Wireless communication basics
are subject to unpredictable behavior due to environmental dependencies and various types of interference. Conditions continuously fluctuate on different timescales due to interference and motion in the environment. Wi-Fi's use of unlicensed spectrum enables many new applications but further magnifies interference-related variability.
Yet it's possible to effectively mitigate many of these impairments through continuous intelligent selection of system operating parameters and a sufficiently agile antenna system. For seamless coverage, smart antenna nodes thus need to alter system transmission and reception parameters in reaction to changing environmental conditions, with the goal of maximizing overall system performance.
802.11 wireless channel is also a shared medium. In enterprise, campus, MDU, hotel and other deployments, lots and lots of data for many users has to get to its destination. Successful communication occurs as long as the data is riding on top of a strong enough signal, such that the receiver can decode it.
The transmission/reception range is the distance over which signal power is above the minimum reception threshold. Once received, the packet is passed up to the medium access (MAC) layer, which decodes and acknowledges the packet's reception, assuming the channel is clear and there is no interference from other endpoints. Otherwise, the channel is set to the collision state and all nodes again contend for the link.
The quality of the received signal is measured by the signal to interference and noise ratio (SINR), defined as the ratio of the power in the signal of interest to the total power of received interference and noise.
See Figure 4 (Source: Sensitivity Tables for minimum required values. Thus, successful reception is a function not only of the distance between sender and receiver but also distance from unrelated transmitters. Nodes within the interference range are subject to possible collisions and packet loss.
Click here for Figure 4.
Figure 4. Minimum required SINR for various 802.11n modulation and coding combinations).
As a transducer, the antenna is an often overlooked but key determinant of performance in any radio system; this applies more so to nodes using the 802.11n standard.
A smart antenna concentrates energy in a particular direction during transmission, while during reception, it accepts energy from a particular direction while rejecting unwanted network noise. An increase in available RF configurations on a per packet basis results in significant communication performance gains.
Proper "de-correlation" of the individual antenna elements is necessary for 802.11n multi-stream communications. Antenna correlation is a measure of similarity of radio signals within a multi-antenna array as well as how these antennas interact.
802.11n leverages this interaction by combining multiple data streams to gain fundamental performance improvements. Due to reflections, RF signals take different paths while en route from the transmitter to the receiver. Each RF path has a slightly different delay between the time the signal is transmitted and received.
This multi-path is what permits the 802.11n increase in throughput through spatial multiplexing. If the paths between the sender and receiver are too similar, then the transmitter reduces the bit rate by reducing the number of "spatial" streams.
802.11n multi-radio transmissions also create more interference. Performance improvements can be limited with omni antennas since omni arrays have no control over the multi-path environment and contain no mechanism by which to reject network and other interference.
About the author
Victor Shtrom earned his PhD in communications theory from University of Cincinnati in 1996. He went to work at ArgoSystems (a Boeing division) designing smart antenna interference cancellation systems for cellular applications. Shtrom subsequently worked on several satellite air interfaces at Boeing. In 1999 he joined Gigabit Wireless, which later became Iospan Wireless, where he was instrumental in the design and development of the world's first commercially available MIMO-OFDM silicon and systems. In 2002, he co-founded Ruckus Wireless.