To be become a LTE Expert?

Here, i am trying to describe whole Lte sequence in my way when UE is powered on............................... 

LTE a terminal must perform certain steps before it can receive or transmit data. These steps can be categorized in cell search and cell selection, derivation of system information, and random access. The complete procedure is known as LTE Initial Access



Successful execution of the cell search and selection procedure as well as acquiring initial system information is essential for the UE before taking further steps to communicate with the network. For this reason, it is important to take a closer look at this fundamental physical layer procedure.

but I strongly recommend you to try to have some big picture of the whole process. Whenever you have some issues or something for you to work, try to ask your self "Where is the current issue located in the whole picture ?".

• Channel bandwidth in terms of Resource Blocks
• SFN (System Frame Number)
• PHICH configuration (used for HARQ ACK/NACK)

• Contention based RAP
• Non-contention based RAP

DL Link Adaptation



1.UE reports CQI, PMI, RI in PUCCH (or PUSCH)
2.Scheduler at eNB dynamically allocated DL resources to the UE (PDCCH)
3.eNB sends user data in PDSCH
4.UE attempts to decode the received packet and sends ACK/NACK using PUCCH (or PUSCH)

UL Scheduling – w/o resource



1.If UE does not have UL-SCH resources, UE sends SR on PUCCH (In absence of PUCCH resources, UE must complete a RACH procedure to request UL-SCH resources.)
2.Scheduler at eNB allocates resources (PRBs and MCS to be used) to UE through “uplink grant” on PDCCH
3.UE sends user data on PUSCH
4.If eNB decodes the uplink data successfully, it sends ACK on PHICH

UL Scheduling – modifying resource



1.UE sends BSR (Buffer Status Report) & PHR (Power Headroom Report) to network on PUSCH
2.Scheduler at eNB dynamically adjusts resources assigned to UE - Grant on PDCCH is adjusted
3.Based on the adjusted grant, UE sends user data on PUSCH
4.If eNB decodes the uplink data successfully, it toggles NDI (New Data Indicator) on PDCCH, and sends ACK on PHICH

•Most scheduling strategies need information about:
–Channel condition
–Buffer status and priorities of the different data flows
–Interference situation in neighboring cells


Remember few major things while designing:
•Throughput
•Efficiency
•QoS Support
–Different types and levels of QoS, respective for different service applications
–Attributes such as bandwidth, delays, error rate and jitter
–Need to serve each subscriber at a certain minimum QoS based on his/her Service Level Agreement (SLA)
•Fairness
–Is a measure of customer satisfaction.
–Neglecting subscribers unfairly in order to increase throughput may lead to high churn rates

By reading the PDCCH (this is valid for both UL scheduling grants and DL scheduling assignments).

PDCCH contains DCI(DL control information), which indicate 3 different messages:-

1. Uplink scheduling grants for PUSCH
2. Downlink scheduling assignment for PDSCH
3. TPC command for PUSCH and PUCCH

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

• UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR";
• UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR"
•  retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"
• periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

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

• From the primary and secondary SCH, the mobile has the Cell Identity
• It can then calculate the (unique) Reference Signal used in the cell
• Reference Signals provide a reference for amplitude, phase and timing
• They are distributed over frequency and time in the Resource Block
– Hence, the mobile can compensate for variation in amplitude and phase over time and frequency

• SCH reference signals in the centre of the band to allow for variable channel bandwidths
– Sent on central 62 subcarriers twice per frame
• Primary SCH
– Signal correlates to 1 of 3 cell identity sequences
– Provides subframe timing and frequency references
• Secondary SCH
– Identifies 1 of 168 cell identity groups
– Provides frame synchronisation
• Hence, Cell Identity is determined

The narrowest channel bandwidth is 72 subcarriers (6 Resource Blocks) but the SCH uses 62 since the processing for reception is simpler and hence quicker.
The primary SCH provides one of 3 possible sequences - the secondary SCH then gives the group Identity. Hence the mobile can then determine the specific cell identity from the 3 x 168 (504) possibilities.

• Sent by the mobile upon request of the eNodeB to allow uplink channel estimation when no other
transmissions are scheduled (on PUSCH or PUCCH)
– Periodicity and subframe offset are configurable
– Sent in the last SC-FDMA symbol of a subframe

• Provides synchronisation and uplink channel
estimation
• Separate DMRS for PUSCH and PUCCH
– Sent once every timeslot for PUSCH
– Repeated 3 times every timeslot for PUCCH

• Timing and frequency offset from Primary SCH
• Cell Identity within the group from P-SCH
• Frame timing from Secondary SCH
• Unique Physical Layer Cell Identity from SCH
• Reference Signals facilitate equalisation
• System Information from PBCH and PDSCH
• RRC Connection
• Attach procedure
• RRC Reconfiguration and Bearer assignment

There is quite a lot of useful information and most of it is premium but a lot is free as well.

