LTE Channel

Physical Channel

Downlink Channel

1. PBCH (Physical Broadcast Chanel)- It is used to broadcast MIB using BCH as transport and BCCH as logical channel.- The PBCH broadcasts a limited number of parameters essential for initial access of the cell such as

downlink system bandwidth, the Physical Hybrid ARQ Indicator Channel structure, and the most significant eight-bits of the System Frame Number.

As shown in the figure, PBCH is downlink only channel. It occupies 72 subcarriers belong to first 4 OFDMA symbols of second slot of every 10ms radio frame. Pls. note that reference signal resource elements(REs) are excluded from the PBCH allocation. Hence PBCH will occupy about (72 x 4) - 48 = 240 REs. As PBCH uses QPSK modulation, it leads to about 480bits per 240 REs.

- PBCH is downlink only channel.- It occupies 72 subcarriers belong to first 4 OFDMA symbols of second slot of every 10ms radio

frame. Pls. note that reference signal resource elements(REs) are excluded from the PBCH allocation. Hence PBCH will occupy about (72 x 4) - 48 = 240 REs. As PBCH uses QPSK modulation, it leads to about 480bits per 240 REs.

- These parameters are carried in - a Master Information Block (MIB) (14 bits long). This information is required to decode other physical channels.

- The PBCH is designed to be detectable without prior knowledge of system bandwidth and to be accessible at the cell edge also.

- The MIB is coded at a very low coding rate and mapped to the 72 center sub-carriers (6 RBs, each RBs = 12 sub carriers, so 6x12 = 72 subcarriers) of the OFDM structure.

- PBCH transmission is spread over four 10 ms frames (over subframe 0) to span a 40 ms period.- Each subframe is self decodable which reduces latency and UE battery drain in case of good signal

quality, otherwise, the UE would soft-combine multiple transmissions until the PBCH is decoded.- After the successful execution of the cell-search procedure, UE decodes the PBCH (MIB/SIBs).

Overhead Generated by LTE PBCH channel

LTE PBCH represents an overhead which reduces the number of REs available for user plane data. Overhead is less for larger bandwidth and more for extended CP(cyclic prefix). As mentioned in the table below LTE PBCH overhead is in-significant for larger channel BW and significant for small channel BW.

2. Physical Control Format Indicator Channel (PCFICH)The Physical Control Format Indicator Channel (PCFICH) is used at the starting of each 1ms subframe. It provides information about number of symbols used for PDCCH transmission. The signalling values for PCFICH depends upon channel bandwidth. The same is mentioned in the following table-1 for different LTE channel bandwidths.

As mentioned 1.4MHz requires more time domain symbols compare to other channel bandwidths due to less carriers in frequency domain. Signalling value depends on eNodeB RRM(Radio Resource Management). It depends on number of active connections. Hence PDCCH signalling increases as per increase in number of active connections.

As shown in the figure, LTE PCFICH channel occupies 16 REs(Resource Elements) in first OFDMA symbol of each 1ms frame. PCFICH uses QPSK modulation and hence 16 REs will occupy 32 bits. This 16REs are divided into 4 quadruplets. The position of which in first OFDMA symbol depends onChannel BW and Physical layer cell identity.As mentioned each quadruplet is mapped to REG(Resource Element Group) with subcarrier index k = k' and is as per following equation.

k'= (Nsc per RB /2) * (NCellID mod2 NDL-RB )

NcellID  = Physical Cell id

NDLRB = Number of resource blocks per bandwidth

NRBSC = Number of frequency carriers per Resource block

Lets suppose

Physical Cell id = 20

Bandwidth = 10Mhz  (NDLRB = 50)

Then according to 3GPP Formula

k_Bar = (12/2).(20 mod 2*50) = 6*20 = 120

Then the four PCFICH mapping values are below 

120

120 + (50/2)*(12/2) = 270

120 + 2*(50/2)*(12/2)  = 420

120 + 3*(50/2)*(12/2) = 570

The rest of three quadruplets are mapped to REGs spaces at intervals of (NDL-RB/2) * (Nsc per RB /2) from the first quadruplet and each other. This way LTE PCFICH channel information is spread across entire subframe as shown.

