Backoff indicator in RACH processing

From the LTE MAC spec TS 36.321, section 5 explains about the Random Access procedure which will be used for the UE's UL synchronization.

Backoff Parameter:

If the random access attempt of a UE fails, either because the preamble sent by the UE was not detected by the eNB or the UE lost the contention resolution, the UE has to start the process over again. To avoid contention and overload, the eNB can signal the UEs that they have to wait a certain time before they try to
connect again. The parameter that controls this is called the backoff parameter (B) and is signaled by the eNB in the random access response. The actual time the UE should backoff is chosen uniformly by the UE in the interval [0,B]. As mentioned, the backoff parameter is sent in the RA response, but all RA responses can however be read by all UEs who sent a preamble in step 1 of the random access procedure. This means that also a UE that did not get a random access response. with its own preamble, i.e., was not detected, can receive the backoff parameter and use it.

Backoff:

The eNB can force the UE to wait a certain time before it tries to connect again.  The maximum length of the backoff time is signaled to the UE by the eNB with the backoff parameter B. One possible scenario is that the backoff only is activated when there is an overload in the system. Therefore it would be interesting to study how the observations of AD (Access Delay) are affected by different values on B, during different conditions of the system. If the AD observers cannot be upgraded to accurately estimate an eventual backoff it would mean that the eNB is depending on AD reports from the UEs.
Backoff Indicator :


When it comes to backoff there is only one parameter that affects how long the UEs wait until they can try again. This is the backoff parameter, which is indicated by an index sent from the eNB to the UE. These backoff indicators have been defined below.
 Backoff parameter values as defined
       

Index	Backoff Parameter value (ms)
0	0
1	256
2	512
3	1024
4	2048
5	4096
6	8192
7	16384
8	32768
9	65536
10	131072
11	262144
12	524288
13	Reserved
14	Reserved
15	Reserved

The actual time the UE waits is called the backoff time and is defined as below.

Definition  Backoff Time :

The backoff time is defined as the time a UE waits after a random access attempt has been declared unsuccessful until the UE is free to try again. The backoff time is chosen uniformly by the UE in the interval [0,B].



In Case of NB-IOT, Backoff Parameter values are defined as below.

Index	Backoff Parameter value (ms)
0	0
1	256
2	512
3	1024
4	2048
5	4096
6	8192
7	16384
8	32768
9	65536
10	131072
11	262144
12	524288
13	Reserved
14	Reserved
15	Reserved

Regards
Venu

LTE Visualisation Tool available to download

Displays the mapping of downlink Signals and Physical Channels onto the set of Resource Elements. Calculates throughputs for each modulation scheme and a range of coding rates. Allows exporting to a csv file for subsequent reading into Excel. Free to download from www.lte-bullets.com

The link is http://www.lte-bullets.com/visualisation_tool.html

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Why we have the concept of VRB in LTE?

The mapping from VRBs to physical resource blocks (PRBs) allows for downlink distributed transmission in frequency domain .Downlink distributed transmission consists of two separate steps:

 (1) a mapping from VRB pairs to PRB pairs such that consecutive VRB pairs are not mapped to frequency-consecutive PRB pairs and
(2) a split of each resource-block pair such that the two resource blocks of the resource-block pair are transmitted with a certain frequency gap in between. The second step can be seen as frequency hopping on a slot basis.

Lot of documents explain you the importance of VRB. Please go throuh them.

Regards
Venu

Circuit Switched Fallback (CSFB)

CS domain services are the services that can be offered today in GSM-UMTS networks. Examples of such services are: voice and its supplementary services (e.g. call waiting, call forwarding), USSD, LCS, SMS, E911, LI, and even CS DUI video, etc. This rich set of CS domain features and capabilities are the result of years of standardization works in 3GPP and operators investments to their GSM-UMTS network.

In EPS, richer features/services can be offered to the end-user together with voice via IMS. While this is the case for EPS, it is challenging for some operators to launch EPS with data and voice/IMS from day one. Hence, these operators need a migration path to allow them to start from EPS with data only and allow the reuse of CS domain services until they get to the point where IMS voice can be added to the EPS.

