Energy Efficient Turbo Decoder with Reduced State Metric Quantization

In the field of mobile communications, the energy issue of a turbo decoder becomes an equivalent constraint as through-put and performance. This paper describes a technique to reduce the internal bitwidth of the state metrics, and hence, to decrease the entire energy dissipation of a turbo decoder. This approach is based on the saturation of the state metrics. Two cases are investigated: saturation outside the ACS recursion loop and saturation inside the ACS recursion loop. The targeted system is the Universal Mobile Telecommunications System (UMTS) with an 8-state turbo decoder using the Max-Log-MAP algorithm. When received symbols and extrinsic informations are respectively 4-bit and 6-bit quantized, the internal bitwidth of the state metrics can be reduced from 7 bits downto 4 bits. This reduction is paid by a loss of 0.1 dB at a Bit Error Rate (BER) of 1^-6. In addition, when 40 SISO decoders perform in parallel, the proposed optimization yields to a reduction of memory area by 10% and leads to an energy reduction of 24% for a 70 nm technology.