Error Protection

The goal of high erasure error protection (heep) is to provide effective error protection schemes for directional links, which are suffering under strong packet erasures. Such links are for example optical wireless point-to-point links which operate on high data-rates, but are disturbed by slow fading. This fading is mostly produced by temporary changes of atmospheric propagation conditions. Furthermore, mobile links can additionally be degraded by temporary transceivers misalignment or line-of-sight obscurations.

High-rate Gigahertz microwave links and especially free-space optical (FSO) links can be degraded significantly by long duration (relative to the symbol-length) link outages. Typical physical-layer coding schemes will fail in recovering long outages because of an insufficient constraint length. Packet-based transmission can help to overcome this problem, because "super blocks" with an inherent transmission time much longer than outage times can be implemented without great effort. Therefore in the case of long outages it is preferable to implement an error protection scheme on packet-layer.

On simplex links or in unidirectional communication scenarios only proactive error correction (forward error correction, FEC) is possible in order to protect the information against data loss. The method Codex GmbH & Co. KG implemented and tested for this scenario is based on an open source large block FEC codec. It implements the LDPC-Staircase and LDPC-Triangle large block FEC codes. In the contrary to LDPC codes working at the physical layer (e.g. for DVB-S2), this is a higher layer FEC scheme working over Ethernet frames. In FSO channels the behavior of Ethernet frame transmission equals a packet erasure channel. This is because of the frame check sequence, which is used by Ethernet, frames are either delivered correctly or get lost in signal fades. Typically errors are correlated. That means that the packet erasure channel has typically a large memory of several hundred frames.

The figure below gives an overview on our encoder implementation. The implementation is able to capture all kinds of Ethernet traffic from a local network (LAN) and builds very large codewords (super blocks) from the collected frames by adding parity information. These super blocks are then transmitted via an Ethernet p2p link to the receiving decoder. Depending on the super block size a super block transmission duration of several seconds can be achieved. This enables the code to overcome outages with durations of several hundred milliseconds. This code makes it possible to communicate over a high-rate communication channel, which is suffering from slow fading without decrease in quality of service.

Schematics of the Codex packet layer FEC system
Schematics of the Codex packet layer FEC system