Conformity Magazine - April 2008 - (Page 43)

multiple copies of the same user data in an effort to combat channel impairments. Proper system testing of any of these techniques can only be accomplished using multiple antenna connections over a real or emulated channel. Furthermore, as the antennas have some correlation on the devices under test, a test system that accurately considers correlation factors like line of site components, angle of arrival and departure, angular speed and cross correlation is essential. WiMAX systems will utilize AAS techniques and many antennas at the base station to maximize the range of the system and reduce the number of base stations required to cover a given area. A WiMAX MISO system with four antennas, and MIMO-enabled Wi-Fi systems with three or more antennas, are common today. At mobile stations/clients, STC (MISO) helps improve performance and mobility without impacting device battery life. Effective channel emulation needs to offer scalable system support to be effective, which can mean up to four antennas (4x4) to handle the many modes that are being deployed today, and to be ready for technologies deployed in future WiMAX and MIMO-enabled Wi-Fi devices. True Device Performance Assessment Drives Exceptional Specifications from the Test Equipment WiMAX and MIMO-enabled Wi-Fi communications technologies present very demanding requirements on RF dynamic range and fidelity. Modern radio systems employ advanced digital modulation technologies to increase capacity or the bits per symbol. For example, WiMAX 64QAM (quadrature amplitude modulation) offers a capacity of six bits per symbol. High order modulations, such as WiMAX 64QAM, demand wide dynamic range and excellent signal linearity. To operate properly, an OFDM 64QAM signal is capable of a high signal-to-noise ratio (SNR) (> 26 dB), and a peak-to-average power ratio (PAPR) of 13 dB. Products that employ advanced modulation will have some dynamic rate adaptation that allows the device to change to a less aggressive modulation and coding scheme when the conditions do not support a more aggressive scheme. The implementation of rate adaptation, combined with transmit power control, results in a signal power that can change over a significant range (> 10 dB) during normal radio operation. The summation of all these factors requires a test device that has a very wide dynamic range of operation. For OFDM 64QAM, 26 dB (to maintain adequate SNR) + 13 dB (PAPR) + 20 dB (rate adaptation and power control) equals a dynamic range for the expected input signal of at least 59 dB. To provide a true assessment of device performance, test equipment should minimize the distortion of signals that it passes to the device under test. As a signal is “processed” and passed by the channel emulator to the device under test, avoiding the introduction of unwanted signal distortions requires a channel emulator that offers significantly better RF fidelity than the system under test. IEEE 802.16, the standard for WiMAX, specifies that a WiMAX transmitter should have an output fidelity of not worse than -31 dB, as described by error vector magnitude (EVM). The emulator must pass the waveform with little significant degradation. EVM is often treated as a noise power quantity. If the emulator EVM is equal to that of the DUT, the overall EVM at the emulator output will be 3 dB worse than the DUT (noise is twice as big). However, with an emulator EVM 10 dB better than the DUT, the overall EVM is -30.6 dB, much closer to the originally transmitted signal. Another consideration for emulator performance is the output noise floor. If this floor is too high, it can cause DUT receiver circuitry to falsely detect the presence of a signal. A high noise floor can be reduced using an attenuator, but this also lowers the signal power and reduces the effective output dynamic range. Ideally, the noise floor of a channel emulator should be low enough without the attenuator to prevent false triggering. The inherent thermal noise (at room temperature) in a 20 MHz wide channel (20 MHz is the maximum defined WiMAX April 2008 Conformity 3 http://www.maruwa-g.com/e/index.html http://www.maruwa-g.com/e/index.html

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Conformity Magazine - April 2008 NewsBreaks Editor’s Note Mandatory Analog Cellular Service Ends FCC Releases Quarterly Reports on Consumer Inquiries and Complaints New Standards List Released for The EU’s Low Voltage Directive FDA Issues Notification on Unretrieved Device Fragments FDA Offers Guidance on Waivers for In-Vitro Diagnostic Devices ESD Open Forum Emerging Issues in Standards Challenges in Testing Design Fundamentals Focus on... New Products Product Reviews Premarket Approval Application Guidance Issued by FDA New Jersey Enacts Electronic Waste Recycling Regulations A New U.S. Standard for Workstation and Computer Design Might the CPSC Hold Retailers Accountable for Safe Products? CPSC Loses “Sole Toy Tester” CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - April 2008

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