Did you ever try to understand the logic/mechanism behind the LTE technology?
Don't you have a basic idea of what exactly is it?
If this was new to you, don't worry!! We gonna learn it with the basics.
Note: All the information and resources are taken from different dedicated websites and channels like TutorialsPoint, TechPlayon, LTE video Tutorial, and many more.
What is LTE?
LTE stands for Long Term Evolution. It was a project started by a telecommunication body, 3GPP(Third Generation Partnership Project) in 2004 to increase the speed and capacity of wireless data networks using new DSP(Digital Signal Processing) techniques.
What is 3GPP?
3GPP is a partnership project bringing together national SDOs(Standards Development Organizations) from around the globe initially to develop technical specifications for the 3rd generation of mobile, cellular telecommunications, UMTS/3G(Universal Mobile Telecommunication System).What is DSP(Digital Signal Processing)?
It is a processor dedicated to number-crunching digital signals like audio. The technology is found inside headphones, smartphones, speakers etc.
LTE Network Architecture
The network architecture of LTE is comprised of the 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).*
UE (User Equipment)
It is any device used directly by an end-user to communicate. It can be a hand-held telephone, a laptop computer equipped with a mobile broadband adapter, or any other device.
In the standard, they were referred to as MS(Mobile Stations).
Downlink(DL): The communication from the base station to UE is called Downlink.
Uplink(UL): The communication from UE to the base station is called Uplink.
Base Station: A base station is a radio receiver/transmitter that serves as the hub of the local wireless network.
E-UTRAN
E-UTRAN stands for Evolved UMTS Terrestrial Radio Access Network. It is the combination of E-UTRA(Evolved Universal Terrestrial Radio Access) and **eNBs **or eNodeB(Evolved Node B/E-UTRAN Node B).
The E-UTRAN handles the radio communications between the mobile and the EPC(Evolved Packet Core).
Officially, E-UTRAN governs the base station, while E-UTRA defines the mobile device side. E-UTRAN uses the OFDMA(Orthogonal Frequency Division Multiple Access) modulation methods for the downlink and SC-FDMA(Single-carrier Frequency Division Multiple Access) for the uplink.
FDM(Frequency Division Multiplexing) - It is a multiplexing method used to divide a channel into many non-overlapping sub-channels. It also allows multiple users to share 1 single link.
Multiplexing: It is a process that combines the input signals into one signal to increase the efficiency of signal transmission.
- OFDM(Orthogonal Frequency Division Multiplexing) - In this method, a set of orthogonal(the peak point of a sub-carrier occurs at the NULL point of others.) sub-carriers overlap in the frequency domain, to increase the channel efficiency and reduce bandwidth consumption. Provide different frequencies at different intervals of time.
Subcarrier : It is a secondary modulated signal frequency, modulated into the main frequency (the carrier) to provide an additional channel of transmission.
- OFDMA(Orthogonal Frequency Division Multiple Access) - It is a special case of OFDM system that allow multiple users to share the available bandwidth and transmit simultaneously. To overcome the multipath fading, OFDMA is used.
Multipath fading occurs when signals reach a receiver via many paths & their relative strengths & phases change.
- SC-FDMA(Single-carrier Frequency Division Multiple Access) -FDMA has a very high Peak to Average Power Ratio (PAPR) that requires expensive and inefficient power amplifiers which drains the battery faster. To overcome this problem, the resource blocks are arranged in such a way that reduces the need for linearity and so power consumption.
What is E-UTRA?
It is a wireless communication technology/air interface that provides a high data rate, low latency communication link between mobile phones and base stations. E-UTRA was developed as a replacement to UMTS/3G, and HSDPA(High-speed packet access) standards.
Latency : the time, it takes for data from your device to be uploaded and reach its target.
HSPA (High-Speed Packet Access) is a 3G UMTS network enhancement that offers peak download speeds.
eNB : It is the radio network node for LTE networks, to which UE connects.
Note : NB is for 3G, eNB is for 4G, gNB is for 5G.
The eNodeB performs following functions :
Radio Resource Management(RRC) - It plays a significant role in controlling power consumption.
**Radio Bearer Control **- It is used for the transfer of RRC and NAS(Non Access Stratum) signaling messages.
Signaling : Used for call and messaging services.
