504rfd3x

8/8/2019 504rfd3x

http://slidepdf.com/reader/full/504rfd3x 1/4

36 www.rfdesign.com April 2005

Broadcast/Satellite Communications

DVB-H architecture for mobile communicationssystems

The availability of new low-power, small form-factor digital video broadcasthand-held (DVB-H) demodulator and silicon tuner solutions now enable thereception of digital TV signals on hand-held devices where small displays andlimited battery power are a fact of life. The requirement for improved videodisplay functionality and support for multimedia applications makes DVB-H[1] anattractive complementary technology to existing 3G in-band solutions.

By Stuart Pekowsky and Khaled Maalej

he deployment of handsets with inte-

Tgrated support for the reception of digi-

tal TV signals is now a reality. Until now, thefeasibility of such support has been limited byseveral key factors, namely power consump-tion, performance and size. Most attempts ataddressing the hand-held receiver markethave been based on adapting existing homeset-top box solutions to the demanding envi-ronment of mobile reception. Experience hasshown that this is not possible due to theconstantly changing receiver environment.The quality of the received signal is affectedby the receiver’s ability to manage adjacent-channel rejection, low signal-to-noise ratios

and Doppler compensation. Mobile DVB-Treceivers have implemented antenna diver-sity technology but at the cost of increasingpower consumption and size. While smallform-factor diversity solutions do exist, inpractice they are not feasible for deploymentin cell phones since antenna diversity requiresa minimum distance between the twoantennas that is not readily available.

The DVB-H stan-dard, actually an ex-tension to the digitalvideo broadcast ter-restrial (DVB-T) stan-

dard, has specified theuse of time-slicing toreduce receiver powerconsumption andmultiprotocol encap-sulation forward errorcorrection (MPE-FEC) to provide anadditional layer of error correction inorder to provide amore robust signal inmobile environmentswhere diversity solu-tions are typically thenorm. In addition,

DVB-H provides for the use of services based

on Internet protocol (IP) datacasting. Anexample of a DVB-H broadcast configura-tion is shown in Figure 1. Figure 2 depictspotential head-end architectures for provi-sion of content in a DVB-H network.

This article focuses on the receiver (spe-cifically, the silicon tuner and demodulatormodules) and discusses the methods of addressing the demands placed on thereceiver in the mobile environment withoutsacrificing performance on hand-held devices.

Adaptations to DVB-T towardDVB-H

DVB-H specifies the use of the followingto address hand-held constraints:s Time-slicing to reduce power consump-tion.s IP datacasting for lower resolution videostreaming.sMPE-FEC for more robust signal reception.s Introduction of the 4K carrier mode fornetwork optimization.

DVB-H front-end: Silicon tuner

plus demodulatorIn order to be able to integrate the DVB-H

front-end into a cell phone, it was necessaryto address several important issues: powerconsumption, performance and mechanicalsize. All of these problems are solved bythe new generation of silicon tuners anddemodulators.

Silicon tuner characteristicsThe silicon tuner is the most recent type of

tuner to evolve from the so-called “in-can”tuners commonly found in set-top boxes andTV tuner PC peripherals for home use. The

performance of the “in-can” tuner can bequite good and is used as a baseline formeasuring the performance of silicon tuners.

The interface between the tuner and thedemodulator was changed from intermediatefrequency (IF) (for DVB-T) to baseband (forDVB-H). This offers the possibility to do theadjacent-channel filtering in baseband andintegrate it into the silicon tuner. Doing so

Figure 1. DVB-H broadcast network.

8/8/2019 504rfd3x



contributes to the reduction of the footprintof the complete solution. As a result, thepower consumption of such tuners has been

reduced to 300 mW. DVB-H has beenconsidered for use in the UHF band but hasbeen adapted for use in other frequncy rangesas well (e.g., 1.672 GHz for U.S. trials).

