Antenna diversity types
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Antenna diversity types
Switched Diversity
Multiple-input, multiple-output (MIMO)
Space-time Block Coding (SBTC)
Cyclic Shift Diversity (CSD)
Transmit Beamforing (TxBF)
Maximal Ratio Combining
Spatial Multiplexing
Switched Diversity
- To help compensate for the effects of multipath, antenna diversity is implemented in wireless networking equipment such as access points.
- Antenna diversity exists when an access point has two or more antennas with a receiver functioning together to minimize the negative effects of multipath.
- Because the wavelengths of 802.11 wireless networks are less than 5 inches long, the antennas can be placed very near each other and still allow antenna diversity to be effective.
- When the access point senses an RF signal, it compares the signal that it is receiving on both antennas and uses whichever antenna has the higher signal strength to receive the frame of data.
- This sampling is performed on a frame-by-frame basis, choosing whichever antenna has the higher signal strength.
- Most pre-802.11n radios use switched diversity. When receiving incoming transmissions, switched diversity listens with multiple antennas.
- Multiple copies of the same signal arrive at the receiver antennas with different amplitudes. The signal with the best amplitude is chosen, and the other signals are ignored.
- The AP will use one antenna as long as the signal is above a predefined signal level. If the signal degrades below the acceptable level, then the AP will use the signal received on the other antenna. This method of listening for the best received signal is also known as receive diversity.
- Switched diversity is also used when transmitting, but only one antenna is used. The transmitter will transmit out of the diversity antenna where the best amplitude signal was last heard. The method of transmitting out of the antenna where the last best received signal was heard is known as transmit diversity.
- When an access point has two antenna ports for antenna diversity, the antennas should have identical gain and should be installed in the same location and with the same orientation. You should not be running antenna cables to antennas in opposite directions to try to provide better coverage. Remember, when diversity is used, the transceiver will switch between the antennas; therefore the antennas need to provide essentially the same coverage. The distance between the antennas should be a factor of the wavelength (1/ 4, 1/ 2, 1, 2).
- The half-duplex nature of the RF medium, when antenna diversity is used only one antenna is operational at any given time. In other words, a radio card transmitting a frame with one antenna cannot be receiving a frame with the other antenna at the same time.
Multiple-input, multiple-output (MIMO)
- MIMO is another, more sophisticated form of antenna diversity.
- MIMO can receive or transmit using multiple antennas concurrently. Complex signal-processing techniques enable significant enhancements to reliability, range, and throughput in MIMO systems. These techniques send data by using multiple simultaneous RF signals, and the receiver then reconstructs the data from those signals.
Space-time Block Coding (SBTC)
- The same transmit data is sent out of multiple antennas
- STBC communication is possible only between 802.11n devices
Cyclic Shift Diversity (CSD)
- CSD diversity signals can be received by either 802.11n or legacy devices.
Transmit Beamforing (TxBF)
- The same signal is transmitted over multiple antennas and the antennas act like a phased array.
Maximal Ratio Combining
- A type of receive diversity technique where multiple received signals are combined, thus improving sensitivity.
Spatial Multiplexing
- Transmitting multiple streams of data increases throughput, taking advantage of multipath.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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