Study and Realization of Joint Source and Channel Coding Techniques for Satellite Based Multimedia Communication

Abstract

The rapid growth of multimedia communication is pushing the present communication systems to their boundaries.There is an ever-increasing demand,put up by a plethora of receiving devices,for fast and efficient multimedia information transfer. For the applications like E -learning, D igital Libraries and R epositories,N E W S B roadcasting Services and D isaster W arning Services; multiple users attempt to access a com- mon source of information viz. image, video, audio and text data. E ach user irrespective of its geographic location and receiving unit's capabilities asks for a certain quality of service (Q oS). The G lobal communication infrastructure need to evolve in the manner so that it can keep pace with the rapidly changing trends of multimedia ex- change. The communication infrastructure needs to enhance its ca- pabilities so as to provide global penetration and support high data rate communication. It should be capable to provide cost-efficient and robust communication to multitudes of receivers and full Fill their Q oS requirements. Satellite technology when considered in conjunction with present wire- less communication infrastructure would be in a position to provide the necessary augmentation. Such a synergy would shift the global in- frastructure towards heterogeneous commucation networks and would reap the benefits of both the worlds viz. satellite and terrestrial infrastructure. H owever, fusion of satellite technology with terres- trial infrastructure would as well put up a novel set of challenges. E ach component of such heterogeneous communication infrastructure would bring along individual limitations also. C onventionally,when it comes to satellite based systems, major design e ort goes in com- bating the effect of AWGN noise. Similarly, when wireless systems are considered, issues related to fading are addressed with a certain rigor. Reception in presence of Doppler is analyzed separately for mo- bile receivers. Whereas, terrestrial broadcast networks concentrates its efforts on adaptive resource(rate/power/time) management at the transmitter-end so that it can offer a certain QoS against uctuating channel quality. The way in which the technologies are converging, it becomes essen- tial that all the above mentioned issues are addressed on a common platform. When it comes to ensure QoS with a certain guarantee for multimedia information in a heterogeneous environment , it is not enough to merely allocate resources. QoS provisioning mechanism needs to encompass source heterogeneity offered by multimedia in- formation, channel heterogeneity offered by heterogeneous networks and receiver heterogeneity offered by receivers with various sensitivity, decoding capability and mobility. It implies that there is a need to tailor the QoS to suit the requirement of individual user/application. A QoS provisioning mechanism is to be incorporated that adapts its efforts in tune with changing network/ channel conditions. This calls forth a transmission scheme which is capable to offer a wide variety of QoS classes. The scheme should have inherent exibility to manage adaptive QoS provisioning in a satellite based broadcast environment. In addition, it should also have a QoS monitoring mechanism which would facilitate the required adaptation. Such modern communication systems can no longer rely on Shan- non's Separation Principle. It seems to fail on many fronts for these sophisticated communication systems. P ower constraints in satellite systems, limitation on bandwidth due to frequency reuse, multiuser environment, non-ergodic multimedia sources as well as unknown and time varying channels pose a big hindrance in achieving optimum performance while incorporating a source coder and channel coder designed on the principle of Separation. A joint design of source and channel coder is needed to handle such practical limitations. A source coder which devices its coding strategies in tune with the vari- ations in the channel states and a channel coder which matches its e orts to suit the varying source statistics; will not only yield an opti- mum utilization of resources but also facilitate an efficient and robust communication in a constrained heterogeneous environment. Besides achieving optimum performance under various constraints, multime- dia communication systems are also expected to support high data rate communication. It needs to employ spectrally-e cient multilevel modulation schemes that can facilitate high data rate communication over congested RF spectrum of satellites. However, special e orts are required to ensure robust detection of these higher order/hierarchical constellations. In order to maneuver through various heterogeneities, a system needs to be exible enough so that provision of QoS can be managed adap- tively . Scalable transmission can induce the required exibility. A multiresolution(M R) source coding in conjunction with unequal er- ror protection provides judicious utilization of resources like rate and power, and provides necessary scalability to adapt to source and chan- nel variations. M R source coding takes care of source heterogeneity, U EP deals with channel heterogeneity and the combination of the two makes it possible to offer different class of QoS to