Filtering Interference From Cellular Receivers

May 1, 2003
Band-reject filters can be applied quite effectively in reducing or removing unwanted IMD interference signals from the operating band of a cellular receiver system.

Interference in cellular receive systems can stem from a variety of causes. Generally, the interference starts at a base-station transmitter (Tx), either from the same cellular system or from a nearby Tx. The interference can result in dropped calls, decreased receiver (Rx) sensitivity (and range), increased Rx noise figure and desensitization of receive-system active components. This last situation can lead to dropping all calls in an individual cellular sector. For code-division-multiple-access (CDMA) handset operators far from a base station (in the far field) with elevated noise floors, the base station will command the mobile units to increase output power. This will lead to higher handset emissions and reduced handset battery life. Fortunately, selective filters can be used in the base station to reduce the deleterious effects of interference on cellular systems.

Tx-based intermodulation distortion (IMD) consists of third, fifth, and higher-order signal products caused by the nonlinear mixing of two or more transmitted carriers, and is a well-understood phenomenon. These IMD products can block the reception of desired signals when they fall into the receive band and exceed the level of the desired receive signals (typically 70 to 100 dBm) at the front end of the base-station Rx (Fig. 1).

Passive components in the transmit signal path can, and do, create IMD products whenever two or more transmit carriers are present. The key to the severity of the interference is the level of these products relative to the base-station Rx sensitivity; ideally, these IMD products should be limited to a level that is below the usable sensitivity threshold of the receive system. Therefore, the integrity of a cellular Tx system is the level of IMD products when multiple carriers are present.

This type of interference to the Rx system can only be solved by improving the intermodulation characteristics of the Tx signal-path components, or by filtering the output of the Tx with a low IMD filter. Since the filter increases signal loss and affects the downlink signal budget, the preferred approach is to control IMD in the transmit-path components, such as antennas, filters/duplexers, power amplifiers (PAs), combiners, cables, connectors, couplers and any other passive or active components in the signal path. IMD in the passive components can be limited through the use of high-quality silver (Ag) or gold (Au)-plated contacts; nickel (Ni)-plated or passivated contacts should be avoided. Tx components should be specified in terms of maximum allowable IMD under specified conditions. Once IMD radiates though the antenna into free space, it cannot be filtered from the receive system without also attenuating the levels of desired signals.

Rx desensitization occurs when a co-located or nearby Tx's high signal strength enters an Rx's signal chain and causes IMD that interferes with or completely blocks the reception of the intended low-level signals. At high enough levels, this IMD can cause the Rx's active components to enter saturation, increasing the noise floor of the Rx's front end low-noise amplifier (LNA) dramatically. In CDMA cellular systems, this type of interference will also cause a base station to send commands that increase the transmit power of far-field handsets (to overcome the rise in noise floor caused by the IMD), leading to higher handset emissions and greatly reducing handset battery life.

Fortunately, there are several solutions for this second type of IMD interference. The first solution is to ensure that the active Rx stages use amplifiers, mixers, and other active or passive circuitry with enhanced dynamic range. Dynamic range can be defined as the difference (in decibels) from the minimum to the maximum signal level over which a component (and ultimately the Rx being composed of components) will function. The LNA is usually the first active stage in an Rx signal chain and, therefore, usually the most critical in determining the effective dynamic range of the receive system including the system noise figure.

There are two basic ways to increase the dynamic range of a receive system, short of changing transmission intermediate-frequency (IF) bandwidths and other factors. The first is to design or select an LNA with the lowest possible noise figure. This improves the Rx sensitivity when an input signal is at minimum levels, and effectively improves the sensitivity of the Rx system for far-field handset users. The second method is to increase the third-order-intercept point (IP3) of the LNA. The IP3 specification is a figure of merit for an amplifier's ability to handle high signal levels without odd-order IMD products (third, fifth, etc.) increasing beyond an acceptable level, and falling into the receive band, causing receive system interference. By increasing an LNA's IP3 characteristics, the receive system's dynamic range increases for higher-level signals. In general, designers select LNAs with as much dynamic range (lowest noise figure and highest IP3) as possible within cost and power budgets.

