Brief Analysis and Establishment of Mobile Phone Shielding System in Supervised Places
1. Background overview
In recent years, prisons, labor camps, detention centers and other supervision places across the country have invested funds to build or rebuild mobile phone shielding systems, hoping to eliminate illegal use of mobile phones, strengthen mobile phone management in supervision places, and ensure the smooth development of supervision work. For example, the Shanghai Prison Bureau and the Tianjin Labor Reeducation Bureau have determined practical shielding technical requirements through a large number of investigations, and established a stable and reliable shielding system through public bidding, which has achieved gratifying results. However, most of the shielding systems that have been built or are under construction across the country cannot achieve the expected shielding effect due to uneven technology, or have serious external interference and have received a large number of complaints, and cannot operate normally, resulting in a double waste of financial funds and time and energy. .
2. Introduction and current situation of the three major shielding technologies
(1) Conventional shielding technology: Sweeping electronic interference is mainly performed on the frequency band or time slot of the downlink signal of the mobile phone, so that the mobile phone cannot identify the login information command assigned by the base station to the mobile phone, so that the mobile phone cannot log in to the network, so as to achieve the purpose of shielding.
1. Primary stage: The jammer with their own antennas are scattered and arranged. This technology can only be used in a small area and a short period of time.
2. Intermediate stage: mainly install high-power shielding equipment outdoors and use directional antennas. When the signal is not too strong and there is no mobile 3G signal coverage, it may barely reach a wide range of use. (The key to success or failure is the technical level of equipment and installation and debugging personnel) The system can be equipped with equipment power failure alarm, remote control power switch and event record query function.
3. Advanced stage: increase on the basis of intermediate
1) For extremely strong signals, digital point frequency, frequency hopping, and pilot frequency shielding technology ensure shielding effect and reduce electromagnetic radiation.
2) The multi-frequency composite combining technology is adopted to improve the shielding efficiency, simplify the system structure, and distribute the shielding power reasonably and evenly.
3) According to the time-division characteristics of mobile 3G, the pilot frequency synchronization technology is adopted to prevent interference with the base station. 4) When the alarm control system is upgraded to shield the failure of a module inside the device, it can alarm
(2) Mobile phone detection technology It mainly detects when the electromagnetic wave signal emitted by the mobile phone when the mobile phone is working causes the electromagnetic power in the area where the mobile phone is located to increase to the set value, so as to determine that there is a mobile phone working. The technology is very advanced at first sound, but it is actually just a very superficial understanding. When the technology is proposed, the following issues are not considered:
1. Uncertainty of mobile phone transmit power The specific base station through which the mobile phone enters the network during operation is determined by the dynamic deployment of the traffic volume of the surrounding base stations. When the mobile phone is logged in and used, if the nearby base station is busy, it will log in to a distant idle base station to work, and the transmission power of the mobile phone will be adjusted according to the link loss of the radio wave from the mobile phone to the base station. Usually, the mobile phone receives the base station. The stronger the signal, the weaker the transmit power.
2. The distance between the mobile phone and the detector (link loss) is uncertain. The distance between the mobile phone and the detector, and whether there are obstacles in the middle will affect the strength of the signal received by the detector.
3. The intensity of electromagnetic interference in space is uncertain Common lightning, fluorescent lamps, electronic equipment, etc. are all interference sources. At the same time, the out-of-band stray is very large when the mobile phone is working. Although the detector is used for the SAW filter on the mobile phone, the out-of-band suppression of the filter is not enough, and it is easy to produce cross-frequency phenomenon. When used for detection, other interference signals cannot be effectively filtered out, which makes the detection system in chaos.
4. The sensitivity of detector settings is uncertain. Generally, the sensitivity settings of detectors do not have a specific quantitative value. During debugging, dial the mobile phone by continuously adjusting the position to observe whether the detector detects it and adjust the sensitivity.
5. False positives and false negatives The shielding system using detection technology is mainly manifested as extremely serious false positives and false negatives, which cannot achieve practical purposes.
(3) Detection + shielding technology The switch of the shielding device is controlled by the detector, and the shielding device is turned on immediately when a mobile phone is detected, hoping to achieve the purpose of immediate shielding. However, the reason why the shielding device cannot be activated immediately in actual operation is that the shielding device mainly achieves the shielding effect by preventing the mobile phone from logging in to the network, and the login command sent by the base station to the mobile phone is issued intermittently. It has already been called. During the period when the base station does not send the mobile phone to change the original login command, the mobile phone can generally still talk normally. This phenomenon is more obvious in China Unicom's 3G and WCDMA networks. Unless the shielding power can be dozens or even hundreds of times that of conventional shielding equipment, the effect of immediately interrupting mobile phone communication can be achieved. Such power is not only difficult to achieve in terms of technology and cost, but also more unsafe due to its strong radiation. At present, the application status of detection + shielding technology seems to be superior in application principle, and it is especially favored and respected by users in the southeast and northeast of my country, but in fact, there is no successful case so far, and it has caused considerable Misleading, has brought no small adverse effects to the entire industry. If cavity filtering and code stream analysis are used, the above shortcomings can basically be solved, but because the cost is too high, it is difficult to be accepted by users and loses market significance
(4) Pseudo base station technology It transmits a signal similar to the base station, so that the mobile phone in the area mistakenly enters its network, so as to achieve the purpose of supervising and controlling the mobile phone. At present, it has been applied by public security and other departments on the GSM network.
