Agilent Measurement Journal - Issue 5 - 2008 - (Page 20)

Table 2. LTE downlink physical signals and physical channels Downlink signals P-SCH* S-SCH* Full name Primary synchronization signal Secondary synchronization signal RS Reference signal (pilot) Purpose Used for cell search and identiﬁcation by the UE. Carries part of the cell ID (one of three orthogonal sequences). Used for cell search and identiﬁcation by the UE. Carries the remainder of the cell ID (one of 168 binary sequences). Used for downlink channel estimation. Exact sequence derived from cell ID (one of 3 x 168 = 504). Purpose Carries cell-speciﬁc information Carries the multicast (MCH) transport channel Scheduling, ACK/NACK Payload Deﬁnes number of PDCCH OFDMA symbols per sub-frame (one, two or three) Carries HARQ ACK/NACK Downlink channels PBCH PMCH PDCCH PDSCH PCFICH PHICH Full name Physical broadcast channel Physical multicast channel Physical downlink control channel Physical downlink shared channel Physical control format indicator channel Physical hybrid ARQ indicator channel * Note: There are no formal acronyms to describe the primary and secondary synchronization signals. The terms P-SCH and S-SCH come from earlier 3GPP technical reports and are still used informally despite their suggestion of “channel” rather than “signal.” LTE downlink structure The LTE physical layer supports the use of paired and unpaired spectrum with frequency-division duplex (FDD) and time-division duplex (TDD) modes, respectively. Each of these modes has its own frame structure, described shortly. First, however, we will consider the composition of the downlink which comprises physical signals and physical channels as described in the 3GPP speciﬁcations.3 Downlink physical signals are generated in Layer 1 and used for system synchronization, cell identiﬁcation and radio channel estimation. The primary synchronization signal (P-SCH) and secondary synchronization signal (S-SCH) encode the cell identiﬁcation data, allowing the UE to identify and synchronize with the network. Downlink reference signals (RS), known as pilot signals in other standards, are used by the UE receiver to estimate the phase and ﬂatness of the received signal. Errors in the received signal are the combination of errors in the transmitted signal and further imperfections in the radio channel. Without the use of reference signals spaced across the channel bandwidth, phase and amplitude shifts in the received signal would make demodulation unreliable, particularly at high modulation depths such as 16QAM or 64QAM. With these high modulation depths, even a small error in the received signal amplitude or phase can cause demodulation errors. Alongside the physical signals are physical channels, which carry data such as control, scheduling and user payload to and from the higher layers. Since LTE is a packet-only system there is no need to deﬁne dedicated channels since all data is carried on the shared channel. The function of each LTE physical signal and channel is summarized in Table 2. Table 3 shows the modulation schemes allowed for the downlink signals and channels. Table 3. Modulation schemes for the LTE downlink Downlink signals P-SCH S-SCH RS Modulation scheme One of three Zadoff-Chu sequences Two 31-bit BPSK M-sequence Complex I + jQ pseudo random sequence (length-31 gold sequence) derived from cell ID Modulation scheme QPSK QPSK, 16QAM, 64QAM QPSK QPSK, 16QAM, 64QAM QPSK BPSK modulated on I and Q with spreading factor 2 or 4 Walsh codes Downlink channels PBCH PMCH PDCCH PDSCH PCFICH PHICH 20 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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