RFFDD vs TDD: Understanding the Key Differences in Wireless Communication
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FDD (Frequency Division Duplex) and TDD (Time Division Duplex) are fundamental concepts used in frame structures of mobile wireless communications and other telecom networks. FDD employs two separate frequency bands for uplink and downlink, while TDD uses the same frequency band for both, but at different time instants or slots.
These approaches are widely implemented in wireless communication systems such as WLAN, Fixed and Mobile WiMax, 4G LTE, 5G NR, and more.
Let’s delve into a comparison of FDD vs TDD to understand the key differences between these two methodologies used in mobile wireless communication.
What is FDD?
In FDD, the available frequency spectrum is divided into two distinct bands: one for transmitting data and the other for receiving data. These bands are separated by a guard band to prevent interference.
As illustrated in the figure, two different frequency bands, Fc1 and Fc2, are used by the transmit and receive paths simultaneously.
To better understand FDD, consider the communication between a base station (BS) and a subscriber station (SS) in a cellular wireless system.
In FDD, both uplink and downlink transmissions are assigned separate frequencies, Fc1 and Fc2, respectively. Both utilize the same time slot (‘t1’) to transmit. Uplink refers to transmissions from SSs to BS, while downlink refers to transmissions from BS to SSs.
In FDD, transmit and receive operations occur simultaneously on different frequencies, providing a constant and continuous data transmission experience. However, FDD systems generally require more power compared to TDD systems due to the need for separate hardware RF modules for transmit and receive chains operating at different frequencies.
What is TDD?
In a TDD system, the same frequency band FC is used by both transmit and receive paths, but at different time instants. The communication channel switches rapidly between transmitting and receiving modes, with devices taking turns transmitting and receiving during specific time intervals.
TDD dynamically divides the available time into alternating time slots for both uplink and downlink transmission. The key advantage of TDD is its flexibility in allocating resources based on varying traffic demands. However, it requires careful synchronization to avoid interference between devices.
As shown in the figure, uplink and downlink transmissions are arranged sequentially on the time scale. For example, uplink is transmitted at ‘t1’ and downlink at ‘t2’, where t2 equals t1 plus some time duration. Both uplink and downlink transmissions occur at the same RF carrier frequency (Fc).
TDD systems are typically cheaper than FDD systems due to the requirement of fewer RF modules, such as synthesizers, local oscillators, and filters. The MAC layer in a TDD system is more complex compared to an FDD system because it must handle accurate time synchronization between the transmit and receive systems.
TDD is often favored over FDD for advanced antenna techniques like beamforming and AAS (Adaptive Antenna System) due to channel reciprocity between the uplink and downlink paths.
Difference between TDD and FDD
The following table highlights the differences between TDD and FDD topologies.
| Parameters | FDD | TDD |
|---|---|---|
| Full Form | Frequency Division Duplexing | Time Division Duplexing |
| Spectrum Allocation | Uses separate frequency bands for uplink (UL) and downlink (DL). | Uses the same frequency band for both uplink and downlink but allocates different time slots for them. |
| Guard band | Requires to prevent interference between UL and DL | Not required as time slots separate transmission and reception |
| Simultaneous operation | Possible as it can transmit and receive on separate frequencies. | Not possible; it alternates between transmit and receive using unique time slots. |
| Synchronization | FDD requires less complex synchronization between devices since they operate on separate frequencies. | TDD requires precise synchronization between devices to ensure accurate switching between transmit and receive slots |
| Hardware complexity | FDD requires specific hardware for both uplink and downlink | TDD often uses the same hardware for both transmit and receive, which simplifies design. |
| Channel asymmetry | FDD is more suitable for asymmetric traffic in DL and UL | TDD can handle channel asymmetry more effectively due to its dynamic time slot allocation |
| Frequency planning | FDD requires careful frequency planning | TDD requires less strict frequency planning |
| System latency | FDD may have slightly lower latency due to simultaneous transmit and receive operations | Latency in TDD can be influenced by time slot duration |
| Interference management | Less prone to self interference due to separate frequencies | Requires careful coordination to avoid interference between devices sharing the same band |
Conclusion
Both FDD and TDD offer unique advantages and are suitable for different scenarios. LTE systems often utilize both in their frame structures to harness the benefits of both topologies.
