Florence Danilo-Lemoine

Channel Estimation for SC-FDE Systems Using Frequency Domain Multiplexed Pilots

We investigate channel estimation for single-carrier-frequency domain equalization (SC-FDE) system using the techniques typically used for an orthogonal frequency domain multiplexing (OFDM) system. Two techniques of frequency domain multiplexed (FDM) pilot insertion using interleaved frequency domain multiple access (IFDMA) signal with a Chu sequence are considered. One called frequency domain superimposed pilot technique (FDSPT) scales data-carrying tones and then superimposes them with pilot tones. This technique preserves spectral efficiency at the expense of performance loss. The other, called frequency expanding technique (FET), shifts groups of data frequencies for multiplexing of pilot tones at the expense of spectral efficiency. Our results show that both techniques increase peak to average power ratio (PAPR) although it is still lower than that of an OFDM system. The application of FDSPT is limited by the pilot overhead ratio, resulting from the removal of data frequencies for pilot frequencies. It is shown that channel estimation using conventional time domain multiplexed pilots and FET pilot tones produce the same BER, while the FDSPT requires about 1.5 dB more power for the same performance. Using FDM pilots in SC system facilitates flexible and efficient assignment of signals to available spectrum.

Iterative frequency domain channel estimation for dft-precoded ofdm systems using in-band pilots

We consider two techniques of in-band frequency domain multiplexed (FDM) pilots using interleaved frequency domain multiple access (IFDMA) signal with a Chu sequence for DFT-precoded OFDM (or single-carrier (SC)) systems. One, called frequency domain superimposed pilot technique (FDSPT), superimposes pilot tones onto scaled or deleted data tones, which preserves spectral efficiency at the expense of a slight performance loss. The other, called frequency expanding technique (FET), multiplexes pilot tones by displacing data tones, which slightly reduces spectral efficiency. Using FDM pilots in SC systems facilitates flexible and efficient assignment of signals to available spectrum. We propose an iterative frequency domain decision-directed interference cancellation technique to reduce the intersymbol interference level of SC signals with FDSPT pilots (resulting from the suppression of data tones). Moreover, we propose a low complexity frequency domain iterative decision-directed channel estimation (IDDCE) technique for SC systems using FDM pilots. Using IDDCE, the frame error rate (FER) performance for coded SC systems using FET and FDSPT pilots with interference cancellation is found to be about 0.2 dB and about 0.5 dB, respectively, away from the FER performance with known channel frequency response at FER=10-2. FDSPT pilots can also be used for OFDM systems with channel coding. It is found that an extra 1 dB of SNR is required at FER=10-2,compared with that using the conventional FET pilots for OFDM systems.

Power backoff reduction techniques for generalized multicarrier waveforms

Amplification of generalized multicarrier (GMC) signals by high-power amplifiers (HPAs) before transmission can result in undesirable out-of-band spectral components, necessitating power backoff, and low HPA efficiency. We evaluate variations of several peak-to-average power ratio (PAPR) reduction and HPA linearization techniques which were previously proposed for OFDM signals. Our main emphasis is on their applicability to the more general class of GMC signals, including serial modulation and DFT-precoded OFDM. Required power backoff is shown to depend on the type of signal transmitted, the specific HPA nonlinearity characteristic, and the spectrum mask which is imposed to limit adjacent channel interference. PAPR reduction and HPA linearization techniques are shown to be very effective when combined.

A Postfix Synchronization Method for OFDM and MIMO-OFDM Systems

This paper proposes a synchronization method that uses as preamble two identical training parts (Chu sequences) followed by a flipped postfix (FlP) for OFDM and MIMO-OFDM systems. The proposed method provides symbol timing and carrier frequency offset (CFO) estimators. By using this FlP-based preamble, the accuracy of the symbol timing estimator is highly improved without increasing the computation complexity. The time synchronization method includes both coarse and fine time synchronization. In particular, this paper indicates how to set up the threshold for the fine time synchronization. Performance evaluation of the proposed method, applied to MIMO-OFDM systems, with various diversity combining shows that receive diversity gives more gain than transmit diversity. The proposed method for both OFDM and MIMO-OFDM systems is also compared to various synchronization methods such as the Schmidl-Cox (SC) and Minn-Zeng-Bhargava (MZB)s methods in time invariant and time-variant Rayleigh fading channels in terms of mean and variance of the timing and CFO estimators. Overall, it is seen that the proposed new method offers good advantages compared to existing synchronization methods.

A Low Complexity Frequency Domain Iterative Decision-Directed Channel Estimation Technique for Single-Carrier Systems

A low complexity frequency domain iterative decision-directed channel estimation (FD-IDDCE) technique for single-carrier (SC) systems with frequency domain multiplexed (FDM) pilots is proposed. The tentative hard decisions from either the frequency domain equalizer output or the Viterbi decoder output are used as extra pilots to further improve the initial frequency channel estimates using a cascaded 2 times 1D Wiener filter. The issue of noise enhancement when finding the least square (LS) estimates of the channel frequency response (CFR) using the tentative decisions in the frequency domain is overcome by the so called frequency replacement algorithm, which replaces the noise enhanced LS estimates of the CFR with the corresponding channel frequency estimates in the previous iteration, by comparing with a threshold. Using the proposed FD-IDDCE, the frame error rate (FER) performance for coded SC systems using FDM pilots with frequency expanding technique and frequency domain superimposed pilot technique was found to be about 0.4 dB and about 1 dB away from the FER performance with known CFR at FER=10-2.

