为移动终端提供低功耗、即时数位广播电视信号

Thanks to the DVB-H standard and some clever engineering in TV tuner and demodulator techniques, handset designers can enable their new designs to receive reliable, high-quality broadcast TV signals.

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Multimedia capability in mobile handsets has grown to include data, audio, video, and now real-time, digital TV programming. Currently, there are three approaches to delivering a TV signal to a hand-held device (Table 1): TV over cellular, TV over satellite and TV over broadcast. Of these, the latter holds the greatest promise for delivering high-performance, low power, real-time digital broadcast television to mobile handsets.

TV over cellular is the technology choice for MobiTV® (Sprint, Cingular, Midwest Wireless and Verizon International) and Verizon’s V-Cast in the United States. However, this approach is limited by low resolution and low frame rates, which result in jerky, ‘stop-and-go’ motion. Even more important, high-quality television requires around 10 times the data rate compared with voice service. So, if many users were to use TV services simultaneously, the cellular network could easily run short of capacity causing dropped calls or “network busy” error messages to occur. This means that, ultimately, cellular networks will not be able to provide a range of TV services to each user. Furthermore, TV services typically carry a monthly subscription fee of about $10 while fees for voice services are often several times more.

TV over satellite has been implemented in trials and early rollouts in Korea. Based on the satellite digital multimedia broadcasting (S-DMB) standard, this approach offers improved video quality and avoids increased congestion on the cellular network, but is fundamentally hampered because it requires a direct line-of-site to the satellite. As a result, users of this technology cannot receive TV signals when obstructions such as bridges come between themselves and the satellite. Even receiving the signal within a building is problematic. To supplement the satellite’s signal, a network of terrestrial repeaters can be used to improve coverage, but, in that case, it may be more economical to deploy a broadcast TV network.

TV over broadcast, based on the open digital video broadcasting-hand-held (DVB-H) standard is just such a broadcast TV network and allows for high-quality, real-time, full-motion television. It offers greatly improved quality as compared to TV services over cellular networks, and it does not impact network capacity for voice services.

DVB-H: method for broadcasting television to mobile handsets

As a result, the DVB project drafted and ratified the DVB-hand-held (DVB-H) standard, a superset of the DVB-T standard, in April 2004. DVB-H addresses high-speed, high data-rate reception of IP-based services for hand-held devices, which have unique power, screen size, coverage and reliability requirements. The standard also introduces a number of features and modifications, among them time-slicing and forward error correction techniques, to enable TV reception in mobile, hand-held devices.

In November 2004, DVB-H was adopted as a European Telecommunications Standards Institute (ETSI) standard. Since then, it has attracted significant worldwide interest and has led to heightened industry activity, including product and service development, launches and field trials.

Tuner technical issues

Designers adding high-quality broadcast TV capability to hand-held devices need to overcome technical challenges, many of which are related to the tuner. The key RF technical challenges of a tuner when receiving broadcast digital TV signals in a hand-held, battery-powered device include frequency response, interference, power and maintaining low cost.

Frequency response: A cellular receiver has to process only a very narrow range of frequencies. For instance, a typical GSM receiver only needs to cover 25 MHz or 30 MHz. In contrast, the first DVB-H tuners will need to tune across the UHF band IV-V frequencies 470 MHz to 890 MHz range with 6 MHz, 7 MHz or 8 MHz channel separation in Europe/Asia and/or the L-band 5 MHz channel 1670 MHz to 1675 MHz range in North America (Figure 1).

Such a broad frequency response affects two major sections of the receiver design: frequency generation, or, more specifically, the phase-locked loop (PLL) synthesizer, and the amplification blocks. The main issue for the PLL synthesizer is the fundamental ability to generate the different frequencies while maintaining the small form factor and low-power consumption required for portable devices. Another major concern is meeting lock time requirements — how long it takes to acquire the frequency. For the amplification blocks, the major design concern is susceptibility to a broader range of interfering signals, which can easily result in distortion.

Interference: Cellular phone receivers operate in narrow frequency bands, typically 25 MHz to 30 MHz. To reduce interference in these systems, designers can put a filter in front of the receiver and effectively eliminate potential interferers across a broad spectrum. In contrast, a TV receiver has a wide band. As a result, it is not possible to use a single fixed filter and reject the interferers. In stationary TV sets, this was overcome with specialized filtering techniques and/or increasing linearity significantly, and the resulting tuner consumed >1 W of power.

