Agilent Measurement Journal - Issue 5 - 2008 - (Page 18)

Rather than transmit a high-rate stream of data with a single carrier, OFDM uses a large number of closely spaced, orthogonal subcarriers that are transmitted in parallel. Each subcarrier is modulated with a conventional modulation scheme (e.g., QPSK, 16QAM or 64QAM), at a low symbol rate. The combination of hundreds or thousands of subcarriers enables data rates similar to conventional single-carrier modulation schemes in the same bandwidth. Figure 2 illustrates several key features of an OFDM signal. In the frequency domain, multiple adjacent subcarriers are each independently modulated with data. Guard intervals are inserted between each of the symbols in the time domain to prevent intersymbol interference caused by multipath in the radio environment. The fast Fourier transform (FFT) and its inverse (IFFT) are used to generate and detect OFDM signals. OFDM offers a number of distinct advantages in comparison to CDMA: • OFDM easily scales up to wide channels which are more resistant to fading. • OFDM channel equalizers are much simpler to implement because the OFDM signal is represented in the frequency domain rather than the time domain. • OFDM is resistant to multipath because its long symbols can be separated by a guard interval also known as the cyclic preﬁx (CP). By sampling the received signal at the optimum time, the receiver can remove the time-domain interference between adjacent symbols caused by multi-path delay spread in the radio channel. • OFDM is better suited to multiple antenna techniques such as MIMO. The frequency domain representation of the signal enables easy pre-coding to match the signal to frequency and phase characteristics of the multi-path radio channel. OFDM also has some disadvantages. Its subcarriers are closely spaced, rendering OFDM sensitive to frequency errors and phase noise. This also makes OFDM sensitive to Doppler shift, which causes interference between the subcarriers. In addition, pure OFDM creates high peak-to-average ratio (PAR) signals, which cause difﬁculty with power ampliﬁer design and power consumption. In comparison to CDMA, OFDM is more difﬁcult to operate at the network’s cell edges. Whereas CDMA relies on different scrambling codes to protect against inter-cell interference at the cell edge, OFDM has no such feature. Some form of frequency planning at the cell edge is therefore required. A comparison of CDMA and OFDM is given in Table 1. Table 1. Comparison of CDMA and OFDM Attribute Transmission bandwidth Symbol period CDMA Full system bandwidth Very short – inverse of the system bandwidth OFDM Variable up to full system bandwidth Very long – deﬁned by subcarrier spacing and independent of system bandwidth Primarily frequency and time, although scrambling and spreading can be layered on top Separation of users Orthogonal spreading and scrambling codes Guard intervals FFT 5 MHz bandwidth Sub-carriers Symbols Frequency Time Figure 2. OFDM signal represented in frequency and time, taken from 3GPP Technical Speciﬁcation TS 25.8922 18 Agilent Measurement Journal

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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