Basics of CDMA technology

What is the CDMA standard? This is a technology that allows you to pass multiple code channels at once, which is digital access. At one time, this technology was developed by Qualcomm, which is actively developing not only microprocessors, but also special devices for transmitting signals at different frequencies. Thanks to the technology of encryption and transmission of data at high speeds in digital networks, the CDMA standard has better voice quality. At the same time, the number of installed base stations is significantly reduced, and this already relieves the costs of cellular operators. In addition, the standard includes some additional services, for example, Internet access, mail checking, exchange of short messages, etc.

The technology of this standard is unique, it expands the range of data transmission, but at the same time the voice data is divided into small segments, which are encoded and transmitted over the line, and each call is identified. Therefore, it is possible to admit moments when a huge number of subscribers are on the same site and they are simultaneously talking on mobile phones. Thus, this is the main advantage of the standard, it is possible to pass a lot more signals with the same parameters.

What is GSM?

Also, in addition to CDMA, there is the GSM standard, which is the most widespread throughout the world. It is also a digital data transmission standard, it appeared in 1980 and is now widespread throughout the world, especially in Europe and the CIS countries. GSM is more widespread in the world, as this communication standard is in great demand among ordinary subscribers for whom mobility and a wide range of services are important. GSM has a drawback, because the quality of communication here is much worse due to the technology itself and requires a large number of base stations. However, GSM has a huge advantage in that here you can easily change the mobile phones themselves due to the change of SIM cards, and the mobile phone manufacturers themselves most often release new items on the market with the GSM standard.

What communication standard is better if you take CDMA technology?

If we take different frequency ranges, then in CDMA the frequencies 800 and 1900 have gained the greatest popularity. Many people are interested in which standard is better for speech? In fact, the quality of speech and data transmission is the same on both standards, because here, in any case, CDMA technology is used, it's just that they are different frequencies. Since the CDMA standard does not make it possible to quickly change the phone itself, RUIM cards have appeared recently, they are somewhat similar to standard SIM cards. All CDMA standards can be suppressed with special devices.

For what reasons is CDMA superior to other standards?

The most important advantage of CDMA is its voice quality. The quality with the correct design of the cellular network is even better than over wired channels, since there is absolutely no noise and there are no interrupted sessions. Moreover, the signal itself is very resistant to hacking, it is almost impossible to decipher it. In addition, as mentioned above, there can be more subscribers on one site, and the quality does not suffer from this.

What is the high confidentiality score of the CDMA standard?

The signal transmission in this standard is divided into segments, which are encrypted, and then a code is issued for each segment. The receiving element must receive the code and unblock the segment, which provides excellent protection for conversations.

Where is the CDMA standard used at the moment?

At the moment, the CDMA standard is actively used in the United States, and the development of this standard is also being carried out in Japan, South Korea, Australia, Canada and some other developed countries.

What does CDMA800 mean and what is the exact frequency range?

This is the frequency range in which mobile network signaling operates. It is worth noting that the transmitter may have some other characteristics, in particular, the frequency varies from 824 to 848 MHz, in case it is a transmitted signal. And when receiving a signal, the range from 869 to 893 MHz is used.

Deputy Director for Development Kerimov Rostislav.

What is CDMA?
Code Division Multiple Access. It is a digital technology developed by Qualcomm that delivers crystal clear audio to the next generation of wireless devices and services. Using the digital coding technique of wideband radio signals, CDMA enables better and more cost-effective audio transmission, greater eavesdropping protection, capacity and flexibility than other wireless technologies. At the same time, this technology makes it possible to organize additional services such as the exchange of short messages (SMS), the use of e-mail and access to the Internet.

How does CDMA work?
CDMA uses spread spectrum technology to split a speech signal into small digital segments and encode them to identify each call. Thus, a large number of subscribers can share the same frequency range, which greatly increases the capacity of the network. In other words, CDMA allows wireless carriers to carry more digital signals in a specific radio frequency band.

What is GSM?
Global System for Mobile communications - Global System for Mobile communications. It is a digital communication standard developed in the early 1980s to enable roaming across Europe.

Which standard provides the best audio quality, CDMA / 800 (Cellular) or CDMA / 1900 (PCS)?
High sound quality is provided by the CDMA digital technology itself, and not by radio frequency. Both standards have the same high sound quality.

Why is CDMA better?
Some of the benefits of CDMA are the same or better audio quality than wired transmission, better reception with less background noise, fewer dropped connections, increased privacy and reliability. Also, CDMA allows serving a larger number of subscribers at the same time - a larger network capacity, which gives fewer unsuccessful attempts to make a call by subscribers of the CDMA network.

Tell us about CDMA privacy.
CDMA transmission is digitally encoded. The speech signal is divided into segments, and a code is assigned to each segment. At the receiving device, the speech signal is reconstructed using a code.

