LTE: SYSTEM ACQUISITION

CELL SELECTION PROCEDURE

Glossary

• Frame = 10 x Subframes = 10ms

• Subframe = 2 x Slots = 1ms

• Slot = 7 OFDM symbols = 0.5ms

• Resource Block = 12 subcarriers x 7 OFDM = 180KHz x 0.5ms

• PSCH – Primary Synchronization Channel

• SSCH – Secondary Synchronization Channel

• PBCH – Physical Broadcast Channel

• PCFICH – Physical Control Format Indicator Channel

• PDCCH – Physical Downlink Control Channel

• PDSCH - Physical Downlink Shared Channel

• PHICH - Physical HARQ Indicator Channel

• RSRP – Reference Signal Received Power

• RSRQ - Reference Signal Received Quality

Channel Bandwidth [MHz] 1.4 3 5 10 15 20

No of Occupied Subcarriers including DC 73 181 301 601 901 1201

FFT Size (N) 128 256 512 1024 1536 2048

N. of Resource Blocks (NRB) 6 15 25 50 75 100

SYNCHRONIZATION PROCEDURES

Cell search is the procedure by which a UE acquires time and frequency

synchronization with a cell and detects the cell ID of that cell. The eNodeB

provides all the necessary signals and mechanisms through which the UE

synchronizes

E-UTRA cell search supports a scalable overall transmission bandwidth

corresponding to 6 resource blocks (i.e., 72 subcarriers).

E-UTRA cell search is based on various signals transmitted in the downlink such

as primary and secondary synchronization signals, and downlink reference

signals. The primary and secondary synchronization signals are transmitted over

the center 72 sub-carriers in the first and sixth subframe of each frame.

Neighbor-cell search is based on the same downlink signals as the initial cell

search.

SLOT AND FRAME SYNCHRONIZATION

The UE attempts to acquire the central 1.4MHz bandwidth in order to decode the

Primary sync signal (PSCH), Secondary sync signal (SSCH), and the system

information block (SIB).

The eNodeB transmits this information on the subcarriers within the 1.4MHz

bandwidth consisting of 72 subcarriers, or 6 radio blocks.

In order to perform slot synchronization, the UE attempts to acquire the Primary

sync signal which is generated from Zadoff-Chu sequences. There are three

possible 62-bit sequences helping the UE to identify the start and the finish of

slot transmissions.

Next, the UE attempts to perform frame synchronization so as to identify the

start and the finish of frame transmission. In order to achieve this, Primary sync

signals are used to acquire Secondary sync signals. The Secondary sync signal (a

62-bit sequence) is an interleaved concatenation of two length-31 binary

sequences scrambled with the Primary synchronization signal.

Once PSCH and SSCH are known, the physical layer cell identity is obtained.

There are 504 unique physical layer cell identities.

PSCH AND SSCH

• Created from Zadoff-Chu sequence (Zero

Autocorrelation codes)

• The primary synchronization signal is

transmitted on 72 active subcarriers, centered

around the DC subcarrier.

• Assists subframe timing determination

• Provides a unique Cell ID index (0, 1, or 2) within

a Cell ID group

• Secondary synchronization signal is

transmitted in and only in slots where the

primary synchronization signal is transmitted.

• Provides a unique Cell ID group number among

168 possible Cell ID groups

REFERENCE SIGNALS

Downlink reference signals are predefined signals occupying specific resource elements

within the downlink time–frequency grid. In every sixth subcarrier in the frequency domain a

reference symbol from the generated reference signal pattern is transmitted. In the time

domain, every fourth OFDM symbol transmits a reference symbol

There are 504 different reference-signal sequences defined for LTE, where each sequence

corresponds to one of 504 different physical-layer cell identities

The downlink reference signals help the terminal distinguish between the different

transmission antennas. Where one antenna is transmitting the reference pattern the other

antennas are transmitting nothing. These physical signals are also used to estimate the

quality of the radio channel.

RSSI

RSRP is the average of the power of all resource elements which carry cell-specific reference

signals over the entire bandwidth.

RSRQ is the ratio between the RSRP and the Received Signal Strength Indicator (RSSI),

BROADCAST CHANNEL PHYSICAL BROADCAST CHANNEL

• Carries the primary Broadcast Transport

Channel

• Carries the Master Information Block (MIB),

which includes:

• Overall DL transmission bandwidth

• PHICH configuration in the cell

• System Frame Number

• Number of transmit antennas (implicit)

Transmitted in

• Time: subframe 0 in every frame

• 4 OFDM symbols in the second slot of

corresponding subframe

• Frequency: middle 1.08 MHz (6 RBs)

TTI = 40 ms

• Transmitted in 4 bursts at a very low data

rate

• Same information is repeated in 4

subframes

• Every 10 ms burst is self-decodable

• CRC check uniquely determines the 40 ms

PBCH TTI boundary

• Last 2 bits of SFN is not transmitted

SYSTEM INFORMATION

• The MIB (scrambled with Cell-ID) reception provides

the UE with LTE downlink bandwidth (DL BW),

number of transmit antennas, System Frame Number

(SFN), PHICH duration, and its gap.

• After reading the MIB, the UE needs to get system

information blocks (SIBs) to know the other system-

related information broadcasted by the eNodeB.

• SIBs are carried in the PDSCH, whose information is

obtained from the PDCCH indicated by the Control

Format Indicator (CFI) field.

• In order to get CFI information, the UE attempts to

read the PCFICH which are broadcasted on the first

OFDM symbol of the subframe.

• Once bandwidth selection is successful, the UE

attempts to decode the DCI (DL control information)

to acquaint with SIB Type 1 and 2 to get PLMN id, cell

barring status, and various Rx thresholds required in

cell selection.

