Miniaturization

Contributed by Ramjee Prasad and Luis M. Correia
Edited by Jean-Paul Linnartz

Antennas and batteries play a key role in wireless systems: with the present capability of microelectronics to integrated circuits and to include signal processors into very small chips, antennas and batteries tend to impose the size and weight of mobile terminals. Of course the higher one goes in frequency the less developed technology is, and many problems can still be found in size and weight at the millimetre wave band, due to power consumption for example, but they are likely to be solved in the near future.

Batteries

The number of hours an equipment based on batteries is allowed to work, or to stand by, and the percentage of its weight corresponding to the battery is not a specific problem of WBMCSs. Laptop computers and cellular phones are the most common terminals relying on batteries these days, and huge investments on R&D have been, and are being, done on this technology, so that working time can be extended and weight can be reduced; a 100 g battery corresponding to several hours of continuous work and a few days on stand by are already available on the market, but of course users want to have better numbers on this. Mobile multimedia terminals, certainly those to be used in some applications of WBMCSs, will be an extension of the current cellular phones, therefore one can foresee that the current problems associated with batteries will be transposed to the WBMCSs terminals; the same thing applies to laptop computers.

Antennas

Antennas (size, type, technology, and so on) are not a specific problem of WBMCSs as well, but again they are very much related with the type of systems that will be made available for users. It does not make sense to impose restrictions (like having to point to a certain direction or avoiding some one to pass in between) on the type of mobility a mobile terminal can have, and even for portable terminals (like computers) these restrictions make no sense; hence, there are only two options as far as antennas are concerned: either an omnidirectional antenna (dipole type) or an adaptive array antenna (enabling the use of narrow beams) is used. Either way, patch antennas seem a very promising solution to be of general use in WBMCSs: for the frequency bands under concern, isolated patches or adaptive arrays (with many elements) can be made with a small size (e.g., a credit card), thus enabling the terminal not to be limited in size by the antenna system.

The role of antennas radiation patterns is not negligible when discussing the performance of a system, and their influence on parameters associated with wave propagation. Although the trade-off between an omnidirectional and a narrow beam antenna is not particular of a WBS, it assumes particular importance at micr wave and millimetre waves, because of the characteristics of wave propagation at these bands. Using an omnidirectional antenna, means a lower gain but also the possibility to receive signals from various directions, without the requirement for knowing where the base station is, and allowing to received rays coming from reflections on the propagation scenario; on the other hand, the use of a very directive antenna provides a higher gain, but it has to be pointed to the base station and does not receive reflected waves coming from directions very different from the one to which it is pointed. A low value for the delay spread is desirable, since it limits the maximum data rate that can be transmitted, and even the use of equalizers can not overcome totally the problem, for which narrow beam antenna can be the solution; although it has the additional advantage of the high gain, which can be important if the link budget does not present very large margins for fading, the need for pointing it can be very discouraging, if not a drawback when line of sight does not exist. On the other hand, omnidirectional antennas lead to high values of delay spread, but they may ensure that the link still exists relying on reflections if line of sight is lost.