DVB-H demodulatorcharacteristics

The primary requirement for the demodu-lator is to provide good reception perfor-mance in mobile environments. The Dopplercompensation requirement for a DVB-Hreceiver is at least 100 Hz. The receiver mustbe able to handle a channel with this variationand at the same time maintain the same

signal-to-noise ratio as the one obtained forlow Doppler frequencies for the same qualityof service (QoS). This point is critical forbroadcasters when they do their network plan-ning. The network coverage must stay thesame when the speed of the receiver varies.

In addition to Doppler compensation, thereceiver will face channel profile variations.The different echoes received at the antennawill vary and the distance between them willchange continuously. The receiver mustcompensate for these variations in order toavoid de-synchronization with the transmit-ter and to maintain good reception.

Co-channel interference is an importanttransient disturbance that comes from differ-

ent industrial sources that are generated inthe operating environment and sometimeseven from other neighboring networks. The

receiver must provide immunity from suchlarge disturbances and also be capable of maintaining good reception even if theinterference exhibits power higher than thedesired signal. In a Groupe Spécial Mobile(GSM) cell phone for example, such interfer-ence typically comes from the upstreamGSM channel.

Sensitivity is an important parameter re-garding providing the best geographicalcoverage. The demodulator design must striveto have an implementation with very lowdegradation with respect to the theoreticalvalues. An implementation margin of 0.5 dB

to 1 dB is something that can be achieved.Figure 3 shows the block diagram of aDVB-H receiver with the two main parts:demodulation and media access control(MAC) processing.

In addition to the demodulation, the link layer or MAC is also integrated into thedemodulator. This layer handles the timeslicing, the IP data extraction and the IP dataerror correction. Because of the high bit ratesthat DVB-H must support, the Reed-Solomondecoding is implemented in hardware.

Power consumptionIn general, a DVB-T Moving Pictures

Experts Group (MPEG-2) program stream

can use up to 10 Mbits/s, which is abovewhat a cell phone or hand-held device canreasonably process or display on a quarter

video graphics array (QVGA) screen. Abitstream of less than 500 Kbits/s can providegood quality video resolution on smallscreens. Time slicing (i.e., data bursting) wasintroduced in order to adapt the DVB-Tstream to the cell phone. The receiver isturned on for a short period and then shutoff the rest of the time. In order to receive500 Kbits/s, the receiver can be turned ononly 500/10000 or 5% of the total receiveduty cycle, which will offer a power con-sumption reduction with the same ratio.This calculation is theoretical and thesynchronization time of the receiver must be

added to the total “on” time in order tocompute the power reduction. It is assumedhere that a 90% reduction of the totalon-time power consumption is achievable.The latest DVB-T front-end (tuner plusdemodulator) technology consumes approxi-mately 500 mW). Therefore, a powerconsumption of 50 mW for the completeDVB-H front-end may be calculated in orderto receive 500 Kbits/s.

In addition to reducing power consump-tion, time-slicing supports the seamlessservice hand over between transmitters.The receiver can scan for other availableRF channels while maintaining the currentservice to the user and switch to a new cell

Figure 2. DVB-H sources and encoding.

8/8/2019 504rfd3x



if the signal received from this cell hasa better signal level and provides the sameservice.

IP datacastingDVB MPE[3] was adopted as part of

the specification for video streaming and datatransfer for use in DVB-H systems.Transmission of IP packets over MPEG-2

is based on digital storage media commandand control (DSMCC)[2] sections that aretypically not repeated. The transparenttransmission of IP is accomplished byencapsulating the entire IP packet payloadwithin the payload of a DSMCC section andby mapping the MAC address to the respec-tive header and payload fields in the DSMCCsection. The section format permits fragment-ing datagrams into multiple sections. If the length of the datagram is less than orequal to 4086 bytes, the datagram is sent inone section. In the case of IP, the maximum

frequency of this fading will depend on theDoppler effect the receiver is undergoing.The lower the Doppler frequency, thegreater the time-interleaving that is neededin the system. The time-interleaving thatDVB-H offers is flexible and can be adapted

to the service. Typical time-interleavingperiods can be as high as 500 ms and as lowas 50 ms. These periods can be even lowerbut then it does not make sense at thesystem level.

