Kevin H. Lin

Space-Time Encoded Secure Chaos Communications with Transmit Beamforming

It is shown that the use of chaos shift keying (CSK) with a transmit diversity technique such as beamforming and space-time coding can provide a secure communication link with improvement in the system error performance. Spreading sequences that are used to scramble data in spread spectrum (SS) systems can be generated using a single mathematical relationship of a chaotic generator. These chaotic sequences are very difficult to predict over long-term chaotic pattern unless the exact initial condition of the chaotic generator is known, providing security. On the other hand, beamforming and orthogonal space-time block coding (OSTBC) transmit diversity techniques are known to provide optimal transmitting structures for communication systems, especially if combined. Based on the signal angle-of- arrival (AoA) estimates, the channel correlation matrix can be constructed, and it is shown that signal transmission of OSTBC codes in the eigen-modes of this matrix gives an effective array weighting gain which improves system error performance without sacrificing any diversity and coding gain. A performance study of CSK with beamforming and space-time encoded is carried out in this paper.

Space-time OFDM with adaptive beamforming: performance in spatially correlated channels

Space-time block coding (STBC) has proved to be an effective means of exploring diversity branches and improving system performance. In a previous work we proposed an extension to that where the STBC was combined with adaptive eigenbeamforming, an approach that takes an advantage of spatial channel correlation and can provide further improvement for the overall system performance. In this paper, we investigate this new transmission structure for broadband orthogonal frequency division multiplexing (OFDM) systems in frequency-selective channels. Numerical analysis showed that systems employing this scheme would not undergo any diversity losses as it maintains the maximum achievable diversity advantage of space-time (ST) codes in realistic channel conditions. Simulation results showed that the proposed structure has a significant improvement in bit/symbol error rate performance in a spatially correlated channel over systems that utilize only space-time coding.

Space-Time OFDM with Adaptive Beamforming for Wireless Multimedia Applications

It is well known that the transmission of multimedia data over wireless channels poses significant constraints on the communication system bandwidth, energy, and latency. To overcome these bottlenecks to wireless multimedia communication, various adaptive compression, modulation, and channel coding schemes have been proposed. Space-time block coding (STBC) with adaptive beamforming represents an effective means of improving the performance and capacity of wireless multimedia systems utilizing OFDM. In this paper, we introduce a multimedia simulation framework employing STBC-OFDM for the investigation of system characteristics in the application of multimedia data such as, digital audio/images/video. Simulation results, based on the transmission of compressed images, showed that the performance improvement introduced by STBC-OFDM with adaptive beamforming could readily be observed even without extensive numerical analysis as traditionally expected

Transmission of Compressed Multimedia Data over Wireless Channels using Space-time OFDM with Adaptive Beamforming

The transmission of multimedia data over wireless channels poses significant constraints on the communication system bandwidth, energy, and latency. To overcome these bottlenecks to wireless multimedia communication, various channel coding and transmit diversity schemes have been proposed. In previous work, we have shown that space-time block-coding (STBC) with adaptive beamforming (STBC-OFDM-AB) is an effective technique for improving the error-rate performance and channel capacity of wireless multimedia systems utilizing OFDM. In this paper, we introduce a transmission system for multimedia communication employing STBC-OFDM with adaptive beamforming incorporating a perceptually-based image compression coder - which consists of a 2-D discrete wavelet transform (DWT), an adaptive quantizer (with thresholding) and variable-length entropy encoding. Initial simulation results based on the transmission of compressed images, showed that the performance improvement introduced by STBC-OFDM-AB can be readily observed, and compared to other transmission methods is better suited to wireless multimedia communication.

Adaptive transmit eigenbeamforming with orthogonal space-time block coding in correlated space-time channels

Conventional space-time codes can provide a significant improvement in system performance only if the signal paths are spatially uncorrelated, a condition that is hardly met in practice. In this paper, we mitigate this condition by combining a technique of eigenbeamforming, based on the channel correlation matrix, with orthogonal space-time block codes (O-STBC) at the transmitter side of the link. No feedback information from the receiver (the mobile station) is utilized in the proposed structure. Simulation results using 4-ary PSK signaling showed that this idea outperforms existing techniques in both uncorrelated and correlated channels in terms of bit-error rate and symbol-error rate.

