Convert digital data into analog signal

Convert digital data into digitally converted analog signal: 
Media passes Digitally converted analog signal. So we need to make this digital data into digitally converted analog signal. But directly cannot make it. So first we have to make digital data into digital pulse.

Digital data to Digital pulse:
First we have to clear about some topics

Baseline Wandering: In decoding a digital signal, the receiver calculates a running average of the received signal power. This is called Baseline. A long string of 0s or 1s can cause a drift in the baseline (Baseline Wandering) and make difficult for the receiver to decode correctly. A good line-coding scheme needs to prevent baseline wandering.

DC Components: When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies. These frequencies around zero called DC components. It presents problems for a system that cannot pass low frequencies or a system that uses electric coupling (via a transformer). For example, a telephone line cannot pass frequencies below 200 Hz. Also a long-distance link may use one or more transformers to isolate different parts of the line electrically. For these systems, we need a scheme with no DC components.

Self-Synchronization: To correctly interpret the signals received from the sender, the receiver’s bit intervals must correspond exactly to the sender’s bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signal.

Line-coding schemes:

Several ways to map data bits to signal elements

NRZ (Non-Return to Zero): This is the simplest modulation scheme. Bit “1” is sent as positive voltage signal and bit “0” sent as low voltage signal. It is called NRZ because the signal does not return to zero at the middle of the bit until get a 0 bit.
The main problem with NRZ encoding occurs when the sender and receiver clocks are not synchronized. The receiver does not know when one bit has ended and the next bit is starting. This problem solved by RZ.

RZ (Return to Zero): It uses three values: positive, negative and zero. In RZ the signal changes not between bits but during the bit. The signal return to 0 in the middle of each bit. And it remains there until the beginning of the next bit.
The problem is that it requires two signal changes to encode a bit and therefore occupies greater bandwidth. But here is no DC component problem. Another problem is it uses three levels of voltage, which is more complex to create and discern. For its disadvantages this scheme is not used today. It replaced by manchester and other schemes.

Alternate Mark Inversion Codes: It uses one 1 for positive side and next 1 for negative side.

Manchester: Transition from high to low in middle of interval = 1 and Transition from low to high in middle of interval = 0

These are categories in different schemes

Unipolar Schemes

NRZ codes

Polar Schemes

NRZ-L

NRZ-I

RZ

Biphase Manchester

Bipolar Schemes

Alternate Mark Inversion  (AMI)

Multilevel Schemes

2BIQ (two binary, one quaternary)

8B6T (eight binary, 6 Ternary)

4D-PAM5 (four-dimensional five-level pulse amplitude modulation)

Multiline

MLT-3 (multiline transmission, three level)

In Unipolar scheme, all the signal levels are on one side of the time axis, either above or below. In Polar Schemes, the voltages are on the both sides of the time axis. Voltage level can be positive or negative. In Bipolar encoding sometimes called multilevel binary. There are three voltage levels: Positive, Negative and zero. Multilevel Schemes creates to increase the data speed or decrease the required bandwidth. We can use Multiline Transmission encoding scheme if we have a signal with more than two levels.

Now the fundamental question is, how can we send a digital signal from PC-A to PC-B? We can transmit a digital signal by using one of two different approaches-
* Baseband Transmission
* Broadband Transmission

Baseband Transmission: 
Baseband transmission means sending a digital signal over a channel without changing the digital signal to an analog signal.
Baseband transmission requires a low-pass channel, a channel with a bandwidth that starts from zero. This is the case if we have a dedicated medium with a bandwidth constituting only one channel.
For an example, the entire bandwidth of a cable connecting two computers is one single channel.
Another example, we may connect several computers to a bus, but not allow more than two stations to communicate at a time.
We need to remember that a low-pass channel with infinite bandwidth is ideal, but we cannot have such a channel in real life. 

Low-Pass channel with Wide Bandwidth: 
If we have a dedicated medium with an infinite bandwidth between the sender and receiver that preserves the exact amplitude of each component of the composite signal. This may possible inside the computer such as between CPU and memory. It is not possible between two devices. The amplitudes of the frequencies at the border of the bandwidth are so small that they can be ignored. This means that if we have a medium, such as a coaxial cable or fiber optics, with very wide bandwidth, two stations can communicate by using digital signals with very good accuracy.

Broadband Transmission: 
Broadband transmission or modulation means changing the digital signal to an analog signal for transmission. Modulation allows us to use a bandpass channel – a channel with a bandwidth that does not start from zero. This type of channel is more available than a low-pass channel.

If the available channel is a band-pass channel, we cannot send the digital signal directly to the channel; we need to convert the digital pulse into digitally encoded analog signal before transmission.

Overview:

Digital pulses are contained in and propagate down the cable. But cannot be propagated through a wireless transmission system, such as Earth’s atmosphere or free space.

AT&T developed the first digital transmission system for the purpose of carrying digitally encoded analog signals, such as the human voice, over metallic wire cables between telephone offices. Today, digital transmission systems are use to carry not only digitally encoded voice and video signals but also digital source information directly between computers and computers network.

Advantages of digitally encoded analog signal:

·         Noise immunity. Digital signals are inherently less susceptible than analog signals to interference caused by noise because with digital, it is not necessary to evaluate the precise amplitude, frequency, or phase to ascertain its logic condition. Instead, pulses are evaluated during a precise time interval, and a simple determination is made whether the pulse is above or below a prescribed reference level.

