Day 1 LTE Technology Overview

7© SAMSUNG Electronics Co., Ltd.

The demand for data and high-speed services means that new, smarter wireless

technologies are needed to support such volumes and speeds. For instance, HDTV

demands 6 to 9 Mbps of continuous connectivity – an entire HSPA/WiMax sector.

The surest way to increase data rates is to use more spectrum. But spectrum must

be used efficiently. The ever-increasing capacity requirements necessitate the use of

smaller cell sizes.

New wireless technologies must therefore focus on:

• More spectrum

• More spectral efficiency

• Smaller cell sizes (Femto cells)

• QOS differentiation, incentivizing off-peak traffic


Coordinated multipoint transmission and reception (CoMP)


Shown above is a comparison between the LTE and GSM / WCDMA architectures.

LTE is a completely packet-switched network, requiring none of the costly circuit-

switched components that are necessary in earlier generations to support

conversational services.

The LTE access network consists of a single node, the eNodeB. There is no controller

equivalent of the BSC or RNC. This reduces the latency on the air interface,

significantly improving user experience.

In the LTE core, the user and control planes are separate. This further reduces

latency, eases signaling loads and allows operators to expand traffic capacity

independent of voice.


Shown above is the timeline for important 3rd and 4th generation 3GPP releases

along with the key features in each release. R8 was the first LTE release where

OFDMA and the LTE core network were specified. R9 introduced Self-Organizing

Networks, discussed later, and LTE femtocells or home base stations. Femtocells are

very small cells with low transmit powers meant for homes or small offices. Release

10 introduces Co-operative Multi-antenna Processing (CoMP) which is a feature

where multiple eNodeB’s co-operate to transmit signals to cell-edge UE’s.


Apart from the need for higher data rates, a major driver for LTE has been the need to

reduce network latencies. Packet-switched devices often lapse into a dormant state and

take some time to become fully active again. WCDMA and early HSPA releases suffered from

latency issues that hampered user experience. 3GPP has since paid special attention to this

issue. LTE requirements therefore clearly spell out the maximum allowable latencies in user

and control planes. Separating the user and control planes, and using IP-based ‘single node’

access and core networks help LTE reduce delays and improve user experience.

Control Plane Transit time (RAN only) from non-active to active state. 100 ms for

transition from R6 Idle state, 50 ms for R6 Cell_PCH state. Related to this, 200 terminals in

active state should be supported in 5 MHz and 400 in wider deployments.

User Plane Time to transmit small IP packet to / from RAN edge node measured on IP

layer. 5ms in unloaded network.


Unlike previous generations of cellular standards, LTE supports flexible bandwidth

deployment. LTE systems can use any of the bandwidths listed above. This allows

operators to deploy in a gradual manner, starting with lower bandwidths and

increasing the bandwidth as traffic grows. Deployments with different cell sites using

different bandwidths are also supported. This is particularly useful for high-capacity

hotspots and layering, i.e., micro or pico cells. LTE also supports both paired (FDD)

and un-paired (TDD) spectrum. RJIL will use TD-LTE.


Shown above are bands used by TD-LTE. There are regular additions to this list based

on ITU regulatory meetings. The 2.3 to 2.6 GHz bands are favored in Asia. Samsung /

RJIL will use the 2.3 GHz band.


Before introducing the LTE network architecture, it’s worth re-visiting earlier

generations to get a sense of how the architecture has evolved. Shown above is a 2G

/ 2.5G GSM / GPRS architecture. The GSM, i.e., CS-voice part is handled by the Base

station Controller (BSC) and the Mobile Switching Center (MSC). The MSC is a switch

that connects to a Gateway-MSC (GMSC) which is plugged in to the landline PSTN.

With the introduction of GPRS, the first step toward packet switching was taken. The

Serving GPRS Support Node (SGSN) and the Gateway-GGSN (GGSN) form the packet

core of the GPRS / EDGE and early WCDMA releases. A Packet Controller Unit (PCU)

in the BSC split the traffic to the CS and PS core networks. Still, the CS core remains,

requiring huge investments in switching equipment and infrastructure.


