Qi Ling

Physical layer built-in security analysis and enhancement of CDMA systems

Historically developed for secure communication and military use, CDMA is now serving as one of the most widely used wireless airlink interface and has been identified as a major technique for 3G wireless communications. In addition to the wide bandwidth and low power spectrum density which make CDMA signals robust to narrow band jamming and easy to be concealed within the noise floor, the physical layer built-in information privacy of CDMA system is provided by pseudo-random scrambling. In this paper, first, the physical layer security weakness of the operational IS-95 CDMA airlink interface is analyzed. Secondly, based on the advanced encryption standard (AES), we propose to enhance the physical layer built-in security of CDMA systems through secure scrambling. Performance analysis demonstrates that while providing significantly improved information privacy, CDMA system with secure scrambling has comparable computational complexity and system performance with that of the IS-95 system. Moreover, it is shown that by scrambling the training sequence and the message sequence separately with two independent scrambling sequences, both information privacy and system performance can be further improved. The proposed scheme can readily be applied to 3G systems and IEEE 802.11b WLAN systems

Physical layer built-in security analysis and enhancement algorithms for CDMA systems

Historically developed for secure communication and military use, CDMA has been identified as a major modulation and multiple-access technique for 3G systems and beyond. In addition to the wide bandwidth and low power-spectrum density which make CDMA signals robust to narrowband jamming and easy to be concealed within the noise floor, the physical layer built-in information privacy of CDMA system is provided by pseudorandom scrambling. In this paper, first, security weakness of the operational and proposed CDMA airlink interfaces is analyzed. Second, based on the advanced encryption standard (AES), we propose to enhance the physical layer built-in security of CDMA systems through secure scrambling. Performance analysis demonstrates that while providing significantly improved information privacy, CDMA systems with secure scrambling have comparable computational complexity and overall system performance with that of conventionally scrambled systems. Moreover, it is shown that by scrambling the training sequence and the message sequence separately with two independent scrambling sequences, both information privacy and system performance can be further improved. The proposed scheme can readily be applied to 3G systems and beyond.

A Spectrally Efficient Frequency Hopping System

Frequency hopping systems have been widely used in military communications to prevent hostile jamming, interception and detection. In traditional frequency hopping (FH) systems, the transmitter hops in a pseudo-random manner among available frequencies according to a pre-specified algorithm, and the receiver operates accordingly in exact synchronization with the transmitters hopping pattern. In multiple access systems, a collision may happen when more than one users transmit in the same frequency band simultaneously. Two major limitations with the conventional frequency hopping systems are: strict requirement on frequency acquisition/synchronization, and very low spectral efficiency due to inefficient utilization of the available bandwidth. In this paper, we introduce a new concept - collision-free frequency hopping (CFFH). Based on the OFDM framework and the secure subcarrier assignment algorithm, the proposed CFFH system can achieve high information capacity through collision-free multiple access, and can successfully resolve the strict synchronization limitation. At the same time, as each user still transmits through a pseudo-random frequency hopping scheme, CFFH can maintain the inherent anti-jamming, anti-interception security features of the conventional FH system.

A Spectrally Efficient Frequency Hopping Scheme

Frequency hopping systems have been widely used in military communications to prevent hostile jamming, interception and detection. In traditional frequency hopping (FH) systems, hopping frequency selection at the transmitter end is controlled by a pseudo-random code sequence, and the receiver operates accordingly in exact synchronization with the transmitters hopping pattern. In an effort to meet the ever increasing requirement on information capacity and reduce the burden of synchronization, in this paper, an innovative message-driven frequency hopping (MDFH) system is proposed. By embedding part of information into the process of hopping frequency selection, the spectral efficiency of the FH system can be significantly improved. Quantitative analysis on the proposed scheme is presented to demonstrate its superior performance and enhanced security features.

Spectrally Efficient Spread Spectrum System Design: Message-Driven Frequency Hopping

Originally developed for secure communications in military applications, frequency hopping systems possess anti-jamming and anti-interception features by exploiting time- frequency diversity over large spectrum. However, the spectral efficiency of existing FH systems is very low due to inappropriate use of the total available bandwidth. To improve the system capacity, in this paper, we propose an innovative message-driven frequency hopping (MDFH) scheme. Unlike in traditional FH systems where the hopping pattern of each user is determined by a pre-assigned pseudo-random (PN) sequence, in MDFH, part of the message stream will be acting as the PN sequence for hopping frequency selection. Essentially, transmission of information through hopping frequency control introduces another dimension to the signal space, and the corresponding coding gain increases system efficiency by multiple times. The MDFH scheme can be further enhanced by allowing simultaneous transmissions over multiple frequency bands. Including both MDFH and OFDM as special cases, the enhanced MDFH scheme, named E-MDFH, can achieve high spectral efficiency while providing excellent design flexibility. E-MDFH can readily be extended to a FH-based collision-free multiple access scheme.