The PCFICH carries CFI(Control Format Indicator) which has a value ranging from 1 to 3. This CFI is coded to occupy complete PCFICH capacity of 32 bits.

Actual value = signalled value + 1 (for 1.4 MHz BW) Actual value = signalled value (for all the channel BWs)

Overhead due to LTE PCFICH channelLike LTE PBCH, PCFICH also introduces overhead which reduces number of resource elements neededfor user plane data. As mentioned in table-2 overhead is less for higher bandwidth and more forextended CP.

3. Physical Downlink Control Channel (PDCCH)It is used to carry DCI (Downlink Control Information). We get the information about number of OFDMA symbols used by PDCCH after decoding PCFICH. The symbols are always at the start of each subframe.

The REs(Resource Elements) allocated to PDCCH are grouped into group of 4 REs referred as quadruplets. RE quadruplets are grouped into CCE ( Control Channel Elements). There are 9 quadruplets in one CCE. Hence 36 REs per CCE. PDCCH uses QPSK which provides CCE capacity of about 72 bits.

There are total 4 PDCCH formats as described in document 3GPP TS 36.211. The same is mentioned in table-1 below :

The PDCCH format is used as per required size of DCI. The DCI bits have 16 bit CRC attached prior to rate-1/3 channel coding & rate matching modules. The table-2 below mentions coding rate for each DCI and PDCCH format. coding rate = number of DCI bits after CRC attachment/Capacity of PDCCH.

The number of CCE depends on channel BW and number of OFDMA symbols allocated for PDCCH. The table represents the same assuming no quadruplets have been allocated to PHICH and 4x4 MIMO is not used.

- This channel is used to inform the UE about the resource allocation of PCH and DL-SCH - indicating the modulation, coding and hybrid-ARQ information related to DL-SCH - Generally, a maximum of three or four OFDM symbols can be used for PDCCH - The information carried on PDCCH is referred to as downlink control information (DCI)- Uses QPSK modulation

4. Physical Downlink Share Channel (PDSCH)The PDSCH channel is the main data bearing channel which is allocated to users on a dynamic and opportunistic basis. The PDCH is also used to transmit broadcast information not transmitted on the PBCH which include System Information Blocks (SIB) and paging & RRC signalling messages. PDSCH is also used to transfer application data.

- Paging messages-These are broadcast using PDSCH channel. LTE UE in RRC IDLE mode monitor PDCCH for paging indications. Based on trigeer it will decode the paging message carried in PDSCH RBs.

- Downlink RRC Signalling messages-These are carried by PDSCH. Signalling Radio Bearers(SRB) will use PDSCH. Every connection usually will have its own set of SRB.

For LTE PDSCH channel uses QPSK, 16QAM, 64QAM modulation types. LTE eNodeB selects suitable modulation type based on adaptation algorithm. This further depends on radio channel condition and buffer capacity. QPSK is most robust scheme and hence it is used for transmission of SIB and Paging. As the name suggests PDSCH is a shared channel and hence its RBs are shared among all the active connections.

5. Physical Multicast Channel (PMCH)This channel defines the physical layer structure to carry Multimedia Broadcast and Multicast Services (MBMS). This control channel occupy the first 1, 2, or 3 OFDM symbols in a subframe extending over the entire system bandwidth.For PMCH channel QPSK, 16QAM, 64QAM modulations are used. It carries MCH. Multicast Channel (MCH) characterised by:

- Requirement to be broadcast in the entire coverage area of the cell- Support for MBSFN combining of MBMS transmission on multiple cells- Support for semi-static resource allocation e.g. with a time frame of a long cyclic prefix.

In Downlink, MTCH logical channel can be mapped to DL-SCH and MCH transport channels.

6. Physical Hybrid ARQ Indicator Channel (PHICH)The PHICH is used to carry positive or negative acknowledgements for uplink data transferred on the PUSCH. They are referred as ACK/NACK. Each connection can carry a max. of 1 TB(Transport Block) per subframe on PUSCH. Hence a maximum of 1 PHICH ACK per subframe per connection is needed.

