SSB Part-2

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5G synchronization process:

In a specific protocol stack implementation, the frequency and time synchronization of the first step can be implemented in a process that is followed by the decoding process of the subsequent PSS and SSS. For example, a possible implementation process can be explained as below:

First of all the terminal tunes the RF receiver to a designated frequency point.

Perform cross-correlation detection on the PSS in the time domain to get time domain synchronization and obtain the cell ID (NID2).

In the first symbol time of the SSB, only the PSS signal is in the SSB frequency domain, so it can be correlated. On the contrary, because there is a PBCH in the third symbol time of the SSS, it is impossible to perform time domain correlation detection on it, see below Figure:

As per, the location of the PSS, the location of the SSS can be obtained, and the cross-correlation detection is performed on the SSS in the frequency domain, and the frequency domain synchronization can be obtained and the cell group ID (NID1) is obtained at the same time.

The PCI can be obtained by the cell ID and the cell group ID, and the PBCH DMRS can be further decoded by the PCI to obtain the SSB index and the half frame number, because these are all components of the generated DMRS. (38.211

Finally, using the channel estimation for the DMRS, the terminal decodes the PBCH and obtains the system message MIB.

4G (LTE) synchronization process :

When a device is powered on, it is not aware of the full system bandwidth. So it does a frequency search, and it first syncs with the DC sub-carrier. 

Using this information, the device reads central 6 RBs (72 subcarriers).

The primary and secondary synchronization signals (PSS and SSS) and the PBCH (containing the MIB) all are contained in the central 72 subcarriers (6 resource blocks) of the system bandwidth. Initially, the Network allows the device to demodulate just this central region.

On demodulating this info, the device gets the RSSI value using which it initiates Initial Acquisition process. It decodes PSS and SSS for Time and Frame Synchronization. 

Using this info, the device also finds the PCI (Physical Cell ID). The device subsequently received MIB and SIB from the NW and then initiates the process of cell selection.

UE  synchronization process in SA Case:

In case of SA once UE is powered on, it tunes the specific frequency and also scans the frequency band on sync raster based on which UE performs cell search. The synchronization raster shows the frequency positions of the SSB. This can be used by the UE for system acquisition when explicit signaling of the SSB position is absent.

NOTE: for each band the synchronization raster and the sub carrier spacing of the SSB is defined separately.

If UE fails to detect the SSB (PSS, SSS) it will tune to next frequency. Once UE successfully decode the SSB, now UE has value of PSS and SSS and so it now tries to decode PBCH. From PBCH UE decodes MIB. So now MIB has information to decode RMSI. Now UE gets SIB1 details in which cell search and RACH information etc are present. 

UE synchronization process in NSA case:

If network support MRDC, then network broad casts PLMN_InfoList_r15 IEs in SIB 2. If UE supports MRDC then in attach request message DCNR bit is set. This information including supported security algorithm is in UE network capability. In UE capability enquiry message network asks for EUTRA-NR capability and UE replies the same in UE capability information. After this UE is attached with LTE network.

Network can add secondary node two ways, first base on measurement using B1 event and second eNB instructs to the UE to add secondary node in RRC reconfiguration message. In which channel info RACH parameters, SCS etc information is present for SCG configuration. Then UE attempt RACH and adds SCG.

Physical-layer Cell Identity (PCI) in 5G:

There are 1008 unique PCIs defined in 5G NR, double of that in LTE which is 504.

1008 NR PCIs are divided into 336 unique PCI groups and each group consists of three different identities.

PCI of a cell can be calculated using following equation:
NIDCell = 3 * NID(1) + NID(2) where NID(1) ∈ {0,1, … ,335} and NID(2) ∈ {0,1,2}

The DUT derives physical cell identity group number NID(1) from SSS and physical-layer identity NID(2) from PSS.

The SSB Basic:

As mentioned in previous part to enable devices to find a cell when entering a system, as well as to find new cells when moving within the system, a synchronization signal consists of two parts namely the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS), is periodically transmitted on the downlink from each NR cell. 

The PSS and SSS, together with the Physical Broadcast Channel (PBCH), is jointly referred to as a Synchronization Signal Block or SS block.

The Synchronization signals can also be used by the UE for RSRQ and RSRP measurements.

The PSS is transmitted in the first OFDM symbol of the SSB and occupies 127 subcarriers in the frequency domain. The remaining subcarriers are empty.

The SSS is transmitted in the 3rd OFDM symbol of the SSB and occupies the same set of subcarriers as the PSS. There are in all 8 and 9 empty subcarriers on each side of the SSS.

The PBCH is transmitted in the second and fourth OFDM symbols of the SS block. In addition, PBCH transmission also uses 48 subcarriers on each side of the SSS.( PBCH symbols carry its own frequency-multiplexed DMRS. QPSK modulation is used for PBCH.)

The total number of resource elements used for PBCH transmission per SS block is thus equals 576.

Note:  This includes REs for the PBCH itself but also resource elements for the DMRS(demodulation reference signals -which is also occupying 144REs) needed for coherent demodulation of the PBCH.

Different numerologies can be used for SSB transmission. However, to limit the need for devices to simultaneously search for SSB of different numerologies, there is in many cases only a single SSB numerology defined for a given frequency band.

In Upcoming Part we will learn more about SSB and SSB interview question and its answer. And for more learning please go through our 5G and LTE page.

Stay tune for more Update…………………

Pinal Dobariya…

1) 3GPP TS 38.211,
2) 3GPP TS 38.212,
3) 3GPP TS 38.213,
4) 3GPP TS 38.331, 38.300
5) TR 38.912


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