Beam-Forming Concepts

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Hello readers, warm greetings and in the last post we saw about the communication at the 60 Ghz range, the design considerations and this week the subject that we are focusing is about the beamforming concept.  But before getting into the topic, we should know why we are going for this concept and why not use the old MIMO concept.

Reasons for using the beamforming concept :

We have already seen that one of the main problem with the 60 Ghz band is that it suffers from high signal path loss and when we used the Frii’s equation for finding the losses, the 60 Ghz band suffers from 28 dB more loss than the present 2.4 Ghz band. In order to address this situation we have to use a large number of antennas at the transceiver to exploit the beamforming gain for extending the range.  In general antennas are connected to the baseband processor via seperate RF/analog downconversion chains so that signal processing can be executed at the spatial domain in digital baseband. But multiple analog chains are expensive at this frequency range and sampling consumes a lot of power.

When we are making use of multiple antennas and few analog chains MIMO concepts strikes hard but it is not applicable in this case. Antenna selection was proposed in [1] but the drawback of this technology is that it provides limited transmission range extension. It is for these reason we go for the beam forming concept.

The Directional Beam Pattern to Beam forming Training :

One of the biggest advantage of the directional beam pattern is that it improves the transmission range but it complicates the communication protocol designs, but if the beam patters are not pointing between two communication devices then they can communicate between each other. Therefore, an efficient protocol that discovers the best beam direction pair between devices is very crucial. This protocol is called the Beam forming training. The best direction is determined with the Gain, the direction in which the gain of the antennas are high are considered the best direction.

Beam -Forming Training:

Beam forming training determines the appropriate transmit and receive antenna sectors for a pair of stations.The Beam forming training process begins with the transmitter sending training packets at pre-defined beam directions. The number of beam depends on the number of antennas and the amount of coverage required. Omni-directional transmission are avoided as they are severely limited in range. The beam forming phase is split into two sub-phases.

  • Sector – Level Sweep (SLS)
  • Beam Refinement Phase (BRP)
    Transmit and Receive Sector Training [2].
    Transmit and Receive Sector Training [2].

During SLS, initial coarse-grain antenna sector configuration is determined and the two stations either trains its transmit antenna or the receive antenna sector. This information is vital in the beam-refinement phase which fine-tunes the selected sectors, but this step is optional. So sector-level sweep is the most important phase. During the SLS , each station acts once as transmitter and once as receiver, the station that transmits first is called the initiator and second the responder. Initiator and responder exchanges series of sector sweep frames over different sectors to find the station that provides highest signal quality. The figure below shows the transmit and receive sector training. During the transmitter sector sweep frames are transmitted on different sectors as mentioned before, while the pairing node receives a quasi omni directional pattern. The strongest transmit sector are identified by the transmitter marker  that marks every frame with an identifier for the used antenna and sector.During the receiver sector sweep transmission of the same sector allows to test for optimum receive sector at the pairing node. This is the general principle of Sector-level sweep.  The next section talks about the fundamental elements of the IEEE 802.11 ad with respect to beam forming protocol.

The next post will talk about the beam forming protocol with respect to the standards. Until then thank you for your time.

Thanks and Regards,

Manigandan Siva.

 

References :

[1]. R. Choudhury and N.H. Vaidya, “Deafness: A MAC Problem in Ad Hoc Networks when using Directional Antennas”, In Proceedings of ICNP,October, 2004.

[2] Thomas Nitsche, Carlos Cordeiro et al.,  “IEEE 802.11ad: Directional 60 GHz Communication for Multi-Gbps Wi-Fi” , Communications Magazine, IEEE  (Volume:52 ,  Issue: 12 ), December, 2014..