AT THE HEART of fourth-generation (4G) wireless products and systems like Long Term Evolution (LTE) is multiple-input multiple-output (MIMO) technology. For engineers to accurately characterize the effects of multichannel RF interactions on the conformance, performance, and interoperability of MIMO-based systems in a laboratory environment, channel emulation is needed. In a nine-page white paper titled, "Improving 4G Wireless Broadband Product Design through Effective Channel Emulation Testing," Azimuth Systems delves into the critical requirements for the channel emulators designed for testing MIMO-enabled LTE/4G devices.

When choosing a channel emulator for use with a 3GPP LTE device, one must consider certain aspects associated with input powernamely input power range, peak power, and signal-tonoise margin. When orthogonal frequency division multiplexing (OFDM) is employed, for example, the peak-to-average power ratio (PAPR) can be greater than 10 dB even though the average power may have some maximum value. As a result, the system must accommodate this maximum.

The document also explains how a fading channel emulator is used to provide realistic fast fading conditions. With an OFDM signal as used in 3GPP LTE, for instance, certain subcarriers may be faded or momentarily reduced in amplitude by 20 dB or more due to frequency-selective fading. In complex modulation formats like 64-state quadrature amplitude modulation (QAM), any momentary drop in signal amplitude must be considered relative to the noise floor of the channel-emulator equipment. Assuming 25C and a 10-MHz-wide signal, the noise power spectral density of the test equipment would need to be less than 155 dBm/ Hz in order to ensure that the signal fidelity was maintained during fading conditions.

To handle the many modes that are currently being designed and tested, effective channel emulation requires at least four-transmit-and-fourreceive- antenna (4x4) capabilities. To represent the multitude of different conditions exhibited in the real world, the emulator also must provide long intervals of non-repeating channel conditions. To keep the channel models defined by the industry bodies readily available while allowing the user to develop their own channels, the channel emulator should allow for the easy modification of standard models or the complete design and input of new models. Of course, two key aspects of any channel emulator are the user interface and system usability. Engineers should look at these features together with technical specifications to pick the emulator that is right for them.

Azimuth systems, inc., 31 Nagog Park, Acton, mA 01720; (978) 263-6610, internet: www.azimuthsystems.com.