**Connection Mobility Control **- The process of continually tracking the location of UE that are connected to a network.
Dynamic allocation of resources to LTE UEs in both Uplink and Downlink.
eNBs provides the E-UTRA User plane (PDCP/RLC/MAC/PHY) and **Control plane **(RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the S-GW(Serving Gateway) by means of the S1-U.
LTE Interfaces : The link between every 2 nodes is having a unique interfaces name. This interface name represents the protocols that the nodes are using in the communication between each other.
In order to understand the radio protocol architecture of LTE network, we need to understand the difference between control plane and user plane.
Control Plane : In this side, signaling messages are exchanged between base station and UE that are generated by RRC(Radio Resource Control) protocol.
User Plane : Data packets are processed by protocols such as TCP(Transmission Control Protocol), UDP(User Datagram Protocol), IP(Internet Protocol) that are exchanged between base station and UE.
What is an IP address?
An IP address is a unique address that identifies a device on the internet or a local network.
In both cases, the information is processed by the PDCP(Packet Data Convergence Protocol), the RLC(Radio Link Control) protocol and the MAC(Medium Access Control) protocol, before being passed to the Physical layer for transmission.
LTE Protocol Stack Layers
- PDCP(Packet Data Convergence Protocol) : When data comes from EPC(Evolved Packet Core), it is stored in PDCP buffer at eNodeB.
Header compression and header decompression of IP data packets - Data reaching to the eNodeB from EPC are IP packets. Each IP packet has a header of 40 bytes and this header file of 40 bytes do not have any meaningful information so sending this 40 bytes of header along with actual data on the interfaces is the wastage of resources.
Moreover, in case of web call in which size of actual data is very small compared to IP header. So, we need some mechanism to compress this header. This header is compressed by PDCP at transmitting end using **Robust header compression **(ROHC) protocol whereas at receiving end PDCP decompresses this header.
ROHC(Robust Header Compression) - The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite.
- Maintenance of PDCP sequence number - At transmitting end PDCP assigned sequence number to each packet before sending it to the next layer i.e. RLC(Radio Link Control). This sequence number is used by PDCP at receiving end to align packets in a sequential order before sending them to upper layers.
At the time of handover, PDCP source eNodeB send sequence number of the last packet, successfully received by the target eNodeB. So, the target eNodeB can start transmitting data to UE that packet onward. This way 2 eNodeB can synchronize data flow at the time of handover using PDCP sequence number.
Handover - It is a process where the user established session(e.g. call) must not be interrupted due to the change in cell coverage area when UE moves from one place to another.
- Security of the Data - If your data is not secure on the interface, some external body may interfere this data. So, it is important to secure this data by integrity and ciphering(a secret code, usually one that's created using a mathematical algorithm) before transmitting it on air interface.
2.** RLC**(Radio Link Control) :
- Concatenation, segmentation and reassembly of RLC SDUs(Service Data Unit) - Although header of IP packet is of fixed size i.e. 40 byte but the data part is variable. For example, in case of VoIP(Voice over Internet Protocol), data part is small but in case of video streaming, data part of IP packet is quite big.
Bandwidth available on air interface keeps on changing very frequently because of changing radio conditions and change in number of UEs under eNodeB, so RLC at transmitting end has to concatenate or segment these data packets dynamically whereas RLC at receiving end, does reassembly of this data.
SDU(Service Data Unit) : Packets received by a layer, called SDU.
PDU(Protocol Data Unit) : Packet output of a layer are called PDU.
VoIP(Voice over Internet Protocol) - VoIP sends the voice/data packets through the internet from the sender to the receiver.
- Retransmission of RLC PDUs - If the data gets corrupted on air interface then RLC, then transmitting entity has to send data packets again but this kind of transmission happens when RLC is on Acknowledgement(AM) mode.
For TM(Transparent Mode) of data, there is no addition or removal of header to/from input data.
For AM(Acknowledge Mode), header addition or removal, segmentation and re-assembling, re-transmission of buffer, all things are applicable.
For UM(Unacknowledge Mode), header addition or removal, segmentation and re-assembling are applicable.
- Reordering of RLC data PDUs - Because of multiple transmissions, order in which packets are received at the receiver could be random so RLC has to reorder all the packets before sending them to the upper layer. This happens in both AM(Acknowledgement) and UM(Unacknowledged) mode.
