On November 28, 2003, the FCC released a Notice of Inquiry (NOI) and Notice of Proposed Rulemaking (NPRM) seeking comment on “Interference Temperature”, a proposed new model for quantifying and managing interference. This concept was initially developed by the Commission’s Spectrum Policy Task Force to improve the management of the radio spectrum. Essentially the idea is to measure the “background” interference temperature due to noise and unintentional emitters, add in the temperature due to the proposed transmission, and check whether the total is within some predetermined, non-interfering limit. In the Notice of Proposed Rulemaking (NPRM) portion of the document, the Commission sought comment on the specific technical rules that would establish the interference temperature concept in the 6,525-6,700 MHz and 12,750-13,250 MHz bands used by the fixed service (FS) and fixed satellite service (FSS). One of the outcomes of the proceeding promoted by the Commission was the introduction of unlicensed devices sharing spectrum with licensed systems.

Comsearch provided comments that, while we agree that increased sharing in the 6,525-6,700 MHz and 12,750-13,250 MHz bands is feasible using streamlined coordination methods, we are opposed to the trial implementation of Interference Temperature operation based on Dynamic Frequency Selection (DFS) and Transmitter Power Control (TPC) proposed in the NPRM. Comsearch believes that it would cause harmful interference to fixed service receivers. Instead, we feel that the FCC should consider streamlined coordination approaches that take into account critical factors like the location of the transmitters and receivers, transmitter powers, path distances, antenna pointing directions, antenna discrimination values, and receiver sensitivities.

The existing emission limits above 960 MHz for unlicensed devices allow an EIRP level of -41.25 dBm/MHz.^[1] A device transmitting at this level is capable of degrading the noise floor of a microwave receiver at a distance of several kilometers if the device happens to be located in the main beam of the microwave antenna.^[2] Licensed FS and FSS stations are able to share the spectrum at much higher EIRP levels, despite the corresponding interference potential over greater distances, as a result of the careful planning that occurs during frequency coordination of these stations. Significantly increasing the allowable EIRP for unlicensed devices as proposed in the NPRM, perhaps to +36 dBm, carries with it a huge potential for causing harmful interference unless a spectrum management regime that is as effective as the existing Part 101 coordination process is implemented. Unfortunately, Interference Temperature as proposed in the NPRM would not be effective.

Regarding interference from unlicensed devices into FS receivers, the NPRM raises two major questions that must be addressed:

The FCC’s NPRM state that “Some systems, especially those employing error correction codes and other interference mitigation techniques are highly robust and can operate in the presence of an undesired signal that is considerably higher than the level of the desired signal without experiencing harmful interference.”

While some spread-spectrum systems may be able to operate with an undesired signal higher than the desired signal, FS systems under Part 101 are not able to use such techniques because of the spectrum efficiency requirements imposed by 47 C.F.R §101.141. Considering the communication engineer’s axiom of “trading bandwidth for signal-to-noise ratio”, the rule requires a high data rate for the allowed bandwidth, and therefore a high ratio of signal power to noise and interference is necessary. To meet the required bandwidth efficiency of 4.5 bps/Hz, it is necessary to use high-order digital modulation schemes such as 64 QAM. Instead of being able to operate with a negative C/(N+I), these systems require large positive C/(N+I) values of 25 to 30 dB or more just to demodulate the signal at an acceptable bit error rate of 10^-6. Further, to meet the stringent reliability objectives necessary for these systems, a significant fade margin must be included in the interference objectives to account for periods of time when the desired signal suffers a deep multi-path fade while the interference signal remains constant. Determining the interference objectives by the T/I approach of TIA TSB 10-F limits degradation of the receiver threshold to no more than 1 dB, and typical C/(N+I) objectives for the 6.7 GHz band are in the 55 to 75 dB range. The need for such high C/(N+I) ratios is not the result of poor design of the FS systems but rather is the result of the basic system requirements - the need to transmit a high data rate signal in a narrow radio channel, the fact that multi-path fading is independent among paths in an area, and the necessity to meet stringent path reliability objectives.

