LTE SON Blog

https://lte-son.blogspot.com/

1. Introduction
2. SON Architecture
3. RACH Optimization
4. SON Features 
5. Coverage & Capacity Optimzation

Low Power Wide Area Networks

To support the further growth and development of the Internet of Things (the IoT) the mobile industry is developing and standardizing a new class of GSM technologies. These Low Power Wide Area (LPWA) networks will support devices requiring low mobility, low power consumption, long range, low cost and security.

Low Power Wide Area (LPWA) networks will play an important role in connecting up the billions of new devices making up the IoT. LPWA technologies are expected to serve a diverse range of vertical industries and support a range of applications and deployment scenarios, which existing mobile technologies may not currently be best placed to connect.

Low-Power, Wide-Area (LPWA) technologies complement existing cellular mobile network and short range technologies, enabling lower costs and better power consumption characteristics What that means for you very much depends on who you are, and the impact will vary for different market participants.

Due to the diversity of IoT application requirements, a single technology is not capable of addressing all of the LPWA use cases and so the mobile industry has focused on three proposed complementary licensed 3GPP standards: Extended Coverage GSM for the Internet of Things (EC-GSM-IoT), Long-Term Evolution LPWA technologies in licensed spectrum can be deployed in a simplified manner, without sacrificing key customer requirements, such as battery lifetime and security.

However, since LPWA can be a significant competitive differentiator, very few participants in any Internet of Things (IoT) market can afford to ‘do nothing’. Any company that elects to ‘do nothing’ can expect to be out-competed by a company that has deployed LPWA (and other IoT) technologies to the maximum extent.

A wide range of players and alternative technologies are already engaged in the LPWA space. The most fundamental differences between these technology types includes the radio spectrum that the technologies use (licensed vs license exempt) and the commercial strategies of the companies that deploy them.

Why use licenced spectrum solutions?
Mobile operators already provide reliable, secure end-to-end IoT platforms that allow customers to scale and manage their business requirements. They also have unrivalled global network coverage as well as technical and business support to react to a customer’s changing needs. Mobile operators will bring this capability and expertise to LPWA technologies.
This means solutions deployed in licenced spectrum will:

• support very low in power consumption – a battery life in excess of 10 years for some applications
• be optimized for brief messages – about the length of an SMS
• have a very low device unit cost – the connectivity module will eventually cost a few dollars
• have good coverage outdoors and indoors, enabling connectivity in rural and underground locations
• be easy to install on to current networks, reusing existing cellular infrastructure wherever possible
• be scalable by being able to support large numbers of devices over a wide geographic area
• deliver secure connectivity and support for authentication appropriate to the IoT application
• be able to be integrated into a mobile operator’s unified IoT platform

Specifications for LPWA solutions will be completed by the end of June 2016 and included in 3GPP Release 13, with full commercial solutions following later in the year.

Market Opportunity for Low Power Wide Area networks

As LPWA networks are designed for IoT and machine-to-machine (M2M) applications that have low data rates, long battery lives, are low cost, operate in remote and hard to reach locations they will be easy to deploy across a number of different verticals such as utilities, agriculture, manufacturing, wearables and transport.

LPWA is an emerging area of the IoT and represents a huge market opportunity as the IoT scales. Analyst firm Strategy Analytics anticipate there will be 5 billion LPWA connections by 2022.

Analysys Mason forecasts LPWA technologies will generate US$970 million globally in connectivity revenue in 2018, rising to US$7.5 billion in 2022. By that time, Strategy Analytics estimates network operators could be generating more than $13 billion from LPWA connectivity, as well as significant additional revenues from value-added services, such as data analytics and security.

What is the GSMA doing?

The GSMA launched the ‘Mobile IoT Initiative’ in June 2015 to accelerate the commercial availability of Low Power Wide Area (LPWA) solutions in licensed spectrum. Backed by 53 of the world’s leading mobile operators, OEMs, chipset, module and infrastructure companies, this Initiative facilitates demonstrations, proofs of concept and trials of a 3GPP backed LPWA licensed spectrum technologies or Mobile IoT technologies. It also provides analysis and feedback to assist 3GPP in standardising the technologies

More Inforamtion:

https://www.lora-alliance.org/portals/0/documents/whitepapers/LoRa-Alliance-Whitepaper-LPWA-Technologies.pdf
http://www.gsma.com/connectedliving/mobile-iot-initiative/

Self-Organizing Networks - SON

SON solutions can be divided into three categories: Self-Configuration, Self-Optimisation and Self-Healing. The SON architecture can be a centralized, distributed or a hybrid solution.

SELF-CONFIGURATION

This is the dynamic plug-and-play configuration of newly deployed eNBs. The eNB will by itself configure the Physical Cell Identity, transmission frequency and power, leading to faster cell planning and rollout.

The interfaces S1 and X2 are dynamically configured, as well as the IP address and connection to IP backhaul. To reduce manual work ANR (Automatic neighbour relations) is used. ANR configures the neighbouring list in newly deployed eNBs and is optimizing the list configuration during operation.

Dynamic configuration includes the configuration of the Layer 1 identifier, Physical cell identity (PCI) and Cell global ID (CGID). There are 504 different PCIs available in LTE, the PCI mapping shall fulfill the collision free condition as well as the confusion free. The PCI can be assigned either in a centralized or distributed way.

When centralised assignment is used the OAM system will have a complete knowledge and control of the PCIs. When the distributed solution is used the OAM system assigns a list of possible PCIs to the newly deployed eNB, but the adoption of the PCI is in control of the eNB. The newly deployed eNB will request a report, sent either by User Equipment (UEs) over the air interface or by other eNBs over the X2 interface, including already in-use PCIs, see figure 1. - The eNB will randomly select its PCI from the remaining values.

Figure 1, PCI reporting

ANR is used to minimize the work required for configuration in newly deployed eNBs as well as to optimize configuration during operation. Correct and up-to-date neighbouring lists will increase the number of successful handovers and minimize the number of dropped calls. Before a handover can be executed the source eNB requires the neighbouring information: PCI and CGID of the target eNB.

The PCI is included in every normal measurement report. The mapping between the PCI and CGID parameters can be done by using information from the OAM or reported by UEs decoding the target cell CGID on the broadcast channel in the target cell, see figure 2. The capability of decoding CGID is an optional UE feature.

Figure 2, UE supported reporting of CGID

A network operator can put a cell on a ANR black list, to block certain handover candidates, for example from indoor to outdoor cells. 3GPP has also specified LTE Inter-Frequency and Inter-RAT (Radio Access Technology) ANR.

The functions described above are mainly included in Release 8.

SELF-OPTIMISATION

Functions for self-optimisation are mainly included in Release 9. It includes optimisation of coverage, capacity, handover and interference.

Mobility load balancing (MLB) is a function where cells suffering congestion can transfer load to other cells, which have spare resources. MLB includes load reporting between eNBs to exchange information about load level and available capacity.

The periodicity of the reporting can be requested in the range of 1 to 10 s. The report can contain, hardware load, S1 transport network load and Radio resource status. The Radio resource status reports are separated in Up Link and Down Link reports, including the total allocation guaranteed and non-guaranteed bit rate traffic, the percentage of allocated Physical Resource Block (PRB) and the percentage of PRBs available for load balancing.