PAPR Reduction using Frequency Domain Multiplexed Pilot Sequences

We investigate the feasibility of applying the peak-to-average power ratio (PAPR) reduction method using pilot sequences, originally proposed for orthogonal frequency division multiplexing (OFDM) signals, for single-carrier (SC) signals with frequency domain multiplexed (FDM) pilots. The idea is to select the FDM pilot sequence with which the transmitted signal produces the lowest PAPR. We also investigate the applicability of the sum of square error (SSE) selection rule for high order modulation of SC signals. The SSE rule selects the pilot sequence which produces the minimum SSE between the transmitted signal and a pre-defined threshold, proportional to the saturation level of a high power amplifier (HPA). It is found that for both SC and OFDM systems, the PAPR reduction capabilities of orthogonal Walsh-Hadamard (W-H) sequences and cyclic shifted Chu (CS-Chu) sequences are similar for small block size, but not for large block size. Using CS-Chu sequences produces better PAPR reduction capability. With an appropriate choice of the value of input backoff power of a HPA, the SSE selection rule produces similar results as that of the minimum PAPR selection rule. The effects of out-of-band radiation for SC and OFDM signals with PAPR reduction using FDM pilot sequences after HPA depends on the amount of non-linearity portion of the HPA. The out-of-band radiation improvement is obvious for a HPA that approximates a linear clipper.

Design of Time and Frequency Domain Pilots for Generalized Multicarrier Systems

By the generalized multi-carrier (GMC) principle a unified framework to describe various multi-carrier as well as single carrier approaches is established. In this paper the GMC principle is extended to pilot design and channel estimation, so that a unified description of pilot aided channel estimation (PACE) by interpolation in time and frequency is established. This applies to frequency domain pilots which are embedded in the GMC signal, as well as pilots sequences time multiplexed with data-bearing GMC blocks. A comparative performance evaluation of the two different GMC variants OFDM and single carrier with various pilot allocation schemes is carried out, also taking into account practical constraints such as the peak to average power ratio (PAPR) and required power amplifier back-off. It is found that a serial modem with time or frequency multiplexed pilots could be designed with a power amplifier with about 2 dB lower maximum power rating than that of a corresponding OFDM modem.

Antenna array training and adaptation techniques in an unpredictable and uncontrolled interference environment

We investigate the performance of an array processing technique based on sample matrix inversion (SMI) and the effects of channel estimation (using various training schemes) to combat intermittent interference. Most current applications of broadband wireless communication systems use short data block lengths. Therefore, antenna weights estimation is usually done only once and these weights are used for the whole packet length. Through simulations, we show that, with an intermittent kind of interference, weights estimation based on a preamble or post-amble only scheme fails to track suddenly appearing interferers, resulting in a degradation to the output SINR. Two new, training-based, channel estimation techniques are presented which show superior performance over the training schemes in this type of environment. For simulating intermittent interference traffic streams, a batch Poisson traffic model is used.

PIC Assisted IBDFE Based Iterative Spatial Channel Estimation with Intra and Inter-Cell Interference in SC-FDE System

In this paper, the issue of channel estimation in Single Carrier Frequency Domain Equalization (SC-FDE) uplink Space Division Multiple Access (SDMA) systems in the presence of intra and inter-cell interference is addressed. It is shown that in such an interference-prone system, a combination of orthogonal pilots, spatial processing and Iterative Channel Estimation (ICE) is effective. Adaptive Iterative Block Soft Decision based Feedback Equalizer (IBDFE) with Parallel Interference Cancellation (PIC) assisted Decision Feedback Iterative Channel Estimation is employed. It is shown that such a combination is a promising way to suppress uplink inter-user interference.

A Novel Postfix Synchronization Method for OFDM Systems

This paper presents a novel synchronization method for OFDM systems that uses preambles containing two identical training parts followed by a flipped postfix (FlP). Specifically, the proposed preamble for AWGN channels consists of a flipped cyclic prefix (FCP) and two identical training parts followed by a FlP The preamble for ISI and fading channels is similar except that the FCP is replaced by a zero padding prefix (ZPP). By using these FlP-based preambles, the accuracy of the timing offset estimator is highly improved without increasing the computation complexity. Performance of the proposed method is evaluated by simulation over AWGN, ISI and fading channels in terms of the mean and variance of the time offset and carrier frequency offset (CFO) estimators. Results show the new time offset estimator has a quite small variance and its mean value is quite close to the exact start point. Comparison to Schmidl-Cox and Minns classical synchronization methods illustrates the advantages of the proposed method.