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Multiple Access: Definition and It’s Types
Multiple Access is positive as the detachment of a communication channel through local combination at a common point. In several cases, however, the communication channel must be efficiently shared among numerous users who are geographically detached and who attempt sporadically to communicate at random times. Three systems have been developed to share a single channel under these conditions efficiently. These are known as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). These skills can be used alone or together in telephone systems and are well illustrated by the more forward-thinking mobile cellular organisms.
Frequency-Division Multiple Access
In FDMA, the goal line is to split the regularity spectrum into slots and then distinct the signals from different users by placing them in separate frequency slots. The trouble is that the frequency spectrum is limited and that there are typically many more potential communicators than available frequency slots. To use the communication channel efficiently, a system must be developed to manage the available slots. In the Progressive Mobile Phone System (AMPS), the cellular system used in the US, different callers use separate frequency slots over FDMA.
When a phone call is completed, a network management computer in the mobile base station assigns the freed frequency slot to a new caller. An essential goal of the AMPS system is to reuse the frequency slots whenever possible to accommodate the most significant number of callers. Frequency slots can be reused locally within a cell when corresponding calls are terminated. Additionally, the frequency slots can be used simultaneously by multiple callers in separate compartments. The batteries must be geographically far enough apart that the radio pointers from one cell are sufficiently tempered at the location of the other cell using the same frequency slot.
Multiple Access Time-division
In TDMA, the goal is to share time into time slots and to separate signals from changed users by placing the messages in single time slots. The trouble is that requests to use a single communication channel occur randomly so that sometimes the amount of applications for time slots is higher than the number of available time slots.
In this case, the information must be buffered or stored until time slots are available to transmit the data. The buffering causes a delay in the system. In the IS54 cellular order, three ordinal signals are interleaved using TDMA and then spread in a 30-kilohertz frequency slot that would be occupied by an analogue signal in AMPS. The buffering and temporal interleaving of digital messages creates an additional delay, but it is so short that it is generally invisible during a call. The IS54 system uses aspects of TDMA and FDMA.
Code Division Multiple Access
In CDMA, signals are sent simultaneously on the same frequency band. Signals are selected or rejected at the receiver by recognizing a custom signature waveform that is built from an assigned spreading code. The IS95 cellular system uses CDMA technology. In IS95, an analogue voice signal to be sent to a cell site is first quantized and then organized into one of several digital frame structures. In a frame structure, a frame with a duration of 20 milliseconds contains of 192 bits. Of these 192 bits, 172 represent the talking motion itself, 12 form a cyclic dismissal check that can be used for error discovery, and 8 forms an encoder “tail” that allows the decoder to function correctly.
These bits are formed into an encoded data stream. After interleaving the encoded data stream, the bits are arranged in sets of six. Each group of six bits indicates which of the 64 possible waveforms will be transmitted. Both of the waveforms that are transmitted has a specific pattern of alternating polarities and occupies a particular part of the high-frequency spectrum.
However, before either waveform is sent, it is multiplied by a code sequence of alternating polarities at a rate of 1.2288 megahertz, thus extending the bandwidth occupied by the signal and approximately 1.23 megahertz (after being filtered at the transmitter) employed by the high-frequency spectrum. At the cell site, a user can be selected from among multiple users with the same 1.23 megahertz bandwidth based on the assigned code sequence.
CDMA
CDMA is sometimes called spread spectrum multiple access (SSMA) because by multiplying the signal by the code sequence, the power of the transmitted signal is spread over a more full bandwidth. Frequency management, a necessary feature of FDMA, is removed in CDMA. When another user wants to use the communication channel, it is assigned a code and sent immediately, instead of being stored until a frequency slot is opened.