[Editorial]
CDMA is the abbreviation of Code Division Multiple Access (CDMA), which is a new and mature wireless communication technology developed on the spread spectrum communication technology, a branch of digital technology.The principle of CDMA technology is based on the spread spectrum technology. CDMA technology is based on spread spectrum technology, whereby information data with a certain signal bandwidth to be transmitted is modulated with 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 modulated by the carrier and sent out. The receiving end uses the exact same pseudo-random code, and the received bandwidth signal for correlation processing, the broadband signal into the original information data of the narrow-band signal that is the de-expansion, in order to realize the information communication.
Background of CDMA technology
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The emergence of CDMA technology stems from the human need for higher quality wireless communications. CDMA technology was researched and developed due to the needs of war during the Second World War, and its original idea was to prevent the enemy from interfering with its own communications, and was widely used in military anti-jamming communications during the war, and was later updated into a commercial cellular telecommunication technology by Qualcomm in the U.S.A. In 1995, after the first commercial CDMA system was in operation, the theoretical advantages of the CDMA technology were put to the test in practice, and thus the CDMA technology was used in North and South America. After the operation of the first commercial CDMA system in 1995, the theoretical advantages of CDMA technology have been tested in practice, which has led to its rapid popularization and application in North America, South America and Asia. Many countries and regions around the world, including Hong Kong, China, South Korea, Japan, and the United States have built CDMA commercial networks. In the United States and Japan, CDMA has become the main mobile communication technology in the country. In the U.S., 7 out of 10 mobile communication operating companies have chosen CDMA, and by April this year, 60% of the population in South Korea had become CDMA users. In the 27th Olympic Games hosted by Australia, CDMA technology played an important role.
CDMA technology standards
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The standardization of CDMA technology has gone through several phases. IS-95 was the first of the cdmaONE series of standards to be released, and the first CDMA standard to be widely used around the globe was IS-95A, which supported 8K encoded voice services. Subsequently were published 13K voice encoder TSB74 standard, support for 1.9GHz CDMA PCS system STD-008 standard, which 13K encoded voice service quality has been very close to the voice quality of wired phones. With the growth of demand for data services in mobile communications, in February 1998, Qualcomm announced the IS-95B standard for use on the CDMA base platform. IS-95B can provide CDMA system performance and increase the data traffic of the user's mobile communications equipment, providing support for 64kbps data services. Subsequently, cdma2000 became the standard for the transition of narrowband CDMA systems to third-generation systems. cdma2000 proposed 1X and 3X development strategies in the early stages of the standards study, but subsequent studies have shown that 1X and 1X enhanced technologies represent the future direction of development.
The standardization of CDMA technology has advanced the application of this technology worldwide. Currently, in the United States, South Korea, Japan and other countries, CDMA technology has gained large-scale application. In some European countries, some operators have also built CDMA networks. According to CDG (World CDMA Development Group) statistics, CDMA users at the end of 1996 was only 1 million; to March 1998 has grown rapidly to 10 million; as of September 1999, the number of users has more than 40 million. 2000 the total number of global CDMA cell phone subscribers has exceeded 50 million in early 2000, the number of users in one year growth rate reached 118%. CDG said that Asia has become the main driving force behind the growth of the CDMA market, the number of CDMA subscribers in the Asian region has increased by 88% compared to a year ago, reaching 28 million. The U.S. region's growth rate is as high as 143% to 16.5 million, but the absolute number of users to be lower than in Asia, in the Asia-Pacific region, Hong Kong, China, Japan, South Korea, Australia, Thailand, India, the Philippines, New Zealand, Bangladesh, and many other countries and regions have been built CDMA commercial network, the number of subscribers has been more than 21 million households. The third highest growth rate is in Central and South America, where the number of CDMA subscribers has reached 5 million. CDG also said that the Chinese mainland has the greatest potential for growth in the global CDMA market in the future, and estimated that the number of subscribers in the Chinese mainland market in 2003 could reach 40 million.
CDMA is the direction of development of mobile communication technology. In the 2G stage, CDMA enhanced IS95A and GSM in the technical system in the same generation of products, to provide roughly the same service. However, CDMA technology has its own unique features, in terms of good call quality, less dropped calls, low radiation, health and environmental protection and other significant features. In 2.5G stage, CDMA2000 1X RTT and GPRS have obvious difference in technology, 1X RTT is higher than GPRS in transmission rate, 1X RTT is more mature than GPRS in new service bearing, and it can provide more medium and high speed new services. From 2.5G to 3G technology system transition, CDMA2000 1.X to CDMA20003.X transition is smoother than GPRS to WCDMA transition.
