Agilent Measurement Journal - Issue 4 - 2008 - (Page 23)

-1, 1 Q 1, 1 I 1, 1 -1, -1 -1, 1 1, -1 1, 1 -1, -1 -1, 1 1, -1 Sequence of QPSK data symbols to be transmitted -1, -1 1, -1 QPSK modulating data symbols V V A DM ol OF ymb s CP e Tim A DM l -F bo SC sym CP Tim e A DM ol OF ymb s 15 kHz Frequency fc A DM l -F bo SC sym 60 kHz Frequency fc OFDMA Data symbols occupy 15 kHz for one OFDMA symbol period SC-FDMA Data symbols occupy N x 15 kHz for 1/N SC-FDMA symbol periods Figure 2. Comparison of how OFDMA and SC-FDMA transmit a sequence of QPSK data symbols The LTE downlink uses traditional OFDMA methods and differs from other systems such as UMB and WiMAX only in details of the OFDM numerology (that is subcarrier spacing, symbol length, bandwidth, etc.). On the left side of Figure 2, N adjacent 15 kHz subcarriers — already positioned at the desired place in the channel bandwidth — are each modulated for the OFDMA symbol period of 66.7 µs by one QPSK data symbol. In this simple four-subcarrier example, four symbols are taken in parallel. These are QPSK data symbols so only the phase of each subcarrier is modulated and the subcarrier power remains constant between symbols. After one OFDMA symbol period has elapsed, the CP is inserted and the next four symbols are transmitted in parallel. For visual clarity, the CP is shown as a gap; however, it is actually ﬁlled with a copy of the end of the next symbol, meaning the transmission power is continuous but has a phase discontinuity at the symbol boundary. To create the transmitted signal, an inverse FFT is performed on each subcarrier to create N time-domain signals that are vector summed to create the ﬁnal time-domain waveform used for transmission. SC-FDMA signal generation begins with a special precoding process but then continues as with OFDMA. Before outlining the generation process it is helpful to first describe the end result as shown on the right side of Figure 2. The most obvious difference between the two schemes is that OFDMA transmits the four QPSK data symbols in parallel, one per subcarrier, while SC-FDMA transmits the four QPSK data symbols in series at four times the rate, with each data symbol occupying N x 15 kHz bandwidth. Visually, the OFDMA signal is clearly multi-carrier and the SC-FDMA signal looks more like single-carrier, which explains the “SC” in its name. Note that OFDMA and SC-FDMA symbol lengths are the same at 66.7 µs; however, the SC-FDMA symbol contains N “sub-symbols” that represent the modulating data. It is the parallel transmission of multiple symbols that creates the undesirable high PAR of OFDMA. By transmitting the N data symbols in series at N times the rate, the SC-FDMA occupied bandwidth is the same as multi-carrier OFDMA but — crucially — the PAR is the same as that used for the original data symbols. This should make heuristic sense without delving into the mathematics: Adding together many narrowband QPSK waveforms in OFDMA will always create higher peaks than would be seen in the wider-bandwidth single-carrier QPSK waveform of SC-FDMA. As the number of subcarriers N increases, the PAR of OFDMA with random modulating data approaches Gaussian noise statistics but, regardless of the value of N, the SC-FDMA PAR remains the same as that used for the original data symbols. Agilent Measurement Journal 23

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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