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.