Backoff Indicator

Backoff Indicator is a special MAC subheader that carries the parameter indicating the time delay between a PRACH and the next PRACH. There are cases where a UE has to send another PRACH after it already sent a PRACH. 

The most common cases are as follows.

 i) UE sent a PRACH but didn't get a RAR for some reason. 

 ii) UE sent a PRACH and got RAR, but the RAPID in the RAR is not for the UE. 

If the random access attempt of a UE fails, either because the preamble sent by the UE was not detected by the eNB or the UE lost the contention resolution, the UE has to start the process over again. To avoid contention and overload, the eNB can signal the UEs that they have to wait a certain time before they try to connect again. The parameter that controls this is called the backoff indicator (BI) and is signaled by the eNB in the random access response. The actual time the UE should backoff is chosen uniformly by the UE in the interval [0,B]. As mentioned, the backoff parameter is sent in the RA response, but all RA responses can however be read by all UEs who sent a preamble in step 1 of the random access procedure. This means that also a UE that did not get a random access response. with its own preamble, i.e., was not detected, can receive the backoff parameter and use it. 

The eNB can force the UE to wait a certain time before it tries to connect again. The maximum length of the backoff time is signaled to the UE by the eNB with the backoff parameter B. One possible scenario is that the backoff only is activated when there is an overload in the system. Therefore it would be interesting to study how the observations of AD (Access Delay) are affected by different values on B, during different conditions of the system. If the AD observers cannot be upgraded to accurately estimate an eventual backoff it would mean that the eNB is depending on AD reports from the UEs.

Services Provided by Physical (L1) Layer

The physical layer offers data transport services to higher layers. The access to these services is through the use of transport channels via the MAC sub-layer. A transport block is defined as the data delivered by MAC layer to the physical layer and vice versa. Transport blocks are delivered once every TTI (Transmission Time Interval).

The physical layer is expected to perform the following functions in order to provide the data transport service:

- Error detection on the transport channel and indication to higher layers

- FEC encoding/decoding of the transport channel

- Hybrid ARQ soft-combining

- Rate matching of the coded transport channel to physical channels

- Mapping of the coded transport channel onto physical channels

- Power weighting of physical channels

- Modulation and demodulation of physical channels

- Frequency and time synchronisation

- Radio characteristics measurements and indication to higher layers

- Multiple Input Multiple Output (MIMO) antenna processing

- Transmit Diversity (TX diversity)

- Beamforming

- RF processing. (Note: RF processing aspects are specified in the TS 36.100)

Reference signals

Five types of downlink reference signals are defined:

- Cell-specific reference signals (CRS)

- MBSFN reference signals

- UE-specific reference signals (DM-RS)

- Positioning reference signals (PRS)

- CSI reference signals (CSI-RS)

There is one reference signal transmitted per downlink antenna port.

Cell-specific reference signals

Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting PDSCH transmission. Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3. Cell-specific reference signals are defined for f 15 kHz only.

The number of Resource Elements (REs) within each Resource Element Group (REG) and the number of REGs within an OFDM symbol is affected by the number of cell-specific reference signals present on all antenna ports. The number and location of cell specific reference signals are dependent on the number of antenna ports and the type of cyclic prefix used. Each antenna port has a unique cell specific reference signal associated with it. As the REG arrangement is affected by cell specific reference signals, the REG arrangement for a one or two antenna port configuration or four antenna port configuration is different. The REG arrangement for each resource block within a subframe and for every antenna port is identical.

To facilitate the estimation of the channel characteristics LTE uses cell specific reference signals (pilot symbols) inserted in both time and frequency. These pilot symbols provide an estimate of the channel at given locations within a subframe. Through interpolation it is possible to estimate the channel across an arbitrary number of subframes.

Cell-specific RS is transmitted in each physical antenna port. It is used for both demodulation and measurement purpose. Its pattern design ensures channel estimation accuracy.

Cell-specific reference signals are used for…

– cell search and initial acquisition,

– downlink channel estimation for coherent demodulation/detection at the UE,

– downlink channel quality measurements.

UE-specific Reference Signals

UE-specific reference signals are supported for transmission of PDSCH and are transmitted on antenna port(s) p 5 , p 7 , p 8 or p 7,8,..., 6 , where is the number of layers used for transmission of the PDSCH. UE-specific reference signals are present and are a valid reference for PDSCH demodulation only if the PDSCH transmission is associated with the corresponding antenna port according to Section 7.1 of [4]. UE-specific reference signals are transmitted only on the resource blocks upon which the corresponding PDSCH is mapped.

