CDMA professional definition
Background of CDMA technology
CDMA technical standard
Advantages of CDMA
Durability of CDMA technology
Classification of mobile communication technology
On the radiation of GSM and CDMA mobile phones
Knowledge of CDMA system
CDMA related question and answer
CDMA related question and answer
[Edit this paragraph ]CDMA professional definition
CDMA is the abbreviation of Code Division Multiple Access, which is a brand-new and mature wireless communication technology developed on spread spectrum communication technology, a branch of digital technology. The principle of CDMA technology is based on spread spectrum technology, that is, the information data to be transmitted with a certain signal bandwidth is modulated by a high-speed pseudo-random code with a bandwidth much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded and then transmitted by carrier modulation. The receiver uses the same pseudo-random code to correlate with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, thus realizing information communication.
[Edit this paragraph ]CDMA technical background
The emergence of CDMA technology stems from the human demand for higher quality wireless communication. During the Second World War, CDMA technology was studied and developed for the needs of war. Its original intention is to prevent the enemy from interfering with their own communications. It was widely used in military anti-jamming communication during the war, and was later updated as a commercial cellular telecommunication technology by Qualcomm. 1995, after the first commercial CDMA system was put into operation, many theoretical advantages of CDMA technology were tested in practice, thus it was rapidly popularized and applied in North America, South America and Asia. Many countries and regions in the world, including China, Hongkong, South Korea, Japan and the United States, have established commercial CDMA networks. In the United States and Japan, CDMA has become the main mobile communication technology in China. In the United States, 7 out of 65,438+00 mobile communication operators choose CDMA. By April this year, 60% of South Korea's population had become CDMA users. CDMA technology played an important role in the 27th Olympic Games held in Australia.
[Edit this paragraph ]CDMA technical standards
The standardization of CDMA technology has gone through several stages. IS-95 is the first released standard among cdmaONE series standards, and the first CDMA standard that is really widely used in the world is IS-95A, which supports 8K coded voice services. Subsequently, TSB74 standard of 13K speech encoder came out respectively, which supported STD-008 standard of CDMA PCS system of 1.9GHz, and the voice service quality of 13K coding was very close to that of wired telephone. With the increasing demand for data services in mobile communication, Qualcomm announced in February 1998 that the IS-95B standard will be used in CDMA infrastructure platform. IS-95B can provide CDMA system performance, increase the data flow of users' mobile communication devices, and provide support for 64kbps data services. Since then, cdma2000 has become the transition standard of narrowband cdma system to the third generation system. At the initial stage of standard research, cdma2000 put forward the development strategies of 1X and 3X, but subsequent research shows that the enhancement technologies of 1X and 1X represent the future development direction.
The standardization of CDMA technology promotes its worldwide application. At present, CDMA technology has been widely used in the United States, South Korea, Japan and other countries. In some European countries, some operators have also built CDMA networks. According to the statistics of CDG (World CDMA Development Group), the number of CDMA users at the end of 1996 was only 1 10,000; By March of 1998, it had rapidly increased to100000; By September, 1999, the number of users had exceeded 40 million. At the beginning of 2000, the total number of CDMA mobile phone users in the world exceeded 50 million, and the number of users increased by 1 18% within one year. CDG said that at present, Asia has become the main driving force for the growth of CDMA market, and the number of CDMA users in Asia has increased by 88% compared with a year ago, reaching 28 million. The growth rate of the United States is as high as 143%, reaching16.5 million, but the absolute number of users is lower than that of Asia. In the Asia-Pacific region, China, Hongkong, Japan, South Korea, Australia, Thailand, India, the Philippines, New Zealand, Bangladesh and other countries and regions have established CDMA commercial networks with more than 26,543,800 users. The third growth rate is in Central America and South America, with 5 million CDMA users. CDG also said that Chinese mainland is the region with the greatest growth potential in the global CDMA market in the future, and it is estimated that the number of users in Chinese mainland market will reach 40 million in 2003.