Where is CDMA technology used?
CDMA covers most of the US. CDMA is also used in other countries around the world.

What is CDMA / 800 (Cellular)?
CDMA / 800 (Cellular) uses the 800 MHz radio frequency band.

What is the exact frequency range of CDMA / 800 (Cellular)?
Transmitter 824.01-848.97 MHz Receiver 869.01-893.97 MHz.

What is CDMA / 1900 (PCS)?
CDMA / 1900 (PCS) (Personal Communicatoins Services) uses a different radio frequency band than CDMA / 800 (Cellular). Basically, it uses the 1900 MHz band.

What is the exact CDMA / 1900 frequency range (PCS)?
Transmitter 1850-1920 MHz Receiver 1930-1990 MHz.

Code Division Multiple Access (CDMA) is a digital technology that opens the door to a new generation of wireless data services and facilities. Using digital coding and a "wide range" of radio frequency (RF) technologies, CDMA provides higher voice quality, better privacy, higher system capacity and flexibility than other wireless technologies, and provides a wide range of advanced services. such as live messaging, e-mail and Internet access.

Developed for commercial use by QUALCOMM Incorporated, the IS-95 CDMA is internationally recognized and is now the backbone of the cdmaOne ™ family of industrial wireless systems. Today, commercial CDMA networks serve millions of subscribers worldwide.

Outstanding audio and conversation quality. CDMA provides the same high quality voice transmission as conventional cable. In addition, CDMA eliminates background noise, concurrent channel conversations and interference, enhancing confidentiality and communication quality. Less energy consumption. CDMA phones use significantly less data transmission power than phones using other technologies, resulting in increased battery life, talk time and standby time. By using smaller batteries, manufacturers can make smaller, lighter phones.

Fewer dropped calls. CDMA increases system throughput by virtually eliminating busy signals, parallel calls, and dropped calls resulting from system congestion. By using a proprietary cell-to-cell call transfer technique called "soft handoff", CDMA has significantly reduced the likelihood of call dropouts or call interruptions during hands-free operation.

Extended coverage. Covering a wider spectrum, CDMA signal provides greater coverage than other wireless technologies, both indoors and outdoors. In addition, CDMA is used by other types of telecommunication systems, providing a single, wide area of ​​coverage and consistency. With improved signal characteristics and a wider range, CDMA has expanded the area in which messages can be received and transmitted. CDMA networks only need a subset of the network nodes required by other wireless technologies to cover a given area. With fewer radio access network nodes, service providers can reduce their initial investment as well as operating and maintenance costs.

Security and privacy of information. In addition to eliminating parallel conversations and extraneous noise, a wide range of transmission of messages encoded in digital form, CDMA technology protects against eavesdropping. CDMA voice encryption also protects against tampering and other types of fraud.

Additional services. The use of a CDMA digitally controlled channel allows users to access a wide range of new services, including caller identification, instant messaging (such as pager messages) and data transmission. With CDMA, simultaneous voice and data transmission is possible.

Flexibility. CDMA is the only wireless technology that delivers both fixed and mobile services within a single platform, supporting two revenue streams, allowing providers to work with their customers on a one-phone basis. In addition, CDMA networks are less expensive to develop and design than other types of wireless systems, making them easier to expand and reconfigure.

Higher throughput. CDMA provides 10 to 20 times the bandwidth of analog wireless technologies and 3 times the bandwidth of other digital technologies, allowing service providers to serve more subscribers and more wireless traffic within a given radio frequency range. With the rapidly increasing number of wireless subscribers and usage times, bandwidth is a critical factor.

Fast commissioning. CDMA systems can be deployed and expanded more quickly and cost effectively than most wireless networks. And because they require fewer network nodes, CDMA networks can be deployed faster than other types of wireless networks.

Service quality. The superior voice quality achieved with CDMA and value-added services including wireless data transmission gives service providers a significant competitive advantage in attracting and supporting customers.

Choice. With broad support from telecom equipment leaders and cost savings, service providers can choose from a wide range of advanced, cost-competitive CDMA products.

The phones of the latest years of release provide many functions, including the ability to use different communication standards.

Not all mobile phone users have a sufficiently confident knowledge of what communication standards they use to connect with other subscribers. Most consumers of communication services may need this information only when they have to choose a new phone.

An impressive list of options and characteristics is replete with many mysterious designations and abbreviations, the meaning of which is either hard to guess, or even remains a secret behind seven seals. What lies behind the CDMA and WCDMA letter ciphers, what are they used for and can we do without them today?

What is CDMA in a phone?

As you know, there are several different standards for data transmission in mobile communications. It has become a generally accepted standard for our country and European countries, however, as an alternative for it back in the 90s, the CDMA standard, or Code Division Multiple Access (code division multiple access technology) was proposed.

