IEEE COMSOC MMTC E-Letter

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Distinguished Position Paper Series

Trends in Multimedia Communications Over Mobile Networks

Hamid. Gharavi (IEEE Fellow), National Institute of Standards and Technology,

100 Bureau Drive, Gaithersburg, MD USA 20899

gharavi@nist.gov

Increasing demand for high quality multimedia services has been a driving force in

the technological evolution of high bandwidth wireless/mobile communications systems

and standards. The most challenging aspect is the support of higher quality video, which

requires a higher bandwidth. Although the operational third generation (3G) wireless

systems may be capable of handling low data rate video [1], [2], the problem is that

mobile cellular technology is not ready to offer reliable real-time video services. In

addition, as the proliferation of mobile video accelerates, the next generation wireless

communication systems must aim at providing higher per user data rate services to

support higher quality real-time audio/video services, especially as new applications,

such as ubiquitous on line journalism and live citizen reporting, are emerging.

It is important to note that in mobile environments a higher per user bandwidth does

not necessarily guarantee a higher video quality reception. Thus, the major technical

challenges will be to cope with frequency-selectivity fading due to the use of larger

bandwidths. As far as transmission at the physical layer is concerned, schemes such as

OFDM (Orthogonal Frequency Division Multiplexing) and MC-CDMA (Multi CarrierCode Division Multiplexing), which are capable of providing frequency diversity, can

indeed enhance robustness to frequency selective fading.

In addition, recent technological breakthroughs in Multiple Input Multiple Output

(MIMO) techniques [3], [4], i.e. providing space-time diversity, have already made a

significant impact to transmission over mobile channels in terms of reliability and

throughout. Needless to say, a deep fade in all the wireless channels may still erase some

of the information and consequently, the use of error control coding - particularly for

real-time multimedia - remains a major research topic in mobile communications.

Error control coding methods, such as error detection and error correction have been

traditionally applied at the physical layer in most digital cellular communication systems

(also referred to as channel coding). In the case of the IEEE 802 standard for Wireless

Local Area Networks (WLAN), for instance, the physical and link layers are responsible

for handling error control coding for IP packets. This includes a 16-bit CRC (cyclic

redundancy check) error detection field at the physical layer and packet retransmissions

via the MAC (Medium Access Control) sub-layer.

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However, in a packet based wireless network environment where communications

may need to be integrated into the framework of the OSI (Open Systems Interconnection)

model, the error control coding strategy is a challenging issue. This is mainly a

consequence of the layering structure in the OSI protocol stack, where the protection

against packet loss is handled separately and independently for each layer. For instance,

in addition to the link and physical layers, error control coding in the form of Forward

Error Correction (FEC) is also applied at the application layer. For video transmission

particularly, a combination of multi-layer coding and FEC with a differing level of

protection for each layer (also known as unequal error protection: UEP), has been an

effective approach for transmitting video over multipath fading mobile channels [5], [6].

Nonetheless, when the channel condition is unknown and there are multiple receivers,

the use of a fixed rate error control coding could be wasteful and unreliable. Recently, a

new generation of rate-less codes, such as raptor codes [7], has been considered for file

download in the Digital Video Broadcast for Handheld (DVB-H) standard [8]. In the

raptor code, which is a category of the Fountain code, the encoder can generate as many

encoded symbols as needed from a block of data on the fly. Raptor codes have the

advantage over traditional fixed rate erasure codes, such as Reed-Solomon codes, for

their ability to manage the overhead when channel conditions are unknown.

The flexibility and reliability of the Raptor code for UEP has been studied by a

number of researchers in recent years [9], [10]. IETF RFC 5053 [11] defines procedures

for generating the Raptor FEC and its application for the reliable delivery of data objects.

In addition, for Digital Video Broadcast two-layer protection for real-time multimedia

data has been recently proposed, where the first layer (base layer) is protected by the 1-D

interleaved parity code, and the enhancement layer is produced by the Raptor code [12].