Such migration path is possible with CS Fallback (CSFB) feature. CSFB is introduced in 3GPP Rel-8 to allow an UE in EPS to reuse CS domain services by defining how the UE can switch its radio from EUTRAN access to other RAT (e.g. GERAN/UTRAN/1xRTT access) that can support CS domain services. In addition, CSFB specification TS 23.272 also defines how the SMS is transferred to the UE natively via EPS from the MSC. It should be noted that this type of SMS delivery mechanism is defined in CSFB specification but the UE is not falling back to GERAN/UTRAN/1xRTT access.

With CSFB, UE under EPS can enjoy the fast PS data access and can switch over to GERAN/UTRAN/1xRTT access for CS domain services when needed. In addition, UE can also utilize the SMS feature supported by CSFB architecture.

UE, which wants to use CSFB, must first register itself to the CS domain via EPS. For GSM-UMTS CSFB feature, UE performs a combined EPS/IMSI Attach/TAU procedure. In the EPS Attach/TAU response message, the network indicates back to the UE whether CSFB (including SMS) is supported, “SMS-only”, “CSFB Not Preferred”, or none of these features are supported. “CSFB Not Preferred” is an indication to allow data centric devices to continue reside in EPS and to allow CSFB (including SMS) features to be used. On the other hand, a voice centric device receiving “CSFB Not Preferred” or “SMS-only” will assume CSFB is not supported in this network and will try to reselect to other networks (i.e. 2G or 3G) to obtain voice services. In 1xRTT CSFB features, the UE is aware that the network supports 1xCSFB by examining the system information broadcast information over E-UTRAN access and performs the 1xCS registration to the 1xRTT MSC via the CDMA2000 signaling tunnel between the UE (via EPS) and 1xCS IWS. This 1xCS registration request and response is transparent to the EPS.

After the UE has successfully registered itself to the CS domain (and has received positive response from MME that CSFB is possible in GERAN/UTRAN case), it can then request the MME to perform CSFB procedures whenever it wants to use CS domain services (e.g. originating a voice call or answer to a terminating voice call). Besides voice call, USSD, MO-LR, MT-LR, NI-LR, and call-independent Supplementary Services procedures (e.g. activates CFB) can also trigger CSFB procedures. In the CS terminating scenario, an active UE has the ability to reject terminating call request while it still resides in EPS. This is particularly useful when the end-user is watching a streaming video under EPS and does not want to answer a call from an unknown number to avoid any streaming disruption in the streaming video due to unwanted CSFB procedures.

For the GSM-UMTS CSFB feature, EPS can perform the CSFB procedure with PS handover procedure, RRC connection release with redirection information, or cell change order with NACC (for GERAN only). This is based on network configuration and deployment option. For 1xRTT CSFB feature, CSFB can be done with RRC connection release with redirection information or 1xSRVCC based signaling (known as enhanced 1xCSFB). 1xRTT CSFB UE may also have dual-Rx/dual-Tx or Dual-Rx/Single-Tx capability. Dual-Rx/dual-Tx 1xRTT CSFB UE can simultaneously transmit and receive on both EPS and 1x at the same time. This allows the UE to obtain 1x voice service from 1xRTT system while maintaining the data stream over EPS at the same time. This is also based on network configuration and deployment option, and UE capability. Dual-Rx/Single-Tx 1xRTT CSFB UE allows simplification in EPS network deployment because there is no coordination is required between the E UTRAN and 1xRTT network (i.e. S102 is not required).

After the UE is redirected to GERAN/UTRAN/1xRTT access via one of the above procedures, the existing CS setup procedure is taken over for the remaining of the call.

