5G-NR: Channels

5G-NR: Channels

Introduction:

         In this blog, we will discuss all types of changes, their functionality, and channel mapping in short. Like LTE, NR channels are the same.

Mainly there are three types of channels :

1- Logical channels

2- Transport channels

3- Physical channels 




1- Logical Channels:

           Logical channels are functioning between RLC and MAC layers. There are 5 types of logical channels. Logical channels are further divided into two groups, Fist is control channels and second is traffic channels. below are the logical channels and their short description.

1- BCCH (Broadcast control channels)

2- PCCH (Paging control channels)

3- CCCH (Common control channels)

4- DCCH (Dedicated control channels)

5- DTCH (Dedicated traffic channels)  => Traffic channel

1- BCCH (Broadcast control channels):

             The network always transmits the BCCH over the air in the downlink. This is downlink broad channels.(gNB --> UEs).  it is used to transmit the system information messages like SIBs and MIB in the downlink. 

=> In 5G NSA(non-standalone mode), system information is not transmitted over BCCH channels. it is transmitted via master note i.s. LTE eNodeB.

=> in 5g SA(Standalone mode), system information(MIB and SIBs) transmitted through the BCCH channel.

2- PCCH (Paging control channels):

             PCCH is also a downlink channel and it is used for transmitting paging information from the network to devices. whenever the network needs to find out the devices, then the network uses the PCCH channel for paging information.

             It can also be used to transmit the system information change notification and an indication of ongoing PWS (public warning system) broadcast.

=> In 5G NSA(non-standalone mode): paging is not transmitted over PCCH channels. it is transmitted via master note i.s. LTE eNodeB.

=> in 5g SA(Standalone mode), paging transmitted through the PCCH channel.

3- CCCH (Common control channels):

             CCCH is used by a device to establish or re-establish an RRC(radio resource control) connection. this is referred to as SRB(Signaling radio bearer) 0.

4- DCCH (Dedicated control channels):

            This is a two-way channel for the transfer of control information when the device has an RRC connection. SRBs when DCCH is activated including 

SRB -1: This is used for RRC message

SRB -2: This is used for NAS(non-access stratum) messages and has a lower priority than SRB -1.

SRB -3: This is newly introduced in 5G NSA(non-standalone mode ). this is used to configure measurements, MAC, RLC, physical layer parameters as well as RLF(radio link failure) parameters.

5- DTCH (Dedicated traffic channels):

              This is a point to point channel that may exist in the uplink and downlink. It is a part of DRB(Data radio Bearer) assigned to the devices. this channel is mainely used for transfring the userdata

2-Transport channel:

           Transport channels are functioning between MAC layer and Physical layer. there are 5 types of Transport channels. 

1- BCH  (Broadcast channel)

            This is a broadcast channel that is a part of the SS(Synchronization signal) block. it includes the MIB.

2- DL-SCH  (Downlink chared channel)

            This channel supports dynamic scheduling and dynamic link adaptation by varying the antenna mapping, modulation, coding scheme, and resources/power allocation. In addition, it supports HARQ (Hybrid automatic repeat request) operation to improve performance.

3- PCH  (Paging Channel)

            This channel is used to carry the PCCH, It utilizes DRX(discontinuous reception) to improve the battery life.

4- UL-SCH  (Uplink shared channel)

             This is similar to DL-SCH, it used for uplink data transmission. It also supports the DRX algorithems for device power saving.


5- RACH (Random access channel)
            This channel carries limited information and is used in configuration with physical channel and preamble to improve contention resolution procedure.

This channel is also defined in the transport channels, although it does not carry transport block.










3- Physical Channels:
      Physical channels are used to transmit the signals on the air. There are 6 types of Physical channels. in which three channels are downlink channels and the other three channels are uplink channels.

1-PBCH     (Physical broadcast channel)

2-PD-SCH  (Physical downlink shared channel)

3-PDCCH   (Physical downlink control channel)

4-PRACH   (Physical Random access channel)

5-PU-SCH  (Physical uplink shared channel)

6-PUCCH   (Physical uplink control channel)

1-PBCH(Physical broadcast channel)
            The network always transmits the BCCH over the air in the downlink. This is downlink broad channels.(gNB --> UEs).  it is used to transmit the system information messages like SIBs and MIB in the downlink. 

=> In 5G NSA(non-standalone mode), system information is not transmitted over BCCH channels. it is transmitted via master note i.s. LTE eNodeB.

=> in 5g SA(Standalone mode), system information(MIB and SIBs) transmitted through the BCCH channel.

