The 60 GHz Band

A consensus has been reached that in order to achieve wireless communication systems at rates of tens of Gigabits per second (Gbps), the use of higher frequencies and channels of larger bandwidth is a necessity, and the 60 GHz is the starting point [1].

The use of the frequency spectrum is a very complicated matter, because every country has the right to regulate and license it as it sees fit. Traditional Wi-Fi systems work around the 2.4 GHz or 5 GHz bands, but the remaining unlicensed portions of the spectrum below 6 GHz are practically non-existent. The 60 GHz band, on the other hand, has more bandwidth available than all the lower unlicensed bands combined. Up until recent years, the 60 GHz band went largely unused due to the high oxygen absorption that signals in this band experiment, and lack of low cost RF technology [6]. Figure 4 shows the worldwide availability of this band, and it can be appreciated that it ranges from 3 to 9 GHz wide. To realize how impressive this is, it should be noted that the 5 GHz unlicensed band has around 500 MHz of usable bandwidth, while the 2.4 GHz band has less than 85 MHz in most regions [1] [4].

Figure 4. Worldwide spectrum availability at the 60 GHz band [4]

Figure 4. Worldwide spectrum availability at the 60 GHz band [4]

But having access to this much wider bandwidth does not come without challenges, and any new technology also has to be able to exploit the increased resources at a low cost.

The Friis equation is used to calculate the path loss from transmitter to receiver:

friis

where Pr is the received power, Pt is the transmitted power,  Gt is the transmitter antenna gain,  Gr is the receiver antenna gain,  λ is the wavelength, and R is the range from transmitter to receiver.  Considering that the received power is affected proportionally by the square of the wavelength, and that a signal around 60 GHz has a substantially smaller wavelength than a signal around 2.4 GHz, it becomes obvious that the power requirements are much more stringent for the newer technology (there is a loss of about 21 to 28 dB relative to the 2.4 and 5 GHz bands).

To make matters worse, the total noise from the wider band is much higher. Therefore it has been calculated that in general, 60 GHz systems will operate at 10 dB higher received power than IEEE 802.11n systems [4]. Even with this measure, 60 GHz systems will be intended for short range applications, which actually suits well to the interests of many entertainment producing companies. As mentioned in the previous post, the most prominent use of this technology will be the replacement of Wired Digital Interfaces (WDIs), and they are more than often used to stream copyrighted multimedia. The short range nature of 60 GHz systems puts at ease any worry of enabling copyright infringement, and more importantly, it decreases the amount of interference caused to other devices.

The more stringent power demands and increased aggregate noise make it harder to achieve low cost solutions, but on the other hand, technologies at the 60 GHz band have an advantage over those at 2.4 and 5 GHz: unlike the latter, the 60 GHz unlicensed band is more less the same all around the world, so that the manufacturers do not have to redesign transmitter and receiver antennas separately for each country with different regulations and differently located unlicensed bands, and this lowers the cost of production considerably [5].

Even when 60 GHz systems use higher transmit power than IEEE 802.11n systems, and the range of operation is restricted to smaller ranges, the requirement arises for high gain directional antennae to compensate for the much larger path loss. Again, the 60 GHz band proves to be an excellent choice when stopping to consider that for a given antenna aperture , the gain  scales inversely with the square of the wavelength, so for a perfectly efficient antenna system [1] [4]:

gain

This makes the production of small antennae for the 60 GHz band feasible.

A unique feature of any standard in the 60 GHz band is the absolute need for a Beamforming (BF) protocol. Older systems used omnidirectional antennae, but we have just argued for the use of high-gain antennae, which produce beams of much narrower width (i.e., with much more directivity). A BF protocol would automatically point the antennae so that they can find each other to coordinate operation and optimize antenna settings in an efficient, interoperable manner, and this helps to achieve the necessary link budget [1] [4].

The 60 GHz radio propagation channel has been studied for more than two decades, but the research and development of beamforming and beamsteering algorithms are much more recent. Channel models to test their performance are still being investigated, along with statistical shadowing models [7].

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