Agilent Measurement Journal - Issue 4 - 2008 - (Page 26)

Figure 5. Analysis of a 16QAM SC-FDMA signal Multipath resistance with short data symbols? At this point it is reasonable to ask, “How can SC-FDMA still be resistant to multipath when the data symbols are still short?” In OFDMA, the modulating data symbols are constant over the 66.7 µs OFDMA symbol period but an SC-FDMA symbol is not constant over time since it contains N sub-symbols of much shorter duration. The multipath resistance of the OFDMA demodulation process seems to rely on the long data symbols that map directly onto the subcarriers. Fortunately, it is the constant nature of each subcarrier — not the data symbols — that provides the resistance to delay spread. As shown earlier, the DFT of the time-varying SC-FDMA symbol generated a set of DFT bins constant in time during the SC-FDMA symbol period even though the modulating data symbols varied over the same period. It is inherent to the DFT process that the time-varying SC-FDMA symbol — made of N serial data symbols — is represented in the frequency domain by N time-invariant subcarriers. Thus, even SC-FDMA with its short data symbols beneﬁts from multipath protection. It may seem counterintuitive that N time-invariant DFT bins can fully represent a time-varying signal. However, the DFT principle is simply illustrated by considering the sum of two ﬁxed sine waves at different frequencies: The result is a non-sinusoidal time-varying signal — fully represented by two ﬁxed sine waves. Examining a real SC-FDMA signal Figure 5 shows some of the measurements that can be made on a typical SC-FDMA signal. The constellation in trace A (top left) shows this is a 16QAM signal. The unity circle represents the RS (every seventh symbol), which do not use SC-FDMA but are phase modulated using an orthogonal Zadoff-Chu sequence.3 Trace B (lower left) shows the signal power versus frequency. The frequency scale is in 15 kHz subcarriers numbered from –600 to 599, which represents a bandwidth of 18 MHz. From this we can conclude this must be a 20 MHz channel and the allocated signal bandwidth is 5 MHz towards the lower end. The brown dots represent the instantaneous subcarrier amplitude and the white dots the average over 10 ms. In the center of the trace, the spike represents the LO leakage (IQ offset) of 26 Agilent Measurement Journal

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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