2-PD-SCH(Physical downlink shared channel)
          This channel supports dynamic scheduling and dynamic link adaptation by varying the antenna mapping, modulation, coding scheme, and resources/power allocation. In addition, it supports HARQ (Hybrid automatic repeat request) operation to improve performance.
it is also used for--
- Unicast data transmission
- Random access responce message
- delevery part of System information messages

3-PDCCH(Physical downlink control channel)
            This channel is used for downlink control information(DCI). downlink control information is necessary for proper reception and decoding of downlink user data.

4-PRACH(Physical Random access channel)
          This channel carries limited information and is used in configuration with physical channel and preamble to improve contention resolution procedure.
            This channel is also defined in the transport channels, although it does not carry transport block.

5-PU-SCH(Physical uplink shared channel)
        This is similar to DL-SCH, it used for uplink data transmission. It also supports the DRX algorithms for device power saving.

6-PUCCH(Physical uplink control channel)
                      This channel is used for uplink control information(DCI). uplink control information is necessary for scheduling and HARQ procedure.


 Channel Mapping:

5G-NR: Carrier Aggregation (CA)

5G-NR: Carrier Aggregation (CA)

Description:

        when there is no CA in the picture,  UE will receive and transmit data on a single carrier, this carrier is called primary component carrier and the corresponding cell is called a primary serving cell. In case of carrier aggregation, one or more component carriers are aggregated with the primary component carrier in order to support wider transmission bandwidth. 

Carrier Aggregation:

       Carrier Aggregation feature is introduced in the initial version of Release-15 of 3GPP Specifications. 5G New Radio uses carrier aggregation of multiple Component Carriers (CCs) to achieve high-bandwidth transmission (and hence high data rate). 

In LTE, you can aggregate a maximum up to five carriers that is one primary component carrier and four secondary component carriers. But in 5G NR supports aggregation of up to 16 components carriers.

    Carrier aggregation is designed to support aggregation of a variety of different arrangements of CCs, including CCs of the same or different bandwidths, adjacent or non-adjacent CCs in the same frequency band, including CCs of the same or different numerologies and CCs in different frequency bands. Each CC can take any of the transmission bandwidths, namely (5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 90, 100) MHz for FR1 & (50, 100, 200, 400) MHz for FR2 respectively.

 A UE that is configured for carrier aggregation connects to one Primary Serving Cell (known as the ‘PCell’ in MCG or ‘PSCell’ in SCG) and one or more Secondary Serving Cell (known as ‘SCell’).

All RRC connections and Broadcast signalings are handled by the Primary serving cell. The primary Serving cell is the master of the whole procedure. Primary serving cell decides that which serving cell need to be aggregated or added and deleted from the Aggregation.

      
Now we will look into the role of Primary serving cell and secondary serving cell in terms of carrier aggregation.

1- Role of Primary serving cell: followings are the role of primary serving cell.

=> Dynamically add or remove the secondary component carriers.
=> Dynamically activate and deactivate the secondary cell.
=> Handle all RRC(Radio resource control) and NAS(non-access stratum) procedures.
=> Receive measurement reports and control mobility of UE.

Note: Primary serving cell can be changed only at the time of handover.


2- role of Secondary serving cell: followings are the role of secondary serving cell.

=> An UE can aggregate maximum up to 16 component carrier where 1 is primary component carrier and 15 are secondary component carrier. (In case of LTE it is 1PCC and 4 SCC).
=>Actual number of secondary serving cell that can be allocated to UE is dependents on UE capability.

Note: It is not possible to configure an UE with more UL CCs than DL CCs, while revere of this can be possible.

==================================================================
There are mainly three ways by which component carriers can be allocated.

1- Intra Band Contiguous:
 In this Primary component carrier and secondary component carrier is configured with same band but they are contiguous.



2- Intra Band Non-Contiguous:

   In this Primary component carrier and secondary component carrier is configured with same band but they are not contiguous.



3-inter band Contiguous:

 In this Primary component carrier and secondary component carrier  are allocated on two frequency band.





By using the above configuration, infinite combinations are possible. But 3GPP has defined allowed combinations

Denoting Band combination:

CA_X:

        Denotes intra band contiguous CA

        e.g CA_10(band)

CA_X-X:

        Denotes intra band non-contiguous CA

        e.g CA_10-10

CA_X-Y:

        Denotes inter band contiguous CA

        e.g CA_10-20

Precondition for CA:

     UE can be configured CA only when it is capable to support CA. UE informs its  capability to the network during registration procedure in "UE capability information" message to network.

5G NR: DCI Formats

5G NR: DCI Formats

Introduction: 

         In this blog of our 5g series, we discussed downlink control information or DCI. we will look at its content on how it is encoded and modulated then mapped to the 5g new radio slot etc.

