We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured nondirected indoor infrared channels with intersymbol interference. PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However, the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD.

Decision-feedback equalization of pulse-position modulation on measured nondirected indoor infrared channels

Because of the optimality of the pulse position modulation (PPM) technique for the infinite-bandwidth channel, it has been suggested as a technique for the bandwidth-limited channel. The use of differential pulse position modulation (dPPM) as an alternative coding scheme to PPM is proposed. In dPPM each new word begins at the end of the previous word. To avoid synchronization problems, a simple scheme that decreases the throughput of the dPPM channel but is easy to analyze is chosen. For some parameter values this restricted dPPM scheme has greater throughput (in terms of capacity and cutoff rate) than PPM. >

Differential PPM has a higher throughput than PPM for the band-limited and average-power-limited optical channel

In this paper, we present the first indoor light-based detection and localization system that builds on concepts from radio detection and ranging (radar) making use of the expected growth in the use and adoption of visible light communication (VLC), which can provide the infrastructure for our LiDAL system. Our system enables active detection, counting and localization of people, in addition to being fully compatible with existing VLC systems. In order to detect human (targets), LiDAL uses the visible light spectrum, it sends pulses using a VLC transmitter and analyses the reflected signal collected by a photodetector receiver. Although we examine the use of the visible spectrum here, LiDAL can be used in the infrared spectrum and other parts of the light spectrum. We introduce LiDAL with different transmitter-receiver configurations and optimum detectors considering the fluctuation of the received reflected signal from the target in the presence of Gaussian noise. We design an efficient multiple input multiple output (MIMO) LiDAL system with wide field of view (FOV) single photodetector receiver, and also design a multiple input single output (MISO) LiDAL system with an imaging receiver to eliminate the ambiguity in target detection and localization. We develop models for the human body and its reflections and consider the impact of the colour and texture of the cloth used as well as the impact of target mobility. A number of detection and localization methods are developed for our LiDAL system including cross correaltion, a background subtraction method and a neural network based method. These methods are considered to distinguish a mobile target from the ambient reflections due to background obstacles (furniture) in a realistic indoor environment.

LiDAL: Light Detection and Localization

We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured non-directed indoor infrared channels with intersymbol interference (ISI). PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Detailed performance results for 2-, 4-, 8-, and 16-PPM links at bit rates of 10 Mb/s and 30 Mb/s over 46 actual measured indoor infrared channels are presented for these two DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD.

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Decision-feedback equalization of pulse-position modulation on measured non-directed indoor infrared channels

Digital Pulse Position Modulation (digital PPM) is a modulation format that codes n bits of PCM into a single pulse that occupies one of 2n time slots. Various studies over the last three decades have shown that such a scheme can offer an improvement in receiver sensitivity of 5-11 dB when compared to Pulse Code Modulation (PCM). Such
an increase in sensitivity can be exploited beneficially in both long haul applications and
the multi-user environment. However, this improvement results in a considerable increase
in the final data rate of the original PCM, and this makes implementation difficult.


Alternative methods have been proposed, such as multiple PPM, dicode PPM, differential PPM and overlapping PPM, that reduce transmission bandwidth while maintaining an increased sensitivity. Dicode and multiple PPM (MPPM) are the most bandwidth efficient of these formats and MPPM, the subject of this thesis, offers the best
sensitivity without the large bandwidth increase. In this scheme, multiple pulses per frame are used, with the pulse positions being determined by the original PCM word.


The main concern of this thesis is a full and detailed investigation of an Optical MPPM link operating over a dispersive optical channel. As the analysis of any (X/Y) multiple PPM system, in which X denotes the number of data slots and Y the number of pulses, is extremely time-consuming, a novel automated solution was designed to predict the equivalent PCM error rates of specific sequences and simplify the task. An original mathematical formulation is developed using the Maximum Likelihood Sequence
Detection (MLSD) scheme. Using the equivalent PCM error rates of specific sequences generated from software, a full simulation of an MPPM optical link can be produced and the results show the effectiveness of the MPPM format over PPM. A measure of coding quality is proposed that accounts for efficiency of coding and bandwidth expansion. Original results are presented for a (12/Y) MPPM system, considered as very efficient and examined by many authors, showing that the most efficient systems are in the middle of
the family. A methodology to predict the Bit Error Rate (BER) of any MPPM system is also proposed. The results obtained confirmed the results obtained from the full mathematical analysis.


The effects of linear increment, linear decrement, Gray code and random mapping of data on ther performance of a (12/Y) multiple PPM system are also examined. Simulations show that the Gray code is the most effective as it minimizes the Hamming distance between adjacent multiple PPM words.


Further experiments showed that system performance can be obtained exclusively with the use of software making the analysis simpler and minimizing the time consumption. A novel algorithm is presented and results obtained using this method, agree with those obtained using a full mathematical model.


Certain mappings can either enhance or degrade the final total error probability of the system and hence affect the sensitivity of the MLSD scheme. In this thesis the author also suggests a methodology of how to predict and generate an optimum or close to optimum mapping. The methodology is based on minimizing the occurrence of dominant error sequences. Detailed results show the effectiveness of this mapping routine.


For the first time also, high order MPPM codes are considered for analysis. All the experiments completed for the (12/Y) MPPM system are repeated for a range of MPPM systems (with 4, 7, 15, 17, 22, 28 and 33 slots operating over a plastic optical fibre (POF) channel showing again that the most efficient systems are in the middle of
the family. Close to optimum mappings are also presented for these MPPM systems. The estimated mappings were found to be far superior to the efficient Gray codes, linear coding and a series of random mappings. To measure these optimum mappings a very efficient mapping is also considered. This mapping minimises the Hamming distance between all MPPM codewords. This mapping allows repetitions of MPPM codewords and cannot be used in a MLSD scheme. Therefore, it is only used for comparisons with the (close to) optimum mappings. It is shown that the optimum mappings are close to ideal.


Other correlation techniques are also considered for optimised detection in the MLSD scheme. Results obtained showed that raised cosine filtering can enhance detection.

An investigation of an optical multiple pulse position modulation link over a dispersive optical channel

The design and performance of optical PPM communication systems with ideal (rectangular) pulses have been well documented. However, many optical channels (fibers, atmosphere, clouds) are extremely dispersive to narrow pulses, producing receiver pulses that are spread (stretched) in time. This stretching produces interslot interference within a PPM frame, and if severe enough, can stretch over several frames (intersymbol interference). In this paper, laser pulse stretching in optical PPM formats is investigated in terms of performance degradation and decoder design alternatives. Several methods are considered for combating the pulse stretching, including pulse equalization, extended pulse integration, and pulse shape matching. Performance of these methods is compared for the case of exponential stretching and Gaussian statistics.

G. Prati

Papers

Decoding with Stretched Pulses in Laser PPM Communications