How to Meet the Testing Challenges of Multi-Antenna Systems[Copy link]
With the rapid growth of applications with higher data rates such as 5G, wireless systems are faced with the requirements of wider bandwidth and wider network coverage. Multi-antenna technologies, such as multiple-input multiple-output (MIMO) and beamforming, use methods such as diversity, multiplexing and increasing antenna gain to improve spectrum efficiency and signal-to-noise ratio (SNR) to cope with the pressure of limited spectrum resources.Testing multi-antenna systems requires a test solution that can provide multiple signals and maintain a certain phase relationship between them. Different strategies for configuring the generation of phase-coherent signals will lead to different measurement results.1. What is phase coherence?
If two signals always have a constant relative phase, they are coherent (Figure b). When two signals are present, the coherent signals will combine cogeneratively or destructively depending on their relative phase.When characterizing multi-channel components, such as phased array antennas, it is necessary to precisely control the phase relationship between the individual channels (Figure c). For digitally modulated signals, phase coherence means that the two baseband signal generators are time synchronized and the RF carriers are coherent (see Figure d).Similarly, radar pulses require precise timing of the pulse bursts to simulate the corresponding spatial delays (see Figure e).2. Why is phase coherence important?Multi-antenna technology in wireless communication systems can increase channel capacity and improve channel reliability. The main multi-antenna technologies include spatial diversity, spatial multiplexing and antenna arrays.Space diversitySpatial diversity can be achieved through channel switching, signal weighting, time delay or transmit diversity.
Spatial multiplexingSpatial multiplexing uses multiple-input multiple-output (MIMO) transmission technology, which does not require additional bandwidth or additional transmission power. Therefore, it is a very effective means to increase channel and system capacity.
Antenna array-beamformingBeamforming can form a signal beam in a specific direction by precisely adjusting the signal phase of the coherent antenna unit, so that the signal energy is concentrated in the direction of the transmitting/receiving end, thereby improving the spectrum utilization efficiency and increasing the system capacity.
Strategies for generating multi-path phase-coherent signals and their advantages and disadvantages
In order to simulate multi-channel test scenarios, the phases between the test signals must be coherent and controllable. Different implementation strategies have different impacts.
1
Independent local oscillator
The simplest way to achieve phase stability between multiple signals is to lock their respective 10MHz frequency references together and synchronize different signal sources through a trigger signal and a common 10MHz time base.
The two signal sources have their own independent local oscillators and phase-locked loops, which will cause phase drift between them. In most cases, the phase-locked loop can lock the phase drift within the loop bandwidth, but it cannot fully track the higher-level response.In MIMO systems, slow phase drift between channels is not a big problem and this test method can provide acceptable performance.Phase NoiseThe phase noise between the signal sources locked to the frequency reference is uncorrelated, which brings corresponding phase errors to the test system. Test instruments with highly stable time base and excellent phase noise performance will reduce the phase drift and phase error of the system, and are suitable for MIMO and spatial diversity testing. This method will not work when accurately calibrating device characteristics. It is necessary to share the same local oscillator to obtain more accurate results.
2
Sharing the same LO
This two-way phase-coherent test system uses the local oscillator of the upper instrument to provide two signal sources respectively, thus achieving complete coherence of the two signal sources. The phase error between the two is less than 1 degree.
Phase ShiftEven if the same local oscillator is shared, the system will still produce phase deviation and phase shift between different channels due to the influence of cable length and adapters.
3
Direct Digital Synthesis (DDS)
Direct Digital Synthesizer technology generates analog waveforms by generating time-varying signals in digital form and then performing digital-to-analog conversion. The DDS architecture provides ultra-low phase noise and fast frequency switching speed, with extremely high frequency tuning resolution. Different DDS systems are phase-aligned with each other through synchronous reset.
Keysight Technologies' new generation of dual-channel microwave vector signal generator VXG (M9383B/M9384B) uses DDS technology to provide two coherent signal channels. Without touching any hardware, it can maintain time alignment between channels with an accuracy of <10ps.
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