Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology [J. Res. Natl. Inst. Stand. Technol. 106, 455–469 (2001)] Video Transmission for Third Generation Wireless Communication Systems Volume 106 Number 2 March–April 2001 H. Gharavi This paper presents a twin-class unequal broken into two streams, is unequally protected video transmission system over protected. We use a very simple error cor- National Institute of Standards and wireless channels. Video partitioning based rection coding scheme for illustration and Technology, on a separation of the Variable Length then propose more sophisticated forms of Gaithersburg, MD 20899-8920 Coded (VLC) Discrete Cosine Transform unequal protection of the digitized video (DCT) coefficients within each block is signals. We show that this strategy results considered for constant bitrate transmission in a significantly higher quality of the re- and S. M. Alamouti (CBR). In the splitting process the frac- constructed video data when it is transmit- tion of bits assigned to each of the two par- ted over time-varying multipath fading Cadence Design Systems Inc, titions is adjusted according to the re- channels. San Jose, CA 95134 quirements of the unequal error protection scheme employed. Subsequently, parti- Key words: multimedia communications; tioning is applied to the ITU-T H.263 cod- [email protected] third generation mobile systems; ing standard. As a transport vehicle, we WCDMA; wireless video. have considered one of the leading third generation cellular radio standards known as WCDMA. A dual-priority transmission Accepted: November 1, 2000 system is then invoked on the WCDMA system where the video data, after being Available online: http://www.nist.gov/jres 1. Introduction The wireless revolution in the 1980s was primarily is expected that in the next decade the number of users driven by market demands for mobile radio voice com- will reach the one billion mark. munications. The first generation analog wireless com- The second generation wireless systems were intro- munication systems used old radio technologies com- duced in the 1990s and were primarily an evolutionary bined with novel cellular network planning [1] to step towards improving the capacity of cellular systems provide transparent ubiquitous mobile access to users. through digitization of voice and efficient digital modu- Initially, the cellular market was a niche market targeted lation schemes. These systems also provided additional at business people. As the price of the user terminals features such as security, short messaging, and circuit- and services dropped due to an unprecedented exponen- switched data. tial increase in demand and related economies of scale, We are now in the midst of another evolution planned cellular radios became the communications tool of for deployment in the 2000s. This evolution is once choice for the masses. Today, there are in the neighbor- again driven by the need for greater bandwidth in antic- hood of 300 million mobile phone users in the world. It ipation of further demand for voice services. However, 455 Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology the new wireless standards will also provide a pipeline 2. Review of WCDMA Radio Standards for broadband services such as enhanced high rate data and multimedia services. The global growth of interest It seems that a common international standard for the in the Internet and in digitized audio and video, and the third generation wireless communications systems may demand for such services through the fixed communica- soon be adopted. The international standard may have tions networks, is an important factor. Although there three radio access schemes and is currently being har- are currently no applications with great mass market monized by the Third Generation Partnership Project appeal that require broadband wireless access, it is an- (3GPP). The three radio access schemes are the Fre- ticipated that the popularity of these services in fixed quency-Division-Duplex (FDD) mode, the Time-Divi- networks will eventually impact the market for wireless sion Duplex (TDD) mode and the Multi-Carrier (MC) communications. mode. The FDD mode is a direct sequence (DS) CDMA The next generation wireless systems are required to scheme initially proposed by Japan’s Association of Ra- have voice services of wireline quality and to provide dio Industries and Businesses (ARIB) and the European high bit rate data services of 144 kbit/s to 2 Mbit/s Telecommunications Standards Institute (ETSI). The depending on the radio environment. At the same time, original proposed radio standard was called the UMTS they are to operate reliably in different types of environ- Terrestrial Radio Access or UTRA which included a ments: macro, micro, and pico cellular; urban, subur- TDD mode for deployment in unpaired bands. The ban, and rural; indoor and outdoor. In other words, the multi-carrier mode is