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Long Term Evolution
Long Term Evolution (LTE) is a 4G wireless broadband technology
developed by the Third Generation Partnership Project (3GPP). "Long Term
Evolution" because it represents the next step (4G) in a progression
from GSM, a 2G standard, to UMTS, the 3G technologies based upon GSM.
- Easily deployable network technology.
- High speed, low latency.
- Low coverage range.
- Can work on different frequency bandwidth.
LTE and 4G are two different things. LTE deployed in india first was not
fully 4G instead its technical term was “3.9G”. It’s because full 4G speed
and services are quite difficult to achieve in india due to various
interruptions and LOS issue with the geographical conditions not supporting
LTE is a completely different technology but it is compatible with previous
versions of networks.
Before moving on to the core part of LTE we must discuss how the
evolution of network took place.
The main parts of the 2g network being BTS, BSC, MSC.
BTS dealing with the radio link protocols and handling the radio interface,
BSC managing the radio resource for one or more BTS and MSC dealing
with the Authentication, location and all about the subscriber profile.
Works at a frequency band of 900MHz and 1800MHz. Uses TDMA and
FDMA, gives speed of 9.6kbps and has a carrier frequency of 200KHz.
HSCSD dedicates 4 timeslots for data connection, is inefficient as it ties up
resources even when nothing sent i.e. channel wasted.
Is known for voice services.
Then came the idea of packet switching, in accordance with the voice
network, a packet switched network was implemented with it that only
focused on data network hence increasing the performance by load sharing.
It dedicates 8 time slots and resources are not tied up all the time. Once the
data usage was not in use, the channel resources would be released and can
be used by others in need. Offers speed up to 115kbps, based on CDMA
only. Carrier frequency remaining same.
- Also known as EDGE (enhanced data rates for global evolution).
- Uses 8PSK modulation,
- 3x improvements in data rate on short distances.
- Can fall back to GMSK for greater distances.
- Combine with GPRS (EGPRS)~ 384 kbps.
In 3G network, BTS is replaced by node B, BSC by RNC (radio network
control) while the rest remaining same. Node B is enhanced as it uses low
voltage, less size and more users occupied. The RNC connects to the Circuit
Switched Core Network through Media Gateway (MGW) and to
the SGSN (Serving GPRS Support Node) in the Packet Switched Core
Network, while also managing the radio transceivers in the node b
equipment, as well as management tasks like soft handoff thus doing work
of both the BSC and PCU.
This tech. is more flexible as it supports 5 major radio technologies:-
FDMA, CDMA accounts for IMT-DS(direct speed) and IMT-MC(multi
carrier),TDMA accounts for IMT-TC(time code) and IMT0SC(single
It is based on WCDMA technology. Each UE is allocated a downlink
‘Spreading-code’ such that no two UE’s have a mutual common component
in the spreading code i.e. they will always be orthogonal to each other.
Hence, data bits can be transmitted to multiple UE’s within the same time
slot in the downlink, as they are mutually orthogonal.
This allows better usage of spectrum as now it can serve many more UEs
within the same frequency.
3G is faster than 2G because it uses more bandwidth.
GSM uses carrier of 200KHz and the 50MHz band used is divided into two:
25MHz each for uplink and downlink. 25MHz/200KHz gives 125 ARFCN
channels where one is reserved for guard band at start and stop.
Under one frequency band there are 8 time slots and these frequency bands
are guarded by additional and reserved bandwidth of 100KHz at start and
stop of the FDMA frame.
It is not to be believed that CDMA and WCDMA are almost same and can
be used in each other technologies. WCDMA is not derived from CDMA
and is only used for 3G. As indicated by the word wideband, WCDMA uses
a much wider bandwidth than that of CDMA. WCDMA uses frequency
bands that are 5MHz wide compared to CDMA where each frequency band
is only 1.25MHz wide. Despite this, both technologies still use code
division to create a greater number of channels within the same given
bandwidth and only the algorithms used vary and not the basic concept
In 3G it uses WCDMA and HSPA at a freq band of 5MHz unlike GSM
(200KHz). This frequency band is given to all users which are
differentiated by unique spreading codes.
Data rate = Spectral efficiency (bits/Hz) x available bandwidth (Hz)
As we can see, with the available carrier bandwidth of 5Mhz and ability to
accommodate more users and increasing spectral efficiency for a same
modulation scheme, 3G will be 25 times faster than 2G.
One of the important differences between 2G and 3G is that while on a call
in a 2G network it does not supports data connectivity simultaneously, i.e.
only one at a time. ‘E’ sign goes off as soon as we switch to a voice call.
In 3G, we can use both at a same time, data can be accessed while we are
on a call.
H+ or 3.5G
High Speed Packet Access (HSPA) is an amalgamation of two mobile
telephony protocols, High Speed Downlink Packet Access (HSDPA) and
High Speed Uplink Packet Access (HSUPA) that extends and improves the
performance of existing WCDMA protocols
3.5G introduces many new features that will enhance the UMTS technology
- Adaptive Modulation and Coding: The modulation scheme and coding are
changed on a per-user basis, depending on signal quality and cell usage.