After time-interleaving, a Reed-Solomonencoder is applied in order to protect thedata. The code rate is flexible and can also beadapted to the service. A typical code ratemay be ¾ but rates as low as ½ or as high as7 / 8 may be used.

Figure 4 shows the advantage thatMPE-FEC brings to the system. The Dopplerfrequency is given by the x-axis while they-axis shows the required carrier-to-noiseratio (C/N) that is required to reach a certain

transmission unit (MTU) is set to 4086 bytesor less so that the datagrams will never befragmented. The MAC address has beendivided into 6 bytes that are located in twogroups[3]. Bytes 5 and 6 are mapped to thetable_id_extension field of the DSMCC

section, while bytes 1, 2, 3 and 4 are mappedto the payload area of the DSMCC section.This mapping was done to utilize thelimited capabilities of the first generationof demultiplexers.

IP video streaming to devices with smalldisplays is ideal for providing low-resolution,low bit-rate data transfer.

MPE forward error correction(MPE-FEC)

MPE-FEC was added to the DVB-H speci-fication in order to implement time-interleav-ing and error correction. In a mobile environ-ment, the signal will be subject toattenuation and, therefore, fading. The

Figure 3. DVB-H demodulator block diagram.

8/8/2019 504rfd3x



is an IP stream that is sent to the deviceapplication processor. The applicationprocessor is typically responsible for thedecoding and rendering of video and audioand the integration of these services withinthe context of the device type, such as a

cell phone or personal digital assistant. RFD

References1. ETSI EN 302 304 V1.1.1 (2004-11)

Digital Video Broadcasting (DVB); Trans-mission System for Hand-held Terminals(DVB-H).

2. ISO 13818-6 Extension for DigitalStorage Media Command and Control.

3. ETSI EN 301 197 DVB Specificationfor Data Broadcasting.

QoS. The blue curve shows the performancewithout MPE-FEC while the red one showsthe performance with MPE-FEC and at acode rate of ¾.

One can see from the graph that the C/Ncurve is flat up to a Doppler frequencyof 120 Hz. This result helps simplify thenetwork planning for the broadcasters.

4K modeThe 4K mode was introduced (in addition

to the existing 2K and 8K modes) in orderto offer a better compromise between

Figure 4. C/N vs. Doppler frequency.

ABOUT THE AUTHORS

Stuart Pekowsky holds a BSEE fromArizona State University and MSEE fromNew York Polytechnic University. Hehas been a hardware and softwaredeveloper for the development of such diverse projects as air combatmaneuvering systems, real-time stock market information systems, GSMtelephone handset and switchingsystems and DVB playout and receiversystems. He is currently new businessdevelopment manager for DiBcom S.A,

providers of silicon solutions for mobileand portable digital TV reception. Hecan be reached via e-mail [email protected].

Khaled Maalej graduated from theEcole Polytechnique, Palaiseau, France,and received an advanced degree intelecommunications from the EcoleNationale Supérieure des Télécommun-ications in Paris. He spent five yearsat SAT/Sagem, within the signal pro-cessing group where he co-developedseveral integrated circuit designs and

optimization code for satellite andcable network reception. He participatedin the design of the first QAM digitaldemodulator for DVB-C receivers. Hewas also technical manager of thebroadband communication businessunit within the Atmel Corporation andparticipated in the design of a 1024-QAM demodulator in which he concen-trated on the carrier synchronizationand equalization. He is currently CTOof DiBcom S.A., providers of siliconsolutions for mobile and portabledigital TV reception. He can be reached

via e-mail at [email protected].

mobility and network robustness in termsof echoes. For the same useful bit rate,the 8K carrier number doubles the level of protection of the system against echoes as

compared to 4K.Of the existing DVB-T networks, 90%

utilize the 8K mode. It is field proven andoffers the right performance for Doppler andecho protection when used in the ultrahigh-frequency (UHF) band.

Application processingThe output of the DVB-H demodulator

Circle 28 or visit freeproductinfo.net/rfd

Documents

504rfd3x