The physical layer provides transport channels to the L2. These transport channels differ in their characteristics how data is transmitted and are mapped to different logical channels provided by the MAC layer. Logical channels describe which type of data is conveyed.
The logical channels can be divided into control channels and traffic channels.
Control Channels:-
• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE.
• Common Control Channel (CCCH): This channel is used by the UEs having no RRC connection with the network. CCCH would be used by the UEs when accessing a new cell or after cell reselection.
• Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting MBMS scheduling and control information from the network to the UE, for one or several MTCHs. After establishing an RRC connection this channel is only used by UEs that receive MBMS.
• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection.
Traffic Channels:-
• Dedicated Traffic Channel (DTCH): A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
• Multicast Traffic Channel (MTCH): A point-to-multipoint downlink channel for transmitting traffic data from the network to the UEs using MBMS.
TRANSPORT CHANNELS:-
An effort has been made to keep a low number of transport channels in order to avoid unnecessary switches between different channel types, which are found to be time consuming in UMTS. In fact there is currently only one transport channel in downlink and one in uplink carrying user data, i.e., channel switching is not
needed.
Downlink:
• Broadcast Channel (BCH): A low fixed bit rate channel broadcast in the entire coverage area of the cell. Beamforming is not applied.
• Downlink Shared Channel (DL-SCH): A channel with possibility to use HARQ and link adaptation by varying the modulation, coding and transmit power. The channel is possible to broadcast in the entire cell and beamforming may be applied. UE power saving (DRX) is supported to reduce the UE power consumption. MBMS transmission is also supported.
• Paging Channel (PCH): A channel that is broadcasted in the entire cell. DRX is supported to enable power saving.
• Multicast channel (MCH): A separate transport channel for multicast (MBMS). This channel is broadcast in the entire coverage area of the cell. Combining of MBMS transmissions from multiple cells (MBSFN) is supported.
Uplink:
• Uplink Shared channel (UL-SCH): A channel with possibility to use HARQ and link adaptation by varying the transmit power, modulation and coding. Beamforming may be applied.
• Random Access Channel (RACH): A channel used to obtain timing synchronization (asynchronous random access) and to transmit information needed to obtain scheduling grants (synchronous random access). The transmission is typically contention based. For UEs having an RRC connection there is some limited support for contention free access.
PHYSICAL CHANNELS:-
The physical layer offers services to the MAC layer in the form of transport channels. User data to be transmitted is delivered to the physical layer from the MAC layer in the form of transport blocks. The MAC layer at the transmitter side also provides the physical layer with control information necessary for transmission and/or reception of the user data.
Downlink :-
• Synchronization signals:- 504 different physical layer cell IDs are composed of 168 different physical layer ids within the group.
There are Primary synchronization signals based on frequency domain Zad-Off chu sequence indicating the physical layer ids and Secondary synchronization signals indicating the physical layer cell id group.
• DL reference signal:- DL reference signals are used for DL Channel quality measurements and DL channel estimation for coherent demodulation at the UE. They are like P-CPICH in WCDMA.
• PBCH:- PBCH carries system information.In order to lower system overhead and improve coverage only minimum information, which is the MIB is sent on PBC whereas SIB's are sent on PDSCH.MIB contains DL system bandwidth, PHICH configuration, which is the PHICH group number and the SFN (System frame number).
• PCFICH:- CFI ( Control format indicator) from higher layer indicates how many OFDM symbols per subframe are for PDCCH and the number could be 1,2,3 and 4. The CFI carried by 32 bits of information, which occupies 16 RE's distributed in frequency.
• PDCCH:- PDCCH contains DCI (downlink control information) which indicate 3 different messages.
1. Uplink scheduling grant for PUSCH
2. Downlonk schedulink assignment for PDSCH
3. TPC command for PUSCH and PUCCH
• PHICH:- PHICH carries the HARQ ACK/NACK, which indicates whether the eNodeB has correctly received transmission on PUSCH.
• PDCSH:- The physical resource elements that not assigned by the channels/signals described above can be used to transmit information of higher layers, i.e. control/user data, on the PDSCH. The PDSCH resource
will be scheduled on TTI basis by means of the DCIs.Modulation schemes QPSK,16QAM,64QAM are supported.
Uplink:-
• PRACH:-PRACH is used for contention based random access procedure when the UE initiates and attach procedure, a TAU, or UE wants to send data and needs transition from ECM-IDLE to ECM-CONNECTED. The uplink synchronization is achieved through this procedure at the same time.
• PUCCH:- PUCCH carries UCI from higher layers.To achieve frequency diversity, frequency hopping is used from one edge of the bandwidth to the other edge.So PUCCH is like a kind of guard band for the UL transmission .
• PUSCH:- Physical uplink shared channel carries user data from UL-SCH and L1/L2 control information including ACK/NACK for HARQ, CQI, Precoding matrix indicatoe(PMI) and rank indicator(RI).
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