  The ever-increasing demand to extract temporal correlations across sequential
data and perform context-based learning in this era of big data has led to the
development of long short-term memory (LSTM) networks. Furthermore, there is an
urgent need to perform these time-series data-dependent applications including
speech/video processing and recognition, language modelling and translation,
etc. on compact internet-of-things (IoT) edge devices with limited energy. To
this end, in this work, for the first time, we propose an extremely area- and
energy-efficient LSTM network implementation exploiting the passive resistive
random access memory (RRAM) crossbar array. We developed a hardware-aware LSTM
network simulation framework and performed an extensive analysis of the
proposed LSTM implementation considering the non-ideal hardware artifacts such
as spatial (device-to-device) and temporal variations, non-linearity, noise,
etc. utilizing an experimentally calibrated comprehensive phenomenological
model for passive RRAM crossbar array. Our results indicate that the proposed
passive RRAM crossbar-based LSTM network implementation not only outperforms
the prior digital and active 1T-1R crossbar-based LSTM implementations by more
than three orders of magnitude in terms of area and two orders of magnitude in
terms of training energy for identical network accuracy, but also exhibits
robustness against spatial and temporal variations and noise, and a faster
convergence rate. Our work may provide the incentive for experimental
realization of LSTM networks on passive RRAM crossbar arrays.
