EMM, ECM, RRC States, TAU and Handovers

Its very interesting to observe the connection between 5 terms mentioned in the subject line.

Refer here and here for details on EMM, ECM and RRC states.

EMM has two states:- EMM Registered and EMM De-registered. When a UE is connected to the network it moves from EMM De-registered to EMM Registered. Also the ECM state moves to ECM connected from ECM idle. Before these two happening the RRC state in UE is moved from RRC idle to RRC connected. So when UE is actively connected to the network the states in UE/network are RRC Connected, ECM Connected and EMM Registered.

When UE moves to Idle mode first the RRC connection is released. That is RRC state moves from RRC connected to idle. Then ECM state moves to ECM idle from connected. This mean eNB has released the RRC connection and network has released the UE bearer context information but EMM state is Registered. This is termed as S1 release in 3GPP TS 23.401. It would be interesting to know that ECM idle state is valid only on S1 and S11 interface. The UE information is actively maintained in PGW but is released in eNB and MME. Also the behavior of network in ECM idle mode is different in different cases. (future posts). The basic idea of idle mode is UE is not know to the network and in connected mode UE is known to the network. The term "known" refers to UE location, bearer information etc.

TAU - Tracking Area Update. A tracking area is a group of cells that are being tracked by a SGW. When UE is connected to the network, a list of tracking areas are sent to it. When a UE moves to new tracking area it will trigger a tracking area update procedure as defined in 3GPP TS 23.401. A TAU can be triggered when UE is in idle mode or connected mode.

Now at any given point of time UE can be in idle or connected mode. If UE is in connected mode and moving from one place to other then handovers take place. Note that handovers are transparent to UE except that UE now camps on to a new eNB. Also handovers are network initiated. This means a handover can happen when the UE is known to the network, i.e when UE is in connected state. Also after the handover if UE finds that it is present in new tracking area then it can initiate a tracking area update.

But a UE need not be in a connected state all the time. What happens when a UE is in idle mode and is moving from one place to other? When UE is in idle mode, MME will have the UE's last know location. So if some data arrives to UE, while it is in idle mode, MME will initiate paging process to find UE. Now if UE has moved from the last know location how will the MME know the UE's new location. This is where TAU becomes extremely important. In this case it is the responsibility of UE to inform about its location to network. This is done by sending TAU. As I said when UE is attached to the network MME sends a list of tracking areas. So if MME wants to find UE i.e is in idle mode, it will simply send paging message to the eNB and eNB will page for the UE. But if UE is in new tracking area, that is not in the list received during initial attach, it will initiate a TAU. Once network receives TAU it will store the UE's location info.

To initiate a TAU UE should have a RRC connection. First thing it does is it camps on to a nearest eNB by doing cell re-selection process. After RRC connection is established UE may send TAU to network. Note that ECM state in network is still idle for that UE. But TAU may move the ECM state in network from idle to connected. (future post)

Thus if we look at overall picture a handover can happen when UE is in EMM Registered, ECM connected and RRC connected state. A TAU may be initiated by UE when it is in EMM Registered, ECM idle and RRC idle state. Both the procedures help UE in its movement from one place to other.

LTE Initial Setup

Reference:

3GPP TS 36.213: EUTRAN Physical Layer Procedures

3GPP TS 36.331: EUTRAN RRC Protocol Specification

LTE Cell Search:

When the UE is powered up it needs a network to attach itself. The first towards it is Cell search. Cell Search is a procedure by which a terminal can find a potential cell to attach too.

As a part of cell search procedure the terminal obtains the identity of cell and estimates the frame timing of the identified cell. LTE supports 510 different cell identifiers divided into 170-cell identity group of 3 identities each.

LTE provides two signals in downlink;

-       Primary Synchronization Signal

-       Secondary Synchronization signal.

In first step of cell search, UE uses primary sync signal to find the timing on 5 ms basis. This signal is transmitted twice in each frame(as LTE frame is of 10 ms).

Terminal can use this signal to identify the frame timing with a 5 ms ambiguity. Here terminal locks it local oscillator frequency to the base station carrier frequency. The terminal also finds an identity within the cell. It also obtains partial knowledge about reference signal structure.

In the next step terminal detects the cell identity group and determines the frame timing using secondary synchronization signal.

Random Access Procedure

To transmit data terminal needs a connection setup with the network. So a terminal has to ask for one. Random access procedure is used to establish uplink and unique terminal ID.

-       First step consists of UE transmitting a Random Access Preamble allowing the eNB to estimate the transmission timing of the terminal.

-       In the next step network transmits a Random Access Response. This consists of timing advance command to adjust the terminal transmit timing, based on timing measurement received in the first step. In addition to establish uplink synchronization this step also assigns uplink resources to be used in next steps to the terminal. Temporary identity is also assigned to UE for further communication with the network. This response is sent on PDCCH.

-       Third step consists of transmission of mobile terminal identity to the network using UL-SCH. The exact content of this signal depends on the state so of terminal whether the network previously knows it or not. (RRC_IDLE)

-       4^th step consists of contention resolution message from network to terminal on DL-SCH.

RRC Procedures

There are two RRC states in LTE. RRC_Idle & RRC_Connected.

In RRC_Idle there is no signaling radio bearer established, that is there is no RRC connection.

In RRC_Connected there is a signaling radio bearer established

Signaling Radio Bearers(SRB) are defined as Radio bearers that are used only to transmit RRC and NAS messages. SRB’s are classified into

Signaling Radio Bearer 0: SRB0: RRC message using CCCH logical channel.

Signaling Radio Bearer 1: SRB1: is for transmitting NAS messages over DCCH logical channel.

Signaling Radio Bearer 2: SRB2: is for high priority RRC messages. Transmitted over DCCH logical channel.            

