LTE Short Question - Part-2

>> SIB1 ->

SIB 1 Transmitted on BCCH->DL-SCH->PDSCH

After initial synchronization of UE with the network and obtaining the MIB, the UE starts to listen System Information Block Type 1 (SIB1) which is carrying cell access related information. SIB1 is transmitted through BCCH -> DL-SCH -> PDSCH channels. It is always sent in sub-frame #5 of the Radio-Frame, and continues re-transmitting the same message in each 20 ms with different redundancy versions. Each 80ms a new SIB1 is transmitted.

Here the list of carried information from the network to UE through SIB1:

PLMN Identity List: up to 6 PLMN can be identified in the list. First one is primary PLMN.

PLMN Identity: identifies operator global identity, combination of MCC and MNC.

Cell Reserved For Operator Use: Reserved or not Reserved. 

In case of multiple PLMNs indicated in SIB1, this field is specified per PLMN inverse of *Cell Barred.

Tracking Area Code: identifies a tracking area for paging the users.

Cell Identity: identifies a cell within PLMN.

Cell Barred: identifies the cell access status, if it is barred then it is not allowed the user to access to the cell. In case of multiple PLMNs listed in SIB1, this field is common for all PLMNs.

Intra Frequency Reselection: identifies the intra-frequency reselection permission status if it is allowed or not.

CSG Indication: False or True, identifies Closed Subscriber Group.

CSG Identity: is a limited set of users with connectivity access, in case of “true” only those UEs can access to the CSG cell which are in the group.

q_RxLevMin: minimum required received RSRP level for cell selection. Actual value in dBm is obtained by multiplying by two.

q_RxLevMinOffset: indicates the offset to the minimum required signal level.

p-Max: defines the maximum allowed uplink transmit power for the UE.

Freq Band Indicator: indicates LTE frequency band.

SI Periodicity: periodicity of System Information Blocks except than SIB1.

SIB Mapping Info: list of mapped SIBs.

SIB Type: type of transmitted SIB message in the list.

SI Window Length: a window is defined to enable multiple transmissions of the SI message within the window. SI Window Length can be set to 1, 2, 5 10, 15, 20 or 40 ms.
System Info Value Tag: if a change has occurred in the SI messages. UEs can use System Info Value Tag to verify if the previously stored SI messages are still valid.

SIB1 carried information:

Example for SIB1, taken from drive-test tool:

plmn-IdentityList
PLMN-IdentityList : 
[0 ] : 
plmn-Identity
mcc
MCC : 
[0 ] : 1
[1 ] : 1
[2 ] : 1
mnc
MNC : 
[0 ] : 0
[1 ] : 2
cellReservedForOperatorUse : notReserved
trackingAreaCode : 38441 (0x9629)
cellIdentity : 113491462 (0x6C3BE06)
cellBarred : notBarred
intraFreqReselection : allowed
csg-Indication : False
q-RxLevMin : -60
p-Max : 23
freqBandIndicator : 3
schedulingInfoList
SchedulingInfoList : 
[0 ] : 
si-Periodicity : rf16
sib-MappingInfo
SIB-MappingInfo : 
[0 ] : 
extensionBit0 : 0
Optionalitem : sibType3
[1 ] : 
si-Periodicity : rf32
sib-MappingInfo
SIB-MappingInfo : 
[0 ] : 
extensionBit0 : 0
Optionalitem : sibType5
si-WindowLength : ms40
systemInfoValueTag : 8

The following two tabs change content below.

SRS Vs DMRS

There are two types of reference signals used in LTE uplink, to estimate uplink channel quality. Which allow eNB to take smart decisions for resource allocation for uplink transmission, link adaptation and to decode transmitted data from UE .

SRS
The Sounding Reference Signal (SRS) is a reference signal transmitted by the UE in the uplink direction which is used by the eNodeB to estimate the uplink channel quality over a wider bandwidth. The eNodeB may use this information for uplink frequency selective scheduling.

Why we need this kind of signal ? As you know, in LTE eNodeB often allocates only a partial section of full system bandwidth for a specific UE and at a specific time. So it would be good to know which section across the overall bandwidth has better channel quality comparing to the other region. In this case, Network can allocate the specific frequency region which is the best for each of the UEs. (If we always have to use full bandwidth, we may not need this kind of reference signal since there is no choice even when there is a better or worse spots within the bandwidth).

