RFFDMA Vs TDMA: Key Differences Explained
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FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) are both multiple access techniques used for wireless communications in mobile computing devices such as smartphones, tablets, laptops etc. These techniques allow multiple users to share a common communication medium, such as a wireless channel or a wired link.
FDMA divides the available frequency spectrum into multiple non-overlapping channels in which each user is allocated a specific frequency channel for the entire duration of their communication. TDMA divides time into multiple time slots in which each user is allocated specific time slot for their communication.
What is FDMA?
In FDMA scheme, entire frequency spectrum is divided into multiple RF channels or carrier frequencies. Each RF carrier is allocated to different users for entire duration of their communication. Each user’s signal occupies its designated frequency channel without overlapping with other users’ channels. FDMA is commonly used in analog communication systems and early digital systems.
Example:
- In GSM system, entire frequency band of 25 MHz is divided into 124 RF carriers of 200 KHz bandwidth each.
- In Satellite communication system, entire transponder band of 500 MHz is divided into 24 channels having 40MHz bandwidth each (36 MHz useful BW, 4MHz guard band).
There are two main types of FDMA scheme used in satellite network: SCPC (Single channel per carrier) and MCPC (multiple channel per carrier). MCPC uses FDM or TDM as multiplexing scheme.
Following are the FDMA types:
- FAMA: The full form is Fixed Assignment Multiple Access. In FAMA, frequencies are pre-allocated to users or subscribers or VSATs.
- DAMA: The full form is Demand Assignment Multiple Access. In DAMA, frequencies are dynamically allocated based on requests.
What is TDMA?
In TDMA scheme, entire bandwidth is shared among different subscribers at fixed pre-determined or dynamically assigned time intervals (i.e. time slots). TDMA divides time into multiple time slots, and each uses is allocated a specific time slot for their communication. The user take turns to transmit or receive data within their assigned time slots and channel is shared among multiple users by dynamically allocating time resources.
TDMA uses TDM multiplexing technique. TDMA is widely used in digital communication systems, especially in cellular networks like GSM (2G) and some satellite communication systems. It is suitable for applications where users can share the same frequency band efficiently by utilizing different time slots, allowing more users to be served simultaneously.
Example:
In GSM each RF carrier is used by 8 users at different time slots. Total duration of 4.615 ms frame is divided among 8 time slots having 577 µs each on all the 124 channels. Figure depicts channel number 124.
Let us compare FDMA vs TDMA and mentions the difference between FDMA and TDMA techniques.
| Parameters | FDMA | TDMA |
|---|---|---|
| Full form | Frequency Division Multiple Access | Time Division Multiple Access |
| Principle | Divides frequency spectrum into multiple channels, each user is allocated dedicated channel for communication. | Divides time span into multiple time slots, each user is allocated dedicated slot for communication. |
| Resource allocation | Frequency based , users are assigned specific frequencies for entire duration | Time based, users are assigned specific time slots for duration of communication |
| Simultaneous communication | Possible using different frequency channels | Not possible, users take turns to communicate on their assigned time slots |
| Interference | More susceptible to interference especially in crowded channel environments | Better interference management since users only transmit in their unique dedicated slots reducing likelihood of collision. |
| Synchronization | Users need to synchronize to specific assigned frequency | Users need to synchronize to timing of their assigned slots |
| System flexibility | More rigid since each user is tied to specific frequency channel | More flexible as time slots can be allocated dynamically based on demand |
| Efficiency in handling users | It becomes less efficient when number of users increases. | It is efficient in handling large number of users due to dynamic allocation |
| Duplex communication | FDMA can operate in either half duplex or full duplex | TDMA can operate in full duplex mode |
| Guard spacing | Requires guard bands to separate channels | Requires guard intervals to avoid overlap |
| Handover complexity | More complex due to different frequency channel assignements | More straightforward if same time slots are used in neighboring cells |
| Implementation complexity | More complex | Less complex |
| Resource wastage | It is observed in FDMA systems if some channels remain under-utilized while others are heavily loaded. | It is not observed in TDMA as time slots can be allocated dynamically as per demand. |
| Examples | Analog AMPS (Advanced Mobile Phone System), Analog TV broadcasting | IS-136 (Digital AMPS), GSM (2G) etc. |
Conclusion
FDMA and TDMA techniques enable efficient utilization of the communication medium while allowing multiple users to communicate concurrently. FDMA provides continuous and dedicated access to channels where as TDMA allows users to share the same band by utilizing different time slots. The specific usage of these techniques depend on the requirements and constraints of the wireless communication system being deployed.
Both FDMA and TDMA schemes are used together in satellite communication and cellular mobile communication to offer several advantages, including increased system capacity, reduced interference between users and their ability to support large number of users across a wide coverage area. OFDMA (Orthogonal Frequency Division Multiple Access) utilizes both FDMA and TDMA principles simultaneously. OFDMA is a multi-user communication technique commonly used in modern wireless communication systems, particularly in 4G LTE and 5G cellular networks.
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