Full duplex vs half duplex over RF

A recent comment on these forums and an ongoing thread in another forum got me thinking about people’s perceptions / interpretations of full vs half duplex. An online web site describing a typical FM repeater as “full duplex” because it receives on one frequency and transmits on another raised further questions of how deep this issue goes.

I know what I was taught in school, and I know how industry has used the definition of full duplex. But there seems to be some documented discrepancies to that.

So, what is your description of full duplex on radio?

T!

(edit) I am talking about traditional data / voice comms over RF, not the new fangled techniques for single frequency full duplex RF operations.

communicating simultaneously

Full duplex

Here is my perspective on full duplex vs. half duplex.

Full duplex refers to simultaneously transmitting and receiving. For traditional radio technology it means use of two separate frequencies: one to receive and one to transmit. A typical radio repeater uses full duplex. It receives on frequency f1 and simultaneous transmits on frequency f2.

Half duplex refers to communication where you (your radio) either transmit or receive, but not simultaneously. A typical two-way radio works in a simplex or half duplex mode of operation. The radio receives and transmits on one single frequency. It alternates between transmitting and receiving; can't do both at the same time.

Jim41

Jim41 said:

Here is my perspective on full duplex vs. half duplex.

Full duplex refers to simultaneously transmitting and receiving. For traditional radio technology it means use of two separate frequencies: one to receive and one to transmit. A typical radio repeater uses full duplex. It receives on frequency f1 and simultaneous transmits on frequency f2.

Half duplex refers to communication where you (your radio) either transmit or receive, but not simultaneously. A typical two-way radio works in a simplex or half duplex mode of operation. The radio receives and transmits on one single frequency. It alternates between transmitting and receiving; can't do both at the same time.

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Simultaneous transmitting and receiving is only part of the requirement for full duplex, the other part is to be capable of multiple parties simultaneously (can't communicate with a party of one).

An FM repeater typically cannot carry two conversations at one time. A modulation mode independent translator may be able to.

Taken to extremes, your basic simplex radio does exactly the same thing as a repeater, it receives one frequency (audio frequency, your voice) and transmits it at the same time on another (RF frequency). In the other direction it receives on one frequency (RF) and transmits on another (AF), simultaneously.

T!

Simplest terms, if you push the button to talk and cannot hear the other party attempting to talk to you without releasing the button, you are not in full duplex mode.
I spent too many years installing and servicing RCC to let a semi-duplex repeater system be called full duplex.

The base on a semi duplex if wire-tied to the repeater/base can hear if the mobile attempts break-in.
Most emergency communications will use this type of semi-duplex operation.

Split frequencies are often seen in Taxi comms. The mobiles transmit on one freq (can not hear while transmitting) and the base (usually wire-lined) transmits on a separate frequency similar to a repeater/base however the mobile units cannot hear anyone but the base.

The cost of a duplexer in the mobile is usually prohibitive for most agencies to try to go full duplex. The size of the duplexer makes hand-held operation on any but microwave freqs pretty clumsy.

Cell phones are a good example of full duplex operation. A hand-held on the local club repeater is a good example of semi-duplex.

and that is worth exactly what you paid for it ;-)

Bruce

Token said:

Simultaneous transmitting and receiving is only part of the requirement for full duplex, the other part is to be capable of multiple parties simultaneously (can't communicate with a party of one).

An FM repeater typically cannot carry two conversations at one time. A modulation mode independent translator may be able to.

Taken to extremes, your basic simplex radio does exactly the same thing as a repeater, it receives one frequency (audio frequency, your voice) and transmits it at the same time on another (RF frequency). In the other direction it receives on one frequency (RF) and transmits on another (AF), simultaneously.

T!

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Slow down a minute and re think what you just said. Your normal radio might be able to transmit and receive on different frequencies to use a repeater. But that mobile or portable radio can only do one at a time. It either can transmit or it can receive. It can not do both at the same time. This is simplex operation.

Duplex operation is being able to receive and transmit at the same time on different frequencies. This is just like talking on your home telephone. It is rare to find a radio package that is full duplex. Back in the days of MTS (mobile telephone service) years ago, you did have the ability to run full duplex. Don't know of any service that is operating like that any more.

If you really pay attention how a cell phone works, you will find that only one person can talk at a time. It is not a true full duplex mode of operation. Only one person can talk at a time.

The only full duplex radio operation that I knew of was the old IMTS (Improved Mobile Telephone Service) that was available years back on VHF and UHF. Normally it was originally supplied by the telephone company. Those frequencies have mostly been sold off and used as paging frequencies. Some were then sold again for different services.

jim202 said:

The only full duplex radio operation that I knew of was the old IMTS (Improved Mobile Telephone Service) that was available years back on VHF and UHF. Normally it was originally supplied by the telephone company. Those frequencies have mostly been sold off and used as paging frequencies. Some were then sold again for different services.

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Add RCC to that. Back in the early 1970's, I worked at Harris/RF on their VHF/UHF products. We built both IMTS and RCC radios. The radios themselves were virtually identical, but control heads and frequencies were slightly different. The frequency blocks were close enough together that we used the same duplexers (Sinclair) in both units.

