Providing mobility will cause prices to
be raised, at which point WLL operators will be providing a more expensive and inferior service than the PTO and a generally inferior service than the cellular operator. However, that does not need to stop the WLL operator from marketing a mobile service by offering home base stations and dual-mode phones (although the PTO also could market a similar service).
The provision of WLL is a complex matter, with WLL networks being used for a range of tasks from provision of telecommunications to areas that never have had communications, through shortening waiting lists in Eastern European countries to providing competition to the PTO in developed countries. The different environments require a range of different approaches, different technologies, and different services that will be the subject of further papers. Despite the wide range of networks, none of them would benefit from the provision of mobility. A WLL operator that wants to offer mobility should seek a cellular license.
Table 15.2
The Current Competitive Environment
Operator Mobility Quality Price Services Market
PTO None Excellent Low Wide
ranging
All homes
Cellular Full Variable High Limited All
individuals Cordless Limited Good to
excellent
Medium Limited City
workers and dwellers
WLL None
(except via home base stations)
Needs to be excellent
Low Needs to be
wide ranging
All homes in coverage area
232 Introduction to Wireless Local Loop
16
Developing the Business Case
T is probably the single most important document in the process of deploying the network. With decisions having been made as to the technology, the density of the cells, the interconnect method, and the market niche, the business case is the point where all those come together to determine whether the project will be successful.
As well as being vital internally, the business case will be shown to investors and, possibly in a shortened form, to the government as part of the license application.