The WCDMA R99 architecture took some further steps towards independence of the

Radio Access Network (RAN) and the core. A Radio Network Controller (RNC) took

on some of the functions of the MSC. The inter-RNC interface was introduced in

order to support the CDMA soft handover feature. The base station was called

NodeB. Later releases, R5 HSDPA and above, made the NodeB capable of fast

dynamic scheduling but some of the more executive functions (e.g. admission

control) continued to rest with the RNC.

On the core side, there was little development in R99, largely to allow upgrades

using existing core networks. R4 did introduce the MSC Server or soft-switch and

Media Gateway (MGW) functionalities. This was the first time that control and user

planes were separated in the core. R5 allowed full IP Multimedia Subsystem (IMS)

support, an all IP network. Still, most operators continued to rely on CS core for

voice.


The LTE access network is a single node RAN. The LTE base station, eNodeB, is

completely responsible for all the access functionalities. There is no controller node

analogous to the 2G Base Station Controller or the 3G Radio Network Controller. This

minimizes the number of interfaces required. In earlier generations, a failure in a

centralized controller for multiple base stations could potentially degrade service for

the entire area. LTE reduces such single-point failures.

The LTE network is completely packet-based end to end. No traffic is circuit

switched. LTE supports real-time conversational traffic by ensuring appropriate QoS

treatment for it. Every node in the LTE traffic path is QoS aware, making QoS an end-

to-end feature. Different QoS treatment is given to control, user and administrative

traffic.


Work on the LTE core was called System Architecture Evolution (SAE). The result was

an Evolved Packet Core (EPC). The LTE core is a single logical node, though the

control and user plane nodes are separate. Since IP is used end-to-end, it is often

referred to as a flat architecture.

EPC supports interworking with a wide range of other networks – GSM / GPRS,

WCDMA / HSPA, CDMA and WiFi – with appropriate interfaces defined for each

scenario. Since LTE is the technology of choice for both 3GPP and 3GPP2 operators,

interworking with both legacy cellular systems is a must. Initial LTE deployments are

usually in urban pockets. So most users will require handover to a legacy network

when they travel out of LTE coverage.


The LTE access network is an evolved UMTS Terrestrial Radio Access Network, or E-

UTRAN. It consists of a single eNodeB, the LTE base station, but no controller node.

The eNodeB’s are connected to each other via the X2 interface. Since there is no

controller, the eNB’s coordinate with each other.

The EPC consists of a Mobility Management Entity (MME) that handles all control

plane functionality, and two gateways – Serving Gateway (S-GW) and PDN Gateway

(P-GW) – in the user plane. The S-GW and P-GW can be combined into a single

physical node called the SAE-GW. The separation of user and control planes allows

operators to expand one independently of the other. It also reduces unwanted

signaling loads. The functions of each node are explained in some detail in the

following slides.


Being the single node in the E-UTRAN, the eNB performs all the Radio Resource

Management (RRM) functions. Thus it sets up radio bearers and admits or disallows

services based on the radio resource availability. An important function of the eNB is

to control mobility within the radio network. During handovers or cell reselection, it

coordinates with other eNB’s using the X2 interface to control user mobility. The eNB

is also responsible for dynamic radio resource allocation on the LTE time-frequency

resource grid. It handles scheduling of resources as well as paging and broadcast

functions. Encryption and header compression were carried out by controller nodes

in earlier generations, but they are done by the eNB in LTE.

An eNB is connected to one or more MME’s via the S1-MME interface. When a user

attaches to the network, the eNB must select an MME from an MME-pool. It sends

signaling information such as measurements and messages to the MME.


Universal platform type A Management board Assembly

LTE eNB Channel card board Assembly

Shown above are the Samsung Digital Unit (DU) and Remote Radio Unit (RRU). The

DU is installed in a cabinet at the bottom of the tower or the rooftop floor. It consists

of a UAMA board, which is the controller, and an L9CA board which is the channel

card. A single L9CA board can support up to 3 sectors.

