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We are starting new batch for LTE (4G) Protocol testing and log analysis from 05/11/2020. Interested candidate can take a demo session.

Batch timing 7.30AM to 8.30AM
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Hey guys

We are starting new batch for LTE (4G) Protocol testing from 14 September 2020. Interested candidate can take a demo session.

Batch timing 8.30PM to 9.30 PM
Regular batch

For more details
Call us on
8805948995

Timing Advance (TA) in LTE:

In GSM system MS sends its data three time slots after it received the data from the BTS. This is ok as long as MS-BTS distance is small but increasing distance requires consideration of propagation delay as well. To handle it Timing advance (TA) is conveyed by network to MS and current value is sent to the MS within the layer 1 header of each SACCH. BTS calculates the first TA when it receives RACH and reports it to the BSC and BSC/BTS passes it to UE during Immediate Assignment.

In UMTS Timing Advance parameter was not used but in LTE Timing Advance is back.

In LTE, when UE wish to establish RRC connection with eNB, it transmits a Random Access Preamble, eNB estimates the transmission timing of the terminal based on this. Now eNB transmits a Random Access Response which consists of timing advance command, based on that UE adjusts the terminal transmit timing.

The timing advance is initiated from E-UTRAN with MAC message that implies and adjustment of the timing advance.

3GPP TA Requirements

Timing Advance adjustment delay
UE shall adjust the timing of its uplink transmission timing at sub-frame n+6 for a timing advancement command received in sub-frame n.

Timing Advance adjustment accuracy
The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal to ±4* TS seconds to the signalled timing advance value compared to the timing of preceding uplink transmission. The timing advance command is expressed in multiples of 16* TS and is relative to the current uplink timing.

Maintenance of Uplink Time Alignment

The UE has a configurable timer timeAlignmentTimer which is used to control how long the UE is considered uplink time aligned

when a Timing Advance Command MAC control element is received then UE applies the Timing Advance Command and start or restart timeAlignmentTimer.
when a Timing Advance Command is received in a Random Access Response message then one of following action is performed by UE
- if the Random Access Preamble was not selected by UE MAC then UE applies the Timing Advance Command and starts or restarts timeAlignmentTimer.

- else if the timeAlignmentTimer is not running then UE applies the Timing Advance Command starts timeAlignmentTimer; when the contention resolution is considered not successful then UE stops timeAlignmentTimer.

- else ignore the received Timing Advance Command.

when timeAlignmentTimer expires UE flushes all HARQ buffers, notifies RRC to release PUCCH/SRS and clears any configured downlink assignments and uplink grants.
Timing Advance Command MAC Control Element

The Timing Advance Command MAC control element is identified by MAC PDU subheader with LCID value = 11101 (Timing Advance Command) .

It has a fixed size.

Timing Advance Command MAC control element has following fields.

R: reserved bit, set to "0"
Timing Advance Command: This field indicates the index value TA (0, 1, 2… 63) used to control the amount of timing adjustment that UE has to. The length of the field is 6 bits.

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RNTI's in LTE:

RNTI stands for Radio Network Temporary Identifier. RNTIs are used to differentiate/identify a connected mode UE in the cell, a specific radio channel, a group of UEs in case of paging, a group of UEs for which power control is issued by the eNB, system information transmitted for all the UEs by the eNB etc…

There are a several RNTI types in LTE such as SI-RNTI, P-RNTI, C-RNTI, Temporary C-RNTI, SPS-CRNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI, RA-RNTI, and M-RNTI. Each RNTI’s usage, its value range etc…are discussed in detail below

SI-RNTI (System Information RNTI)
SI-RNTI is used for broadcast of system information.
It is a common RNTI meaning that, it is not allocated to any UE explicitly.
SI-RNTI is of 16-bit in length and its value is fixed to 65535 (0xFFFF). A single SI-RNTI is used to address SystemInformationBlockType1 as well as all SI messages
Broadcast of System Information uses BCCH logical channel which is then mapped to DL-SCH transport channel which intern mapped to PDSCH physical channel. The UEs should know the scheduling information for PDSCH which is carrying System Information. The required scheduling information is contained in DCI (Downlink Control Information) whose CRC is scrambled by SI-RNTI
DCI Formats which carries scheduling information for System Information are DCI Format 1A and DCI Format 1C in common search space
The UE starts decoding PDCCH scrambled with SI-RNTI at the start of SI-Window (for the concerned SI message) until the end of the SI-window, or until the SI message was received excluding the following subframes.
- subframe #5 in radio frames for which SFN mod 2 = 0
- any MBSFN subframes
- any uplink subframes in TDD

