Chun Sum Ng

On the SER analysis of A-law companded OFDM system

Orthogonal frequency division multiplexing (OFDM) has established itself as a very appealing technique for high-rate data transmission over dispersive channels. The main disadvantage of OFDM is that the signal exhibits Gaussian-like time-domain waveform with high peak-to-average power ratio, which limits the efficiency of the transmitter amplifier and degrades the received signal to noise ratio. The companding technique can be employed to mitigate quantization noise and the peak-to-average-power ratio of OFDM system. The symbol error rate of the companded OFDM systems is investigated. The performances of the systems with and without companding are compared.

Adaptive error coding using channel prediction

In this paper, we construct a finite-state Markov chain model for a Rayleigh fading channel by partitioning the range of the received signal envelope into K intervals. Using a simulation of the classic two-ray Rayleigh fading model, a Markov transition probability matrix is obtained. Using this matrix to predict the channel state, we introduce an adaptive forward error correction (FEC) coding scheme. Simulation results are presented to show that the adaptive FEC coding scheme significantly improves the performance of a wireless communication system.

End-to-end outage minimization in OFDM based linear relay networks

Multi-hop relaying is an economically efficient architecture for coverage extension and throughput enhancement in future wireless networks. OFDM, on the other hand, is a spectrally efficient physical layer modulation technique for broadband transmission. As a natural consequence of combining OFDM with multi-hop relaying, the allocation of per-hop subcarrier power and per-hop transmission time is crucial in optimizing the network performance. This paper is concerned with the end-to-end information outage in an OFDM based linear relay network. Our goal is to find an optimal power and time adaptation policy to minimize the outage probability under a long-term total power constraint. We solve the problem in two steps. First, for any given channel realization, we derive the minimum short-term power required to meet a target transmission rate. We show that it can be obtained through two nested bisection loops. To reduce computational complexity and signalling overhead, we also propose a sub-optimal algorithm. In the second step, we determine a power threshold to control the transmission on-off so that the long-term total power constraint is satisfied. Numerical examples are provided to illustrate the performance of the proposed power and time adaptation schemes with respect to other resource adaptation schemes.

Improved weighted least squares algorithm for the design of quadrature mirror filters

This paper presents an improved weighted least squares (WLS) algorithm for the design of quadrature mirror filters (QMFs), First, a new term is incorporated into the objective function that effectively prevents an optimization algorithm from producing suboptimal QMFs. These suboptimal QMFs exhibit a transition band anomaly; the frequency responses of the filters have large oscillatory components in the transition band. The new term can be applied to the WLS design of any FIR filter to prevent a similar transition band anomaly. Next, we present an algorithm to obtain the QMF coefficients that minimize the objective function incorporating the new term. The computational requirement of this algorithm is also briefly discussed. Last, we include a set of practical design rules for use with our algorithm. These rules simplify the design process by providing good estimation of the design parameters, such as the minimum filter length, to meet a given set of QMF specifications.

Closed-form error probability formula for narrow-band FSK, with limiter-discriminator-integrator detection, in Rayleigh fading

We present a new closed-form bit error probability formula for narrow-band FSK with limiter-discriminator-integrator detection in Rayleigh fading channels. This formula eliminates the tedious numerical integrations required in previous publications. The computed results are verified by experiment. We provide physical explanations, on how the system parameters such as fading, IF filter bandwidths and FM carrier frequency deviations affect the error performance, not previously available in the literature. >

On the error rates of differentially detected narrowband pi /4-DQPSK in Rayleigh fading and Gaussian noise

Closed-form formulas for the probability density function (PDF) and cumulative distribution function (CDF) of the differential phase of a phase-modulated carrier in Rayleigh-fading and correlated noise is derived and applied to evaluate the bit-error-rate (BER) of pi /4-DQPSK and conventional DQPSK (differential quaternary phase-shift keyed) systems. Under very narrow IF band-limitation and at high SNR, the pi /4-DQPSK outperforms the conventional DQPSK because the former has less spectral splatter during phase transitions between symbol. The performances of these two systems approach one another as the bandwidth parameter BT increases. >

DSTBC Impulse Radios with Autocorrelation Receiver in ISI-Free UWB Channels

For transmitted-reference impulse radio, it has been experimentally shown that multiple transmit antennas can provide an energy boost in the received signal. Here, we propose a 2-transmit and Q-receive differential space-time block coded impulse radio with autocorrelation receiver. A mathematical model for predicting the system bit-error-rate (BER) performance is derived for intersymbol interference-free transmission over an ultra-wideband channel, and validated with computer simulated results. From the results, it is observed that in migrating from 1-transmit to 2-transmit antennas, a signal-to-noise ratio gain of 3 dB, 2 dB and 1 dB at BER = 10-5 can be achieved, respectively, for the case of 1-, 2- and 4-receive antennas.

Improved signal constellations for differential unitary space-time modulations with more than two transmit antennas

In this paper, we propose new and improved unitary signal constellations for differential unitary space-time modulations utilizing more than two transmit antennas. The proposed unitary designs are constructed from fundamental building blocks which comprise the generator matrices of diagonal cyclic codes, and the 2/spl times/2 and 3/spl times/3 rotational matrices. The performances of the proposed codes are superior to those of previously proposed codes.

Further results on the bit error probabilities of MDPSK over the nonselective Rayleigh fading channel with diversity reception

This paper presents a fundamental approach for deriving the bit error probability of BDPSK and QDPSK over the nonselective Rayleigh fading channel for a receiver with an arbitrary IF filter, and for a fading process with an arbitrary Doppler spectrum with arbitrary Doppler bandwidth. The results generalize those published earlier which were restricted to matched filter reception and to a fading process with a small Doppler bandwidth compared to the symbol rate. This allows the error probability to be studied in the presence of varying degrees of ISI due to the bandlimitation of the received signal by the IF filter, and in the presence of fading fluctuations of various rates. The analytical approach presented is simple, and yet powerful in that it can handle the case of diversity reception. This is a great advantage over the alternative approach of using the distribution of the differential phase of the received signal over a symbol interval. The bit error probability results apply to both conventional BDPSK and QDPSK, as well as /spl pi//2-2DPSK and /spl pi//4-4DPSK, and allow the irreducible bit error probability as well as the SNR at which this irreducible value sets in to be studied as a function of the Doppler bandwidth and IF filter bandwidth. The computed results are applicable to the design of digital cellular mobile communication systems.

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d2TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.