The advantages of CDMA
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(1) Large system capacity
Theoretically, in the use of the same frequency resources, the CDMA mobile network than the analog network capacity of 20 times, the actual use of analog networks than 10 times larger than the GSM to be 4-5 times larger.
(2) Flexible Configuration of System Capacity
In a CDMA system, an increase in the number of subscribers is equivalent to an increase in background noise, resulting in a degradation of voice quality. However, there is no limit to the number of subscribers, and the operator can make a compromise between capacity and voice quality. In addition, automatic equalization between multiple cells is possible based on traffic and interference.
This feature is related to the mechanism of CDMA, which is a self-interference system in which all mobile users occupy the same bandwidth and frequency, for example, imagine the bandwidth as a big house, and all people will enter the only big house. If they use completely different languages, they can hear their peers clearly with only some interference from other people's conversations. Here, the air in the house can be imagined as a broadband carrier wave, and the different languages are treated as codes, and we can keep adding users until the whole background noise limits us. If we can control the signal strength of the users, we can accommodate more users while maintaining high quality calls.
(3) Better call quality
TDMA's channel structure can only support up to 4Kb voice encoders, it cannot support more than 8Kb voice encoders. Whereas CDMA's structure can support 13kb voice encoder. Thus it can provide better call quality.The vocoder of CDMA system can dynamically adjust the data transmission rate and select different level level transmissions according to the appropriate threshold value. Also the threshold value changes according to the background noise so that better call quality can be obtained even in the presence of high background noise. In addition, TDMA adopts a kind of hard handover, users can obviously feel the interruption of the call, in the user-intensive, base station-intensive city, this interruption is especially obvious, because in such areas will occur every minute 2 to 4 times the handover situation. The CDMA system "drop call" phenomenon is significantly reduced, the CDMA system uses soft switching technology, "first connected and then disconnected", which completely overcomes the shortcomings of hard switching easy to drop calls.
(4) Simple frequency planning
Users are differentiated by different sequence codes, so different CDMA carriers can be used in neighboring areas, flexible network planning and simple expansion.
(5) Low network construction cost
CDMA technology simplifies the planning of the entire system by using the same frequency in each part of each cell, reducing the number of required stations without reducing the volume of traffic and thus reducing the deployment and operating costs. the CDMA network has a large coverage area, high system capacity, and fewer base stations are required, which reduces the cost of network construction.
CDMA digital mobile technology is different from the now well-known GSM digital mobile system. Analog technology is known as the first generation of mobile telephony technology, GSM is the second generation, CDMA is part of the second generation and a half of mobile communications and is more advanced than GSM.
CDMA Technology Holding Points
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1. CDMA is a kind of spread spectrum communication, he has the following characteristics of spread spectrum communication:
(1) Strong anti-jamming ability. This is the basic feature of spread spectrum communication, which is incomparable to all communication methods.
(2) Wideband transmission with high anti-fading capability.
(3) As a result of broadband transmission, the power of the useful signal transmitted in the channel is much lower than the power of the interfering signal, so the signal seems to be hidden in the noise; i.e., the power word density is relatively low, which is conducive to signal concealment.
(4) The correlation of spread spectrum code is utilized to obtain the user's information, which is highly resistant to interception.
(5) Multiple users receive and transmit at the same time.
2. In the spread spectrum CDMA communication system, due to the use of new key technologies and has some new features:
(1) The use of a variety of diversity. In addition to the traditional spatial diversity. Since it is broadband transmission plays the role of frequency diversity, at the same time in the base station and mobile station using the RAKE receiver technology, equivalent to the role of time diversity.
(2) Voice activation technology and sectorization technology are adopted. Because the capacity of the CDMA system is directly related to the interference received, the adoption of voice activation and sectorization technology can reduce interference and can make the capacity of the whole system increase.
(3) Mobile station-assisted soft switching is adopted. Through it, seamless switching can be realized, which ensures the continuity of calls and reduces the possibility of dropped calls. The mobile station in the switching area receives signals from multiple base stations through diversity, and can reduce its own transmit power, thus reducing the interference to the surrounding base stations, which is conducive to improving the capacity and coverage of the reverse link.
(4) Power control techniques are used so that the leveling transmit power is reduced.
