The importance of phase-coherent RF signals

As the number of higher-throughput applications grows, so does the need for a wider bandwidth and network coverage in wireless systems. Given limited spectrum allocation, wireless communication engineers must look for ways to improve spectral efficiency and the signal-to-noise ratio (SNR) of systems. Multiple-input multiple-output (MIMO) and beamforming can help RF designers achieve diversity, multiplexing, and antenna gain to improve spectral efficiency and SNR.

Testing multi-antenna systems requires a test system capable of providing multiple signals and a constant phase relationship between the signals. This article provides an overview of phase coherence and why it matters. It also offers tactics for generating phase-coherent signals.

What Is phase coherence?

Two signals are coherent if they have a constant relative phase at all times. Figure 1a illustrates two non-coherent signals with phase variances, and Figure 1b shows coherent signals with fixed phase offset. When present together, signals will combine constructively or destructively, depending on their relative phase.

In cases where a multichannel component such as a phased-array antenna are characterized, precise control of the phase angle relationship between the channels is needed (Figure 1c). For digitally modulated signals, phase coherence indicates both timing synchronization between baseband generators and phase coherence between RF carriers (Figure 1d). Similarly, radar pulses require precise timing of the pulse bursts to simulate the appropriate spatial delays (Figure 1e).

Figure 1 Phase relationships between two signals including (a) non-coherent, (b) coherent, (c) controllable phase relationships, (d) configurable modulation, and (e) triggerable pulses. Source: Keysight

Why phase coherence matters

Most wireless systems, whether in commercial applications or aerospace and defense, use multi-antenna techniques at the receiver, transmitter, or both to improve overall system performance. These techniques include spatial diversity, spatial multiplexing, and beamforming. Engineers use multi-antenna techniques to achieve diversity, multiplexing, or antenna gains. Through these gains, wireless systems can increase a receiver’s data throughput and SNR.

Spatial diversity

When multipath signals arrive at a receiver, they combine constructively or destructively, depending on their relative phase. The quality and reliability of a wireless link can be improved by using two or more antennas. This can be accomplished with channel switching, signal weighting, time delay, or transmit diversity.

In any case, the goal of spatial diversity is to provide multiple paths for a radio signal to reach a receiver’s antenna. Figure 2 illustrates that not all methods require multiple antennas at the receiving side.

Figure 2 Spatial diversity techniques for receiver diversity and transmitter diversity including (a) channel switching, (b) signal weighting, (c) time delay, and (d) transmit diversity. Source: Keysight

Spatial multiplexing

The system splits transmitted data into multiple encoded data streams. Then it transmits all data streams simultaneously over the same radio channel through different antennas. In order to recover the original data at the receiver, MIMO systems use computationally inverse channel property estimation algorithms. To simulate the MIMO multipath signals for spatial multiplexing performance tests, multiple signal generators and channel simulators are needed. They emulate the multipath scenarios and inject additive white Gaussian noise (AWGN) to emulate the desired SNR.

Spatial multiplexing is a transmission technique for a MIMO system. The system splits transmitted data into multiple encoded data streams. It transmits all data streams simultaneously over the same radio channel through different antennas. To recover the original data at the receiver, MIMO systems use computationally inverse channel property estimation algorithms.

Figure 3 represents a 2×2 (two transmitters and two receivers) MIMO diagram where two symbols (b1 and b2) transmit simultaneously for double the data throughput.

Figure 3 A 2×2 MIMO system diagram where two symbols (b1 and b2) transmit simultaneously for double the data throughput. Source: Keysight

A simple formula appears in Equation 1:

where r is the received signal, s is the source signal, and h is the wireless channel response.

The receiver can perform channel estimation (the h matrix above) using training sequence algorithms. Transmit signals (s1 and s2 ) can be recovered through signal processing using the formula in Equation 2:

The calculation in Equation 2 uses timing-aligned signals and a common local oscillator (LO) to upconvert and downconvert multichannel signals. This technique increases test challenges for simulating multichannel RF signals and the channel matrix, as most commercial signal generators have an individual baseband generator and LO. To simulate the MIMO multipath signals for spatial multiplexing performance tests, multiple signal generators and channel simulators are needed. They emulate the multipath scenarios and inject AWGN to emulate the desired SNR.

