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**1 - 2**of**2**### “14-ch07-285-336-9780123744838 ” — 2011/3/9 — 22:03 — page 285 — #1 CHAPTER

"... parallel data transmission in the frequency domain and mainly owes its success to the easy equaliza-tion for linear time-invariant (LTI) frequency-selective channels. In OFDM systems, the data symbol stream is split into L parallel flows, which are transmitted on equispaced frequencies called subcar ..."

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parallel data transmission in the frequency domain and mainly owes its success to the easy equaliza-tion for linear time-invariant (LTI) frequency-selective channels. In OFDM systems, the data symbol stream is split into L parallel flows, which are transmitted on equispaced frequencies called subcar-riers, each one characterized by a transmission rate that is 1/L times lower than the original data rate. This is obtained by splitting the original data stream into multiple blocks, which are transmit-ted in consecutive time intervals, where each symbol of a block is associated to a specific subcarrier. This frequency-domain multiplexing can be efficiently performed by means of fast Fourier transform algorithms. Due to the use of orthogonal (equispaced) subcarriers, OFDM systems with LTI frequency-selective channels avoid the so-called intercarrier interference (ICI) among the data symbols of the same OFDM block. Differently from conventional frequency-division multiplexing, a frequency overlapping among the spectra associated to different substreams is permitted, resulting in a significant reduction of the bandwidth requirements. Moreover, for LTI frequency-selective channels, the absence of ICI allows an easy channel equalization, which can be performed on a per-subcarrier basis by means of scalar divisions. The intersymbol interference (ISI)1 among data symbols of different OFDM blocks, induced by multipath propagation, is avoided by a suitable cyclic extension of each OFDM block, usually

### Low Complexity Joint Semiblind Detection for OFDM Systems over Time-Varying Channels

"... Abstract—Orthogonal frequency division multiplexing (OFDM) modulation is widely used in communication systems to meet the demand for ever increasing data rates. In this paper, a low complexity joint semiblind detection algorithm for OFDM systems over time-varying channels is proposed based on the ch ..."

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Abstract—Orthogonal frequency division multiplexing (OFDM) modulation is widely used in communication systems to meet the demand for ever increasing data rates. In this paper, a low complexity joint semiblind detection algorithm for OFDM systems over time-varying channels is proposed based on the channel correlation and noise variance. The problem is relaxed to a continuous non-convex quadratic programming problem. Then an iterative method is utilized to deduce a sequence of reduced-size quadratic programming problems. These are solved by limiting the search in the 2-dimensional subspace. Furthermore, a low-bit descent search is employed to improve the system performance. Results are given which demonstrate that the proposed algorithm provides comparable performance with lower computational complexity than that of a sphere decoder. I.