As depicted in Figure 1,^[3] the interference temperature concept depends on the idea that interference signals often prevent receivers from operating down to the threshold they could achieve based on thermal noise alone. In this scenario, there is an appreciable likelihood of existing interference above the thermal noise floor, and setting an Interference Temperature limit will allow unlicensed devices to cause similar interference somewhat above the receiver thermal noise floor while at the same time placing a cap on the total interference that a receiver should be expected to suffer. Conceptually, this situation may apply to, for example, land mobile and broadcasting bands below 1 GHz where there is significant noise floor degradation as a result of man-made interference sources, but it does not apply to microwave bands above 1 GHz where there is no such widespread noise floor degradation.

Because widespread noise floor degradation does not exist in the 6.7 and 13 GHz bands, there is no natural “margin” to be captured for use by an unlicensed underlay service. Setting an Interference Temperature limit that allows more than 1 dB of degradation of the threshold of microwave receivers would directly harm licensed microwave systems in favor of the new unlicensed service. The Commission should set the Interference Temperature limit at 6 dB below the noise floor of the FS receivers. Table 1 shows typical interference limits for several 6.7 GHz and 13 GHz microwave systems. There may be a large number of unlicensed devices contributing to the interference into a FS receiver, and the total interference from these devices should be expected to meet the objectives in Table 1.

Higher levels of interference should only be allowed if:

1. The FS receiver has excess fade margin. Threshold degradation in excess of 1 dB may be acceptable if the receiver still meets its reliability objective based on the appropriate path fading models.
2. The interference is intermittent rather than constant. This situation may apply if the unlicensed devices transmit with less than full duty cycle; on the other hand there may be no advantage if there are a large number of devices. An approach such as Fractional Degradation of Performance (FDP)^[4] could be used to quantify this interference. FDP of 25% corresponds to 1 dB degradation from a constant interference source.
3. The interference is removed when the FS receivers fade. In other words, the FS receivers control the unlicensed transmitters.

In the NPRM, the Commission states that “in light of the great disparity in magnitude between permissible emission levels for licensed versus unlicensed devices, sound engineering judgment intuitively suggests that the 6,525-6,700 MHz and 12.75-13.25 GHz bands can support expanded unlicensed operations enabled by an interference temperature approach without detrimental impact to incumbent operations.” The Commission thus appears to suppose that FS receivers will be protected from harmful interference despite a 77 dB increase in the EIRP allowed to unlicensed devices, because the allowed EIRP would still be 49 dB below the EIRP level allowed to licensed FS transmitters. Lower EIRP of unlicensed devices by itself is not enough to protect FS receivers. Part 15 presently allows +36 dBm EIRP in the 5,725-5,850 MHz band, and because no directional antenna requirements are included with the Commission’s proposals, we presume that the NPRM contemplates allowing the same operation in the 6.7 GHz and 13 GHz bands: EIRP of +36 dBm using an omni-directional antenna. Typical FS operation in the 6.7 GHz band might involve a 1 W transmitter connected to an 8-foot diameter parabolic antenna for a maximum main beam EIRP of about +70 dBm. Considering the pattern of the Andrew PAR8-65 antenna, the EIRP of this setup would be less than +36 dBm for all angles more than 3.5 degrees from the boresight direction, and would be just +10 dBm for a 100 degree sector behind the antenna. The Commission must recognize that the area of potential interference created by a +36 dBm EIRP omni-directional unlicensed device may be significantly larger than that of a +70 dBm EIRP FS transmitter using a typical directional microwave antenna. Introducing omni-directional unlicensed transmitters to the bands has a huge potential for causing interference to FS receivers, even if the EIRP level allowed to the unlicensed transmitters is much lower than that of the FS transmitters.