If in GSM communication the digitized information packets are divided in time, then in the CDMA standard not only time, but also coded division is used. Voice packets are encrypted in one way, personal data packets in another, and the Internet connection uses a third encryption method. Thanks to this, all data can be transmitted simultaneously without interfering with each other.


If the phone has not only GSM, but also CDMA, it means that you have a two-standard phone that can work in networks with different signal coding principles. In fact, CDMA is a better, faster and more reliable standard. Adoption of GSM communication as the main standard for European countries is considered by many experts to be a serious mistake.

What is WCDMA in a phone?

Another communication standard that is widely used today in the construction of 3G networks is WCDMA. This abbreviation stands for Wideband Code Division Multiple Access, i.e. broadband CDMA standard.

As the name implies, WCDMA is a variant of the CDMA standard that uses broadband communication access. It is the main one for mobile operators in Japan, thanks to which wireless Internet appeared and became publicly available in this country earlier than others.

Today, on the principle of WCDMA, construction is carried out, which provide high speed and reliability of data exchange (at short distances up to 2 Mbit per second, at a considerable distance from base stations - up to 384 Kbit per second).


WCDMA uses broadband access, which has a range of 5 MHz. The WCDMA technology has significantly broader capabilities compared to the GSM standard: it allows simultaneous transmission of a voice signal, video signal and digital information packets.

One of the advantages of WCDMA is the lack of binding to a specific territorial location of base stations. Using this standard, you do not notice that you move from one zone to another while driving - for example, when traveling by car or on a train. You can even cross the border between countries, but this will not affect the received signal.

Most modern phones use the WCDMA standard, which provides high-quality, fast and reliable wireless Internet and other wireless communication services, including regular mobile telephony.

Do you need CDMA and WCDMA in your phone today?

The CDMA standard is still used for the operation of some wireless networks operating in Russia - the most famous of which is "SkyLink". CDMA is extremely common in China, where it is used almost on a par with GSM.


Many operators in various countries of the world have already managed to make sure of its reliability and prospects. As for WCDMA, it is already used in Russia to provide 3G services. Therefore, when choosing a new phone, be sure to look for a mention of the presence of the WCDMA standard in its characteristics.

1. Basic principles of CDMA
2. Differences between CDMA and other standards
3. Services in CDMA networks
4. General characteristics and principles of operation
5. Multi-access technology
6. Development and prospects of the CDMA standard

1. Basic principles of CDMA

In order to overcome the above disadvantages, manufacturing companies had to turn to fundamentally different digital systems made using the technology multi-station code division multiple access (CDMA) or, as it is called all over the world, CDMA (Code Division Multiple Access) that use spread spectrum noise-like signals. Of course, everything new is a good share of the old, and in our case, not at all forgotten.

Code division multiple access technology based on orthogonal signal division has been known for a long time. In the USSR, the first work devoted to this topic, called "Foundations of the theory of linear selection" was published in the collection LEIS back in 1935, and its author was Dmitry Vasilievich Ageev. And after the war, for a long time, CDMA technology was used in military communication systems both in the USSR and in the USA, since it had many valuable advantages for such systems, which will be discussed below.

The very principle of CDMA is to spread the spectrum of the original information signal (in our case, speech), which can be produced by two different methods, which are called as follows: "frequency hopping" and "direct sequence".

The so-called "frequency hopping" (or FH - Frequency Hopping) is implemented as follows: the carrier frequency in the transmitter constantly changes its value within certain specified limits according to a pseudo-random law (code), individual for each voice channel, at relatively short time intervals. The receiver of the system behaves in a similar way, changing the frequency of the local oscillator according to exactly the same algorithm, ensuring the selection and further processing of only the desired channel. With the help of FH, attempts are now being made to improve the technical characteristics of narrowband digital cellular systems, in particular, GSM.

The second method is "direct sequence" (or DS - Direct Sequence), which is based on the use of noise-like signals and is used in most working and future CDMA systems. It provides for modulation of the information signal of each subscriber with a single and unique pseudo-random noise-like signal (in this case, it is the code), which expands the spectrum of the original information signal. It should be noted right away that the number of variants of such codes reaches several billion, which would make it possible to create a personal connection on the scale of our planet. As a result of the described process, the narrow-band information signal of each user expands over the entire width of the frequency spectrum allocated for network users (the signal base becomes much greater than 1). At the receiver, the signal is reconstructed using the identical code, as a result of which the original information signal is reconstructed. At the same time, the signals of other users for this receiver continue to remain expanded and are perceived by it only as White noise which is the most soft interference that will least interfere with the normal operation of the receiver.

In order to popularly explain the principle of such a system, we will use one very apt allegory, which, when explaining the basics of CDMA technology, Motorola usually offers “for experts and not so much”. Imagine a room in which many pairs of people are talking to each other at the same time, and in different languages. Each of them understands their interlocutor well, and all extraneous conversations are perceived as a kind of background and do not particularly interfere with the conversation.

