INTRODUCTION

Millimeter-Wave Frequencies Meet Broadband Data

Keysight has seen dramatic changes in mobile network devices and infrastructure design, development, and deployment in the transition from 4G to 5G. One substantial challenge affecting circuit design is frequencies extending into the 70 GHz millimeter-wave (mmWave) band. High millimeter-wave frequencies and the drive towards miniaturization directly impact the design of the circuits and systems.

These design trends increase the density and complexity of system integration because they include mixed fabrication technologies and phased array antennas in RF modules. Engineers must assemble multitechnology structures and perform the circuit, electromagnetic (EM), and electrothermal analysis across technology boundaries.

Sophisticated digital modulation schemes are enabling millimeterwave broadband data transmission. This process impacts the design of components and systems because it now requires new figures of merit like error vector magnitude (EVM) for RF, microwave, and mmWave applications. Your existing RF and microwave electronic design automation tool may not fulfill these new critical requirements to assemble, simulate, and verify multi-technology RF modules.

Four Secrets to Mastering m-Wave Communications Circuit Design.

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An analysis of the spectrum potential for next-generation wireless systems.

Link to the GSA Member white paper: KeySight – Exploring the 6G Spectrum Landscape

In each generation of cellular communications, new spectrum has been key to delivering more services, more capacity, and higher data throughput to end users. 5G benefited from large contiguous bandwidths of millimeter-wave (mmWave) spectrum, known as frequency range 2 (FR2). And 5G benefited from the reallocation and unlocking of midband spectrum (3.4 to 4.9 GHz) with its more favorable propagation characteristics. The spectrum that will be available for 6G is unclear, but three frequency ranges are under discussion, including the upper midband (sometimes called midband or, unofficially, FR3) from 7 to 24 GHz and sub-terahertz bands from roughly 90 to 300 GHz. The third range involves maximizing spectrum below 7 GHz through refarming, new band allocation, and increased spectral efficiency.

Each of the proposed bands has benefits and drawbacks. The bands below 7 GHz provide the best coverage. But the spectrum in this range is already allocated, and getting access to additional spectrum requires moving incumbents somewhere else or refarming. Research into ways to increase spectral efficiency and make the most of what is available must continue. The bands between 7 and 24 GHz cannot provide the same coverage as those below 7 GHz. Still, this range is under significant research and is a useful and necessary candidate to expand capacity. The millimeter bands between 24 and 90 GHz provide high capacity and low latency for local deployments. 5G introduced these bands, but they remain underused. These bands may not make headlines related to 6G, but they will likely be a part of the final makeup and will help deliver services when very high capacity is necessary in dense urban areas. The sub-terahertz bands could meet extreme capacity needs in hyperlocal deployments. Figure 1 shows the relative bandwidth available in each of these areas. It is becoming clear that 7 to 24 GHz is the most popular target for new spectrum 6G deployments. While still of interest, the sub-terahertz bands are under scrutiny because of the challenges the industry faces with FR2. But they remain an option for the second phase or later rollouts.

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Exceeding 100 Gbps Data Throughput with a Sub-THz Testbed for 6G Research

6G research is in its very early stages. The vision for what the International Telecommunication Union calls Network 2030 continues to take shape. While the industry is years away from starting the standards development process, sub-terahertz (sub-THz) territory is a focus of active research.

Getting to 100 gigabits per second (Gbps) to 1 terabit per second (Tbps) data throughput is a key objective and an active area of research for 6G. This extreme data throughput could evolve into a Key Performance Indicator (KPI) for 6G. However, it poses significant challenges, both from an RF perspective and baseband perspective.

There are three fundamental approaches to increasing data throughput.

One approach involves using higher-order modulation schemes such as 64 QAM to increase the number of bits transmitted for each symbol. Given a fixed and finite spectrum bandwidth, increasing the modulation order from QPSK (transmitting two bits for each symbol) to 64 QAM (transmitting six bits for each symbol) would increase the data throughput by a factor of three, if channel conditions and radio performance allow. A 1 GHz QPSK symbol rate would result in a 2 Gbps theoretical raw calculated data throughput without forward error correction (FEC) coding rate redundancy. However, increasing the modulation order to 64 QAM would result in a 6 Gbps data throughput, while using the same spectrum occupied bandwidth. Page 2

The second approach uses more spectrum bandwidth and increases data throughput by using a higher symbol rate. For example, the 1 GHz symbol rate, the occupied channel bandwidth is approximately 1.22 GHz, assuming a 0.22 root-raised cosine filter alpha (or excess bandwidth). Increasing the symbol rate by a factor of ten to 10 GHz would increase the QPSK data throughput to 20 Gbps, but would use a much wider swath of spectrum (approximately 12.2 GHz). Increasing the modulation order to 64 QAM could increase the data throughput to 60 Gbps, but supporting higher order modulation schemes at these extreme modulation bandwidths becomes much more challenging due to reduced Signal-to-Noise (SNR) ratio, greater linear amplitude and phase impairments, and other technical challenges [1].

A third approach transmits multiple and independent streams of data using multiple antenna techniques such as multiple-input/multiple-output (MIMO). MIMO exploits the channel and simultaneously transmits and receives multiple and independent data streams for higher data throughput. For the 1 GHz symbol rate with QPSK, by using MIMO encoding/decoding and transmitting two independent streams of data simultaneously, you may increase the data throughput. However, the actual increase in data throughput, would depend on the channel conditions and system overhead.

This whitepaper will discuss using the first two approaches at H-band (220-330 GHz) from an RF physical layer perspective, and show that it is possible to exceed 100 Gbps using 64 QAM modulation with an occupied bandwidth of 30 GHz.

6G-Going Beyond 100 Gbps to 1 Tbps

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