SS Block in 5G-NR

References:

1) 5G NR The Next Generation Wireless access Technology by ERIK DAHLMAN, STEFAN PARKVALL, JOHAN SKOLD.

2) 3GPP TS 38.211 version 15.2.0 Release 15 ,5G; NR; Physical channels and modulation.

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 consisting of two parts, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS), is periodically transmitted on the downlink from each NR cell. The PSS/SSS, together with the Physical Broadcast Channel (PBCH), is jointly referred to as a Synchronization Signal Block or SS block.

The SS block serves a similar purpose and, in many respects, has a similar structure as the PSS/SSS/PBCH of LTE. However, there are some important differences between the LTE PSS/SSS/PBCH and the NR SS block. At least partly, the origin of these differences can be traced back to some NR-specific requirements and characteristics including the aim to reduce the amount of “always-on” signals, and the possibility for beamforming during initial access.

As with all NR downlink transmissions, SS-block transmission is based on OFDM. In other words, the SS block is transmitted on a set of time/frequency resources (resource elements) within the basic OFDM grid. Fig below illustrates the time/frequency structure of a single SS block transmission. As can be seen, the SS block spans four OFDM symbols in the time domain and 240 subcarriers in the frequency domain.

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

2nd OFDM symbol in SSB consists of PBCH+DMRS.The SSS is transmitted in the third OFDM symbol of the SS block and occupies the same set of subcarriers as the PSS. There are eight and nine empty subcarriers on each side of the SSS used for guard which are filled as nulls.

The PBCH is transmitted within the second and fourth OFDM symbols of the SS block. In addition, PBCH transmission also uses 48 subcarriers on each side of the SSS. The total number of resource elements used for PBCH transmission per SS block thus equals 576 (240+240+48+48). Note that this includes resource elements for the PBCH itself but also resource elements for the demodulation reference signals (DMRS) needed for coherent demodulation of the PBCH.

PBCH DMRS : PBCH DMRS is a special type of physical layer signal which functions as a reference signal for decoding PBCH. In LTE (at least in TM1, 2, 3, 4), we don’t need this kind of special DMRS for PBCH because we can use CRS(Cell Specific Reference Signal) for PBCH decoding.

Table below lists the different numerologies applicable for SS-block transmission together with the corresponding SS-block bandwidth and time duration, and the frequency range for which each specific numerology applies. Note that 60 kHz numerology cannot be used for SS-block transmission regardless of frequency range. In contrast, 240 kHz numerology can be used for SS-block transmission although it is currently not supported for other downlink transmissions. The reason to support 240 kHz SS-block numerology is to enable a very short time duration for each SS block. This is relevant in the case of beam-sweeping over many beams with a corresponding large number of time multiplexed SS blocks.

The SS block bandwidth is simply the number of subcarriers used for SS block(240) multiplied by SS block subcarrier spacing.

Let’s take an example of Numerology of 15 kHZ.

SSB Bandwidth = 15*20*12

= 3.6 MHz.

Here one Question comes in my mind as below:

Assuming FR1 operating band in 5g with 5 Mhz Channel bandwidth and 30 Khz carrier spacing is not able to cell search or we can say not able to get the initial access. What can be the reason?

The answer to the above question is :- The Synchronization signals are transmitted as a part of SS block which occupies 20 Resource Blocks. This means that if we have 5Mhz of channel bandwidth in FR1 which is using 30 Khz of carrier spacing then 20 RB cannot accomodate within 5Mhz because Calculation = 30* 20* 12= 7.2 Mhz. So whenever channel bndwidth is 5Mhz only 15Khz (15*20*12= 3.6Mhz) can accomodate and Synch signals can be transmitted in SSB.

SS Block Periodicity:

The SS block is transmitted periodically with a period that may vary from 5 ms up to 160 ms. However, devices doing initial cell search, as well as devices in inactive/idle state doing cell search for mobility, can assume that the SS block is repeated at least once every 20 ms. This allows for a device that searches for an SS block in the frequency domain to know how long it must stay on each frequency before concluding that there is no PSS/SSS present and that it should move on to the next frequency within the synchronization raster.

Next Question comes into mind is what is Synchronization raster.

So we have two kinds of raster one is Channel raster and Synchronization raster.

