31-Jan-2014 Fanny Mlinarsky octoScope, Inc. Testing MIMO Radios Day 5: Benchmark Testing of MIMO Performance 31-Jan-2014 Fanny Mlinarsky octoScope, Inc.

Wireless Test Challenges Why is getting repeatable and consistent measurements is next to impossible in open air conditions? Modern wireless devices are designed to automatically adapt to the changing channel conditions. Adaptation algorithms programmed into the baseband layer of these radios are complex and sometimes get into unintended states. Wireless environment is time-, frequency- and position- variable in terms of path loss, multipath, Doppler effects and interference, often stumping the decision logic of the adaptation algorithms.

Benchmark Testing Goals of benchmark testing is to objectively compare performance of devices from different vendors Under best conditions Under challenging conditions

Ideal Benchmarking Testbed Ideal benchmarking testbed must: Create conditions under which throughput of the DUT is maximized Emulate a range of realistic conditions, including path loss, multipath, noise and interference Guarantee repeatable measurements reproducible at labs around the world

Evolution of Wireless Test Methods MIMO OTA Multi Path Emulator (MPE) SISO conducted Isolation box New generation wireless testbeds must support MIMO OTA testing to accommodate MIMO and multi-radio devices with internal antennas.

Factors Impacting MIMO Throughput Explanation/Impact Notes MIMO channel correlation Function of several variables including device antenna spacing, antenna polarization and multipath The lower the correlation the higher the throughput Angular spread of the received signal Related to correlation and strongly influenced by multipath in the channel Multipath causes signal to bounce around and arrive at different angles, thereby widening the angular spread at a receiver. Typically, the wider the angular spread the higher the MIMO throughput. Device antenna spacing and orientation Related to angular spread and correlation MIMO throughput will vary vs. device orientation and antenna spacing. Typically, the wider the antenna spacing the lower the correlation and the higher the throughput. Antenna polarization Vertical, horizontal or circular Cross-polarization (vertical and horizontal) is sometimes used to lower MIMO correlation, thus enabling spatial multiplexing. Multipath reflections can alter polarization. Noise and interference High noise power with respect to signal power results in low SNR (signal to noise ratio) MIMO devices can adapt to the environment by selecting the most suitable mode of operation (e.g. TX diversity in low SNR conditions; spatial multiplexing in high SNR, low correlation conditions). Motion of devices or multipath reflectors Causes Doppler spread of the signal OFDM signaling is sensitive to Doppler spread. Throughput should be measured in a variety of Doppler environments. Delay spread of reflections Causes clusters of reflections to arrive at the receiver at different times Delay spread is higher for larger spaces (e.g. outdoors) than for smaller spaces (e.g. home environment)

Maximizing MIMO Throughput in a Small Anechoic Chamber Factors Corresponding chamber capabilities MIMO channel correlation Low correlation can be achieved via wide spacing of test antennas Angular spread of the received signal (1) Test antenna array is close to the DUT antennas, widening the angular spread of the LOS signal; (2) chamber geometry creates -20 dB reflections surrounding the DUT. Both of these factors maximize angular spread of the test channel thereby maximizing MIMO throughput. Device antenna spacing and device orientation Test antenna spacing can be adjusted. DUT can be rotated with respect to test antennas. Antenna polarization Test antennas can be cross-polarized. However, reflections inside the chamber and metal surfaces in the DUT (ground planes, batteries, etc.) will also alter polarization of the signal that reaches the DUT antennas. Noise and interference Can be injected in a controlled manner via extra RF ports. Motion of devices or multipath reflectors Can be implemented via phase dithering of the signal or by using a fader. Delay spread of reflections octoBox MPE module models IEEE standard delay spread.

Factor: Angular Spread Shorter distance = wider angular spread = higher throughput 4” 8” 8’ Open air - throughput decreases with distance Source of plot: SmallNetBuilder.com measurement (linksys_ea6500_5ghz_80mhz_up_mpe_vs_openair.png)

Wide Angular Spread in a Small Anechoic Chamber DUT Test antenna spacing can be controlled octoBox chamber

Multipath in a Small Chamber Realistic magnitude of reflections in a typical house. Reflections surround the DUT from walls, ceiling and floor, creating a wide-angular- spread environment modeling non-line-of- sight (NLOS) multipath reflections to help achieve maximum throughput. Line of sight (LOS) transmission between the test antennas and the DUT is a function of the proximity of test antennas to the DUT. -20 dB absorption on frequencies 1-6 GHz -15 dB absorption on frequencies down to 700 MHz 2.25” gradient absorptive foam covers inside surfaces But what about delay spread of the multipath?

