capacity bits per channel use
Those four are definition (in information and comm. theory) which you have to understand within the context.
1. Bits per channel use: Number of bits a transmiter send at one time to a receiver via a channel. You can imagine there is a pipe with a certain diameter (ex: 10 cm). Let say one end of the pipe is at a roof, and the other end is close to the ground. You are at the roof and try to "send" apples to the ground through the pipe. Let assume also that an apple has dimension of 4 cm. So at one time (i.e. per pipe use) you can only "transfer" 2 apples. If you have oranges, then you might be able to transfer more than 2 oranges per pipe use.
2. Bits per complex dimension.
A signal can be represented as a point in signal space which is a complex plane. For example a QPSK signal has 4 points at +1, -1, +j, -j. For QPSK, each point corresponds to two bits, i.e. +1 -> 00, +j -> 01, -1 -> 11, -j -> 10 (with gray coding). So for QPSK you have 2 bits per complex dimension.
3. Bits per symbol.
One symbol (M-ary modulation) corresponds to several bits. Ex.: QPSK, one symbol (+1 or -1 or +j or -j) corresponds to two bits.
4. Bits per second.
This must be clear, i.e. how many bits in one second. You can transmit one bit per second with the period of each bit is 1 second, or you can transmit 2 bits per second with period for each bit is 1/2 second, or you may transmit one QPSK symbol in 1 second (1 QPSK symbol corresponds to 2 bits).
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