LTE: QoS and Bandwidth

I want to touch base on two concepts here. I am quite not clear on things at this moment, may be some one can help!

Concept 1:

LTE QoS: Default bearers, Dedicated bearers, TFT and Bearer QoS (QCI, ARP, MBR, GBR) are the QoS variables in LTE (any more?). I am very much aware how these behave in EPC, but I am unable to map the same with UE. Assume a default bearer has been established. Note that there is no TFT associated with default bearer, but there is a bearer level QoS present in default bearer creation. This QoS might limit the bit rate on the network side. And these QoS values are indicated to the UE through a NAS message (Activate Default Bearer Context Request?). So far so good. Now UE wants to have a dedicated bearer for particular application. UE requests for a dedicated bearer using Bearer Resource Allocation NAS message which contains the TFT, or to say traffic flow aggregates. Once this message reaches EPC, PGW consults PCRF and allocates a QoS value to this particular TFT. The QoS allocated for the TFT is signaled to UE in Activate Dedicated Bearer Context Request NAS message. Now UE has the QoS values. The big confusion is how are these QoS values limiting the data flow from UE?

My theory: Basically there is QoS concept in air interface (what?). All the QoS rules are imposed by eNB, which means the bandwidth is controlled on the eNB side. So even if the UE gets all the spectrum it will not be able to do much as the bandwidth on the network side is limited. I believe the QoS for TFT is indicated to UE so that it can also regulate the usage of spectrum. Right? Totally out of mind?

Concept 2:

Considering the above "my theory" to be correct lets try and understand how service provider can offer services. In India there is no 3G yet (except for BSNL) on mobile phones. How ever there are mobile operators whose networks are 3G ready and have started offering 3G speeds over USB sticks (TATA and Reliance). When I spoke to their customer care executives I was told the USB sticks come with a limited data plan. They also told me that this restriction was imposed by the government. The restriction can be explained using "my theory". If we start offering unlimited data service over USB sticks people might start consuming all the spectrum all the time leaving nothing for others to use. Base stations are always throttling. This can be avoided by placing data usage limit or time limit. Good move.

On the other hand, all the Blackberry's in my office run on EDGE. These handsets are provided with unlimited data usage plans. This means I can connect a Blackberry to laptop and start using it as a modem and get unlimited access to internet. Only problem as of now is these devices are running on EDGE which means low speeds.

The authorizing body is imposing data limit usage on USB sticks while allowing unlimited access on mobile phones. What will happen when a 3G phone arrives? If the network is 7.2 Mbps ready and so is the device then people can get unlimited access using mobile phone at high rates. Its just a matter of USB cable which can give high rates on laptop too. So what will the plans be when 3G arrives here? I ask my boss, "Hey with 3G you will get 3 Mbps speed but you will be able to do only 3 GB per month". This actually surprised him and he said I would rather stay on EDGE and get unlimited access. People using blackberry's dont want to fall into limited usage schemes as its frustrating to keep monitoring how much data they have used all the time. These things seem quite contradictory.

I have no idea how things are going to turn up. Can anybody enlighten me? Thanks!

New revision of 3GPP Specs

Folks, September revision of LTE specs are out. I could download latest revision of 3GPP TS 29.274 - v8.3.0 (GTPv2) spec but looks like rest of the specs are not on 3GPP website yet. I am guessing all the specs should be available in next two days as some final discussions are going on in email threads. The CR list for this spec is huge, will try and see what all has changed. If you are working on LTE then it is the time to start looking at these new specs.

NSN Finlad Folks!

Any one from NSN Finland reading the blog? Would really like to have a word. Thanks. My email ID can be found in About Me section.

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 Radio Interface

LTE is not complete without the radio interface. It has been my burning desire to understand the radio network of LTE. I did some research and this is the second post on radio side of the network.

LTE Radio Interface User Plane protocols

In downlink data from SAE will enter eNB. The data is an IP packet. The IP packet is several protocols and is passed to UE.

LTE Radio Interface Control Plane Protocols

The control has two more layers over PDCP. RRC layer is terminated at eNB, while NAS layer goes all the way to MME.

Lets take a look at each layer individually: -

NAS: Non-Access Stratum (3GPP TS 24.301)

NAS is responsible for EPS bearer management, authentication, paging and mobility handling in ECM IDLE state.

RRC: Radio Resource Control (3GPP TS 36.331)

This layer is responsible for Broadcast and paging. It also takes care of RRC connection management, radio bearer control, mobility functions and UE measurement reporting and control.

PDCP: Packet Data Control Protocol (3GPP TS 36.323)

This layer is responsible for IP header compression to avoid unnecessary overhead in the payload. This layer is also responsible for ciphering and integrity protection check.

RLC: Radio Link Control (3GPP TS 36.322)           

RLC is responsible for segmentation/concatenation, retransmission handling and in sequence delivery of messages to higher layers. RLC offers services to PDCP in form of radio bearer. These radio bearers are mapped to EPS bearers in EPC.

MAC: Media Access Control (3GPP TS 36.321)

Mac handles ARQ, uplink and downlink scheduling. The scheduling functionality is located in eNB. There is one MAC entity per cell for both uplink and downlink. The HARQ is present in both UE and eNB. MAC offers services to RLC inform of logical channels.

