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As most signals broadcast from telecommunication antennas are not perfect tones, the PIM and harmonic responses are not impulses. The responses are spread more widely through the frequency spectrum (Fig. 2). This aspect suggests that the PIM product doesn’t need to be exactly within the reception band to cause distortions. Rather, it must be within the receive filter’s passband (Fig. 3).

New Modulation Schemes Raise PIM, Fig. 2

As the LTE and LTE-A standards require higher-throughput data, maintaining a low noise floor is a chief concern. Telecommunications systems in less dense spectrum may only consider third-order intermodulation distortion as significant. This is not the case with very dense spectrum, as the other-order products could be within the reception band of neighboring carriers. Other-order products also may increase the noise floor throughout the system, as they may be retransmitted.

New Modulation Schemes Raise PIM, Fig. 3

Commonly used cellular telecommunications bands are affected by PIM products beyond just third-order intermodulation distortion. The antenna front ends and antennas for these complex digital modulation standards also are increasing in frequency performance and density. For example, increased frequency-handling capability is being designed into the latest mobile antennas. These antennas transmit and receive efficiently at many different bands and switch between these bands rapidly.

This approach would not cause enhanced PIM issues in an isolated system. When these antenna systems are distributed throughout an environment to ensure good signal reception, the proximity of these antennas decreases. This densification of antennas and broadcast frequencies has led to a scenario in which the redundant transmission of the antennas could induce significant PIM levels within their companion structures. PIM then extends beyond the simple two-tone frequency case and multiplies the frequencies mixing throughout the signal environment (Fig. 4).

New Modulation Schemes Raise PIM, Fig. 4

The products for multi-tone frequency mixing are more numerous than with just two tones. In a two-tone mixing, there are only two third-order intermodulation-distortion odd-order products that are considered of primary concern. With three-tone, mixing there are four third-order intermodulation-distortion odd-order products of similar power levels (Table 2). Modulation schemes that use the same frequency for transmit and receive (like time-domain LTE and WiGig, for example) could exhibit degraded performance with multi-tone PIM generators nearby.

New Modulation Schemes Raise PIM, Table 2

This problem is compounded when considering distributed antenna systems (DASs). DASs are predominantly used for commercial and industrial areas, where enclosed spaces limit larger cell broadcasting. These areas are often more prone to having PIM generators in the environment and near the antennas. PIM generators could be any material that is electrically responsive in the environment—with either nonlinear properties or the potential for such properties when in contact with other materials.

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