Bit Error Rate(BER) basics and measurement
This page describes Bit Error Rate(BER) basics and explains Bit Error Rate measurement in VSAT system.
BER or Bit Error rate is used as performance measurement of any system used to transmit digital data in the form of bits from one end to the other. It includes both wire line and wireless system.
Bit Error Rate (BER) = Number of bits in Error/Total number of bits transmitted
As mentioned above in the BER equation, BER calculation is done by comparing received bits with known reference bits at receiver and bits received in error is found out. Once we have bits in error it has been divided by total number of bits to obtain BER (Bit Error rate).
For Example, if 106 bits are transmitted and we find that there are 50 bits received in error than 50 x 10-6 is the BER of this case.
Fig.1 shows typical BER versus SNR curve, it shows BER of 10-2 at SNR of 10 dB.
Often this reference bits are predefined pattern or constant pattern of all 1s or 0s or preamble in the frame or CRC or header check sequence.
For BER (Bit Error rate) measurement of complete end to end wireless system using BER test equipment is 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. BER test equipment is interfaced with MUX-DEMUX unit and same PN sequence is set. The same is shown in the figure below.
Fig.2 depicts HP 4951C protocol analyzer interfaced with MUX/DEMUX to measure performance of VSAT 1-satellite-VSAT 1
in self-loopback at RF to check Bit Error Rate (BER) of the system.
Here in Protocol analyzer data pattern is set which is used as reference to
compare with received bits to determine bits in error. To make it work in loop
back modulator frequency and demodulator frequency in Satellite modem is set to the same value.
VSAT 1 Modulator Frequency- 70 MHz, Demodulator Frequency- 70 MHz
Fig.3 depicts HP 4951C protocol analyzer interfaced with MUX/DEMUX to measure performance of VSAT 1-satellite-VSAT 2 and again VSAT-2-satellite-VSAT 1. Here loop back is made manually at VSAT 2 MUX-DEMUX using special cable made with respect to data interface standard. This checks Bit Error Rate (BER) of the end to end system. Here following setting are done.
VSAT 1 Modulator Frequency- 70 MHz, VSAT 2 Demodulator Frequency- 70 MHz
VSAT 2 Modulator Frequency- 71 MHz, VSAT 1 Demodulator Frequency- 71 MHz
For wireless system such as WiMAX the same concept of measuring the performance of digital
system is defined as PER (Packet Error Rate). In PER based performance measurement number
of packets in error is found out. In some systems FER or frame error rate is also determined.
Refer BER Measurement➤➤.