The initial scheme is quadrature phase-shift keying (QPSK), but in good
radio conditions 16QAM and 64QAM can significantly increase data
throughput rates. With 5 Code allocation, QPSK typically offers up to
1.8 Mbit/s peak data rates, while 16QAM offers up to 3.6 Mbit/s.
Additional codes (e.g. 10, 15) can also be used to improve these data rates
or extend the network capacity throughput significantly.
- Fast Scheduling: Each user device continually transmits an indication of
the downlink signal quality, as often as 500 times per second. Using this
information from all devices, the base station decides which users will be
sent data in the next 2 ms frame and how much data should be sent for each
user. More data can be sent to users which report high downlink signal
- Backward compatibility with 3G
- Enhanced Air Interface
Uses evolved HSPA or HSPA+ via beam forming and MIMO.
Beam forming: Focuses the transmitter power of an antenna in a beam
towards the user’s direction.
MIMO: Multiple input multiple outputs use multiple antennas at sending
and receiving sides.
More over 2G has a very good coverage area as compared to others. It
works on a frequency band of 900MHz and 1800MHz.
3G works on 2100 MHz frequency band.
The problem with the operational frequency band is that, the higher the
frequency the less will be the coverage area of that network.
This is why 2G network is widely available in india.
Frequency is inversely proportional to the wavelength of the signal.
800-900 MHz frequency band is the best band available for usage. The
band below this range is occupied by the army and ministry.
This is why 3G and 4G towers are implanted a near distances and more
frequently with each other as they work on a higher frequency band thus
providing less coverage. However, if 3g and 4g networks are given a low
frequency band, they won’t require to install more towers for radiation
which can be less harmful.
It is very difficult to do so as 2g network is still mostly used in india
because still now a large number of people use handsets that are not
compatible with the 3g and 4g networks.
Once every UE is capable of adapting between 3g and 4g, 2g network will
be shifted on a higher frequency band.
It is to be remembered that by ITU standards LTE is in fact 3.9G, in other
words still a 3G technology but a very fast and advanced one, approaching
but not achieving all the 4G requirements.
- Uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink.
- Uses Single Carrier Frequency Division Multiple Access (SC-FDMA) for
- Uses Multi-input Multi-output (MIMO) for enhanced throughput.
- Reduced power consumption.
- Higher RF power amplifier efficiency (less battery power used by
- Promises data transfer rates of 100 Mbps
- Based on UMTS 3G technology
- Optimized for All-IP traffic
EPS = EUTRAN + EPC
EUTRAN: Evolved Universal Terrestrial Radio Access Network
It involves UE and eNodeB.
EPC: Evolved Packet Core
It involves MME, SGW, P-GW, HSS and PCRF
Evolved NodeB (eNB)
The primary difference between 2g/3g and E-UTRAN architectures is the
absence of a radio network controller (RNC)/base station controller (BSC).
Those functionalities are now moved to eNBs. It is responsible for following
- Radio resource management (RRM) functionality eg. radio bearer
- IP header compression and encryption of the user data stream.
- Uplink/downlink radio resource allocation.
- Transfer of paging messages over the air.
- Transfer of broadcast control channel (BCCH) info over the air.
- Selection of MME during a call.
- Mobility control in the connected state.
- Control and processing of RF measurements.
An eNB is able to communicate with different MME in order to enable load
sharing and redundancy.
Mobile Management Entity (MME)
MME is the key control node for LTE access network. It is responsible for
tracking and paging procedure including retransmissions, and also for idle
mode of User Equipment (UE). MME is also involved in bearer activation
and its deactivation procedures, to its task also belongs choosing the SGW.
The MME tracks and maintains the current location of all the UE’s that have
registered with the LTE network. It also involves in the target MME
selection for inter-MME handovers.
MME is also termination point of ciphering and integrity protection for NAS
It is also responsible for playing a vital role in user authentication and
communicating with HSS, which enables the transfer of subscription and
authentication data to the MME for authenticating data to the MME for
authenticating a user’s access to the network.
MME functions include:
Tracking Area list management;
Mapping from UE location (e.g. TAI) to time zone, and signalling a
UE time zone change associated with mobility;
PDN GW and Serving GW selection;
MME selection for handovers with MME change;
SGSN selection for handovers to 2G or 3G 3GPP access networks;
Roaming (S6a towards home HSS);
Bearer management functions including dedicated bearer
Lawful Interception of signalling traffic;
Warning message transfer function (including selection of appropriate
Serving Gateway (SGW)
Serving GW is the gateway which terminates the interface towards E-
UTARN. For each UE associated with the EPS, at given point of time, there
is a single Serving GW.
SGW is responsible for handovers with neighboring eNodeB's, also for data
transfer in terms of all packets across user plane. To its duties belongs taking
care about mobility interface to other networks such as 2G/3G. SGW is
monitoring and maintaining context information related to UE during its idle
state and generates paging requests when arrives data for the UE in
downlink direction. (E.g. somebody's calling).
