The Importance Of Phase-Coherent RF Signal

As the number of higher-throughput applications grows, so does the need for 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 helps you understand 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 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 you characterize a multichannel component such as a phased-array antenna, you need to precisely control the phase angle relationship between the channels (figure 1c). For digitally modulated signals, phase coherence indicates both timing synchronization between baseband generators and phase coherence between RF carriers (see figure 1d). Similarly, radar pulses require precise timing of the pulse bursts to simulate the appropriate spatial delays (see figure 1e).

Fig. 1: Phase relations between two signals.

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. You can improve the quality and reliability of a wireless link by using two or more antennas. You can accomplish this 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.

Fig. 2: Spatial diversity techniques for receiver diversity and transmitter diversity.

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, you need multiple signal generators and channel simulators. 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. 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:

Equation 1

The receiver can perform channel estimation (the h matrix above) using training sequence algorithms. You can recover the transmit signals (s1 and s2) through signal processing using the formula in equation 2:

Equation 2

Fig. 3: A 2×2 MIMO system diagram.

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, you need multiple signal generators and channel simulators. 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 wavefront 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.

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

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

Conclusion

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, you must accurately re-create the operational environment. To accomplish this, you must create signals 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. Our N5182B MXG vector signal generator is used as an example in our technical document 3 Tactics for Configuring Phase-Coherent RF Signal Generation.