New USB4 Specification to Enable 80Gbps Performance

Thu Dec 01 16:38:45 CST 2022

0_USB4-V2-80Gbps-HornmicLink

New USB4 Specification to Enable 80Gbps Performance

USB-IF released the latest USB4 Ver2.0 specification on October 18, 2022, again doubling USB bandwidth, providing 80Gbps performance and DisplayPort 2.1 support via USB Type-C cable and connector. This will benefit higher performance displays, storage devices, USB hubs, etc.

USB4 Gen4 Introduction

With the newly released USB4 Ver2.0 specification, the transfer rate is doubled again to 80 Gbps (40G bps x2) and is denoted as USB4 Gen4. The electrical layer is encoded with PAM3 (Pulse Amplitude Modulation 3-level) signals, as shown in Figure 1, and utilizes three voltage states for transmission, so two eye diagrams are formed at the top and bottom.

1_PAM3-Eye-diagram

Each channel transmits at 25.6 GBaud, and the transmitter encodes the binary bit signal in an 11-bits to 7-trits (trinary) mapping configuration, and transmits it as a PAM3 signal to achieve a dual-channel 80 Gbps transmission speed; this is a different transmission method from the previous one. On the receiver side, Gen4 requires BER TER (Trit Error Ratio) to be received at 1E-8 or lower without FEC (Forward Error Correction).

The number of bits that can be transmitted per Symbol is 1 / 1.58 / 2 for PAM2 / PAM3 / PAM4, respectively.

The following figure shows the comparison of PAM2/PAM3/PAM4.

Comparison-of-PAM2-PAM3-PAM4

USB4 Gen4 uses PAM3 signal encoding for the following reasons.

For USB4 Gen4 to reach 80 Gbps and follow the same PCB and cable as Gen3, new encoding methods such as PAM3 or PAM4 must be adopted and considered from two main directions.

  • Total signal loss

If the signal is transmitted at 40 Gbps using NRZ, the transmission loss will exceed 40 dB at 20 GHz, and the IC cannot compensate for this excessive loss, resulting in incorrect signal reception. The total transmission loss of PAM4 and PAM3 at Gen4 Nyquist frequencies of 10 GHz and 12.8 GHz is about 23 dB and 28 dB respectively, and the IC can compensate for this loss.

  • Transmission BER

PAM3 transmits with half the eye height of NRZ and PAM4 transmits with 1/3 the eye height of NRZ, increasing the difficulty of signal restoration at the receiver side, while PAM3 outperforms PAM4 in signal-to-noise distortion ratio (SNDR).

Bandwidth optimization for USB4 Gen4 specification

  • USB4 Gen4 supports "Asymmetric Link" for speeds up to 120 Gbps

Gen4 adds Asymmetric Link. Only Gen4 can support asymmetric transfer, Gen2 and Gen3 only support symmetric transfer. Asymmetric transmission means that the number of TX channels (Lane) is the same as the number of RX channels.

Gen4's asymmetric transmission allows for up to 120Gbps (40Gbps x3) in one direction while maintaining 40Gbps in the other direction. The conversion from symmetric to asymmetric transmission is controlled by the Connection Manager.

3_Symmetric-and-Asymmetric-USB4-Link

  • USB4 Gen4 Adds USB3 Gen T Optimized Bandwidth Utilization

USB4 adds support for USB3 Gen T tunneling protocol, mainly to allow USB3 tunneling to make better use of USB4 transmission bandwidth. USB3 Gen X and USB3 Gen T are new definitions in USB4 Ver2.

4_USB3-Gen-X-and-USB3-Gen-T-Tunneling-Compare



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By HornmicLink_Henry @221201 16:46