At the same time, a high degree of protection against active and passive interference is provided, which makes it possible to operate at low values ​​of the signal-to-noise ratio (3-5 dB) with a significantly lower transmitted signal power. Thus, information signals of a large group of users are simultaneously transmitted in the same radio frequency channel.

It should also be said that CDMA is widely used in military communication systems for a reason, since the expansion of the signal spectrum allows us to counteract deliberate artificial interference. If you expand the base of the radio signal to very large values, then you can make it below the noise level that a potential enemy can observe. On the receiving side, the original signal will be restored. Thus, such systems could be used (and such systems exist) without interfering with the operation of other radio equipment using the same radio frequency range. However, this is not used in existing commercial CDMA cellular systems.

2. Differences between CDMA and other standards

In frequency division systems (both FDMA and TDMA), there is a problem of so-called "reuse" of frequency channels. In order not to interfere with each other, neighboring base stations must use different channels. Thus, if a BS has 6 neighbors (the most often considered case, while the zone of each BS can be represented as a hexagon, and everything looks like a honeycomb), then the number of channels that this BS can use is seven times less than the total number of channels in the range allocated for the network. This leads to a decrease in the capacity of the network and the need to increase the density of the BS installation in densely populated areas. For CDMA, there is no such problem at all. All BSs operate on the same channel. Thus, the frequency resource is used more fully. The capacity of a CDMA network is usually several times higher than that of TDMA, and an order of magnitude higher than that of FDMA networks.

In order for the phones located close to the base station not to clog more distant subscribers with their signal, CDMA provides for a smooth power control, which leads to a significant reduction in the power consumption of the phone near the base station and, accordingly, an increase in the phone's operating time without recharging.

One of the nice features of CDMA networks is the ability to "soft" transition from one BS to another (soft handoff). In this case, a situation is possible when one subscriber is "led" by several BSs at once. The subscriber simply will not notice that he has been “transferred”; another BS. Naturally, in order for this to become possible, precise synchronization of the BS is required. In commercial systems, this is achieved by using time signals from the GPS (Global Positioning System) of the US satellite positioning system.

CDMA is an almost entirely digital standard. Usually, all conversions of the information signal occur in digital form, and only the radio part of the device is analog, and much simpler than for other groups of standards. This allows almost the entire phone to be made in the form of a single microcircuit with a high degree of integration, thereby significantly reducing the cost of the phone.

The digital nature of CDMA is highly conducive to using this technology for wireless data transmission. In the example considered above, we set a not very high speed, however, the existing CDMA implementations allow us to multiply the data transfer rate, albeit at the expense of reducing the network capacity.

CDMA standards use a more modern codec for digitizing speech, which subjectively improves the quality of analog signal transmission compared to current TDMA standards.

Of the disadvantages of CDMA, it is possible to note the need to use a fairly wide and continuous band, which is not always possible in the modern environment of a shortage of frequency resources and the great complexity of the implementation of this technology in hardware.

RESULTS:

1. Higher communication quality compared to other communication standards.
2. Higher data transmission rate and, accordingly, wider possibilities of using CDMA terminals.
3. Less Energy consumption terminals that extend the life without recharging.
4. Large network capacity (more complete use of the frequency resource).
5. The CDMA standard (IS-95) is more adapted to the transition to the third generation.

3. Services in CDMA networks

We can talk for a long time about the advantages of this or that technology, but for a commercial telecommunications operator the set of services for which the subscriber will pay money is paramount. The main advantage of CDMA networks is a very high quality of voice transmission, and this, you see, is the main service of all telephone networks.

Speech in the CDMA system is converted into a digital stream by means of a special vocoder with a variable rate depending on the speech intensity. This makes it possible to economically use the bandwidth of the radio path. That is, while you are silent, the resource of the radio channel can be used by another subscriber, who is also talking at the moment.

IS-95 systems use vocoders with maximum bit rates of 8 kbps and 13 kbps. Accordingly, data transmission can be carried out at speeds of 9.6 kbps and 14.4 kbps, respectively. Note that the last digit is not available on other networks.

Voice quality with a 13 kbps vocoder is close to that of digital cable lines. But, of course, such vocoders reduce the network capacity somewhat. Therefore, for subscribers who are only interested in voice communication, the so-called improved 8 kbps vocoders with the same high speech quality. It should be noted that the topic of vocoders continues to be relevant in the sense of further improving the quality with further decreasing the bit rate. The price of the issue is a further increase in network capacity. And this forces many specialists to continue such work.

