Single-carrier Block Transmission Research Articles

Low cost VLSI architecture of resisting long echo channel estimation for DTMB system

Digital television/terrestrial multimedia broadcasting (DTMB) was announced as the Chinese digital television terrestrial broadcasting (DTTB) standard in August, 2006. It adopts TDS-OFDM (time-domain synchronous-orthogonal frequency division multiplexing) as the baseline modulation technology. There is not pilot information in order to avoid reduction of channel throughput. It uses pseudo-noise (PN) sequence as guard interval (GI) and the training sequence for both multi- and single-carrier block transmissions which makes the synchronization faster than CP-OFDM (cyclic prefix-orthogonal frequency division multiplexing) systems. It can resist the inter-symbol interference (ISI) and can be used for channel estimation and synchronization. In this paper, a full-mode channel estimation scheme for DTMB demodulator is developed. This method utilizes the PN sequence to obtain channel estimation and equalization both in three kinds of frame head (FH) mode as well as in single and multi-carrier modulation transmission mode. To implement on hardware realization, interference cancellation is predigested, then equalization and filter for channel impulse response are optimized. A low cost VLSI architecture for channel estimation and equalization is proposed which works effectually and achieves low complexity as well. Computational simulation results show that the proposed scheme has little BER loss in performance.

New Combining Scheme in STBC Single Carrier Block Transmission System

A new diversity combining algorithm is presented in this paper with high performance in the STBC single carrier (SC) block transmission MISO system with two transmit antennas and one receive antenna. The decision on the transmitted signal is made by STBC-based SC frequency domain equalizer (STBC-SC-FDE), and then the line of sight (LOS) component under Rician fading channels or the component with highest power under Rayleigh fading channels in the received signal is obtained by cancelling the multipath signals reconstructed by the initial detecting solution and the channel impulse response (CIR) from the received signal. Then, the LOS or the strongest component is combined using STBC-like combining scheme. The new algorithm can achieve the performance advantages dramatically over STBC-SC-FDE which is verified by computer simulations carried out in the SUI-4 and TU wireless communication link.

Asymptotically optimal low-complexity SC-FDE in data-like co-channel interference

In this paper, the design of a low-complexity linear equalizer is considered for single-carrier (SC) block transmission in the presence of inter-symbol interference (ISI) and data-like co-channel interference (CCI). Unlike the linear minimum mean-squared error (LMMSE) frequency-domain equalizer (FDE) designed to suppress ISI only, the LMMSE FDE suffers from high computational complexity due to the CCI component in the signal correlation matrix. Motivated by the fact that the double Fourier transform of the autocorrelation function of a wide-sense cyclostationary process consists of impulse fences with equal spacing, a low-complexity FDE is proposed that approximates the frequency-domain correlation matrix of the CCI plus noise by a block matrix with diagonal blocks. It is shown that the proposed FDE is asymptotically optimal in the sense that the average mean-squared error (MSE) converges to that of the LMMSE FDE as the block length tends to infinity. It is also shown that the proposed FDE is more numerically stable than the LMMSE FDE when the receive filter is matched to the transmit filter and its output is over-sampled to better capture the cyclostationarity of the CCI. Discussions and numerical results include SC block transmission systems with unique word instead of cyclic prefix, and systems with multiple receive antennas.

Asynchronous CDMA Under Single-Carrier Block Transmission—Architectures and Signal Models

Single-carrier (SC) block transmission with frequency-domain equalization (FDE) is an efficient scheme that avoids complex time-domain equalization in multipath channels. There are also several systems that are proposed that apply SC-FDE in code-division multiple access (CDMA). However, the present CDMA systems with FDE may not be suitable for asynchronous users for lack of a signal model that can suitably express the despread signal in the frequency domain. In this paper, we first propose circularly sliding (CS) despreading that can attain preliminary separation of interference among asynchronous users. Then, we derive a signal model that represents spreading, channel effect, and despreading as a series of circular convolutions. This signal model helps to derive a new FD minimum mean-square error (MMSE) equalization algorithm for SC block transmission asynchronous CDMA. We further extend the signal model by including equalization as another circular convolution. The extended signal model indicates several equivalent architectures of the receiver. For these architectures, despreading and equalization can be implemented in either the time or frequency domain, and the order is exchangeable. Based on the proposed signal model, we also derive a RAKE combiner that can be implemented in the frequency domain. Our architectures and signal model can be applied for further development of multiuser-detection algorithms.

Efficient Multipath Diversity Receiver for STBC Block Transmission System

A space-time and multipath diversity combining algorithm is presented for STBC single carrier block transmission system with two transmit and one receive antennas. The initial solution is achieved by an STBC-based frequency domain equalizer, and the multipath components in the received signal are decoupled by this initial solution and channel state information. Finally, STBC combining is carried out on each decoupled multipath component separately, and then the single carrier output branches are combined further using the maximal ratio combining (MRC) algorithm.

