Showing papers on "SC-FDE published in 2005"

Frequency domain channel estimation for SC-FDE in UWB communications

Abstract: Recently, single carrier block transmission with frequency domain equalization (SC-FDE) has been shown to be a promising candidate for ultra-wideband (UWB) communications. In this paper, we address the channel estimation problem for SC-FDE transmission over UWB channels. A mean square error (MSE) lower bound for the frequency domain LMMSE channel estimator is derived and the optimal pilot sequence that achieves this lower bound is obtained. Further simplification leads to a frequency domain channel estimator with reduced computational complexity. The performance of the simplified estimator for SC-FDE over UWB channels is evaluated and compared with that of perfect channel state information. The effects of non-optimal and optimal pilot symbols are also investigated. Our results show that the proposed frequency domain channel estimator performs well over UWB channels with only small performance degradation compared to the perfect channel estimation

Maximum diversity in single-carrier frequency-domain equalization

Abstract: In broadband wireless communications, multipath propagation often results in an overall channel with a long impulse response that could span tens or even hundreds of symbol intervals. To equalize such long channels, conventional single-carrier time-domain equalization becomes infeasible due to high computational complexity. Relying on the use of fast Fourier transform, single-carrier frequency-domain equalization (SC-FDE) offers low-complexity equalization as well as other desirable features. In this correspondence, we prove that SC-FDE is also capable of collecting full multipath diversity even without channel coding. As far as we are aware, this is the first analytical proof of the diversity gain of SC-FDE. This conclusion justifies those existing simulation results regarding the performance comparison between SC-FDE and orthogonal frequency-division multiplexing (OFDM), and offers important guidelines for further improving SC-FDE and OFDM

Iterative detection for pretransformed OFDM by subcarrier reconstruction

Abstract: In this paper, an iterative detection method for an uncoded pretransformed (PT) orthogonal frequency division multiplexing (OFDM) system where the channel is not known at the transmitter is proposed. The iterative detection starts with linear detection. The noiseless received signal at the weakest subcarrier (corresponding to the smallest channel amplitude) is estimated based on all the detected data symbols using a hard or soft decision. Then, the actual received signal at the weakest subcarrier is replaced by the estimated one. This process is referred to as reconstruction here. After reconstruction, linear detection is carried out again to generate the next set of symbol estimates. The whole process proceeds iteratively to reconstruct the received signal at the next-weakest subcarrier. The transform coefficients and the iterative process are designed to maximize the minimum signal-to-noise power ratio. Under the assumption that the previous detection is error free, it is shown analytically that the iterative method achieves a diversity advantage of i+1 in the ith iteration, thus providing an explanation of its superior performance. Due to the flexibility of the transform design, the analysis conducted is applicable for other common systems as well. Simulations in realistic channels are carried out, and the bit-error rate performance of the iterative detection is superior as compared to that of the conventional detectors for the PT-OFDM or OFDM system.

MIMO extensions for SC/FDE systems

Abstract: Several efforts are currently underway in investigating multiple-input multiple-output (MIMO) communication systems MIMO technology promises high data rate wireless communications and/or the potentiality of exploiting transmit and receive diversity schemes For WLAN (wireless local area networks) scenarios most investigations focus on OFDM (orthogonal frequency division multiplexing) transmission In this paper we discuss multiple antenna diversity and spatial multiplexing schemes for SC/FDE (single carrier transmission with frequency domain equalization), a single carrier technology closely related to OFDM due to its frequency domain signal processing characteristics Equalizer structures are derived and analyzed, and their performance is demonstrated using BER-simulations

A comparision of OFDM and non-linear SC/FDE signals: Non-linear amplification

Abstract: One of the most influential parameters in today's wireless communications market is the battery lifetime, especially for mobile devices. In contrast to OFDM (Orthogonal Frequency Division Multiplexing), SC/FDE (single carrier transmission with frequency domain equalization) allows to apply signal formats with constant envelope, such as CPM (Continuous Phase Modulations) or at least significantly lower Peak to Average Power Ratios (PAPRs) such as Offset-QPSK. This allows the signal to drive the power amplifier into its maximum power efficient region. As a consequence the batteries lifetime is extended and its size is reduced. In this work, we compare non-linear modulated, block-oriented SC/FDE signals with OFDM signals in terms of bit error rate and spectrum disturbances at the non-linear region of the power amplifier.

A Multi-user SC-FDE-MIMO System for Frequency-Selective Channels

Abstract: This paper proposes a multi-user multiple input-multiple output (MIMO) system based on a singlecarrier frequency-domain equalization (SC-FDE) technique for frequency-dispersive multipath environments, where the signals are corrupted by both co-channel interference (CCI) and intersymbol interference (ISI). The proposed scheme effectively suppresses both CCI and ISI simultaneously with relatively low complexity. Furthermore, it does not impose any restriction on the number of available antennas. Simulation results show that the proposed scheme offers improved bit error rate and throughput performance and is superior to multi-carrier systems. The proposed scheme will make the multi-user MIMO system for frequency-selective channels more practically feasible.

Performance of SC-FDE in UWB communications with channel estimation errors

Abstract: We analyze the performance of single carrier transmission with frequency domain equalizations (SC-FDE) over UWB communications with least-square (LS) channel estimation. The probability density functions (PDF's) of the frequency domain minimum mean-squared error (MMSE) equalizer taps are derived. The error probability of SC-FDE over UWB channels with LS channel estimation is presented and evaluated numerically. Simulation results validate the use of Gaussian approximation method in the performance analysis of the SC-FDE system with LS channel estimation.

Space diversity schemes for STBC-based SC/FDE systems and SC/Pre-FDE systems

Abstract: In wireless communications, multiple receive antennas can be used with orthogonal frequency division multiplexing (OFDM) or single-carrier (SC) frequency domain equalization (FDE) and frequency domain pre-equalization (Pre-FDE) to significantly improve system capacity and performance. However, the complexity of such combination can be very large because multiple discrete Fourier transform (DFT) blocks, each for one receive antenna, are required to fully take advantage of the space diversity. A receive space diversity architecture was proposed in previous paper for OFDM systems using orthogonal designs. Using this architecture, the number of DFT blocks can be reduced but performance degradation will be introduced. In this paper, we propose two space diversity schemes for SC systems; one uses pre-FDE technique and the other uses FDE and space-time block coding (STBC) technique to compose a multiple-input multiple-output (MIMO) system. Using these schemes, the total number of Fourier transform (FT) blocks can be further reduced but the performance can be significantly improved. Simulation results show that the proposed SC system schemes exceed the corresponding OFDM schemes at high signal-to-noise ratio (SNR).

A TDMA scheme for SC-FDE UWB communications

Abstract: We propose a time division multiple access (TDMA) scheme for the BPSK modulated single carrier block transmission with frequency domain equalization (SC-FDE) over ultra-wideband (UWB) communications. The transmitter and receiver matrices for the proposed TDMA SC-FDE system are derived to achieve the multiple access interference (MAI) free and interblock-interference (IBI) free transmission without the insertion of cyclic prefix (CP) between blocks, circumventing the need for long CP and large FFT size in high speed UWB communications. The performance of the proposed TDMA SC-FDE system over UWB channels is investigated and compared with that of the block spread code division multiple access (CDMA) scheme. The effects of oversampling diversity and imperfect channel estimation are also investigated. Results show that the proposed TDMA scheme can effectively eliminate the error floor in CDMA due to insufficient CP, and it is more efficient to accommodate larger number of users than CDMA in high data rate transmission