The RRU is installed close to the antenna. The two are connected using an optical

cable. This minimizes losses along the length of the tower and allows high data rate

information exchange.


Channel-dependent scheduling is an important aspect of an LTE eNB. Radio

conditions within the cell vary with time and location. The eNB scheduling

mechanism exploits this by preferentially scheduling UE’s in better radio conditions

while keeping a fair amount of resources for UE’s in bad radio conditions, thereby

improving throughput and quality.

Since LTE radio resources are basically frequencies, one of the key challenges in LTE

networks is managing inter-cell interference. Neighboring NB’s can operate on the

same bandwidths. Some co-ordination is therefore required to reduce interference

in the network. The Samsung Smart Scheduler Server addresses this problem by

providing UL & DL co-ordination among the eNB’s it controls. It constantly keeps

eNB’s informed of neighbor resource allocations, so that their own allocations can

minimize inter-cell interference. This results in increased cell-edge throughput.

Additionally, the Smart Scheduler also provides O&M functionalities.


The MME is the heart of the EPC as far as signaling is concerned. It is responsible for

the setup and management of bearers, or logical connections, with the user. When

user equipment (UE) is idle, the MME needs to generate a paging message to reach

the UE. This means that the MME must have some idea of the UE location. So the UE

updates the MME of change in location at suitable times. The MME is also

responsible for authentication.

Since it does setup, the MME is responsible for selecting gateways for user plane

traffic. This could be based, for example, on load considerations. The MME also has

an S3 interface to connect with other 3GPP core networks, e.g. to an SGSN. When a

user travels out of LTE coverage into 2.5/3G coverage, the MME can select the

appropriate SGSN. Within an LTE network, inter-MME mobility is handled via the S10

interface. The MME checks authorization and roaming issues via S6a, its interface to

the Home Subscriber server (HSS). It also supports lawful intercept for signaling

messages.


The Samsung MME offers high reliability and capacity. It supports mobility

management functions such as tracking area updates, paging and handover. It can be

co-located with the S-GW and the P-GW.

The MME consists of three kinds of cards:

LEMA: The LTE EPC Management Board Assembly consists of the switch and the

management functionality.

LENA: All external connections to the MME terminate on the LTE EPC Network

Assembly board.

LESA: The core MME functionality resides in the LTE EPC Session management board

Assembly.

Samsung MME supports high redundancy. The LEMA and LENA boards support 1:1

redundancy, while the LESA board operates in 2:1 redundancy mode.


The S-GW is the local mobility anchor. It is analogous to the 2G SGSN. It ‘anchors’ the

UE data connection during inter-eNB handovers. Packets may arrive for the UE when

it is idle. The S-GW then buffers them until the UE is active and ready to receive data.

It also initiates a network triggered service request in such cases. Besides, the S-GW

can also handle handovers to other 3GPP networks via the S4 interface. It facilitates

re-ordering of packets by sending an “end marker” to the eNB or source SGSN / RNC

after handover.

The S-GW helps in inter-operator charging via the S5 / S8 interface to the P-GW. It

supports transport level QoS functions such as IP Differentiated Services and

supports lawful interception of data packets.

A UE has one and only one S-GW at a time.


The P-GW is the EPC entry and exit point for packets to travel in and out of the LTE

network. It is analogous to the GGSN. Thus, the P-GW is connected to the external

Packet Data Network (PDN), for example the Internet, and to all other external

networks such as 3GPP, 3GPP2.

The P-GW allocates IP addresses to UE’s and performs all the policy control and

charging related functions. Naturally, it also performs DHCP functions. It plays an

important role in QoS and also supports IP Differentiated Services. The S-GW and P-

GW can be and often are combined into one physical node, called the SAE Gateway.


