NOTE: This is the third article in a multi-part series on MPEG-2.

Converting the MPEG-2 transmission from the high bandwidth satellite signal to a lower bandwidth cable signal can best be accomplished by a combination of Quaternary Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM).

What is QPSK?

Quaternary Phase Shift Keying (QPSK) is a signal transmission technology that is ideal for satellite transmission, with low power transmissions that are compensated by high gain antennas. Because noise is the biggest weakness of satellite transmissions, QPSK is well suited, as it is highly noise resistant.

QPSK works by using two carrier signals, each separated by 90 degrees of phase, providing maximum phase separation and delineation between signals. This distinct separation of signals allows for a 2:1 compression ratio, doubling the efficiency of the circuit and allowing for larger transmissions.

An example of QPSK transmission quality is below:

Figure 1A
Figure 1B
Figure 1C

In Figure 1A, you can see a clear QPSK signal. The dot in each quadrant is very defined and there is no leakage into other quadrants or symbols. The next figure represents a QPSK signal at threshold. Note how the dots are undefined, and leaking into other quadrants' symbols. The "hazy" dots are actually lost bits or errors, resulting in frequent screen blackouts and/or blocks and mosaic patters on the TV image. If this were an Internet data transmission, it would be unusable because of the high error rate. The third figure is a QPSK signal that is significantly below threshold.

The more defined the dot is in each quadrant, the better the signal and the fewer the errors. Conversely, the more hazy the dot is in each quadrant, the more unreliable the signal.

What is QAM?

Quadrature Amplitude Modulation (QAM) is a sophisticated modulation technique, using variations in signal amplitude that allow data-encoded symbols to be separated into equal quadrants. Derived from QPSK, QAM contains multiple symbols per quadrant. For example, a 128 QAM signal contains 32 symbols in each quadrant (Four quadrants times 32 symbols each equals 128). In the same way, a 256 QAM signal contains 64 symbols per quadrant. Thus, QAM is a very adaptable, robust modulation technique.
Below is an example of a 64 QAM signal (Four quadrants of 16 symbols each equals 64 total symbols).

Figure 2

In cable transmission, noise is much less a factor than it is with satellite transmissions. However, bandwidth is always an issue. The Multiple System Operator (MSO) or Private Cable Operator (PCO) wants to transmit the highest signal quality, while using the least amount of bandwidth. This can only be accomplished with QAM modulation, due to the phase modulation.

The robustness of the QAM signal, as compared to QPSK, is illustrated in the figures below.

Figure 3A
Figure 3B
Figure 3C

Note each illustration of the 64 QAM signal contains 16 dots in each quadrant. Figure 3A illustrates a perfect QAM signal, with very well defined dots. The second figure illustrates a QAM signal that is still very good, but the dots or symbols are hazier. In the third figure, the QAM signal has a very high noise ratio, but the signal is still above threshold, even through the dots seem to be leaking into the next quadrant. Because there are multiple dots in each quadrant, unlike QSPK, the majority of the dots are not seeping into the next quadrant. This keeps the signal above threshold.

The Benefits of QPSK-QAM Transmodulation

The single biggest drawback of upgrading an existing MDU property's Satellite Master Antenna Television (SMATV) system is the cost and inconvenience of recabling. QAM allows the MPEG2 data stream to be seamlessly converted from a satellite signal to a cable signal and transmitted over the existing network. The signals are simply demodulated at the cable head end and then remodulated for cable distribution using QAM to reduce the bandwidth of the multiplexed signal to comply with cable's narrower bandwidth constraints. For example, DirecTV™ transmits most of its programming from 32 transponders using 1GHz of bandwidth over the satellite and to the DBS dish at the home. By converting these QPSK signals to QAM, it is possible to deliver all of the 32 transponders over the coaxial distribution networks using only 192MHz of bandwidth.

Some of the benefits to QPSK-QAM transmodulation are:
· The QPSK signal can carry more bandwidth. The bandwidth of a QAM MCPC carrier is approximately 8 MHz, compared to a QPSK MCPC carrier, which has a bandwidth of 33-36 MHz.
· The QAM signal can be inserted into any existing Master Antenna (MATV) system. It can use any part of the frequency band up to 860 MHz.
· In the QPSK/QAM conversion, the Network Information Table (NIT) and the Conditional Access system are not modified in any way, allowing for transparent modulation.
· The QAM signals can be inserted in the lower frequency range of a cable system, thus eliminating the problems of high frequency losses in broadband coaxial systems.
· QAM is expressly designed for cable network distribution of digital signals. QAM headends use Nyquist filtering to smooth out signal distribution problems, such as poor return loss and non-linearity.

Calculating the Transformation From QPSK to QAM
In transforming the signal from QPSK to QAM, the following conversion calculation is used:

2 X Symbol Rate QPSK X Code Rate (FEC)
Symbol Rate QAM = Bits Per Symbol

In converting a 64 QAM symbol, it would be as follows:

2 X 29473 X 3/4
Symbol Rate 64 QAM = 6

Therefore, the Symbol Rate from QPSK to 64 QAM is 7368.25.

Costs

Using the QPSK to QAM signal modulation, the existing cable system is able to deliver the maximum video signals to each viewer, without rewiring or upgrading the distribution plant. The only cost associated with this deployment is at the Headend. The typical cost of a QPSK to QAM transmodulator is $500. To calculate the Headend cost, multiply the number of satellite transponders by $500. With the combination of QPSK and QAM, costs are low and the signal's quality remains high - making it an appealing solution for providers and residents alike.