If the SI message was not received by the end of the SI-window, the UE repeats reception at the next SI-window occasion for the concerned SI message

P-RNTI (Paging RNTI)
P-RNTI is used by the UEs for the reception of paging.
It is a common RNTI meaning that it is not allocated to any UE explicitly.
P-RNTI is of 16-bit in length and its value is fixed to 65534 (0xFFFE).
Paging message is carried by PCCH logical channel which is mapped to PCH transport channel. The PCH transport channel is mapped to PDSCH physical channel. The eNB scrambles PDCCH’s CRC with P-RNTI for transmission of PDSCH that carries paging information
DCI Formats which carries scheduling information for paging are DCI Format 1A and DCI Format 1C in common search space

RA-RNTI (Random Access RNTI)
As part of Random Access procedure, the eNB’s MAC generates Random Access Response (RAR) as a response to the Random Access Preamble transmitted by the UE. RAR is transmitted on DL-SCH transport channel which intern is mapped to PDSCH. The eNB scrambles PDCCH’s CRC with RA-RNTI for transmission of PDSCH that carries RAR(s).
RA-RNTI can be addressed to multiple UEs, i.e., multiple UEs might decode PDCCH scrambled by the same RA-RNTI.
RA-RNTI unambiguously identifies which time-frequency resource was utilized by the UE to transmit the Random Access preamble (explained below)
RA-RNTI is of 16-bit in length and its value is derived from the below equation where t_id is the index of the first subframe of the specified PRACH (0≤ t_id

What happened when UE switched ON

Frequency Search (Scan the entire frequency bands)
2) Cell Search – will search for Normally a UE would find multiple cells in this process
3) a) MIB decoding (b) SIB decoding
4) Cell Selection/ Cell Reselection
5) Initial Process >

The complete procedure is known as LTE Initial Access.

Although there can be many algorithms defined for this procedure the basic sequence is as follows:

A list of preferred earfcn is maintained in UE sim. UE measures RSSI of each element of this list.
From the list of 1. all channels with RSSI > a threshold value is determined..
UE decode sync/reference signals and find Physical Cell Id of each candidate from 2.
Of all the cells of step 3. UE decode MIB and SIB. Now, UE has a list of frequency, PCI and PLMN of the filtered cells.
From all of the above information, UE makes a decision of on which of these cells it would camp on.
Now, let us get into details.

UE tune to every channel it supports and measure RSSI:-
In the equipment, operator stores all the frequency bands it supports. For each band, preferred earfcn values are also stored in order. Based on that information UE will scan all those channels and measure the RSSI. RSSI is Received Signal Strength Indicator. It is total signal power of each resource element including interference or noise. This is included here in measurement since UE currently does not have any information about network and measuring this parameter doesn’t require any channel coding process.

Suppose an enodeb corresponds to one operator, say airtel, working in band 3, earfcn 1400 and operating bandwidth 10 Mhz (1820 to 1830 Mhz). Another enodeb in same cell corresponds to different operator, say vodafone, in the band 3 with earfcn 1500 and operating bandwidth 10MHz(1830 to 1840). Now, suppose the UE supports band 3 (1805 to 1880 Mhz) which has a overall bandwidth 75Mhz and band 7 of overall bandwidth 70 MHz.. It starts RSSI scanning from band 3 on a 100khz raster basis so overall it will scan 750 Earfcn in Band 3. The preferred earfcn list is orderly maintained with UE as- 1400, 1500…. This list can contain multiple earfcn for 1 operator but the list is maintained according to preference. Out of these, earfcn that have a RSSI value greater than a certain threshold value will be shortlisted and maintained in order.
From the list of 1. all channels with RSSI > a threshold value is determined.
The thresholds are implementation dependent

