Bit Error Rate (BER) Basics and Measurement Techniques

This page describes the basics of Bit Error Rate (BER) and explains how to measure it, particularly within a VSAT (Very Small Aperture Terminal) system. BER is a key performance indicator for any system that transmits digital data (bits) from one point to another, whether it’s a wired or wireless connection.

What is Bit Error Rate (BER)?

BER is defined as:

Bit Error Rate (BER) = Number of bits in Error / Total number of bits transmitted

In essence, it’s the ratio of incorrectly received bits to the total number of bits sent. To calculate BER, the received bits are compared to a known reference sequence at the receiver. The number of bits received in error is then determined.

Example:

If 1,000,000 (10⁶) bits are transmitted and 50 bits are received in error, the BER would be:

50 / 1,000,000 = 50 x 10^-6

This means the BER is 50 in a million, or 0.00005.

Reference Bits:

The reference bits used for comparison are often a predefined pattern, a constant sequence of all 1s or 0s, a preamble within a data frame, a Cyclic Redundancy Check (CRC), or a header check sequence.

BER vs. SNR

Fig.1: BER versus SNR curve

Figure 1 illustrates a typical BER versus Signal-to-Noise Ratio (SNR) curve. This curve demonstrates that a higher SNR generally leads to a lower BER. For example, the curve shows a BER of 10^-2 at an SNR of 10 dB.

BER Measurement in a VSAT System

To measure the BER of a complete end-to-end wireless system, such as a VSAT link, a BER test equipment is used. This is typically done by looping back the other side MUX-DEMUX or looping back the same side by setting modulator and demodulator frequencies to the same value.

The BER test equipment is connected to the MUX-DEMUX unit, and the same Pseudo-random Number (PN) sequence is configured on both the transmitting and receiving ends.

Fig.2: Bit Error Rate measurement

Figure 2 shows an example where an HP 4951C protocol analyzer is interfaced with a MUX/DEMUX unit to measure the performance of a VSAT system in a self-loopback configuration at RF. This configuration allows you to check the BER of the system itself.

In this setup, the protocol analyzer is configured with a specific data pattern, which serves as the reference for comparison with the received bits to identify errors. To enable loopback operation, the modulator and demodulator frequencies in the satellite modem are set to the same value.

VSAT 1 Modulator Frequency- 70 MHz, Demodulator Frequency- 70 MHz

Fig.3: Bit Error Rate measurement in VSAT system

Figure 3 illustrates measuring BER in a VSAT 1 - satellite - VSAT 2 configuration, and then VSAT 2 - satellite - VSAT 1. Here, the loopback is manually created at the VSAT 2 MUX-DEMUX using a special cable designed for the data interface standard. This allows for checking the BER of the complete end-to-end system.

Here are the settings used in this example:

• VSAT 1 Modulator Frequency: 70 MHz
• VSAT 2 Demodulator Frequency: 70 MHz
• VSAT 2 Modulator Frequency: 71 MHz
• VSAT 1 Demodulator Frequency: 71 MHz

Packet Error Rate (PER) and Frame Error Rate (FER)

For wireless systems like WiMAX, the concept of measuring the performance of a digital system is often referred to as Packet Error Rate (PER). PER measures the number of packets received in error. In some systems, Frame Error Rate (FER) is also measured.