Wookwon Lee

Carrier Frequency Offset Estimation for OFDM Systems with Null Subcarriers

In this paper, we propose an iterative CFO estimator using a specific pattern of alternating pilots and null subcarriers in training OFDM block. One major advantage of the proposed estimator is its simple structure. The proposed method estimates the CFO in an iterative fashion based on the signals at the null subcarriers and use of the estimated CFO to correct the carrier frequency for the next iteration. It is found that with the proposed scheme, the final estimation accuracy depends only on the signal to noise ratio and is insensitive to the initial frequency offset. Simulation results demonstrate that the algorithm can achieve relatively high estimation accuracy within a few iterations in frequency-selective fading channels.

On use of traditional M/G/1 model for IEEE 802.11 DCF in unsaturated traffic conditions

For analysis of the IEEE 802.11 distributed coordination function (DCF), one of the key assumptions commonly accepted is that every node always has at least one frame to transmit. In practice, however, this assumption may or may not be valid, particularly under unsaturated traffic conditions. In this paper, we assess the accuracy in using the traditional M/G/1 queueing model for the DCF as an alternative model at an individual node, and analyze its performance in unsaturated traffic conditions. Analytical and simulation results are compared to demonstrate a reasonable accuracy of the proposed approach for varying traffic loads at an individual node

Channel estimation with power-controlled pilot symbols and decision-directed reference symbols [OFDM systems]

In this paper, a new channel estimation technique is presented that can improve the performance of channel estimation with fewer pilot symbols transmitted than would be normally required in the conventional techniques. In the proposed technique, temporary estimates of CSI are initially obtained based on the transmitted pilot symbols and used to create additional decision-directed reference symbols (referred to as virtual pilots) with the least-square estimation technique. The proposed technique is differentiated from existing decision-directed channel estimation techniques in that the blind decision process for data symbols is incorporated to increase the total number of pilot symbols, instead of directly using the decision-directed CSI estimates. In order to enhance the quality of the demodulated data symbols that may be selected as virtual pilot symbols, transmit power control can be employed on the transmitted pilot symbols. From computer simulation, it is observed that the proposed method outperforms the conventional method and achieves about 3 dB gain for QPSK and more than 5 dB for 16-QAM at an SER of 10/sup -2/ in the channel model considered.

A low-complexity channel estimator for OFDM systems in multipath fading channels

The minimum mean-square error (MMSE) criterion is often used for channel estimation in wireless OFDM systems. The MMSE channel estimators perform better than the least-square (LS) criterion-based techniques but their computational complexity is one of the main drawbacks for practical implementation, especially when the number of subcarriers is large. To address this issue while maintaining the performance advantage, we present a new simplified MMSE channel estimator for OFDM systems in multipath fading channels. The proposed method is primarily based on the rank-reduction of the channel correlation matrix in the frequency domain. Simulation results show that the proposed method achieves almost the same performance as the full-rank MMSE method, while significantly reducing the computational complexity.

Carrier frequency offset estimation for OFDM systems with null subcarriers

An accurate estimation of carrier frequency offset (CFO) is a crucial task for orthogonal frequency-division multiplexing (OFDM) systems but should be performed with affordable computational complexity for practicality. In this paper, the authors derive two new estimation algorithms (one for the integer part and the other for the fractional part of CFO), each of which utilizes only one OFDM block with null subcarriers but offers improved accuracy and reduced complexity. The proposed estimator for fractional CFOs operates iteratively and is insensitive to the initial CFO. The other proposed estimator for integer CFOs employs a pseudonoise binary random sequence to assist in subcarrier arrangement and accurate estimation of the integer CFO. To demonstrate the efficiency of the proposed methods, numerical results are provided from computer simulation and analysis, and comparisons are made with other existing methods in the literature

Channel estimation for high-speed packet-based OFDM communication systems

In orthogonal frequency division multiplexing (OFDM) based packet communication systems, it is desired to minimize the number of pilot symbols for better spectrum utilization. Also, accurate channel estimation is required for coherent demodulation of OFDM signals, particularly when modulated by multilevel modulation schemes. In this paper, we present a pilot-assisted decision-directed channel estimation technique suitable for packet transmission with relatively short OFDM symbols. Simulation has shown that our approach can save about 20% of the number of pilot symbols required in the conventional least square estimator for a situation considered in this paper.

A study on effect of interference in cooperative communication

The motivation behind cooperative communication has been to exploit the benefits of multiple-input multiple-output (MIMO) systems in an otherwise unlikely scenario. In a system where implementing multiple antennas is impractical, cooperative communication allows a form of transmit diversity to occur by cooperation among other single antenna users, thus creating a virtual antenna array. However, little work has been done that deals with the effect of interference on such systems. This paper explains how a transmit diversity scenario of unequal path gains for the same interferer occurs when participating nodes have the ability to cancel interference, thereby increasing system performance in terms of total link SER.

On channel estimation for OFDM systems in multipath environments with relatively large delay spread

The accuracy of channel state information (CSI) is critical for demodulation of multi-level modulated OFDM signals in multipath environments where the received signal experiences fading both in amplitude and phase. CSI estimates are commonly obtained from a priori known signal either appended as the training symbols in the beginning of a stream of OFDM symbols or inserted as the pilot tones in certain subcarriers of an individual OFDM symbol. In both cases, conventional channel estimation techniques process signal samples from the useful part of OFDM symbols. However, in excessively time-dispersive multipath channels where the receiver might be synchronized to a path in the middle of the overall channel impulse response, conventional channel estimation techniques may not capture some of the path components or its accuracy is degraded due to ISI. In this paper, we propose an improved channel estimation technique based on the use of a white preamble and the cyclic time-shifting property of the Fourier transform. Our computer simulation results show that the proposed scheme using a white preamble can improve the BER performance by more than 4 dB in a scenario considered in this paper.

Performance of closed-loop power control for a multiple-channel mobile station in the cdma2000 system

We investigate the reverse link power control of the cdma2000 system where multiple traffic channel signals of different data rates are code-multiplexed and transmitted in a multipath Rayleigh fading environment. In particular, we evaluate the performance of a closed-loop power control scheme utilizing one power control command for all traffic channels. Simulation results show that the use of one common power control command may not be optimal for all code-multiplexed channel signals of different data rates. This is largely due to the different variations in the effective bit signal-to-interference-plus-noise ratios (SINRs) for a given fading rate in a multipath fading channel. With an observation of the performance improvement by more than 1 dB, we propose to use independent power control for each channel signal.

Clean-and-Forward Approach in Cooperative Wireless Networks

This paper proposes a new cooperative strategy for cooperative communication among distributed nodes in a wireless communication network. In the proposed strategy referred to as the clean-and-forward, each node in a set cooperating with the source node cleans the received signal from the source, which may be corrupted by channel-induced and/or multiple-access interference as well as noise, and then forwards it to the destination. To justify the benefit of the proposed strategy, the overall link performance is illustrated in terms of symbol error rate and compared with that of a hybrid decode-and-forward cooperative technique whose performance is known to be comparable to or better than that of another strategy known as the amplify-and-forward