UE-specific reference signal are transmitted using resource elements with the same index pair k, l regardless of their antenna port p . UE-specific reference signals are denser in frequency but only transmitted when data is transmitted on the corresponding layer.

CSI Reference Signals

CSI reference signals are transmitted on one, two, four or eight antenna ports using p 15 , p 15,16 , p 15,...,18 and p 15,...,22 , respectively. CSI reference signals are defined for f 15 kHz only.

Feedback of channel-state information (CSI) is based on a separate set of reference signals – CSI reference signals. CSI reference signals are relatively sparse in frequency but regularly transmitted from all antennas at the base station.

The CSI reference signal is transmitted in each physical antenna port or virtualized antenna port and is used for measurement purposes only

A cell can be configured with one, two, four or eight CSI-RS. The exact CSI-RS structure, including the exact set of resource elements used for CSI-RS in a rosource block, depends on the number of CSI-RS configured within the cell and may also be different for different cells. More speifically, within a resource-block pair there are 40 possible positions for the reference symbols of CSI-RS and, in a given cell, a subset of corresponding resource elements is used for CSI-RS transmission.

MBSFN reference signals MBSFN reference signals shall be transmitted in the MBSFN region of MBSFN subframes only when the PMCH is transmitted. MBSFN reference signals are transmitted on antenna port 4. MBSFN reference signals are defined for extended cyclic prefix only.

Synchronization Signals

There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity (2) ID (1) ID cell ID N 3N N is thus uniquely defined by a number (1) ID N in the range of 0 to 167, representing the physical-layer cell-identity group, and a number (2) ID N in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group.

Primary synchronization signal and Secondary synchronization signal

Both primary and secondary synchronization signals are designed to detect all type of UEs. The synchronization signals always occupy the 62 sub-carrier of the channel, which make the cell search procedure same regardless of channel bandwidth. Although 72 subcarriers (6 RB) are available, only 62 sub-carriers are used so that the UE can perform the cell search procedure. The primary synchronization signal subcarriers are modulated using a frequency domain Zadoff-Chu Sequence. Each subcarrier has the same power level with its phase determined by the root index number in sequence generator as defined in 36.211

The secondary signal is used to identify cell-identity groups. The number and position of subcarrier are same as for the primary synchronization signal: that is the central 62 sub carriers. The sequence generation function utilizes an interleaved concatenation of two length 31 binary sequences as defined in 36.211. The secondary synchronization signal gives a cell-identity group number from 168 possible cell identities N (1, ID).

Radio Channel Quality Feedback

CQI (channel quality indicator)

CQI is an indication of the downlink mobile radio channel quality as experienced by this UE. Essentially, the UE is proposing to the eNodeB an optimum modulation scheme and coding rate to use for a given radio link quality, so that the resulting transport block error rate would not exceed 10%. 16 combinations of modulation scheme and coding rate are specified as possible CQI values. The UE may report different types of CQI. A so-called “wideband CQI” refers to the complete system bandwidth. Alternatively, the UE may evaluate a “sub-band CQI” value per sub-band of a certain number of resource blocks which is configured by higher layers. The full set of sub-bands would cover the entire system bandwidth. In case of spatial multiplexing, a CQI per code word needs to be reported.

The time and frequency resources used by the UE to report CQI are under the control of the eNB. CQI reporting can be either periodic or aperiodic. A UE can be configured to have both periodic and aperiodic reporting at the same time. In case both periodic and aperiodic reporting occurs in the same subframe, only the aperiodic report is transmitted in that subframe.

For efficient support of localized, distributed and MIMO transmissions, E-UTRA supports three types of CQI reporting:

- Wideband type: providing channel quality information of entire system bandwidth of the cell;

- Multi-band type: providing channel quality information of some subset(s) of system bandwidth of the cell;

- MIMO type: open loop or closed loop operation (with or without PMI feedback).

Periodic CQI reporting is defined by the following characteristics:

- When the UE is allocated PUSCH resources in a subframe where a periodic CQI report is configured to be sent, the periodic CQI report is transmitted together with uplink data on the PUSCH. Otherwise, the periodic CQI reports are sent on the PUCCH.