Channel raster:- The channel raster defines a subset of RF reference frequencies that can be used to identify the RF channel position in the uplink and downlink. TheRF reference frequency for an RF channel maps to a resource element on the carrier. For each operating band, a subset of frequencies from the global frequency raster (The global frequency raster defines a set of RF reference frequencies FREF. The RF reference frequency is used in signalling to identify the position of RF channels, SS blocks and other elements) are applicable for that band and forms a channel raster with a granularity ΔFRaster, which may be equal to or larger than ΔFGlobal.

Synchronization raster :- The synchronization raster indicates the frequency positions of the synchronization block that can be used by the UE for system acquisition when explicit signalling of the synchronization block position is not present.

A global synchronization raster is defined for all frequencies. The frequency position of the SS block is defined as SSREF with corresponding number GSCN (Global Synchronization Raster Channel).

The 20 ms SS-block periodicity is four times longer than the corresponding 5 ms periodicity of LTE PSS/SSS transmission. The longer SS-block period was selected to allow for enhanced NR network energy performance. The drawback with a longer SS-block period is that a device must stay on each frequency for a longer time in order to conclude that there is no PSS/SSS on the frequency. However, this is compensated for by the sparse synchronization raster discussed above, which reduces the number of frequency-domain locations on which a device must search for an SS block.

Even though devices doing initial cell search can assume that the SS block is repeated at least once every 20 ms, there are situations when there may be reasons to use either a shorter or longer SS-block periodicity: • A shorter SS-block periodicity may be used to enable faster cell search for devices in connected mode.

• A longer SS-block periodicity may be used to further enhance network energy performance. A carrier with an SS-block periodicity larger than 20 ms may not be found by devices doing initial access. However, such a carrier could still be used by devices in connected mode, for example, as a secondary carrier in a carrier-aggregation scenario.

SS BURST & SS BURST SET

SS Burst :- An SS burst, consisting of multiple SS/PBCH blocks, can be generated by creating a larger grid and mapping SS/PBCH blocks into the appropriate locations, with each SS/PBCH block having the correct parameters according to the location.

SS Burst Set :- The set of SS blocks within a beam-sweep is referred to as an SS burst set.

Again question comes into mind that What is Beam sweeping?

Beam sweeping (with example) :- Beam Sweeping is a technique to transmit the beams in all predefined directions in a burst in a regular interval. For example, the first step in mobile terminal attach procedure is Initial Access, which is to synchronize with system and receive the minimum system information broadcast. So a “SS Block” carries the PSS, the SSS and the PBCH, and it will be repeated in predefined directions (beams) in time domain in 5ms window, this is called a SS burst, and this SS burst will be repeated in 20ms periodicity typically.

By applying beam-forming for the SS block, the coverage of a single SSblock transmission is increased. Beam-sweeping for SS-block transmission also enables receiver-side beam-sweeping for the reception of uplink random-access transmissions as well as downlink beam-forming for the random-access response.

Although the periodicity of the SS burst set is flexible with a minimum period of 5 ms and a maximum period of 160 ms, each SS burst set is always confined to a 5 ms time interval, either in the first or second half of a 10 ms frame.

Also it is to be noted that the maximum number of SS blocks within an SS burst set is different for different frequency bands. • For frequency bands below 3 GHz, there can be up to four SS blocks within an SS burst set, enabling SS-block beam-sweeping over up to four beams;

• For frequency bands between 3 GHz and 6 GHz, there can be up to eight SS blocks within an SS burst set, enabling beam-sweeping over up to eight beams;

• For higher-frequency bands (FR2) there can be up to 64 SS blocks within an SS burst set, enabling beam-sweeping over up to 64 beams.

TS 38.211 Section 7.4.3.1 defines the Synchronization Signal / Physical Broadcast Channel (SS/PBCH) block as 240 subcarriers and 4 OFDM symbols containing the following channels and signals:

  • Primary synchronization signal (PSS)
  • Secondary synchronization signal (SSS)
  • Physical broadcast channel (PBCH)
  • PBCH demodulation reference signal (PBCH DM-RS)

The Primary Synchronization Sequence (PSS) :- The PSS is the first signal that a device entering the system will search for. At that stage, the device has no knowledge of the system timing. Furthermore, even though the device searches for a cell at a given carrier frequency, there may, due to inaccuracy of the device internal frequency reference, be a relatively large deviation between the device and network carrier frequency. The PSS has been designed to be detectable despite these uncertainties. Once the device has found the PSS, it has found synchronization up to the periodicity of the PSS. It can then also use transmissions from the network as a reference for its internal frequency generation, thereby to a large extent eliminating any frequency deviation between the device and the network.

