Thursday, July 05, 2012

From Kwerel & Williams to Interference Limits

Evan Kwerel & John Williams have proposed that future allocations should self-protect against projected adjacent band interference by assuming that they will receive only the “protections provided between flexible use bands” (Kwerel & Williams 2011, references at the end). The slide deck in Kwerel & Williams (2012) provides more detail: when a new allocation is being established next to a band likely to be repurposed for flexible use, the new allocation must (1) protect existing systems and future flexible use systems in that adjacent band, and (2) self-protect against interference from those systems, where flexible use systems is defined as “a dense deployment of base, mobile and fixed transmitters operating at fully functional power levels typical of a modern wireless cellular architecture.”

 Requirement (2) bears on the receivers of the new allocation. It resembles a qualitative interference limit based on the resulting energy from a “modern wireless cellular architecture.” A key selling point of this approach is that it doesn’t go beyond familiar parameters already used in regulation, like transmitter EIRP, compared to over interference limits that introduce probability distributions of resulting signal strength.

Thinking about a cellular deployment in the adjacent band is a very useful starting point. However, I do not believe it is precise enough to be useful in regulation, and particularly in enforcement. If one removes the studied ambiguity of the Kwerel & Williams proposal, the apparent familiarity and resemblance to existing rules evaporates, and one ends up with interference limits.

The requirement that receivers should self-protect against interference from flexible use systems doesn’t provide enough specificity to resolve interference disputes. Imagine that an operator Rae claims that her receivers are suffering harmful interference from Tom’s transmitters. Tom might not dispute that Rae’s receivers are being degraded, but could argue that his transmitter deployment is, in fact, equivalent to a “flexible use system” as defined above. Rae would dispute that, but deciding between them is problematic since the definition is rife with undefined terms like “dense” deployment, “fully functional” power levels, and “modern” wireless cellular architecture; further, base, mobile and fixed transmitters deliver very different interference patterns, and Rae and Tom will each draw attention to the scenario that suit their case best. Even if these terms had a commonly accepted meaning reflecting the state of the art at a particular moment, the state of the art constantly evolves – how is the adjudicator supposed to pick the vintage?

A bare “cellular next door” requirement is also insufficient if the FCC decides to design or mandate receiver performance requirements (aka receiver standards). The path from a requirement to protect oneself against interference from flexible use systems to a receiver standard has many steps – and the path leads through interference limits. First, one has to infer the undesired adjacent signal strength the receiver has to tolerate by choosing a transmitter topology; i.e. one has to determine an interference limit. Then one has to stipulate minimum and maximum desired signal strengths and criteria for successful operation in order to define the tests that will establish interference rejection ratios. Even more may be needed; receiver standards may specify implementation-specific selectivity tests like image rejection that only apply to superhet designs, not direct conversion receivers.

More precision is clearly required to operationalize the Kwerel & Williams proposal. One first needs to distinguish between base and mobile transmitters, since the resulting field strength on the ground delivered by base stations is much larger than that for handsets. As can be seen in the simulations in Ofcom/Transfinite (2008, Tables 10 and 14), the power flux spectral density exceeded at fewer than 5% of locations at 10 meter height is -42.2 dB(W/m2) per MHz for an IMT-2000 downlink, but only -63.2 dB(W/m2) per MHz for the corresponding uplink. The regulatory requirement should therefore specify whether the interference resulting from base stations or handsets is the test case.

Next, even if one specifies base stations, and provides the qualification of transmission at the highest allowed power, this does not enable one to characterize the interference suffered by a receiver in an adjacent band since the probability distribution of resulting field strength on the ground will depend on the base station antenna height, base station separation distance, and antenna downtilt.

To my knowledge, the FCC rules specify at most maximum height, and never minimum height, let alone antenna downtilt. For example Part 27.50(d) doesn’t seem to place any height limit on the AWS-1 downlink in 2110-2155 MHz. Further, Part 27.50(d)(2)(B) specifies different allowed transmitted power depending on population density (1,640 watts/MHz versus and 3,280 watts/MHz in sparsely populated areas, a 3 dB difference).

Even if additional specifications are added to EIRP, the transmitted power doesn’t give any indication of the resulting interfering signal that a receiver has to deal with. A femto-cell access point running a quite low power just down the hallway may deliver much more interference than a macro-cell running at maximum allowed power, but a mile away. To be usable, the Kwerel & Williams approach would also have to specify the geographical layout (e.g. inter-transmitter spacing) in some detail; since terms like macro-cell and femto-cell have no agreed meaning, one cannot use them.

After all this refinement, the approach no longer resembles current regulations very much, and its main selling has been lost. To be usable, the system to be self-protected against must be framed among other things as a base station architecture operating at maximum power at a stated minimum height.

In the end, it’s the resulting field strength on the ground that matters. What one could do is to assume (say) a collection of macro-cell base stations with a specific height, downtilt and inter-transmitter distance running at some transmit power, and then calculate the resulting field strength distribution. This then becomes the criterion for self-protection: the new allocation cannot claim harmful interference unless the resulting field strength, whether from a macro-cell or femto-cell, exceeds this limit.

Thus, one ends up operationalizing the Kwerel & Williams approach by modeling the probability distribution of the field strength that results from a particular base station power, height and distribution, as in the Ofcom/Transfinite study, section 4.3. For example, an IMT-2000 downlink in the assumed topology delivers signals below 85.6 dB(μV/m) per MHz at 95% of locations. For 22.7 dBW EIRP base stations at 30 m height with transmitters separated by 1.86 km, using a 5 MHz channel at 826 MHz, the in-band power flux density measured at 10 meters was calculated to be -42.2 dBW/m2 per MHz or less at 95% of locations. That’s equivalent to an OOB interference limit of 103.6 dBμV/m per MHz not exceeded at more than 5% of locations (converted using dBμV/m = dBW/m2 + 145.76).

The “cellular neighbor” case has been proposed as a starting point for rulemaking (presentation by Dennis Roberson, FCC TAC meeting, 27 June 2012 at timecode 80:00; disclosure: I was a member of the group whose work Dennis was reporting). However, the interference limits may need to be adjusted up or down if the neighboring service is unlikely ever to be cellular-like, particularly if one is skeptical that Coasian bargaining will allow neighboring services to negotiate a change in the promulgated limit.


Kwerel, E. and Williams, J. (2012). Solving the receiver problem without receiver standards. Slide presentation. FCC Workshop on Spectrum Efficiency and Receivers (Day 2). Retrieved from

Kwerel, E. & Williams, J. (2011). Forward-looking interference regulation. Journal on Telecommunications and High Technology Law, 9(2):516-518. Retrieved from

Transfinite Systems (2008). Derivation of power flux density spectrum usage rights. Tech. rep., Ofcom. Retrieved from

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