Packet Data Network Gateway (PGW)
It is the node that connects the UE to external PDNs and acts as the UE’s
default router. A UE may be connected to multiple PDNs via one or more
PGW is responsible to act as an "anchor" of mobility between 3GPP and
non-3GPP technologies. PGW provides connectivity from the UE to external
PDN by being the point of entry or exit of traffic for the UE. The PGW
manages policy enforcement, packet filtration for users, charging support
Possible to use non-3GPP technologies are: WiMAX, CDMA 1X and
The PDN gateway is responsible for allocation of an IP address to the UE
during default EPS bearer set up.
Home Subscriber Server (HSS)
The HSS is a user database that stores subscriber information to support
other call control and session management entities. It is a storehouse for user
identification, numbering and service profie.
It is mainly involved in user authentication and authorization. During
registration, the MME talks to the HSS via S6a interface. The HSS generates
security information for mutual authentication, integrity check and ciphering
and can also provide information about the user’s physical location.
Policy and Charge Rules Function (PCRF)
It is the main QoS control entity in the network.
The PCRF functionalities include policy control decision and flow-
based charging control.
It is responsible for building the policy rules that will apply to a user’s
services and passing the rules to the P-GW via the Gx interface.
The policy rules indicate whether the P-GW should grant resource
reservation requests and if it is allowed to process packets for a given
The PCRF may use the subscription information as a basis for the
policy and charging control decisions.
Talking about the interfaces now, there are two types of interface:
Control plane- that is used for signaling and specific radio functionality.
User plane- used to transmit or exchange information.
2G, 3G and LTE Inter-Connectivity
The following are LTE Interfaces:
S1-MME :- Reference point for the controlplane protocolbetween E-
UTRAN and MME.
S1-U:- Reference point between E-UTRAN and Serving GW for the
per bearer user plane tunnelling and inter eNodeB path switching
S3:- It enables user and bearer information exchange for inter 3GPP
access network mobility in idle and/or active state.
S4:- It provides related control and mobility supportbetween GPRS
Core and the 3GPP Anchor function of Serving GW. In addition, if
Direct Tunnel is not established, it provides the user plane tunnelling.
S5:- It provides user plane tunnelling and tunnel management between
Serving GW and PDN GW. It is used for Serving GW relocation due
to UE mobility and if the Serving GW needs to connect to a non-
collocated PDN GW for the required PDN connectivity.
S6a:- It enables transfer of subscription and authentication data for
authenticating/authorizing user access to the evolved system (AAA
interface) between MME and HSS.
Gx:- It provides transfer of (QoS)policy and charging rules from
PCRF to Policy and Charging Enforcement Function (PCEF) in the
S8:- Inter-PLMN reference point providing user and controlplane
between the Serving GW in the VPLMN and the PDN GW in the
HPLMN. S8 is the inter PLMN variant of S5.
S9:- It provides transfer of (QoS)policy and charging control
information between the Home PCRF and the Visited PCRF in order
to supportlocal breakout function.
S10:- Reference point between MMEs for MME relocation and MME
to MME information transfer.
S11:- Reference point between MME and Serving GW.
S12:- Reference point between UTRAN and Serving GW for user
plane tunnelling when Direct Tunnel is established. It is based on the
Iu-u/Gn-u reference point using the GTP-U protocolas defined
between SGSNand UTRAN or respectively between SGSNand
GGSN. Usage of S12 is an operator configuration option.
S13:- It enables UE identity check procedure between MME and EIR.
SGi: - It is the reference point between the PDN GW and the packet
data network. Packet data network may be an operator external public
or private packet data network or an intra operator packet data
network, e.g. for provision of IMS services. This reference point
correspondsto Gi for 3GPP accesses.
Rx: - The Rx reference point resides between the AF and the PCRF in
the TS 23.203.
SBc: - Reference point between CBC and MME for warning message
delivery and control functions.
Few technicalterms and advantages ofLTE:
- EPS is a connection-oriented transmission network and, as such, it
requires the establishment of a “virtual” connection between two
endpoints (e.g. a UE and a PDN-GW). This virtual connection is called
as a bearer. It provides a “bearer service”, i.e. a transport service with
specific QoS attributes.
- In 2g/3g in casethe MSC fails due to overload or anything, the whole
data connection breaks down with the network while in LTE after
authorization from MME once the connection is made with SGW it will
always stay on regardless of the failure of MME
The data traffic will then be handled on S1-U interface between E-
UTRAN and S-GW.
- LTE is all IP based. It’s all data is routed via IP addressing. Each UE
gets a unique IP address while using LTE.
- Currently LTE doesn’t support calls it is only used for data services.
Whenever we make a call being on LTE network it switches back to 3g
or 2g network and this is called as CSFB (circuit switching fall back).
- One of the major and important factors in LTE is latency, i.e. packet
transmission and arrival is very less and fast. Also the jitter is very low.
It is all possible because of the less process time in the network
architecture. Since IP addressing requires NATTING for IPv4 addresses,
it take a lot of time processing and converting public IP’s to private and
The idea of removing the NATTING from this process will increase the
latency and hence less jitter. It is under process and possible due to
availability of IPv6 addresses which won’t require NATTING as every
UE will get its own public/private IP addresses. This is an also important
feature that is going to support voice over LTE network (VoLTE).