CDMA is the development direction of mobile communication technology. In the 2G stage, CDMA enhanced IS95A and GSM are the same generation products in technical system, providing roughly the same services. However, CDMA technology has its unique features, such as good call quality, less dropped calls, low radiation, health and environmental protection. At the 2.5G stage, CDMA2000 1X RTT is obviously different from GPRS in technology. In terms of transmission rate, 1X RTT is higher than GPRS, and in terms of new service bearer, 1X RTT is more mature than GPRS, which can provide more new services with medium and high rates. Transition from 2.5G to 3G and from CDMA2000 1. X to CDMA20003. X is smoother than GPRS to WCDMA.
[Edit this paragraph] Advantages of ]CDMA
(1) Large system capacity
Theoretically, under the condition of using the same frequency resources, the capacity of CDMA mobile network is 20 times larger than that of analog network, 10 times larger than that of analog network in practical use, and 4-5 times larger than that of GSM.
(2) Flexible configuration of system capacity
In CDMA system, the increase of users is equivalent to the increase of background noise, which leads to the decline of voice quality. But there is no limit to the number of users, and operators can compromise between capacity and voice quality. In addition, multiple cells can be automatically balanced according to traffic and interference.
This characteristic is related to the mechanism of CDMA. CDMA is a self-jamming system, and all mobile users occupy the same bandwidth and frequency. For example, think of bandwidth as a big house, and everyone will enter the only big house. If they use completely different languages, they can clearly hear the voices of their peers, with only some other people's conversations interfering. Here, you can think of the air in the room as a broadband carrier and treat different languages as codes. We can keep adding users until the whole background noise limits us. If we can control the signal strength of users, we can accommodate more users while maintaining high-quality calls.
(3) The call quality is better.
The channel structure of TDMA can only support a 4Kb speech encoder at most, but it cannot support a speech encoder above 8Kb. The structure of CDMA can support 13kb speech coder. Therefore, better call quality can be provided. The vocoder of CDMA system can dynamically adjust the data transmission rate and select different levels for transmission according to the appropriate threshold. At the same time, the threshold changes according to the change of background noise, so that even in the case of large background noise, better call quality can be obtained. In addition, TDMA adopts hard handoff, so users can obviously feel the interruption of the call, especially in cities with dense users and dense base stations, because there will be 2 to 4 handovers every minute in such areas. However, the phenomenon of "dropped call" in CDMA system is obviously reduced. CDMA system adopts soft handover technology, "connect first and then disconnect", which completely overcomes the shortcoming that hard handover is easy to drop calls.
(4) Simple frequency planning (? )
Users are distinguished by different sequence codes, so adjacent cells can use different CDMA carriers, and the network planning is flexible and the expansion is simple.
Although the frequency planning of CDMA system is simple, there are PN short codes in CDMA system, and the planning of PN short codes is not necessarily simpler than the frequency planning.
Generally speaking, CDMA planning is not simple. On the contrary, it is more complicated than GSM system.
(5) The network construction cost is low.
CDMA system has a large capacity and its working frequency is lower than that of GSM. Therefore, in CDMA planning, the distance between stations is generally sparse. Therefore, the network construction cost can be better saved.
(6) Network environment protection
Technical system average transmission power maximum transmission power
Gsm 125mw 2w
CDMA 2 MW 200 MW
As can be seen from the above data, CDMA mobile phones are 2/ 125 of the average transmission power of GSM mobile phones.
CDMA mobile phones are more green and environmentally friendly.
[Edit this paragraph] Key points of ]CDMA technology
1.CDMA is a spread spectrum communication with the following characteristics:
(1) has strong anti-interference ability. This is the basic feature of spread spectrum communication, which is incomparable to all communication methods.
(2) Broadband transmission with strong anti-fading ability.
(3) Because of the broadband transmission, the power of the useful signal transmitted in the channel is much lower than that of the interference signal, so the signal seems to be hidden in the noise; That is, the power density is relatively low, which is beneficial to signal hiding.
