Shigang Tang

Analysis on LUT Based Predistortion Method for HPA with Memory

Most existing methods for HPA predistortion with memory effects are based on polynomial or indirect learning structures. Few papers concern about the memory predistortion with LUT based techniques. Based on the previously presented method, we propose an improved two-dimensional LUT indexing method for HPA predistortion with memory. Computer simulations and corresponding analysis demonstrate that the proposed method has more effective predistortion capabilities but lower computational complexity

Iterative Channel Estimation for Block Transmission with Known Symbol Padding - A New Look at TDS-OFDM

This study proposes an iterative channel impulse response (CIR) estimation scheme for multicarrier block transmission with known symbol padding, in the context of the time-domain synchronous OFDM (TDS-OFDM) system. It is shown that the TDS-OFDM still fulfils the circular convolution relationship even if pseudo-noise (PN) sequence is used as guard interval. While conventional methods perform the channel estimation based on the cyclic prefix reconstruction or residual intersymbol interference cancellation in the time domain, by processing the data part and the known symbol padding in one TDS-OFDM block separately, the new proposal uses the one- tap frequency-domain equalization to separate the data part and the known symbol padding and the CIR can be estimated in an iterative manner. In a typical example for 3780-point FFT based TDS-OFDM system with 16QAM constellations, the mean square error (MSE) of of the proposed channel estimator can be reduced to about 10-5/2 after two iterations when the channel signal-to-noise ratio (SNR) is larger than 8 dB, without the initial CIR estimation. The BER performance of a QPSK TDS-OFDM system shows that the signal-to-noise-ratio (SNR) degradation based on the proposed channel estimator is less than 0.3 dB compared with the lower bound for both Brazil A and B channels.

Intercarrier Interference Cancellation with Frequency Diversity for OFDM Systems

The implementation of OFDM system suffers from the effect of phase noise (PHN) generated by local oscillator (LO), which disturbs the orthogonality among subcarriers and causes intercarrier interference (ICI). In frequency domain, the received signal can be formulated as the circular convolution of the transmitted signal and the ICI weighting function. In this paper, the symmetric conjugate property of the ICI weighting function is investigated. Based on that, the ICI self-cancellation of symmetric data-conjugate method is studied to suppress PHN. A new ICI cancelling demodulation (ICD) scheme with frequency diversity combining is proposed. Under flat fading channel, the carrier-to-interference ratio (CIR) improvement from this proposed method is larger than 22 dB, and can tolerate a phase noise with standard deviation of 7 deg when BER is 10-3. In frequency selective fading channel, the proposed method is shown to be able to offer significant diversity gain

Novel Decision-Aided Channel Estimation for TDS-OFDM Systems

Accurate channel estimation is essential for coherent signal transmission. This paper presents a novel channel estimation algorithm for time-domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) systems with identical pseudo-noise (PN) sequence padding. Simple cyclic reconstruction and tail cancellation methods are proposed to estimate the channel impulse response (CIR). Phase equalization and partial hard decision on the received data symbols are used to improve the estimation performance. Simulation results show that the proposed channel estimation algorithm brings good BER performance (approaching the ideal case) with fast convergence (no iteration required) over frequency-selective slowly fading channels.

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.

Loop Delay Correction for Adaptive Digital Linearization of Power Amplifiers

As one of the most common linearization techniques, adaptive digital predistortion minimizes the difference between the input signal of the predistorter and the feedback signal from the power amplifier. The loop delay between these two signals must be controlled precisely so that the convergence of the predistorter should be assured. In this paper a novel two-stage loop delay correction scheme is presented. In the first stage, the integer delay between the input signal and the feedback signal is estimated based on the cross-correlation of them. In the second stage, the fractional delay between these two signals are corrected by a delay lock loop (DLL) circuit, in which a novel timing error detector used as the discriminator is proposed. The proposed loop delay correction scheme is applied to an adaptive digital predistorter in an orthogonal frequency-division multiplexing (OFDM) system. Simulation results show that the proposed loop delay correction method eliminates the loop delay perfectly, so that nearly optimal power spectral density (PSD) performance is achieved and a high degree of linearization is realized.

Embedded Transmission of Multi-Service Over DTMB System

To efficiently support the transmission of terrestrial and mobile services within the same spectrum over the Digital Television Terrestrial Multimedia Broadcasting (DTMB) system, a flexible multi-service datacasting scheme in a backward compatible manner is proposed. Using this scheme, the conventional DTMB receivers work as usual by simply selecting the specific terrestrial DTV programs on the basis of the package identifier (PID) checking, while only the desired mobile service data are processed by the mobile receivers via the designed control signals. Both theoretical analysis and computer simulation show that, at acceptable spectral efficiency penalty, the proposed method not only supports the embedded transmission of multi-services with no reception performance degradation for the traditional terrestrial broadcasting service, but also flexibly provides much better mobile reception performance.

Self-Cancellation of Intercarrier Interference in OFDM Systems with Phase Noise

The implementation of OFDM system suffers from the effect of phase noise generated by local oscillator (LO), which disturbs the orthogonality between subcarriers and causes intercarrier interference (ICI). In this paper, the symmetric conjugate-negative property of the ICI weighting function is found. Based on that, the ICI self-cancellation of symmetric data-conjugate method is studied to suppress phase noise. The carrier-to-interference ratio (CIR) improvement by this method is shown to be larger than 22 dB in flat fading channel. For a BER of 10-3, this method can tolerate a phase noise standard deviation of 7 deg in flat fading channel. Moreover, this method could offer frequency diversity gain in frequency selective channels. Both analytical and numerical results show that this method outperforms the conventional schemes

Channel estimation for cyclic postfixed OFDM

Novel channel estimation algorithm is presented in this paper for the recently proposed cyclic postfix based orthogonal frequency division multiplexing (OFDM) systems. Phase equalization with the erasure decision is used to reduce both the channel estimation error and the computational complexity. Simulation results show that the proposed channel estimation algorithm can effectively estimate the channel impulse response (CIR) and the performance of the proposed phase equalization with erasure decision is comparable with the minimal mean square error (MMSE) equalization but offers less computational complexity.

Phase Noise Suppression in OFDM systems in Presence of IQ Imbalance

The implementation of OFDM system suffers from the effect of phase noise and IQ imbalance, which disturb the orthogonality between subcarriers and cause intercarrier interference (ICI). When treated separately, effective algorithms, such as the ICI self-cancellation scheme, exist for suppression of phase noise as well as IQ imbalance. However, with both effects present, such algorithms do not lead to effective suppression. In this paper, we propose a novel technique to jointly combat the IQ imbalance and phase noise at baseband. An IQ imbalance estimation and compensation module, which is robust against phase noise, is added in the ICI self-cancellation based OFDM system. After the IQ imbalance has been compensated in the time domain, the ICI self-cancellation scheme of data-conjugate method is used to suppress the phase noise and the residual IQ imbalance. This scheme has a fast convergence and a nearly optimal BER performance even for large IQ imbalance in AWGN channel. In multipath channel, even for large IQ imbalance, this joint suppression scheme offers frequency diversity gain and as a result, the BER performance surpasses that of the conventional OFDM system without IQ imbalance and phase noise