Physical Layer: (3GPP TS 36.201)

It handles coding/decoding, modulation/demodulation, multiple antennas etc. It offers services to MAC layer inform of transport channels.

LTE Channels Over view: (3GPP TS 36.300)

LTE Physical Channels: Downlink Channels

-       Physical Broadcast Channel: PBCH

-       Physical Control Format Indicator Format: PCFICH

o   This informs UE about number of OFDM symbols used for the PDCCH’s.

o   This is transmitted in downlink.

-       Physical Downlink Control Channel: PDCCH

o   Informs UE about resource allocation of PCH & DL-SCH and HARQ information related to DL-SCH

o   PCH: Paging channel. DL-SCH: Downlink Synchronization Channel.

-       Physical Hybrid ARQ Indicator Channel: PHICH

o   Carries Hybrid ARQ Ack/NAK’s in response to uplink transmission

-       Physical Downlink Shared Channel: PDSCH

o   Carries DL-SCH and PCH

-       Physical Multicast Channel: PMCH

o   Carries Multicast channel (MCH).

Uplink Channels

-       Physical Uplink Control Channel: PUCCH

o   Carries HARQ ACK/NAK in response to downlink transmission

o   Carries scheduling request.

-       Physical Uplink Share Channel: PUSCH

o   Carries UL-SCH

-       Physical Random Access channel: PRACH

o   Carries random access preamble.

LTE Transport Channels

The physical layer offers information transfer services to MAC and higher layers. The physical layer transport services are described by how and with characteristics data is transferred over the radio interface. (What kind of data is transferred is dealt in logical channels)

Downlink Transport Channels:

-       Broadcast channel: BCH

o   This channel is used to broadcast info in the entire cell.

o   It has fixed and pre defined Transport Format (not aware of TF’s yet)

-       Downlink Shared Channel: DL-SCH

o   This channel is used for transmitting downlink data.

o   It supports HARQ, dynamic link adaptation.

o   It I can be used to broadcast data in entire cell.

o   It supports UE discontinuous reception (DRX) to enable power saving in UE.

o   It also supports MBMS transmission.

-       Paging Channel: PCH

o   Used for transmitting paging information.

o   PCH supports DRX so that UE can sleep and wakeup to receive PCH in specific time intervals.

-       Multicast Channel: MCH

o   This channel is used to support MBMS.

Uplink Transport Channels:

-       Uplink Shared Channel: UL-SCH

o   Supports HARQ

o   Counter part of DL-SCH

-       Random Access Channel: RACH

Transport and Physical Channel Mapping

Downlink Channels:

Uplink Channels:

LTE Layer 2:

LTE layer 2 is split in MAC, RLC and PDCP.

Layer 2 Structure of downlink

Layer 2 Uplink Structure

The communication between two sub-layers is marked with circles. These are called Service Access Points (SAP). SAP between Physical layer and MAC sub-layer provides the transport channels. The SPA’s between MAC and RLC provide logical channels. Multiplexing several logical channels (i.e radio bearers) to same transport channel is preformed by MAC sub-layer.

Logical Channels:

MAC sub layer offers different kind of data services to RLC inform of logical channels. Logical channels define what type of data is transferred between UE and eNB. Logical Channels are classified into Control Channels (for control plane information transfer) and Traffic Channels (for transfer of user plane data)

Control Channels:

-       Broadcast Control Channel: BCCCH

o   This channel is used of broadcasting system control information.

o   This is downlink channel.

-       Paging Control Channel: PCCH

o   Downlink channel.

o   Transfers paging information and system information change notification.

o   This channel is used for paging when the network does not know the location cell of the UE.

-       Common Control Channel: CCCH

o   Channel of transmitting control information between UE and network.

o   This channel is used for UE’s having no RRC connection with the network.

-       Multicast Control Channel: MCCH

o   Point to Multi point downlink channel used for transmitting MBMS control information from the network to UE.

o   This channel is only used by UE’s that receive MBMS.

-       Dedicated Control Channel: DCCH

o   A point-to-point bi directional channel that transmits dedicated control information between a UE and the network.

o   Used by UE’s having an RRC connection.

Traffic Channels:

-       Dedicated Traffic Channel: DTCH

o   Uplink and downlink channel.

o   Point-to-point channel dedicated to one UE for transfer of user data.

-       Multicast Traffic Channel: MTCH

o   Point-to-Multipoint downlink channel for transmitting traffic data from network to UE.

Mapping logical and transport channels:

Uplink

Downlink

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.

LTE Handovers

Below are possible handover scenarios in LTE. Any more scenarios you can think of? Let me know.

X2 Based handover without SGW change

X2 Based Handover with SGW changed

S1 Based Handover

S1 Based Handover -II

The above are the scenarios I can think when it comes to handovers in LTE. However all the scenarios may not make any sense during initial LTE deployments. My guess is that we will mostly see the first scenario during the initial stages, followed by the fourth scenario. Considering the first scenario, it will be very important to see how many sessions can a MME and SGW/PGW can handle. What would be performance numbers of the devices? These performance numbers are what drives the number of devices. My guess would be 1 SGW should be able to take care of say a Million general users. If we consider corporate networks and etc it may go beyond 1. I have no clue this is just a guess. Can anybody throw some light in this direction?

Excuse me for just posting the figures, I am working on something and its taking longer than I expected. More to come soon.