How SMS works in LTE

SMS stands for short message service.The “short” part refers to the maximum size of the text messages: 160 characters (letters, numbers or symbols in the Latin alphabet). For other alphabets, such as Chinese, the maximum SMS size is 70 characters.

There are two ways to implement SMS in LTE. The ideal solution would be doing SMS chat using IMS.IMS over LTE is specified to transfer any form of data (e.g, voice, SMS and any other form of multi media data), IMS is still in its early stage as many network operators have not implemented IMS over their networks.

The other solution is called SG LTE. (It is like we have CS Fallback as an interim solution before they fully implement voice call over IMS).

The implementation logic of SG-SMS is very similar to WCDMA SMS. In WCDMA, we injected the SMS message into a DCCH channel and send it to the destination. It means that we carried the message over a control channel, not over a data channel. SG SMS is also using a similar concept, we send the message over a control channel. For Example,
i) UE <– NW : RRC Connection Reconfiguration
ii) UE –> NW : RR Connection Reconfiguration Comlete
iii) UE <– NW : dlInformationTransfer (embedd the SMS message – CP Data – into this message)
iv) UE –> NW : ulInformationTransfer (embedd CP-ACK into this message)

So the SMS is actually encapsulated in the NAS signaling protocol used between the UE and MME, and forwarded to and fro the MSC/VLR whereby the UE was registered in the combined EPS/IMSI attach procedure. This new interface between MME and MSC/VLR is called the Sgs interface. All entities supporting CSFB (MME, MSC and UE) are required to support SMS via Sgs although the reverse is not true (entities supporting SMS via Sgs not necessarily have to support CSFB).

SMS via Sgs is relatively easy to implement, involving only the MME, MSC and UE, out of which only the MSC belongs to the legacy CS network. SMS via Sgs also do not have known issues or deployment complexity.

EPS – The Network Architecture


Nodes Description

Evolved UTRAN

The evolved RAN for LTE consists of a single node, i.e., the eNodeB (eNB) that interfaces with the UE.  The eNB hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.


Serving Gateway (SGW) The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state UEs, the SGW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.

Mobility Management Entity (MME)

The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN)
node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.

Packet Data Network Gateway (PDN GW)
The PDN GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs. The PDN GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening. Another key role of the PDN GW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).

PCRF – Policy and Charging Rules Function

Policy and Charging Rules Function (PCRF) is the software node designated in real-time to determine policy rules in a multimedia network. This particular component of LTE is responsible for supporting the detection of service data flow, the charging system based on this data flow, and policy enforcement.Unlike earlier policy engines that were added on to an existing network to enforce policy, the PCRF is a software component that operates at the network core and accesses subscriber databases and other specialized functions, such as a charging system, in a centralized manner.Because it operates in real time, the PCRF has an increased strategic significance and broader potential role than traditional policy engines. This has led to a proliferation of PCRF products since 2008.

Orthogonal Frequency-Division Multiple Access (OFDMA)

With Orthogonal Frequency-Division Multiple Access (OFDMA) muliple access is achieved by assigning subsets of sub-carriers to individual users. This allows simultaneous low data rate transmission from several users.OFDMA simultaneously supports multiple users by assigning them specific subchannels for intervals of time.So in short,

  • Each terminal occupies a subset of sub-carriers
  • Subset is called an OFDMA traffic channel
  • Each traffic channel is assigned exclusively to one user at any time
  • The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique
  • OFDMA allows different users to transmit over different portions of the broadband spectrum (traffic channel)
  • Simpler Reciever – Only FFT processor is required
  • Bit Error Rate performance is better only in Fading environment

Orthogonal Frequency Division Multiplexing – OFDM

OFDM – Orthogonal Frequency Division Multiplex, the modulation concept being used for many wireless and radio communications radio applications from DAB, DVB, Wi-Fi and Mobile Video.OFDM is a special case of Frequency Division Multiplexing.Orthogonal frequency division multiplexing (OFDM) has been shown to be an effective technique to combat multipath fading in wireless communications.

Concept – Divide and Rule..

  • An OFDM signal consists of a number of closely spaced modulated carriers.
  • When modulation of any form – voice, data, etc. is applied to a carrier, then sidebands spread out either side.
  • It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data.As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM.
  • Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another.

Understand.. How it works?

The main concept is orthogonality. 

Area Under a sine wave is always 0
Area Under a sine wave is always 0

You would recall that the functions, sin(nx), cos(nx) : n = 1, 2, 3, … are orthogonal with respect to Riemann integration on the intervals [0, 2π], [-π, π], or any other closed interval of length 2π. This fact is a central one in Fourier series. Corresponding to it the area under a sine(or a cosine) wave will be always 0.

So by using the same concept we can transmit, using a single transmitter, a set of frequency multiplexed signals with the exact minimum frequency spacing, to make them orthogonal so that they do not interfere with each other.In OFDM, the subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the subchannels is eliminated and intercarrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver (No Filters and No Guard bands.. well theoretically ). Unlike in conventional FDM, a separate filter for each subchannel is not required.

Data in OFDM

  • The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier.
  • The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.