| Wireless CommunicationChapter: Wireless Propagation Channels |
Contributed by Peter E. Leuthold and
Pascal Truffer
In principle, three different domains determine the radio signal
transmission:
Table 1: Generic scheme of effects to be considered for
stochastic radio channel model (SCRM)
Of course, the deterioration of the transmission quality is strongly depending on the signal processing, i.e. modulation, coding, detection etc., but these aspects are beyond the scope of a mere channel model.
The operation scenario implies some fundamental data like frequency range, system bandwidth and environments, i.e. urban, rural, indoor etc. which even impress the general character of a wireless transmission link due to the relationship between the topographical features and the wavelength or the time resolution, respectively. Dispersion in frequency, time, direction and polarization is a crucial aspect of radio communication. We have to distinguish between multipath propagation where each path suffers from a multiplicity of well-known effects as well as temporal fluctuations which can be split off in short-term or small-scale fluctuations (fast fading) and long-term or large-scale fluctuations comprising gradual and sudden changes of path parameters mainly because of movements either of the mobile terminals (MT) or reflectors and scatters, respectively. Transceiver characteristics contributing to the SCRM are the parameters which describe the MT movement and the antenna configuration with its radiation pattern and diversity properties.
Concluding from this table, the consideration of all aspects leads to a rather high computational complexity of the SRCM. The dedication of the SRCM to a certain class of operation scenario, i.e. the delivery of indoor services with a given maximum bit rate (bandwidth) at a prescribed frequency range, is a first step toward a simplified model approach. Moreover, the identification of a few types of environment allows a remarkable complexity reduction. Thus, four important indoor situations have been chosen [2]:
Obviously, the allocated frequency range plays a key role with respect to the path loss while the room dimensions influence directly the delay spread (DS). Referring to the uncertainty relation the system bandwidth is inversely proportional to the time resolution and, therefore, determines the necessary sampling rate of the model.
Furthermore, the question arises if it would be possible to neglect those effects that are expected to have only little impact on the system performance. Indeed, different levels of model complexity can be introduced according to the transmission constellation or the needed accuracy, respectively. As an example, two reduced complexity levels are proposed [2]:
a) Minimum complexity :
b) Medium complexity : Minimum complexity according to a) plus