For the DVB-H standard, the tuner must select either a single 5 MHz, 6 MHz, 7 MHz or 8 MHz channel out of the entire band of channels (example: countries using an 8 MHz channel separation will have a total of 48 channels present in the passband of the tuner for UHF band IV-V). The rest of the frequency spectrum in the UHF band IV-V will be made up of an unknown combination of analog (ex. PAL), and/or DVB-T TV signals at potentially large amplitudes relative to the desired DVB-H signal.

All of these other TV signals are seen as interfering signals in the receiver (Figure 2). Depending on where the cellular subscriber is, the receive system will need to adjust and be able to pick up the maximum signal level, which could be -25 dBm or maybe even higher, as well as picking the minimum signal level right out of the noise floor, then amplifying it, and passing it through the system in the presence of many other distorting signals that could be much stronger than the desired minimum signal level signal. The relative undesired signal power to the desired signal power ratio can be >45 dB and as much as 56 dB for certain conditions. This means that the receiver needs to be able to amplify the desired signal in the presence of an interfering signal that is >45 dB higher.

To meet this challenge, tuner designers have had to develop some new techniques. At Microtune Inc., the company calls its process ClearTune™ technology, which was developed out of the company’s expertise in cable set-top box, traditional television, and automotive TV systems (Figure 3).

Microtune’s patent-pending ClearTune technology provides significant immunity to interfering signals. This technology will provide immunity to in-band interfering signals within the UHF bands IV-V and add to the rejection of any external filtering that may be developed for the input of the chip to be placed on the printed circuit board. It is noteworthy to mention that this technology will significantly reduce the impact of the cell phone GSM 900 MHz power amplifier signal that generates a blocking signal inside the phone that will be a major interferer for the tuner. This technology will not add any benefit for close-in interfering signals but for far-away interfering signals (i.e., >50 MHz), an aspect that is overlooked in the industry. This technology will show its benefit in real-world applications where the spectrum is not controlled as in laboratory settings.

Power: While >1 W of power in a tuner is considered low for a stationary television, it is not low enough for use in a handset. The challenge, then, is to handle the interferers of broadcast television by using a limited amount of power. The DVB-H standard begins to address this challenge with its “time-slicing” technique. When this technique is combined with careful block-by-block optimization, it is possible to achieve adequate linearity without sacrificing the power budget.

DVB-H takes on power

In order to support a range of TV content without severely draining the battery or impacting the ability to receive cellular calls, DVB-H uses a technique called “time slicing,” which allows up to 10 hours of TV viewing on a single battery charge. With time slicing, each TV program is broadcast at a different point in time, so, when a user selects a program, the handset only receives that TV signal and can power down in between transmissions of that channel’s content.

With time slicing, the terrestrial transmitter is always on, sending different programs that are staggered in time. First it broadcasts program No. 1, then No. 2, then No. 3, up until n programs, then the cycle repeats. The receiver knows when its program is being transmitted and is only turned on when the desired program is being sent. By doing this, the receiver can be powered down for the majority of the time (Figure 4).

For example, the standard channel bandwidth in a European DVB-H system is 8 MHz, and the expected modulation type is 16 QAM. This bandwidth and modulation scheme results in about a 30 Mbits/s raw data rate. After accounting for Reed Solomon encoding, interleaving, forward error correction, and so on, the remaining data “payload” that is left is approximately 10 Mbits/s. (In the United States, the 1670 MHz to 1675 MHz system has a 5 MHz channel width, so the result would be approximately 6 Mbits/s.)

Keep in mind that a hand-held screen measures approximately two inches to four inches (5 cm to 10 cm) and features one-quarter VGA (320 × 240) resolution. This means that it requires approximately 320 kbits/s to receive good quality audio/video. Therefore, with time slicing, it is possible to get up to 27 programs multiplexed (multiple programs staggered in time) with the tuner selecting only the desired program, turning on only when needed, and resulting in a savings of up to 90% of the power consumption. A system with a 5 MHz channel width then could accommodate about 15 multiplexed programs.

