multimedia communication techniques for remote

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solutions to the problems concerning the real-time multimedia data transmission in generic cable-based AVS applications. ... transmission over upstream noisy channels have been performed in order to ... ingress-noise [5], that is an impulsive disturbance due to ... image compression coding techniques [7] allows to lower.
MULTIMEDIA COMMUNICATION TECHNIQUES FOR REMOTE CABLEBASED VIDEO-SURVEILLANCE SYSTEMS Claudio Sacchi and Carlo S. Regazzoni University of Genoa, Department of Biophysical and Electronic Engineering (DIBE) Via Opera Pia 11/A I-16145 Genoa (ITALY) Phone: +39-010-3532674, Fax: +39-010-3532134 Abstract This paper1 aims at presenting a study of costs and performances concerning different multimedia communication techniques that can be employed in the context of the implementation of 2nd generation remote video-surveillance systems using wired Cable TV networks as communication means. The multimedia information is acquired by video sensors and transmitted through the CATV network to a remote control centre, where a PC-based image processing architecture performs the image processing tasks needed by the implementation of the foreseen system functionalities. The paper will show some possible analogue and digital solutions to the problems concerning the real-time multimedia data transmission in generic cable-based AVS applications. Laboratory simulations of multimedia data transmission over upstream noisy channels have been performed in order to test the performances in terms of quality of the received images provided by the most advanced high-speed commercial cable modem systems .

1.

Introduction

The technical evolution of the advanced video-based surveillance (AVS) systems has lead from old FirstGeneration CCTV systems (1960-1980) to the SecondGeneration PC-based systems (1980-today) [1], where digital image processing architectures have been introduced in order to assist the human operator in the video-surveillance tasks. The opportunity of exploiting low-cost PC-based technologies explicitly targeted to AVS (e.g. surveillance cameras with digital signal processing, image acquisition board, low-level image processing software libraries, etc.) has favourably increased the market of surveillance products for highsecurity indoor applications. Moreover, the 2nd generation

1

This work has been partially supported by EC-DGIII ESPRIT HPCN TTN Network within the AVS-RIO (Advanced Video Surveillance – Cable Television Based Remote Video Surveillance System for Protected Sites Monitoring) project.

AVS systems can exploit the most recent developments concerning image compression coding and digital transmission techniques, in order to consider the practical implementation of remote monitoring systems for outdoor applications. A remote AVS system should transmit continuously digitised and compressed image sequences to a remote control centre, where such sequences are decoded and processed by a PC-based image processing system. The problems to be faced in such kind of applications mainly concern with the limited bandwidth resources provided by the existing wireless and wired network technologies for continuous transmission of multimedia information from the guarded sites to the network head-end [2] (i.e. upstream transmission). The transmission of JPEG-compressed RGB colour image sequence at a frame-rate of 5 fps, with a 512x512x8 resolution, and compression rate equal to 16 requires an upstream transmission rate of about 1.9 Mb/s. It is shown in [2] that only the UMTS technologies for wireless LAN’s [3] and the cable modem technologies [3] for wired Hybrid-Fibre-Coax (HFC) network are able to support so high transmission rates. Up to now, the definitive standardisation of UMTS technologies is quite close, whereas the high-speed cable modems technologies are already available on the market. In this paper, an analysis in terms of costs and performances of some available solutions for multimedia data transmission in the context of 2nd generation AVS systems for remote applications, using wired Cable TV networks as communication means, has been performed and the related results shown.

2.

Multimedia information transmission over CATV networks

The most relevant problem concerning the multimedia transmission in the 2nd generation remote videosurveillance applications is the asymmetrical upstream/downstream availability characterising the existing wireless and wired network infrastructures [2]. The upstream bandwidth available is generally much smaller than the downstream one. This fact is due to the nature of the applications for which the existing networks are addressed. The CATV networks were originally

