Interleaving Domain Multiple Access

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Interleaving Domain Multiple Access , or IDMA for short, is a method for signal and message transmission. It is a further development of the CDMA - multiplex transmission method for digital signal transmission.

development

Transmission of a data stream

Classic digital data transmission protects your data by channel-coding it accordingly . The code scheme is adapted to the channel accordingly. In order to be able to detect errors , so-called parity bits are added, which represent a kind of checksum for a certain area of ​​the data. If the data and the checksum do not match, the corresponding data are recognized as incorrect. If you arrange several checksums crosswise around a data block, you can localize individual errors within a block and thus also eliminate them. However, since data transmission links do not only interfere with individual bits , but usually several bits in a row, the data are redistributed in such a way that a disruption of a certain length disrupts the data stream in such a way that adjacent areas within the data block are never affected. A block disorder is thus divided into many individual disorders. The corresponding process is called scrambling or interleaving . Using special procedures such as B. Turbo codes , interleaving can be applied not only to blocks, but also to data streams. Using a maximum-likelihood - decoder such. B. the Viterbi algorithm, the original data stream can then be reconstructed.

Transmission of multiple data streams

Frequency division multiplexing

The oldest method for the transmission of multiple data streams is the frequency division multiplexing (FDM; Engl. Frequency-division multiplex ), wherein the data on a plurality of discrete frequencies are transmitted. This process has already been used with analog data ( telephone network ; radio network ; voice radio ).

Time division multiplexing

From time-division multiplexing (TDM, Eng. Time-division multiplex ) occurs when multiple stations their data sequentially and send a transmission medium to share. This technique is used all the time in our natural communication , for example when one participant speaks in a discussion while the others listen. In the wireless technology of the time division multiplexing has been achieved in that the radio equipment with a transmission button were provided, which had to be able to speak the transmitter operate.

Combined time-frequency multiplex method

With the introduction of digital cellular networks , processes were implemented in which both time and frequency multiplex are operated. To ensure that each transmission link within the system has an exclusive channel available, there is a control entity that assigns the individual senders and receivers their frequencies and time intervals in which they must send and receive. The current GSM networks work according to this procedure .

Spread band method

In military radio technology, modulation methods were required that could be hidden from the enemy . This made use of the fact that the noise level is a function of the bandwidth of a receiver and looked for ways to transmit the signal in a narrow band , but at the same time to spread it over a wide band. If you use a narrow-band signal and vary its frequency continuously by means of a "pseudo-random generator", you can follow the signal if the receiver knows the sequence of frequencies. If you do not know the spreading sequence, you can receive part of the signal within a narrow band for a short time, but not the rest. If you use a broadband receiver, the narrowband signals are not recognized in the noise. The signal using the spread spectrum method remains invisible.

Code Division Multiple Access (CDMA)

When using a data interleaver it turned out that it was possible to operate several transmitters with different spreading codes in the same frequency band without disrupting the data transmission. It is true that the individual transmitters transmit at the same frequency at times and thus generate error blocks. However, this disruption is so short that it is broken down into individual errors again by means of interleaving, which in turn can be corrected. If the probability that two transmitters are transmitting on the same frequency becomes too high, the errors increase and can therefore no longer be corrected.

Currently used CDMA procedures:

Further development to IDMA

In the past, CDMA and TD-FD systems were developed in parallel. In CDMA systems, the main focus was on the spreading method, while in TD-FD systems emphasis was placed on the error correction mechanisms . IDMA is the consistent merging of both lines of development . With CDMA, the data was saved by means of interleaving and then spread. In TD-FD systems, the entire signal was protected by means of interleaving. IDMA is now following the step of protecting the entire signal by means of interleaving. The interleaving is now extended to the chip level. This reduces the influence of other data channels, since the chip level is now also protected. Further investigations have shown that it is possible to replace the pseudo-random sequences (gold codes) with different interleaving patterns. This has resulted in the new name Interleaving Domain Multiple Access .

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