Now you might be thinking what if all UEs transmit the SRS with same interval and periodicity or in other words how eNB distinguish the UE specific SRS in case of overlapped SRS transmission? 

Well in that case using transmission_comb and cyclic shift parameters present in RRC Connection setup and RRC Connection Reconfiguration, eNB distinguish and decode different UE specific SRS.

Demodulation reference signal (DMRS)
Demodulation reference signal (DMRS) in uplink transmission is used for channel estimation and for coherent demodulation which comes along with PUSCH and PUCCH. If DMRS is bad or by some reason not decoded properly by base station , PUSCH or PUCCH will be not decoded as well. Hence DMRS is not optional like SRS.

DMRS only state channel quality of frequency region in which PUSCH or PUCCH is being transmitted.

So what about positioning of DMRS in resource grid, is this fixed ? Answer is Yes and No both. So, when DMRS sent by UE with PUCCH, position of reference signal vary according to PUCCH format indicator. But in case of PUSCH it is always the center symbol of a slot (3rd symbol of slot0 and 10th symbol of slot1).

To support a large number of UEs (User terminal), a large number of DMRS sequences needed and it is achieved by cyclic shifts of a base sequence. As we know in LTE -Advance we will have concept of MIMO in uplink as well, hence DMRS have to enhance for MIMO transmission and each UE will use different DMRS sequences.

> What if SRS and CQI coincide on the same subframe?
Well in that case a UE shall not transmit SRS whenever SRS and PUCCH format 2/2a/2b transmissions (CQI, CQI with 1 or 2 bit HARQ ACK/NACK) happen to coincide in the same subframe [3GPP 36.213 Section 8.2].

PMI (Precoding Matrix Indicator) 

PMI (Precoding Matrix Indicator)  PMI (Precoding Matrix Indicator), UE indicates to eNB , which precoding matrix should be used for downlink transmission which is determined by RI. Precoding matrix is used to map data stream from no of layer to no of antenna.

Precoding matrix indicator (PMI) The precoding matrix determines how the individual data streams (called layers in LTE) are mapped to the antennas. They are applicable to closed loop transmission modes

• Transmission Mode-4:Closed Loop Spatial Multiplexing.

• Transmission Mode-5:Multi-User MIMO.

• Transmission Mode-6:Closed Loop Spatial Multiplexing using a single layer.

The LTE UE use PMI information to signal preferred set of weights to be applied during the precoding process. UE does this in order to maximize the downlink S/N ratio.

Table-1 mentions complex weights. Based on following configurations one out of 4 is used by LTE UE.
Antenna ports-2, RI =1 , PMI = {0,1,2,3}

Codebook Index	0	1	2	3
Weight for antenna-1	0.7071	0.7071	0.7071	0.7071
Weight for antenna-2	0.7071	-0.7071	j*0.7071	-j*0.7071

Table-1: Closed Loop Spatial Multiplexing Weights(single layer & 2 Antenna Ports)

Table-2 mentions complex weights for following configurations. This configured is used by UE to select 1 of 2 sets of complex weights.
Antenna ports=2, RI=2, PMI={0,1}.

	Antenna-1		Antenna-2
Codebook Index	Codeword-1	Codeword-2	Codeword-1	Codeword-2
1	0.5	0.5	0.5	-0.5
2	0.5	0.5	j*0.5	-j*0.5www

Table-2 Closed Loop Spatial Multiplexing weights(2 layers & 2 antenna ports)

For antenna ports of 4, PMI of value {0,1,....14,15} can be used to indicate 1 of 16 sets of complex weights.

LTE PMI can be transmitted using PUSCH or PUCCH channel.

RI (Rank Indicator) 

UE indicates to eNB, the number of layers that should be used for downlink transmission to the UE.

LTE RI stands for Rank Indicator.

RIs are applicable for open loop transmission and closed loop transmission modes. These modes use more than a single layer between layer mapping and precoding modules.

• Transmission mode-3 indicates open loop spatial multiplexing
• Transmission mode-4 indicates closed loop spatial multiplexing

In LTE system, UE uses RI to inform about number of layers required during layer mapping.