What, in fact, do you mean when talking about the high quality of voice transmission in CDMA networks? This is, first of all, clear sound and the absence of extraneous noise, which is unusual for radiotelephones. But here, as they say, it is better to hear once than to see ten times in various literature. Well, now you don't have to go somewhere, for example, to the USA or Korea. CDMA networks (IS-95) are already operating in a number of regions of Russia. It was possible to talk on CDMA radio telephones and at the Moscow forums SVIAZ-EXPOCOMM-97 and -98, where such demonstration systems installed by Qualcomm and Personal Communications were operating. And nowadays it is not so exotic at all, because there are already quite a few IS-95 networks built in Russia.

And, of course, there is a legitimate interest in ways of accessing the INTERNET via IS-95 cellular networks. Of course, a CDMA subscriber can simultaneously talk on a cell phone and work on a computer connected to it on the INTERNET. Among the standards for mobile communications existing today, the best position here is with IS-95 due to the provision of the highest access speed (14.4 kbps). And the manufacturing companies promise to increase this speed in the near future (fortunately, in the “common pipe” of the CDMA radio channel, you can redistribute the resource allocated to each subscriber). By the way, the presentation of the data transmission system over the CDMA network was organized by Qualcomm during the Moscow exhibition SVYAZ-EXPOCOMM-98 based on the network of the Moscow operator “Personal Communications”.

Of course, subscribers will be able to exchange fax messages and communicate by e-mail. And in the foreseeable future, the emergence of transmission rates of 64 kbps and even 144 kbps is expected.

As for the various additional services that are provided to subscribers of digital cellular networks (call forwarding, voice mail, identification of numbers, IN services, etc., including roaming), all of them are determined by the network switching equipment (controller or switching center) and not depend on the type of used radio interface... What the developers put into the switch will work out (the main thing is that you can install an additional soft on it). In general, what the operator considered necessary to purchase for its network (based, for example, marketing research), that the subscribers will be able to use. And if something is missing now, then it is quite possible to buy it later.

Historically, Europeans have developed the most detailed various applications (in terms of additional services) for the GSM standard (but this refers more to the functions of the switch, and not radio interface). But on February 20, 1998, Qualcomm announced the successful completion of tests with the British operator Vodafon of a CDMA base station (IS-95) included in the GSM mobile switching center. This means that now nothing is impossible and there is a real opportunity to combine the numerous services developed within the GSM project and adapted to European telephone networks with the advantages radio interface IS-95.

4. General characteristics and principles of operation

The principle of operation of cellular communication systems (CCS) with code division multiplexing can be illustrated by the following example.

Let's say you are sitting in a restaurant. There are two people at each table. One couple speaks to each other in English, another in Russian, the third in German, etc. It turns out that in a restaurant everyone speaks at the same time on the same frequency range (speech from 3 kHz to 20 kHz), while talking with your opponent, you understand only him, but you hear everyone.

Likewise, in the CDMA standard, information transmitted over the air from a base station to a mobile or vice versa gets to all network subscribers, but each subscriber understands only the information that is intended for him, i.e. Russian understands only Russian, German understands only German, and the rest of the information is eliminated. The language of communication is currently a code. In CDMA, this is organized by using the encoding of the transmitted data, more precisely, the Walsh function multiplication unit is responsible for this.

Unlike the GSM standard, which uses TDMA (Time Division Multiple Access - multi-station code division access, that is, several subscribers can talk on the same frequency as in CDMA, but unlike CDMA, at different times), the IS-95 standard uses the frequency range more economically.

CDMA is called a broadband system and the signals transmitted over the air are noise-like. Broadband - because it occupies a wide bandwidth. Noise-like signals - because when several subscribers are working on the air at the same frequency at the same time, the signals are superimposed on each other (you can imagine noise in a restaurant when everyone is talking at the same time). Interference-resistant - because when an interference signal occurs in a wide frequency band (1.23 MHz), a narrow range (<150кГц), сигнал примется почти неискаженный. За счет помехоустойчивого кодирования потерянные данные система восстановит, см. рис 1, где показан полезный сигнал и помеха (СЗС - селективная помеха).

And in the GSM standard this will not work. Due to the fact that GSM is initially narrowband itself. The bandwidth used is 200 kHz.

Qualcom's CDMA system is designed to operate in the 800 MHz frequency range. The CDMA system is based on the method of direct spreading of the spectrum of frequencies based on the use of 64 types of sequences formed according to the law of Walsh functions. For the transmission of voice messages, a speech-converting device with the CELP algorithm with a conversion rate of 8000 bit / s (9600 bit / s in the channel) was selected. Possible modes of operation at speeds of 4800, 2400, 1200 bit / s.

CDMA channels use rate convolutional coding? (on channels from the base station) and 1/3 (on channels from the mobile station), soft decision Viterbi decoder, interleaving of transmitted messages. The total bandwidth of the communication channel is 1.25 MHz.