A PHICH ASK is identified by two parameters : - PHICH Group- PHICH Orthogonal sequence index within PHICH group

As shown in the figure, three REGs that support a PHICH group are evenly distributed across LTE system BW. This provides frequency diversity. As mentioned here PCFICH always appear and will occupy first symbol of each subframe. PCFICH occupy 4REGs irrespective of channel bandwidth.

The number of PHICH groups is a function of downlink channel bandwidth and PHICH group scaling factor. Both the downlink channel BW and PHICH group scaling factor are tranmitted within MIB and carried by LTE PBCH channel. The table-1 below mentions number of PHICH groups for various scaling factor,CPs and channel BWs.

The no. of PHICH groups become double when using extended CP because no. of orthogonal sequences for each group became halved.

- Normal CP has 8 orthogonal sequences per PHICH group - Extended CP has 4 orthogonal sequences per PHICH group

A UE selects its PHICH group and orthogonal sequence index as per following equation:

PHICH Group = ( Ilowest + nDMRS ) *mod( N group )

PHICH Sequence Index = ( [Ilowest / N group] + nDMRS ) * mod(Nseq)

Where ;

Ilowest = Lowest physical RB index allocated to slot-1 of PUSCH nDMRS = Demodulation RS cyclic shift signalled within LTE DCI-0N group = No. of PHICH groupsNseq = No. of orthogonal sequences per PHICH group (Normal CP: 8 and Extended CP:4)

Each PHICH ACK ismodulated using BPSK modulation scheme and occupies :- 12 REs when using normal CP- 6 REs when using extended CP

PHICH ACK belongs to same group will occupy same set of REs and are differentiated using orthogonal sequences. In extended CP case, PHICH groups are paired and each pair of groups occupy 12 REs.

The Set of 12 REs allocated to each PHICH group is divided into three quadruplets. MIB on PBCH indicates whether PHICH uses normal CP or extended CP. A normal CP means PHICH uses first OFDMA symbol belongs to a subframe. A extended CP means PHICH uses first 3 OFDMA symbols belong to a subframe.

Uplink Channel

1. Physical Uplink Control Channel (PUCCH)This LTE channel is used to carry UCI(Uplink Control Information). UCI can also be transported using PUSCH channel. An LTE UE can never transmits both PUCCH and PUSCH during the same subframe.

- If UE has application data OR RRC signalling then UCI is carried over PUSCH- If UE does not have any application data OR RRC signalling then UCI is carried over PUCCH

This is a stand-alone uplink physical channel. This PUCCH control signaling channel comprises following :- HARQ ACK/NACK- CQI-channel quality indicators- MIMO feedback - RI(Rank Indicator),PMI(Precoding Matrix Indicator)- Scheduling requests for uplink transmission- BPSK or QPSK used for PUCCH modulation

PUCCH consists of 1 RB/transmission at one end of the system bandwidth which is followed by another RB in the following slot(at opposite end of the channel spectrum). This makes use of frequency diversity with 2dB estimated gain. A PUCCH Control Region comprises every two such RBs.

The standard specifies 6 LTE PUCCH formats as mentioned in the table-2 below. As mentioned PUCCH format 2a and 2b are not applicable for extended CP.

The LTE PUCCH channel is allocated 2 RBs at the edges of channel BW. Each PUCCH transmission occupy 1 RB on each side of the channel bandwidth. These two RBs are distributed across two time slots. RB numbering for PUCCH starts on outside edges and increases inwards. PUCCH is allocated RBs at the edge of channel BW to avoid fragmenting RBs available to PUSCH.

2. Physical Uplink Shared Channel (PUSCH)This channel is used to carry RRC signalling messages, UCI (uplink Control Information) and application data. Uplink RRC messages are carried using PUSCH. SRB use PUSCH and each and every connection will have its unique SRB.

- LTE PUSCH channel contain user information data.- The PUSCH carries both user data as well as control signal data. Control information carried

can be MIMO related parameters and transport format indicators. - The control data information is multiplexed with the user information before DFT spreading

module in the uplink SC-FDMA physical layer.- PUSCH supports QPSK,16QAM and 64QAM(optional). The LTE eNodeB selects suitable

modulation based on adaptation algorithm.