I think full duplex in the 900mhz band or higher would be pretty sweet! I'm going to look in to this..

thought yall might find this interesting.
i sure do.

A team of Columbia Engineering researchers has invented a technology—full-duplex radio integrated circuits (ICs)—that can be implemented in nanoscale CMOS to enable simultaneous transmission and reception at the same frequency in a wireless radio. Up to now, this has been thought to be impossible: transmitters and receivers either work at different times or at the same time but at different frequencies. The Columbia team, led by Electrical Engineering Associate Professor Harish Krishnaswamy, is the first to demonstrate an IC that can accomplish this. The researchers presented their work at the International Solid-State Circuits Conference (ISSCC) in San Francisco on February 25.

full article @ New technology may double radio frequency data capacity

jim202 said:

Duplex operation is being able to receive and transmit at the same time on different frequencies....It is rare to find a radio package that is full duplex.

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I'll agree that it is rare to find a radio package that can receive and transmit at the same time on different frequencies in the same band. But, radios that can do that on different bands are fairly common.

Quote" 'll agree that it is rare to find a radio package that can receive and transmit at the same time on different frequencies in the same band. But, radios that can do that on different bands are fairly common. Quote

Not really you hold one in your hand every day, your cell phone is one.

Full duplex is like cell phones or even the old IMTS VHF/UHF mobile phone system. Both base station and mobiles require duplexers to have a true duplex system.

While I havvn't seen the documentation, but what I think Columbia Engineering has done is more on the order of rapid switching of transmit and receive like radar.More than likely a digital format, but just in a lower band, as physics wouldn't allow anything else.

jim202 said:

Slow down a minute and re think what you just said. Your normal radio might be able to transmit and receive on different frequencies to use a repeater. But that mobile or portable radio can only do one at a time. It either can transmit or it can receive. It can not do both at the same time. This is simplex operation.

Duplex operation is being able to receive and transmit at the same time on different frequencies. This is just like talking on your home telephone. It is rare to find a radio package that is full duplex. Back in the days of MTS (mobile telephone service) years ago, you did have the ability to run full duplex. Don't know of any service that is operating like that any more.

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You responded to me, but I think you were thinking of someone elses comment. I was not saying a normal ham radio, or even a traditional FM repeater, was full duplex.

What I was saying was in response to Jim41, when he said a repeater was full duplex because it can receive (on the input) and transmit (on the output) simultaneously. My point was that simply transmitting and receiving at the same time is not full duplex, although at RF that has traditionally been part of the solution to get to full duplex.

I used the extreme example of a regular simplex radio, it receives data at audio frequencies and simultaneously transmits it at RF frequencies, this is simply cross band repeat at the extreme; receive on one set of frequencies while simultaneously transmitting on another. The fact that one set of frequencies is a sound wave and the other set is an electromagnetic wave does not change the basic function. Despite it receiving and transmitting simultaneously it still only supports one part of one conversation at a time and is not full duplex.

Discussions similar to his response, that a traditional FM repeater is full duplex because it receives on one frequency while simultaneously transmitting on another, are part of the reason I started this thread. It is not the first time I have heard this, sometimes from very knowledgeable people. But it does not fit what I was taught as full duplex, lacking the simultaneous capability of two way communications.

T!

n0nhp said:

Simplest terms, if you push the button to talk and cannot hear the other party attempting to talk to you without releasing the button, you are not in full duplex mode.

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^^^^^^^ This.


I have always said it as "if you do not have the ability to hear the other person while you are talking, or they do not have the ability to hear you while they are talking, you are not full duplex".

T!

Interesting discussion.

If a radio had dual receive and dual transmit wouldn't duplex be possible? Basically two radios stuffed into one box. Listen on one frequency and transmit on another. Then you could talk and listen at the same time.

Maybe since it's actually two radios on different frequencies it wouldn't qualify as "a" duplex radio?

Brasso, that would be full duplex. A repeater is a full duplex radio, it receives on one frequency and transmits on another at the same time. Some of them, in fact most of them, use a device called a 'duplexer' so that they can use the same feed line and antenna.

Digital muddies the water somewhat...

"Full duplex" is a brain perception more than it is a system topology.

In full duplex, you are communicating with someone else - or a group of people - and all of them can talk and listen at the same time, providing and demanding instantaneous feedback and interaction.

Back in the day, that was accomplished with transmitters always transmitting and receivers always receiving, along with duplexers or helical resonators filtering out the other signal. In radio systems, this was almost always one person to another person, usually through the public switched telephone network.

Years ago, I worked with a guy in two-way who said it was possible to do full duplex on a simplex frequency. I was like, "Bull...!" but he was an engineer and he knew about the technologies that wouldn't emerge until very recently. It is possible for a vocoder to constantly sample your voice, break it up into data bursts and transmit it in between receiving data bursts from a base or other mobile - even on a single frequency. In practice they are usually done with a talk-in and talk-out frequency pair, but modern cellular, Nextel, DMR, TETRA, and OpenSky are all like that. They transmit bursts in a timeslot (or in time and frequency domain) and receive synchronized with the base.

