Question

Compare and contrast how different Line Coding Schemes works in Digital to Digital communication. Also mention the best line coding scheme in your opinion with valid reasons?

Answer 1

Compare and contrast how different Line Coding Schemes works in Digital to Digital communication.

Difference between Unipolar, Polar and Bipolar Line Coding Schemes

Data as well as signals that represents data can either be digital or analog. Line coding is the process of converting digital data to digital signals. By this technique we converts a sequence of bits to a digital signal. At the sender side digital data are encoded into a digital signal and at the receiver side the digital data are recreated by decoding the digital signal.

We can roughly divide line coding schemes into five categories:

1. Unipolar (eg. NRZ scheme).
2. Polar (eg. NRZ-L, NRZ-I, RZ, and Biphase – Manchester and differential Manchester).
3. Bipolar (eg. AMI and Pseudoternary).
4. Multilevel
5. Multitransition

But, before learning difference between first three schemes we should first know the characteristic of these line coding techniques:

• There should be self-synchronizing i.e., both receiver and sender clock should be synchronized.
• There should have some error-detecting capability.
• There should be immunity to noise and interference.
• There should be less complexity.
• There should be no low frequency component (DC-component) as long distance transfer is not feasible for low frequency component signal.
• There should be less base line wandering.

Unipolar scheme –
In this scheme, all the signal levels are either above or below the axis.

• Non return to zero (NRZ) – It is unipolar line coding scheme in which positive voltage defines bit 1 and the zero voltage defines bit 0. Signal does not return to zero at the middle of the bit thus it is called NRZ. For example: Data = 10110.

  

  But this scheme uses more power as compared to polar scheme to send one bit per unit line resistance. Moreover for continuous set of zeros or ones there will be self-synchronization and base line wandering problem.

Polar schemes –
In polar schemes, the voltages are on the both sides of the axis.

• NRZ-L and NRZ-I – These are somewhat similar to unipolar NRZ scheme but here we use two levels of amplitude (voltages). For NRZ-L(NRZ-Level), the level of the voltage determines the value of the bit, typically binary 1 maps to logic-level high, and binary 0 maps to logic-level low, and for NRZ-I(NRZ-Invert), two-level signal has a transition at a boundary if the next bit that we are going to transmit is a logical 1, and does not have a transition if the next bit that we are going to transmit is a logical 0.

  Note – For NRZ-I we are assuming in the example that previous signal before starting of data set “01001110” was positive. Therefore, there is no transition at the beginning and first bit “0” in current data set “01001110” is starting from +V. Example: Data = 01001110.

  

  Comparison between NRZ-L and NRZ-I: Baseline wandering is a problem for both of them, but for NRZ-L it is twice as bad as compared to NRZ-I. This is because of transition at the boundary for NRZ-I (if the next bit that we are going to transmit is a logical 1). Similarly self-synchronization problem is similar in both for long sequence of 0’s, but for long sequence of 1’s it is more severe in NRZ-L.

• Return to zero (RZ) – One solution to NRZ problem is the RZ scheme, which uses three values positive,negative,and zero. In this scheme signal goes to 0 in the middle of each bit.
  Note – The logic we are using here to represent data is that for bit 1 half of the signal is represented by +V and half by zero voltage and for bit 0 half of the signal is represented by -V and half by zero voltage. Example: Data = 01001.

  

  Main disadvantage of RZ encoding is that it requires greater bandwidth. Another problem is the complexity as it uses three levels of voltage. As a result of all these deficiencies, this scheme is not used today. Instead, it has been replaced by the better-performing Manchester and differential Manchester schemes.

• Biphase (Manchester and Differential Manchester ) – Manchester encoding is somewhat combination of the RZ (transition at the middle of the bit) and NRZ-L schemes. The duration of the bit is divided into two halves. The voltage remains at one level during the first half and moves to the other level in the second half. The transition at the middle of the bit provides synchronization.

  Differential Manchester is somewhat combination of the RZ and NRZ-I schemes. There is always a transition at the middle of the bit but the bit values are determined at the beginning of the bit. If the next bit is 0, there is a transition, if the next bit is 1, there is no transition.

  Note –
  1. The logic we are using here to represent data using Manchester is that for bit 1 there is transition form -V to +V volts in the middle of the bit and for bit 0 there is transition from +V to -V volts in the middle of the bit.
  2. For differential Manchester we are assuming in the example that previous signal before starting of data set “010011” was positive. Therefore there is transition at the beginning and first bit “0” in current data set “010011” is starting from -V. Example: Data = 010011.

  

  The Manchester scheme overcomes several problems associated with NRZ-L, and differential Manchester overcomes several problems associated with NRZ-I as there is no baseline wandering and no DC component because each bit has a positive and negative voltage contribution.

  Only limitation is that the minimum bandwidth of Manchester and differential Manchester is twice that of NRZ.

Bipolar schemes –
In this scheme 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.

• Alternate Mark Inversion (AMI) – A neutral zero voltage represents binary 0. Binary 1’s are represented by alternating positive and negative voltages.
• Pseudoternary – Bit 1 is encoded as a zero voltage and the bit 0 is encoded as alternating positive and negative voltages i.e., opposite of AMI scheme. Example: Data = 010010.
  

