Concatenated peak-to-average power ratio reduction scheme with threshold limited selection for coded orthogonal frequency-division multiplexing

The authors propose a concatenated scheme to reduce the peak-to-average power ratio (PAPR) in coded orthogonal frequency-division multiplexing (OFDM) systems. First, they employ a label-bits-inserted encoder of a random-like code to achieve selected mapping (SLM). Then they set a threshold at the selector to limit the number of candidate sequences. Both analytical and numerical results show that the complexity of the SLM implemented by the label-bit-inserted encoder can be significantly reduced by threshold limited selection. With the same complexity, the performance of PAPR reduction is improved. The proposed concatenated PAPR reduction scheme enjoys many advantages including low-complexity, small overhead, no side information transmission and no performance loss or additional complexity at the receiver.     

Reduced complexity detection technique for layered space time block coded multiple-input multiple-output orthogonal frequency division multiplexing

The combination of vertical Bell Labs layered space time (V-BLAST) and space time block coding (STBC), known as a Layered STBC (LSTBC) system, offers high spectral efficiency with a higher order of diversity. The system structure, computational complexity and error performance of the V-BLAST and LSTBC multiple-input multiple-output orthogonal frequency division multiplexing systems are analysed and compared. It is shown that, compared with V-BLAST, the overall diversity order of LSTBC increases two fold. This diversity gain is achieved at the expense of a four?fold increase in the computational complexity of the QR decomposition (QRD) algorithm, required at the receiver of both LSTBC and V-BLAST. The authors propose a modified QRD algorithm which reduces this four-fold complexity to two fold.     

Peak-to-average power ratio reduction in OFDM using cyclically shifted phase sequences

Two approaches for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) are proposed that are relied on a set of cyclically shifted phase sequences (CSPS) and implemented using the time domain circular convolution. After multiplying CSPS with the frequency domain data, the signal candidates can be expressed as weighted sum of the circularly shifted OFDM time domain data in the first method, which is called CSPS method. In the second method, weighted coefficients for generating the signal candidates in CSPS method are optimally selected to improve its performance; thus, the second method is referred to as optimised CSPS (OCSPS) method. The performances of the CSPS and OCSPS methods are evaluated using simulated data and compared with those of selective mapping (SLM) and partial transmit sequences (PTS). The simulation results show that both the CSPS and OCSPS methods can reduce the PAPR effectively, and that the OCSPS performs even better than the CSPS. The OCSPS can achieve the same performance as compared to the PTS. A distinct feature of the proposed methods is that only one inverse discrete Fourier transform is needed, and thus, the candidates can be calculated in time domain directly.     

Restricting the orthogonal frequency division multiplexing peak-to-mean envelope power ratio by a new set of sequences of length 16

A new set of 136 binary sequences of length 16 applied to reducing the peak-to-mean envelope power ratio of orthogonal frequency division multiplexing systems is reported. A non-systematic encoder is developed. This set of sequences can be used with the existing 384 binary Golay sequences of 16. The resultant set will consists of 512 sequences, an integer power of 2, which expedite implementation.     

Analytical performance of orthogonal frequency division multiplexing systems impaired by a non-linear high-power amplifier with memory

The performance of modern communication transmitters utilising multi-carrier modulation techniques is highly sensitive to non-linear distortions arising mainly from the high-power amplifier (HPA). In addition, the wideband characteristics of multi-carrier signals result in frequency-dependent distortions, typically known as memory effects. This study aims to analytically evaluate the impact of the distortion induced by non-linear HPA with memory on a multi-carrier signal exemplified by orthogonal frequency division multiplexing (OFDM). The behavioural model of HPA considered for theoretical analysis is a memory polynomial model, which is a truncated form of the Volterra series. In the theoretical framework developed, this study shows that memory in HPA considerably degrades the performance of OFDM in terms of symbol error rate (SER), and the distortion itself can be canonically characterised by a complex attenuation component and a non-linear noise component. Closed-form expressions for SER in additive white Gaussian noise channel are derived and the SER for fading channel is approximated by the adaptive Gauss-Kronrod quadrature method. Simulation results are shown for a realistic HPA, based on the Wiener-Hammerstein model, and compared with the analytical results to validate the proposed analysis.     

Impact of second-order PMD in a coherent optical orthogonal frequency division multiplexing system

The second-order, polarization-mode dispersion (PMD) effect for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system in long-haul transmission with single-mode fiber was analyzed in detail. The closed-form expression is presented for the time impulse response of an OFDM signal affected by second-order PMD. Analysis and simulation results show that, compared with polarization-dependent chromatic dispersion (PCD), CO-OFDM system performance is mainly influenced by depolarization rate (DR) which can moderate phase noise induced by chromatic dispersion. For a fiber link of 800 km and DR of 30 ps, the system Q value is improved by about 1.3 dB over the case of no DR.     