(5) It has soft capacity characteristics. It is possible to increase the number of available channels by increasing the error frame rate during peak traffic periods. When the loads of neighboring cells are light and heavy, the heavily loaded cell can make the burden sharing by reducing the transmit power of the guide frequency so that the marginal users of the cell can switch to the neighboring cells due to the insufficient strength of the guide frequency.
(6) Good compatibility. Due to the CDMA bandwidth is very large, the power is distributed over a wide spectrum, the power word density is low, the interference to the narrow-band analog system is small, so the two can **** exist. That is, good compatibility.
(7) COMA has high frequency utilization and does not require frequency planning, which is one of the characteristics of CDMA.
(8) CDMA's highly efficient OCELP voice coding. Voice coding technology is an important topic in digital communications.OCELP is a signal that utilizes a code table vector to quantize the difference and produce a signal with variable output rate depending on the degree of voice activation. This coding five-horse method is considered to be the most efficient coding technique available, which greatly increases the capacity of the system while ensuring that there is good voice quality. The vocoder has two rate sequences, 8kbit/S and 13kbit/S. The 8kbit/S sequence is variable from 1.2kbit/s to 9.6kbit/s, while the 13kbit/S sequence is variable from 1.8kbt/s to 14.4kbt/s. Recently, an 8kbit/S sequence has been developed, which is a very efficient way to encode the voice. Recently, another 8kbit / sEVRC type encoder was introduced, also has 8kbit / s vocoder capacity, voice quality has also improved significantly.
Mobile communication technology classification
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Mobile communication system has a variety of classification methods. For example, according to the nature of the signal, can be divided into analog, digital; according to the modulation method, can be divided into frequency modulation, phase modulation, amplitude modulation; according to the multiple access connection method, can be divided into:
Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA).
At present, China Unicom and China Mobile are using the GSM mobile phone network, which is a combination of FDMA and TDMA. GSM has great advantages over analog cell phones, but the spectral efficiency is only three times that of the analog system, and the capacity is limited; it is difficult to reach the level of wired phones in terms of voice quality; and the TDMA terminal access rate can only reach a maximum of 9.6 kbit/s; and it can only reach the level of 9.6 kbit/s in terms of voice quality. 9.6kbit/s; TDMA system has no soft switching function, so it is easy to drop calls, affecting the quality of service. Therefore, TDMA is not the best wireless access to modern cellular mobile communications, and CDMA multiple access technology is fully suitable for modern mobile communication network requirements of high capacity, high quality, integrated services, soft switching, etc., is being more and more operators and users of the favor.
Currently, China Unicom owns the CDMA service.
About GSM and CDMA cell phone radiation
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It is well known that due to the use of fast reverse power control, soft switching, voice activation and other technologies in the CDMA (IS-95) system, as well as IS-95 specifications on the maximum power of the cell phone transmitter limitations, so that the CDMA cell phone in the process of communication with the small radiation power and enjoy the "green phone". CDMA cellular phones enjoy the reputation of "green cellular phone". However, some recent reports have questioned the "green phone" claim, arguing that GSM phones are comparable to CDMA phones in terms of radiation, based on the idea that GSM phones only radiate one-eighth of the time, so the SAR values (the amount of RF power absorbed per unit of mass) of GSM phones are roughly the same as those of CDMA phones.
In fact, GSM cell phones and CDMA cell phone radiation power who is big and who is small or how much difference, in order to get the actual objective comparison results, by an internationally renowned CDMA technology authority and a well-known domestic GSM network optimization company engineers and technicians in early December 2001 along the entire line of the second ring road in Beijing, CDMA and GSM network in the cell phone transmission power of the test. The test results show that in the Second Ring Road The test results show that the average transmit power of CDMA cellular phone is 2.4 dBm (1.72mW) and the average transmit power of GSM cellular phone is 28.9dBm (773 mW) on the Second Ring Road. Considering that the GSM cellular phone transmits only in one-eighth of the time, the equivalent average transmit power of GSM cellular phone in the time can be reduced to 19.85dBm (96.63mW). Thus, the average transmit power of a CDMA cell phone is 1.78% of the equivalent average transmit power of a GSM cell phone over time.
