Low-Complexity Equalisation and Channel Estimation over Fast Fading Channels
- Publication Type:
- Thesis
- Issue Date:
- 2022
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The next generation wireless communication systems aim to achieve high capacity and low latency with high-mobility scenario as an important channel condition for various new applications. With the significantly increased data rate and Doppler frequency shift, the systems' ability to cope with fast channel variations is of significant importance. This thesis develops effective and efficient solutions to improve the performance of both conventional and emerging modulations over fast fading channels.
The recently proposed orthogonal time frequency space (OTFS) modulation shows outstanding performance over fast fading channels. However, existing research on OTFS is mostly focused on its delay-Doppler domain structure. In this thesis, channel and system models in different signal domains are firstly derived in both continuous and discrete forms, providing the basis for exploiting the full potential of OTFS with low complexity. Particularly, a circular stripe diagonal structure in the frequency-Doppler domain channel matrix for arbitrary multipath delays and Doppler shifts is identified through analyses and simulations, paving the way for low-complexity techniques to be adopted to combat fast channel fading.
Exploiting the circular stripe diagonal nature of the frequency-Doppler channel matrix, a low-complexity frequency-domain minimum mean-square-error (MMSE) equalisation for OTFS systems with long signal frames and fully resolvable Doppler spreads is then formulated. It is also demonstrated that the proposed MMSE equalisation is applicable to conventional modulations with short signal frames and partially resolvable Doppler spreads.
The diversity performance analyses for OTFS are further provided under both maximum likelihood and linear equalisations. Inspired by the frequency-domain precoding structure, an adaptive transmission scheme with frequency-domain precoding matrix composed of the eigenvectors of the channel matrix is proposed to improve the system performance under MMSE equalisation, and its optimised performance is derived with simple analytical expressions. Considering two extreme channel conditions, the lower and upper bounds for the diversity performance of the adaptive transmission scheme are also derived. The derived performance bounds can serve as performance benchmarks for OTFS and other precoded OFDM systems.
Based on the re-formulation of OTFS as precoded-OFDM, three variants of the original OTFS system for low-complexity channel estimation over fast fading channels are finally proposed in this thesis. They enable one-dimensional channel estimation and corresponding equalisation to be applied in either frequency or time domain. Simulation results demonstrate that the proposed frequency-domain pilot aided OTFS scheme is the most effective transmission technique for high-mobility wireless communications in terms of diversity performance, signalling overhead, and power efficiency.
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