The Phase Noise Challenge Pacing the Race to 5G

The wireless communication industry’s move toward mmWave frequencies, driven by 5G cellular, is posing a challenge to existing oscillator technologies, particularly phase noise. New techniques and approaches may be required.

5G is pushing the state-of-the-art in virtually every area of cellular radio technology, including higher channel frequencies. To meet the most ambitious 5G goals, including peak data rates of 10 Gbps, cell edge data rates of 100 Mbps and 1 ms end-to-end latency, near-mmWave frequency bands above 20 GHz are needed in the U.S. One of the many challenges posed by near-mmWave frequencies is managing radio link noise and interference. In 1948, Claude Shannon showed that radio system capacity depends not only on the signal strength and bandwidth, but also on the radio link noise level.¹

Radio link noise has two broad sources: internal and external. External noise, also termed interference, is related to the environment, while internal noise is related to the radio system’s electronic circuitry. The main interest of this article is the internal noise generated in the local oscillator (LO), i.e., the phase noise. From Shannon’s Law, it is the key limiter of radio channel capacity. Quoting James Buckwalter, a professor at the University of California Santa Barbara, “Oscillator phase noise is a silent killer in interference limited systems.”²

Oscillator phase noise is defined as the oscillator’s short-term instability resulting in random fluctuation in the frequency or phase of its output (see Figure 1). Phase noise is measured as the power spectral density for each 1 Hz frequency of a single sideband relative to the power spectral density of the oscillator’s central frequency, in dBc/Hz. A well-known empirical model for phase noise, the Leeson equation was developed to describe and predict LC tank circuit phase noise performance.³

Figure 1 Oscillator phase noise.

Figure 2 Typical QAM (I/Q) modulator.

In Leeson’s equation, F is an empirically determined constant for curve fitting, k is Boltzmann’s’ constant, T is the absolute temperature in Kelvin, P_sig is the tank power dissipation, ω0 is the oscillation frequency, Q is the loaded oscillator quality factor, Δω is the offset from the oscillation frequency and ω₁ is the corner frequency between the 30 dB/decade and 20 dB/decade slope regions.

EFFECT OF LO PHASE NOISE

LO phase noise performance is critical to modern radio system performance, especially to high data rate orthogonal frequency division multiplexing (OFDM) systems.⁴ OFDM is the data modulation technique used by most data transmission systems today, including LTE (4G cellular), Wi-Fi, cable and DSL networks. OFDM enables transmission systems to operate close to the Shannon theoretical capacity, overcoming frequency specific interference but susceptible to oscillator phase noise.

In an ideal OFDM modulator, the data stream is mixed with the ideal oscillator frequency (labeled carrier oscillator in Figure 2) to produce ideal modulated symbols (see Figure 3a). In real life, however, the LO generates the carrier frequency and additional close-in frequencies called additive phase noise. These frequencies, i.e., the carrier plus the additive phase noise, are mixed with the data to produce the modulated signal. The addition of the phase noise around the central carrier frequency produces an error in the phase angle of the resulting symbol, called its error vector magnitude (EVM)⁵, leading to a shift in the placement of the symbol in the constellation (see Figure 4). Too great an error obscures the symbol, making demodulation questionable or impossible, as shown in Figure 3b.

The EVM, an expression of the symbol position in the decoded constellation relative to ideal, is an important specification that cellular system equipment must meet to qualify for commercial use⁶ (see Table 1). Symbol position errors, measured as EVM, have multiple causes. The most important source of vector error for high data rate OFDM radio communications is LO phase noise.⁷ Symbol vector errors lead to inter-symbol interference, which is measured as symbol error rate. Symbol errors, which corrupt the data stream, slow the data rate by forcing some data to be resent, degrading link performance. In this way, LO phase noise degrades radio link performance.