[Paper Review] Co-channel Interference Cancellation for Space-Time Coded OFDM Systems Using Adaptive Beamforming and Null Deepening

Raungrong Suleesathira|arXiv (Cornell University)|Feb 17, 2010

Advanced Wireless Communication Techniques9 references3 citations

TL;DR

This paper proposes an adaptive beamforming and null deepening technique to cancel co-channel interference (CCI) in space-time coded OFDM (STC-OFDM) systems. Using a least mean square error (LMMSE)-based adaptive beamformer without requiring prior knowledge of direction-of-arrival (DOA), the method enhances interference suppression while preserving the space-time decoder structure, achieving significant performance gains over conventional null steering beamformers in frequency-selective fading channels.

ABSTRACT

Combined with space-time coding, the orthogonal frequency division multiplexing (OFDM) system explores space diversity. It is a potential scheme to offer spectral efficiency and robust high data rate transmissions over frequency-selective fading channel. However, space-time coding impairs the system ability to suppress interferences as the signals transmitted from two transmit antennas are superposed and interfered at the receiver antennas. In this paper, we developed an adaptive beamforming based on least mean squared error algorithm and null deepening to combat co-channel interference (CCI) for the space-time coded OFDM (STC-OFDM) system. To illustrate the performance of the presented approach, it is compared to the null steering beamformer which requires a prior knowledge of directions of arrival (DOAs). The structure of space-time decoders are preserved although there is the use of beamformers before decoding. By incorporating the proposed beamformer as a CCI canceller in the STC-OFDM systems, the performance improvement is achieved as shown in the simulation results.

Motivation & Objective

• To address co-channel interference (CCI) in space-time coded OFDM (STC-OFDM) systems, which degrades performance due to signal superposition at receivers.
• To develop an interference cancellation method that does not require prior knowledge of direction-of-arrival (DOA) of interferers.
• To maintain compatibility with existing space-time decoder structures while enhancing interference suppression.
• To improve spectral efficiency and bit error rate (BER) performance in frequency-selective fading channels.

Proposed method

• An adaptive beamforming algorithm based on the least mean square error (LMMSE) criterion is employed to estimate interference suppression weights.
• Null deepening is applied to enhance the depth of nulls in the beamforming response, improving rejection of co-channel interferers.
• The beamformer is placed before the space-time decoder, preserving the original decoder architecture.
• The system operates without requiring direction-of-arrival (DOA) information, enabling blind interference cancellation.
• The beamformer adapts in real time to changing interference environments using error feedback from the receiver.
• The method is evaluated in a frequency-selective fading channel with multiple co-channel interferers.

Experimental results

Research questions

• RQ1 Can co-channel interference in STC-OFDM systems be effectively canceled without prior knowledge of interferer directions?
• RQ2 How does adaptive beamforming with null deepening compare to conventional null steering beamformers in terms of BER performance?
• RQ3 To what extent does the proposed method preserve the structure and functionality of existing space-time decoders?
• RQ4 What is the impact of null deepening on interference null depth and system robustness in fading channels?

Key findings

• The proposed adaptive beamforming with null deepening achieves superior bit error rate (BER) performance compared to the null steering beamformer, especially in low SNR regimes.
• The method effectively suppresses co-channel interference without requiring prior knowledge of direction-of-arrival (DOA), enabling blind operation.
• Null deepening significantly enhances the depth of interference nulls, improving interference rejection capability.
• The space-time decoder structure remains unaltered, ensuring compatibility with existing STC-OFDM systems.
• Simulation results confirm that the proposed technique improves spectral efficiency and system reliability in frequency-selective fading environments.