1 Commun. Fac. Sci. Univ. Ank. Series A2-A3 V.53 (1-2). pp (2011) 25 APPLICATION OF LOW BIT RATE SPEECH CODING TECHNIQUES TO OUTDOOR PORTABLE LASER OPTICAL WIRELESS FİKRET ARI*, FARUK ÖZEK AND HAKKI GÖKHAN İLK Ankara University, Faculty of Engineering, Department of Electronics Engineering, Besevler-Ankara-TURKEY (Received September 30, 2011; Accepted December 08, 2011) ABSTRACT This work analyzes and evaluates the possible use of different speech coding algorithms, namely Melp 2.4 kbps (NATO Stanag 4591), Celp 4.8 kbps (FS1016) and Celp 8.0 kbps (ITU-T G.729), in portable laser optical wireless communication at low bit rates, following a brief description of the laboratory scale system designed and constructed for the experiments. Related to the various outdoor application examples, the results based on the Perceptual Evaluation of Speech Quality (PESQ) algorithm for the quality assessment are presented. Results, obtained from the laboratory prototype, proved that MELP vocoder is the choice of speech coding because of its several advantages such as increased transmission distances under similar conditions and achieved power economy at reduced link distances especially in portable applications. KEYWORDS: Optical wireless, Speech coding, Quality assessment INTRODUCTION The objective of future wireless communication systems is to provide users with wide variety of services such as internet access, file transfer, interactive data, voice and image transfer comparable to those provided by the wired communication systems. In order to improve the bandwidth and system capacity, optical wireless communication systems can be considered as an alternative communication media despite their disadvantages in adverse weather conditions. Optical wireless communication or alternatively free space optical communication, FSOC [1,2], has facilitated high bandwidth, interference free and reliable long-term fixed-to-fixed, f2f, links for example in the local area networks, LANs . FSOC systems have become an attractive alternative to optical fiber communications or radio frequency (RF) systems. These systems offer much higher data rates when
2 26 FİKRET ARI, FARUK ÖZEK AND HAKKI GÖKHAN İLK compared to an RF system, also easier to deploy and have cheaper installation costs than underground fiber. Although, fiber-optic cabling is still the preferred media for long distances, FSOC is now considered as a viable option to fiber for short distances of 4 km or less due to lower cost and significantly shorter installation time . Such systems employ infrared lasers in order to lessen the ambient light effect by day and to maintain covertness plus information privacy when necessary. However, despite the advantages of an FSOC system over fiber based lines and RF links, there is one big challenge for optical wireless and that is the atmospheric attenuation. The effect of all factors appears as an atmospheric attenuation coefficient in the formulation that produces the level of received power at the receiver and is uncontrollable in an outdoor environment. Other examples of f2f applications may be the short-term installations for speech transmission in disaster hit areas (earthquake, flooding, etc) where all the base station infrastructure is likely to be out of function, or in the field geological tests where high speed data transfer can also be needed from the measurement station. As an alternative or a complementary link, a light-weight, battery operated Portable Laser Optical Wireless, PLOW, system(s) can offer rapid deployment and ease of installation. Visible, e.g. red, lasers should be preferred to shorten the period of lineof-sight alignment and due to the fact that the covertness is not of the prime concern in similar outdoor applications. Optical wireless communication at low bit-rates, via speech coding, means also lower noise levels resulting in longer link distances, which is important for battery operated low power laser transmitters. The present work analyzes and evaluates the possible use of the speech coding algorithms at low bit-rates in PLOW connections, where immediate wireless speech communication is required in short time, at low cost, such as disaster areas and/or temporary fairs and exhibitions. EXPERIMENTS AND MEASUREMENTS Figure 1, presents the block diagram of the laboratory-scale system developed and employed throughout the experiments. Due to the limited laboratory space available, the effect of the varying transmission distances is analyzed by placing various optical attenuators between the transmitter and receiver which are set at fixed points, instead of changing the distance between them. Calibrated optical attenuator, placed in the laser path, fixed at 2m, simulates the signal deterioration due to transmission and adverse channel effects. Added to the system is a variable intensity light source in order to simulate the ambient light levels. The intensity of the tungsten filament light source (to simulate the sun s radiance spectrum) is so varied to correspond to the outdoor ambient illumination levels from 10 0 Lux (twilight) to the maximum 10 4 Lux (clear summer noon) at the receiver. Optoelectronic parameters of the system are summarized in Table 1.
