Vtyjkyuk

Is there an existing protocol or modulation method where multiple data bits are sent over a single wire at once or maybe an additional ground line (like serial communication)?

I know there are methods like PSK or FSK where phase or frequency of carrier is altered to represent different bits or states of signal, but those changes in phase or frequency are transmitted one after another i.e. serially and not at once.

Is there an existing communication or modulation method or protocol which can send multiple data bits at once and not one after another using the shifting done in PSK or FSK?

16-QAM transmits 4 bits simultaneously by modulating both the phase angle and the carrier amplitude: -

At the receiving end, the noise added during transmission propagation may make the bits look like this: -

But, providing there is still a gap between data received and the halfway point between symbols you can detect it.

So, if you understand the noise in your channel and your channel bandwidth is accommodating, you will be able to send more than one bit simultaneously (as suggested by the Shannon-Hartley theorum): -

Sure. PSK and FSK (and other modulation methods, for that matter) can have more than two choices for the phase or frequency. If you have four choices, you can send two bits at once.

Advanced telephone modems (before we all switched to broadband) could encode as many as 8 to 10 bits at a time, using 256 to 1024 different signalling states.

QAM-256 diagram (from here)

This is just sort of a overarching meta-answer, because I haven't seen the word "symbol" highlighted as much as I'd like. In typical communications systems, you only send one symbol at a time, but you may have more than 1 bit per symbol.

A symbol is a logical concept which is mapped to some physical manifestation. For example, in Dave Chapman's answer there are 4 symbols, mapped to the physical voltage levels of 0V 1.25V 2.5V and 3.75V. In the 16QAM example from Andy aka's answer there's 16 symbols, mapped to a combination of amplitudes and phases.

You can then define your mapping of symbols onto bits. If you have a simple digital lane with 2 symbols: 0V and 5V, you might map those symbols onto the bits 1 and 0. If you have 4 symbols (like Dave's voltage answer), you might map the onto pairs of bits, 00, 01, 10, 11. If you have 16 symbols, like 16QAM does, you might map it to 4 bit groups 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, and 1111.

Thus the more symbols you have, the more bits you can transmit at the same time. Of course, more symbols also means more its harder to distinguish which symbol was transmitted later.

Its also possible to send more than one symbol on a wire, if your physical manifestations of those symbols are easy to separate. For example, cable sends data whose symbols fit into very nice narrow frequency bands (one per channel). The symbols sent on each of these channels can be handled independently.

I know there are methods like PSK or FSK where phase or frequency of carrier is altered to represent different bits or states of signal, but those changes in Phase or Frequency are transmitted one after another i.e. Serially and not at once.

This is not necessarily true. If your FSK modulation scheme has 4 or 8 or 16 different frequencies that can be transmitted instead of just two, you can transmit 2 or 3 or 4 bits per symbol.

Any modulation scheme that offers more than 2 different symbol choices in each baud interval is transmitting more than 1 bit per symbol.

So I want to know is there any existing communication or modulation method or protocol which can send multiple data bits at once and not one after another using shifting done in PSK or FSK?

For example, pulse-amplitude modulation (PAM, currently a hot topic in fiber optic data communications) and quadrature amplitude modulation (QAM) are typically designed with more than 1 bit per baud.

After writing this answer, I noticed the question is tagged as "digital electronics": my answer requires analogue components so I don't know if it will be useful. I will leave it regardless in case it is.

As a Control Systems Engineer, I would like to propose a simpler solution.

If you can control either your current or voltage in an analogue fashion with high accuracy, you can pick a high and low reference value, lets say 0-16v for simplicity's sake. From here, if you have a resolution of 1v for your control you can transmit up to 4 bits of information simultaneously by picking the decimal representation of the bit field as your voltage.

For example:

0v => 0000
1v => 0001
7v => 0111
etc.

Then if you set it to a clock, you can understand that this value is updated at x Hz so your programs can respond even if the value hasn't changed.

The only limit to this is the level of precision with which you can control your voltage/current transmission.

There are standardised protocols for this such as PAM16 which is used in ethernet. This picks 16 values between -1v and 1v. Thanks to the comments for this information.

There is a fairly standard method called the "dibit", which sends two bits
in a given time slot. The bits are encoded as an analog voltage, like this:

Voltage Data

0.00 V - 00

1.25 V - 01

2.50 V - 10

3.75 V - 11

This system uses a D/A converter to send, and an A/D converter to receive.
Similar systems exist for "tribits" and quadbits". After that, not so good.
The problem, obviously, is that as you go to smaller and smaller distinctions
between bit patterns, you become more vulnerable to noise.

In fact, this is why digital data transmission was invented in the first place.

Bottom line, you can do this, but there are trade-offs.

A way to transmit several signals over single wire or medium is by using multiplexing, the two major types are FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing).

In FDM basically each signal modulates a different carrier, and all the signals are transmitted in the same medium at once, at the reciever side, there's usually some kind of filter which selects the frequency range of interest and demodulates the signal.

In TDM each signal is transmitted in different time slots, imagine a line of 8 signals where every signal has its own turn, during a small time slot signal 1 will be transmitted, then signal 2, then signal 3 and so on, the cycle will repeat and start with signal 1 again.

Also look at CDMA (Code Division Multiple Access), from Wiki:

CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code).

A variant of FDM is OFDM (Orthogonal Frequency Division Multiplexing)