Efficient bad equalizer in two-ray multipath fading channels

An efficient bidirectional arbitrated decision feedback (BAD) equalizer is presented in single-carrier block transmission system in the two-ray multipath fading channels, where the output from the bidirectional equalizers are combined together directly using maximal ratio combining (MRC) rule to improve the signal-to-noise ratio (SNR) before demodulation. The computational complexity of the BAD equalizer presented is linear with the channel length, which is the same as conventional decision feedback equalizer (DFE) and is significantly lower than that of conventional BAD equalizer as well as the maximum likelihood (ML) algorithm. While the performance of the new scheme depends on the specific channel characteristics, it is shown by simulation results that the performance of the new BAD can surpass the one of DFE dramatically in the minimum or non-minimum phase Two-ray multipath fading channels.

Polynomial-Based Noise Variance Estimation for MIMO-SCBT Systems

In this paper, we present a polynomial-based noise variance estimator for multiple-input multiple-output single-carrier block transmission (MIMO-SCBT) systems. It is shown that the optimal pilots for noise variance estimation satisfy the same condition as that for channel estimation. Theoretical analysis indicates that the proposed estimator is statistically more efficient than the conventional sum of squared residuals (SSR) based estimator. Furthermore, we obtain an efficient implementation of the estimator by exploiting its special structure. Numerical results confirm our theoretical analysis.

Time domain spreading and frequency domain maximal ratio combining reception for frequency diversity enhancement in single carrier UWB communication systems

Ultra-wideband technology is a promising candidate for short-range broadband communication systems. Ultra-wideband channels are characterized by dense multiple paths that provide multipath diversity, also referred to as frequency diversity. Single carrier systems with frequency domain equalization have attracted much interest for high data rate communications. In this paper, frequency diversity in single carrier ultra-wideband systems is enhanced by employing simple frequency domain maximal ratio combining (FDMRC) reception with time domain complex quadratic spreading (CQS) sequences. Since ultra-wideband channels exhibit dense multiple paths and strong frequency selectivity, the proposed system can attain significant performance improvement over traditional single carrier frequency domain equalization (SC-FDE) systems. Through the comparison of the impact of the cyclic prefix length, the proposed system shows stronger robustness to inter-block interference than the conventional SC-FDE system. Compared with multicarrier systems, like orthogonal frequency-division multiplexing and multi-carrier code division multiple access, the proposed receiver complexity is reduced greatly. To accommodate multiuser communication, block spreading is further introduced to combine with the frequency diversity enhancement technique for single carrier block transmission UWB systems, realizing multiuser interference-free reception.

Comparison of Frequency Offset and Timing Offset Effects on the Performance of SC-FDE and OFDM Over UWB Channels

In this paper, the effects of carrier frequency offset (CFO) and sampling time offset (STO) on the performance of single-carrier block transmission with frequency-domain equalization (SC-FDE) and orthogonal frequency-division multiplexing (OFDM) over ultrawideband (UWB) channels are investigated. The signal-to-interference-plus-noise ratio (SINR) of SC-FDE in the presence of CFO is derived and compared with that of OFDM. The effects of CFO on the bit error rate (BER) performance of both systems are also simulated and compared. Two forms of STO are considered, i.e., a constant timing shift from its optimum sampling timing instant and a random timing jitter. We show through analysis that although SC-FDE is reasonably robust to a constant timing offset, it is fairly sensitive to random timing jitter. The BER performances of SC-FDE in the presence of a constant STO and random timing jitter are simulated and compared with OFDM.

Novel channel estimation method based on PN sequence reconstruction for Chinese DTTB system

Digital television/terrestrial multimedia broadcasting (DTMB), announced as the Chinese digital television terrestrial broadcasting (DTTB) standard, uses pseudo-noise (PN) sequence as guard interval (GI) and the training sequence for both multi- and single-carrier block transmissions. The conventional channel estimation methods can be performed through either the subtraction of the PN sequence or the cancellation of residual inter-symbol interference (ISI). In this paper, a novel iterative method is introduced to estimate the overlapping part of PN sequence, meanwhile the channel impulse response (CIR) estimation can be obtained and updated via the reconstruction of the PN sequence in an iterative manner. Simulations show that the proposed algorithm can effectively reduce the ISI and improve the symbol error rata (SER) of DTMB system over slowvarying broadcasting multipath channel even with long delay spread. In practice, the proposed algorithm can be applied to the DTMB receiver-chip directly with low-complexity cost.