(4) Using the correlation of spreading codes to obtain user information has strong anti-interception ability.
(5) Multiple users receive and transmit simultaneously.
2. In the spread spectrum CDMA communication system, it has some new features due to the adoption of new key technologies:
(1) adopts various diversity modes. In addition to the traditional spatial diversity. Because broadband transmission plays the role of frequency diversity, and the base station and mobile station adopt RAKE receiving technology, which is equivalent to time diversity.
(2) Adopt voice activation technology and sectorization technology. Because the capacity of CDMA system is directly related to interference, using voice activation and sectorization technology can reduce interference and increase the capacity of the whole system.
(3) Soft handover assisted by mobile station. It can realize seamless handover, ensure the continuity of calls and reduce the possibility of dropped calls. The mobile station in the handover area can reduce its own transmission power by receiving signals from multiple base stations in diversity, thus reducing the interference to surrounding base stations, which is conducive to improving the capacity and coverage of the reverse link.
(4) Using power control technology, the collimated emission power is reduced.
(5) It has soft capacity characteristics. The number of available channels can be increased by increasing the frame error rate during peak traffic hours. When the load of neighboring cells becomes heavier and heavier, the heavily loaded cell can reduce the transmission power of the pilot, so that the edge users of the cell switch to neighboring cells due to insufficient pilot strength, thus sharing the burden.
(6) Good compatibility. Because CDMA has a large bandwidth, the power is distributed in a wide spectrum, the power density is low, and the interference to narrowband analog systems is small, so both can be stored. That is, good compatibility.
(7)CDMA has a high frequency utilization rate and does not need frequency planning, which is also one of the characteristics of CDMA.
(8) Effective OCELP speech coding by 8)CDMA. Speech coding technology is an important subject in digital communication. OCELP quantizes the difference signal by using the code table vector, and generates a signal with variable output rate according to the degree of voice activation. This five-horse coding method is considered to be the most efficient coding technology at present, which greatly improves the capacity of the system on the premise of ensuring good voice quality. This vocoder has two rate sequences: 8 kilobits per second and 13 kilobits per second. The 8 kbit/sec sequence can be changed from 1.2 kbit/sec to 9.6 kbit/sec, and the 13 kbit/sec sequence can be changed from 1.8 kbit/sec to 14.4 kbit/sec .. Recently, an 8 kbit/sevr has been developed.
[Edit this paragraph] Classification of mobile communication technologies
There are many classification methods for mobile communication systems. For example, according to the nature of the signal, it can be divided into analog and digital; According to modulation methods, it can be divided into frequency modulation, phase modulation and amplitude modulation; According to the multiple access connection mode, it can be divided into:
Frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA).
At present, the GSM mobile phone network used by China Unicom and China Mobile adopts the combination of FDMA and TDMA. GSM has great advantages over analog mobile phones, but its spectrum efficiency is only three times that of analog systems, and its capacity is limited. It is also difficult to reach the level of wired telephone in voice quality; The maximum access rate of TDMA terminal can only reach 9.6 kbit/s; TDMA system has no soft handover function, so it is easy to drop calls and affect the quality of service. Therefore, TDMA is not the best wireless access for modern cellular mobile communication, while CDMA multiple access technology is completely suitable for large capacity, high quality, integrated services and soft handover required by modern mobile communication networks, and is being favored by more and more operators and users.
At present, China Unicom has CDMA service. However, from 1 Oct/Oct, 2008, China Unicom's CDMA service was handed over to China Telecom.
[Edit this paragraph] About the radiation problem of GSM and CDMA mobile phones.
As we all know, CDMA (IS-95) system adopts fast reverse power control, soft handover, voice activation and other technologies. IS-95 specification limits the maximum transmission power of mobile phones, which makes CDMA mobile phones enjoy the reputation of "green mobile phones" because of their low radiation power during communication. Recently, however, some reports have questioned the "green mobile phone", arguing that the radiation of GSM mobile phones and CDMA mobile phones is the same. The basic idea is that GSM mobile phones only produce radiation for one eighth of the time, so the SAR value (radio frequency power absorbed by human body per unit mass) of GSM mobile phones and CDMA mobile phones is roughly the same.
