An Improved OFDM Channel Estimation Based on CMMB

In this paper, we introduce a channel parameter tracking method for adaptive OFDM MMSE channel estimation. Even though the MMSE estimation is one of the most accurate channel estimation methods, it requires several channel information including Doppler frequency, RMS (root mean squared) delay spread, and noise power. To implement the MMSE estimation, tracking of such parameters should be preceded. We first propose methods to track such parameters focusing on the estimation of Doppler frequency, and mention their performance and complexity. We then implement and simulate the adaptive MMSE estimation using these tracking schemes and analyze the estimator's performance in terms of channel tracking accuracy and BER performance. We conclude that the proposed channel parameter estimator tracks channel condition reliably in various channel conditions without significant increase in computational complexity and that BER performance of the adaptive MMSE estimation is better than that of a robust MMSE estimator by 5 dB in average.

In OFDM systems, pilots are usually sent along with the data carriers to calculate channel impulse response (CIR) for channel estimation. However, transmitted signals are normally distorted by many environmental factors when propagating through underwater communication channels. Thus, the original pilots seem to be not enough to recover “good” CIR for channel estimation in underwater acoustic OFDM (UWA-OFDM) systems. Recent studies have developed data pilot-aided (DPA) strategies, known as STA and CDP, for time-variant channels like vehicle-to-vehicle (V2V) communications. These methods work on the assumption of high cross-correlation of CR between two adjacent data symbols and the support of two long training symbols in the IEEE 802.11p packet. These conditions are not provided in the UWA-OFDM systems, which leads to be infeasible to apply existing DPA strategies. Therefore, this paper introduces a novel pilot enrichment (PE) method for improving the performance of the channel estimation in UWA-OFDM systems. The core idea is that some data subcarriers are extracted under a condition related to constellation mapping for creating supplementary “pilots”. The “pilots” are used together with the original pilots for channel estimation. Our method is evaluated in comparison to the MMSE estimation on various modulation schemes. The experimental results illustrate that the PE estimator can perform better than the MMSE estimator in terms of bit error rate (BER), especially on medium and high SNR values.

A low flicker noise and high linearity passive mixer in 0.18 μm CMOS is proposed. The mixer, which consists of gm-stage, AC coupled LO switch-pair and low noise trans-impedance amplifier (TIA), features a low flicker noise corner frequency of 5 kHz. The mixer is designed for direct conversion receivers in China Mobile Multimedia Broadcasting (CMMB) system. Simulation results show that the passive mixer achieves a voltage conversion gain (CG) of 21.2 dB, intercept point (IIP3) of +13 dBm, double sideband (DSB) noise figure (NF) of 18.9 dB @ 5 kHz, white DSB NF of 18.1 dB, and output DC offset of less than 3 mV. The mixer dissipates 7.2 mA (including I path and Q path) from a 1.8 V supply.

High data rate requirements and reliable performances over fast fading channels are concerned more and more for wireless systems, especially when it comes to high-speed railway. This paper gives a detailed analysis of high moving wireless channel characteristics of non-widesense stationary uncorrelated scattering(Non-WSSUS), including other main influencing factors and time-varying conditions, and establishes the scattering channel model, which describes the approximate operation scenarios of high-speed rail line. Considering the OFDM technology, we propose an OFDM scattering channel system. This work has great potential application values in multimedia data transmission. Finally, in order to verify this model, based on a given OFDM system's specifications, we use the conventional least square (LS) algorithm to simulate the channel estimation, which extends the research field of the OFDM channel estimation under the environment of high-speed railway.

The performance of a decision-directed channel estimation scheme is analyzed in the context of both OFDM and MC-CDMA systems. A difficulty associated with the employment of the least squares (LS) approach to the problem of a posteriori channel estimation in the context of a MC-CDMA system is described and a suitable MMSE-based estimator is proposed instead. It is demonstrated that the computational complexity associated with the MMSE estimation method proposed is relatively high in comparison to that of the conventional LS technique. Thus a low-complexity version of the MMSE estimator is proposed. The MC-CDMA system using a low-complexity MMSE estimator proposed is shown to outperform the corresponding OFDM-based scheme.