The Samsung SAE-GW supports P-GW functionalities such as Policy & Charging

Enforcement Function (PCEF). User and service based QoS control as well as flow

based charging are supported. In addition advanced features such as Deep Packet

Inspection (DPI) and HTTP Header Enhancement (HHE) are also supported. DPI refers

to the opening of a packet all the way up to the application layer, allowing operators

to monitor services. HHE refers to enhancements to the HTTP protocol header

allowing the operator to provide subscriber information (e.g. IMSI) to application

servers.

The LEMA and LENA cards have the same functions as in the MME. The LTE EPC Data

Processing Assembly (LEDA) performs the core SAE-GW functions of the user plane.


Shown here is a schematic of the RJIL 4G network deployment. Different levels of

Aggregation routers (AG’s) are deployed. Each eNodeB is connected via backhaul to

an Aggregator node, AG1. An AG1 pair (for redundancy) consists of 4 rings with 5

eNodeB’s per ring, i.e., a total of 20 eNB’s per AG1 pair. 10 AG1 rings (up to 4 AG1’s

per ring) connect to the AG2 level. The centralized scheduler is located at the AG2

level. Finally, an AG3 node supports 16 AG2 pairs. The EPC and IMS nodes are

deployed at the AG3 level.


The RJIL plan consists of 18 EPC’s for 22 circles. These 18 EPC’s are managed by one

of 4 regions – north, south, east and west. While each circle will have a Media

Gateway (MGW), most of the core IMS nodes will be deployed at the regional level.

At the highest level are 2 zones – Mumbai for the West and South zones, and Delhi

for the North and East zones. High level functionalities such as IMS applications, OSS

and billing will reside at the zones.


Spectrum is a scarce and costly resource. Continually increasing the spectrum cannot

be the only way to obtain high data rates. Cellular systems therefore rely on other

techniques to increase data rates:

• Link Adaptation: Modifying link transmission parameters to handle variations in

radio link quality. A well known example is Adaptive Modulation and Coding (AMC),

where UE’s in good radio conditions are given better data rates by modifying the

modulation and coding used

• Channel-dependant Scheduling: Minimize the amount of resources used keeping

the required QOS in mind

• Hybrid ARQ: Transmitters add redundant bits to the data (precaution before

transmission) and receivers request retransmission of erroneously received data

(improvement after transmission)

• Multiple Antennas: Diversity, beam-forming, MIMO


Multiple Input Multiple Output (MIMO) refers to the use of multiple antennas on the

transmit and receive side. It was first introduced in HSPA+, but has been part of LTE

since R8. In fact one of the strengths of OFDMA compared to WCDMA is that it

makes MIMO implementation feasible. MIMO is a general term often used to

describe two possibilities. The first one is Spatial Multiplexing, wherein two data

streams are sent on two antennas simultaneously, thereby increasing data rates. This

is typically used for UE’s in good radio conditions. It is often described as N x M

MIMO, where N is the number of transmitters and M the number of receivers.

Typical values are 2 x 2 and 4 x 4.

For UE’s that are at the cell-edge, the dominant problem is poor signal quality due to

interference. In this case, two copies of the same information can be sent to the UE.

This is Transmit Diversity and it helps reduce the interference experienced by the UE

.


LTE uses Orthogonal Frequency Division Multiple Access (OFDMA). The total carrier

bandwidth is divided into mutually orthogonal sub-carriers of fixed width – 15 KHz.

The small sub-carrier spacing implies large bit periods, making OFDMA robust

against multipath. This multiple access technique allows for allocation of different

bandwidths to different services.

The sub-carriers are orthogonal because the LTE pulse is designed so that, in

frequency domain, the peak power of one sub-carrier coincides with zeroes of all the

other sub-carriers as shown above. One can imagine a large number of finely tuned

radio stations transmitting simultaneously without interfering with each other. A

single OFDM symbol is constructed by parallel transmission of multiple sub-carriers

during a symbol time interval. A single sub-carrier during one OFDM symbol is called

a Resource Element (RE). This is the smallest physical resource in LTE.


