RACH Procedure

RACH Procedure Flow

1. RRC Layer sends Access Req for RRC Connection Request/RRC Coonection Re-establishment Request to MAC and starts T300 timer
[020/008/012] OTA LOG 00:00:21.032 UL_CCCH / RRCConnectionRequestRadio Bearer ID: 0, Freq: 2050, SFN:0
[9504/0002] MSG 00:00:21.032 LTE MACCTRL Start Access Request for reason=0, RAID=2551
[9501/0002/0010] MSG 00:00:21.069 LTE RRC   T300 timer internally restarted with 400ms

2. MAC sends Start_RACH_Req to ML1
[9509/0001/0010] MSG 00:00:21.032 LTE ML1  ML1: LTE_CPHY_START_RACH_REQ rcvd

3. ML1 sends the first preamble and starts RA response timer
[0xB167] LOG 00:00:21.069 LTE Random Access Request (MSG1) ReportLength: 0032 
[9509/0002] MSG 00:00:21.073 LTE ML1 Processing LTE_CPHY_RA_TIMER_STARTED_IND

4. If MAC could decode RAR before ML1 RA timer expiry and send the same to ML1, ML1 will go ahead with MSG3 transmission
[9509/0002] MSG 00:00:21.073 LTE ML1 Processing LTE_CPHY_RA_TIMER_STARTED_IND
[0xB168] LOG 00:00:21.077 LTE Random Access Response (MSG2) ReportLength: 0012

5. If MAC could not give RA parameter request to ML1 before ML1 RA timer expiry, then ML1 sends RA timer expiry indication to MAC
6. Here MAC starts another RACH attempt (with another preamble sequence with increased preamble power); MAC repeats this procedure till it sends maximum number of preambles (MAX_RACH attempts)
7. If MAC reaches MAX_RACH attempts, i.e., maximum number of preambles reached, and still could not get RAR, then MAC sends Random Access Problem indication to RRC and continues RACHing
8. MAC will stop RACHing only when T300 expired in RRC and RRC sends Abort request

VOLTE VOIP IMS SIP CSFB

Often people are confused between VOLTE ,VOIP, IMS , SIP and CSFB.
To be clear between all this item lets explain them and understand one by one.
We will start with CSFB and then VOIP and then VOLTE and IMS.

CSFB - CSFB is CS Fall back.
             This was introduced at the very early stages of 4G/LTE to make CALL's as because LTE has
              been PS from the beginning and over PS LTE was not having any standard offerings for
              calls. Because of not having call support 3GPP standard decides to use to use CS over
              legacy network.
VOIP - Now VOIP is Voice Over IP. Frankly speaking there is no relation between VOIP and LTE.
             VOIP can be used over PS and it could be from 2G , 3G or LTE or may be Wi-Fi even over
             the LAN as well.
VOLTE - VOLTE is Voice Over LTE. The much talked feature of LTE without which LTE was not
                complete Conceptually it it Voice-Over-IP On LTE , but there is a lot more to it than just
                VOIP.
IMS - IMS is IP Multimedia Subsystem. This is a concept for an integrated network.
          This is not a protocol at all.
SIP - Session initiation protocol. This is an application layer protocol. This can establish modify and
         terminate multimedia session.


So in one line VOLTE is an VOIP over LTE which uses SIP protocol over IMS architecture. 

SRVCC Failure

What happens when SRVCC fails at the NW side?
How UE is notified about it and the action it should take?

Notification Procedure (ESM Procedure)
The network can use the notification procedure to inform the UE about events which are relevant for the upper layer which is using an EPS bearer context or has requested a procedure transaction.
If the UE has indicated that it supports the notification procedure, the network may initiate the procedure at any time while a PDN connection exists or a procedure transaction is ongoing.

When the UE receives a NOTIFICATION message, the ESM protocol entity in the UE shall provide the notification indicator to the upper layer.
The notification indicator can have the following value:
#1: SRVCC handover cancelled, IMS session re-establishment required.

The following abnormal case can be identified:
a) Lower layer indication of non-delivered NAS PDU due to handover
If the NOTIFICATION message could not be delivered due to an intra MME handover, then upon successful completion of the intra MME handover the MME shall re-transmit the NOTIFICATION message. If a failure of the handover procedure is reported by the lower layer and the S1 signalling connection exists, the MME shall re-transmit the NOTIFICATION message.