LTE RI can be transmitted using PUSCH or PUCCH.

Number of layers = Number of codewords (for Antenna elements=2),
Here UE can signal RI equal to 0/1 to indicate 1 or 2 layers as preferred one

Number of layers >= Number of codewords (for Antenna elements=4),
Here UE can signal RI equal to {0,1,2 or 3} to indicate 1,2,3 or 4 layers as preferred one.

Q. In TDD, why special sub-frame is used for switching Downlink (DL) to Uplink (UL) . Why not for UL to DL ? What’s the reason Please clarify.

There are 2 versions of the answer , some people will get satisfied with version 1, others with version 2. Both are correct, but that’s how we all learn , what convinces us.

Version 1:

For TDD, in the Downlink , due to path distance DL signal can be delayed. Due to path distance , path delay is created. Delay can cause collision between UL and DL signals. The guard period provides enough time for DL delayed signal to arrive due to path distance, and also gives enough opportunity for (User Equipment)UE to receive UL timing advance command from the Base station.

Follow up Question. Why special sub-frame only in DL to UL, why not from UL to DL ?

UE always transmit in UL after receiving grant from the BS. BS can advance or retard the UL timing command as needed. Once DL signal is received completely then UE can send command /signal in return with respective timing ( UE cannot transmit on its own before receiving a complete response from the DL) . So we need a guard period for DL to UL switching, to avoid collisions. Whereas in case of UL to DL switching there is no need of Guard period, as eNodeB has timing advance feature plus there will be minimum chance of collision in UL to DL case.

In DL to UL switching , a Guard Period is needed , ” To avoid the time advanced UL to collide with the delayed DL”



Version 2:

A simple answer is in TDD because there is no duplex Frequency and all management must be done on the same path, then UE should first receive info about up-link(most important one is Time scheduling ). Therefore, Downlink should always be done before Uplink in TDD mode.

If you have anything else to add , feel free to write it in the comments below.

How Special subframe configuration impact Cell Size?

In TDD , there are 9 special subframe configuration, each have different number of OFDM symbols for DwPTS,GP and UpPTS. Special subframe guard period impact the size of cell, it represent how much propagation delay it can compensate. Longer guard period can compensate more propagation delay result a longer cell size.

Timing Advance

 

Timing Advance is a MAC CE that is used to control Uplink signal transmission timing. Network (eNodeB in this case) keep measuring the time difference between PUSCH/PUCCH/SRS reception and the subframe time and can send a 'Timing Advance' command to UE to change the PUSCH/PUCCH transmission to make it better aligned with the subframe timing at the network side. If PUSCH/PUCCH/SRS arrives at the network too early, network send a Timing Advance command to UE saying "Transmit your signal a little bit late", If PUSCH/PUCCH/SRS arrives at the network too late, network send a Timing Advance command to UE saying "Transmit your signal a little bit early".

 

MAC PDU for Timing Advance is as follows. It is one byte data and the first two bits are reserved and set to be always 0. The remaining 6 bits carries Timing Advance command value ranging from 0 to 63.

 

As you see in the following figures, for Rel 8,9,10 there is no special tag for each component carrier, meaning that even in Carrier Aggregation single Timing Advanced value apply to all the component carriers. But in Rel 11, the first 2 bits are allocated to indicate whether the value is for PCC or SCC. If TAG id is 0, it means it is for PCC.

 

< 36.321 Rel 8,9,10 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

 

< 36.321 Rel 11 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

 

 

Then how to translate each value of TA(Timing Advance) value to physical 'time' delay or advance value. It is described in detail in 36.213 4.2.3 Transmission timing adjustments. Simply put, the UL transmit timing is controlled by following equation.

 

UL Transmission Time = (UL Transmittion Time for Previous subframe) + (TA value - 31) x 16 samples.

                   , where 1 sample is about 0.033 us and 16 samples is about 0.52 us.

 

By this calcuation, you can see that the maximum timing change by single TA value (0 or 63) is about 16.7 us (I hope my calculation is right. please let me know if this calculation is wrong).

 

The Timing Advance is equal to 2 x propagation delay assuming that the same propagation delay value applies to both downlink and uplink directions. So, UE1 needs to start it’s uplink at t1+2δ1 whereas UE2 should start it’s uplink at t1+2δ2. This will ensure that both of the uplink transmissions (from UE1 and UE2) reach the eNodeB at the same time which means that at the eNodeB, both uplink and downlink subframes are time aligned.

If the Timing Advance is not applied, then the start of uplink transmission from UE2 for subframe #n+1 will overlap with the end of uplink transmission from UE1 for subframe #n. Assuming that same resource blocks are assigned to UE1 in subframe #n and UE2 in subframe #n+1, this overlap creates interference which causes reception failures at the eNodeB. If a proper value of Timing Advance is applied, then these subframes won’t collide.

Timing Advance Command in LTE

The eNodeB estimates the initial Timing Advance from PRACH sent by the UE. PRACH is used as timing reference for uplink during UE’s initial access, radio link failure, during Handover etc…The eNodeB sends Timing advance command in Random Access Response (RAR). Once the UE is in connected mode, the eNodeB keep estimating Timing Advance and sends Timing Advance Command MAC Control Element to the UE, if correction is required.

The UE shall adjust the timing of its uplink transmission at subframe #n+6 for a Timing Advance Command received in subframe #n

The UE shall adjust the timing of its transmissions with a relative accuracy better than or equal to ±4*T_S seconds to the signalled timing advance value compared to the timing of preceding uplink transmission

The timing advance command indicates the change of the uplink timing relative to the current uplink timing as multiples of 16T_s.

N_TA is the timing offset between uplink and downlink radio frames at the UE, expressed in units of Ts. where T_s = 1/(2048x15000) = 1/30720000 sec

    Timing Advance Command in MAC RAR

The Timing Advance Command in RAR takes 11 bits and it can indicate an index value T_A (0, 1, 2… 1282) which used to control the amount of timing adjustment that UE has to apply. The amount of the time alignment is given by N_TA = T_A × 16. The Timing Advance obtained via RAR is always positive

Example1 (T_A = 0): When the received T_A = 0 ⇨ N_TA = 0 so no timing adjustment required.

Example2 (T_A = 1): If T_A = 1 ⇨ Timing Adjustment = N_TA = 16 Ts = 16/30720000 sec = 0.5208 μs ⇨ Distance = (3x10⁸x0.5208x10^-6)/2 = 78.12m which is the minimum

Example3 (T_A = 1282): When the received T_A = 1282 ⇨ N_TA = 1282x16Ts = 1282x16/30720000 sec = 667.66 μs ⇨ Distance = (3x10⁸x667.66x10^-6)/2 = 100.15Km which is the maximum propagation distance

The maximum distance value (of slightly above 100Km) would facilitate cell radius of up to 100Km.

    Timing Advance Command MAC CE

In the case of Timing Advance Command MAC CE, it indicates relative Timing Advance which is 6-bit index value T_A (0, 1, 2… 63). In this case, N_TA,new = N_TA,old + (T_A  − 31)×16 where N_TA,old  is the current timing adjustment and N_TA,new indicates new value. Here, adjustment of N_TA value by a positive or a negative amount indicates advancing or delaying the uplink transmission timing by a given amount respectively

Timing Advance command in MAC RAR and MAC CE are illustrated below

Uplink Time Alignment

It was discussed above how the eNodeB controls Timing Advance that each UE has to apply. Once the UE gets a Timing Advance Command, UE applies it and now the question is for how long the UE uses the received Timing Advance value?

The eNodeB provides the UE with a configurable timer called timeAlignmentTimer. TimeAlignmentTimer is used to control how long the UE is considered uplink time aligned

The value of the timer is either UE specific (timeAlignmentTimerDedicated) and managed through dedicated signalling between the UE and the eNodeB, or cell specific (timeAlignmentTimerCommon) which is indicated in SIB2. In both cases, the timer is normally restarted whenever a new Timing Advance is given by the eNodeB. At the time of restart, the timer is restarted to a UE specific value if configured; otherwise it is restarted to a cell specific value

The UE starts/restarts the TimeAlignmentTimer based on the condition when it received the Timing Advance Command.

• The Timing Advance Command is received in MAC RAR but timeAlignmentTimer is not already running: This case may arise in situations like, timeAlignmentTimer has expired (connected mode), Initial access from RRC_IDLE, during RRC Connection Re-establishment procedure etc…After the reception of RAR, the UE shall apply the Timing Advance Command value received in RAR and start timeAlignmentTimer. If the contention is not resolved/successful, then the UE stops timeAlignmentTimer, else, the UE continues running the timer
• The Timing Advance Command is received in MAC RAR as part of non-contention based Random Access procedure (ex: PDCCH Order):  After the reception of RAR, the UE shall apply the Timing Advance Command value received in RAR and starts/restart the timeAlignmentTimer
• The Timing Advance Command is received in MAC RAR as part of contention based Random Access procedure in connected mode and timeAlignmentTimer is already running: This could be in situations like UE is requesting for uplink resources but UE doesn’t have valid PUCCH resources for SchedulingRequest etc…After the reception of RAR, the UE shall ignore the received Timing Advance Command value and shouldn’t restart the timeAlignmentTimer
• When a Timing Advance Command MAC CE is received, the UE shall apply the received value of Timing Advance Command value and start/restart timeAlignmentTimer

As discussed before, the eNodeB continuously measures timing of uplink signal from the UE and adjusts the uplink transmission timing by sending the Timing Advance Command to the UE. If the UE has not received a Timing Advance Command until the expiry of timeAlignmentTimer, the UE assumes that it has lost the uplink synchronization. Hence, prior to any PUSCH/PUCCH/SRS transmission in the uplink, an explicit timing-re-alignment phase using the random access procedure must be performed to restore the uplink time alignment

The UE shall perform the following actions upon the expiry of timeAlignmentTimer:

• Flush all HARQ buffers;
• If configured, release PUCCH resources of Periodic CQI and Scheduling Request, and also SRS configuration. By doing so, the UE doesn’t perform transmission of SRS/PUCCH while timeAlignmentTimer is not running. The eNodeB has to configure these parameters again in order for the UE to transmit SRS/Periodic CQI/Scheduling Request after UE starts timeAlignmentTimer
• Clear configured downlink assignments and uplink grants. i.e., release SPS grant (uplink) and assignment (downlink) if configured

The UE shall not perform any uplink transmission except the Random Access Preamble transmission when timeAlignmentTimer is not running.

LTE eNodeB Scheduler and Different Scheduler Type

Scheduling is a process through which eNodeB decides which UEs should be given resources (RBs), how much resource (RBs) should be given to send or receive data .In LTE, scheduling is done at per subframe basis i.e. every 1 mili second. The entity which is govern this is know as scheduler.

A LTE scheduler performs following function for efficient scheduling:

• Link Adaptation: It selects the optimal combination of parameters such as modulation, channel Coding & transmit schemes i.e. Transmission Mode (TM1/TM2/TM3/TM4) as a function of the RF conditions.
• Rate Control: It is in charge of resource allocation among radio bearers of the same UE which are available at the eNB for DL and at the UE for UL.
• Packet Scheduler: It arbitrates access to air interface resources on 1ms-TTI basis amongst all active
  Users (Users in RRC Connected State).
• Resource Assignment: It allocates air interface resources to selected active users on per TTI basis.
• Power Control: Provides the desired SINR level for achieving the desired data rate, but also controls
  the interference to the neighbouring cells.
• HARQ (ARQ + FEC): It allows recovering from residual errors by link adaptation.

LTE MAC supports the following three types of Scheduling Mechnaism:

• Dynamic Scheduling
• Persistent Scheduling
• Semi-Persistent Scheduling

1. Dynamic Scheduling: Every TTI, MAC checks for the UEs to be scheduled, the Data Availability for each UE to be scheduled and the feedback from the UE on the Channel conditions. Based on these data, it can schedule the resources for the UE through the PDCCH. If data is not available, UE will not get scheduled. All Services can be scheduled using Dynamic Scheduling, but at the expense of the Control signalling [PDCCH Usage – a scarce resource].
2. Persistent Scheduling: In this case, Packets are scheduled on a fixed basis, similar to the Circuit Switched fashion. Here, it does not depend on the Channel Condition. The Resource allocation remains constant for the period of the call.
3. Semi-Persistent Scheduling: It is a Hybrid way of scheduling, which tries to overcome the drawbacks of the Dynamic Scheduling and the Persistent Scheduling.

SPS - Semi-Persistent Scheduling 

In LTE SPS feature is designed to reduce the control channel overhead for VoIP based services. Since VoLTE require persistent radio resource allocation at regular interval (one packet in 20ms from AMR speech codec).  To support large number of VoIP calls there is huge overhead on control signalling.

VoIP periodically generates small sized packets at short and regular intervals. To avoid lot of downlink assignment and uplink grant, SPS feature significantly reduces heavy load on PDCCH by doing minimum downlink assignment and uplink grant. SPS allocates radio resources for a long period of time.
In SPS the UE is pre-configured by the eNB with the SPS-RNTI (Instead of the regular C-RNTI) and a periodicity. Once configured the UE receives the DL/UL data at the configured periodicity.

For Eg.a UE is configured with SPS-RNTI and  periodicity of 20sf. Then UE receives the data every 20ms with SPS-RNTI and normal DL data in other subframes.

The Semi Persistent Scheduling RNTI (SPS-RNTI) is allocated using an RRC Connection Setup or RRC Connection Reconfiguration message. It is applicable when resources are allocated for more than a single subframe, i.e. SPS reduces the control overhead by allowing a single resource allocation to be re-used during multiple subframes. The SPS-RNTI can be used to address a UE by scrambling the CRC bits belonging to DCI formats 0, 1, lA, 2, 2A, 2B or 2C

SPS-C-RNTI is must to configure SPS in either UL or DL or Both directions. The UE monitors the PDCCH in every TTI to check the activate/re-activate/release SPS procedures since eNB can do the above procedures at any subframes after configuring the SPS.
DCI format 1/1A/2/2A/2B/2C are used to activate SPS in DL while DCI format 0 is used to activate SPS in UL. But DCI format 1A is used to release SPS in DL.

In Persistent scheduling, UE gets the grant in all subframes to send the UL data all the time, but in non-persistent scheduling UE needs the grant from the network to send the data. For dynamic uplink scheduling check LTE: Uplink Scheduling. Dynamic scheduling gives the network full flexibility to assign the resources to the UE as compared to persistent scheduling where it gives resource allocation is every subframe. Based on the channel conditions dynamic scheduling varies the resource allocation to the UEs. Persistent scheduling is rarely used in LTE.

Different Types of Schedulers:

• Round Robin: The RR scheduler selects and schedules UEs in a round robin manner, thereby creating an equal resource share. The disadvantage of this approach is that UEs with sub-optimalCQIs may be allocated Physical Radio Resources (PRBs), thus reducing the overall cell throughput.
• Max CQI : The max-CQI scheduler selects the schedulable UEs based on the experienced CQI. The UEs with the highest CQI therefore become candidates for scheduling thereby increasing the overall cell throughput. The disadvantage of this approach is that UEs with lower CQI are denied scheduling instances, thus being starved for throughput and leading to degraded user experience.
• Proportional Fair: The PFS is expected to strike a balance between the traditional Round Robin (RR) scheduler and the max Throughput Scheduler (also known max-CQI (Channel Quality Indicator) scheduler). The PFS scheduler performs in such a manner that it considers resource fairness as well as maximizing cell throughput (in addition to other possible
  performance metrics).

For a Max C/I scheduler, the Sector throughput improves while cell edge throughput drops compared to a PF scheduler where sector throughput may not be as good as Max C/I but cell edge throughput thoroughly improves.

Scheduler Type	Max C/I	Round Robin	Proportional Fair (PF)
How it works	Allocates resources to the user with the instantaneous best RF conditions. UE with the best channel conditions is always prioritized	Resources are shared across users over time regardless of the RF conditions.	Sharing the cell throughput but as a function of RF conditions and bearer priorities
Pros	Very Good Throughput	Resources shared in an equal manner	Trade-off between fairness and cell throughput
Cons	Cell Edge UEs starved of scheduling instances leading to degraded user experience.	UEs with sub optimal CQI conditions will reduce the cell throughput	Implementation complexity and overall cell throughput will not be the highest