When we talk about multi-user communication, there are many users transmitting at the same time and same frequency. Based on the way how the signals are transmitted and received, different channels are defined in the literature.
Let us consider a simple case when there are two transmitters and two receivers. At any time instance, both the transmitters transmit and both the receivers receive. However, all the data streams that have been transmitted may not be desired by both the receivers. Also, all the channels linking a particular transmitter and the receiver may not be of the equal strength. Those channels that carry the unwanted data streams or those channels that are linked between the receiver and the undesired transmitters are called the undesired channels or the interference channels.
Let us consider a simple case when there are two transmitters and two receivers. At any time instance, both the transmitters transmit and both the receivers receive. However, all the data streams that have been transmitted may not be desired by both the receivers. Also, all the channels linking a particular transmitter and the receiver may not be of the equal strength. Those channels that carry the unwanted data streams or those channels that are linked between the receiver and the undesired transmitters are called the undesired channels or the interference channels.
In the figure shown below,Transmitter T1 has message m1 intended to Receiver R1 and Transmitter T2 has message intended to receiver R2 and they are transmitting at the same time and with the same frequency. This causes interference to the Receiver R1 from T2 and and interference to the Receiver R2 from T1, these are indicated by the dotted lines and the solid lines show the channels between the desired transmitters and the receivers in the figure. Such a channel is referred to as Interference Channel.
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| Interference channel |
In the Z channel, the Transmitter T1 has message intended to Receiver R1only thus interfering to the receiver R2 but the Transmitter T2 has the messages intended to both the Receivers R1 and R2 or can be the other way. In the figure below, the interfering channel is shown by the dotted line and there is only one interference channel while remaining 3 are the desired channels which are shown by the solid lines. Here Receiver R1 should decode two messages from T1 and T2 but the receiver R2 should decode only one message from T2.
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| Z channel |
The other channel is the X channel. In the X channel, both the Transmitters T1 and T2 have messages intended for both the receivers R1 and R2. It means R1 has to decode both the messages from T1 and T2 and R2 also has to decode both the messages from T1 and T2. It seems pretty interesting to have X channel, if we can send the messages at the same time to different users and also the receivers can receive the messages at the same time from different users, unlike broadcast and multiple access channels. X channel has special significance because of its higher and non-integer degrees of freedom, and hence higher capacity. The X channel is shown in the diagram below, here mij is the message from transmitter j to receiver i.
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| X channel |
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