encompass different user-devices as well as different applications. As this work focuses its efforts on satellite based broadcast communi- cation, it needs to device an open loop adaptation scheme. The Joint Coding effort put up by the transmitter side is supported by dynamic rate adaptation on the receiver side. A generic two tier scheme is investigated which would enable cost-effective and robust multime- dia communication and ensure a guaranteed QoS under uctuating channel quality and frequency- at fading conditions for multimedia communication in a satellite based broadcast environment. At First, attempt is made to guarantee QoS against abruptly varying channel conditions. The First tier employs resolution scalable (mul- tiresolution) source and applies unequal error protection (UEP) to dif- ferent resolution levels. This infuses a scalable transmission which can be tuned in accordance to the changing SNR of the channel. UEP is achieved through a hierarchical (HR) constellation which constitutes of an M- QAM constellation riding over a baseline 4-PSK constella- tion. A more reliable transmission channel i.e. a better error protec- tion is applied to the coarser-resolution information by mapping it to the baseline 4-PSK constellation. The ner-resolution information is mapped to the M-QAM constellation. This joint coding effort facili- tates transmission of two QoS classes viz. HP(high protection) class and LP(low protection) class. In the case when the receiver has the information regarding the re- ceived SNR, it can adapt the rate and full Fill its own QoS requirements. A stronger (more sensitive) receiver or the receiver having a better quality of reception can avail guaranteed QoS with full resolution. Whereas, a w eaker (less sensitive) receiver or the receiver observing poor quality of reception would avail guaranteed QoS at reduced res- olution. In order to access the quality of reception, the receiver is made to derive an estimate of SNR. It obtains a blind estimate of SNR at baseband level and reduces the rate by dropping of the over- riding QAM constellation whenever the SNR falls below a prescribed threshold. A blind moment based SNR estimation technique is em- ployed which obtains periodic estimates of SNR for AWGN channel. Secondly, the service quality guarantee is provisioned to ensure ro- bust detection of hierarchical constellation under the in uence of slow, at, narrowband fading conditions. Multiplicative distortion caused by this non-dispersive fading could implicate time-varying distortion in amplitude and a time-varying disturbance in phase and can cause serious concern for achieving coherent demodulation of Hierarchical constellation. The second tier of the scheme deals with this issue. The receiver derives channel state information regarding fading by utiliz- ing the pilot symbols which are embedded in the information symbols at the transmitter. These pilot symbols probe the fading channel and provide an estimate of amplitude and phase reference that are used to adjust the level and angle of the incoming symbols to the refer- ence constellation. As a part of the process of channel sounding it also draws an inference regarding channel SNR. The location of pilot symbols in the stream of incoming data estimation is identified using a synchronization process. The outcome of this synchronization pro- cess is indicative of channel SNR. Hence, in case of fading channel, SNR estimation so obtained is utilized to set a ag for rate-reduction in adverse channel conditions. Performance of this Pilot Symbol As- sisted detection scheme is verified for various fade-rates, with various interpolation order, pilot length and inter-pilot spacing. The performance of this generic Joint Source and Channel Coding scheme is analyzed for two QoS parameters viz. reproduction error and latency. The end-to-end distortion of this transmit-receive chain is evaluated for various SNR values.The distortion contribution of each individual components is analyzed. Quantization distortion, dis- tortion involved in mapping a secondary constellation on a baseline QPSK constellation, AWGN noise and the distortion due to channel estimation error are main contributors in the total reproduction er- ror. Results are obtained for both HP class as well as LP class of information. It is observed that, due to unequal error protection, LP class of information is more susceptible to corruption. And hence in adverse channel conditions, QoS can be ensured by retaining only the HP class of information. The impact of the process of SNR estimator and fading channel es- timator is also visualized. The target applications are Digital image repositories, Disaster warning and other such applications which do not have very stringent latency requirements. Neither do they have very stringent loss requirements. Therefore, these block based adap- tation techniques can be utilized, where the service is compromised for the duration of estimation. Thus, this open loop scheme employs joint coding scheme and joint effort put up by both the transmitter and receiver, so that the QoS provisioning can be customized for in- dividual receiver in a broadcast environment. It suggests minimalist modification in the receivers which puts in effort to program QoS as per its own requirement. The scheme also provides graceful degrada- tion in deteriorating channel conditions.