Discrete Multitone Signalling for Wireline Communication

For serial wireline communication beyond 56 Gb/s, bandwidth-efficient modulation is needed. Four-level pulse amplitude modulation (4-PAM) has become the standard technique at 56–64 and 112 Gb/s. However, whereas decision feedback equalization (DFE) with 5 taps or more was common for 2-PAM links at lower data rates, the speculative look-ahead techniques required to satisfy a DFE's timing requirements at 56 Gb/s increase power consumption exponentially with the number of taps and the number of modulation levels. Thus, 4-PAM DFEs at 56 Gb/s are generally limited to 2 taps or fewer. This limitation, in turn, necessitates the use of long (10 taps or more) finite impulse response (FIR) feed-forward equalization (FFE) to accurately eliminate the intersymbol interference in long reach (LR) channels. Thus, LR receivers at 56–64 and 112 Gb/s comprise an analog-to-digital converter (ADC) followed by digital equalization, and the transmitters increasingly use a digital-to-analog converter (DAC) preceded by a digital filter. Discrete multitone (DMT) signalling obviates the need for a long FIR FFE and DFE, and has demonstrated better spectral efficiency than 4-PAM above 50 Gb/s. In this work, we consider the potential of DMT for wireline communication beyond 100 Gb/s. For example, a spectral efficiency of 2.5 bits/sample is achievable over an IEEE P802.3ck channel with 18 dB loss at 40 GHz, affording an aggregate data rate of 200 Gb/s at 80 GS/s with 150 fs of jitter and 1.26 mV of noise at the input to the receiver. Significant improvement in bit error rate (BER) is obtained by increasing DAC resolution to 8 bits.