The challenges of DVB-H mobile TV design and how to deal with them - Embedded.com

The challenges of DVB-H mobile TV design and how to deal with them

There are at least seven existing and emerging mobile TV standards(Table 1 below ), excluding thecellular standards Multimedia Broadcast/Multicast Services (MBMS) and Broadcast/MulticastServices (BCMCS). DVB-H builds on its backwardcompatibility with DVB-T, which allows DVB-Htransmission on existing DVB-T channels.

Table1: There are at least seven existing and emerging mobile TV standards.

Significant additions to DVB-H are the 4k FET mode, in-depthinterleaving for 2k and 4k orthogonal frequency division multiplexing (OFDM ),another forward error correction layer for multiprotocol encapsulateddata (MPE-FEC) and time-slicing to save power. Commercial service hasstarted in Italy and more are planned in other countries in 2007 and2008.

Forward Link Only (FLO)technology, on which MediaFLO is based, was developed by Qualcomm andis planned to be used in North America. Similar to DVB-H, FLO minimizespower consumption through time-slicing.

Terrestrial Digital Multimedia Broadcasting (T-DMB), which was developed andintroduced in South Korea, is based on the Eureka 147 DAB standard.

The Korean mobile satellite system S-DMB is one of few systems notbased on OFDM; instead it uses code division multiplexing (CDM) operating at 25MHz wideS-Band channel.

Japan started providing mobile TV based on ISDB-T in 2006. The standard hasone segment of a 6MHz channel being used for mobile applications. Chinarecently released its own standard for mobile broadcast called ChinaMultimedia Mobile Broadcasting (CMMB).

A key component of CMMB is Satellite Terrestrial InteractiveMultiservice Infrastructure (STiMi), an OFDM-based PHY. Chinaalso announced the adoption of Digital Multimedia BroadcastTerrestrial/ Handheld (DMB-TH) DTV standard, which can be used forhandhelds.

Last February, the DVB Steering Committee approved the SatelliteServices to Handhelds (DVB-SH) specification.

DVB-SH is designed to deliver services to mobile devices via ahybrid satellite- and-terrestrial network. It combines satellitedirect-to-mobile transmissions and terrestrial repeaters.

It is available in two flavors: SH-A, in which the satellite andterrestrial links are both COFDM-based; and SH-B, which uses TDM forthe satellite link and COFDM for the terrestrial link.

Figure1: Zero-IF direct conversion receivers have become the mainstreamarchitecture for DVB-H as they offer the least amount of externalcomponents and reduce power consumption.

Global dominance
DVB-H is currently considered the globally dominant standard for mobileTV. Zero-IF direct conversion receivers (Figure 1 above ) have become themainstream architecture for DVB-H because they meet specifications forthe analog and RF section of the receiver by offering the least amountof external components and reducing power consumption.

Low-IF architectures as an alternative suffer from stringentimage-rejection requirements caused by the N±1 blocker.

The use of advanced CMOS technologies enables various mixed-signalcalibration and circuit techniques to overcome the shortcomings ofzero-IF direct conversion architectures, such as the DC-offsets.

The high bandwidth and large number of sub-carriers used for DVB-Tand DVB-H allow for the reduction of several kilohertz of spectrumaround DC without noticeably degrading performance.

Furthermore, recent products have shown it is possible to integratethe LNA on a pure CMOS tuner despite the wide input frequency rangerequired for DVB-H. System noise figures below 3.5-4dB at the RFreference point are feasible, achieving the needed sensitivity.

Mobility, integration issues The mobile nature of the terminals maycause fast time-varying channels and Doppler frequency shifts of thereceived signal and its echoes.

Receiver performance under these conditions is largely determined bythe modulation used and protection offered by the standard, as well asthe performance of the signal processing algorithms.

DVB-H is able to address these impairments and offers a variety ofconfigurations to adjust performance trade-offs to actual requirements.The impact of the analog tuner section is typically small provided agood AGC concept and sufficient C/N headroom are available.

