NOTE : This note is about a tempary 5G specification that was implemented and tried before 5G specification is finalized. I keep this note for study purpose.

As mentioned in Overview page, one of the critical differences in terms of waveform design between 5G Pretrial and current LTE is that in 5G Pre Trial the subcarrier spacing is 75 Khz as opposed to 15 Khz in current LTE.  It means Subcarrier spacing of the Pre-Trial is 5 times wider than LTE subcarrier spacing. As a result, the OFDM symbol length of 5G Pre Trial gets 5 times shorter than LTE symbol length. This in turn gives result that the length of one subframe in Pretrial spec gets 5 times shorter than LTE subframe length. However, 5G Pretrial set the length of one Radio Frame to be same as in LTE.  It means that in 5G Radio Frame are 5 times more subframes than LTE radio frame.

Confusing ???

I know it is confusing :). Let me put these in tabular format.

Followings are the items that will be briefly described in this page. I will create separate pages for each of the Physical channels for the details, but you would have bird's eye view over overal frames in single page here.

• Subframe Types
• Radio Frame Structure - Downlink
• PSS - Primary Synchronization Signal
• SSS - Secondary Synchronization Signal
• ESS - Extended Synchronization Signal
• xPBCH
• BRS - Beam Reference Signal
• BRRS - Beam Refinement Reference Signal
• ePBCH and ePBCH DMRS
• xREG - Resource Element Group
• xPDCCH and xPDCCH DMRS
• xPDSCH PNRS - Phase noise reference signal
• Comparison with the NR Frame Structure

Subframe Types

Both trial specification defines four different subframe structure (type) as follows. Following illustration shows the possible structure of one subframe (0.2 ms) and each label stands for :

• Dc : Downlink Control
• Dd : Downlink Data
• Uc : Uplink Control
• Ud : Uplink Data
• GP : Gap

a. Subframe including DL control channel and DL data channel

b. Subframe including DL control channel, DL data channel and UL control channel

c. Subframe including DL control channel and UL data channel

d. Subframe including DL control channel, UL data channel and UL control channel.

The four drawings share two rules. Every subframe starts with Dc, so the UE can read a downlink or an uplink grant at the start of every 0.2 ms subframe. A GP appears wherever the subframe changes from downlink to uplink, because the UE needs time to switch its radio from receive to transmit.

So the direction of a subframe is not fixed by a TDD configuration table, as it is in LTE. It follows the subframe type and the scheduling of that subframe. Types b and d also end with Uc. With type b, the UE can send uplink control, such as HARQ feedback, in the same subframe that carried its downlink data.

• Every subframe starts with downlink control : the UE finds its grant at the start of each 0.2 ms subframe.
• A gap separates downlink from uplink : GP appears only where the direction changes.
• The direction changes with scheduling : the type of each subframe is not fixed by a TDD table, as it is in LTE.

Radio Frame Structure - Downlink

As mentioned above, in Pre trial specification one radio frame consists of 50 subframes. One example of the radio frame is shown below.  Subframe 0 and 25 are a kind of special subframe which carries synchronization signals (PSS, SSS, ESS) and Beam Forming Reference signal (BRS). The contents in all other subframes varies depending on subframe type and scheduling information.

Matlab Code :  TestFrame

In the example above, each column is one subframe, numbered 0 to 49, and the vertical axis is frequency. Subframes 0 and 25 look different from all the others, because they carry the synchronization block in the middle of the band. Two synchronization subframes per frame means the UE gets a synchronization opportunity every 5 ms, since 25 subframes of 0.2 ms make 5 ms.

As mentioned above, subframe 0 and 25 is special ones that carries various synchronization signal,reference signal and xPBCH. If I magnify the subframe 0, 25, it would look as follows.

< Structure of Subframe 0 and 25 >

The magnified part shows the centre of the band, between subcarrier 450 and 750. From the bottom, ESS, PSS and SSS sit in three separate blocks, and each block runs through all 14 OFDM symbols. BRS and xPBCH use the subcarriers above and below the synchronization block. The sections below take these signals one at a time.

• One radio frame holds 50 subframes : subframes 0 and 25 carry the synchronization signals, xPBCH and BRS.
• Synchronization repeats every 5 ms : 25 subframes of 0.2 ms separate the two special subframes.
• The synchronization block sits in the centre of the band : ESS, PSS and SSS each occupy their own block in all 14 symbols.

PSS - Primary Synchronization Signal

The purpose of Primary synchronization Signal is same as PSS in LTE. That is, it is used for Radio Frame Synchronization. It is defined in 211-6.8.1 Primary synchronization signal (PSS).

