JPL's Wireless Communication Reference Website

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


Quadrature Amplitude Modulation

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

One can modulate one complex symbol in each time slot on each useful carrier. These symbols are elements of a symbol set. If we put all the symbols of a symbol set in a complex plane, we obtain what is called the signal constellation. Typically, a signal constellation contains M = 2^m signal points, which means that each symbol carries m bits of information. Hence, M should be large to obtain a large transmission rate. On the other hand if M is large, the required signal-to-noise ratio to obtain a desired error rate is also large. The choice of a signal constellation is made according to a trade-off between transmission rate and required signal-to-noise ratio.

For Digital Terrestrial Television Broadcasting, DTTB, usually three modulation schemes are considered; 4-PSK (or 4-QAM), 16-QAM and 64-QAM, with M = 4, 16 and 64, respectively. To obtain a sufficiently low error rate for these modulation schemes we need signal-to-noise ratios (E_s/N_0) on the order of 6 dB, 12 and 18 dB and dB, respectively.

Signal Constellations

Signal constellations with added noise at critical SNR values are depicted below.


Figure: 4-QAM signal constellation with AWGN at E_s/N_0 = 6 dB.


Figure: 16-QAM signal constellation with AWGN at E_s/N_0 = 12 dB.


Figure: 64-QAM signal constellation with AWGN at E_s/N_0 = 18 dB.

The bits are usually mapped into the constellation points using Gray mapping. In this case, an error event will cause a minimum number of bit errors.

Digital Terrestrial Television Broadcasting Modulation

Digital Terrestrial Television Broadcasting (DTTB) will use OFDM transmission with QAM subcarrier modulation. Some DTTB proposals foresee the option of hierarchical transmission. In this case, the modulation and error protection are organized such, that at the receiver side, different bit streams can be extracted from the received signal, each with a different a priori reliability. For example, we can use non-uniform QAM signal constellations, transmitting m_HP high priority (HP) bits and m_LP low priority (LP) bits per symbol. On bad channels, a receiver will only be able to recover the HP bits reliably, while on a good transmission channel, also the LP bits will be detected with a low error probability.



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