The PSS extends over 127 resource elements.There are three different PSS sequences x0 , x1 , and x2, derived from different cyclic shifts of a basic length-127 M-sequence.

Secondary Synchronization Sequence (SSS) :-Once a device has detected a PSS it knows the transmission timing of the SSS. By detecting the SSS, the device can determine the PCI of the detected cell. There are 1008 different PCIs. However, already from the PSS detection the device has reduced the set of candidate PCIs by a factor 3. There are thus 336 different SSSs, that together with the already-detected PSS provides the full PCI. Note that, since the timing of the SSS is known to the device, the per-sequence search complexity is reduced compared to the PSS, enabling the larger number of SSS sequences. The basic structure of the SSS is the same as that of the PSS that is, the SSS consists of 127 subcarriers to which an SSS sequence is applied.

PBCH :- While the PSS and SSS are physical signals with specific structures, the PBCH is a more conventional physical channel on which explicit channel-coded information is transmitted. The PBCH carries the master information block (MIB), which contains a small amount of information that the device needs in order to be able to acquire the remaining system information broadcast by the network.

Information carried by PBCH

1) The SS-block time index is provided to the device as two parts: • An implicit part encoded in the scrambling applied to the PBCH.

• An explicit part included in the PBCH payload. Eight different scrambling patterns can be used for the PBCH, allowing for the implicit indication of up to eight different SS-block time indices. This is sufficient for operation below 6 GHz (FR1) where there can be at most eight SS blocks within an SS burst set. For operation in the higher NR frequency range (FR2) there can be up to 64 SS blocks within an SS burst set, implying the need for three additional bits to indicate the SS-block time index. These three bits, which are thus only needed for operation above 10 GHz, are included as explicit information within the PBCH payload.

2) The CellBarred flag consist of two bits:

• The first bit, which can be seen as the actual CellBarred flag, indicates whether or not devices are allowed to access the cell;

• Assuming devices are not allowed to access the cell, the second bit, also referred to as the Intra-frequency-reselection flag, indicates whether or not access is permitted to other cells on the same frequency.

If detecting that a cell is barred and that access to other cells on the same frequency is not permitted, a device can and should immediately re-initiate cell search on a different carrier frequency. It may seem strange to deploy a cell and then prevent devices from accessing it. Historically this kind of functionality has been used to temporarily prevent access to a certain cell during maintenance.

3) The 1st PDSCH DMRS position indicates the time-domain position of the first DMRS symbol assuming DMRS Mapping Type A.

4) The SIB1 numerology provides information about the subcarrier spacing used for the transmission of the so-called SIB1, which is part of the system information.The same numerology is also used for the downlink Message 2 and Message 4 that are part of the random-access procedure. Although NR supports four different numerologies (15 kHz, 30 kHz, 60 kHz, and 120 kHz) for data transmission, for a given frequency band there are only two possible numerologies. Thus, one bit is sufficient to signal the SIB1 numerology.

5) The SIB1 configuration provides information about the search space, corresponding CORESET, and other PDCCH-related parameters that a device needs in order to monitor for scheduling of SIB1.

6) The CRB grid offset provides information about the frequency offset between the SS block and the common resource block grid.The frequency-domain position of the SS block relative to the carrier is flexible and does not even have to be aligned with the carrier CRB grid. However, for SIB1 reception, the device needs to know the CRB grid. Thus, information about the frequency offset between the SS block and the CRB grid must be provided within the PBCH in order to be available to devices prior to SIB1 reception. Note that the CRB grid offset only provides the offset between the SS block and the CRB grid. Information about the absolute position of the SS block within the overall carrier is then provided within SIB1.

7) The half-frame bit indicates if the SS block is located in the first or second 5 ms part of a 10 ms frame. The half-frame bit, together with the SS-block time index, allows for a device to determine the cell frame boundary.

All information above, including the CRC, is jointly channel coded and ratematched to fit the PBCH payload of an SS block. Although all the information above is carried within the PBCH and is jointly channel coded and CRC-protected, some of the information is strictly speaking not part of the MIB. The MIB is assumed to be the same over an 80 ms time interval (eight subframes) as well as for all SS blocks within an SS burst set. Thus, the SS-block time index, which is inherently different for different SS blocks within an SS burst set, the half-frame bit and the four least significant bits of the SFN are PBCH information carried outside of the MIB.



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