Signal processing techniques for ensuring fidelity of back-end signal transmission in flash memory based solid state drives

Solid State Drives (SSDs) use Flash memory for its high speed performance for random reads. An SSD controller has a front end interface which communicates with the host and a back end interface which communicates with the media. A popular media interface is the Open NAND Flash Interface (ONFI). As throughputs increase, the ONFI bandwidths have to increase. ONFI 4.0 supports data rates exceeding 2.5 GHz. At these rates, the ONFI channel is far from ideal. The channel becomes frequency selective and the channel impulse response exhibits a notch. Since inter-symbol-interference (ISI) sets in, a scheme for ISI cancellation is required or some other means to eliminate the effect of ISI becomes mandatory. In this work, we propose a filter bank approach to solve the problem of notch cancellation. Since a frequency band is completely lost, over-sampling of the signal becomes mandatory. One scheme maybe to use the excess bandwidth to replicate the frequency contents which are in the notch region. At the receiver, the replicated contents are translated back to their correct spectral location. However, this method has the drawback that the high-pass and low-pass filters have high order. Since data rates exceed 2.5GHz, it is difficult to implement high order filters at such rates. Hence we propose a filter bank based approach which consists of digital filters which have low order and still permit perfect reconstruction. The idea of filter banks is to permit aliasing at an individual filter basis, but when the signal from different filters is combined, the aliasing component from the different banks cancel each other. An extra set of constraints are also required to ensure that the overall transfer function is atmost a delay. We propose using a biorthogonal filter bank as is used in the discret multi-tone modulation (DMT) system for asymmetric digital subscribe line (ADSL) applications. Fast Fourier Transform (FFT) filter banks are commonly used for these applications, but a general biorthogonal filter bank does serve the purpose. A general biorthogonal filter bank can be used to design filters which have non-uniform band pass characteristics. Simulation results show that use of the biorthogonal filter bank leads to complete recovery from the notch. The filter bank also permits equalization of the channel response in the other than the notch frequency region to create an ideal channel for signal transmission.