designed for analogue and/or digital TV broadcasting, which is a mono-directional kind of transmission [4]. For this reason, the most of the available CATV bandwidth is reserved for allocating broadcast TV channels. The frequencies that are not used for TV broadcasting are generally the ones belonging to the range 5-50 MHz. Such frequencies are strongly affected by the so-called ingress-noise [5], that is an impulsive disturbance due to the electromagnetic emissions of electrical and electronic devices operating inside the residential sites. For this reason, the lowest frequency range is exploited as upstream channel for two-way communication in asymmetric broadband services, such as residential INTERNET providing [3], where the amount of information transmitted in the upstream direction is much smaller than the one transmitted in the downstream one and the quality constraints are quite relaxed. However some more professional applications, such as the remote and outdoor AVS ones, are characterised by different communication requirements. The upstream transmission in remote video-surveillance applications needs a much higher bit-rate than the downstream one, as a continuous information flow must be transmitted in real time from the residential guarded sites to the network head-end. Moreover the constraints on the received signals in terms of low bit-error-rates are obviously much more severe.

3.

Multimedia information transmission in cable-based AVS applications: proposed solutions

3.1

Analogue video transmission

The conceptually simplest and lowest-cost solution is surely the full analogue transmission. The analogue baseband TV signal, provided as output by the cameras is sent over the upstream CATV channel by using a frequency converter. Afterwards a transverter located at the network head-end attends to the broadcast retransmission of the signal over a free 6 MHz. downstream TV channel. An analogue receiver located at the remote control centre is tuned at the downstream retransmission frequency. This kind of solution has some advantages mainly related to the low cost of the needed infrastructures (order of costs 1,000-2,000 USD). The disadvantages concern with the inefficient management of the upstream bandwidth resources (only few cameras can transmit information simultaneously over the upstream channel), the lot of power expense due to maintain a good quality of the received images also in presence of harsh upstream noise, and with the necessity of introducing efficient analogue encryption algorithms.

3.2

Digital transmission of image sequences

Many problems concerning the multimedia transmission for the video-surveillance applications considered in this

work could be solved by using advanced digital transmission techniques over Hybrid-Fibre-Coax (HFC) networks. During these last years, some commercial cable modem systems has been developed, which can exploit the available bandwidth for efficient high-bit-rate transmission over upstream and downstream channels [3]. One of the most promising research topics in cable modem technology concerns with the provision of symmetrical bandwidth availability over the two communication direction. Some commercial products already available on the market can ensure a perfectly symmetrical communication, with a fixed bandwidth occupation of 6 MHz. The bit-rates allowed by commercial cable modem systems are generally comprised within 2 Mb/s and 10 Mb/s [3], however the incoming launch of products capable of symmetric 30 Mb/s communications has been already announced. The centralised management of the channel allocation at the network head-end, together with the exploitation of advanced source and channel coding technique allow to such products to support very high upstream bit-rate with very good bit-error-rate (BER) performances. The use of image compression coding techniques [7] allows to lower the transmission rate, thus meeting the cable modem upstream rate constraints. The main advantage of using digital upstream transmission consists in the improved flexibility provided to the AVS system, allowing multiple access to a lot of users (up to 1000 per 6MHz channel). On the other hand, the cost required for the HW/SW head-end equipment needed by the cable modem systems is not as low as the one required by the full analogue solution, especially when advanced technical solution for improving the robustness and the security of the upstream transmission are considered (cost 50,000 USD). 

4.

Narrowband and wideband upstream digital transmission techniques

The upstream transmission in commercial cable modem systems is generally performed by using narrowband BPSK or QPSK digital modulations [8], together with convolutional Forward Error Correction (FEC) coding (Trellis coding [8]). Typical code-rate values employed in commercial cable modem products are 3/4 and 7/8. In the most common cable modem systems, the multiple access is managed by TDMA-based protocols. A block diagram of a 3/4 Trellis-coded QPSK CATV upstream transmission system is depicted in Figure 1. An interesting alternative approach with respect to the above exposed one is the Synchronous-CDMA (S-CDMA) wideband transmission system, patented by Terayon Corporation. The S-CDMA transmission system is an innovative application of Spread Spectrum-based DS/CDMA techniques [9] to upstream CATV transmission. The basic idea of S-CDMA cable modem system is the multiplexing of a digital bitstream into K

( 1 K 128 ) 64 Kb/s streams (see Figure 2), each of one trellis-coded, interleaved and modulated by a 16QAM with a BPSK DS/SS orthogonal spreading [6]. The FEC coding, digital modulation and spreading are performed by a Pseudo Noise (PN) encoder, whose block diagram is depicted in Figure 3. Each S-CDMA cable transmitter has 128 PN encoders, some of which allowed for data transmission. The number K of active encoders sets the net payload rate of each residential user (from 64 Kb/sec to 8 Mb/sec). The number of active encoders per user is managed by the system at the head-end level. In every case, the PN signal transmitted over the upstream channel has a bandwidth equal to 4.6 MHz. [6]. 