In order to get the real and objective comparison results, engineers and technicians from internationally renowned authoritative CDMA technology companies and domestic well-known GSM network optimization companies tested the transmission power of mobile phones along the second ring road in Beijing in early February of 20001. The test results show that the average transmission power of CDMA mobile phone on the second ring is 2.4 DBM (1.72 MW), and that of GSM mobile phone is 28.9dBm(773 mW). Considering that GSM mobile phone only transmits one eighth of the time, the equivalent average transmission power of GSM mobile phone in time can be reduced to 19.85dBm(96.63mW). In this way, the average transmission power of CDMA mobile phone is equivalent to 1.78% of the equivalent average transmission power of GSM mobile phone in time.
Comparison between 1.CDMA and GSM systems on mobile phone transmission power requirements
Let's first understand the requirements of CDMA and GSM related technical specifications for mobile phone transmission power. At present, the maximum transmission power of GSM mobile phone in 900MHz band and 1800MHz band is 2W (33dBm), and the specification requires that the minimum transmission power of GSM mobile phone in the communication process should be not less than 5dBm and 0dBm respectively. The CDMA IS-95A specification requires that the maximum transmitting power of mobile phone is 0.2 W ~ 1 W (23 DBM ~ 30 DBM). At present, the network actually allows the maximum transmission power of mobile phones to be 23dBm (0.2W), and the specification does not require the minimum transmission power of CDMA mobile phones.
In the actual communication process, at a certain time and place, the actual transmission power of the mobile phone depends on many factors, such as the environment, the requirements of the system for communication quality, voice activation, etc., which actually depends on the link budget of the system. In the usual network design and planning, for basically the same frame error rate requirements, GSM system requires that the carrier-to-interference ratio of mobile phone signals arriving at the base station is usually around 9dB. Because CDMA system adopts spread spectrum technology, the gain of spread spectrum of full-rate coding is 2 1 dB, which is greater than other low-rate coding, so the requirement of equivalent carrier-to-interference ratio of signal before despreading is less than-1.4 dB! (CDMA systems usually need to despread signals with a value of about 7dB).
Let's compare the initial value determination and power control mechanism of GSM and CDMA mobile phones. 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 GSM system, there is no power control before the mobile phone enters the dedicated mode in the random access stage. In order to ensure successful access, the mobile phone transmits at the maximum power allowed by the system (usually the maximum transmission power of the mobile phone). After the dedicated channel (SDCCH or TCH) is allocated, the mobile phone will adjust the transmission power of the mobile phone according to the instruction of the base station, and the adjustment step is usually 2dB. Adjust the frequency once every 60 milliseconds.
For CDMA system, in the random access state, the mobile phone will estimate a smaller value as the initial transmission power of the mobile phone according to the received base station signal level, and send the first access probe. If the response information from the base station is not received within the specified time, the mobile phone will increase the transmission power and send the second access probe. If the response information from the base station is not received within the specified time, the mobile phone will increase the transmission power again. This process is repeated 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, ordering the mobile phone to increase or decrease the transmission power in steps of 1dB.
From the above comparison, it can be seen that other unique technologies of CDMA system are considered, such as soft handover, diversity effect of RAKE receiver on multipath, and improvement of uplink budget by powerful forward error correction algorithm. The requirement of CDMA system for mobile phone transmission power is much smaller than that of GSM system. However, GSM mobile phones transmit at maximum power when accessing, and the power control speed is slow when talking, so the probability of mobile phones transmitting at high power is higher. The unique random access mechanism and fast reverse power control of CDMA mobile phone can keep the average transmit power of mobile phone at a low level. The above qualitative analysis conclusions have been verified in the actual measurement.
