When two devices are connected via wireless, one device sends a character using its TX line, and the character is received by the other device on its RX line, and vice versa.

My question is, RF signals reflect, get absorbed etc. Does the signal reflects and go back to transmitter sometimes ? If it does, how the transmitter recognizes it and discard the character ?


First of all, it is important to distinguish between different layers of networking. A well-implemented RF stack is made up of different layers that provide what is called orthogonality between functionality. This means that you can distinguish the function of each 'layer' in the stack exclusively. For instance, a typical RF networking stack may look like:

  • Bottom layer: the (analogue) RF transceiver that just carries a signal on top of some carrier frequency and radiates it out or amplifies it in from an antenna
  • Next layer: the binary encoding layer, that decides how data transmitted and received should be interpreted (i.e. little/big endian, ECC, encryption)
  • Next layer: protocol layer. This interprets the data as data, commands, etc.
  • etc.
  • Last layer: application layer. This takes the data and does something useful with it.

In your example, there is of course some possibility that a sent RF signal gets bounced back and received as data again. A well-written stack will have in its protocol some kind of way to distinguish between packets that are, and data that isn't aimed at that specific device. For instance a header in the protocol layer that says 'from: node1, to: node2'. For instance USB 2.0 works this way: all devices on a hub controller receive all data, but only those devices with the right address will do something with it.

However, I should also note that your example is very unlikely to happen. For all but the most advanced transceivers it is not possible to send and receive at the same frequency at the same time. And when you bounce those signals around in a building or even in the direct neighborhood of your transceiver, they still move at the speed of light and will - for the timescale of a transceiver - arrive instantaneously back. If they do not, the most likely event would simply be some kind of interference pattern and not something strong enough to be interpreted as data. Most transceivers use slightly different carrier frequencies for simultaneous transmit and receive, just so they can more easily distinguish between the two. If they even have simultaneous (full-duplex) tranasmit and receive capabilities - this is a relatively new feature in consumer RF implementations.

  • \$\begingroup\$ Off topic : I'm trying to use signal reflection for some reason. You've stated advanced transceivers. Are Broadcom Radio SoC's in that category ? I'm okey with layers. If it is theoretically possible, then i will look into their open source drivers to go to PHY layer and try to get the reflected bit , if any. But here is a problem. The reflected part must be a portion of the signal right ? will it still be a meaningful thing ? \$\endgroup\$ – cowboysaif Oct 12 '13 at 9:17
  • \$\begingroup\$ I don't know of specific wireless radios doing this, but for instance DOCSIS 3.0 cable modems and VDSL2 modems use simultaneous QAM send/receive, using different quadrants for send and receive instead of using different carriers. This is extremely similar technology to wireless techniques, even using very similar carrier frequencies. If you'd stick a suitable power amplifier on top of such a modem and hook it up to an antenna you can use it as a wireless modem with simultaneous RX/TX. This is for instance how high-speed wireless internet is done in places without landlines. \$\endgroup\$ – user36129 Oct 12 '13 at 9:34

Here are a few bits of information to help you understand what is going on. I'm dumbing this down because clearly the OP doesn't understand some of the concepts: -

  • A transmitter outputs power from its antenna. While it is transmitting it won't be receiving at that frequency.
  • A transmitter output stage is like a loudspeaker - it has no idea of the meaning of anything it outputs and just continues to try and output power irrespective of what might be coming back at it.
  • "Reflections" are used constructively in some antennas to obtain higher antenna gain and directionality (by design).
  • "Reflections" can occur destructively which can misshape the antennas characterisitics (random objects getting close to the antenna).
  • Meaningful reflections happen at the speed of light and can upset an antenna if the objects causing them are about one wavelength (or less) away.
  • To detect one bit of data (one small part of an 8 bit character) I'd say at least ten cycles of the transmission needs to have occured and this puts it way beyond the "within one wavelength" scenario.

Does this help you understand what you're asking?

  • \$\begingroup\$ cleared my basic concept. I am interested in programming Broadcom radio system on a chip that are used in smartphones / routers. I think this SoC's has duplex feature. It can send and receive simultaneously. But you have a strong point. To send and receive simultaneously frequencies doesn't have to be same. Then reflections are out of question, unless transmitter and receiver uses some sort of different antenna. Again my guess. \$\endgroup\$ – cowboysaif Oct 12 '13 at 9:06
  • 1
    \$\begingroup\$ Simultaneous Tx and Rx need to use different frequencies and, the receiving circuits need to totally block the power transmitted or they will likely be destroyed or have their reception performance badly limited. A typical receiver might be able to decode a power level of -100dBm and its transmitter might output +10dBm - the receiver is very sensitive and the transitter represents a massive potential problem! You can use a loudspeaker to emit one tone and use it as a microphone to receive a different tone but think about the different power levels involved. \$\endgroup\$ – Andy aka Oct 12 '13 at 9:12

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