IEEE 802.11 ad Beamforming Protocol

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Hello all, and in the previous post we saw about the general beamforming training which could be associated into different medium access schemes and the association process.  This post talks about the association beamforming training associated with the 802.11 ad and the enhancement that has been made to  connect with the PCP/AP without preceding coordination.


The beamforming training between the PCP/AP and an unassociated station cannot rely on coordination preceding to the beam training. The PCP/AP uses its beacon sweep during the BTI, as an initiator sector sweep for all stations to overcome the challenges of directional link setup. The Sector Sweep (SSW) frame specific control fields are added to the beacon frame. To allow multiple stations to respond to a beacon sweep without coordination, the A-BFT interval does two things, one it implements a contention base response period and secondly the A-BFT reserves channel time for multiple responder sector sweeps from the stations. The figure below shows an overview for the association beamforming training during BTI and A-BFT is shown in the upper left part of the figure.

The A-BFT slot consists of fixed time allocation for number of SSW frames. The contention process that takes place during an A-BFT does not apply carrier sensing, but a collision is detected by a missing SSW feedback frame from PCP/AP and also a station might be unable to finish its sweep because its sectors exceed the number of SSW frames per slot. Thus to handle such situations, several measures could be taken.

  • The PCP/AP can answer an incomplete sweep with SSW feedback frame, forcing the selection of a sub-optimal transmit sector.
  • A station might contend for more slots during the A-BFT in the same slot or a following slot.

Because of this process congestion occurs and to resolve the problem of congestion of the association beam forming training interval, a station has to take additional amount of back-off slots when it retries in order to sustain the same slot.


Beamforming training during the DTI can be initialized following two different methods. First, the initiator can
directly begin a sector level sweep when it gains control over the channel. This method is required during CSMA/CA
access. Second, the PCP/AP can convey beam training parameters between two nodes, during dynamic or pseudostatic channel allocation. Using the second mechanism, the PCP/AP learns about the pending beam training and can integrate that information into the scheduling process.


Beamforming Training
Beamforming Training[1]

For direct beam training initialization, a station that seized the channel initiates the beamforming process with a transmit sector sweep to the responder. However, if the initiator intends to start a receive antenna training, additional signaling is necessary. In that case, the initiator inquires the number of receive sectors at the responder via the PCP/AP or higher level protocols. Then, to initialize the SLS, a Grant/Grant-ACK exchange is used to request a receive sector sweep. Following that, both nodes start the training after the Grant-ACK frame. During contention based access, short inter frame spacing between beamforming frames ensures no other node wins a transmit opportunity and causes interference. Beam training via the PCP/AP during pseudo static channel allocation is requested with the initial traffic specification that is transmitted. The beam training parameters are included by the PCP/AP in the extended schedule element that announces the first allocation, which causes both nodes of a traffic stream
to commence training at the beginning of their first allocation. To initiate beam training via the PCP/AP during dynamic
channel allocation, a node requests an allocation to the beam training partner. In its corresponding SPR frame, the initiator
indicates the parameters for the intended training. When granting the corresponding allocation request, the PCP/AP
includes the beam training parameters into the Grant frames sent to both stations involved in the allocation.

Beam refinement during the DTI typically follows immediately after a SLS. The initiator uses the SSW ACK frame to request transmit or receive training as described in Section VI.B. A station that seized the channel can also initiate a standalone BRP using a BRP setup phase. To request mandatory beam refinement transactions only, the setup phase comprises a single BRP frame initiating the refinement sequence.

This posts sums up the technology behind the beamforming concept and the next post will be about why IEEE 802.11 ad standardization sounds better than the other contenders and followed by the conclusion for the 60 Ghz band standardization.


Manigandan Siva.