MLB can also be used between different Radio Technologies. In case of inter-RAT the load reporting RAN Information Management (RIM) protocol will be used to transfer the information via the core between the base stations of different radio technologies. A cell capacity class value, set by the OAM-system, will be used to compare and weigh the different technologies radio capacities against each other.

A handover due to load balancing is carried out as a regular handover, but it may be necessary to amend parameters so that the User Equipment (UE) does not return to the congested cell. The amendment must take place in both cells, so that the handover settings remain coherent in both. The eNBs need to estimate how much the cell border needs to be shifted, expressed in dB, to avoid a quick return of the UE.

Mobility robustness optimization (MRO) is a solution for automatic detection and correction of errors in the mobility configuration. In Release 9 the focus is on errors causing Radio link failure (RLF) due to too late or early handover, or handover to an incorrect cell.

Figure 3, Late Handover, the UE does not receive the RRC Handover command, due to weak signal

In case of late handover, see figure 3, the handover procedure in the source cell is initialized too late, since the UE is moving faster than the Handover (HO) parameter settings allow. Hence when the RRC HO command from the serving cell is transmitted the signal strength is too weak to reach the UE, now located in the target cell, connection is lost. The UE attempts a connection re-establishment, containing PCID and C-RNTI belonging to the source cell, but received by the target cell. The target eNB will then inform the source cell about RLF to adjust Handover parameters.

Figure 4, Handover too early, the signal strength in the target cell is too weak, and the connection is lost almost immediately

It is a bit more complicated to detect a too early handover, see figure 4. The UE has successfully been handed over from source cell A to target cell B, but since it was triggered too early the connection will drop almost immediately due to too poor radio conditions in the target cell B. The UE will then try to re-establish the connection, which will now take place in the original source cell, cell A, since this cell is the strongest one. The UE will use the PCID and RNTI from the target cell B and the source cell A will then consider this as a Radio Link Failure due to too late handover and send an indication to the target cell B. But the target cell B will now recognize the parameters in the indication, as given to a mobile that had just completed a handover to cell B from cell A now indicating failure. The target cell B will send back a report about too early Handover to adjust Handover parameters, to the source cell A.

In order to save energy some cells can be switched off when capacity is not needed. The power consumption in a base station is not only related to load, a number of functions requires power even if there are no users to serve. But, if a cell is switched off, in the legal operator license there are still requirements on coverage, the coverage must be maintained at all times! The suspension of the cell may occur when the last user leaves the cell, all incoming handovers during this period of time will then be rejected. The cells that remain on, providing coverage, can wake up a suspended cell when traffic load increase. This can be done with a wake-up call to the sleeping cell.

RACH optimisation aims to minimise the number of attempts on the RACH channel, causing interference. The UE can be polled by the eNB for RACH statistics after connection. The number of preambles sent until successful RACH completion, and the number of contention resolution failure are in the statistics. But PRACH configuration parameters can also be distributed amongst eNBs, like zero correlation configuration, root sequence, high speed flag and PRACH frequency offset.

SELF-HEALING

Features for automatic detection and removal of failures and automatic adjustment of parameters are mainly specified in Release 10.

Coverage and Capacity Optimization enables automatic correction of capacity problems depending on slowly changing environment, like seasonal variations.

Minimization of drive tests (MDT), is enabling normal UEs to provide the same type of information as those collected in drive test. A great advantage is that UEs can retrieve and report parameters from indoor environments.

Reference: http://www.3gpp.org/technologies/keywords-acronyms/105-son

LTE Radio Quality Indicator's

The LTE standard defines three quality indicators that serve as a benchmark for the transmission quality in the downlink: CQI, PMI and RI (channel state information – CSI). The user equipment (UE) can measure all three and transmit the information in the uplink to the base station (BS), which then adapts the signal transmission in the downlink accordingly, although this is not mandatory. To actually improve transmission quality through a modification in the downlink, the statistical properties of the channel must remain constant between the time a quality indicator is reported to the eNB and the time the transmission is modified (coherence time). 

Channel quality indicator (CQI)

The CQI indicates the highest modulation and the code rate at which the block error rate (BLER) of the channel being analyzed does not exceed 10 %. The CQI accepts discrete values between 0 and 15. Index 0 indicates that the UE has not received any usable LTE signals and that the channel is inoperable. The CQI report for the UE has a wide variety of settings. As an example, the UE can use one of two methods to send the CQI value to the eNB via the uplink:

- periodically via the PUCCH or PUSCH channels,
- aperiodically via the PUSCH channel.

In this case, the eNB explicitly requests the UE to send a CQI report. In addition, the frequency domain resolution in the CQI report can be varied. Apart from the wideband CQI for the entire channel bandwidth, there are different subband CQIs, each of which indicates the transmission quality for a specific frequency subrange. 

The CQI index reported to the eNB by the UE is derived from the quality of the downlink signal. In contrast to other mobile radio systems such as HSDPA, the LTE CQI index is not directly associated with the measured signal-to-noise ratio. Instead, it is also influenced by the signal processing in the UE. With the same channel, a UE featuring a powerful signal processing algorithm is able to forward a higher CQI index to the BS than a UE that has a weak algorithm. 

Precoding matrix indicator (PMI) 

The precoding matrix determines how the individual data streams (called layers in LTE) are mapped to the antennas. Skillfully selecting this matrix yields a maximum number of data bits, which the UE can receive together across all layers. However, this requires knowledge of the channel quality for each antenna in the downlink, which the UE can determine through measurements. If the UE knows what the allowed precoding matrices are, it can send a PMI report to the eNB and suggest a suitable matrix. 

Rank indicator (RI) 

The channel rank indicates the number of layers and the number of different signal streams transmitted in the downlink. When using a single input multiple output (SIMO) or a transmit diversity configuration, only one layer is utilized. In contrast, 2×2 MIMO (multiple input multiple output) with spatial multiplexing uses two layers. The goal of an optimized RI is to maximize the channel capacity across the entire available downlink bandwidth by taking advantage of each full channel rank. RI is not the sole benchmark for the state of the channel when using LTE. CQI and PMI are taken into account as well, since the value of the RI also influences the allowed precoding matrices and CQI values. In contrast, the eNB can only utilize the CQI reporting to adapt the downlink channel (assuming the RI does not change such as in pure SIMO mode). The eNB is not forced to react to the feedback from the UE and modify the signal in the downlink accordingly. In most cases, it nevertheless makes sense to do this in order to reduce the error rate and increase the data throughput. However, inaccurate feedback from the UE regarding the state of the channel can lead to exactly the opposite situation. For this reason, it is vital to ensure that the UE accurately indicates the state of the channel by means of the CQI, PMI and RI parameters. 

Backoff Indicator

Backoff Indicator is a special MAC subheader that carries the parameter indicating the time delay between a PRACH and the next PRACH. There are cases where a UE has to send another PRACH after it already sent a PRACH. 

The most common cases are as follows.

 i) UE sent a PRACH but didn't get a RAR for some reason. 

 ii) UE sent a PRACH and got RAR, but the RAPID in the RAR is not for the UE. 

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 indicator (BI) 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. 

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.

Services Provided by Physical (L1) Layer

The physical layer offers data transport services to higher layers. The access to these services is through the use of transport channels via the MAC sub-layer. A transport block is defined as the data delivered by MAC layer to the physical layer and vice versa. Transport blocks are delivered once every TTI (Transmission Time Interval).

