LTE Reference & Synchronization Signals

December 22, 2017 | By Astro Ahmed @ GrEEK CAMPUS (m.rahm7n@gmail.com)

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Pilots/Reference Signals are an Important factor regarding estimating the channel status. It can be shown that in an OFDM-based system an equidistant arrangement of reference symbols in the lattice structure achieves the minimum MSE, mean squared error, to estimate the radio channels, where Channel Estimation is an important factor regarding the ease of processing the received packets by the receiver and estimate the channel effects based on the reference signals which is allocated by the transmitter in order to recover the transmitted information correctly by removing the channel effects.

Some PILOTS are allocated in Down link like [CRS, CSI, MBSFN, PRS (Positioning RS)] and for Up Link [Demodulation Reference Signals (DM-RS), Sounding Reference Signal (SRS)].

Here is a brief overview about each of them.

DL: CRS Cell-specific
Used for EVM Minimization and Equalizer Training, and can be used for synchronization. used as the power level reference for the rest of the component carrier's channels.

DL: CSI Channel State Information
Used by the UE to estimate the channel and report channel quality information (CQI) to the base station.

DL: MBSFN Multicast/Broadcast Single Frequency Network
Used for equalization of PMCH transmissions.

UL: DMRS
Used in UL transmission for channel estimation and for coherent demodulation.

Synchronization Signals Classifications
PSS (Primary Synchronization Signals)
&
SSS (Secondary Synchronization Signals)

Used by the UE to obtain the cell identity and frame timing.

This arrangement creates 504 unique physical cell identities.

You can obtain N_ID(2) by successfully demodulating the PSS.
The SSS can then be demodulated and combined with the knowledge of N_ID(2) to obtain N_ID(1).
Once you establish the values you can determine the cell identity (CELL ID) N_ID(CELL).

PSS Mapping

The PSS is mapped into the first 31 subcarriers either side of the DC subcarrier. Therefore, the PSS uses six resource blocks with five reserved subcarriers each side.