1

3 3:1 MUX

Convolutional FEC coding and interleaving

I, Q

4 QPSK map

s (t )

b(t)

Figure 1: Block diagram of a Trellis-coded QPSK upstream transmission system PN encoder 1

Input bitstream

PN encoder 2

5.

Upstream digital transmission simulations

In order to test the quality of service provided by different digital multimedia transmission techniques over CATV network, some simulations concerning digital transmission techniques over upstream noisy channels have been performed. Both simulations of the narrowband QPSK and wideband S-CDMA upstream transmission systems have been performed by using the MATLAB® SIMULINK® libraries. An additive upstream noisy channel, affected both by Gaussian noise and by impulsive ingress-noise has been simulated in order to provide a credible channel model for the transmission simulations. The variance of the Gaussian noise has been set up in order to obtain a SNR of 6.5 dB in all the simulations considered. Such value is quite in line with the experimental transmission trials performed in the upstream transmission testing [10]. The ingress-noise model chosen in the considered simulations is the sum of CW sinusoidal pulses with Poisson-distributed times of arrival, suggested by Middleton [11]. The formulation of the ingress-noise considered in the present work, which is derived by a slightly simplified version of Middleton’s formulation is given by:







To the upstream channel

PN encoder K

(t )







AI cos 2 f I (t 





k

)







T

(t 

k

)

(4.1)

k 1

The time of duration of each pulse is T 2TS , where TS 60 sec is the time of duration of an encoded four







Figure 2: S-CDMA system global scheme

bit symbol. 

T

(t ) is a rectangular pulse. The upstream



1

3 3:1 MUX 64Kb/s bitstream

Convolutional FEC coding and interleaving

I, Q

4

s k (t )

16 QAM map

ak (t ) Orthogonal PN code generator

transmission frequency is 8 MHz. The frequency of the ingress-noise pulses is a random variable uniformly distributed in the range comprised between 5 and 11 MHz. Also the pulse amplitude is a random variable, uniformly distributed in such way that the transmission signal-to-ingress noise ratio SNRI ˆ PS AI2 , where PS is the transmission power for each 4-bit symbol, is uniformly distributed between –10 dB and 10 dB. This choice has been suggested by the experimental upstream measurement of SNR I presented in [10]. The times of 

Figure 3: Block diagram of a single PN encoder The upstream transmission of the PN signals coming out from the different PN encoders is forced to be synchronous by the channel allocation manager located at the network head-end, thus minimising the CDMA multiaccess interference (MAI) [9]. Due to the choice of orthogonal spreading sequences, the MAI can be identically zero when an ideal synchronism is maintained. The S-CDMA system can provide a very efficient multiple access to the upstream channel. Indeed SCDMA system can support up to 2000 modem transmitting at different rates per 6 MHz channel [6], ensuring a very low bit-error-rate (BER = 10 9 @ SNR = 15 dB [6]). Moreover the PN channel coding provides the typical Spread Spectrum privacy to the transmitted information, that is already encrypted without adding any further redundancy. 

