An RF Power Measurement Solution for Multi-antenna MIMO Transmission

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  • Author: Chin Aik Lee, Agilent Technologies, Inc.

Multi Input Multi Output (MIMO)technology has been widely applied to today’s high data throughput wireless connectivity transmitters and receivers to improve data throughput as well as system efficiency.  However, designing a MIMO system presents complicated challenges including interferences among the channels that should be solved with accurate RF power measurement. To provide a standardized method for RF power measurement, European Telecommunications Standards Institutes (ETSI) defined new RF power measurements for multi-channel RF power measurements with the revision of the ETSI EN 300 328 v1.8.1 and EN 301 893 v1.7.1 test standards.

Conforming to these new RF power measurement requirements presents the following test challenges:

• Up to 4 measurement channels must be synchronous at all antenna outputs

• At least a 1-second measurement duration must be provided for non-frequency hopping spread

spectrum (FHSS) devices

• At least a 1 MSa/s measurement rate must be used

• Up to 4 million data samples of detection power of the burst signal must be obtained.

In this paper, a new RF power measurement solution that addresses the revised EN 300 328 v.1.8.1 and EN 301 893 v1.7.1 requirements is discussed. This solution features the USB peak and average power sensor and USB modular simultaneous data acquisition unit.

Test Requirements

The European Telecommunications Standards Institute (ETSI) is an independent, non-profit organizationwhose mission is to produce telecommunications standards for today and the future. ETSI is responsible for the standardization of information and communication technologies (ICT) within Europe. These technologies include telecommunications, broadcasting, and related areas such as intelligent transportation and medical electronics. Manufacturers of wideband data transmission systems used in wireless local area networks (WLAN) and Bluetooth devices have to prove compliance to the specifications defined by ETSI.

The EN 300 328 v1.8.1 (2.4 GHz) and EN 301 893 v1.7.1 (5 GHz) test standards specify new MIMO, beamforming, and adaptivity test requirements. More specifically, these standards defined new multi-channel synchronous power measurement procedures for a single test system, as well as procedures to support test cases and dynamic frequency selection (DFS). As such, R&D and test labs need a regulatory test system for wireless devices operating in the 2.4 GHz and 5 GHz bands.

EN 300 328 v1.8.1 RF Power Measurement Challenges

Version V1.8.1 of the ETSI standard specifies a special type of power measurement. The power  measurement must be fully time-synchronized and in compliance with the standard for up to four channels and have up to 4 million data samples of detection power of the burst signal, allowing it to characterize devices with multiple input multiple output (MIMO) and beamforming capability. The power measurement rate must be > 1 MSa/s and for non-frequency hopping spread spectrum (FHSS) devices support at least a 1 s measurement time.

Paragraph 5.3.2 in the standard defines a power measurement procedure where the following parameters have to be measured: RF output power, duty cycle, Tx sequence, Tx gap, and the medium utilization (MU) factor based on the RF power samples data within burst analysis. Currently there is no existing test systems ready to meet the requirement of this ETSI standard.

Test Solution Block Diagram

To address the challenges presented by the new power measurement procedures, there is a new test solution that performs the certification tests in line with the ETSI EN 300 328 V1.8.1 standard. Using this solution during the design and development stage, manufacturers of wideband data transmission systems can now verify whether their new products will meet the new certification requirements.

Figure 1 shows a block diagram of the test solution. It includes a custom power test set (see Figure 2)equipped with four USB peak and average power sensors and two USB modular data acquisition units in order to form a four simultaneous digitizer with 14-bit resolution and a sampling rate of up to 2 MSa/s/ch with 4 MB/ch memory. This allows the synchronized recording of the four analog signal outputs from the peak power sensors, fulfilling the new ETSI requirement to perform fully time-synchronized power measurements for up to four channel antenna ports.

In addition, the new test solution software supports calibrated power (dBm) from digitizer voltage, and records power versus time for further RF burst analysis. The RMS and maximum RF output power, duty cycles, the maximum Tx sequence, the minimum Tx gap, and the medium utilization factor also can be automatically measured and calculated.

                                 Figure 1. Test solution configuration to support new ETSI requirements

 

                         Figure 2. The custom power test set included in test solution for meeting new ETSI standards

 

The following are examples of the measurement results provided by this test solution.

 

                                                          Figure 3: RF output power UI (combined MIMO 2X2)

 

                                                                 Figure 4: RF output power synchronous.

 

                                                                 Figure 5: Non-adaptive testing parameters

 

                                                              Figure 6: MIMO power raw data results.

 

MIMO Synchronous Power Measurement

MIMO is the simultaneous use of multiple antennas at both the transmitting and receiving ends of transmissions to improve spectral efficiency. MIMO technology offers tremendous performance improvement for wireless local area networks (WLANs) and other cellular technologies. MIMO synchronous power measurement requires multiple, time-synchronized power measurements be obtained in order to comply with the ETSI EN 300 328 V1.8.1 standard requirement. The new RFpower measurement solution with a combination of four units of USB peak and average power sensors and two unit of USB modular data acquisition does 1 MSa/s with up to 4 seconds synchronous record length under MIMO conditions. The test software has a built-in user interface (UI) that allows users to select Config1, 2X2, or Config2 3X3 or 4X4 MIMO power measurement (see Figure 3).

However, the new 300-328 v.1.8.1 test standard regulates the 2.4 GHz band that covers both WLAN (MIMO) and Bluetooth (single in, single out (SISO)) and necessitates that both connectivity technologies have larger memory to support power measure over a longer length of time. Using the USB peak and average power sensor and USB modular data acquisition solution is still the best way to cover this test requirement.

                                        Figure 7.MIMO 2X2, and 3X3 and 4X4 power measurement configurations

 

USB Peak and Average Power Sensor

USB peak and average power sensor is a high performance USB power sensor with an additional 30 MHz video bandwidth capability for measuring average, peak, and peak-to-average power burst signals. An external trigger enables accurate triggering of small signals close to the signal noise floor. The USB peak and average power sensor come with built-in trigger in/out connections. This feature allows the USB power sensors to connect to an external trigger signal from a signal source or device-under-test (DUT) via a standard BNC-to-SMB cable. The sensors also come with recorder/video-output features, providing multi-channel power measurement synchronization capabilities, accuracy,and flexibility.

Meeting the new power measurement requirement to capture 1 MHz sampling waveforms from the power sensor requires at least 1 MB memory per second. However, the USB sensor has a 96 K memory with an 80 MSa/s capability, limiting its ability to capture un-decimated data only upto 1.2 ms.

The USB power peak and average power sensor has an analog video output and automatically converts the AC level without the trigger, thus the USB modular data acquisition units are used to record the data. The USB power sensors capture the maximum, peak, and average power points where as the USB modular data acquisition measure duty cycle and timing sequence check.

 

USB Modular Data Acquisition (DAQ)

USB modular simultaneous sampling multifunction data acquisition (DAQ)provides analog input sampling rate coverage of up to 2 MSa/s for each channel, and up to four channels of 14-bit resolution. Two USB modular DAQunits support four multi-channel measurements with 1 MSa/s for each channel, thus complying with the new standard. The dedicated analog-to-digital conversion (ADC) capabilityof USB modular DAQ also allows the simultaneous sampling of data to be carried out for power measurement analysis

 

Conclusion

The new requirements for RF power measurement of multi-antenna MIMO transmissions can be addressed with combination of USB peak and average power sensor(four units) and USB modular data acquisition (two units).This solution meets the measurement requirements defined according to the revision of the ETSI EN 300 328 v1.8.1 test standard.