Current Hot Topic in Wireless Communication: 60 GHz unlicensed band

Wireless Communication in 60 GHz
Unlicensed (ISM) Band

Hi Friends, As you might have heard from your colleagues or friends that many people are doing
projects in 60 GHz. There are lots of projects going on all over the world at this frequency.
What is special in this frequency? Is this kind of ISM band with some special feature??? Yes!!!

The 60 GHz band is of much interest since this is the band in which a massive amount of spectral space (5 to 7 GHz) has been allocated for dense wireless local communications.
There exists an ever increasing supply of, and demand for, broadband multimedia applications calling for an ever increasing capacity of wireless networks. Finally, this will cause a demand for wireless transfer capacity far in excess of what can be accommodated in the currently used bands at 2.4-2.5 and 5.2-5.8 GHz

An obvious solution to this problem is to resort to the 60 GHz band, where bandwidth is abundantly available. In particular, for dense local communications, the 60 GHz band is of special interest because of the specific attenuation characteristic due to atmospheric oxygen of 10 to 15 dB/km. The 10-15 dB/km regime makes the 60 GHz band unsuitable for long-range (> 2 km) communications so that it can be dedicated entirely to short-range (< 1 km) communications.

For the small distances to be bridged in an indoor environment (<50m) the 10 to 15 dB/km attenuation has no significant impact. The specific attenuation in excess of 10 dB/km occurs in a band-width of about 8 GHz centered around 60 GHz. Thus, from physical point of view, there is about 8 GHz bandwidth available for dense wireless local communications. This makes the 60 GHz band of utmost interest for all kinds of short range wireless communications.

In the United States, the Federal Communications Commission (FCC) set aside the 59-64 GHz
frequency band for general unlicensed applications This was the largest contiguous block of radio spectrum ever allocated. FCC rules allow 10 Watts of equivalent isotropic radiated power in this band which complies with a maximum power density of 9 μW/cm2 at 3 meters distance. This means that 20 dBm transmit power would be the legal power limit with an antenna having 20 dBi gain. Commercial power amplifier GaAs MMICs are now available that can produce 16 dBm of transmit power with good linearity. In Japan, there was a new regulation in August 2000 for high speed data communication. The frequency range is 54.25-59 GHz for licensed use with a maximum output power of 100 mW and a minimum antenna gain of 20 dBi and 59 – 66 GHz for unlicensed use with a maximum output power of 10 mW and a maximum antenna gain of 47 dBi. In Europe, frequency is allocated for mobile in general in the 59-66 GHz band.

Currently, there is only one standard addressing the 60 GHz band and that is the IEEE 802.16 standard for Wireless MAN which covers 10 to 66 GHz . It concerns a last-mile broadband wireless connectivity alternative to fiber-based DSL. This saves tremendous initial investments in the deployment of last-mile networking technology.

CHOICE OF RF TECHNOLOGY:
Cost efficient RF solutions for high data rate transmission at 60 GHz still have to be determined. In this respect, some important choices have to be made which might by crucial for commercial success:
A. choice of the 60 GHz radio front-end architecture,
B. choice of technology in which the radio front-end
should be implemented: GaAs, InP, Si, or SiGe

A. Front-end architecture
With respect to the choice of the architecture of the 60 GHz front-end radio there are, in principle,
three options:
• employing sub sampling,
• employing direct conversion (i.e., “zero IF”),
• employing super heterodyning


The Popular approach is super heterodyne architecture because of some advantages over other two options. low noise amplifier (LNA) and a mixer which down converts to IF. The transmit branch consists of a mixer, a power amplifier (PA) and the transmit antenna. The antennas are (integrated) patch antennas.
The mixers are image rejecting mixers. They need not to be IQ mixers. The IF is taken at 5 GHz with the idea that, with appropriate modifications, IEEE 802.11a. The oscillator circuit could be a voltage controlledoscillator (VCO) controlled by an (off-chip) frequency synthesizer

B. Front-end technology
For such a high frequency it is obvious that MMIC (Microwave Monolithic IC) technology is preferred.
From cost point of view it is mandatory to reduce the number of components on the PCB board as much as possible. This reduction of the number of RF chips to a minimum is also important for minimizing losses in chip interconnection which can become easily significant at high frequencies. So, the level of integration should be as high as possible.
The cheapest semiconductor technology is based on silicon CMOS. But due to disadvantages like high mobility & getting higher Transition frequency (Ft) , people go for GaAs based MMICs.
In principle, an RF frond-end can be composed having considerable performance: 16 dBm transmit power, 5 dB noise factor and –100 dBc/Hz @ 100 kHz phase noise.
GaAs-based 60 GHz devices such as low-noise amplifiers, high power amplifiers, multipliers and
switches can nowadays be ordered in large quantities in die form at prices in the order of 15 € a piece


Table I lists GaAs and SiGe cost estimates


In the short term GaAs as well as InP technology will be applied since these are already mature technologies providing excellent performance. Because of the cost
advantage, however, it is likely that SiGe technology will become the ultimate solution for low-cost high volume 60 GHz front-end MMIC chips.

Antennas:
60 GHz antennas should feature the properties such as low fabrication cost, readily amenable to mass production, light weight, low volume, high efficiency which implies low-loss feed, easily integrated with MMIC RF front-end circuitry, covering the 59 – 66 GHz frequency band, eventually, circular polarization.

For the shorter term, the only viable solution that remains is the use of micro strip antennas. Micro strip patch antennas feature all of the properties listed in the aforementioned list of required properties. Linear polarizations are possible with a straight forward feed structure. Patch antennas can also have circular polarization. The application of circular polarization is considered because there are strong indications that channel delay spread is substantially lower in case circular polarization is used instead of linear polarization. Feed lines and matching networks can be fabricated simultaneously with the antenna structure. Finally, dual-frequency and dual polarization antennas can be easily made Patch antennas have some disadvantages like narrow bandwidth and associated tolerance problems, radiation from feeds and junctions etc.

Conclusion
In the United States as well as in Japan a large contiguous block of radio spectrum has been allocated for unlicensed use. Up to now Europe did not follow although it is recognized by CEPT
( European Conference of Postal and Telecommunications Administrations) that “the high frequency re-use achievable in the oxygen-absorption band reduces the requirement for sophisticated frequency planning techniques and offers the possibility of a pan-European deregulated telecommunications environment for various low-power, low cost,
short-range applications”. The message herewith posed to CEPT is that it would be wise to follow the US and Japan by opening a large part of the 60 GHz band for unlicensed use. The resulting world-wide license-free band would be a clear incentive for the industry to develop standards and 60 GHz radio technology with the prospective of a new mass market




References:
1) 60 GHz radio: prospects and future directions by smulders
2) Google.com :)