RRC Procedures:

-       Paging

o   To transmit paging info/system info to UE in RRC_IDLE state.

-       RRC Connection Establishment

o   The purpose is establishing SRB1.

o   This procedure is initiated by UE when upper layers requests of a signaling connection when UE is in RRC_IDLE mode.

-       RRC Connection Reconfiguration

o   The purpose is to establish/modify/release radio bearers.

o   Also to perform handovers

o   Network initiated procedure(?)

-       RRC Connection Re-Establishment

o   To re-establish RRC connection which involves SRB1 resumption and reactivation.

-       Initial Security Activation

o   Activate security upon RRC establishment.

o   eNB initiated procedure.

RRC release procedure.

LTE: Physical Layer

I couldn't control my desire to learn the LTE physical layer, so I pushed everything aside and started reading 36 series specs. The Radio network of LTE looks fairly simple at a glance but the complexity increases as we go deep, just like any other system.

LTE Uu interface is what I am looking at. eNB behaves like a relay mapping the radio network to the IP network. The IP side consists of an interface towards MME over S1_MME and towards SGW over S1_U. The radio side communication also has two planes, user plane and control plane.

The above figure shows EUTRAN user plane. As we see we have a MAC layer, RLC and PDCP. Individual protocols shall be dealt with later. The user plane looks fairly simple as data from UE goes to eNB and eNB maps this data over GTP tunnel and sends it to SGW over S1_U. MAC, RLC and PDCP are at Layer 2 in UE and eNB.

However control plane adds few more things. L3 comes into picture for NAS signaling. This NAS signaling is carried all over to MME inside RRC signal.

Over PDCP we have RRC layer which is responsible for paging,RRC connection management, mobility functions etc etc. RRC is terminated in eNB. But NAS is terminated in MME. NAS is responsible for EPS bearer management, Authentication, security etc. Attach Request is a NAS signal which is carried all the way to MME.

Stepping few layers below we have PHY which is physical layer. This is where the actual engineering is. The whole concepts of high speeds come into picture because of sophisticated physical layer. Its no secrete what technologies are used here. OFDMA with 64 QAM and 2x2 MIMO is the most discussed combination for LTE. How does this combination give us such high speeds?

QAM : Quadrature Amplitude Modulation

Going back to engineering basics, we have a simple modulation scheme called PSK. Phase shift keying, which is analog to digital modulation scheme(transmitter side). In PSK we have 1 bit per symbol .0 and 1. Each bit is associated with a Phase shift. with 4 Phase shifts we can transmit 2 bits per symbol. As with 64 QAM we shall be able to transmit 6 bits per symbol. If we look at this scheme in the given bandwidth, by changing the modulation scheme, we are able to transmit more and more bits. This is resulting in increase of data rates.

Time to look at Shannon's theorem :

As I said above, changing the modulation scheme gives you more throughput. However hight modulation schemes can be only be used when the signal to noise ratio is high. From above theorem, channel capacity is bandwidth multiplied by logarithm of SNR. Higher the SNR higher is the channel capacity which means more throughput.

Second factor which increases channel capacity is bandwidth. Now bandwidth is directly proportional to symbol rate. Higher the symbol rate then higher is the bandwidth. But again, increasing the symbol rate doesn't increase the channel efficiency as channel bandwidth is fixed because available spectrum is finite. So there is a trade off between symbol rate and channel throughput. The basic idea is keeping on increasing the symbol rate(modulation scheme) doesn't always improve the efficiency. So considering these factors I think 64 QAM should be a suitable choice for LTE.

OFDM : Orthogonal Frequency Division Multiplexing

With above in mind lets head to OFDM. The theory behind OFDM is little confusing. Lets understand the below figure (FDMA).

Consider we have X amount of spectrum. This can be divided into channels of each Y amount of bandwidth. Each channel is separated by Guard band to avoid interference. This is basic idea in normal multiplexing schemes. I believe in CDMA we identify each channel by a code (?). So what is happening is we have equally spaced channels occupying the entire bandwidth. Note that these channels are non overlapping. Each channel has a subcarrier(?).

In OFDM: With OFDM systems, it is possible to increase throughput in a given channel without increasing channel bandwidth or the order of the modulation scheme. This is done using digital signal processing methods that enable a single channel to be created out of a series of orthogonal subcarriers. As below figure illustrates, subcarriers are orthogonal to one another such that the maximum power of each subcarrier corresponds with the minimum power (zero-crossing point) of the adjacent subcarrier. In a typical system, the bit stream for a channel is multiplexed across various subcarriers. These subcarriers are processed with an inverse Fourier transform (IFT) and combined into a single stream. As a result, multiple streams can be transmitted in parallel while preserving the relative phase and frequency relationship between them.

This way we can include more number of subcarriers in a given bandwidth thus increasing the overall system throughput.

MIMO : Multiple Input Multiple Output

The Shannon's theorem above is assumed to have 1 transmitter and 1 receiver antenna. If we consider multiple antennas then the theorem could be modified as

Thus in theory increasing the antennas will effectively increase the channel capacity without any change in available bandwidth. Now what we can do with MIMO is increase SNR by transmitting a unique bit stream using multiple antenna in the same channel. This is called Spatial Multiplexing.

With MIMO systems, the bit stream is multiplexed to multiple transmitters without changing the symbol rate of each independent transmitter. Thus, by adding more transmitters, we can increase the throughput of the system without affecting the channel bandwidth.

Thus the combination of OFDMA, MIMO and QAM will give us more bandwidth and higher data rates in LTE. The source for this post comes from various places and it would be stupid of me to post the names of the text books. Next, the above is my understanding of the system, kindly correct me if there are any mistakes. Will appreciate it.

Hope, this was helpful, more to come soon and comments are greatly welcomed.