3.MAC(Medium Access Layer) :
Prioritisation among various data streams for a given UE - Data packets of different streams have different priority based on their type. For example, packets of VoIP call are of highest priority where available bandwidth on the interface is not enough to carry all the data streams. So, there should be some entity that selectively sends data which belongs to highest priority stream on air interface. This role is performed by MAC.
- Error correction through HARQ(Hybrid Automated Repeat Request) -
ARQ(Automated Repeat Request) : It is an error control mechanism for data transmission which uses ACK(Acknowledgement) or NACK(Negative Acknowledgement) and timeouts to achieve reliable data transmission over an unreliable communication link.
In the process, the receiver used an error detection code, typically a Cyclic Redundancy Check (CRC), to detect whether the received packet is in error. If no error is detected in the received data, the transmitter is notified by sending a positive acknowledgement. If an error is detected, the receiver discards the packet and sends a negative acknowledgement to the transmitter, and requests a re-transmission.
An Acknowledgement (ACK) or Negative Acknowledgement (NACK) is a short message sent by the receiver to the transmitter to indicate whether it has correctly or incorrectly received a data packet, respectively.
Timeout is the time interval after the sender sends the data packets.
CRC(Cyclic Redundancy Check) - CRC is an error-detecting technique commonly used in digital networks and storage devices to detect accidental changes to raw data.
HARQ(Hybrid Automated Repeat Request) : It is the combination of high rate FEC(Forward Error Correction) and ARQ(Automated Repeat Request) error control. Inspite of discarding the packets, it stores in buffer and send request for re-transmission from the sender and then combine the new received data with previous error data.
FEC(Forward Error Correction) : FEC method is used to protect data against errors in the transmission over unreliable or noisy communication channels, such as mobile wireless channel.
PHY(Physical Layer) :
Encodes raw data before modulation - Raw data go through encoding process before modulation. This process is needed so that receiver can detect any error in the data caused during the transmission on air interface. If FEC(Forward Error Correction) techniques are used, the receiver can crack the data at its end, if there is some corruption in the data. If the data is recorrected at the receiver end then there is no need to retransmit the data.
Measures the air interface to know channel quality - Due to mobility, multiple reflections from the surroundings and noise on air interface, available bandwidth on the interface keeps on changing very frequently. So, there is need of some mechanism to measure the air interface quality to know the current channel bandwidth.
RRC(Radio Resource Control) : It initialises other layers like PDCP, RLC, MAC and PHY then actual transmission takes place.
- Broadcast of the system information - In a cell, MIBs(Master Information Block)/SIBs(System Information Block) are broadcast by RRC layer. MIB and SIB contains all the cell and network related information, whenever an UE is down or moved in an IDLE(Integrated Development and Learning Environment) mode, it decodes its broadcast information to get all the cell parameters such as bandwidth, etc.
Note : When UE does not have any RRC connection, it is said to be in IDLE(Integrated Development and Learning Environment) mode.
MIB(Master Information Block) : It is a very important message or information that is broadcasted by the LTE eNodeB irrespective of any user presence.
SIB(System Information Block) : It is one important message that the UE will be looking to decode from eNodeB's broadcast.
- RRC connection control - RRC layer of UE triggers RRC connection with eNodeB.
Note - It is obligatory for UE to have RRC connection before sending data packets to network.
- State Transition - RRC layer in UE, triggers state transition of UE from IDLE mode to connected mode and vice versa. When there's an activity in data flow for a particular UE for a long time, RRC layer of eNodeB triggers RRC connection release of such UE and sends such UE in IDLE state. State transition is performed to save battery and air interface resources.
Connected Mode : In the Connected Mode, the UE constantly communicates with the network and so eNodeB knows its location at the cell level.
- Paging - Whenever there is an empty voice call or data to be sent to any UE, RRC layer of eNodeB sends paging message for that UE in the UE tracking area.
Paging : It is one-to-one communication between the mobile and the base station. In this process, the network uses to find out a subscriber's location before actual call establishment and then send an alert to the mobile station of an incoming call.
- Initial security activation - At the time of RRC connection setup, security keys are exchanged with RRC layer of eNodeB and UE.
Measurement configuration and reporting - In connected mode, an UE has to do many types of measurement such as inter-frequency, intra-frequency and internet measurement. All these measurement are configured in UE by RRC layer of eNodeB. RRC layer of UE reports all the measurements to RRC layer of eNodeB, based on these measurements RRC layer of eNodeB takes all the handovers related decisions.
NAS(Non Access Stratum) : It is a part of control plane, composed of EMM(EPS Mobility Management) and ESM(EPS Session Management) layers.
- EMM(EPS Mobility Management) - It takes care of ability related scenarios such as tracking area control, paging, security mode control and authentication.
Authentication : Authentication is a process by which UE and Network check, if the other party has right authority to communicate each other.
- ESM(EPS Session Management) - It takes care of bearer related scenario such as default EPS bearer context activation, EPS bearer conetxt modification, PDN connectivity procedures, PDN disconnect procedures.
Classification of Channels in LTE
Broadly in LTE Channel are divided into three categories named as below:
**Logical channels **(What type of Information)
Transport channels (How this information is transported)
Physical Channels (Where to send this information)
These all three types of channel are present in Downlink as well as Uplink direction.
(A) Logical Channel : It define the data-transfer services offered by the MAC layer. Data and signaling messages are carried on logical channels between the RLC and MAC layers.
Logical channels further can be divided into two categories :
Control channels - Carry signaling messages in the control plane.
Traffic channels - Carry data in the user plane.
There are 10 logical channels : 7 in Downlink and 3 in Uplink.
(a) Downlink
Broadcast Control Channel (BCCH) – It Used for broadcasting MIBs/SIBs.
Paging Control Channel (PCCH) – It is used for paging the UE.
Common Control Channel (CCCH) -It is Common to multiple UE’s.
Dedicated Control Channel (DCCH) – It used to transmit dedicated control information for a particular UE.
Multicast Control Channel (MCCH) – It is used for transmit information for Multicast.
Multicast : This technology helps content owners to develop a customized viewing experience for a mass audience.
Dedicated Traffic channel (DTCH): Dedicated Traffic for a particular UE.
Multicast Traffic Channel (MTCH): Used to transmit Multicast data.
(b) Uplink
**Common Control Channel **(CCCH) -It is Common to multiple UE’s.
Dedicated Control Channel (DCCH) – It used to transmit dedicated control information for a particular UE.
Dedicated Traffic channel (DTCH): Dedicated Traffic for a particular UE.
(B) Transport Channels : Data and signaling messages are carried on transport channels between the MAC and the physical layer.
There are 6 Transport channels.
(a) Downlink
Broadcast Channel (BCH) : This LTE transport channel maps to Broadcast Control Channel (BCCH) and carries information like used for MIB and send information to Physical Broadcast channel (PBCH).
**Downlink Shared Channel **(DL-SCH) : It is the main channel for downlink data transfer. It is used by many logical channels like BCCH, CCCH, DCCH DTCH,MCCH,MTCH and send its information to Physical Downlink Shared Channel (PDSCH). The information carried by this channel is SIB.
Paging Channel (PCH) : To convey the PCCH information and mapped to Physical Downlink Shared Channel (PDSCH) and carries Paging Information.
Multicast Channel (MCH) : It is used to transmit MCCH information to set up multicast transmissions. This channel is mapped to Physical Multicast Channel (PMCH).
(b) Uplink
Uplink Shared Channel (UL-SCH) : It is the main channel for uplink data transfer. It is used by many logical channels like CCCH, DCCH DTCH.
Random Access Channel (RACH) : This is used for random access procedure.
(C)Physical Channel : Both data and signaling messages are carried on physical channel in LTE.
It is further classified into 2 channels -
Physical Data channels (DL, UL)
Physical Control Channels (DL,UL)
(a) Downlink
Downlink physical Data Channel (PBCH, PDSCH,PMCH)
2. Downlink Physical Control Channel (PCFICH,PHICH,PDCCH)
**Physical Broadcast Channel **(PBCH) : It carries system information(MIB) for UEs requiring access to the network.
**Physical Downlink Shared Channel ** (PDSCH): The PDSCH can carry DL-SCH or PCH. It carries SIB information, Paging Information and user plan Data.
