Koishi (47) Code of the Mobile Maker | Mynavi News Mynavi News Mynavi

CDMA services used in 3G mobile phones will end this year both in Japan and the United States.The method called W-CDMA also ended in 2026 (docomo) and 2024 (Softbank).The first CDMA method (IS-95) introduced in 2G was used mainly in the United States.Although CDMA technology was conducted in the 1950s, it was rarely used in the private sector until the 1980s because it was used as a military technology.Although there are aspects of digital technology and semiconductor evolution, CDMA technology is also used for GPS, and the use of GPS private use has triggered the use of CDMA technology in the private sector.

A method of sharing a certain band in multiple communications is called "Multiple Access".For multiple connections by wireless communication, there are "Time Division Multiplay Access. TDMA)," Frequency "(FREQUENCY ~. FDMA)," Code "(CODE ~. CDMA)," Space "(SPACE ~There are SDMA).TDMA and FDMA are relatively easy to understand because they are divided by time and frequency.On the other hand, CDMA is hard to grasp the image in the first place.

Simply put, the CDMA sends 1 bit of the data to be sent as a bit pattern consisting of multiple bits.At this time, to use a radio signal that spreads throughout the band, it is also called "diffusion modulation" or "spectral diffusion".This bit pattern satisfies special conditions, and can be divided by calculation later even if the bit length is mixed with the same pattern.The CDMA communication method can use this principle to make multiple communications in a certain frequency band.At this time, the receiving side uses the same bit pattern as the sender.However, if you do not know the bit pattern, you will not be able to receive it.This kind of confidentiality is the reason why it was used in military technology.

CDMA seems to be difficult, but it is surprisingly simple to actually calculate.I have tried to understand this with Excel.The only formula used was multiplication and addition.All of this figure was created in Excel (God Excel?).

First, it is a bit pattern, but it is necessary to satisfy some special properties. In CDMA, this bit pattern is called "diffusion code" or "pseudo random noise", but mathematically, it is called "orthogonal sign" (there are multiple methods). This orthogonal code can create a long bit pattern with a simple calculation (Fig. 01). In the case of 8 bits, there are only 8 out of 256 combinations (seven accurately because the patterns that are all 1 are not available). In W-CDMA, this is called the OSVF code, repeating the same pattern, and repeating the reversed pattern, can create a pattern that double the bit length. For this reason, no matter how long patterns, there is no need to record on the table, so it can be easily calculated. In the case of hardware, it can be created with a relatively simple electronic circuit.

In the CDMA method, 1 and 0 of the digital signal are represented by 1 and -1 (or vice versa).This is to make the digital signal look like a wave.The diffusion cord created in this way has a mysterious nature.In a residential calculation with yourself (multiplying and taking the sum of the sum), the maximum value and the minimum value only when the phase is the same or completely reversed (Fig. 02).In any case, the same bit length is the maximum or minimum value in any case (Fig. 03).Other bit patterns that are not diffusion codes do not have the minimum or maximum value.This is related to the characteristic that the number of 1 and -1 is the same in the pattern.

With this property, it can be separated later even if communication of different diffusion cords is mixed, and can be restored even if noise is entered.

Here, we will try to send 3 -bit data using 8 -bit diffusion code.There are two terminals here, each as channel 1 and channel 2.On the sender, a different diffusion code (pattern 3 and pattern 6) is used for channel 1 and channel 2.

Submit the bit and the diffusion code to be submitted, and obtain 24 -bit diffusion from the original data of one bit (Fig. 04).This is sent, but the two signals are mixed because the same frequency band is used.Also, noise is mixed here.

The receiving side knows the pattern of channel and diffusion code in advance.The original transmission data can be obtained by using the appropriate diffusion code pattern to the mixed radio signal, and by examining the code.

In the actual CDMA, it is necessary to match the reception signal and the starting position of the diffusion code pattern.This is called synchronization.In the same period, there are things to be performed when the mobile phone terminal and the base station are first connected, and when the mobile phone terminal side moves and the base station is switched (handover).Here, we have been synchronized to make it easier.

In this figure, only the Sign function is used to check the multiplication, addition, and numerical signs.It looks complicated, but the operation is surprisingly easy.For example, the "diffusion result" of the "sender" is the result of the above two cells.The "synthetic signal" of the "radio signal" is a added result of channel 1 and 2.The "collation processing with the diffusion code" of the "receiving side" is a multiplication of the same bit of the spread code pattern (the top table in the figure) and the "signal with noise" in the "radio signal".The cell on the right is only a horizontal sum (SUM function).The "integration processing" is a great name, but it only combines the value of the cell above.

The mechanism is simple, but in this principle, many terminals can be connected to one base station at the same time.It is as if you are looking at some tricks that multiple communications can be separated just by adding them with the diffusion code.

The story of this title is the original title "Code of the Lifemaker" of J. Hogan's "Creator's Rule".In this work, a "evolved" "life" appears until the civilization is held.CDMA was widespread in GPS, 2G, and 3G, but 4G was replaced with other technologies such as OFDM.While TDMA and FDMA are digital communication, the concept of analog -like technology, while digital data, can be said to have evolved into successive technologies such as OFDM.