Saturday, January 2, 2010

Defining Interference or "Think Receiver"

The Interference Protection Working Group of the FCC's Spectrum Policy Task Force recommends that the FCC should consider using the "interference temperature" metric to quantify and manage interference. "Interference temperature" is a measure of radio frequency (RF) power (power generated by other emitters and noise sources) available at a receiving

antenna to be delivered to a receiver. More specifically, it is the temperature equivalent of the RF power available at a receiving antenna per unit bandwidth, measured in units of degrees Kelvin. As conceptualized by the FCC, the terms "interference temperature" and "antenna temperature" are synonymous. The term "interference temperature" is more descriptive for interference management.

Interference temperature can be calculated as the power received by an antenna (watts) divided by the associated RF bandwidth (hertz) and a term known as Boltzman's Constant (equal to 1.3807 wattsec per ┬║Kelvin). Alternatively, it can be calculated as the power flux density available at a receiving antenna (watts per meter squared), multiplied by the effective capture area of the antenna (meter squared), with this quantity divided by the associated RF bandwidth (hertz) and Boltzman's Constant. An "interference temperature density" can also be defined as the interference temperature per unit area, expressed in units of ┬║Kelvin per meter squared and calculated as the interference temperature divided by the effective capture area of the receiving antenna (determined by the antenna gain and the received frequency). Interference temperature density can be measured for particular frequencies using a reference antenna with known gain. Thereafter, it can be treated as a signal propagation variable independent of receiving antenna characteristics.

As illustrated in Figure 1, interference temperature measurements can be taken at receiver locations throughout the service areas of protected communications systems, thus estimating the real-time conditions of the RF environment.

(Source: FCC)

Figure 1: Interference temperature illustrated

Forms of Interference

Interference can be classified into two broad categories: co-channel (CoCh) interference (internal) and out-of-channel interference (external). These forms of interference manifest themselves as shown in Figure 2.

(Source: IEEE)

Figure 2: Forms of interference

Figure 2 illustrates a simplified example of the power spectrum of the desired signal and CoCh interference. Note that the channel bandwidth of the CoCh interferer may be wider or narrower than the desired signal. In the case of a wider CoCh interferer (as shown), only a portion of its power will fall within the receiver filter bandwidth. In this case, the interference can be estimated by calculating the power arriving at the receive (Rx) antenna and then multiplying by a factor equal to the ratio of the filter's bandwidth to the interferer's bandwidth.

An out-of-channel interferer is also shown. Here, two sets of parameters determine the total level of interference. First, a portion of the interferer's spectral sidelobes or transmitter output noise floor falls CoCh to the desired signal, that is, within the receiver filter's passband. This can be treated as CoCh interference. It cannot be removed at the receiver; its level is determined at the interfering transmitter. By characterizing the power spectral density (psd) of sidelobes and output noise floor with respect to the main lobe of a signal, this form of interference can be approximately computed similarly to the CoCh interference calculation, with an additional attenuation factor due to the suppression of this spectral energy with respect to the main lobe of the interfering signal. Figure 3 details the relationship of these lobes to the transmitter.

Figure 3: Main lobe, side lobes, and back lobe

Second, the receiver filter of the victim receiver does not completely suppress the main lobe of the interferer. No filter is ideal, and residual power passing through the stopband of the filter can be treated as additive to the CoCh interference present. The performance of the victim receiver in rejecting out-of-channel signals, sometimes referred to as blocking performance, determines the level of this form of interference. This form of interference can be simply estimated in a manner similar to the CoCh interference calculation, with an additional attenuation factor due to the relative rejection of the filter's stopband at the frequency of the interfering signal.

Cofrequency/Adjacent-Area Case Operators are encouraged to arrive at mutually acceptable sharing agreements that would allow for the maximum provision of service by each licensee within its service area. Under the circumstances where a sharing agreement between operators does not exist or has not been concluded and where service areas are in close proximity, a coordination process should be employed.

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