Redefining Signal Integrity in mmWave Communication

Empowering 5G and Beyond with Ultra-Low Jitter VCXOs

Overview

Achieving Unprecedented Signal Integrity in mmWave Communication with Ultra-Low Jitter VCXOs

The advent of 5G and beyond has driven significant advancements in millimeter-wave (mmWave) communication technologies. These systems, operating at frequencies above 24 GHz, offer vast bandwidth and enable high data rates crucial for applications like augment- ed/virtual reality, high-resolution video streaming, and autonomous driving. However, realizing the full potential of mmWave communication requires meticulous attention to signal integrity, particularly in the clocking domain. This article explores the critical role of ultra-low  jitter Voltage-Controlled Crystal Oscillators (VCXOs) in achieving the stringent phase noise requirements for high-order modulation schemes like 1024-QAM in mmWave systems.

Image Source: Yumpu. ‘QAM Demodulation & Wireless Communication’.

Challenges

Signal Integrity Challenges in mmWave Communication

mmWave signals are inherently susceptible to various impairments due to their high frequencies. These include:

• Path Loss : Signal attenuation increases significantly with frequency, necessitating sophisticat- ed antenna designs and higher transmit power
• Atmospheric Absorption : Atmospheric gases, particularly oxygen and water vapor, absorb mmWave energy, impacting signal propagation
• Multipath Propagation : Reflections and scattering from objects create multiple propagation paths, leading to signal distortion and interference
• Phase Noise : Phase noise in the local oscillator (LO) and clock signals directly translates to phase jitter in the transmitted and received signals, degrading modulation accuracy and increasing bit error rates (BER)

For high-order modulation schemes like 1024-QAM, which encode 10 bits per symbol, the constellation points are densely packed. This makes them extremely sensitive to phase noise. Even small amounts of jitter can cause the received signal point to stray outside its decision boundary, leading to errors. Therefore, minimizing phase noise is paramount for achieving the required Error Vector Magnitude (EVM) and BER performance for 1024-QAM.

Efficiency

Why 1024-QAM?

1024-QAM is a high-order modulation scheme that allows the transmission of 10 bits per symbol, making it highly efficient for data-intensive applications. By increasing the number of bits encoded per symbol, 1024-QAM significantly boosts spectral efficiency, enabling higher data rates within the same bandwidth. This is particularly advantageous for 5G mmWave systems, where spectrum is a valuable and limited resource. However, the dense constella- tion of 1024-QAM also requires stringent signal integrity, as even minor phase noise or jitter can lead to errors.

For example, in a typical mmWave communication system operating under MCS 11 (1024-QAM) with a 400 MHz channel bandwidth, the theoretical data transmission rate can reach up to 7.2 Gbps. Achieving this rate requires precise clocking and ultra-low jitter to maintain the required modulation accuracy and minimize bit error rates.

Image Source: ScienceDirect. ‘QAM Modulation Techniques and Their Application in Modern Communication Systems’.

Clocking

Clock Tree Architecture and the Need for Jitter Cleaning

mmWave communication systems typically employ complex clock tree architectures to distribute clock signals to various modules, including the baseband processor, RF transceiv- er, and antenna module. A typical clock tree might originate from a stable master clock source, which is then multiplied and divided to generate various frequencies required by different subsystems.

In our specific case, the network synchronizer generates a 122.88 MHz RF_Clock signal, which serves as the clock source for the antenna module. This RF_Clock is then converted up to 26.5 GHz to serve as the carrier frequency. Any jitter present on the RF_Clock directly impacts the phase noise of the 26.5 GHz carrier.

Several factors can introduce jitter into the clock tree:

• Clock Multiplication and Division : Frequency synthesis operations like multiplication and division can amplify existing jitter
• Clock Distribution Network : The physical layout of the clock distribution network, including trace lengths, impedance mis- matches, and crosstalk, can introduce additional jitter
• Power Supply Noise : Noise on the power supply rails can modulate the clock signal, contributing to jitter

To achieve the stringent phase noise requirements for 1024-QAM, jitter cleaning is essential. This typically involves using a low-noise clock source, such as a VCXO, in conjunction with a jitter attenuator or phase-locked loop (PLL) to filter out unwanted jitter components.

For MCS level 11 (1024-QAM) operation, the phase noise requirement is extremely tight. Typi- cal system requirements demand phase noise levels that translate to sub-100fs integrated jitter. Therefore, a high-performance, ultra-low jitter VCXO is crucial at the RF_Clock stage to provide a clean reference for the up conversion process.

Advantages

The Benefits of Ultra-Low Jitter VCXOs

Taitien’s VL series is uniquely designed VCXOs offer significant advantages for mmWave com- munication systems targeting 1024-QAM:

• Ultra-Low Jitter Performance : Our VCXOs are engineered to deliver exceptionally low phase noise, resulting in integrated jitter performance of less than 20 fs. This significantly exceeds the requirements for 1024-QAM and ensures robust signal integrity
  
• Simplified System Integration : The superior phase noise performance of our VCXOs reduces the burden on subsequent jitter cleaning stages, potentially eliminating the need for complex and pow- er-hungry jitter attenuators or allowing them to operate at lower performance levels with less power consumption. This simplifies system design and reduces cost
• Proven Environmental Robustness : The VCXO has been a proven solution for many years, designed to serve varying environmental conditions such as fluctuating temperatures or mechanical vibrations. This ensures reliable performance even in challenging operational environments
• Improved System Performance : By minimizing phase noise, our VCXOs con- tribute to improved EVM, lower BER, and higher data throughput, enabling mmWave systems to achieve their full potential
  

Conclusion

Achieving the stringent signal integrity requirements of mmWave communication systems, particularly for high-order modulation schemes like 1024-QAM, demands careful consider- ation of the clocking architecture. Our ultra-low jitter VCXOs provide a crucial building block for these systems, offering unparalleled phase noise performance, simplified integration, and improved overall system performance. By minimizing phase noise at the source, our VL series enable mmWave systems to deliver the high data rates and reliability required for next-gen- eration applications.