The standard uses separate processing of reflected signals arriving with different delays, and their subsequent weight addition, which significantly reduces the negative influence of the multipath effect. With separate processing of beams in each receive channel, 4 correlators operating in parallel are used at the base station, and 3 correlators at the mobile station. The presence of correlators operating in parallel makes it possible to implement a soft “handover” mode when switching from cell to cell.

Soft handoff occurs by controlling a mobile station with two or more base stations. The transcoder, which is part of the main equipment, evaluates the quality of signal reception from two base stations sequentially frame by frame. The process of choosing the best frame leads to the fact that the resulting signal can be formed in the process of continuous switching and subsequent “gluing” of frames received by different base stations participating in the “handover”.

The handshaking protocols in CDMA, as well as in the AMPS standards, are based on the use of logical channels.

In CDMA, channels for transmission from a base station are called Forward, for reception by a base station - Reverse. The channel structure in CDMA in the IS-95 standard is shown in Fig.

Direct channels in CDMA:

Pilot channel

Used by the mobile station for initial synchronization with the network and monitoring base station signals in time, frequency and phase.

Sync channel

Provides base station identification, pilot emission level, and base station pseudo-random sequence phase. After the completion of the indicated synchronization steps, the connection establishment processes begin.

Call channel

Used to call a mobile station. After receiving the call signal, the mobile station transmits an acknowledgment signal to the base station, after which the call establishment and channel assignment information is transmitted to the mobile station via the call channel. The paging channel starts working after the mobile station has received all the system information (carrier frequency, clock frequency, clock delay).

Direct access channel

Designed to transmit voice messages and data, as well as control information from a base station to a mobile one.

Return channels in CDMA:

Access channel

Allows the mobile station to communicate with the base station when the mobile station is not yet using the traffic channel. The access channel is used to establish calls and responses to messages sent on the call channel, commands and requests to register with the network. Access channels are combined (combined) by call channels.

Return traffic channel

Provides the transmission of voice messages and control information from the mobile station to the base station.

The structure of the transmission channels of the base station is shown in Fig.

Each logical channel is assigned its own Walsh code. There can be 64 logical channels in one physical channel. there are 64 Walsh sequences to which logical channels are assigned, each of which has a length of 64 bits. Of all 64 channels, the first Walsh code (W0) is assigned to the 1st channel, which corresponds to Pilot channel, the next channel is assigned the thirty-second Walsh code (W32), the next 7 channels are also assigned their own Walsh codes (W1, W2, W3, W4, W5, W6, W7) to which the call channels correspond, and the remaining 55 channels are intended for data transmission over Direct traffic channel.

When the sign of the data message bit is changed, the phase of the used Walsh sequence is changed by 180 degrees. Since these sequences are mutually orthogonal, there is no mutual interference between transmission channels of one base station. Interference on the transmission channels of the base station is created only by neighboring base stations that operate in the same radio frequency band and use the same bandwidth, but with a different cyclic shift.

The order of the passage of voice data in a mobile station before sending it on the air.

Let's take a closer look at the structural diagram of the reverse traffic channel. In the forward and reverse channels, this pattern is repeated; depending on which channel is currently in use, some blocks of this scheme are excluded.

1. The speech signal is fed to the speech codec. At this stage, the speech signal is digitized and compressed using the CELP algorithm.
2. Then the signal goes to the error-correcting coding unit, which can correct up to 3 errors in the data packet.
3. Then the signal goes to the signal interleaving unit.
   The block is designed to deal with bursts of errors on the air. Bursts of errors - distortion of several bits of information in a row.
   Principle... The data stream is written to the matrix row by row. As soon as the matrix is ​​filled, we begin to transfer information from it in columns. Consequently, when several bits of information are distorted on the air in a row, when receiving a burst of errors, passing through the inverse matrix, it is converted into single errors.
4. Then the signal goes to the coding unit (from eavesdropping).
   A 42-bit mask (sequence) is superimposed on the information. This mask is classified. With unauthorized interception of data on the air, it is impossible to decode the signal without knowing the mask. The method of iterating over all possible values ​​is not efficient, since when generating this mask, iterating over all possible values, you will have to generate 8.7 trillion masks with a length of 42 bits. A hacker, using a personal computer, passing a signal through each mask and converting it into an audio file, then, recognizing it for the presence of speech, will spend a lot of time.
5. Walsh code interleaver.
   The digital data stream is multiplied by a sequence of Walsh-generated bits.
   At this stage of signal encoding, the frequency spectrum is spread, i.e., each bit of information is encoded with a sequence built according to the Walsh function, 64 bits long. That. the data flow rate in the channel is increased by 64 times.
   Therefore, in the signal modulation block, the signal manipulation speed increases, hence the broadening of the frequency spectrum. The Walsh function is also responsible for filtering out unnecessary information from other subscribers.
   At the moment of the beginning of the communication session, the subscriber is assigned the frequency at which he will operate and one (out of 64 possible) logical channel, which determines the Walsh function. At the moment of reception, the signal goes in the opposite direction according to the scheme.
   The received signal is multiplied by the Walsh code sequence. The correlation integral is calculated from the result of the multiplication. If the Z threshold satisfies the limit value, then the signal is ours.
   The sequence of Walsh functions are orthogonal and have good correlation and autocorrelation properties, so the probability of confusing your signal with someone else's is 0.01%.
6. The block for multiplying the signal by two M-functions (M1 - 15 bits long, M2 - 42 bits long) or they are also called PSP - pseudo-random sequences.
   The block is intended for mixing the signal for the modulation block. Each assigned frequency is assigned a different M-function.
7. Signal modulation block.
   The CDMA standard uses phase modulation PM4, OFM4.

