Video goes mobile
DVB-H test techniques promise to bring reliable TV reception to handheld devices.
Simone Gerstl, Rohde & Schwarz -- Test & Measurement World, 6/1/2005
DVB-H (see, "Digital video terminology") is ideal for mobile television transmission, as it meets the two main challenges that occur in mobile television:
- First, DVB-H ensures stable reception even in difficult environments by incorporating additional error correction called MPE-FEC, which is not available in DVB-T. MPE-FEC operates at the Internet Protocol level to allow secure reception even if many packets are lost.
- Second, to extend a mobile receiver's battery life, DVB-H cuts power consumption by as much as 90% compared to DVB-T; to do that, it employs time-slicing technology that transmits data in bursts instead of continuously, allowing the receiver to switch off between those bursts.
Another enhancement to the DVB-T standard is the 4k modulation mode in DVB-H (Figure 1). This compromise between 8k (which allows big single frequency networks [SFNs] but only at limited speed) and 2k (which allows very high speed but only on small SFNs), ensures stable mobile reception at high speed. (In this context, the "size" of an SFN refers to the maximum distance between two transmitters within the SFN.) Transmission parameter signaling (TPS) bits indicate to the receiver whether DVB-H-specific features are used and, if so, which ones.
| Figure 1. a) A 4k modulation scheme represents a compromise between 2k and 8k versions. b) TPS signaling indicates DVB-H-specific features. |  |
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DVB-H network setup
As DVB-H is based on DVB-T, the network setups are similar. A playout center (Figure 2) consists of a multiplexer that has MPEG-2 transport streams (TS) as inputs. A new feature in DVB-H is that at least one TS comes from a DVB-H IP encapsulator, such as the R&S DIP010, which encapsulates incoming IP data in order to pack it in a transport stream.
Figure 2. A playout center incorporating a multiplexer can combine DVB-H and DVB-T services.
The output TS of the multiplexer feeds the DVB-H-compatible modulator/transmitter, such as those in the R&S 7000 series, capable of supporting the 4k modulation mode and the DVB-H-specific TPS-bits. Because this technical setup is similar to that of DVB-T, broadcasters can combine DVB-T and DVB-H services within a network sharing one multiplexer. This makes it possible to offer DVB-H services using an existing network, thus lowering the cost for the operator and reducing the risk of investing in DVB-H technology. Especially for the trial networks, this is an advantage for network operators, who can use existing equipment instead of procuring new systems.
As new technologies evolve, the main demands for test and measurement equipment can be found within the R&D process. In the case of DVB-H, there are mainly two R&D applications: R&D for DVB-H chips and R&D for DVB-H-compatible mobiles.
DVB-H testingIn a setup for testing a DVB-H compatible RF chip within an R&D lab (Figure 3), a signal source provides a baseband signal (DVB-H stream) that feeds a test transmitter. From there, the RF signal goes into the device under test (DUT). The output signal of the DUT goes to a baseband DVB-H analyzer.
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| Figure 3. A setup for testing DVB-H-compatible chips can test multiple functions within a DUT, including MPE-FEC and demodulation stages. |
For testing the different functional groups within an RF receiver chip, the incoming signal can be varied. In principle, it is possible to test all the functionalities of a chip by changing the respective parameters in the input signal. At the signal-generation stage, for example, the MPE-FEC can be switched on and off in order to test the performance of a chip's MPE-FEC section. The test transmitter can change the transmission mode (2k, 4k, 8k) or make variations in the parameters (for example, modulation level), insert transmission noise, and simulate moving receivers by introducing fading. Thus, the receiver performance can be easily checked by means of a BER (bit-error-rate) measurement.
For testing the performance of a mobile terminal, you establish a real transmission by using a DVB-H test transmitter. To simulate real-life situations (Figure 4a), you add calibrated Gaussian noise to an ideal signal from the test transmitter and apply the combined signal to the receiver section of the mobile terminal under test. BER tests will show how well the receiver can handle those noisy situations.
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| Figure 4. a) To test a mobile terminal, you use a DVB-H test transmitter to establish a transmission and add calibrated Gaussian noise to simulate real-life situations. b) Closed-loop testing evaluates the reverse link. |
For a mobile terminal, you must use channel simulation, or fading simulation, to test how the unit behaves when it is on the move. Fading reflects the fact that transmitted signals reach a moving receiver over different signal paths. The receiver must cope with the fact that these paths have different levels and phases and also different delays. The test transmitter also performs the fading.
After checking the DVB-H implementation in the mobile terminal, you can use a closed-loop test to evaluate the reverse channel (Figure 4b). Combining a DVB-H test transmitter with a radio communication tester provides for the necessary transmission simulations. The circled numbers in Figure 4b show the sequential steps:
1. A DVB-H tester generates packets over the air.
2. A DVB-H mobile terminal's RF downconversion and demodulation function demodulates the test signal and issues demodulated packets to the terminal's mobile radio stage.
3. The radio transports packets over-the-air on the reverse link.
4. A mobile tester examines the loop-back packets and calculates packet error rate.
The DVB-H section of the mobile phone receives DVB-H signals from the test transmitter. An application running in the mobile phone initializes the wireless return path, over which it will transmit, for example, a summary of the data it has received via the DVB-H channel. A mobile radio tester receives and analyzes these signals. Communication with the DVB-H test transmitter closes the loop and allows the calculation of a closed-loop packet error rate. Inserting Gaussian noise into the DVB-H signal thus allows the test system to determine receiver limitations.
Network operator considerations
When running a DVB-H network (both pilot networks and real operational networks), an operator must be able to analyze the transmitted signal. For this purpose, you can use a test receiver to demodulate the DVB-H signal. The output baseband signal can then feed an MPEG-2 analyzer, which can carry out tasks such as analyzing the content and structure of the services and performing timing and data-rate measurements. In addition, an MPEG-2 analyzer can export the IP content of a DVB-H stream to an arbitrary IP address. An application for this is to play out the content in the service with an MPEG player or H.264 player.
One open question about DVB-H is exactly who the network operators will be, with business issues now surpassing technological ones as potential impediments (see "DVB-H business models"). Trials that are underway or are imminent will address the business questions, with effective solutions emerging to match the technological ones.
| Digital video terminology |
| Abbreviation | Term | Description |
| ASI | Asynchronous serial interface | A method for transferring digital TV data as a single-program TS. |
| DVB-H | Digital Video Broadcasting—Handhelds | |
| DVB-T | Digital Video Broadcasting—Terrestrial | |
| FEC | Forward-error correction | Algorithm to correct transmission errors on the receiving end. |
| IP | Internet Protocol | Specification of packet format and address scheme. |
| MPE | Multi protocol encapsulation | Methods to encode IP datagram stream onto TS. |
| MPE-FEC | Multi protocol encapsulation forward-error correction | |
| PSI/SI | Program specific information | Data required by the receiver to de-multiplex and decode the various programs in the TS (e.g., NIT, INT). |
| SFN | Single frequency network | |
| TPS | Transmission parameter signaling | Signaling of parameters related to the transmission scheme (e.g., to channel, coding, and modulation). |
| TS | Transport stream |
| Author Information |
| Simone Gerstl, Dipl. Wirtsch.-Ing., is a product manager for terrestrial transmitter systems at Rohde & Schwarz in Munich, Germany |
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