Second, the road test description and result analysis
The road test experiment tests the transmission power of CDMA and GSM mobile phones in the actual communication process. The CDMA test mobile phone and GSM test mobile phone dial 186 1 at the same time, and the car radio is adjusted to the appropriate volume to simulate two-way call. The speed is about 40km per hour. GSM mobile phones are sampled every 480ms, and CDMA mobile phones are sampled every 20ms. The experimental results show that the linear average transmission power of CDMA mobile phone is 2.4dBm (1.72 mW), and the probability of transmitting at the maximum power (23dBm, 0.2 W) is 0.2%. The linear average transmission power of GSM mobile phone is 28.9dBm (773 mW), and the probability of transmitting at the maximum power (2 W) is 2 1.8%. It is worth noting that Beijing Mobile's GSM network in Beijing is quite mature at present, and the distance between base stations is small, so GSM mobile phones can transmit with less power, while CDMA network is in the development stage. After the optimization of the network, the requirements for the transmission power of CDMA mobile phones will be smaller.
Three, mobile phone safety radiation standards and mobile phone transmission power
The influence of mobile phone radiation on human body is still under constant observation and research. There are a large number of contradictory research reports abroad, and there is no comprehensive scientific conclusion at present. At present, the international standards (including FCC, NCRP and CENEIEC in Europe) are specific absorption rate, which refers to the radio frequency power absorbed by human body per unit mass. (Formula omitted)
Because the mobile phone is very close to the human brain when talking (without headphones), the distance between the radiation antenna of the mobile phone and the human brain is usually less than 15cm. The human brain is in the near field of antenna radiation. Because of the complexity of human tissue structure, it is very difficult to calculate the relationship between antenna radiation power and field intensity distribution in human body theoretically. However, according to the electromagnetic field theory, one thing is certain. When the antenna structure and the relative position between the mobile phone and the human body are fixed, the greater the antenna output power, the higher the electric field intensity formed in the human body, and the greater the RF radiation power absorbed by the human body. At present, an important method to measure SAR value is to use the equivalent model of human tissue to measure the actual field strength in the human body radiated by radio frequency with a probe.
The FCC standard is stricter on SAR, and two types of radiation standards are recommended for 30MHz- 15GHz band:
1. Controlled radiation limit:
0.4mw/g (average value of human body), with a peak value of 8mw/g (average value of arbitrary 1 g human tissue), with an average time of 6 minutes;
2. Uncontrolled radiation limit
0.08mw/g (average value of human body), peak value 1.6mw/g (average value of arbitrary 1 g human tissue), with an average time of 30 minutes.
Mobile phone radiation belongs to uncontrollable radiation that people can't control the RF source.
In particular, at present, the SAR test results of mobile phones are all obtained when the mobile phone is moving at the maximum transmission power and full rate. The maximum transmission power of CDMA mobile phone is 0.2W, and that of GSM mobile phone is 2W. The GSM mobile phone only transmits at 1/8, and the measurement of SAR value is the average value for a long time. So in this case, it is not surprising that the SAR values of GSM mobile phones and CDMA mobile phones are almost the same. Just because the SAR values of CDMA mobile phones and GSM mobile phones are equivalent in this extreme situation, it cannot be arbitrarily assumed that the radiation of CDMA mobile phones and GSM mobile phones is similar in the actual communication process. Because in the actual communication process, neither GSM mobile phone nor CDMA mobile phone will always transmit at maximum power, especially CDMA mobile phone will transmit at full speed, and the probability of maximum power is extremely small. According to the statistical results of the previous road test, the probability of high-power transmission of GSM mobile phones is far greater than that of CDMA mobile phones, and the average transmission power of CDMA mobile phones is far less than the maximum transmission power of CDMA mobile phones and the average transmission power of GSM mobile phones. Therefore, the actual SAR value radiated by CDMA mobile phone to human body in the actual communication process will be far lower than the nominal SAR value of CDMA mobile phone and the actual SAR value of GSM mobile phone.
