Submitted by Keysight Technologies
Overview and motivation
Increasing the data throughput on high-speed digital interfaces can be accomplished in at least three ways:
- Using multiple parallel signals
- Increasing the symbol rate
- Using higher order modulation schemes that carry more than 1 bit per symbol (such as NRZ)
Traditionally, #1 and #2 are used because NRZ data transmission is very well understood and can be realized in a cost-effective way, both on the transmit side as well as the receive side. However, there are limitations.
Using a large number of parallel signals requires a higher pin-count on chips and more traces on backplanes, cables and boards, which makes the overall solution more expensive and eventually impractical.
Increasing the symbol rate works well up to a certain point. But board traces have significant attenuation at higher frequencies. Depending on the length of the trace, increasing the symbol rate beyond ~32 Gbaud is very difficult. For technologies such as chip-on-glass used in smart-phone displays, the threshold is at even lower symbol rates of 1 to 2 GBaud.
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