How using two or more RF modules at same place is it possible ?

Hi
I have FSK 915Mhz Transmitter and Receiver module i wrote simple program with StartID and StopID for increase accuracy everything is good no problem at all.
now if i buy three Transmitter and Receiver at same frequency (915Mhz) are they work in a constant place (e.g. my house) with different Start and Stop ID ?
Now i want know data will not corrupted or added or subtracted in space when transmitters send together ?
Devices works separated (may three Transmitter send together) if i need to change frequency or change coding method or you have any idea please guide me. (See Attached image)
I'm Using "RS232- 1200 8-bit no-parity inverted", i had tested even parity but didn't works on this modules:
Receiver Module Datasheet:
"HM-R(HM-R)":http://roboeq.com/PDF/0501017.pdf
Transmitter Module Datasheet:
"HM-T(Hm-T)":http://roboeq.com/PDF/0501016.pdf
My collage master said this is not possible anybody tested two or more modules together in constant place ?
Thanks

You need different channels otherwise you are going to interfere with the others and you will wind up with data corruption

Think of how the Cell site works Also think in terms of Radio control You cant have two radios on the same frequency side by side hence why you change the crystals

FSK consists of two carrier frequencies. The frequency changes depending on the symbol 1 or 0 used. Mathematically can be described as:
s(t) = f1(t) (2*pi*fc1*t +phase) + f2(t) (2*pi*fc2*t +phase)
So, in power spectrum, there are two signals f1(t) and f2(t) + side bands

You would need to work out what bandwidth you require in order to space your channels on the RF spectrum so that you don’t get cross channel interference

See attached as well

Hope this explains this for you

Or you could take a cue from the Ethernet protocol. Transmit in bursts and listen for collisions. If a collision occurs, the have the two transmitters start a timer with a random delay. That way, it is more likely that one transmitter will transmit at a different time than the other.

Or you could do some sort of time division multiplexing. Transmitter 1 transmits, then transmitter 2, then transmitter 3, then transmitter1, etc. You would need to encode each transmission with an address, so the recievers know which transmission to pay attention to.

Then, there is a new kind of transmission that allows many transmissions to occur simultaniously on the same frequency, but it's such a new technology, that there's nothing "off the shelf" that you could apply--you'd have to implement the technology yourself: http://www.bbc.co.uk/news/science-environment-17221490

OR

The best way to deal with what this gentleman wants is by using multiplexing that way he only needs one transmitter , one receiver but his hardware would have to run via hardwire cable and he would need to have a de multiplexer the other side In other words TDM “Time division Multiplexing”

http://en.wikibooks.org/wiki/Communication_Systems/Time-Division_Multiplexing

Or

TDD: Time Division Duplex

Time Division Duplexing (TDD) refers to a transmission scheme that allows an asymmetric flow for uplink and downlink transmission which is more suited to data transmission. In a Time Division Duplex system, a common carrier is shared between the uplink and downlink, the resource being switched in time. Users are allocated one or more timeslots for uplink and downlink transmission.

See below for better understanding of this scheme and others

http://www.radio-electronics.com/info/cellula...te-long-term-evolution/lte-fdd-tdd-duplex.php

Mark

www.harrington.force9.co.uk

Mark's amendments will only work if there is only one transmit location. If the reason for having three transmitters is that there are three data sources that are topographically removed, then, unless there is a way to convey the data from the other two locations over to third location, three transmitters will be necessary.

In which case, TDM can be achieved in one of the following ways:
1. One of the three transmitters becomes the master and the other two transmitting stations have receivers. The master transmitter broadcasts a synchronizing signal that sets the timing at the other two stations. Each station will have a time slot for sending their data. Transmitter # one would go first, sending it's data during it's alloted time slot, then transmitter two would follow, sending it's data, then transmitter three. Any of the transmitters can be transmitter one, and the other two transmitting stations would also have a receiver so they can monitor transmitter #1.
2. If a highly stable timebase is available (such as a timebase synchronized by WWV or by GPS, etc.) then, method 1 could be used without the synchronizing signal--which would eliminate the two receivers. The transmitters would need only to be synced one time (unless a power failure/solar flare/etc, interrupted this synchronization, in which case they would need to be synced again or, they could sync on time of day, of something like that)
3. If the data needs to be conveyed as quickly as possible, then each transmitter would need a receiver so it can monitor the other transmitters and then transmit it's data when none of the other transmitters is transmitting. This can be done synchronously or asynchronously.
a. Synchronously: Transmitter # one transmits. Transmitter two hears transmitter one transmitting and gets ready to follow transmitter one. When transmitter one finishes, transmitter two transmits and transmitter three gets ready, etc.
b. Asynchronously: When any one of the transmitters has data to transmit, it goes ahead, which signals the other two transmitters to wait. If two transmitters try to transmit at the exactly the same time (should be a rare occurrence), they both stop transmitting and each sets a random delay, which should have one transmitter start before the other.
c. Asynchronously without three receivers: This would only work if data events are sparse and small. When a transmitter has data, it sends it more than once, each time with a different delay period between transmissions. The duration of these delays is a multiple of the set data transmission time frame. Lets call them slots. So, if it typically takes 1 second to transmit a packet of data, then a slot would be 1 second long. Transmitter # one would transmit it's data (for 1 second) then wait 1 time slot (or 1 seconds), transmit the same data again, and wait 2 time slots (or 2 seconds), then transmit again and wait 3 time slots (i.e. 3 seconds) minimum before transmitting any new packets. Transmitter two would transmit it's data, wait 2 slots, transmit again, wait 1 slot, transmit, wait a minimum of 3 slots before transmitting any new packets. Transmitter # 3 would transmit, wait 3 slots, transmit, wait 1 slot, transmit, then wait a minimum of 2 time slots. That way, even if two or more transmitters transmit at the same time, only some portions of their transmission session would overlap. The receiver would match checksums, or do error detection (and perhaps even correction), or whatever is necessary to determine which of the packets is valid. Also, each dataset would be identified by a serial number, so the receiver can discard duplicates.

Hi Steve

Yes I can see that working well thankyou for your correction and advice in otherwords from what I understand of this TDD

Nice explanation


I've learnt something new today as well

Mark

i've learnt many things about TDD. RF and FSK systems. thanks a lot Steve and Mark

Very good reference thanks.