EDNE April 2012 - (Page 18)

A mo insight intonthly T&M techn the latest o the pages logy from Measurem of Test & (www.tmw ent World orld.com ) ® Test & Measurement World is the leading monthly magazine for engineers and managers in the electronics testing industry. Test MIMO Wi-Fi and LTE radios over the air ota testing can siMulate conditions such as reFlections and Fading in a controlled environMent. by Fan n y M l i na rs k y • p r e s i d e n t o F o c to s c o p e ® Radio technologies such as IEEE 802.11 and 3GPP (3rd generation partnership project) LTE rely on MIMO (multiple input, multiple output) techniques to increase the range and data rates of radio transmissions. Using digital-signal processing, MIMO radios sense the conditions in the channel on a packet-by-packet basis and make instantaneous decisions on whether to employ 802.11 or LTE techniques. Tests of MIMO radios require OTA (over-the-air) measurements and simulations of transmission channels that are more complex than those needed for single-radio systems. MIMO technology uses multiple synchronized radios—up to four for 802.11n and LTE and up to eight for the emerging 802.11ac—to adapt to continuously changing conditions in the wireless channel. MIMO radios use any of four techniques to extend range and data rates: • TX (transmit) and RX (receive) diversity, which adds robustness when channel conditions are challenging (e.g., low signal-to-noise ratio or high multipath conditions); • Spatial multiplexing, which increases throughput by sending multiple simultaneous streams when channel conditions are favorable; • Beamforming, which extends the range or enables multiple users to share the wireless channel; or • MU-MIMO (multi-user MIMO), which enables multiple stations to transmit simultaneously in the same frequency channel by focusing the antenna pattern. In the past, engineers could test ra- dios in a conducted environment by disconnecting the antennas and replacing them with coaxial cabling that guided the signal to the controlled test circuitry (Figure 1, left side). Today’s sophisticated MIMO techniques, including TX diversity and beamforming, require OTA test methods (Figure 1, right side). An IEEE 802.11 task group has developed a draft document that specifies test metrics and methods for conducted and OTA test environments (Ref. 1). C o n tr o l l ed and u nC o ntr o l l e d ota test m eth o ds Many engineers consider conduct- ed test environments to be controlled environments and consider OTA environments to be uncontrolled. OTA testing can, however, be performed under both controlled and uncontrolled conditions. Uncontrolled OTA test methods include using a typical house or outdoor setting to measure the throughput and range of the devices. Controlled OTA testing is typically performed in an anechoic chamber. Uncontrolled environments result in measurements that can vary over time, while controlled environments (conducted or OTA), when properly implemented, produce repeatable measurements (Figure 2). FIGURE 1. (Left) A traditional conducted test setup involves disconnecting the antenna from the DUT (device under test) and connecting coaxial cabling to the antenna port of the DUT. (Right) An OTA test setup requires coupling the DUT antenna field into the test equipment with measurement antennas. 18 EDN EUROPE | APRIL 2012 www.edn-europe.com http://www.tmworld.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE April 2012

Cover
Contents
International Rectifier
Contents
RS Components
Masthead
Comment
International Rectifier
Pulse
Analog Devices
FTDI
Digikey
Mouser
Baker's best
Digikey
Test & Measurement
Agilent Technologies
Digikey
Protect POE systems
Hirose
Coilcraft
Mesago
Cover Story
Digikey
Signal Integrity
RS Components insert
Balancing GBWP and quiescent current
Noise wars
Design Idea
Product Roundup
Tales from the cube

EDNE April 2012