GSM? CDMA? Which is lower?

Although it is now 3G era, the user base is still dominated by 2G users. The network standard of 2G mobile phones is mainly divided into CDMA and GSM, and the impact of mobile phone radiation on the body is also passed down from 2G era. Then in the end, which of CDMA and GSM networks is lower in radiation? Today we take a scientific look to find out!

The radiated power of GSM mobile phones and CDMA mobile phones is much smaller or smaller. In order to arrive at the actual objective comparison results, an internationally renowned CDMA technology authority company and a well-known GSM network optimization company engineering and technical personnel in China in 200112 In early January, the testing of mobile phone transmission power in CDMA and GSM networks was carried out along the entire Second Ring Road of Beijing. The test results show that the average transmit power of the CDMA mobile phone on the second ring road is 2.4 dBm (1.72mW), and the average transmit power of the GSM mobile phone is 28.9dBm (773 mW). Considering that the GSM mobile phone only transmits in one eighth of the time, the GSM mobile phone The equivalent average transmit power over time can be reduced to 19.85 dBm (96.63 mW). From this point of view, the average transmit power of a CDMA mobile phone is equivalent to 1.78% of the equivalent average transmit power of a GSM mobile phone over time.

First, the CDMA and GSM system requirements for mobile phone transmission power comparison Let us first understand the CDMA and GSM related technical specifications on the mobile phone transmission power requirements. Currently, the maximum transmit power of the 900MHz band used in GSM mobile phones is 2W (33dBm), and the maximum transmit power in the 1800MHz band is 1W (30dBm). At the same time, the specification requires that for the GSM900 and 1800 bands, the minimum transmit power of mobile phones during communication should not be lower than 5dBm respectively. And 0dBm. The CDMA IS-95A specification requires a maximum transmit power of the mobile phone to be 0.2W to 1W (23dBm to 30dBm). At present, the network actually allows the maximum transmit power of the mobile phone to be 23dBm (0.2W). There is no requirement on the minimum transmit power of a CDMA mobile phone.

In the actual communication process, at a certain point in time, the actual transmit power of the mobile phone depends on the environment, the system requirements on the communication quality, voice activation and many other factors actually depend on the system's link budget. In the usual network design and planning, the GSM system requires that the carrier-to-interference ratio of mobile phone signals arriving at the base station is generally about 9dB for the same error frame rate requirement. Because the CDMA system uses spread spectrum technology, the spread spectrum gain encodes the full rate. The gain is 21dB (the gain for other low-rate encoding is larger), so the equivalent carrier-to-interference ratio of the signal before despreading is less than -14dB! (The CDMA system usually has a value of about 7dB for the despread signal.)

Let's compare the initial value of the transmit power of GSM and CDMA mobile phones and the power control mechanism. The communication between the mobile phone and the system can be divided into two phases. One is the access phase and the other is the traffic communication phase. For the GSM system, the mobile phone has no power control before entering the dedicated mode in the random access phase. To ensure successful access, the mobile phone transmits at the maximum power allowed by the system (usually the maximum transmit power of the mobile phone). After assigning the dedicated channel (SDCCH or TCH), the handset will adjust the transmit power of the handset according to the instruction of the base station, and the adjustment step is usually 2dB. The adjustment frequency is 60ms once.

For a CDMA system, in the random access state, the mobile phone will estimate a smaller value as the initial transmit power of the mobile phone according to the signal level of the received base station, and send the first Access Probe if the base station is not obtained within the specified time. The response message, the mobile phone will increase the transmission power, send a second Access Probe, if the base station has not received the response within a specified time, the mobile phone will increase the transmission power. This process is repeated until the base station receives a response or reaches the maximum number of attempts. In the call state, the base station sends a power control command message to the mobile phone every 1.25 ms to instruct the mobile phone to increase or decrease the transmission power in steps of 1 dB.

From the above comparison, it can be seen that CDMA is taken into consideration when considering other technologies unique to CDMA systems, such as soft handover, RAKE receiver diversity effect on multipath, and strong forward error correction algorithm to improve the uplink budget, etc. The system's requirement for the transmit power of the mobile phone is much smaller than the requirement of the GSM system for the transmit power of the mobile phone. The GSM mobile phone is transmitted with the maximum power during the access process, and the power control speed during the call is slow, so the chance of the mobile phone transmitting with high power is relatively large. The unique random access mechanism and fast reverse power control of CDMA mobile phones can keep the average transmit power of mobile phones at a low level. The above qualitative analysis conclusions are verified in the actual measurement.

Second, the road test test description and results analysis Road test experiment conducted CDMA and GSM mobile phone in the actual communication process of the transmission power test. CDMA test handsets and GSM test handsets were dialed at the same time as the 1861, and the car radio was adjusted to the appropriate volume to simulate two-way conversations. The speed is about 40km. The GSM mobile phone samples every 480ms, and the CDMA mobile phone samples every 20ms.



