CDMA without secrets.

CDMA without secrets.

CDMA without secrets

So, CDMA technology continues to strengthen its position in the world. It has gained the greatest popularity in the USA, South Korea and Japan, which are among the countries with a high level of telecommunications development. The main areas of application have been determined: mobile cellular communications and wireless access (WLL). The time is approaching when satellite communications will join them in the form of the global low-orbit system Globalstar. As for Russia, WLL networks based on CDMA have already been deployed in several cities and their number is planned to increase in the near future.

Strictly speaking, code division multiplexing systems are not a fundamentally new word in science and technology. They have been known for a long time. For example, systems with spectrum spreading, using complex (noise-like) signals, have been and are widely used in such areas as radar, telemetry systems, radio navigation and communications, especially when solving special problems. These systems were valued, first of all, for their secrecy and noise immunity, which was achieved due to the wideband and low spectral density of the signals. The task of ensuring the simultaneous operation of many channels in a common frequency band was usually not set. In the first generation communication systems, ptumon-like signals were often used, that is, long sequences of ones and bullets, in which the symbols alternated according to a law close to random. These include M-sequences and newly formed sequences (in foreign literature — Gold sequences).

Due to the great practical importance of complex signals, their research has been carried out in parallel in many countries of the world for a long time. In particular, the most active work was carried out in the USA and the former USSR. As a result, a significant amount of information was accumulated, thanks to which the transition to more advanced systems of the new generation became possible.

One of the widely proclaimed advantages of CDMA over other systems is the more efficient use of the allocated frequency resource. It is reported, for example, that compared to AMPS, the efficiency can be 30 times higher! Before reacting violently to such success, we can try to analyze its causes. To begin with, let us remember that the maximum throughput of a communication channel can be estimated using the Shannon relation (see inset).

All existing communication systems, including the cellular systems known today, are to varying degrees far from the Shannon threshold. There are many reasons for this. One of them is related to the choice of one of three access methods: frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). All of them, as well as their combinations, are used in existing cellular communication standards: FDMA in AMPS, TDMA in GSM, and CDMA in the IS-95 standard. The access method determines the order of using the common frequency band allocated for the multi-channel communication system. It is known that any multiple access method is inferior in total throughput to the case when one high-speed channel is organized in the allocated band, occupying the entire band.

With FDMA, this is due, in particular, to the need to introduce protective frequency intervals to prevent mutual interference between channels. In exactly the same way, protective time intervals are introduced with TDMA.

As for CDMA, since all channels use the same frequency-time resource, there is also a source of mutual interference. When several channels operate simultaneously, each of them is allocated an individual code. Each information symbol (O or 1) is transmitted by a corresponding code sequence, the length of which can reach several tens, hundreds and even thousands of characters. The length of the sequence depends on the code class and the number of channels. Each subscriber receiver contains a device that optimally processes (compresses) its own signal and suppresses foreign signals.

Quantitative estimates of the efficiency of various radio systems can be obtained based on the basic principles of information theory. For example, the potential capacity of a communication channel V [bit/s], its bandwidth DF [Hz] and the signal-to-noise ratio P c/Psh [eg.] are related by the ratio:

V=DP • log2(1+Pc/Psh)

Unfortunately, in the first generation systems it was impossible to achieve complete suppression, alien signals increased the noise level and reduced the overall efficiency. The interference power increased with the number of simultaneously operating channels (approximately proportional to their number). Comparison of different access methods demonstrated the highest efficiency of time division TDMA, and the lowest — code division CDMA. As for FDMA, it occupies an intermediate position. But what about the constantly emphasized advantages of CDMA? The essence of the novelty of the new generation of systems with code division lies in the explanation of this paradox.

Unlike their predecessors, these systems belong to the class of systems with synchronous code division. The concept of synchronicity is key here and means that code sequences are transmitted not at arbitrary moments in time, but are tied to a single synchronizing time grid. This made it possible to use a new class of codes based on a set of Walsh functions, which in mathematics are classified as orthogonal functions. Their main feature is that with optimal processing, an alien signal can be completely suppressed. Thus, mutual interference between channels is completely eliminated. As for the volume of the ensemble (that is, the number of different signals of this class), it is limited only by the capabilities of technical implementation and the needs dictated by a specific practical task.

Let us remind you once again that a positive effect can be achieved only with careful synchronization of all transceivers in the communication network, the number of which, as is known, can reach many thousands. In addition, the devices for generating and optimally processing coded signals become more complex. All this requires the use of digital VLSI specially developed for these purposes. At the same time, the equipment of base stations becomes more complex, their careful binding to the terrain is required, etc. Thus, the price for the advantages of modern CDMA systems was the complexity of the base and subscriber equipment, which could not but affect the cost. Their distribution became possible, first of all, due to the success of microelectronics. As has happened more than once, the improvement of operational characteristics was determined by technological progress. In fact, the IS-95 standard describes the equipment of the next generation in comparison with the current AMPS and GSM systems. This is its strength and its weakness. Further success will largely depend on how quickly a balance between cost and quality can be achieved.

The efficiency of the system is largely determined by its noise immunity. It is generally recognized that CDMA has clear advantages in this regard. This is especially relevant for urban conditions, where the electromagnetic environment is difficult and there are problems with frequency distribution, specific conditions for the propagation of radio waves, as well as many narrow-band interferences of various origins. When processing broadband signals in an optimal receiver, effective interference suppression occurs due to decorrelation. This makes it possible to reduce the transmitter power and increase the cell radius. There is also an additional effect — a decrease in the subscriber's radiation level during a conversation. Although there is still no consensus on the effect of electromagnetic radiation from a cell phone on human health, it is psychologically reassuring.

The question arises: if CDMA really has such advantages, then how long will the other standards «last»? It is not easy to answer. The fact is that in technology, perhaps, there is nothing more conservative than standards. Changing them is associated with replacing a huge amount of equipment, material costs and inconvenience for users. Therefore, extremely compelling reasons are needed to switch to a new standard.

Different countries solve this problem differently. For example, in the USA, where analog AMPS networks were deployed everywhere at one time, the need for an update has become urgent. It is a different matter in Europe, where the main standard is the relatively advanced digital GSM standard. Many experts there treat CDMA with understandable caution. In their opinion, the technology based on frequency-time division TDMA and FDMA has not yet said its last word. 06 This is evidenced, in particular, by the successful promotion of microcellular systems based on DECT technology around the world.

As is well known, making forecasts is a thankless task. And yet we will risk assuming that in the near future, systems with code and time division will develop in parallel and divide the world communications market, and rather on a geographical basis. In the meantime, Qualcomm at a press conference in Moscow confirmed its intention to continue developing a series of new CDMA standards, including for the transmission of multimedia information (115 kbps), and a high-speed standard with a transmission speed of 2 Mbps.

At the same time, it is impossible not to note certain efforts to bring CDMA and TDMA closer together. First of all, this concerns CDMA developers, who are forced to take into account the fact that their equipment is entering the communications market, where other standards dominate. Successful work on coupling CDMA with AMPS and GSM is reported. These achievements should not be overestimated, since they are still of a private nature. For example, in the case of GSM, the possibility of using a GSM SIM card in a CDMA telephone was demonstrated. Thus, we are talking about the unification of auxiliary equipment.

As for the standards of digital mobile communications of the next generation, the tendency to combine various access methods is more clearly manifested here. For example, in the promising UMT-2000 standard, developed in Europe and claiming international status, along with frequency-time division, code division is also used. This achieves increased efficiency in the use of the radio spectrum.