**Physical Multicast Channel **(PMCH) : This channel type is used to carry MCH and mainly used for MBMS Services.
Physical Control Format Indicator Channel (PCFICH) : It informs the UE about the format of the signal being received. It indicates the number of OFDM symbols used for the PDCCH channel, whether 1, 2, or 3.
The information within the PCFICH is essential because the UE does not have prior information about the size of the control region (PDCCH). A PCFICH is transmitted on the first symbol of every sub-frame and carries a Control Format Indicator (CFI).
Physical Downlink Control Channel (PDCCH) : It carries information known as the Downlink Control Information (DCI) . It carries the control information for a particular UE or group of UEs.
Physical Hybrid ARQ Indicator Channel (PHICH) : This channel is used to report the Hybrid ARQ(Automated Repeat Request) status. It carries the HARQ ACK(Acknowledgement)/NACK(Negative Acknowledgement) signal indicating whether a transport block has been correctly received.
(b) Uplink :
Uplink physical Data Channel ( PUSCH,PRACH)
Uplink Physical Control Channel (PUCCH)
Physical Uplink Shared Channel (PUSCH) : This physical channel is used for Uplink data transmission by the UE. They may also carries the uplink control information sometimes.
Physical Random Access Channel (PRACH) : this is used for random access procedure called RACH procedure. UE does RACH procedure to get the Uplink synchronization.
Physical Uplink Control Channel (PUCCH) : This provides the various control signaling. These signaling are known as Scheduling request, Downlink data ACK/NACK and CQI(Channel Quality Indicator) information.
EPC
EPC(Evolved Packet Core) : The EPC is divided into different parts :-
- MME(Mobility Management Entity) : It is the main component of the SAE(System Architecture Evolution).
SAE(System Architecture Evolution) - It is a new network architecture designed to simplify LTE networks and establish a flat architecture similar to other IP based communications networks.
- Handles UE mobility in IDLE mode - When connected in IDLE mode, eNodeB and MME can locate the tracking area of the UE.
Tracking Area : It is a logical grouping of cells in LTE networks.
Maintains UE context during RRC IDLE state - Transition from IDLE to connected mode or vice versa is local to UTRAN and is entirely control by eNodeB. MME does not know anything about this state transition. But during all the transitions, UE context information about all the services used by it is maintained by MME.
NAS signaling and NAS signaling security - MME is responsible for NAS signaling and security of NAS messages.
Does bearer management for the UE
Bearers are the tunnels used to connect the UE to Packet Data Networks(PDNs) such as the Internet.
2.SGW(Serving Gateway) :
**Acts as mobility anchor for the data bearers **when UE moves between eNodeB's.
Buffers the downlink data when UE is in IDLE mode - When UE is in IDLE mode then there is no connection between UE and eNodeB. At the time, SGW first downlink packages coming from PDN Gateway.
3.PGW(PDN Gateway) : PDN stands for Packet Data Network.
Allocation of IP address to the mobile - When UE is connected to the LTE network, an IP is allocated to the UE. The IP is allocated by PGW.
Filtering of downlink user IP packets into different QoS based Bearers - Different services in UE such as VoIP call, browsing, video streaming, etc have different delay requirements. For example, VoIP call is very delay sensitive whereas browsing is not very delay sensitive. So, PDN Gateway filters the data coming from the internet based on their delay requirements.
QoS(Quality of Service) : It refer to the measurement of the overall performance of a service experienced by the users of the network.
4.** PCRF**(Policy and Charging Rule Function) :
- Takes charging enforcement decisions - PCRF charge subscriber based on their volume of uses of high bandwidth applications. Also lowers the bandwidth of the wireless subscribers using heavy bandwidth apps during peak usage time.
5.** HSS**(Home Subscriber Service) : This part manage subscriber's data and authentication in a flexible way across all the technologies(2G, 3G, 4G, Wifi-network), while also delivering a high QoS to their subscribers and ensuring quick and smooth introduction of new and innovative services.
IP Network
It comprises of IMS(IP Multimedia Subsystem) and Internet.
IMS(IP Multimedia Subsystem) : This framework is used for delivering multimedia communication services such as voice, video and text messages over IP network. Data exchange occurs in the form of packets, even for voice calls.
The END!!!
I hope this BLOG is INFORMATIVE to you.
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