Jinesh P. Nair

An Iterative Channel Estimation Method using Superimposed Training in OFDM Systems

In this work an iterative time domain least squares (LS) based channel estimation method using superimposed training (ST) for an orthogonal frequency division multiplexing (OFDM) system over frequency selective fading channels is proposed for the IEEE 802.16e 2005 standard. The estimate of the channel is generalized to provide scope for exploiting the coherence time and the coherence bandwidth for an improved performance. The performance of the channel estimator is analyzed in terms of the Mean Square Estimation Error (MSEE) and its impact on the uncoded Bit Error Rate (BER) of the OFDM system is studied. The selection criterion for the training sequences is proposed by jointly optimizing the MSEE and the BER of the OFDM system. Such a sequence is required to have an impulse like autocorrelation for lags extending at least up to the order of the estimator and also ensure a fair distribution of the residual interference due to the training sequence on the data symbols prior to data detection in all the used subcarriers. The effectiveness of the mathematical analysis presented is demonstrated through a comparison with the simulation studies. Also, the proposed scheme is applied to the standard, its suitability is examined and a case is made with the required design of the sequence.

Optimal superimposed training sequences for channel estimation in MIMO-OFDM systems

In this work an iterative time domain Least Squares (LS) based channel estimation method using superimposed training (ST) for a Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system over time varying frequency selective fading channels is proposed. The performance of the channel estimator is analyzed in terms of the Mean Square Estimation Error (MSEE) and its impact on the uncoded Bit Error Rate (BER) of the MIMO-OFDM system is studied. A new selection criterion for the training sequences that jointly optimizes the MSEE and the BER of the OFDM system is proposed. Chirp based sequences are proposed and shown to satisfy the same. These are compared with the other sequences proposed in the literature and are found to yield a superior performance. The sequences, one for each transmitting antenna, offers fairness through providing equal interference in all the data carriers unlike earlier proposals. The effectiveness of the mathematical analysis presented is demonstrated through a comparison with the simulation studies. Experimental studies are carried out to study and validate the improved performance of the proposed scheme. The scheme is applied to the IEEE 802.16e OFDM standard and a case is made with the required design of the sequence.

Channel estimation and equalization based on implicit training in OFDM systems

Implicit training (IT) based channel estimation exploits the first order statistics in the received data, induced by superimposing periodic training sequences with good correlation properties, along with information symbols. Hence the need for additional time slots for training the equalizer is avoided. In this paper we investigate on the applicability of this technique to orthogonal frequency division multiplexing (OFDM) systems over a frequency selective fading channel. Based on the estimate, a zero forcing solution is used to equalize the channel. The effect of the deterministic mean of the data on the channel estimate is considered. By exploiting the periodicity of the training sequences in the frequency domain, improved estimates of the channel coefficients are obtained. The performance of the estimator is presented in terms of the mean square estimation error (MSEE) and uncoded bit error rate (BER). All the improvements come at the cost of a loss in bandwidth which makes this scheme similar to the comb type pilot based channel estimation scheme for OFDM

An optimal superimposed training sequence for channel estimation in OFDM systems

In this work an iterative time domain least squares (LS) based channel estimation method using superimposed training (ST) for an orthogonal frequency division multiplexing (OFDM) system over frequency selective fading channels is proposed. The performance of the channel estimator is analyzed in terms of the mean square estimation error (MSEE) and its impact on the uncoded bit error rate (BER) of the OFDM system is studied. The selection criterion for the training sequences is proposed by jointly optimizing the MSEE and the BER of the OFDM system. A chirp based sequence satisfying the selection criterion is proposed and compared with the other sequences found in the literature. The sequence is generalized to incorporate the use of guard subcarriers in the OFDM based system. The proposed sequence offers fairness through providing equal interference in all the data carriers unlike earlier proposals. Experimental studies are carried to study the performance of the proposed scheme and also its comparison with the existing schemes is presented. Also, the proposed scheme is applied to the IEEE 802.16e OFDM standard, its suitability is examined and a case is made with the required design of the sequence.

A bandwidth efficient channel estimation method using superimposed training for MIMO-OFDM systems

In this work an iterative time domain least squares (LS) based channel estimation method using superimposed training (ST) for a multi input multi output(MIMO) orthogonal frequency division multiplexing (OFDM) system over frequency selective fading channels is proposed. The estimate of the channel is generalized to provide scope for exploiting the coherence time and the coherence bandwidth for an improved performance. The performance of the channel estimator is analyzed in terms of the mean square estimation error (MSEE) and its impact on the uncoded bit error rate (BER) of the MIMO-OFDM system is studied by considering a two branch transmitter diversity system employing Alamouti space time coding. The selection criterion for the training sequences is proposed by jointly optimizing the MSEE and the BER of the OFDM system. Such sequences, one for each transmitting antenna are required to form an orthogonal set and also ensure a fair distribution of the residual interference due to the training sequence on the data symbols prior to data detection in all the used subcarriers. The effectiveness of the mathematical analysis presented is demonstrated through a comparison with the simulation studies. Also, the proposed scheme is applied to the IEEE 802.16e OFDM standard, its suitability is examined and a case is made with the required design of the sequence.