Multipath in a Small Chamber octoBox DUT AP Test Antennas Master Client Traffic TX/RX Remote Desktop Attenuators Ethernet Filter Multipath segment Testbed being used for benchmarking http://www.smallnetbuilder.com/wireless/wireless-howto/32082-how-we-test-wireless-products-revison-7 MPE = multi path emulator

octoBox MPE Response vs. IEEE Model B

octoBox MPE Frequency Response

Beware of Adaptation Algorithms Initiation algorithm 4” 8” 8’ Open air - throughput decreases with distance

Beware of Adaptation Algorithms Example of throughput measurement of an 802.11ac link using IxChariotTM. In this example the test conditions are static, but it appears that the adaptation algorithm of the TX DUT keeps making adjustments resulting in throughput fluctuations vs. time.

Benchmarking of MIMO Throughput Source: www.smallnetbuilder.com http://www.smallnetbuilder.com/wireless/wireless-howto/32082-how-we-test-wireless-products-revison-7

SmallNetBuilder AP Testbed Client: ASUS PCE-AC66; driver Win 7 6.30.95.26* Test client machine has Win Defender and Firewall disabled; runs no antivirus; does not run Windows update. It is dedicated solely to being a test client. Traffic generator/analyzer: IxChariot Measured throughput strongly influenced by the IxChariot test file size. SmallNetBuilder uses 2,000,000 to 5,000,000 Byte file size. The high performance throughput script uses 10,000,000 Byte file size by default. * Subject to change with new standards and technology releases

Isolating DUTs in a Wireless Testbed Loss of testbed dynamic range due to crosstalk

How to Select RF Isolation Chamber Two issues to be aware of: Isolation specifications often don’t include the impact of data and power cables that must penetrate the walls of the chamber to power and control the DUT inside during the test. OTA support requires high isolation, absorption and special conditions to enable high MIMO throughput. Choose peel-and-stick gasketing that’s easy to replace Plastic rails and hardware to avoid reflections OTA = over the air

Wireless Performance Metrics Metrics for wireless devices and systems include Throughput Packet error rate (PER) Range Roaming speed Mesh operation Application specific operation, for example DSRC automotive networks Test should be performed with and without channel impairments DSRC = direct short range communications

Wireless Testbed Building Blocks Testbeds should be versatile – made up of building blocks suitable for configuring a variety of test topologies A single large screen room only supports a single airlink

Roaming Test Configuration RF attenuators Controller or Ethernet switch Isolation chambers RF port Spare combiner ports for connecting a sniffer AP1 ATTEN1 Traffic source/sink Ethernet AP2 1:4 RF combiner ATTEN2 Traffic source/sink Client/bridge under test Feed-through Ethernet filters USB octoBox Stackable

octoBox Roaming Test Script Programmable attenuator ramp Ping request-response delay Connection outage due to roaming

Testbed Considerations Extra RF ports are useful to inject interference or connect monitor probes An extra filtered USB connections can house a USB module for traffic monitoring

Wireless Mesh Testing Wireless Mesh Metrics Self-healing, self-forming Throughput, QoS vs. hops Throughput, QoS vs. range Routing efficiency Dealing with interference

Wireless Mesh Test Configuration Maximize attenuators to force auto-re-routing of traffic flow to test self-healing 1 RF splitters used to direct signal to multiple neighboring devices Radios are paced in isolation chambers Fixed attenuators to set traffic flow via one branch or another to test self configuration octoBox quadStack isolation enclosures with built-in RF combiners and attenuators

Connected Car Testbed Emulate groups of connected vehicles traveling over a busy highway interchange

Example Large-scale DSRC Testbed Emulate motion of groups of cars with respect to other groups 8-port wireless channel emulator Motion Multipath Noise Up to 24 radio modules in each chamber 192 radios in the testbed

Summary Wireless technology is spreading into many applications Different benchmarks will be needed to ensure proper operation of complex radios used in hostile airlink conditions Now is a good time to think about a versatile wireless testbed that can accommodate needed tests and provide reproducible measurements in labs around the world.

For More Information To download white papers, presentations, test reports and articles on wireless topics, please visit http://www.octoscope.com/English/Resources/Articles.html www.octoscope.com