1. Comparison of the transmission power requirements of mobile phones between CDMA and GSM systems
Let’s first understand the requirements for mobile phone transmission power in the relevant technical specifications of CDMA and GSM. The maximum transmit power of the currently commonly used GSM mobile phones in the 900MHz band is 2W (33dBm), and the maximum transmit power in the 1800MHz band is 1W (30dBm). At the same time, specifications require that for the GSM900 and 1800 frequency bands, the minimum transmit power of the mobile phone during communication cannot be less than 5dBm. and 0dBm. The CDMA IS-95A specification requires the maximum transmit power of mobile phones to be 0.2W to 1W (23dBm to 30dBm). Currently, the network actually allows the maximum transmit power of mobile phones to be 23dBm (0.2W). The specification does not require the minimum transmit power of CDMA mobile phones.
In the actual communication process, at a certain time and place, the actual transmit power of the mobile phone depends on the environment, the system's requirements for communication quality, voice activation and many other factors. In fact, it depends on the system. Link budget. In normal network design and planning, for basically the same frame error rate requirements, the GSM system requires that the carrier-to-interference ratio of the mobile phone signal arriving at the base station is usually about 9dB. Since the CDMA system uses spread spectrum technology, the spread spectrum gain is sufficient for full-rate coding. The gain is 21dB, (the gain for other low-rate codes is greater), so the requirement for the equivalent carrier-to-interference ratio of the signal before despreading is less than -14dB! (CDMA systems usually require the value of the signal after despreading to be about 7dB) .
Let’s compare the initial value determination and power control mechanism of GSM and CDMA mobile phone transmit power. The communication between the mobile phone and the system can be divided into two stages, one is the access stage, and the other is the traffic communication stage. For the GSM system, there is no power control before the mobile phone enters the dedicated mode during 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 allocating a dedicated channel (SDCCH or TCH), the mobile phone will adjust its transmit power according to the instructions of the base station. The adjustment step is usually 2dB. The frequency of adjustment is every 60ms.
For the CDMA system, in the random access state, the mobile phone will estimate a smaller value as the initial transmit power of the mobile phone based on the received base station signal level, and send the first Access Probe. If no response information is received from the base station within the specified time, the mobile phone will increase the transmission power and send a second Access Probe. If no response information is received from the base station within the specified time, the mobile phone will increase the transmission power. This process repeats until a response from the base station is received or the set maximum number of attempts is reached. In the call state, the base station will send a power control command message to the mobile phone every 1.25ms, instructing the mobile phone to increase or decrease the transmit power, with a step size of 1dB.
As can be seen from the above comparison, taking into account other unique technologies of the CDMA system, such as soft handover, RAKE receiver’s diversity effect on multipath, and powerful forward error correction algorithm on the uplink Due to the improvement of the road budget, etc., the CDMA system has much smaller requirements on the transmission power of mobile phones than the GSM system. The GSM mobile phone transmits at maximum power during the access process, and the power control speed is slower during the call, so the probability of the mobile phone transmitting at high power is greater. 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 were verified in subsequent actual measurements.
2. Road test description and result analysis
The road test experiment tested the transmission power of CDMA and GSM mobile phones during the actual communication process. The CDMA test mobile phone and the GSM test mobile phone dial 1861 at the same time, and the radio in the car is adjusted to the appropriate volume to simulate a two-way call. The speed is about 40km. GSM mobile phones sample every 480ms, and CDMA mobile phones sample every 20ms.
The results of the test are: The linear average transmit power of the CDMA mobile phone is 2.4dBm (1.72 mW), and the probability of transmitting at the maximum power (23dBm, 0.2 watt) is 0.2%; the linear average transmit power of the GSM mobile phone is 28.9dBm (773 mW), the probability of transmitting at maximum power (2 watts W) is 21.8%. It is worth noting that the current Beijing Mobile GSM network in urban Beijing is quite mature. The distance between base stations is small. GSM mobile phones can transmit with lower power, while the CDMA network is in the development stage. After the network is optimized, the requirements for the transmission power of CDMA mobile phones will be higher. Small.