While the time-slicing technique reduces power consumption, it does not directly solve the problems of interference, both from other transmissions in the bandwidth or from the nearby cellular power amplifier (PA). Time slicing, however, does make it possible for tuner manufacturers to develop a device that addresses these interferers, meets the mobile and portable DVB-T/H radio access interface specification (MBRAI) DVB specification, and works within the power budget.

DVB-H takes on interference

In addition to time slicing, the use of coded orthogonal frequency division multiplexing modulation (COFDM) in DVB-H is of great importance. COFDM uses subcarriers that are responsible for transmitting small amounts of information. So, if one carrier is lost or destroyed during transmission due to some type of interferer, then only a small amount of the signal payload is lost. The more subcarriers used, the greater the immunity to interference.

In traditional COFDM designers could select from 2000 or 8000 subcarriers. With the DVB-H standard, system designers can select 2000, 4000 or 8000 subcarriers. Using 8000 carriers offers greater resistance to interference, but with the 2000 carriers, there is greater protection from Doppler shift because the frequency spacing between the subcarriers is larger. Doppler shift is crucial for mobile TV signals, because subscribers will want to receive signals while they are in fast-moving vehicles. So, DVB-H has to be functional at autobahn speeds of 100 mph to 120 mph or perhaps even higher. As a result, early designers of the DVB-H standard proposed 4000 carriers as a compromise between the two original COFDM scenarios.

Table 1. Techniques for delivering television to mobile handsets. TV Over Cellular Example: MobiTV
Low-quality, low frame rate, ‘stop and go’ motionTV Over Satellite Standard: S-DMB
Improved quality, but depends on line-of-sight transmissionTV Over Broadcast * Open Standard: DVB-H
* High-quality, full-motion real-time broadcast
* Superset of DVB-T digital standard
* Supported by handset manufacturers (Nokia, Motorola, Samsung); IC suppliers (Microtune, DiBcom, Freescale, Philips, TI) and operators (Cingular, Crown Castle, France Telecomm, O2, T-Mobile, T-Systems)

Designers of DVB-H systems, then, have a lot of choice and flexibility. They can select which level of COFDM subcarrier they want to use. And, they can select from several different code rates, guard intervals, and modulation types (including QPSK, 16 QAM and 64 QAM).

Cost: For mobile TV functionality in portable devices, cost is essentially a bill of materials issue. By selecting a highly integrated solution, handset designers can save on cost, footprint and reduce the number of required external components.

Most current tuner offerings for DVB-H include a discrete design that uses an external low-noise amplifier (LNA) and transformer balun. Microtune recently introduced a single-chip integrated silicon tuner, the MT2260, that requires neither and can be used to receive broadcast DVB-H TV signals in either the 470 MHz to 890 MHz range in Europe and Asia or 1670 MHz to 1675 MHz range in North America (Figure 5).

Interface to demodulator

When selecting a tuner, it is important to consider complete bill of materials (BOM) costs, footprint and flexibility in handling the two frequency ranges in use for DVB-H. In addition, it is important to select a tuner and demodulator that can interface with each other. For instance, some key questions include: Can the tuner provide the intermediate frequency (IF) that the demodulator requires? Can the designer direct couple or ac couple? How is the automatic gain control (AGC) circuitry functioning between the demodulator and tuner?

By selecting a direct-conversion architecture, it is possible to simplify the receiver design by eliminating the need for external filtering. In a direct-conversion scenario, the tuner converts the desired channel down to 0 Hz and passes it directly to the demodulator. This allows for a streamlined demodulator design that does not need on-chip frequency downconversion and only needs to handle low sampling frequencies (for low power consumption).

Emerging DVB-H marketplace

Participants in the DVB-H standards development process continue to work together to ensure interoperability of their devices and to promote new services using this open global standard. Successful launch and uptake of DVB-H requires mobile phones with DVB-H functionality, available programming content, content aggregators, and cellular operators that support the service. As progress continues on the technological and product front, there is a growing need for content providers and companies that can supply the infrastructure and engineering services required to deliver the content to the cellular handset. Many of the major broadcasting companies are working to develop content, and infrastructure and engineering service companies in Europe, Asia, and North America are already heavily immersed in the technology.

In the meantime, designers looking to integrate broadcast television into their mobile designs already have tuners available to them that were specifically optimized to satisfy the DVB-H standard and the unique concerns of mobile television.