Low overhead pilot-aided synchronization for single carrier modulation with frequency domain equalization

The aim of the paper is to compare multicarrier and single carrier modulation schemes for radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. In the case of OFDM (orthogonal frequency division multiplexing), the inverse FFT transforms the complex amplitudes of the individual subcarriers at the transmitter into the time domain. At the receiver the inverse operation is carried out. In the case of single carrier modulation, the FFT and its inverse are used at the input and output of the frequency domain equalizer in the receiver. Different single carrier and multicarrier transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In the case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used.

In this paper, a pseudo minimum mean square error (P-MMSE) frequency domain equalizer (FDE) is proposed to combat deep fading channel for single carrier (SC) modulation under IEEE 802.11ad and IEEE 802.15.3c standard. This FDE has resistance like MMSE to deep fading channel and maintain low complexity like zero-forcing (ZF) equalizer. In order to calculate bit error rate (BER) performance, the proposed P-MMSE integrating with our previous dual-mode baseband receiver [1] to do simulation. P-MMSE FDE can provide about 1dB improvement for 16QAM and 5 dB improvement for QPSK and BPSK when BER at 10−-2 in serious deep fading channel. Besides, it enhances the BER floor from 4 × 10−-3 to 10−-3 in 16-QAM Moreover, P-MMSE FDE maintains the same complexity as ZF FDE.

In this paper, main single carrier (SC) and multicarrier (MC) systems with frequency domain equalization (FDE) are reviewed with the objective to identify key features of the evolution path of air interfaces towards 4G. In particular, this paper shows that the FFT/IFFT operation can be seen as the concatenation of two operations: S/P conversion and spreading. We show that the transmitter of an OFDMA system performs a S/P conversion and a multiplication with specific complex spreading codes. By replacing the orthogonal complex spreading codes of this equivalent OFDMA system, with traditional orthogonal spreading codes that are used in DS-CDMA systems, for instance Walsh-Hadamard codes, we achieve a novel multicode-CDMA system. The comparison of this novel SC transmission scheme with other SC and MC systems shows that the novel SC system has almost the same performance as OFDMA and also similar PAPR. On the other hand, it has a pretty different behavior wrt other SC systems such as IFDMA and DS-CDMA with FDE.

In this paper we consider the use of single carrier (SC) modulations combined with frequency‐domain equalisation (FDE) in future broadband wireless systems. We propose iterative receiver structures with joint equalisation and carrier synchronisation. The proposed receivers can be regarded as modified frequency‐domain turbo equalisers where we perform a decision‐directed frequency offset estimation within each iteration of the turbo equaliser.Our performance results show that the proposed receiver structure has good bit error rate (BER), even with moderate frequency offsets and in severely time‐dispersive channels. Moreover, our receiver has a relatively low implementation complexity, due to its fast Fourier transform (FFT)‐based frequency‐domain implementation. Copyright © 2010 John Wiley & Sons, Ltd.

The application of frequency domain equalization technique makes single carrier modulation a valuable alternative resisting the ISI caused by the multipath channel in the broadband HF communication. This paper does a research on the principle and characteristics of Single Carrier systems with Frequency Domain Equalization (SCFDE), and gives the performance of SC-FDE based on different equalization schemes under multipath channel. Keywords-HF channel; ISI; Single carrier; FDE; BER

The application of frequency domain equalization techniques makes single carrier modulation a potentially valuable alternative to OFDM, especially in regard to its robustness to RF implementation impairments. In this overview paper we survey recent results on single carrier systems with frequency domain equalization (SC-FDE), including comparisons and compatibilities with OFDM, and extensions via decision feedback equalization and overlap-save processing.

Frequency domain equalization (FDE), as a linear equalizing method, is widely adopted in many applications with either OFDM or single carrier (SC) modulation. This paper analyzes the the noise transfer effect, a case for noise colorization, after FDE. This process is investigated for CP-OFDM, ZP-OFDM and SC systems where SC is treated as a multi-carrier modulation with the number of subcarriers equal to 1. Based on this analysis and by assuming the de-mapping independence among subcarriers, the average channel capacity for the systems is derived. We have proved that the capacity of the CP-OFDM system is at least equal to that of ZP-OFDM in both low and high SNR scenarios and that the capacity of the SC system is no greater than that of either ZP-OFDM or CP-OFDM systems. In a multi-path channel, if the IDFT size of the OFDM symbol is not equal to the DFT size of the equalizer, the leakage of the enhanced noise in a deeply faded sub-channel will cause the loss of capacity in adjacent sub-channels and the overall average capacity is thus reduced.

This paper investigates two typical frequency-domain turbo equalization (FDTE) schemes, i.e., frequency-domain equalization (FDE) with parallel interference cancelation (FDE-PIC) and FDE with frequency-domain decision feedback (FDE-FDDF) in single-carrier (SC) multiple-input–multiple-output (MIMO) systems. These two FDTE schemes are proven to be equivalent under the condition that we derived. We discover that the covariance matrix of the low-complexity FDE-PIC scheme and the correlation matrix of the FDE-FDDF scheme satisfy a unique relationship for constant-modulus modulations, and the original FDE-FDDF scheme suffers numerical instability for non-constant-modulus modulations because the estimation of the correlation matrices leads to the inversion of ill-conditioned matrices. By proposing a new estimation method of the correlation matrices, we guarantee that the non-constant-modulus FDE-FDDF scheme is numerically stable and that the relationship between these two FDTE schemes holds for both constant-modulus and non-constant-modulus modulations. Furthermore, we prove that the FDE-PIC and FDE-FDDF schemes are equivalent under this relationship. The equivalence of these two FDTE schemes is also verified through numerical simulations in terms of bit error rate (BER) at each turbo iteration and the estimation of the correlation matrices.

