PDCCH and PUCCH

Physical Downlink Control Channel (PDCCH)

The downlink control signalling (PDCCH) is located in the first n OFDM symbols where n ≤ 4 and consists of:

- Transport format and resource allocation related to DL-SCH and PCH, and hybrid ARQ information related to DL-SCH;

- Transport format, resource allocation, and hybrid-ARQ information related to UL-SCH;

Transmission of control signalling from these groups is mutually independent. Multiple physical downlink control channels are supported and a UE monitors a set of control channels. Control channels are formed by aggregation of control channel elements, each control channel element consisting of a set of resource elements. Different code rates for the control channels are realized by aggregating different numbers of control channel elements.

QPSK modulation is used for all control channels. Each separate control channel has its own set of x-RNTI. There is an implicit relation between the uplink resources used for dynamically scheduled data transmission, or the DL control channel used for assignment, and the downlink ACK/NAK resource used for feedback

Physical Uplink Control Channel (PUCCH)

The PUCCH shall be mapped to a control channel resource in the uplink. A control channel resource is defined by a code and two resource blocks, consecutive in time, with hopping at the slot boundary. 

Depending on presence or absence of uplink timing synchronization, the uplink physical control signalling can differ. In the case of time synchronization being present, the outband control signalling consists of:

- CQI;

- ACK/NAK;

- Scheduling Request (SR).

The CQI informs the scheduler about the current channel conditions as seen by the UE. If MIMO transmission is used, the CQI includes necessary MIMO-related feedback.

The HARQ feedback in response to downlink data transmission consists of a single ACK/NAK bit per HARQ process. PUCCH resources for SR and CQI reporting are assigned and can be revoked through RRC signalling. An SR is not necessarily assigned to UEs acquiring synchronization through the RACH (i.e. synchronised UEs may or may not have a dedicated SR channel). PUCCH resources for SR and CQI are lost when the UE is no longer synchronized.

Ref. 36.300

RSRP and RSRQ

In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells. In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).

RSRP is a RSSI type of measurement. It measures the average received power over the resource elements that carry cell-specific reference signals within certain frequency bandwidth. RSRP is applicable in both RRC_idle and RRC_connected modes, while RSRQ is only applicable in RRC_connected mode. In the procedure of cell selection and cell reselection in idle mode, RSRP is used.

RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. It is defined as (N*RSRP)/(E-UTRA Carrier RSSI), where N makes sure the nominator and denominator are measured over the same frequency bandwidth;

The carrier RSSI (Receive Strength Signal Indicator) measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks. The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc.

The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both.

Ref. 3GPPP 36.214

UE Identity's in LTE

• Globally Unique Temporary Identity (GUTI)
  
  GUTI is allocated to the UE by the MME and has two components GUMMEI (Globally Unique MME ID) and the M-TMSI (MME-TMSI). The GUMMEI identifies the MME.When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session. The Globally Unique MME Identifier (GUMMEI) is constructed from the MCC, MNC and MME Identifier (MMEI).

• Temporary Mobile Subscribe Identity (M-TMSI)
  
  The M-TMSI identifies the UE within the MME. An M-TMSI identifies a user between the UE and the MME. The relationship between M-TMSI and IMSI is known only in the UE and in the MME.This value is allocated by MME.

• Temporary Mobile Subscriber Identity (S-TMSI)
  
  For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI. S-TMSI = MMEC + M-TMSI. It uniquely identify's the UE within an MME group. It is also included in RRC Connection Request.

• International Mobile Subscriber Identity (IMSI)
  
  IMSI is used for subscriber identification and stored in the Subscriber Identity Module (SIM). IMSI is usually 15 digits long. The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (Europeanstandard) or 3 digits (North American standard). The remaining digits are the mobile station identification number (MSIN) within the network's customer base.

• International Mobile Equipment Identity (IMEI)
  
  The IMEI number is used by the network/operators to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network. It is usually found printed on the phone.

Radio Network Temporary Identifier (RNTI) is used as UE identifiers within E-UTRAN and in signalling messages betweeen UE and E-UTRAN.

• Cell RNTI (C-RNTI)
  
  The C-RNTI provides a unique UE identification at the cell level identifying RRC Connection. Each RRC connection is associated with C-RNTI.
• Random Access RNTI (RA-RNTI)
  
  The RA-RNTI is assigned by the eNB to a particular UE after this UE has sent a random access preamble on the Physical Access Channel (PRACH). If this random acccess preamble is received by the eNB and network granted, the base station sends an acquisition indication back to the mobile and this acquisition indication message contains the RA-RNTI. In turn the UE will use the RA-RNTI to send RRC connection request message on the radio interfac UL and the parameter will help to distinguish messages sent by differnt UEs on the Random Access Channle (RACH). This procedure is called as contention based random access procedure.

• System Information RNTI (SI-RNTI)
  
  The SI-RNTI is sent on the PDCCH. It does not stand for a particular UE identity. Instead it signals to all mobiles in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH). This is necessary since the PDSCH is used to transport both broadcast system information for all UEs and singaling/payload for particular mobiles. In other words, the SI-RNTI indicated which DL resource blocks are used to carry SIBs.
• Paging RNTI (P-RNTI)The P-RNTI is derived from the IMSI of the subscribed to be paged and constructed by the eNB. For this reason IMSI is transmitted in a S1AP paging message fromk the MME to eNB. To receive paging messages from E-UTRAN, UEs in idle mode monitor the PDCCH channel for P-RNTI value used to indicate paging.
• Temporary Cell RNTI (TC-RNTI)
  
  When the UE does not have allocated C-RNTI then Temporaru C-RNTI is used. A temporary identity, the TC-RNTI, used for further communication between the terminal and the network. If the communication is successful then TC-RNTI is promoted eventually to C-RNTI in the case of UE not having a C-RNTI.