arrival of the interfering pulses k is a Poisson distributed random variable. A simulated realisation of the global noise affecting the upstream transmission (i.e. AWGN noise plus ingress-noise) is reported in Figure 4. An MPEG2 compressed image sequence has been used as input for the above mentioned upstream transmission simulations. A compression rate equal to 32 has been chosen in order to provide a quite satisfactory quality of the received image sequences after decoding. Two transmission simulations have been considered: (a) Wideband S-CDMA transmission at 1 Mb/s (i.e. K = 16 PN encoders allowed for transmission). Orthogonal Hadamard-Walsh PN codes of length 128 have been employed for spreading [8], (b) Narrowband QPSK 

transmission at 1 Mb/s. The upstream QPSK transmission system is the one depicted in Figure 1. Figure 5 shows a first interesting output of the simulations, i.e. the I temporal evolution of the in-phase ( SNR out ) output signal-to-noise ratio, achieved by the matched-filter receivers of the S-CDMA (after de-spreading) and QPSK detectors. The length of the time observation window considered is equal to 125TS . The effects of the ingressnoise on the S-CDMA transmission are quite negligible, I as the values of SNRout are slightly displaced from the optimal theoretical value of 13 dB. On the contrary, the QPSK upstream transmission is much more degraded by the ingress-noise, as evidenced by the deep collapses of I corresponding to the ingress-noise pulse hits. In SNR out Figures 6 (a), and (b), two received and MPEG2-decoded frames achieved at the end of the S-CDMA upstream transmission simulation and the corresponding one achieved at the end of the QPSK upstream transmission simulation respectively are shown. The better quality achieved by S-CDMA transmission techniques is clearly resulting from the comparison of the two images. In the context of high-security applications, the degradation of the quality of the received image sequences encountered in the QPSK transmission simulations can severely compromise the performances of the image processing modules at the remote control centre. For this reason, wideband S-CDMA cable modem techniques are surely more attractive for cable-based AVS systems. Narrowband QPSK modems could also be adopted only if related cable modem technologies employ advanced signal processing techniques at the head-end level explicitly devoted at limiting the detrimental effects of the ingress-noise.

(a)

Figure 6: Received and MPEG-decoded frames after S-CDMA (a) and QPSK (b) upstream transmission simulation

References [1]

[2]

[3]

[4]

[5]

[6]

[7]

Figure 4: Simulated realisation of upstream noise (AWGN noise plus ingress noise)

[8] [9]

[10]

[11]

Figure 5: Output in-phase SNRs achieved by SCDMA (solid line) and QPSK (dashed line) transmission simulations

(b)

C. S. Regazzoni, “Multimedia Video Surveillance Systems: Requirements, Existing Solutions and Open Problems”, Lecture presented at University of Oulu (SF), February, 23, 1999, Part I. P. Mahonen, “Integration of Wireless Networks and AVS”, in Advanced Video Based Surveillance Systems, C.S. Regazzoni, G. Vernazza and G. Fabri, eds., Kluwer Academic Publishers, Norwell (MA): 1999, Chapter 4, pp. 144-153. “Cable Data Modems, A Primer for Non-Technical Readers”, CableLabs Publications, http available at: http://www.cablelabs.com/. J. R. Palmer, “CATV Systems: Design, Philosophy and Performance Criteria as the Basis for Specifying Equipment Components”, IEEE Trans. on Broadcasting, Vol. 13, No. 2, April 1967, pp. 57-68. R.P.C. Wolters, “Characteristics of the Upstream Channel Noise in CATV networks”, IEEE Trans. on Broadcasting, Vol. 42, No. 4, December 1996, pp. 328-332. “Advanced Modulation Systems Technology Reports – Terayon Cable Modem Transmission Test Results”, CableLabs Publications, http available at: http://www.cablelabs.com/. B. Furth, S.W. Smoliar, H. Zhang, “Video and Image Processing in Multimedia Systems”, Kluwer Academic Publishers, 1995. J. G. Proakis, “Digital Communications”, 3rd Edition, McGraw-Hill International Editions: New York 1995. R. Pickholtz, D. L. Schilling, and L. B. Milstein, “Theory of Spread-Spectrum Communications – A Tutorial”, IEEE Trans. on Comm, Vol. COM-30, No. 5, May 1982, pp. 855-884. D. Jaeger, “BER Performances of the DVB-RC Upstream Signal – Physical Layer”, Publication of the Telecommunication Institute of Technical University of Braunschweig (D). D. Middleton, “Statistical-Physical Models of Urban Radio-Noise Environments – Part I: Foundations”, IEEE Trans. on EMC, Vol. 14, No. 2, May 1972, pp. 38-56.