It is shown in ITU-R M.1652 that if an unlicensed device cannot detect a power level above -62 dBm on a particular channel, then its transmissions at +30 dBm EIRP on that channel would not cause harmful interference to radar systems.^[5] Based on such analysis, the DFS thresholds set for the 5.25-5.35 GHz and 5.470-5.725 GHz bands, -62 dBm for devices of up to 23 dBm EIRP and -64 dBm for devices of 23 dBm to 30 dBm EIRP, are appropriate to protect radars from harmful interference. DFS is a workable solution for radar interference because:

• To overcome the path losses experienced as the signal travels to and returns from an object, radars must transmit extremely high power levels, and these signals are correspondingly easy for an unlicensed device to detect.
• The path the radar signal travels from the radar transmitter to the unlicensed device is the same as the path of potential interference from the unlicensed device to the radar receiver
• Radars use antenna scanning to direct the main beam in multiple directions, increasing the ability of unlicensed devices to detect the signal.

Conversely, DFS is not a workable solution to prevent harmful interference to the fixed service because:

• FS transmitters use only moderate EIRP levels and directional antennas that greatly suppress the radiated EIRP in all but the direction of the main beam. This makes the signal very difficult or impossible for an unlicensed device to detect.
• The path the signal travels from FS transmitter to the unlicensed device is not the same as the path of potential interference from the unlicensed device to the FS receiver.
• Because the “detection path” and “interference path” are different, there is no connection between the level that an unlicensed device may detect on a channel, and the interference effect of its transmissions on that channel. While unable to detect the FS transmitter signal, an unlicensed device could be in the main beam of the FS receiver antenna and capable of causing catastrophic interference to the receiver. A device may not hear any use of the channel but cause catastrophic interference, or may hear a relatively high signal on the channel but not cause harmful interference.

Comsearch believes that the Commission’s sharing analysis between the FS and unlicensed devices^[6] is flawed because of several inaccurate assumptions. First, the Commission assumes that there would always be at least 20 degrees antenna discrimination from a FS receive antenna towards an unlicensed transmitter. However, at much greater distances than the 100 meter distance the Commission was considering, it is very possible that an unlicensed transmitter could be in the main beam of the FS receive antenna. Furthermore, harmful interference is quite possible at these greater distances because of the sensitivity of the FS receivers. Second the Commission’s analysis does not consider the effect of discrimination from the FS transmit antenna towards the unlicensed device, or the effect of the use of ATPC by the FS transmitter. This discrimination and ATPC power reduction make it difficult or impossible for the unlicensed device to detect FS use of the channel. Finally, the Commission’s analysis assumes that “unwanted emissions received by the FS receiver will be dominated by the emissions from the closest [unlicensed] device^[7].” Instead, devices that are farther away but in the main beam of the FS receive antenna may dominate unwanted emissions received by the FS receiver.

In Case 1, an unlicensed device is located 1 km behind the transmitting antenna of a 20 km FS link and in the main beam of the receiving antenna of the link. In this configuration, the discrimination of the transmitting FS antenna makes it difficult or impossible for the unlicensed device to detect the FS use of the channel. At the same time, the transmissions of the unlicensed device would cause severe interference to the FS receiver. 

In Table 2 a link budget for Case 1 based on realistic parameters shows that the level of the signal of the FS transmitter at the unlicensed device would be -115.5 dBm referenced to a 0 dBi antenna, while the level of the interference signal from the unlicensed device at the FS receiver would be -62.9 dBm. It is questionable whether the unlicensed device could hear the FS signal, since the level would be comparable to the noise floor of a conventional receiver, while the interference of the unlicensed device would take nearly all the fade margin of the FS receiver and reduce the link reliability from 99.9999% to 96%. To avoid harmful interference in Case 1, either the DFS threshold would have to be set at -116 dBm so the unlicensed device would not transmit, or the unlicensed device EIRP would have to be limited to -6 dBm so the interference level would meet the objective. Such limits do not appear to allow for a viable unlicensed underlay service. Furthermore, if the unlicensed device were 4 km instead of 1 km behind the FS transmit antenna, the FS signal at the unlicensed device would be reduced 12 dB to -127 dBm0 while the interference signal of the unlicensed device at the FS receiver would only be reduced 1 dB. At some point the unlicensed device would become unable to detect the presence of the FS transmitter and would have no basis upon which to make its transmit or do not transmit decision.

