Wireless Pathways

Have you stared at the bars on your wireless phone or the strength of your Wi-Fi signal and wondered why it keeps changing and what could make your bits fly a little faster. As much as I love being un-tethered and the freedom of using my laptop or phone anywhere, I sometimes miss the predictable performance of the trusted thin blue Ethernet cable. How do the laws of physics explain the difference between the two mediums? What are the considerations to establish a reliable wireless link?

First the channel of communication over a wired connection operates in a more shielded (for interference) environment that attenuates at a slower rate. Attenuation is defined in Wikipedia as the gradual loss in intensity of any kind of flux (energy) through a medium (either cable or wireless). (The reference to flux reminded me immediately of Doc Brown’s invention of the Flux Capacitor used in the DeLorean Time Machine from the film, Back to the Future.(http://en.wikipedia.org/wiki/DeLorean_time_machine#Flux_capacitor but I digress …) Wireless channels of communication are generally in a shared environment exposed to interference and also increased “attenuation”. My respect for the blue thin cable just went up.

To understand a wireless channel, I had to first visualize the path a wireless signal takes from transmitter to receiver. There isn’t a single path dedicated to the signal but is defined in a two spatial scales of multi-path fading. The first is large scale fading of the signal based on path loss (attenuation) and shadowing. The second is small scale fading based on multi-path fading and Doppler effects.

(Source: digitalradiotech.co.uk)

Path loss in this context (or path attenuation) is defined as the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss may be due to many effects of the signal, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. As can be observed in the figure path loss is also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium the distance between the transmitter and the receiver, and the height and location of antennas. Path loss can be estimated through various models – simple ones and complex ones. The base model which is also called “free space” loss model is considered too simplistic for use and the original Maxwell’s equations are too impractical for the real world conditions. The models in use are hybrids of empirical models and theoretical models with many approximations.

Wikipedia also defines deterministic methods based on the physical laws of wave propagation are also used; ray tracing is one such method. These methods are expected to produce more accurate and reliable predictions of the path loss than the empirical methods; however, they are significantly more expensive in computational effort and depend on the detailed and accurate description of all objects in the propagation space, such as buildings, roofs, windows, doors, and walls. For these reasons they are used predominantly for short propagation paths.

Professor Rajaraman’s CSG 250 course materials from Northeastern University (www.ccs.neu.edu/home/rraj/Courses/.../TransmissionFundamentals.ppt) describe the effect of multi-path fading in two ways. First, multiple copies of a signal may arrive at different phases. If phases add destructively, the signal level relative to noise declines, making detection more difficult. Second, Intersymbol interference (ISI) is caused when one or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit.