LTE - MFBI

MFBI - Multi Frequency Band Indicator

EnodeB will inform UE about Multiple Overlapping bands it supports in the SIB1 message provided if MFBI is enabled in the EnodeB.

Consider an example of Serving Band 4 by an EnodeB in a particular Cell. The corresponding Overlapping Band in the FDD is Band 10.

Frequency info:

Band 4:

DL Bw: 2110 to 2155 Mhz
UL Bw: 1710 to 1755Mhz

Band 10:

DL Bw: 2110 to 2170 Mhz
UL Bw: 1710 to 1770 Mhz

So from the Serving BW's its clear that both the Band 10 & 4 are overlapping each other in a range of 45Mhz

Why MFBI?

Now a UE is trying to latch on the Band 4, But the UE wont support band 10, due to the enabling of MFBI, UE will unknowingly latch on to the Band 10 as well.

Overlapping Bands in FDD & TDD:

E-Utra Operating Bands	Overlapping E-Utra Bands	Duplex Mode
2	25	FDD
3	9	FDD
4	10	FDD
5	18, 19, 26	FDD
9	3	FDD
10	4	FDD
12	17	FDD
17	12	FDD
18	5, 26, 27	FDD
19	5, 26	FDD
25	2	FDD
26	5, 18, 19, 27	FDD
27	18, 26	FDD
33	39	TDD
38	41	TDD
39	33	TDD
41	38	TDD

PHY - PUCCH

PUCCH - Physical Uplink Control Channel

PUCCH carries uplink control information. PUCCH will never be transmitted along with PUSCH in the same subframe.

There are multiple PUCCH formats: (Source 3GPP spec 36.211 Rel 9)

We will see how PUCCH RB allocation happens in the upcoming blog updates.

RRC STATES

RRC STATES

RRC states in LTE are very simple. Its just 2 states as explained in the 3GPP spec 36.331

1. RRC_IDLE
2. RRC_CONNECTED

RRC_IDLE:

1. A UE specific DRX may be configured by upper layers.
2. UE controlled mobility;
    The UE:
     i. Monitors a Paging channel to detect incoming calls, system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;
     ii. Performs neighbouring cell measurements and cell (re-)selection;
     iii. Acquires system information
     iv. Performs logging of available measurements together with location and time for logged measurement configured UEs.

RRC_CONNECTED:

Transfer of unicast data to/from UE.

At lower layers, the UE may be configured with a UE specific DRX.
1. For UEs supporting CA, use of one or more SCells, aggregated with the PCell, for increased bandwidth;
2. Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC) to GERAN;

The UE:
1. Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system
information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification;
2.  Monitors control channels associated with the shared data channel to determine if data is scheduled for it;
3. Provides channel quality and feedback information;
4. Performs neighbouring cell measurements and measurement reporting;
5. Acquires system information.

You can find the RRC state change Diagram across LTE/UMTS/GPRS due to inter RAT handovers here:

Source: 3GPP Spec 36.331

DCI

DCI - Downlink Control Information

DCI is transmitted in the PDCCH. It tells the UE how to decode the PDSCH for getting the DL data transmitted to it in the same subframe.

There are many DCI formats which tells specific predefined downlink data formats to be used by UE.

So without decoding the DCI, UE will never come to know about the DL data sent to it by the eNodeB.

What DCI informs to UE?

UE will get the following informations from DCI after successful decoding of the DCI in PDCCH.

1. Number of RB's allocated (This will differ with channel Bandwidth)
2. Modulation & Coding scheme (MCS) used in PDSCH for the DL data.(QPSK or 16QAM or 64QAM)
3. Redundancy version - RV
4. New Data Indicator
5. HARQ process number
6. TPC command for PUCCH

Note: 
Incase of MIMO DCI formats like DCI 2 & 2A, 3 Transport blocks informations will be present in DCI for TB1 & 2. Each contails MCS, RV & NDI informations in it.

CRC added to diff CDI formats is of 16 bits & its scrambled with UE-RNTI.
After that 1/3 Rate matching will happen.

So DCI carries all those above data in it. One quick question which comes to the mind now is? How UE's will come to know whether the DCI is intended for a particular UE?

While the process of encoding, DCI will be padded with the CRC bits scrambled with the UE-RNTI. So, once the UE, descrambles the CRC of DCI, it will come to know whether the intended UE is that or not. If the UE-RNTI passed in DCI CRC not matches with the decoded UE, then UE will simply ignore it.

