Baseband modulation refers to the process of transmitting digital signals over a communication channel without changing the frequency of the signal to a higher-frequency carrier. The signal is transmitted directly without frequency translation, making it suitable for short distances, such as within a local area network (LAN).
Baseband Amplitude Shift Keying (ASK)
For Binary Amplitude Shift Keying (BASK), binary bit '0' can be represented as lower level voltage and bit '1' as higher level voltage. For example, you can map binary bit '0' to 0 and bit '1' to 5 volts. Then, you can transmit the signal to the wire to the destination.
Baseband Frequency Shift Keying (FSK)
For Binary Frequency Shift Keying (BFSK), you can map binary bit '0' to 'j' and bit '1' to '1'. Here, signals are in phase. But it is not necessary for BFSK. For baseband representation, we can map the bits like these for various purposes, such as calculating power spectral density (PSD), etc. In the case of transmitting through a wireless medium, we can modulate signal 'j' with a lower carrier frequency and signal '1' with a higher carrier frequency.
Baseband Phase Shift Keying (PSK)
In binary phase shift keying (BPSK), the phase of the carrier signal is altered to convey data. For example, a phase shift of 0 degrees could represent a binary '1', while a shift of 180 degrees could represent a binary '0'. For example, we can map binary bit '0' with '-1' and bit '1' with '+1'. In the constellation diagram of BPSK, you can see constellation points are '-1' and '+1', and the phase difference between them is 180 degrees.
Further Reading