UE decode sync/reference signals and find Physical Cell Id of each candidate from 2:-
There are 504 Physical Cell Identities(PCI). These identities are divided into 168 unique cell layer identity groups in the physical layer, in which each group consists of 3 physical layer identities.
This information is transmitted using two different signals. The two signals, carrying the physical layer identity and the physical layer cell identity group, are the primary and the secondary synchronization signals respectively

The UE first looks for the primary synchronization signal (PSS) which is transmitted in the last OFDM symbol of the first time slot of the first subframe. This enables the UE to acquire the slot boundary independently from the chosen cyclic prefix used in this cell. PSS is transmitted twice per radio frame, so it is repeated in subframe 5 (in time slot 11). This enables the UE to get time synchronized on a 5 ms basis, This was done because if a UE starts reading cell from between a subframe, it can get time synchronized.

In FDD, PSS is broadcast using the central 62 subcarriers.

The PSS is used to:
– Achieve time syncronization.
– Identify the center of the channel bandwidth in the frequency domain
– Finds which 1 of 3 Physical layer Cell Identities (PCI), cell belongs
PCI are organised into 168 groups of 3 so the Primary Synchronisation Signal identifies the position of the PCI within the group but does not identify the group itself.

Secondary Syncronization symbol (SSS) is located in the symbol before PSS, transmitted twice per subframe. The two transmissions of the SSS are different so the UE can detect which is the first and which is the second. This sequence alternates in even and odd subframes, ex In subframe1 sequence A is transmitted in symbol a and sequence A’ is transmitted in symbol b, then in subframe 2 sequence A is transmitted in symbol b and sequence A’ is transmitted in symbol a. This way subframe syncronization is achieved.

SSS is also transmitted in central 62 subcarriers in the symbol before PSS.

The SSS is used to:
– achieve radio frame synchronisation
– Find which 1 of 168 Physical layer Cell Identity (PCI) groups is used,
Hence,the PCI could be deduced when combined with the pointer from the PSS. Following formula is used:-

PCI = 3*physical layer id group(from SSS)+phy layer cell id(from PSS).

Of all the cells of step 3. UE decode MIB and SIB. Now, UE has a list if frequency, PCI and PLMN of the filtered cells:-
Here, I am not explaining the structure of MIB and SIB. At this point UE knows the PLMN from SIB1 and can select the cell to camp on.

UE finds a suitable cell. A suitable cell is one that fulfills cell selection criteria.
Criteria for Cell Selection are:-

– Cell must transmit power strong enough to be detected by UE.

i.e Srxlev > 0
Where,
Srxlev = Qrxlevmeas – (Qrxlevmin+Qrxlevminoffset) – Pcompensation

Qrxlevmeas = RSRP measured by UE in dBm
Qrxlevmin = min. required RSRP signalled within SIB1
Qrxlevminoffset = usually included in SIB1. If not included than value of zero is set.
Pcompensation = MAX(PEMAX – PUMAX, 0)

These values are decoded from SIB.

– Cell must not be barred.

– PLMN saved in UE sim must match with cell’s PLMN.

There are 2 types of cell selection:-

– Initial cell selection,

– Stored procedure cell selection.

In the initial cell selection procedure, as described above, no knowledge about RF channels is available at the UE. In that case the UE scans the supported E-UTRA frequency bands to find a suitable cell. Only the cell with the strongest signal per carrier will be selected by the UE.

The second procedure uses information about carrier frequencies and optionally cell parameters received and
stored from previously-detected cells. If no suitable cell is found using the stored information the UE starts with the initial cell selection procedure.

LTE Frame Structure

Time duration for one frame (One radio frame, One system frame) is 10 ms. This means that we have 100 radio frame per second.

Number of subframe in one frame is 10
Number of slots in one subframe is 2. Each subframe is of 1ms. Each subframe consist of 2 time slot and each time slot occupies 0.5 ms in time domain. This means that we have 20 slots within one frame.

Each radio frame is numbered using SFN . SFN is of 10 bit which provides range from 0 to 1023.

The frame structures for LTE differ between the Time Division Duplex, TDD and the Frequency Division Duplex, FDD modes as there are different requirements on segregating the transmitted data

There are two types of LTE frame structure:

Type 1: used for the LTE FDD mode systems.
Type 2: used for the LTE TDD systems.

Type 1: FDD Frame Structure
As LTE FDD is full duplex system, means both the downlink and uplink transmission happens at the same time at different frequencies.

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