As our society's hunger for data grows—not only more data, but more data delivered faster—older modulation schemes based on NRZ-type encoding grow increasingly inadequate. We need to get data from point A to point B as efficiently as possible, whether that means between chips on a PC board or from one end of a long-haul optical fiber to the other. A modulation scheme that's gaining favor in many quarters is PAM4, and in this post we'll look at the basics of PAM4 before turning to the test and analysis challenges it poses.
For quite some time, NRZ-type encoding has been the mainstay modulation scheme for data transmission. In an NRZ scenario, we take a binary pattern, say 0011010, and encode that into a series of fixed voltage levels, with the lower voltage being a zero and the higher voltage being a one (see data stream M in the Figure 1). We'll assume a given bit rate of, say, 28 Gb/s.
Figure 1: PAM4 doubles the number of bits in serial data transmissions by increasing the number of levels of pulse-amplitude modulation, but does so at the cost of noise susceptibility.
If we look at that NRZ signal as an eye diagram, it will have a bit period, T, and amplitude, A. The required bandwidth for this signal is related to the bit period (1/T). The faster the bit rate, the shorter the bit period and the higher the bandwidth.
There is also a signal-to-noise (SNR) ratio requirement, which is related to the amplitude. The smaller the eye diagram becomes vertically, the more difficult it becomes to maintain a SNR that allows us to interpret the signal at the receiving end of the link.
Fundamentally, what we'd like to do is to double the number of bits we send from point A to point B. One way to achieve this goal is by adding a second lane or channel. In this channel, we might like to send a different bit pattern, say 0101100 (see data stream L in the Figure 1). But there's a downside to this approach, which is that we now need two transmitters, two receivers, and two channels. We might not have the luxury of the additional real estate or power consumption, so we look for another solution.