SINR CQI RSRP RSSI RSRQ

SINR - Signal to Interference plus Noise Ratio

Signal to Interference plus Noise Ratio (SINR) is measured by UE on Resource Block (RB) basis.
UE computes SINR on each RB, converts it to CQI and reports it to eNodeB where it is used to select the most suitable MCS for user data transmission in particular RB. SINR value defines the MCS to be used for a RB i.e. the number of bits per modulation symbol to be sent i.e. throughput to be achieved for that particular RB as well as the number of RBs to be allocated by eNodeB to user. SINR can be defined as the ratio of the signal power to the summation of the average interference power from the other cells and the background noise.


CQI - Channel Quality Indicator

CQI is a quantized and scaled version of the experienced SINR. The process of adapting MCS depending on current channel conditions is termed as Link Adaptation. If the SINR is good, higher order MCS (e.g. 64QAM) can be selected implying that more bits per modulation symbol can be transmitted and higher throughput can be achieved. If the SINR is poor, lower order MCS (i.e. QPSK) should be selected implying fewer bits per symbol are transmitted which in turn results in lower throughput.


RSRP - Received Signal Received Power

Reference Signal Received Power (RSRP) is a cell-specific signal strength related metric that is used as an input for cell resection and handover decisions. For a particular cell, RSRP is defined as the average power (in Watts) of the Resource Elements (REs) that carry cell-specific Reference Signals (RSs) within the considered bandwidth.
RSRP measurement, normally expressed in dBm, is utilized mainly to make ranking among different candidate cells in accordance with their signal strength. Generally, the reference signals on the first antenna port are used to determine RSRP, however, the reference signals sent on the second port can also be used in addition to the RSs on the first port if UE can detect that they are being transmitted



RSRQ - .Reference Signal  Received Quality

Reference Signal Received Quality (RSRQ) measurement is a cell-specific signal quality metric. Similar to the RSRP measurement, this metric is used mainly to provide ranking among different candidate cells in accordance with their signal quality. This metric can be employed as an input in making cell reselection and handover decisions in scenarios (for example) in which the RSRP measurements are not sufficient to make reliable cell-reselection/handover decisions.
It is defined as
RSRQ = ( N.RSRP )/(LTE Carrier RSSI )
where, N is the number of Resource Blocks (RBs) of the LTE carrier Received Signal Strength Indicator (RSSI) measurement bandwidth.



RSSI - Received Signal Strength Indicator

Received Signal Strength Indicator (RSSI) is the linear average of the total received power observed only in OFDM symbols carrying reference symbols by UE from all sources, including co-channel non-serving and serving cells, adjacent channel interference and thermal noise, within the measurement bandwidth over N RBs. RSSI is used as an input to compute the LTE RSRQ measurement discussed above.


Reference- http://airccse.org/journal/jwmn/7415ijwmn09.pdf

RB RE Speed

Resource Element:
Covered by 1 sub carrier and one symbol period i.e 1 Symbol

Resource Block:
Covered by 12 Subcarriers and 6 or 7 symbols(Based on Cyclic prefix)

We can calculate the capacity if we know the Bandwidth allocated,Modulation scheme used and Cyclic Prefix type.
Consider 
Bandwidth Allocated = 5MHz
Modulation Scheme = QPSK(2 its per symbol)
Cyclic prefix used = Normal Cyclic Prefix (7 symbols in a slot)

Capacity = No of bits transferred in a sub frame / Duration of the Subframe(i.e 1milli sec)

Calculation No of Bits transferred in a Subframe

Total number of Resource block in a slot = Bandwidth / (Each subcarrier bandwidth * no of subcarriers in a Resource Block) 
Bandwidth = 5Mhz
Each sub carrier width = 15Khz
No of sub carriers in a Resource Block = 12 (As 12 Subcarrier is 1 Resource block )

Calculation of total no of resource blocks in 5MHz Bandwidth

so Resource block in one slot = 5 Mhz / (15 KHz *12 ) = 27.77 (But used RBs will be 25 for 5MHz bandwidth)
so Total Resource block in one subframe = 2 * Resource blocks in a slot = 50 RBs

Calculation of total no of symbols/Resource Elements in 5MHz Bandwidth
Total no of symbols / Resource Elements = 50 * no of symbols(REs) in a Resource block
No of symbols(REs) in a resource block = 12 subcarrier * 7 symbols = 84 symbols(REs)
Total symbols(REs ) in 50 Resource blocks = 50 * 84 = 4200 Resource Elements.

****Calculation of total no of bits sent in 5MHz Bandwidth in one subframe ****

In one symbol we can send no of bits at a time which depends on the modulation scheme (like BPSK,QPSK ,64QAM
BPSK - 1 bit,QPSK - 2 bit ,64 QAM - 6bits in a Symbol/ Resource elements
We will consider QPSK Scheme so in one symbol 2bits we can send.
So 1 Symbol = 2 bits.

So total no of Bits sent in a subframe = 2 * 4200 = 8400 bits.

So capacity = 8400 / 1ms = 8.4 MBPS

So capacity in 5MHz Bandwidth with QPSK Modulation with normal cyclic Prefix is 8.4MBPS.

if Modulation scheme is BPSK, 1 bits in a symbol = 1 * 4200 / 1ms = 4.2 MBPS
if Modulation scheme is 64QAM , 6 bits in a symbol = 6 * 4200 / 1ms = 25.2 MBPS.

Frame Structure in LTE

FDD Down-link Frame Structure in LTE

Source: http://www.sharetechnote.com/image/36_211_Fig4_1_1_FDD_DL_FrameStructure.PNG


1 Frame = 1 ms
1 Frame = 10 Sub-frame

1 Slot = 0.5 ms
2 Slot = 2*0.5 ms = 1 ms
2 Slot = 1 Sub-frame

1 Frame = 10 (Sub-frame) * 2  slots = 20 Slots





Source: http://www.sharetechnote.com/image/FDD_DL_FrameStructure_Symbols.PNG


1 Slot = 7 Symbols
(In case of Normal Cyclic Prefix, 6 in case of Extended Cyclic Prefix)

1 Symbol = 0.5(Duration of 1 slot) / 7 (Number of Symbol per slot) ms =  ~71.4 us
1 Symbol =  Cyclic Prefix + Data

First OFDM symbol is longer than the rest of the OFDM symbol in that slot.

First Symbol = 71.9 us  = 5.2 us+ 66.7 us
Rest Symbol = 71.3 us  = 4.7 us + 66.6 us

First Symbol (time) + 6*Rest Symbol (time) = 71.9 us  + 6*71.3 us = 71.9 us + 427.8 us = 499.7 us = ~500 us = 0.5 ms

The Number of sample we see here in below illustration is based on the sampling rate is 30.072 M Samples/Sec and 2048 bins/IFFT(N_ifft). Since real sampling rate and N_ifft varies depending on system BW, we need to scale this number according to a specific BW.




Source: http://www.sharetechnote.com/image/FDD_DL_FrameStructure_Subframe_01.PNG












Source :http://www.sharetechnote.com/image/PHY_CH_DL.PNG













Source: http://www.sharetechnote.com/image/LTE_DL_FrameStructure_01.png

LTE Procedure's

All the Procedure's for an LTE UE in CS and PS mode along with multi RAB combination are as below.