On the other hand, objectively speaking, the widely used SAR standard may not fully reflect the influence of mobile phone radiation on human body. Because the standard is based on the thermal effect of electromagnetic radiation on human body. In fact, people pay more and more attention to the non-thermal effects of electromagnetic waves, especially low-frequency pulsed electromagnetic waves. The low-frequency pulse electromagnetic wave emitted by GSM mobile phone has affected the precision medical equipment, and whether the normal use of hearing aid equipment is harmful to human body is still inconclusive. In order to avoid the above shortcomings 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.
Due to the different technical systems between CDMA and GSM, such as the requirements for the transmission power of CDMA and GSM mobile phones, the determination of initial transmission power and power control mechanism, the average transmission power of CDMA mobile phones is much lower than that of GSM mobile phones in the actual communication process. The actual measurement of the current network proves that the average transmission power of CDMA mobile phone is more than 500 times smaller than that of GSM mobile phone. Considering that GSM mobile phones only transmit in one eighth of the time, the energy radiated by CDMA is more than 60 times less than that radiated by GSM mobile phones in the same time.
The safety standard SAR value of mobile phone radiation is obtained when the mobile phone transmits at maximum power. In this case, the SAR values of GSM mobile phones and CDMA mobile phones are relatively normal. Because the average transmission power of CDMA mobile phone in the actual communication process is far less than the maximum transmission power of CDMA mobile phone and the average transmission power of GSM mobile phone, the actual radiation of CDMA mobile phone to human body is far lower than the SAR value under the maximum transmission power of mobile phone, and it does not radiate low-frequency radio waves during use. CDMA mobile phone is a veritable "green mobile phone"!
[Edit this paragraph ]CDMA system knowledge
The concept of 1 CDMA system
CDMA system is a communication system based on code division technology (spread spectrum technology) and multiple access technology. The system assigns a specific address code to each user. Address codes are quasi-orthogonal to each other, so they can overlap in time, space and frequency; The information data with a certain signal bandwidth to be transmitted is modulated by a pseudo-random code with a bandwidth much larger than the signal bandwidth, which expands the bandwidth of the original data signal, and the receiver carries out the opposite process to expand, thus enhancing the anti-interference ability.
CDMA system belongs to sub-interference system.
2 CDMA system time:
System zero time: define1October 6th 1980 1: 00 as the system start time. The zero-offset long code and short code are in the initial state at the same time.
All base stations will take the start time of GPS time every even second (or an integer multiple of 80ms later) as the initial state of the 0-offset PN code (with a period of 80/3 ms), that is, just before that, output1"1"and15 "0".
All base stations need to take 1980 1: 00 of 1 October 6th (the GPS start time) as the initial state of the M-sequence long code (just before that, a "1"chip and 4 1 consecutive "0" chips were output.
Advantages of using GPS timing: fast switching and simple synchronization.
3 Disadvantages of CDMA system:
The spread spectrum sequences from different users in asynchronous CDMA network are not completely orthogonal, which causes multiple access interference.
Because of using the same carrier frequency, many users * * * use one channel, and strong signals have obvious inhibitory effect on weak signals, thus producing a "near-far" effect and affecting users' calls.
Using power control technology to solve the "far-in" effect in CDMA system
4 China CDMA system frequency use planning
The carrier frequencies occupied by CDMA in Unicom's new space-time are uplink (825MHz-835MHz) and downlink (870MHz-880MHz).
Carrier frequency calculation:
Uplink: carrier frequency =0.030MHz* carrier frequency+825.000mhz..
Downlink: carrier frequency =0.030MHz* carrier frequency+870.000mhz..
Carrier Frequency Number Channel Number Uplink (MHz) Downlink (MHz)
7 283 833.49 878.49
6 242 832.26 877.26
5 20 1 83 1.03 876.03
4 160 829.80 874.80
3 1 19 828.57 873.57
2 78 827.34 872.34
1 37 826. 1 1 87 1. 1 1
CDMA network operator replacement
On June 3, 2008, China Unicom and China Telecom reached a framework agreement on C-network sales, and both parties agreed to protect users' services and rights from being affected.
