Why a High Data Rate?

Data rate is really what broadband is about. For example HIPERLAN allows physical channel bit rates up to 23.5 Mb/s, and in MBS it can go as high as 155 Mb/s. Such high data rates impose large bandwidths, thus pushing carrier frequencies for values higher than the UHF band: HIPERLAN has frequencies allocated in the 5 GHz and 17 GHz bands; MBS will occupy the 40 GHz and 60 GHz bands; even the infra-red band is being considered for broadband WLANs. However, many people argue if there is a need for such high capacity systems, having in mind all the compression algorithms that are being developed and the type of applications that do require tenths of Mb/s. One can face this issue from another perspective.

Not too many years ago (really not too many), people would be happy to buy a Personal Computer with a 33 MHz clock, 4 MB of RAM and 100 MB of hard disk (of course, lower numbers can be presented here, depending how further back one goes in time); word-processors, spread-sheets and other computer applications were used quite successfully, with the drawback of taking some time to run and being limited by the reduced available memory, however creating files of some kB. The wish of every user was the increase of frequency clock, RAM and hard disk memories to run those same applications. But as hardware has been improved in the last years, also software has increased enormously its needs: one can buy now a Personal Computer with a clock higher than 1 GHz, a RAM higher than 128 MB, and a hard disk with a capacity higher than 15 GB (all these values are orders of magnetude higher than the previous ones), but after installing the operative system, the word-processor, the spread-sheet, and all the other applications, several hundreds of MB are already occupied; moreover, it is very easy to have a file occupying 10 MB, after including all the figures, graphs and so on.

This need for high capacity systems has been recognized by the Visionary Group, put together by the European Commission, to give a perspective of what should be the "hot topics" in the area of Telecommunications for research in the next European programmes (following RACE and ACTS). In this visionary perspective of the road to follow, in order to go along the needs of society in the years to come as far as communications is concerned, capacity is referred as one of the major issues to be developed, due to the foreseen increase in demand for new services (specially those based on multimedia); together with this, personal mobility will impose new challenges to the development of new personal and mobile communication systems.

A conclusion can be drawn from this: even if at a certain point in time it may look "academic" to develop a system for a capacity much higher than what it seems reasonable (in the sense that there are no applications requiring such high capacity), it is worthwhile to do it, since almost for sure in the future (which may be not very far) applications will come out that need those capacities and even more. The story of fibre optics is elucidative on that.

Services and Applications

The applications and services of the two systems are also different. HIPERLAN is mainly intended for communications between computers (thus being an extension of wired LANs); nevertheless it can support real-time voice and image signals, and users are allowed to some mobility and can have access to public networks. MBS services are quite more wide in range: Fig. 2 shows several of the possible applications, according to their data rate and mobility; of course these applications are not exclusive of MBS, and many, if not all, can be seen as possible applications of WBMSs in general. Some of these applications are already available for fixed and/or narrowband systems, thus WBMS represents only some extended upgrade in mobility or data rate; others are really specific of WBMS, and can only be made available through this type of system. Moreover, some of the applications require the existence of an operator, while others can be implemented on a private basis; the future, in the sense of market impositions, will dictate which ones will have success as key start services.

European R&D initiatives in mobile systems, and the corresponding efforts to standardize the upcoming systems has lead to the definition of UMTS, which is considered as a third generation system. However, there are differences between the foreseen features and applications of UMTS and MBS, Table 2, the latter being capable of providing more advanced services, but with some possible restrictions on coverage.