Agilent Measurement Journal - Issue 4 - 2008 - (Page 27)

the signal; the large image to the right is an OFDM artifact deliberately created using 0.5 dB IQ gain imbalance in the signal. Both the LO leakage and the power in non-allocated subcarriers will be limited by the 3GPP speciﬁcations. Trace C (top middle) shows a summary of the measured impairments including the error vector magnitude (EVM), frequency error and IQ offset. Note the data EVM at 1.15 percent is much higher than the RS EVM at 0.114 percent. This is due to a +0.1 dB boost in the data power as reported in trace E, which was ignored (for illustration) by the receiver to create data-speciﬁc EVM. Also note the RS power boost is reported as +1 dB, which can also be observed in the IQ constellation because the unity circle does not pass through eight of the 16QAM points. Trace D (lower middle) shows the distribution of EVM by subcarrier. The average and peak of the allocated signal EVM is in line with the numbers in trace C. The EVM for the non-allocated subcarriers reads much higher although the way this in-channel impairment will be speciﬁed will be as a power ratio between the wanted signal and the unwanted signal. The ratio for this signal is around 30 dB as can be seen in trace B. The blue dots in trace D also show the EVM of the RS, which is very low. Trace E (top right) shows the ability to measure EVM by modulation type from one capture. This signal uses only the RS phase modulation and 16QAM so the QPSK and 64QAM results are blank. Finally, trace F (lower right) shows the PAR — the whole point of SC-FDMA — in the form of a complementary cumulative distribution function (CCDF) measurement. It is not possible to come up with a single ﬁgure of merit for the PAR advantage of SC-FDMA over OFDMA because it depends on the data rate. The PAR of OFDMA is always higher than SC-FDMA even for narrow frequency allocations; however, when data rates rise and the frequency allocation gets wider, the SC-FDMA PAR remains constant but OFDMA gets worse and approaches Gaussian noise. A 5 MHz OFDMA 16QAM signal would look very much like Gaussian noise. From the white trace it can be seen at 0.01 percent probability the SC-FDMA signal is 3 dB better than the Gaussian reference trace, and as every ampliﬁer designer knows, even a tenth of a decibel shaved from the peak power budget is money in the bank. Conclusion In essence, SC-FDMA means “create a single-carrier waveform and shift it to the desired part of the frequency domain.” After a careful consideration of the characteristics of OFDMA and the new SC-FDMA, we can conclude that SC-FDMA provides the advantages of OFDMA — especially robust resistance to multipath — without the problem of high PAR. The use of SC-FDMA in LTE, however, is restricted to the uplink because the increased time-domain processing would be a considerable burden on the base station, which has to manage the dynamics of multi-user transmission. It will be interesting to see if LTE — the latest of the three new OFDMA cellular standards — has indeed identiﬁed a superior solution for the uplink or whether the pure OFDMA used in WiMAX or the OFDMA/CDMA combination used in UMB prove to be just as successful when all the factors are taken into account. Today, the experts disagree so we will have to wait on the ultimate arbiter, time, before we ﬁnd out for sure. References 1. 3GPP TS 36.201 v8.0.0 section 4.2.1 2. 3GPP TS 36.211 v8.0.0 subclause 5.6 3. 3GPP TS 36.211 v8.0.0 subclause 5.5 “WiMAX,” “Mobile WiMAX” and “WiMAX Forum” are trademarks of the WiMAX Forum. Correction In Issue Three of Agilent Measurement Journal, the article “What Next for Mobile Telephony?” presents the “G factor” equation (page 35) in its general form as G = Îor / (Îor + Ioc ). The article should have used the 3GPP form of the equation, which is G = Îor / Ioc. Agilent Measurement Journal 27

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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