Channel estimation with iterative discrete Fourier transform‐based smoothing for orthogonal frequency‐division multiplexing systems with non‐uniformly spaced pilots in channels with long delay spread

Abstract

An orthogonal frequency-division multiplexing (OFDM) system with non-uniformly spaced pilots, such as worldwide interoperability for microwave access, voids virtually all discrete Fourier transform (DFT)-based interpolation methods for channel estimation, because they all require pilots to be uniformly spaced. Therefore, we propose two iterative algorithms to solve this problem. In the proposed algorithms, the inverse discrete Fourier transform and DFT are iteratively used to smooth the channel frequency response estimated by linear interpolation (LI) or other means. To be complete, we also propose a method to truncate the channel impulse response for the proposed algorithms. The simulation results show that the proposed approach has much lower mean square error in a long delay channel than the LI method has, and it also outperforms a decision-directed approach at high speed and/or lower signal-to-noise ratio environments. Overall, the proposed approach is a promising approach for channels with long delay spread in an OFDM system with non-uniformly spaced pilots.

Iterative channel estimation method for long‐term evolution downlink transmission

“…For type-A symbols, the estimation strategy is similar to the one proposed in [9], in which IFFT and FFT are iteratively employed as a type of smoothing filter. To further improve the estimation accuracy, decision feedback is also implemented.…”

“…It is shown that the performance of this approach with only a small number of iterations is comparable to that of the decision-directed approach in some cases [9], but without the burden of constructing an encoder inside the receiver. Considering these factors, we use the…”

“…There are numerous channel estimation methods available in the literature. These estimation methods can be roughly divided into the following categories: functional (linear or higher‐order function) interpolation [1–3], discrete Fourier transform‐based (DFT‐based) interpolation [4–8], fast Fourier transform‐based (FFT‐based) smoothing [2, 9], filter‐based interpolation [10–13], decision‐directed approach [14, 15], and iterative approach [7, 9, 16]. The following briefly reviews each of the methods.…”

“…This method, though effective, may have difficulty in truncating the computed CIR, if there are too few pilots in a symbol (and therefore too short CIR). To overcome this problem, a modified approach which uses inverse FFT (IFFT) and FFT for the task of smoothing but not interpolation was proposed [2, 9]. In this approach, a full‐length CIR is obtained by applying IFFT to the interpolated CFRs (for example, by 2D linear interpolation) of all subcarriers instead of only to the CFRs of the pilots.…”

“…The interpolated CFRs are then obtained by filtering the CFR sequence using a low‐pass filter. Unfortunately, this approach sometimes is not very effective [9].…”