Turbo Product Codes (TPCs) are a powerful method of forward error correction (FEC) coding and are well suited to Wireless LAN applications, such as the 802.11 standardized equipment. Although TPCs are not specified in the standard, the standard is open for user defined forward error correction. The technology is emerging from the world of proprietary implementations and becoming an open solution in many diversified applications from mobile hand held PCs to emergency rooms in hospitals. Wireless LAN systems utilize radio frequency signals in a line of sight fashion for purposes of either avoiding the issues of cable and wire installations or allowing mobile communications. One major challenge facing wireless LAN applications is signal corruption that usually results in transmission delays. Turbo Product Code (TPC) error correction offers a superior method of improvement netting a coding gain of more than 9dB at a packet error rate of 1/10E6 typically in systems that previously used no FEC coding.
The traffic sent over Wireless LANs is packet data ranging from about 28 Bytes to 2.25 K Bytes. A single TPC implementation can easily handle multiple packet sizes. Remaining efficient when transmitting small packets requires a minimum latency between the sender and receiver. This constrains any block oriented error correction method to have a small packet size. The decoder for a block code must receive the entire codeword before it can determine the corrected value for any bits in the codeword. The minimum latency for a block code with encoded packet size N (bits), transmitted over a system with channel rate C (bits/sec), is equal to N/C. This is a lower bound on the latency as it does not take into account the time required to actually decode the packet. Turbo Product Codes can be built with packet sizes as low as 64 bits. In addition, the decoder for a TPC can be constructed such that the decoding time is much less than the time to receive one packet. Therefore, the latency through the decoder is slightly more than N/C.
The causes of errors in Wireless LANs are usually classified as Gaussian or Impulse noise. Errors caused by Gaussian noise are effectively corrected by adding FEC to the system. If the Impulse noise causes burst errors that are less than seven times the X-axis code length for a square 2D product code, then TPC with helical interleaving enabled can correct the errors. For longer bursts a large interleaver is required to spread the errors across multiple blocks.
The flexibility of Turbo Product Codes allows a single decoder to support a wide range of code rates. This can be used during poor channel conditions to decrease the bit error rate (BER) out of the decoder at the expense of bandwidth. This also allows for multiple channel systems to multiplex a single decoder, with different codes for each channel. Figure 1 is a performance graph showing both Bit Error Rates (BER) and Packet Error Rates (PER) for a (64,57)x(64,56) enhanced TPC code shortened to fit into a 188 Byte packet size. This results in a (2141,1504) bit, which is a 188 Byte code, with code rate of approximately 0.7.
Figure 1, TPC Bit Error Rate and Packet Error Rate Performance
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