Along with OFDMA, LTE also uses fast (of the order of 1ms), dynamic time-domain

scheduling done at the LTE base station, called the eNodeB. This allows for a highly

flexible allocation of radio resources on a time-frequency grid as shown above. The

basic unit of allocation is a Resource Block (RB) consisting of 12 sub-carriers (12 x 15

= 180 KHz) and a single time slot, 0.5 ms.

The fact that radio resources are managed completely by the eNodeB makes LTE a

single-node access network, reducing latencies and improving throughputs.


A problem with OFDMA systems is the high Peak-to-Average Power Ratio (PAPR).

The transmitted power in OFDM is the sum of the powers of all the subcarriers. Due

to large number of subcarriers, the peak to average power ratio (PAPR) tends to have

a large range. The higher the peaks, the greater the range of power levels over which

the power amplifier is required to work. This is a problem for use with mobile

(battery-powered) devices.

A new, OFDMA-based scheme called single carrier frequency division multiple access

(SC-FDMA) was developed for the LTE uplink. By restricting uplink transmissions to

smaller, contiguous parts of the carrier, SC-FDMA enables a lower UE peak-to-

average power ratio (PAPR) which eases amplifier design in the mobile devices.

43

Shown above is a time-frequency view of OFDMA and SC-FDMA. As can be seen,

OFDMA sends one modulation symbol on one sub-carrier in one symbol duration.

SC-FDMA sends one modulation symbol on N sub-carriers (4 in the above example)

in 1/Nth symbol duration. The number of sub-carriers N depends on the bandwidth

allocated to the UE.

44

The LTE UE is a stand alone device performing all the functions that are performed in

the network. At the lowest physical layer, it encodes and decodes data, performs

OFDM, modulation and demodulation, as well as supports multiple receive /

transmit. The UE updates the network of its location and sends measurements to

allow the network to make informed decisions, especially for handovers. The UE

must setup, maintain and teardown IP sessions with the LTE network. It must also

keep a record of the identities given to it by the eNB and the EPC.


LTE UE categories define the standards by which a particular handset, dongle or

other equipment will operate. Based on their maximum data rates and number of

multiple transmit / receive antennas, UE’s are assigned categories. Shown above are

the R8 / R9 UE categories.

There are five different LTE UE categories defined. As can be seen in the table above,

the different LTE UE categories have a wide range in the supported parameters and

performance. LTE category 1, for example does not support MIMO, but LTE UE

category five supports 4x4 MIMO. Note that despite supporting the same

modulation and MIMO, categories 2,3 and 4 support different maximum data rates.

This is because they differ in the number of maximum bits they can decode in each

TTI.

It is also worth noting that UE class 1 does not offer the performance offered by that

of the highest performance HSPA category. Additionally all LTE UE categories are

capable of receiving transmissions from up to four antenna ports.

51

PoC: Push to talk over cellular

Different services have different requirements of the network. Quality of Service

(QoS) is a way to differentiate between services based on these requirements so that

the network can serve users better. Shown above are some typical services used on

cellular networks. Each of these has its own QoS requirements. Generally, QoS is

understood in terms of three key factors: bit-rate, delay and error. Services are then

categorized based on their requirements of these three. For instance, real-time (RT)

services are strict about delay. If the RT service happens to be video, it also requires

high data rate and has low tolerance for errors. Web-browsing, on the other hand,

can tolerate some delay but is strict about error rates.

Since LTE supports a wide variety of services, it needs to have a QoS mechanism in

place to satisfy service requirements.


Before discussing the LTE QoS mechanism, it is important to understand bearers. A

bearer is essentially a logical connection that carries some data. The UE is logically

connected to different entities in the LTE network. With the E-UTRAN (i.e. eNB), the

UE has a radio bearer. With the EPC, the UE is connected to the P-GW via an EPS

bearer that is used to carry IP traffic with a certain QoS. Each EPS bearer is

associated with a set of QoS parameters.

A PDN connection is the IP session link between the UE and the external data

network (e.g. the internet). This is the level at which the UE is known by an IP

address. All EPS bearers belonging to one PDN connection share the same IP

address. While a PDN connection is equivalent to a session, in 2G terms, an EPS

bearer is equivalent to a PDP context.


