The mobile phone signal shielding device, in essence, implements electromagnetic interference on the wireless network on which the mobile phone communication depends, so that the mobile phone cannot access the network, and thus cannot receive the information transmitted by the mobile network, and cannot send out information through the network, but It has no effect on the signals generated by the phone.

 

The principle of mobile phone (radio communication) is as follows: In a certain frequency range, the mobile phone and the base station are connected by radio waves, and the data and sound are transmitted at a certain baud rate and modulation mode.

 

 

In response to the above communication principle, the mask scans from the low-end frequency of the forward channel to the high-end at a certain speed during operation. The scanning speed can form garbled interference in the message received by the mobile phone, and the mobile phone cannot detect the normal data sent from the base station, so that the mobile phone cannot establish a connection with the base station. Mobile phones are characterized by search networks, no signals, no service systems.

 

Since there are many channels in any frequency band of mobile communication, and each channel can be used as a BCH, as to which channel in a certain cell is used as: BCH, it is completely determined by the MSC according to the distribution of channels in the entire network area. . Generally, in order to prevent mutual crosstalk between channels and channels, the frequencies between channels in the same cell should be significantly different. For example, after using channel 5, do not use channels 4 and 6, adjacent. The same channel and adjacent channels cannot be used in the cell. Among the channels allocated in each cell, one channel is designated as BCH, and the remaining channels are traffic channels.

 

Cell phone signal shielding works

 

Due to the unfixed nature of the BCH, the uncertainty of the use of the shield, and the possibility that there are two or more BCHs in the adjacent area of ​​the cell, and therefore interference with the signal masker may occur in the area. All channels of the mobile communication network are interfered with, that is, all frequency signals in several frequency bands of GSM, DCS, PHS, CDMA, SCDMA, WCDMA, TDS-CDMA should be transmitted, so as to achieve interference to all mobile phone signals, as a whole The shielding of the received signal of the mobile phone, cutting off the connection between the mobile phone and the base station.

 

Cell phone jammers have been widely misused in society in recent years. This paper briefly analyzes the working principle of the mobile phone jammer and the interference caused by the mobile communication network when it is used, and further explains with the actual interference case, in order to fully understand the setting of the mobile phone jammer to the mobile communication network and the airwave order. Seriously harmful.

 

 

 

I. Introduction

 

    In recent years, some units in the society have developed, produced and set up mobile phone jammers (also known as mobile phone blockers, venue cleaners, conference information security machines, etc.). This is an interference device that transmits strong interference signals in the frequency band of mobile communication (GSM/DCS/CDMA), thereby blocking the communication between the mobile communication handset and the base station. Although the installation of such equipment has achieved the purpose of banned mobile phones in public places, at the same time, it is a heavy cost to interfere with the mobile communication network and disturb the order of airwaves. It is a practice of sacrificing the overall interests in exchange for local interests. The radio administration has banned this practice. As we all know, since radio waves are freely propagating in the air, it is impossible to strictly interfere with a specific space without affecting the normal operation of the mobile communication network by using radio waves to interfere with mobile communications.

 

 

 

Second, 1. The typical mobile phone jammers (cell phone jammers) work principle

 

    At present, mobile phone jammers are classified according to interference methods: full-band scanning interference, uplink signal interference,

 

Downstream signal interference, noise interference, etc. One of the most typical and popular ones is the mobile phone jammer using full-band scanning interference. The working principle is simple, the cost is low, and the harmful interference "good effect" is widely used.

 

 

 

1.1 Its technical indicators:

 

    Working frequency band: 860-960MHz, 1800-1880MHz

 

    Output power: 0-3W adjustable

 

    RF output: pure carrier without modulation

 

    Output impedance: 50Ω

 

1.2 Block diagram of its working principle:

 

1.3 Its working principle:

 

    The NE555 circuit generates a sawtooth low-frequency oscillating circuit, which is modulated on the variable-capacitance diode of the high-frequency oscillating circuit to directly generate a radio frequency scanning signal, and then performs power amplification. Without any processing circuit, the principle is simple, the cost is low, and the scanning interference can be implemented on the entire mobile communication frequency band, which is extremely harmful.

 

 

 

Third, 2. Downstream signal interference analysis

 

2.1 Mobile communication interference level definition

 

    According to the definition of interference level by the GSM system CMRC (Operation and Maintenance Center), we divide the interference to mobile communication into five levels:

 

    Level Interference signal strength (range)

 

     0 —110dBm---- —100dBm

 

     1 —100dBm-----95dBm

 

     2 —95dBm ---- —90dBm

 

     3 —90dBm ---- —85dBm

 

     4 —85dBm ---- —47dBm

 

    5 > —47dBm

 

    According to the experience of mobile communication operation and maintenance, level 4 interference will have a very large impact on mobile communication, which is manifested by the difficulty of dialing in and out of the call, the call is interrupted, the voice quality is seriously degraded, and the interference of level 5 or above will make the mobile phone out of service. Zone, causing the base station to receive congestion.

