Lunds universitet/Institutionen för elektro- och informationsteknik
Abstract
Due to computation-intensive nature of LDPC decoders, a lot of research is going towards efficient implementation of their original algorithm (SPA). As "Min-Sum" approximation is basically an overestimation of SPA, this thesis investigates more accurate, yet area efficient, approximations of SPA, to select an optimum one. In a general comparison between main approximation methods (e.g. LUT, PWL, CRI), PWL showed the most area-efficiency. Studying different mathematical formats of SPA, Soft-XOR based format with forward-backward scheme was chosen for hard- ware implementation. Its core function (Soft-XOR) was implemented with CRI approximation, which achieved the highest efficiency, compare to other approxi- mations. Using this core function, a flexible, pipe-lined, Soft-XOR based CNU (the computational unit of LDPC decoders) with forward-backward architecture was developed in 18nm CMOS. The implemented CNU’s area and speed can eas- ily be changed in instantiation. A SPA decoder based on the developed CNU was estimated to have an area of 1.6M as equivalent gate count and a throughput of 10Gb/s, with a frequency of 1.25GHz and for 10 iterations. The decoder uses IEEE 802.11n Wi-Fi standard with flooding schedule. The BER/SNR loss, com- pare to floating-point SPA, is 0.3dB for 10 iterations and less than 0.1dB for 20 iterations.You have to get lost before you can be found, a quote by Jeff Rasley goes very well for Low Density Parity Check (LDPC) codes. First invented by Gallager in 1962 but kind of lost during the journey of evolution of telecommunication networks because of their high complexity and demanding computations, which technology was not so advanced to handle, at that time. However, during late 1990s, success of turbo codes invoked the re-discovery of Low Density Parity Check (LDPC) codes. Recently it has attracted tremendous research interest among the scientific com- munity, as today’s technology is advanced enough and to make LDPC decoders completely commercial. In a wireless network, the information is not just sim- ply sent, but first encoded. In a sense, all the transmitted bits are tied together, according to some mathematical rules. Therefore, if noise destructs parts of the information while traveling, the LDPC decoder at the receiver side, can automat- ically detect and retrieve those parts, based on the other parts. Here, our main focus is on the decoder. For actual hardware implementation of the decoder, some level of approximation of the ideal algorithm is always necessary, which reduces the accuracy depending on the approximation. Ericsson is developing the next-generation wireless network for 5G, and already possesses the "Min-Sum" approximation of the LDPC decoder. As the current requirements demand more accurate decoders, the goal of this thesis is to evalu- ate a more accurate but more costly version of the LDPC decoder, as well as its flexibility. Thus, several candidates were selected and evaluated based on their complexity, cost, and their accuracy towards error correction. After performing several trade-offs, an approximation method is chosen and the corresponding cost is derived. With this acquired data, a trade-off between accuracy and cost can be made, depending on the application