When a UE first attaches to the network and establishes a new PDN connection, LTE

sets up a Default Bearer that remains active for the lifetime of that connection. A

Default Bearer is essentially a way to provide always on connectivity. Recall that

packet switched user devices frequently go into dormant modes. Having a Default

Bearer is a way to minimize the signalling required to start data transfer when

coming out of inactivity. This helps LTE achieve low signalling latencies.

Depending on services that the UE requests, a Dedicated EPS bearer may be set up.

This is essentially demand based. Since the Default bearer is associated with a set of

QoS parameters, any service that has different QoS requirements may require a

Dedicated Bearer.

From the QoS perspective, bearers can be Guaranteed Bit Rate (GBR) or non-GBR. A

GBR bearer is a way to commit a certain bit rate to the user. Default bearers are

always non-GBR. Dedicated bearers can be GBR or not, depending upon the service

QoS.


SDF: An aggregate set of packet flows that matches a set of filters.

http://wired-n-wireless.blogspot.in/2010/09/sdf-qci-and-dedicated-bearers.html

Since QoS is essentially a function of the service or application being invoked by the

user, LTE uses the notion of a Service Data Flow (SDF). An SDF is basically a set of IP

flows corresponding to a service. The P-GW receives IP flows from an external PDN

and performs Filtering, i.e., maps these IP flows to SDF’s. Each SDF corresponds to a

particular QoS treatment applied by the Policy Control & Enforcement Function

(PCEF), which is a P-GW functionality. An SDF thus corresponds to a QoS treatment.

An SDF aggregate is a combination of one or more IP flows. Since an SDF

corresponds to a QoS treatment and so does an EPS bearer, it follows that an EPS

bearer can carry only one SDF aggregate, i.e., QoS treatment remains the same

within a bearer. This is how the LTE network maps services to bearers based on QoS.


QCI- QoS Class Identifier

QoS in LTE networks operates at different levels. At the service level SDF’s are used

to apply QoS and other policies such as traffic conditioning to the IP flows. At the

session level, QoS is based on Access Point Names (APN’s) defined in P-GW. APN’s

correspond to the external network path that the P-GW will use for the service (e.g.

voice APN, Mobile TV APN). Typically, an APN will consist of multiple bearers. For

non-GBR bearers, the network uses an Aggregate Maximum Bit Rate (AMBR) to

enforce QoS. As the name suggests, AMBR regulates the total maximum bit rate of

multiple bearers set up on a given APN.

At the bearer level, the policy uses Binding. Binding essentially ties a bearer to a

Quality Class Indicator (QCI), which is a numeric value for QoS class. QCI is discussed

in the next slide.

Finally, at the UE level, similar to an APN, a per-UE AMBR is used to regulate the

QoS. Since the radio resource allocation and scheduling are done by the eNB, UE

level QoS is enforced by the eNB.


The QCI is a scalar mapping of a set of transport characteristics - bearer

with/without guaranteed bit rate, priority, packet delay budget, packet error loss

rate. It is used to infer node-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. 9 QCI levels are defined in the

Release 8 version of the specifications according to the level of service required by

the application. Each QCI implies a Packet Delay Budget (PDB) between the UE and

P-GW and a Packet Error Loss Rate (PELR).The QCI, together with Allocation-

Retention Priority (ARP) and, if applicable, Guaranteed Bit Rate (GBR) and Maximum

Bit Rate (MBR), determines the QoS associated with an EPS bearer.


Shown above are the key players in the PCC architecture.

PCRF: The Policy & Charging Rules Function decides on polices and charging based

on flows and conveys these decisions to the PCEF.

PCEF: The Policy & Charging Enforcement Function, residing in the SAE-GW, enforces

policies such as gating and QoS control on behalf of the PCRF.

BBERF: The Bearer Binding & Event Reporting Function, also in the SAE-GW, binds a

service flow to an IP bearer (in effect a QoS). It also reports events such as request

for a new service, especially to charging systems.

