FDD vs TDD in a Phone: Duplexer or Switch at the Antenna?

Two ways to share one antenna

A phone has one antenna path per band, but it must both send and receive. There are two basic ways to let the transmitter (TX) and receiver (RX) coexist.

• FDD (frequency-division duplex): TX and RX operate at the same time on different frequency bands.
• TDD (time-division duplex): TX and RX use the same frequency band but take turns in time.

Everything about the front-end hardware follows from this one difference. In FDD the two signals are separated by frequency, so a filter can tell them apart. In TDD they are separated by time, so a switch can do the job.

FDD: the duplexer

A duplexer is a three-port filter device that lets a transmitter and receiver share one antenna in FDD. The three ports are:

1. the antenna port, shared by both directions;
2. the TX port, which takes the power amplifier output;
3. the RX port, which feeds the receiver input.

Inside, the TX side passes the transmit band and rejects the receive band. The RX side does the reverse. Because the two bands are different, each filter can reject the other direction's signal.

The key requirement is isolation. In FDD the strong transmit signal leaks toward the receiver, so the duplexer must provide high transmit-to-receive isolation. The receiver is built to handle very weak signals. The transmitter is many orders of magnitude stronger. If too much TX power reaches the RX port, the receiver front end can be overloaded or desensitized.

Isolation in dB is the difference between the TX power applied and the leakage at the RX port:

So the leakage level is:

In S-parameter terms, with the antenna matched, this is the transmission from the TX port to the RX port, expressed as a loss. The picture below shows the two-port version of the idea. A duplexer has three ports, so it has a 3 × 3 set of parameters instead of 2 × 2, but each pair reads the same way.

TDD: the switch

In TDD a switch can connect the antenna alternately to the transmit path and the receive path. While the phone transmits, the switch connects the antenna to TX. While it receives, the switch connects the antenna to RX.

The switch does not need to separate two signals by frequency, because only one path is active at any moment. The same band is used both ways, so one band-limiting filter can serve both paths. A switch has a lower insertion loss burden than a pair of duplexer filters in concept, because it is not a filter with a rejection requirement at a nearby band. Even so, the switch still adds some insertion loss on the active path, and that loss matters on the receive side, where it adds directly to the noise figure.

Why a switch is not enough for FDD

The common mistake is to assume that a good switch gives enough isolation for FDD. It does not, for two reasons.

The transmitter is on while the receiver listens. In TDD, a switch that connects the antenna to TX also disconnects RX. Its isolation only has to protect the receiver from a transmitter that is part of the same time slot plan. In FDD both paths are connected to the antenna at the same time. A switch cannot be in both positions. If it were used to join them, it would act as a combiner, not as a separator.

A switch has no frequency selectivity. A duplexer rejects the TX frequencies at the RX port by filtering. A switch only offers a finite amount of isolation between its open and closed throw, and that isolation is the same for any frequency in its range. It cannot reject the transmit band while passing the receive band, which is what continuous FDD operation requires.

Filter response: what selectivity looks like

A duplexer is built from band-pass responses, but the idea of passband and rejection is easier to see on a simple low-pass example. The plot shows S21 (transmission) and S11 (reflection) of an ideal 5th-order Butterworth low-pass filter with a 1 GHz cutoff. It is a computed ideal response, not measured data. Below cutoff, S21 is near 0 dB. Above cutoff, S21 falls steeply. A duplexer arm needs that same kind of fall, placed at the other direction's band.

For the difference between insertion loss in the passband and the rejection outside it, see Return Loss vs Insertion Loss: What Each Number Tells You.

Worked example: leakage at the RX port

The numbers below are assumed for illustration only. They are not data for any real part.

Assume 20 dBm of TX power enters the TX port. Assume the receiver can tolerate at most −30 dBm at its input.

Required isolation:

I = P_TX − P_leak = 20 − (−30) = 50 dB

Now suppose a switch with an assumed 25 dB of isolation were used. The leakage would be:

P_leak = 20 − 25 = −5 dBm

That is 25 dB above the assumed −30 dBm limit. In power terms, 25 dB is a factor of 10^(25/10) ≈ 316. And that leakage is present continuously, because in FDD the transmitter never stops while the receiver listens. A duplexer with at least 50 dB TX-to-RX isolation meets the assumed limit.

Summary of hardware

	FDD	TDD
TX and RX in time	Simultaneous	Alternate
TX and RX in frequency	Different bands	Same band
Antenna sharing element	Duplexer (three-port filter device)	Switch
Isolation comes from	Frequency filtering	Switch position

In plain words · for students and beginners

Duplex operation in plain words

An everyday analogy

Think of a phone call. In FDD, you and the other person talk at the same time, but you each use a different pitch, like one low voice and one high voice. A good listener can pick out one voice from the other. In TDD, you use the same voice but take turns: you talk, then you listen.

The analogy breaks down in one place. Real people can ignore their own voice. A receiver cannot. Its own phone's transmitter is far louder than the signal it wants to hear, so the hardware must work to keep that loud signal out.

The same example, in simple steps

The numbers are made up to show the idea.

1. The transmitter sends 20 dBm. dBm is just a way to write power on a log scale.
2. The receiver can only accept up to −30 dBm without trouble.
3. The gap is 20 − (−30) = 50 dB. So we need 50 dB of isolation.
4. A switch with 25 dB of isolation would let through 20 − 25 = −5 dBm.
5. That is far too much. A duplexer that gives at least 50 dB is needed.

Mistakes beginners make

• Thinking a switch is a small duplexer. A switch picks one path at a time. A duplexer lets two paths work at the same time by using filters.
• Mixing up FDD and TDD. FDD means two different frequencies at once. TDD means one frequency, taking turns.
• Forgetting that isolation is a ratio. It is measured in dB, and bigger is better. A small number means a lot of leakage.

Remember

• FDD: TX and RX are on together at different frequencies, so use a duplexer with high TX-to-RX isolation.
• TDD: TX and RX share one band and take turns, so a switch can connect the antenna to one path at a time.
• A switch cannot replace a duplexer in FDD, because it has no frequency selectivity and both paths are active at once.

Related pages on RF View

• Return Loss vs Insertion Loss: What Each Number Tells You