# <center><i class="fa fa-edit"></i> Learning the Fundamentals : 4G LTE Systems </center>
###### tags: `Pre-Internship`
:::info
**Goal:**
To gain a basic understanding of the basics of Cellular Networking by observing how 4G-LTE works
- [x] How 4G Works
- [x] How 4G Differs from Previous Generations of Technologies
- [x] How Radio Access Network is implemented in 4G
- [ ] How 5G improves from 4G

**Resources:**
[LTE Tutorial](https://www.tutorialspoint.com/lte/index.htm)
[LTE Youtube Channel](https://www.youtube.com/channel/UCf5srFJ-JofnE8r-bn1o1VA)
:::

## Background

LTE stands for Long Term Evolution and it was started as a project in 2004 by telecommunication body known as the Third Generation Partnership Project (3GPP). SAE (System Architecture Evolution) is the corresponding evolution of the GPRS/3G packet core network evolution. The term LTE is typically used to represent both LTE and SAE.

LTE evolved from an earlier 3GPP system known as the Universal Mobile Telecommunication System (UMTS), which in turn evolved from the Global System for Mobile Communications (GSM). Even related specifications were formally known as the evolved UMTS terrestrial radio access (E-UTRA) and evolved UMTS terrestrial radio access network (E-UTRAN). First version of LTE was documented in Release 8 of the 3GPP specifications.

## 4G Key Parameters
Here's the summary of 4G key Parameters compared to 3G :


### Frequency Range (2 - 8 GHz)
Is the unit on what frequencies does the antenna for signal transmission works. Is taken into consideration in Networking because frequency is directly related to how much [Path Loss](https://www.electronics-notes.com/articles/antennas-propagation/propagation-overview/free-space-path-loss.php#:~:text=It%20is%20possible%20to%20calculate,of%20the%20frequency%20in%20use.) is taken after the signal travels in a certain distance, which relates to the optimal **operating range**, and also the [Data Rate](https://www.flukenetworks.com/blog/cabling-chronicles/bandwidth-and-data-rates#:~:text=The%20relationship%20of%20speed%20to,scheme%20of%20the%20active%20equipment.), which directly relates to Networking Speed.


### Bandwidth ( 5 - 20MHz)
The bandwidth determines how much capacity is available on a certain channel to send or receive data (and is also correlates to data rate). The correlation between bandwidth and capacity can be approximated by the Nyquist Formula :
Needless to say, its almost always better to have higher bandwidth.

### Modulation (QPSK, 16QAM, 64QAM With OFDM)

LTE uses the popular orthogonal frequency division multiplex (OFDM) modulation scheme. It provides the essential spectral efficiency to achieve high data rates but also permits multiple users to share a common channel. OFDM divides a given channel into many narrower subcarriers. The spacing is such that the subcarriers are orthogonal, so they won’t interfere with one another despite the lack of guard bands between them. This comes about by having the subcarrier spacing equal to the reciprocal of symbol time. All subcarriers have a complete number of sine wave cycles that upon demodulation will sum to zero.


In LTE, the channel spacing is 15 kHz. The symbol period therefore is 1/15 kHz = 66.7 µs. The high-speed serial data to be transmitted is divided up into multiple slower streams, and each is used to modulate one of the subcarriers. For example, in a 5-MHz channel, up to 333 subcarriers could be used but the actual number is more like 300. A 20-MHz channel might use 1024 carriers. The modulation on each can be quadrature phase-shift keying (QPSK), 16-phase quadrature amplitude modulation (16QAM), or 64-state quadrature amplitude modulation (64QAM) depending on the speed needs.


## 4G Architecture
The high-level network architecture of LTE is comprised of following three main components:

1. The User Equipment (UE).
2. The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
3. The Evolved Packet Core (EPC).

The evolved packet core communicates with packet data networks in the outside world such as the internet, private corporate networks or the IP multimedia subsystem. The interfaces between the different parts of the system are denoted Uu, S1 and SGi as shown below:


***The User Equipment (UE)***
The internal architecture of the user equipment for LTE is identical to the one used by UMTS and GSM which is actually a Mobile Equipment (ME). The mobile equipment comprised of the following important modules:

1. Mobile Termination (MT) : This handles all the communication functions.
2. Terminal Equipment (TE) : This terminates the data streams.
3. Universal Integrated Circuit Card (UICC) : This is also known as the SIM card for LTE equipments. It runs an application known as the Universal Subscriber Identity Module (USIM).

A USIM stores user-specific data very similar to 3G SIM card. This keeps information about the user's phone number, home network identity and security keys etc.

***The E-UTRAN (The access network)***

The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN) has been illustrated below:


The E-UTRAN handles the radio communications between the mobile and the evolved packet core and just has one component, the evolved base stations, called eNodeB or eNB. Each eNB is a base station that controls the mobiles in one or more cells. The base station that is communicating with a mobile is known as its serving eNB.

LTE Mobile communicates with just one base station and one cell at a time and there are following two main functions supported by eNB:

1. The eNB sends and receives radio transmissions to all the mobiles using the analogue and digital signal processing functions of the LTE air interface.
2. The eNB controls the low-level operation of all its mobiles, by sending them signalling messages such as handover commands.
3. Each eNB connects with the EPC by means of the S1 interface and it can also be connected to nearby base stations by the X2 interface, which is mainly used for signalling and packet forwarding during handover.

A home eNB (HeNB) is a base station that has been purchased by a user to provide femtocell coverage within the home. A home eNB belongs to a closed subscriber group (CSG) and can only be accessed by mobiles with a USIM that also belongs to the closed subscriber group.

***The Evolved Packet Core (EPC) (The core network)***

The architecture of Evolved Packet Core (EPC) has been illustrated below. There are few more components which have not been shown in the diagram to keep it simple. These components are like the Earthquake and Tsunami Warning System (ETWS), the Equipment Identity Register (EIR) and Policy Control and Charging Rules Function (PCRF).


Below is a brief description of each of the components shown in the above architecture:

The Home Subscriber Server (HSS) component has been carried forward from UMTS and GSM and is a central database that contains information about all the network operator's subscribers.

The Packet Data Network (PDN) Gateway (P-GW) communicates with the outside world ie. packet data networks PDN, using SGi interface. Each packet data network is identified by an access point name (APN). The PDN gateway has the same role as the GPRS support node (GGSN) and the serving GPRS support node (SGSN) with UMTS and GSM.

The serving gateway (S-GW) acts as a router, and forwards data between the base station and the PDN gateway.

The mobility management entity (MME) controls the high-level operation of the mobile by means of signalling messages and Home Subscriber Server (HSS).

The Policy Control and Charging Rules Function (PCRF) is a component which is not shown in the above diagram but it is responsible for policy control decision-making, as well as for controlling the flow-based charging functionalities in the Policy Control Enforcement Function (PCEF), which resides in the P-GW.

The interface between the serving and PDN gateways is known as S5/S8. This has two slightly different implementations, namely S5 if the two devices are in the same network, and S8 if they are in different networks.

## Explaining 4G with Correlation to OSI Layer
### Layer 3
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.

**Non Access Stratum**
The Highest stratum of the **control plane**. The following functions exist in the non-access stratum:
1. Mobility management: maintaining connectivity and active sessions with user equipment as the user moves
2. Call control management
3. Session management: establishing, maintaining and terminating communication links
4. Identity management

In other words , The layer supports traffic and signalling messages between the CN and UE.

**IP**
Contains user data generated by the upper layers. In other words, the origin of **User Plane Data**.

**Radio Resource Control**
The main services and functions of the RRC sublayer include broadcast of System Information related to the non-access stratum (NAS), broadcast of System Information related to the access stratum (AS), Paging, establishment, maintenance and release of an RRC connection between the UE and E-UTRAN, Security functions including key management, establishment, configuration, maintenance and release of point to point Radio Bearers.

More on RRC states can be read [Here](https://www.3gpp.org/ftp/tsg_ran/WG2_RL2/TSGR2_06/Docs/Pdfs/r2-99807.pdf)

### Layer 2
The data link layer, or layer 2, is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the physical layer.

**Packet Data Convergence Control**
PDCP receives User Traffic, or PDCP SDUs from the IP layer, and performs IP compression by removing IP Header and adding tokens of 1-4 bytes ([More on IP compression](http://www.effnet.com/pdf/Whitepaper_Header_Compression.pdf)). PDCP in LTE has additional functions like integrity protection and ciphering.