 

 

 

2.2 Downstream signal jammer power calculation

 

Taking the 935-960MHz downlink signal of the GSM system as an example, setting a conference room of a certain building needs to interfere with the GSM downlink signal within a range of 50 meters. According to the design principle of the mobile phone jammer, in order to block the mobile phone call, the interference signal strength received by the mobile phone must be ≧-47dbm. (See "Mobile Communications", 2003, Issue 4). According to the flat space path loss formula:

 

    Lp(db)=40lgD-20lgHb-20lgHm

 

    Lp----flat space path loss (dBi)

 

    D----- Propagation distance, here is set to 50 meters

 

    Hb----effective height of the launch pad, here refers to the effective height of the jammer, set to 2 meters.

 

    Hm----The effective height of the receiving station, here refers to the height of the mobile phone, set to 0.5 meters.

 

    Then, the path loss Lp=40lg50-20lg2-20lg0.5=67.96dBi

 

    Interferer output power = 67.96 + (-47) = 21dBm

 

The antenna interface loss, antenna gain, etc. are ignored here, because the smaller does not affect the overall calculation result, and the actual interference interference power is 200mW.

 

 

 

2.3 Interferer downlink interference range calculation

 

 Below we analyze the distance range of 4 levels of interference around the 200mWw mobile phone signal jammer installed in a conference room. According to the interference level definition, the power at the critical point of the 4-level interference is -85dbm, and its path loss

 

Lp (dB) = 23 - (-85) = 108 dBm.

 

According to formula one,

 

    108=40lgd-20lg2-20lg0.5

 

    Finally d = 500 meters

 

That is to say, for the downlink signal jammer, if the interference in the 50m range is to be generated in the conference room on the first floor, it must be installed to the mobile phone jammer with a low 200mW. The jammer generates 4 levels of interference to the downlink signal of the base station within 0.5 km around. If the jammer is raised, or the interference power is increased, the range of interference is larger, up to 2-3 km. The interference of the downlink signal will affect the reception of the BCCH broadcast signal by the mobile phone to the base station, so that the mobile phone in the entire interference zone cannot make a call.

 

 

 

4, 3 uplink signal interference analysis

 

The analysis of interference from the jammer to the uplink signal of mobile communication is analyzed in the following two cases:

 

3.1 Set up a 200mw jammer to cause 4 levels of interference to the base stations in the range of kilometers around.

 

Set the jammer height Hb = 2 meters, and the base station receive antenna height Hm = 30 meters.

 

According to formula 1, get 108=40lgd-20lg2-20lg30

 

d=3900 meters

 

That is, the mobile phone jammer with a height of 2 meters and 200mW will cause 4 levels of interference to the 30 meter high mobile communication base station within 3.9 kilometers.

 

 

 

3.2 Suppose that the above 200mW jammer is installed 2km away from the base station, and how much interference intensity is caused to the 30m high base station.

 

Pm(dB)=Pb(dB)-Lp(dBi) (Formula 2)

 

    =23-(40lg(d)-20lg(Hb)-20lg(Hm))

 

    =23-40Lg(2000)+20lg(2)+20lg(30)

 

    =-73 dBm

 

Pm(dB)---- Base station receiving antenna power (dBm)

 

Pb(dB)---- jammer transmit power (23dBm)

 

Lp(dB)---- Flat space loss (dBi)

 

That is to say, the mobile phone jammer with a height of 2 meters and 200mW will be 30 meters high within 3.9 kilometers.

 

The base station causes 4 levels of interference. Setting the above 200mW uplink signal jammer 2 kilometers away from the base station will cause interference of -73 dBm to the base station of 30 meters high.

 

 

 

V. 4. Actual interference cases

 

    According to my long experience, some manufacturers have set the interference power of the jammer to 1W in order to enhance the interference effect, causing serious interference to the mobile communication in the surrounding 1-2 km range, disturbing the normal mobile communication and airwave order. For example, multiple interference devices are installed in the college entrance examination classroom, causing serious interference to the base stations around the school. The interference power measured at the base station is -42dbm/150KHz. For example, the transmission power of the jammer in an important conference room is 1W, which seriously affects the connection of mobile phones within the range of 1 kilometer.

 

 

 

5 Conclusion

 

Due to the characteristics of the free propagation of radio waves in the air, it is decided that the purpose of mobile phone jammers to block mobile communication in a specific area will inevitably interfere with the normal operation of the surrounding mobile communication, which will inevitably lead to confusion of the order of the radio waves. This has been fully proved from the above theoretical and practical interference cases.