Currently, the equipment of the CDMA standard is the newest and most expensive, but at the same time the most reliable and most protected. The European Community, within the framework of the RACE research program, is developing a CODIT project to create one of the variants of the Universal Mobile Telecommunications System (UMTS) based on the principle of code division multiple channels using wideband direct spread spectrum (DS-CDMA) signals.

The main difference of the CODIT concept will be the efficient and flexible use of the frequency resource. As we explained earlier, wideband CDMA signals are largely unaffected by narrowband interference. Due to this property, the CODIT standard will additionally use guard intervals between carriers for data transmission.

5. Multi-access technology

Digital radio communication is characterized by the ability multi-station access or multi-access, which implies the simultaneous transmission of information through one device by many users in a common channel. In this case, the division of the common channel can be performed by frequency (FDMA), time (TDMA) and code (CDMA), which can be illustrated as shown in Fig.:

Frequency division splits the transmission spectrum into sections for different users. This is the only method that can be used for analog communication. All analog standards of cellular communication are based on this method: NMT, AMPS, TACS, etc. The disadvantages of such systems are now obvious: poor noise immunity and the associated low quality of speech transmission, ineffective use of the scarce radio spectrum, lack of protection from eavesdropping, etc. It should also be said that the peak of its development of analog systems passed in 1993, after which a steady decline in the number of their subscribers is observed. The most widespread analog standard in the world was and still is AMPS. The other two methods are used in digital technology and are usually combined with frequency division. In the case of time division multiple access, multiple subscribers transmit their messages on the same radio frequency, but at different times, which allows an increase in the volume of voice traffic and a number of other advantages typical for digital communication systems. This method is based on such narrowband digital cellular standards as GSM and its variant DCS, as well as D-AMPS, which became a logical continuation of the AMPS standard.

6. Development and prospects of the CDMA standard

The really proven advantages of CDMA technology have led to the orientation of all vendors developing third generation radiotelephone communication systems (including those for mobile communications) exclusively towards it, or rather towards various options for broadband CDMA.

Earlier, international regulatory bodies developed recommendations for systems of the third generation (Third Generation or 3G), referred to conventionally as IMT-2000. There is also an organization in the USA CDMA Development Group(or CDG), coordinating the activities of the developers. This is, of course, also the ITU-R - the international body dealing with radio communications.

The IMT-2000 concept provides for the possibility of interoperability with other existing terrestrial and satellite communication networks (in particular GSM), voice, data and video transmission. There will be transmission speeds of 64 kbps, 144 kbps, 2 Mbps: something for subscribers with limited mobility, something for highly mobile subscribers. The main areas of work here are as follows: high-speed data transmission, multimedia applications, interaction with satellite networks, global roaming, compatibility with second generation networks. For generation III systems, radio frequencies are allocated around 2 GHz.

And analogue systems should quietly leave the world stage in 2006-2008, about which the world community has already agreed in the first half of the 90s.

Many developers have tackled the problems formulated in the IMT-2000 concept, but if they were not so similar to crayfish, swan and pike. The choice of most of them, as already mentioned, fell on the technology of broadband CDMA. And in the choice of technology, Europeans, Americans, Japanese, Koreans and many others suddenly became united. But!

The Europeans had two promising but incompatible standards TD / CDMA and W-CDMA (which they will still technically combine under the general name W-CDMA or UMTS), and the North Americans - WB-CDMA (which is now called cdma2000), as a further development of today's cdmaOne ... So far, all this is still a very crude development, requiring running-in and fine-tuning. But tests are underway, investments are being mastered, but, unfortunately, there is no proper coordination.