It should be noted that in this AL-FEC approach, the source packets are carried in

separate RTP (Real-time Transport Protocol) streams. The next layer in the protocol

stack is the transport layer. Between the two popular transport protocols, TCP and UDP

(User Datagram Protocol), UDP is the preferred protocol. Nonetheless, this protocol,

unlike the connection oriented TCP, is the best effort transport protocol. Since UDP

cannot provide reliable packet transmission, additional error control coding may be

needed to prevent any significant loss of video quality.

Indeed, further coding can be accomplished at the next lower layer in the protocol

stack, which is the network layer. This layer is responsible for routing RTP/UDP/IP

packets to their destinations. In the case of mobile ad-hoc networks (MANET), these

autonomous networks are not quite capable of reliably distributing RTP/UDP/IP packets

[13], [14]. One major obstacle is a problem with the dynamically changing network

topology. This manifests itself in a frequent route change, consequently causing a

potentially long delay [15]. Co-channel interference from other users is another important

factor that is problematic and can severely impact the end-to-end throughout

performance. While mitigating the effect of interference continues to be an active

research topic, a new approach known as network coding, has recently emerged [15]. Its

concept is based on performing coding operations in the interior network, rather than just

being received and forwarded by the intermediate nodes (routers). By intelligently mixing

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packets in multicast routing it is possible to enhance the network throughput

performance. Although network coding appears to work well for wired networks, the

debate on its suitability for unreliable multihop environments for real-time multimedia

services is now becoming a hot research topic.

Finally it is worth noting that in contrast to fixed wireless communications, existing

mobile networks are still incapable of supporting reliable, interactive, and high quality

video services. This is mainly due to a number of factors, such as increasing demands for

live video transmissions, bandwidth limitations, co-channel interferences, and everchanging channel conditions. Developing methods such as space-time diversity for

cooperative transmission for multihop networks is becoming the most active research

topic in combating multipath fading [16], [17]. In addition, in such environments error

control coding techniques still continue to play a major role in supporting high quality

multimedia services for the next generation of wireless/mobile networks. As far as their

deployment is concerned, the major problem is that they are separately applied within

each OSI layer without any attention to the tradeoffs between overhead, latency and

performance. This will involve the issue of cross layer optimization in order to maximize

their efficiency in accordance with service quality requirements.

References

1. H. Gharavi and S. M. Alamouti, “Multi priority Video Transmission for Third Generation Wireless

Communication Systems,” Proceedings of the IEEE, Vol. 87, No. 10, October 1999, pp1751-1763.

2. L. Hanzo, P. Cherriman, J. Streit, “Video Compression and Communications,” H.261, H.263, H.264,

MPEG4 and Proprietary Codecs as well as HSDPA-Style Adaptive Turbo-Transceivers, John Wiley

and IEEE Press, September 2007

3. S. M. Alamouti. A simple transmit diversity technique for wireless communications. IEEE Journal on

Selected Areas in Communications, 16(8): 1451–1458, 1998.

4. V. Tarokh, H. Jafarkhani, and A. R. Calderbank. Space-time block codes from orthogonal designs.

IEEE Transactions on Information Theory, 45(5): 1456–1467, 1999.

5. R. Stedman, H. Gharavi, L. Hanzo, and R. Steele, "Transmission of Coded Images via Mobile

Channels," IEEE Transactions on Circuits and Systems for Video Technology, CSVT, vol.3, NO. 1,

pp.15-26, February 1993.

6. H. Gharavi, "Pilot Assisted 16-QAM for Video Communication," IEEE Transactions on CSVT, Vol.

12, No. 2, February 2002, pp. 77-89.

7. A. Shokrollahi, “Raptor Codes”, IEEE Trans. Inf. Theory, Vol. 52, No. 6, June 2006, pp. 2551–2567.

8. ETSI, TS 102 472 V1.2.1, “Digital Video Broadcasting (DVB); IP Datacast over DVB-H: Content

Delivery Protocols,” Dec. 2006.

9. N. Rahnavard, B. N. Vellambi, and F. Fekri, “Rateless codes with unequal error protection property”,

IEEE Trans. Inf. Theory, Vol. 53, No. 53, pp. 1521–1532, April 2007.

