Telecom Channels Notes Prepared By Shahzada Arslan Ahmed +923036677718
Notes Prepared By
Shahzada Arslan Ahmed
3G
LTE channel types
There are three categories into
which the various data channels may be grouped.
- Physical channels: These are transmission channels that carry user
data and control messages.
- Transport channels: The physical layer transport channels offer
information transfer to Medium Access Control (MAC) and higher layers.
- Logical channels:
Provide services for the Medium Access Control (MAC) layer within
the LTE protocol structure.
3G
LTE physical channels
The LTE physical channels vary
between the uplink and the downlink as each has different requirements and
operates in a different manner.
- Downlink:
- Physical Broadcast Channel (PBCH): This physical channel carries system
information for UEs requiring to access the network. It only carries what
is termed Master Information Block, MIB, messages. The modulation scheme
is always QPSK and the information bits are coded and rate matched - the
bits are then scrambled using a scrambling sequence specific to the cell
to prevent confusion with data from other cells.
The MIB message on the PBCH is mapped onto the central 72 subcarriers or six central resource blocks regardless of the overall system bandwidth. A PBCH message is repeated every 40 ms, i.e. one TTI of PBCH includes four radio frames.
The PBCH transmissions has 14 information bits, 10 spare bits, and 16 CRC bits. - Physical Control Format Indicator Channel (PCFICH) : As the name implies the PCFICH informs the UE
about the format of the signal being received. It indicates the number of
OFDM symbols used for the PDCCHs, whether 1, 2, or 3. The information
within the PCFICH is essential because the UE does not have prior
information about the size of the control region.
A PCFICH is transmitted on the first symbol of every sub-frame and carries a Control Format Indicator, CFI, field. The CFI contains a 32 bit code word that represents 1, 2, or 3. CFI 4 is reserved for possible future use.
The PCFICH uses 32,2 block coding which results in a 1/16 coding rate, and it always uses QPSK modulation to ensure robust reception. - Physical Downlink Control Channel (PDCCH) : The main purpose of this physical channel is
to carry mainly scheduling information of different types:
- Downlink resource scheduling
- Uplink power control instructions
- Uplink resource grant
- Indication for paging or system information
The PDCCH
contains a message known as the Downlink Control Information, DCI which carries
the control information for a particular UE or group of UEs. The DCI format has
several different types which are defined with different sizes. The different
format types include: Type 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3, 3A, and 4.
- Physical Hybrid ARQ Indicator Channel (PHICH) : As the name implies, this channel is used to
report the Hybrid ARQ status. It carries the HARQ ACK/NACK signal
indicating whether a transport block has been correctly received. The
HARQ indicator is 1 bit long - "0" indicates ACK, and
"1" indicates NACK.
The PHICH is transmitted within the control region of the subframe and is typically only transmitted within the first symbol. If the radio link is poor, then the PHICH is extended to a number symbols for robustness. - Uplink:
- Physical Uplink Control Channel (PUCCH) : The Physical Uplink Control Channel, PUCCH
provides the various control signalling requirements. There are a number
of different PUCCH formats defined to enable the channel to carry the
required information in the most efficient format for the particular
scenario encountered. It includes the ability to carry SRs, Scheduling
Requests.
The basic formats are summarised below:
PUCCH
Format
|
Uplink
Control Information
|
Modulation
Scheme
|
Bits
per Sub-frame
|
Notes
|
Format 1
|
SR
|
N/A
|
N/A
|
|
Format 1a
|
1 bit HARQ ACK/NACK with or
without SR
|
BPSK
|
1
|
|
Format 1b
|
2 bit HARQ ACK/NACK with or
without SR
|
QPSK
|
2
|
|
Format 2
|
CQI/PMI or RI
|
QPSK
|
20
|
|
Format 2a
|
CQI/PMI or RI and 1 bit HARQ
ACK/NACK
|
QPSK
+ BPSK
|
21
|
|
Format 2b
|
CQI/PMI or RI and 2 bit HARQ
ACK/NACK
|
QPSK
+ BPSK
|
22
|
|
Format 3
|
Provides support for carrier
aggregation.
|
- Physical Uplink Shared Channel (PUSCH) : This physical channel found on the LTE uplink
is the Uplink counterpart of PDSCH
- Physical Random Access Channel (PRACH) : This uplink physical channel is used for
random access functions. This is the only non-synchronised transmission
that the UE can make within LTE. The downlink and uplink propagation
delays are unknown when PRACH is used and therefore it cannot be synchronised.
The PRACH instance is made up from two sequences: a cyclic prefix and a guard period. The preamble sequence may be repeated to enable the eNodeB to decode the preamble when link conditions are poor.
LTE
transport channels
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:
- 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:
- Uplink Shared Channel (UL-SCH) : This transport channel is the main channel for
uplink data transfer. It is used by many logical channels.
- Random Access Channel (RACH) : This is used for random access requirements.
LTE
logical channels
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 channels:
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 (MCCH) : This control channel is used for Information
needed for multicast reception.
- Dedicated Control Channel (DCCH) : This control channel is used for carrying
user-specific control information, e.g. for controlling actions including
power control, handover, etc..
- Traffic channels: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.
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