UCI is transmitted using PUSCH instead of PUCCH when there is RRC and application data to be transferred at the same time instant.

Modulation type is conveyed to UE using PDCCH DCI format-0. This CI also signals RB allocation and TB size. LTE PUSCH channel uses QPSK when TTI bundling is enabled. If eNodeB directs UE to use 64QAM, but if UE does not support it then 16QAM modulation type is selected.

3. Physical Random Access ChannelThis page on LTE PRACH describes LTE Physical Random Access Channel(PRACH).It mentions links for WCDMA PRACH and GSM RACH channel basics.This channel is used to carry random access preambles used for initiation of random access procedure. The basic structure is mentioned in the figure. As shown a random access preamble includes a CP, a sequence and a guard time.This carries the random access preamble. The RACH transport channel is mapped to this.

- Carries the random access preamble a UE sends to access the network- It consists of 72 sub-carriers in the frequency domain- There are 4 different RA(random access) preamble formats defined in LTE FDD

specifications. The same have been mentioned in the table-1 below. It consists of different preamble and CP duration to accommodate different cell sizes.

The preamble format to be used in a specific cell is informed to the UE using PRACH configuration index. This is broadcasted in SIB-2. PRACH configuration index also indicates SFN and subframes. This gives the exact position of random access preamble. Table-2 beloe mentions LTE PRACH channel configuration index, preamble format, allowed SFN and allowed subframes.

The preamble uses subcarrier spacing of 1.25KHz instead of 15KHz. The random access preamble occupies 1,2 or 3 subframes in the time domain(1,2,3ms) and 839 subcarriers in frequency domain(1.05MHz). There will be 15KHz guard band on both the sides and hence it uses total of 1.08MHz (equal to 6 RBs). The position of LTE random access preamble is defined by PRACH frequency offset parameter carried in SIB-2.

There is a max. of 1 random access preamble in a subframe but more than one UEs can use it. Multiple UEs using same preamble resource allocations are differentiated by their unique preamble sequences.

As mentioned in table-2 max. of 64 preamble sequences are divided into group-A and group-B. LTE UE selects the sequence from these two groups based on size of uplink packet and radio conditions. This helps eNodeB to calculate PUSCH resources needed for UE uplink transfer. Sequences in Group-A are used for smaller size packets or larger size packets in poor radio conditions. Sequences in Group-B are used for larger size packets in good radio conditions.

Transport Channel

The LTE transport channels vary between the uplink and the downlink as each has different requirements and operates in a different manner. Physical layer transport channels offer information transfer to medium access control (MAC) and higher layers.

Downlink Channel- Broadcast Channel (BCH)

The LTE transport channel maps to Broadcast Control Channel (BCCH)- Downlink Shared Channel (DL-SCH)

This transport channel is the main channel for downlink data transfer. It is used by many logical channels.

- Paging Channel (PCH)To convey the PCCH

- Multicast Channel (MCH)This transport channel is used to transmit MCCH information to set up multicast transmissions.

Uplink Channel- Uplink Shared Channel (UL-SCH)

This transport channel is the main channel for uplink data transfer. It is used by many logicalchannels.

- Random Access Channel (RACH)This is used for random access requirements.

Logical Channel

The logical channels cover the data carried over the radio interface. The Service Access Point, SAP between MAC sublayer and the RLC sublayer provides the logical channel.

Control Channel : these LTE control channels carry the control plane information:- Broadcast Control Channel (BCCH)

This control channel provides system information to all mobile terminals connected to the eNodeB.

- Paging Control Channel (PCCH)This control channel is used for paging information when searching a unit on a network.

- Common Control Channel (CCCH)This channel is used for random access information, e.g. for actions including setting up a connection.

- Multicast Control Channel (MCH)This control channel is used for Information needed for multicast reception.

- Dedicated Control ChannelThis control channel is used for carrying user-specific control information, e.g. for controlling actions including power control, handover, etc

Traffic Channel : These LTE traffic channels carry the user-plane data:- Dedicated Traffic Channel (DTCH)

This traffic channel is used for the transmission of user data.- Multicast Traffic Channel (MTCH)

This channel is used for the transmission of multicast data.