Channel Estimation in Multicarrier Communication Systems

The data rate and spectrum efficiency of wireless mobile communications have been significantly improved over the last decade or so. Recently, the advanced systems such as 3GPP LTE and terrestrial digital TV broadcasting have been sophisticatedly developed using OFDM and CDMA technology. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer which requires knowledge on the channel impulse response (CIR).This is primarily provided by a separate channel estimator. Usually the channel estimation is based on the known sequence of bits, which is unique for a certain transmitter and which is repeated in every transmission burst. Thus, the channel estimator is able to estimate CIR for each burst separately by exploiting the known transmitted bits and the corresponding received samples. In this thesis we investigate and compare various efficient channel estimation schemes for OFDM systems which can also be extended to MC DS-CDMA systems.The channel estimation can be performed by either inserting pilot tones into all subcarriers of OFDM symbols with a specific period or inserting pilot tones into each OFDM symbol. Two major types of pilot arrangement such as block type and comb type pilot have been focused employing Least Square Error (LSE) and Minimum Mean Square Error (MMSE) channel estimators. Block type pilot sub-carriers is especially suitable for slow-fading radio channels whereas comb type pilots provide better resistance to fast fading channels. Also comb type pilot arrangement is sensitive to frequency selectivity when comparing to block type arrangement. However, there is another supervised technique called Implicit Training (IT) based channel estimation which exploits the first order statistics in the received data, induced by superimposing periodic training sequences with good correlation properties, along with the information symbols. Hence, the need for additional time slots for training the equalizer is avoided. The performance of the estimators is presented in terms of the mean square estimation error (MSEE) and bit error rate (BER)

LPTV-Aware Bit Loading and Channel Estimation in Broadband PLC for Smart Grid

Power line communication (PLC) has received steady interest over recent decades because of its economic use of existing power lines, and is one of the communication technologies envisaged for Smart Grid (SG) infrastructure. However, power lines are not designed for data communication, and this brings unique challenges for data communication over power lines. In particular for broadband (BB) PLC, the channel exhibits linear periodically time varying (LPTV) behavior synchronous to the AC mains cycle. This is due to the time varying impedances of electrical devices that are connected to the power grid. Another challenge is the impulsive noise in addition to power line background noise, which is due to switching events in the power line network. In this work, we focus on two major aspects of an orthogonal frequency division multiplexing (OFDM) system for BB PLC LPTV channels; bit and power allocation, and channel estimation (CE). First, we investigate the problem of optimal bit and power allocation, in order to increase bit rates and improve energy efficiency. We present that the application of a power constraint that is averaged over many microslots can be exploited for further performance improvements through bit loading. Due to the matroid structure of the optimization problem, greedy-type algorithms are proven to be optimal for the new LPTV-aware bit and power loading. Significant gains are attained especially for poor (i.e. high attenuation) channel conditions, and at reduced transmit-power levels, where the energy per bit-transmission is also low. Next, two mechanisms are utilized to reduce the complexity of the optimal LPTV-aware bit loading and peak microslot power levels: (i) employing representative values from microslot transfer functions, and (ii) power clipping. The ideas of LPTV-aware bit loading, complexity reduction mechanism, and power clipping are also applicable to non-optimal bit loading schemes. We apply these ideas to two additional sub-optimal bit loading algorithms that are based on even-like power distribution for a portion of the available spectrum, and demonstrate that similar gains in bit rates are achieved. Second, we tackle the problem of CE for BB PLC LPTV channels. We first investigate pilot based CE with different pilot geometry in order to reduce interpolation error. Block-type, comb-type, and incline type pilot arrangements are considered and a performance comparison has been made. Next we develop a robust CE scheme with low overhead that addresses the drawbacks of block-type pilot arrangement and decision directed CE schemes such as large estimation overhead for block-type pilot geometry, and difficulty in channel tracking in the case of sudden changes in the channel for decision directed approaches. In order to overcome these drawbacks, we develop a transform domain (TD) analysis approach to determine the cause of changes in the channel estimates, which are due to changes in the channel response or the presence of impulsive noise. We then propose a robust CE scheme with low estimation overhead, which utilizes pilot symbols placed widely apart and exploits the information obtained from TD analysis as a basis for switching between various CE schemes. The overhead of the proposed scheme for CE is low, and sudden changes in the channel are tracked affectively. Therefore, the effects of the LPTV channel and the impulsive noise on CE are mitigated. Our results indicate that for bit and power allocation, the proposed reduced complexity LPTV-aware bit loading with power clipping algorithm performs very close to the optimal LPTV-aware bit loading, and is an attractive solution to bit loading in a practical setting. Finally, for the CE problem, the proposed CE scheme based on TD analysis has low estimation overhead, performs well compared to block-type pilot arrangement and decision directed CE schemes, and is robust to changes in the channel and the presence of impulsive noise. Therefore, it is a good alternative for CE in BB PLC

Performance Evaluation of Channel Estimation in OFDM System for Different QAM and PSK Modulations

To recover accurate transmitted data at the receiver end, the information regarding channel state derived from channel estimation methods play a very important role in any communication system. In this paper the performance evaluation of different types of QAM and PSK modulations with three different channel estimation methods in OFDM system for wireless communication in frequency domain for slow fading channel is compared. The results must be useful in OFDM based applications like IEEE 802.16(d) and equivalent standards.DOI:http://dx.doi.org/10.11591/ijece.v1i2.14

Papr analysis and channel estimation techniques for 3GPP LTE system

High data rates and secured data communication has become an unavoidable need of every mobile users. 3G technology provided greater data speed and secured networks compared to its predecessor 2G or 2.5G. The highest bit rates in commercially deployed wireless systems are achieved by means of Orthogonal Frequency Division Multiplexing (OFDM) [1]. The next advance in cellular systems, under investigation by Third Generation Partnership Project (3GPP), also anticipates the adoption of OFDMA to achieve high data rates. But a modified form of OFDMA i.e. SCFDMA (Single Carrier FDMA) having similar throughput performance and essentially the same complexity has been implemented as it has an edge over OFDMA having lower PAPR (peak to average power ratio) [2]. SCFDMA is currently a strong candidate for the uplink multiple access in the Long Term Evolution of cellular systems under consideration by the 3GPP. In our project we have worked on PAPR analysis of OFDMA, SCFDMA and various other SCFDMA (with different subcarrier mapping). Though SCFDMA had larger ISI it has lower PAPR which help in avoiding the need of an efficient linear power amplifier. We have analyzed various modulation techniques and implemented various kinds of pulse shaping filters and compared the PAPR for IFDMA, DFDMA and LFDMA (kinds of SCFDMA). Like other communication systems, in SCFDMA we encounter many trade-offs between design parameters (such as roll-off factor) and performance. The project report also constitutes the channel estimation techniques implemented in OFDM systems. Due to multipath fading the channel impulse response fluctuates for different subcarriers in different time slots. But with channel estimation OFDM systems can use coherent detection instead of differential. For MIMO system like OFDM channel information is vital for diversity combining and interference suppression [3]. So we need to estimate the channel as accurately as possible. As we have taken a slow Rayleigh fading channel in our study we used block type pilot arrangement channel estimation which uses LS (least square), MMSE (minimum mean square error) estimator. Due to higher complexity of the MMSE estimator, modified MMSE is implemented where tradeoff is made with performance. Here we have compared various channel estimation techniques used in OFDM systems. There are various other adaptive estimation techniques like LMS and RLS for estimating blind channels and comb type pilot arrangement estimation techniques for fast fading channels