**Radio Link Control**
Radio Link Control/ RLC layer does segmentation of these SDUS to make the RLC PDUs. RLC adds header based on RLC mode of operation. RLC submits these RLC PDUs (MAC SDUs) to the MAC layer.

Segmentation of RLC can be defined as logical categorization of data into their respecting channels (**logical channels**), there are 5 channels for control plane data, and 2 channels for user plane data with each forwarding different types of message to the MAC Layer :
1. Paging Control Channel (PCCH) : Contains paging message (when the eNB wishes to contact an idle UE).
2. Broadcast Control Channel (BCCH) : For Broadcasting System Control Information , including system bandwidth, antenna configuration, reference signal power. Contains a lot of Signalling Data.
3. Common Control Channel (CCCH) : Used by UE to establish connection with eNB when the device is switched on.
4. Dedicated Control Channel (DCCH) : After the connection has been setup with UE, DCCH is used to transmit control info.
5. Multicast Control Channel (MCCH) : Broadcasting Control Information for all UEs in the same cell.
6. Dedicated Traffic Channel (DTCH) : Carries actual traffic User Data.
7. Multicast Traffic Channel (MTCH) : Carries broadcasting message, such as Real Time Broadcasting of Sports.


**Medium Access Layer**

MAC layer is responsible for Mapping between logical channels and transport channels, Multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels, de multiplexing of MAC SDUs (which is RLC PDUs) from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels, Scheduling information reporting, Error correction through HARQ, Priority handling between UEs by means of dynamic scheduling, Priority handling between logical channels of one UE, Logical Channel prioritization.

There are 6 **Transport Channels**, which the MAC Layer maps into :
1. Broadcast Channel : MAC forwards Master Information Block data (contains system critical info)from BCCH into this channel.
2. Downlink Shared Channel : MAC Multiplexes data from System Information Block of BCCH, Downlink Data of DCCH, and MCCH, Downlink data of DTCH, and MTCH.
3. Paging Channel : MAC forwards data from PCCH into this channel.
4. Multicast Channel : Forwards data from MTCH.
5. Uplink Shared Channel : Contains Uplink Data from all Uplink logical channels, which is CCCH, Uplink DCCH, and DTCH.
6. Random Access Channel : Has no corresponding logcal channel, is used when user is not synchronized in UPLINK direction, used to establish synchronization.


Besides Multiplexing the different datas from logical channel, other functions of MAC Layer are :
1. Logical Channel Prioritization (MAC Layers to prioritize these Transport channels)
2. HARQ Retransmission (Automatic retransmission of incomplete datas)

### Layer 1
The physical layer or layer 1 is the first and lowest layer. This layer may be implemented by a PHY chip.The physical layer defines the means of transmitting raw bits over a physical data link connecting network nodes. This is the layer where data is modulated and transmitted through AIR interface.

Physical Control Channels are listed in the below table:

From Layer 3 to Layer 1 , the overall route can be illustrated in this image :

## The Brain of 4G : MAC Scheduler

Sitting just above the Physical layer, the MAC Scheduler assigns bandwidth resources to user equipment and is responsible for deciding on how uplink and downlink channels are used by the eNodeB and the UEs of a cell. It also enforces the necessary Quality of Service for UE connections. QoS is a set of rules that come from the Policy and Charging Rules Function (PCRF) in the core network. These rules define priority, bit rate and latency requirements for different connections to the UE. They is usually based on the types of applications using the UE connection. For example, the QoS requirements for a VoLTE call are different from those for checking the e-mail.

As seen in the image below, the MAC scheduler has control over the OFDM modulation in the sense that it decides, according to information received from other LTE network components, how much bandwidth each UE receives at any given moment. In this figure, the resource element (sub-carrier) is represented on the frequency axis, while the sub-frames are represented on the time axis.

Most Importantly, MAC scheduler also determines the type of modulation used (QPSK, 16QAM, or 64QAM) by processing the radio channel conditions.