Understanding FDD vs. TDD Microwave Systems

Don’t Get Burned with Half-Duplex Capacity Claims

It is finally time to uncover the mystery between FDD and TDD, specifically the importance of FDD as a key element in network performance.

FDD stands for Frequency Division Duplex, and TDD stands for Time Division Duplex. Both FDD and TDD are two spectrum usage techniques, both forms of duplex, used in mobile or fixed wireless broadband links. It is essential to these links that transmission can occur in both directions simultaneously so that data can flow downlink (DL) and uplink (UL) at the same time without any noticeable interruptions.

FDD needs two separate frequency bands or channels. A sufficient guard band needs to separate the transmitting and receiving channels, so they do not interfere with one another and guarantee clear and uninterrupted transmission. A large guard band does not impact capacity. The frequency allocation for the UL /DL capacity is predetermined based on the system needs so that it is the same in either direction. It is not possible to dynamically change capacity. Continuous transmission and high performance are guaranteed with FDD.

TDD systems use a single frequency band for both transmit and receive. A system shares the same band and assigns alternative time slots for transmit and receive operations. Any data that is transmitted could be 1 byte long or a frame of multiple bytes. Time slots could be dynamically allocated and variable in length based on network needs. A guard period is needed to ensure that UL and DL transmissions do not collide. Swapping capacities in UL/DL degrades the performance of the network.

What does this mean for actual capacity?

TDD radios claim capacity numbers in half-duplex, meaning the quoted numbers are half in one direction and a half in the other. So 1Gbps of capacity is actually 500Mbps transmit and 500Mbps receive since the bandwidth is shared in a TDD system.

FDD radios claim capacity numbers in full-duplex, meaning the quoted numbers are available in both directions. Therefore, 1Gbps of capacity is 1Gbps transmit and 1Gbps receive since the bandwidth has separated allocated frequencies for each direction in an FDD system.

What does this mean for latency?

Because TDD radios share the same spectrum for send and receive communications, latencies can be high and variable. Latencies will depend on the time allocation of the spectrum for send/receive and may also be dependent on the size of the packets. FDD systems, on the other hand, which have dedicated, unshared spectrum, have much lower and more predictable latencies.

Where does Aviat technology fit?

All Aviat’s radios use FDD technology to ensure high performance and uninterrupted transmission, which is important for critical business applications. When comparing radios and specifications, it’s important to understand the basic technology (FDD vs. TDD) to compare apples to apples from both a capacity and latency standpoint.

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