Time Slicing (DVB-H) in Mobile TV solutions
1). Comparison of DVB-H and DVB-T
Comparison of the reception of time-sliced DVB-H services with that of multiplexed DVB-T (in Figure)
§ The principle of time slicing is to transmit data in high-data-rate bursts corresponding to a single service rather than continuously multiplexed with other services. Between two consecutive bursts, no data is transmitted for the service.
2). Enhancement in Layers (Physical and Link) for DVB-H
2.1). DVB-H Link Layer Enhancement: (from DVB-T)
§ Time slicing: One mobile TV channel has a fairly low bit rate compared to the total bit rate of a multiplex. To reduce drastically the power consumption, the terminal is only turned on to receive data bursts containing streams of a selected TV channel. Thus, the receivers will be turned off most of the time but have to know when to turn on. To do that, each data burst indicates in its header the start time for the following burst of the same TV channel.
§ FEC Multi-Protocol Encapsulated–Forward Error Correction (MPE-FEC) improves C/N and Doppler performance in mobile channels and tolerance to impulse interferences, but its use is optional.
2.2). DVB-H Physical Layer Enhancement: (from DVB-T)
§ Transmisson Parameter Signaling (TPS): Some additional signaling bits have been introduced to indicate whether DVB-H services are present in the multiplex, and if MPE-FEC is in use.
§ FFT for 4K mode: This additional transmitter mode gives more flexibility for network design. Still, 2K and 8K transmission modes of DVB-T can be used to transmit DVB-H services.
§ Symbol interleaver: The option of an in-depth interleaver is available now for 2K and 4K modes. Thus, bits are interleaved over one, two, or four OFDM symbols, improving robustness and tolerance to impulse noise.
§ 5 MHz channel bandwidth: A DVB-H multiplex can be allocated outside traditional broadcasting bands using a frequency channel of 5 MHz.
3). Time Slicing Overview for DVB-H
§ Time slicing (mandatory for DVB-H) to reduce the average power consumption of the receiving terminal and enable smooth and seamless frequency handover.
§ It is claimed in the DVB-H literature that time slicing can reduce the power consumption up to 90 percent with respect to a non-time-sliced transmission.
§ From less optimistic conclusions, it still found reduced power dissipation of up to 80 and 50 percent on average.
§ IP datagrams are transmitted as data bursts in small time slots.
§ Each burst may contain up to two megabits of data (including parity bits).
§ There are 64 parity bits for each 191 data bits, protected by Reed-Solomon codes.
§ The front end of the receiver switches on only for the time interval when the data burst of a selected service is on air.
§ Within this short period of time, a high data rate is received which can be stored in a buffer.
§ This buffer can either store the downloaded applications or playout live streams.
§ Therefore, the inclusion of specific provisions in the technology itself so as to restrict the power consumption of the devices is required. This is the first purpose of the time-slicing data transmission technique that is implemented in DVB-H.
§ The second purpose of time slicing is to optimize handovers. Indeed, the mobility of the users of DVB-H devices introduces the requirement of cell handovers, that is, the ability to change the frequency and data stream to receive the same content in another radio cell.
4). Support of Handover
§ A service handover may also be required in the case where a neighboring radio cell provides a better signal for the same content than the current cell. To ensure a high quality of service, such handovers should be seamless, that is, have no noticeable influence on the quality of the display on the handheld device.
§ As the receiver finds itself in the “between burst” state most of the time, the handover process can be initialized almost as soon as it becomes necessary. The fact that it is always performed between bursts prevents it from disturbing data reception in any way.
§ The use of DVB-H signaling that provides a fast way to access signaling, and intelligent methods can be used to reduce the number of signals to check.
§ This ensures that the frequency-checking process only needs to last for a small fraction of the total time separating the transmission of two successive bursts, ensuring a lower bound to the potential loss in power saving.
5). Characteristics of the Bursts
§ The concept of time slicing is to send data in bursts using a significantly higher bitrate compared to the bitrate required if the data was transmitted continuously.
§ Within a burst, the time to the beginning of the next burst (delta-t) is indicated.
§ Between the bursts, the data of the elementary stream is not transmitted, allowing other elementary streams to use the bitrate otherwise allocated. This enables a Receiver to stay active for only a fraction of the time, while receiving bursts of a requested service.
§ If a constant lower bitrate is required by the mobile handheld terminal, this may be provided by buffering the received bursts.
§ The bandwidth of one burst is less than the total available bandwidth of the TS, other elementary streams can be transmitted during its on-time
6). A burst is defined by the following parameters
§ Burst size:
§ Burst bandwidth:
§ Burst duration:
§ On-time or burst duration: