Synchronous vs. Self-Synchronizing Scramblers: A Comparison

This page compares Synchronous and Self-Synchronizing scramblers, highlighting the differences between them and their applications in data communication. We’ll cover their advantages and disadvantages.

Introduction

In wireless communication, a scrambler is employed to eliminate long sequences of ones and zeros by randomizing the data. This process takes place before Forward Error Correction (FEC) encoding, modulation, or line encoding. The scrambler resides on the transmitter side.

A descrambler is used at the receiver to recover the original bit pattern from the randomized data. This process is illustrated in the physical layer of WiMAX, according to the IEEE 802.16 standard.

The scrambler serves two main functions:

• Ensuring Frequent Transitions: It introduces more transitions in the data by breaking up long strings of identical data bits (1s and 0s). This facilitates accurate timing recovery at the receiver.
• Energy Dispersion: It spreads the energy across the carrier signal, thereby reducing Inter-Carrier Interference (ICI).
• DC Balance: The concept of scrambling is also used in R8ZS and HDB3 coding techniques to maintain DC balance.

Synchronous Scrambler (Additive Scrambler)

A synchronous scrambler uses a Linear Feedback Shift Register (LFSR) to generate a Pseudo-Random Binary Sequence (PRBS). Alternatively, a pre-stored PRBS can be used.

The synchronous scrambler transforms the data stream by performing a modulo-2 sum of the input data and the PRBS sequence. Therefore, it is also known as an additive scrambler. The circuit configuration usually follows a PRBS generator based on a specific polynomial, such as 1 + X¹⁴ + X¹⁵.

Figure 1 illustrates a typical synchronous scrambler and descrambler circuit.

The same SYNC word is used for both the scrambler and descrambler at the transmit and receive ends. This ensures synchronous operation.

Advantages of Synchronous Scrambler

• Error Detection: Offers error detection capabilities.
• Improved Synchronization: Removes long strings of zeros and ones, providing more transitions in the data pattern, making synchronization easier without the need for a separate clock signal.
• No DC Component: Does not introduce any DC components.

Disadvantages of Synchronous Scrambler

• Worst-Case Scenario Failure: Can fail to generate random sequences in worst-case conditions.
• SYNC Word Dependence: Must be reset using the SYNC word, or significant error propagation will occur.

Self-Synchronizing Scrambler (Multiplicative Scrambler)

The scrambler and descrambler circuit for a self-synchronizing scrambler are illustrated in the figure below. Unlike synchronous scramblers, it doesn’t require a frame SYNC word for synchronization. This is why it’s called a self-synchronizing scrambler.

This type of scrambler effectively multiplies the input by a transfer function in the Z-space, hence the alternative name: Multiplicative Scrambler.

Figure 2 shows a self-synchronizing scrambler and descrambler circuit.

These scramblers are often defined by a polynomial, like 1 + X¹⁴ + X¹⁵. The multiplicative scrambler is recursive, whereas the descrambler is non-recursive.

Advantages of Self-Synchronizing Scrambler

• No SYNC Word Required: Doesn’t need a SYNC word (also called a “SEED”) for operation, unlike synchronous scramblers.
• Similar Advantages: Shares similar advantages to those listed for synchronous scramblers.

Disadvantages of Self-Synchronizing Scrambler

• Worst-Case Scenario Failure: Can fail to generate random sequences in worst-case conditions.
• Error Multiplication: A single bit error at the descrambler input can lead to error multiplication at the output. One error can result in “W” errors, where “W” is the number of taps used in the scrambler.