It is made up of 62 data based on Zadoff-Chu sequence and occupy 62 subcarriers, but in resource allocation total 72 sub carrier (6 RB) is allocated for the PSS. It means 5 sub carriers on both side of PSS is reserved (not used) as a kind of gap.

One big difference you see in comparision to current LTE would be that in 5G Pretrial the PSS is transmitted in all OFDM symbols whereas PSS is transmitted in only one OFDM symbol in LTE. This is because Network in PreTrial is transmitting PSS for 14 different antenna ports as indicated below.

Matlab Code :  PSS

The magnified view shows the 62 PSS subcarriers in every OFDM symbol of the subframe. The labels mark antenna port 300 on symbol 0 and antenna port 313 on symbol 13. So each symbol carries the PSS on its own antenna port, and the network can send each port on a different beam. A UE receives the PSS best in the symbol whose beam points toward it.

This also shows why the ESS is needed. The PSS looks the same in every symbol, so a UE that detects it still does not know which of the 14 symbols it received.

• PSS repeats in all 14 symbols : in subframes 0 and 25, once per antenna port from 300 to 313.
• Each symbol can use a different beam : the repetition works as a beam sweep over the 14 symbols.
• PSS alone does not give the symbol index : the sequence is the same in every symbol.

SSS - Secondary Synchronization Signal

The purpose of Secondary synchronization Signal is same as SSS in LTE. That is, it is used for Radio Frame Synchronization. It is defined in 211-6.8.2 Secondary synchronization signal (SSS).

It is made up of 62 data based on Gold Sequence and occupy 62 subcarriers, but in resource allocation total 72 sub carrier (6 RB) is allocated for the SSS. It means 5 sub carriers on both side of SSS is reserved (not used) as a kind of gap.

One big difference you see in comparision to current LTE would be that in 5G Pretrial the SSS is transmitted in all OFDM symbols whereas SSS is transmitted in only one OFDM symbol in LTE. This is because Network in PreTrial is transmitting SSS for 14 different antenna ports as indicated below.

Matlab Code :  SSS

The SSS follows the same pattern as the PSS, with one symbol per antenna port from 300 to 313. In frequency, the SSS sits directly above the PSS, as the subframe 0 picture shows. So after the UE detects the PSS, it knows where to look for the SSS. As in LTE, PSS and SSS together give the physical cell identity.

• SSS uses the same 14 port sweep as PSS : ports 300 to 313, one per symbol.
• SSS sits above PSS in frequency : each takes 62 subcarriers in a 72 subcarrier allocation.
• PSS and SSS give the cell identity : the same division of work as in LTE.

ESS - Extended Synchronization Signal

ESS is a new type of Synchronization signal that is not in current LTE. The purpose of Extended synchronization Signal is to identify the OFDM symbol index and transmitted in symbol 0-13 in subframes 0 and 25. It is defined in 211-6.8.3 Extended synchronization signal.

It is made up of 62 data based on Zadoff-Chu sequence and occupy 62 subcarriers, but in resource allocation total 72 sub carrier (6 RB) is allocated for the ESS. It means 5 sub carriers on both side of ESS is reserved (not used) as a kind of gap.

The ESS is transmitted in all OFDM symbols. This is because Network in PreTrial is transmitting ESS for 14 different antenna ports as indicated below.

Matlab Code :  ESS

The ESS solves the problem that PSS and SSS leave open. Both of them look the same in all 14 symbols, so they cannot tell the UE which symbol it received. The ESS is different in each symbol, so it gives the OFDM symbol index. With that index, the UE finds the subframe boundary, even when it can hear only one beam.

One number needs checking. The picture above labels the ESS block as 63 sub carriers, while the paragraph above it gives 62. The exact length is in 211-6.8.3.

• The ESS gives the OFDM symbol index : PSS and SSS repeat unchanged, so they cannot.
• The ESS is what makes the beam sweep usable : a UE that hears one beam still finds the subframe timing.
• The ESS sits below the PSS : it is the lowest of the three synchronization blocks.

xPBCH

xPBCH is broadcasted in subframe 0 and 25 and it carries broadcasting message (MIB) at each and every OFDM symbols using the same beams used for beam reference signals in each OFDM symbol.

Matlab Code :  xPBCH

The picture shows why xPBCH and BRS are described together. Outside the 18 RBs reserved for PSS, SSS and ESS, the RBs are split between them. In the magnified RBs, xPBCH takes the lowest four subcarriers of each RB, and the rest are reserved for BRS. xPBCH runs through all 14 symbols, so the MIB is sent once on every beam.