The discussion of mobile multimedia broadcasting technologies and service implementation in this chapter is limited to the broadcasting network based scheme can be received by portable or hand-held devices with a small screen. Similar to DTTB systems, digital mobile multimedia broadcasting systems are designed for high-data-throughput (to support multiple low-data-rate services) transmission through wireless channels (VHF/UHF bands in terrestrial, S- or L-band for satellite networks). The chapter summarizes the features of DVB-H, Advanced Television Systems Committee (ATSC)-M/H, China Multimedia Mobile Broadcasting (CMMB), and the new released DVB Next Generation Broadcasting System to Handheld (NGH) standards with the focus on the technical merits of reliable transmission under wireless, mobile environment and the robust reception using the portable or hand-held devices. ATSC-Mobile/Handheld (ATSC-M/H) is the standard for the DTV mobile transmission developed by the ATSC in 2007. The ATSC-M/H system shares the same RF channel of the traditional ATSC broadcast services.

In this paper, we consider OFDM channel estimation in the presence of frequency offset and phase noise. In the literatures, most channel estimation methods assume perfect frequency synchronization and the knowledge of channel statistics. Phase noise and residual frequency offset cause inter-carrier interference (ICI), which consequently impairs the accuracy of channel estimation. The lack of knowledge of channel statistics can make channel estimation much harder. To resolve these problems, we propose with the aid of cyclic prefix (CP) based frequency offset estimation statistics-independent channel estimation. We iteratively search for the most likely channel impulse response (CIR) length, and use it not only for the optimum compensation of frequency offset, but also for finding the optimum window to filter the least square (LS) channel estimate which further suppress the effects of ICI and noise. The proposed scheme is compared with conventional methods for both non-interpolation and interpolation cases. Numerical results are presented to illustrate the effectiveness of the proposed scheme.

Pilot assisted channel tracking (PACT) has been very popular for channel estimation in OFDM systems. However, as the mobility in the system increases and one has to maintain a high accuracy in channel estimation, the pilot overhead typically has to be increased. Emerging cellular OFDM standards are expected to use about 6 - 12% pilot overhead per stream, and any possible reduction in pilot overhead would be useful. Decision directed channel tracking (DDCT) can help reduce pilot overhead, but is known to suffer from error propagation at high fade rates. In emerging broadband wireless systems promising peak bit rates of 50 Mbps or more, saving on pilot overhead by using DDCT schemes would be highly attractive provided: (a) Such a DDCT approach does not suffer from error propagation and has an error rate performance comparable to (or better than) PACT schemes even at high fade rates; (b) The computational complexity of such a DDCT approach is not significantly more than that of the MMSE based PACT scheme. In this work, we propose a low complexity DDCT method which exploits the structure of the regression matrix in conjunction with robust statistics to mitigate the effect of error propagation. It has a much lower computational complexity and a better error rate performance than the decision directed EM-Kalman and other existing robust statistics based DDCT schemes. The proposed method also outperforms a 12.5% pilot overhead based PACT scheme with only a modest increase in computational complexity.

This paper study the channel estimation based on the exponentially weighted (EW) RLS algorithm. The advantages of the proposed estimator arise from its implementation in the time-domain, whereas fewer channel parameters are required to be estimated compared with the frequency-domain channel coefficients. In addition, the matrix inversion operation required in LS and LMMSE estimators is avoided here by recursively updating the channel estimates. Therefore, the computational complexity is highly reduced compared to frequency-domain based channel estimation. Furthermore, the proposed estimator has good tracking capability due to exploiting the time correlation between the path gains. This paper provides analysis, evaluation and computer simulations in MATLAB. The performance is evaluated in terms of the MSE of the channel estimate and BER for different Doppler frequencies (correspond to different mobility speeds) and Monte Carlo simulations are performed and the MSE and BER performance versus SNR are obtained by averaging over 10000 channel realization. For comparisons, the BER performance is also presented for perfectly known channel at the receiver. In all the simulations, perfect synchronization between the transmitter and the receiver is assumed.

