The intermodulation interference of communication system and its solution are explained in detail
Intermodulation interference refers to the phenomenon where signals of different frequencies, when passing through nonlinear circuits, generate frequency combinations that are identical or similar to the useful signal frequencies, thus causing interference to communication systems. According to the IS95 standard and the testing criteria of the National Radio Commission, the spurious and intermodulation signals generated by GSM repeaters should be less than 9 KHz-1GHz-36dBm, less than-30dBm at 1GHz-12.75GHz. In mobile communication system, there are three reasons for intermodulation: transmitter intermodulation, receiver intermodulation and external effect.
The spurious and intermodulation products in repeaters mainly originate from the power amplifier modules inside the repeater. Intermodulation in transmitters occurs when multiple transmitters (carriers) operate simultaneously, and due to insufficient isolation in the combiner system, signals couple with each other. The interference signals penetrate into the final power amplifiers of the transmitters, synthesizing with the useful signals to form intermodulation products that are then transmitted along with the useful signals,causing interference. Receiver intermodulation is primarily caused by the nonlinearity of the high-frequency stage and the first mixer circuit. Intermodulation due to external effects mainly results from poor contact in passive circuits such as transmitter feed lines and high-frequency filters, as well as nonlinear contact between different metals, which cause strong electric field divergent signals to induce intermodulation, generating interference sources.
When multiple frequency signals pass through a nonlinear circuit, they will modulate each other, producing intermodulation distortion, with second-order and third-order distortions being the most significant; the higher the order, the less severe the distortion. Second-order intermodulations such as fa+fb and fa-fb, due to their frequencies being far from the dominant signal frequencies fa and fb, can be disregarded: the two models of third-order intermodulations, 2fa-fb and fa+fb-fc, because their frequencies are close to or equal to the dominant signal frequency, have the greatest impact on communication; intermodulations above the third order have smaller amplitudes and can also be disregarded. Mobile communication equipment primarily considers the effects of third-order intermodulation.
At the same location, if there are more than two transmitters, intermodulation interference may occur. The RF signal fA emitted by transmitter A travels through the antenna of transmitter B and enters the power amplifier stage of transmitter B, where it is modulated with the transmission frequency fB of that machine, generating a third frequency fC. Conversely, fD is also produced. Therefore, at this point, the two transmitters emit four frequency points of RF power signals. Among them, fC and fD are intermodulation products (see Figure 1).
一. The harm of intermodulation interference
(1) The impact of intermodulation interference on the system:
Intermodulation products are also radio frequency energy signals emitted by transmitters. When these signals are intermodulated with another transmitter, they generate additional intermodulation products. Therefore, in the airspace above base stations with multiple transmitters, there is a vast amount of unordered spectrum energy, which some people refer to as background noise. These signals have a very high probability of matching the frequencies of other differential receivers or paging BP machine frequencies. At the same time, they severely interfere with civil aviation navigation signals and broadcast television signals. Thus, addressing the spatial radio wave disorder caused by intermodulation interference is an urgent necessity.
When an operator (whether it be China Mobile or China Unicom) sets up a repeater with high spurious and intermodulation signals, these signals fall outside the band of the operator and can cause co-channel interference to the downlink signal of another nearby operator. For example, if Operator A plans to install a repeater on the fourth floor, with spurious and intermodulation levels at-36dBm (meeting regulatory standards), and both the spurious and intermodulation signals and the useful signal are transmitted through a service antenna with a gain of 17 dBi, then the output intensity of the spurious and intermodulation signals at the front of the antenna is-18 dBm. According to the free-space wireless propagation formula, the attenuation is approximately 50dB at a distance of 10 meters, about 70dB at 100 meters, and around 90dB at 1 kilometer; this allows for calculating the co-channel interference on other operators' downlink signals. In an unobstructed environment, the co-channel interference within 100 meters in front of the antenna is greater than-88 dBm. If the signal strength of another operator is below-79 dBm, resulting in a carrier-to interference ratio below 9, it will lead to disconnection issues. Therefore, when undertaking indoor or outdoor repeater projects, controlling the distances between devices and antennas of different operators is crucial for avoiding interference.
