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Adjacent channel interference


Carrier-to-interference ratio, also known as carrier interference protection ratio, should be expressed as C/I; co-channel interference protection ratio should be expressed as C/Ic; adjacent channel interference protection ratio should be Expressed as C/Ia. The so-called adjacent channel interference refers to the interference caused by the adjacent channel power of the interfering station falling within the passband of the receiving adjacent channel receiver. Due to frequency planning, there are channels adjacent to the working channel of the cell in adjacent cells, or the coverage area of ​​the base station cell is larger than the design requirement due to some reason, which will cause adjacent channel interference. When the carrier-to-interference ratio C/I of the adjacent channel is less than a certain value, it will directly affect the call quality of the mobile phone. In severe cases, the call will be dropped or the mobile phone user will not be able to establish a normal call.

C/A, adjacent frequency interference protection ratio means that in frequency reuse mode, adjacent frequencies will interfere with the frequency used by the serving cell, and the ratio between the two signals is C/A. In the GSM specification, C/A>-9dB is generally required, and a 3dB margin is generally added in engineering, that is, C/A>-6dB is required.

Near-far effect

Because the distance between the mobile station and the base station is different, the power of the transmitted signal when it arrives is also different. The signal power in the short distance is high, and the power in the long distance is small, which will interfere with each other. This effect is usually called the near-far effect.

If the adjacent channel users transmit within a very close range from the user receiver, and the receiver wants to receive the baseband signal using the preset channel, this problem will become serious. That is, a nearby transmitter captured the user's receiver. When a mobile station very close to the base station uses a weak signal and the mobile station uses an adjacent channel, the near-far effect will also occur.


There are two main factors that affect the size of adjacent channel interference, the interference suppression ratio of adjacent channel coexistence is better than ACIR and the offset between base stations. Therefore, the following methods can be used to reduce adjacent channel interference: (1) Improve the ACLR and ACS requirements of base stations and mobile stations. From the relationship between ACIR and ACLR and ACS, it can be seen that increasing the ACLR and ACS requirements of base stations and mobile stations can increase the ACIR value, thereby effectively reducing adjacent channel interference.

(2). Increase the frequency guard band between systems. Research results show that for the coexistence of two WCDMA systems, if the 5MHz guard band is increased to 10MHz, the ACIR can be increased by 10dB accordingly, thereby effectively reducing inter-system interference.

(3) Through good engineering planning, minimize the base station offset between systems and suppress inter-system interference. But this makes network planning subject to great restrictions.

In actual mobile network construction, a compromise should be considered between system capacity loss, frequency protection band, and implementation complexity.

Frequency planning

Adjacent channel interference mainly involves frequency planning. When the mobile phone occupies the A frequency point in the call state, and at the same time you can receive the B frequency point (-200kHZ) adjacent to the A frequency point in your primary cell, and the B frequency point level value is 9dB higher than the A frequency point At this time, it is the so-called adjacent channel interference. As for the frequency of -400kHz, it is unlikely that the frequency point of -400kHz is higher than the frequency point of A by 41dB. Therefore, when planning the frequency point, it is only necessary to analyze whether there are adjacent to the frequency point A around the main cell. The frequency point number may be entered.

In principle, frequency planning means that the cell where the user is located will not receive signals at the same or adjacent frequency as the configured frequency of the cell. Call quality above 6dB will become very poor.

Power control is mainly to solve the problem of the transmission power of the mobile phone when the distance between the MS and the BTS changes, with the ultimate goal of saving power. The last two values ​​are the adjacent frequency point number and the power value of the frequency point.

Adjacent channel interference ratio

For GSM, TETRA and other systems, there are adjacent channel interference protection ratios. For example, the adjacent channel interference protection ratio of GSM is -6dB, and the adjacent frequency of TETRA The interference protection ratio is -45dB. : Although there is an interference protection ratio in the engineering regulations, in actual optimization, it is necessary to avoid the situation of the same adjacent frequency as much as possible, and it is not that there is no problem within this protection ratio.

Same frequency interference, adjacent frequency interference, cross-frequency interference

Same frequency, as the name implies, is signal interference with the same frequency. For example, the frequency of Chengdu Radio Station is 91.4MHZ, if 91.4MHZ appears again The signal, he interfered with the normal operation of the radio station, Gu became a co-channel interference.

The adjacent frequency is the interference of the similar frequency. 91.4MHZ, 91.45 and other signals with similar frequency interfere with it. Generally, there is a certain interval between radio frequencies to prevent adjacent frequency interference.

Intermodulation interference means that when two or more different frequency signals act on a non-linear circuit, they will modulate each other to produce a new frequency signal output. If the frequency falls within the working channel bandwidth of the receiver, It constitutes interference to the receiver, and we call this interference intermodulation interference. Intermodulation interference is mainly the base station that has the index and mode co-sites, and the interference to the digital base station is caused by the influence of the transmitter of the analog base station. The direct consequence of this kind of interference is that the time slot cannot be allocated, which causes a waste of base station resources and also causes call drops.

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