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The second path to improving Rx desensitization is by limiting the out-of-band or interfering signals from reaching the Rx. This can be accomplished by decreasing the beamwidth of the base-station antenna (and increasing antenna and link gain). However, this is usually not possible since the base-station antenna beamwidth must support coverage within a (typically) 60-to-120-deg. sector for multiple users. Because of the wide beamwidth, interfering signals can easily gain entry to an Rx system. So, rather than modifying the antenna, or employing extremely expensive smart antenna systems, highly selective RF filters or duplexers are typically used to reject IMD-based interference.

Both bandpass and band-reject (notch) filters can be used to limit receive-band interference. As a general rule, high-selectivity receive filters and duplexers usually employ a bandpass structure. A highly selective duplexer (which combines filters for both transmit and receive bands) can provide high receive-path rejection above and below the passband of interest, thus rejecting by a specified level potential or existing interference signals. Because of antenna site/zoning constraints, duplexers are attractive since they allow antennas to be consolidated or reduced at a given site. Because of zoning restrictions, most sites require duplexers for receive and transmit signals. High-performance duplexers offer a suitable solution for new sites or those undergoing major upgrades. For sites already employing "generic" duplexers, which have operated successfully for many years, newly installed or co-located competitive Txs can unveil the shortcomings of these generic duplexers. For example, if a duplexer has been optimized for transmit rejection, and uses a wide receive filter bandwidth with inadequate rejection of IMD interference, the Rx can become desensitized by sufficiently high-level IMD interference signals that are allowed to pass through the duplexer (Fig. 2).

Fortunately, a band-reject or notch filter offers a novel solution to Rx desensitization. Although available for decades, the application of this type of filter in cellular systems has been limited to extreme spot interference or test-system requirements. The unique advantage of the notch filter is its ability to reject only the specific band of interest, and pass the intended receive and transmit bands with very little insertion loss. Its operation is the inverse of a bandpass filter. Instead of rejecting most frequencies while passing a narrowband, the notch filter rejects a narrowband segment and passes a broad range of frequencies.

As an example, K&L Microwave offers cellular AMPS band notch filters for both A and B Band operators that attenuate the Specialized Mobile Radio (SMR) transmit band (851-to-866-MHz) BTS transmit band by at least 37 dB with low loss in receive and transmit passbands. The filters are available as a single-notch device (model WSN-00099 for B-band operators) or as a dual rack-mount assembly (Fig. 3).

Bandpass filters have the disadvantage of affecting the isolation of the receive system relative to one's own Tx. As a result, extensive re-engineering and re-specification may be required for a system in which bandpass filters are used to meet the original equipment manufacturers' (OEMs) transmit rejection. The notch-filter solution rejects only the interfering band of interest, and allows the otherwise properly functioning generic duplexer and cellular transceivers to remain in place. The notch-filter approach offers very low signal loss in both receive and transmit passbands, and allows for "in-line" installation in both duplexed or receive-only antenna feeds. For duplexed systems where the transmit-signal levels can be high, the notch filters mentioned above have been designed and tested to handle very high RF power levels, to 500 W CW and 10 kW for instantaneous peaks.

As an example of how notch filters can provide an effective solution to cellular Rx desensitization, dual notch filter assemblies (model WSA-00129) were installed in sectors at various cellular sites in a major metropolitan (B-band) market, where the dropped-call rate was consistently approaching or even exceeding 20 percent due to SMR interference. Each of these sites employed a generic duplexer with 25-MHz passbands; each site faced interference from newly constructed SMR sites, either close to (within 0.5 mile) or co-located with the providers equipment. Figure 4 shows the noise-floor performance improvement from installing the notch filters (an approximate 30-dB improvement in the noise floor). Dropped calls improved to less than 2 percent.

Another approach to the cellular Rx desensitization problem is the use of high-performance LNAs integrated with a selective receive-band filter. An example of this type of assembly (from K&L Microwave) employs high IP3 LNA and an integrated duplexer to achieve 2-dB noise figure and >30-dB gain with less than 5-MHz passband and tunability over the full AMPS cellular band (Fig. 5). One of the LNAs features 15-dB gain and output IP3 of +37 dBm while the higher-gain LNA offers 37-dB gain and +42 dBm output IP3.

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