Multiple transmit and receive antennas (MIMO) have been used in OFDM systems for capacity improvement. In practice, channel state information has to be estimated for diversity combining or space-time decoding. Previous work on channel estimation assumes that MIMO channels are independent and identically distributed (i.i.d.). In certain propagation environments, there exists spatial correlation among channels corresponding to different pairs of transmit and receive antennas. The spatial correlations can he exploited to improve channel estimation. In this paper, we develop a minimum mean-square-error (MMSE) channel estimator for MIMO-OFDM systems that can make full use of the spatial correlation. We also design optimum training sequences that minimize the channel estimation error. When MIMO channels are i.i.d., the training sequences for different transmit antennas are orthogonal and with equal power. However, when MIMO channels are spatially correlated, the power allocation for training sequences can be further optimized. Our simulation results show that the proposed MMSE estimator can exploit spatial and frequency correlations of MIMO channels in OFDM systems and therefore has good performance.

A novel on-line channel estimation and tracking method for wireless OFDM systems based on comb-type pilots is developed. Long training sequences are used to estimate the channel response in the frequency domain, and a scalar Kalman filtering scheme is designed to refine the channel estimation. To track the channel variations during a long burst packet, pilots estimated via vector Kalman filtering are used to interpolate linearly and modify the estimated channel response. Both channel AWGN and state variable noise variance are adaptively estimated. Furthermore, to overcome phase changes across symbols owing to the frequency offset estimation error, a pilot based phase compensation scheme is incorporated. The complete algorithm can be processed in real time with much better performance than training sequence based or pilot based channel estimation methods. Simulation results based on the IEEE 802.11a wireless LAN standard are presented.

This work proposes symbol and block level adaptive channel estimation schemes, based on the least mean squares (LMS) and block-LMS (BLMS) approaches, respectively, for multiuser-MIMO (MU-MIMO) systems. The proposed schemes do not require knowledge of the first and second order statistics of the time-varying MU-MIMO channel, while also having a lower computational complexity in comparison to the Kalman filter based channel estimation approaches present in the existing literature. Another important aspect of the proposed MU-MIMO framework is that channel estimation is carried out at the base station (BS), which simplifies the receiver architecture. Analytical expressions are derived for the error covariance matrix at each time instant and the asymptotic mean square error (MSE) of the proposed LMS and BLMS frameworks. Further, a superimposed pilot (SIP) framework for MU-MIMO channel estimation been developed, which transmits data symbols to a group of selected users during the training phase, thus leading to a significant improvement in the sum-rate performance. Simulation results are presented to demonstrate the improved sum-rate and MSE performance of the proposed schemes and also to verify the analytical results.

In orthogonal frequency division multiplexing (OFDM) systems, channel variations within an OFDM block destroy orthogonality between subcarriers, resulting in intercarrier interference (ICl), which increases an error floor in proportional to normalized Doppler frequency. To mitigate the effects of channel variations, many schemes have been proposed, but they are computationally complex or at the price of sacrificing spectral efficiency. In this paper, we first provide an ICI analysis in both time- and frequency-domain while existing literatures analyze the ICI effects mainly in the frequency domain. Based on this analysis, we then propose a novel ICI suppression technique, which uses iterative ICI cancellation in time domain and has a computational complexity that is linear in the OFDM symbol length. Channel estimation in rapidly time-varying scenarios is critical for ICl suppression and coherent demodulation. We also propose a scheme for estimating the channel parameters varying within an OFDM block. Along with the channel estimation scheme, we investigate the issue of optimum pilot tone placement. Theoretical analysis and simulation results show that the proposed ICl suppression method and channel estimation scheme have low computational complexity and good performance for most Doppler frequency of practical importance.

Simulation and Comparison of Channel Estimation Based on Block-type Pilot Frequency in OFDM System

This paper presents a new method for frame rate up‐conversion using sub‐sampled bilateral motion estimation, motion vector smoothing, simplified bidirectional motion compensated interpolation, and complex motion detection. The proposed method produces improved image quality and requires less computational complexity compared to conventional motion estimation. The average PSNRs of the proposed method for frame rate up‐conversion outperform those of the benchmark frame rate up‐conversion technique by up to 4.39 dB for the test sequences. Furthermore, the motion estimation of the proposed FRUC method reduces the required computational complexity to 12.5% of that required for full‐search block matching motion estimation.