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Thursday, 9 September 2021

Satellite Multiple Access Techniques

MULTIPLE ACCESS TECHNIQUES IN SATELLITE COMMUNICATION NOTES


Mobile users share the wireless spectrum at the same time. The sharing might be based on frequency, time, or code. Wireless telephony employs duplexing technology, which permits talking and listening at the same time, similar to FDD frequency. Division Duplexing provides each user with two bands of frequencies. Because a simplex channel is made up of a forward and a reverse channel, each duplex channel is made up of one simplex channel.


The forward and reverse channels are separated by a frequency range across the system. TDD, or time division duplexing, uses time instead of frequency to have identical forward and reverse channels.


Both FDD and TDD have advantages and drawbacks. In wireless communication systems, these multiple access methods utilizing any of these approaches are utilized.


DIFFERENT MULTIPLE ACCESS TECHNIQUES:


THE THREE important multiple access techniques are:


1 TIME DIVISION MULTIPLE ACCESS

2 FREQUENCY DIVISION MULTIPLE ACCESS

3 CODE DIVISION MULTIPLE ACCESS


Time Division Multiple Access (TDMA)


The time slots of the whole available time are shared between several people. To achieve bidirectional communication, each duplex channel (TDD) includes distinct forward and reverse time slots. The figure represents a TDD time slot with the same frequency:

 

This TDD mode allows any transceiver to function as both a transmitter and a receiver. TDD is commonly seen in cordless phones. The TDMA concept is shown. In a TDMA system, the entire spectrum is split into time slots, with each time slot allowing just one user to utilize the radio channel.


Digital data transmission and digital modulation are permitted in TDMA. Many customers can tune in to their favorite station within their designated time intervals. As demonstrated, several users' transmissions are interlaced into a single frame. 

The synchronization information and address information are included in the preamble field in this way. The guard bits are used to allow separate receivers to be synchronized across different frames and time periods.


It is believed that there are N time slots for N users, with each user accessing the channel during their allotted time period.


The following are some of the characteristics of TDMA: 1)Each user of the TDMA multiple access systems has the same carrier frequency, but no time periods overlap.


Frequency Division Multiple Access in Mobile Communication


All users share the satellite at the same time using FDMA, but each broadcasts in its frequency band. When using analog modulation, when signals are present all of the time, this is the most typical technique used. As shown in the diagram, the available transponder bandwidth is shared among the users, allowing them to all broadcast at the same time.


The most basic type of multiple access is FDMA. It assigns a carrier frequency (or many carrier frequencies for busy stations) and a bandwidth around that carrier frequency to an earth station permanently. All of the station's outgoing traffic is frequency modulated on that carrier, regardless of destination.


Each carrier is allocated a specific frequency band for the uplink in fixed-frequency operation, and other carriers share that band. Several carriers share frequency bands for demand multiple access (DMA), with a specific band assigned based on availability at the time of need.


In an FDMA system, each carrier spectra must be sufficiently separated from one another to avoid carrier crosstalk. The nearby low-power carrier will be affected by a high-power carrier. Excessive separation, on the other hand, causes satellite bandwidth to be squandered. For accessing, FDMA uses extremely easy frequency adjustment and provides virtually independent channel off operation.


A satellite's frequency plane is made up of the carrier frequencies and bandwidth allotted to all ground stations. Every FDMA station must be able to receive at least one carrier from all of the other stations in the network.


Bidirectional communication is feasible in conversational telephonic systems at the same time, and it is also necessary for cellular communication. A duplexer is a device that allows you to talk and listen at the same time. Frequency multiplexing/duplexing is referred to as frequency division duplexing (FDD) since it is done with frequency.


The duplex channel in FDD has two simplex channels, forward and reverse, as shown in the diagram. The forward frequency band transmits radio traffic from the base station BTS to the mobile unit, whereas the reverse frequency band transmits radio traffic from the mobile unit to the base station BTS. To enable simultaneous conversion, a duplex device is kept in both the mobile unit and the base station.


Some features of the FDMA scheme:


1) To eliminate adjacent channel interferences ACI, FDMA requires appropriate filtering on the receiver side.

2) In the FDMA scheme, if a channel is not in use, it is considered idle and is not used by other users. As a result, there is a risk of resource waste.

3) A single phone circuit can be handled by an FDMA channel at a time.


Types:


1) fixed assignment multiple access FAMA

2) Demand assignment multiple access DAMA


Non-linear effects in FDMA SCHEME OF MULTIPLEXING:


Multiple radio channels share the same antenna at the base station in this system. For optimum power and efficiency, power amplifiers are run at saturation, and they are non-linear. Intermodulation frequency production is caused by the dispersion of signals throughout the whole frequency domain. It will amplify interferences in the actual transmission, therefore IM should be kept to a minimum.


Code Division Multiple Access (CDMA)


Many earth stations broadcast orthogonally coded spread-spectrum signals in the same frequency range at the same time via CDMA. Decoding systems receive several stations' mixed broadcasts and retrieve one. Users can broadcast at the same time and share the frequency allotment under the CDMA system. The diagram represents the block diagram of the CDMA System.

 

Several users utilize the whole transponder bandwidth in this system at all times. Signals from various users are encoded so that information from a single transmitter can only be recognized and retrieved by a correctly synchronized receiving station that understands the code. That is, each receiving station has its code, referred to as its address, and anytime a transmitting station intends to send a message to that receiver, it simply modulates its broadcast with the desired receiver's address.


At an earth station, carrier separation is accomplished by identifying the carrier with the correct address. These addresses are often in the form of a periodic binary sequence that modulates the carrier or changes the carrier's frequency state. The carrier correlation procedure is used to identify addresses. For code generators, a digital address is obtained.


A station address generator repeats its address sequence, which is overlaid on the carrier alongside the data.


CDMA is better suited to military tactical communication environments where numerous small groups of mobile stations are only connected for a short period at irregular intervals.

There may be an issue of near-far impact since the same channel is used by multiple users. When compared to other multiple access methods, the major advantage of CDMA is the lower degree of interference. At the receiving end, the receiver selectively adjusts to hearing the indented signal of the users since each user/subscriber is assigned a distinct pseudo-random codeword that is orthogonal to all other pseudo-random codewords of remaining users. To avoid near-far issues, CDMA uses proper power control methods.


Some features of CDMA multiple access schemes:


1) In CDMA, if the spreading sequences are not perfectly orthogonal from one user to another, there is a risk of self-jamming. As a result, this spreading sequence or pseudo-random noise code must be meticulously prepared before being multiplied by the message signal.

2) Compared to TDMA and FDMA, CDMA has a higher soft capacity limit.

3) The RSSI (radio signal strength indicator) is used in CDMA to improve power control.


Types


1) Fixed assignment multiple access

2) Demand assignment multiple access

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