JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Broadcast Systems, Digital Video Broadcasting (DVB), Digital Terrestrial Television Broadcasting (DTTB)


Synchronization for Digital Terrestrial Television Broadcasting

Contributed by Paul G.M. de Bot and Flavio Daffara

Reliable reception of DTTB signals is only possible if good frequency and time synchronization is achieved. To enhance synchronization performance, the DFT blocks, or OFDM symbols, are organized in frames. In the European Digital Video Broadcasting (DVB) project, a frame containing a number of 96 OFDM symbols is proposed, including a silent period (null symbol). This null symbol is used for coarse time synchronization. Immediately after the null symbol, a reference symbol having good autocorrelation properties in the frequency domain is transmitted. This reference symbol is used for coarse frequency synchronization. Since the transmitted content of the reference symbols is known a priori at the receiver side, the receiver is able to estimate the frequency domain transfer function. Hence, the receiver can calculate the time domain transfer function, and is consequently able to determine the optimal observation window position (fine time synchronization). The reference symbol in its turn is followed by a Transmission Parameter Signalling (TPS) symbol, which contains transmission mode information, such as the used signal constellation and convolutional code rate. The TPS symbol is modulated and protected such that it can be received even under very bad channel conditions.

Oscillator Phase Noise

Since oscillator stability of the receiver can be a limiting factor, the frequency synchronization needs to be extremely accurate. A small frequency error causes a fixed rate of phase rotation in each QAM signal, or cell, as well as cross-talk between the subcarriers. To support more accurate frequency synchronization, the remaining 93 symbols in a frame contain a certain amount of pilots, which do not contain data and from which the receiver can estimate the actual frequency offset. In order to minimize the cross-talk, this frequency error signal is being fed back and compensated for, prior to the DFT. Since OFDM is very sensitive to frequency jitter and phase noise, the local oscillator in the receiver front-end needs to have a very high frequency accuracy.

Example

In the frame structure described above, 93 of the 96 OFDM symbols in a frame are used for data transmission. However, in these 93 symbols, about 12.5 % of the carriers are used to transmit pilot symbols. Hence, the net symbol rate equals about (93/96)(7/8) times the gross symbol rate.

For a DTTB system with the above parameters the symbol rate is 5.14 Mbaud.

Channel Estimation

Channel amplitude and phase estimation are required for enabling coherent detection of the QAM signals, and for providing reliability information to the soft-decision Viterbi decoders. This channel estimation is performed by using the reference symbol and pilot cells. To obtain information about the frequency domain characteristics of the CCI the null symbol is analyzed. More accurate estimation of the S/(I + N) ratio in each subcarrier can be obtained by examining the statistics of the received signal.



JPL's Wireless Communication Reference Website © Paul G.M. de Bot, Flavio Daffara and 1993, 1995.