The physical layer is expected to perform the following functions in order to provide the data transport service:

- Error detection on the transport channel and indication to higher layers

- FEC encoding/decoding of the transport channel

- Hybrid ARQ soft-combining

- Rate matching of the coded transport channel to physical channels

- Mapping of the coded transport channel onto physical channels

- Power weighting of physical channels

- Modulation and demodulation of physical channels

- Frequency and time synchronisation

- Radio characteristics measurements and indication to higher layers

- Multiple Input Multiple Output (MIMO) antenna processing

- Transmit Diversity (TX diversity)

- Beamforming

- RF processing. (Note: RF processing aspects are specified in the TS 36.100)

Reference signals

Five types of downlink reference signals are defined:

- Cell-specific reference signals (CRS)

- MBSFN reference signals

- UE-specific reference signals (DM-RS)

- Positioning reference signals (PRS)

- CSI reference signals (CSI-RS)

There is one reference signal transmitted per downlink antenna port.

Cell-specific reference signals

Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting PDSCH transmission. Cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3. Cell-specific reference signals are defined for f 15 kHz only.

The number of Resource Elements (REs) within each Resource Element Group (REG) and the number of REGs within an OFDM symbol is affected by the number of cell-specific reference signals present on all antenna ports. The number and location of cell specific reference signals are dependent on the number of antenna ports and the type of cyclic prefix used. Each antenna port has a unique cell specific reference signal associated with it. As the REG arrangement is affected by cell specific reference signals, the REG arrangement for a one or two antenna port configuration or four antenna port configuration is different. The REG arrangement for each resource block within a subframe and for every antenna port is identical.

To facilitate the estimation of the channel characteristics LTE uses cell specific reference signals (pilot symbols) inserted in both time and frequency. These pilot symbols provide an estimate of the channel at given locations within a subframe. Through interpolation it is possible to estimate the channel across an arbitrary number of subframes.

Cell-specific RS is transmitted in each physical antenna port. It is used for both demodulation and measurement purpose. Its pattern design ensures channel estimation accuracy.

Cell-specific reference signals are used for…

– cell search and initial acquisition,

– downlink channel estimation for coherent demodulation/detection at the UE,

– downlink channel quality measurements.

UE-specific Reference Signals

UE-specific reference signals are supported for transmission of PDSCH and are transmitted on antenna port(s) p 5 , p 7 , p 8 or p 7,8,..., 6 , where is the number of layers used for transmission of the PDSCH. UE-specific reference signals are present and are a valid reference for PDSCH demodulation only if the PDSCH transmission is associated with the corresponding antenna port according to Section 7.1 of [4]. UE-specific reference signals are transmitted only on the resource blocks upon which the corresponding PDSCH is mapped.

UE-specific reference signal are transmitted using resource elements with the same index pair k, l regardless of their antenna port p . UE-specific reference signals are denser in frequency but only transmitted when data is transmitted on the corresponding layer.

CSI Reference Signals

CSI reference signals are transmitted on one, two, four or eight antenna ports using p 15 , p 15,16 , p 15,...,18 and p 15,...,22 , respectively. CSI reference signals are defined for f 15 kHz only.

Feedback of channel-state information (CSI) is based on a separate set of reference signals – CSI reference signals. CSI reference signals are relatively sparse in frequency but regularly transmitted from all antennas at the base station.

The CSI reference signal is transmitted in each physical antenna port or virtualized antenna port and is used for measurement purposes only

A cell can be configured with one, two, four or eight CSI-RS. The exact CSI-RS structure, including the exact set of resource elements used for CSI-RS in a rosource block, depends on the number of CSI-RS configured within the cell and may also be different for different cells. More speifically, within a resource-block pair there are 40 possible positions for the reference symbols of CSI-RS and, in a given cell, a subset of corresponding resource elements is used for CSI-RS transmission.

MBSFN reference signals MBSFN reference signals shall be transmitted in the MBSFN region of MBSFN subframes only when the PMCH is transmitted. MBSFN reference signals are transmitted on antenna port 4. MBSFN reference signals are defined for extended cyclic prefix only.

Synchronization Signals

There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity (2) ID (1) ID cell ID N 3N N is thus uniquely defined by a number (1) ID N in the range of 0 to 167, representing the physical-layer cell-identity group, and a number (2) ID N in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group.

Primary synchronization signal and Secondary synchronization signal

Both primary and secondary synchronization signals are designed to detect all type of UEs. The synchronization signals always occupy the 62 sub-carrier of the channel, which make the cell search procedure same regardless of channel bandwidth. Although 72 subcarriers (6 RB) are available, only 62 sub-carriers are used so that the UE can perform the cell search procedure. The primary synchronization signal subcarriers are modulated using a frequency domain Zadoff-Chu Sequence. Each subcarrier has the same power level with its phase determined by the root index number in sequence generator as defined in 36.211

The secondary signal is used to identify cell-identity groups. The number and position of subcarrier are same as for the primary synchronization signal: that is the central 62 sub carriers. The sequence generation function utilizes an interleaved concatenation of two length 31 binary sequences as defined in 36.211. The secondary synchronization signal gives a cell-identity group number from 168 possible cell identities N (1, ID).

Radio Channel Quality Feedback

CQI (channel quality indicator)

CQI is an indication of the downlink mobile radio channel quality as experienced by this UE. Essentially, the UE is proposing to the eNodeB an optimum modulation scheme and coding rate to use for a given radio link quality, so that the resulting transport block error rate would not exceed 10%. 16 combinations of modulation scheme and coding rate are specified as possible CQI values. The UE may report different types of CQI. A so-called “wideband CQI” refers to the complete system bandwidth. Alternatively, the UE may evaluate a “sub-band CQI” value per sub-band of a certain number of resource blocks which is configured by higher layers. The full set of sub-bands would cover the entire system bandwidth. In case of spatial multiplexing, a CQI per code word needs to be reported.

The time and frequency resources used by the UE to report CQI are under the control of the eNB. CQI reporting can be either periodic or aperiodic. A UE can be configured to have both periodic and aperiodic reporting at the same time. In case both periodic and aperiodic reporting occurs in the same subframe, only the aperiodic report is transmitted in that subframe.

For efficient support of localized, distributed and MIMO transmissions, E-UTRA supports three types of CQI reporting:

- Wideband type: providing channel quality information of entire system bandwidth of the cell;

- Multi-band type: providing channel quality information of some subset(s) of system bandwidth of the cell;

- MIMO type: open loop or closed loop operation (with or without PMI feedback).

Periodic CQI reporting is defined by the following characteristics:

- When the UE is allocated PUSCH resources in a subframe where a periodic CQI report is configured to be sent, the periodic CQI report is transmitted together with uplink data on the PUSCH. Otherwise, the periodic CQI reports are sent on the PUCCH.

Aperiodic CQI reporting is defined by the following characteristics:

- The report is scheduled by the eNB via the PDCCH;

- Transmitted together with uplink data on PUSCH.

When a CQI report is transmitted together with uplink data on PUSCH, it is multiplexed with the transport block by L1 (i.e. the CQI report is not part of the uplink the transport block).The eNB configures a set of sizes and formats of the reports. Size and format of the report depends on whether it is transmitted over PUCCH or PUSCH and whether it is a periodic or aperiodic CQI report.

PMI (precoding matrix indicator)

PMI is an indication of the optimum precoding matrix to be used in the base station for a given radio condition. The PMI value refers to the codebook table. The network configures the number of resource blocks that are represented by a PMI report. Thus to cover the full bandwidth, multiple PMI reports may be needed. PMI reports are needed for closed loop spatial multiplexing, multi-user MIMO and closed-loop rank 1 precoding MIMO modes.

RI (rank indication)

RI is the number of useful transmission layers when spatial multiplexing is used. In case of transmit diversity, rank is equal to 1.

RRM functions

Radio Bearer Control (RBC)

The establishment, maintenance and release of Radio Bearers involve the configuration of radio resources associated with them. When setting up a radio bearer for a service, radio bearer control (RBC) takes into account the overall resource situation in E-UTRAN, the QoS requirements of in-progress sessions and the QoS requirement for the new service. RBC is also concerned with the maintenance of radio bearers of in-progress sessions at the change of the radio resource situation due to mobility or other reasons. RBC is involved in the release of radio resources associated with radio bearers at session termination, handover or at other occasions. RBC is located in the eNB.

Radio Admission Control (RAC)

The task of radio admission control (RAC) is to admit or reject the establishment requests for new radio bearers. In order to do this, RAC takes into account the overall resource situation in E-UTRAN, the QoS requirements, the priority levels and the provided QoS of in-progress sessions and the QoS requirement of the new radio bearer request. The goal of RAC is to ensure high radio resource utilization (by accepting radio bearer requests as long as radio resources available) and at the same time to ensure proper QoS for in-progress sessions (by rejecting radio bearer requests when they cannot be accommodated). RAC is located in the eNB.

Connection Mobility Control (CMC)

Connection mobility control (CMC) is concerned with the management of radio resources in connection with idle or connected mode mobility. In idle mode, the cell reselection algorithms are controlled by setting of parameters (thresholds and hysteresis values) that define the best cell and/or determine when the UE should select a new cell.

Also, E-UTRAN broadcasts parameters that configure the UE measurement and reporting procedures. In connected mode, the mobility of radio connections has to be supported. Handover decisions may be based on UE and eNB measurements. In addition, handover decisions may take other inputs, such as neighbour cell load, traffic distribution, transport and hardware resources and Operator defined policies into account. CMC is located in the eNB.

Dynamic Resource Allocation (DRA) - Packet Scheduling (PS)

The task of dynamic resource allocation (DRA) or packet scheduling (PS) is to allocate and de-allocate resources (including buffer and processing resources and resource blocks (i.e. chunks)) to user and control plane packets. DRA involves several sub-tasks, including the selection of radio bearers whose packets are to be scheduled and managing the necessary resources (e.g. the power levels or the specific resource blocks used). PS typically takes into account the QoS requirements associated with the radio bearers, the channel quality information for UEs, buffer status, interference situation, etc. DRA may also take into account restrictions or preferences on some of the available resource blocks or resource block sets due to inter-cell interference coordination considerations. DRA is located in the eNB.

Inter-cell Interference Coordination (ICIC)

Inter-cell interference coordination has the task to manage radio resources such that inter-cell interference is kept under control. ICIC mechanism includes a frequency domain component and time domain component. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. The preferred ICIC method may be different in the uplink and downlink. The frequency domain ICIC manages radio resource, notably the radio resource blocks.

For the time domain ICIC, Almost Blank Subframes (ABSs) are used to protect resources receiving strong inter-cell interference. MBSFN subframes can be used for time domain ICIC when they are also included in ABS patterns. The eNB cannot configure MBSFN subframes [4] as ABSs when these MBSFN subframes are used for other usages (e.g., MBMS, LCS). ICIC is located in the eNB.

PDCCH and PUCCH

Physical Downlink Control Channel (PDCCH)

The downlink control signalling (PDCCH) is located in the first n OFDM symbols where n ≤ 4 and consists of:

- Transport format and resource allocation related to DL-SCH and PCH, and hybrid ARQ information related to DL-SCH;

- Transport format, resource allocation, and hybrid-ARQ information related to UL-SCH;

Transmission of control signalling from these groups is mutually independent. Multiple physical downlink control channels are supported and a UE monitors a set of control channels. Control channels are formed by aggregation of control channel elements, each control channel element consisting of a set of resource elements. Different code rates for the control channels are realized by aggregating different numbers of control channel elements.

QPSK modulation is used for all control channels. Each separate control channel has its own set of x-RNTI. There is an implicit relation between the uplink resources used for dynamically scheduled data transmission, or the DL control channel used for assignment, and the downlink ACK/NAK resource used for feedback

Physical Uplink Control Channel (PUCCH)

The PUCCH shall be mapped to a control channel resource in the uplink. A control channel resource is defined by a code and two resource blocks, consecutive in time, with hopping at the slot boundary. 

Depending on presence or absence of uplink timing synchronization, the uplink physical control signalling can differ. In the case of time synchronization being present, the outband control signalling consists of:

- CQI;

- ACK/NAK;

- Scheduling Request (SR).

The CQI informs the scheduler about the current channel conditions as seen by the UE. If MIMO transmission is used, the CQI includes necessary MIMO-related feedback.

The HARQ feedback in response to downlink data transmission consists of a single ACK/NAK bit per HARQ process. PUCCH resources for SR and CQI reporting are assigned and can be revoked through RRC signalling. An SR is not necessarily assigned to UEs acquiring synchronization through the RACH (i.e. synchronised UEs may or may not have a dedicated SR channel). PUCCH resources for SR and CQI are lost when the UE is no longer synchronized.

Ref. 36.300

RSRP and RSRQ

In cellular networks, when a mobile moves from cell to cell and performs cell selection/reselection and handover, it has to measure the signal strength/quality of the neighbor cells. In LTE network, a UE measures two parameters on reference signal: RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality).

RSRP is a RSSI type of measurement. It measures the average received power over the resource elements that carry cell-specific reference signals within certain frequency bandwidth. RSRP is applicable in both RRC_idle and RRC_connected modes, while RSRQ is only applicable in RRC_connected mode. In the procedure of cell selection and cell reselection in idle mode, RSRP is used.

RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. It is defined as (N*RSRP)/(E-UTRA Carrier RSSI), where N makes sure the nominator and denominator are measured over the same frequency bandwidth;

The carrier RSSI (Receive Strength Signal Indicator) measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks. The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc.

The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both.

Ref. 3GPPP 36.214

UE Identity's in LTE

• Globally Unique Temporary Identity (GUTI)
  
  GUTI is allocated to the UE by the MME and has two components GUMMEI (Globally Unique MME ID) and the M-TMSI (MME-TMSI). The GUMMEI identifies the MME.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. The Globally Unique MME Identifier (GUMMEI) is constructed from the MCC, MNC and MME Identifier (MMEI).

• Temporary Mobile Subscribe Identity (M-TMSI)
  
  The M-TMSI identifies the UE within the MME. An M-TMSI identifies a user between the UE and the MME. The relationship between M-TMSI and IMSI is known only in the UE and in the MME.This value is allocated by MME.

• Temporary Mobile Subscriber Identity (S-TMSI)
  
  For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI. S-TMSI = MMEC + M-TMSI. It uniquely identify's the UE within an MME group. It is also included in RRC Connection Request.

• International Mobile Subscriber Identity (IMSI)
  
  IMSI is used for subscriber identification and stored in the Subscriber Identity Module (SIM). IMSI is usually 15 digits long. The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (Europeanstandard) or 3 digits (North American standard). The remaining digits are the mobile station identification number (MSIN) within the network's customer base.