Aperiodic CQI reporting is defined by the following characteristics:

- The report is scheduled by the eNB via the PDCCH;

- Transmitted together with uplink data on PUSCH.

When a CQI report is transmitted together with uplink data on PUSCH, it is multiplexed with the transport block by L1 (i.e. the CQI report is not part of the uplink the transport block).The eNB configures a set of sizes and formats of the reports. Size and format of the report depends on whether it is transmitted over PUCCH or PUSCH and whether it is a periodic or aperiodic CQI report.

PMI (precoding matrix indicator)

PMI is an indication of the optimum precoding matrix to be used in the base station for a given radio condition. The PMI value refers to the codebook table. The network configures the number of resource blocks that are represented by a PMI report. Thus to cover the full bandwidth, multiple PMI reports may be needed. PMI reports are needed for closed loop spatial multiplexing, multi-user MIMO and closed-loop rank 1 precoding MIMO modes.

RI (rank indication)

RI is the number of useful transmission layers when spatial multiplexing is used. In case of transmit diversity, rank is equal to 1.

RRM functions

Radio Bearer Control (RBC)

The establishment, maintenance and release of Radio Bearers involve the configuration of radio resources associated with them. When setting up a radio bearer for a service, radio bearer control (RBC) takes into account the overall resource situation in E-UTRAN, the QoS requirements of in-progress sessions and the QoS requirement for the new service. RBC is also concerned with the maintenance of radio bearers of in-progress sessions at the change of the radio resource situation due to mobility or other reasons. RBC is involved in the release of radio resources associated with radio bearers at session termination, handover or at other occasions. RBC is located in the eNB.

Radio Admission Control (RAC)

The task of radio admission control (RAC) is to admit or reject the establishment requests for new radio bearers. In order to do this, RAC takes into account the overall resource situation in E-UTRAN, the QoS requirements, the priority levels and the provided QoS of in-progress sessions and the QoS requirement of the new radio bearer request. The goal of RAC is to ensure high radio resource utilization (by accepting radio bearer requests as long as radio resources available) and at the same time to ensure proper QoS for in-progress sessions (by rejecting radio bearer requests when they cannot be accommodated). RAC is located in the eNB.

Connection Mobility Control (CMC)

Connection mobility control (CMC) is concerned with the management of radio resources in connection with idle or connected mode mobility. In idle mode, the cell reselection algorithms are controlled by setting of parameters (thresholds and hysteresis values) that define the best cell and/or determine when the UE should select a new cell.

Also, E-UTRAN broadcasts parameters that configure the UE measurement and reporting procedures. In connected mode, the mobility of radio connections has to be supported. Handover decisions may be based on UE and eNB measurements. In addition, handover decisions may take other inputs, such as neighbour cell load, traffic distribution, transport and hardware resources and Operator defined policies into account. CMC is located in the eNB.

Dynamic Resource Allocation (DRA) - Packet Scheduling (PS)

The task of dynamic resource allocation (DRA) or packet scheduling (PS) is to allocate and de-allocate resources (including buffer and processing resources and resource blocks (i.e. chunks)) to user and control plane packets. DRA involves several sub-tasks, including the selection of radio bearers whose packets are to be scheduled and managing the necessary resources (e.g. the power levels or the specific resource blocks used). PS typically takes into account the QoS requirements associated with the radio bearers, the channel quality information for UEs, buffer status, interference situation, etc. DRA may also take into account restrictions or preferences on some of the available resource blocks or resource block sets due to inter-cell interference coordination considerations. DRA is located in the eNB.

Inter-cell Interference Coordination (ICIC)

Inter-cell interference coordination has the task to manage radio resources such that inter-cell interference is kept under control. ICIC mechanism includes a frequency domain component and time domain component. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. The preferred ICIC method may be different in the uplink and downlink. The frequency domain ICIC manages radio resource, notably the radio resource blocks.

For the time domain ICIC, Almost Blank Subframes (ABSs) are used to protect resources receiving strong inter-cell interference. MBSFN subframes can be used for time domain ICIC when they are also included in ABS patterns. The eNB cannot configure MBSFN subframes [4] as ABSs when these MBSFN subframes are used for other usages (e.g., MBMS, LCS). ICIC is located in the eNB.

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.