Qingchong Liu

Physics-based pulse distortion for ultra-wideband signals

Pulse distortion is difficult to model by the state of the art statistical measurements. In this paper, we propose a physics-based deterministic model for urban environments consisting of high rise buildings. We first derive the channel impulse response of the renowned Wolfisch-Bertoni model in closed form. This model captures many properties that are not available in the existing statistical models such as the IEEE 802.15.4a model. The closed form time domain solutions are in good agreement with the inverse fast Fourier transform (IFFT)-based frequency domain solutions. These solutions are important to understanding the system principle.

Throughput analysis of CDMA systems using multiuser receivers

Throughput bounds are attained for random channel access multichannel code-division multiple-access (CDMA) systems and spread slotted Aloha systems employing multiuser receivers. It is shown that the normalized throughput of these two systems reaches 1.0 exponentially fast in the region r/K 0. The maximum throughput of the random channel access multichannel CDMA systems is found as K-/spl radic/(1-(1/M))KlogK-O(logK), where M is the number of channels in the system. The maximum throughput is reached when the average number of simultaneous users is r/sub m/=K-/spl radic/((1-(1/M))KlogK))+O(/spl radic/(K/logK)). The maximum throughput of the spread slotted Aloha systems is K-/spl radic/(KlogK)-O(log K). The maximum throughput is reached when the packet arrival of Poisson distribution has the arrival rate /spl lambda//sub m/=K-/spl radic/(KlogK)+O(/spl radic/(K/logK)).

Performance analysis for optical wireless communication systems using subcarrier PSK intensity modulation through turbulent atmospheric channel

This paper compares the performance of on-off keying (OOK) modulation and subcarrier phase shift keying (PSK) modulation through an optical wireless channel. The bit error rate (BER) for a turbulent channel is derived for optical wireless communication systems employing OOK or subcarrier BPSK. A lower bound is derived for BER performance when fixed-threshold OOK is used. It is also shown that the BER performance of the optical wireless communication system, employing subcarrier PSK modulation, is more than 3 dB better than that of a compatible system employing fixed-threshold OOK modulation at BER=10/sup -6/ and with turbulence parameter /spl sigma/=0.1. When /spl sigma/ is 0.2 and fixed-threshold OOK is employed, the BER performance never falls down to 10/sup -4/. The BER performance degradation of optical wireless communication systems employing subcarrier BPSK between /spl sigma/=0.1 and /spl sigma/=0.2 is only 2.5 dB at BER=10/sup -6/.

PSK communications systems using fully saturated power amplifiers

Quasiconstant envelope phase-shift keying (PSK) is analyzed to assess its ability to overcome nonlinearities caused by fully saturated RF power amplifiers in communications systems. These modulations can achieve bit error rate (BER) performance comparable to linear BPSK in additive white Gaussian noise (AWGN) channel. Quasiconstant envelope offset quadrature PSK (OQPSK) is presented as a design example. At a BER = 10/sup -5/, the SNR degradation caused by fully saturated power amplifiers is 0.1 dB. The simulated BER matches analytically derived results. For a communications system employing the quasiconstant envelope OQPSK and a rate 1/2 convolutional code with K = 7, the demodulation performance is degraded by 0.25 dB at a BER = 10/sup -5/ when a fully saturated power amplifier is employed.

Complexity of Verdu optimum multiuser detection algorithm in multichannel CDMA systems

A statistical characterization of the complexity function of the Verdu optimum multiuser detection (VOMD) algorithm is presented for a communication system employing a finite number of randomly accessed orthogonal channels and a finite number of simultaneous users. Multichannel code-division multiple-access (CDMA) systems are proposed. It is proved that the probability, in which the individual channel complexity is greater than A/sup r(1+/spl alpha/)/, approaches zero exponentially fast as the average number of simultaneous users in each channel increases, where A is the modulation alphabet size and /spl alpha/>0. When the number of simultaneous users is large, the complexity of applying the VOMD algorithm to each individual channel is negligible when compared with the complexity of applying the same algorithm directly to the traditional single-channel CDMA system supporting the same number of simultaneous users. The probability distribution of the joint complexity function of the aggregate system is found. It is shown that when the number of simultaneous users is large, the joint complexity function is negligible compared with applying the VOMD algorithm directly to the traditional single-channel CDMA system supporting the same number of simultaneous users. Therefore, a multichannel CDMA communication system can support a comparable population of simultaneous users to the traditional single-channel CDMA system of comparable bandwidth, while reducing the complexity of optimum multiuser detection to a practical level.

Subcarrier PSK intensity modulation for optical wireless communications through turbulent atmospheric channel

This paper studies the coded performance of optical wireless communication systems employing subcarrier PSK modulation through atmospheric turbulence channel. Convolutional code and Viterbi decoding are used for optical wireless communication system with subcarrier BPSK and a comparable system with fixed-threshold OOK. It is shown that an interleaver can help to handle bursty errors and improve the coded BER performance.

User registration in broadband slotted Aloha networks

This paper proposes an algorithm for user terminals in broadband slotted Aloha networks to jointly succeed in time synchronization, power adjustment, and registration to the network control center. The performance is analyzed considering nonnegligible initial time uncertainty, nonnegligible power uncertainty, and minimizing cochannel interference as well as binary feedback from the central receiver. User terminal initial power setting can be performed using the method developed in this paper to make sure the interference generated in the registration stage is negligible. The algorithm is sufficient to keep the false registration probability less than any number required by the network. It can help broadband slotted Aloha networks to significantly reduce system cost through employing a large number of inexpensive user terminals with non-negligible time uncertainty and nonnegligible power uncertainty.

Doppler measurement and compensation in mobile satellite communications systems

The frequency errors dominated by the dynamic Doppler is analyzed for mobile satellite communication systems employing fast-moving satellites. Methods are proposed and analyzed for the basestations to measure and compensate the dynamic Doppler of large range. It is shown that the proposed methods give the minimum uncertainty of the dynamic Doppler for the modem to handle. Therefore, these methods are optimal for frequency synchronization and Doppler measurement in mobile satellite communications systems.

Quasi-constant envelope OQPSK through nonlinear radio and AWGN channel

A quasi-constant envelope OQPSK (QCEOQPSK) signal is formed using a linear OQPSK modulator to overcome the nonlinearity caused by fully saturated RF power amplifiers. The performance of the QCEOQPSK signal in a AWGN channel is studied for a variety of filters in the transmitter and matched linear filters followed by hard decision in the demodulator. It is shown that at -43 dB the mainlobe of the power spectral density of the QCEOQPSK is only 77% of the bandwidth corresponding to the mainlobe of the MSK power spectrum density. At a bit error rate of 10/sup -5/, the SNR degradation caused by fully saturated power amplifiers is 0.1 dB in the AWGN channel compared with the ideal BER of BPSK using linear power amplifier. This is 2.3 dB better than the bit error rate of QPSK using linear power amplifier in the broadband satellite communications systems. Employing rate 1/2 convolutional code with constraint length K = 6 and the Viterbi (1966) decoder, the SNR degradation is only 0.25 dB at BER = 10/sup -5/. Therefore, QCEOQPSK can be employed in broadband wireless or broadband satellite communication systems to achieve higher spectral efficiency, near optimal demodulation performance and significantly reduce terminal cost.

Binary Sequences with Optimal Correlations and Large Linear Span

A family of binary sequences is presented and proved to have optimal correlations and large linear span. It includes the small set of Kasami sequences, No sequence set and TN sequence set as special cases. A lower bound on the linear span of the family is provided. With suitable choices of parameters, it is proved that the family has exponentially larger linear spans than both No sequences and TN sequences.