• International Mobile Equipment Identity (IMEI)
  
  The IMEI number is used by the network/operators to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network. It is usually found printed on the phone.

Radio Network Temporary Identifier (RNTI) is used as UE identifiers within E-UTRAN and in signalling messages betweeen UE and E-UTRAN.

• Cell RNTI (C-RNTI)
  
  The C-RNTI provides a unique UE identification at the cell level identifying RRC Connection. Each RRC connection is associated with C-RNTI.
• Random Access RNTI (RA-RNTI)
  
  The RA-RNTI is assigned by the eNB to a particular UE after this UE has sent a random access preamble on the Physical Access Channel (PRACH). If this random acccess preamble is received by the eNB and network granted, the base station sends an acquisition indication back to the mobile and this acquisition indication message contains the RA-RNTI. In turn the UE will use the RA-RNTI to send RRC connection request message on the radio interfac UL and the parameter will help to distinguish messages sent by differnt UEs on the Random Access Channle (RACH). This procedure is called as contention based random access procedure.

• System Information RNTI (SI-RNTI)
  
  The SI-RNTI is sent on the PDCCH. It does not stand for a particular UE identity. Instead it signals to all mobiles in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH). This is necessary since the PDSCH is used to transport both broadcast system information for all UEs and singaling/payload for particular mobiles. In other words, the SI-RNTI indicated which DL resource blocks are used to carry SIBs.
• Paging RNTI (P-RNTI)The P-RNTI is derived from the IMSI of the subscribed to be paged and constructed by the eNB. For this reason IMSI is transmitted in a S1AP paging message fromk the MME to eNB. To receive paging messages from E-UTRAN, UEs in idle mode monitor the PDCCH channel for P-RNTI value used to indicate paging.
• Temporary Cell RNTI (TC-RNTI)
  
  When the UE does not have allocated C-RNTI then Temporaru C-RNTI is used. A temporary identity, the TC-RNTI, used for further communication between the terminal and the network. If the communication is successful then TC-RNTI is promoted eventually to C-RNTI in the case of UE not having a C-RNTI.

SC-FDMA Modulation

Modulation symbol mapping

The transmitter of an SC-FDMA system converts a binary input signal to a sequence of modulated subcarriers. Todo so, it performs the signal processing operations shown in the Figure. Signal processing is repetitive in a few different time intervals. Resource assignment takes place in transmit time intervals (TTIs). In 3GPP LTE, a typical TTI is 0.5 ms. The TTI is further divided into time intervals referred to as blocks. A block is the time used to transmit all of subcarriers once.

At the input to the transmitter a baseband modulator transforms the binary input to a multilevel sequence of complex numbers Xn in one of several possible modulation formats including Binary Phase Shift Keying (BPSK), quaternary PSK (QPSK), 16-level Quadrature Amplitude Modulation (16-QAM) and 64-QAM. The system adapts the modulation format, and thereby the transmission bit rate, to match the current channel conditions of each terminal.

The type of modulation format used often depends on the signal-to-noise level of the received signal and the receiver ability to decode them correctly. These modulated symbols are then mapped to subcarriers. An inverse-FFT (IFFT) is used to transform the modulated subcarriers in frequency domain to time domain samples.

In general, the same modulation format is used in all the subcarriers to keep the control information overhead small. However, it is possible to have different modulation formats over multiple subcarriers, and it is in fact advantageous in harsh and time varying channel conditions. In a broadband system, the channel is frequency selective over its large system bandwidth, meaning the signal fading on each subcarrier is independent. The interference level on each subcarrier can also be different and vary uniquely with time. It results in a different signal-to-impairment level on each of the subcarriers. Hence, having an appropriate modulation format on these subcarriers would help to maximize the overall system throughput. OFDM system inherits an adaptation of modulation formats to each of the subcarriers depending on channel conditions, and this is called Channel-dependent scheduling.

A cyclic prefix block copies a portion of the samples at the end of the time domain samples block (at the IFFT output) to the beginning. Since the DFT/FFT outputs are periodic in theory, copying the samples to the beginning will make the signal continuous. The length of the cyclic prefix depends on the channel delay spread, and is preferably longer than the length of the channel response. At the receiver, the prefix part of the symbol is thrown away as it may contain ISI from its previous symbol. Hence, it removes the effect of ISI caused by the multipath signal propagation. However, the prefix is the overhead in an OFDM system, as it does not carry any useful information.

PAPR analysis SC-FDMA offers similar performance and complexity as OFDM. However, the main advantage of SC-FDMA is the low PAPR (peak-average-power ratio) of the transmit signal. PAPR is defined as the ratio of the peak power to average power of the transmit signal. As PAPR is a major concern at the user terminals, low PAPR makes the SC-FDMA the preferred technology for the uplink transmission. PAPR relates to the power amplifier efficiency at the transmitter, and the maximum power efficiency is achieved when the amplifier operates at the saturation point. Lower PAPR allows operation of the power amplifier close to saturation resulting in higher efficiency. With higher PAPR signal, the power amplifier operating point has to be backed off to lower the signal distortion, and thereby lowering amplifier efficiency.

As SC-FDMA modulated signal can be viewed as a single carrier signal, a pulse shaping filter can be applied to transmit signal to further improve PAPR. PAPR comparison between OFDM and SC-FDMA variations such as interleaved SC-FDMA and localized SC-FDMA has been done in [2]. With no pulse shaping filters, interleavedSC-FDMA shows the best PAPR. Compared to OFDM PAPR, the PAPR of interleaved SCFDMA with QPSK is about 10 dB lower, whereas that of localized SC-FDMA is only about 3 dB lower. With 16-QAM, these levels are about 7 dB and 2 dB lower respectively. Therefore, interleaved SC-FDMA is a preferred modulation technique for lower PAPR. Pulse shape filtering of SC-FDMA in fact degrades the PAPR level of interleaved SC-FDMA whereas it shows no effect with localized SC-FDMA.

LTE SC-FDMA

For the LTE uplink, a different concept is used for the access technique. Although still using a form of OFDMA technology, the implementation is called Single Carrier Frequency Division Multiple Access (SC-FDMA).

Single carrier frequency division multiple access (SC-FDMA) has been adopted by the third generation partnership project (3GPP) for uplink transmission in technology standardized for long term evolution (LTE) of cellular systems.SC-FDMA was chosen because it combines the low PAPR techniques of single-carrier transmission systems, such as GSM and CDMA, with the multi-path resistance and flexible frequency allocation of OFDMA.

One of the key parameters that affects all mobiles is that of battery life. Even though battery performance is improving all the time, it is still necessary to ensure that the mobiles use as little battery power as possible. With the RF power amplifier that transmits the radio frequency signal via the antenna to the base station being the highest power item within the mobile, it is necessary that it operates in as efficient mode as possible. This can be significantly affected by the form of radio frequency modulation and signal format. Signals that have a high peak to average ratio and require linear amplification do not lend themselves to the use of efficient RF power amplifiers. As a result it is necessary to employ a mode of transmission that has as near a constant power level when operating. Unfortunately OFDM has a high peak to average ratio. While this is not a problem for the base station where power is not a particular problem, it is unacceptable for the mobile. As a result, LTE uses a modulation scheme known as SC-FDMA - Single Carrier Frequency Division Multiplex which is a hybrid format. This combines the low peak to average ratio offered by single-carrier systems with the multipath interference resilience and flexible subcarrier frequency allocation that OFDM provides.

SC-FDMA is a modified form of OFDM with similar throughput performance and complexity. This is often viewed as DFT-coded OFDM where time-domain data symbols are transformed to frequency-domain by a discrete Fourier transform (DFT) before going through the standard OFDM modulation. Thus, SC-FDMA inherits all the advantages of OFDM over other well-known techniques such as TDMA and CDMA.

The major problem in extending GSM TDMA and wideband CDMA to broadband systems is the increase in complexity with the multipath signal reception. The distinguishing feature of SC-FDMA is that it leads to a singlecarrier transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme which makes it suitable for broadband systems.

In SC-FDMA as well as OFDM, equalization is achieved on the receiver side after the FFT calculation, by multiplying each Fourier coefficient by a complex number. The advantage is that FFT and frequency domain equalization requires less computation power than the conventional timedomain equalization.

QoS Class Identifier (QCI)

The need for supporting a broader variety of applications requiring higher bandwidth and lower latency led 3GPP to alleviate the existing QoS principles with the introduction for EPS of a QoS Class Identifier (QCI).

The QCI is a scalar denoting a set of transport characteristics (bearer with/without guaranteed bit rate, priority, packet delay budget, packet error loss rate) and used to infer nodes specific parameters that control packet forwarding treatment (e.g., scheduling weights, admission thresholds, queue management thresholds, link-layer protocol configuration, etc.).

Each packet flow is mapped to a single QCI value (nine are defined in the Release 8 version of the specifications) according to the level of service required by the application. The usage of the QCI avoids the transmission of a full set of QoS-related parameters over the network interfaces and reduces the complexity of QoS negotiation.

The QCI, together with Allocation-Retention Priority (ARP) and, if applicable, Guaranteed Bit Rate (GBR) and Maximum Bit Rate (MBR), determines the QoS associated to an EPS bearer. A mapping between EPS and pre-Release 8 QoS parameters has been defined to allow proper interworking with legacy networks.

The QoS architecture in EPC enables a number of important capabilities for both operators and users:

• VoIP support with IMS. QoS is a crucial element for providing LTE/IMS voice service. 
• Enhanced application performance. Applications such as gaming or video can operate more reliably. 
• More flexible business models. With flexible, policy-based charging control, operators and third-parties will be able to offer content in creative new ways. For example, an enhanced video stream to a user could be paid for by an advertiser. 
• Congestion control. In congestion situations, certain traffic flows (e.g., bulk transfers, abusive users) can be throttled down to provide a better user experience for others.

IP address allocation

A UE performs the address allocation procedures for at least one IP address (either IPv4 address or IPv6 prefix) after the default bearer activation if no IPv4 address is allocated during the default bearer activation.

One of the following ways is used to allocate IP addresses for the UE:

a) The HPLMN allocates the IP address to the UE when the default bearer is activated (dynamic or static HPLMN address);

b) The VPLMN allocates the IP address to the UE when the default bearer is activated (dynamic VPLMN address); or

c) The PDN operator or administrator allocates an (dynamic or static) IP address to the UE when the default bearer is activated (External PDN Address Allocation).

The IP address allocated for the default bearer is used for the dedicated bearers within the same PDN connection. IP address allocation for PDN connections, which are activated by the UE requested PDN connectivity procedure, is handled with the same set of mechanisms as those used within the Attach procedure.

PDN types IPv4, IPv6 and IPv4v6 are supported. An EPS Bearer of PDN type IPv4v6 may be associated with one IPv6 prefix only or with both one IPv4 address and one IPv6 prefix. PDN type IPv4 is associated with an IPv4 address. PDN type IPv6 is associated with an IPv6 prefix. PDN types IPv4 and IPv6 are utilised for the UE and/or the PDN GW support IPv4 addressing only or IPv6 prefix only; or operator preferences dictate the use of a single IP version only, or the subscription is limited to IPv4 only or IPv6 only for this APN. In addition, PDN type IPv4 and IPv6 are utilised for interworking with nodes of earlier releases.

The way that the UE sets the requested PDN type may be pre-configured in the device per APN. Unless otherwise configured (including when the UE does not send any APN), the UE sets the PDN type during the Attach or PDN Connectivity procedures based on its IP stack configuration as follows:

- A UE which is IPv6 and IPv4 capable shall request for PDN type IPv4v6.

- A UE which is only IPv4 capable shall request for PDN type IPv4.

- A UE which is only IPv6 capable shall request for PDN type IPv6.

- When the IP version capability of the UE is unknown in the UE (as in the case when the MT and TE are separated and the capability of the TE is not known in the MT), the UE requests for PDN type IPv4v6.

The HSS stores one or more PDN types per APN in the subscription data. During the Attach or UE requested PDN connectivity procedure MME compares the requested PDN type to the PDN type in the subscription records for the given APN and sets the PDN type as follows:

- If the requested PDN type is allowed by subscription, the MME sets the PDN type as requested.

- If the requested PDN type is IPv4v6 and subscription data only allows PDN type IPv4 or only allows PDN type IPv6, the MME sets the PDN type according to the subscribed value. A reason cause shall be returned to the UE indicating that only the assigned PDN type is allowed. In this case the UE shall not request another PDN connection to the same APN for the other IP version.

- If the requested PDN type is IPv4 or IPv6, and neither the requested PDN type nor PDN type IPv4v6 are subscribed, the PDN connection request is rejected.

- If the requested PDN type is IPv4v6, and both IPv4 and IPv6 PDN types are allowed by subscription but not IPv4v6, the MME shall set the PDN type to IPv4 or IPv6 where the selection between IPv4 and IPv6 is implementation specific. The UE should then initiate the UE requested PDN connectivity procedure to this APN in order to activate a second PDN connection with the other single address PDN type which was not allocated by the network.

NOTE 1: If the MT and TE are separated, the UE might not be able to use reason cause "single address bearers only" as a trigger for activating a second single-stack EPS bearer.

The PDN GW may restrict the usage of a PDN type IPv4v6 as follows.

- If the PDN GW receives a request for PDN type IPv4v6, but the PDN GW operator preferences dictate the use of IPv4 addressing only or IPv6 prefix only for this APN, the PDN type shall be changed to a single address PDN type (IPv4 or IPv6) and a reason cause shall be returned to the UE indicating that only the assigned PDN type is allowed. In this case the UE shall not request another PDN connection to the same APN for the other IP version.

- If the PDN GW receives a request for PDN type IPv4v6, but the MME does not set the Dual Address Bearer Flag due to the MME operator using single addressing per bearer to support interworking with nodes of earlier releases the PDN type shall be changed to a single IP version only and a reason cause shall be returned to the UE indicating that only single IP version per PDN connection is allowed. In this case the UE should request another PDN connection for the other IP version using the UE requested PDN connectivity procedure to the same APN with a single address PDN type (IPv4 or IPv6) other than the one already activated.

During inter-RAT mobility between E UTRAN and UTRAN/GERAN, an EPS bearer with PDN type IPv4v6 is mapped one-to-one to PDP type IPv4v6.

During inter-RAT mobility between E-UTRAN and UTRAN/GERAN, an EPS bearer with PDN type IPv4 is mapped one-to-one to a PDP context of PDP type IPv4. An EPS bearer with PDN type IPv6 is mapped one-to-one to a PDP context of PDP type IPv6.

