# Full duplex shared frequency radio?

Normally shared frequency analogue1 radios (CB, aviation, etc.) can't receive when transmitting, because the signal being transmitted is much louder and will jam any signal from different transmitter.

Is it possible to somehow subtract the transmitted signal to be able to pick out signals from other transmitters even when transmitting? Is there any published design that allows receiving while transmitting or at least detecting that there is another transmission in progress?

1 Or digital, but with short packets collisions are less disruptive and there are other ways around the problem like slot allocation.

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Stop and think about the enormous ratio between the voltage the antenna is being driven with while transmitting relative the tiny signal it picks up while receiving. –  Olin Lathrop Mar 18 at 18:13
Actually I think the first point to consider is what modulation you are using. For AM you would need a large dynamic range. For FM, its pretty much impossible by design. But in the digital domain, lots of methods exists. My favourite of which is CDMA, which uses orthogonal signal vectors (similar to how different frequencies are orthogonal under FT). –  Aron Mar 19 at 2:22
@Aron: That's why I mentioned analogue and gave frequency-modulated examples. –  Jan Hudec Mar 19 at 5:50
@Aron I'm a fan of CDMA, too, but popular CDMA implementations (like IS-2000 and IS-856) use the technology to pick out the desired received signal from all the other (i.e. intended for other receivers) received signals, while using separate frequencies for forward (downlink) and reverse (uplink) communications. –  drewbenn Mar 19 at 7:07
@drewbenn I believe that next gen will begin to use SDMA, which is really cool. Its already starting to show up in consumer Wifi. –  Aron Mar 19 at 7:10

## 3 Answers

This isn't trivial for a number of reasons, but it is possible. Part of the problem is that the receiver has to have a huge dynamic range to be able to receive the 1W signal and the tiny signal from afar just above the noise floor, and understand both without losing data.

One system that can help, though, is to remove the transmitted signal from the antenna before the receiver:

Some research points out that this can be made to work.

However, the point is that what you're asking is possible, but it's very difficult to do well.

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+1 that's a good find dude and 73dB of cancellation is pretty good going although it's only a claim on their part. –  Andy aka Mar 18 at 20:45
@Andyaka Yes, I'll be interested to see others duplicate or improve on this work. –  Adam Davis Mar 18 at 21:01
It's going to get harder as you go higher in frequency because the link-loss between two radios is proportionally higher with increased frequency. Also, local reflections being variable are going be curved balls that might be a spoiler. –  Andy aka Mar 18 at 21:07
@Andyaka how do you figure link loss increases with frequency? –  Phil Frost Mar 19 at 11:26
S@Phil it's the Friis equations not me that figured this out. –  Andy aka Mar 19 at 11:37

Likely received power level at (say) 27MHz, 1km from a 1 watt transmitter can be calculated from the link loss equations of Friis. In free space and with isotropic antennas the link loss is: -

Link Loss (dB) = 32.5 + 20log$_{10}$(MHz) + 20log$_{10}$(kilo metres) = 32.5dB + 28.6dB + 0 = 61dB.

If the power output is 1 watt from that distance antenna (30dBm), the received power is -31dBm or 794 nW.

Trying to disentangle this tiny bit of power from the 1 watt you maybe transmitting would be a nightmare and this is the free-space scenario - over land and with objects in the way and trying to accommodate fades, the likely received power will be another 30dB lower.

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Typically full duplex radios utilize different RX/TX frequencies to avoid the mentioned problems of picking out the tiny received signal among the blasted transmission.

If you use something like time-slots, then it's not technically full duplex at that point. You can do that to get the illusion of full-duplex however. That depends on your application. For something like voice, which is low bandwidth, you can definitely time share the connection to provide the appearance of full-duplex.

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How is time-slots any less full duplex than frequency slots? Surely any cell phone qualifies as "full duplex". Are TDMA phones actually not? –  Phil Frost Mar 18 at 19:17
@PhilFrost: Time-slots are not full duplex. And separate frequencies are not shared frequency. The question was explicitly not about either. –  Jan Hudec Mar 18 at 19:46
In an RF context, full duplex means transmitting while receiving at the same time instant. @jan-hudec sorry, somehow I missed the 'shared frequency' bit of your question. –  Ryan Johnson Mar 18 at 21:08
So, TDMA phones are not full-duplex communication devices, while CDMA or FDMA phones are? That does not make much since to me, since I can talk and listen simultaneously on any of these devices. –  Phil Frost Mar 18 at 22:30
@PhilFrost TDMA is like (bad analogy alert!) watching television: your TV actually displays about 30 different still pictures every second, but your brain combines them so you "see" a moving picture. Just because your phone "sounds" to you like it is full-duplex, doesn't mean it technically is. –  drewbenn Mar 19 at 6:56