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Multipath fading can degrade the performance of a wireless communication system significantly when the data rate is as high as those systems beyond Long-Term Evolution Advanced (LTE-A). To combat the multipath-fading effects, orthogonal frequency-division multiplexing (OFDM) techniques have been extensively studied and used in existing systems. However, another effective method to combat multipath fading is a code-division multiple access (CDMA) scheme, which has not been extensively investigated for relay network communication systems. This paper considers uplink CDMA relay networks (consisting of multiple sources, multiple relays, and a single destination) and downlinks (consisting of a combined source, multiple relays, and multiple destinations) under multipath fading. The objective of this paper is to propose a unique, simple, and effective method of finding optimum nonbinary pseudo noise (PN) spreading and despreading sequences employing a maximum signal-to-interference-plus-noise ratio (SINR) criterion and channel state information (CSI). This paper also examines the sensitivity of the proposed schemes to 1-bit quantization, mistiming alignment, wideband jamming, and imperfect CSI. Simulation results verify that the proposed method shows much faster convergence in finding optimum PN sequences and better performance than existing schemes under the same environment. [ABSTRACT FROM AUTHOR]

This paper proposes to encode pilot bits together with information bits by using a systematic encoder and applying them to an iterative receiver, where channel estimation and decoding are iteratively performed. This paper then shows why and how the encoded pilot bits can give a positive gain over a conventional uncoded pilot system. Specifically, this paper considers an orthogonal frequency-division multiplexing system that is encoded by a low-density parity-check code under a frequency-selective fading channel and shows theoretical analysis as well as simulation results. [ABSTRACT FROM AUTHOR]

As channel time-variation increases, orthogonality among subcarriers in an orthogonal frequency division multiplexing (OFDM) symbol can be destroyed because of the relatively long symbol period, whereupon intercarrier interference (ICI) appears, and hence an irreducible error floor occurs. It was shown recently that grouping pilot tones into a number of equally spaced clusters can yield better channel estimation against the doubly selective channel than placing each pilot tone in an equally spaced manner. However, a random pilot sequence was used in the literature, and an optimal sequence has not yet been studied. This paper presents how to optimize the clustered pilot sequence. First, this paper (a) proves the existence of an optimum pilot sequence and (b) suggests a guideline for finding an optimum sequence. Second, this paper shows that an optimum sequence is independent of the signal-to-noise ratio and Doppler rate, and the sequence is generally optimum for any channels. Third, this paper verifies through analysis and simulation that the coded-OFDM system with an optimized cluster pilot sequence can yield a smaller mean square channel estimation error and lower bit error rate than the system with an equidistance pilot or a random cluster pilot under a rapidly time-varying channel. [ABSTRACT FROM AUTHOR]

An inter-carrier interference (ICI) occurs in an orthogonal frequency division multiplexing (OFDM) system when a channel is rapidly time-varying. The proposed scheme employs a polynomial cancellation code (PCC) to suppress this ICI, and uses a linear complex field (LCF) code as a transmit-diversity technique to make the transmission rate compatible up to {n_T} / {(n_T+1)}, where n_T is the number of transmit antennas used. The conventional orthogonal code (OC), such as the Alamouti code, with accompanying PCC cannot achieve this rate. Both analysis and simulation results verify that the proposed scheme can achieve higher channel capacity as well as lower bit error rate (BER) than the other schemes in rapidly time-varying channels, even with a linear receiver and inaccurate channel estimation. [ABSTRACT FROM PUBLISHER]