Agilent Measurement Journal - Issue 4 - 2008 - (Page 24)

Having seen what SC-FDMA looks like, we will now explain the precoding process that brings it about. Figure 3 shows the ﬁrst steps, which create a time-domain waveform of the QPSK data sub-symbols. Using the four color-coded QPSK data symbols from Figure 2, the process creates one SC-FDMA symbol in the time domain by computing the trajectory traced by moving from one QPSK data symbol to the next. This is done at N times the rate of the SC-FDMA symbol such that one SC-FDMA symbol contains N consecutive QPSK data symbols. For simplicity, we will not discuss time-domain ﬁltering of the data symbol transitions even though such ﬁltering will be present in any real implementation. Having created an IQ representation in the time domain of one SC-FDMA symbol, the next stage is to represent it in the frequency domain using a discrete Fourier transform (DFT; Figure 4). The DFT sampling frequency is chosen such that the time-domain waveform of one SC-FDMA symbol is fully represented by N DFT bins spaced 15 kHz apart, with each bin representing one subcarrier with amplitude and phase held constant for 66.7 µs. There is always a one-to-one correlation between the number of data symbols to be transmitted during one SC-FDMA symbol period and the number of DFT bins created — and this in turn becomes the number of occupied subcarriers. This should make intuitive sense: When an increasing number of data symbols is transmitted during one SC-FDMA period, the time-domain waveform changes faster, generating a higher bandwidth and hence requiring more DFT bins to fully represent the signal in the frequency domain. Note from Figure 4 that there is no longer a direct relationship between the amplitude and phase of the individual DFT bins and the original QPSK data symbols. This is quite different from the OFDMA example in which data symbols directly modulate the subcarriers. The next stage is to shift the baseband DFT representation of the time-domain SC-FDMA symbol to the desired part of the overall channel bandwidth. Because the signal is now represented as a DFT, frequency shifting is a very simple process achieved by copying the N bins into a larger DFT space that can be up to the size of the system channel bandwidth — of which there are six to choose from in LTE, spanning 1.4 MHz to 20 MHz. The elegance of the DFT lets us position the signal anywhere in the channel bandwidth, thus executing the frequency-division multiple access (FDMA) essential for efﬁciently sharing the uplink between multiple users. *This explains the origin of the latter portion of “SC-FDMA.” V(I) V(Q) -1, 1 Q 1, 1 +1 I -1 -1 One SC-FDMA symbol period One SC-FDMA symbol period +1 Figure 3. Creating the time-domain waveform of an SC-FDMA symbol -1, -1 1, -1 Figure 4. Baseband and shifted frequency domain representations of an SC-FDMA symbol Frequency Frequency *Although 3GPP did consider a distributed form of subcarrier allocation for the uplink that would have alleviated susceptibility to narrowband fading, it instead opted for the adjacent allocation described here combined with the possibility of frequency hopping at the slot (0.5 ms) level. 24 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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