Agilent Measurement Journal - Issue 4 - 2008 - (Page 48)

In Figure 1, voice data is carried on the logical dedicated trafﬁc channel (DTCH; green box), and signaling data is carried on the dedicated control channel (DCCH; blue box). Each logical channel is channel coded and interleaved, then segmented to conform to the physical layer’s 10 ms frame structure and rateadjusted to match the physical layer data-block size. The trafﬁc and control channels are then multiplexed together to form the coded composite transport channel (CCTrCH; pink box). After a second interleaving, this transport channel is mapped onto the physical data channel (DPDCH; blue path), which is then spread using orthogonal variable-spreading-factor (OVSF) codes to attain the desired 3.84 Mbps rate. The pilot, power control and other control data are mapped onto the physical control channel (DPCCH; red path), which is also spread to 3.84 Mbps and scaled to be –6 dB relative to the DPDCH. The composite spread signal, containing data on the in-phase (I) path (blue) and control information on the quadrature (Q) path (red), is scrambled using a complex function called hybrid phase-shift keying (HPSK). HPSK scrambling offers an important beneﬁt: It reduces the zero-crossing transitions in the IQ plane. This reduces the peak-to-average power ratio of the signal and ultimately simpliﬁes mobile design. Not shown in Figure 1 is the ﬁnal physical channel in which the data is passed through a root-raised-cosine (RRC) ﬁlter and an IQ modulator. The ﬁltered modulated signal is then applied to the RF carrier. The downlink is created in a similar manner with two exceptions: the DPDCH and DPCCH are time-multiplexed and a different channelization process is used. Please see Reference 3 for a detailed look at W-CDMA downlink signals. For more information on W-CDMA uplink signals and HPSK, please see Reference 2. Paraphrasing, the 3GPP speciﬁcations for terminal conformance deﬁne EVM as follows: The error vector magnitude measures the difference — the error vector — between reference and measured waveforms. Both waveforms pass through a matched RRC ﬁlter with 3.84-MHz bandwidth and a = 0.22 rolloff. Both waveforms are then modiﬁed by selecting frequency, absolute phase, absolute amplitude and chip-clock timing values that minimize the error vector. The ﬁnal EVM result is the square root of the ratio of the mean error-vector power to the mean reference power expressed as a percentage (Figure 2).4 The error vector shown in the ﬁgure represents the quantity measured for each chip. The individual chip-error vectors are then combined as deﬁned in the EVM equation to yield the EVM measurement result. N–1 EVM = γ=0 N–1 γ=0 S Z'(γ) – R’(γ) 2 x 100% R’(γ) 2 S Magnitude error Q Error vector (Z-R) Measured vector (Z) Phase error ø Reference vector (R) I Overview of EVM It takes a wide array of measurements to characterize a transmitter — and each measurement provides a different insight into transmitter performance. As an example, a signal’s constellation can be used to evaluate the accuracy of an RF waveform modulated by a complex waveform. A symbol is the smallest data unit being transmitted and each one has a known location within the constellation of a particular data pattern. From this, a reference signal can be created to compare the expected and actual constellations. This comparison — the EVM — is a critical measure of W-CDMA link performance. Figure 2. Deﬁnition of EVM For single-channel EVM (QPSK EVM), the baseband I and Q signals are recovered and passed through an RRC ﬁlter. The obtained samples are then passed through a QPSK decoder at the chip-timing rate to determine the correct chip location for each sample. An assumption is made about the error of the true chip that allows the sampled chip to be placed in the correct quadrant. Once all of the chips have been decoded, they are QPSK encoded and passed through a raised-cosine ﬁlter to create the reference signal. This reference signal is then compared to the measured signal to produce an EVM result. 48 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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