The recovery of the mobile TV signal in mobile environments issubject to various impairments that include specific system integrationissues.

Considering the wide bandwidth of a UHF receiver along with the sizelimitation of the antenna for the mobile terminal, the antenna designis quite challenging. If the antenna is designed for broadband, it willnaturally have a relatively low realized gain cutting deeply into thelink budget. Although low-noise receiver designs can alleviate thisproblem, the antenna is still the best “low-noise amplifier” in thesystem.

Hence, it makes sense to look for alternative antenna approaches.Tuned or resonant antennas can exhibit significantly betterperformance. However, nonlinear tuning elements, such as varactordiodes, can degrade the wanted signal in the presence of the strong GSMtransmitter signal, since the isolation between cellular and DVB-Hantennas is only about 10-20dB.

Blocking is another design challenge on several levels. The widespectrum that a mobile TV receiver needs to cover translates to apotentially high number of unwanted signals that need to be addressedby the receiver's dynamic range.

This requires high linearity in conjunction with a good noisefigure, as well as a wide-band RF gain control loop to protect themixer and succeeding stages from being saturated by far-off blocker.

The first two requirements ultimately set the power consumption ofthe analog receiver section. Hence, single-frequency applicationsrequiring somewhat less linearity have the advantage of being furtheroptimized with respect to power consumption and noise figure.

Receivers are typically designed to fulfill all linearity and noisespecifications even though practical situations may not present theseconditions.

As an example, if the wanted signal is high enough or if nointermodulation scenario is present, the receiver's power consumptioncan be reduced significantly. This offers another way to save power orsmart algorithms if the right handles are provided in the analogsection.

Figure2: The GSM transmitter presents a serious additional blocker besidesthe in-band analog and DTV blocker.

GSM Tx blocker
As shown in Figure 2, above ,in GSM, the transmitter (GSM900 Tx) presents a serious additionalblocker besides the in-band analog and DTV blocker. If the spectrum isused up to 750MHz, the distance to the GSM Tx is only about 130MHz.This also implies that the antenna isolation is quite small, in theorder of 10-20dB, depending on the implementation.

The GSM Tx affects a mobile TV receiver in two distinct ways. First,it is a blocker transmitted with +33dBm. If we assume about 10dBantenna isolation, the blocker at the input terminal is still in theorder of 3dBm.

For this reason, a GSM rejection filter needs to be placed directlyafter the antenna to lower the GSM Tx level. Even with the 50dBrejection, the input power to the receiver would still be in the orderof -27dBm. Together with an in-band blocker halfway between the GSM Txand wanted signal, this can create a severe intermodulation scenario.To alleviate these problems, new devices such as the TUA9000 add extraGSM Tx attenuation in particular for wanted frequencies below 700MHz.

The second way GSM Tx affects the mobile TV receiver is through thePA noise. If the typical noise power caused by the PA in the DVB-Hfrequency range is, for example, around -85dBm measured in 100kHzbandwidth, then about -76dBm noise in an 8MHz channel is seen at the RFreference point considering about 10dB antenna isolation.

This noise cannot be filtered by the mobile TV receiver. Instead, ithas to be filtered by inserting a high-pass filter after the GSM Tx, asshown in Figures 1 and 2. Some 30dB attenuation is required in thiscase. This, of course, will eat into the precious Tx power budget ofthe PA, reducing the PAE.

The wide frequency range of DVB-H also increases susceptibility toother interference sources, such as application processors or othercomponents in the system whose clock and processing frequencies mayfall right into the DVB-H frequency range.

With careful system and application design, DVB-H can enable theintegration of power, area and cost-efficient terminals, enablingmobile TV, a great new feature for mobile devices.

Michael Flath is Senior StaffEngineer, Tuner Devices, at Infineon Technologies AG. To read a PDFversion of this story, go to “Addresschallenges in DVB-H receiver design.”

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