For those who are aware of the categories of the UE's being used in practice may be aware that the most common ones have been 'Category 3' with 100Mbps max in DL and 50Mbps max in UL. The new 'Cat. 4' devices are becoming more common as more manufacturers start bringing these devices to the market. They support 150Mbps max in DL and 50Mbps max in UL. Neither of them supports Carrier Aggregation.
A lot of Cat. 4 devices that we may use in testing actually supports carrier aggregation. The next most popular devices soon to be hitting the market is Cat. 6 UE's with 300Mbps max in DL and 50Mbps max in UL. Category 6 UE's support 2 x 20MHz CA in downlink hence you can say that they can combine 2 x Cat. 4 UE's in DL but they do not support CA in uplink hence the UL part remains the same as Cat. 4 device.

• The IMSI is a permanent identity assigned by the Service Provider
• It is valid as long as the Service is Active with the Service Provider
• It is stored on the USIM card and on the HSS (Home Subscriber Server)
• It globally and uniquely identifies a user on any 3GPP PLMN (Public Land Mobile Network)

• The IMEI is a permanent identity assigned by the Device Manufacturer
• Valid as long as the Device is in Use
• Stored on the Device hardware and on the HSS (Home Subscriber Server)

• Dynamic Identity assigned by the eNodeB
• Valid as long as the UE is Connected to the eNodeB that assigned the C-RNTI
• Stored in the UE and the eNodeB

• Dynamic Identity assigned by the MME (Mobility Management Entity)
• Valid as long as the UE is Registered with the EPC (Evolved Packet Core) and Attached to the MME that assigned the GUTI
• Stored on the UE and the MME

• Dynamic Identity assigned by the PGW
• Valid as long as the UE is Registered with the EPC (Evolved Packet Core)
• Stored in the UE and the PGW and any other node "north" of the PGW

• o   New data arrives in previously empty buffers: Assuming we are at the “beginning” of UL data transmission when all data buffers are empty, if data becomes available for transmission in the UE for any radio bearer a BSR is triggered. 
• o   Higher Priority data arrives: If the UE has already sent a BSR and is waiting for a grant but then higher priority data becomes available for transmission, the eNB needs to know this and therefore a new BSR is triggered. Note that this happens even when the triggering RB is in the same LCG for which there is an outstanding BSR.
• o   To update the eNB about the current status of buffers: If, for example, a UE is uploading a file, the data is arriving in the UE transmission buffer asynchronously with respect to the grants it receives from eNB. Consequently there is an ongoing need to keep the eNB updated as to the amount of data still to be transmitted. For this purpose the UE keeps a timer. When the periodicBSR-Timer expires, a BSR is triggered. The timer, configured by RRC, ranges from 5ms up to 2.56 seconds. It can be disabled by setting it to infinity, which is also the default.
• o   To provide BSR robustness: The LTE standard provides a mechanism to improve the robustness of buffer status reporting. We want to avoid deadlock situations which may occur when the UE sends a BSR but never receives a grant. A BSR retransmission mechanism is built into the UE implementation. The UE keeps a retxBSR-Timer which is started when a BSR is sent and stopped when a grant is received.  If the timer expires, and the UE has still has data available for transmission, a new BSR is triggered. The retransmission timer, configured by RRC, ranges from 320ms up to 10.24 seconds. Unlike the periodic timer it cannot be disabled. The default is 2.56 seconds.

• Interestingly, there is no direct relationship between the BSRs sent by the UE and how it processes a grant from eNB. Resource grants are allocated by the UE to radio bearers on a logical channel priority basis. Membership in a particular LCG is not relevant.  For example, let’s say a UE requests resources for LCG 2 in order to send a HTTP request. Before the grant was received an RRC message becomes ready to send.  Then when the grant is received the RRC message gets priority and uses up as much of the resource as it needs. The HTTP request will get the leftovers, if any. Note that RRC messages are sent on SRBs which are assigned to LCG 0 by default.

1. Transmit Diversity
2. MIMO one layer
3. MIMO two layers

1. Maximum number of RRC connections supported per cell (parameter or feature)
2. Maximum number of RRC connections active per cell
3. Average number of RRC connections active per cell
4. Maximum number of users per TTI supported per cell (parameter or feature)
5. Maximum number of users scheduled per TTI in the cell(s) of interest
6. Average number users scheduled per TTI in the cell(s) of interest