Your perception, though, is still people being able to do full break-in to your speech, even though the radio transmits and receives in alternating bursts - it's never transmitting and receiving at the same time.

Full duplex also demands more attention from your brain than semi-duplex or simplex does. In full duplex, people say something and expect interaction right away. They demand you give them priority regardless of what you're doing (like driving or walking through the crowd in a supermarket). That's why researchers have equated driving while cellphone - with hands-free devices or not - to equate to being at the impairment threshold of intoxication. In semi-duplex or simplex, you say something, they can't reply until you stop, and vice versa. This is also the argument why two-way radio should not fall under cellphone laws - because it does not consume more brain processing resource. It's less taxing on the human brain's ability to process information (and our brains are quite limited in being able to multitask efficiently - read Kahneman's Attention and Effort [1973]).

jackj said:

Brasso, that would be full duplex. A repeater is a full duplex radio, it receives on one frequency and transmits on another at the same time.

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Please explain how a traditional FM repeater is full duplex and how all parties can talk and simultaneously hear everyone else at the same time.

T!

A repeater transmits and receives at the same time. That is what makes a repeater full duplex. No repeater will allow everyone to talk and listen at the same time. You can not stand in the middle of a room full of people and understand all of the conversations going on around you at once but you can pick out one conversation and follow it. But a repeater is a device in a system, not the system itself. The system is NOT full duplex, it's half duplex. Only the repeater is full duplex.

jackj said:

A repeater transmits and receives at the same time. That is what makes a repeater full duplex.

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So by your definition full duplex is simply the ability to receive and transmit simultaneously with no other requirement.

jackj said:

No repeater will allow everyone to talk and listen at the same time. You can not stand in the middle of a room full of people and understand all of the conversations going on around you at once but you can pick out one conversation and follow it.

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The breakdown of the system you are describing is in you, not the system carrying the data to you. While an individual human may not be able to understand everyone at the same time it would be technically possible to understand everyone at the same time.

The point here is that it is possible, even if you cannot do it yourself. More importantly to the discussion of full duplex is the fact you can pick any one conversation and talk with the other person, and while they are talking you can also talk, with no destruction or loss of data. They can hear and understand you despite the fact they are talking themselves.

jackj said:

But a repeater is a device in a system, not the system itself. The system is NOT full duplex, it's half duplex. Only the repeater is full duplex.

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I would argue the other side of that. A traditional FM repeater by itself is NOT full duplex, although when combined with correctly configured other hardware it might be a node in a full duplex system.

Lets define a traditional FM repeater.

A traditional FM repeater monitors one frequency (F1). When a signal meeting either the power level threshold requirement or a control tone requirement is present on F1 that signal is received or the squelch is opened and the audio is demodulated to base band. The base band audio is then applied to a transmitter that is keyed on by the same detection criteria as originally opened the receiver squelch (say a COR held active by the received tone or received signal level). The transmitter then transmits the base band audio on a separate frequency (F2).

In normal use stations A and B, using the FM repeater, will have a half duplex radio that transmits in FM on F1 and receives on F2. Station A keys up his radio and talks. If, before station A finishes talking and unkeys, station B keys up and tries to talk to A, two things happen. One, station B can no longer hear station A. And station A cannot hear station B. A third station, monitoring F2, will typically only hear one station, the stronger station that captures the FM receiver, or may hear an unusable mash/mix of the two stations.

So lets look at the definition of full duplex.

"Reference Data for Radio Engineers", 6th edition, defines full duplex as “Full Duplex: A type of operation that permits simultaneous communication in both directions between the called and the calling parties.”

"Buchsbaum's Complete Handbook of Practical Electronic Reference Data", Second Edition, defines full duplex as "Devices which transmit in one direction at a time are said to be operating in half-duplex mode. Devices which operate in both directions simultaneously are said to be operating in a full-duplex mode." and "In a full duplex circuit, simultaneous communications in both directions is possible."

"IEEE Standard Dictionary of Electrical and Electronics Terms", Third Edition. Defines full duplex as "Full Duplex (telecommunications). Method of operation where each end can simultaneously transmit and receive. Note: Refers to a communications system or equipment capable of transmission simultaneously in two directions."

Many online references put it the same way, but a quick look at Wikipedia (shudder) says “In a full-duplex system both parties can communicate to the other simultaneously.”

The important take away here is that all of these say essentially the same thing, simultaneous (more than one station at a time) and both directions (to and from each station). I would say a traditional FM repeater as they are typically configured does NOT meet this requirement. An all mode translator, on the other hand, might very well do so if combined with the correct end equipment at each station location.

T!

What Token says +1, not the twaddle that went on before....

A repeater is a full duplex radio

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Rubbish.

it receives on one frequency and transmits on another at the same time.

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True, but this does not make it a duplex repeater. A duplex repeater is supposed to do this in both directions, so you need two receivers, two frequency translators and two transmitters, all going at the same time with no crosstalk.