Traditionally, the Commission has adopted rules that require licensees to use the minimum transmitter power necessary and to use antennas that minimize the impact on other users. Such rules are rightly seen as encouraging efficient use of the spectrum. However, allowing unlicensed devices with DFS into the microwave bands would create opposite incentives for FS users. Table 1 shows a front-to-back ratio of 70 dB for the FS transmit antenna. This value corresponds to an ultra-high performance antenna. On the other hand, the Part 101 antenna standards only require front-to-back discrimination of 45 dB for Category B and 55 dB for Category A in the 6.7 GHz band^[8]. If the microwave path used Category B antennas instead of ultra-high performance antennas, the decrease in antenna discrimination would make the FS signal easier to detect at the unlicensed device and increase the chance that the unlicensed device would decide not to transmit. Thus by allowing unlicensed devices with DFS into the FS bands, the Commission would be creating an incentive for FS licensees to use the worst antennas that they could get away with. Similarly, FS licensees would have an incentive to use the highest transmitter power that they could, and an incentive not to use ATPC. If the FS link in Case 1 used an ATPC that reduced the transmitter power 10 dB, the unlicensed device would be that much less likely to detect the FS transmitter, and the interference from the unlicensed device would render the link unavailable even without any fading of the link.

In Case 2, an unlicensed device is located 1 km behind the receiving antenna of a 20 km FS link and in the main beam of the transmitting antenna of the link. Thus the geometry of the case is opposite that of Case 1. Here, the level of the FS transmitter at the unlicensed device is -71.9 dBm0 (much higher than Case 1), while the interference level of the unlicensed device at the FS receiver is -106.5 dBm (much lower than Case 1). In the geometry of Case 2, the unlicensed device would be able to detect the FS transmitter, and the interference caused by the unlicensed transmitter to the FS receiver would meet the interference objective. Since the interference would meet the objective, the DFS threshold of -64 dBm proposed in the NPRM would be acceptable for Case 2. However if the DFS threshold were set low enough to protect the FS receiver for the geometry of Case 1, then the DFS threshold would be exceeded in the geometry of Case 2, and the unlicensed device would decide it could not transmit.

In Case 3, an unlicensed device is located in the middle of a 40 km FS link. Even in this geometry, the level of the FS transmitter at the unlicensed device, -68.7 dBm0, meets the proposed DFS threshold. The device would decide it could transmit, but the interference level at the FS receiver, -59.7 dBm, would render the link unavailable.

From this we conclude there is no connection between the level that an unlicensed device may receive from an FS transmitter, and the interference that that device’s transmissions may cause to the associated FS receiver. There is no way to select a DFS threshold that is both low enough to protect the FS receivers and high enough to allow a viable unlicensed underlay service.

Comsearch recommends that the FCC abandon the proposed trial implementation of Interference Temperature (DFS and TPC) operation of unlicensed devices in the 6.7 and 13 GHz bands because it has no hope of protecting the FS from harmful interference while at the same time allowing a viable unlicensed underlay service. Instead, we feel the Commission should investigate realistic strategies for allowing unlicensed devices to share the band. Such realistic strategies could include:

• Control of the unlicensed transmitters by the FS receivers
• Mandatory registration of unlicensed devices in frequency coordination databases and up-front interference analysis
• Real-time self-coordination of unlicensed devices by using new software algorithms, GPS positioning, and up-to-date downloads of local FS link data
• Directional antenna requirements for the unlicensed devices

The implementation of Interference Temperature proposed in the NPRM cannot replace the present frequency coordination because it ignores critical factors like the location of the transmitters and receivers, transmitter powers, path distances, antenna pointing directions, antenna discrimination values, and receiver sensitivities. Automating frequency coordination must involve detailed interference calculations that take these factors into account. An oversimplified solution as the Commission has proposed in this NPRM can only lead to harmful interference to the FS, unreasonably restrictive limits on the unlicensed service, or both.