Single carrier (SC) modulations employing frequency domain equalization (FDE) have been shown to be appropriate for severe time-dispersive channels, having similar or better performances than OFDM modulations, while offering the same complexity and lower peak-to-average power ratio. In this paper we introduce a class of low complexity linear frequency domain precoders (multiple-beamformers) for spatially correlated frequency selective multiple-input-multiple-output MIMO systems employing SC modulations. The specific designs targets minimization of a lower bound on the mean-square-error (MSE) under the assumption that only second order statistics of channel (covariance matrix of channel and noise) is available at the transmitter. These precoders are designed for different wideband receivers such as BLAST, MIMO-DFE, linear FDE. The proposed designs are assessed and compared for relatively large correlations and severely frequency selective channels.

The performance of multicarrier and single carrier modulation schemes is compared for wideband mobile radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. The transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. In particular, the systems are compared with respect to the degradation due to pilot-aided channel estimation and carrier frequency synchronization.

Single carrier frequency division multiple access (SC-FDMA) which utilizes single carrier modulation at the transmitter and frequency domain equalization at the receiver is a technique that has similar performance and essentially the same overall structure as those of an OFDMA system. One prominent advantage over OFDMA is that the SC-FDMA signal has lower peak-to-average power ratio (PAPR). SC-FDMA has drawn great attention as an attractive alternative to OFDMA, especially in the uplink communications where lower PAPR greatly benefits the mobile terminal in terms of the transmit power efficiency. SC-FDMA is currently a working assumption for the uplink multiple access scheme in 3GPP Long Term Evolution (LTE). To achieve low PAPR, a novel scheme, SC-OFDMA with super frame using intermediate mapping (SF-IFDMA), is proposed in this paper. In our proposed scheme, B frames with N subcarriers will be combined into one super frame, then SC-OFDM technique is applied to reduce the PAPR of the super frame, the super frame with lowered PAPR will finally be divided into B OFDM frames for the transmission. The combining and dividing process is equivalent to the concatenation process. The proposed scheme requires no side information and can significantly reduce PAPR while maintaining small number of subcarriers.

The first optical single-carrier (SC) transmission system to employ frequency-domain equalisation (FDE) is proposed; 25 Gbit/s SC with FDE is demonstrated in severe chromatic dispersion conditions. The results show that SC with FDE is very tolerant of the chromatic dispersion.

Recently, single carrier block transmission (SCBT) has received much attention in high-rate phase-coherent underwater acoustic communication. However, minimum-mean-square-error (MMSE) linear FDE may suffer performance loss in the severely time dispersive underwater acoustic channel. To combat the channel distortion, a novel multi-channel receiver with maximum ratio combining and a low complex T/4 fractional iterative frequency domain equalization (FDE) is investigated to improve diversity gain and the bit error rate (BER) performance. The proposed method has been verified by the real data from a lake underwater acoustic communication test in November 2011. At 1.8 km, the useful data rates are around 1500 and 3000 bits/ s for BPSK and QPSK respectively. The results show the improvements of system performance. Compared with MMSE FDE system, the output SNR improvement is 6.9 dB, and the BER is from 10-3 to no error bits for BPSK. The output SNR improvement is 5.3 dB, and the BER is from 1.91×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2 to 2.2×10-4</sup> for QPSK.

Modified fast Fourier transform in FBMC for satellite communications

Long-haul mode-division multiplexing (MDM) employs adaptive multi-input multi-output (MIMO) equalization to compensate for modal crosstalk and modal dispersion. MDM systems must typically use MIMO frequency-domain equalization (FDE) to minimize computational complexity, in contrast to polarization-division-multiplexed systems in single-mode fiber, where time-domain equalization (TDE) has low complexity and is often employed to compensate for polarization effects. We study two adaptive algorithms for MIMO FDE: least mean squares (LMS) and recursive least squares (RLS). We analyze tradeoffs between computational complexity, cyclic prefix efficiency, adaptation time and output symbol-error ratio (SER), and the impact of channel group delay spread and fast Fourier transform (FFT) block length on these. Using FDE, computational complexity increases sublinearly with the number of modes, in contrast to TDE. Adaptation to an initially unknown fiber can be achieved in ~3-5 μs using RLS or ~15-25 μs using LMS in fibers supporting 6-30 modes. As compared to LMS, RLS achieves faster adaptation, higher cyclic prefix efficiency, lower SER, and greater tolerance to mode-dependent loss, but at the cost of higher complexity per FFT block. To ensure low computational complexity and fast adaptation in an MDM system, a low overall group delay spread is required. This is achieved here by a family of graded-index graded depressed-cladding fibers in which the uncoupled group delay spread decreases with an increasing number of modes, in concert with strong mode coupling.