3. Mobile phone safety radiation standards and mobile phone transmission power
The impact of mobile phone radiation on the human body is still under constant observation and research. There are a large number of conflicting research reports abroad. At present, There is no comprehensive scientific conclusion yet. The standard currently adopted internationally (including the US FCC, NCRP, and Europe's CENEIEC) is the SAR value (SPECIFIC ABSORPTION RATE), which refers to the radio frequency power absorbed by the human body per unit mass. (Formula omitted) Since the mobile phone is close to the human brain during a call (without headphones), the distance between the mobile phone's radiation antenna and the human brain is usually less than 15cm. The human brain is in the near field radiated by the antenna. 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. But according to the electromagnetic field theory, one thing is certain. When the antenna structure and the relative position of the mobile phone and the human body are certain, the greater the output power of the antenna, the higher the intensity of the electric field formed in the human body, and the greater the radio frequency radiation power absorbed by the human body. . Currently, an important method for measuring SAR values ??is to use an equivalent model of human tissue and use a probe to measure the actual field strength value in the human body exposed to radio frequency radiation.
The FCC standard that has stricter requirements for SAR recommends two types of radiation standards for the 30MHz-15GHz frequency band:
1. Controlled radiation limits:
0.4mw/g (average human body), peak value 8mw/g (average for any 1 gram of human tissue), average time 6 minutes;
2. Uncontrolled radiation limit
0.08mw/g (human average), peak value 1.6mw/g (average for any 1 gram of human tissue), average time 30 minutes.
Mobile phone radiation is uncontrolled radiation that humans cannot control from radio frequency sources.
It should be pointed out that the results of the current mobile phone SAR tests are all obtained when the mobile phone moves at maximum transmit power and full speed. The maximum transmit power of CDMA mobile phones is 0.2W, and the maximum transmit power of GSM mobile phones is 2W. However, GSM mobile phones only transmit in 1/8 of the time, and the measurement of SAR value is an average over a longer period of time. Therefore, the difference between GSM mobile phones and CDMA mobile phones is It is not surprising that the SAR values ??in this case are similar. We cannot arbitrarily believe that the radiation of CDMA mobile phones and GSM mobile phones are similar during the actual communication process just because the SAR values ??of CDMA mobile phones and GSM mobile phones are similar in this extreme case. Because in the actual communication process, GSM mobile phones and CDMA mobile phones will not always transmit at maximum power. In particular, the probability of CDMA mobile phones transmitting at full rate and maximum power is extremely small. Judging from the statistical results of the previous road test, the probability of GSM mobile phones transmitting at high power is much greater than the probability of CDMA mobile phones transmitting at high power. The average transmit power of CDMA mobile phones is much smaller than the maximum transmit power of CDMA mobile phones, and is also much smaller than that of GSM mobile phones. The average transmit power of mobile phones, therefore, the actual SAR value of human body radiation from CDMA mobile phones during actual communication will be much lower than the nominal SAR value of CDMA mobile phones, and also much lower than the actual SAR value of GSM mobile phones.
On the other hand, objectively speaking, the currently widely adopted SAR standard may not be able to fully reflect the impact of mobile phone radiation on the human body. Because this standard is based on the thermal effect of electromagnetic radiation on the human body. In fact, the non-thermal effects of electromagnetic waves, especially low-frequency pulse electromagnetic waves, on human body radiation are increasingly attracting people's attention. The low-frequency pulse electromagnetic waves emitted by GSM mobile phones have affected the normal use of precision medical equipment and hearing aids, and are they also harmful to the human body? , the jury is still out.
In order to avoid the above-mentioned defects of GSM mobile phones, the terminal equipment of the third generation mobile communication system will emit continuous radio waves like CDMA mobile phones instead of pulse waves.
Because the technical systems of CDMA and GSM have different requirements for the transmission power of CDMA and GSM mobile phones, as well as the determination of the initial transmission power value and the power control mechanism, in the actual communication process, the average transmission power of CDMA mobile phones Far lower than the average transmit power of GSM mobile phones. Actual measurements on the current network have confirmed that the average transmission power of CDMA mobile phones is more than 500 times smaller than that of GSM mobile phones. Considering that GSM mobile phones only transmit in one-eighth of the time, in the same time, the energy radiated by CDMA is greater than that radiated by GSM mobile phones. The energy is more than 60 times smaller. CDMA, the full name of code division multiple access, is a relatively advanced communication technology. It is characterized by high frequency utilization, low power consumption of mobile phones, and good voice quality.
The safety standard SAR value of mobile phone radiation is It is obtained when the mobile phone transmits at maximum power. In this case, it is completely normal for the SAR values ??of GSM mobile phones and CDMA mobile phones to be equivalent. Since the average transmission power of CDMA mobile phones during actual communication is far less than the maximum transmission power of CDMA mobile phones and also much less than the average transmission power of GSM mobile phones, the actual radiation of CDMA mobile phones to the human body is far lower than the maximum transmission power of mobile phones. SAR value, and does not radiate low-frequency radio waves during use, CDMA mobile phones are truly green mobile phones!