Reduction In Bandwidth Efficiency Research Articles

A modified Hadamard based SLM without side information for PAPR reduction in OFDM systems

High peak-to-average power ratio (PAPR) is a concern in orthogonal frequency division multiplexing (OFDM) systems. Hadamard based selected mapping (HSLM) which uses Hadamard code as the phase sequence in selected mapping (SLM) is an attractive technique to reduce PAPR. But it requires sending side information (SI) to the receiver for each data block, and this results in a reduction in bandwidth efficiency. In this paper, we proposed a modified PAPR reduction method called semi-Hadamard based selected mapping (semi-HSLM) to decouple the phase information matrix into a phase rotation matrix for PAPR reduction and a SI matrix for side information hiding. We proposed a semi-hadamard matrix generation method to generate the phase rotation matrix, and designed a cyclic shift matrix as the SI matrix. Compared with the traditional HSLM, the semi-HSLM saves half of the phase storage and achieves good PAPR reduction performance.

A Fine-Grained Analysis of Time Reversal MU-MISO Systems Over Correlated Multipath Channels With Imperfect CSI

Time reversal (TR) is regarded as a potential future transmission scheme for multi-user (MU) multiple-input single-output (MISO) ultra-wideband (UWB) communication systems. In spite of TR’s good performance in MU-MISO UWB systems, it suffers from performance degradation due to spatial correlation and imperfect channel-state information. In this paper, a comprehensive performance analysis of MU-MISO UWB systems with a TR pre-filter is provided. Both spatial correlation and channel estimation errors (CEE) are taken into account in the propagation channel model, and the system performance is mainly studied in terms of its effective signal-to-interference-plus-noise ratio (SINR), channel capacity, and bit error rate. The novel closed-form expressions for the average effective SINR are derived in order to characterize the influence of propagation conditions such as channel impulse response duration, spatial correlation, and CEE on TR performance metrics. The expressions reveal more precisely to us that spatial correlation and CEE have different effects on the average power of the desired signal, inter-symbol interference, and inter-user interference. Moreover, the impacts of spatial correlation and CEE on the performance of MU-MISO TR system are thoroughly investigated. In particular, the capacity loss of MU-MISO TR system due to spatial correlation and CEE is quantified and fully discussed at the end of this paper. It is shown that high spatial correlation and CEE cause a remarkable reduction in bandwidth efficiency. Finally, all theoretical results are confirmed by numerical simulations.

Carrier Frequency Offset Mitigation in Asynchronous Cooperative OFDM Transmissions

Carrier frequency offset (CFO) mitigation is critical for orthogonal frequency-division multiplexing (OFDM)-based cooperative transmissions because even small CFO per transmitter may lead to severe performance loss, especially when the number of cooperative transmitters is large. In this paper, we show that cyclic prefix (CP) can be exploited to mitigate or even remove completely the CFO. The mitigation performance increases along with the CP length. In particular, long CP with length proportional to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NI</i> , where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> is the fast Fourier transform (FFT) block length and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> is the number of cooperative transmitters, can guarantee a complete CFO removal. While this comes with a reduction in bandwidth efficiency, the long CP in the proposed scheme is exploited to enhance transmission power efficiency in a way similar to spread-spectrum systems, and thus is different from conventional CP that degrades both bandwidth and power efficiency. An efficient CFO-mitigation algorithm is developed that has complexity at most O( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">NI</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), or even linear in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> approximately in some cases. Implemented as a preprocessing procedure independently from cooperative encoding/decoding details, this algorithm makes the CFO problem effectively transparent to and thus has general applications in OFDM-based transmissions.

Intercarrier interference self-cancellation scheme for OFDM mobile communication systems

For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.

Noncoherent detection of preamble-signal-assisted differentially phase-encoded APSK signals

A noncoherent detection scheme for differentially encoded amplitude- and phase-shift-keyed data transmission, assisted with differentially phase-shift-keyed preamble signals, is presented and analyzed for an additive white Gaussian noise channel. It is analytically shown that, despite a controllable reduction in bandwidth efficiency, the proposed scheme yields an error performance approaching that of the quadrature amplitude modulation scheme with ideal coherent detection within a loss of approximately 1 dB.