(2) Impact on adjacent base station cells:
When using high-power broadband repeaters, the intermodulation indicators are relatively high. Since all channels of the repeater share a single power amplifier module, and the subsequent filter is a broadb and filter, it has no effect on suppressing intermodulation signals within the passband. Therefore, it is difficult to meet the IS95 standard requirements for intermodulation signals. If the same frequency signal from a nearby base station of the same operator is received, and the subsequent broadband filter does not suppress them during output, severe co-channel interference will occur.
Assuming the direct repeater output power is 10W, the third-order intermodulation is typically 40dBc, with the actual output level of third-order intermodulation being (0dBm). If the serving base station uses carriers 80 and 84, the third-order intermodulation signals will fall on carriers 76 and 89, which are then transmitted outward through an antenna with a gain of 17dBi. This can result in varying degrees of co-channel interference for the base station using carriers 76 and 89 in front of the service antenna, leading to increased call drop rates and higher handover failure rates.
(3) The effect on its own upward signal
The spurious and intermodulation signals generated by the downlink power amplifier of the repeater enter the uplink channel through the duplexer inside the repeater. If this signal is too high, it will seriously interfere with the uplink signal and reduce the coverage range of the repeater.
For example, the spurious and intermodulation of the repeater is-36dBm (meeting the regulatory requirements), the isolation of the duplexer is 70 dB, and the spurious and intermodulation generated by the downlink power amplifier interfere with the uplink channel at-106 dBm. Adding the noise figure of the repeater, which is 5 dB, the overall noise level reaches-101 dBm. To ensure the minimum carrier-to-noise ratio of 9dB required by GSM, the minimum uplink signal level must be greater than-92 dBm. If the output power of the repeater is 5W (37 dBm) per carrier and the maximum output power of the mobile phone is 2W (33 dBm), then the actual coverage distance of the repeater is-87dBm for the downlink signal level. Below this level, mobile users will not be able to communicate due to insufficient uplink signal-to-noise ratio.Generally, the power measurement of a transmitter uses a direct-reading power meter, which has a certain bandwidth (some up to 1 kHz). Since power is the integral of spectral energy, the power measured by a directreading power meter is the sum of the effective main frequency power and the power of unwanted intermodulation products. For example, if the power meter shows 100W, it may actually be that the power at the main frequency f0 is only 80W. The remaining 20W is the power of intermodulation products and spurious components.
Generally, the power measurement of a transmitter uses a direct-reading power meter, which has a certain bandwidth (some up to 1 kHz). Since power is the integral of spectral energy, the power measured by a direct reading power meter is the sum of the effective main frequency power and the power of unwanted intermodulation products. For example, if the power meter shows 100W, it may actually be that the power at the main frequency f0 is only 80W. The remaining 20W is the power of intermodulation products and spurious components.
二． Solutions
From the above analysis, it can be seen that there are two intermodulation factors affecting the uplink output: the linearity of the equipment itself and the ALC control level. In order to avoid third-order intermodulation, the following methods can be adopted
(1) Select the appropriate frequency combination. Select the channel point group without third-order intermodulation to work at a wide frequency distance, so that the third-order intermodulation will not fall within the frequency used;
(2) Automatic gain (power) control (APC) technology is adopted to reduce the transmission power in real time to reduce the intermodulation level so that it does not fall into the nonlinear region of active devices.
(3) Improve the selectivity of the front end of the receiver, suppress the interference signal; improve the linearity of the input stage of the receiver, improve the intermodulation resistance ratio; improve the selectivity of the power amplifier In GSM networks, the introduction of repeater equipment to control cell wireless coverage and coordinate channel interference across the entire network is essential. During the design and construction of projects, appropriate anti-interference methods (such as equipment selection and optimization of network structure) should be chosen based on specific project conditions. The effectiveness of repeater coverage is primarily reflected in usage performance, equipment performance metrics, stability, and not affecting network parameters. It is not advisable to solely pursue high downlink power; instead, it should be placed in relatively isolated areas and use suitable base stations as signal sources to avoid wireless interference.
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