DCI: 

=> downlink control information or DCI carries control information used to schedule user data PD-SCH on the downlink and PU SCH on the uplink.

=> it is carried by the PDC CH or physical downlink control channel.

=> it indicates the location in time and frequency of the data that is scheduled for transmission.

=> the modulation and coding schemes used the number of antenna ports or layers as well as other aspects such as HARQ.

=> the user equipment needs to decode the DCI before they can decode downlink data or transmit uplink data depending on the content of the DCI.

=> one or more of several formats can be used.

 => Format 0 is for uplink grant meaning that it contains information that pertains to data the UE is about to transmit on the uplink.

=> Format 1 is for downlink allocation this means it includes information about the way data was sent to the UE.

=>For both uplink and downlink information there are two possible formats one with

underscore zero(0) and one with an underscore one(1).

=> The format with underscore zero is called the fallback format it is more compact than the full format with underscore one because it doesn't include all options and therefore it trades off less scheduling flexibility for reduced control overhead.

=> finally format 2 addresses the information needed for groups of UEs and TPC commands.

=> Downlink control information uses polar code for error protection this is the main difference with encoding in LTE where tell binding convolutional encoding was used.

=> another difference with LTE where that the CRC used here is longer at 24 bits instead

of 16 for LTE.

=> the CRC value is crumbled with a UE identifier called the radio network temporary identifier(RNTI) in order to indicate that which UE the message is intended for.

=> After encoding downlink control information is scrambled with QPSK modulated and mapped to resource blocks with a very specific pattern.

=>UE must look for PDCCH and decode the PDCCH to get the required DCI information for further processing.

=> There are several significant differences with LTE:

1-  first the PDC CH may not spend the complete 5g bandwidth whereas in LTE it always does this. it is important because of the bandwidth may be much larger up to 400 Mhz in 5g and UEs in 5G or not required to support large bandwidth.

2- PDCCH in 5g supports device-specific beamforming this means, control information can be beamed toward a particular UE, this is possible because of the PDCCH has associated DMRS or demodulation reference symbols which undergo the same beamforming. it is similar to the concept of EPDCCH that was introduced late in LTE deployment.

    

 note that P DCCH is mapped to a corset or control resource set a concept that defines the location of a control region within the 5G resource grid.

Examples:

let us now look at two concrete examples of DCI usage first for downlink data scheduling.

For Downlink:

=>  The UE looks for the pc CH and if a match is found meaning that a block decoded with a CRC that matches the RNTi of a UE. it passes the DCI and extracts all information about where in time and frequency data is located and how data was sent to the UE, with this information, the UE can grab the relevant parts of the 5g grids.

=>Performs channel estimation equalization inverse rate matching and decoding to

retrieve the downlink data packet.

For uplink:

 => for the uplink transmission downlink control information carrying an uplink grant. comes in response to a scheduling request from the UE when the gNB received the scheduling request, it makes all the decisions about when and how the UE should transmit the data that is ready for transmission.

=> Those parameters include beside the time and frequency location and modulation and coding scheme other information such as precoding which comes in the form of an index that points to a table of possible precoding matrices.

=>After decoding the control information for the uplink grant remember this would be format 0_0 and format 0_1. The UE transmits uplink data according to those parameters.

=> To understand how downlink information is mapped to the 5g grid, we must introduce two new concepts.

1- resource element groups
2- control channel elements or CCE

1-Resource element group:
    The resource element group is simply a block of 12 resource elements by one symbol. this is the basic unit used to define CCEs.

2- control channel elements or CCE

=> one control channel element corresponds to six resource element groups this means that one CCE includes six times 12 resource elements that equals 72.

1CCE = 6x12 = 72 resource elements

54 are available for the PDCCH itself.

18 are reserved for associated DMRS or demodulation reference symbols.

 =>one PDCCH is mapped to one or more CCEs. the standard defines several aggregation

levels as in LTE except for the introduction of a new level of 16 which was not available in LTE. 

=> The higher the aggregation level the more resources are used but the more possibility for redundancy enhance.

DCI_Format 0_0 Decoder:

DCI Format 0_0:

This is used for the scheduling of PUSCH in a single cell and it is shared by gNB to UE in DL direction. UE is required to decode the UL grant on PDCCH, so that the UE can transmit PUSCH on UL Slot in Uplink. DCI 0_0 is mainly used for common search space signaling and UL Grant.