·         Digital signals are also better suited than analog signals for processing and combining using a technique called multiplexing.

The Sine Wave of Analog signal has three characteristics.

i) Amplitude

ii) Frequency

iii) Phase

When we change any one characteristic, we create a different version of that wave. So, by changing one characteristics of a simple electric signal, we can use it to represent digital data. After line coding and error recovery we change digital signal into analog. There are three main mechanisms for modulation digital data into an analog signal. And fourth one is combines changing of amplitude and phase and most efficient among these.

i) Amplitude Shift Keying (ASK)

ii) Frequency Shift Keying (FSK)

iii) Phase Shift Keying (PSK)

iv) Qadrature Amplitude Modulation (QAM)

Amplitude Shift Keying (ASK):

The simplest digital modulation technique is ASK (amplitude-shift keying). Sometimes it is called DAM (digital modulation technique), where after line coding (NRZ / RZ) the binary bit directly modulates the amplitude of an analog carrier. The amplitude of the carrier signal varied and frequency and phase remain constant while the amplitude changes.

Binary ASK (BASK): We know that it can have several levels of signal elements, each with a different amplitude. ASK is normally implemented using only two levels. This is called Binary ASK or on-off keying (OOK).

Advantage: Simplicity

Disadvantage: ASK is a modulation technique most effected by noise. So this technique is not used in wireless.

Use: It is used in fiber optic.

Frequency Shift Keying (FSK):

            After line coding (NRZ / RZ) the binary bit directly modulates the frequency of an analog carrier. The frequency of the carrier signal varied to create signal elements. Both amplitude and phase remain constant while the frequency changes.

Binary FSK (BFSK): Considering two carrier frequencies. One carrier for 0 and another for 1. Actual matter is normally the carrier frequencies are very high and the difference between them is very small.

Implementation: There are two implementations of BFSK.

1.      Non-coherent

2.      Coherent

In non-coherent BFSK there may be discontinuity in the phase when one signal element ends and the next begins. In coherent BFSK, the phase continues through the boundary of two signal elements.

Non-coherent BFSK can be implemented by treating BFSK as two ASK modulations and using two carrier frequencies. Coherent BFSK can be implemented by using one voltage-controlled oscillator (VCO) that changes its frequency according to the input voltage. The input to the oscillator is the unipolar NRZ signal. When the amplitude of NRZ is zero, the oscillator keeps its regular frequency, when the amplitude is 1, the frequency is increased.

Advantage: Though amplitude remains constant so noise can easily separate from original amplitude.

Disadvantage: It needs two carriers. So bandwidth needs more than ASK

Use: Over voice lines, in high-frequency radio transmission

Phase Shift Keying (PSK):

In PSK, the signal element phase change when signal element 0 comes after 1 or 1 comes after 0.

            Binary PSK: The simplest PSK is binary PSK, in which we have only two signal elements (from 1 to 0 and 0 to 1). The phase of the output carrier shifts between two angles that are separated by 180 ^o.

            Implementation: The implementation of BPSK is simple. We use same idea what we used for ASK but we use polar NRZ for BPSK instead of unipolar NRZ signal. The polar NRZ signal is multiplied by the carrier frequency. The 1 bit (positive voltage) is represented by a phase starting at 0 ^o and the 0 bit (negative voltage) is represented by a phase starting at 180 ^o.

            Advantage: Binary PSK is as simple as binary ASK with one big advantage is it is less susceptible to noise than ASK.  It is superior to FSK because we not need two carrier signals.

            Disadvantage: More complex signal detection process than ASK and FSK.

If we can represent 2 bits in one signal then our time will save. It will reduce 50% time and 50% bandwidth. We need such that modulation. There is a modulation, which is doing this. That is Quadrarure PSK.

Quadrature PSK (QPSK):

If we keep two bits 0 and 1 in order it will looks like 00, 01, 10 and 11. Then its corresponding phases will be 0^o, 90^o, 180^o and 270^o. But actually such phase can not create accurately.

If not then receiver will be confused. So, the four possible phases counted as -135^o, -45^o, 135^o, 45^o.

Advantages: we can send more bits in less time with a less bandwidth.

Limitation: bit should not be more that 2. As a result for more than 2 bits increase BER and makes complex so receiver cant read easily.

8-PSK:

3 bits are encoded, forming tribits and producing 8 different output phases are listed below

Bits	Phases
000	-112.5o
001	-157.5o
010	-67.5o
011	-22.5o
100	112.5o
101	157.5o
110	67.5o
111	22.5o

16PSK:

4 bits are encoded, forming quabits and producing 16 different output phases are listed below

Bits	Phases
0000	11.25o
0001	33.75o
0010	56.25o
0011	78.75o
0100	101.25o
0101	123.75o
0110	146.25o
0111	168.75o
1000	191.25o
1001	213.75o
1010	236.25o
1011	258.75o
1100	281.25o
1101	303.75o
1110	328.25o
1111	348.75o

Quadrature Amplitude Modulation (QAM):

            Quadrature Amplitude Modulation is a combination of ASK and PSK.

Bandwidth Capacity:

The relationship between bandwidth and bit rate also applies to the opposite situation. For a given bandwidth (B), the highest theoretical bit rate is 2B. For example, a standard telephone circuit has a bandwidth of approximately 2700 Hz, which has the capacity to propagate 5400 bps through it. Another example if the bit rate is 1 Mbps then the bandwidth is 500 KHz.


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