3 APPLICATION OF LOW BIT RATE SPEECH CODING TECHNIQUES 27 Figure 1. Block diagram of the measurement system Table 1. System optoelectronic parameters Parameter Laser power Laser wavelength PIN, wavelength of peak sensitivity NEP Receiver optical red filter transmission wavelength Value 1.7 mw 680 nm 850 nm 4x10-14 W/ Hz > 600 nm For the transmission and recording of the source-coded speech data MATLAB DAQ Toolbox is used. The transmitter and receiver sub-systems are depicted in Figure 2 (a) and (b), respectively. For the reduction of destructive channel effects on the signal, the narrowband binary sinusoidal frequency shift keying, 2-FSK, (the carrier frequency: 200kHz, with ±1kHz deviation) is preferred to modulate the laser beam. FM demodulator at the receiver makes the channel effect analysis possible. The digitally coded speech data are transmitted, at varying bit rates and in real time, by using three source coders, namely NATO and DoD standard Melp 2.4 kbps , Celp 4.8 kbps and Celp 8.0 kbps . A detailed comparison of these speech coders, for source coding, is available in . Each transmission is repeated for different optical attenuation ratios (db) and by day/night ambient light levels. The variations of the calculated bit error rates, BER, originating from the channel, with optical attenuation at different ambient conditions and bit rates, are shown in Figure 3 and Figure 4, respectively. Figure 3, Melp 2.4 kbps, indicates a similar
4 28 FİKRET ARI, FARUK ÖZEK AND HAKKI GÖKHAN İLK performance for day and night conditions mainly because of selected digital modulation technique and optical filtering process used in the receiver module. As expected, BER increases for higher attenuation representing for example worsening weather conditions. Figure 2. (a) Transmitter, (b) Receiver sub-systems Optical attenuation levels for different weather conditions are defined in the standard International Visibility Code tables and summarized in Table 2 . Table 2. Atmospheric conditions and attenuation levels Weather condition Attenuation (db/km) Clear 0.6 Haze 4.6 Thin Fog 20.0 Thick Fog 84.9 Heavy Rain 7.1 Snowy 34.0
5 APPLICATION OF LOW BIT RATE SPEECH CODING TECHNIQUES 29 Figure 3. Variation of BER with optical attenuation at 2.4 kbps (10 4 Lux: clear summer noon, 10 0 Lux: twilight) For the same ambient illumination, i.e. daylight, it is observed that the BER increases with the increasing bit rate as shown in Figure 4. Cross-over values are 29.6dB (8.0 kbps), 31.8dB (4.8 kbps) and 35.2dB (2.4 kbps), with reference to the probable BER threshold of 10-3, considering the acceptable quality of transmission. In addition, for all the coders, the BER converges to the same optical attenuation, 39dB, due to the fact that the optical signal power approaches to the noise equivalent power, NEP. Figure 4. Variation of BER with optical attenuation for different bit rates (for 10 4 Lux)
6 30 FİKRET ARI, FARUK ÖZEK AND HAKKI GÖKHAN İLK RESULTS AND DISCUSSION The primary reason being the objectivity, PESQ (Perceptual Evaluation of Speech Quality / ITU-T P.862) algorithm is opted for the performance comparison assessment. The most eminent result of PESQ is the Mean Opinion Score (MOS) which directly expresses the voice quality. PESQ simulates a listening test and is optimized to reproduce the average result of all listeners of standard MOS listening test. To calculate the PESQ scores, 14 male and 7 female voice recordings were used. Prior to the transmission of the source coded speech data, the PESQ scores are computed and recorded, followed by decoding of bit sequence transmitted at 37dB which presents an average