The MIB on xPBCH is small, and its main job is to tell the UE where to find the rest. The ePBCH section below shows this link, because the SFN and the subframe of ePBCH come from the configuration in the MIB. So the UE reads the signals of subframes 0 and 25 in a fixed order: PSS, SSS and ESS for timing, then xPBCH for the MIB, and then ePBCH for the system information.

• xPBCH carries the MIB on every beam : it is repeated in all 14 symbols of subframes 0 and 25.
• xPBCH shares each RB with BRS : four subcarriers for xPBCH, eight for BRS.
• The centre 18 RBs are not used : they are reserved for PSS, SSS and ESS.

BRS - Beam Reference Signal

BRS is a special reference signal to let UE knows of the identification of each Beam that the network transmit. It also functions as DMRS for xPBCH. 

It occupies 8 subcarriers(5th~12th subcarrier) in every RB(resrouce bloc) except the 18 RBs at the center of the frequency (Note : The 18 RBs at the center is reserved for PSS, SSS, ESS). It is transmitted at every symbols (i.e, symbol 0 ~ 13) in subframe 0 and 25 as shown below. The data is based on Pseudo Random Data (Gold Sequence).

Matlab Code :  BRS

The BRS picture is the mirror of the xPBCH picture. BRS takes subcarriers 5 to 12 of each RB, and the first four are reserved for xPBCH. BRS is sent in all 14 symbols with the same beams as xPBCH. So the UE can measure the received power of each beam and report the best one. The same fact explains why BRS acts as the DMRS for xPBCH. The UE decodes the MIB in a symbol with the channel that it has just measured on that beam.

• BRS identifies and measures each beam : one beam per symbol, 14 symbols per special subframe.
• BRS doubles as the xPBCH DMRS : both use the same beam in the same symbol.
• BRS takes eight subcarriers per RB : subcarriers 5 to 12, outside the centre 18 RBs.

BRRS - Beam Refinement Reference Signal

BRRS is the signal transmitted by gNB for Beam Refinement and the subframe and the symbols for BRRS is configured by DCI. The location of BRRS in frequency domain varies depending on antenna port as shown below.

In the example, BRRS sits in one OFDM symbol. Eight antenna ports, 600 to 607, take turns every four subcarriers, so each port repeats every 32 subcarriers. This is different from BRS. BRS is broadcast in fixed subframes for every UE. The network schedules BRRS by DCI for a UE that already has a beam from the BRS measurement, and the UE measures the BRRS ports to refine that beam.

The two beam signals form a two-step procedure. In the first step, the UE measures BRS in subframes 0 and 25 and reports the best beams. In the second step, the network sends BRRS around the reported beam, and the UE measures the BRRS ports to choose among them. NR keeps the same two steps, with SSB for the first and CSI-RS with repetition for the second, as the comparison at the end of this page shows.

• BRRS is scheduled, BRS is broadcast : DCI sets the subframe and the symbols of BRRS.
• BRRS carries eight ports : ports 600 to 607, interleaved every four subcarriers in the example.
• BRRS refines a beam that BRS found : the two signals work in sequence.

ePBCH and ePBCH DMRS

ePBCH is the channel that carries SIB (xSystemInformation). It can be transmitted in a specified SFN and a specified subframe depending on the configuration specified in MIB.

In the subframe assigned for ePBCH, it is transmitted in all symbols (symbol 0~13) and transmitted across the whole bandwidth (i.e, in all RBs). It occupies 10 subcarriers in each RB (all the subcarrier except 3rd and 9th subcarriers).

< ePBCH >

Matlab Code :  ePBCH

< DMRS for ePBCH >

Matlab Code :  ePBCH DMRS

< ePBCH and DMRS >

Matlab Code :  ePBCH and DMRS

The pictures above build the same resource grid in steps: ePBCH alone, DMRS alone, and then both together. In the magnified RBs, the DMRS takes two subcarriers per RB, the 3rd and the 9th, in every symbol. So the DMRS subcarriers are six subcarriers apart, and the UE can estimate the channel between them.

ePBCH does not share its subframe with BRS or with the synchronization signals. It uses the whole band, so the system information gets much more room than the MIB on xPBCH.

• ePBCH carries system information : its SFN and subframe come from the configuration in the MIB.
• ePBCH uses the whole band and all 14 symbols : 10 subcarriers per RB carry data.
• The ePBCH DMRS is on every sixth subcarrier : the 3rd and the 9th subcarrier of each RB, in every symbol.

xREG - Resource Element Group

xREG is not a specific physical channel. It is a term indicating a specific locations in control channel area. It is similar to CCE in LTE, but xREG allocation rule is much simpler than LTE CCE. There are 16 xREG within one OFDM symbol (Note : Depending on configuration, we can use only one OFDM symbol or two OFDM symbols for control region). Within one xREG, there are multiple groups of Resource Elements for carrying control channel data (DCI) and multiple groups of Resource Elements allocated to carry DMRS signal as illustrated below.

