Propagation revisited: Wireless multipath
Previously, we had discussed the various behaviors of radio frequency (RF) propagation and introduced the concept of multipath, which is a special case of propagation. Multipath is a phenomenon that occurs when an RF signal travels over many different paths between the transmitter and receiver, arriving at different times at the receiver. When this occurs, the signals combine in various ways to effect signal gain, attenuation, or cancellation (nulling).
While not unique to Wi-Fi, the reception of multiple instances of the same signal is a particular concern in wireless local area networks (WLANs). Multipath occurs when a transmitted signal is reflected, scattered, or otherwise diverted from a direct line of sight between the transmitter and receiver. The signals arrive at the receiver by different and distinct paths, and they also arrive at different time intervals due to the lengthening of the path. All of this conspires to produce interference between the signals, resulting in the received data likely being corrupted.
Multipath does not occur solely from reflections. Refraction, diffraction, or scattering, depending upon the surface characteristics of the object the signal encounters are other factors. In an outdoor environment, signals will encounter walls, cars, planes, and even the earth itself; all will produce some unique propagation effect on the signal as it travels between the transmitter and receiver. In an indoor environment, metal surfaces like filing cabinets and desks will wreak havoc on a signal. Even office dividers will exhibit refraction and absorption effects on the signal, changing its course slightly yet enough to introduce a delay.
Know the 4 signal conditions of multipath
The period of time between the reception of the signals is called the delay spread. This is typically in nanoseconds, but it is enough of a delay to cause problems. Four conditions result from multipath:
1. Upfade — This is an increase in signal strength that occurs when multiple signals are received within the phase angles of 0-120 deg and is considered constructive multipath. The additive effect of the two superimposed signals cannot result in an increase in signal greater than that of the transmitted signal because of free space path loss. The signal will be increased from what would have been received had it traveled in a straight line from the transmitter to the receiver without the effects of multipath.
2. Downfade — Downfade, which is the inverse of upfade, is where the signals combine to produce a lower signal amplitude and decreased signal strength resulting from received signals in the 121-179 phase angle. This is considered destructive multipath.
3. Nulling — This is a partial or complete signal cancellation occurring when signals arrive 180 degrees out of phase with the primary signal and is obviously destructive.
4. Data corruption — When signals exhibit different delay spreads, the differential can cause demodulation problems and overlapping bits, causing a condition known as intersymbol interference, or ISI. This is the most common destructive multipath and the root cause of data corruption.
ISI is the condition that occurs when two signals arrive at slightly different time intervals, causing one signal to "blur" into another. ISI is also caused by impedance mismatch; a reflected signal will cause distortion when combining with the original signal.
Until the introduction of IEEE 802.11n, multipath presented limitations to Wi-Fi propagation. One way in which 802.11a/b/g systems overcame some negative effects of multipath was antenna diversity. Two antennas and one radio would be used to determine the best signal; each antenna is examined to ascertain which signal is of better quality. Pre-IEEE 802.11n access points (APs) used switched diversity, which simply listened on multiple antennas and picked the best signal, ignoring the rest; this was called receive diversity. Because the transmitter had no way of knowing what antenna the receiving AP would acquire the best signal on, the signal would typically be transmitted from the antenna that received the best signal; this was called transmit diversity.
IEEE 802.11n APs take advantage of multipath. A new technology called multiple in, multiple out, or MIMO, utilizes multiple radio "chains," or streams, and ensures that a delay spread is introduced to allow the receiving 802.11n AP to properly differentiate between signals and properly process them. The effect of MIMO is increased system reliability, range, and speed. MIMO will be discussed in detail in the next segment in this series.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
The management, control, and data planes
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.