Difference between Active and passive beamforming mechanisms

Active beamforming is the mechanism used in WiFi access points and clients where the beamforming is done with different chains on a transmitting radio (usually using active components like transistors). Passive beamforming on the other hand is achieved by using passive components (resistors, capacitors, and inductors) to introduce phase and amplitude variations across different path such that we are able to achieve constructive addition in some cases which results in beamforming.

The main differences between these two mechanisms: 

• Butler matrices are a passive beamforming and channel model rendering mechanism.
• Beamforming can be useful against jamming attacks to increase SNR at client.
• Passive beamforming networks are better because active components are more susceptible to change in temperature and other environmental factors.
• Compared to active beamforming networks, passive beamformers can be made electrically stable and reliable across a wide range of operating conditions, although a great deal of flexibility in the physical layout may be needed to accommodate a large variety of connector interfaces for different antenna arrays.

References

* Miscellaneous articles on butler matrices

* Understanding Passive Beamforming Networks http://defenseelectronicsmag.com/military-defense-electronics/understanding-passive-beamforming-networks

WLAN Interference control through Antenna Selection (ASEL)

In this work, we present a novel, robust scheme for high density WLAN deployments. This scheme uses well known selection diversity at the transmitter. We show that our scheme increases the number of simultaneous transmissions at any given time without excessive overhead (compared to other schemes such as Multi-user MIMO). Furthermore, this scheme can be easily implemented using existing standards.

The main contributions that come out of this report are:

1. a simple method to tackle interference in the network by selecting the best antenna during 

transmission. 

2. A high potential to reduce the collisions for hidden node terminals. 

3. Further, we also proposed a mechanism to account for the next wave of 802.11ac by allowing simultaneous transmission from multiple APs on the same channel.

Important Links

Citation information

PDF of pre-print

WiFi: PP-AMSDU versus SPP-AMSDU a brief comparison

Before we jump into the different type of AMSDUs supported, let us do a quick recap of where AMSDUs fit in with the aggregation hierarchy. Here is an image from Wikipedia:

MSDU, MPDU and PPDU

Multiple MSDUs aggregate to form an AMSDU. Multiple of these will form an AMPDU. The different type of AMSDUs are defined in the WGs documents:

3.114n Payload Protected A-MSDU (PP A-MSDU): An A-MSDU that is CCMP protected but does not include the A-MSDU Present field (bit 7 of the QoS control field) in the construction of the AAD. 

3.135n Signaling and Payload Protected A-MSDU (SPP A-MSDU): An A-MSDU that is CCMP protected and does include the A-MSDU Present field (bit 7 of the Qos control field) in the construction of the AAD.