Table of Contents
Multiple Access Techniques
Multiple Accessing is also called Multiple Destinations because the transmission from each earth station is received by all other earth stations in the system. Multiple Accessing implies that more than one user has access to one or more radio channels within a satellite communication system. It is known as Multiple access techniques.
Need of Multiple Access Techniques
- Efficient use of spectrum: Multiple Access Techniques enable multiple users to share the available frequency spectrum without causing interference to each other.
- Support for Multiple Access: In wireless communication, there are often many users who need to access the network simultaneously. Multiple access techniques make it possible for multiple users to transmit and receive data at the same time.
Types of Multiple Access Techniques
In multiple-accessing systems, the method by which a satellite transponder bandwidth is used depends on the multiple accessing methods used. There are three Basic techniques used in multiple accessing and these are:
- FDMA: Frequency Division Multiple Access
- TDMA: Time Division Multiple Access
- CDMA: Code Division Multiple Access
1. Frequency Division Multiple Access (FDMA)
In FDMA, each earth station’s transmitter is assigned specific uplink and downlink frequency bands within an allotted satellite bandwidth which may be, preassigned or demand-assigned. FDMA transmissions are separated in the frequency domain and it is therefore must to share the total available transponder bandwidth as well as the total transponder power.
Advantage of FDMA
- All stations can transmit continuously without waiting their turn.
- The system is not over complicated.
- It efficiently handles smaller groups of users.
Disadvantage of FDMA
- Does not differ significantly from analog systems.
- Require the right RF filtering to minimize adjacent channel interference.
2. Time Division Multiple Access (TDMA)
In TDMA, each earth station transmits a short burst of information during a specific time slot within a TDMA frame. A specific time slot burst is named an epoch. It is a must that bursts are properly synchronized, this is essential so that each station burst arrives at the satellite at a different time. In TDMA it is possible to use the entire transponder bandwidth and power for each transmission but it is done only for the prescribed period.
Advantages of TDMA
- It effectively handles both voice communication and data transmission needs.
- It supports services like video band data, SMS, multimedia applications, and video conferencing.
- It provides the user with extended battery life and talk time.
Disadvantage of TDMA
- Each user has a predefined time slot. When moving from one cell to another, if all time slots in this cell are full the user might be disconnected.
3. Code Division Multiple Access (CDMA)
In CDMA, all earth stations can transmit within the same frequency band and there is no limitation at what time or on which carrier frequency they are to be transmitted. In CDMA, the complete transponder bandwidth is used by all stations continuously. In this case, signal separation is carried out with envelope encryption/decryption technique.
Advantages of CDMA
- The CDMA increases the security of cellular communication networks.
- The CDMA supports simultaneous conversations.
- CDMA handsets are smaller in size.
Disadvantage of CDMA
- Higher complexity of a receiver ( receiver can not fast listen into the medium and start receiving if there is a signal ).
Comparison between FDMA, TDMA, CDMA
| Sr No. | Parameter of Comparison | FDMA | TDMA | CDMA |
|---|---|---|---|---|
| 1. | Bandwidth utilization | Single channel per carrier – SCPC | Multiple channel per carrier – partial allocation. | SCPC, partial or full allocation. |
| 2. | Interference rejection | Limited | Limited with frequency hopping. | Can suppress interference up to noise limit. |
| 3. | Can provide capacity improvement through hopping | Most sensitive | Less sensitive | Least sensitive |
| 4. | Doppler frequency shift | Bandwidth limiting | Burst time limiting | Removed by receiver |
| 5. | Spectrum flexibility | Intermodulation effect | Moderated bandwidth use per carrier | Capacity interminate due to unloading unknown |
| 6. | Capacity | Basic capacity available | Largest demand for contiguous | Multiple channels per carrier – partial allocation. |
| 7. | System requirements | Frequency band assignment | Time slot assignment network synchronization | PN code assignment |
| 8. | Uses the least bandwidth carrier | No | No | Yes, With graceful degradation |
| 9. | Transmit power | Average power | Peak power | Average power |
| 10. | Multipath resistance | Vulnerable | Vulnerable | Inherent |
Frequently Asked Questions (FAQs)
What is the FDMA technique?
It is a Frequency Division Multiple Access. Which spectrum is divided in the form of frequency which is assigned to the user for the whole time.
What is the TDMA technique?
It is defined as Time Division Multiple Access in which time slots are provided to the user. Users can communicate during their turn only.
What is the CDMA technique?
It is Code Division Multiple Access. It is also called spread spectrum access. Transmission can be spread throughout the entire bandwidth. The user can transmit it whenever they want.
What is the purpose of multiple access?
Multiple access like multiplexing, involves sharing communication resources between several users that broadcast their transmissions so that more than one other user may receive them.
Related Posts
- Internet of Things | Functional Block of IoT, Characteristics and Application
- SCADA | Types, Block Diagram and Functions of SCADA System
- 8085 Architecture | Exploring Its Different Units
- What is PLC | Building Block of PLC and their Functions
- UPS | Types | Comparison Between Online and Offline UPS
- Pin Diagram of 8051 Microcontroller
Leave a Reply