To be become a LTE Expert?

As UEs have become smarter and the air interface more complex, the need for more detailed communication between the UE and the network has increased dramatically.  The UE has become the eyes and the ears of the radio access network.  Its main goal is to find the best cell and maximize performance with that cell.

To help the UE accomplish this, the eNodeB gathers several pieces of information from the UE and then adjusts its’ downlink transmissions accordingly. The UE tells the network what cells it can see and how strong it can see them, its current channel conditions, the current state of its’ memory buffer, which antennas should be transmitting in the downlink, how many different transmission streams can be supported simultaneously, acknowledgements when data is received successfully, and any other information that the network wants to know.

This is the first in a series of blogs on these different types of UE feedback.  This blog will focus on the measurement report.  The measurement report is the mechanism used by the UE to tell the network whatever results have been requested. Typically, these are measurements of the surrounding cells.  They can also include requested measurements of block error rate, transmit power and other UE-based parameters. The UE learns the requested information using a measurement configuration.

When a UE is in RRC-CONNECTED mode, this measurement configuration is provided to the UE by means of dedicated signaling; typically using the RRCConnectionReconfiguration message.

The measurement configuration provided to the UE includes the following parameters:

Measurement Objects:  the objects on which the UE shall perform the measurements; i.e. frequencies and cells.  In other words:  who should the UE measure? These include intra- and inter- frequency neighbors, IRAT UMTS neighbors, IRAT GSM neighbors and IRAT CDMA2000 HRPD and 1xRTT neighbors.
Reporting Configurations:  the criteria used by the UE to trigger the transmission of a measurement report and the quantities that the UE includes in the report. In other words: when should the UE send a report?  This trigger can either be periodical or event-based.
Measurement Identities: an identifier that links one measurement object with one reporting configuration.  In other words: the UE needs to keep track of the objects to be measured and their specific triggers.  The measurement identity is used as a reference number in the measurement report.
Quantity configurations: the measurement quantities and associated filtering used for all event evaluation and related reporting per Radio Access Technology.
Measurement gaps:  periods of time that the UE may use to perform measurements while in connected mode.
The UE maintains a single measurement object list, a single reporting configuration list and a single measurement identities list.  Any measurement object can be linked to any reporting configuration of the same RAT type.

As mentioned earlier, a report can be event-triggered or periodical.  An event-based measurement report will be transmitted when the criteria for any of the following events have been met:


A1:  Serving Cell becomes better than a defined threshold

A2:  Serving Cell becomes worse than a defined threshold

A3:  Neighbor cell becomes some offset better than the primary cell

A4:  Neighbor cell becomes better than a defined threshold

A5:  Primary cell becomes worse than a defined threshold and a neighbor becomes better than a second threshold

A6:  Neighbor cell becomes some offset better than the serving cell

B1:  Inter-RAT neighbor becomes better a defined threshold

B2:  Primary cell becomes worse than a defined threshold and inter-RAT neighbor becomes better than a second threshold

Periodical measurement reports are sent based on the reporting configuration.  For instance, it could be configured that the UE report its’ transmit power every 2 seconds or its’ transport channel block error rate every second.  This is operator-specific.

We have discussed the vehicle used by the UE to tell the network what it can see as well as other operator-configured parameters.  Another piece of information that the network needs to be successful is some indication of the channel conditions at the UE.  This will help the network adapt its downlink transmission to match the UE’s capability at that time.   These channel quality indicators or CQI’s will be the subject of the next blog in this series.


Source:  LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (3GPP TS 36.331 version 10.5.0 Release 10)

• Append a 24 bit checksum (CRC) to the transport block. This CRC is used to determine whether the transmission was successful or not, and triggers Hybrid ARQ to send an ACK or NACK, as appropriate
• Segment the transport block into code blocks. A code block must be between 40 and 6144 bits long. If the transport block is too small, it is padded up to 40 bits; if the TB is too big, it is divided into smaller pieces, each of which gets an additional 24 bit CRC.
• Process each code block with a 1/3 turbo coder
• Reassemble the resulting code blocks into a single codeword

• Scramble the contents of each codeword, using a sequence based on the UE's C-RNTI and the cell's Physical Cell ID (PCI) 
• Convert the bit sequences into the corresponding modulation symbols (using QPSK, 16QAM or 64QAM) 
• Assign the modulation symbols to one or more layers, depending on the specific transmission scheme being used

• Apply the required precoding factors to the modulation symbols in each layer
• Map the precoded symbols to the appropriate antenna ports
• Assign the modulation symbols to be transmitted on each antenna port to specific resource elements (the subcarriers and symbols within the resource blocks)
• Generate the final time-domain OFDM signal for each antenna port

Basic Scheduling Operations:
- Purpose
* Efficient SCH(Data) Resources Assignments
- Consideration
* Traffic Volume, QoS (Buffer Status, Priority…)
* Channel Condition
- Scheduling Interval
* Dynamic Scheduling by MAC : One TTI (1ms : One Sub-frame)
* Semi-Persistent for VoIP : Multiple TTIs by RRC
- Resource Assignment : PRBs & Associated MCS

Scheduling information: DCI PDCCH


Resource Assignment
- Per UE # PRBs Assignment
* Channel Dependent Resource Allocation
  •User Assignment based on Channel Quality
- CQI / MCS / TBS

As per 3GPP Rel 9 revision of the LTE standard defines bands of operation, including both paired and unpaired spectrum. 1-32 bands are for paired (FDD) operation, while bands 33-40 are for unpaired (TDD) operation

System Information Block 2 specifies the resources reserved for the PRACH transmissions. The resource may be specified as 1, 2, 3, 5, ... sub-frames within the frame (the set of options depending on frame type and PRACH preamble type). Initially this will be 6 contiguous resource blocks but additional frequencies could be specified once all the time resource has been allocated.
The mobile sends a PRACH preamble in a randomly chosen PRACH resource and waits for the Random Access Response (RAR).
The Random Access Response comprises a resource assignment which is implicitly addressed to the mobile by scrambling the CRC with the RA-RNTI that was used by the mobile in the PRACH transmission. The Downlink resource is used for transmission of the RAR which is addressed to a number of mobiles which have sent PRACH preambles. For each mobile a grant is given which the mobiles use to send their first RRC message.

