www.WirelessCommunication.NL Chapter: Capita Selecta

Effect of User Mobility on Communication Systems

• At the physical layer, channel characteristics vary with the location of the user and because of mobility, vary in time. Because antennas mounted on vehicles have rather unfavourable positions and little 'clearance', the propagation characteristics in the mobile radio channel are notoriously poor. A mobile radio link is hindered by a number of propagation mechanisms, viz., multipath scattering from objects near the mobile antenna, shadowing by dominant obstacles and attenuation mechanisms on the propagation path between transmitter and receiver.

  Multipath reception causes rapid fluctuations of the received signal power, whereas shadowing is experienced as a slow amplitude effect. Adaptive transmit power control can mitigate the adverse effects of individual shadowing to some extent. Since most mobile networks are interference-limited rather than noise-limited, interfering signals are likely to produce substantially greater adverse collective impact if adaptive power control is used to overcome shadowing on their own transmission path. Mobile systems must live with signal fluctuations during transmission. Because of high demands on spectrum efficiency, extensive frequency re-use and mutual interference must be accepted. This means that the classical system analysis inspired by the stationary AWGN channel of conventional information theory may not be applicable.

• At the data link layer, channel coding is to be chosen in accordance with the specific character of the fading radio channel. In contrast to AWGN channels with errors randomly distributed in time, bursts (i.e., clusters of) errors are experienced in the mobile channel. A typical mobile channel allows relatively reliable communication during certain periods, interrupted by other periods of particularly poor communication. The latter are called 'fades'.
• At the network layer, the routing of signals may change from time to time as the user moves through the service area of the network. This may require localisation of the terminal (or localisation of a specific user in more sophisticated 'personal' communication systems) to find optimum routing.
• Transport layer protocols and applications should anticipate to fading and channel throughput fluctuations. This many be at odds with strictly OSI layered protocol stacks.
• At the presentation layer, improved (speech and video) source coding techniques have helped in achieving acceptable spectrum efficiency with digital systems.
• At the application layer (and above), personal mobility and transportation of goods require specific services and facilities.

www.WirelessCommunication.NL (c) Jean-Paul M.G. Linnartz, 1993, 1995.