The current status is:
1. If you want to use it on all networks including CDMA, CDMA2000, WCDMA, TD-SCDMA, and 5G, it will be very difficult to operate without operator support.
2. The equipment cost is too high, the system is too complicated, and it is very difficult to integrate multiple operators and multiple systems.
3. To ensure that peripheral users outside the shielded area are not affected by the pseudo base station, higher technical means are required. 4. There have been individual units in China that have established projects for construction, but there is no news of success.
5. Our company is working with Zhejiang Telecom Company, Mobile Company, Prison Administration and other relevant departments to discuss and demonstrate related technologies on the use of the "Police Connect" function in prisons that have been shielded by similar technologies. There is no practical breakthrough yet.
Third, the current common misunderstandings
1. The higher the power, the better the shielding effect? Many companies take advantage of the fact that ordinary users cannot test the actual power, and deliberately exaggerate the power of the shield for publicity, causing the nominal power index of the equipment to lose its reference value. (For example, the indicators of some low-cost single-board cameras in the security industry may be higher than those of mid-to-high-end box cameras.) In fact, for a large-scale shielding system, what plays a decisive role in the shielding effect is the shielding technology. The advantages offered by advanced shielding technology far outweigh the output power (especially the nominal power) factor of the shielded device. (For example, the lighting brightness brought by 40W ordinary incandescent lamps and 18W brand energy-saving lamps)
2. Can you avoid external interference by using a directional antenna? As everyone knows, the directional antenna covers the main lobe, the side lobe and the back lobe, and the main lobe coverage angle is defined as the angle when the off-axis power from the main lobe drops by 3db. That is, the radiation intensity from the side and rear of the directional antenna is only much lower than the main lobe. Therefore, if the space close to the antenna still has considerable shielding power, coupled with the reflection of the shielded building, it will still cause external interference. Moreover, if it is not applied properly, it will cause uplink interference, and its interference range will be larger. Therefore, whether the equipment technology can meet the conditions, the application of the antenna, and the debugging of the system are far more meaningful than whether the directional antenna is used.
3. Is the shielding distance of the device a hard indicator? 1) The shielding distance is related to the signal strength and quality in space. Simply speaking, the distance is meaningless. If the same shielding power is 100 meters away at the weaker -70db, it is only 12.5 meters at the stronger -52db. 2) The shielding distance is related to the gain of the antenna. The shielding host of the same power uses antennas with different gains, and the distance will be very different. For example, when using an antenna with a gain of 16db, the distance is 100 meters, while when using an antenna with a gain of 4db, the distance is only 25 meters. The above examples are all assumed to be open space (free space theory). If you consider various building environments inside the prison, spatial signal strength, direction and other comprehensive factors, to do a good job of the entire shielding system requires a combination of a large number of application technologies and product technologies. , both are indispensable.
Fourth, the basic conditions to ensure the effect of the shielding project:
1. First of all, a design and implementation plan that conforms to the actual building structure, layout and spatial signal distribution on the site must be adopted.
2. The equipment used is of advanced technology, reliable operation, and has been tested by a large number of successful cases. For extra-strong signals, it should have digital point frequency, frequency hopping, and pilot frequency shielding technology. For mobile 3G signals must have pilot synchronization technology.
3. Design, installation and commissioning personnel must be familiar with the relevant professional (radio communication) theory, have long-term shielding design, installation and commissioning experience, and be equipped with necessary professional testing equipment.
5. Necessary procedures for project establishment and initial operation
1. Adequate research and investigation of product quality, technical means, enterprises and their brand awareness in the market through various channels. At present, the more convenient method is to use the Internet search to find relevant enterprises, conduct preliminary understanding and communicate with relevant technical personnel, and listen to technical explanations.
2. Observing the effect of on-site demonstration After a large number of shielding system-related companies have researched and communicated, a few companies were selected and asked to bring equipment products and related testing instruments to the site for on-site testing and shielding effect demonstrations. Through a simple demonstration process, it will not be difficult to judge what kind of product technology is most suitable for the use of this unit.
3. The actual effect of the on-site inspection of the case When the actual effect is inspected, it is best to bring the mobile phone into the shielding area after the approval of the leader of the case unit to verify the shielding effect.
4. The key technical requirements must be clearly specified in the bidding documents and contract bidding documents (in order to prevent the bidder from making false technical responses), and detailed descriptions, pictures and other materials of the corresponding technical requirements products must be provided in the documents, and these technical products must be brought to the site. The bidding site shall be used for verification by the bidding committee. If necessary, a power-on demonstration shall be carried out, and a technical explanation shall be given. In order to prevent mistakes in bidding and ensure the smooth implementation of the project, the bidding documents can stipulate that: after winning the bid, the experimental installation and commissioning of shielding in a designated area should be completed within a specified time, and the winning bidder should be determined to be valid or contract after the expected effect is achieved. Effective, otherwise it will be re-tendered or the next candidate will win the bid. No payment will be made until the contract takes effect.
