BER
Understanding BER
BER is the bottom-line metric for any digital communication system. All other specifications (NF, gain, EVM, linearity) ultimately determine whether the system achieves the required BER under operational conditions.
BER vs SNR
- BPSK/QPSK: BER = Q(sqrt(2 Eb/N0)). 10^-6 BER at Eb/N0 = 10.5 dB.
- 16-QAM: 10^-6 BER at Eb/N0 = 14.5 dB.
- 64-QAM: 10^-6 BER at Eb/N0 = 18.5 dB.
- With FEC (rate 1/2): Coding gain of 3-8 dB. LDPC codes approach Shannon limit.
BER Measurement
- BERT (Bit Error Rate Tester): Known bit sequence transmitted and compared at receiver.
- Error counting: Requires sufficient observation time for statistical confidence. 10^-6 BER needs > 10^7 bits.
Frequently Asked Questions
What is BER?
BER is the ratio of incorrectly received bits to total bits. It is the fundamental digital link quality metric. Requirements: 10^-3 for voice, 10^-6 for data, 10^-9 for fiber. BER depends on SNR, modulation, and coding.
How is BER measured?
A BERT transmits a known pseudo-random bit sequence and compares the received sequence. The ratio of errors to total bits gives BER. Sufficient observation time is needed: measuring 10^-9 BER requires transmitting at least 10^10 bits.
How does FEC improve BER?
Forward Error Correction adds redundant bits that allow the decoder to correct errors without retransmission. Modern LDPC and turbo codes achieve 6-10 dB coding gain, reducing the required SNR by that amount for the same BER.