Spec: 34.108

7.3.2.3.1 For UE Supporting CS
Step	Direction		Message	Comments
	UE	SS
1			SYSTEM INFORMATION (BCCH)	Broadcast
2			PAGING TYPE1 (PCCH)	Paging (CS domain, TMSI)
3			RRC CONNECTION REQUEST (CCCH)	RRC
4			RRC CONNECTION SETUP (CCCH)	RRC
5			RRC CONNECTION SETUP COMPLETE (DCCH)	RRC
6			PAGING RESPONSE	RR
7			AUTHENTICATION REQUEST	MM
8			AUTHENTICATION RESPONSE	MM
9			SECURITY MODE COMMAND	RRC
10			SECURITY MODE COMPLETE	RRC
11			ACTIVATE RB TEST MODE	TC
12			ACTIVATE RB TEST MODE COMPLETE	TC
13			RADIO BEARER SETUP	RAB SETUP
14			RADIO BEARER SETUP COMPLETE	RRC
15			CLOSE UE TEST LOOP (DCCH)	TC (UE test loop mode set up)
16			CLOSE UE TEST LOOP COMPLETE	TC (confirms that loopback entities for the radio bearer(s) have been created and loop back is activated)
17			OPEN UE TEST LOOP	TC
18			OPEN UE TEST LOOP COMPLETE	TC
19			RRC CONNECTION RELEASE	RRC
20			RRC CONNECTION RELEASE COMPLETE	RRC
7.3.2.3.2 For UE supporting PS only
Step	Direction		Message	Comments
	UE	SS
1			SYSTEM INFORMATION (BCCH)	Broadcast
2			PAGING TYPE1 (PCCH)	Paging (PS domain, P-TMSI)
3			RRC CONNECTION REQUEST (CCCH)	RRC
4			RRC CONNECTION SETUP (CCCH)	RRC
5			RRC CONNECTION SETUP COMPLETE (DCCH)	RRC
6			SERVICE REQUEST	GMM
7			AUTHENTICATION AND CIPHERING REQUEST	GMM
8			AUTHENTICATION AND CIPHERING RESPONSE	GMM
9			SECURITY MODE COMMAND	RRC
10			SECURITY MODE COMPLETE	RRC
11			ACTIVATE RB TEST MODE	TC
12			ACTIVATE RB TEST MODE COMPLETE	TC
13			RADIO BEARER SETUP	RRC (RAB SETUP)
14			RADIO BEARER SETUP COMPLETE	RRC
15			CLOSE UE TEST LOOP (DCCH)	TC (UE test loop mode set up)
16			CLOSE UE TEST LOOP COMPLETE	TC (confirms that loopback entities for the radio bearer(s) have been created and loop back is activated)
17			OPEN UE TEST LOOP TC	TC
18			OPEN UE TEST LOOP COMPLETE	TC
19			RC CONNECTION RELEASE	RRC
20			RC CONNECTION RELEASE COMPLETE	RRC
7.3.2.3.3 For CS+PS multi RAB combination
Step	Direction		Message	Comments
	UE	SS
1			SYSTEM INFORMATION (BCCH)	Broadcast
2			PAGING TYPE1 (PCCH)	Paging (CS domain, TMSI)
3			RRC CONNECTION REQUEST (CCCH)	RRC
4			RRC CONNECTION SETUP (CCCH)	RRC
5			RRC CONNECTION SETUP COMPLETE (DCCH)	RRC
6			PAGING RESPONSE	RR
7			AUTHENTICATION REQUEST	MM
8			AUTHENTICATION RESPONSE	MM
9			SECURITY MODE COMMAND	RRC
10			SECURITY MODE COMPLETE	RRRC
11			PAGING TYPE2 (DCCH)	TMSI(GSM-MAP)/P-TMSI
12			SERVICE REQUEST	GMM
13			SECURITY MODE COMMAND	RRC
14			SECURITY MODE COMPLETE	RRC
15			ACTIVATE RB TEST MODE	TC
16			ACTIVATE RB TEST MODE COMPLETE	TC
17			RADIO BEARER SETUP	RRC CS radio bearer(s) are configured
18			RADIO BEARER SETUP COMPLETE	RRC
19			RADIO BEARER SETUP	RRC PS radio bearer(s) are configured
20			RADIO BEARER SETUP COMPLETE	RRC
21			CLOSE UE TEST LOOP (DCCH)	TC (UE test loop mode set up)
22			CLOSE UE TEST LOOP COMPLETE	TC (confirms that loopback entities for the radio bearer(s) have been created and loop back is activated)
23			OPEN UE TEST LOOP	TC
24			OPEN UE TEST LOOP COMPLETE	TC
25			RRC CONNECTION RELEASE	RRC
26			RRC CONNECTION RELEASE COMPLETE	RRC

Link http://www.etsi.org/deliver/etsi_ts/134100_134199/134108/12.01.00_60/ts_134108v120100p.pdf