based on the Telecommunications next generation systems are supposed to have better Industry Association (TIA) standard known as quality and coverage, be more power and bandwidth CDMA2000 or IS2000. efficient, and be deployed in diverse environments. In the next section, we discuss the physical layer These high data rates make video transmission possible parameters and spreading and modulation for the FDD for a number of applications such as video conferenc- mode of 3GPP radio access scheme and we will refer to ing, emergency medical services, and site surveys. How- it as WCDMA [2]. Please note that the information in ever, since most existing video compression standards this paper may not be consistent with the latest develop- [7,8,9,10,11] have been developed for relatively benign, ments in the standard. We intend to give the reader a nearly error-free environments, they cannot be directly basic understanding of concepts and do not intend this applied in a hostile mobile domain. This is mainly due paper to be a replacement for the detailed specifications, to the extensive employment of variable length coding which may not be finalized. We then simulate the pro- techniques, which are efficient in bitrate reduction posed video coding scheme over the downlink physical terms, but are error-sensitive. A single transmission er- layer of WCDMA and report the performance results. ror may result in an undecodable string of bits. One effective method of protecting the compressed video 2.1 The WCDMA Airlink Parameters signal is to split the coded video signal into a number of separate bitstreams where each can be transmitted via a The modulation chip rate for WCDMA is 3.84 Mega separate channel having a different degree of error pro- Chips Per Second (Mcps)1. The number of chips per tection [12,13,14]. The bitstream splitting can be ac- modulation symbol is called the Spreading Factor (SF). complished by taking into consideration the perceptual The specified pulse shaping roll off factor is 0.22. This significance of coded video, where better protection is results in an effective minimum bandwidth of provided for the transmission of the perceptually more 3.84(1+0.22) which is less than 5 MHz. The nominal important bits. In this paper, such a strategy has been channel spacing for WCDMA 5 MHz. The WCDMA considered for one of the leading third generation cellu- carriers are specified on a grid with a resolution of 200 lar radio standards known as WCDMA. kHz. Although the typical separation between the carri- The paper begins with an overview of the WCDMA ers is 5 MHz, it may vary depending on the deployment radio standard [2]. Then, a video partitioning scheme scenario. The frame length is fixed at 10 ms, which based on the ITU-T H.263 standard, is presented. This corresponds to 38 400 chips. The number of bits or is followed by a very simple error protection coding symbols in a frame may vary depending on the data rate, scheme for the transmission of partitioned video over which is variable. The variable data rates are supported IMT-2000 channels. The transmission system model using variable spreading factors. and its parameters are then discussed. This is followed by simulation results that evaluate the transmission of partitioned video using the downlink WCDMA physical layer as a transport vehicle. Finally, for possible future 1 Chip is defined as a unit representing a bit after spreading every data investigations, more sophisticated forms of unequal pro- symbol. tection for the digitized video signals are discussed. 456 Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology The transmissions from all the physical channels channels are spread by a spreading code with a spread- within a 5 MHz band are code division multiplexed and ing factor (SF) that may vary from 4 to 512 on the transmitted over the same band. We discuss two chan- downlink and 4 to 256 on the uplink, depending on the nels in this paper: i) Dedicated Physical Data Channels data rate of the channel. The spreading factors must be (DPDCH) and, ii) Dedicated Physical Control Channels an integer power of 2. (DPCCH). Table 1 summarizes the bit rates, symbol rates, and the spreading factors for these channels. All 2.2 The WCDMA Downlink The frame structure for the downlink DPCCH/ DPDCH channel is shown in Fig. 1. A super frame is Table 1. The bit rates, symbol rates, and spreading factors for WCDMA dedicated transport channels made up of 72, 10 ms frames. Each frame has 15 equal- length slots. As shown in Fig. 1, the DPCCH and Spreading factor (SF) Symbol rate bit rate kbit/s DPDCH are time multiplexed within the same slot. (thousand symbols per second) The DPCCH channel consists of pilot symbols, trans- mit power-control (TPC) bits and transport format com- 512 (downlink only) 7.5 15 bination indicator (TFCI) bits (please note that TFCI is 256 15 30 128 30 60 optional and is not used for fixed rate services).