Peak-to-average power ratio reduction using second order cone programming based tone reservation for terrestrial digital video broadcasting systems

High peak-to-average power ratio (PAPR) is a critical issue in any multicarrier systems using orthogonal frequency division multiplexing, as terrestrial digital video broadcasting (DVB). It can result in low power efficiency and large performance degradation of systems, due to the nonlinearity of high-power amplifier (HPA). A PAPR reduction method based on tone reservation technique with second-order cone programming (SOCP) approach in terrestrial DVB systems is proposed. The authors first demonstrate the superiority of the SOCP optimisation algorithm compared with an iterative gradient-based algorithm, using the current DVB-T parameters: significant PAPR reduction gains can be achieved with only a very small set of subcarriers in the useful bandwidth, making the proposed method more promising in terms of spectral efficiency. Moreover, the proposed solution presents a very good trade-off between PAPR reduction gain and mean transmitted power increase. An overall study, taking into account the limitation of the power level of the dedicated subcarriers and the evaluation of the performances in presence of a nonlinear HPA, is presented. These performances are given in terms of adjacent channel power ratio and bit error rate. The resulting PAPR reduction gain demonstrates that the relevance of the proposed method for the future DVB-T standard is straightforward.     

Redundancy-free and BER-maintained selective mapping with partial phase-randomising sequences for peak-to-average power ratio reduction in OFDM systems

Selective mapping (SLM) is one of the most effective peak-to-average power ratio (PAPR) reduction techniques proposed for orthogonal frequency-division multiplexing systems. However, the fact that this technique requires the transmission of side information (SI), to enable the receiver to reverse the randomisation process before performing signal demodulation, can make it undesirable for certain applications as it degrades the system's bandwidth efficiency and, more significantly, the bit error rate (BER) performance. A modified SLM technique (MSLM) that can operate without the need for any dedicated SI for both channel-coded and uncoded scenarios while providing similar PAPR reduction performance as the ordinary SLM technique is proposed. It will be demonstrated that the proposed MSLM technique not only improves the system's bandwidth efficiency, but also achieves significantly better BER performance than the ordinary SLM technique.     

Pilot overhead reduction in turbo coded OFDM systems employing an iterative channel estimation under low signal-to-noise ratio environments

The authors evaluate the improved energy and spectral efficiency by pilot overhead reduction of turbo coded orthogonal frequency division multiplexing (OFDM) systems employing an iterative phase estimation algorithm. Developed from the recently proposed iterative phase estimation schemes, the phase estimation and compensation process is embedded into the basic iterative turbo decoding process for the application to OFDM systems with just a slight complexity overhead. At each decoding iteration, sub-carrier phase rotations are estimated from the extrinsic information arranged in each sub-carrier and are compensated for the next decoding iteration. This enables the iterative phase estimation algorithm to successfully work under very low signal-to-noise ratios even without pilot symbols. The pilot symbols are just very rarely inserted only for breaking the erroneous phase estimation propagation frame to frame in case of large residual phase offset beyond reliable decoding range. Simulation results show that the iterative phase estimation algorithm drastically reduces the pilot insertion overhead and thus, it achieves improved spectral efficiency as well as bit error rate (BER) performance by saving pilot energy compared to the conventional method.     

Selective post-IFFT amplitude randomising for peak-to-average power ratio reduction in orthogonal frequency-division multiplexing-based systems

The inherent high peak-to-average power ratio (PAPR) of multicarrier transmission, such as orthogonal frequency-division multiplexing (OFDM) or discrete multi-tone (DMT), can lead to a significant degradation in the transmission power efficiency which is unacceptable especially in battery-powered terminals. Among the most popular PAPR reduction techniques proposed in the literature is the selective mapping (SLM) technique which has been shown to offer PAPR reductions of several decibels. However, the SLM technique requires invoking the inverse fast fourier transform (IFFT) process several times per transmitted OFDM block which increases the system's complexity and hence may result in long latencies and high power consumption. The authors propose a new low complexity post-IFFT PAPR reduction technique that can outperform the SLM technique in terms of PAPR reduction while its operational complexity is orders of magnitude less than that of SLM technique.     

Performance comparison of low density parity check codes using square root Kalman equalisation and orthogonal frequency division multiplexing techniques for broadband fixed wireless access systems

Broadband fixed wireless access (BFWA) systems enable services such as high-speed data communication, high quality voice/video conferencing and high-speed internet access in areas where a wired link is not possible. However, the BFWA channel is a slow-fading channel having deep frequency-selective fading caused by clusters of scatterers in the environment that introduce inter-symbol interference (ISI) at the receiver. Low density parity check (LDPC) codes, optimised for the single-input single-output BFWA channel, are designed using the structured balance incomplete block design method. The use of both quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM) are investigated theoretically. To help overcome the ISI effects of the channel, equalisation techniques are employed separately with LDPC decoding for a system employing QPSK and 16-QAM modulation schemes. The equaliser single carrier approach is then replaced with orthogonal frequency division multiplexing (OFDM) and the performance of these two approaches is evaluated in terms of bit-error rate. The simulation results show that equalisation with LDPC coding has a measurable performance gain over LDPC coding with OFDM.