Field of DCI 0_0:

Decoder

---

Number of BWP RBs :

UL-DCI HexValue      :

Binary value :

DCI Fields	No of bits	Bits	Decimal value
Identifier for DCI formats	1
Frequency domain resource assignment
Time domain resource assignment	4
Frequency hopping flag	1
Modulation and coding scheme	5
New data indicator	1
Redundancy version	2
HARQ process number	4
TPC command for scheduled PUSCH	2
Padding Bits	x
UL/SUL indicator	x

---

5G-NR: MIB

5G-NR: MIB

Introduction:

                   When the UE switches on, it starts listening to the SSB(PSS/SSS/PBCH) for synchronization and tried to get the system information. after the successful cell setup gNB always broadcasting the MIB according to the SSBs occasions.

MIB:

MIB (Master information block) includes the system information and always transmitted on the BCH from network to UE with a periodicity of 80 ms and repetitions made within 80 ms.

it also includes the parameters that are needed to acquire/decode the SIB1 from the cell. 

The first transmission of the MIB is scheduled in subframes defined by [TS 38.211, 7.4.3.2] and repetitions are scheduled according to the period of SSB;

It uses QPSK modulation for transmission from the network to UE It is transmitted on OFDM symbol 1,2,3.

RRC parameters of MIB:

NR PBCH under SSB:

MIB ::=                             SEQUENCE {

    systemFrameNumber                   BIT STRING (SIZE (6)),

    subCarrierSpacingCommon             ENUMERATED {scs15or60, scs30or120},

    ssb-SubcarrierOffset                INTEGER (0..15),  è offset in number of sub carrier between SSB(RB-0) and OffsetToPointA.

    dmrs-TypeA-Position                 ENUMERATED {pos2, pos3},

    pdcch-ConfigSIB1                    PDCCH-ConfigSIB1,

    cellBarred                          ENUMERATED {barred, notBarred},

    intraFreqReselection                ENUMERATED {allowed, notAllowed},

    spare                               BIT STRING (SIZE (1))

}

Description if parameters:

systemFrameNumber:

The 6 most significant bit (MSB) of the 10-bit System Frame Number. The 4 LSB of the SFN is conveyed in the PBCH transport block as part of channel coding (i.e. outside the MIB encoding).

subCarrierSpacingCommon

Subcarrier spacing for SIB1, Msg.2/4 for initial access and broadcast SI-messages. If the UE acquires this MIB on a carrier frequency <6GHz, the value scs15or60 corresponds to 15 Khz and the value scs30or120 corresponds to 30 kHz. If the UE acquires this MIB on a carrier frequency >6GHz, the value scs15or60 corresponds to 60 Khz and the value scs30or120 corresponds to 120 kHz.

scs15or60	scs30or120
FR1	15 Khz	30 Khz
FR2	60 Khz	120 Khz

 

ssb-SubcarrierOffset

Corresponds to k_SSB (see TS 38.213 [13]), which is the frequency domain offset between SSB and the overall resource block grid in the number of subcarriers. (See 38.211).

The value range of this field maybe extended by an additional most significant bit encoded within PBCH as specified in 38.213 [13].

This field may indicate that this beam does not provide SIB1 and that there is hence no common CORESET (see TS 38.213 [13], section 13). In this case, the field pdcch-ConfigSIB1 may indicate the frequency positions where the UE may (not) find a SS/PBCH with a control resource set and search space for SIB1 (see 38.213 [13], section 13)

dmrs-TypeA-Position

Position of (first) DM-RS for in downlink (see 38.211, section 7.4.1.1.1) and in uplink (see 38.211, section 6.4.1.1.3).

cellBarred indicates whether the cell allows UEs to camp on this cell as per specification TS 38.304

intraFreqReselection indicates if Intra frequency cell reselection is Allowed or notAllowed. It controls cell reselection to intra-frequency cells when the highest ranked cell is barred, or treated as barred by the UE as specified in TS 38.304

The call flow of MIB and SIB:

NR and LTE comparison:

Parameters	Long Term Evolution (LTE)	New Radio (NR)
Broadcast Channel	Transport – BCH	Transport – BCH
	Physical- PBCH	Physical- PBCH
Periodicity	40 ms periodicity with 10 ms re-transmission periodicity	80 ms periodicity with repetitions made within 80 ms
Channel Coding	Tail Bit Convolution encoding	Polar Coding
Modulation	QPSK Modulation	QPSK Modulation
Resource Allocation	6 RBs (72 subcarriers) in Frequency domain	It is transmitted on OFDM symbol 1,2,3.
	4 symbols of first subframe second slot symbol 0, 1, 2 and 3.	It uses 0 to 239 subcarriers number on symbol 1 & 3, whereas on symbol 2 it uses sub carried number 0 to 47 and 192 to 239