Published:2011/8/18 2:42:00 Author:Amy From:SeekIC
Stefan Tauschek
MIMO
Increasing antenna gain by a few decibels is by no means the only trick that can be conjured out of the magic box of RF technology Multiple-input, multiple-output (MIMO) technology is also proposed as a means lo significantly increase range and data rates without increasing RF power. What lies behind this name is a sophisticated antenna technology called ’phased-array antennas’, which originals from radar systems. A phased-array antenna consists of a two-dimensional array of dipole elements, which produces a radiation pattern with a strongly focused label. If the dipole* are driven in phase by an RF signal, the antenna radiates a plane wove perpendicular to the array. This yields a considerable increase in range and eliminates multipath effects at the receiver.
If the relative phases of the RF signals fed lo the individual dipoles are modified in a suitable manner (Figure 9), the direction of the lobe can be steered in almost any desired direction without making any mechanical changes. The only thing that is necessary for implementing an antenna that can emit a beam of RF waves in almost any desired direction, similar to the beam from a lighthouse (Figure 10), is the ability to control the phase relationships of the individual elements of the antenna. Although this technique does not increase the total amount of energy radiated by the transmitter antenna (which means it still complies with the regulations), it considerably increases the amount of energy arriving at receiver antenna.
Antenna diversity and receiver combining
Up io now, we have concentrated on the transmitter end in our efforts to boost the performance of our WLAN. However, MIMO also encompasses improvements at the receiver end to raise the total physical capability of the system. The initial objective is to solve the problem of multipath effects. Especially in enclosed spaces, radio waves do not travel to the receiver antenna along only one path, but instead along many different paths that result from multiple reflections. The reflected signals are offset in phase from the main signal, and the net result of superimposing the reflected signals on the main signal can dramatically reduce signal quality and thus decrease the usable range. This makes antenna position a decisive factor, and changing the position of the antenna by only a few centimeters can be enough to make good reception considerably worse or dramatically improve poor reception. For this reason, current WLAN implementations already have two receiver antenna paths, which are demodulated in two independent RF front ends of the WLAN chip set. For each transfer (transmit and receive], the baseband controller checks which of the two paths provides the better result and selects it to be used for further processing. This technique is called ’antenna diversity’.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/18/Wireless_Connectivity__ISM_WLAN_WMAN_Bluetooth_et_al_6.html
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