VersaFEC is a system of short-block, low latency Low Density Parity Check (LDPC) codes designed to support latency-sensitive applications, such as cellular backhaul over satellite. Designed to provide maximum coding gain while minimizing the end-to-end latency, VersaFEC provides an excellent alternative to existing LDPC and DVB-S2 codes.

VersaFEC is designed to:

  • Provide an expanded choice of modulation and code combinations (ModCods). These new combinations provide equivalent coding gains to our existing LDPC offering, and also significantly reduce latency. The existing LDPC codes (as as well as DVB-S2 short-block codes) use 16 kbit blocks, whereas VersaFEC uses blocks ranging from 2 kbits to 8.2 kbits.
  • Support a patent-pending Adaptive Modulation and Coding (ACM) system. The ModCods have been chosen to provide a continuous progression in terms of Eb/No and spectral efficiency, while reducing latency to near-theoretical minimums.

VersaFEC Coding Performance

VersaFEC includes 12 Modulation and Code sets (ModCods):

Modulation Code Rate Spectral efficiency, bps/Hz Block size, bits Typical Eb/No, for BER = 5 x 10-8 Latency at
64 kbps, in milliseconds
Min. Data Rate, CCM mode Max. Data Rate, CCM mode
BPSK 0.488 0.49 2k 2.4 dB 26 18 kbps 5.7 Mbps
QPSK 0.533 1.07 4.1k 2.2 dB 53 20 kbps 10 Mbps
QPSK 0.631 1.26 4.1k 2.7 dB 59 23 kbps 10 Mbps
QPSK 0.706 1.41 4.1k 3.4 dB 62 26 kbps 10 Mbps
QPSK 0.803 1.61 4.1k 3.8 dB 66 28 kbps 12 Mbps
8-QAM 0.642 1.93 6.1k 4.6 dB 89 35 kbps 12 Mbps
8-QAM 0.711 2.13 6.1k 5.2 dB 93 39 kbps 12 Mbps
8-QAM 0.780 2.34 6.1k 5.6 dB 97 43 kbps 12 Mbps
16-QAM 0.731 2.93 8.2k 6.3 dB 125 53 kbps 12 Mbps
16-QAM 0.780 3.12 8.2k 7.0 dB 129 57 kbps 14 Mbps
16-QAM 0.829 3.32 8.2k 7.5 dB 131 60 kbps 14 Mbps
16-QAM 0.853 3.41 8.2k 8.0 dB 132 62 kbps 16 Mbps

The performance of the VersaFEC codes compared with the Shannon Bound is shown in the below Figure. For all ModCods, VersaFEC is within 0.7 to 1.0 dB of the Shannon Bound. This puts the performance of VersaFEC at or near the DVB-S2 performance with 16 kbit blocks.

VersaFEC Codes vs contrained capacity
[larger image]

Compared to Turbo Product Codes, VersaFEC generally provides 1.0 dB or more reduction in Eb/No. This translates into leased bandwidth savings and reduced BUC/HPA sizes.

Low Latency

VersaFEC is specifically designed for low latency applications. As a comparison, our LDPC Rate 2/3 8-QAM and VersaFEC Rate 0.642 8-QAM provide essentially identical spectral efficiency and Eb/No performance. However, at 64 kbps, the latency has been reduced from 350 milliseconds to 89 milliseconds – a reduction by a factor of 4.

Compared to DVB-S2 short block, the VersaFEC provides a latency reduction of anywhere from a factor of 4 to as much as a factor of 10. E.g. DVB-S2 QPSK Rate 2/3 and VersaFEC QPSK 0.631 have near identical spectral efficiency and Eb/No performance. However, at 64 kbps, the VersaFEC latency is 59 ms compared to over 500 ms for DVB-S2 – a reduction by a factor of over 8.

In addition to using smaller block sizes, to further reduce latency, VersaFEC uses non-interleaved systematic LDPC codes. Compared to DVB-S2’s interleaved codes, this achieves a significant reduction in latency. The total end-to-end latency for a high code rate systematic code (such as VersaFEC) asymptotically approaches half the latency of an interleaved code (such as DVB-S2).

VersaFEC® is a Registered Trademark of Comtech EF Data