An ultra-high-speed IC capable of generating 256 Gbps PAM4 signals per lane Expected to facilitate a tenfold increase in the optical transmission capacity of datacenters compared with current levels

1.Research background

Short-range communication technologies such as Ethernet are required for low-cost, compact low-power transceiver equipment in data centers and other such facilities. Conventional short-range communication uses a NRZ (non-return-to-zero) code that transmits data as a pulse signal with two levels. For higher transmission capacity, the use of PAM4 coding is being introduced. This is a modulation scheme where data is transmitted as a pulse signal with four levels (Fig. 1). In PAM4 coding, the transmitted data must be transformed into multilevel analog voltages, but it has been difficult to generate multilevel voltage waveforms of sufficient speed and quality for 256 Gbps transmissions.

2.What we have achieved

By using InP-HBT process technology (Fig. 4) together with our proprietary high-speed DAC architecture and waveform quality enhancement technology (Figs. 2, 3), we have succeeded in fabricating a PAM4 signal generator circuit capable of generating 256 Gbps multilevel voltage signals at a symbol rate of 128 Gbaud (Fig. 5). In 100 Gbps Ethernet technology, which is currently in widespread use, optical signals are multiplexed across four lanes, each with a capacity of 25 Gbps. By doubling the number of signal levels and increasing the signaling speed by a factor of five, we have succeeded in developing the world's first ultra-high-speed IC capable of generating PAM4 signals with a data rate of 256 Gbps per lane (ten times that of current practical technology) (Fig. 6).

3.Future prospects

This ultra-high-speed IC can generate high-quality multilevel voltage signals with a high symbol rate (128 Gbaud), which has so far been difficult to achieve. It is therefore expected to find applications in the R&D of innovative devices with symbol rates of 100 Gbaud to 128 Gbaud for next-generation large-capacity communication systems, and in measuring instrument (signal generators) for system verification. We also expect this technology to lead to developments in fields such as large-capacity optical communication that require high-speed multilevel signals and large-capacity optical communications, including four-lane multiplexing to achieve 1 Tbps transmission. At NTT, by cooperating with our partners, we aim to create new services and industries using ultra-high-speed ICs, and to promote the continued evolution of ultra-high-speed IC technology (*7).

4.Technical points

We have realized a PAM4 signal generator circuit (Fig. 2) by applying our proprietary high-speed DAC architecture technology and waveform enhancement technology. In general, DAC require that multi-bit digital signals are input at the same speed as the symbol rate of the generated analog signal, and this digital input interface has limited the operating speed of the DAC as a whole. Also, the operation of DACs at high speed has given rise to problems such as waveform distortion due to skew (temporal shifts) between bits inside the DAC, and noise due to leakage of the clock signal into the data signals.
In this technology, by applying our proprietary high-speed DAC architecture technology, which uses a high-speed selector circuit (SEL) to provide half-rate inputs, together with our waveform enhancement technology with a distortion compensation delay circuit and noise rejection buffer, we can dramatically improve the operating speed and waveform quality. Following the fabrication of a PAM4 signal generation circuit using our in-house InP-HBT technology, we have succeeded in generating multilevel voltage signals at a rate of 256 Gbps.

Glossary

^*1DAC (Digital to Analog Converter):
A circuit that converts the digital signal output from a digital signal processing circuit into an analog signal.

^*2PAM4 (4-level pulse amplitude modulation):
An intensity modulation scheme that transmits two bits of data (00, 01, 10, 11) at a time using four analog level pulse signals. Compared with a NRZ (non-return to zero) code that transmits a single bit of data (0, 1) via two analog level pulse signals, PAM4 can transmit twice as much information at a time.

^*3Modulation:
Altering a waveform in order to send information. In the case of an electrical signal, for example, the simplest form of intensity modulation involves switching between binary voltage signals. In multilevel intensity modulation, more information is transmitted at a time by sending signals at several voltage levels so that more information can be sent at a time.

^*4InP:
Indium phosphide. A Group Ⅲ-Ⅴ semiconductor consisting of indium and phosphorus, used when implementing functions for applications that require high speed and/or high output.

^*5HBT:
Heterojunction bipolar transistor

^*61 Tbps (1 terabit per second):
A data transfer rate 1,000 times faster than one gigabit per second (1 Gbps). The current 100 Gigabit Ethernet standard delivers 100 Gbps by combining 25 Gbps × 4 lanes. Our latest achievement has raised this limit to 256 Gbps, and allows 1 Tbps transmissions to be achieved by combining four lanes in the same way.

^*7Press release, 7th February 2017:
"Opening of Ultra-high-speed IC Technology Using an Advanced Compound Semiconductor Process" (http://www.ntt.co.jp/news2017/1702e/170207a.html)