A Novel Concept: Message Driven Frequency Hopping (MDFH)

Frequency hopping systems have been widely used in military communications to prevent hostile jamming, interception and detection. In traditional frequency hopping (FH) systems, hopping frequency selection at the transmitter end is controlled by a pseudo-random code sequence, and the receiver operates accordingly in exact synchronization with the transmitters hopping pattern. In an effort to meet the ever increasing requirement on information capacity and reduce the burden of synchronization, in this paper, an innovative message-driven frequency hopping (MDFH) system is proposed. By embedding part of information into the process of hopping frequency selection, the spectral efficiency of the FH system can be significantly improved. Quantitative analysis on the proposed scheme is presented to demonstrate its superior performance and enhanced security features.

Blind identification of the central aortic pressure waveform from multiple peripheral arterial pressure waveforms.

We introduce a blind identification technique to reconstruct the clinically more relevant central aortic pressure waveform from multiple less invasively measured peripheral arterial pressure waveforms. We conducted initial testing of the technique in two swine in which peripheral arterial pressure waveforms from the femoral and radial arteries and reference central aortic pressure were simultaneously measured during diverse hemodynamic conditions. We report an overall error between the estimated and measured central aortic pressure waveforms of 4.8%. Potential clinical applications of the technique may include critical care monitoring with respect to invasive catheter systems and emergency and home monitoring with respect to non-invasive arterial pressure transducers

Spectrally Efficient Frequency Hopping System Design forWireless Networks

Frequency hopping systems have been widely used in military communications to prevent hostile jamming, interception and detection. In traditional frequency hopping (FH) systems, the transmitter hops in a pseudo-random manner among available frequencies according to a pre- specified algorithm, and the receiver operates accordingly in exact synchronization with the transmitters hopping pattern. In multiple access systems, a collision may happen when more than one users transmit in the same frequency band simultaneously. Two major limitations with the conventional frequency hopping systems are: strict requirement on frequency acquisition/synchronization, and very low spectral efficiency due to inefficient utilization of the available bandwidth. In this paper, we introduce a new concept collision-free frequency hopping (CFFH). Based on the OFDM framework and the secure subcarrier assignment algorithm, the proposed CFFH system can achieve high information capacity through collision-free multiple access, and can successfully resolve the strict synchronization limitation. At the same time, as each user still transmits through a pseudo-random frequency hopping scheme, CFFH can maintain the inherent anti-jamming, anti-interception security features of the conventional FH system. The proposed CFFH scheme can be used for both civilian and military applications where secure high speed information transmission is needed.

Blind-Channel Estimation for MIMO Systems With Structured Transmit Delay Scheme

This paper considers blind-channel estimation for multiple-input multiple-output (MIMO) systems with structured transmitter design. First, a structured transmit delay (STD) scheme is proposed for MIMO systems. Unlike existing transmit diversity approaches, in which different antennas transmit delayed, zero padded, or time-reversed versions of the same signal, in the proposed scheme, each antenna transmits an independent data stream, therefore promises higher data rate and more flexibility to transmitter design. Second, second-order statistics based blind-channel estimation algorithms are developed for MIMO systems with STD scheme. Channel identifiability is addressed for both correlation-based and subspace-based approaches. The proposed approaches involve no pre-equalization, have no limitations on channel zero locations, and do not rely on nonconstant modulus precoding. Third, when channel coding is employed, estimation accuracy can be further enhanced through ldquopostprocessingrdquo, in which channel estimation is refined by taking the tentative decisions from the channel decoder as pseudo-training symbols. Simulation examples are provided to demonstrate the robustness and effectiveness of the proposed approaches.

Efficiency Improvement for Alamouti Codes

In this paper, an approach to improve the code efficiency of Alamouti codes is proposed for high data rate communications over multiple-antenna channels. Unlike most of the existing methods which are designed to achieve full-rate and/or full-diversity, we aim at increasing the spectral efficiency. The proposed scheme carries more information bits in each transmission block than the Alamouti code does and still allows maximum likelihood decoding for sub-streams to be decoupled at the receiver. Simulation results also indicate that the full transmit diversity is retained with the improvement of code efficiency. Although the case of two transmit antennas is the main focus of this paper, the same idea can be directly applied to Alamouti codes with more than two transmit antennas.