Designing unique words for near‐optimal training in block transmission systems

AbstractIn this paper, we consider the problem of designing unique words (UWs) in single‐carrier block transmission systems for the purpose of performing bandwidth efficient channel estimation. By first deriving the Cramér‐Rao lower bound (CRLB) for the UW‐based channel estimator, we show that designing optimal UWs reduces to a problem of constructing sequences of length Q ≤ N/2 that, when padded with zeros to make a length‐N sequence, have good periodic autocorrelation properties. We then show that the only class of short (length‐Q) sequences that, when padded with N − Q zeros, have perfect periodic autocorrelation properties is the class of Kronecker delta sequences. Due to the fact that these sequences typically suffer from a high peak‐to‐average power ratio (PAPR), we then formulate two optimisation procedures to find near‐optimal UWs under certain constraints. Finally, we show that the sequences that are found with the proposed technique perform significantly better than Kronecker delta sequences that have been scaled to meet a given PAPR constraint. Copyright © 2008 John Wiley & Sons, Ltd.

Narrowband interference avoidance for ultra-wideband single-carrier block transmissions with frequency-domain equalization

In this article, a novel method of performing interference avoidance in next-generation single-carrier ultrawideband block transmission systems is presented. This method relies on the application of a time-domain window to each data block prior to transmission, where the window is optimized such that the required notch depth is achieved for each transmission. Furthermore, a modification of this technique is also detailed, whereby the complexity can be reduced from O(N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) to O(N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) where N is the number of symbols per block. It is shown that the proposed techniques overcome many of the drawbacks of current methods such as transmit power control, frequency notching, and active interference cancellation.

Robust receiver design for MIMO single-carrier block transmission over time-varying dispersive channels against imperfect channel knowledge

We consider MIMO single-carrier block transmission over time-varying multipath channels, under the assumption that the channel parameters are not exactly known but are estimated via the least-squares training technique. While the channel temporal variation is known to negate the tone-by-tone frequency-domain equalization facility, it is otherwise shown that in the time domain the signal signatures can be arranged into groups of orthogonal components, leading to a very natural yet efficient group-by-group symbol recovery scheme. To realize this figure of merit we propose a constrained-optimization based receiver which also takes into account the mitigation of channel mismatch effects caused by time variation and imperfect estimation. The optimization problem is formulated in an equivalent unconstrained generalized-sidelobe-canceller setup. This enables us to directly model the channel mismatch effect into the system equations through the perturbation technique and, in turn, to further exploit the statistical assumptions on channel temporal variation and estimation errors for deriving a closedform solution. Within the considered framework the proposed robust equalizer can be combined with the successive interference cancellation mechanism for further performance enhancement. Flop count evaluation and numerical simulation are used to evidence the advantages of the proposed scheme.

Robust Frame Synchronization for Chinese DTTB System

Digital terrestrial multimedia/television broadcasting (DTMB), which is the recently released Chinese digital television terrestrial broadcasting (DTTB) standard, uses guard intervals padded with known pseudo-noise (PN) sequences for multi-carrier or single-carrier block transmission. Well designed synchronization algorithms are a prerequisite for achieving high-quality multimedia transmission. In this paper, a novel frame synchronization technique, which is robust to carrier frequency offset (CFO), is proposed for DTMB systems. This technique is derived through segmenting the local PN sequence into M contiguous blocks and performing multiple cross-correlations with the received signal. The metric for peak detection is defined as the non-coherent summation of the M cross-correlation magnitude outputs. Moreover, a low-complexity frame synchronizer based on segment-based full correlation for PN420 mode in DTMB system is proposed. It is shown through computer simulation that the proposed techniques generally outperform the conventional ones when CFO occurs, over both AWGN and frequency-selective fading channels. For a DTMB system using PN420 mode, the maximal allowed CFO to keep system still working normally of our proposed schemes are more than 3 times larger than that of the conventional method.

Performance of SC-FDE system in UWB communications with imperfect channel estimation

Single carrier block transmission with frequency domain equalization (SC-FDE) has been shown to be a promising candidate in ultra-wideband (UWB) communications. In this paper, we analyze the performance of SC-FDE over UWB communications with channel estimation error. The probability density functions of the frequency domain minimum mean-squared error (MMSE) equalizer taps are derived in closed form. The error probabilities of single carrier block transmission with frequency domain MMSE equalization under imperfect channel estimation are presented and evaluated numerically. Compared with the simulation results, our semi-analytical analysis yields fairly accurate bit error rate performance, thus validating the use of the Gaussian approximation method in the performance analysis of the SC-FDE system with channel estimation error.