I. Comparison of CDMA and GSM system requirements for cell phone transmitter power
Let's take a look at the CDMA and GSM specifications for cell phone transmitter power requirements. Currently commonly used GSM cell phone 900MHz band maximum transmit power of 2W (33dBm), 1800MHz band maximum transmit power of 1W (30dBm), at the same time, the specification requirements, for the GSM900 and 1800 bands, the communication process of cell phone minimum transmit power can not be lower than 5dBm and 0dBm. CDMA IS-95A specification for cell phone The maximum transmit power requirement of CDMA IS-95A specification for cell phones is 0.2W ~ 1W (23dBm ~ 30dBm), the current network actually allows the maximum transmit power of the cell phone is 23dBm (0.2W), the specification of the minimum transmit power of the CDMA cell phone has no requirements.
In the actual communication process, at a certain point in time and in a certain location, the actual transmit power of the cell phone depends on the environment, the system requirements for the quality of communication, voice activation, and many other factors, in fact, it is dependent 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 the arrival of cell phone signals at the base station of the carrier to dry ratio is usually about 9dB, due to the CDMA system using spread spectrum technology, spread spectrum gain for full-rate coding gain of 21dB, (for other low-rate coding gain is even greater), so the equivalent of the pre-decompression signal carrier to dry ratio of the requirements of less than -14dB! (CDMA systems usually require a value of about 7dB for the signal after deconvolution).
Let's compare the initial value of transmit power and the power control mechanism of GSM and CDMA cell phones. The communication between the cell phone and the system can be divided into two phases, one is the access phase and the other is the voice communication phase. For the GSM system, the cell phone in the random access stage does not enter the dedicated mode before, there is no power control, in order to ensure the success of the access, the cell phone to the maximum power allowed by the system to transmit (usually the maximum transmit power of the phone). After the dedicated channel (SDCCH or TCH) is assigned, the cell phone adjusts the cell phone's transmit power according to the base station's commands, usually in steps of 2 dB, at a frequency of 60 ms.
For the CDMA system, in the random access state, the cell phone will estimate a smaller value as the initial transmit power of the cell phone according to the received signal level of the base station, and send the first Access Probe, and if it does not get an answer message from the base station within the specified time, the cell phone will increase the transmit power, and send the second Access Probe, and if it does not get an answer message from the base station within the specified time, the cell phone will increase the transmit power, and send the second Access Probe. If there is no answer from the base station within the specified time, the cell phone will increase the transmit power again. This process is repeated until an answer is received from the base station or the set maximum number of attempts is reached. During a call, every 1.25ms, the base station sends a power control command message to the cell phone, commanding the cell phone to increase or decrease the transmit power in 1dB steps.
From the above comparison, we can see that, taking into account the other unique technologies of the CDMA system, such as soft-switching, the diversity effect of the RAKE receiver on the multipath, and the improvement of uplink budget by the powerful forward error correction algorithm, the CDMA system is more effective for the cell phone than other systems. CDMA system is much smaller than GSM system in terms of transmit power requirement for cellular phone. While GSM cell phone transmits at maximum power during access, the power control is slower during call, so the cell phone has a higher chance of transmitting at high power. Whereas, the unique random access mechanism and fast reverse power control of CDMA cell phones can keep the average transmit power of the cell phone at a lower level. The above qualitative analysis conclusions are verified in the actual measurements later.
II. Description of the road test and analysis of the results
The road test experiments were conducted to test the transmit power of CDMA and GSM cell phones during the actual communication process. 1861 was dialed by both the CDMA test phone and the GSM test phone at the same time, and the radio in the car was adjusted to the appropriate volume to simulate a two-way call. The car speed is about 40km, the GSM cell phone is sampled every 480ms, and the CDMA cell phone is sampled every 20ms. The results of the test are: the linear average transmit power of CDMA cell 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 transmit power of GSM cell phone is 28.9dBm (773 mW), and the probability of transmitting at the maximum power (2 W) is 21.8%. It is worth noting that the current Beijing Mobile GSM network in downtown Beijing is quite mature, the base station spacing is small, GSM phones can be less power to transmit, while the CDMA network is in the development stage, the network optimization, the CDMA cell phone transmit power requirements will be even smaller.
