Fixed Wireless Communications at 60GHz Unique Oxygen Absorption Properties
by Shigeaki (Shey) Hakusui, President, Harmonix Corporation
Wireless communication devices operating at higher frequencies, such as 60GHz, allow businesses to link to the fiber easily, without the cost and time delays associated with physical fiber installation.
Due to the increased bandwidth demands and the scarcity of microwave frequency allocations, the wireless communications industry is beginning to focus on higher, previously unallocated portions of the spectrum in the millimeter wave frequencies from 40GHz to 300GHz. Due to the high levels of atmospheric RF energy absorption, the millimeter wave region of the RF spectrum is not usable in long haul, wireless communications segments. However, for short haul, "last mile" segments, the expanded RF data bandwidth available in the millimeter wave region makes it ideal for interference free, fiber speed connectivity.
Figure 1 illustrates the atmospheric absorption for millimeter wave frequencies.
At the millimeter wave frequency of 60GHz, the absorption is very high, with 98 percent of the transmitted energy absorbed by atmospheric oxygen. While oxygen absorption at 60GHz severely limits range, it also eliminates interference between same frequency terminals.
Figure 1: Dry Atmospheric Absorption per Kilometer
The benefit of Oxygen absorption relative to frequency re-use is detailed in figure 2. Figure 2 illustrates the distance relationship between the 60GHz frequency reuse range, the green region, and the traditional range, the blue region. Oxygen absorption makes possible the same-frequency reuse within a very localized region of air space. Operation within the 60GHz millimeter wave spectrum enables very dense interference free deployment of same frequency radio terminals.
Figure 2: Frequency Reuse Source FCC Bulletin 70A
A 60GHz communications system must overcome the effects of oxygen absorption, 16dB/KM. In order to operate reliably at even short ranges, a very focused, narrow-beam antenna must also be employed to increase the level of signal available to the target receiver. This combination of oxygen absorption and narrow beam transmission enhances the security of the 60GHz radio link, minimizing the probability of unauthorized intercept.
Traditional wireless communications systems operating in the lower frequency ranges of 900MHz to 40GHz often interfere with each other when placed too closely together. This interference, due to the dispersion and uncontrolled propagation of RF energy through the atmosphere is minimized by FCC frequency coordination, licensing and through the implementation of interference avoidance techniques such as spread-spectrum modulation. FCC Licensing precludes dense deployment through the limited number of regional licenses granted and spread-spectrum techniques have proven only marginally effective, as the overall noise floor has risen. In the 60GHz region, the effects of oxygen absorption and the use of narrow beam antennae minimize the probability of interference between the radios. Theoretically, 100,000 systems operating at 60 GHz can be co-located in a ten square kilometer area without interference problems.
Weather conditions have an adverse effect on all RF transmissions, especially in the millimeter wave region where severe rainstorms can cause as much as a 20dB loss in signal strength for every kilometer of transmission. As the distance the radio transmission increases, the fade margin needed to compensate for weather effects increases proportionately. Since radios operating at 60GHz transmit only over short distances, the compensation for weather effects is not as great as for systems transmitting one kilometer and beyond.
At 60GHz, the extremely high atmospheric absorption level is due primarily to the molecular composition of the atmosphere. Figure 3 illustrates the atmospheric attenuation characteristics for wavelengths from 3 cm to 0.3 mm. For millimeter waves, the primary absorption molecules are H2O, O2, CO2 and O3. Since the presence of O2 is fairly consistent at ground level, its effect on 60GHz radio propagation is easily modeled for margin budgeting purposes. In addition, the high level of attenuation from oxygen absorption makes even the worst weather-related attenuation insignificant, especially on the short paths where 60GHz systems operate. Even extremely heavy rainfall, 25mm/hr (5dB/KM), will make only a very small percentage contribution to aggregate attenuation in the 60GHz oxygen absorption region.
Figure 3: Atmospheric Attenuation Characteristics for Wavelengths 3 cm. to 0.3 mm.
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