After the National Day in 2008, cdma will become a member of China Telecom family, and keep pace with China Unicom and China Mobile in the field of mobile communication.
China Unicom's CDMA network business and technology were officially handed over to China Telecom on June, 2008.
[Edit this paragraph ]CDMA related questions and answers
Q: Why is the CDMA connection rate high?
Answer: People who surf the Internet are experienced, so when there are few people looking for it, there is less network congestion and it is easy to get through. The same is true for mobile phones. CDMA comes from military anti-jamming system, which has a parameter of "processing gain" much higher than other systems. In addition, CDMA signals occupy the whole frequency band, which is almost seven times as efficient as ordinary narrowband modulation. So generally speaking, for the same bandwidth, the capacity of CDMA system is four to five times that of GSM system, the network congestion is greatly reduced, and the connection rate is naturally high.
-Q: Why is there almost no noise on CDMA cell phones?
A: CDMA adopts advanced digital speech coding technology, and uses multiple receivers to receive signals from different directions at the same time. It seems that you listen to several people telling a story from different aspects, and when you synthesize it, you understand the whole story, and at the same time omit some irrelevant content.
-Q: Why is it difficult to delete messages?
A: We know that the base station is the guarantee for mobile phone calls. When you move to the edge of the coverage area of the base station, the base station should automatically "switch" to protect you, otherwise you will be disconnected. When the CDMA system is switched, the coverage of the base station is "single coverage-double coverage-single coverage", which automatically switches to the adjacent idle base station, that is, the mobile phone is disconnected from the original base station only when it is confirmed to move to the single coverage area of another base station, thus ensuring that the mobile phone will not be dropped.
-Q: Why is it called "green mobile phone"?
A: The transmission power of CDMA system is very small, only 0. 1 ~ 200 MW, so the electromagnetic radiation to human body can be ignored, and there is basically no adverse effect on health; In addition, low power consumption means long life of batteries and phones, which protects the environment, so some people call it "green mobile phone".
-Q: Why is it difficult to eavesdrop on the phone?
A: To eavesdrop on your phone, you must find your code address. But your address is a pseudo-random code, * * * has 4.4 trillion possible permutations, so it is too difficult to crack the password or eavesdrop on the conversation.
Q: What is the function?
A: Six kinds: three parties talking at the same time; Incoming number display; Incoming calls are automatically transferred to your pre-designated phone, pager or mobile phone (unconditional call transfer); When you are on the phone, automatically transfer the incoming call to your pre-designated phone, pager or mobile phone (transfer when busy); If you don't answer the call within 30 seconds, it will be automatically transferred to your pre-designated phone, pager or mobile phone (no answer call forwarding); Call two callers separately (call waiting).
Q: What numbers can I call?
Answer: You can make local calls, domestic long-distance fixed calls, local GSM and domestic GSM mobile phones.
Q: Can CDMA cover the whole world?
A: Usually analog phones can only be used in areas covered by analog networks, while GSM phones can only be used in areas covered by GSM networks. Therefore, all networks are stepping up construction and expanding network coverage to expand the roaming range of users. Although CDMA dual-mode mobile phones can be used in both CDMA and analog network coverage areas, they are currently limited to countries or regions with relatively perfect CDMA network coverage, such as the United States, Canada, South Korea and Hong Kong. And "global communication" is still a wish. At present, the coverage of CDMA in China is only in Beijing, Shanghai, Guangzhou, Xi, Hebei and other cities.
Q: What CDMA mobile phone brands are there in the market?
A: At present, there are not many CDMA mobile phone brands in Beijing, mainly from the United States and South Korea, such as Qualcomm, Samsung, LG and Hyundai. It is reported that CDMA products from Motorola, Ericsson, Toshiba, Sony and other manufacturers will also be listed soon. There are not many varieties, but the styles are not out of date, such as Samsung's watch mobile phone and Qualcomm's PHS Q mobile phone.