10. D. Sejdinovic, D. Vukobratovic, A. Doufexi, V. Senk, and R. Piechocki, “Expanding window fountain

codes for unequal error protection”, Proc. 41st Asilomar Conf., Pacific Grove, pp. 1020– 1024, 2007.

11. M. Luby, A. Shokrollahi, M. Watson, T. Stockhammer,” Raptor Forward Error Correction Scheme for

Object Delivery, IETF, RFC 5053, October 2007 “http://www.rfc-editor.org/rfc/rfc5053.txt.

12. A. Begen and T. Stockhammer, “DVB Application-Layer Hybrid FEC Protection,” draft-ietf-fecframedvb-al-fec-02, August 11, 2009 “http://tools.ietf.org/search/draft-ietf-fecframe-dvb-al-fec-02.

13. H. Gharavi, K. Ban, “Multihop Sensor Network Design for Wideband Communications, ”The

Proceedings of the IEEE, vol. 91, NO. 8, August 2003, pp. 1221-1234.

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14. H. Gharavi ” Control Based Mobile Ad-hoc Networks For Video Communications, IEEE Transactions

on Consumer Electronics, Vol. 52, No. 2, May 2006, pp. 383-391.

15. R. Ahlswede, N. Cai, S. Li, and R. Yeung, “Network information flow,” IEEE Transactions on

Information Theory, vol. 46, July 2000, pp. 1204-1216.

16. S. Katti, H. Rahul, W. Hu, D. Katabi, M. Médard, and J. Crowcroft, “XORs in the air: practical

wireless network coding,” in Proc. ACM SIGCOMM’06, Pisa, Italy, Sep. 11-15, 2006, pp. 243-254.

17. C. Fragouli, J. Widmer, and J.-Y. L. Boudec, “A network coding approach to energy efficient

broadcasting: from theory to practice,” in Proceedings of IEEE INFOCOM, Apr 2006, pp. 1-11.

Hamid Gharavi (Fellow, IEEE) received the Ph.D. degree from

Loughborough University, Loughborough, U.K., in 1980. He joined AT&T

Bell Laboratories, Holmdel, in 1982. He was then transferred to Bell

Communications Research (Bellcore) after the AT&T-Bell divestiture, where

he became a Consultant on video technology and a Distinguished Member of

Research Staff. In 1993, he joined Loughborough University as Professor and

Chair of Communication Engineering. Since September 1998, he has been with

the National Institute of Standards and Technology (NIST), US Department of

Commerce, Gaithersburg, MD.

Dr Gharavi was a core member of the Study Group XV (Specialist Group on

Coding for Visual Telephony) of the International Communications

Standardization Body CCITT (ITU-T). He was selected as one of the six

university academics to be appointed to the U.K. Government’s Technology Foresight Panel in

Communications to consider the future through 2015 and make recommendations for allocation of key

research funds. His research interests include video/image transmission, wireless multimedia, mobile

communications and third generation wireless systems, and mobile ad-hoc networks. He holds eight U.S.

patents related to these topics.

Dr Gharavi received the Charles Babbage Premium Award from the Institute of Electronics and Radio

Engineering in 1986, and the IEEE CAS Society Darlington Best Paper Award in 1989. He has been a

Distinguished Lecturer of the IEEE Communication Society. In 1992 Dr Gharavi was elected a Fellow of

IEEE for his contributions to low bit-rate video coding and research in subband coding for image and video

applications. He has been a Guest Editor for a number of special issues. Dr Gharavi served as a member of

the Editorial Board of the PROCEEDINGS OF THE IEEE from January, 2003 to December, 2008. He is

currently a member of the Editorial board, IET Image Processing. He served as an Associate Editor for the

IEEE Transactions on CAS for Video Technology (CSVT) from 1996 to 2006. He then became the Deputy

Editor-in-Chief of this IEEE Transactions through December 31, 2009. Dr Gharavi was recently appointed

to serve as the new Editor-in-Chief for the IEEE Transactions on CSVT.

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