João Tavares

Size distribution of polycyclic aromatic hydrocarbons in a roadway tunnel in Lisbon, Portugal

Atmospheric aerosols of four aerodynamic size ranges were collected using high volume cascade impactors in an extremely busy roadway tunnel in Lisbon (Portugal). Dust deposited on the tunnel walls and guardrails was also collected. Average particle mass concentrations in the tunnel atmosphere were more than 30times higher than in the outside urban background air, revealing its origins almost exclusively from fresh vehicle emissions. Most of the aerosol mass was concentrated in submicrometer fractions (65%), and polycyclic aromatic hydrocarbons (PAH) were even more concentrated in the finer particles with an average of 84% of total PAH present in sizes smaller than 0.49μm. The most abundant PAH were methylated phenanthrenes, fluoranthene and pyrene. About 46% of the total PAH mass was attributed to lower molecular weight compounds (two and three rings), suggesting a strong influence of diesel vehicle emissions on the production of local particulate PAH. The application of diagnostic ratios confirmed the relevance of this source of PAH in the tunnel ambient air. Deposited dust presented PAH profiles similar to the coarser aerosol size range, in agreement with the predominant origin of coarser aerosol particles from soil dust resuspension and vehicle wear products.

HF channel modeling for real time packet transmission

The recent rapid growth of multimedia communications has efficiently allowed delivering different services, formats and contents over an enormous variety of digital networks with IP acting as an integration protocol. The main objective of this research work is to evaluate the performance of an high frequency (HF) wireless network for transporting multimedia services according to UDP/IP protocol stack. Besides, allowing civil/amateur communications, HF bands are also used for long distance wireless military communications. Therefore, our work is based on NATO Link and Physical layer standards, STANAG 5066 and STANAG 4539, respectively. A typical transmission bandwidth is about 3 kHz resulting in a varying bit rate in the range between 75 and 12800 bps. This very low bit rate by itself imposes serious challenges for reliable real time multimedia communications. This paper discusses optimal combinations of channel coders, modulators and packet sizes in order to achieve the greatest throughput in function of the signal-to-noise ratio and HF channel conditions.

Adaptive joint H.263-channel coding for memoryless binary channels

Our main purpose is to show that when video source coding is dependent on the channel characteristics, better reconstructed video quality can be achieved. Our study considers a fixed overall bit rate transmission of 64 kbit/s over error prone memoryless binary channels and the source coder is the ITU-T H.263 video standard. The variable H.263 source coding rate adapts to a particular channel bit error rate (BER) at any instant of time. We first investigate the error sensitivities of H.263 syntactical elements and group them into several classes of different significance. By identifying the most sensitive elements, we develop a data grouping (DG) technique which, by itself, exhibits improved error resilience. We then design an unequal error protection (UEP) scheme and compare its performance with an equivalent (same source coding bit rate) equal error protection (EEP) scheme, using rate compatible punctured convolutional (RCPC) codes of different rates for forward error protection. Both forward error correction schemes adopted have resulted in PSNR improvements of over 20 dB for BERs higher than of 4/spl times/10/sup -3/. Finally, if during the real time encoding process, the encoder is aware of the channel BER and there is an unlimited number of code rates, the paper shows that UEP and EEP strategies provide quite similar performances.

Multi-decoder digital television platform

The existing analogue transmission systems perfectly match conventional TV sets allowing various independent viewers to watch different programmes at any time. However, digital TV is rapidly growing among our society and its attractiveness lies not only in the higher quality TV images and spectrum efficiency but also in the glimpse of new interactive functionalities that are changing the role of TV in our lives. Due to the widespread conventional TV set usage, a set top box (STB) is required to convert digital into analogue. However, this temporary solution, achieved with commercially available digital STB, is somewhat functionally limited because the latter are only capable of decoding one programme at a time. This limitation appears to be a drawback when considering the traditional analogue case, where various independent users can watch different programmes. In order to overcome this shortcoming, a multidecoder STB was investigated and validated by a particular implementation accounting for three independent simultaneous viewers. This paper describes the basic functionalities of this novel STB.

Audio-visual conference through the ionosphere at 4 kbps

Unlike in data communications, multimedia communications through heterogeneous networks is still far from being achieved due to a lack of solutions at all protocol layers. We are evaluating the performance of a high frequency (HF) wireless network for transporting multimedia services. Beyond allowing civil/amateur communications, HF bands are also used for long distance wireless military communications. Therefore, our research work is based on NATO link and physical layer standards, STANAG 5066 and STANAG 4539 respectively. A typical transmission bandwidth is of about 3 kHz with a throughput bit rate up to 12800 bps, resulting in a very low channel capacity and therefore imposing serious challenges for reliable real time multimedia communications. The paper describes a complete video conference system designed to allow end-to-end communication and discusses the quality of service in terms of packet loss and delay

Error statistical analysis of H.263 syntactical elements

The purpose of our work is to report a solution for the problem of the Joint Source-Channel Coding (JSCC) of the H.263 bit stream to be transmitted through error prone channels. Our investigations led us to classify the H.263 syntactical elements into several classes of different significance. By identifying the most sensitive elements we developed a data partitioning (DP) technique which exhibits improved error resilience. A good reconstructed video quality is obtained for a constant 64 kbit/s transmission rate using Rate Compatible Punctured Convolutional (RCPC) codes of different rates for forward error protection. By exploiting the different syntactical element sensitivities we presented an Unequal Error Protection (UEP) scheme that surpasses the optimal Equal Error Protection (EEP). The forward error correction adopted has resulted in PSNR improvements over 20 dB for bit error rates higher than of 4 X 10-3.

Multi-conference over HF packet radio channels

The main objective of the research prototype is to demonstrate a narrow band multi-conference system over the Internet where one route is a high frequency (HF) wireless link.

Significance analysis of MPEG-4 video syntactical elements

This paper considers the transmission of QCIF resolution MPEG-4 video signals over hostile channels like wireless and mobile at transmission rate of 64 kbps. Although MPEG-4 incorporates some error resilience tools, this standard is still fragile for video transmission over time-variant deep fading channels. We investigated the significance of MPEG-4 video syntactical elements considering MPEG-4 encoded streams, QPSK modulated and delivered through Binary, AWGN and Rayleigh (50 Km/h) channels with SNR range between 0 and 40 dB. We have used four QCIF video test sequences, Foreman, News, Container and Coastguard, and enabled slice resynchronization and data partitioning modes in MPEG-4 video coding. The simulation results show that despite significance of particular elements varies with the channel SNR and with the particular test sequence, we could graphically identify two classes of significance at low channel SNRs. The most significant class includes elements such as VOP header, Video Packet header, DC Marker and DC Coefficients of INTRA-VOP & MB header. At high Rayleigh channel SNR, above 15 dB, AC coefficients of INTRA-VOPs and Motion Vectors were found to be the most significant elements. Any unequal error protection, data partitioning and grouping strategies covering a wide range of SNRs can then be designed based on the significance results presented in this paper.