Agilent Measurement Journal - Issue 5 - 2008 - (Page 19)

Extending OFDM with OFDMA With standard OFDM, low-rate user equipment (UE) transmissions occupy narrow frequency allocations and can suffer from narrowband fading and interference. Because of this, the LTE downlink uses OFDMA, which adds time-division multiple access (TDMA) to basic OFDM. Figure 3 shows that with basic OFDM each user is allocated a ﬁxed set of subcarriers. With OFDMA, subcarriers are allocated dynamically among different users on the channel. The result is a more robust system with increased capacity, attributable to the efﬁciency gained from multiplexing low-rate users and dynamically scheduling users by frequency (according to each user’s instantaneous channel conditions). that the transmission power is continuous but has a phase discontinuity at the symbol boundary. To create the transmitted signal, an IFFT is performed on each subcarrier to create M time-domain signals. In turn, these are vector-summed to create the ﬁnal time-domain waveform used for transmission. -1, 1 Q 1, 1 I -1, -1 1, -1 QPSK modulating data symbols 1, 1 Subcarriers Subcarriers -1, -1 -1, 1 1, -1 -1, -1 1, 1 1, -1 -1, 1 Sequence of QPSK data symbols to be transmitted Symbols (time) Symbols (time) V OFDM User 1 User 2 OFDMA User 3 A DM ol OF ymb s Tim e Figure 3. OFDM and OFDMA subcarrier allocation A DM ol OF ymb s 15 kHz Frequency fc A simpliﬁed OFDMA data transmission is summarized in Figure 4. The example uses four (M) subcarriers over two symbol periods with the payload data represented by QPSK modulation. Real LTE signals are allocated in units of 12 adjacent subcarriers and will be described in more detail shortly. The M adjacent subcarriers are spaced 15 kHz apart and positioned at the desired place in the channel bandwidth. Each subcarrier is modulated for the OFDMA symbol period of 66.7 µs by one QPSK data symbol. In this four subcarrier example, four data symbols are taken in parallel. Since these are QPSK data symbols, only the phase of each subcarrier is modulated, therefore subcarrier power remains constant between symbols. After one OFDMA symbol period has elapsed, a CP is inserted and the next four symbols are transmitted in parallel. The CP is shown as a gap for visual clarity; however, it is actually ﬁlled with a copy of the end of the next symbol. This means Data symbols occupy 15 kHz for one OFDMA symbol period OFDMA Figure 4. OFDMA transmitting a series of QPSK data symbols The OFDMA signal in Figure 4 is clearly multi-carrier and it is this parallel transmission of multiple symbols that creates the undesirably high PAR of OFDM. As the number of subcarriers increases, the PAR of the OFDMA signal (with random modulating data) approaches Gaussian noise statistics. High PAR creates problems for power ampliﬁer design and is a main reason why 3GPP developed SC-FDMA with its lower PAR for the LTE uplink. Agilent Measurement Journal 19

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 5 - 2008

Agilent Measurement Journal Issue 5 Finding a New Path when Assumptions Break Down Contents Testing MIPI D-PHY Protocols Oscilloscope Firmware Update DNA Replication Joining the LTE/SAE Trial Initiative Testing Proteins Praise for DC Power Analyzer Scripting Features for AMDS Testing Hybrid PCBA Systems Improving the Gene Expression System Workflow Turning a “Good Enough” Test Strategy into One That’s Reliable, Repeatable and Traceable Understanding the Effects of Limited-Bandwidth Channels on Digital Data Signals Introducing the 3GPP LTE Downlink Validating the 26 Validating the Physical and Protocol Layers in DDR Memory Interfaces Understanding Total Jitter Measurements of Low Probabilities Using Behavioral-Model Simulation to Accurately Predict First-Order PLL Performance Creating Synchronous High-Frequency Sampling across Multiple Digitizers Overcoming the Challenges of Testing FlexRay Networks Understanding Operating Life and Repeatability of Electromechanical Switches and their Effect on Total Cost of Ownership Implementing Micro and Nano LC/MS Techniques for High Sensitivity Lipidomics Analysis

Agilent Measurement Journal - Issue 5 - 2008

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