  The bipolar scheme is an alternative to NRZ.This scheme has the same signal rate as NRZ,but there is no DC component as one bit is represented by voltage zero and other alternates every time.

>> WHAT IS LINE CODING?

In telecommunication, a line code (also called digital baseband modulation, also called digital baseband transmission method) is a code chosen for use within a communications system for baseband transmission purposes. Line coding is often used for digital data transport.

Binary 1’s and 0’s, such as in PCM signaling, may be represented in various serial–bit signaling formats called line codes.

>> WHY LINE CODING?

There are many reasons for using line coding. Each of the line codes you will be examining offers one or more of the following advantages:

• Spectrum Shaping and Relocation without modulation or filtering. This is important in telephone line applications, for example, where the transfer characteristic has heavy attenuation below 300 Hz.
• Bit clock recovery can be simplified.
• DC component can be eliminated; this allows AC (capacitor or transformer) coupling between stages (as in telephone lines). Can control baseline wander (baseline wander shifts the position of the signal waveform relative to the detector threshold and leads to severe erosion of noise margin).
• Error detection capabilities.
• Bandwidth usage; the possibility of transmitting at a higher rate than other schemes over the same bandwidth.

At the very least the LINE-CODE ENCODER serves as an interface between the TTL level signals of the transmitter and those of the analog channel. Likewise, the LINE-CODE DECODER serves as an interface between the analog signals of the channel and the TTL level signals required by the digital receiver.

>> MOST POPULAR LINE CODES

Some of the most popular line codes are shown below.

The line codes shown above are also known by other names:

Polar NRZ: Also called NRZ–L where L denotes the normal logic level assignment

Bipolar RZ: Also called RZ–AMI, where AMI denotes alternate mark (binary 1) inversion

Bipolar NRZ: Also called NRZ–M, where M denotes inversion on mark (binary 1)

>> TWO MAJOR CATEGORIES OF LINE CODING

There are 2 major categories: return–to–zero (RZ) and nonreturn–to–zero (NRZ). With RZ coding, the waveform returns to a zero–volt level for a portion (usually one–half) of the bit interval.

>> FURTHER CLASSIFICATION

The waveforms for the line code may be further classified according to the rule that is used to assign voltage levels to represent the binary data.

1) Unipolar Signalling:

In positive–logic unipolar signaling, the binary 1 is represented by a high level (+A volts) and a binary 0 by a zero level. This type of signaling is also called on–off keying (OOK).

2) Polar Signaling:

Binary 1’s and 0’s are represented by equal positive and negative levels

3) Bipolar (Pseudoternary) Signaling:

Binary 1’s are represented by alternating positive or negative values. The binary 0 is represented by a zero level. The term pseudoternary refers to the use of 3 encoded signal levels to represent two–level (binary) data. This is also called alternate mark inversion (AMI) signaling.

4) Manchester Signaling:

Each binary 1 is represented by a positive half–bit period pulse followed by a negative half–bit period pulse. Similarly, a binary 0 is represented by a negative half–bit period pulse followed by a positive half–bit period pulse. This type of signaling is also called split–phase encoding.

>> PROPERTIES OF LINE CODES

Each line code has advantages and disadvantages.

For example, the unipolar NRZ line code has the advantage of using circuits that require only one power supply, but it has the disadvantage of requiring channels that are DC coupled (i.e. with frequency response down to f = 0), because the waveform has a non–zero DC value.

The polar NRZ line code does not require a DC coupled channel, provided that the data toggles between binary 1’s and 0’s often and that equal numbers of 1’s and 0’s are sent. However, the circuitry that produces the polar NRZ signal requires a negative voltage power supply as well as the positive voltage power supply.

The Manchester NRZ line code has the advantage of always having a 0 DC value, regardless of the data sequence, but it has twice the bandwidth of the unipolar NRZ or polar NRZ code because the pulses are half the width.

>> DESIRABLE PROPERTIES OF A LINE CODE

• Self–Synchronisation: There is enough timing in formation built into the code so that bit synchronisers can extract the timing or clock signal. A long series of binary 1’s or 0’s should not cause a problem in time recovery.
• Low Probability of Bit Error: Receivers can be designed that will recover the binary data with a low probability of bit error when the input data is corrupted by noise or ISI.
• A Spectrum that is Suitable for the Channel: For example, if the channel is AC coupled, the PSD of the line code signal should be negligible at frequencies near 0. In addition, the signal bandwidth needs to be sufficiently small compared to the channel bandwidth, so that ISI will not be a problem.
• Transmission Bandwidth: This should be as small as possible.
• Error Detection Capability: It should be possible to implement this feature easily by the addition of channel encoders and decoders, or the feature should be incorporated into the line code.

>> DIFFERENTIAL CODING

When serial data is passed through many circuits along a communication channel, the waveform is often unintentionally inverted (i.e. data complemented). This result can occur in a twisted pair transmission line channel just by switching the 2 leads at a connection point when a polar line code is used. (Note: such switching would not affect the data of a bipolar signal)

To eliminate this problem differential encoding is often employed.

Each digit in an differential encoded sequence is obtained by comparing the present input bit with the past encoded bit. A binary 1 is encoded if the present input bit and past encoded bit are of opposite state. A binary 0 is encoded if the states are the same.

Differential coding is often used with Manchester coding. The diagram below shows Manchester coding and differential Manchester coding for a sequence of bits.