In Rel-9, IDLE mode camping mechanism is enhanced in the EPS and GPRS to allow the network to influence the UE’s RAT camping policy so that a CSFB UE will select GERAN/UTRAN access when it is in IDLE condition. The intention is to minimize the occurrence of CSFB procedure from EPS to allow the UE to invoke the CS domain services directly from GERAN/UTRAN as much as possible. On the other hand, this requires additional intelligence in the cell reselection policy in the GERAN/UTRAN access in order to move the UE in active state to EPS to enjoy the fast PS access when appropriate. There are also optimization enhancements to Rel-9 for speeding up the overall CSFB procedure.

As indicated earlier, SMS delivery via CS Domain is also defined as part of the CSFB feature. UE can utilize this feature after it has successfully attached itself to the CS domain. It should be noted that EPS has the option to support only the SMS feature and not the CSFB feature which redirect the UE to another RAT. For GERAN/UTRAN CSFB, MME can indicate this condition by having an SMS-only indicator to the UE during their combined EPS/IMSI Attach/TAU procedure. For 1xRTT CSFB, this indication is not specified, as the 1xCS registration procedure is transparent to the EPS. UE receiving the “SMS-only” indicator will not invoke the CSFB request and should not expect any CS paging coming from EPS.

When interworking with a 3GPP MSC, SMS is delivered via the SGs interface. For MO-SMS, UE first establishes a NAS tunnel to transfer the SMS PDU to MME. MME then transfer these SMS PDU over to MSC via the SGs. MT-SMS works the same way by having the MME establish a NAS tunnel to UE over E-UTRAN access.

When interworking with 1xMSC, the UE establishes a CDMA2000 tunnel with the 1xCS IWS via EPS and SMS is delivered via that tunnel. EPS is transparent to this process.

3GPP also defines the CSFB UE in voice-centric and data-centric mode of operation in TS 23.221. Voicecentric CSFB UE will always attempt to find a RAT where voice services can be supported. In the example of UE receiving an SMS-only or “CSFB Not Preferred” indication from the network during combined EPS/IMSI attach procedure, the voice-centric UE will autonomously switch to UTRAN/GERAN access if coverage is available so voice service is possible to this user. With a data-centric mode of operation, the CSFB UE will not switch to UTRAN/GERAN given the same scenario with the SMS-only indication from the network and will forgo the voice services or CS domain services altogether. This is because the data-centric mode UE wants the best possible PS access and voice is not the determining factor to move away from EPS.

Article from "4G Broadband Evolution: 3GPP Release 10 and beyond"

Knowing GUTI - Globally Unique Temporary ID

In LTE the GUTI is allocated to the UE by the MME and has two components. These are the GUMMEI (Globally Unique MME ID) and the M-TMSI (MME-TMSI). While the GUMMEI identifies the MME, the M-TMSI identifies the UE within the MME.

In GPRS and UMTS the mobile's temporary id was the (P-TMSI) Packet Temporary Mobile Identity. This id is changed on a frequent basis and used instead of the IMSI (The International Mobile Subscriber Identification) in most air interface messages for security reasons.

In LTE, the P-TMSI is now called the Globally Unique Temporary ID, or the GUTI. Some of the digits in the GUTI identify the Mobility Management Entity the mobile was last registered with and they are referred to as the Globally Unique MME Identifier, or the GUMMEI.

When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session.It's also possible to roam between different radio technologies. If the mobile has reselected from a UMTS cell to an LTE cell, a TAU is made and since the mobile does not have a GUTI, the P-TMSI is sent instead.This way, the newly assigned MME can contact the 3G SGSN to request the subscribers current profile (IP address, PDP contexts, etc.).

The same mechanisms apply when the mobile reselects from an LTE cell to a UMTS or GPRS cell. In this case the GUTI is sent in the P-TMSI parameter and the procedure is reffered to as Routeing Area Update (RAU) instead of TAU.

The GUTI has two main components:

• one that uniquely identifies the MME which allocated the GUTI; and
• one that uniquely identifies the UE within the MME that allocated the GUTI.

The Globally Unique MME Identifier (GUMMEI) is constructed from the MCC, MNC and MME Identifier (MMEI). Within the MME, the mobile is identified by the M-TMSI.

Finally the GUTI is constructed from the GUMMEI and the M-TMSI.

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