# Question about digital modulation

So there are several ways to transmit a bitstream but where is the actual info when my carrier is a sinewave?

My basic understanding is we have symbols (phase,amplitude or frequency), we assign bits to these symbols and by modulating or changing those parameters (APSK,BPSK,FSK..) the receiver knows (or tries to decide) what got sent from the transmitter.

So the symbols can be seen as graphical representations of the associated bits?

• A total M unique Symbols, may represent a unique frequency and/or phase and/or amplitude can each represent M bits simultaneously. By using rules to minimize simultaneous bit changes such as Gray Code, BER is improved in conditions of low SNR. Randomizing the bits with Trellis codes distributes the energy of each symbol to all possible symbols rather than being so much bit pattern dependent. Bandwidth/bit rate compression or bits/baud also comes at a cost in terms of higher SNR of carrier or CNR needed. Some are more tolerant to Doppler shifts ( Mobiles), Raleigh and Rician Fading. – Sunnyskyguy EE75 Mar 7 '18 at 15:55
• Unclear what you really are asking. A "graphical representation" would be a picture that you draw on paper or, on a computer screen, in an attempt to explain how the system works. – Solomon Slow Mar 7 '18 at 16:00
• P.S.: One symbol in a digital communication system can stand for more than one bit. E.g., in a simplistic FM system, you could use two different frequencies (one bit per symbol), or you could use four different frequencies (two bits per symbol), or you could use 13 different frequencies (approximately 3.7 bits per symbol on average.) – Solomon Slow Mar 7 '18 at 16:05
• You should read up on modulation in general; Modulation alters a characteristic of the carrier to superimpose information on the carrier. The modulation can be analogue or digital in nature, but digital modulation of an analogue carrier always results in an analogue signal. – Peter Smith Mar 7 '18 at 16:12
• M symbols would be log2(M) bits, not M bits – alex.forencich Mar 7 '18 at 19:38

Your unmodulated carrier is a sine wave. When modulated it can look like this: -

That's called On-Off-keying (OOK) and very similar to morse code. It could look like a version of OOK called ASK (amplitude shift keying): - And below is an image of ASK, FSK and PSK: - So the symbols can be seen as graphical representations of the associated bits?

The words you use do not ring true. Look at the pictures.

You aren't quite correct with how you use bits, symbols and modulation. First you have bits. A number of bits (determined by your encoding/modulation scheme) makes a symbol. A symbol is what is transmitted in one time instant (different modulation schemes will have different time periods for when a symbol is valid). The actual alteration of the carrier wave to represent this symbol is a function of the modulation scheme. The basic schemes are Frequency Shift Keying (FSK) which would alter the frequency of the carrier, Amplitude Shift Keying (ASK) which alters the amplitude and Phase Shift Keying which alters the phase. There are other schemes which are more complicated (QPSK, QAM, etc) but these are rally variations of the basic schemes.

As an example, using on-off Amplitude Shift Keying. A valid 1 is indicated by the presence of a carrier wave above a certain amplitude . A valid 0 is indicated by no carrier wave. The symbol would consist of 1 bit. By altering this to have four amplitude levels you could transmit a symbol consisting of 2 bits (amplitude 0 = "00", amplitude 1 = "01", amplitude 2 = "10", amplitude 3 = "11"). Some modulation schemes can transmit a much greater number of bits in one symbol. If you were to view this signal in the frequency domain you would see your carrier, at a fixed frequency, varying between the fixed amplitude levels.

This basic description holds for the other modulation schemes. In FSK different frequencies would represent a different symbol. This would look like the carrier jumping between the various frequencies.

Just seen Andy's answer. His pictures explain it perfectly.