Pushing the limits of Ethernet speeds and NRZ
The increased demand for more data from consumers and businesses has required faster and faster Ethernet technologies. To that end, a shift from NRZ(Non-Return to Zero or PAM2) to PAM4 (Pulse Amplitude Modulation 4) has now become the answer to increasing data throughput. PAM has never been used at the high speeds we are seeing today and this is where the challenge begins. Margins have shrunk, edges are more critical and error rates increase when working with PAM4.
PAM has never been used at the high speeds we are seeing today and this is where the challenge begins.
Pulse Amplitude Modulation (PAM4) vs. Non-Return to Zero (NRZ/PAM2)
First let’s look at the differences between NRZ (PAM2) and PAM4. NRZ(PAM2) (Fig 1) has two amplitude levels with 1 bit of information per symbol. The real-time eye shows one distinct eye opening with one distinct rise and fall time.
Figure 1: NRZ signal and Eye Diagram
The PAM4 (Fig 2) by contrast has four distinct amplitude levels and two bits of information per symbol. The real-time eye shows three distinct eye openings with six rise and fall times.
Figure 2: PAM4 signal and Eye Diagram
PAM4 has four amplitude levels with two bits of information per symbol, while NRZ(PAM2) has one bit of information per symbol. Symbols are expressed in terms of baud (Bd). So PAM4 has twice the throughput for the same baud rate of NRZ.
Figure 3: Same data expressed as NRZ vs. PAM4
With standard (linear) PAM4 we have the potential for two transitions at the same time. These transitions can cause two bit errors per symbol. If we convert standard PAM4 to gray code, we can cut our bit error down to one bit error per symbol. This reduces our overall bit error in half.
Figure 4: Standard (Linear) PAM4 converted to Gray code
Clock Skew and Eye Vertical alignment
Clock skew can have a significant impact on the vertical alignment of PAM4 eyes. When the upper and lower eyes are skewed to the left relative to the middle eye as shown below, this indicates that the most significant bit (MSB) is early with respect to the least significant bit (LSB). We can imagine a mask (shown in green in Fig 5) that sets a margin for what skew is acceptable. In this case, a quarter unit interval (UI) mask might be a good starting point. As the eyes drift, further past center alignment of the middle eye, symbol errors (SER) will increase and data recovery suffers.
Figure 5: Eye Skew – skew between top, bottom eyes relative to the middle eye
Non-linearity and Amplitude compression
Non-linearity and amplitude compression are also an issue when rise/fall times differ between the upper (MSB) and lower (LSB) eyes or voltage amplitude for various levels are too high or low. In Fig 6 the lower eye is compressed with the upper eye dilated.
Figure 6: Non-linearity and amplitude compression can also effect SER.
To measure the transmitter linearity of the PAM4 signal, we measure the mean signal level transmitted for each PAM4 symbol. Symbol levels are derived from the voltage levels V0, V1, V2 and V3 as shown in Fig 7. Vmid is the halfway point between V0 and V3. To determine how far off we are from the ideal symbol level, we can calculate the effective symbol level (ES) as shown below. Ideally, we would like both ES1 and ES2 to be 1/3 so that the eyes are perfectly symmetrical and our voltage levels are aligned. This all brings us to level separation mismatch ratio (RLM). Ideally, we would want our level separation mismatch ratio to equal 1. This would imply that all our PAM4 eyes are symmetrical and open.
Figure 7: Transmitter linearity measurement and level separation mismatch ratio RLM
Forward Error Correction (FEC)
With all that can go wrong with the signal from the transmitter though the channel and to the receiver, how can we correct for errors along the way? This is where error correction can help to correct at least some of the errors. With error correction, we have the advantage that we don’t need to retransmit the data again. The Reed-Solomon error correction scheme has properties that are well suited for PAM4, as it can correct for burst errors shown below. Reed Solomon error correction treats symbols the same no matter how many bits are contained in the symbol. So, with PAM4 having two bits per symbol compared to NRZ only having one bit per symbol doesn’t penalize the efficiency of the Reed Solomon correction scheme. Of course, with any error correction scheme, parity symbols are sent with the data and this will add to the overhead in our data stream.
Figure 8: Burst Errors showing corrupted bits
What PAM4 brings to the table is an opportunity to overcome NRZ (PAM2) speed challenges while doubling the data throughput at the same baud rate. Keeping PAM4 eyes open, symmetrical and un-skewed when transmitting data brings new challenges to designers. Understanding how these characteristics play together is important for successful implementation of PAM4 solutions. This introduction to PAM4 shows just a taste for what PAM4 has to offer and some of the challenges that must be overcome.
Understanding how these characteristics play together is important for successful implementation of PAM4 solutions. This introduction to PAM4 shows just a taste for what PAM4 has to offer and some of the challenges that must be overcome.
Keysight has been a key contributor to IEEE802.3bs and other Ethernet standards that use PAM4 and understands the test requirements. New test challenges with PAM4 can be overcome, and Keysight has the solutions to overcome these challenges. Keysight has other comprehensive test solutions from design simulation to physical layer testing that includes transmitter, receiver and channel for PAM4.
For more information go to https://www.keysight.com/find/PAM4