Low-complexity direction estimation techniques for fast beamforming training in millimeter wave multiantenna systems

This paper explores a novel approach for beamforming (BF) training in millimeter wave (mmWave) multiantenna communication systems. The contribution of this paper is in the following aspects: i) This paper proposes a fast BF training procedure building on the novel concept of beamcoding for the estimation of the optimal direction of arrival (DoA) and direction of departure (DoD). Briefly, the beamcoding scheme facilitates the identification of DoA/DoD at the receiver/transmitter based on a set of orthogonal codes. ii) Building on the properties of a special class of orthogonal codes called constant amplitude zero autocorrelation(CAZAC) sequences, this paper proposes a low-complexity implementation of beamcoding procedure to facilitate BF training. We emphasize that the proposed beamcoding schemes can be implemented at the transmitter/receiver with the help of only a single radio frequency (RF) chain with a set of analog phase shifters. Notably, the proposed method is not restricted to just mmWave communication systems, and can be applied to any generic system involving multiantenna transceivers. Simulation results are presented to demonstrate the efficacy of our proposed idea.

Method of Generating Multiple Sets of Orthogonal Codes with Wide Choice of Spreading Factors

This paper presents a generic method to construct multiple sets of orthogonal codes for application in direct sequence spread spectrum systems. The plurality of orthogonal sets is generated using any single two-level autocorrelation sequence of period one less than the desired spreading factor of the orthogonal codes. The proposed method hence generates orthogonal codes of all spreading factors for which a corresponding two-level autocorrelation sequence exists. The resulting orthogonal sets with wide choice of spreading factors is desirable in synchronous direct sequence spread spectrum systems. The sets can be generated such that they do not contain the codeword with single polarity as in the case of Walsh-Hadamard codes and orthogonal sets generated from cyclic shifts of two-level autocorrelation sequences. This renders all codes in the generated sets with an effective spreading gain. The generation method also encompasses a procedure to generate N-1 sets with low inter-set correlation for application to oversaturated spread spectrum systems.

An Energy Efficient Sub-Band Based UWB Receiver and Its Performance Improvement by Interference Rejection Filtering

A sub-band based ultra wideband (SUWB) system is proposed for energy efficient short range wireless communications. The proposed technique divides the bandwidth of 500 MHz into number of sub-bands that may be utilized for higher data rate or higher reliability or multi user access depending on system requirements. The energy efficiency is achieved by reducing the sampling rate requirements and also by the use of an orthogonal code based interference rejection and multi-path cancellation receiver. An interference rejection method is also proposed to further enhance the performance of the system for high data rate and/or high delay spread channels. This results in a low power and low complex alternative to improve performance in comparison to the conventional equalization methods. The simulation results in terms of the BER performance of the method for the IEEE 802.15.4a channel models are presented. A significant improvement in SNR of 5-7 dB is reported with the incorporation of the interference rejection filter for low and medium delay spread channels.

Timing synchronization in super-regenerative receivers with a single quench cycle per symbol

Super-regenerative receivers (SRR) are known for their ultra-low power (ULP) consumption. However, its wide frequency selectivity seriously degrades performance especially in the presence of interference. An attractive approach to address this problem is by lowering the quench rate of the SRR to the modulation rate. However, this brings in uncertainty due to lack of oversamples and leads to the timing synchronization problem in demodulation. This paper proposes a timing synchronization method for SRR with a single quench cycle per modulated symbol. A novel asynchronous preamble structure and an associated timing synchronization algorithm are proposed. Significant performance gains are obtained due to the improved frequency selectivity enabling the SRR system to adhere to the constraints set forth by the IEEE 802.15.4 standard while maintaining the ULP advantage.

An iterative channel estimation method using superimposed training for IEEE 802.16E based OFDM systems

An iterative least squares (LS) based channel estimation method for superimposed training (ST) based OFDM systems is proposed. The performance of the method is analyzed in terms of the mean squared estimation error (MSEE) and its impact on the bit error rate (BER) of the OFDM system is studied. A training sequence selection criterion that jointly optimizes the MSEE and the BER of the OFDM system also proposed. The effectiveness of the mathematical analysis presented is demonstrated through a comparison with the simulation studies. Also, the proposed scheme is applied to the IEEE 802.16e OFDM standard and a case is made with the required design of the sequence.