Lu Chao

Channel estimation for indoor diffuse optical OFDM wireless communications

Pilot insertion for channel estimation in orthogonal frequency division multiplexing (OFDM) is a promising technique for broadband/high data rate wireless communications. In indoor diffuse optical wireless links, channel impulse response depends on the location of the receiver. For links to offer mobility tracking is essential which can be complex to implement. In this paper we propose channel estimation to allow receiver mobility. We have examined channel estimation with different interpolation methods for OFDM system and have compared the symbol error rates (SER) performance.

Digitally generated pulse width modulation transmitted over optical fibre

Analogue generated pulse width modulation (APWM) suffers from a non-linearity problem associated with the reference signal when operated at high modulation indices. This problem can be solved by employing digital techniques to generate the PWM waveform. This paper reports preliminary practical results based on a digital PWM system.

Squarewave frequency modulation techniques

Transmission of high-quality analogue sourced signals over copper or optical fibre links and networks demands a critical choice of modulation format to achieve both high performance and to avoid unnecessary bandwidth overhead penalties. Pulse time modulation (PTM) techniques are being increasingly employed to overcome many of the problems associated with traditional analogue and digital methods. Among the family of pulse time modulation techniques squarewave frequency modulation (SWFM) is particularly well suited to the transmission of high-quality wideband instrumentation and video signals, such as high definition TV, due to its attractive combination of performance and simplicity. This paper presents theoretical and practical aspects of SWFM and describes a number of experimental SWFM systems capable of transmitting signals from DC to 30 MHz. >

High-speed pulse time modulation techniques

A principal factor in realizing a high-performance bandwidth-efficient fiber communication system at an acceptable cost is the choice of modulation format on the optical carrier. In this context pulse time modulation (PTM) techniques represent an attractive alternative to purely digital or analogue methods. This paper reviews the PTM family and explores their potential for use in high-speed fiber systems intended for transmission of analogue data.

Squarewave FM optical fibre transmission for high-definition television signals

Transmission of high definition TV signals over optical fibres will require a careful choice of modulation technique in order to avoid unnecessary bandwidth overhead penalties . This paper reports preliminary results from the use of squarewave frequency modulation in two different optical fibre systems intended for operation with such signals.

Optical fibre transmission of a broadband subcarrier multiplexed signal using PTM techniques

Subcarrier multiplexing (SCM) is an attractive multiplexing technique to deliver broadband services over optical fibres. The majority of existing systems employ an analogue signal for optical transmission and therefore the performance of these systems are limited. This paper proposes to improve SCM system performance by converting the analogue SCM signal into a pulse time modulated signal for optical transmission. The pulse time modulation techniques investigated are pulse frequency modulation and square wave frequency modulation. System measurements and theoretical predictions were obtained and they are in close agreement. We have improved the dynamic range and optical power budget considerably, compared to conventional systems.

Pulse time modulation techniques for low cost analog signal transmission systems

Pulse time modulation techniques (PTM) have been proven to be attractive techniques for achieving low cost analog optical transmission systems. To facilitate future design of such transmission systems, a review of these modulation techniques is made. Spectrum expressions of PPM, PWM, PFM, and SWFM are derived and given. The results are used to predict the requirement for minimum carrier frequency and transmission bandwidth as well as the signal to noise ratio performances of optical transmission systems employing such techniques.

Optical FM modulation for coherent and direct detection analog transmission systems

This paper describes the distortion characteristics of optical FM modulated direct and coherent detection analog transmission systems. Numerical solution of the single frequency laser diode rate equations was obtained to analyse the intermodulation distortion introduced by the nonlinear FM modulation process of the laser diode. The results indicate that relatively good distortion performance can be obtained for both coherent and direct detection systems.

Application of subcarrier multiplexing technique in optical FM direct-detection systems

Applying a subcarrier multiplexing technique (SCM) in an optical frequency division multiplexing system (OFDM) will improve utilization of the optical channel, at the same time increasing the flexibility in designing an optical network. In this paper, system examples based on the combination of these two techniques ar discussed. Particular attention is given to the application of SCM in OFDM systems which employs FM direct detection techniques. Component characteristics which affect system performance are discussed and results presented.

Pulse-time modulation for subcarrier-multiplexed systems

Subcarrier multiplexing (SCM) is an attractive technique to deliver broadband services over optical fibers. The existing SCM systems employ an analogous signal for optical transmission and their performance is limited. Pulse time modulation (PTM) is an intermediate technique which offers high performance at a low cost in optical transmission systems. In this paper we present two popular PTM techniques suitable for SCM systems. Theoretical predictions and practical measurements have been used to evaluate system performance. The technique employed has improved the dynamic range and the optical power budget, compared to conventional systems.