It is the HPLMN operator that should define in the subscription whether a dynamic HPLMN or VPLMN address may be used.

The EPS UE may indicate to the network within the Protocol Configuration Options element that the UE wants to obtain the IPv4 address with DHCPv4:

- the UE may indicate that it prefers to obtain an IPv4 address as part of the default bearer activation procedure. In such a case, the UE relies on the EPS network to provide IPv4 address to the UE as part of the default bearer activation procedure.

- the UE may indicate that it prefers to obtain the IPv4 address after the default bearer setup by DHCPv4. That is, when the EPS network supports DHCPv4 and allows that, it does not provide the IPv4 address for the UE as part of the default bearer activation procedures. The network may respond to the UE by setting the PDN Address to 0.0.0.0. After the default bearer establishment procedure is completed, the UE uses the connectivity with the EPS and initiates the IPv4 address allocation on its own using DHCPv4. However, if the EPS network provides IPv4 address to the UE as part of the default bearer activation procedure, the UE should accept the IPv4 address indicated in the default bearer activation procedure.

- if the UE sends no Address Allocation Preference, the PDN GW determines whether to use DHCPv4 or not based on per APN configuration

Both EPS network elements and UE shall support the following mechanisms:

a. IPv4 address allocation via default bearer activation, if IPv4 is supported.

b. IPv6 prefix allocation via IPv6 Stateless Address autoconfiguration according to RFC 4862 [18], if IPv6 is supported;

Furthermore, the Protocol Configuration Options may be used during bearer activation to configure parameters which are needed for IP address allocation. Both EPS network elements and UE may support the following mechanisms:

a. IPv4 address allocation and IPv4 parameter configuration after the attach procedure via DHCPv4 according to RFC 2131 [19] and RFC 4039 [25];

b. IPv6 parameter configuration via Stateless DHCPv6 according to RFC 3736 [20].

c. Allocation of IPv6 prefixes using DHCPv6 according to RFC 3633 [21].

EPS network elements may support the following mechanism:

a. Allocation of a static IPv4 address and/or a static IPv6 prefix based on subscription data in the HSS.

If the static IP address/prefix is not stored in the HSS subscription record, it may be configured on a per-user per-APN basis in the DHCP/Radius/Diameter server and the PDN GW retrieves the IP address/prefix for the UE from the DHCP/Radius/Diameter server. In this case, static IP address/prefix is allocated by the same procedures as the dynamic IP address/prefix allocation.

The following clauses describe how the above listed IP address allocation mechanisms work when GTP based S5/S8 is used. The way of working of the IP address allocation mechanisms for PMIP based S5/S8 can be found in TS 23.402 [2].The procedures can be used both for PLMN (VPLMN/HPLMN) or external PDN based IP address allocation.

NOTE 2: It is transparent to the UE whether the PLMN or the external PDN allocates the IP address and whether the IP address is static or dynamic.

In order to support DHCP based IP address configuration, the PDN GW acts as the DHCP server for HPLMN assigned dynamic and static and VPLMN assigned dynamic IP addressing. When DHCP is used for external PDN assigned addressing and parameter configuration, the PDN GW shall act as the DHCP server towards the UE and it shall act as the DHCP client towards the external DHCP server. The Serving GW does not have any DHCP functionality. It forwards packets, including DHCP packets, between the UE and the PDN GW.

IPv6 Stateless Address autoconfiguration specified in RFC 4862 [18] is the basic mechanism to allocate /64 IPv6 prefix to the UE.

During default bearer establishment, the PDN GW sends the IPv6 prefix and Interface Identifier to the S GW, and then the S GW forwards the IPv6 prefix and Interface Identifier to the MME or to the SGSN. The MME or the SGSN forwards the IPv6 Interface Identifier to the UE. The MME does not forward the IPv6 prefix to the UE. If the UE receives the IPv6 prefix from the SGSN during PDP Context Activation procedure, it ignores it.

NAS Attach Request

The UE initiates the Attach procedure by the transmission, to the eNodeB, of an Attach Request. Following parameters are included in this message.

IMSI or old GUTI, last visited TAI (if available), UE Core Network Capability, UE Specific DRX parameters, Attach Type, ESM message container (Request Type, PDN Type, Protocol Configuration Options, Ciphered Options Transfer Flag), KSIASME, NAS sequence number, NAS-MAC, additional GUTI, P-TMSI signature, Voice domain preference and UE's usage setting) message together with RRC parameters indicating the Selected Network and the old GUMMEI.

The old GUTI may be derived from a P TMSI and RAI.

IMSI is included

• if the UE does not have a valid GUTI or
• if the UE does not have a valid P TMSI available, or
• if the UE is configured to perform Attach with IMSI at PLMN change and is accessing a new PLMN. 

The UE stores the TIN in detached state.

• If the UE's TIN indicates "GUTI" or "RAT-related TMSI" and the UE holds a valid GUTI then the old GUTI indicates this valid GUTI. 
• If the UE's TIN indicates "P TMSI" and the UE holds a valid P TMSI and related RAI then these two elements are indicated as the old GUTI. Mapping a P TMSI and RAI to a GUTI 
• If the UE holds a valid GUTI and the old GUTI indicates a GUTI mapped from a P-TMSI and RAI, then the UE indicates the GUTI as additional GUTI. If the old GUTI indicates a GUTI mapped from a P-TMSI and RAI and the UE has a valid P-TMSI signature associated to it, the P-TMSI signature shall be included.

The UE sets the voice domain preference and UE's usage setting according to its configuration. If available, the last visited TAI shall be included in order to help the MME produce a good list of TAIs for any subsequent Attach Accept message. Selected Network indicates the PLMN that is selected for network sharing purposes. The RRC parameter "old GUMMEI" takes its value from the "old GUTI" contained in the Attach Request.

If the UE has valid security parameters, the Attach Request message is integrity protected by the NAS-MAC in order to allow validation of the UE by the MME. KSIASME, NAS sequence number and NAS-MAC are included if the UE has valid EPS security parameters. NAS sequence number indicates the sequential number of the NAS message. If the UE does not have a valid EPS security association, then the Attach Request message is not integrity protected.

The UE network capabilities indicate also the supported NAS and AS security algorithms. PDN type indicates the requested IP version (IPv4, IPv4/IPv6, IPv6). Protocol Configuration Options (PCO) are used to transfer parameters between the UE and the PDN GW, and are sent transparently through the MME and the Serving GW. The Protocol Configuration Options may include the Address Allocation Preference indicating that the UE prefers to obtain an IPv4 address only after the default bearer activation by means of DHCPv4.

If the UE intends to send PCO which require ciphering (e.g., PAP/CHAP usernames and passwords) or send an APN, or both, the UE shall set the Ciphered Options Transfer Flag and send PCO or APN or both only after authentication and NAS security setup have been completed (see below). If the UE has UTRAN or GERAN capabilities, it sends the NRSU in the PCO to indicate the support of the network requested bearer control in UTRAN/GERAN.

Request Type is included in the ESM message container and indicates "Handover" when the UE has already an activated PDN GW/HA due to mobility with non-3GPP accesses. Attach Type indicates whether it is an EPS attach or a combined EPS/IMSI attach or an Emergency Attach.

For an Emergency Attach the UE sets both the Attach Type and the Request Type to "Emergency" and the IMSI shall be included if the UE does not have a valid GUTI or a valid P-TMSI available. The IMEI shall be included when the UE has no IMSI, no valid GUTI and no valid P-TMSI.

UE Radio Capability Handling

The UE Radio Capability information contains information on RATs that the UE supports (e.g. power class, frequency bands, etc). Consequently, this information can be sufficiently large (e.g. >50 octets) that it is undesirable to send it across the radio interface at every transition from ECM IDLE to ECM CONNECTED. To avoid this radio overhead, the MME stores the UE Capability information during ECM IDLE state and the MME shall, if it is available, send its most up to date UE Radio Capability information to the E UTRAN in the S1 interface INITIAL CONTEXT SETUP REQUEST message unless the UE is performing an Attach procedure or a Tracking Area Update procedure for the "first TAU following GERAN/UTRAN Attach" or for a "UE radio capability update".

If the UE is performing an Attach procedure or a Tracking Area Update procedure for the "first TAU following GERAN/UTRAN Attach" or for "UE radio capability update", the MME shall delete (or mark as deleted) any UE Radio Capability information that it has stored, and, if the MME sends an S1 interface INITIAL CONTEXT SETUP REQUEST message during that procedure, the MME shall not send any UE Radio Capability information to the E UTRAN in that message. This triggers the E UTRAN to request the UE Radio Capability from the UE and upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message.

If the UE is performing a Service Request (or other) procedure and the MME does not have UE Radio Capability information available (or it is available, but marked as "deleted"), then the MME sends an S1 interface INITIAL CONTEXT SETUP REQUEST message to the E UTRAN without any UE Radio Capability information in it. This triggers the E UTRAN to request the UE Radio Capability from the UE and upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message.

NOTE 2: This use of the INITIAL CONTEXT SETUP REQUEST message means that for a signalling only procedure such as a periodic Tracking Area Update, the UE Radio Capability would not be sent to the E UTRAN.

NOTE 3: If a "first TAU following GERAN/UTRAN Attach" Tracking Area Update is performed during ECM-CONNECTED mode, e.g. after an inter RAT handover, no INITIAL CONTEXT SETUP REQUEST is sent and the UE Radio Capability information in the MME will remain deleted until the next ECM-IDLE to ECM-CONNECTED transition (or later, e.g. if the next activity from the UE is another Tracking Area Update).

The UE Radio Capability is not provided directly from one CN node to another. It will be uploaded to the MME when the E-UTRAN requests the UE Radio Capability information from the UE. During handover via the MME (both intra RAT and inter RAT), the radio capability information for the source and target 3GPP RATs (with the possible exception of UTRAN) are transferred in the "source to target transparent container". Information on additional 3GPP RATs is optionally transferred in the "source to target transparent container". Transfer of the radio capability information related to the source and/or additional RATs is beneficial as it avoids the need for the target RAT to retrieve the information from the UE prior to a subsequent inter-RAT handover.

To allow for the addition of future radio technologies, frequency bands, and other enhancements, the MME stores the UE Radio Capability Information even if it is larger than specified in TS 36.331 [37], up to a maximum size of 510 octets.

NOTE 4: The 510 octet value comes from the information element encoding rules described in TS 24.007 [45] and the assumption that the information contained within this UE Radio Capability Information Element stored by the MME is the equivalent of information signalled in two information elements in the GERAN NAS signalling for the case of GERAN to E UTRAN PS handover.

The E UTRAN stores the UE Radio Capability information, received in the S1 interface INITIAL CONTEXT SETUP REQUEST message or obtained from the UE, for the duration of the RRC connection for that UE. Before any handover attempt from E UTRAN to UTRAN, the E UTRAN retrieves the UE's UTRAN Radio Capabilities from the UE.

If the UE's non-UTRAN UE Radio Capability information changes while in ECM-IDLE state (including cases of being in GERAN/UTRAN coverage), the UE shall perform a Tracking Area Update indicating "UE radio capability update" when it next returns to E UTRAN coverage.

Functions of various CSFB architectural elements

Mobility Management Entity (for GERAN/UTRAN CSFB)
o Multiple PLMN selection and reselection for the CS domain
o Deriving a VLR number and LAI from the TAI of the current cell and based on the selected PLMN for CS domain, or using a default VLR number and LAI
o For CS fallback, generating a TAI list such that the UE has a low chance of "falling back" to a cell in a LA different to the derived LAI (e.g. the TAI list boundary should not cross the LA boundary)
o Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE
o Initiating IMSI detach at EPS detach
o Initiating paging procedure towards eNodeB when MSC pages the UE for CS services
o Supporting SMS procedures with UE and MSC via SGs
o Rejecting CS Fallback call request (e.g. due to O&M reasons)

Mobility Management Entity (for 1xRTT CSFB)
o It serves as a signaling tunneling end point towards the 3GPP2 1xCS IWS via S102 interface for sending/receiving encapsulated 3GPP2 1xCS signaling messages to/from the UE
o Handling of S102 tunnel redirection in case of MME relocation
o 1xCS-IWS (terminating S102 reference point) selection for CSFB procedures
o Buffering of messages received via S102 for UEs in idle state

E-UTRAN for GERAN/UTRAN
o Forwarding paging request and SMS to the UE
o Directing the UE to the target CS capable cell via appropriate procedure (i.e. PS handover, RRC release with redirection, CCO w/NACC)
o The configuration of appropriate cell reselection hysteresis at Location Area boundaries (or across the whole E-UTRAN) to reduce Tracking Area Update traffic
o To facilitate the configuration of TA boundaries with LA boundaries, the E-UTRAN can
gather statistics (from the inbound inter-RAT mobility events of all UEs) of the most common LAs indicated in the RRC signaling
o Configuration to permit the operator to choose the target “fallback” RAT and frequency

MSC for GERAN/UTRAN
o Maintaining SGs association towards MME for EPS/IMSI attached UE
o Supporting SMS procedures via SGs to EPS
o In order to speed up the potential LAU procedure during CS fallback the MSC may be configured to lower the frequency of Authentication, TMSI reallocation and Identity check for UEs that are EPS/IMSI attached via the SGs interface

E-UTRAN for 1xRTT
o Provision of broadcast information to trigger UE for 1xRTT CS registration
o Establish CDMA2000 tunnel between the UE and MME and forward 1xRTT messages
o Directing the UE to the target CS capable cell via appropriate procedure (i.e. RRC release with redirection or enhanced 1xCSFB procedure with 1xSRVCC based)
o Release of E-UTRAN resources after UE leaves E-UTRAN coverage subsequent to a page for CS fallback to 1xRTT CS if PS handover procedure is not performed in conjunction with 1xCS fallback
o Invoking the optimized or non-optimized PS handover procedure concurrently with enhanced 1xCS fallback procedure when supported by the network and UE, and based on network configuration.

UE supporting GERAN/UTRAN CSFB
o CSFB procedures for EPS/IMSI attach, update and detach
o CS fallback request/reject and SMS procedures for using CS domain services

UE supporting 1xRTT CSFB
o 1xRTT CS registration over the EPS after the UE has completed the E-UTRAN attachment
o 1xRTT CS re-registration due to mobility
o CS fallback request/reject and SMS procedures for using CS domain services
o Includes enhanced CS fallback to 1xRTT capability indication as part of the UE radio capabilities if it  supports enhanced 1xCSFB
o Includes concurrent 1xRTT and HRPD capability indication as part of the UE radio capabilities if supported by the enhanced CS fallback to 1xRTT capable UE