Hello my dear readers, I couldn’t resist writing about this topic so this is a back to back posts and it will be a continuation of the previous posts where we saw about the 60 Ghz range, why it is important and who are the key players in this frequency band.
This post introduces more about it and specifically we will see about IEEE 802.11 ad which is an amendment to the existing 802.11 standard. We will see about the how this standards new novel techniques helps in highly directional communication than the omni-directional communication that was used in the previous standards. But before getting into it, we will see about the directional communication in detail.
DIRECTIONAL COMMUNICATION :
We have already seen that the 60 Ghz spectrum suffers from high attenuation, so in order to overcome this problem the standard defines a directional communication scheme that takes advantage of the beam forming gain to cope up it. Beam forming results in high directional signal focus on a particular direction. IEEE 802.11 ad sectors can be implemented with multi directional antennas or computed antenna weight vectors for phased antenna array. The good thing about this implementation is that the 60 Ghz millimeter range allows significantly smaller antenna form factors than its predecessors.
Beam forming training is a term used when a sector focuses antenna gain in one particular direction, so that the communicating nodes have to agree on the optimal pair of receive and transmit sectors to optimize signal quality and throughput. This process has the advantage of reducing the search space of possible antenna array by discretized antenna azimuth. (azimuth is a term generally used to define antenna pattern). Higher antenna gain imposes stronger directionality and higher number of narrow antenna sectors.
DESIGN ASSUMPTIONS :
As the standard operates in a very high frequency, there are number of design assumptions that result from the change of frequency band.
- Quasi- Omni – Directional Antenna pattern : A truly omni-directional mm-Wave antenna patterns is not practical and is an ideal case, so in this standard quasi-omni-directional patterns that allow gain fluctuations over the pattern is introduced.
- Inefficient Omni- Directional Communication: As we have already seen in the previous posts, attenuation is directly proportional to the frequency and attenuation is very high in this frequency, it leads to severly reduced transmission range and throughput so the quasi-omni directional antenna pattern are used but it is still needed when the direction of the communication partner is not known (for eg: Receiver) quasi-omni patterns are still needed. Thus directional antenna gain are still added to at least one side of a link to achieve a sufficient range. They are mostly added at the receivers and only in very rare cases it is used in the transmitter.
- Extreme Efficiency loss on poorly trained beams : The efficiency loss relates to the throughput difference between the highest and lowest tranmission scheme and the standard defines it to be 6.5 Gbps . A poorly trained beam that uses a low throughput scheme reduces the system performance and therefore should be avoided at all costs.
- Reduced Interference throughput : The highly directional transmission properties mentioned before helps the 802.11 ad devices for reduced interference outside the beam directions. As the interference property is strong, it benefits the devices to spatially reuse the same frequency band and can significantly increase the system’s overall throughput.
These are the main design assumptions and of course the Directional Communication is one of the main design assumption of this standard.
The next post will talk about the the IEEE 802.11 ad beamforming concept that helps in very high directional communication that makes the communication possible.
Thanks and Regards,
 IEEE 802.11 working group, “IEEE 802.11ad, Amendment 3: Enhancements for Very High Throughput in the 60 GHz Band,”
INSIGHTS OBTAINED FROM
 P. Smulders, “Exploiting the 60 GHz Band for Local Wireless Multimedia Access: Prospects and Future Directions”, IEEE
Communications Magazine, 40(1):140-147, January 2002.
 Thomas Nitsche et al., “IEEE 802.11ad: Directional 60 GHz Communication for Multi-Gbps Wi-Fi”, IEEE Publications..