SPR: The Subscriber Profile Repository provides subscriber specific data to the PCRF

to assist in evaluating policy decisions.

AF: The Application Function represents applications, i.e., it provides application

level information to the PCRF assisting the PCRF in policy decision making.

The Gx, Rx and Sp interfaces are Diameter based.


The PCC functions just discussed are performed based on Rules. A PCC Rule is

essentially a concise way to detect SDF’s and capture PCC parameters for the PCEF to

enforce. An SDF can be identified by a flow descriptor, a 5-tuple consisting of the

source and destination IP and port addresses and the protocol type being used. The

protocol type and port are usually good indicators of the nature of the service and

can be used to give it a certain QoS treatment.


PCC Rules are broadly of two kinds. A Dynamic Rule, as the name suggests, can

change. Its definition comes to the P-GW from the Policy Control and Resource

Function (PCRF) via the Gx interface between the two. A Pre-Defined Rule, on the

other hand, is provisioned directly into the PCEF by the operator. Dynamic rules

allow adaptation to changes in traffic, for instance, a new service type.


Shown here is an example of policies applied to flows. The PCEF filters IP flows by

applying SDF templates, resulting in the grouping of IP flows into SDF’s. Then, using

policy rules, a policy is applied to each SDF. This covers QoS, charging and other

aspects of the service.


Like policy rules, charging has its own components. A Charging Key is used for an SDF

to determine the tariff to be applied to it. A Service Identifier establishes the identity

of the service or service component that the SDF in a rule relates to. This identifier

can then be used to support Flow-Based Charging. Thus, a users transactions on a

banking website are charged differently than streaming content.

An important consideration is whether charging is offline or online, or in fact the

service is free of charge. Also important is the method to measure usage – it can be

based on volume (e.g. per MB), duration (e.g. per unit time), a combination of the

two, or an event (e.g. activation).


Charging systems are broadly either Offline (OFCS) or Online (OCS). OCS’s can affect

services in real-time, especially important for pre-paid services. OFCS’s work offline

in the background, based on billing plans and billing cycles.

OCS (online charging system) provides credit management and grants credit to the

PCEF based on time, traffic volume or chargeable events.

OFCS (off-line charging system) receives events from the PCEF and generates

charging data records (CDRs) for the billing system.


An important development in the 3GPP cellular standards is the introduction of Self-

Organizing Networks (SON’s) in R8. Cellular networks have increased in complexity.

Multiple technologies, heterogeneous networks (i.e. macro, micro, pico and femto

cells), multiple vendors and multiple services in 3G and 4G make optimization that

much harder. SON’s address these issues.

Broadly, the processes taken on by SON fall under the two categories shown above.

Routine or repetitive processes such as configuration can be streamlined by

automating them. This leads to reduced OPEX and faster deployments. There may be

processes that are good candidates for automation because they are too fast or too

‘fine’ (e.g. per user, per flow) for manual intervention. Measurements from various

network devices, monitoring tools and users, along with algorithms that react to

these measurements can improve network performance. Such improvements (e.g.

adding of a missing neighbor) can happen fast, in near real-time, not being

dependent on manual intervention. They also Minimize Drive Testing (MDT), a costly

and time consuming exercise.


Shown here is a high-level view of the Samsung eNodeB self-configuration

mechanism. Samsung eNB’s support plug-and-play with IPV6.

1) The eNB detects an open port and acquires the eNB IP and the Samsung LTE

System Manager (LSM) IP addresses from the DHCP server on the universal OAM

VLAN.

2) Next, it connects to the LSM and gets the software image and configuration from

the LSM.

3) Once it has its configuration, the eNB now sets up two additional VLAN’s – one

for the S1-Control plane connection with the MME and a second one for the S1-

User plane and X2 interfaces with the SAE-GW and neighboring eNB’s,

respectively.


4) On the VLAN for S1-C, the eNB establishes an S1-C connection with the MME.

5) On the S1-u and X2 VLAN, it establishes the S1-U connection with the SAE-GW.

