Online Shaping for ISI Channels with a Limited Number of ADC Bits

Abstract: An online shaping technique for high performance communication over Gaussian channels with Inter-Symbol Interference (ISI) and receiver Analog to Digital Converter (ADC) noise is presented. The technique uses online transmitter precoding over Pulse Amplitude Modulation (PAM) constellation, designed to shape the symbols distribution so that peak power constraint at the channel output is satisfied. An iterative decoder shares information between a modified M-BCJR module, which computes online the trellis transition probabilities of the shaped distribution, and turbo decoder. The result is a reduction in the required ADC Effective Number Of Bits (ENOB), which is in particular attractive in modern high-speed wireline links. Theoretical bounds are analytically derived which enable to assess the possible gain using shaping. On practical scenarios aim to transmit 200 Gbps and 400 Gbps over printed circuit board, we demonstrate in simulations an overall ENOB gains as high as 1.43 bit and 1.78 bit, respectively, compared to uniform 4-PAM transmission with turbo equalization at the receiver side.