Samsung Reveals GDDR7 Memory Uses PAM3 Signalling to Achieve 36 Gbps Data-Rate

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The next-generation GDDR7 memory standard is shaping up nicely, to double bandwidth and density over the current GDDR6. In a company presentation detailing upcoming memory technologies, Samsung revealed that GDDR7 uses PAM3 signalling. While ones and zeroes are stored in DRAM memory cells, it is transmitted between devices (such as the DRAM chip and the GPU) in electrical waveforms known as “signals.” Ones and zeroes are interpreted by patterns in the signal waveform.

Conventional GDDR6 memory uses NRZ (non-return to zero) or PAM2 signalling to achieve data-rates starting from 14 Gbps, with 24 Gbps expected to be the fastest production GDDR6 memory speed on offer, however some of the faster GDDR6 speeds such as 18 Gbps, 20 Gbps, and 22 Gbps couldn’t hit production soon enough for the development phase of the GeForce RTX 30-series “Ampere” GPU, and so NVIDIA and Micron Technology co-developed the GDDR6X standard leveraging PAM4 signalling, to offer speeds ranging between 18 Gbps to 23 Gbps (or higher) several quarters ahead of this faster JEDEC-standard GDDR6.

Conventional NRZ signalling provides 1 bit per cycle transmission rate, while PAM4 does 2 bits per cycle. PAM3 increases this to 3 bits per cycle using a more advanced waveform with many more “eyes” (gaps created by intersections of waves that are interpreted as bits). Samsung states that PAM3 is 25% more efficient than NRZ signalling, and that GDDR7 will be 25% more energy efficient. PAM3 signalling is also used by the 80 Gbps per-direction Thunderbolt 4 standard, and the upcoming USB4.

As for performance, the Samsung slide references 36 Gbps data-rate, which confirms that GDDR7 will bring a generational doubling in data-rates over GDDR6, much like GDDR6 did over GDDR5. A typical GPU with a 256-bit memory bus, when using 36 Gbps-rated GDDR7 memory, will enjoy 1152 GB/s of memory bandwidth. High-end GPUs with 384-bit memory interfaces will do 1728 GB/s. Mainstream GPUs with 128-bit interfaces get 576 GB/s on tap.