Antenna array—beamforming

An antenna array is a set of antenna elements used to transmit or receive signals. Coherently driven antennas with the appropriate phase delay between antenna elements can form signal beams. The uniform wave front allows a group of low-directivity antenna elements to behave like a highly directional antenna. The phase delays between the channels ultimately decide the antenna pattern, as seen in Figure 4.

Figure 4 A phased array of antennas forms a beam by adjusting the phase between coherent antennas. Source: Keysight

When the number of antenna elements by a half wavelength separation is increased, the antenna beamwidth gets narrower. By applying a 90-degree phase shift to the signal at each antenna, the direction of the beam can be changed. As phase shifts change between elements in different amounts, the beam can be steered in a range of directions. To simulate such multichannel signals, precise control of the phase difference between the channels for both transmitter and receiver tests is needed.

Generating multiple phase-coherent signals

Testing multi-antenna systems such as spatial diversity, spatial multiplexing, and antenna arrays requires a test system capable of providing multiple signals with stable phase relationships between them. However, a commercial signal generator has an independent synthesizer to upconvert an intermediate frequency (IF) signal to an RF signal. To simulate the multichannel test signals, the phase between test signals must be coherent and controllable. Let us explore different tactics to generate multichannel signals and assess the pros and cons of these tactics.

Independent and shared local oscillators

The easiest way to achieve a certain amount of phase stability between signal generators is to use two signal generators with synchronized baseband generators, a triggering signal, and a common 10 MHz frequency reference. Using an independent LO that other signal generators share prevents phase drift caused by the machines having their own phase-locked loops. Phase noise is another factor a shared LO helps. Phase drift and phase error can be improved by using high-quality, stable references and instruments with low phase noise.

Alternatively, if an independent LO is unavailable, multiple machines can share the internal LO of one signal generator. Figure 5 represents two Keysight MXG N5182B vector signal generators that are set up for a phase-coherent test system. The system takes the LO of the top signal generator, splits it, and uses it as the LO input (see red lines) for both signal generators. With this configuration, the RF paths of the two signal generators are fully coherent. The fully coherent configuration appears on the left side of Figure 5, while the right side shows that the phase difference between the two signal generators is less than 1 degree.

Figure 5 Setup for two phase-coherent RF channels with a common LO. The fully coherent configuration (left) and the phase difference between the two signal generators is less than 1 degree (right). Source: Keysight

When using a shared LO, some static time and phase skew between instrument channels will still be encountered. Cable lengths and connectors cause static time and phase variations. The delays and phase shifts skew the phase relationship between the channels. Correction of these offsets is needed to ensure that the measured differences come from the device under test and not from the test system.

Direct digital synthesis

Direct digital synthesis (DDS) produces an analog waveform by generating a time-varying signal in digital form and then performing a digital-to-analog conversion. The DDS architecture provides an optimal path to low phase noise, fast frequency switching speed, and extremely fine frequency tuning resolution.

DDS maintains a fixed phase relationship between its output for each frequency. The synchronization requires initial clock alignment using a common reference clock. Synchronous reset to the phase accumulator achieves the phase alignment. This reset can be applied on every frequency update. The synchronous reset of the phase produces a fixed and repeatable phase relationship for each channel.

Generating phase-coherent and phase-stable signals

As multi-antenna technology matures and the demand for diversity, multiplexing, and antenna gains grows, test systems require tightly aligned channels for accurate tests. When performing a characterization test, the operational environment must be accurately re-created. To accomplish this, signals must be created in such a way that they will coherently combine to simulate their real-world behavior.

There are different tactics for generating phase-coherent or phase-stable signals for various multi-antenna test applications and requirements. Always strive to minimize errors that various tactics may cause. In addition, ensure that test instruments are phase coherent and phase controllable for the test applications, such as beamforming and phased-array antennas.

TJ Cartwright is a product marketing manager focused on analog and digital RF signal generators at Keysight Technologies. He has spent time in markets for medical, pro audio and video, a variety of wireless communication protocols, semiconductor design, and several industrial applications. He is currently expanding into a deeper knowledge base in GNSS, 5G NR, and Quantum.

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