Friday, November 21, 2008

An overview of different digital modulation schemes

There are 3 main ways that digital TV is modulated and transmitted nowadays: 8VSB, QAM and OFDM. Here's a little bit of introduction into each:

I've talked about 8VSB quite a bit on this blog. It is 8 level Vestigial SideBand modulation. Reducing it to its most basic description, you take a carrier and amplitude modulate it with a square wave that has 8 different legal amplitude levels. The result is a tremendously wide double-sideband signal. You then pass that signal through a Nyquist filter that reduces the signal down to 6 MHz of bandwidth and that's what 8VSB is. In actual fact, most modern 8VSB modulators don't actually work that way. Instead, they directly synthesize the equivalent waveform instead, but the net result is the same. The symbol rate of 8VSB is about 10 megabaud. And this is fundamentally why 8VSB is vulnerable to problems with multipath. There are 10 million symbols per second, meaning that the sample interval is only 100 microseconds long. In general, the shorter the sample interval is, the tighter the tolerances are.

QAM stands for Quadrature Amplitude Modulation. With QAM, you take a carrier and amplitude modulate it and phase modulate it at the same time. It's thus a combination of AM and FM that happen simultaneously. As with 8VSB, there is a sampling rate when the receiver must determine where in two dimensions the signal exists. In general, there are 2^n different amplitude and phase shift values in a "square" QAM constellation that has n different points on each axis, which yields n bits per baud. The more complex the constellation, the lower the symbol rate can go for the same bit rate. But the more complex the constellation, the higher your requirements for S/N become so that you can distinguish the different points from each other. Most cable companies run QAM-256, which is quite a complex constellation - each point encodes 8 bits of data. They can get away with 38 MB/s in a 6 MHz channel because of the lack of multipath and the high S/N ratio typical of cable delivery. You can run QAM over the air, but you would typically do so with only a 16 point constellation, which would net you a much lower channel bit rate than QAM-256, all else being equal. That said, if you were to attempt QAM-16 at ATSC's 19 MB/s data rate, the baud rate would be lower than 8VSB because each baud encodes 4 bits of data instead of only 3.

You might ask about 8VSB and its "constellation." With 8VSB only the amplitude of the signal is used to encode information. By coercing the waveform into a narrow bandwidth, we must give up any semblance of control over the signal's phase. As a result, when plotted on a constellation display, 8VSB's constellation consists of 8 vertical lines. Thus, each baud contributes 3 bits of information, which is why the baud rate of 8VSB must be so high. In theory, you could reduce both the baud rate of 8VSB and the width of the Nyquist filter at the same time, but doing so would make for a mode that wouldn't be compliant with ATSC specifications. By contrast, the DVB specifications incorporate many different modulation schemes, going all the way from 5 to 8 MHz wide.

The third method for delivering bulk digital data over RF is OFDM. OFDM stands for Orthogonal Frequency-Division Multiplexing. OFDM is a divide-and-conquer scheme. The incoming datastream is divided amongst a number of relatively closely spaced RF channels, each of which is either sent with traditional Phase Shift Keying or perhaps a low constellation QAM mode (PSK, however, at its heart is simply a variant of QAM - without any information on the amplitude axis). Because each individual carrier only has a small portion of the data, the resulting baud rate for each carrier is quite low. It does, however, complicate the receiver quite a bit, because it has to be able to receive and decode a number of digital streams in parallel. This is actually less of a big deal than it might seem, however, since it is relatively routine for multiple receivers to operate within a single chip. For example, there exist single-chip multichannel GPS receivers. And, of course, multiple methods exist to synthesize an OFDM "fugue," so to speak. The big downside of OFDM, however, is in its transmitter linearity and overhead requirements (particularly because of its much higher peak-to-average ratio), and its increased S/N requirements for receivers.

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