Difference Between Unipolar, Polar, and Bipolar Line Coding Schemes

Unipolar Vs. Polar Vs. Bipolar Line Coding Schemes: Know the Difference Between Unipolar, Polar, and Bipolar Line Coding Schemes

The data and signals representing data can be either analog or digital. The process of line coding converts the digital data into digital signals. Before we dive into the difference between Unipolar, Polar, and Bipolar Line Coding Schemes, let us first know a bit more about Lide Coding.

Using the Line Coding technique, we can easily convert a given sequence of various bits into a digital signal. The sender-side encodes the digital data into the digital signals, while the receiver-side recreates this digital data by decoding the received digital signal.

These coding schemes can be roughly divided into five major categories:

Multitransition
Multilevel
Bipolar (ex., Pseudoternary and AMI)
Polar (ex., RZ, NRZ-I, NRZ-L, and Biphase- differential Manchester and Manchester)
Unipolar

We will learn the difference between the last three schemes in this article. But before we do that, here are the characteristics of these line coding techniques:

Self-synchronizing should be there. It means that the clocks of both- the sender as well as the receiver should be synchronized.
There must be some capability of error detection.
The immunity to interference and noise must be present.
The complexity must be very low.
No component of low frequency must be there (DC-component) because it is not feasible for a low-frequency component signal to have a long-distance transfer.
The base-line wandering must be very less.

What is a Unipolar Scheme?

All the signal levels are present either below or above the axis in the case of a unipolar scheme.

NRZ (Non-return to Zero) – It is a type of Unipolar scheme in which the bit 0 gets defined by the zero voltage and the bit 1 gets defined by the positive voltage. The signal here never returns to zero while in the middle of a bit. Hence, it is known as NRZ. Example: Data = 10110.

This scheme utilizes more power compared to the polar scheme for sending a single bit per unit line resistance. Also, for a continuous set of ones or zeros, there will be a problem of base-line wandering and self-synchronization.

What is a Polar Scheme?

In the case of Polar Schemes, we have voltages on both given sides of an axis.

NRZ-I and NRZ-L – Both of these are kind of similar to the unipolar scheme of NRZ, but the difference is that in this case, we use two voltage/ amplitude levels. In the case of NRZ-Level (NRZ-L), the value of the bit gets determined by the level of the voltage. Here, binary 0 maps to a low logic level, while binary 1 maps to a high logic level. On the other hand, in the case of NRZ-Invert (NRZ-I), the two-level signal consists of a transition at the boundary only if the bit that is going to be transmitted next is logical 1. This transition is not at all present if the next bit to be transmitted is logical 0.
NRZ-I and NRZ-L: Comparison between them – Both of these have a problem of baseline wandering, but it is twice as bad for NRZ-L as it is for NRZ-I. It is because of the transition present at the NRZ-I boundary (it happens if the next bit to be transmitted is a logical 1). In a similar way, the problem of self-synchronization is the same for a long sequence of various 0s. But for a long sequence of various 1s, the self-synchronization is way more severe in the case of NRZ-L.
RZ (Return to Zero) – The RZ scheme is one solution to the problem of NRZ- that uses three values- negative, positive, and zero. In the mid of each bit, the signal gets to 0 in this scheme signal. The RZ encoding requires greater bandwidth, which is its primary disadvantage. Complexity is another problem because it uses three different voltage levels. Due to all these deficiencies, we don’t use this scheme anymore today. It has now been replaced with differential Manchester and Manchester schemes now – that perform much better.
Biphase (Differential Manchester and Manchester) – The Manchester encoding is kind of a combination of the NRZ-L and RZ (transition occurring at the mid of the bit) schemes. Here, the overall duration of a bit is present in two halves. During the first half, the voltage persists at one level. In the second half, this voltage moves to the other level. The transition present in the middle of the bit implements synchronization.
The Differential Manchester is basically a combination of the NRZ-1 and RZ schemes. A transition always occurs in the middle of a bit. But we determine the bit values at the very beginning of that bit. A transition occurs if the next bit turns out to be 0, but there is no transition if the next bit turns out to be 1.
Pros and Cons of Differential Manchester and Manchester Schemes – The Manchester scheme provides a solution to various problems that tag along NRZ-L, and the differential Manchester scheme provides a solution to various problems that tag along NRZ-I. It happens because every bit consists of a negative and a positive voltage contribution. Thus, there’s no DC component, and baseline wandering is also not present.
The only limitation, in this case, is that the minimum bandwidth required in differential Manchester, as well as Manchester, is twice as compared to that of the NRZ.

What is a Bipolar Scheme?

In the case of a bipolar scheme, we have three voltages: negative, positive, and zero. Here, the voltage level is zero for one data element. While for the other element, the voltage level alternates between negative and positive.

AMI (Alternate Mark Inversion) – The Binary 0 gets represented by a neutral voltage. On the other hand, alternating negative and positive voltages represent the Binary 1s.
Pseudoternary – Here, we encode the bit 1 as a zero voltage, while we encode the bit 0 as alternating negative and positive voltages. It means that this one is exactly the opposite of the AMI scheme. For example, Data = 010010.

This scheme (bipolar) acts as an alternative to the NRZ. The signal rate in this scheme is very similar to that of the NRZ. But here, we have no DC component, because the voltage zero represents one bit while the other one keeps alternating every time.