The test results show that the linear average transmit power of CDMA mobile phones is 2.4 dBm (1.72 mW), the probability of transmitting with maximum power (23 dBm, 0.2 watts) is 0.2%, and the linear average transmit power of GSM mobile phones is 28.9 dBm (773%) mW), the probability of launching at maximum power (2 watts W) is 21.8%. It is worth noting that the Beijing mobile GSM network in the Beijing area is quite mature and the spacing between base stations is relatively small. GSM mobile phones can be launched with relatively low power. The CDMA network is at a stage of development. After the network is optimized, the requirements for the transmission power of CDMA mobile phones will be even greater. Primary Three, Safety Radiation Standards for Mobile Phones and Cellular Transmitter Power The influence of cell phone radiation on human body is constantly under observation and research. There are a large number of contradictory research reports abroad, and there is no comprehensive scientific conclusion at present. The current international standard (including the US FCC, NCRP, and European CENEIEC) is commonly used as the SPECIFIC ABSORPTION RATE, which refers to the radio frequency power absorbed by the body's unit mass.

Since the mobile phone is close to the person's brain (without headphones) during the call, the distance between the mobile antenna and the human brain is usually less than 15cm. The human brain is in the near field of antenna radiation. Due to the complexity of the human tissue structure, it is very difficult to theoretically calculate the relationship between the antenna radiation power and the field strength distribution in the human body. However, according to the theory of electromagnetic fields, one thing is certain. In the case of a certain structure of the antenna and the relative position of the mobile phone and the human body, the greater the output power of the antenna, the higher the electric field intensity formed in the human body, and the greater the radio frequency radiation power absorbed by the human body. . An important method for measuring the SAR value is to use a human tissue equivalent model and use a probe to measure the actual field strength in a person exposed to radio frequency radiation.

The stricter requirement for SAR is the FCC standard, and two types of radiation standards are recommended for the 30MHz-15GHz band:

1, controlled radiation limit:

0.4mw/g (human average), peak 8mw/g (average for any 1g body tissue), average time 6 minutes;

2. Uncontrolled radiation limit 0.08mw/g (human average), peak 1.6mw/g (average for any 1g body tissue), average time 30 minutes.

Mobile phone radiation belongs to non-control radiation where people cannot control radio frequency sources.

It needs to be pointed out that the results of the current mobile phone SAR tests are all obtained when the mobile phone is moving at the maximum transmission power and full rate. The maximum transmit power of CDMA mobile phones is 0.2W, and the maximum transmit power of GSM mobile phones is 2W, but GSM mobile phones only transmit at 1/8 of the time, and the determination of SAR value is a long-time average. Therefore, GSM mobile phones and CDMA mobile phones are The similar SAR value in this case is not surprising. We cannot arbitrarily assume that CDMA mobile phones and GSM mobile phones radiate similarly in the actual communication process because the SAR values ​​of CDMA mobile phones and GSM mobile phones are equivalent in this extreme case. Because in the actual communication process, neither the GSM mobile phone nor the CDMA mobile phone will always transmit at the maximum power. In particular, the probability that the CDMA mobile phone transmits at the full rate and maximum power is extremely small. According to the statistical results of the road test, the probability of a GSM mobile phone transmitting with high power is far greater than that of a CDMA mobile phone. The average transmit power of a CDMA mobile phone is far less than the maximum transmit power of a CDMA mobile phone, and is far less than that of the GSM mobile phone. The average transmit power of the mobile phone, therefore, the actual SAR value of CDMA mobile phone radiation to the human body in the actual communication process will be much lower than the nominal SAR value of the CDMA mobile phone, and is also far lower than the actual SAR value of the GSM mobile phone.

On the other hand, objectively speaking, the currently widely used SAR standards may not be able to fully reflect the impact of mobile phone radiation on the human body. Because the standard is based on the thermal effects of electromagnetic radiation on the human body. In fact, the non-thermal effects of electromagnetic waves, especially low-frequency pulsed electromagnetic waves on the human body, are also attracting more and more attention. The low-frequency pulsed electromagnetic waves generated by GSM mobile phones have affected the normal use of precision medical equipment and hearing aids and are harmful to humans. At present, there is no definitive conclusion. In order to avoid the above-mentioned defects of the GSM mobile phone, the terminal devices of the third generation mobile communication system will all transmit continuous radio waves like CDMA mobile phones instead of pulsed waves.

Conclusion Since the technical systems of CDMA and GSM require the transmit power of CDMA and GSM mobile phones, and the initial transmit power values ​​and power control mechanisms are different, the average transmit power of CDMA mobile phones is much lower than that of GSM mobile phones in the actual communication process. The average transmit power. The actual test confirmed that the average transmit power of CDMA mobile phones is more than 500 times lower than that of GSM mobile phones. Considering that GSM mobile phones only transmit in one eighth of the time, the energy of CDMA radiation is smaller than that of GSM mobile phones in the same time. 60 times more.

The safety standard SAR value of mobile phone radiation is obtained when the mobile phone is transmitting at the maximum power. In this case, the SAR values ​​of the GSM mobile phone and the CDMA mobile phone are quite normal. Since the average transmit power of CDMA mobile phones in the actual communication process is far less than the maximum transmit power of CDMA mobile phones, it is also much smaller than the average transmit power of GSM mobile phones. Therefore, the actual radiation of CDMA mobile phones to the human body is far lower than the maximum transmit power of mobile phones. SAR value, and does not radiate low-frequency radio waves during use, CDMA mobile phones are veritable "green phones."