6) If it has a Neighbor Relations Table, the eNB then establishes X2 connections

with neighboring eNB’s on the S1-U and X2 VLAN.


The Samsung Automatic Neighbor Relations (ANR) functionality begins at

deployment of the eNB. During the initial configuration, as discussed earlier, the eNB

obtains a Neighbor Relations Table (NRT) from the LSM. Based on this, it sets up X2

connections with its defined neighbors.

Once operational, the Samsung eNB also performs self-optimization. This can be UE-

based, wherein the UE assists in obtaining neighboring cell information by reporting

it to the LSM. If such reporting is unavailable, the network, via the LSM, itself

recommends the identity and IP address of the neighbor. Based on handover

statistics, the Samsung eNB can rank as well as remove neighbors.


Earlier cellular generations used circuit-switched core networks for voice. LTE,

however, is completely packet-based. The routing of voice traffic in LTE therefore

requires approaches that are different from the earlier generations. Shown above

are two approaches to voice in LTE.

Circuit Switched Fallback simply leans on the earlier networks (2G / 3G) to handle

voice. As such, it is a temporary fix, typically used during early stages of deployment.

Voice over LTE, or VoLTE, uses the IP Multimedia Subsystem (IMS) network to offer

completely packet-switched voice to LTE users. This is the long-term solution for

voice. Another term often used in the literature is Single Radio Voice Call Continuity

(SR-VCC). ‘Single radio’ refers to the VoLTE advantage that the UE doesn’t have to

switch radios when making a voice call. ‘Continuity’ refers to the feature that when

an LTE user on a voice call moves out of LTE coverage, the handover to 2G / 3G

should be seamless.


Shown above is a view of the IP Multimedia Subsystem architecture and its

interfaces with the EPC. The signaling protocol used for VoIP is Session Initiation

Protocol (SIP). Below is a brief description of each node.

P-CSCF: The Proxy Call Session Control Function handles bearer establishment and

related functions. Being a proxy, it is the first point of contact for an IMS user device.

S-CSCF: The Serving CSCF holds user-specific data such as details of some services

etc., downloaded from the HSS. It terminates call/session signaling from UE and

interacts with the application & services area.

I-CSCF: Interrogates the HSS for calls terminating on the UE to determine which S-

CSCF will cater to the UE.

BGCF: Breakout Gateway Control Function is a SIP proxy that processes routing

requests from S-CSCF based on telephone numbers.


MGCF: Media Gateway Control Function controls the Media Gateway (MGW) using

the H.248 protocol. The MGW forms the media plane while the MGCF is the

signaling plane. The MGCF translates from SIP (packet switched) to ISUP (circuit

switched) and communicates with the PSTN via a Signaling Gateway (SiGW). The

MGW translates from RTP (Real-Time Transport Protocol) to circuit switched voice

(e.g. AMR).

MRFx: The Media Resource Function (MRF) provides media functionalities and is

also split into the Media Resource Function Control (MRFC) which forms the

signaling plane and the Media Resource Function Part (MRFP) which is the media

plane. The MRFC interprets signaling from the S-CSCF, while the MRFP processes

media.

Application Environment: The application servers consist of the Voice Call Continuity

(VCC), the Rich Communications Suite (RCS) and the Telephony Application Services

(TAS) servers.


Shown above is an overview of the steps involved in UE registration with the IMS.


Shown above is a step by step overview of the VoLTE call setup using IMS.


A natural question about VoLTE is what are its advantages over applications that

already provide VoIP services. First, VoLTE is more QoS aware. This means that it

enables prioritization of voice over other data by using the LTE QoS classes. Over the

Top (OTT) VoIP treats voice like any other data stream.

Secondly, VoLTE being native to the handset can leverage some of its capabilities

such as wideband codecs and dual microphone to enhance audio perception. It can

also use the native address book to offer Rich Communication Suite (RCS) services

such as Presence. Such advantages are not available to OTT VoIP.


Day 1 LTE Technology Overview

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