• The Access burst comprises a preamble and a few bits of payload data
– Several preamble formats (lengths) are specified
• Initial message from the mobile use nonsynchronised timing
• The eNodeB supplies the required Timing Advance
• Any subsequent transmissions can use synchronised PRACH

Initially, the time delay between the base station and the mobile is not known accurately, so the PRACH transmissions are not synchronised. Subsequently, the timebase at the mobile is adjusted and the transmissions are synchronised with uplink transmissions from other mobiles in the cell.
There are several preamble sequences defined for a cell, the mobile selects one randomly and this is used to identify the mobile in the response sent from the base station.

• For transmission of multicast and broadcast information
• Format is similar to the PDSCH but it is for reception by several mobiles
• Sub-channel spacing is 7.5 kHz and symbol length is doubled
• Modulation QPSK, 16 QAM or 64 QAM

The longer symbol length means a longer cyclic prefix permitting good reception over large cells or for combination of signals broadcast simultaneously over a set of cells.

• Carries DL-SCH - user data and higher layer (RRC, NAS) signalling
• Time sharing of data transmission to mobiles
• Carries the PCH - Paging of mobiles
• Also carries the System Information
– The System Information Blocks are carried on the PDSCH so the transmission bandwidth used and
repetition schedule can be varied
• Modulation QPSK, 16 QAM or 64 QAM

• Carries the Ack/Nack responses to the uplink transmissions
– This relates to the HARQ mechanism implemented within MAC
– 1/3 Repetition coding so Ack = 1,1,1, Nack = 0,0,0
• Modulation QPSK

• Provides the mobile with details of the number of OFDM symbols used on the PDCCH
– Expressed in the Control Format Indicator (CFI) range 1 to 3 symbols within a sub-frame
• Modulation QPSK

• CRC generation depends on UE Identities -
implicitly addresses Resource Assignments
– SI-RNTI = FFFF Assignments for System Information
– P-RNTI = FFFE Assignments for Paging messages
– RA-RNTI based on subframe number in which PRACH was received
Assignment for Random Access Response
– C-RNTI the identity given during RRC Connection Assignment for DLSCH or ULSCH (uplink grant)

• Allocation may be Dynamic - single TTI or Semi-persistent - periodically repeating
• UE is sent a bitmap to assign the uplink Resource Blocks
– Bitmap type 2 - Assignment of a set of contiguous Resource Blocks

The grant in FDD mode relates to the uplink sub frame which is 4 sub frames delayed from that in which the resource allocation is included to allow the mobile time to process the information. In TDD the delay is different.

• Allocation may be Dynamic - single TTI or Semi-persistent (periodically repeating)
• UE is sent a bitmap to assign the downlink Resource Blocks in the same TTI
– Direct bitmap - each bit assigns one resource block
– Bitmap type 0 - assigns Resource Block Groups (sets of consecutive Resource Blocks)
– Bitmap type 1 - assigns individual resource blocks (for frequency diversity) from the Resource Block Groups
– Bitmap type 2 - several sets of contiguous blocks (no segmentation of band into Resource Block Groups)

Semi-persistent allocation of Resource Blocks is useful for real-time applications such as VoIP where the transfer of data is constant and repetitive.
The semi-persistent allocation is provided to the mobile’s C-RNTI so if the mobile does not see any further allocation, it may use this repetitive allocation.
If it does see an allocation to its C-RNTI, this takes precedence.
The direct bitmap is only used for up to 10 resource blocks (10 bits) otherwise the bitmap size would become too large.
Assignments are usually made which cover transmissions in both halves of the subframe, but it is also possible to have separate assignments for each half of the subframe.

• Carries Downlink Control Information (DCI), resource block assignments for transmissions
– Assignment information is sent every subframe
• Sent in a small set of Control Channel Elements
– So UE does not need to decode all the PDCCH
– Space for Control channel assignments is known to all
– Space for Dedicated assignments is per mobile
• CRC of the Assignments depends on the mobile’s active identity (implicit addressing)
• Modulation QPSK

• Carries the BCH - System Information
– Only the Master Information Block is carried on the PBCH
– (The System Information Blocks are sent on PDSCH)
• From SCH and BCH the mobile can determine the cell identifiers
• Sent on central 72 subcarriers (6 resource blocks) once a frame (10ms)
• Modulation QPSK for reception over the cell

Reference symbols are added to the downlink transmissions for:
Channel quality measurements
Channel estimation and equalisation over the frequency band to allow demodulation of the received signal
Hence the reference symbols are distributed in time and frequency.
If a second antenna is used (MIMO operation) this will transmit reference signals in the alternate resource elements.
In the example above, we assume frame structure type 1 and a short cyclic prefix so there are 7 symbols in the timeslot