Space-time block coded OFDM with adaptive modulation and transmitter beamforming

We propose an optimum transmitting structure consisting of adaptive modulation, space-time block coding (STBC), and transmitter beamforming for orthogonal frequency division multiplexing (OFDM) systems to improve the data throughput and error performance. Using correlative properties of multi-input multi-output (MIMO) channels, we maximize the channel-to-noise ratio (CNR) based on a water-filling principle to optimally select the number of antenna beams and the power splitting ratio. The adaptive modulator employed is based on a lookup matrix adaptive bit and power allocation (LM-ABPA) scheme. As compared to non-adaptive systems, simulation results demonstrated that the proposed configuration achieves spectrally more efficient transmission under a constant error rate as well as better error rate performance under a constant data rate

Multimedia Transmission over Wireless Space-Time-Frequency coded OFDM Systems with Adaptive Beamforming

The transmission of multimedia data over wireless systems generally require network devices designed with a high communication bandwidth, power, and processing resources. To deal with these bottlenecks commonly associated with multimedia transmission, various diversity coding schemes have been proposed. We have previously shown that wireless OFDM systems based on space-time block-coding with adaptive beamforming (STBC-OFDM-AB) are well suited to multimedia communication. Recent studies have shown further performance gains in systems utilizing space-time-frequency (STF) coding. In this paper, we introduce a transmission system which combines STF coding with adaptive beamforming (STF-OFDM-AB). Simulation results based on the transmission of compressed images showed that the performance improvements introduced by STF-OFDM-AB can be readily observed

Adaptive Modulation with Space-Time Block Coding for MIMO-OFDM Systems

Space-time block coding (STBC) scheme has proved to be efficient in enhancing the performance of communication systems in terms of both bit-error-rate (BER) and symbol-error-rate (SER). Adaptive bit and power allocation (ABPA), on the other hand, is an adaptive modulation scheme that assigns an optimum symbol constellation and power to all subcarriers in a multi-carrier system to improve spectral and power efficiency. In this work, we combine a lookup matrix based ABPA algorithm with subchannel grouping (LM-SG-ABPA) and STBC for orthogonal frequency division multiplexing (OFDM) systems to further enhance the performance and support high data rate applications. Numerical results showed that the proposed transmission structure achieves not only a significantly higher spectral and power efficiency over systems that do not employ any ABPA schemes, but also a better error probability performance in a multiple-input multiple-output (MIMO) channel

Performance of STF Coded OFDM with Transmit Eigenbeamforming in Correlated Fading Channels

A new transmit diversity configuration that combines transmit eigenbeamforming with space-time-frequency (STF) coding for orthogonal frequency division multiplex (OFDM) systems is investigated. Based on the signal angle-of-arrival (AoA) estimates, channel correlation matrix is constructed. It is shown that signal transmission of STF codes in the eigen-modes of this matrix gives an effective array weighting gain which improves system error performance without sacrificing any diversity and coding gain. For imperfect AoA estimation, error probability loss due to incorrect estimation is quantified and the required accuracy of AoA estimation for securing the performance gain is shown for different angular spreads

An Unequal Modulation Scheme for the Transmission of Compressed Multimedia Data over Adaptive MIMO-OFDM Systems

Modern video/image source coders employ data compression techniques which encode information that are not equally important. Transform based or subband coders, compress data into their respective low-frequency and high-frequency components. In wireless/mobile communication systems, data representing the low-frequency components are more sensitive to the time-varying nature of channel conditions and propagation environments. To deal with this problem, we propose an optimum transceiver structure for a combined source-modulation coded MIMO-OFDM system with adaptive eigen-beamforming. Using an unequal adaptive modulator, we maximize the channel-to-noise ratio (CNR) based on a lookup matrix-adaptive bit and power allocation (LM-ABPA) scheme to sort and allocate subcarriers with the highest SNR to the low-frequency components of the compressed data, and adjusting the signal constellation/modulation type respectively. In comparison to other transmission systems, simulation results based on the application of compressed images showed that the proposed unequal adaptive modulation scheme achieves significant performance gains under a constant data rate load