Third, the cell phone safety radiation standards and cell phone transmitter power
The impact of cell phone radiation on the human body is still under constant observation and research, there are a large number of conflicting foreign research reports, there is no comprehensive scientific conclusions. At present, the international (including the U.S. FCC, NCRP, Europe's CENEIEC) commonly used standard is the SAR value (SPECIFIC ABSORPTION RATE), which refers to the human body per unit of mass absorbed by the radio frequency power. (Formula omitted)
As the cell phone is close to the human brain (without headset), the distance between the cell phone radiation antenna and the human brain is usually less than 15cm. the human brain is in the near-field of the antenna radiation, due to the complexity of the human body's organizational structure, the theoretical calculation of the antenna radiated power and the distribution of the field strength of the human body in the relationship between the field strength of the body is very difficult. However, according to the electromagnetic field theory, one thing is certain, in the antenna structure and the relative position of the cell phone and the human body must be the case, the higher the antenna output power, the higher the electric field strength formed in the human body, the higher the human body absorbed RF radiation power. An important method of measuring SAR is to use a human tissue equivalent model and a probe to measure the actual field strength in the human body exposed to RF radiation.
The more stringent SAR requirement is the FCC standard, which recommends two types of radiation standards for the 30MHz-15GHz band:
1. Controlled radiation limits:
0.4mw/g (averaged over the human body), with a peak of 8mw/g (averaged over any 1 gram of human tissue) and an averaging time of 6 minutes;
2. Uncontrolled Radiation Limit
0.08mw/g (human average), peak 1.6mw/g (averaged over any 1 gram of human tissue), averaged over 30 minutes.
Cell phone radiation belongs to the uncontrolled radiation that a person cannot control the RF source.
It should be noted in particular that the results obtained from the cell phone SAR tests that are currently being conducted are obtained when the cell phone is moving at its maximum transmitter power and full rate. the maximum transmitter power of a CDMA cell phone is 0.2 W, and the maximum transmitter power of a GSM cell phone is 2 W. However, the GSM cell phone is only transmitting 1/8 of the time, and the determination of the SAR value is an average over a long period of time. Therefore, it is not surprising that GSM and CDMA cell phones have similar SAR values in this case. We can not because in this extreme case CDMA cell phone and GSM cell phone SAR value is comparable and arbitrarily think that in the actual communication process CDMA cell phone and GSM cell phone radiation is also similar. Because in the actual communication process, GSM cell phone and CDMA cell phone will not always transmit at maximum power, especially CDMA cell phone at full rate, the probability of maximum power transmission is very small. From the statistical results of the previous road test, the probability of GSM cell phone with high power emission is much larger than the probability of CDMA cell phone high power emission, the average emission power of CDMA cell phone is much smaller than the maximum emission power of CDMA cell phone, is also much smaller than the average emission power of GSM cell phone, therefore, in the actual communication process of CDMA cell phone on the human body radiation of the actual SAR value will be much lower than the nominal SAR value of CDMA cell phone. Therefore, the actual SAR value of CDMA cell phone radiation to human body in the actual communication process will be much lower than the nominal SAR value of CDMA cell phone, and also much lower than the actual SAR value of GSM cell phone.
On the other hand, objectively speaking, the current widely used SAR standard may not be able to fully reflect the impact of cell phone radiation on the human body. This is because the standard is based on the thermal effect of electromagnetic radiation on the human body. In fact, electromagnetic waves, especially low-frequency pulsed electromagnetic waves on the human body radiation of the non-thermal effect has also become increasingly concerned about the low-frequency pulsed electromagnetic waves generated by the GSM cell phone emissions have affected the normal use of precision medical equipment, hearing aids, whether it is also harmful to the human body, is not yet conclusive. In order to avoid the above defects of the GSM cell phone, the third generation of mobile communication system terminal equipment will be like the CDMA cell phone as continuous radio waves rather than pulse waves.
Due to the CDMA and GSM technology system of CDMA and GSM cell phone transmit power requirements and the initial transmit power value of the setting and power control mechanism is different, in the actual communication process, the average transmit power of CDMA cell phone is much lower than the average transmit power of GSM cell phone. The current network measurement confirms that the average transmit power of CDMA cell phone is more than 500 times smaller than the transmit power of GSM cell phone. Considering that the GSM cell phone only transmits in one-eighth of the time, the energy radiated by CDMA is more than 60 times smaller than that radiated by GSM cell phone in the same period of time.
The safety standard SAR value for cell phone radiation is derived from the case where the cell phone is emitted at maximum power, in which case it is completely normal for the SAR values of GSM and CDMA cell phones to be comparable. Due to the CDMA cell phone in the actual communication process of the average transmission power is far less than the maximum transmission power of the CDMA cell phone, but also far less than the average transmission power of the GSM cell phone, so the CDMA cell phone on the human body of the actual radiation is far lower than the cell phone under the maximum power of the SAR value, and in the use of the process of low-frequency radio waves do not radiate, the CDMA cell phone is a veritable "green phone"!