NOTE : This example shows the case where two OFDM symbols are allocated for downlink control region. But there can be cases where only one OFDM symbol is allocated for downlink control region.

The picture marks one xREG with a red box. In this example, one xREG covers 72 subcarriers, or 6 RB, in one OFDM symbol. 16 xREGs then cover 1152 subcarriers, and the top of the band above them is not part of the control region. Inside each xREG, some REs are reserved for xPDCCH DMRS, as the arrows show, and the rest carry DCI.

This is the simplification compared with LTE. In LTE, one CCE is made of nine REGs that are interleaved over the whole control region. In PreTrial, one xREG is one contiguous block in one symbol.

• One xREG is a contiguous block : 72 subcarriers in one OFDM symbol in this example.
• 16 xREGs fill one control symbol : the control region takes one or two symbols.
• Each xREG carries its own DMRS : some REs in every xREG are reserved for xPDCCH DMRS.

xPDCCH and xPDCCH DMRS

xPDCCH is the physical channel that carries DCI data and xPDCCH DMRS is the reference signal that helps UE to decode xPDCCH. The location of these data is allocated to a specific xREG which is explained in previous section.

NOTE : This example shows the case where two OFDM symbols are allocated for downlink control region. But there can be cases where only one OFDM symbol is allocated for downlink control region.

< Location (Resource Elements) for xPDCCH DMRS >

< Location (Resource Elements) for xPDCCH and DMRS >

The two pictures above use the same control region as the xREG picture. The DMRS picture shows the reference signal REs alone, repeated at a regular spacing through the control region. The combined picture adds the xPDCCH data REs around them. Because the DMRS sits inside every xREG, the UE can estimate the channel for one xREG on its own.

This matters at 28 GHz. The network sends the control channel on a beam toward the UE, and a cell-wide reference signal would not go through that beam. A DMRS inside the xREG goes through the same beam as the DCI.

• xPDCCH carries DCI in xREGs : the location of each xPDCCH is given in xREGs.
• The DMRS travels with the DCI : both use the same beam, so the channel estimate fits.
• No cell-wide reference signal is needed : unlike LTE, where the UE decodes PDCCH with CRS.

xPDSCH PNRS - Phase noise reference signal

At 28 GHz, the oscillators of the UE and the base station add phase noise that changes from one OFDM symbol to the next. A DMRS at the start of the data region cannot follow that change. So the xPDSCH carries an extra reference signal, the PNRS, over the rest of the subframe.

The picture below shows the PNRS for antenna ports 60 and 61 on two neighbouring subcarriers. In the magnified RBs, both ports start a few symbols into the subframe and run to its end, and the number of symbols is configurable. Across the band, the PNRS lines repeat at a regular spacing.

So PNRS is dense in time and sparse in frequency. Phase noise rotates all subcarriers of one symbol by about the same amount. A few subcarriers are therefore enough to measure it, but the UE has to measure it in almost every symbol.

The same need appears in the uplink. The overview page lists a phase noise reference signal for xPUSCH as well, because the oscillator of the UE adds phase noise to its own transmission. NR keeps the idea as PT-RS, in both the downlink and the uplink.

• PNRS tracks phase noise over the subframe : it is present in most symbols of the xPDSCH.
• PNRS uses ports 60 and 61 : on two neighbouring subcarriers in the example.
• PNRS is sparse in frequency : the phase rotation is common to the subcarriers of one symbol.

Comparison with the NR Frame Structure

Most elements on this page reappear in NR, often under a new name. The table below maps each PreTrial element to its closest NR counterpart. The match is in the concept rather than the numbers, because the NR numerology and resource grid are different.

The largest change is in synchronization. PreTrial gives one symbol to each beam inside two fixed subframes, and it needs the ESS to tell the symbols apart. NR puts PSS, SSS and PBCH into one SS/PBCH block of four symbols and sends one block per beam. The block index then replaces the ESS, so NR needs one signal less.

The control channel changed in the other direction. The PreTrial xREG is a simple block, while NR builds a CCE from six REGs and can interleave them over a CORESET. The DMRS inside the control region remained, because NR also beamforms the PDCCH.

• Most PreTrial signals have an NR counterpart : BRS, BRRS and PNRS became SSB, CSI-RS and PT-RS.
• The SS/PBCH block replaced the 14 symbol sweep : one block per beam, and the block index replaced the ESS.
• ePBCH became SIB1 on PDSCH : NR sends system information as ordinary scheduled data.
• The control channel kept its own DMRS : NR also sends PDCCH on a beam.