Different DCI formats:

There are as many as 9 DCI formats as of now in Rel8 3GPP. They are:

DCI - 0
DCI - 1
DCI - 1A
DCI - 1B
DCI - 1C
DCI - 1D
DCI - 2
DCI - 2A
DCI - 3
DCI - 3A

In those formats, DCI 0 is always used to indicate Uplink Data Transmission.

DCI formats 1 to 2A are used to indicate DL data transmission format in PDSCH

DCI formats 3 & 3A are used for Transmit Power Control information for PUCCH & PUSCH.

PHY - PDCCH

PDCCH - Physical Downlink Control Channel

PDCCH is the most important downlink channel which carries the control information to the UE's. It tells the UE's 2 most important things.

1. Where the Downlink Data is in the current subframe for the UE.
2. Which UE need to transmit the UL data now.

PDCCH is transmitted in PDCCH Quadruplets of 4 RE's each. So 9 PDCCH Quadruplets or 9 REG's comprises 1 CCE

So the number of PDCCH Quadruplets transmitted will differ based on 2 factors.

1. Channel Bandwidth
2. Number of UE's & amount of control information to be sent for each UE.

In general Higher the BW, higher the PDCCH Quadruplets usage. We will see more about the CCE's & DCI carried by the PDCCH in the upcoming blogs.

Important notes on PDCCH:

1. PDCCH will always be carried on 1st the symbol on every subframe.
2. Depending on the PDCCH format, the number of symbols used for PDCCH will change in every subframe.
3. PDCCH format 1, 2, 3 used for all the Channel Bandwidth's. In addition format 4 which uses the 4th OFDM symbol in a subframe is used only in the 1.4Mhz channel BW.
4. PDCCH always uses QPSK modulation.

PHY - PCFICH

PCFICH - Physical Control Format Indicator Channel

Physical channels carries Control signals as wells as Data signals. PCFICH is the channel which carries the control signals in DL.

PCFICH carries the control format indicator CFI, which will tell about the PDCCH usage. So without decoding the PCFICH, its not possible for the UE to decode the PDCCH & in turn PDSCH.

How PCFICH RE's are allocated?

PCFICH is carried in the 1st OFDM symbol of every Subframe in 4 REG's each having 4 RE's.

So total RE's in a PCFICH = Number of Resource Element groups * Number if RE's per group

                                           = 4 * 4 = 16 RE's in every Subframe

This implies, PCFICH will be carried on the 1st OFDM symbol of every subframe over 16 subcarriers in the frequency domain.

One important info to be noted down is, All the 16 subcarriers are not continuous. Lets see how the Resource Element Grouping made for PCFICH.

PCFICH Positioning:

PCFICH positioning can be identified by the UE, with the help of 2 parameters, which UE already knows about it.

1. CELL ID
2. Channel Bandwidth

Both the parameters UE's would have got it from the PSS & SSS decoding. Now its to make use of it & decode other channels in the treasure hunt.

3GPP Spec 36.211 talks about the PCFICH decoding.

The 4 REG's are mapped to 4 different values calculated from the above formula.

You can find the PCFICH resource allocation in the Grid for the Cell ID = 0 & Ch BW = 1.4 Mhz below:

Similary the REG spacing will differ for different bandwidth. PCFICH plays an important role in the Physical cell allocation of the live LTE network for better cell edge signalling & throughput. we will discuss more about it in the later blogs.

LTE - PSS & SSS

LTE - PSS & SSS

Primary Synchronisation Signal (PSS):

PSS is the signal which is first decoded by the UE to access the LTE Cell. Lets see how the PSS is decoded & where exactly the PSS is transmitted.

1. PSS is always transmitted on 2 slots in every frame.

Slot 0 & Slot 10 will contain the PSS. i.e., 1st slots of Subframe 0 & Subframe 5.

Where it will be tranmitted in the Slots? 

Its always on the last OFDM symbol of the slot. Which means, out of 7 OFDM symbols in a slot, 7th Slot will carry the PSS.

What UE will achieve by decoding PSS? 

1. Subframe synchronisation.
2. Slot synchronisation.
3. Symbol synchronisation.

All three in the Time Domain.

4. To identify the center of the Channel BW in  Frequency Domain,

5. To deduce the PCI from 3 available PCI's

Note: The PSS cannot be used to achieve radio frame synchronisation

Secondary Synchronisation Signal (SSS):

Once the PSS is decoded successfully the next task for the UE is to decode the SSS. So where SSS will come in Radio frame?

SSS will be carried in the 6th OFDM symbol of the Slot 0 & 10. Which means, like PSS, SSS also be tranmitted twice in a Radio Frame, exactly just before the PSS.

What UE will achieve by decoding SSS? 