Efficient Channel Estimation for MIMO Single-Carrier Block Transmission With Dual Cyclic Timeslot Structure

We investigate channel estimation for timeslot-structured single-carrier block transmission (SCBT) over space-, time-, and frequency-selective fading multiple-input multiple-output (MIMO) channels. A MIMO-SCBT with a dual cyclic timeslot structure is presented first. Then, an optimal channel estimation in the minimal mean square error (MMSE) sense on the timeslot basis is investigated. It is shown that the optimal pilots for the timeslot-based MMSE channel estimation are related to the statistical channel state information in eigenmode. Under the assumption that the transmit correlation is unknown at the transmitter, the optimal pilots satisfy the same condition as reported for the block-based least-square (LS) channel estimation in literature, and the channel estimation can be simplified to initial block-based LS channel estimation followed by space-time postprocessing. Particularly, for spatially uncorrelated channels, the space-time postprocessing can be reduced to pathwise processing. A new design of the pilot sequences is given, which leads to an efficient implementation of the channel estimation. Later on, a more efficient implementation for the initial channel estimation is obtained by using the structure of the pilot sequences, and discrete cosine transform (DCT)-based implementation is developed for the space-time postprocessing to approximate the optimal solution with low implementation complexity. Finally, the performance of the proposed channel estimation is verified via simulations.

Interference Cancellation Schemes for Single-Carrier Block Transmission with Insufficient Cyclic Prefix

This paper proposes intersymbol interference (ISI) and interblock interference (IBI) cancellation schemes at the transmitter and the receiver for the single-carrier block transmission with insufficient cyclic prefix (CP). The proposed scheme at the transmitter can exterminate the interferences by only setting some signals in the transmitted signal block to be the same as those of the previous transmitted signal block. On the other hand, the proposed schemes at the receiver can cancel the interferences without any change in the transmitted signals compared to the conventional method. The IBI components are reduced by using previously detected data signals, while for the ISI cancellation, we firstly change the defective channel matrix into a circulant matrix by using the tentative decisions, which are obtained by our newly derived frequency domain equalization (FDE), and then the conventional FDE is performed to compensate the ISI. Moreover, we propose a pilot signal configuration, which enables us to estimate a channel impulse response whose order is greater than the guard interval (GI). Computer simulations show that the proposed interference cancellation schemes can significantly improve bit error rate (BER) performance, and the validity of the proposed channel estimation scheme is also demonstrated.

On the Capacity of Ultra Wideband Block Transmission Systems with Narrowband Interference Avoidance

In this letter, the theoretical capacities of single-carrier and multi-carrier block transmission systems that employ narrowband interference avoidance are derived. It is shown that the capacity of an OFDM system that implements active interference cancellation is degraded by approximately one bit per second per Hz at high SNR when 10% of the band is nulled. In contrast, the capacity of a single-carrier system that uses envelope scaling in the time domain to create deep frequency notches is not significantly affected by the nulling process.

Subspace algorithm for blind channel identification and synchronization in single-carrier block transmission systems

A blind channel identification and synchronization method for single-carrier block transmission system over frequency-selective fading channel is proposed. We employ cyclic prefix (CP) insertion to introduce the redundancy at the transmitter side. As a result, the channel impulse response (CIR) could be estimated in a blind fashion by means of subspace algorithm based on second-order statistics through the exploitation of the redundancy. The condition for identifying CIR along with the proof of uniqueness of CIR estimate is provided regardless of CP length relative to channel order as well as data frame size. We further develop a sorting approach to avert timing offset issue in block synchronization. Numerical results presented performance comparison with orthogonal frequency-division multiplexing (OFDM) systems in terms of bit error rate (BER) and channel estimation error. We also compare with traditional zero-padding OFDM systems in terms of the failure rate of block synchronization.

Spectrum Shaping and NBI Suppression in UWB Communications

This paper investigates spectrum shaping in ultra-wideband (UWB) communications in order to introduce spectral nulls to limit interference with narrowband signals. Each transmitted symbol is represented by a "coded Gaussian monocycle pulse" in which Gaussian monocycles are weighted, delayed and summed in accordance with a designed codeword. The use of the Gaussian monocycle ensures that the UWB spectrum mask established by the Federal Communications Commission (FCC) is met, and the codeword is designed to generate a spectral null at the frequency or frequencies being used by existing narrowband devices. Signals obtained with different spectrum shapings (e.g., Butterworth, Chebyshev, elliptical) and by introducing nulls at multiple interference frequency bands are discussed. This approach can be used in various systems; as one application, we simulate the performance of a coded monocycle UWB system with a spectral null in the presence of narrowband interference (NBI) using single carrier block transmission with frequency domain equalization (SC-FDE), and compare its performance with that of an uncoded SC-FDE UWB system using a single Gaussian monocycle. Our results show that NBI can be effectively suppressed by transmitting and matched filtering the pulse with a spectral null at the interference frequency, therefore improving the robustness of UWB systems to NBI