LTE Channels
Dr. Eng. Mostafa Mohamed
PMP – ITIL – IBDL
RHCE-MCSA-CCNA-CCAI-CSCU
Channel Mapping
LTE Channels
Logical Channels DL (What) (Between RLC&MAC)
A logical channel is defined by the type of information
it carries. Generally classified as:
A control channels (carry control data like RRC sig.)
A traffic channels (carry user plane data)
Transport Channels DL (How)
Defined by how and with what characteristics the
information is transmitted over the radio interface
The transport channels from the MAC layer are
mapped to the physical channels
Downlink Physical Signals and Channels (Where)
Downlink Physical Signals
Reference Signals
Synchronization Signals (primary , secondary)
Reference Signals
Channel estimation & Equalization, cell search, channel
quality measurements, coherent demodulation.
4 reference signal per RB.
One of 504 complex pseudorandom sequence.
Location in frame is PCI dependent mod(PCI,6).
Each antenna has its own CRS and the antenna does
not transmit when the other antennas are transmitting
their CRSs.
Reference signals position in time domain is fixed (symbol 0
and 4 in the slot for Type 1 Frame) whereas in frequency
domain it depends on the Cell ID (PCI).
Antenna 0/1 : transmitted on two OFDM symbols in every
slot.
Antenna 2/3 : transmitted on one OFDM symbols in every slot
If 1 Tx antenna: 4 Reference Signals per RB
If 2 Tx antenna, there are 8 Reference Signals per RB
If 4 Tx antenna, there are 12 Reference Signals per RB
PSS
Defines the Cell Identity by one of three Zadoff-Chu root
sequences.
It is sent on the last symbol of slot 0 and 10 of every frame
(subframe 0, 5)
It is sent over the central 72 sub carriers, using 62 symbols
and blanking the remaining 10 symbols (middle 1.08 MHz)
PSS detection allows for:
Symbol, slot, subframe synchronization in time domain.
Uses QPSK modulation.
SSS
Carries one of 168 pseudo-random Gold sequences in the
first frame occurrence and a different one in the second frame
occurrence. (cell identity groups)
SSS purpose :
Frame synchronization
Defines the Physical Cell Identity (PCI).
PSS , SSS :Occupies 144 Resource Elements per frame (20 slots)
i.e. (62 subcarriers +10 DTx) * 2 times/frame.
Downlink Physical Channels
– Physical Downlink Shared Channel (PDSCH)
– Physical Control Format Indicator Channel (PCFICH)
– Physical Hybrid-ARQ Indicator Channel (PHICH)
– Physical Multicast Channel (PMCH)
- Physical Downlink Control Channel (PDCCH)
- Physical Broadcast Channel (PBCH)
Physical Channels
Data area : PDSCH: Carries user data, paging data, SIBs
(cell status, cell IDs, allowed services…)
Data area: PMCH: For multicast traffic as MBMS services
Control area: PHICH: Carries H-ARQ Ack/Nack messages
from eNB to UE in response to UL transmission.
Control area: PCFICH: Format indicator
Control area: PDCCH: Carries the DCI (DL control
information), schedule uplink resources on the PUSCH or
downlink resources on the PDSCH.
Control area: PBCH: Broadcast system information
Definitions
Resource Element Group (REG)
Set of 4 contiguous REs (ignoring RSs), (called quadruplet)
Control Channel Element (CCE)
Set of 9 REGs
PBCH
Broadcast the MIB (Master Information Block) which Carries
system BW, system frame number (for this frame), PHICH
configuration.
Occupies the central 72 subcarriers across 4 symbols (1.08 MHz)
once a frame.
Transmitted during second slot of each 10 ms radio frame
on all antennas (subframe 0- symbol 0,1,2,3).
Mapped into four frames within 40 ms.
PDCCH
Occupies the REs within the first 1 to 3 symbols of each TTI
If PDCCH data is small then it only occupies the 1st symbol
It uses QPSK modulation.
Carries Downlink Control Information (DCI)
DCI: Informs how data is mapped in the TTI (10 formats in LTE
R 8).
DCI contains MCS index parameter (from 0 to 31) which
mapped to modulation and TBS.
PCFICH
Indicates how many symbols in the beginning of each subframe
are allocated for PDCCH (1,2,3 or 4 depends on BW)
Carries details of PDCCH format (CFI – control format indicator)
Occupies up to 16 RE per TTI (32 bit)
Set of 4 REGs (4 RE = 1 REG)
PHICH
Carries ACK/NACK for UE messages.
Normal CP: Tx during 1st symbol of each TTI
Number of groups varies with bandwidth.
PHICH group is a set of 3 REGs (12 RE)
PDSCH
Occupy the RE not used by signals or other physical
channels
Uplink Physical Signals and Channels
• Uplink Physical Signals
Demodulation Reference Signals (DRS):
Used for channel estimation in the eNB receiver to demodulate
control and data channels.
Located in the 4th symbol (3rd symbol- extended CP) of each
slot and spans the same bandwidth as the allocated uplink data.
Sounding Reference Signals (SRS)
Sent in different parts of the bandwidth where no uplink data
transmission is available.
SRS is sent to evaluate the RF channel performance outside the
RBs used for UE transmission
SRS uses resources normally available for PUSCH
Multiple UEs can send the SRS at the same time using different
resources.
SRS is transmitted in the last symbol of the subframe.
Uplink Physical Channels
PUSCH
Carries signaling and user traffic
Data is allocated in multiples of 1 RB. Only factors of 2, 3
and 5 RBs are allowed
PUCCH
Carries uplink control information (UCI) when no PUSCH is
available.
If PUSCH is available UCI are multiplexed with user data.
UCI carries scheduling request SR, ACK/NACK info related to
DL data packet, CQI , and MIMO feedback (RI, PMI)
UCI has many formats (1 - SR, 1a – ACK/NACK, 1b, 2,2a,2b)
Number of control region is variable (higher layer configurable).
PRACH
SI contains a list of allowed preambles (64 per cell)
RACH operation uses around 1.08 MHz bandwidth, this is
equal to 6 resource blocks (in frequency domain).
Used for initial access, handover, re-establishment.
Channels Modulation Schemes
Thanks
Dr. Mostafa Elgamala
For more info:
/https://www.facebook.com/groups/MobileCommunicationAndIT
Whats: +201025191022
NID(1)= (168 cell identity groups)
NID(2)= (3 cell identity- sector)