1. Radio Frame synchronisation.
2. To deduce one of the 168 PCI groups.

So at the End of SSS decoding, UE will come to know the Complete Cell ID.

RE Allocation's for PSS & SSS:

 6 RB's are allocated for PSS & SSS irrespective of the Channel Bandwidth.

So Total RE's = 6 * 12 = 72 Resource Element's.

Out of this 72 RE's, 1st 5 & the Last 5 RE's are not used for Transmissions, i.e., DTX

In case of 1.4Mzh BW, only 6 RB's are present so PSS & SSS will occupy the entire BW.

What will happen if the Channel Bandwidth is 10 Mhz? Where PSS & SSS will occupy?

Since PSS & SSS will occupy the central channel BW, here it will occupy @ the center. IN 10Mhz BW, we have 50 RB's are present.

Total Subcarrier Freq's in 10Mhz = 50 * 12 = 600

So it will start from Subcarrier 264  & end at Subcarrier number 335. Remember the subcarriers starts with 0 to 500 here.

Pls find the Example Pic for PSS & SSS below.

PHY - PBCH

PBCH - Physical Broadcast Channel

PDCH is a downlink only channel. After the successful cell search, UE needs to decode the PBCH to know the few vital informations like

1. System Bandwidth   ------ 3 Bits
2. PHICH information ------- 3 Bits
3. System Frame Number (SFN) ---- 8 Bits

So total 14 Bits of Useful content. Along with that 10 reserved bits added upto 24 Bit Data, comprises a MIB in LTE

MIB will always be carried in the 4 OFDM symbols in the 2nd Slot of the every 10ms frame. So lets calculate the RE's allocated in each MIB in every frame.

Consider the following before we go for the calculations of MIB RE'S.

1. LTE is with FDD with Channel Bandwidth of 10 Mhz.

2. 50 Resource Blocks are supported in the 10Mzh channel Bandwidth.

3. Normal cyclic prefix is followed.

As we know very will, in LTE, 

1 Frame -- 10 ms

1 Subframe -- 1 ms

1 Slot - 0.5 ms

In a time domain, we have 14 OFDM symbols per RB

In Frequency domain, we have 12 Subcarriers per RB

A OFDM symbol in a Time domain mapped with a Single Subcarrier comprise a Resource Element, RE.

So Total available RE's in 1 resource block = 12 * 14 = 168 RE's.

So for a 10Mhz BW, total RE's = 168 * 50 = 8400 RE's

Since, MIB is occupying 4 OFDM symbols in the 2nd Slot in a Frame for 6 RB's, the total Resource Elements allocated for MIB is:

Total RE's for MIB = No of RB's * Number of Subcarriers * Number of OFDM symbols.

                                = 6 * 12 * 4

                                = 288 RE's

So in terms of Bits, how do we calculate? As we know PBCH uses QPSK modulation, it needs 2 Bits to transfer a symbol. 

So, Total number of Bits required to Transmit a MIB = Bits used for QPSK * Number of RE's

                                                                                     = 2 * 288 = 576 Bits

  One more Interesting thing about MIB transmission is about the Duplication Transmissions. Why do we need the Duplicate transmissions? If the UE decodes the MIB in the 1st Frame successfully, why the duplicate Transmission is required for the next 3 Frames?

Example of PBCH decoding for MIB:

LTE - Phy layer

PHY LAYER:

The most interesting part of the LTE is the PHY layer functionality. If any one asks how the bit stearms are carried in PHY layer? I say it by Channels!

The Phy channels posses interesting individuality in its own aspect. what are the channels present? There are many. Few works for UL control & Data, Few for DL control & Data.

Lets discuss about the DL channels 1st...

There are as many as 6 DL channels present in PHY.

1. PBCH
2. PCFICH
3. PHICH
4. PDCCH
5. PDSCH
6. PMCH

What about UL channels in PHY? They are as follows:

1. PUCCH
2. PUSCH

We will see each & every channel in detail in the upcoming pages...

LTE - Overview

LTE - Long term evolution came as a solution for the need of a higher data rates of the internet users. LTE will give a whooping data rate of around 300Mbps in Downlink & 150 Mbps in Uplink. Such a high data rate in DL & UL in our smart phones, tablets will leave us in a merry. But how about looking in to the things which makes it possible? Whats the engineering behind this high end technology? There is a vast change right from UE's to EnodeB's then to the core network elements.

Let's sperate the Topics of research to 3 different elements.

1. Access part
2. Core network
3. ISP & billing.

We will see parallely everyday all the 3 aspects of the buddling technology & its implementations day by day in the most easiest approach. Lets start the game - LTE...