Power efficiency: as small as possible for required data rate and error probability

Error detection/correction

Suitable power spectral density, e.g., little low frequency content

Timing information: clock must be extracted from data

Transparency: all possible binary sequences can be transmitted

Line Coding Requirements:

Unipolar

Only two voltage levels are used.

All the signal levels are on one side of

the time axis, either above or below.

The bit rate same as data rate.

DC component is present in the

encoded signal and there is loss

of synchronization for long

sequences of 0’s and 1’s.

Unipolar: NRZ (Non-Return-to-Zero)

Traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) scheme in which the positive voltage defines bit 1 and the zero voltage defines bit O.

It is called NRZ because the signal does not return to zero at the middle of the bit.

5 Categories of Line Coding:

Unipolar

(NRZ)

Polar

(NRZ, RZ, Biphase)

Bipolar

(AMI, Pseudoternary)

Multilevel

(2BIQ, 8B6T, 4D-PAMS)

Multiline

(MLT3)

**Digital-to-Digital Conversion**

**Line Coding**

**Khristine Shara Lee M. Teves, David Miguel b. dela Cruz**

CPE, ECE Dept., DLSU

CPE, ECE Dept., DLSU

Line Coding

The process of converting binary data

into a digital signal.

Figure 1. Unipolar NRZ

Polar

Polar encoding technique uses two voltage levels – one positive and the other one negative.

Polar: NRZ (Non-Return-to-Zero)

Most common and easiest way to transmit digital signals is to use two different voltage levels for the two binary digits.

There are two encoding schemes in NRZ: NRZ-L and NRZ-I. In polar NRZ encoding,

In the first variation, NRZ-L (NRZ-Level), the level of the voltage determines the value of the bit.

In the second variation, NRZ-I (NRZ-Invert), the change or lack of change in the level of the voltage determines the value of the bit. If there is no change, the bit is 0; if there is a change, the bit is 1.

Figure 2.a. Polar NRZ

Polar: RZ (Return-to-Zero)

In RZ, the signal changes not between bits but during the bit. Key characteristics of the RZ coding are:

Three levels

Bit rate is double than that of data rate

No dc component

Good synchronization

Main limitation is the increase in bandwidth

Figure 2.b. Polar RZ

Unipolar: Biphase - Manchester

The idea of RZ (transition at the middle of the bit) and the idea of NRZ-L are combined into the Manchester scheme.

In Manchester coding the mid-bit transition serves as a clocking mechanism and also as data.

In the standard Manchester coding there is a transition at the middle of each bit period. A binary 1 corresponds to a low-to-high transition and a binary 0 to a high-to- low transition in the middle.

Figure 2.c. Polar Manchester and Differential Manchester

Bipolar

In bipolar encoding (sometimes called multilevel binary), there are three voltage levels: positive, negative, and zero.

The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative.

Bipolar: AMI (Alternate Mark Inversion)

This scheme uses three voltage levels.

Unlike RZ, the zero level is used to represent a 0 and a binary 1’s are represented by alternating positive and negative voltages

Figure 3. Bipolar AMI and Pseudoternary

Bipolar: Pseudoternary

This is same as AMI, but alternating positive

and negative pulses occur for binary 0 instead

of binary 1.

Key characteristics are:

Three level

No DC component

Loss of synchronization

Lesser bandwidth

Multilevel: 2BIQ

A mBnL scheme means two binary, one quaternary (2BIQ), uses data patterns of size 2 and encodes the 2-bit patterns as one signal element belonging to a four-level signal. In this type of encoding m =2, n = 1, and L =4 (quaternary).

Figure 4.a. Multilevel 2BIQ

Unipolar: Biphase - Differential Manchester

The ideas of RZ and NRZ-I are combined into Differential Manchester scheme.

In Differential Manchester, inversion in the middle of each bit is used for synchronization.

The encoding of a 0 is represented by the presence of a transition both at the beginning and at the middle and 1 is represented by a transition only in the middle of the bit period.

Key characteristics are:

Two levels

No DC component

Good synchronization

Higher bandwidth due to doubling of bit rate with respect to data rate

Multilevel: 8B6T

A very interesting scheme is eight binary, six ternary (8B6T).

The idea is to encode a pattern of 8 bits as a pattern of 6 signal elements, where the signal has three levels (ternary). In this type of scheme, we can have 28 =256 different data patterns and 36 =478 different signal patterns.

Figure 4.b. Multilevel 8B6T

Multilevel: 4D-PAMS

The last signaling scheme in this category is called four dimensional five-level pulse amplitude modulation (4D-PAM5). The 4D means that data is sent over four wires at the same time.

It uses five voltage levels, such as -2, -1, 0, 1, and 2. However, one level, level 0, is used only for forward error detection

Figure 4.c. Multilevel 4D-PAMS

Multiline: MLT3

If we have a signal with more than two levels, we can design a differential encoding scheme with more than two transition rules. MLT-3 is one of them.

The multiline transmission, three level (MLT-3) scheme uses three levels (+v, 0, and - V) and three transition rules to move between the levels.

1. If the next bit is 0, there is no transition.

2. If the next bit is 1 and the current level is not 0, the next level is 0.

3. If the next bit is 1 and the current level is 0, the next level is the opposite of the last nonzero level.

Figure 5. Multiline MLT3

References

Catalan, J.

Line Coding

(PDF Document).

Line Coding for Digital Communication

(PDF Document). Retrieved from

http://web.stanford.edu/class/ee179/handouts/slide18.pdf

Kharagpur.

Data Communication Fundamentals: Transmission of Digital Signal

(PDF Document). Retrieved from

http://nptel.ac.in/courses/106105080/pdf/M2L4.pdf

Forouzan, B. (2007).

Data Communications and Networking

(4th ed.). New York, NY:

The Mcgraw-Hill Company, Inc.