In European developments, both Japanese and Koreans are involved in projects. But there is also an American version, in which almost all of the local manufacturers participate. It is clear that the American version will live on, because it is created on the basis of the experience gained during the actual operation of many cdmaOne networks. But Europeans are also known for their comprehensive and pedantic attitude to the creation of cellular communication systems, which they have already demonstrated during the development of GSM. An experimental W-CDMA system was recently supplied by Ericsson to the Japanese operator NTT DoCoMo, and an agreement has been reached to test a similar system in Sweden with the local operator Telia

Now let us rise for a moment above the earthly vanity into near-earth space. There are now deployed global satellite communications systems on LEO satellites. One of them, Iridium, uses TDMA technology, the other, Globalstar, uses CDMA technology (close to the one discussed above). These systems have their own peculiarities in terms of equipping spacecraft, the number of earth stations, etc., however, they must provide communication in “blank spots” where there is no terrestrial cellular communication. The first system has already started functioning, the second will start working next year. Despite the fact that the recent accident of the Ukrainian launch vehicle, which buried several Globalstar satellites, will somewhat postpone the implementation of the network of the same name, we wish them success and note that preliminary information on tariffs indicated that the cost of Globalstar services is almost an order of magnitude lower (there is, apparently, this is the merit of the CDMA technology itself). And note that this is not its only application, since most promising satellite projects also rely on the use of CDMA technology.

We go back down. Due to its high technical and consumer characteristics, the WLL system, called MGW, manufactured by Tadiran, which uses the FH-CDMA method and is capable of operating in the radio frequency ranges from 800 MHz to 3.5 GHz, exists and is actively being introduced on the Russian market. But although this is a relative, but a distant one.

But what about the wireless communication systems of the future? Their sprouts are already making their way in the current market. That is, CDMA is now a broad concept and is not at all limited to the IS-95 standard. Therefore, everything that is based on the specified standard is now called “cdmaOne” (that is, “they were the first”), and this name appears now in various literature and on telecommunication forums.

Consumers are already being offered fixed communication systems based on broadband CDMA (B-CDMA) technology, which uses radio channels with a width of 5, 10 and even 20 MHz and working radio frequencies in the ranges of 2, 3, 4 GHz. Worked over all of these Lucent Technologies, Samsung, Interdigital, Siemens and etc . What do they offer to the consumer? Of course, even higher noise immunity, even greater communication confidentiality and, of course, high bandwidth. Now subscribers can receive ISDN services and even transmit video information.

Of course, these are very different types of systems that do not have any common standard, although a number of companies collaborated with each other at the stage of equipment development. And do not make the mistake of mistaking them (whatever the suppliers say) for the third generation radiotelephone communication systems. Their ideology, of course, is similar, but the technical implementation of interfaces, a set of services, etc. differ.

Let us mention that almost all manufacturers and developers of CDMA equipment maintain contacts with Qualcomm, which is not only the holder of a number of patents for this technology, but also has a staff of qualified specialists in this area. Although Bell Labs, Samsung, Nortel, Motorola and many North American operators now have the latter, they often do not consider it shameful to consult each other.

Such an offensive of a new technology, about which until very recently nothing was known, was not to the liking of those who had previously invested in the development of TDMA technology systems. As a result, for several years there was a heated debate in the press about the "supposed" advantages of CDMA. The opponents were Europeans, who relied on GSM systems (and D-AMPS, which they traded in other countries). Suffice it to recall how recently the Swedish Ericsson convinced everyone of the practical unrealizability theoretical advantages of CDMA / IS-95 and at the same time sued Qualcomm over patents for this technology (you cannot give a better rationale for the attitude to IS-95 in different countries). Ironically, today it is Ericsson who is forced to solve the problems of cdmaOne and release the corresponding network equipment, because Qualcomm sold its respective divisions to it. God works in mysterious ways…

With the introduction of many commercial cdmaOne networks around the world, this topic has shifted towards next-generation communications systems. And already there, it seems, not CDMA and TDMA systems will compete, but CDMA systems of various modifications.

A mobile phone is designed for autonomous operation in a cellular network, which is in demand and is dynamically developing. For users, it has become a necessary means of communication. This is a high-tech device that identifies a subscriber using a Sim card. There are many types of phones.

In comparison with each other, they differ in technical characteristics, functionality, design. The cost directly depends on the capabilities of the device, its manufacturer, quality and form factor. There are main types of mobile devices:

• monoblock with keyboard;
• slider;
• cot;
• a mobile phone with a touch screen.

Modern technologies

Progress does not stand still, and now the smartphone has become the most in demand. This is an “intelligent” mobile phone that functions on a par with a personal computer. It has an operating system, and also works with a variety of programs, applications, it has WIFI and GPS modules. This is how it radically differs from a simple telephone.

The phone catalog contains the latest models of devices. The main characteristics of modern smartphones:

• operating system;
• the amount of RAM, built-in memory;
• permission;
• camera;
• monoblock type.

The higher these indicators are for a smartphone, the higher its price will be.

Where can I buy

Online stores offer a wide selection of device models from the most popular, proven global brands: Samsung, Alcatel, Fly, Lenovo, HTC, Nokia, IPhone and others. New, improved models appear. Prices for phones, additional accessories, components, spare parts can be compared using the Aport website. In addition, the site will help evaluate market offers and choose the best seller.