Gradient based Adaptive Channel Estimation for Orthogonal Time Frequency Space (OTFS)
Upalekar, Divya Subhash
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The next generation wireless system with high mobility requirements brings the challenge to mitigate the effect of a time-varying channel. Conventional multicarrier systems like Orthogonal frequency division multiplexing (OFDM) are designed to mitigate the multipath effects that cause Inter Symbol Interference (ISI), Since OFDM is highly sensitive to inter carrier interference (ICI), it is not wellsuited for the high mobility scenarios with significant Doppler shifts and frequency dispersion. As the Doppler spread and phase noise leads to the inter carrier interference (ICI). Unlike the traditional time-frequency domain schemes,the OTFS system transmits the information symbols in the delay-Doppler domain. OTFS converts a doubly-dispersive time-frequency channel into a nearly static channel in the delay-Doppler domain by means of the Symplectic Fourier transform . In the delay-Doppler domain, the information symbols experiences constant fading, thus the OTFS system performs better than the OFDM system even in the presence of high Doppler. One of the channel estimation schemes for the OTFS system is pilot based estimation in which pilots are transmitted in the delay-Doppler domain. In this method, the delay-Doppler coordinates are estimated using the spreading of the pilot output in the time-frequency domain due to transformations. The channel coefficients estimated by these method are accurate in the absence of noise, but in the presence of noise the channel path gain value estimates were affected. Additional pilot power is required for this method to get the accurate estimate. In the proposed channel estimation algorithm, these slowly varying channel path gain values are estimated using the Gradient based adaptive algorithms. In this method the pilot based approach is combined with the adaptive methods to gain the advantages of both the methods. This method uses a single pilot symbol surrounded by some guard symbols, for the estimation of the delay-Doppler domain channel. In this method the additional pilots were inserted to support the adaptive algorithms, but it can give accurate results even in presence of noise.