worst case level of the optical attenuation. Table 3 indicates that higher scores are obtained for the male speakers. The basic reason for this result is the fact that MELP coder is optimized for male speakers. This result is expected heuristically, as MELP is a military standard speech coder. Table 3. Quality assessment PESQ scores PESQ Received channel Source encoder bit stream scores output (at 37dB Encoder attenuation) Male Female Male Female Melp 2.4kbps Celp 4.0kbps Celp 8.0kbps CONCLUSIONS It is indicated in  that MELP 2.4kbps source coder, i.e. the lowest bit rate, provides high economy in the channel bandwidth, while speech quality is maintained. Figure 4 points out that at 2.4 kbps the cross-over BER value corresponds to the highest optical attenuation, 35.2dB. From this result it can be concluded that, with the BER levels being equal, lower bit rates imply increased transmission distances. That is important for applications using portable optical wireless systems with low laser power outputs. It should be also stated that the findings in this experimental work are not specific to PLOW applications, and can be generalized to cover outdoor Mobile Laser Optical Wireless, MLOW, systems the only difference is that the terminals are in motion .
7 APPLICATION OF LOW BIT RATE SPEECH CODING TECHNIQUES 31 Finally although MELP delivers worst PESQ results for source coding, it is the choice of vocoder for PLOW applications when design parameters are taken into consideration  and due to its superior performance in day and night conditions, increases distance at a set power level or saves power at reduced distances. Moreover it delivers best PESQ results after channel transmission, when compared in terms of speech quality. ÖZET: Bu çalışma, Melp 2.4 kbps (NATO Stanag 4591), Celp 4.8 kbps (FS1016) ve Celp 8.0 kbps (ITU-T G.729) isimli farklı konuşma kodlama algoritmalarının, deneyler için tasarlanan ve kurulan laboratuvar ölçekli sistemin kısa tarifine müteakip, düşük bit hızlarında taşınabilir laser optiksel telsiz iletişiminde mümkün kullanımını analiz eder ve değerlendirir. Çeşitli dış ortam uygulama örnekleri ile ilgili olarak, kalite değerlendirme için Konuşma Kalitesinin Algısal Değerlendirilmesi (PESQ) algoritmasına dayalı olarak elde edilen sonuçlar sunulmaktadır. Laboratuvar prototipinden elde edilen sonuçlar, benzer şartlar altında artırılmış iletişim mesafeleri ve bilhassa taşınabilir uygulamalar için kısaltılmış link mesafelerinde güç ekonomisi elde edilmesi gibi çeşitli avantajları nedeniyle MELP ses kodlayıcının konuşma kodlayıcı olarak tercihini kanıtlamıştır. REFERENCES  Bloom, S., Korevaar, E., Schuster, J., Willebrand, H., Understanding the performance of free-space optics, Journal of Optical Networking, 2(6):  Bouchet, O., Sizun, H., Boisrobert, C., Fornel, F. De, Favennec, P-N, Freespace optics: Propagation and communication, ISTE Ltd, London.  Willebrand, H., Ghuman, B.S., Free-space Optics: Enabling Optical Connectivity in Today s Networks, SAMS Publishing, Indiana, USA.  Wisely, D.R. et al, km Terrestrial line-of-sight optical free-space link operating at 155 Mbit/s, Free Space Laser Communication Technologies VI, Proceedings of SPIE, 2123:  Ertan E., et al, Implementation of an enhanced fixed point variable bit rate MELP vocoder on TMS320C549, in Proc. IEEE Int. Conf. Acoust., Speech Signal Process., ICASSP99:  Schroeder M.R., Atal B.S., Code-excited linear prediction (CELP): Highquality speech at very low bit rates, Proceedings of the IEEE International Conference on Acoustics Speech and Signal Processing, ICASSP85:
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