SYSTEM INFORMATION

RANDOM ACCESS PROCEDURE (RACH)

RAP is required for uplink synchronization, the two types of RACH procedure are:

1. Contention-Based Random Access Procedure:

I. The transmission of a random-access preamble, allowing the eNodeB to estimate the transmission

timing of the terminal.

II. The network transmits a timing advance command to adjust the terminal transmit timing, based

on the timing estimate obtained in the first

III. The transmission of the mobile-terminal identity to the network using the UL-SCH similar to

normal

IV. scheduled data.

V. The transmission of a contention-resolution message from the network to the terminal on the DL-

SCH.

PRACH resources found in SIB-2

RANDOM ACCESS PROCEDURE (RACH)

2. Non-Contention-Based Random Access Procedure: The network initiates this procedure, when the UE is already in communication with the

eNodeB, by transmitting an allocated preamble to the UE. There are no collisions with

other UEs because the eNodeB controls the procedure and hence has the necessary

information to support a non-contention-based RAP

Contention-free random access can only be used for re-establishing uplink

synchronization upon downlink data arrival, handover, and positioning. Only the first two

steps of the previous procedure are used.

RANDOM ACCESS PROCEDURE (RACH)

The preamble format determines the length of the Cyclic Prefix and Sequence.

FDD has 4 preamble formats (for different cell sizes), 16 PRACH configurations

are possible.

Each configuration defines slot positions within a frame (for different

bandwidths). Each random access preamble occupies a bandwidth corresponding

to 6 consecutive RBs.

CELL SELECTION

S is the criterion defined to decide if the cell is still suitable . This criterion is

fulfilled

When the cell selection receive level is Srxlev > 0. Srxlev is computed based on

Equation

Srxlev

= Qrxlevmeas

– (Qrelevmin

+ Qrxlevminoffset

) – Pcompensation

[dB]

where Pcompensation

= max(PEMAX

– PUMAX

,0) [dB]

• Qrxlevmeas is the measured receive level value for this cell. This measured value is the linear average over the power of the resource elements that carry the cell-specific reference signals over the considered measurement bandwidth. Consequently, it depends on the configured signal bandwidth.

• Qrxlevmin is the minimum required receive level in this cell, given in dBm. This value is signaled as Q-RxLevMin by higher layers as part of the System Information Block Type 1 (SIB Type 1). Qrxlevmin is calculated based on the value provided within the information element (-70 and -22) multiplied with factor 2 in dBm.

• Qrxlevminoffset, is an offset to Qrxlevmin that is only taken into account as a result of a periodic search for a higher priority PLMN while camped normally in a Visitor PLMN (VPLMN). This offset is based on the information element provided within the SIB Type 1, taking integer

by a factor of 2 in dB. The offset is defined to avoid ping-pong different PLMNs. If it is not available then Qrxlevminoffset is assumed to

be 0 dB.

CELL SELECTION

PCompensation is a maximum function. Whatever parameter is higher, PEMAX-PUMAX or 0, is the value used for PCompensation.

PEMAX [dBm] is the maximum power a UE is allowed to use in this cell, whereas

PUMAX [dBm] is the maximum transmit power of an UE according to the power class the UE belongs too.

At the moment only one power class is defined for LTE, which corresponds to Power Class 3 in WCDMA that specifies +23 dBm. PEMAX is defined by higher layers, PEMAX can take values between -30 to +33 dBm. Only when PEMAX > +23 dBm PCompensation is it considered when calculating Srxlev.

SYSTEM ACQUISITION SUMMARY

UE performed a rough frequency

synchronization

(UE has found a good carrier candidate

with strong 72 (6x12) subcarriers which

might carry the Sync signals and PBCH)

UE is

switched on

UE searches for a strong cell in

the DL band

UE determined: -Exact carrier frequency -Cell ID index within a Cell ID group -Subframe timing -Cyclic Prefix Length (by trial and error method)

UE knows: -Frame timing -Cell ID group (1 out 168)

UE acquired most essential system information. UE can read PDCCH/PDSCH and register in the system.

PBCH is time aligned with the

Sync channels

UE can read PBCH channel now

UE looks for the (PSS)

Attempts to match one out of three

possible primary Sync signals (Cell ID

index within a Cell ID Group)

PBCH is time aligned with the

Sync channels

UE can read PBCH channel now

UE attempts to detect (SSS)

Tries to match 1 out of 168 possible

secondary Sync signals (Cell ID Groups)

UE looks for the (PSS)

Attempts to match one out of three

possible primary Sync signals (Cell ID

index within a Cell ID Group)

SYSTEM ACQUISITION SUMMARY

DL Sync and Bandwidth

Detection

SIB Type 1 Acquisition

SIB Type 2 Acquisition

Initial Access Procedure

PLMN ID

Match

Cell Barred

Rx-Levmin

Threshold

Acquire another cell

PLMN ID Acquired

No

Yes

No

Yes

No

Yes

TEST CRITERIA

• Primary Synchronization (Slot Timing, PHY Layer ID)

• Secondary Synchronization (Radio Frame Timing, Cell ID FDD\TDD

detection)

• Reference Signal Detection (Calculation of RSRP, RSRQ) – SISO, MIMO

• PBCH Detection (MIB and SIB reading)

• Cell Selection

• Random Access Procedure

• Open Loop Power Control

REFERENCES

1. LTE E-UTRAN and its Access Side Protocols (By: Suyash Tripathi,

Vinay Kulkarni and Alok Kuma)

2. 3GPP TS 36.213 V8.8.0 (2009-09)

3. 3GPP TS 36.211 V8.9.0 (2009-12)

THANK YOU