Microwave Engineering Europe - April 2008 - (Page 14)

14 TEST & MEASUREMENT WiMAX and Wi-Fi channel emulation highs and lows: high dynamic range, low noise ﬂoor Data communications technologies, as employed in WiMAX and Wi-Fi, present very demanding requirements on system dynamic range and ﬁdelity. Most modern radio systems employ advanced digital modulation technologies to increase capacity, as deﬁned by bits per symbol. An example is 64QAM (Quadrature Amplitude Modulation) that offers a capacity of six bits per symbol. But such high order modulations also demand high dynamic range and linearity. An OFDM 64QAM signal is capable of a peak to average power ratio (PAPR) of 13 dB and requires a high signal-to-noise ratio (SNR) (> 26 dB). As is the case with products that employ advanced modulation, most will have some rate adaptation that allows the device to change to less aggressive modulation when the conditions do not support a more aggressive modulation. 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, 13 dB (PAPR) + 26 dB (to maintain adequate SNR) + 20 dB (rate adaptation and power control) = a dynamic range of at least 59 dB for the expected input signal. As the device signal is then “processed” by the channel emulator, the high dynamic range of the signal requires even greater ﬁdelity on the channel emulator to avoid introducing any unwanted distortions. IEEE 802.16, the technical standards body which deﬁnes the technical requirements for WiMAX, speciﬁes that a WiMAX transmitter should have output ﬁdelity, as described by error vector magnitude (EVM) of -31 dB. It can be shown mathematically that if a channel emulator offers the same ﬁdelity as the device under test, the ﬁdelity of the signal out of the channel emulator will be 3 dB less, -28 dB. Ideally, test equipment will introduce as little distortion as possible when passing the signal. Test equipment should minimize the distortion of the signals that it passes. A channel emulator that has an EVM of -41 dB, 10 dB better than a device at the speciﬁcation mentioned above, will pass a signal at -30.6 dB, very close to the original signal. With the burst nature of WiMAX and WiFi transmissions, devices go from offering full power output to no power output on a transmission by transmission basis. This requires a relatively high dynamic range and low noise ﬂoor for proper receiver evaluations. The channel emulator should Figure 2: Plot shows throughput reduction during changing channel conditions. Control commands can facilitate efﬁcient and effective troubleshooting. faithfully reproduce this range. The inherent thermal noise in a 20 MHz wide channel (20 MHz is the maximum deﬁned WiMAX channel bandwidth and the mandatory Wi-Fi channel bandwidth) is given by -174 dBm/ Hz * 20 MHz = 101 dBm in 20 MHz (at room temperature). A receiver with a real noise ﬁgure of 10 dB would then have a noise threshold or noise ﬂoor of -91 dBm at 20 MHz. The test equipment should not be the limitation in receiving low power signals. Channel control aids troubleshooting of bad behaviors As previously discussed, effective WiMAX and Wi-Fi testing requires running channel models for long periods of time, often several hours or even days. As the channel evolves over time, events may occur long into a run of the channel model that cause a signiﬁcant loss in signal throughput. Investigating the conditions that created this loss in throughput requires advanced control of the emulator. If an engineer’s control of the emulator is limited to the simple act of starting it, the engineer would have to wait possibly several hours or days until the speciﬁc channel condition is recreated. A channel emulator that provides the user with channel control commands such as fast forward, rewind, pause and play, enable the engineer to start the emulator and immediately fast forward to the time period of interest (which may have occurred hours or even days earlier in the test run). Similarly, if a test is in progress and an anomaly is observed as having occurred in the past, pause and rewind controls allow the engineer to pause the emulator and rewind to the time period of interest. Once the channel conditions under investigation are being run, the engineer may need to debug the radio. Advanced channel controls such as looping on a point in time, single stepping the channel and observing the actual channel parameters during those steps, signiﬁcantly aid the engineer in debugging the issue and provides data that can be used in system simulations. Figure 2 shows the results of a throughput versus time performance test conducted on a SISO device. The test plot identiﬁes signiﬁcant throughput reductions throughout the test. Using the advanced channel emulator controls the user “plays” the speciﬁc time period of interest to help understand the cause(s) of the throughput reductions. Accurate and repeatable emulation with a controlled RF environment The value of wireless device design and quality assurance testing increases when test results are both accurate and repeatable. Accurate and repeatable testing of wireless equipment is achieved in a controlled RF environment that is not subjected to external RF interference. To obtain accurate and repeatable results when using a channel emulator that has high dynamic range, the channel emulator must be integrated with a controlled RF environment. Cost effective channel emulation in a controlled RF testing environment can Microwave Engineering Europe ● April 2008 ● www.mwee.com 011-012-014-016_MWEE.indd 14 26/03/08 17:56:54 http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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